Elevons - NCAIRncair.in/newsletters/NewsletterV5.1.pdf · Welcome to the present edition of the NCAIR ‘Elevons’ newsletter. This edition of the newsletter is a combined edition

July 2015 . Newsletter . Volume 5 . Issue 1

COMBINED EDITION

ISSUES IN ULTRA-PRECISION MACHINING (UPM)

OF TITANIUM ALLOYS

8 FEATURED ARTICLE

Elevons

14R & D ARTICLE

Fabric Permeability Computation with Varying Fiber Distribution

and Packaging

Selective Laser Melting (SLM): Process and Applications

32TECHNOLOGY UPDATES

CONTENTS Editorial, NCAIR Events and News Updates 1-7

FEATURED ARTICLE

Issues in Ultra-precision Machining (UPM) of Titanium Alloys – Abhishek Bihari , Prof. Rakesh G. Mote 8

R&D UPDATES

Fabric Permeability Computation with Varying Fiber Distribution and Packaging 14

Comparison of various Liquid Composite Moulding (LCM) processes 19

Modeling and Simulation of process induced deformations in fiber reinforced polymer (FRP) matrix composites 24

TECHNOLOGY UPDATES

Selective Laser Melting (SLM): Process and Applications 32

AEROSPACE NEWS BRIEFS

NASA's 10-engine electric plane completes flight test 38

A huge breakthrough in nuclear fusion 38

Carbon-fiber epoxy honeycombs mimic the material performance of balsa wood 38

Composite plane life cycle assessment shows lighter planes are the future 38

Unidirectional carbon fiber prepreg tapes just 15 gsm 38

Acknowledgement

We extend our sincere gratitude to the faculty members of IIT-Bombay for their support. We are also thankful to the students and staff of NCAIR, for their valu able articles and other support.

Editors : Prof. Suhas Joshi and Prof. Asim Tewari Asst. Editors : Dr. Sarbani Banerjee Belur Ms. Vani K. Sreedhara Contact details : [email protected] Articles that fall under the purview of NCAIR Newsletter are always welcome.

Editorial

Welcome to the present edition of the NCAIR ‘Elevons’ newsletter. This edition of the newsletter is a combined edition of the December 2014 and March 2015 issues. We regret for the inordinate delay. We anticipate and expect the year 2015 would be a promising one for NCAIR. We look forward to NCAIR’s continued growth and achievement of its ambitious goals and milestones.

We welcome you to Volume 5, Issue 1 of the NCAIR newsletter. The important aim of this newsletter is to keep the readers informed of the ongoing developments, and innovations in research being a part of NCAIR.

In this issue of the newsletter, we present some interesting contributions related to aerospace applications and manufacturing processes. The featured article of this issue discusses various facets of ultra-precision machining (UPM) of titanium alloys.

Research and development is an important aspect of NCAIR. This is reflected in the various R&D updates, which has articles on fabric permeability computation with varying fiber distribution and packaging, comparison of various liquid composite moulding processes, modeling and simulation of process induced deformations in fiber reinforced polymer matrix composites .

The technology update section focuses on recent technological advancements in the arena of aerospace manufacturing globally. In this issue, we present Selective Laser Melting (SLM): process and its applications. The newsletter also carries important news briefs in the aerospace domain.

We would like to thank Prof. Rakesh Mote, Ir. Bey Vrancken, Prof. Dr Ir. Jan Van Humbeeck and Dr. Ir. Akhilesh K. Swarnakar for their contribution to this newsletter. We extend our gratitude, to the various students and staff of NCAIR, for their articles and contributions. Also we would like to acknowledge Prof. Mandar Rane, IDC, IIT Bombay and his team for designing ‘Elevons’.

Hope you will find this newsletter interesting. Please feel free to provide us with any feedback, including things that you would like to see being featured in the forthcoming editions through email: [email protected]

For an online edition of this newsletter and more news about NCAIR, please visit our website http://www.ncair.in

Kind regards,

Suhas S. Joshi & Asim TewariEditors

1.

