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Vol. 22, No.2, August 2011A Bulletin of the Indian Laser Association
Special Issue on
Interaction Meet on Utilization of Laser Technology in Industry & Medicine at RRCAT
President
Prof. P. K. Gupta (RRCAT, Indore)
Vice President
Prof. S. K. Sarkar (BARC, Mumbai)
Gen. Sec. I
Prof. V. P. M. Pillai
Univ. of Kerala, Thiruvananthapuram
Gen. Sec. II
Prof. K.S. Bindra (RRCAT, Indore)
Treasurer
Mr. P. Saxena (RRCAT, Indore)
Regional Representatives
Dr. S. K. Bhadra,
CGCRI, Kolkata
Prof. M. P. Kothiyal,
IIT Madras, Chennai
Prof. D. Narayana Rao,
University of Hyderabad, Hyderabad
Prof. Hema Ramachandran,
Raman Research Institute, Bangalore
Dr. A. K. Razdan,
Laser Science & Technology Centre, Delhi
Editor
Prof. Manoranjan P. Singh
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. Pankaj Misra
Mr. H.S. Patel Dr. S. Verma
Dr. G.J. Singh Dr. B.N. Upadhyay
Dr. C.P. Singh
Guest Editor
Shri R. Kaul (RRCAT, Indore)
Dr. S. Sendhil Raja (RRCAT, Indore)
(RRCAT,Indore)
ILA Executive Committee Editorial Team of
Cover Photo :
Image (top left) of Laser Rapid Manufacturing (LRM) System at LMPD, RRCAT and porous structure (top right), made by cross thin wall strategy using LRM (details on page 13). Image in middle shows the laser cutting process designed and developed at SSLD, RRCAT (details on page 3). Image of Laser Surf-Check instrument (left bottom) developed at LPTD, BARC, Mumbai (details on page 24) and the laser line triangulation setup (right bottom) developed at LBAID, RRCAT, Indore (details on page 28).
A Bulletin of the Indian Laser Association
Contents
Vol. 22, No. 2, August 2011
Page No.
From The Editor 1
From The Guest Editors 2
1. High Power Nd:YAG Lasers in Indian Nuclear Power Plants 3B. N. Upadhyaya, S. C. Vishwakarma, R. Arya and S. M. Oak
2. Laser Rapid Manufacturing of Engineering Components 13C P Paul, P Bhargava, S K Mishra, C H Premsingh and L M Kukreja
3. Metallurgical Characterization of Laser Fabricated Structures of Engineering Alloys 18P Ganesh, Rakesh Kaul, Harish Kumar, C H Premsingh, S K Mishra and L M Kukreja
4. Laser Based Instruments for Measurement Applications 24Aseem Singh Rawat
5. Laser based Instrumentation 28Ishant Dave, Rohan Bhandare, Brijesh Pant, Sendhil Raja and P K Gupta
6. Application of Laser Processing of Materials for High Temperature 33Molten Chloride EnvironmentA. Ravi Shankar, Ravikumar Sole, Jagdeesh Sure and U. Kamachi Mudali
Report
7. Interaction Meet on Utilization of Laser Technology in 38Industry & Medicine at RRCAT
Announcement
8. DAE-BRNS National Laser Symposium 39
1
I feel honoured to be editor of Kiran. It has served very well in bringing
together the Indian laser community. I thank all my predecessors for their
efforts. I am sure that with active contributions from its members/readers
Kiran will attain greater heights in future.
The immense progress in laser technology in recent times has witnessed a
rapid increase in the usages of lasers in all spheres of science and
technology. In order to translate this to the well being of common man a
meaningful collaboration between the scientists/researchers and those in
the industry and Medicine is imperative. As an attempt towards this and
also to mark fifty glorious years of lasers an interaction meet on
Utilization of Lasers in Industry and Medicine was organized by Indian
Laser Association on 28th and 29th April 2011 at Raja Ramanna Centre
for Advanced Technology (RRCAT), Indore. This issue of Kiran is based
on the lectures and poster presentations during the meeting. I am thankful
to my colleagues Shri. Rakesh Kaul and Dr. Sendhil Raja for agreeing to
be the guest editors for this issue.
I look forward to receiving articles and your suggestions for further
improvement.
Best warm regards,
Manoranjan P. Singh
From the Editor....
2
Lasers have come a along way since their inception in 1960 to mature into
a reliable tool for many industrial and medical applications. The outgoing
year, being the golden jubilee year of laser invention, assumed significant
importance in the history of science and technology. In order to celebrate
50 years of invention of laser, Indian Laser Association organized a two-
day interaction meet on Utilization of Lasers in Industry and Medicine on
28th and 29th April 2011 at Raja Ramanna Centre for Advanced
Technology (RRCAT), Indore. The interaction meet was organized with
an objective to provide a platform to showcase indigenous laser
technologies developed for industrial and medical applications in major
academic and research institutions of the country and to promote closer
interaction between academic/research institutions of the country and
Indian industry. About 30 participants from 25 different companies
attended the meet. Some of the notable participants were Tata Motors Ltd.,
Bharat Heavy Electricals Ltd., Larsen & Toubro Ltd. The two day event
witnessed informative presentations, interactive technology showcase
sessions and lively group discussion sessions.
In this special issue of Kiran we are presenting selected articles based on
presentations made in the interaction meet. The selected articles cover the
application of laser in the demanding Indian nuclear industry, rapid
manufacturing of engineering components, surface treatment of plasma
sprayed thermal barrier coating for improved performance in high
temperature molten salt environment and laser based instrumentation.
We hope that you will find this issue of Kiran both interesting and useful.
Rakesh Kaul & Sendhil Raja S
From the Guest Editors....
3
radiation is within the pump band of Nd:YAG and rest goes as heat. Out of this 10% of lamp radiation is absorbed by Nd:YAG rod, only about 5-6% is emitted as laser output, the rest heats the Nd:YAG rod. In diode pumped Nd:YAG lasers, it is normally pumped at 808 nm by diode source and emission occurs at 1064 nm leading to thermal effect mainly due to quantum defect of ~ 24%. Thus, it is necessary to effectively cool the pump cavity and rod to remove the heat load. Closed loop water is circulated through the pump cavity to remove the heat load from lamp, rod and reflector. As the circulating water is in contact with the rod surface, under steady state conditions the rod center is at a higher temperature as compared to rod surface and due to temperature gradient from rod center to rod surface, the rod acts as a thermal lens. The dioptric power of pumped Nd:YAG rod increases linearly with pump power and hence acts as a
3focusing element of variable refractive power . Thus, the parameters of passive resonator are modified by the active lens element and the resonator has to be designed taking into consideration of variable lens element. Thus, it is challenging to design a resonator with single or multi-rod configuration for a long range of pump operation and to develop high power Nd:YAG lasers with a good beam quality for its fiber optic beam delivery. Then, it becomes convenient and easy to carry out material processing applications with fiber optic beam delivery and a small material processing head.
The industrial Nd:YAG laser activity of Solid State Laser Division (SSLD), Raja Ramanna Centre for Advanced Technology (RRCAT) has developed different types of highly efficient lamp pumped Nd:YAG lasers. These lasers have been extensively used to carry out various material processing applications in Indian nuclear power plants. In co-ordination with various units of Department of Atomic Energy (DAE) and Nuclear Power Corporation of India Limited (NPCIL), RRCAT explored the possibility to carry out various material processing tasks with high power Nd:YAG lasers at various nuclear power plant sites. Initially, different material processing techniques were established with high power Nd:YAG laser and based on feedback, a robust industrial laser system of 250 W average power, 2-20 ms pulse duration and 1-200 Hz repetition rate having 5 kW peak power and 100 J maximum pulse energy with time-shared multi-port optical beam delivery system was developed. This laser system is remotely operable and has been engineered for
Abstract
High power Nd:YAG lasers with fiber optic beam delivery have tremendous potential in material processing applications in the field of nuclear energy due to non-contact nature of the process, low secondary waste generation, and remote operation with flexible beam delivery through optical fibers. Robust high power Nd:YAG lasers and innovative laser material processing techniques developed at RRCAT have been successfully utilized in Indian nuclear power plants on industrial scale to enormously reduce MANREM consumption, time and cost. It has also been brought out that high power Nd:YAG lasers have potential applications in new reactor installations and in maintenance operation of running nuclear plants.
Introduction
Lamp pumped and diode pumped Nd:YAG lasers with fiber optic beam delivery have been exploited commercially for various material processing applications such as cutting, welding, drilling, etc. in
1-3harsh environments . In order to enhance quality and range of material processing applications, it is necessary to deliver the beam through an optical fiber with core diameter and numerical aperture as low as possible. Thus, to cope up with the need of material processing applications, higher and higher power Nd:YAG lasers with improved beam quality are being developed. The basic configuration of a lamp pumped Nd:YAG laser consists of a pump cavity containing a flash lamp and an Nd:YAG rod within a gold coated elliptical reflector or a close coupled diffuse reflector and an optical resonator. Similarly, the configuration of a diode pumped Nd:YAG laser consists of a Nd:YAG rod and diode pump source for end or side pumping. These lamp pumped and diode pumped systems suffer from low beam quality due to thermal lensing and stress induced birefringence. The composite effect of thermal lensing and birefringence is to limit the fundamental mode spot size within the rod and hence the beam quality. The main effort is towards reduction of thermal lensing and stress induced birefringence to improve the beam quality or alternatively to go for birefringence compensation. About 50% of the electrical input supplied to flash lamp goes as heat and the rest 50% of the electrical input is emitted as optical radiation. As the flash lamp emits in a broad spectrum, only about 9-10% of the emitted
High Power Nd:YAG Lasers in Indian Nuclear Power Plants
B.N. Upadhyaya*, S.C. Vishwakarma, R. Arya and S.M. Oak Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013
*E-mail : bnand@rrcat.gov.in
4
Application of High power Nd:YAG lasers in Nuclear power plants
High power Nd:YAG lasers with fiber optic beam delivery have been utilized for various material processing applications in nuclear power plants for maintenance operations. Some of the important massive applications and related developments are as described as below:
its robustness with proper fixtures and toolings for various material processing operations on industrial scale related to nuclear field. This system is pumped with 5 kW input electrical power and provides an electrical to laser conversion efficiency of about 5%, which is the highest as compared to any commercially available lamp pumped
4Nd:YAG laser . This fiber coupled Nd:YAG laser system has four time-shared fiber ports, each of them has a fiber
having 600 mm (400 m optional) core diameter, 0.2 NA and 150 m length. Specially designed material processing nozzles of diameter in the range 13 mm to 25 mm with gas flow through the same tube containing optical fiber were developed for applications having space restrictions in nuclear power installations. Using this, cutting of stainless steel sheets up to 14 mm and welding up to depth 2 mm were established. Now, it has been scaled to 500 W average power with 2-40 ms duration and 1-100 Hz rep.
rate with 10 kW peak power and 400 mm fiber optic beam delivery for laser cutting of up to one inch SS and weld
5depths in SS up to 5 mm . A lab model of 1 kW average power and 20 kW peak power Nd:YAG laser has also been developed. Development of industrial model of 1 kW average power Nd:YAG laser with fiber optic beam delivery and its application in deep penetration welding and concrete cutting is under progress. High power lamp pumped CW Nd:YAG laser has also been developed with an output power of 880 W having 4.4% electrical to laser
6conversion efficiency . CW Nd:YAG lasers with kW level power scaling using multi-cavity design and modulation is useful in deep penetration keyhole welding and laser rapid manufacturing. Development of compact high power CW fiber lasers with all fiber integration and compact footprint has also been taken up. In initial efforts, development of 120 W single transverse mode CW fiber laser with an optical-to-optical efficiency of 75% has been carried. Its power scaling to achieve kW level will be highly useful in nuclear maintenance operations. Fig.1, 2, & 3 shows a typical view of industrial Nd:YAG lasers developed at SSLD, RRCAT. About 20 systems of industrial Nd:YAG laser have been commissioned in different DAE units for various material processing applications.
m
Fig. 1: 250 W average power and 5 kW peak power industrial Nd:YAG laser with time-shared fiber-optic beam delivery.
Fig. 2: Industrial Nd:YAG laser with 500 W average power and 10kW peak power.
Fig. 3: Industrial Nd:YAG laser with 1 kW average power and 20 kW peak power.
Fig. 4: Simplified flow diagram of PHWR with Calendria and coolant channels.
5
designed material processing nozzles of diameter 1/2 inch with gas flow through the same tube containing optical fiber were developed for applications having space restrictions in nuclear power installations. Using this system, laser cutting of 612 bellow lips during EMCCR of NAPS-1, NAPS-2 and KAPS-1 reactors has been performed successfully in May 2006, Nov. 2008 and Feb. 2009. A miniature fiber coupled laser cutting head with 1/2” diameter is mounted on the fixture in such a way that it takes care of position tolerance of bellow lip and diameter of coolant channel. It is desired to separate the bellow rings in such a way that the outer ring can be reused for welding at the time of re-commissioning. This required grooving of the ring at weld location up to a depth of ~ 4 mm. It is easy to cut through and through using laser beam while it is very difficult to make grooves in a material. The laser grooving technique for carbon steel was established specially for this purpose.
