Vol.24 No.2

Vol. 24, No.2, August 2013A Bulletin of the Indian Laser Association

Special Issue on

Nano-Texturing to Rapid Manufacturing using Lasers

Editor

Prof. Manoranjan P. Singh RRCAT,Indore

Guest Editors

Prof. J. Dutta Majumdar IIT, Kharagpur

Dr. C.P. Paul RRCAT, Indore

Editorial Board

Prof. A.K. Gupta SCTIMST,

Thiruvananthapuram

Dr. A.K. Maini LASTEC, New Delhi

Prof. S. Maiti TIFR, Mumbai

Prof. S.C. Mehendale RRCAT, Indore

Prof. V.P.N. Nampoori CUSAT, Kochi

Prof. B.P. Pal IIT, Delhi

Prof. Reji Phillip RRI, Bangalore

Prof. Asima Pradhan IIT, Kanpur

Prof. B.P. Singh IIT, Bombay

Prof. B.M. Suri BARC, Mumbai

Prof. C. Vijayan IIT, Madras

Editorial Committee (RRCAT, Indore)

Dr. C.P. Paul Dr. C.P. Singh

Mr. H.S. Patel Dr. S. Verma

Dr. G.J. Singh Dr. B.N. Upadhyay

Dr. Pankaj Misra Dr. S. Sendhil Raja

ILA Executive Committee Editorial Team of

Cover Photo (Left to Right) :

Top row: Cut surfaces of underwater laser cutting at two different parameters (page No. 19), online laser rapid manufacturing, laser rapid manufactured impeller (page No. 49).

Middle row: Laser clad surface (page No. 44), direct laser ablated surface (page No. 39), slurry eroded surface (page No. 47), cross-section of laser clad (page No. 34).

Bottom row: Laser drilled combustion liner, laser drilling setup, laser drilled nozzle guided vanes (page No. 12).

President

Prof. S.K. Sarkar BARC, Mumbai

Vice President

Prof. L.M. Kukreja RRCAT, Indore

Gen. Sec. I

Prof. P.K. Dutta IIT, Kharagpur

Gen. Sec. II

Prof. K.S. Bindra RRCAT, Indore

Treasurer

Dr. S. Verma RRCAT, Indore

Regional Representatives

Dr. S.K. Bhadra CGCRI, Kolkata

Prof. M.P. Kothiyal IIT Madras, Chennai

Prof. D. Narayana Rao Univ. Hyderabad

Prof. H. Ramachandran RRI, Bangalore

Dr. A.K. Razdan LASTEC, New Delhi

Web Committee

Chairman:

Prof. P.A. Naik RRCAT, Indore

Webmaster:

Mr. Rajiv Jain RRCAT, Indore

A Bulletin of the Indian Laser Association

Contents

Vol. 24, No. 2, August 2013

Page No.

From the Desk of Editor 1

From the Desk of Guest Editors 2

1. Applications of Laser Processing of Materials 3

G. Padmanabham and Ravi Bathe

2. Some Recent Material Processing Studies with High Power Fiber Laser 15

3. Recent Advances in Laser Microwelding 24

Swarup Bag

4. Refurbishment of AISI H13 Die Materials by Laser Cladding 33

5. Laser Assisted Nano-Texturing of Amorphous and Multicrystalline Silicon Wafers 38

for Photovoltaic Device Applications

6. Slurry Erosion Wear Characteristics of Laser Clad Surfaces 43

7. Laser Rapid Manufacturing: A Pursuit of Unorthodox Manufacturing 49

A.K. Nath, S. Mullick, Y.K. Madhukar, S.S. Chakraborty, K. Maji and D.P. Karmakar

G. Telasang, J. Dutta Majumdar, G. Padmanabhan and I. Manna

I.A.Palani and N.J. Vasa

Satish More and G.R.Desale

C.P. Paul, Atul Kumar, P. Bhargava and L.M. Kukreja

1

Laser material processing has come of age now and it is therefore

natural that many top research and development institutions in India

are pursuing this with a purpose to take it to the next level. This special

issue of Kiran is an attempt to take stock of the developments in this

very important area of research. We are grateful to Prof. Jyotsna Dutta

Majumdar and Dr. Christ P. Paul for agreeing to be Guest Editors of

this issue and doing a commendable job of getting articles from all the

leading institutes under a coherent theme of "Nano-Texturing to Rapid

Manufacturing using Lasers".

We are sure this issue will find appreciation from people in research as

well as in industry.

Manoranjan P Singh

From the Desk of Editor....

Vol. 24, No.2, August 2013

2

Despite formidable challenges, innovations in the field of laser

material processing are accelerating faster than ever and as a result,

amazing diversity has been witnessed during last few decades. This

special issue presents a glimpse of country's R&D activities in the area

of laser material processing at different academic institutes and

national laboratories. The present issue is an amalgam of vivid fields

spreading from nano-texturing to rapid manufacturing using lasers.

The issue starts with an article presenting extensive experience of the

close-to industry technology development with selected results for

automotive and power generation industries at Center for Laser

Processing of Materials (CLPM), Hyderabad. Next article presents

the applications of high power fiber lasers in advanced materials

processing, including cutting and underwater processing at IIT

Kharagpur. Enormous applications of laser processing in surface

engineering and repairing is brought together in separate two articles

from IIT Kharagpur and CSIR-National Chemical Laboratory, Pune.

The role of mathematical model in understanding the basic

phenomena of laser micro-welding process is introduced in the article

from IIT Guwahati. Improving serviceability and functionality of

components by laser surface nano-texturing, in particular, towards

efficient photovoltaic device development is also presented in the

article from IIT Indore and IIT Madras. Laser rapid manufacturing: a

pursuit of unorthodox manufacturing and its novel applications, being

developed at RRCAT Indore is briefly presented as last article of the

issue.

We congratulate all the authors and thank them for sending articles in

time for this special issue. We believe that the readers will find this

issue both interesting and informative.

Jyotsna Dutta Majumdar &

C.P. Paul

From the Desk of Guest Editors....


3

Laser based techniques have the following decisive advantages over conventional processing methods:

• Excellent beam control and easy conversion to automatic operation;

• No contact, zero force processing;

• No tool wear;

• Low thermal heat input on the work piece, due to very high energy density at the processing point;

• High processing speed combined with excellent reproducibility of the processing results;

• Allows processing of very hard, brittle, or soft materials;

• In combination with a suitable system technology, any desired/complex contour can be processed; and

• Easy integration into conventional manufacturing processes.

Depending on the laser power used and dimensions of the processed feature/component, laser processing is categorised as microprocessing and macroprocessing. Some of the established laser macroprocessing applications include, laser welded tailored blanks for automotive bodies, laser hardened steam turbine blades, laser-hybrid welded ship hulls, laser-welding of

Abstract

The Center for Laser Processing of Materials (CLPM) is a unique high-power industrial lasers-based R&D facility in the country, aimed at promoting and providing laser-based materials processing solutions for industrial application. Several laser based applications - surface engineering, welding, drilling, cutting have been demonstrated at the center in the recent past. Development experience and results of selected applications, including laser surface hardening of automotive compressor crankshaft; laser cladding of burner tip plates of thermal power plant boiler; laser deposition repair of turbo-machinery shaft; laser welding of tailor welded blanks; and laser drilling of aero-engine components are presented here.

Keywords: laser hardening; laser cladding; laser welding; laser drilling.

Introduction

Laser as a clean, spatially and temporally precise, intense heat source is extensively used as a manufacturing tool in several industrial sectors. Lasers processing of materials includes, manufacturing processes such as cutting, welding, cladding, surface hardening, drilling, machining, microprocessing, texturing, shock peening, marking etc. as shown in figure 1.

Applications of Laser Processing of MaterialsG. Padmanabham* and Ravi Bathe

Center for Laser Processing of Materials (CLPM), International Advanced Research Center for Powder Metallurgy and New Materials (ARCI),

Balapur PO, Hyderabad 500 005, India*E-mail: [email protected]

Fig. 1: Manufacturing processes due to laser materials interaction


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they are operations in the melting regime. Keyhole welding needs higher intensities as metal vaporization is required to form and sustain the keyhole. Laser shock hardening/glazing, drilling and ablation techniques require high intensities combined with short interaction times of the laser beam as vaporistation is the predominant phenomena. In lasers featuring continuous operation, variation of intensities may be effected by adjusting the power or by focusing laser beam; interaction times are varied either by changing the scanning speed of laser on the material or using pulsed power with different pulse duration. In pulsed lasers, variation of the power, duration, pulse energy, repetition rate, and pulse shaping, offer additional possibilities of tailoring a given material processing method to specific requirements on the material.

The Centre for Laser Processing of Materials at ARCI has been set up in late 1990s by the Government of India with an aim to demonstrate and promote application of laser based solutions in manufacturing. Currently three industrial laser systems: a continuous wave (CW) CO 2

laser (DC 035, Rofin; maximum power 3.5 kW), a pulsed Nd:YAG laser (JK704, GSI Lumonics, maximum average power ~ 400 W, peak power 20 kW), and a high power diode laser (LDF 1000-6000, Laser Line, maximum power 6 kW are available at our lab). All these systems are integrated with computer controlled CNC/robotic work stations. The possible processing regimes with these three lasers at CLPM are highlighted in the laser processing map shown at figure-3. Recently, the center has initiated efforts to extend its processing capability regime into ultra-short interaction time and very high intensities required for nano/micro processing by setting up ultrafast Ti-Sapphire micromachining laser facilities. The dotted lines in the figure 3 indicate these extended regimes of laser processing envisaged at CLPM. Based on the currently available processing capabilities, R&D was carried out at the center to address several industrial requirements. Each of the applications had certain processing and metallurgical challenges to be met for accomplishing the performance requirements. Some of the application development activities at CLPM are described in this paper.

Laser Hardening

The principle of laser hardening is similar to conventional hardening, wherein the material to be hardened is heated to a temperature above austenitisation and quenched to form martensite. In laser hardening a laser beam is scanned over the component / location to be hardened. Figure 4 shows the laser surface hardening process. At the beam-material interaction location, the temperature raises and as the laser beam spot moves away from the location, the heat is extracted out by self-

stiffeners to aircraft fuselage skin, laser drilling of cooling holes in aero engine components. Some of the typical and well-established applications of microprocessing are micro cut/micro welded medical implants (stents, catheters), slicing and edge isolation of solar cells, microlithography etc. The laser systems market has seen a steady growth over the years and with the advent of more robust and energy efficient lasers, the stage is set for a significant jump in their application in manufacturing. Industrial laser revenue in the year 2011 crossed the US $ 2 billion mark (figure 2).

The effect of laser beam on the work piece is dependent on the material, beam characteristics and the interaction time. Each of the processes requires the laser intensity and interaction time with the material being processed to be within a certain regime. In conjunction with materials processing, the values shown in figure 3 represent commonly used interaction times and laser beam intensities.

Relatively low intensities and long interaction times are required for laser hardening as a comparatively large material volume is to be heated to effect the metallurgical transformation and keep the process within heating regime. Higher intensities and slightly longer interaction times are required for joining and cladding processes as

Fig. 2: Industrial laser revenues a) Yearly revenues; and b) Sector-wise revenue in the year 2011 [1]

Fig. 3: Laser beam intensity and interaction time for the processing of materials and capability of lasers available at the CLPM


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coils of different configurations matching the geometry. Further, in order to correct the distortion, post-machining is carried out. The material used is low alloy high strength steel (0.52%C, 0.4%Cr) forged, hardened & tempered to 300-320 VHN followed by finish grinding. Desired requirements are: a uniform case depth with hardness of 550 – 600 VHN; minimal or negligible distortion; no surface melting; no post-processing; and high productivity. A single laser beam spot can be tailored to achieve the hardening at selected locations by scanning it in the identified locations using a robotic motion system or a CNC workstation.

A 6 kW CW fiber coupled diode laser integrated with 6-axis robot and a turn and tilt table (figure 6) was used for the purpose. The laser system is equipped with an Emaqs camera for temperature feedback from the processing zone and LompocPro software which regulates the laser power to keep the surface peak temperature at specified level. Initially, lab-scale coupons of the crankshaft material are laser hardened using a 17 mm x 2 mm

quenching action of the component bulk. Surface hardening of the component at a particular location takes place.

The nature of the hardened layer, viz., level of hardness, depth of hardness, width of hardness etc. are dependent on the steel composition, peak temperatures attained during the process, energy distribution in the scanning beam spot and the interaction time between the beam and surface. Almost all steels and cast irons are hardenable. Major advantages of laser hardening include, highly localized and precise hardening with minimal heat input, fine and hard microstructures due to high cooling rates, negligible distortion, no requirement of quenching medium, no surface damage and versatility in terms of processing a wide range of steels and profiles using automation. Due to these advantages, laser hardening has found a number of applications such as, steam turbine blades, torsion springs, press tools, gears, locomotive cylinder liners etc. Recently, laser hardening process has been developed at CLPM for a compressor crankshaft [2] as described below. Crankshaft is an important engine component. Even a slight surface wear can affect the engine performance. It is prone to wear at five locations (A, B, C, D and E in figure 5b)) viz., in the bearing seat areas, pin and flange areas. Conventionally, this is done by induction hardening in multiple steps using induction

Fig. 4: Laser surface hardening principle

Fig. 5: Typical crankshaft (A, B, C, D and E on schematic – locations to be hardened)

(a)

(b)Fig. 6: a) Fiber coupled diode laser system; and b) Effect of laser power (kW) and scan speed (mm/min) on hardening behavior of En18D steel

(a)

(b)


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roughness also measured. No distortion was observed and surface roughness changed very insignificantly as shown in figure 8. The laser hardened crankshafts were subjected to overload test specified by the user and were found to meet the performance requirements. In summary, the laser hardening process developed has demonstrated single setting hardening of all locations with requisite case depth, without distortion and damage to surface, making it suitable for productionisation.

The other laser hardening applications developed at CLPM are shown in figure 9. It is evident that the process is suitable for very thin section components like piston rings to very large components with complicated profiles like the cast iron sheet metal hemming bed.

Laser Cladding

Laser cladding is a process in which material to be coated is fused using a laser beam and deposited on a substrate. The laser energy also causes melting of substrate and a shallow melt pool is created on the substrate into which the molten coating material is deposited. The coating material can be fed in the form of powder, using a co-axial or an off-axis nozzle. This method is called the blown-powder method. In coaxial method, the powder and the laser beam are fed coaxially and the powder focus meets with the laser focus on the substrate where the powder melts and forms a clad. In off-axis method, the powder and the laser beam are fed in a certain angle with each other. Other methods include, pre-placing of the powder or feeding coating material in the form of a wire. Schematics and nozzles are shown in figure 10. As a very thin layer of the substrate gets melted and mixes with the molten coating material, good metallurgical bonding occurs, which is a major advantage of laser cladding process compared to thermal spray processes. As the heat input can be precisely controlled, the base metal dilution into coating material can be kept below 2%. Because of this the original properties of the coating material can be retained in one layer of coating itself. The process uses minimal heat input compared to other such coating processes such as plasma transferred arc and weld overlay processes. Hence, the distortion effects are minimal and the molten material experiences very rapid cooling and solidification resulting in fine microstructures. However, this may cause cracking when there is substantial mis-match in the thermophysical properties of the coating material and the substrate material and when some very hard phases are forming.

rectangular spot to identify the processing window in terms of power (P) and scan speed (V) to achieve the required hardness and case depth without melting. Figure 6 shows the effect of P/V ratio on hardening behavior.

At P/V of 125, melting of the surface occurred. As melting induces tensile stresses, they are not permitted in rotating components like crankshafts. Hence, a P/V ratio of 100 was chosen which yielded a hardness of 650-700 HV, throughout its depth of 375 microns. Residual stress measurements indicated compressive stresses in the hardened region. In terms of effect of geometry, when there are edges, hole-regions, corners etc. there can be undue rise of temperature due to heat accumulation resulting in melting, which is undesirable. Depending on the location of scanning, ramping up/ramping down of power to control the surface temperature was adopted. Using these optimum conditions laser hardening of the actual component was carried out. Typical cross section of the hardened case in the flange-bearing seat corner is shown in figure 7. With the help of robot programming, all the five locations could be sequentially hardened one after other in one setting. The laser treated crankshafts' run-out was checked using a dial gauge and surface

Fig. 7: Cross section of the laser hardened case

Fig. 8: Surface roughness profile a) before laser treatment; and b) after laser treatment

(a)

(b)

Piston ring Cam Shaft Hemming bedFig. 9: Laser hardened components using diode laser


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CLPM for power plant applications [3], which is described below. In power plants, degradation of various components like coal nozzle tips, boiler tubes, burner spreaders, etc. due to various modes of wear, erosion and corrosion, is a common problem leading to their replacement during maintenance schedules. Shutdown due to such problems severely affects the power production. Nozzle tip failures continue to be the most important one in coal fired power plants to increase the over-haul to over-haul service periods to two years. The pulverized coal slurry passing through the burner tip plates causes erosive wear. A typical burner tip and type of erosion it experiences is shown in figure 11. In addition, marginal corrosive attacks due to the presence of corrosive elements in the fuel, and exposure to

0temperatures as high as 900 C further deteriorate the components by severe wear resistance. The burner tips are fabricated from SS 310 steel and the surfaces are coated by weld overlay method. Improved life of these burner tip plates to two years will save substantial costs for the power producers.

Considering the wear conditions cermet powders viz., hard tungsten carbide particles in a tough nickel matrix was chosen (NiCrBSi + WC). Initial experiments were performed on lab-scale coupons of SS 310. Ni alloy and WC powders were fed separately using a twin powder feeder system. This enabled variation of amount of WC particles in the matrix material. 6-kW fiber coupled diode laser was used and the powder was fed through a 1.0 mm

0off-axis nozzle (45 ) into a 1.5-mm focused beam spot. Processing variables like laser power, scanning speed and powder-feed rate were varied to optimize the effect of process parameters on the single-track clads to produce 1.5 mm thick crack-free continuous clad of NiCrBSi with sound interface and minimal dilution possible. After optimizing NiCrBSi cladding, effect of addition of WC in the powder mixture for obtaining hard clad layers was studied. Figure 12 shows the microhardness profile of the

Sometimes, fast cooling may also result in entrapment of gases if any, and cause porosity. The aim in laser cladding is to achieve good quality clad layers viz., without cracks, without porosity, good metallurgical bond at the interface and low dilution of the coating material by the substrate. The main factors governing the process are the laser power, powder feed rate, beam spot shape and the interaction time between the beam and powder/substrate materials. Depending on cladding material, particle size, feeding method and coverage/geometry the amount of energy transferred and metallurgical reactions vary. The feeding nozzles and available power, decide the particle size. Several commercially available powders Fe-based, Co-based, Ni-based, SS and cermets are amenable for laser cladding.

Laser cladding, due to above mentioned advantages finds numerous industrial applications. Due to low deposition rates and the advantage of precision deposition, most of the early applications were of fine cladding such as valve seats of engines, turbine blades, die repair and correction. However, with the advent of high power lasers and good feeding systems the applications have expanded to heavy engineering as well such as earth moving equipment, shafts, and power plant components. Further, with availability of suitable processing heads, even internal bores are being cladded with the process.

Recently, laser cladding process has been developed at

Fig. 10: Laser cladding a) Coaxial powder feeding; and b) Off-Axis Feed in Laser Cladding

Fig. 11: Eroded boiler burner tip of thermal power plant


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some more experiments were conducted using laser power pulsing also to retain as many hard WC particles as possible and further improvement in wear performance was observed [4]. The wear plates were assembled in actual burner tip and after few months, the cladded plate was analysed for performance. The plate almost disappeared from the tip, with a few portions left. When closely investigated it was found that the plate cracked at the weld location and wherever the welding was intact, the clad performed well. Hence, it was concluded that appropriate welding methods while assembling the plate is important. In the second field trial, laser cladded plates were fabricated with optimized processing conditions with some compositional changes and assembled in the burner tip with due care during welding. The plates remained intact even after 15 months of service. The technology is now ready for implementation on production basis after further improvement of productivity.

Laser Clad Repair

Laser cladding technique, due to low heat input and possibility to deposit material in a precise manner enables repair of expensive engineering components. One such

clads made with different WC content. It can be seen that increase in WC content increases the hardness of the clad. This is due to presence of increased amount of hard WC particles in the matrix as seen in the cross sectional microstructures shown in figure 13. The hardness in laser-clad region varied between 750 HV and 1010 HV depending on the microstructural fineness and dendritic network of carbides of W and Cr as shown in figure-13. However, the clads need to be checked for erosive wear performance, as hardness alone is not sufficient to improve the erosive wear resistance.

Dry-sand erosion wear tests conducted on optimized laser-clad coating at high temperature (Figure 14), indicated improvement in erosive wear resistance of laser-clad coating as compared to weld-overlay coated counterparts when tested under similar conditions. NiCr-25% WC clads showed the best performance. Even though the 50%WC-NiCr clads showed higher hardness, the wear performance is not so good due to presence of porosity. Based on this laboratory erosion test results, NiCr-25% WC was used and actual plates were cladded and fitted into the burner tip for field testing. Further,

Fig. 12: Microhardness variation from top of the clad to base metal with varying contents of WC in the Ni-matrix

Fig. 13: Microstructure of laser clad of NiCrBSi on SS 310 plate with varying contents of WC a) 0%; b)10%; c)25% and d)50%

Fig. 14: Laser cladded layers a) Erosive wear performance; and b) Optimised laser cladded plate fitted in actual burner tip


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One of the most popular applications of laser welding is Tailor welded blanks (TWB). TWBs use two or more dissimilar blanks of steel to form one blank. The idea is to "tailor" the blank with different properties at different locations as per performance requirements. This concept of tailoring a blank with multiple steel sheets precisely where they are needed offered the designers, flexibility in design and to reduce vehicle mass and reduce total cost [6]. Consider the example of a car door inner. A single thickness blank with thickness sufficient to meet stiffness requirement at the hinge is formed into the component shape. However, the same thickness is not needed on the latch side. The TWB concept adapts a two material blank which has a thicker sheet towards the hinge side and thinner sheet towards the latch side. Fig.16 shows a schematic of fabricating a TWB for door inner. A 1.6 mm thick sheet is welded to a 0.8 mm thin sheet and sheet metal blank fabricated.