NCAIR EVENTStechnical Communication workshop, November 1, 2014, iit BombayThis was a one-day workshop that focused on reading, writing and speaking skills. The purpose of the workshop was to enhance communication skills amongst students in IIT Bombay. In this workshop, participants engaged in various exercises, activities and group discussions thus laid emphasis on their critical analysis and reflective thinking.

one-day Workshop on advances in ion and Electron Beam techniques in applied Sciences, November 13, 2014, iit-BombayIn our endeavour to provide end-to-end support to Indian small and medium scale industries to enhance their ability to participate in aerospace manufacturing, NCAIR organised a workshop to talk about the advances in ion and electron beam techniques in applied sciences on November 13, 2014 at IIT Bombay.

The important highlights of this workshop was talks by eminent faculty from IIT Bombay, renowned industry experts from companies like Carl Zeiss and Oxford instruments and participants having an opportunity to interact with experts during the panel discussion.

The workshop was inaugurated by Prof. Subhasis Chaudhuri, Deputy Director (AIA), IIT Bombay. The chief guest for the workshop was Mr. Daniel Sims, Managing Director, Carl Zeiss India. The industry academia collaboration was presented by Prof. P.M. Mujumdar, Dean R&D, IIT-Bombay.

2.

1.1 - Participants at the Technical Communication workshop, November 1, 2014.

2.1 - Participants of Workshop on Advances in Ion and Electron Beam Techniques in Applied Sciences, Nov 13, 2014.

2.2 - Mr. Daniel Sims, Managing Director, Carl Zeiss India at the one day workshop on Advances in Ion and Electron Beam Techniques in Applied Sciences

1.1

2.1 2.2

one day Composites Convention, February 16, 2015 at VMCC Conference room, iit BombayA one-day composite convention hosted by NCAIR was held at IIT-Bombay to brainstorm the scenario of advanced composite manufacturing technologies and discuss about the implications of it in the current scenario of the aerospace ecosystem of India.

Dr. P.Mangalagiri, consultant at in the National Programme on Smart and Micro-Systems at ADA chaired this convention. Eminent personalities like Dr. Ramesh Sundaram, Dr. C.M., Manjunatha, Mr. M.K. Sridhar from NAL, Mr. A. Gnanasekar, Mr. P. Mukhopadhyay from HAL, Mr. J. Dhanasekaran Mr. BVSR Murthy from ASL, Shri. A. Rajarajan fromVSSC attended this convention and gave their opinion on the further need of development in the field of composites. They were joined by Prof. P.M. Mujumdar, Dean R&D, Prof. Asim Tewari PIC, NCAIR, Prof. Naik, Adjunct Professor, Aerospace Department, Prof. sushil Mishra, Prof. Ramesh Singh, Prof. Anirban Guha from Mechanical Department, Prof. Sauvik Banerjee from Civil Department, Prof. Guruprasad and Prof. Mira Mitra from Aerospace department, Prof. A.R. Bhattacharya, Prof. Aparna Singh, from Materials and Metallurgical Department, IIT Bombay to assess the academician view of the research and development in the area of composite manufacturing.

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3.1 - Participants of the Composite Convention along with NCAIR team

3.2 - Group photo of the participants of the Composite Convention L-R: Mr. A. Gnanasekhar, HAL, Ms. Swetha Sridhar, NCAIR, Ms. Vani Sreedhara, NCAIR, Dr. Aparna Singh, IITB, Dr. Sudhho, IITB, Dr. Sushil Mishra, IITB, Dr. Asim Tewari, IITB, Dr. P.D. Mangalgiri, ADA, Dr. Ramesh Sundaram, NAL, Mr. M.K. Sridhar, NAL, Mr. A. Rajarajan, VSSC, Dr. C.M. Manjunatha,CSIR-NAL, Dr. Guruprasad, IITB, Mr. Varun Sharma, Brahmos, Mr. Taha Khot, NCAIR, Mr. Gautham Salkar, NCAIR, Dr. Sushrut Vaidya, NCAIR, Mr. P. Mukhopadhyay, HAL.