Two industrial Nd:YAG lasers with four port time shared fiber optic beam delivery and 150m long fiber optic cable were deployed for cutting of bellow lip, one on each north and south vaults of 220MW reactors and in-situ bellow lip cutting was performed and separation was ensured for all the 612 bellow lips in each reactor. The fixing of tool on any of the coolant channels requires about one minute and the cutting process takes ten minutes for each bellow lip, and total operation was completed within a few days of laser operation. Laser cutting of 18 Nos. of shock absorber yoke studs having a diameter of 16 mm was also performed during EMCCR activity to access bellow lip weld for laser cutting. These studs were jammed and could not be opened by any mechanical means. This resulted in a large MANREM saving as compared to conventional technique and also time saving of at least six months with enormous cost saving. Fig.6 shows the fixture mounted on a coolant channel performing the cutting process in mock-setup. Fig.7 shows the laser cut and separated bellow lip. Fig. 8 shows fixture mounted on end face of coolant channel for cutting of HPFC stud in mock up and fig. 9 shows laser cutting of HPFC stud. Fig.10 (a) & (b) shows the fixture mounted on E-face of one of the coolant channels in NAPS-1 and NAPS-2 reactors respectively. Fig. 10(c) shows laser cut shock absorber studs from NAPS-2 and Fig. 11 shows welded bellow lip. The same fixture was utilized for re-welding of bellow lip during re-installation of coolant channels. This fixture is able to hold laser welding nozzle as well as TIG welding torch.
Prior to bellow lip weld cutting, it is necessary to remove obstruction of all the 612 shock absorber yoke assembly and its 1224 studs. In previous EMCCR campaign at NAPS-1, it was found that in serious attempts to open a few jammed shock absorber studs, shock absorber yoke
Development of laser cutting technique and in-situ laser cutting of bellow lips during en-masse coolant channel replacement (EMCCR) campaign at NAPS-1, NAPS-2 and KAPS-1 reactors
For an introduction of nuclear applications of Nd:YAG lasers, we first look at the design of Pressurized Heavy Water Reactors (PHWRs). It is characterized by natural uranium fuel, heavy water as moderator, pressure tube containment of primary coolant, fuel bundles and ON POWER refueling. Each Reactor has typically 306 coolant channels, which are mounted horizontally within a horizontal cylindrical vessel, called Calandria and surrounded by low pressure, low temperature heavy water moderator. Fig.4 shows a simplified flow diagram of PHWR together with side-view of Calendria & coolant channels. A single coolant channel is a composite structure of end fitting, liner tube and a pressure tube. These pressure tubes, which contains fuel bundles, is made up of Zr-2 or Zr-2.5% Nb alloy and is attached with SS-403 liner tube and end fitting by means of rolled joints. Further, each end fitting is connected to a coolant pipe (feeder) by hub joint with a seal ring and high pressure feeder coupling (HPFC) studs. Annular space around the coolant channel is sealed by a metallic bellow and CO gas is circulated in it (see Fig.5). It is essential to 2
replace the pressure tubes in PHWR type of nuclear reactors after a life of 10-15 years and this replacement is performed during EMCCR campaign of such reactors. This is a complicated process due to space restrictions and high MANREM involvement. The 306 coolant channels placed in a matrix are very close to each other and bounded to the core of the reactor by means of two shrink fit welded bellow attachment rings, made up of carbon steel, one on each face of reactor core located at a distance of about 945 mm from E-face of end fittings i.e., from end point of coolant channel. These coolant channels can be replaced, if the welded bellow rings are detached at the welding point on each end. This requires grooving at the welding point up to the depth of welding (~3-4 mm) and then pulling the channel. Although, single point mechanical cutters can be utilized for this operation, but these mechanical cutters are bulky, require their frequent replacement and take long time to cut, which results in higher MANREM involvement.
The mechanism for laser cutting of bellow lip developed at RRCAT consists of a motorized circumferential rotary arrangement, which can be mounted on the E-face of coolant channel and can be fixed on it just by tightening
7of a single bolt . The tool is designed to fit on E-face of end fitting using bore of the end fitting. The locking of this tool is based on tapered ball locking grip at sealing plug position of end fitting. Tightening of a box nut of size M32x2.5 can lock the fixture at E-face. Specially
6
Fig. 5: A sketch of bellow lip cutting fixture mounted on E-face of coolant channel.
Fig. 6: Bellow lip cutting mock-up.
Fig. 7: Separated bellow lip.
Fig. 8: Fixture mounted on coolant channel for
laser cutting of HPFC studs.
Fig. 9: Laser cutting mock-up of HPFC studs.
Fig. 10(a) & (b): A site view of laser based bellow lip cutting in NAPS-1 & NAPS-2 reactors.
(b)
(a)
©
Fig. 10(c): A site view of laser cut shock absorber studs from NAPS-2 reactor.
7
miniature shielded cutting nozzle has also been developed which has a collimating and focusing lens
Oalong with a 45 bending mirror and a window to protect optics from damage during cutting operation. The cutting fixture consists of mounting, locking & holding arrangement for the laser cutting nozzle along with a motorized traversal arrangement for linear cutting of the nut. The whole mechanism has been miniaturized in size to accommodate all the components only in a total width of 27 mm, which can be easily mounted on position on the shock absorber assembly. The locking mechanism has been made to work on cam arrangement, which generates forces in a triangular frame. The cam is slightly shifted from the position of the triangular plane; due to this shift, the force generated by the cam works as a moment force, which is responsible for locking the fixture rigidly in three dimensions with the shock absorber assembly. In order to prevent yoke sock absorber stud damage, the motion of cutting nozzle has been limited by means of limit switches fitted on both of the J-hooks. Creep adjustment of H10 coolant channel at RAPS-3 was being carried out from north vault since last three biennial shutdowns due to jamming of shock absorber rear nuts, whereas creep adjustment of all the other coolant channels was being carried out from south vault. This was resulting in large MANREM consumption in creep adjustment process. The jammed nuts were cut at two locations from the same side in such a way that a piece of nut can be removed across stud diameter. Both the up and down rear nuts of H10 channel were cut successfully and creep of H10 could be adjusted from the same side after cutting of jammed nuts. Fig. 12(a), (b), (c) and (d) show the developed laser cutting fixture, a mock up of fixture mounted on yoke assembly of stud and a view of laser cut samples of rear nut.
Laser cutting of FBTR spent fuel fuel subassembly
Laser cutting for dismantling of highly radioactive fuel subassemblies of FBTR was succesfully carried out in hot cell at IGCAR, Kalpakkam using in-house build fiber coupled industrial Nd:YAG laser (250 W average power and 5 kW peak power). The Pu-U carbide fuel rods have undergone a burn-up of 154 GWd/t and had a radiation
7 level of 10 rad/hour. This fuel assembly was precisely cut at a gap of 5 mm from the position of the fuel pins for Post–Irradiation Examination (PIE) of burnt fuel with total cutting time of the subassembly ~ 2 minutes, cutting
assembly studs were broken from end shield. This required a lot of effort to make threads at proper location in end shield during re-commissioning process of reactor coolant channels. To avoid this difficulty, laser cutting technique and fixture for 18mm diameter shock absorber yoke assembly studs was also developed at KAPS site (in a similar fashion as at NAPS-2) to cut these studs near the bellow lip weld joint to remove obstruction for bellow lip weld cutting and also to avoid damage of stud threads in end shield. Out of 1224 studs, a total of 78 were found jammed and using laser technique, these studs were cut successfully and safely near the bellow lip weld joint during EMCCR campaign at KAPS-1. Now, the laser based cutting technique for EMCCR of PHWRs has been matured and can be exploited in future EMCCR campaigns.
Laser cutting of shock absorber rear nut at RAPS-3 reactor for creep adjustment
A laser based cutting technique for shock absorber rear nuts in PHWRs has also been developed. This technique has been successfully used for in-situ laser cutting of H10 rear nuts at RAPS-3 reactor in Sept. 2008. The technique consists of a motorized compact fixture, which holds a fiber optic beam delivery cutting nozzle and can be
8operated remotely .
The laser cutting system consists of our home-build 250 W average power fiber coupled industrial Nd:YAG laser having multi-port time shared fiber optic beam delivery and a specially developed remotely operable compact laser cutting fixture. The laser beam was delivered
through a 400 mm optical fiber to achieve larger depth of focus and to have high power density for grooving up to a depth of 13 mm (equal to the thickness of nut). A
Fig.11: Laser welded bellow lip.
Fig. 12: (a) Rear nut cutting fixture, (b) Mock up of fixture mounted on yoke assembly, (c) & (d) Axial and transverse view of cut sample.
(a) (b) (c) (d)
8
at IGCAR since past three years for PIE data. Fig. 13 (a), (b), and (c) show cutting of hexagonal FBTR spent fuel during mock up, cut sample and actual cutting in hot cell.
This laser was also deployed at NFC to extract fuel from rejected fuel pins of PHWRs and fuel from about 65 tons of storage was extracted within a period of one year.
Laser welding of High dose rate brachytherapy assembly
Treatment of cancer by using radiation emitted from the radio-isotopes is in practice for decades. Teletherapy and Brachytherapy are widely used for this purpose. In Teletherapy, the cancerous volume is irradiated by gamma rays emitted by radio-isotopes. Brachytherapy is one of the most efficient ways of treating cancers such as localized uterus cancer and cancers of the head and neck. Brachy is from a Greek word for "short", hence, Brachytherapy approximately means short distance therapy. This is essentially a supplementary radiotherapy, where a radioactive source is placed inside or next to the area requiring treatment. High Dose Rate (HDR) Brachytherapy is a common brachytherapy method used for treatment of a large number of cancer patients. Applicators in the form of catheters are arranged on the patient. A high dose rate source (often Iridium- 192) is then driven along the catheters on the end of a wire by a machine while the patient is isolated in a room. The source remains in a preplanned position for a preset time to allow controlled doses of radiation to be delivered to the cancerous tissues, without damaging the healthy tissues. The capsules that hold the radioactive 'seed' are only a few millimetres long, and about a millimeter in diameter and have a wall thickness of less than 150µm.
The welds that join the capsules together (five weld joints) need to produce a hermetic seal, with a smooth weld bead. Presently, hospitals in India engaged in providing Brachytherapy, use imported HDR source
9speed ~120 mm/minute, cut width of 400 m . Compared to the conventional mechanical methods there are several advantages in laser cutting like: it is fast, does not lead to contamination and secondary waste generation, does not create shape deformation and stress on surface, which is important for measuring swelling, cracks, and stress of a burnt subassembly at different locations.
The laser beam was delivered through a 400 m optical fiber with a focused spot size of 400 m on the job to minimize waste generation. The laser system has a dual port time shared fiber optic beam delivery, with one fiber port for optimization of cutting process outside the hot cell, and another for cutting inside the hot cell. A compact, shielded cutting nozzle assembly of 20 mm outer diameter containing beam delivery fiber and coaxial flow of the assist gas was specially developed for insertion through the S-bend in hot cells. As the fuel subassemblies were hexagonal in shape and sodium (the liquid coolant in the FBTR) was stuck on the inner wall of the fuel with some swelling after a huge burn-up, a loaded roller was attached to the nozzle to maintain the beam focus position. Due to the presence of highly active sodium, cutting was carried out with nitrogen as an assist
2gas at a pressure of 8 kg/cm . Now, this technique of laser cutting of FBTR spent fuel subassembly is in regular use
Fig. 13(a): Cutting of hexagonal FBTR spent fuel bundle.
Fig. 13(b): Cut sample.
Fig. 13(c): Cutting of FBTR spent fuel bundle in hot cell.
9
which includes cutting of 4 mm thick liner tube and 12 mm thick end fitting made up of SS. Total cutting time was 12 min. This was cut to generate data on Zr-2.5%Nb pressure tubes used for the first time in KAPS-2 reactor after a life of 8 years.
KAPS-2 is the first reactor in which Zr-2.5%Nb pressure tubes were used and it was required to generate data on these kind of pressure tubes. It was decided to take out one of the pressure tubes after about eight years of reactor operation. To extract pressure tube, it was required to cut liner tube and end fitting from inside due to space restrictions. This cutting was performed remotely by laser cutting fixture specially designed with several innovative ideas. The coolant tube cutting fixture mechanism consisted of two disks of Alluminium, one of them gets attached at E-face and the other disk is inserted inside end fitting through a dual rod handle which comes out from two diametrically opposite holes in the first disk and holds the two disks together and can also fix the
11separation of the two disks . Fig.15 shows the developed fixture.
assembly, which consists of radioactive material, miniature housing with cover and metallic wire ropes. BARC with the help of RRCAT has developed indigenous HDR source assembly for BRIT. The quality of the indigenously developed HDR source assembly is matching with the imported ones, and will be considerably less expensive. As compared to other welding techniques, laser welding is advantageous in terms of heat affected zone(HAZ), pointed and localized heating with better bead quality. A typical HDR source assembly, has four miniature SS micro machined components viz.; machine end terminal, rope joining sleeve, source retaining capsule and cover & two SS wire ropes (dia. 0.91 and 0.73 mm). There are five laser welded joints between SS wire ropes and miniature components. Laser welding of miniature components has
10been performed without any damage . A laser welding
system with 200 mm fiber optic beam delivery and required arrangement has been developed at RRCAT, which has been commissioned at BRIT for regular production of HDR brachytherapy assembly. Fig. 14(a) and (b) show the brachytherapy assembly and laser welded sample of this assembly.
Development of laser cutting technique for in-situ cutting of a single coolant channel at KAPS-2 reactor
Laser cutting technique along with fixture was developed and deployed successfully in Jan. 2005 for in-situ cutting of single coolant channel S-7 from inside of the channel,
Fig. 14 (a): Brachytherapy Assembly having HDR Source.
Fig. 14 (b): Welded brachytherapy assembly.
Fig. 15: Coolant channel cutting fixture.
Fig. 16: Coolant channel cutting mock-up.
Machine End Terminal SS Wire Rope (dia 0.91 mm) SS Wire Rope (dia 0.73 mm)
Cover
Ir-132 Source
LW2LW3 LW5
LW4LW1
Rope Joining Sleeve
Source Retaining Capsule
10
nozzle consists of a rotating disk (gear) supported by ball bearings on a vertical bracket (plate). It is having a central hole of 110mm diameter through which the tube is brought into the position by an existing ram. The tube is located and gripped by a V-block based pneumatically actuated two-piece gripper cum locator. The gripper is mounted on the backside of stationary bracket the cutting head is mounted on a compact (rectangular piston) cylinder. Rotary encoder and proximity switches monitor the position of locating cylinders, cutting head cylinders and rotary disk. The actual cutting process has been performed by initial longitudinal cutting through an axial pulling of the tube up to a length of 2.7 m, then pulling was stopped, and cutting head started rotation in circumferential direction.