The benefits of using TWBs include: weight reduction; fewer parts, consequently fewer dies and spot welds; design flexibility leading to improved structural integrity, safety and dimensional accuracy; reduced design and development time; lowered manufacturing costs due to reduced and optimal material use. Tailor welded blanks are used for body side frames, door inner panels, centre pillar inner panels, and wheelhouse/shock tower panels. The TWB concept has been in application since 1990s in Europe, USA and Japan. Several companies around the world such as Thyssen Krupp A.G, Posco, including some of the automakers themselves such as Volkswagen etc., produce TWB forms in large scale. But, in India efforts towards development of TWBs and their usage started only in the recent past. The welding process for fabrication of TWBs is crucial as the blanks inspite of having a weld should exhibit formability similar to that of the base steels chosen. The welding process should be such that defect-free welds should be made with high repeatability and productivity. Even though several processes have been evaluated, laser beam welding and mash seam welding are preferred processes employed worldwide for the production of TWBs. Laser beam welding is the most preferred process due to high welding speeds resulting in narrow weld beads with high productivity.

In India, CLPM developed the laser welding process for joining of dissimilar steel/thickness sheets on a coupon

example, demonstrated at CLPM is an air-blower shaft used in turbomachinery. Figure 15 a) shows a shaft with worn out bearing seat area.

The shaft is made of high strength low alloy steel and is a very expensive component. The requirement is to repair the worn out area by deposition of suitable material with no porosity or cracks in the material. Stellite-6 was chosen and laser cladding was carried out on the worn out area (figure 15 b) after preparing the surface by machining. After cladding, the cladded region was machined to remove about 0.6 mm to obtain smooth surface as shown in figure 15 c). The cladded region was subjected to ultrasonic testing and it passed the requisite standards. The shaft has been then put into service and found to be performing well without any problems. Laser cladding technique has been successfully applied to reclaim an expensive component at a fraction of its cost due to advantages like excellent metallurgical bond between the coating and the substrate, no distortion and defect free and refined microstructure coatings. Several other components can be successfully repaired using laser cladding such as, diesel engine cylinder heads, die tools, bladed disks of aeroengines etc.

Laser Welding

Laser welding and laser assisted joining techniques find application in several sectors including automobile, electronic, energy, aerospace, ship-building and oil pipelines. Out of all the laser materials processing systems sold worldwide, 12% of them were laser welding systems [5]. With the advent of higher power and robust lasers the application domain is only expected to increase further. Conventionally, laser welding means autogenous laser keyhole welding, which makes use of high energy density available with laser beams. Laser keyhole welding yields deep and narrow fusion zone with aspect ratios 10:1 and can weld thicker sections also with one-side accessibility. As the weld zone is small, shielding is easy. High welding speed, simple joint design, low heat input, small heat-affected zone, low thermal distortion are other major advantages of laser welding.

Fig. 15: Air-blower shaft a) worn out bearing seat area; b) Laser clad repair in progress; and c) Laser cladded bearing seat area in the as-cladded and post machined condition

Fig. 16: Schematic of tailored blank fabrication and pressed door inner from a TWB


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the beam accurately runs in the middle of the edges of the two sheets to be joined. IF steel of 0.8 mm sheet thickness was successfully welded to DP 590 and SPC 440 steel of 1.6 mm autogenously at a welding speed of 5 m/min and the blanks were subjected to Erichsen cup testing. This is a qualitative test to ascertain the integrity of the weld and the change in formability. A typical TWB weld cross section, cup tested specimen are shown in figure 18 a) and b). Subsequently, specimens made with optimised parameters are subjected to extensive formability testing viz., limiting dome height test (LDH) and formability limit curves (FLC) were generated as shown in figure 18 c). It is clear from the figures that good welds with formability limits in the range of two base steels could be achieved.

Based on these results it is now possible to productionise this TWB fabrication technology using a suitable welding system integrated with weld process monitoring tools such as seam tracker, weld geometry monitors and weld defect monitors. A very cost effective system can be built for this purpose.

The materials that are widely applied for making TWBs include: C-Mn steel, Extra Deep Drawable steel (EDD steel), Interstitial free steel (IF steel); High strength Low alloy steels (HSLA); Bake Hardenable steel. In the recent past advanced high strength steels (AHSS) such as Dual Phase (DP) steels, Transformation Induced Plasticity Steels are also being considered for the TWB application. However, laser weldability of these alloys is being actively investigated at CLPM. [7-9].

Other laser welding applications developed at the CLPM include hermetical sealing of solenoid valve (magnetic and non-magnetic material joining); fast response digital thermometer assembly, SAW sensor button to flex plates;

level for further use in fabrication of actual blanks. This was done under a sponsored project of Core Group on Automotive R&D (CAR) programme. Accomplishing consistently good quality blanks requires development of suitable welding procedures. For example, during experimentation it was found that preparation of edges of the sheets to be welded proved to be critical in obtaining defect free welds. The effect of edge preparation on the defect levels in the laser welds of TWBs is shown in figure 17

Process optimisation also includes, laser power, spot size, welding speed and integration of seam tracker to see that

Ground edges and weld joint

Sheared edges and weld joint

Fig. 17: Effect of edge preparation on defects in laser welded TWBs

Fig. 18: TWB coupons a) weld cross section; b) cup tested specimen; and c) Formability limit curves


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events in the context of laser drilling are schematically shown in figure 20 a).

The energy required to remove material by melting is about 25% of that needed to vaporize the same volume, so a process that removes material by melting is often favored. Whether melting or vaporization is more dominant in a laser drilling process depends on many factors, with laser pulse duration and energy playing an important role. Generally speaking, ablation dominates when a Q-switched Nd:YAG laser is used. On the other hand, melt expulsion, the means by which a hole is created through melting the material, dominates when a flash lamp pumped Nd:YAG laser is used. A Q-switched Nd:YAG laser normally has pulse duration in the order of nanoseconds, peak power in the order of ten to hundreds

2of MW/cm , and a material removal rate of a few micrometers per pulse. A flash lamp pumped Nd:YAG laser normally has a pulse duration in the order of hundreds of microseconds to a millisecond, peak power

2in the order of sub MW/cm , and material removal rate of ten to hundreds of micrometers per pulse. For machining processes, ablation and melt expulsion typically coexist. A schematic of features of laser-drilled holes is shown in figure 20 b). Barrelling is the effect of energy trapped inside the workpiece to form a cavity, the formation of the barrel can guide the ejected material as it passes through the hole, forcing the molten material around the hole to come away from the sides. Resolidified material indicates the amount of material that had vaporized or melted during drilling, but had not escaped from the hole and so had resolidified on the internal surface. Taper is the measure of overall taper of the hole sides. The surface debris is an assessment of the amount of resolidified materials appearing on the surface of the hole. In terms of process technology, four different types of laser drilling may be distinguished: single pulse drilling, percussion drilling, trepanning drilling and helical (twist) drilling. The simplest way is to removal of material through a single laser pulse, the hole is created in a single laser pulse. This technique is mainly used for drilling narrow (< 1 mm) holes through thin (< 1 mm) plates. The drilling process in which the laser operates in a repeated manner, with short pulses, is called laser percussion drilling. In this way the laser builds up energy and operation in this manner allows for large bursts of energy and material is removed from the hole as vaporized material and as an ejection of the liquid melt. This melt is ejected up the sides of the walls of the hole, driven by the vapor pressure, which develops within the hole. It's a lot harder to control hole quality using this method. Probably the most popular is trepanning, used to drill wider (< 3 mm) holes in plates (< 10 mm). This is actually a cutting technique. The laser beam pierces the work piece just inside the perimeter of the hole and then track outward to

Lithium-ion battery casings etc. Some of them are shown in figure 19.

One of the recent variants of laser based joining is the laser-arc hybrid welding process. This process combines the advantages of laser welding and arc welding. For example, the deep penetration capability of laser and the edge-bridging and filler addition capability of MIG arc. At CLPM, a laser-MIG hybrid welding system has been built in 2010 and the process developed for single-pass welding of thick plates of mild steel (12 mm thick), 9 Cr- 1 Mo type and other ferritic-martnesitic steels used in fusion reactors [10]. The process is now being tried on thick section Ni-based alloys for super-critical boiler applications and maraging steels.

Laser drilling

Laser drilling of holes generally occurs through melting and vaporization (also referred to as "ablation") of the work piece material through absorption of energy from a focused laser beam. Melt expulsion arises as a result of the rapid build-up of gas pressure (recoil force) within a cavity created by evaporation. For melt expulsion to occur a molten layer must form and the pressure gradients acting on the surface due to vaporization must be sufficiently large to overcome surface tension forces and expel the molten material from the hole. Some of these

Fig. 19: Laser welding applications developed at CLPM

Fig. 20: Laser drilling a) Schematic of the physical effects; and b) features of drilled holes


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in pulse duration for TBC/IN718 material. Based on the above studies laser drilling process for aeroengine turbine components such as combustion liners, nozzle guide vanes and shroud segments was taken up. In these components, often the holes are drilled at different angles, in large numbers through the thermal barrier coating on the substrate. Systematic investigation of the process behavior during percussion drilling on IN718 and TBC/IN718 materials was carried out. The penetration depth is more in case of bare superalloy as compared to TBC coated superalloy drilled at same parameter. This indicates that the drilling process is significantly more efficient when drilling from the IN718 side than TBC side. In holes drilled normal to the surface of the material, minimal delamination is observed. In contrast holes drilled at a higher angle of incidence reveal a substantial increase in delamination at top coat/bond coat interface. In precision drilling, along with the beam intensity the focal plane position of the laser beam has a significant effect on the resultant hole geometry.

Figure 23 shows laser-drilled components of gas turbine at CLPM. In order to efficiently manipulate the

the circumference. Then either by rotating the work piece or the laser beam a hole is cut out to the correct diameter. This technique can produce high quality holes. The roundness and hole variation are good as good as CNC control. The taper of the hole can also be of a reasonable quality. Helical drilling is special version of trepanning drilling. In this case, in addition to the x-y circular motion, the focus position is shifted inside the work piece, describing helical path. The four techniques are depicted schematically in figure 21.

Laser drilling offers techno-economic advantages over other drilling techniques. A wide range of materials can be laser drilled, as the hardness of the material does not influence this non-contact technique and the electrical conductivity, too, is not a concern. Drilling through these TBC/Superalloys by conventional drilling or punching methods is very difficult. Therefore, turbine engine manufacturers are forced to look at alternative drilling methods. Although laser drilling is gaining widespread acceptability, high variability in properties and quality due to very complex phenomena involved in the laser drilling process is a concern. Therefore, it is very important to investigate these phenomena in order to control and optimize the overall hole quality in the final product. Major concerns in laser drilling are hole taper, circularity, spatter formation, barreling, micro-crack, delamination (in case of layered structure), recast layer, heat affected zone, and surface debris. Control of the drilling process for given materials is accomplished through appropriate selection of various parameters such as beam energy (power), pulse width, pulse repetition rate, focal spot size, focal position and assist gas and its pressure. Variations or improper setting of any of the above parameters will result in unacceptable hole quality. Several investigations have been conducted at CLPM on laser drilling of important materials such as superalloys and thermal barrier coated (TBC) superalloys (used in hot sections of gas turbine engine) [11-13]. Figure 22 shows the entrance hole diameter as a function of laser power density at various pulse widths (0.5, 1, 2, and 3 ms). As the pulse duration increases, significant changes in diameter were observed as a function of power density. Also as the power density increases, the hole diameter was found to increase at constant pulse duration. Interestingly, it was observed that at constant flux, the hole diameter increases and taper decreases with increase

Fig. 21: Schematic of the different laser drilling processes

Fig. 22: Influence of Pulse width on Hole Diameter and Taper angle

Laser drilled combustion liner

Laser drilling of Nozzle Guided Vane

Laser drilled Deflector Plates

Fig. 23: Laser drilled component at CLPM

Laser drilled Shroud Segment


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burrs and cracks, changes the morphology etc.) occurs during the process. In this context, femtosecond pulses offer great advantages over the nanosecond and picosecond pulses in their ability to deposit energy into a material and remove or modify it in a very short time, before thermal processes originate. As a result – the heat affected zone is reduced significantly. The ultrafast lasers essentially vaporize matter without generating heat (“cold” ablation). The energy deposition occurs on a timescale that is short compared to atomic relaxation processes. Also, the intensity of a femtosecond pulse is high enough to drive highly non-linear absorption processes in materials that do not normally absorb. At

13 2higher intensities (typically > 10 W/cm ) of femtosecond lasers, multiphoton ionization becomes significantly stronger. Also the high peak intensities of femtosecond pulses, new kinds of laser-matter interactions become possible. Smaller feature sizes, greater spatial resolution, and better aspect ratios can be achieved. With these advantages over conventional laser microprocessing, a wide variety of applications have opened up for femtosecond laser processing.

In the recent past, several demands for laser microprocessing have come from Indian industry as well as R&D institutions in view of this at CLPM, it has been targeted to build up capabilities to process metals, semiconductors, polymers, and ceramics in bulk or thin film form with feature sizes ranging from 10s of micrometers to sub-micron. Accordingly, a high pulse energy ultrafast laser system has been conceived and being developed jointly with National Research Council (NRC) of Canada under a collaborative agreement. The system is built around Ti:Sapphire regenerative amplifier laser source, with shorter pulse duration (<120 fs); high average power (> 12 W) and pulse energy (1.2 mJ) at high repetition rate (10 kHz). It is integrated with a high resolution object positioning system (XYZ linear stages, AB tip/tilt stages) synchronized with galvanoscanners and a uniquely designed wavelength selector unit. Ultrafast variable attenuator enables full control in space, time and energy domains, allowing fabrication of difficult geometries with sub-micron resolution and repeatability. Also it is built with multiple beam delivery systems, which consists of four beam paths: 800 nm, 400 nm, 266 nm and pump laser beam (532 nm) with flip-mirrors arrangement which enables changing of beam path easily. Workstation vision system composed of high resolution CCD camera is synchronized with linear stages and gives vision for whole system, allowing acquiring position, shape or any feature of the object and adjusting the beam. Overall it has been conceived as a versatile system for micro/nano machining research and production. Feasibility studies can be carried out to solve unique micro/nano machining needs. Some of the R&D

complicated geometry components accurately under the stationary laser beam, several fixtures have been developed. The nozzle guided vanes as well as the combustion liners have passed all the required tests before usage in the actual machine.

Ultrafast Laser Micro/Nano Processing

Laser microprocessing is one of the most flexible manufacturing technologies to create features in the sub-micron sizes. Its ability to accurately and reproducibly produce structures in a wide range of materials makes it an indispensible technology in a wide array of applications such as, microcutting of cardiac stents, microdrilling of PCB vias, microscribing of silicon/thin film solar cells, microlightography of electronic chips, microscribing/cutting of electrodes in flat panel displays, surface microtexturing of automotive engine parts, precision hole drilling in fuel injection nozzles, microfabrication of MEMS devices etc. The application domain of laser microprocessing is increasing steadily with more advanced ulltrafast lasers with millijoule/ femtosecond combination at kHz repetition rates are becoming commercially available. By the year 2015, the laser micromachining market size is expected to be $725 M [14]. In laser micromachining, material removal is by ablation, which relies on strong absorption of laser photons by the material being processed. Ablation occurs by vaporization, molecular dissociation and/or ionization depending on the wavelength, pulse duration and fluence of the laser beam. A broad range of lasers is currently employed for laser micromachining such as carbon dioxide, solid state (Nd:YAG and Ti:sapphire), copper vapor, fiber, diode and excimer lasers. Most common wavelengths for microfabrication applications range from 1064 nm (fundamental Nd:YAG) to 248 nm (KrF excimer) and those provided by frequency doubled and tripled Nd:YAG (532 nm and 355 nm) and Ti:sapphire (800 nm) lasers. Two fundamental laser micromachining techniques exist, direct writing and mask projection. Direct writing method uses a focused beam as a pen to write structures on the material as it is moved over the surface. To achieve good results, a high spatial coherence from a Gaussian TEM beam is required to produce a 00

very small spot size of a focused beam. Typically Nd:YAG and Ti:sapphire lasers are employed in direct write machining. In contrast, mask projection makes use of multimode, spatially incoherent beams such as those of excimer lasers. Beam shaping and homogenizing is usually required prior to any mask plane, with a projection lens reducing the mask pattern to the required size. Conventionally, lasers with pulse durations in the range of nanoseconds to microseconds are used. However, the level of precision and quality is limited due to thermal and mechanical damage (melting, formation of


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3. S.M. Shariff, M. Tak, S. Shanmugam and G. Padmanabham, Laser Surface Hardening of Crankshaft, SAE 2009-28-0053, Proc. Int. conf. Surface Modification Technologies (SMT-23), Chennai (2009).

4. M. Tak, S.M. Shariff, V. Sake, G. Padmanabham, stProc. 31 Int. Congress of Laser & Electro optic

(ICALEO), 515 (2012).

5. D.A. Belforte, Industrial Laser Solutions for Manufacturing, p. 8, (2007),

6. Mombo-Caristan, J-C et al., The Industrial Laser Handbook, Springer-Verlag, New York, p 89 (1992).

7. B. Shanmugarajan, J.K. Sarin Sundar and G. Padmanabham, Laser Welding of Advanced High Strength Steels for Tailor Welded Blank (TWB) Applications, SAE 2009-28-012

8. G. Padmanabham, Y. Krishna Priya and B. Shanmugarajan, Proc. ASM Int. Conf. on Materials and Manufacturing Technologies Pune (2011).

9. K.V. Phani Prabhakar, Venkateswaran Perumal, M. Shome and G. Padmanabham, Int. Symp. on Joining of Materials (SOJOM-2012), Welding Research Institute and the Indian Welding Society, BHEL-Trichy, India, 19-22 January (2012).

10. G. Padmanabham, B. Shanmugarajan and K.V. Phani Prabhakar, Indian Welding Journal 45, 29 (2012)

11. S Nirmala, R. Bathe and A.S. Joshi, Lasers in Eng. 17, 361 (2007).

12. S. Bandyopadhyay, H. Gokhale, J.K. Sundar, G. Sundararajan and S.V. Joshi, Opt. Lasers Eng. 43,163 (2005)

13. S. Bandyopadhyay, J.K. Sundar, G. Sundararajan, and S.V. Joshi, J. Mat. Proc. Tech. 127, 83 (2002)

14. The worldwide market for lasers market review and thforecast 2012 report, Strategies Unlimited, 5

Edition (2012).

activities planned with this system include:

• Surface structuring: The microstructuring of surfaces is interesting for several applications. One example is the structuring of cylinder walls in combustion engines. Small cavities are serving as a reservoir for the oil, preventing a breakdown of the oil-film. This results in significantly reduced particle emission.

• Sub-micron material processing: Precision hole drilling, cutting and milling (Laser Scribing of transformer steel (cold-rolled grain oriented steel (CRGO)), Laser Drilling of Fuel Injector Nozzle, Laser cutting/drilling of Nb-alloys, Ti-alloys, Ni-alloys, Si, Glass, GaAs, etc.)

• Displays and solar: Solar cell edge isolation, P1-P3 processing, thin-film ablation

• Photonics devices: Machining of optical waveguides in bulk glasses or silica, and inscription of grating structure in fibers

• Microfluidics: Microfluidic channels and devices

Summary

The technological processes addressed at CLPM are: laser surface engineering including hardening, cladding and alloying; laser deposition based repair and reclamation of metallic components; laser welding, including autogenous and laser-arc hybrid welding; and laser drilling. Based on the understanding and expertise developed in the past few years a number of applications have been successfully developed and are ready for know-how/technology transfer to industry. Laser microprocessing using ultrafast processing capabilities also are being built up currently.

Acknowledgments

Several people contributed to the work described in this paper. The authors gratefully acknowledge Dr. G. Sundararajan, Dr. S. V. Joshi, J K Sarin Sundar, S. M Shariff, B. Shanmugarajan, Manish Tak, K. V. Phani Prabhakar, S T Gururaj and E. Anbu Rasu for inputs/contributions. Thanks are due to research fellows, project students and trainees of the center.

References

1. D.A. Belforte, Annual Economic Review and Forecast, Industrial Laser Solutions, p4 (2011).

2. S.M. Shariff, M. Tak, S. Shanmugam, G. Padmanabham, Laser Surface Hardening of Crankshaft, SAE 2009-28-0053, Proc. Int. conf. Surface Modification Technologies (SMT-23), Chennai (2009).


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surface hardening, alloying, cladding, and texturing; metal forming and rapid manufacturing.High power CO 2

lasers and Nd:YAG lasers have been the workhorse for these applications. The advent of high power fiber laser with its better beam quality, higher efficiency, relatively compact size, and virtually maintenance-free operation is bringing change in this scenario. Because of its wide applications in manufacturing a 2 kW Fiber laser integrated with a 5-axis CNC workstation was procured and commissioned in the Mechanical Engineering Department, IIT Kharagpur in 2009 with the funding from DST, Government of India under FEST programme. Since then this has been used for carrying out undergraduate and postgraduate laboratory experiments and various material processing research studies. Figure 1 shows the laser system along with the CNC workstation. Some of the material processing studies carried out include the cutting and drilling of carbon reinforced polymer sheets, cutting of thick metal sheets with the objective of improving cut quality, welding of dissimilar materials and dissimilar thickness, metal forming, paint stripping, surface hardening, alloying, cladding and texturing [1-6]. This has been also used to develop a novel water-jet assisted underwater cutting process [7]. In this article a brief over-view of some of these material processing studies is presented.