3.2

3.1

New Colleagues at NCAIR Omanath Pawar

He has his Bachelor of Engineering (B.E.) degree in 2010 in Mechanical Engineering from Pune Vidhyarthi Griha’s College of Engineering and Technology, Pune. After graduating, he worked as Quality Assurance Engineer in Elica PB India Pvt Ltd. For one year. He completed his M.Tech with specialisation of Materials, manufacturing and modelling (MMM) from IIT Bombay in 2014. He has joined NCAIR as a Research Associate.

Sunil Kumar

He is pursuing his Masters of Technology and Bachelors of Technology (Dual

Half day tech talk by Mr. rick albrecht, Executive Section Manager for advanced Mechanical design team at GE-aviation, March 13, 2015, at VMCC, iit-BombayThis talk focused on why innovation is paramount and its role in positioning any organization as a market leader. Also, the talk was aimed at various applications of composites to fan case structure of GEnx engine, developed high pressure ratio compressors over time, TAPS combustor & CMC technology to withstand higher temperatures and produce low emissions.

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4.2 - Mr. Rick Albrecht, Executive Section Manager for Advanced Mechanical Design team at GE-Aviation.

NCAIR NEWS UPDATES Continued support from Boeing to NCair• BoeingsignedanextensionagreementwithNCAIRvouchingfortheircontinuedsupport.

• DMC 125 FD duoBLOCK universal milling/turning machining centre from DMG Mori has been received at NCAIR, IIT-Bombay.

4.1 - Participants for the Tech Talk

4.1 4.2

Degree) in Aerospace Engineering with specialization in Structure Optimization from Indian Institute of Technology Bombay, Mumbai. He has joined NCAIR as a Research Associate.

J. Bhagyaraj

He received his PhD degree in Materials Science and Engineering from Indian Institute of Technology Kanpur. He has joined IITB in Mechanical Engineering department as a Post-Doctoral Fellow and will be working on projects carried out at NCAIR.

Nausheen Azam

She has pursued her Computer Engineering from Mumbai University and has joined NCAIR as a Research Assistant. Her area of interest includes programming in different programming languages. She would be working as a project staff on NCAIR Project.

Sushrut Vaidya

He has his PhD in Civil Engineering, with a specialization in Applied Mechanics, at the University of Connecticut, USA. He joined the Mechanical Engineering Department of IIT Bombay as a Post-Doctoral Fellow in March 2015, and will be working with the Microstructure Modeling Group at NCAIR.

Harshil Shah

He has done his Bachelor of Engineering in Mechanical Engineering from K.J.Somaiya College of Engineering, Mumbai. He is a motorsports and aerospace enthusiast. He has joined NCAIR as a Research Assistant and is working mainly on manufacturing research specifically with 3D modeling and simulation.

Kunal Pralhad Barhate

He has pursued his Master of Technology degree in Mechanical Engineering from Nirma University, Ahmedabad (Gujarat, India). He has joined NCAIR as a research associate and working on computational modelling of composite.

Amol More

He is pursuing his Master of Technology degree in CAD/CAM from VIT University,Vellore (Tamil Nadu) India. He has joined NCAIR as a Project Research Associate.

Gautam Salkar

He has done his Master of Science degree in Mechanical Engineering from North Carolina State University at Raleigh, NC in USA. He has worked previously as a Test Engineer at Cummins. He has joined NCAIR as a Project Research Associate and is working on computational modelling of composites.

Arun Nair

He has done his Bachelor of Engineering degree in Mechanical Engineering from University of Pune, Maharashtra (India). He has joined IIT-B in Mechanical Engineering department as a Research Assistant and will be working on 5-Axis DMG Mori Seiki Machine.and is working mainly on manufacturing research specifically with 3D modeling and simulation.