Laser cutting of fuel bundle end plates
Dismantling of spent fuel bundles from PHWRs by cutting end plates is also being carried out regularly in hot cell at BARC by using laser from RRCAT. Figure 19 (a) shows a view of laser cutting of end plate of fuel bundle in hot cell. Fig. 19 (b) & (c) shows intact fuel bundle and dismantled fuel bundle.
There is a third long screw, which passes through the first disk and is attached to the second disk. Tightening of this third screw pushes movement of a button out of the disk diameter and helps in locking this disk with the inner diameter of the end fitting and the whole fixture. The motion of nozzle for circumferential cutting from inside of the tubes has been motorized by means of a DC motor and a geared coupling of fixture with the motor. Tool fixing time was about one minute and total cutting time was four minutes for liner tube and ten minutes for end fitting with enormous MANREM, time and cost savings. Fig.16 shows a mock-up of coolant channel cutting.
Development of laser cutting technique for easy storage of pressure tubes removed from reactors during EMCCR
Laser cutting technique was developed and successfully deployed for cutting of 7 pressure tubes made up of zircaloy material, which were removed from MAPS-1
12reactor . These tubes of 5m length were cut in two pieces to establish laser cutting technique for reduction in storage space.
Pressure tubes in PHWR's are about 5m in length and are highly radioactive. After EMCCR operations, these tubes are stored as such and require a large space. For initial study, a laser based cutting fixture was designed and deployed for cutting of seven pressure tubes removed from MAPS-1 in two halves to reduce storage space and found to be very useful in reducing storage space. This will be further deployed in mass cutting of pressure tubes by slotting the pressure tube linearly in three pieces using three nozzles simultaneously at 120 with each other and then cutting it circumferentially after a certain length. Fig.17 shows pressure tube cutting fixture and cutting mock up and Fig.18 shows a cut samples from pressure tube.
In this case job was fixed and laser beam was moved circumferentially. Rotation arrangement for laser cutting
Fig. 17: Pressure tube cutting fixture and mock-up.
Fig. 18: Cut sample from pressure tube.
Fig. 19: (a) A view of laser cutting of end plate of PHWR fuel bundle in hot cell, (b) intact fuel bundle, (c) dismantled fuel bundle using laser.
11
Laser cutting technique and fixture for these steam generator tubes made up of Inconel-80 from inside of the tube was developed and deployed successfully in Jan. 2009 for cutting of one of the SG tubes at SG-3 location in NAPS-2 reactor at a distance of 783 mm from the base of the tube. The base of this SG tube is welded on a base in man hole of steam generator assembly. The critical issue in cutting of SG tube was to cut blindly from inside at the desired location and that the nearby tubes should not damage. This laser cutting technique was found to be an easy technique and can be utilized in future for such leaky SG tube cutting purposes in PHWRs. Fig. 20 shows miniature cutting nozzle inserted through a dummy SG tube and fig. 21 shows laser cut sample.
Underwater cutting with laser
Underwater laser cutting and welding has many applications in nuclear facilities and is a promising technique for maintenance/dismantling operations as well as for collecting sample pieces for post-irradiation
13-14examination . In the field of nuclear decommissioning also, underwater cutting of nuclear facilities is desirable. For such operations, it is highly useful to deliver the laser beam through optical fiber because of its flexibility. During dry laser cutting process, a high-power laser beam is focused on the job so that the material reaches its melting temperature and a high-pressure active or inert gas is used to remove the molten material. During this process, a considerable amount of energy is conducted into the work piece resulting in changes in the material properties and the microstructure of the material leading to large heat affected zone (HAZ). In addition, debris and metal vapour from the cut kerf is spread in air. In cutting of irradiated material, debris and metal vapour creates airborne activity, which may be harmful for people working nearby, whereas, underwater cutting is advantageous in terms of a narrow HAZ adjacent to the laser cut surface providing better samples for the analysis of irradiated material with minimum thermal damage and effective reduction in debris spread in air.
There are several requirements from NPCIL to cut nuclear components underwater in water pool at a depth of about 8-10 m. In Dhruva reactor also, it is required to cut Aluminium racks of 3 mm thick. In this regard laser cutting technique using fiber optic beam delivery has been developed for cutting of SS up to a thickness of 12 mm and aluminium of thickness 4 mm. Development of fixture for underwater cutting is under progress. Fig. 22 shows a view of undercutting mock-up of zircaloy.
Conclusion
In conclusion, in-house built industrial Nd:YAG lasers were deployed successfully for refurbishing and maintenance operation of nuclear power plants on
Laser cutting of steam generator (SG) tube at NAPS-2 reactor
Steam generator tube assembly consists of a large number of SG tube matrix made up of Inconel-80 with inner diameter 14 mm, 1.5 mm thickness and nearby tubes are separated by a gap of 6 mm only. It is required to cut and extract leaky SG tubes for analysis purposes.
Fig. 20: Miniature cutting head inserted through SG tube.
Fig. 21: Cut sample from SG tube.
Fig. 22: Underwater cutting mock-up for 4.2 mm thick zircaloy.
12
8. S. C. Vishwakarma, R. K. Jain, B. N. Upadhyaya, Ambar Choubey, D. K. Agrawal, S. M. Oak, “Development of In-situ Laser based Cutting Technique for Shock Absorber Rear Nut in Pressurized Heavy Water Reactors”, DAE-BRNS National Laser Symposium 2007, MSU, Vadodara.
9. Ambar Choubey, D. K. Agrawal, S. C. Vishwakarma, B. N. Upadhyaya, Sabir Ali, R. K. Jain, S. K. Sah, R. Arya, Jojo Joseph, and K. V. Kashivishwanathan, S. M. Oak, “Laser cutting of Fast Breeder Test Reactor fuel subassembly in hot cell”. DAE-BRNS National Laser Symposium 2007, M. S. University of Baroda, Vadodara, Gujrat, India, Dec. 17-20, 2007, pp. 59.
10. B. N. Upadhyaya, M. K. Mishra, S. C. Vishwakarma,R. K. Jain, A. Choubey, D. K. Agrawal, D. N. Badodkar, Manjit Singh, K. V. S. Sastry, B. N. Patil, and S. M. Oak, “Laser micro-welding of Brachytherapy assembly having high dose rate source” DAE-BRNS National Laser Symposium (NLS-08), Jan. 7-10, 2009, LASTEC, New Delhi, India, SA7-022, pp. 75-76.
11. B. N. Upadhyaya, S. C. Vishwakarma, R. K. Jain, Ambar Choubey, Pankaj Gupta, S. K. Chadda, and T. P. S. Nathan, “Development of In-situ Laser Cutting Technique for Coolant Channels in Pressurised Heavy Water Reactors”, DAE-BRNS National Laser Symposium-2005, Vellore, Dec. 7-10, 2005, pp. 61.
12. B. N. Upadhyaya, S. C. Vishwakarma, P. Gaure, R. K. Jain, Amber Choubey, G. Mundra, Pankaj Gupta, S. K. Chadda and T. P. S. Nathan, “Development of Laser based Cutting Machine for Easy Storage of Irradiated PHWR Pressure Tubes”, DAE-BRNS National Laser symposium-04, BARC, Mumbai, Jan.10-13, 2005, pp. 258-261.
13. A. Kruusing, Underwater and water-assisted laser processing: Part 1—General features, steam cleaning and shock processing. Optics and Lasers in Engineering, 41(2), 307-327 (2004).
14. R. K. Jain, D. K. Agrawal, S. C. Vishwakarma, A. K. Choubey, B. N. Upadhyaya, and S. M. Oak, "Development of underwater laser cutting technique for steel and zircaloy for nuclear applications", PRAMANA Journal of Physics, 75 (6), 1253-1258 (2010).
industrial scale along with medical applications and there is a lot of scope for development of various laser systems and laser based processes which can be utilized as an advanced technique to save MANREM, time and cost for Indian nuclear power plants in future.
References
1. T. Ishide, O. Matumoto, Y. Nagura, and T. Nagashima, “Optical transmission of 2 kW CW YAG laser and its practical applications to welding”, SPIE 1277, 1990, pp.188-198.
2. W. Koechner, Solid state laser engineering, 5th ed. Berlin, Springer,1999.
3. Y. Shimokusu, S. Fukumoto, M. Nayama, T. Ishide, S. Tsubota, A. Matsunawa, and S. Katayama, "Application of 7 kW class high power yttrium–aluminum–garnet laser welding to stainless steel tanks", J. Laser Applications, 14(2), 68-72 (2002).
4. B. N. Upadhyaya, S. C. Vishwakarma, A. Choubey, R. K. Jain, Sabir Ali, D. K. Agrawal, A. K. Nath, “A highly efficient 5 kW peak power Nd:YAG laser with time-shared fiber optic beam delivery”, Optics and Laser Technology, 40(2), 337-342 (2008).
5. Ambar Choubey, S. C. Vishwakarma, B. N. Upadhyaya, R. Arya, R. K. Jain, Sabir Ali,D. K. Agrawal, S. M. Oak, “Development of 500W average power fiber coupled pulsed Nd: YAG Laser”, DAE-BRNS National Laser Symposium (NLS-09), BARC, Mumbai, India, Jan. 13-16, 2010, CP-01-11, pp. 46-47.
6. B. N. Upadhyaya, S. C. Vishwakarma, Sabir Ali, V. Bhawsar, S. K. Sah, R. Arya, D. K. Agrawal, R. K. Jain, A. Choubey, S. M. Oak, “Development of an Efficient 880 W CW Nd:YAG Laser”, DAE-BRNS National Laser Symposium 2007, M. S. University of Baroda, Vadodara, Dec. 17-20, 2007, pp. 52.
7. S. C. Vishwakarma, R. K. Jain, B. N. Upadhyaya, Ambar Choubey, D. K. Agrawal, Pankaj Gupta, S. K. Chadda, and T. P. S. Nathan, “Development of Laser based System for Cutting of Bellow lip during En-masse Coolant Channel Replacement in PHWR type of Nuclear Reactors”, DAE-BRNS National Laser Symposium-2005, Vellore, India, Dec. 7-10, 2005, pp. 61.
13
Laser Rapid Manufacturing System consists of high
power laser, integrated with a beam delivery system,
powder feeder and a 5-axis CNC workstation in a
optional controlled atmospheric chamber. A defocused
laser beam of desired diameter (0.1 - 3 mm) is used for
metal deposition at fabrication point. The metallic
powder is fed into the molten pool using either one or
both powder feeder through a co-axial powder-feeding
nozzle. Argon gas is used as shielding and powder carrier
gas. The fabrication point is moved as per the required
shape using standard numerical codes to fabricate the
component. Table I summarizes some typical examples
of industrial applications of LRM and Figure 2 shows
some of the complex shaped components laser rapid
manufactured in our laboratory.
Recent advances in high speed computers, computer
aided design (CAD), laser technologies and
layered/additive manufacturing techniques have led to
the next generation fabrication methodology, involving
''feature-based design and manufacturing''. This
fabrication procedure has been termed as Laser Rapid
Manufacturing (LRM). In this, a fully functional near-net
three dimensional (3D) object can be fabricated directly
from its CAD model by adding metallic materials (in the
form of powder) into the design domain through
sequential deposition tracks. Each track is deposited by
simultaneous laser melting and rapid solidification of fed
metallic powder on a thin wetted layer of the moving
substrate/pre-deposit surface in a predetermined shape
and dimensions. Additive nature as well as special
attributes mainly resulted from unique laser beam
characteristics has made LRM a potential candidate for
various applications such as rapid prototyping, cladding,
and parts repair especially for prime components. Small
heat affected zone (HAZ), minimal dilution, direct
deposition, and integration of CAD tools with the
production process are some of the main features of LRM
that can eliminate many manufacturing steps compared
to conventional methods, and also overcome the
limitations of existing metal manufacturing technologies
in terms of as materials-machine planning, man-machine
interaction, intermittent quality checks, reduction of
production time, enhancement of thermal controllability,
and production of functionally graded parts
(heterogeneous structures) [1]. Figure 1 illustrates one of
the LRM systems at our laboratory.
Laser Rapid Manufacturing of Engineering Components
C.P. Paul*, P. Bhargava, S.K. Mishra, C.H. Premsingh and L.M. KukrejaLaser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013
*E-mail: paulcp@rrcat.gov.in
Fig. 1: Photograph of recently commissioned Laser Rapid Manufacturing System.
Table I. Examples of Industrial applications of LRM
14
deposits, slow cooling was a prerequisite to obtain crack-
free bushes. It was achieved by placing base plate,
undergoing laser deposition, in a special sand bath
maintained at an elevated temperature of 673 K. The sand
bath consisted of an electrically heated copper plate
buried in the sand. Temperature of the bath was measured
and automatically controlled with the help of a
temperature controller. Laser rapid manufacturing of
cylindrical Colmonoy-6 bushes, with dimensions of 20
mm outer diameter, 2.5 mm wall thickness and 40 mm
length, involved depositing circular clad tracks one over
the other. Figure 3(a) shows the photograph of LRM of
Colmonoy-6 bush in progress. After LRM, the fabricated
parts were left buried in the sand bath for more than 8
hours to achieve slow rate of cooling. The resultant
Colmonoy-6 bushes were found to be crack free and their
mechanical properties were at par with the
conventionally processed bushes. The measured
dimensional tolerance using three-point method was 0.2 -
0.5 mm, while surface roughness (R ) was in the range of a
25 - 40 µm [2].