Laser surface hardening (LSH) [1,2]

LSH gained popularity because the process is very selective and does not require any external quenching. However, the main limitation of this process is the depth of hardening that can be achieved. The onset of surface melting does not allow increasing either the laser power

ABSTRACT

High power fiber laser because of its excellent beam quality, high efficiency, and versatility is finding wide applications in materials processing. Several laser material processing modalities such as surface hardening, metal forming, underwater cutting and paint stripping have been investigated using a 2 kW fiber laser. It has been established through theoretical and experimental studies that depth of hardening can be increased by repetitive laser pulse irradiation. Parametric study of laser forming of curved surfaces and process optimizations were carried out experimentally, and process modelings were done by Finite element method, statistical regression analysis and soft computing techniques. While correcting the angle in pre-bent AISI 304 stainless steel samples though temperature gradient mechanism by laser forming, it was found that laser irradiation at convex surface produced relatively higher bending angle than that at concave surface. A novel underwater laser cutting process using water-jet as an assist instead of gas-jet to remove molten material was developed. This process does not produce much turbulence in water and aerosols in surrounding environment, and therefore, will be attractive for underwater cutting of radioactive materials. An online underwater laser cutting monitoring system based on acoustic signal has been also developed. Water-jet assisted laser processing has been extended to remove paint, and compared to conventional laser removal process which usually leaves ash on the surface; this removes paint completely without any trace of paint or ash. A brief account of these processes is presented in this article.

Keywords: fiber laser; surface hardening; metal forming; underwater cutting; acoustic signal.

Introduction

Laser is a modern, efficient and elegant tool of high precision and power which is finding ever-increasing applications in physics, chemistry, biology, medicine, defence and all branches of engineering. High power lasers are being used regularly in many manufacturing industries to process a wide variety of materials in many interesting ways like cutting, drilling, welding, scribing,

Some Recent Material Processing Studies with High Power Fiber Laser

A.K. Nath*, S. Mullick, Y.K. Madhukar, S.S. Chakraborty, K. Maji and D.P. KarmakarMechanical Engineering Department, IIT Kharagpur, Kharagpur-721302, India

*E-mail: [email protected]

Fig. 1: (a) 2 kW Fiber Laser coupled with a (b) 5-axis CNC workstation


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or the laser-material interaction time to enhance the hardening depth. It was reported that the depth of hardening can be increased by repetitive pulse hardening [8]. A detailed analysis was carried out solving analytically the one-dimensional heat conduction equation for temperature variation during the heating and cooling cycles in repetitive laser pulse (RLP) irradiation, and the effects of pulse on-time, pulse off-time, number of laser pulses, beam diameter and laser peak power on the temperature distribution and depth of hardening were studied. Eq. 1 is the analytical solution of 1-D heat conduction equation derived for the temperature rise during heating with n number of laser pulses in RLP irradiation [1,2].

(1)

Similarly, the temperature during the cooling cycle after ththe (n+1) laser pulse can be given by eq.2,

(2)

Figure 2a and bshow the typical temperature profiles measured experimentally with a noncontact IR temperature sensor and calculated using Eq. 1 & 2. As the temperature signal of IR sensor depends on the emissivity of coated surface and measuring arrangement, instead of comparing the absolute peak values of the measured and calculated temperature profiles, the depth of temperature modulation during the cyclic heating were compared, which agreed reasonably well.

Investigation was done in a wide frequency range starting from 1 Hz to 1000 Hz. It was seen that the average heating rate reduced and the soaking time, for which a surface layer is maintained above the phase transformation temperature, increased significantly in case of RLP heating compared to CW heating for a constant laser power density. This facilitated homogenization of austenite and increase in the depth of hardening. Typical experimental results of micro-hardness profiles along depth obtained in AISI 1055 steel specimens in CW and repetitive pulse modes, and comparison with theoretical results are presented in Figure 3. It was established that the depth of hardening increased with the number of laser pulses incident at low repetition frequency in spot

2Fig. 2: Surface temperature for laser intensity=10kW/cm , laser beam diameter=3mm, 100Hz, DC=50%; (a) experimentally monitored by a non-contact IR temperature sensor (b) calculated temperature profile at surface and 0.5mm depth

Fig. 3: Measured microhardness profiles and comparison with predicted profiles at (a) different CW laser powers, laser scan speed=18mm/s, (b) different peak laser powers, repetition frequency=100Hz, duty cycle=50%, scan speed=12mm/s

(a)

(b)

(a)

(b)


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was studied [10]. Figure 5 shows the focused laser beam propagation through water. Absorption and scattering of laser beam at the focal volume is apparent.

The absorption of laser power depends on water temperature, reducing with increasing water temperature. At high laser intensities the absorption becomes nonlinear, i.e. it depends on laser intensity. The nonlinear absorption coefficient can be expressed as [10].

where, P is the laser power, L (=L +L ) is the total water 0 1 2

column height, L is the focal point distance from water 1

surface, L is the focal point distance from the painted 2

surface, ω and ω are the beam waists at water surface and s 0

focal plane respectively, α is the linear absorption L2coefficient, M is the laser beam quality factor, and λ is the

laser wavelength (1.07µm). The value of α , β and γ were L-1 -6experimentally determinedas 0.135cm , 2.5 × 10 cm/W

hardening, Figure 4B [1].

Underwater Laser Cutting

Underwater laser processing, viz. cutting, welding, cladding, and shock peening are often used in the maintenance, repairing and dismantling of nuclear reactor components. In underwater laser cutting usually a high pressure gas jet is used to displace water from the process zone and cutting is done in the dry condition [9]. The high pressure gas jet produces very high turbulence in water and a lot of aerosols as it bubbles out of water. In case of cutting radioactive materials the gas can carry some amount of radioactive particles along with it as aerosols and cause contamination in the surrounding atmosphere. In order to minimize the turbulence in water and aerosols emerging out of water, a novel underwater laser cutting technique was developed in which a high pressure water-jet is used coaxially along with a high power laser beam to remove the molten material from the cutting front [7]. The water-jet reduced the turbulence in water considerably; however it introduced some convective heat loss in the cutting process.

Measurement of absorption of laser power in water [10]

In order to design underwater laser cutting head, the absorption characteristics of focused laser beam in water

Fig. 4: (A) Measured microhardness profiles for different number of laser pulses (B) Optical macrographs of laser surface hardened zone (a) single laser pulse (b) 3 laser pulses and (c) 5 laser pulses; Laser power =550W, t =200ms, t = 400mson off

Fig. 5: Absorption and scattering of the focused laser beam in water

Fig. 6: Variation in absorption coefficient with (a) water temperature, water column height = 40mm, laser power = 345

−4 −1W, Eq. (1) (linear curve fitting), α = (0.308−5.7 × 10 T)cm , −1Eq. (2) (semiempirical), α = 52.8/[T(1-exp(−360/T))]cm ; (b)

laser power for unfocused and focused laser beam


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Various materials such as AISI 304 steel sheets up to 2.5 mm thickness, mild steel sheets up to 1.2 mm thickness and aluminum sheet up to 0.5 mm thickness were cut at 1800 W CW laser power. Due to large spherical aberration of lens of F no.=1, the minimum laser beam focus spot diameter was nearly 600 µm, which limited the cutting performance in the present experimental setup.

Figure 8a shows the variation of maximum cutting speed with laser powerfor a constant standoff distance (SOD) of 1.0 mm and water-jet speed of 18 m/s for different sheet

3thickness. The specific energy, S (J/mm ) which is a measure of process efficiency is also plotted in Figure 8a. The specific energy is defined as the laser power required for removing a unit volume of material and is estimated using the following relation:

(1)

Here, P is the incident laser power, vis the cutting speed, L

and t and w are the sheet thickness and kerf-width respectively.

The maximum cutting speed increased with laser power at constant water-jet speed and SOD, and the rate of increase is more for thinner sheets, Figure8a. The specific energy was also high at the lower range of laser power and it decreased sharply and then became constant with the increase of laser power. Specific energy was higher for thicker sheets; however, as laser power was increased the specific energy for different sheet thicknesses decreased and tended to converge. It is expected that the maximum cutting speed at which the specimen is just separated out, will increase with laser power. The decrease in specific energy and its convergence to a near constant value indicates that the cutting efficiency improves with the increase of laser power and corresponding increase in cutting speed. This could be because of the reduction in heat loss at higher cutting speeds.

-8 2and 1.65 × 10 cm /W respectively [10]. The variations of absorption coefficient on water temperature and laser power are shown in Figure 6a and b respectively. The predictions by theoretical models are also shown in Figure 6a. While designing the underwater laser cutting head the length of water column through which laser beam has to travel before striking the work-piece surface was kept at a minimum.

Water assisted underwater laser cutting[6]

A water-jet assisted underwater laser cutting experimental setup consisting of a underwater laser cutting head, and a high pressure water flowing system through cutting nozzle was developed and placed on 5 axis CNC machine to which high power fiber laser beam is delivered. The design of the underwater laser cutting head is schematically shown in Figure 7a. This incorporates a plano-convex lens of 25 mm diameter and 25.4 mm effective focal length, an optical window and a nozzle of 1.5 mm orifice diameter.

Figure 7b shows the underwater laser cutting process in a 1 mm thick steel sheet with water-jet assist at ~18 m/s water-jet speed and 1800 W laser power. The gentle nature of the cutting process is evident from the relatively short length of melt ejection shower compared to what is usually observed in a gas-assisted underwater laser cutting process. For a comparison the gas-assisted underwater laser cutting process is also shown in Figure 7c,which was carried out by replacing water with high pressure N gas. 2

Fig. 7: (a) Design of the underwater laser cutting head; Photograph of the (b) water-jet assisted underwater laser cutting (c) gas assisted underwater laser cutting of 1 mm thick stainless steel sheet


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From the experimental study it was observed that the underwater laser cutting performance are similar to the conventional gas assisted laser cutting characteristics; however, the convective heat loss by water-jet reduces the process efficiency. The process efficiency improves with increasing cutting speed at higher laser powers. Since this process produces less turbulence and gas bubble than the gas-assist underwater laser cutting, this can be attractive for underwater processing of radioactive components.

Development of a modified water nozzle [11]

A modification was done with regard to the inlet water flow into the cutting head to increase the stable water jet length. Numerical simulation of flow stream lines through outlet nozzle for vertical and horizontal water inlets revealed that the former configuration produced parallel stream lines at the nozzle outlet that supported longer cylindrical water-jet than the later one, which produced diverging streamlines, Figure10a and b. Due to the geometrical constrains, vertical water inlet was not feasible to incorporate, therefore the outlet nozzle design was modified such that the water entering through horizontal inlet turns around to flow out in vertical direction, along the outlet axis. Figure 11a and b show the original nozzle and the modified nozzle respectively. The simulated water streamlines at nozzle outlet of modified nozzle is shown in Figure 10c which shows that the water streamlines are mainly along the nozzle axis. The water-jet flow through straight, straight-divergent and modified nozzle designs are shown in Figure 12. Among various nozzle designs the modified nozzle provided steady cylindrical water jet of maximum length and this was in 6-10 mm range, decreasing with the increase of water stagnant pressure in 1-10 bar range.

As mentioned in section 2.2 the spherical lens of 25.4 mm focal length and F no. = 1 introduced large spherical aberration and produced very large focus spot size. In order to reduce spherical aberration a pair of lens of 50 mm focal length and F no. = 2 is being incorporated in the cutting head with modified nozzle. This produced laser focus diameter of nearly 250 µm. The modified nozzle design will be used for further studies on underwater laser cutting of thick metal sheets.

The effects of stand-off distance on maximum cutting speed and the minimum specific energy for cutting of 1 mm thick sheet by varying SOD in the range of 0.5 – 1.5 mm at a constant water-jet speed of 18 m/s, are depicted in Figure 8b. The maximum cutting speed tended to reduce and the specific energy tended to increase with the increase of SOD. The effect of water-jet speed on the cutting speed and specific energy was not significant within the range of water-jet speed of 15 - 19 m/s.

The measured kerf width value for the underwater cutting process was in the range of 0.5 to 0.8 mm for the sheet thickness of 0.5-1.5 mm with the laser focus spot diameter of nearly 600 µm.

The macrographs of the cut surfaces presented in Figure 9 show the striation patterns at different cutting speeds in 1mm thick specimens. At a lower cutting speed the material removal is almost complete with a little dross sticking at the bottom edge, Figure 9a, but at a higher cutting speed the molten material is not completely ejected and a lot of dross remains stuck at the bottom edge, Figure 9c.

Fig. 8:Variation of the cutting speed and specific energy with laser power for (a) different sheet thickness at 1.0 mm SOD and (b) at different SOD for 1.0 mm thick stainless steel sheet at 18 m/s water-jet speed

Fig. 9: Macrographs of the cut surfaces at 1500 W laser power, 0.5 mm SOD, 18 m/s water-jet speed and different cutting speeds, (a) 400 mm/min (b) 700 mm/min and (c) 1600 mm/min


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coefficient of the time domain signal were employed to distinguish the AE signals corresponding to through-cut or failed- cut. It was found that the RMS value and Kurtosis coefficient were smaller and Skewness was larger for through-cuts than those for failed cuts.

Figure 14 shows the signals in frequency domain corresponding to the through- and failed cuts. The average value of amplitude over the entire frequency range as well as in a selected frequency range of 673- 2355 Hz was found to be smaller for through-cuts than that for failed-cuts.

Thus, the through-cut and failed-cut can be successfully distinguished by the signal parameters like RMS value, Skewness, Kurtosis Coefficient of the time domain acoustic signals and also by the mean amplitude of frequency domain signals of entire frequency range or a selected frequency range.

Laser Forming [3-5,13-17]

Since the successful demonstration of the process for bending a 22 mm thick steel plate using a 15 kW CO laser 2

by Kitamura in 1983 laser forming has received a considerable research attention for having the advantages like absence of hard tooling, flexibility and amenability to automation, etc [12]. Laser forming is a non conventional technique that is used to deform sheets made of metallic materials such as stainless steels, light alloys of aluminium, magnesium and titanium and also the composites and brittle materials like glass, ceramic etc. There are mainly three laser forming mechanisms – temperature gradient mechanism (TGM), buckling mechanism (BM) and upsetting mechanism (UM).

TGM is dominant when steep temperature gradient is set in between the top and bottom surfaces of a sheet and causes out-of-plane bending of sheets towards the laser beam. UM is dominant when temperature across the sheet thickness is nearly uniform and produces almost uniform plastic deformation through the thickness, i.e. in-plane shrinkage. Under similar conditions, thin sheets with smaller bending moment of inertia experience buckling, and bending either towards or away from the laser beam can be obtained depending on other conditions like initial

Online detection of water-jet assisted under-water laser cutting of stainless steel sheet using acoustic sensor

In underwater laser cutting the direct visualization of cutting process may not be always feasible especially when cutting is done deep in water and it not in line of sight. Therefore, some online detection system is required for distinguishing the through-cut from incomplete or failed-cut. Towards the development of an online detection system based on acoustic sensor an exhaustive study was carried out to determine parameters that can differentiate the through- and failed cutting. The acoustic signals generated during water-assisted underwater laser cutting process were recorded with the help of an acoustic sensor (hydrophone) and Dolptinear Gram software at 22 kHz sampling frequency. Recorded signals were further analyzed and processed using MATLAB software. The acoustic sensor (hydrophone) was mounted on the cutting head and it was moving with the head. Figure 13 shows a set of typical acoustic signals detected during through and failed cutting. Several signal properties like RMS value, Skewness, Kurtosis

Fig. 10: Simulated water streamlines with different water inlets. Red to blue is with increasing horizontal velocity component of water streamlines

Fig. 11: Water nozzle outlet (a) original design (b) modified design to ensure water flow along the nozzle axis

Fig. 12: Water-jet through nozzle with 1.0 mm opening (a) straight, (b) straight divergent, (c) straight modified, at 10 bar pressure


21

An experimental study was conducted to investigate the effects of laser parameters (viz. laser spot diameter, laser power and scan speed) on the bending and thickening of circular blanks of AISI 304 for different circular and radial scan schemes to form bowl shape surface [3]. TGM and UM were ensured for circular and radial scans respectively to obtain bending as well as thickening for 3D forming. The combination of the smallest laser spot diameter, highest laser power and minimum scan speed produced the highest amount of bending with in circular scans and highest thickness increment with radial scans. Figure 15 shows typical 3D laser formed bowl and dome shaped surfaces.

One of the important applications of laser forming in automotive, aerospace and ship building industry is the correction of bending angle of work pieces bent mechanically or using gas flame. An experimental study to investigate the effects of initial bending angle (mechanically formed), side of laser scan (convex or concave side of bent edge), Fourier number and laser spot diameter on the correction of bending angle of mechanically bent samples of stainless steel AISI 304 was conducted [13]. Less correction in bending angle was obtained when the laser was scanned at the concave side of bent edge. The main reason for this could be the Bauschinger effect. The correction in bending angle decreased and then increased with the increase in initial bending angle in 20-85 range. For the same Fourier number correction in bending angle increased with increasing laser spot diameter and for the same laser spot diameter it decreased with the increase in Fourier number [13].

Laser paint striping [7]

In many cases the paint used on the surface for preventing atmospheric contamination needs to be removed for various reasons. Conventional methods like mechanical or chemical cleaning, usually employed for the purpose are difficult to control. The major drawbacks of conventional cleaning techniques are subsequent damage in painting texture, reaction with pigments or base metal due to non–controlled penetration, and environmental issues. High power lasers have been used in paint removal with several advantages over the conventional techniques. Specifically, selective removal, no substrate

stress state of the sheet, etc. For TGM to occur Fourier 2number viz. κ.d/s .v, (where k, s, v and d are the thermal

diffusivity of material, sheet thickness, scan speed and laser spotdiameter, respectively) should be much lesser than unity whereas for BM and UM to occur it is the opposite. Laser forming encompasses bending about a straight line, 2D forming utilizing bends about multiple straight lines and more complicated 3D forming for generating surfaces, curved about multiple axes [3].

In order to develop a better understanding of the effects of various laser processing parameters and optimize them for maximum bending angle theoretical and experimental studies on 2 kW fiber laser were performed [3-5, 13-17]. It was shown that the pulsed laser bending produced more bending angle as compared to continuous laser bending at constant line energy [4]. Finite element method, statistical regression analysis and soft computing techniques were successfully implemented for modeling and analysis of the laser forming process and the performances of the developed models were found to be satisfactory in predicting the deformations and process parameters. Soft computing-based methods were also carried out to analysis and synthesis (inverse analysis) of laser forming process to obtain a class of shapes [5, 14].

Fig. 13: AE Signal corresponding to through cut and failed cut

Fig. 14: Frequency domain plot of the through cut and failed cut signals

Fig. 15: Typical 3D laser formed (a) bowl and (b) dome shaped surfaces


22

The laser paint removal process usually leaves behind traces of combustion product i.e. ashes on the surface. An additional post-processing such as light-brushing or wiping by some mechanical means is required to remove the residual ash. In order to remove the paint completely from the surface in a single step, a water-jet assisted laser paint removal process was investigated. The laser beam was delivered on the paint-surface along with a high speed water-jet to remove the paint and residual ashes effectively. SEM images and EDX spectra of bare substrate, ash adhered surface with GJAL paint striping and completely paint removed surface with WJAL are presented in Fig17 (a) and (b), (c) and (d), and (e) and (f) respectively. Relatively high percentages of Si, Al, Ca and O in GJAL paint striped surface (Figure17d) compared to that in bare surface (Figure17b) and in WJAL paint striped surface (Figure17f) indicate the presence of ash adhered on the surface. The specific energy was found to be marginally more than that for the gas-jet assisted laser paint removal process; however, complete paint removal was achieved with the water-jet assist only. The relatively higher specific energy in case of water-jet assist is mainly due to the scattering of laser beam in the turbulent flow of water-jet.

Conclusion

A brief overview of various laser material processing modalities carried out with the high power fiber laser has been presented. It has been demonstrated through theoretical and experimental studies that the limitation of

damage, fast rate of cleaning are the key favourable factors in laser paint removal. Laser can serve the purpose efficiently in hazardous places like in nuclear industries, marine industries, large size bridges and walls, and also in paint stripping from aircraft and automobile bodies, storage tank, rail cars etc. The mechanism of laser paint removal can be one or more of the process such as vaporisation, ablation, combustion, multi-photon absorption, shock removal etc., depending upon laser wavelength, laser power density, pulse duration and type of paint.

The laser paint removal behaviour with the continuous wave (CW) beam and repetitive pulses has been investigated using high power Yb: fiber laser. The specific energy defined as laser energy required to remove unit volume of paint, was found to be dependent on the laser processing parameters. In CW mode the specific energy reduced with the increase of laser scan speed and corresponding increase of laser power. In case of repetitive pulsed mode the specific energy was found to depend on the pulse on-time as well as on the time interval between two successive pulses. Figure16a and b shows a single laser paint removed track and a typical pattern made by selective paint removal by laser. Figure 16c shows the variation of specific energy at different laser modulation frequencies and duty cycles.

During the laser paint irradiation a plume of burning fume was formed over the surface and the variation in specific energy with laser processing parameters has been attributed to the absorption of laser radiation in the plume. Over all, the process efficiency was found to be maximum at 2 kW laser power, 150 Hz modulation frequency, 5% duty cycle and 50% overlap between two successive pulses [7].