Colmonoy-6 Bushes
”Nickel-based alloys “Colmonoy , due to their
outstanding wear resistance, high hardness at elevated
temperatures and low induced radioactivity, find
applications for hardfacing of austenitic stainless steel
components of nuclear power plants. In the event of
complicated component geometry providing limited
access for hardfacing, pre-fabricated Colmonoy-6 bushes
can be used in place of local hardfacing. Conventionally,
these bushes are made by casting/weld deposition
followed by machining. However, high capital cost for
the low volume of fabrication makes it a prohibitive
option. These customized Colmonoy-6 bushes were
prepared at our laboratory by LRM as an alternative to
conventional processing. Laser rapid manufacturing of
Colmonoy-6 bushes was carried out on a sandblasted 12
mm thick plate of type 316L stainless steel as base. In
view of poor cracking resistance of Colmonoy-6
,
Fig. 2: Laser rapid manufactured (a) simple cage of Inconel-625and (b) and multiple-vane impeller of type 316L stainless steel.
(a)
(b)
Fig. 3: (a) Laser Rapid Manufacturing of Colmonoy-6 bush and (b) bushes after final machining.
(a)
(b)
15
detrimental Therefore, the selection of laser processing
parameters and material's composition play a critical role
in LRM of WC-Co. Laser rapid manufactured WC-Co
under optimized parameters were found to be free from
bulk defects such as micro-cracks, intermetallic phases
and inclusions etc. Figure 4 (a) shows the microstructure
of laser rapid manufactured WC-Co deposit showing
uniform dispersion of un-melted WC particles in Co-
matrix. The micro-hardness in the laser clad zone (1250
– 1700 HV at 1000g load) was found to be comparable to
that of conventional WC - Co specimens [3]. Laser rapid
manufacturing with optimized parameters, was
subsequently used for the fabrication of low cost tools.
Figure 4(b) shows a typical laser rapid manufactured
multi-point cutting tool. Such fabricated tools were used
for cutting of type 316 stainless steel and the cut quality
produced with these tools was found to be at par with
associated tool life of more than 80%.
Porous Structures
Until recent past, porosity was considered as one of the
harmful defects that impeded efficiency or functional
properties of the manufactured products, limiting its
applications to non-load bearing components. However,
if the porous materials could be produced with adequate
mechanical strength, they would find direct applications
as lightweight structures, functional materials,
transportation materials etc. This encouraged us to
undertake research towards the development of porous
structures with adequate mechanical strength. Laser rapid
manufacturing, being a layer-by-layer additive
manufacturing technique, has a unique capability to
selectively deposit materials at the desired points. The
loci of these desired points have been termed by us as
“LRM strategy”. Different LRM strategies can be used to
fabricate same material with different porosity contents
or the materials with same porosity content but different
mechanical properties. Various strategies are being
investigated for LRM of porous materials including
cross thin wall fabrication method, recursive ball
deposition method etc. In cross thin wall fabrication
method, clad tracks in each layer are deposited in a
direction orthogonal to its preceding layer.
Figure 5 shows optical macrographs of representative
porosities on three different cross-sections viz. plane
normal to scanning direction (X-axis), plane normal to
transverse traverse direction (Y-axis) and plane normal to
build-up direction (Z-axis) of laser rapid manufactured
structure of Inconel-625 under different deposition
.
,
,
Low-cost Cemented Carbide Tools
In the realm of the hard materials, cemented carbide
(WC-Co) is a popular choice for tools, dies and wear
prone parts that find wide applications in machining,
mining, metal cutting, metal forming, construction etc.
There is a need to develop a low cost repair technology or
new fabrication techniques for WC cutting tools because
of high cost and increasing demand of tungsten carbide
(WC) powder. In WC-Co system, the bulk hardness is
governed by WC particles, while the toughness and
strength of these materials can be tuned by adding an
adequate amount of Co. Absorption of laser radiation by
WC particles is about 1.4 times stronger than that in Co
for 1.064 µm wavelength. As a result of excessive
heating and partial melting of WC particles, WC phase
may undergo dissociation causing carbon deficiency in
WC-Co composite and precipitation of carbon as
graphite. This graphite reacts with atmospheric oxygen to
form CO and CO , which often appear as gas porosity 2
whereas availability of free carbon in the matrix leads to
formation of a brittle ternary eutectic phase of W, Co and
C; often referred as “eta” phase. Formation of both,
graphite and eta-phase in WC-Co composite, is
Fig. 4: (a) Back scattered electron image of the microstructure of WC-Co deposition, and (b) laser rapid manufactured multi-point cutting tool.
(a)
(b)
16
micrograph of cross-section corresponding to region
OA shows clearly visible pores with nearly circular
tracks. On the other hand, the macrostructure
corresponding to region AB shows elongated pores and
tracks due to compression. The pores are compressed
and almost filled due to material flow. This flow of the
material gives rise to plateau regime in the stress strain
curve. The slope and length of the curve in plateau region
depends on the rate of densification of the material, which
is primarily governed by the dynamics of compression of
pores and walls in a correlated manner within the porous
structure. The optical macrograph corresponding to
region BC shows almost completely compressed
structure. The neighboring tracks are compressed
together with completely deformed fine pores.
,
,
conditions. As seen in this figure, resultant laser rapid
manufactured specimens have pores, arranged in the
form of regular arrays. The location of these pores is at
the junctions of adjacent tracks and adjoining layers,
specifically at the track overlap region. The size of the
pores is not uniform at various locations within the same
sample and it can be seen that the average bulk porosity
increases due to increase in the pore size. The shape and
size of the pores are different on three different planes,
indicating that the resultant porous structures will have
anisotropy in mechanical properties. The shape and size
of the pores on the planes normal to X and Y axes are
nearly the same. Therefore, it is expected that the
mechanical properties along these two axes are largely
similar.
Figures 6 (a) and (b) present photograph of laser rapid
manufactured porous structure, made by cross thin wall
strategy and typical engineering stress-strain curve
obtained during compressive testing of the same,
respectively. The initial part of the curve (OA) involves
sharp increase in stress with small compressive strain.
This is a region of elastic deformation with small amount
of plastic deformation. The associated plastic
deformation in this region is responsible for mechanical
damping. After the initial sharp increase in stress, there is
a change over to a regime of plastic deformation
predominantly associated with closure of porosity where
small increase in stress is accompanied by larger
compressive strain (AB). After extended plateau regime,
the slope of the curve (BC) increased which is indicative
of densification of material in the previous regime (AB).
At this stage, the porosity is negligible with neighboring
tracks completely touching each other. The optical
,
,
Fig. 5: Optical macrographs of representative porosities on three different cross-sections of laser rapid manufactured structure of Inconel-625.
Fig. 6: (a) Laser rapid manufactured Inconel-625 porous structure and (b) typical engineering stress-strain curve obtained during compression testing of laser rapid manufactured porous structure.
17
References
1. J Lawrence, J Pou, D K Low, E Toyserkani (Eds.),
Advances in Laser Materials Processing
Technology, Research and Applications, CRC Press
and Woodhead Publishing Ltd, Cambridge, UK,
First Edition. (2010).
2. C P Paul, Amit Jain, P Ganesh , J Negi and A K Nath,
Laser Rapid Manufacturing of Colmonoy
Components, Laser and Optics in Engineering, 44,
1096-1109,(2006).
3. C P Paul, H Alemohammad, E Toyserkani, A
Khajepour, S Corbin, Cladding of WC-12Co on low
carbon steel using a pulsed Nd:YAG laser. Material
Science and Engineering-A, 464, 170-176, (2007).
4. C P Paul, S K Mishra, C H Premsingh, P Bhargava, P
Tiwari and L M Kukreja, Parametric investigations
on laser rapid manufactured porous structures of
Inconel-625 using cross-thin-wall fabrication
strategy, Int. J. Adv. Mfg. Technol. (under review).
The value of compressive yield stress is 226 MPa along
the scanning and transverse traverse directions, while it is
254 MPa along the build-up direction for laser rapid
manufactured specimens of around 12% porosity. This
difference in value is due to the LRM strategy adopted in
the present experiments. The reported tensile yield
strength of the conventionally processed Inconel-625
was in the range of 414 - 758 MPa, 414 - 655 MPa and 290
- 414 MPa in as rolled, annealed and solution treated
conditions respectively [4].
Conclusions
Our recent research presented above shows a glimpse of
the small section of the vast domain of potential
application of LRM. The results of studies demonstrated
that LRM can be adopted as an alternative fabrication
method to fabricate functional metal parts and
components. Mechanical properties of laser
processed/fabricated components are reported to be at par
or in some cases even better than their wrought
counterparts. Repair of worn metal components like
turbine blades and shafts etc may lead to large economic
incentives to various industries like power generation,
aeronautics and chemical processing etc.
Acknowledgements
The authors thankfully acknowledge the technical
support of members of Laser Materials Processing
Division, RRCAT for carrying out laser processing
experiments and characterizations.
18
manufactured structures of Inconel-625 (IN-625) and SS
316L. Different stages involved in fabrication of compact
tension (CT) test specimens are shown in Fig. 1. Fatigue
crack growth rate (FCGR) tests were conducted on the 12
and 25 mm thick Compact Tension (CT) specimens, as
per ASTM E647 standard. Subsequent to FCGR testing,
the same specimens were used for evaluation of fracture
toughness, as per ASTM E1820 standard [1].
The results of FCGR obtained in the present study were
compared with the reported data for corresponding
wrought materials. Laser rapid manufactured specimens
of IN-625 and SS 316L exhibited steady state crack
growth, referred as stage II crack growth, in the
investigated stress intensity range of 14-38 MPaÖm for
IN- 625 and 11.8-24 MPaÖm for SS 316L. Fatigue crack
growth rates for laser fabricated IN-625 were found to be
lower than the reported values in the DK range of 14-24
MPaÖm and above this range they tended to coincide as
seen in Fig. 2(a). On the other hand, FCGR in laser rapid
fabricated specimens of SS 316L were quite close to the
Abstract
Laser based manufacturing is an emerging fabrication
methodology with many unique features due to low heat
input, minimal distortion and capability to fabricate near-
net shape three dimensional (3D) components. By
adopting suitable processing methodologies and
controlling the laser processing parameters, complex
structural components (either monolithic or multi-
material) can be fabricated. The article presents the
results of the studies carried out at laser materials
processing division, RRCAT on the metallurgical
characterization of laser fabricated Inconel 625 and
bimetallic structures involving Stellite-21 and Type
316L. Laser clad composite joints of Stellite-21 and SS
316L fabricated with and without compositional grading
at joint interface were characterized to study the
influence of grading on the fracture behavior of clad joint.
Results of tensile and instrumented impact tests are
discussed in light of the compositional grading at the
substrate-clad interface.
Mechanical Properties of Laser Rapid Manufactured
Structures of Inconel 625 and Type 316L SS
A nickel based super-alloy Inconel-625 (IN-625),
because of its high temperature oxidation resistance,
mechanical strength and wide use in high temperature
applications, was used for fabrication of specimens by
LRM for evaluation of mechanical properties like tensile,
impact, fatigue and fracture toughness. An in-house
developed high power CO laser, integrated with beam 2
delivery system and CNC work station was used for
specimen fabrication. For industrial acceptability of any
new fabrication process (laser based fabrication in the
present case), it is essential to generate the structural
integrity qualification data. Present study was undertaken
to characterize the critical mechanical properties like
fatigue crack growth rate, fracture toughness and impact
toughness of laser rapid manufactured structures. Fatigue
and fracture toughness tests were performed with
compact tension (CT) and single edge notched bend
(SENB) specimens, extracted from laser rapid
Metallurgical Characterization of Laser Fabricated Structures of Engineering Alloys
P. Ganesh*, Rakesh Kaul, Harish Kumar, C.H. Premsingh, S.K. Mishra and L.M. KukrejaLaser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013
*E-mail: ganesh@rrcat.gov.in
Fig. 1. Different stages involved in LRM of compact tension test specimens: (a) initial SS block with V-groove; (b) filling of groove by LRM; (c) machined CT specimen extracted from laser deposited SS block [1].
19
extensive crack branching. The crack plane followed a
tortuous path due to the layered deposition and associated
rastering pattern involved in laser fabrication process.
Instrumented Charpy impact testing of IN-625
specimens, exhibited impact energy of 46.5 - 49 J while a
post deposition annealing treatment at 1223 K brought
about 10 % improvement in impact energy. Gradual fall
in load after the peak load in load-displacement plots is
representative of ductile nature of crack propagation in
these specimens. Reported Charpy impact energy (for
keyhole specimens) of IN-625 in the as rolled condition
was in the range of 65-70 J [5].
Laser Rapid Manufacturing (LRM) of Bimetallic wall
and Tubular Bush
A bimetallic wall and a tubular bush were fabricated by
LRM with an in-house developed CW CO laser, coupled 2
with a CNC work-station. The bimetallic wall comprised
of SS 316L on one side and Stellite-21 (St-21) on the
other side, whereas the tubular structure consisted of St-
21 on the inner side and SS 316L on the outer. Chemical
composition of powders used is presented in Table I.
Table I: Chemical composition (weight %)
of powders used for LRM
Figure 3 presents schematic illustration of methodologies
adopted for fabrication of bimetallic wall and tubular
structures, along with their photographs and associated
macrostructures. Laser rapid manufacturing of bimetallic
wall involved alternate deposition of two adjacent clad
tracks of SS 316L and St-21, with overlap at the center, as
shown in Fig. 3(a) [6]. Two separate powder feeders,
positioned on either side of the laser beam, were used to
feed SS 316L and St-21 powders during the experiment.
Photograph of the bi-metallic wall and its cross-sectional
macrostructure are presented in Figs. 3(b) and 3(c),
respectively. Etching contrast between clad layers on
opposite sides of the bimetallic wall, as seen in Fig. 3(c),
is indicative of the difference in their chemical
compositions. LRM of bimetallic tube employed a co-
axial powder feeding nozzle to deposit four partly
overlapping concentric circular clad tracks in each layer.
The two inner clad tracks were deposited with St-21
powder whereas the two outer clad tracks were made with
SS 316L, as shown in Fig. 3(d). Dimensions of bimetallic
tube shown in Fig. 3(e) were: 25 mm inner diameter with
ones reported for their wrought counterparts as shown in
Fig. 2 (b)
The J-integral fracture toughness (J ) values for laser Ic
rapid manufactured specimens of IN-625 and SS 316L 2were found to be in the range of 194-254 kJ/m and 143-
2259 kJ/m , respectively. Crack tip opening displacement
(CTOD) fracture toughness values for both these
materials were found to be in the range of 0.28-0.54 mm.
Fracture toughness values (both J and CTOD) of laser IC
rapid manufactured specimens of SS 316L, although
lower than that of its wrought counterpart, are in close
agreement with the reported values for corresponding
weld metal [2].
Charpy impact energy of laser rapid manufactured
specimens of SS 316L was found to be in the range of 90-
110 J, which is at par with the wrought material in the
annealed condition[4]. Fracture surface of impact tested
specimen exhibited mixed mode fracture features with
+Fig. 2: Comparison of experimental and reported [3 , 4*] fatigue crack growth rate results for Inconel 625 and type 316L SS [1].
Material C Cr Ni Mn Si Mo Fe Co P S
SS 316L 0.025 18 12 1 0.5 2 Bal - 0.03 0.02
St-21 0.26 26.3 2.8 0.65 1.88 5.53 1.4 Bal - -
20
3.8 mm wall thickness. Sharp etching contrast developed
on the cross-section of the bimetallic tube as seen in Fig.
3(f) indicates large difference in chemical composition at
the interface.
In addition, an SS tube (post-machined dimensions: 34
mm ID and 2 mm wall thickness) with an internal step of
St-21 (height: 1.5 mm and width: 6.5 mm) has also been
fabricated by LRM, as shown in Fig. 4. This kind of
structure will be useful for fabricating components where
an insert is required to provide an internal hard-faced
lining at selective places. This demonstrates the
capability of LRM to add functional overhanging
features.
Fig. 3: Schematic illustration of methodologies adopted for LRM of (a) bimetallic wall and (d) bimetallic tube, with the images of structures (b&e) and associated macrostructures (c&f)
(a) (b)
(c)
(d) (e) (f)
Fig. 4: Laser rapid manufactured tube of SS with an internal step of Stellite 21.
21
miniature tests as they are conducted on a very local
region by compression and hence resultant values may be
higher as compared to conventional results. However the
BI test results of specimens made with 316L by LRM and
laser welding, matched well with the reported results. The
test results along with experimentally measured tensile
test results were used to formulate the empirical relations
for estimation of tensile properties of materials of
specific composition based on ShP test data from a small
volume of material [7]. It is established from the present
study that miniature specimen test techniques like BI
testing and ShP testing can be successfully used for
estimation of gradient of tensile parameters in the case of
multi-material components, where the material available
is not sufficient enough for fabrication of conventional
tensile specimens.
Laser Rapid Manufacturing of Compositionally
Graded Structures
A study on the influence of compositional grading on
fracture behavior of laser clad joint of SS 316L and St-21,
was performed with specimens fabricated using an in-
house made CW CO laser [8]. For the deposition of 2
Metallurgical characterization
Laser rapid manufactured bimetallic structures exhibited
regular pattern of clad layers of 0.6 - 0.8 mm thickness.
These components exhibited significant transition in
chemical composition and micro-hardness across their
wall thickness, as shown in Figs. 5 and 6. With respect to
bimetallic wall, tubular bush recorded gradual transition
in chemical composition and micro-hardness across its
wall thickness. Bimetallic structures of this kind may
find application in Fast Breeder Reactor, where internal
lining of Stellite is required on tubular SS components for
enhanced resistance against galling. Miniature specimen
test techniques like ball indentation (BI) testing and shear
punch (ShP) testing were used to evaluate the variation of
tensile characteristics due to associated compositional
variation across the cross-section of bimetallic structures
fabricated by LRM. Tensile strength of Stellite-21,
estimated using BI test method was about 30% higher
than the results measured from conventional tensile tests
and close to reported values for the wrought material.
Defects/discontinuities if any may show their effect in
conventional tensile tests giving a gross measure of
strength whereas no such details can be known from
Fig. 5. (a) EDS concentration profiles of Co and Fe and (b) micro-hardness profiles across wall thickness of bimetallic wall fabricated by LRM (D - distance from substrate/clad interface)
Fig. 6. (a) EDS concentration profiles of Co, Cr and Fe and (b) micro-hardness profile across wall thickness of bimetallic tube fabricated by LRM.
(a)
(b)
(a)
(b)
22
contrast, fracture surface of notched “graded clad”
specimen exhibited quasi-cleavage type fracture as seen
in Fig. 8(d).
Instrumented Charpy impact testing of laser clad
composite specimens brought out significant difference
in fracture behavior of composite specimens induced due
to compositional grading. The impact specimens were
fabricated in such a way to facilitate crack propagation
from SS 316L to St-21, as shown in the inset of figure 9.
Although, fracture of both “direct clad” and “graded
clad” specimens consumed largely similar impact
energies (32-37 J and 35-37 J, respectively), load-
displacement traces of the two specimens exhibited
distinct difference in associated modes of crack
propagation after general yield as shown in figure 9.
Failure in “direct clad” specimens was associated with
abrupt drop from peak load as crack propagated across
sharp interface between SS and St-21. On the other hand,
fracture of “graded clad” specimens was marked with
graded overlays, chemical composition of clad layers was
controlled by using pre-mixed powders of St-21 and SS
316L in predetermined ratios. Graded overlay of three
layers was deposited by cladding with premixed powders
of St-21 and SS 316L in the ratios of 30:70, 70:30 and
100:0, respectively. In the subsequent part of the text, SS
316L specimens clad with St-21 deposits and graded St-
21 deposits are referred as “direct clad” and “graded clad”
specimens, respectively. Figure 7 compares
microstructures of the interface region and associated
composition profiles of “direct clad” and “graded clad”
specimens. The cross-sections of laser clad specimens
exhibited typical cast microstructure with distinct etching
contrast with underlying base metal (SS 316L),
signifying transition in chemical composition across SS
316L-St-21 clad interface. With respect to “direct clad”
specimens, “graded clad” specimens exhibited diffused
interface involving transition from wrought
microstructure of base metal (SS 316L) to cast
microstructure of the clad layer, as shown in Figs. 7(a)
and 7(b), respectively. With respect to direct clad
specimens, graded clad specimens recorded more
gradual built up of chemical composition along the
thickness of the clad deposit. Laser rapid manufactured
composite specimens of SS 316L and St-21 were
characterized by tensile, impact and fatigue testing for
evaluating the influence of compositional grading on the
fracture behavior.
The specimens for tensile testing of “direct clad” and
“graded clad” specimens were fabricated in such a way
that the substrate/clad interface was normal to the loading
axis. In addition to smooth specimens, notched
specimens were also tested to restrict plastic deformation
to the zone of interest. The failure of smooth specimens
took place in the softest zone (viz. wrought SS) at a stress
of 600 - 630 MPa with significant amount of plastic
deformation, as manifested by its dimpled fracture
surface shown in Fig. 8(a). On the other hand, specimens
with notch in the St-21 clad region suffered brittle
fracture along inter-dendritic boundaries (refer Fig. 8(b))
at a higher stress of 950 - 968 MPa. Failure of the
specimens with notch at the interface region of took place
at an intermediate stress level (720-750 MPa) with
distinctly different modes of crack propagation in “direct
clad” and “graded clad” specimens. Fracture surface of
notched “direct clad” specimens exhibited randomly
distributed regions of ductile fracture in Fe-rich regions
(represented by dimples) and brittle fracture along inter-
dendritic boundaries in Co-rich regions (Fig. 8(c)). In
Fig. 7. Microstructure of interface region and associated EDS concentration profiles for direct clad (a, c) and graded clad (b,d) specimens. Arrows mark partially melted zone (PMZ) in figure 7(a).
Fig. 8: SEM fractographs of tensile tested specimens with SS/St-21 joint: (a) smooth specimen - failure in wrought SS; (b) notched specimens - failure in St-21 region; (c) notched “direct clad” and (d) notched “graded clad” specimens with failure in the interface region.
(a) (b)
(c) (d)
23
process which can enhance the service performance of bi-
material structures.
References
1. P. Ganesh, R. Kaul, C.P. Paul, Pragya Tiwari, S.K. Rai, R.C. Prasad, L.M. Kukreja: Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures, Materials Science and Engineering-A 527(29-30) (2010) 7490–7497.
2. W. J. Mills, in: S. R. Lampman et al, (Eds.), ASM handbook Vol. 19: Fatigue and fracture, ASM international, materials park, OH, 1997, pp.733-735.
3. h t t p : / /www.ascgenoa . com/ma in /news le t t e r / 9 /%5B2%5D6-07_AIAA_2007_2381-meta l -Fatigue.pdf
Bahram Farahmand, Charlie Saff, De Xie and Frank Abdi, Estimation of Fatigue and Fracture Allowables For Metallic Materials Under Cyclic Loading, report No. AIAA-2007-2381, American Institute of Aeronautics and Astronautics.
4. S Lampman, in: S. R. Lampman et al, (Eds.), ASM handbook Vol. 19: Fatigue and Fracture, ASM international, materials park, OH, 1997, pp.725-727.
5. http://www.specialmetals.com/documents/Inconel %20alloy%20625.pdf on June 29, 2010
6. P. Ganesh, R. Kaul, S. Mishra, P. Bhargava, C.P. Paul, Ch. Prem Singh, P. Tiwari, S.M. Oak and R.C. Prasad: Laser rapid manufacturing of bi-metallic tube with stellite-21and austenitic stainless steel, Transactions of The Indian Institute of Metals 62(2) (2009) 169-174.
7. P. Ganesh, V. Karthik, R Kaul, C. P. Paul, P. Tiwari, S. K. Mishra, C. H. Prem Singh, T. Reghu, S. S. Sheth, K. V. Kasiviswanathan, R. C. Prasad and L.M. Kukreja: Fabrication of Multi-material Components by Laser Rapid Manufacturing and their Characterization, Proc. International symposium on Processing and fabrication of Advanced Materials-XVII, Vol. I, N Bhatnagar and T. S. Srivatsan, Eds. (I.K. Internal publishing house, New Delhi, India, December-2008) pp. 97-108.
8. P. Ganesh, A. Moitra, P. Tiwari, S. Sathyanarayanan, H. Kumar, S.K. Rai, R. Kaul, C.P. Paul, R.C. Prasad, L.M. Kukreja: Fracture behavior of laser-clad joint of Stellite 21 on AISI 316L stainless steel, Materials Science and Engineering-A 527(16-17) (2010) 3748–3756.
Acknowledgements
Authors are thankful to the members of Laser Materials Processing Division, RRCAT for their technical support for carrying out laser processing experiments. Thanks are due to Smt Pragya Tiwari of ISUD, RRCAT for extending SEM facilities. Authors gratefully acknowledge the support from Shri V. Karthik and Dr A Moitra from IGCAR Kalpakkam for extending the testing facilities for carrying miniature specimen tests and instrumented impact tests.
gradual drop in load from its peak as crack propagated
through graded interface, indicating plastic deformation
accompanying crack propagation. Compositional
grading across SS/St-21 interface brought about an
increase in the fraction of crack propagation energy at the
expense of initiation energy. In the light of the results of
the study it is inferred that compositional grading brought
about a change in the mode of crack propagation (from SS
to St-21) from initiation-controlled fracture in “direct
clad” specimens to propagation-controlled fracture in
“graded clad” specimens. Scanning electron microscope
examination of the associated fracture surfaces revealed
ductile fracture in the SS 316L clad region and brittle
fracture along inter-dendritic boundaries similar to that in
tensile fracture surface (Fig 8b).
Conclusions
The present article briefly reviewed the recent results of
the metallurgical characteristics of the laser fabricated
engineering alloys. It is evident from the results of FCGR
and fracture toughness that the laser fabricated structures
of IN-625 and SS 316L possessed adequate toughness
and fatigue crack growth rate at par with the wrought
counterparts. The results of the study on laser rapid
manufacturing of bi-metallic structures demonstrated
that near-net-shaping multi-material metal parts with
engineered compositional heterogeneity is feasible with
proper control on processing parameters. The use of
miniature specimen test techniques like ball indentation
testing and shear punch testing was found to be effective
in estimating the gradient in tensile properties of multi-
material structures fabricated by LRM. The results of
instrumented impact tests conducted on direct and graded
clad SS- St-21 Charpy specimens revealed that
compositional grading at substrate-clad interface can
alter the mode of crack propagation and delay the fracture
Fig. 9: Load-displacement plot of instrumented Charpy impact specimens (shown in inset) [8].
24
s = optical RMS roughness o
l = Wavelength of the incident beam
a = incident angle
Ra is calculated from s assuming sinusoidal profile for o
normal machined surfaces and using calibration curve.
This technique is useful for surfaces whose roughness is
very less than the wavelength, l, of the incident beam.
Thus if a diode laser at 670 nm is used then surfaces of Ra
less than 250 nm i.e. 0.25 µm can be measured.
The instrument consist of three parts(Fig 1)
(I) Sensor head unit
(ii) Monitoring Unit
(iii) Power Supply unit
The sensor head unit carries a Laser diode as light source
and two photodiodes for measurement of incident and
specularly reflected intensities. An amplifier card is also
there to amplify the photodiode signals. The monitor unit
receives the output from the sensor head. It carries a
microcontroller (87c552) which has integrated 10 bit
ADC. The two inputs are fed directly to the analog inputs
of µc , which converts them into digital value for use by
the microcontroller for calculation of s & Ra values o
using look-up table. The µc is also interfaced to LCD and
keyboard for user interface. Through keyboard it is
possible to select either Ra or s for display or voltages o
corresponding to specular and incident intensities. The
power supply unit carries linear regulated power supply
for the Laser diode and for electronic circuit. + 3 V for
Laser diode and ± 5 V for electronic circuit.