Fig. 16: Photographs of stainless steel samples, (a) complete paint removal at 300W CW laser power and 9000mm/min of scan speed, (b) Typical Pattern made by selective removal of paint (c) variations of specific energy at different frequencies and duty cycle

Fig. 17: SEM image and EDX spectra of (a) and (b) bare substrate, (c) and (d) ash adhered surface with GJAL paint striping, (e) & (f) completely paint removed surface with WJAL


23

6. S. Mullick, Y.K. Madhukar, S. Roy, S. Kumar, D.K. Shukla, A.K. Nath, Int. J. Mach. Tool Manu. 68, 48 (2013).

7. Y.K. Madhukar, S. Mullick, D.K. Shukla, S. Kumar, A.K. Nath, Appl. Surf. Sci. 264, 892 (2013).

8. T. Miokovic, V. Schulze, O. Vohringer, D. Lohe, Acta. Mater. 55, 589 (2007).

9. R.K. Jain, D.K. Agrawal, S.C. Vishwakarma, A.K. Choubey, B.N. Upadhyaya and S.M. Oak, Pramana 75, 1253 (2010).

10. S. Mullick, Y.K. Madhukar, S. Kumar, D.K. Shukla, and A.K. Nath, Appl. Opt. 50, 6319 (2011).

11. S. Mullick, Y.K. Madhukar, S. Roy and A.K. Nath, World Academy of Science, Engineering and Technology 78, 221 (2013).

12. N.B. Dahotre, S.P. Harimkar, Laser Fabrication and Machining of Materials, Springer (2008).

13. S.S. Chakraborty, K. Prashanth, P. Singh, Y.K. Madhukar, A.K. Nath, Processing and Fabrication of Advanced Materials XXI 2, 274, December 10-13, IIT Guwahati (2012).

14. K Maji, D.K. Pratihar, A.K. Nath, Accepted Opt. Lasers Eng. (2013).

15. K. Maji, A.K. Nath, D.K. Pratihar, Experimental and numerical study on multi-scan laser forming of stainless steel sheet, ICAMMP Conference, Dec. 9-11, Kharagpur, (2011).

16. K. Maji, D.K. Pratihar, A.K. Nath, Modeling of pulsed laser bending of sheet metal using neuro-fuzzy system, AIMTDR Conference, Dec. 14-16, Kolkata (2012).

17. K. Maji, R. Shukla, A.K. Nath, D.K. Pratihar, Finite element analysis and experimental investigations on laser bending of AISI304 stainless steel sheet, IConDM Conference, July 18-20, Chennai, (2013).

depth of hardening in laser surface hardening can be alleviated by using repetitive laser irradiation. Water-jet assisted underwater cutting process has been successfully demonstrated. This produces very little turbulence in water and aerosols in surrounding atmosphere and therefore, will be attractive for cutting radioactive component underwater in nuclear industry. An online monitoring system based on acoustic signal has been also demonstrated for underwater laser cutting.Paint stripping by water-assisted laser irradiation was found to have better efficacy than gas assisted laser process. Laser forming of 3D surfaces of various shapes were investigated by experimental parametric studies and also with statistical and numerical analyses. The variety of manufacturing modalities carried out with the fiber laser only shows its capability and versatility in laser material processing applications.

Acknowledgments

Authors would like to acknowledge undergraduate students Aniruddha Gupta, Frederick Benny and ChanakyaHridaya whose works have formed a part of this article. They would also like to acknowledge Prof. D K Pratihar and Prof. V. Racherla for their guidance in statistical and numerical analyses of laser forming process respectively.

References

1. A.K. Nath, A. Gupta and F. Benny, Surf. Coat. Tech. 206, 2602 (2012).

2. A. Gupta and A.K. Nath, Appl. Mech. Mater. 110-116, 823 (2012).

3. S.S. Chakraborty, V. Racherla, A.K. Nath, Opt. Laser. Eng. 50, 1548 (2012).

4. K. Maji, D.K. Pratihar and A.K. Nath, Opt. Laser Technol. 49, 18 (2013).

5. K. Maji, D.K. Pratihar and A.K. Nath, Soft. Comput. 17, 849 (2013).


24

aluminium garnet) and CO lasers. YAG is a solid-state 2

laser where lasing action is confined to a YAG rod that is doped with Nd atoms. YAG lasers are used in both pulsed and CW (continuous wave) modes. However, the production of CW Nd:YAG is not as straightforward as pulse mode and the pulse mode of Nd:YAG is preferred. On the other hand, the CO laser is a gas laser and is 2

primarily used in the CW mode. The YAG lasers have a wavelength of 1064 nm which is exactly ten times smaller than CO lasers and can be used for producing very fine 2

spot. The YAG lasers work efficiently on metals but their wavelengths are not easily absorbed by many other materials such as wood, acrylic, plastics and fabrics. A CO laser beam has much more latitude and can be 2

absorbed easily by many organic materials [2].

More recent advances in laser technology is fiber laser that are solid-state and focus the laser light through an optical fiber. Fiber lasers produce beams of very high beam quality and thus can be focused down to a very small spot size. Fiber lasers are pumped with laser diodes and offer higher efficiency than other lasers used for similar applications, like the Nd:YAG laser and CO laser 2

[3]. Another recent development is the direct diode laser where light from a bank of laser diodes is directly used for welding. Diode lasers are characterised by their ability to be focussed into very fine spots on account of their small wavelength of 800-976 nm. They can be used very efficiently for the welding of plastic materials. Disk lasers are also making inroads with their high beam quality and CW operation. Moreover, YAG wavelengths can be piped down a fiber for convenient delivery over considerable distances and is one the reasons for the popularity of YAG lasers. Laser light coming out of the CO laser cannot be transmitted through a fiber and hence 2

has to be transmitted through air and diverted with help of mirrors and hence is referred to as direct optics [4].

It is thus obvious that individual laser is having their merits and demerits in application point of view. In microwelding system, the fiber or direct diode laser is preferred for precise focus diameter and controlling of energy transfer. However, the Nd:YAG is widely used in industry for flexibility of pulse shaping in highly conductive materials or dissimilar materials joining. Hence the significant issues of laser microwelding process are documented in this article to provide an overall view of the subject by creating individual sections

Abstract

The advancement of laser technology along with controlled non-contact mode of heat energy transfer makes laser as primary choice for precision manufacturing. Recently, laser microwelding has gained significant attention in miniature applications where other welding process draws the limitation. Present article describes state-of-the-art literature review on laser microwelding process. The main focus of this article is the possible direction on the development of mathematical model considering the effect of various physical phenomena involved in microwelding process. The mathematical model can be used beneficially for fundamental understanding of underlying mechanism of the process. A sophisticated model based on integrating scientific principles alone can replace costly experimental conductance. The usability or application of laser in micro scale welding process is discussed first. Recent development of microwelding techniques are presented thereafter. The flexibility of pulse shaping in micrwelding brings the applicability of difficult-to-weld materials. The results and discussions of various microwelding processes using the developed numerical model are presented finally. Current application on various materials and geometric configuration shows the growth of the process. Till this process is in initial stage and has potentiality for future development in various aspects of pulse modulation and application in dissimilar or difficult-to-weld materials joining.

Keywords: laser transmission welding; ultra-shot pulse; non-fourier heat conduction; Fluid flow.

Introduction

The term micro joining commonly refers to the process where at least one dimension of the part being processed is less than 100 μm. The difference between traditional and microwelding is that the latter is performed at extremely low power in combination with fine control of the source along with the aid of a high-powered microscope [1]. Lasers have been used as a tool for precise materials processing due to its non-contact mode of operation, ability to focus it to a small size and in joining of various metals as well as non-metallic materials. A wide variety of laser beam sources are used for microwelding. In welding industry, the two most common are Nd:YAG (neodymium-doped yttrium

Recent Advances in Laser MicroweldingSwarup Bag

Department of Mechanical Engineering, Indian Institute of Technology, Guwahati-781039, Assam, India

E-mail: [email protected]


25

converted into thermal energy. This happens at the interface of the two joining parts due to conductive heat transfer. The lower part needs to have the ability to absorb the light energy to create heat. Most thermoplastics naturally transmit infrared laser radiation so absorptive properties are realized by adding soot or pigmentation. The most common and best absorbing additive is carbon black doped at a rate of about 0.5%.

Welding of high conductive materials such as copper, silver, and gold is problematic or nearly impossible with traditional 1064 nm pulsed Nd:YAG laser. A recent innovation of 532 nm green Nd:YAG pulsed laser is capable an order-of-magnitude increase in material absorption over the 1064 nm wavelength expanding the use of lasers in conductive microwelding applications. Green laser beam in the visible spectrum is better absorbed by highly reflective materials that cannot be done with a traditional 1064 nm YAG laser.

In conventional pulse welding process, every single pulse melts the material which quickly solidifies at the end of the pulse and hence more energy is pumped into the workpiece. Moreover, in pulse shaping for highly reflective difficult-to-weld materials, the change of absorptivity is limited and small changes in reflectance lead to poor reproducibility of pulsed mode seam welding. The idea of continuous wave welding for micro parts using temporal shape of the laser pulse leads to recent development of SHADOW technique to realize the extended weld seams with a pulsed laser using single pulse and sweeping the laser beam over the surface during the pulse [11-12]. SHADOW stands for Stepless High Speed Accurate and Discrete One Pulse Welding. This method produces smooth continuous surface as compared to pulsed shaping methods. The application of SHADOW technique is mainly found in joining of metals and alloys in repairing of watch components i.e. joining between stainless steel and highly conductive copper.

The latest developments in the field of polymer welding using high brilliance laser sources together with advanced irradiation strategies based on the local and temporal modulation of the laser beam is TWIST – Transmission Welding by an Incremental Scanning Technique [11]. For this process a dynamic periodic beam deflection is applied to control the fusion and the solidification process where high dynamic beam oscillations are overlapped to the normal welding direction along the welding contour. High dynamic oscillations prevent the thermal material damage a more homogeneous energy input across the weld seam width can be achieved.

Grewell et al. [13] devised the diffractive optics to reshape lasers for welding of plastics. By using inverse

of their own merits. Section 2 provides with system development in laser microwelding process. The significant findings towards the efficient use of available laser source in microwelding application are described. Section 3 presents the background of pulse shaping during welding of difficult-to-weld materials. Section 4 demonstrates the current application area of laser microwelding processes. Section 5 provides the mathematical background for the conduction based, transport phenomena based heat transfer and fluid flow analyses, and Non-Fourier heat conduction based numerical models. Section 6 demonstrates the results for different microwelding processes using the developed numerical model. The future prospect is outlined in section 7 and the concluding remarks are described in section 8.

Advances in System Development

The laser microwelding process is widely appreciated in the tool, die, and plastic injection moulding industries. More and more applications are coming out in repair weld without distorting the surrounding areas. However, the precision and accuracy is controlled by CNC laser in the automated process [5]. Micro welding of steel, copper, aluminium, plastic and metal-plastic are possible now-a-days with high precision [6-10]. The laser microwelding process can be used to join cracks. The energy that is transferred is locally distributed and with the duration being short it can be conveniently treated with precision. Laser microwelding technology is advantageous in respect of high productivity due to the high speed of the welding process. This also minimizes the time loss and adopts the changes in new product design and development. Moreover, the welding can be done on different types of materials with less distortion and shrinkage.

The general configuration of laser micro welding is similar to the conventional process. The components to be joined are lapped or butted together and a highly focused beam is propagated along the joint with certain scanning speed. Another configuration used by researchers is the laser transmission welding which is promising technology for many industries such as the automotive, electronic, and biomedical engineering. The basic principle of this joining method is passing or transmitting laser radiation through one piece of transparent material to create a weld. Unlike standard welding where the energy is applied at the surface of the materials, transmission welding aims to apply the energy in between two plastic pieces at their interface. The upper layer needs to be transparent to laser wavelengths in the infrared and near-infrared spectrum. Typical laser wavelengths are of 808 nm or 980 nm. Once the laser beam passes through the upper layer it still needs to be


26

crack sensitive materials. It is thus obvious that pulse shaping provides added capacity to overcome some intrinsic problems during joining of materials.

The instrumentation of pulse shaping over millisecond level is well established and a considerable work has been progressed in that direction. Hugger et al. [18] successfully conducted the microwelding of aluminium and copper using pulse pulsed Nd:YAG laser where the surface roughness and distortion are reduced by pulse shaping. There is also significant development on optical waveform synthesis or pulse shaping of ultra-shot laser in the picosecond and femtosecond range [19]. However, the production of state-of-the-art pulse shape at micro-nano-pico-femto-second level demands a complicated and extended experimental set-up which is still a challenging task. Moreover, the modulation of pulse shaping in ultra-shot pulse laser is some extent arbitrary and difficult to produce the shape like rectangular pulse. Therefore, the influence of pulse shaping in ultra-shot pulse laser welding is an open area of research for precision joining of materials at micro scale.

Current Applications

The use of pulse Nd:YAG laser is preferred over CO 2

laser for joining different grade steels and aluminum foils [15-17]. This type of laser provides good weld during dissimilar welding between different grades of aluminium alloy. However, the quality of the joint depends on the proper control of laser pulse. The shape and size of fusion zone is very much sensitive to gap between foils [9]. The microwelding of aluminium is very sensitive to the temporal pulse shaping [10]. Now –a-days a direct diode laser without an optical fiber and low power fiber lasers are promising alternatives of Nd:YAG laser in microwelding process [20]. The continuous wave (CW) single mode fiber laser is more suitable for stainless steel and titanium alloys up to 1 mm thick foil. However, to weld reflective materials like aluminium and copper base alloys and dissimilar materials, pulsed Nd:YAG laser is better choice. The welding of thin foil of stainless steel using direct diode laser at high laser scanning speed needs low power density than CO or Nd:YAG laser [21]. A superior pulse-2

to-pulse energy stability is possible for fiber lasers [22-24]. In microwelding, fiber laser is characterised by high peak power, high repetition rate with single-mode beam quality, low pulse energy whereas Nd:YAG laser is characterised by low peak power, long high-energy pulses but poorer beam quality. Overall, the smaller size substrate material is most favourly joined by fiber or diode laser whereas pulsed Nd:YAG laser is suitable for high thermal conductive materials.

One of the major challenges in microwelding process is to

Fourier transformations diffractive lenses are designed and fabricated to shape the beam into predetermined patterns. This method can obviously be applicable to both modes of laser welding of plastics i.e. surface heating and through transmission infrared welding. The welding technique uses diffractive optics for laser beam shaping using mask, not the concept of scanning.

Schmitt et al. [14] developed miniaturized scanner-based laser microwelding system by integrating a beam deflection option for welding of metal as well as plastic materials. Highly dynamic oscillation techniques lead to the stabilization of the welding process of difficult-to-weld materials with an increased process velocity and reduced joining geometry. The process requirement is the suitable beam deflection system and the design of miniaturized laser processing optics. However, the selection and integration of highly dynamic scanner systems and processes enables further process improvements of laser beam micro-welding.

It is thus obvious that the system or technology development in laser microwelding processes actually confines to efficient or ingenious use of laser beam by adding extra equipment as compared to conventional process. However, the absorptivity of laser for a specific material mainly depends on the wavelength. Hence the shape of temporal pulse having some role to alter the absorptivity irrespective of laser wavelength.

Pulse Shaping in Laser Microwelding

Pulse shaping is a technique used to temporally distribute energy within a single laser pulse that provides the user an added degree of control over the heat delivered to the laser material interaction zone. The modulation of pulse in time domain enables to endorse keyhole or conduction mode welding, optimization of penetration depth and welding of highly reflective materials in conventional pulsed laser welding process [15]. In conduction mode laser welding, typically a low peak power and a long pulse time is used whereas keyhole mode welding is characterized by high peak power at a short pulse time. In full depth penetration, the pulse shape has typically a low peak power over a certain time and the peak power is raised suddenly to melt the second material and finally the power is reduced gradually to stable the melt and to cool down. A high peak power is used to melt highly reflective materials and a rapid reduction in peak power is followed to avoid overheat. Therefore, the melt grows under low peak power and long pulse time [16]. A typical pulse shape for crack sensitive materials has an upslope at the beginning of the pulse to prevent thermal shock and a down slope at the end of the pulse to have a controlled cooling down period. By controlling the cooling down phase the thermal shrinkage stresses are reduced [17] for


27

Development of Mathematical Model

It is obvious during the development of mathematical model for laser microwelding process, the phenomenological effect of laser comprises of two broad categories. Fourier heat conduction model for continuous or pulse laser welding is valid for the pulse width of the order of micro or millisecond. In this case, the thermal energy transport may be influenced by the momentum transport of liquid metal within small weld pool. Hence a coupled mass, momentum and energy transport phenomena based heat transfer and fluid flow model is suitable for laser microwelding. In ultra-shot pulse (nano-pico-femtoseconds) laser microwelding, the non-Fourier heat conduction model is suitable where thermal relaxation time in applying the heat flux or temperature development is important [36]. Moreover, the electron-phonon coupling in two temperature model is also suitable for ultra-shot pulse laser microwelding [37].

The interaction of laser with substrate material may form either keyhole or conduction mode welding. However, the conduction mode heat transport is feasible for laser microwelding. At the same time the flow of liquid material within the weld pool may influence the weld shape and size. Hence the conservation of mass and momentum within weld pool and energy balance for whole solution domain is maintained with the interaction of various physical phenomena through boundary or over the volume. The driving force for liquid metal movement in laser welding is the surface tension force that acts on the top surface of weld pool and the buoyancy force over volume of molten pool. Energy balance is maintained by heat flux on top surface of the specimen and loss of heat by convection and radiation from the surface. To analyse transport phenomena in laser micro welding, the conservation of mass is expressed as [38]

..(1)

where v denote the velocity components along x (i =1), i 1

x (i = 2) and x (i = 3) directions. The liquid molten 2 3

material is assumed as an incompressible, Newtonian, and laminar flow. Hence the momentum conservation equation is expressed as [38]

..(2)

where P is the pressure, ρ is the density, F is the body i

force component, δ is knocker delta, µ is the viscosity of ij

molten material. It is also assumed that laser is moving at a speed of v along x direction. The conservation of w 2

energy is represented as [38]

join dissimilar materials. The primary issue for dissimilar material welding is the possibility of misalignment in the physical properties and poor metallurgical affinity between two materials that lead to formation of crack and brittle intermetallic phases [6]. This can be overcome by adding a third material as filler that helps to reduce brittle intermetallic phases. In joining between aluminum and copper the use of silver or tin as filler materials yields a considerable improvement of the static and dynamic mechanical stability of welded joints [6].

There is tremendous application of laser to weld non-metallic materials such as glass, silicon, and plastics [25-29]. These non-metallic materials are generally joined by transmission welding. The wavelength of laser source is chosen in such a manner that one of the substrate material acts as transparent and ultra-shot pulse (femtosecond) laser serves this purpose. The microwelding of glass and silicon have potentiality in electronics industry since they are semiconductor materials. However, an intermediate layer of the order of micrometer is used for laser transmission welding where the layer absorbs the laser irradiation. Sometimes a shadow mask of reflective material is used to control local heating in desired area in case of nanosecond pulsed Nd:YAG laser. It is noteworthy that Nd:YAG laser is not so precise as ultra-shot pulse laser, but it is possible to reshape the pulse according to the application. In joining of plastic components, near-infrared laser radiation is generally used by transmission welding. The welding between polyethylene (PE) film and black PE sheets are performed using diffractive optics to reshape laser beams of YAG laser [13].

Recently, the transmission welding of biocompatible materials such as polyimide (PI) and titanium (Ti) are observed in medical implant devices and neural implant devices [30-33]. Fiber laser or near-infrared diode laser is the choice where the formation of the joint is a result of the creation of strong chemical bonds between Ti and certain polymeric functional groups. The microwelding between pin-to-plate using pulsed Nd:YAG laser are observed in dot matrix printing pin joint [34]. An interesting application of microwelding between copper ball and borosilicate glass using a nanosecond pulse Nd:YAG laser is observed in measurement hardware, solar batteries, and touch panels [35].

It is thus observed that microjoining process has tremendous application in the area of micro-electro-mechanical systems (MEMS), medical devices, packaging technology, lightweight automotive structures and sensor industry where highly integrable products are joined with accuracies of few microns. However, as device becomes smaller, researchers faced challenges to join these miniature geometries robustly by avoiding excessive damage.


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transmission welding the amount of heat flux decreases as it reaches the interface of the two joining materials. Therefore, the heat flux incident on the surfaces of the two materials is given as

..(8)

where Q refers to laser power, r is effective radius of eff

laser beam on the work piece surface, d is the power density distribution factor of heat source, T is the fraction t

of laser energy transmitted through the top material. The heat transfer between the two parts relies on the gap conductance at the interface. The model consider the contact conductance at the weld interface reflecting the influence of contact pressure, surface roughness, and elastic modulus.

The primary assumption of classic Fourier heat conduction is that the heat transport is instantaneous with the propagation of heat in molecular level at infinite speed. This assumption is valid for the macro scale modeling of heat transport where the basic governing equations are developed over the continuum [36]. However, classic Fourier heat conduction analysis fails to realize the effect of high frequency short duration pulse heating. Therefore, at high pulse frequency within short duration of pulse on-time the hyperbolic nature of heat conduction is observed. The theoretical foundation of the problem is based on phase lag. Cattaneo [39] proposed a thermal wave model with single phase time lag, in which the temperature gradient established after certain duration. This modified non Fourier heat conduction equation is given by

..(9)

where q is heat flux, τ is relaxation time, k is thermal s

conductivity of material, T is temperature and t is time. The transient energy equation in absence of internal heat source is expressed as

..(10)

where ρ is density and c is specific heat capacity of the p

material. is the velocity vector of the heat source. Using eqs (9) and (10), the governing equation for non-Fourier heat conduction is expressed as

..(11)

where α is the thermal diffusivity and τ is the natural property of the material and usually constant, V is the y

laser scanning speed along or parallel to y axis. Since the laser pulse duration is of the order of picoseconds, heat

where T is the temperature, C is the specific heat of material, the rate of internal heat generation, and k the ij

component of thermal conductivity tensor. The body force in laser welding consists of buoyancy force and is acting along Z-direction (i = 3). The equation of body force is written as

..(4)

where β is the coefficient of thermal expansion, g is the gravitational acceleration and T is the reference o

temperature. The boundary interaction for energy transport is expressed mathematically as

..(5)

where q the heat flux on the top surface followed s

Gaussian distribution, n indicates normal to the surface, h is the heat transfer coefficient, T is the ambient c 0

temperature, and ε is the emissivity. To avoid non-linearity that arise due to fourth order of temperature variable in radiation term, a lumped heat transfer coefficient combination of convection and radiation is expressed as

..(6)

where ε is the emissivity of the surface and h is explicitly eff

function of temperature.

It is noteworthy that the solution of temperature distribution is the ultimate objective of different laser welding process. The basic governing equation is Fourier heat conduction irrespective of continuous or pulse mode of laser. The only variation is the boundary conditions for different type of lasers. Hence a dedicated heat source model in the form of surface flux or volumetric heat density is required to represent laser as a source of heat. In laser welding the heat flux varies in exponential way along the depth direction. Assuming the condition mode laser welding, the heat source model is defined by

..(7)

where Q refers to laser power, r is effective radius of eff

laser beam on the work piece surface, d is the power density distribution factor of heat source, p is the weld depth, and t is the thickness of plate. R and α are reflectivity and absorption coefficient (per unit length) of the material. The term actually used in eq. (7) is volumetric heat source term and is incorporated in eq. (3) through internal heat generation term. In laser

..(3)


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commercial software such as ABAQUS is used. Using these developed numerical models, the different mode of laser microwelding is analysed.

Figure 1 depicts the computed temperature and velocity fields for single pulsed fiber laser welding of SS304. The temperature field is depicted by isotherm contour (red colour) and velocity of liquid metal is indicated by the arrow vector line. The solidus isotherm of SS304 defines the weld pool shape and size. The figure indicates that the aspect ratio (penetration/width) is very high for laser welding. The driving force for liquid metal movement is the buoyancy force and surface tension force. The temperature coefficient of surface tension dγ/dT is considered as positive since the material contains surface active element. Therefore, the molten metal flows from outward periphery to the centre of the liquid pool. The buoyancy force acts in the upward direction. As a result, the fluid flow makes a circulation loop in the anti-clockwise direction. The magnitude of velocity vector is more at the top of weld pool and it reduces gradually along the depth direction. The maximum order of magnitude of velocity in laser microwelding is ~ 350 mm/s. It is noteworthy that flow field is important when material contains surface active elements and is not possible to analyse using heat conduction only.

Figure 2 depicts the influence of relaxation time on temperature distribution of 0.1 µm thick gold film. The applied load is 2660 W for the pulsed duration of 100 fs with 76 MHz pulse repetition rate. The solution domain for analysis is 10 µm x 20 µm x 0.1 µm. The reference temperature for the process is taken 300 K. Since the correct estimation of relaxation time is difficult for material, it is necessary to investigate the effect of relaxation time on temperature distribution. It is obvious that the peak temperature increases with increase in

losses from the surfaces are negligible. Hence the solution domain is subjected to heat flux only. The speed at which the thermal wave propagates through the material is expressed as

..(12)

It is noteworthy that if C approaches to infinity i.e. τ t

approaches to zero; it becomes classic Fourier heat conduction equation. Since the eq. (11) corresponds to single relaxation time, it is also called single phase lag model and τ is the phase lag time for heat flux. The governing eq. (11) is discretized in special domain using Galerkin's weighted residue technique. Considering the contribution from all elements over the solution domain the final algebraic equation can be written as

..(13)

ewhere the elemental matrix [K ] accounts thermal conductivity term which is modified by the laser

e escanning speed, [M ] accounts relaxation time, [C ] eaccounts specific heat capacity, and F accounts the

imposed heat flux. To get temperature distribution famous Newmark algorithm is used for time domain discretization [20]. The temperature at time t+Δt is calculated by

..(14)

where β and δ are parameters that can be determined to obtain accuracy and stability. The matrix relation gives

1temperature vector T at the end of time step ∆t in terms 2of known temperature given as vector T at the start of

the time step and we can proceed for subsequent steps. However, T and T at time t+Δt are calculated by

..(15)

..(16)

Presently, Galerkin method (unconditionally stable) i.e. δ = 3/2, β = 8/10 is used to find the temperature distribution at the end of each time step.

Results and Discussions

The governing equations along with boundary conditions are solved using mathematical tool, here the finite element method. An in-house developed computer code is used for transport phenomena based heat transfer and fluid flow analysis. Also a 3D non-Fourier heat conduction model is developed using Intel FORTRAN compiler since this module is not readily available in commercial software. For laser transmission welding, the

Fig. 1: Temperature and velocity field in laser microwelding in presence of surface active element


30

indicates the fusion zone dimensions corresponding to melting isotherm of 1324 K. The dimensions of solution domain are considered as 100 µm, 200 µm, and 200 µm along x, y and z- axis respectively. The heat flux is applied at a depth of 50 µm from the top surface where the laser is focused. It is obvious from the figure that the heat is concentrated in a narrow zone and may result in high thermal stress over a small zone.

Theoretical investigation by mathematical modelling of the process provides better understanding of underlying physics involved in LTW. The LTW is being applied in joining various thermoplastics, various dissimilar materials, plastics to metals etc. Thus process is being extended to joining of micro parts for applications in various industries. Also the clear weld technique for joining two clear thermoplastic parts even introduced some great flexibility into laser welding. Figure 5 depicts three dimensional computed temperature distributions in laser transmission welding of two polypropylene samples corresponding to velocity of 2 mm/s and power density of

22.8 W/mm at two different locations. The melting isotherm for the material is 448 K. It is obvious that the maximum interface temperature is ~ 600 K which ensure that there is melting of polypropylene joining is performed by the application pressure. Figure 6 depicts the cross-section view of temperature distribution for LTW. It is obvious that the lower surface subjected to higher temperature as compared to upper surface. The upper surface is transparent to laser wavelength and the heat energy releases in the second material. There is thermal contact resistance at the interface of two materials resists the heat flow by conduction. Hence the temperature distribution is different in two materials.

Future Prospects

There is enormous scope for the development of this process either through experiments or development of mathematical models. The feasibility of the process for new materials, more dissimilar material joining and various geometric configurations weld are future

relaxation time from 0.01 ps to 0.07 ps. However, there is a delay to achieve peak temperature at lower value of relaxation time. Figure 3 describes the temperature contour of gold film at fifth pulse of pulse train at 67.789 ns for the pulse duration of 100 fs for stationary heat source.

It is observed from literature that the thermal wave speed of glass is around 0.9 m/s and hence the relaxation time

-7for glass is ~ 3.7 x 10 which is relatively more than that of gold (in picosecond level). Since the laser pulse duration is of the order of femtosecond which is also comparable with thermal relaxation time, there exists a thermal non-equilibrium state between electrons and metal lattice. Therefore, in this case it is worthwhile to apply non-Fourier heat conduction as compared to classic Fourier heat conduction analysis. Figure 4 depicts the temperature simulation during joining of borosilicate glass using non-Fourier heat conduction. The laser is defocused at a certain depth from the top surface of the plate and is moving at a speed of 1 mm/s. The simulation of temperature profile is performed under the condition of ultra-shot laser with 400 fs pulse duration, 1MHz, pulse repetition rate and 0.5 µJ pulse energy. The red zone

Fig. 2: Influence of thermal relaxation time on temperature distribution of gold film of 0.1 µm thick.

Fig. 3: Three dimensional temperature distribution of thin film gold plate of 10 x 20 0.1 µm dimension

Fig. 4: Three dimensional temperature distribution of thin film gold plate of 10 x 20 0.1 µm dimension


31

in this article. The following conclusions can be drawn from the perspective of current development in laser microwelding.

There are several applications of laser microwelding for joining various types of materials. Fiber laser, direct diode laser or Nd:YAG laser are generally used for micro scale applications. However, the pulse shaping of Nd:YAG laser or beam diffraction techniques are used to get more precise and efficient welding subject to a specific material. The use of ultra-shot pulse laser in welding of relatively brittle material is also promising technology in microwelding process. The SHADOW and TWIST technics are remarkable achievements in this direction.

The development of mathematical model helps to realize the acquaintance with physical insight of the process and differential influence of process parameters on final weld joint quality. This actually reduces the costly experimental work. Different microwelding processes are analysed by developing heat transfer models for predicting the temperature profiles during the welding process. The inclusion of momentum transport within small weld pool actually helps to modify the heat transport and the effect of surface active elements even in micro scale welding process. However, in ultra-shot pulse laser welding the non-Fourier heat conduction model predicts better temperature distribution by taking into the effect of thermal relaxation time for high frequency short duration pulse heating. The welding process can be precisely controlled by pulse shaping to produce a good quality joint even in ultra-shot pulse laser.

Acknowledgements

The author gratefully acknowledge the financial support provided by SERB (Science & Engineering Research Board), INDIA (grant no. SERB/F/0797/2013-2014 dated 20.05.2013) to carry out this work. The support from author's students R.S. Desai, M. Baruah, P. K. Sahu and Bipul Das to prepare this manuscript is gratefully acknowledged.

References

1. V.V. Semak, G.A. Knorovsky and D.O. MacCallum, J. Phys. D: Applied Physics 36, 2170 (2003).

2. Y. Zhou, Microjoining and nanojoining, Woodhead Publishing Limited (2008).

3. K.G. Nichols, Review of laser microwelding and micromachining, Proc IEE, 116, 2093 (1969).

4. Y. Zhou, A. Hu, M.I. Khan, et al. J. Phys.: Conf. Ser. 165, 012012-1(2009).

prospects for this joining process. The real challenge lie in welding of dissimilar materials is due to wide difference of material properties and the nature of contact surface. Moreover, the success of the microjoint depends on the accurate design and control of input laser power. Proper identification of process parameters is necessary to avoid insufficient joint strength or damage of material. The geometric precision and cost of equipment is future challenge for mass production of laser microwelding processes.

The numerical model mainly confines to thermal analysis. A sophisticated numerical model that can integrate all involved process physics may reduce the experimental cost with the help of computational cost. The estimated time–temperature history requires verification whether there forms keyhole. Moreover, present heat transfer model also needs to be enhanced to account the free surface profile of weld joint. A dedicated thermo-mechanical-metallurgical model is necessary to predict the plastic deformation and residual stress. A more general finite element model for laser transmission welding process is underway including the effect of contact resistance due to the presence of any air gap at the weld interface.

Conclusions

The practical aspect as well as theoretical development of the heat transfer model in laser microwelding is discussed

Fig. 6: Cross-sectional view of temperature distribution for laser transmission welding

Fig. 5: Temperature distribution in laser transmission welding at scanning velocity of 2 mm/s


32

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33. T. Mahmood, A. Mian, M.R. Amin, et al. J. Mater. Proces. Technol. 186, 37 (2007).

34. C.K. Chung and Y.C. Lin, Microsyst. Technol. 12, 104 (2005).

35. A. Utsumi, T. Ooie, T. Yano and M. Katsumura, JLMN-Journal of Laser Micro/Nanoengineering, 2, 133 (2007).

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5. K.W. Guo, Recent patents on Nanotechnology, 3, 53 (2009).

6. I. Mys, M. Schmidt, Laser micro welding of copper and aluminum, Proc of SPIE, 6107, 610703-1 (2006).

7. B. Majumdar, R. Galun, A. Weisheit and B.L. Mordike, J. Mater. Sci. 32, 6191 (1997).

8. A. Horn, I. Mingareev and A. Werth, JLMN-J. Laser Micro/Nanoengineering, 3, 114 (2008).

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11. A. Olowinsky, A. Boglea and J. Gedick, Laser Technik Journal 5, 48 (2008).

12. A.M. Olowinsky, K. Klages and J. Gedicke, SHADOW a new welding technique: basics and applications, Proc SPIE, 5662, 291 (2004).

13. D. Grewell, D. Ditmer, D. Hansford and A. Benatar, Beam shaping with diffractive optics for laser micro-welding of plastics, Proc. 63rd Annual Technical Conference for the Society of Plastic Engineers, Brookfield (2005).

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33

AISI H13 hot work tool steel (5% Cr) is the most common die material used in metal forming and casting industries. Dies are prone to damage prematurely due to severe hot-working process conditions like thermo-mechanical fatigue, high speed molten melt erosion and high temperature wear [8,9]. Processes based on fusion welding i. e. tungsten inert gas welding, metal inert gas welding, plasma arc welding, and laser-beam welding, are frequently adopted for deposition of hard and wear resistant overlays, for rebuilding of worn or cracked surfaces and to modify the shape of existing tools [10-13]. In an earlier attempt, a detailed study of the effect of laser variables (applied energy density, pulsing, etc.) on the microstructures, phases, stress distribution and hardness distribution of single track laser clad H13 tool steel was undertaken. In addition, the effect of post clad heat treatment operation on the microstructures and microhardness of the clad zone was also studied [14]. In the present study, the effect of laser cladding and post clad tempering treatment on the microstructure and wear behavior of laser clad AISI H13 tool steel on AISI H13 tool steel has been undertaken in details.

Experimental

In the present study, hot working AISI H13 tool steel (0.4 wt.% C, 1.0 wt.% Si, 0.4 wt.% Mn, 5.2 wt.% Cr, 1.5 wt.% Mo, 1.0 wt.% V, Fe balance) plates with dimensions of 100 mm × 100 mm × 12 mm in hardened (austenitized to 1030 °C for 10 min followed by gas quenching) and tempered (at 620°C for 2 h) conditions were used as substrate for laser cladding operation. Precursor powder (feeding material) used in laser cladding was AISI H13 tool steel of similar composition (0.36 wt. % C, 0.95 wt. % Si, 0.4 wt. % Mn, 5.0 wt. % Cr, 1.1 wt. % Mo, 0.86 wt. % V, Fe balance) as that of substrate with spherical particle morphology with average particle size around 100 µm. Laser cladding was carried out using a 6 kW continuous wave diode laser (wavelength 915-980 nm) coupled with fiber delivery and optic head system mounted on 6-axis robot with a spot diameter of 3 mm with top-hat intensity distribution. A co-axial powder feeding nozzle assembly was used to feed the powder co-

Abstract

The present study aims at development of AISI H13 tool steel clad layer on conventionally hardened and tempered AISI H13 hot work die material by high power diode laser (6 kW) and a detailed investigation of its microstructure and mechanical properties (wear resistance) for the possibility of its application for repairing of die. Laser cladding was carried out using a fiber coupled diode laser equipped with a coaxial powder feeder with a laser

2energy density of 133 J/mm and powder density varying -3 2from 10.6 to 16 × 10 g/mm . Laser cladding showed the

presence of martensite, retained austenite and carbides in the microstructure with the hardness of 600-650 VHN. Wear resistance of laser surface clad samples (evaluated by fretting wear testing) showed a superior behavior as compared to conventional hardened and tempered H13 tool steel.

Keywords: laser cladding; die-casting dies; H13 tool steel; laser deposition; die refurbishment.

Introduction

Laser cladding is a process of deposition of material with similar or dissimilar composition by high power laser assisted melting and simultaneously applying it on a substrate with adequate wetting but limited dilution at the interface to achieve good metallurgical bonding [1-3]. Precision repairing of the damaged spot or surface is obviously more cost effective than total replacement. Laser cladding provides the localized and controlled heat input and reduces distortions and cracking during material deposition [4-7]. Though laser cladding is an emerging technique for the precision repairing of the damaged part, optimization of laser parameters is essential to ensure a defect free microstructure with a minimum residual stress and near-equivalent/improved properties of the clad zone as compared to the base substrate. In the past, laser cladding was successfully applied to deposit high-performance material or the same material to recover high-value part damaged due to machining errors and to refurbish original geometries of a damaged part of high value component [1].

Refurbishment of AISI H13 Die Materials by Laser Cladding

1, 2 1 2 1, 3,*G. Telasang , J. Dutta Majumdar , G. Padmanabhan and I. Manna1Department of Metallurgical and Materials Engineering,

Indian Institute of Technology, Kharagpur 721 302, WB, India,2Center for Laser Processing of Materials, International Advanced Research Center (ARCI)

for Powder Metallurgy & New Materials, Hyderabad 500005, AP, India,3Indian Institute of Technology, Kanpur 208016, UP., India.



34

were indentified.

Figs. 2 (a,b) show the scanning electron micrographs of the cross section of multi-track laser clad showing (a) the center of the clad, (b) overlapped zone, processed with a

2laser energy density of 133 J/mm and powder density of -3 213.3×10 g/mm . Fig 2 (a) shows that the microstructure

of the clad zone consists of the presence of prior austenite dendrites along with the presence of martensite and retained austenite. Due to successive single track cladding with overlap, clad zone of the previous clad gets tempered leading to dissolution of interdendertic precipitates resulting into development of tempered microstructure with the presence of a low area fraction of carbides in the inter-dendritic zone along with the presence of martensite. Resolidification of over-lapped region leads to development of columnar dendritic structure growing from the solid-liquid interface (Fig. 2b). The decreased area fraction of carbides in the clad zone as compared to the overlapped zone is attributed to reheating of clad zone to a temperature above austenitizing temperature followed by rapid quenching during progress of the successive clad track.

A detailed X-ray diffraction analysis of the top surface of laser clad H13 tool steel and the same after tempering was undertaken to understand the microstructures and phase distribution in clad zone. Fig. 3 compares the X-ray diffraction profile of H13 tool steel substrate (in hardened and tempered condition) (profile 1), and multi-track laser

axially with laser beam on the substrate surface placed at 14 mm from the nozzle tip (working distance). Argon (Ar) gas was used as carrier gas (at 1.5 bar pressure) to feed the powder with a 4 bar pressure and 7.5 l/min flow rate. Laser parameters were presented in terms of combined parameters like laser energy density (= laser power / (beam diameter × speed)) and powder density (= powder feed rate / (60 × beam diameter × speed)). Powder

-3 -3 2density was varied from 10.6 × 10 to 16 × 10 g/mm to form single layer multi-track overlap claddings with a 45% overlap to obtain 0.85 – 1.3 mm thick clad layer, covering 90 mm x 90 mm surface area of AISI H13 tool steel substrate.

Following laser cladding, a detailed characterization of the clad surface and cross section was undertaken by scanning electron microscopy (Hitachi Model S-4300SE/N, Japan) coupled with energy dispersive spectroscope. The phases present in the microstructure was determined using an X-ray diffractometer (PANalytical Xpert-PRO, Almelo, Netherlands) with Cu-K radiation, operating at 45 kV, 40 mA and a scan rate of α

0.02°/s. The microhardness of the laser treated surface was measured by a Vickers microhardness tester (Walter UHL, VMHT 104) using 200 g applied load. Finally, the wear resistance of clad surface was evaluated against WC ball (of diameter 5 mm) by fretting wear testing at an applied load of 20 N, stroke length of 1 mm and oscillating frequency of 10 Hz.

Results and discussion

Figs. 1 (a-c) show the optical micrographs of the cross section of multi-track laser clad lased with an applied

2laser energy density of 133 J/mm and powder density of -3 2 -3 2 -3(a) 10.6×10 g/mm (b) 13.3×10 g/mm and (c) 16×10

2g/mm with 45% overlap between two successive clads. From Fig. 2 it is observed that with varying powder density for same applied laser energy density and same inter-clad overlap, the height of clad layer increased with increasing powder density. As the single clad height and width changed with powder density, during overlap multi-track cladding the same will induce surface roughness (waviness) as evident from the Fig. 1(a-c). Post clad machining of the multi-track layer surface will remove the roughness to yield the smooth surface with effective clad layer height as shown in Fig 1(c). Hence, the effective clad height measured for different applied

-3 2 -3 2powder densities 10.6×10 g/mm , 13.3×10 g/mm and -3 216×10 g/mm are 0.85 mm, 1 mm and 1.3 mm

respectively. From Fig 1 it may be noted that the clad zone is continuous and defect free with the presence of a very shallow heat affected zone with an average depth of 0.5 mm. Depending on the microstructures obtained across clad layer different zones like clad zone and overlap zone

Fig. 1: Optical micrographs of the cross section of multi-track laser clad lased with an applied laser energy density of 133

2 -3 2 -3J/mm and powder density of (a) 10.6×10 g/mm (b) 13.3×10 2 -3 2g/mm and (c) 16×10 g/mm

Fig. 2: Scanning electron micrographs of the cross section of multi-track laser clad (a) center of the clad, (b) overlapped

2zone, processed with a laser energy density of 133 J/mm and -3 2powder density of 13.3×10 g/mm


35

A detailed study of fretting wear behavior of as received, laser surface clad and post-clad tempered samples was carried out using fretting wear testing machine against tungsten carbide (WC) ball with an applied load of 20 N. Fig. 5 shows the fretting wear behavior in terms of scar depth profile measured across transverse direction of the scar on hardened and tempered H13 tool steel substrate (curve 1), laser clad processed with laser energy density

2 -3 2of 133 J/mm and powder density of 13.3 X 10 g/mm (curve 2), at an applied load of 20 N. A detailed comparison of the depth and width of wear scars of laser surface clad samples with that of hardened and tempered H13 tool steel substrate shows that there is a significant reduction in wear loss in laser clad surface as compared to the as received substrate. Improved wear resistance of

clad H13 tool steel processed with laser energy density of 2 -3 2133 J/mm and powder density of 13.3 X 10 g/mm

(profile 2). From Fig. 3 it is evident that the phases present in both conventionally heat treated H13 tool steel substrate and as-clad H13 tool steel surface consists of mainly martensite. There is no signature of presence of retained austenite or carbides in the X-ray diffraction profiles, which possibly is due to its presence in a very low mass fraction. However, a careful analysis of the XRD peaks shows that there is marginal broadening of the peaks which is mainly due to the presence of lattice strain and refinement of crystallite size. A detailed analysis of the broadened peaks shows that there is a marginal decrease in crystallite size due to cladding (to 19 nm) as compared to hardened and tempered H13 tool steel (22 nm).