Salient features
Measurement Range : Average roughness (Ra) :
0.05 µm to 0.22 µm
RMS roughness (σ ) :0
0.05 µm to 0.15 µm
Accuracy : ±20 %
Measurement time : < 1 sec
Laser source : Diode Laser at 670 nm, 4 mW
Beam spot size : 1 mm
Measurable surface : Flat
shape
The following three instruments have been developed at
Laser and Plasma Technology Division, Bhabha Atomic
Research Centre, Mumbai and can be taken up by
industry for production:
LASER SURF-CHECK
Introduction
It is a Laser based non-invasive, hand-held, stand-alone
Roughness measuring instrument. The measuring probe
here is a low power Laser beam from diode Laser, which
falls on the sample surface. The incident beam and the
specularly reflected beam intensities are measured and
the ratio of the two is used to calculate the optical RMS
(σ ) and the average roughness (Ra) of the sample. In the 0
mechanical stylus based roughness measuring
instrument, a diamond tip on a stylus moves along the
surface and leaves a mark on the surface while measuring
its profile. This spoils a soft surface of very low
roughness value. Also it is very slow. Many times it is not
required to know the fine details but to obtain the average
parameter like Ra or RMS roughness of the surface. In
such cases, Laser based parametric technique like the one
of specular reflectance measurement used in Laser Surf-
Check gives directly the roughness parameter i.e. Ra or
σ , very fast.0
Being non-invasive in nature, it is very useful for
roughness measurement of soft surfaces. The fast
measurement capability makes it ideal for routine
comparison of similar surfaces.
Method
This optical technique for roughness measurement is
based on measurement of specular reflectance and then
using following equation:
2 Ix/Io = exp [ - (4ps /l) cos a] o
Where,
Ix = specularly reflected intensity from relatively
smooth surface
Io = specularly reflected intensity from perfectly
smooth surface
2
Laser Based Instruments for Measurement Applications
Aseem Singh RawatLaser and Plasma Technology Division, BARC, Mumbai - 400 085
E-mail : aseem@barc.gov.in
25
mechanical) are placed at known separation along the
path of the projectile and time taken by the projectile to
move the distance from one sheet to another is measured.
When optical sheets are used for measurement, it
becomes non-contact and non-destructive in nature.
The Laser Velocity Meter instrument uses diode lasers
with optics to generate light sheets. It has microcontroller
based circuit for velocity calculation along with LCD and
switches for user interface. It can be used as a tabletop
standalone single instrument or with two parts having
sensor unit near the projectile path and display unit at
control room connected through fiber optic link. It has
measurement range from 25 m/sec to 5000 m/sec with an
accuracy better than ±2 % of measured value.
Method
In LVM, two parallel light sheets are generated using
diode lasers at 670 nm with line generating optics. They
are separated, by fixed known distance and kept
perpendicular to the direction of projectile motion. When
a horizontal moving projectile crosses the light sheet,
shadow is generated on photodiode detector at each sheet
(Fig 2) that creates electrical pulses. The time between the
two pulses is electronically measured using high speed
Type of surface finish : Polished, Grinded
Surface material : Electroformed nickel alloy
For other materials, calibration is required
Micro-controller based circuit
LCD display
Keyboard interface for Selecting Ra or σ for display ,0
Diagnostic testing by selecting intensity values for
display
Resetting the instrument
RS 232 interface for data logging in PC
Fig 1 Photograph of Laser Surf-Check instrument
Application areas
• For actual measurement of roughness of metallic
mirrors.
• For routine comparison of similar surfaces of any
finish process like grinding, lapping, polishing.
LASER VELOCITY METER
Introduction
Measurement of speed of a projectile is required in
various material study experiments for determination of
parameters like relationship between material surface
deformation with impact, coefficient of restitution,
viscosity of a liquid, testing quality of a bullet in firing
range etc.
Laser Velocity Meter (LVM), is a non-contact, stand-
alone velocity measuring instrument based on time of
flight principle. In time of flight based measurement, the
time taken by a projectile to move a known distance is
measured and used to calculate the speed. For
determining time, two parallel sheets (optical or Fig. 3 : Block Diagram of LVM from top
Fig. 2 Light sheet Generation ( front view)
26
Applications
1) For study of material properties like deformation
with known impact
2) To find Coefficient of restitution of a material
3) In Defense material test labs (One unit has been
supplied to DMRL, Hyderabad)
REBOUND VELOCITY METER
Introduction
Rebound Velocity Meter (RVM), is a non-contact, stand-
alone velocity measuring instrument based on time of
flight principle. It is similar to LVM, except that here
instead of two light sheets, only one light sheet is used due
to which measurement range has reduced. It can be used
as a tabletop standalone single instrument. It has
measurement range from 1 m/sec to 400 m/sec with an
accuracy better than 2 % of measured value.
Method
Rebound velocity meter is also a velocity measuring
instrument based on time of flight principle, but here the
difference is in its time measurement method, unlike the
conventional method of measuring time taken by the
object to travel between two optical sheets separated by a
known distance, here a single narrow light sheet is used.
For known dimension of the object, the time taken by the
object to cross the thin sheet ( Fig 5) is measured to
calculate its velocity. The advantage of this method is that
the optical sheet can be placed very near to target and it
measures more accurate velocity for smaller object at
lower speed than two sheet method. The other important
feature of Rebound Velocity meter is that the electronic
circuit in it is designed such that instead of single velocity
it can measure two consecutive velocities and display
them.
Salient features
Measurement range : 1 m/sec to 400 m/sec
Accuracy : 2% of measured value
Size/Shape of : Preferably spherical, of diameter
counter to give time of flight. Thus time taken by a
projectile to move from one sheet to another sheet
measured electronically is used to calculate the velocity
of the projectile. The instrument has microcontroller
based circuit for velocity calculation(Fig 3).
Salient Features
Measuring Range : 25-5000 m/sec
Accuracy : better than ±2% of
measured value
Measurement Time : 10 msec
(max.)
Projectile Size : > 2 mm
Laser Source : Diode Laser (670nm, 5mWatt)
Width of light sheet : 50 mm
Thickness of : 1 mm
light sheet
Distance between two parallel light sheets : Adjustable
Detector used : PIN Photodiode
Remote operation possible with sensor unit near the
target and monitor unit with display can be upto 50 meters
away
Sensor unit & monitor unit connected using optical fiber
cable to avoid EMI
Stand-alone : Microcontroller based circuit
instrument
Display on monitor : Liquid Crystal Display unit
Speed display in m/sec.
PC-Connectivity : Serial Port, RS-232 interface at
1200 baud
Fig 4 Photograph of Sensor Unit of Laser Velocity Meter
Fig.5 Block diagram of RVM
27
RS 232 interface
Power input : 230 Vac, 20 mA
Power consumption : < 5 Watts
Dimensions : 74cmx15cmx16cm
Fig 6 Photograph of Rebound Velocity Meter
Applications
1) For study of material properties like deformation
with known impact
2) To find Coefficient of restitution of a material
3) In Defense material test labs
between 2 - 40 mm
Measurement time : Few msecs (depends on speed and
size of projectile)
Laser Source : Diode Laser at 670 nm of
5 mW power
Thickness of : 1 mm
light sheet
Width of light sheet : 40 mm
Suitable for opaque as well as transparent objects
Can be configured for horizontal and/or vertical moving
projectiles
Stand alone instrument with microcontroller based
circuit and LCD display (Fig 6)
Keyboard interface for
Resetting the instrument (required after each reading to
avoid multiple readings)
Entering projectile dimensions (if different from default )
Function key to select display the ratio of rebound to
falling velocity
Function key to select decimal resolution of reading
LED indication for function selected
Switch selectable measurement of falling velocity only or
with rebound
the projectile
28
The PSD consists of a uniform resistive layer formed on the surface of a high resistivity semiconductor substrate, and a pair of electrodes formed on both ends of the resistive layer for extracting the position signal currents. The schematic of the PSD is shown in figure 2.
The active area, which is a resistive layer generates photocurrent when light falls on it. When the laser spot is incident on the PSD, an electric photo-current is generated at the incident position. This electric photo-current generated is divided by the resistive layer and collected by output electrodes X and X as photocurrents 1 2
Ix and Ix , while is divided in inverse proportion to 1 2
distance between incident position and each electrode. The two current outputs of PSD Ix and Ix are converted 1 2
into voltages V and V by trans-impedance amplifiers. 1 2
Thus we have X = (V -V )/(V +V )×Lx/2A 2 1 2 1
The calculated position X is proportional to displace-A
ment of target from reference point x and is given by
x = sinγ × X /M × sinαA
where γ is angle between focal plane of focusing lens and plane of PSD while α is angle between translation axis of target and focal plane of focusing lens and M is the magnification of the lens.
Introduction
Lasers due to their unique properties find ready applications in metrology and inspection of precision mechanical components. Moreover laser based metrology and inspection systems are non-contact in nature making them suitable for use in the nuclear industry where the components to be measured or inspected could be radioactive. Various laser based instruments and systems have been developed at RRCAT for the metrology and inspection of nuclear fuel components. Some of the techniques developed for these applications could be adapted for use in industrial applications. We present here three such developments that could find wide application in industrial use. The first is a laser triangulation probe developed for non-contact profiling of precision mechanical components, the second is a laser line-triangulation based 3D digitization system developed for accurate form measurement and the third is a fiber optic proximity sensor that can be used for position measurement at high speeds such as in turbine and motor shaft position and rpm.
Triangulation Sensor
The triangulation sensor is a position sensing system that uses a non-contact measurement technique to determine position of an object with respect to certain reference point. The schematic of the sensor is shown in figure 1. The laser diode module shown in the schematic illuminates the target whose position is to be measured. The light scattered from the target is re-imaged on to a position sensing detector (PSD) using a lens assembly with a magnification M. The laser diode module, imaging lens assembly and position sensing detector all are encased in a sensor head and forms the triangulation senor probe. As the target position changes along the axis shown, the scattered light is imaged on different positions along the PSD. The PSD generates two outputs in form of currents which are proportional to the position of incident spot. The trans-impedance amplifier converts these currents into voltages. These voltages are digitized and processed by a microcontroller to determine the position of the target. The calculated position is displayed on LCD after linearization and multiplied by the calibration factor.
Laser based InstrumentationIshant Dave*, Rohan Bhandare, Brijesh Pant, S. Sendhil Raja and P.K. Gupta
Laser Biomedical Applications and Instrumentation Division
Raja Ramanna Centre for Advanced Technology, Indore - 452 013
*E-mail : ishantdave@rrcat.gov.in
Fig. 1: Schematic of a laser triangulation sensor probe.
Fig. 2: Schematic of a typical position sensitive detector
29
IDE provided by Keil development Tools. The compiler for code was provided by Keil development Tools. It generates both object file (OMF2 format) as well as hex file. The generated hex file is downloaded into flash of micro-controller using USB debug adaptor provide by Silicon Laboratories. The microcontroller C8051F120 contains an on chip industrial standard JTAG interface for in circuit testing. Due to this provision the microcontroller device can be debugged in actual application system without the use of external hardware.
For calibration, the system was mounted on an optical bread board. The target was mounted on a linear translation stage with digital readout, in front of the sensor. Zero reading of the stage was preset at stand-off distance from the sensor. The target was moved over the sensor range (±10mm) and the stage position from the digital readout and the sensor readings were noted. The calibration curve is shown in figure 4 below. For more accurate results a second order curve was fitted and same equation was implemented in firmware for compensation of the PSD non-linearity and to extend the linear range of the sensor.
The developed sensor can be used for position or displacement measurement in systems where non contact measurement is necessary, like in nuclear industry, where conventional techniques like LVDT fails. The sensor in conjunction with a X-Y positioning stage can be used for
Thus position of the target surface x is given by
x = sinγ/Msinα × (V -V )/(V +V )×Lx/22 1 2 1
The two current outputs of the PSD are converted into voltages by current to voltage converters using op-amps as shown in the circuit diagram in figure 3. These two voltages are digitized by an on chip ADC of the microcontroller. The logged values are averaged for 1000 samples to reduce the scatter in the position data due to random noise in the signal. The microcontroller processes these outputs using the formula discussed above to calculate the position of target .The calculated value is displayed on a liquid crystal display, with an update rate of 2Hz. To minimize the error due to ambient light the PSD output is measured with the laser diode in the “on” condition and in the “off” condition to correct for the positional error due to ambient light. The TTL modulation input of the laser diode module is connected to port pin of microcontroller, pulling this pin high or low switches laser diode on or off. The laser diode is switched on and off with square a wave generated by microcontroller on the pin with a frequency of 2 KHz. The micro controller logs both the outputs of PSD with laser diode on and off. Then it subtracts the value with laser diode off from laser diode on and the subtracted value is averaged for 1000 samples.
The system also compensates for varying reflectivity of target using an automatic gain control technique. The on chip DAC output of the microcontroller is amplified and used to control the analog modulation input of the laser diode module. The output of DAC is adjusted such that sum of current outputs of PSD remains within the specified band of predefined values irrespective of the reflectivity of the target surface.
The firmware for the system is developed in C language. The editing and listing of code was done in an integrated development environment provided by Silicon Laboratories. It can also be done using Keil Micro-vision
Fig. 3: Schematic of the front end circuit of the sensor
Fig. 4: The calibration curve for the developed sensor
Fig. 5: Photograph of the developed sensor head & the display unit
30
illuminated by the laser module. The object is moved along different orientations, either rotated or linearly translated to obtain the desired 3D information. Alternatively the laser line can be scanned across the object to generate the 3D data points. In the current system scanning is achieved using a FPGA based stepper motor controller card which rotates a mirror for the purpose of scanning the laser line output as shown in figure 7, a photograph of the setup. The developed system is portable and tripod mountable and can be used to scan objects of size 100mm X 100mm X 100 mm with a resolution of 10 microns. This system needs one time calibration for mapping the coordinate frames and generating the mathematical framework which is needed for extracting the 3D Data.