Fig. 4 shows the microhardness profile across the multi-track laser clad surface processed with laser energy

2 -3density of 133 J/mm and powder density of 10.6 × 10 2 -3 2g/mm (curve a) 13.3 × 10 g/mm and (curve b), and 16 ×

-3 210 g/mm (curve c). All laser clad layers showed the uniform hardness profile across the multi-track laser clad with enhanced hardness value ranging between 600-650 VHN compared to hardened and tempered H13 tool steel (480 VHN). The increased hardness of the clad surface is attributed to presence of fine supersaturated martensite and carbides in the microstructure (cf. Fig. 2 a and b). Hardness variation across the clad layer may be attributed to reheating and subsequent rapid cooling effect on previous clad while successive next clad is being deposited, due to which thermal gradient will be generated in the previous clad leading to the development of gradient microstructure with martensite of different degree of super saturation.

Fig. 3: X-ray diffraction profiles of H13 tool steel substrate (in hardened and tempered condition) (profile 1), multi-track laser clad H13 tool steel processed with laser energy density of 133

2 -3 2J/mm and powder density of 13.3 X 10 g/mm (profile 2)

Fig. 4: Microhardness profile across the multi-track laser clads 2surface processed with laser energy density of 133 J/mm and

-3 2powder density of (curve a) 10.6 × 10 g/mm (curve b) 13.3 × -3 2 -3 210 g/mm and (curve c) 16 × 10 g/mm

Fig. 5: Vertical cross-sectional depth profile of the fretting wear scar (transverse section) developed on hardened and tempered AISI H13 tool steel substrate (curve 1) and the laser clad

2processed with laser energy density of 133 J/mm and powder -3 2density of 13.3 × 10 g/mm (plot 2), at applied load of 20 N


36

samples with a significant difference in the coefficient of friction value. Hence, from the coefficient of friction values at a higher load it may be concluded that the degree of oxidation and the nature of oxide scale (which is responsible for the reduction in coefficient of friction) are same for all the samples under fretting wear at a higher applied load. A comparison of the coefficient of friction values of all samples under steady state shows that coefficient of friction is significantly reduced due to laser cladding as compared to as-received substrate, with a minimum value achieved in laser clad surface. The reduced coefficient of friction due to laser cladding is attributed to its improved hardness and supersaturation of the matrix with alloying elements, particularly, chromium.

Summary and Conclusions:

The present study aims at a detailed investigation of the effect of laser cladding of AISI H13 tool steel on its microstructure and tribological properties (wear resistance). The major conclusions drawn from this study are:

1. The microstructure of the clad zone shows the presence of coarse prior austenite dendrites with the inter-dendritic spacing covered with carbides, along with the presence of martensite and retained austenite.

2. An improvement in microhardness value to a level of 600-650 VHN was observed in as-clad surface as compared to 480 VHN for hardened and tempered H13 tool steel.

3. Significant improvement in fretting wear behavior was observed in clad zone against WC surface by fretting wear testing as compared to hardened and tempered H13 tool steel substrate.

4. Laser surface engineering reduces the coefficient of friction under steady state (0.35) considerably as compared to as-received substrate (0.60) under fretting wear testing at 20 N load.

Acknowledgements:

Partial financial assistance for the present study from the Department of Science and Technology, New Delhi, Board of Research in Nuclear Science, Bombay, Aeronautic Research and Development Board, New Delhi, and J. C. Bose Fellowship (to I. Manna) are gratefully acknowledged. Dr. G. Sundararajan, Director and Dr. S. V. Joshi, Additional Director, ARCI – Hyderabad acknowledged for their support throughout the course of this investigation. Thanks to Mr. Ramesh Reddy K. of ARCI for his help in X-ray diffractometry and surface profilometry.

laser clad surface is attributed to improved hardness, uniformity in microstructure and a superior microstructural integrity due to laser surface cladding.

Fig. 6 shows the variation of co-efficient of friction measured during fretting wear against WC ball at an applied load of 20 N of H13 tool steel substrate (curve 1) and laser clad processed with laser energy density of 133

2 -3 2J/mm and powder density of 13.3 x 10 g/mm (curve 2). Fig. 6 reveals that there is an initial increase in co-efficient of friction to a higher value which suddenly drops within a short distance of 100 m of sliding and then gradually increases to reach a steady value. The initial rise in coefficient of friction is attributed to a stronger adhesion between the ball and sample surfaces due to interaction at the local contact points occurring at surface asperities causing hindrance against relative motion between the mating surfaces. The drop in coefficient of friction may arise due to dislodgement or fracture of the adhesive joints caused by shearing force applied at the contact points. Subsequent fragmentation and accumulation of worn debris is likely to change the mode of wear from two bodies to three bodies wear situation between the rubbing surfaces. A detailed comparison of the coefficient of friction values of all samples under steady state wear shows that the coefficient of friction of all samples decreases with increase in applied load. The decreased coefficient of friction with increase in applied load is attributed to formation of a stable oxide scale on the surface which acts as a lubricant and barrier for diffusion between the mating surfaces. Application of a higher load (20 N) reduces the coefficient of friction in both hardened and tempered H13 tool steel and laser clad

Fig. 6: The variation of co-efficient of friction measured during fretting wear of H13 tool steel substrate (curve 1), laser clad

2processed with laser energy density of 133 J/mm and powder -3 2density of 13.3 × 10 g/mm (curve 2) at applied load of 20 N


37

8. G. Roberts, G. Krauss, R. Kennedy, Tool steels, Metals Park, Ohio, USA. American Society of Metals (ASM), 5th edn. (1998).

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38

laser assisted texturization and the advantages of laser assisted techniques over the conventional for increasing the solar cell efficiency.

Need for Texturing on a Photovoltaic Device

The texturization of the surface of silicon wafers is a standard process in the production of silicon solar cells. Texturization has the two important benefits of reducing light reflection from the surface and increasing the average path length of weakly absorbed, long-wavelength light within the silicon as shown in figure 1. The latter benefit more important as the thickness of silicon wafers or films from which the cells are made is reduced. The technology of laser texturization of multicrystalline solar cells with laser texturization steps [5-7].

There are three different kinds of texturization techniques for multicrystalline silicon solar cells which are currently under investigation form implementation in a production line:

a) Acid Texturingb) Reactivate ion etching c) Mechanical texturization

Each of these techniques has some advantage and drawbacks. The application of etches based on HF-HNO 3

induces difficulty in reproducible results due to the random distribution of grains of different crystallographic orientation on the surface of the multicrystalline silicon. Precise control of temperature as a well as composition of etches is highly complex. Reactive ion etches creates a needle like surface, on which screen printing is difficult, but this problem can be

Abstract

Development of texturization on the surface of the crystalline silicon is a major step in the manufacturing process of the solar cell. The texturization on the silicon surface is mainly done to trap the maximum amount of sunlight onto the surface so as to increase the efficiency of the solar cell. Wide varieties of research have been pursued on different texturization techniques of the solar cell. The techniques mainly includes acid texturing, Reactive ion texturing, Mechanical texturing, laser assisted texturing etc. Among these techniques, laser assisted texturing has high flexibility to develops different textured surface for efficient photovoltaic device development. This paper gives a detail review about the different laser assisted texturization and their overall advantages and limitation towards efficient photovoltaic device development.

Keywords: laser; nano-texturing; photovoltaic; surface processing; dry etching; wet etching.

Introduction

Photovoltaic cells based on silicon are increasingly considered as one of the attractive renewable energy sources. Solar cells have been used in situations where electrical power from the grid is unavailable, such as remote area power systems, earth orbiting satellites, consumer systems, hand held calculators, wrist watches, remote radiotelephones, mobile phones, and water pumping applications. Today's world wide electronics and photovoltaic market is dominated by the crystalline silicon (c-Si) technology to achieve high efficiency [1]. Wide variety of research groups are highly focusing towards improving the efficiency of the solar cell, either by tuning tailoring the properties of material [2]. However trapping the light incident on the surface of the photovoltaic cells will also lead to an increase in efficiency of the solar cells, there are several methods, such as the deposition of antireflection coatings and texturing of a surface of the silicon wafers have been used to reduce the surface reflectance of the solar cells. There are wide variety of texturization techniques to improve the efficiency of the solar cell, however laser assisted surface texturing techniques have their own advantages as compared to the other conventional one [3-4]. This article gives a details review about the different types of

Laser Assisted Nano-Texturing of Amorphous and Multicrystalline Silicon Wafers

for Photovoltaic Device Applications1* 2I.A.Palani and N.J. Vasa

1 Mechatronicsand Instrumentation lab, Discipline of Mechanical Engineering, IIT Indore, Indore, India 2Opto-Mechatronics Laboratory, Department of Engineering design, IIT Madras


Fig. 1: Laser textured surface on poly Si


39

laser operating at a wavelength of 308 nm. The XeCl laser has a typical energy of 500 mJ per pulse and 25 ns in pulse duration. The laser fluence investigated ranged from 0.2

2 to 0.7 J/cm

The simple way to improve the silicon based solar cell efficiency is to irradiate the silicon surface with series of nano-second laser pulses, in the presence of a argon gas atmosphere. This will generate micro spike on the silicon surface that strongly reduces the incident solar light reflection. Figure 3 shows the SEM micrograph shows the pillar type structure formed by the capillary waves when silicon wafers are treated with a nano-second laser. The periodicity of the capillary wave center is usually close to the laser wavelength, they are formed by the interference between the incident beam and the light scatters by the minor surface defects. Ablation and melt formation occurs at non-uniform depths; after re-solidification, the ripple structure is frozen in place and acts as a precursor for the formation of the beads cones and spikes.

For higher energy densities and number of pulses those capillary waves tend to collapse to form a more hydro dynamically stable structure. The absorption of light on these beads is not uniform: beads which tends to amplify the phenomena and creates more erected structures (“penguin-like” structures), when increasing the energy density and the number of laser pulses.

However without using the carrier gas, these kinds of spikes were not observed. The height of these structures is approximately 10 µm, with a spacing of 2.5 µm as measured by SEM. These structures tend to trap the incident light by increasing the amount of mirror-like and diffuse reflections [10].

Laser based wet etching (LIBWE)

Laser Induced Back side wet etching is a non thermal micro-etching in which the etchant plays a major role on surface etching, however interaction of laser have their own effect in etching. Figure 4(a) shows the working principle of LIBWE in which a liquid layer is kept in contact with the silicon and the silicon acts as a transparent medium when the Co laser is made to pass 2

thorough the surface, the laser focused on the absorbed liquid medium resulting in formation of laser induced

overcome by an additional wet chemical etching step. However the additional alkaline etching step brings the disadvantage of a reduced gain in reflectance. Mechanical Texturing may be effective but has some limitations related to textured materials. It cannot be applied specially for thin, wrapped and fragile materials [8].

The possible way to overcome this problem is the use of laser processing. Strongly coherent and monochromatic laser beam focused to small spot produces high power density. High quality laser beam makes it possible to use the laser technology for processing, which is impossible to carry out with any different techniques. The wide range of applied power, power densities available from lasers and the possibility of accurate laser beam control are the key features which contribute to its successful application in many different aspects of surface processing [9].

Laser Texturing of Semiconductor Materials

Laser based dry surface etching

Laser processing of silicon has received significant attention in the last several decades. L.A. Dobrzanski et, al has done texturization n on silicon wafers by means of diode pumped pulsed Nd:YAG laser (Neodymium doped Yetrium aluminum garnet) operating at wavelength of 1064 nm [8]. Laser settings were adjusted experimentally by producing different textures

The texture consisting of parallel grooves with spacing of 50 Ω were produced. The successive grooves were scribed with the constant spacing within consecutive scanning the wafer surface by laser beam, in the opposite directions as shown in figure 2 (a). The texturization was carried out by directing a pulsed laser beam onto the

-7bottom capacitor plate in a vacuum of 10 Torr prior to the deposition of the cell dielectric Using a XeCl excimer

Fig. 2: (a) Laser based surface etching (b) Generation of periodic structures through laser

(a)

(b)

Fig. 3: Silicon pillar formation through direct laser ablation


40

extent. Figure 5 shows pictorially the overlap of the consecutive laser beams and the three regions, namely irradiated regions corresponding to the preceding and succeeding laser pulses; and the overlapping interface region. To maintain uniformity, the overlapping was done along the Y-axis also after completing the processes along the X-axis for a length of 1.5 cm. the procedure is shown in Fig 5.

In the study, to induce texturization two trials were conducted in which the samples were annealed with each subsequent laser beam overlapped at 50% and 90% in terms of the diameter of the laser spot. To investigate the influence of the laser fluence on the annealing and the subsequent texturing, a-Si films coated on glass and crystalline silicon substrates were treated with different fluence values by altering the laser beam energy.

To understand the phenomena of laser assisted texturing with beam overlap, Marangoni effect may be considered. Marangoni convection occurs when the surface tension of an interface depends on the temperature distribution or the concentration of the species [11-12].

In the case of temperature dependence, the Marangoni effect is called thermo capillary convection. Various stages of peak generation in the interface region are shown in Figs. 6 (a) to (c). The Marangoni effect is the mass transfer along an interface due to the surface tension

plasmas. Confining these plasma may lead to etch silicon from rear side.

Figure 4(b) shows the SEM micro graph of the random nanostructures generated on the silicon wafer, during a preliminary run. Figure 4(c) shows the proposed V-groove periodic structure for trapping sunlight in a photovoltaic cells for improved efficiency.

Laser based surface annealing combined texturing

Laser annealing combined texturing a recently developed technique to convert low cost amorphous silicon to high quality textured polycrystalline silicon for photovoltaic applications. In the case of laser annealing combined with laser texturing, the laser pulse is first made incident on the surface of the a-Si. After the phase transformation is completed, the next laser spot is made to overlap with the previously irradiated spot to a certain predetermined

Fig. 4: (a) Laser Induced Backside Wet Etching (b) Silicon wafer etched using LIBWE (c) Proposed structure through LIBWE for photovoltaic device

(a)

(b)

(c)

Fig. 5: Laser annealing and subsequent texturing using spot overlap

Fig. 6: (a) molten Si flow during laser texturing (b) molten Si flow during cooling c) growth of crystalline Si peak


41

Fig. 7 (b) shows spectroscopy measurements of the % reflection between the untreated and the laser textured samples at 50 and 90% overlap in the region of visible spectrum and Fig. 7 (a) shows the reflection in the region of infrared region, in both these regions, laser textured samples at 90% overlap showed a reduction in reflection. At a typical wavelength of 1100 nm, the absorption factor for the untreated a-Si film was around 0.5. On the other hand, the absorption factor value was improved to 1.25 and 1.68 for samples textured with 50% overlap and 90% overlap, respectively [15].

Line Beam Texturing for Faster Manufacturing Rate

Conventionally laser's are with a circular spot which may result in low scanning rate, hence as an alternative approach, laser beams are transformed to produce a line beam with plano convex cylindrical lens. These line beams can overcome the limitations of laser beam with a circular spot. The line beams can be directly used for laser annealing with a faster manufacturing rate. In this section an attempt has been made to convert a circular beam to a line beam using a cylindrical lens and the feasibility study on laser annealing and subsequent texturing is investigated. Line beam generation is expected to allow fast scanning of the laser beam over a-Si film. A cylindrical lens was used in converting the circular shape laser beam into a line beam.

Figure 8 (a) shows an experimental setup for simultaneous laser annealing and texturing with a line shape laser beam. The laser considered for the

3+experiment was a solid-state pulsed Nd : YAG laser with a wavelength of 355 nm.

The laser pulse was controlled for single-shot treatment. Laser beam of Gaussian profile was made to incident on the plano-convex cylindrical lens of focal length f = 300 mm. The length and width of the line beam were 5 mm and 0.5 mm, respectively. Laser fluence values of 240,

2 360 and 560 mJ/cm were used with 90% beam overlap. Experiments were conducted with a-Si film deposited on glass substrate. The a-Si sample was mounted on an X-Y stage for line beam annealing.

To confirm the crystallanity and texturing of a-Si film coated on glass substrate, Raman spectroscopy was performed on the laser treated samples. In case of laser-

2treated a-Si film at 360 mJ/cm a typical Raman shift at -1521 cm was observed which confirmed the

crystallization characteristics. At the laser fluence of 560 2 -1mJ/cm , a Raman shift of 513 cm was observed which

confirms the ablation characteristics.

In addition to crystallization characteristics the samples were analyzed through AFM to investigate the surface characteristics. Fig. 8(b) shows the AFM image of the textured peak. A maximum Rq value of 420 nm was observed [16-17].

Fig. 7: Shows spectroscopy measurements of the % reflection between the untreated and the laser textured samples at 50 and 90% overlap

gradient. Since a liquid with a high surface tension pulls more strongly on the surrounding liquid, the presence of gradient in the surface tension will naturally cause the liquid to flow away from the region of low surface tension. During the heating cycle, the Marangoni convection results in a vortex motion and liquid movement towards the region of low temperature as shown in Fig. 6 (a). On cooling, due to the decrease in temperature and the surface gradient, the convective motion originating from the Marangoni effect will be damped and the hydrodynamic motion will then evolve as shown in Fig. 6 (b). Capillary waves will be generated due to the surface tension. These capillary waves will exist in the liquefied region until the fused silicon congeals. During cooling, the amplitude of the surrounding surface will first decrease and transforming to its mirror image by passing a phase where there is practically a smooth surface. This may result in splashing of material along the liquid trench axis as shown in Fig. 6 (c).

Hence the formation of peaks depends on the initial temperature of the liquid and the size of the liquid zone. This leads to the formation of a rough pattern at the interface [13-14].


42

much material removal/wastage

4. The concept of laser assisted annealing combined texturing was also demonstrated using a line beam laser for a faster scan rate.

Acknowledgement:

The authors would like to acknowledge Prof. M. Singaperumal from IIT Madras and Prof. T. Okada from Kyushu University for their constant encouragement and support to bring out this work.

References

1. M.J. Ariza, F. Martin and D. Leinen, Appl. Phys. A 75, 579 (2001).

2. R.B. Bergmann, Appl. Phys. A 69, 187 (1999).

3. N. Bianco. and O. Manca, Int. J. Thermal Sci. 43, 611 (2004).

4. M. Birkholz., B. Selle, E. Conrad, K. Lips and W. Fuhs, J. Appl. Phys. 88, 4376 (2000).

5. S. Ishigame, K. Ozaki, T. Sameshima and S. Higashi, Solar Energy Materials and Solar Cells 66, 381 (2001).

6. S.W. Park, J. Kim, J. Korean Phys. Soc. 43, 423 (2003)

7. S.H. Lee, J. Korean Phys. Soc. 39, 369 (2001)

8. L.A. Dobrzaeski, A. Drygaaa, J. Achievements in Materials and Manuf. Eng. 17, 321 (2006).

9. L.A. Dobrzaeski, A. Drygaaa, P. Panek, M. Lipieski, P. Zicba, J. Achievements in Materials and Manuf. Eng. 24, 179 (2007).

10. L.A. Dobrzaeski and A. Drygaaa, J. Achievements in Materials and Manuf. Eng. 31, 77 (2008).

11. I.A. Palani, N.J. Vasa, M. Singaperumal, Mat. Sci. and Semiconductor Processing 11, 107 (2008).

12. I.A. Palani, N.J. Vasa, M. Singaperumal, T. Okada, J. Laser Micro and Nano Eng. 5, 150 (2010).

13. I.A. Palani, N.J. Vasa, M. Singaperumal, T. Okada. IEEE Region 10 Annual International Conference, Proceedings/TENCON, 5686429: 1909 (2010).

14. I.A. Palani, N.J. Vasa, M. Singaperumal, T. Okada, Thin Solid Films 518, 4183 (2010).

15. I.A. Palani, N.J. Vasa, M. Singaperumal, T. Okada, Proc. of SPIE, 7584: 758410-8 (2010).

16. G. Amutha, I.A. Palani, N.J. Vasa, M. Singaperumal, T. Okada, J. Solid Mechanics and Materials Eng. 7, 206 (2013).

17. N.J. Vasa, I.A. Palani, M. Singaperumal, T. Okada, J. Optics and Precision Eng. 19, 2263 (2010).

Conclusion

The concept of trapping the incident sunlight on the surface of the photovoltaic solar cell is a focussed theme of different research group. For the past few years texturization of the multi crystalline silicon has been performed using acid texturing, Reactive ion etching, Mechanical texturing etc. However these techniques have their own advantages and limitations. Hence as an alternative approach laser assisted surface texturing has been adopted due to its flexibility. Laser assisted texturing of photovoltaic cell can be adopted in four different ways

1. Laser based dry surface etching: Laser with different frequencies and pulse width is made to interact on the surface of the silicon to develop periodic nanostructures and Nano pillars for efficient trapping of light

2. Laser Induced Backside Wet Etching (LIBWE) is viable method to develop V grooves on the surface of the multicrystalline silicon

3. Laser assisted annealing combined texturing is a recently developed technique, basically meant to convert amorphous silicon to highly efficient polycrystalline silicon. This techniques could develop random texture on the surface, without

Fig. 8: (a) Line beam laser annealing (b) AFM image of the roughness pattern on a crystalline silicon


43

fluid or solid particles. Solid particle erosion is the loss of material that results from repeated impact of solid particles, suspended in the flowing fluid, at any target surface [1-2]. A schematic view of solid particle erosion is shown in Fig.1.