The calibration of the scanner is done using a calibration grid as proposed by Zhang et al. The camera parameters and the extrinsic parameters viz. translation and rotation for the object frame are calculated using the images shown below in figure 8.
surface profiling of various components for form measurement. The sensor can be used as a replacement for a mechanical dial gauge to measure thickness, height and run out etc.
The developed laser based triangulation sensor is shown in figure 5. The developed sensor has a linearised operating range of ±10 mm and a resolution of 20µm. The sensor operates with a stand-off distance of 100mm.
Laser based 3D Digitization System
Three dimensional digitization of real objects is of interest in various fields such as engineering design & proto-typing, reverse engineering, metrology, orthopaedics and dentistry. The field is ever-growing since the advancement of laser based digital scanners which are capable of generating very accurate data at high scanning rates. The system developed here has a typical range of 100x100x100mm with a resolution of 10microns.
The laser triangulation probe described in the previous section can be extended to the other dimension by replacing the laser spot with a laser line and the PSD with a CCD camera as shown in figure 6. In laser line triangulation we have a plane described by the laser line generator and a unique direction identified by the camera, the intersection of which is a unique point in three dimensions Intersection of a line and a plane is a unique point which can be easily computed if the equations of the line and the plane are known. In three dimensions, solving for the coordinate of the point implies solving three equations in three respective dimensions. The system developed consists of a laser line generator designed with reflective optics which illuminates the object of interest, a high resolution CCD Camera (1200 X 1600 pixels is used to grab frames of the object
Fig. 6: Schematic of a Laser line triangulation setup
Fig. 7: Photograph of the laser line triangulation setup
Fig. 8: Calibration images used for the laser line triangulation setup
31
show the point cloud generated from the scanning of a dental mold and the subsequent meshed point cloud to generate the CAD model of the object.
Reconstruction of 2.2 million points is done in a minute by the developed software. The system can be scaled to digitize large objects such as in archeological artifacts or other similar objects of interest.
Fiber-optic Proximity Sensor: Fiber optic sensors are very popular in instrumentation where there are constraints such as contactless, non-electrical in nature, noise/radiation immune, small in size etc. A fiber optic based sensor probe was developed for measurement of vibrations and RPM (Revolutions Per Minute) of turbo machinery. It is possible to measure the displacements in the range of about 3 to 5 mm with a resolution better than 10 µm and RPM of up to 2,50,000. The output is transmitted as an industry standard 4 to 20 mA current signal used in industrial PLC control.
The schematic of the probe is shown in figure.11 above. The probe tip contains a bundle of multimode fibers in the centre called the illuminating fibers. These are coupled to a high brightness LED at the end. The illuminating fiber illuminates the measuring surface. A bundle of fibers concentric to the illuminating bundle collects the back reflected light. These fibers are coupled and focused onto a photo detector which converts the optical signal into an electrical signal. The intensity of the reflected light depends on the distance between probe tip and measuring surface, which is measured in terms of displacement.
The laser line is scanned across the object of interest and the point cloud for the illuminated pixels is calculated. The image shown in figure 9 is one of hundreds of images which are acquired with the help of the rotating mirror scanner and the synchronized CCD camera of the laser line triangulation setup.
The point clouds generated from the laser line scanning are post-processed in a 3D meshing software viz. 3D Reshaper from Technodigit Corp. which is capable of meshing point clouds, in order to generate a CAD file of the acquired point cloud data. The images 10a and 10b
Fig. 9: A typical image of the laser line triangulation output.
Fig. 10a: The point cloud generated by the system for a dental mold.
Fig. 10b: The rendered CAD model generated by the system for a dental mold.
Fig 11: Schematic of the fiber optic proximity sensor
Fig. 12: Position-response characteristic
curve of the developed probe.
32
The figure 14 shows the electrical interface diagram for both RPM and displacement measurement. The back coupled light from receiving fibers is focused on a photodiode. The Op Amp based signal conditioning circuitry provides necessary gain and offset adjustments. The processed signal is transmitted as a standard instrumentation signal of 4 to 20 mA. For RPM measurement a comparator circuit converts the optical pulses into a train of electrical pulses. The pulses are counted by the CPLD based counter in specified time and hence the frequency/RPM is measured. It also generates necessary interface signal for a “serial 4 to 20mA” transmitter. The threshold and hysteresis adjustment is provided to compensate for surfaces of different reflectivity and roughness
There are several limiting factors in use of this kind of fiber optic displacement sensors. The measuring surface must be scattering type (Lambertian surface) as the reflected light is measured in terms of distance. The surface and tip of the probe must be free from any contaminations which obstructs light. Any change in reflectivity or any obstruction of light to and from the sensor affects the accuracy of the measurement. Due to typical characteristics of the probe it produces the same output for two different locations on each slope which may be misleading. Inspite of these limitations due to the probes simplicity and robustness it can find applications in health monitoring of rotating machines where contact probes wear out frequently and electrical sensors are prone to EMI noise from the motors. The probe can also be used over a large temperature variation such as in turbo-machinery since the probe front-end has no electronic components.
Conclusion: We have presented here three different laser based instruments developed as spin-offs from the various instruments developed for the nuclear industry. The developed instruments are to be considered as complementary to the existing mechanical techniques and not as replacements for them. The use of laser based instruments comes at an added penalty of requiring a greater level of sophistication of the operators and cleaner operating environment. The instruments should be used where the benefits offered such as high accuracy, wear-free, robustness etc by the laser based instruments greatly offset the penalties. The three instruments described here have been prototyped and tested in field applications and are at a level where they can be readily transferred to interested industrial partners who are interested in taking up production of these instruments.
The figure.12 shows the typical characteristic curve of the developed probe. The intensity of the reflected light measured by the photodiode is plotted against the distance of the probe tip from the measuring surface. It depicts two semi-linear regions with two different slopes called the “front slope” and “back slope” with an optical plateau in between. The shape of the characteristic curve depends on various parameters like distance between illuminating and receiving fibers, diameter of the fibers, numerical aperture of the fibers, arrangement of the fibers etc and can be manipulated by the appropriate design of the probe.
The figure.13 shows how both displacement and RPM of a rotating spindle can be measured using this kind of probe. Above portion shows the displacement part in which any deflection of a spindle changes the distance between tip and surface causing change in intensity as shown in shaded region. The change in intensity is measured in terms of displacement. For RPM measurement a notch is provided on the spindle. Every time the notched passes the tip the distance changes. This produces optical pulses at the output which is converted into electrical signal and counted in terms of RPM.
Fig. 13: Displacement and RPM measurement with the probe
Fig. 14: Schematic of the front-end signal conditioning circuit.
33
corrosive environments. A successful coating should be
refractory, chemically inert, possess good mechanical
strength and thermal shock resistance, have low thermal
conductivity and exhibit similar thermal expansion
coefficient to that of the substrate [4-6]. These
requirements have led to the development of partially
stabilized zirconia coatings as TBC's for its high thermal
expansion coefficient close to that of many metals and
alloys used in engine applications [5,7]. Plasma sprayed
yttria stabilized zirconia coatings (PSZ) are considered as
one of the potential options for the salt purification vessel
and electrorefiner application in reprocessing plant and
the coating has shown excellent corrosion resistance in molten LiCl-KCl salt [8,9]. Therefore ceramic oxide
coatings increase inertness, corrosion resistance and
durability in aggressive molten LiCl-KCl salt. The
plasma spray process is an economical method for
producing reproducible and durable thick thermal barrier
coatings. It is a well established technique for applying
ceramic coatings to protect the surface of engineering
components against corrosion, erosion and wear at high
temperatures [10]. During the plasma spraying process,
residual stresses generated within the coating system by
the rapid cooling of molten droplets are relieved by
through-thickness microcracking [3]. The presence of
micro cracks and interconnected porosity in the coating
affects the mechanical properties and deteriorates the
oxidation and corrosion resistance. These micro cracks
and pores are considered to be path for molten salts and
corrosive gases to attack the TBC system [2,3,11]. Lasers
have been used for the modification of surfaces and
elimination of such defects.
Lasers are promising technological tool for surface
modification [12], due to their characteristics speedy
treatment, simple process control, and delivery of high
energy density to a localized surface area without significantly heating up the whole body [12,13]. Laser
processing has been attempted for surface modification
of PSZ coatings to improve the wear resistance, thermal- shock resistance [2], corrosion resistance [2,14], and life
Abstract
Thermal barrier coatings are well known for the protection of high temperature components. Plasma
sprayed yttria stabilized thermal barrier zirconia coating
(PSZ) has been proposed for protection of various
components in pyrochemical reprocessing plants
involving molten LiCl-KCl salt for temperatures up to o600 C. The pores and micro-cracks present in the as-
sprayed coatings may cause corrosion of the substrate as
molten LiCl-KCl salt can penetrate through these defects
on prolonged exposure. Laser processing has been
attempted as a promising technique for surface
modification of as-sprayed PSZ coatings. Laser
remelting of the as-sprayed PSZ coatings resulted in a 50
µm thick densified layer on the surface and eliminated
microstructural inhomogeneities like pores and voids,
however, segmented cracks were formed. The
microhardness of the laser remelted surface increased
and surface roughness was reduced. The beneficial non-
transformable tetragonal phase was formed after laser re-
melting while the as sprayed coating consisted of
insignificant monoclinic phase. Distinct polygonal grains
with interface separating fine and coarse grains were
observed. Laser processing by addition of silica
decreased segmented cracks; however, further
optimisation is required to achieve dense and crack-less
surface. The paper highlights the results obtained for
application of laser remelting on plasma sprayed PSZ
coating.
Introduction
Thermal barrier coatings (TBC's) are being increasingly
used in the turbine section of advanced gas turbine
engines and aerospace industry [1-3]. For pyrochemical
reprocessing of spent metallic fuels, by electrorefining
process, molten LiCl-KCl salt is used as electrolyte at o600 C. The material used for fabrication of
electrorefining vessels and components should have high
corrosion resistance. High performance corrosion
resistant coatings are essential for such severely
Application of Laser Processing of Materials for High Temperature Molten Chloride Environment
A. Ravi Shankar, Ravikumar Sole, Jagdeesh Sure and U. Kamachi Mudali*Corrosion Science and Technology Group
Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102
*E-mail: kamachi@igcar.gov.in
34
employed are given in Table 3. The as sprayed and laser
remelting samples were characterised by XRD, Optical
microscopy and SEM.
Results and Discussion
The surface morphology of as-sprayed partially
stabilised zirconia (PSZ) coating applied over 316L SS is
shown in fig 2a. The typical morphology of as sprayed
coating consists of completely melted splat regions,
cracks within the splats, unmelted particles, and pores
between the splats (fig 2a). These types of complex
microstructures form due to rapid solidification of molten
and semi molten particles impinging on the substrate
during plasma spraying. It has been reported that thermal
barrier PSZ coating by plasma spraying posess upto 10%
pores, and form microcracks due to splat type of melting
and solidification, consisting of unmelted particles,
partially melted particles etc. [18] which are detrimental
for corrosion. The SEM micrograph of laser remelted
surfaces is shown in figures 2b-d. The porous as sprayed
coating shown in fig 2a became smooth and dense after
laser remelting as shown in fig 2b-d. After laser melting,
of the coating [15]. Laser remelting has been considered
as a potential process for the improvement of plasma
sprayed TBC properties by reducing surface roughness,
eliminating open porosity on the surface [3,16], and
generating a fully dense layer with homogeneous
microstructure [2]. In order to reduce the corrosion attack
and to seal open pores on the TBC surface [17], and to
enhance the thermal shock resistance and life time of
zirconia coatings, laser remelting process is a promising
technique [1]. Numerous studies on laser surface
treatment of zirconia were performed to eliminate cracks
and porosity using CW and multimode, CO laser, high 2
power diode laser, Nd: YAG laser, etc. [1-3,12,13]. A four
fold improvement in life after laser glazing zirconia
based TBC's was observed in cyclic corrosion tests [11].
In addition to eliminate cracks and porosity on the
surface, lasers have been used to improve resistance to
mechanical erosion and chemical corrosion of plasma
sprayed zirconia coatings [13]. The present paper
discusses laser remelting of plasma sprayed partially
stabilized zirconia surface on type 316L SS to achieve a
smooth and pore free surface.
Experimental Details
AISI Type 316L SS discs of 25 mm diameter and 5 mm in
thickness were used as substrates. A bond coat of 50 µm
thick NiCrAlY was coated on 316L stainless steel
substrates to provide good adhesion between substrate
and ceramic coat. Over the bond coat 300 µm thick
partially stabilised zirconia (ZrO -8 wt %Y O ) was 2 2 3
coated by Air Plasma Spraying (APS) process. The
coated layers were produced by M/s Plasma Spray
Processors, Mumbai, with a METCO 9MB type plasma
gun with optimized spray parameters. The laser remelting
process was carried out with Continuous Wave (CW)
multi beam CO laser (wave length 10.64 µm) with a 3 2
axis Computer Numerical Controller (CNC) workstation
as shown in fig 1. The laser processing parameters
Fig. 1. Schematic diagram of multi beam CW CO laser system.2
Table 1. Laser processing parameters.
Laser processing parameters
CO laser active CO + N + He gases 2 2 2
medium (15: 26: 61 mbar)
Material ZrO + 8 wt% Y O2 2 3
Power (W) 50, 75, 100
Beam diameter (mm) 1.5
Scan speed (mm/sec) 1, 2.5, 5
Interaction time (sec) 1.5, 0.6, 0.3
Track shift (mm) 0.5
Shield gas Argon
Fig 2. SEM micrographs of partially stabilized zirconia coating over type 316L SS (a) as-sprayed (b-d) laser remelted.