Slurry erosion occurs when surface material suffers from impingement of the solid erosion particles suspended in the carrier liquid. Slurry erosion has gained importance in second half of 20th century as a number of slurry transportation systems came into existence during that period. The flow of solid-liquid mixture causes erosion wear of equipments/ components, which reduces their service life. The slurry erosion problem generally occurs in petroleum extraction wells [3], coal liquefaction processes [4-8], solids handling systems [9-11], Hydraulic turbines [12].

Erosion wear determines the service life of the equipments/ components handling solid–liquid mixtures. Different methods like heat-treatment, surface coatings, etc. were employed to improve the surface characteristics of the target material to minimize the erosion wear. Among these methods, the laser cladding process has many advantages over conventional ones such as good bonding between clad layer and substrate, low heat input and distortion of the substrate, etc. [13].

Zhanget al. [14] have fabricated Ni-alloy clad layer on martensitic stainless steel and evaluated its erosion wear with and without heat-treatment using a slurry pot tester. They found around 37% improvement in erosion resistance of the clad surface compared to that of the substrate. Similar improvement in the cavitations erosion resistance of Ni–Cr–Fe–WC clad layer was observed by Tam et al. [15]. They attributed this improvement to the formation of Ni-richmatrix reinforced by precipitation of carbides and tightly bound WC particles with proper selection of laser cladding parameters. Przybylowicz and Kusinski [16] have developed T-400 cladding of high quality and low dilution. The improvement in hardness of the clad layer compared to the substrate has shown significant improvement in the cavitations erosion resistance.

Iwai and Nambu[11] have used TiN coating on high-speed steel by single layered and multi-layered PVD process. They found that the PVD coating gives 50–91%

Abstract

In present study slurry erosion behaviour of substrate material AISI SS304L steel and Tribaloy T-700, PAC 718 and METCO 41 C clad surfaces were carried out using slurry erosion test rig. The substrate material was cladded using a 2kW continuous fiber laser with coaxial powder feeding nozzle to investigate the improvement in slurry erosion characteristics. The Tribaloy T-700 and PAC 718 cladded surfaces show 34% and 23.5% improvement in erosion resistance compare to substrate material, respectively. However, METCO 41 C laser clad surface has shown little improvement in erosion resistance compare to the substrate material. The SEM micrographs of worn out Cladded Surfaces at shallow impact angles show that the material is removed mainly by, the platelet mechanism and material displaced in the direction of flow which increases with increase in the impact angle. Whereas, at normal impact angle the indentation craters with rim are observed and gets flattened and finally fractured due to repeated impacts.

Keywords: laser cladding; slurry pot tester; erosion wear; quartz.

Introduction

Wear is defined as the progressive volume loss of material from a target surface. It may occur due to corrosion, abrasion and erosion. The wear due to corrosion is caused by chemical reactions, which can be prevented by adopting suitable measures; whereas the wear due to abrasion and/or erosion can only be minimised by controlling the affecting parameters. Erosion may take place due to mechanical interaction between the target surface and fluid, multi-component

Slurry Erosion Wear Characteristics of Laser Clad Surfaces

Satish More and G.R.Desale*CSIR-National Chemical Laboratory, Pune


Fig. 1: Erosion wear by solid particle impact [2]


44

EZEECUT NXG CNC EDM wire cutting machine. Similarly, the wear test specimens were cut from the substrate material. These test specimens were polished by #1000 emery paper to achieve initial identical conditions for the cladded and substrate specimens.

Range of Parameters

To investigate the erosion wear behaviour of laser cladded surfaces, Indian Standard sand of 550 m particle size with water was used. The range of parameter for erosion testing is given in Table 1. The substrate the cladded wear specimens were tested at 3.71 m/s velocity in sand-water mixture of 10% by weight concentration for 90 minutes duration. The wear specimens were oriented in the range of 15 to 90 angles with respect to the direction of its rotation inside the slurry pot tester.

Erosion Wear Testing

A pot tester of approximately 7 litre capacity, as shown in Fig. 3 (a–c) has been used in the present investigation. The test specimens of substrate and cladded surfaces were polished to achieve identical condition. Wear specimens were cleaned with tap water, rinsed in acetone and dried with hot air blower before and after each test. Mass loss of the wear specimens after each test was measured by an electronic balance within ±0.1mg. Experiments were performed with solid–liquid mixtures of 10% by weight concentration. The solid–liquid mixture was prepared by mixing solid particles in water. A predetermined mass of sand was poured first in the pot and then it was closed by tightening the acrylic cover. Known quantity of water was then added through the hole at the top of the cover to completely fill the pot. The propeller shaft was then rotated in down-pumping mode at suspension speed of 340 rpm, which was predetermined to achieve nearly uniform distribution of solids in the pot. As shown in Fig. 3(b), the fixtures were oriented using angular plate (Fig. 3 (c)) at 15°, 22.5°, 30°, 45°, 75°and 90°angles with respect to the peripheral direction of their rotation. The specimens were rotated at 404rpm speed in a direction opposite to that of the propeller to achieve an average peripheral velocity of 3.71 m/s. The speed of each shaft was monitored by using

higher erosion resistance than that of the substrate material. Mann [12] has used coating for mitigation of slit effect in hydro-turbines of hydropower station located in Himalayan region. He observed minimum volume loss for borided T410 steel followed by D-gun sprayed tungsten carbide, borided 13Cr–4Ni steel, hard chrome plating, plasma nitriding and D-gun coated chromium carbide steels. Zhaoet al. [17] have applied ceramic coatings to protect the pump impeller due to slurry erosion–corrosion. The test specimens were tested in jet-in-slit rotating tester for erosion. They found that the ceramic coatings are effective in preventing the erosion damage to the pump impeller. Tu et al. [18] have fabricated TiN coatings on commercial-Ti alloy and tested in jet-in-slit tester with angular silica sand at velocities between 6.4 and 15.2 m/s. They observed that the wear specimens show high slurry erosion resistance particularly at low velocities. Speyer et al. [19] have applied aluminium base coatings on AISI 1020 steel for improvement in erosion resistance. They observed that aluminium does not increase the erosion resistance but alloying with silicon improves its performance. They reported that erosion resistance increases within crease in the micro-hardness, particularly at 30° impact angle compared to 90° impact angle. Deuiset al. [20] have observed that the aluminium–silicon alloys and aluminium-based MMCs containing hard particles offer superior operating performance and resistance to wear.

In Present work attempts have been made to investigate the erosion wear behaviour of laser cladding of different hard facing powders on AISI SS304L substrate. The processing parameters for single pass Laser clad were optimised and used to get 50% overlap clad layers on the large surface of the substrate and further used for mechanical and metallurgical testing. These cladded samples were used to prepare the test specimens for erosion wear testing. The erosion wear data of cladded surfaces and substrate material are graphically presented for understanding the erosion wear behaviour. Additionally, the eroded surfaces were examined under the scanning electron microscope to understand the mechanism of material removal.

Experimental Procedure

Cladded Wear Test Specimens

The Tribaloy T-700, PAC 718 and METCO 41 C powders were deposited on the stainless steels by 50% overlapped clad tracks to cover a large surface area for preparing wear specimens. The photographic views of the cladded surfaces are shown Fig.2. The clad surface was machined to remove the waviness and un-melted solid particles. The substrate material was then machined to obtain a 2 mm thick plate. From this 2 mm thick plate, the wear specimens of size 30 mm x 5 mm were cut using

Fig. 2: Photopraphic vew of 50% overlap clad layer


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specimen, m/s; W -Measured mass loss, kg; α - L

Orientation angle of wear specimen, degree.

Results and Discussion

The main objective of the present investigation is to develop a hard facing surface using laser cladding. The erosion behaviour of the clad surface is evaluated in the slurry pot tester at different orientation angles and compared with that of the substrate material. This could help to enhance the life span of the equipments handling solid-liquid mixture.

Erosion Wear Behaviour

The wear behaviour of both the substrates and the claddings of Tribaloy T-700, PAC 718 and METCO 41 C has been evaluated using slurry pot tester at 3.71 m/s velocity with quartz-water particulate mixture of 550 μm size particles. The solid concentration was kept as 10% by weight and the orientation angle was varied in the

Orange of 15 to 90 . The measurement of mass loss in known time period was used to evaluate the erosion rate.

a non-contact type tachometer. For each wear test, mass loss of the each of the two wear specimens was measured individually over a known time period and the average mass loss value of the two specimens was used in further analysis.

The average mass loss of two wear specimens is used to evaluate the erosion rate according to the relationship proposed by Bree et al. [21] which is as below,

(1)

(2)

Where, ρ - Mass density of the solid particle material, S 3kg/m ; A - Surface area of the wear specimen subjected SP

2to erosion, m ; Cv- Solid concentration by volume, fraction; Cw- Solid concentration by weight, fraction; Ew- Total erosion rate, g/g; T - Time over which mass loss has been measured, sec; V - Peripheral velocity of wear SP

Fig. 3: Schematic diagram of slurry pot tester [13]


46

Effect of Target Material Hardness

The erosion wear rates of AISI SS304L and three claddings are graphically presented with respective micro-hardness in Fig.5. It is observed that at all orientation angles the erosion rate continuously decreases with increasing the surface hardness of the target materials. The similar trend has been observed and reported by many investigators [23-25]. The increase in the surface hardness increases the resistance to platelet formation and thus reduces the size of craters (i.e. depth, width and length), see SEM micrographs of eroded surfaces in Figs.6-8. This reduction in the size of crater is responsible for less material removal and thus the reduction in the erosion rate with increase in surface hardness is obvious. However, in spite of higher surface hardness of METCO 41 C cladding, it shows higher erosion rate than substrate at 75° and 90° orientation angles. This can be attributed to the higher degree of

Further, the material removal mechanism of cladded surface at different angles is studied with the help of SEM of worn out specimens.

The average mass loss data of two wear specimens at different orientation angles in the quartz-water mixture for substrate material and three cladded surfaces the erosion rate is calculated for the experimental conditions i.e. at 3.71 m/s velocity, 550 μm particle size and 10% weight concentration. The variation of the wear rate with orientation angle is graphically presented in Fig. 4 for three cladded surfaces and compared with the erosion data of substrate material. It is seen that the maximum

Owear rate is observed at around 37.5 orientation angle, which decreases continuously with further increase in the angle till 90.

It is seen that AISI SS304L shows increase in erosion rate with increase in the orientation angle showing maximum

-8erosion rate as 5.282 X 10 g/g at 37.5 orientation angle, Owhich decreases with further increase in the angle till 90 .

The Tribaloy T-700 clad layer also shows similar variation of the wear with the angle showing maximum

-8 Owear as 3.473 X 10 g/g at 37.5 orientation angle. It is observed that Tribaloy T-700clad shows improvement in erosion wear resistance in the range of 13.5 to 34.25% compared to the substrate material i.e. AISI SS304L steel

Oover the range of orientation angles. At 37.5 orientation angle, the cladding shows the maximum improvement of 34.25% in erosion wear resistance which may be due to the increase in clad layer surface hardness (534 HV (load 0.981 N)) reducing the cutting wear contribution.

The erosion rate (g/g) of PAC 718 and METCO 41 C claddings with orientation angle is also presented graphically in Fig. 4. The PAC 718 and METCO 41 C claddings also show similar variation of the wear with the

Oangle showing maximum wear at 37.5 orientation angle. It is observed that PAC 718 clad shows a 23.26%

Omaximum improvement in the erosion resistance at 37.5 impact angle as compared to the substrate material. Whereas, METCO 41 Clad shows a 16.27% maximum

Oimprovement in the erosion resistance at 45 impact angle as compared to the substrate material.

Thus, it can be observed from Fig.4, that the all cladded surfaces show significant improvement in the erosion wear resistance at shallow impact angles compare to higher impact angles. However, METCO 41 C clad does not show any significant improvement in erosion wear resistance at higher impact angles compared to that of the

O Osubstrate material. While at 75 and 90 impact angles, the erosion of METCO 41 C clad layer increases compare to substrate. A possible reason for no improvement in erosion wear resistance or higher material removal rate of clad layer may be the higher degree of dilution and insignificant improvement in micro-hardness of the cladded layer [12].

Fig. 4: Variation of erosion rate with orientation angle for substrate and three cladded surfaces (d = 550 µm, Cw = 10 %, V = 3.71 m/s)

Fig. 5: Effect of surface micro-hardness on erosion wear rate. (AISI SS 304 L = 204 HV (0.981 N), METCO 41 C = 294 HV (0.981 N), PAC 718 = 321 HV (0.981 N), Tribaloy T -700 = 534 HV (0.981 N))


47

dilution in the clad layer which may change in the grain structure [12].

SEM Examination

The surface morphologies of worn out surfaces of the substrates and the clad specimens (Tribaloy T-700, PAC 718 and METCO 41 C) after erosion with quartz-water particulate mixture are shown in Figs. 6-8. The cladded surfaces subjected to erosion wear show two distinct erosion mechanisms namely, micro-cutting or plastic deformation and brittle fracture at different orientation angles.

The SEM micrographs of worn out surfaces of the Osubstrate and three cladded specimens at 15 orientation

angle are presented in Fig. 6 (a-d). It is observed from micrographs that the worn out surfaces of substrate and other cladded material show ploughing type craters at low impact angle. These craters show the prominent leading edge lip formed along the flow direction. The craters observed on the eroded surface of substrate material are comparatively larger in size than the craters present on the cladded surfaces. This may be attributed to the higher hardness of the cladded surface than the substrate material hardness. The increase in the surface hardness increases the resistance to platelet formation and thus reduces the size of craters (i.e. depth, width and length). This reduction in the size of crater is responsible for less material removal and thus the reduction in the erosion rate with increase in surface hardness is obvious.

The parallel and normal components of velocity of impacting solid particles are responsible for displacing the material and deep indentation, respectively. At acute angles, the normal component of particle velocity is smaller than the parallel component, thus the particle unable to penetrate deep in the cladded surface or to deform it plastically as a combined effect it results in long size crater. Thus in overall, shallow deep and long craters

Oare observed at acute (15 ) impact angle. Additionally, at this angle the particles are having higher residual kinetic energy while leaving the target surface and results in lower material removal compare to slightly higher impact angle. This can be understood from Fig. 7, where the impacting particle is having higher normal component of velocity results in deep craters and lower parallel component of velocity results in shorter in length craters. Apart from the impacting particle parameters, the properties of target material, namely, hardness and toughness are playing important role to decide the size and shape of crater.

Increasing the impact angle of particle increases the normal component of velocity and material is removed by plastic deformation due to indentation. The SEM micrographs of substrate and other three cladded surfaces eroded at normal impact angle are presented in Fig. 8. It is

(a) Worn out AISI SS304L surface (b) Worn out Tribaloy T-700 clad Surface

(c) Worn PAC 718 clad surface (d) Worn out METCO 41 C clad surface

Fig. 6: Mechanism ofmaterial removal for AISI SS 304L and three different cladded surfaces at 15° impact angle (erodent material: IS Sand, d=550 µm, Cw=10 %, V = 3.71 m/s)



Fig. 7: Mechanism ofmaterial removal for AISI SS 304L and three different cladded surfaces at 37.5° impact angle (erodent material: IS Sand, d=550 µm, Cw=10%, V=3.71 m/s)

Fig. 8: Mechanism ofmaterial removal for AISI SS304L and three different cladded surfaces at 90º impact angle (Erodent material: IS Sand, d=550 µm, Cw=10%, V=3.71 m/s)




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7. G.W. Govier, K. Aziz, The flow of complex mixtures in pipes, Van Nostrand Reinhold Company, New York, (1992).

8. K.C. Wilson, G.R. Addie, R. Cliff, Slurry transport using centrifugal pumps, Elsevier Science Publishers Ltd., New York(1992).

9. E.J. Wasp, J.P. Kenny, R.L. Gandhi, Solid-Liquid flow slurry pipeline transportation, Series in Bulk Solids Handling, Vol. 1 (1975/77), No. 4, Trans Tech Publications, New York(1977).

10. Y. Zhong, K. Minemura, Wear, 199, 36(1996).

11. Y. Iwai, K. Nambu, Wear210, 211(1997).

12. B.S. Mann, Wear237, 140(2000).

13. G.R. Desale, C.P. Paul, B.K. Gandhi, S.C. Jain, Wear 266, 975(2009).

14. D.W. Zhang, T.C. Lei, J.G. Zhang, J.H. Ouyang, Surf. Coat. Tech. 115, 176 (1999).

15. K.F. Tam, F.T. Cheng, H.C. Man, Mat. Res. Bull.37, 1341(2002).

16. J. Przybylowicz, J. Kusinski, Surf. Coat. Tech.125, 13(2000).

17. H.X. Zhao, H. Goto, M. Matsumura, T. Takahashi, M.A. Yamamoto, Surf. Coat. Tech. 115, 123(1999).

18. J.P. Tu, L.P. Zhu, H.X. Zhao, Surf. Coat. Tech. 122, 176 (1999).

19. A.J. Speyer, R.J.K. Wood, K.R. Stokes, Wear250, 802(2001).

20. R.L. Deuis, C. Subramanian, J.M. Yellup, Wear201, 132(1996).

21. S.E.M. deBree, W.F. Rosenbrand, A.W.J. deGee, On the erosion resistance in water-sand mixtures of steels for application in slurry pipelines, Hydrotransport 8, BHRA Fluid Engineering, Johannesburg, (S.A.), Paper C3(1982).

22. K. Shimizu, T. Noguchi, H. Seitoh, E. Muranaka, Wear 233, 157(1999).

23. J.B. Zu, I.M. Hutchings, G.T. Burstein, Wear140, 331(1990).

24. F.Y. Lin, H. Shao, Wear 141, 279(1991).

25. G.P. Tilly, Wear14, 241(1969).

26. R. Jr. Bellman, A. Levy, Wear, 70, 1(1981).

observed that the long platelet ploughing type craters (smear type crater) disappears which are replaced by indentation craters, which is well know mechanism of erosion at this angle [26]. The rounded indentation craters are observed where the material is displaced by plastic deformation around the crater and forms a rim. Due to further impact of the particles, the rim gets flattened and repeated impacts results in fractured.

Concluding Remarks

The experimental investigations on erosion wear behaviour of AISI SS304Lsubstrate and laser cladded surfaces have been carried out systematically. Following broad conclusions can be drawn from this investigation:

1. The substrate material AISI SS304L shows the maximum material removal or peak at 37.5° impact angle and minimum at normal impact angle. Similar erosion wear pattern has been observed for all three cladded surface. Among them Tribaloy T-700 clad surface shows improvement in erosion resistance as compared to the substrate AISI SS304L. While, other two cladded surfaces (PAC 718 and METCO 41 C) show only marginal improvement in erosion resistance at shallow impact angles and poor erosion resistance at normal impact condition compared to the substrate AISI SS304L.

2. In entire range of orientation angles, the erosion wear resistance increase with increasing the surface hardness. However, METCO 41 C shows higher erosion rate than the substrate material at normal impact angle, which can be attributed to the higher dilution of clad layer and grain structure.

3. The SEM micrographs of eroded surfaces depict the mechanism of material removal due to erosion. At shallow impact angles, the platelet mechanism is dominating and material removal is due to micro-cutting and material displaced in the direction of flow. The material removal mechanism for the substrate and clad surface remains same except the size and shape of the craters. While at normal impact angle the platelet (smear) craters disappears and indentation craters with rim are observed.

Acknowledgement

The authors gratefully acknowledge financial support from CSIR-NCL Pune and thankful to Dr.Sourav Pal, Director, and Dr.VivekRanade, Chair, CEPD Division, CSIR-NCL, Pune.

References

2. D.H. Buckley, Surface effects in Adhesion, Friction, Wear and Lubrication, in: Tribology Series

1. T. H. Kosel, ASM Handbook, 18, 199 (1992).


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new layer as per shape and dimensions defined in NC code. A number of such layers deposited one over another and it results in three dimensional (3D) components directly from the solid model. Fig. 1 presents the general scheme of LRM technique.

LRM eliminates many manufacturing steps such as materials-machine planning, man-machine interaction, intermittent quality checks, assembly and related human errors etc. Therefore, LRM offers many advantages over conventional subtractive techniques, such as reduced production time, better process control and capability to form functionally graded parts. It is also an attractive candidate for refurbishing applications because of low heat input, limited dilution with minimal distortion and capability of adding finer near-net shaped features to the components.

Manufacturing techniques, similar to LRM, are being developed with different names at various laboratories around the world [1]. At Sandia National Laboratory,

TMUSA, Laser Engineered Net Shaping (LENS ) is being developed with prime focus on creating complex metal parts in single day. National Research Council, Canada is developing Freeform Laser Consolidation for manufacturing of structural components for advanced robotic mechatronic systems. Automated Laser Fabrication (ALFa) is being developed to produce low cost tungsten carbide components at the University of Waterloo, Canada. Selective Laser Cladding (SLC) at the University of Liverpool, UK and Direct Metal Deposition at the University of Michigan, USA are being used for depositing critical surfaces on prime components. Laser Powder Deposition (LPD) at the University of Manchester UK and Direct Metal Deposition/Laser Additive Manufacturing at Fraunhofer Institute, Germany are being augmented for the fabrication of high

Abstract

Last decade witnessed substantial advancements in high speed computers, computer aided design (CAD) and laser technologies. The convergence of throughput and performance of these advances led to the emergence of new unorthodox technologies including Laser Rapid Manufacturing (LRM). This additive manufacturing process is capable of fabricating engineering components directly from a solid model. LRM is directed to the evolution of next generation “feature based design and manufacturing” and also finds applications in repair of existing prime components at lower cost with improved functionality. This article deals with various aspects of LRM and provides an insight into the underlying basic principles, applications and future prospects.