35
showed well delineated columnar grain structure
[8,19,20]. As the solidification was directional and
vertically from the substrate to the remelted area,
formation of columnar dendritic structure takes place. In
comparison to the lamellar structure resulting from
thermal spraying, the columnar structure provides a
better thermomechanical resistance during thermal
cycling. Figure 4b shows the interface separating coarse
and fine grain structure [20]. The formation of sharp
interface with development of coarse grains was
attributed to the (additional heat entrapment) reheating of
the solidified material where the ripples coalesce, during
rastering of the laser beam with a shift. The combination
of coarse and fine grain structure offers optimum
properties. Fig 5. SEM micrographs of partially
stabilized zirconia coating (a) laser remelted (b) laser
remelted with silica overlay (c & d) laser co-deposition of
silica and corresponding EDX [20,21].
Chen et.al, [17] reported that a dense and crack-less thin
layer can be achieved on the surface of 3wt% SiO -doped 2
ZrO coating using laser re-melting. Based on the 2
literature laser remelting with ZrO + SiO overlay and 2 2
laser co-deposition of silica were carried out [20,21]. The
porosity was eliminated completely and smooth and
dense surface was obtained. SEM micrograph after laser
treatment of the coating showed preferred well delineated
grains of zirconia (fig 2 c&d). However, the segmented
crack morphology was observed in the laser treated
region (fig 2b). The well defined segmented crack
network all along grain boundaries (fig 2d) was formed
due to shrinkage and relief of thermal stresses during
cooling [8,19].Fig 3. Cross section micrographs of
partially stabilized zirconia coating over type 316L SS
(a&c) as-sprayed (b&d) laser remelted.
The cross section SEM micrograph of as-sprayed and
laser remelted partially stabilised zirconia (PSZ) coating
over 316L SS with intermediate bond coat is shown in fig
3 a&b respectively. A 50 µm dense laser remelted layer is
clearly seen in the micrograph (fig 3b). Fig 3 c&d shows
the cross section optical micrographs of as-sprayed and
laser remelted regions respectively depicting the porosity
present. Area percentage porosity of coatings were
determined as per ASTM E 2109 method A, which is a
manual, direct comparison method utilizing standard
images given in the standard which depict typical
distributions of porosity in TBCs. The porosity in the
partially stabilized zirconia coating is reduced from 10%
in the as-sprayed coating (fig 3c) to 0.5% after laser
treatment (fig 3d). The microhardness measurement
indicated that the hardness increased significantly from
664 VHN in the as-sprayed region to 1230 VHN in the
laser treated region (fig 3d) probably due to significant
decrease in the porosity and microstructural changes.
The optical micrographs showing laser remelted
microstructure with fully dense coarse and fine polygonal
grains of zirconia is shown in fig 4. The optical
micrograph of the laser remelted PSZ coated surface
Fig 3. Cross section micrographs of partially stabilized zirconia coating over type 316L SS (a&c) as-sprayed (b&d) laser remelted.
Fig 4. Optical micrographs of laser remelted partially stabilized zirconia coating over type 316L SS.
Fig 5. SEM micrographs of partially stabilized zirconia coating (a) laser remelted (b) laser remelted with silica overlay (c & d) laser co-deposition of silica and corresponding EDX [20,21].
36
Typical splat type of surface morphology of plasma
sprayed Al O -40wt%TiO coating on high density 2 3 2
graphite is shown in fig 7a [22]. Plasma spray coatings
contain completely melted splats, unmelted and partially
melted particles and in addition to that phase separation
could occur in Al O -40wt%TiO which results in 2 3 2
inhomogeneous surface as shown in fig 7b containing
60%Ti. In order to achieve homogeneous surface and
reduce roughness plasma sprayed Al O -40wt%TiO 2 3 2
coatings were subjected to melting using a micro pulsed
Nd:YAG laser at RRCAT, Indore, with power densities of 20.64 and 0.8 MW/cm . Microstructural examination
showed that inhomogeneities were eliminated and a
smooth surface was achieved (fig 7c). However, network
of cracks was formed irrespective of power density. XRD
results indicated that stable α-Al O and Al TiO phase 2 3 2 5
was more predominant in laser remelted coatings [22].
Conclusions
Laser consolidation of plasma sprayed PSZ coatings over
316L SS has been attempted to produce dense and smooth
surface for application in molten chloride environment.
Laser remelting of the coatings resulted in smooth and
glossy surface with fully dense well delineated columnar
grain. The porosity in the as-sprayed coating decreased
and hardness increased significantly after laser treatment.
Segmented crack network along grain boundaries in laser
treated samples are formed due to shrinkage and relief of
thermal stresses. The formation of coarse and fine grains
with sharp interface was attributed to the reheating of the
solidified material. The addition of silica during laser
processing decreased the segmented cracks and resulted
in dense and pore free surface, however further
optimisation of parameters are required. Beneficial non-
transformable tetragonal phase was present in the
remelted layer as determined by XRD analysis. Laser
remelting of Al O -40wt%TiO on high density graphite 2 3 2
eliminated surface inhomogeneities and smooth and
dense surface was formed.
optical microstructure of laser re-melted PSZ surface is
shown in fig 5a while fig 5b shows the optical
microstructure after giving an overlay of ZrO + SiO and 2 2
then laser re-melting. The purpose of SiO is to alter the 2
surface composition and thereby decrease the segmented
cracks [20]. As shown in fig 5b, there is decrease in the
segmented cracks with formation of large cells, however,
the cross section optical micrograph indicated partial
delamination of the ZrO + SiO layer. In order to achieve 2 2
dense and crack less layer, laser parameters and silica
content need to be optimised. An attempt was made by
laser co-deposition of silica with 3.5 kW CO laser at 2
RRCAT, Indore, in pulse modulated mode. Figure 5 c&d
shows the surface morphology of silica deposited sample
with corresponding EDX spectra which shows PSZ
containing 40 wt% silica with dense and fine spherical
agglomerate deposited during laser processing without
any cracks [21]. This type of dense structure without
pores and cracks are desirable for molten chloride
application and further fine tuning of parameters are
required to achieve completely dense and crack free
surface.
The presence of minor amounts of detrimental
monoclinic phase present in the as sprayed coatings as
shown in fig 6 is eliminated on laser remelting. X-ray
diffraction pattern of the as sprayed and laser remelted
surfaces indicated the presence of beneficial metastable
tetragonal phase in the laser treated sample (fig 6)
[8,19,20] which is in good agreement with the literature.
The presence of t' phase in the thermal barrier coatings
was expected to be highly beneficial because of its
thermal stability and structural hardening.
Fig 6. XRD spectra for as coated and laser re-melted PSZ at 50W at varying scan speeds.
Fig 7. (a & b) Surface morphology of plasma sprayed Al O -40 2 3
wt%TiO coating (c) after laser remelting.2
37
10. R. Siva Kumar, and B.L. Mordike, Surf. Eng., 3
(1987) 299-305.
11. I. Zaplatynsky, Thin Solid Films, 95 (1982) 275.
12. Sang Ok Chwa and Akira Ohmori, Surf. Coat.
Technol., 148 (2001) 88-95.
13. Xing Wang, Ping Xiao, Marc Schmidt and Lin Li,
Surf. Coat. Technol., 187 (2004) 370-376.
14. C. Batista, A. Portinha, R.M. Ribeiro, V. Teixeira,
C.R. Oliveira, Surf. Coat. Technol., 200 (2006)
6783-6791.
15. P. C. Tsai, C. S. Hsu, Surf. Coat. Technol., 183
(2004) 29-34.
16. K.M. Jasim, R.D. Rawlings and D.R.F. West, J.
Mater. Sci., 26 (1991) 909-916.
17. H.C. Chen, E. Pfender, and J. Heberlein, Thin Solid
Films, 315 (1998) 159-169.
18. J. R. Davis, Davis and Associates, Microstructural
Characterization of Thermal Spray Coatings, in
'ASM handbook', 9 (2004) 1038.
19. A. Ravi Shankar, B. Jagdeesh Babu, Ravikumar
Sole, U. Kamachi Mudali and H.S. Khatak, Surf.
Eng., 23 (2007) 147-154.
20. A. Ravi Shankar and U. Kamachi Mudali, Surf.
Eng., 25 (2009) 241-248.
21. A. Ravi Shankar et.al unpublished (2011).
22. Jagadeesh Sure et al, unpublished (2011).
Acknowledgements
The authors thank scientists at RRCAT, Indore for their
support in part of the laser processing work carried out at
RRCAT, Indore.
References
1. G. Antou, G. Montavon, F. Hlawka, A. Cornet, C.
Coddet and F. Machi, Surf. Coat. Technol., 172
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2. A. Petitbon, L. Boquet and D. Delsart, Surf. Coat.
Technol., 49 (1991) 57-61.
3. C. Batista, A. Portinha, R.M. Ribeiro, V. Teixeira,
M.F. Costa and C.R. Oliveira, Surf. Coat. Technol.,
200 (2006) 2929-2937.
4. J. R. Brandon and R. Taylor, Surf. Coat. Technol.,
39/40 (1989) 143-151.
5. R. Taylor and J.R. Brandon, Paul Morrell, Surf.
Coat. Technol., 50 (1992) 141-149.
6. J.F. Li, H.L. Liao, C.X. Ding and C. Coddet, J.
Mater. Process. Technol., 160 (2005) 34-42.
7. P. Scardi, M. Leoni and L. Bertamini, Thin Solid
Films, 278 (1996) 96-103.
8. A. Ravi Shankar, U. Kamachi Mudali, Ravikumar
Sole, H.S. Khatak and Baldev Raj, J. Nucl. Mater.,
372 (2008) 226-232.
9. A. Ravi Shankar, U. Kamachi Mudali, Mater.
Corros., 59 (2008) 878- 882.
IONS-India
IIT Delhi student chapter of OSA is happy to host IONS (International OSA Network of Students) conference for the first time in India at Indian Institute of Technology Delhi (IIT Delhi) 1-2 December 2011. Program information will be posted at
http://ions-project.org/?id=4&topic=ionsdelhi.
For more informationcontact Ms. Kanchan Gehlot at
gehlot.kanchan@gmail.com
Important Information
38
Atomic Energy to industrial partners. In the afternoon,
the technology showcase session provided a platform for
one-on-one technical discussions between researchers
and industrial delegates. This was followed by
presentations by industrial participants. Interesting
presentations were made by delegates from Tata Motors
Ltd. Pune, Bharat Heavy Electricals, Hyderabad, Larsen
& Toubro, Mumbai and Archaeological Survey of India.
The proceedings of the day were concluded with a group
discussion session chaired by Dr P. K. Gupta. The
session witnessed lively discussions with industrial
delegates on the ways to strengthen interaction between
laser research community and indigenous industries. On
the second day the first talk was delivered by Prof B. D.
Gupta of Indian Institute of Technology, Delhi on optical
sensors for process monitoring. This was followed by a
presentation by Dr Sunita Belgamwar of Nexus
Mechatronics, Pune on lasers in therapy. Subsequent
talks on optical spectroscopy and imaging for bio-
medical diagnosis were delivered by Dr. Diwakar Rao &
Dr. S. K. Majumder of RRCAT and on laser based
metrology & inspection by Dr. Sendhil Raja of RRCAT.
These presentations were followed by an informative
presentation by Dr P. S. Raju of Technology
Development Board (TDB) of Department of Science
and Technology (DST) on various funding schemes
offered by TDB. The meet was concluded with a group
discussion session chaired by Dr. P. S. Raju.
By:
Rakesh Kaul & S. Sendhil Raja.Raja Ramanna Centre for Advanced Technology, Indore
On the occasion of 50th year of invention of laser, Indian
Laser Association organized a two-day interaction meet
on Utilization of Lasers in Industry and Medicine on 28th
and 29th April 2011 at Raja Ramanna Centre for
Advanced Technology (RRCAT), Indore. The
interaction meet aimed to provide a platform to showcase
indigenous laser technologies developed for industrial
and medical applications in major academic and research
institutions of the country and to promote closer
interaction between academic/research institutions of the
country and Indian industry.
The interaction meet was inaugurated by Dr. P. K. Gupta,
President, Indian Laser Association. The inaugural talk
on technology generation and incubation was delivered
by Shri A. M. Patankar Head TT& CD of BARC. The key
features of the meet were presentations by laser experts
explaining rudiments of laser applications, presentations
by industrial delegates, outlining their experiences and
prospective requirement of laser technology in their
industry and technology showcase sessions involving
presentations of indigenous laser technologies. The meet
was attended by about 30 participants from 25 different
companies. About 35 posters on various laser-based
technologies developed at different R&D centers in India
were presented in the meet.
In forenoon session of the first day, presentation made by
Dr. L. M. Kukreja and Shri Rakesh Kaul included
fundamentals of laser material processing and overview
of related presentations in technology showcase session.
Shri A. M. Patankar, delivered an informative talk on the
modalities of Technology transfer from Department of
Interaction Meet on Utilization of Laser Technology in Industry & Medicine at RRCAT, Indore
Report
MEMBERSHIP FORM
INDIAN LASER ASSOCIATION
Announcement
For circulation among ILA members only.(Not for sale)
Printed by : Rohit Offset Pvt. Ltd. Indore. 2422201-02A Bulletin of the Indian Laser Association
Prof. B. D. Sharma, Dr. Sunita Belgamwar, Dr. V.K. Saxena and Dr. P. S. Raju delivering their lectures during the meet.
Technical Sessions of Interaction Meet on Utilization of Laser Technology in Industry & Medicine, RRCAT, Indore, April 28-29, 2011.
Participants of the meet discussing during the tea break and the poster session.
Recommended