Keywords: laser rapid manufacturing; additive manufacturing; solid structures; porous structures; hard surfaces.

Introduction

The realm of manufacturing was dominated by subtractive or material removal process for many centuries. It included not only the machining processes, but also the formative processes like- casting and forming. The introduction of “Streolithography” in 1987 broke this trend and a new generation of the processes “3D printing” or “additive manufacturing” or “rapid manufacturing” came into existence for prototyping applications. It revolutionized the very concept of manufacturing and brought a “bottom-to-top” fabrication approach. Laser rapid manufacturing is one of such advanced manufacturing processes that is capable of fabricating engineering components directly from a solid model by material addition. In this technique, a solid model of the component to be fabricated is made either by 3D imaging system or by designer using computer aided design (CAD) software or by math data as an output of numerical analysis. Thus obtained model is sliced into thin layers along the vertical axis. The thin layers are converted into corresponding numerical controlled (NC) code and are sent to LRM station in suitable format (e.g. G&M code). LRM station employs a laser beam as a heat source to melt a thin layer on the surface of the substrate/deposited material and fed material to deposit a

Laser Rapid Manufacturing: A Pursuit of Unorthodox Manufacturing

C.P. Paul*, Atul Kumar, P. Bhargava and L.M. KukrejaLaser Materials Processing Division

Raja Ramanna Centre for Advanced Technology, Indore INDIA*E-mail: [email protected]

Fig. 1: General Scheme of Laser Rapid Manufacturing


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angular tilt of V axis is ± 110⁰, whereas W axis is capable

of 360⁰ continuous rotation. The manipulator is interfaced with standard computer numerical controller for manipulating workstation. The laser head coupled with manipulator is mounted in a glove box. The glove box is essentially required for controlling atmospheric conditions during processing. Oxygen and moisture are the main impurities in atmosphere which affects the properties of the deposited bulk materials. Therefore, the system is integrated with oxygen and moisture analyzers.

performance materials. The researchers at Tsinghua University, China are working on diverse area and evaluating the potential of technology for the development of graded Ti alloys for aeronautical, Nickel alloys for power plants and various in-situ repair applications. Thus, the ongoing global research is spearheading towards the deployment this novel fabrication technology for improving qualities of the products, possibilities to engineer integrated multi-materials and multi-functional components and enhancing economic or procedural benefits.

Laser Rapid Manufacturing System

Laser Rapid Manufacturing system consists of the following three primary subsystems.

a. High power laser system

b. Material feeding system

c. Computerized Numerically Controlled (CNC) workstation.

A laser rapid manufacturing system was integrated at authors' laboratory. The schematic arrangement of the system is shown in Fig. 3. It consists of a 2 kW fiber laser system, a 5 axis workstation in a glove box, a computerized numerical controller, a coaxial nozzle, a twin powder feeder, gas analyzers and cameras with image processing hardware. Procured 2 kW fiber laser has output power range 50 W to 2050 W with emission wavelength of 1080 nm. The system has switching on/off time of 80 μs and output modulation rate of 5 kHz. The laser beam is randomly polarized and has product parameter lower than 4. The system has delivery fiber with core diameter of 50 µm. The overall size of the laser system is 1.2 m x 1.2 m x 0.75 m. The output laser beam is passed through refractive focusing optics and indigenously developed compact co-axial nozzle is employed for laser rapid manufacturing. The system is integrated with 5 axis workstation for job manipulation. The 5 axes are X, Y, Z, V and W. X, Y and Z axes are linear traverse axes mutually perpendicular to each other, whereas V is tilt axis about Y axis for tilting laser head and W is continuous rotational axis about Z axis. The effective stroke length of linear axes is 250 mm. The

Fig. 2: presents the various options used for primary LRM subsystems

Fig. 3: Schematic arrangement of LRM system at authors' laboratory

Fig. 4: Typical geometries fabricated using LRM system at authors' laboratory

Table 1: Some typical examples of industrial applications of LRM [2]


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testing at 10 MHz revealed no surface and internal defects respectively. The multi-layer structure of Colmonoy-6 bushes could be very clearly seen during optical microscopy (refer Fig. 6 (a)). The microstructure of colmonoy-6 bush showed a uniform dendrite growth in the direction of build-up. It featured primary dendrite phases and inter-dendrite constituent as shown in Fig. 6 (b).

The compressive testing as per ASTM E9 standard revealed that the compressive strength of LRM fabricated Colmonoy-6 specimens are at par with their wrought counterparts (refer Fig. 7(a). The microhardness profile for the longitudinal and transverse section of laser rapid manufactured Colmonoy-6 is presented in Fig. 7(b).

The desired purity levels are achieved by purging high purity grade Argon gas. In case there is increase in the impurity level, the high purging rate is used to reinstate the indented purity level in the glove-box. The purity level of the glove box is retained by keeping the differential pressure just above the atmospheric pressure.

The typical geometries fabricated using LRM system at authors' laboratory is shown in Fig. 4. Fig. 4 (a) shows the 1.5 mm thick laser clad surface of 50 mm x 50 mm with Stellite-6 on SS316L substrate. Fig. 4 (b) shows typical cross thin walled porous structure of Inconel-625 on substrate having 75 mm base diameter and 12 mm height. Fig. 4 (c) is simple cage of Inconel-625 (size: 40 mm base diameter x 60 mm height), laser rapid manufactured with pulse mode operation of laser.

Typical Applications of LRM

The list of applications of LRM is appending due to global research efforts. Some typical examples of industrial applications of LRM are summarized in Table 1. In the following section, some of the LRM applications developed at our laboratory for various in-house and industrial applications are briefly described.

Colmonoy-6 Bushes

Nickel-based alloys “Colmonoy” are preferred for hardfacing applications in nuclear power plants due to their outstanding wear resistance, high hardness at elevated temperatures and low induced radioactivity. Pre-fabricated Colmonoy-6 bushes are used as substitute to local hardfacing at complicated component geometry having limited accessibility. Conventionally, these bushes are made by casting/weld deposition followed by machining [3]. However, high capital cost for the low volume of fabrication makes it a prohibitive option. Therefore, these customized Colmonoy-6 bushes were fabricated by LRM at our laboratory as an alternative to conventional processing.

To qualify the bushes for the targeted application, laser rapid manufactured Colmonoy-6 bushes were subjected to a number of tests. Dye penetrant testing and ultrasonic

Fig. 5: (a) Laser Rapid Manufacturing and (b) Finally machined and ground Colmonoy-6 bushes

Fig. 6: (a) Different layers of multi-layered colmonoy-6 bush (longitudinal section) and (b) Microstructure of a typical cross-section of colmonoy-6 bush

Fig. 7: (a) Typical compressive stress and crosshead travel curve for laser rapid manufactured and wrought Colmonoy-6 and (b) Longitudinal and transverse micro-hardness profiles of laser rapid manufactured Colmonoy-6 bush


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while formation of cellular microstructure has helped in keeping ductility intact.

Conventional methods of fabricating porous structures, such as furnace sintering technique, space holder technique, replication technique, combustion synthesis technique ferromagnetic fiber arrays technique and vapor deposition technique, have limitations in fabrication of the porous structures with engineered mechanical properties due to inability to control precisely a number of parameters, like- pore-size, shape, volume fraction, pore-distribution, contaminations and their phases etc. Moreover, they cannot be used for generating functionally designed porous structures with graded porosity. Such structures can be potentially fabricated using laser rapid manufacturing [7].

At our laboratory, the porous structures were laser rapid manufactured using cross-thin-wall fabrication strategy and structures up to 60% porosity was achieved. The mechanical properties of these structures were also investigated. In cross thin wall fabrication strategy, the porous material is fabricated by depositing the material in mutually orthogonal directions in successive layers. Fig. 9 (a) presents a schematic of the cross thin wall fabrication strategy. The microstructural studies of the porous structures indicates that the resultant laser rapid manufactured specimens have pores, arranged in the form of regular arrays (refer Fig. 10 (a)). The location of these pores is at the junctions of adjacent tracks and adjoining layers, specifically at the track overlap region (refer Fig. 10 (b)).

This example demonstrated that LRM can be used as a cost effective alternative technique for the fabrication of Colmonoy-6 bushes [4]. LRM has significant advantage over other techniques, in terms of saving of expensive Colmonoy-6 material and reduced machining of the hard material [3].

Solid and porous structures of Inconel-625

Inconel-625 is one of nickel-chromium based alloys, which is widely used for various naval, aerospace and nuclear applications. It has an outstanding fatigue and thermal-fatigue strength; good oxidation and corrosion resistance; excellent resistance to stress corrosion cracking and pitting resistance at elevated temperature; and excellent characteristics for welding and brazing. Considering the wide spread applications and an oxidation resistant property, the studies on laser rapid manufactured structures (solid and porous) of Inconel-625 was carried out at our laboratory. A number of samples at various processing parameters were fabricated and their mechanical and metallurgical properties were evaluated using standard characterization techniques, such as tensile testing, Rockwell hardness testing, Charpy impact testing etc. Table 2 presents the comparison of some of the important mechanical properties of laser rapid manufactured and hot finished & annealed Inconel-625. The properties of laser rapid manufactured Inconel-625 was found to be at par the conventionally processed Inconel-625.

Table 2: Comparison of Some of important mechanical properties of laser rapid manufactured and hot finished & annealed Inconel-625.

Fig. 8 (a) and (b) show the cross section of a typical multilayer overlap LRM fabricated Inconel-625 deposit and its microstructure. The microstructure examinations revealed that there were finely intermixed dendritic and cellular microstructures with high dislocation density. The direction of dendrite growth was along the direction of deposition. The fine dendrite formation was due to inherent rapid cooling rate during LRM, while cellular microstructure is attributed to relatively lower cooling rate during the multi-layer deposition. The fine microstructures with high dislocation density are responsible for higher mechanical strength and hardness,

Fig. 8: (a) Typical cross section exhibiting multilayer overlap deposition and (b) Microstructure of LRM fabricated Inconel-625

Fig. 9: (a) Schematics of Cross thin wall fabrication strategy and (b) laser rapid manufactured porous structure of ~20% porosity


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availability of more WC facilitates the absorption of a greater fraction of the incident laser power into WC, resulting excessive heating and partial melting of WC particles. This excessive heating or partial melting of WC results in carbon deficiency in WC-Co alloys due to precipitation of carbon as graphite. 2WC ↔ W C + C. 2

This graphite reacts with atmospheric oxygen and forms CO and CO ; it often appears as gas porosity in WC-Co 2

system. While the carbon deficiency in the matrix leads to the formation of a brittle ternary eutectic phase of W, Co and C (often referred as eta phase). Eta phase can exists in both forms, either M C ranging from Co W C to 6 3.2 2.8

Co W C or M C of fixed composition Co W C. In case of 2 4 12 6 6

laser processing, the presence of M C is more probable 6

due to rapid cooling rate. Both, graphite and eta-phase are detrimental for the mechanical properties of WC-Co deposits. Therefore, the selection of laser processing parameters and material composition play critical roles in the LRM of WC-Co. We have carried out comprehensive LRM experiments with various WC-Co compositions. It is observed that WC-6 wt% Co and WC-12 wt% Co are good for uniform continuous deposit. But as the thickness was increased, the process became unstable, leading to bulk-defect in the material. Therefore, WC-17 wt% Co was selected for the laser fabrication in the present study.

Fig. 11 (a) shows the macro-morphology of a multi-layer overlapped laser rapid manufactured material. It can be seen that the multi-layer overlapped track gave a smooth rippled surface topography. It indicates that the powder particles were melted and the deposit formed was strongly bonded together. The samples were subjected to dye-penetrant test and it confirmed that the fabricated material was mostly free from surface defects, like- porosity and cracks with the exception in few samples, where micro-cracks were observed at the isolated locations. Fig. 11 (b) shows the backscattered electron image of the laser rapid manufactured WC-Co sample. It could be seen that there was a continuous film of cobalt that separates the carbide particles. There was no partial or full melting of WC particles, as the sharp edges of the WC were clearly visible. The WC-Co matrix was continuous and distribution of WC particles was uniform. The average grain size of the WC particles was about 10 microns. The microstructure observed was comparable

The above case study demonstrates the feasibility of laser rapid manufacturing of solid and porous structures of Inconel-625 alloy. The fabrication of porous structures with control on porosities and yield strengths has vast potential in various prosthetic and engineering applications.

Cemented carbide components

In the realm of the hard materials, tungsten carbide (WC) is a popular choice for tools, dies and wear prone parts and it is widely used for machining, mining, metal cutting, metal forming, construction, and other applications. It has unique combination of high strength, high hardness, high toughness, and moderate modulus of elasticity, especially with fine grained WC and finely distributed cobalt. The most commonly produced commercial straight grades of WC-Co have cobalt contents ranging from 4 to 30% by weight, with grain sizes ranging from 0.5 to 10 microns [8]. Conventionally, WC-Co is sintered at about 1450°C to shape the components using powder metallurgy technology. In WC-Co system, Co plays the most significant role as binder and is responsible for densification through wetting, spreading and formation of agglomerates during liquid phase sintering. A uniform distribution of metal phase in a ceramic is beneficial for improved mechanical properties of the composite. The thermo-physical and the optical properties of the WC-Co are listed in Table 3.

Table 3: Thermo-physical and the optical properties of the WC-Co.

It indicates that the laser absorption of WC powder particles is about 1.4 times more than that of Co for 1.064 µm wavelength. In WC rich powder mixture, the

Fig. 10: (a)Micrograph indicating the pores and (b) the location of pores in the laser rapid manufactured structure

Properties Unit WC Co Steel3Density kg/m 15800 8900 7800

Melting point Deg.C 2687 1495 1435

Thermal W/m K 84 100 68conductivity

Coeff. of m/m K 4.3 14 13.7thermal exp.

Hardness HRA 93 67 28

Absorption - 0.82 0.58 [email protected] μm

Fig. 11: (a) Multi-layer overlapped tracks and (b) backscattered electron image of laser rapid manufactured material


54

material enters in the plastic zone. With further increased in the load, the slope becomes negative and lead to material failure. During the test, it was also observed that a crack was generated at the bottom face of the specimen and it propagated to the top, leading to complete failure. There are few valleys in the curve, specifically in plastic zone, showing propagation of cracks in the direction opposite to that of loading. It seems that these cracks are stopped by ductile cobalt matrix, leading to recovery in loading with increasing displacement. This shows that the laser fabricated material is more resistance to abrupt loading and it is likely to get blunt, rather than break during extreme loading condition. XRD analysis of WC-17wt% Co laser clad sample was carried out using CuKα source at 50 kV and 40 mA. The results of the test are presented in Fig. 12 (b). The analysis confirms the presence of WC and Co phases in the laser deposited zone, while the peak for higher metallic carbides, like- W C, Co W C, Co W C, etc. is absent. In case of 3 3 3 6 6

conventional WC-Co sintering and continuous wave (CW) laser cladding, it is difficult to fabricate WC-Co deposits without the formation of higher carbides due to uncontrolled local heating and longer interaction time [9]. In laser rapid manufacturing, it could be achieved due to very low heat input, controlled local heating and inherent rapid cooling. Laser rapid manufacturing was deployed for the fabrication of low cost tools by depositing WC-Co on mild steel (refer Fig. 13). The performance of these tools was found to be 80% of the conventionally processed tools.

Conclusion

Laser rapid manufacturing is an extremely flexible technique with application in multiple areas from repair of large scale components to manufacturing of component with specific end application. It is now crossing the barriers of conventional component fabrication and entering into new era of “feature based design and manufacturing”. The scope of this technology

with that of produced with conventional sintering process [8]. It may be noted that powder particle size between 0.1 microns and 10 microns are used in conventional sintering of cemented carbide and maximum transverse rupture strength is achieved when the powder particle size is in the range of 4 microns. Dynamic blowing of powder particles is very difficult, as it often results in clogging and uncontrolled flying of the metal powders. When pre-placed bed technique, like- Selective Laser Sintering, is tried, fully dense material could not be achieved [2].

Typical result of three-point flexural testing for conventionally sintered material and laser fabricated material is presented in Fig. 12 (a). In load-displacement curve, there is a higher slope for the laser fabricated material as compared to the conventionally processed material. It confirms that the laser fabricated material possess more stiffness as compared to the conventionally sintered material. As per curve, the general behavior of the laser fabricated material is more towards ductile material, while it is brittle for the conventionally processed material. The load-displacement curve for the laser fabricated material exhibits a linear increase in the displacement with increasing load, initially. As the load is further increased, the slope of the curve decreases and

Fig. 12: (a) Typical load displacement curve of conventionally processed and laser rapid manufactured material during three-point flexural testing and (b) XRD analysis of laser rapid manufactured material

Fig. 13: Laser rapid manufactured low cost cemented carbide end mill


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Acknowledgments

The authors express their sincere gratitude to Dr. P. D. Gupta, Director RRCAT for his constant support and encouragement. Thanks are due to our collaborators Prof A. K. Nath of Indian Institute of Technology, Kharagpur, India. During the experimental work presented above, the technical support of Dr. P. Ganesh, Mr. S. K. Mishra, Mr. C. H. Prem Singh, Mr. M.O. Ittoop, Mr. Abrat Varma, Mr. Anil Adbol and Mr. S. K. Perkar is thankfully acknowledged.

References

1. C.P. Paul, Atul Kumar, P. Bhargava and L.M. Kukreja, Laser assisted manufacturing: fundamentals, current scenario and future applications in: Nontraditional Machining Processes, J. Paulo Davim (Ed.), Springer-verlag, London UK, pp.1-34 (2013).

2. C.P. Paul, P. Bhargava, Atul Kumar, A.K. Pathak and L.M. Kukreja, Laser Rapid Manufacturing: Technology, Applications, Modeling and Future Prospects, in: Lasers in manufacturing, J. Paulo Davim (Ed.), Wiley-ISTE, London UK, pp.1-67 (2012).

3. C.R. Das, S.K. Albert, A.K. Bhaduri, G. Kempulraj, J. Mat. Processing Tech. 141, 60-66 (2003).

4. C.P. Paul, A. Jain, P. Ganesh, J. Negi, A.K. Nath, Opt. Lasers in Eng. 44, 1096–1109 (2006).

5. http://www.specialmetals.com/documents/ Inconel%20alloy%20625.pdf last accessed on May

th30 , 2013.

6. C.P. Paul, P. Ganesh, S.K. Mishra, P. Bhargava, J. Negi, A.K. Nath, Opt. Laser Tech. 39, 800 (2007).

7. C.P. Paul, S.K. Mishra, C.H. Premsingh, P. Bhargava, P. Tiwari, L.M. Kukreja, Int. J. Adv. Manuf. Tech. 61, 757 (2012).

8. G.S. Upadhyaya, Cemented Tungsten Carbides Production, Properties and Testing, Noyes Publications, New Jersey, 1998.

9. C.P. Paul, A. Khajepour, Opt. Laser Tech. 40, 735 (2008).

is being extended to more complex multi-material and multi-functional components with high degree of control on fabrication. Availability of compact high power lasers, advanced CAD/CAM systems with faster computing speeds, and advanced diagnostic and control systems have provided a new dimension to manufacturing and laser rapid manufacturing is one of such development. Technical factors, such as – advancement in sub-systems, and economic factors, such as falling price of lasers and other sub-systems, will further alleviate the deployment of LRM technology to manufacturing.

The present research trend indicates that a deeper understanding of involved phenomena is being generated through experimental and modeling. More and more materials with potential applications are being investigated to generate a comprehensive database of laser rapid manufactured materials, their microstructure, mechanical and other properties through the intensive collaborations among universit ies, research organizations and industries. The fabrication of Colmonoy-6 bushes proves the importance of LRM for strategic applications. Manufacturing of low cost tools using LRM is another niche area with future scope for industrial applications. The mechanical and metallurgical properties of the components fabricated through LRM are somewhat poorer than or in some cases nearly identical to those of their wrought counterparts but these limitations can be suitably exploited for certain specific applications, such as fabrication of porous structures for prosthetics.

With the increased interests from various industries, LRM is likely to lead in the fields of aerospace, medical devices and tooling. In combination with innovative design and planning, the capabilities of LRM have been established to fabricate complex components with delicate details that are very difficult or even impossible to make using conventional manufacturing processes. It does not mean that LRM is a threat to the existence of conventional manufacturing processes, but it is simply going to augment the industries with advanced manufacturing technology to address unresolved complex geometrical and material issues. In combination with other conventional manufacturing processes, LRM is going to provide a unique cost effective solution to next generation “feature based design and manufacturing” using the strength of virtual and remote manufacturing. Hence, it is a novel pursuit of unorthodox manufacturing.


(a) Laser Rapid Manufacturing and (b) Finally machined and ground Colmonoy-6 bushes (See Page No. 52)

5-axis CNC workstation. (See Page No. 14) Schematic arrangement of LRM system. (See Page No. 51)

Water-jet through nozzle with 1.0 mm opening (a) straight, (b) straight divergent, (c) straight modified, at 10 bar pressure. (See Page No. 19)

Laser drilled component at CLPM. (See Page No. 11)

Issues of Kiran and Membership form are available at www.ila.org.inFor circulation among ILA members only.

(Not for sale)Printed by : Burhani Offset Printers, Indore. M- 99811-74052.A Bulletin of the Indian Laser Association

Typical crankshaft (A, B, C, D and E on schematic - locations to be hardened) (See Page No. 4)

Laser clad repair in progress (See Page No. 8)

Typical geometries fabricated using LRM system at authors' laboratory (See Page No. 51)

Temperature and velocity field in laser microwelding in presence of surface active element. (See Page No. 28)

Temperature distribution in laser transmission welding at scanning velocity of 2 mm/s. (See Page No. 29)

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Vol.24 No.2