5.

PublicationsPapers publishedOmanath A. Pawar, Yogesh S. Gaikhe, Asim Tewari, Ramesh Sundaram, and Suhas S. Joshi; Analysis of hole quality in drilling GLARE fiber metal laminates, Composite Structures, vol. 123, pp. 350-365, 2015. http://www.sciencedirect.com/science/article/pii/S0263822314007223

Omanath A. Pawar, Yogesh S. Gaikhe, Asim Tewari and Suhas S. Joshi; Effect of point angle in single-shot drilling of AA2024/CFRP/Ti6Al4V stacks, Conference proceedings, MEGRES, 2015, IIT Bombay.

Papers presented in conferencesISAMPE National Conference on Composites (INCCOM-13) was held on November 14th and 15th, 2014 at VSSC, Thiruvananthapuram, Kerala. Following are the papers that were presented:

� Effect of Single shot Drilling on Multilayered Metallic-Composite Stack - Yogesh Gaikhe

� Fabric Permeability Computation with Varying Fiber Distribution and Packaging - Kashyap Mohan and Sajan Joshi

� Multi-physics Simulation of Process Induced Deformation in Composites Incorporating Effects of Flow Compaction - Vinayak Khandare

NCAIR exhibited its research work in the recently concluded Sheet Metal Forming and Research Association Conference 2014 which was held on 27-28 November, 2014 at Victor Menezes Convention Center, IIT-Bombay. Posters of ongoing research projects were presented at the exhibition which was well received by all the participants.Idea contest at IIT Bombay

Idea contest at IIT Bombay General Electric (GE) and IIT Bombay organised one day seminar on Industry Academia Collaboration on Technology, IACT at IIT Bombay campus.

Jigar Goda, Ashish Saxena and Sagar Telrandhe from NCAIR has won the winners prize of Rs. 10,000 each.

6.

Visits at NCAIRProfessor Sir Mike Gregory, Head, Institute for Manufacturing University of Cambridge, UK visited NCAIR on September 13, 2014.

Professor Robert Allison, Vice Chancellor and President Loughborough University, UK visited NCAIR on November 7, 2014.

Dr. Prakash D. Mangalgiri, Consultant, NPMASS, Aeronautical Development Agency, Bengaluru visited NCAIR on November 26, 2014.

Dr. Prasad Potluri, Professor of Robotics and Textile Composites, The University of Manchester visited NCAIR on December 24, 2014.

Dr. Vahan, VP - Global Sales of X-ray Microscope, Carl Zeiss visited NCAIR on February 5, 2015.

Mr. Tim Hughes, Development Director, Delcam, visited NCAIR on March 16, 2015.

7.

5.1

6.1

5.2

5.1 - Visit to NCAIR office: L-R: Ms. Swetha Manian, Ms. Vani Sreedhara, Mr. Taha Khot, Prof. Sir Mike Gregory, Prof. Asim Tewari, Mr. Jigar Goda, Mr. Sumit Katekar.

6.1 - Visit to NCAIR office: L-R: Mr. Taha Khot, Mr. Sumit Katekar, Ms. Swetha Manian, Dr. Prasad Potluri, Mr. Swarnendu, Prof. Anirban Guha, Ms. Vani Sreedhara.

5.2 - Visit to NCAIR office: L-R: Prof. Rakesh Mote, Prof. Sridhar Balasubramanian, Prof. Asim Tewari, Prof. Sir Mike Gregory, Prof. Prita Pant, Prof. Sreedhara Sheshadri, Prof. Sushil Mishra.

FEATURED ARTICLE

Issues in Ultra-precision Machining (UPM) of Titanium AlloysAbhishek Bihari, M.Tech. Student, Department of Mechanical Engineering, IIT Bombay and Rakesh G. Mote, Assistant Professor, Department of Mechanical Engineering, IIT Bombay. Email: [email protected]

introductionThe high quality surfaces and microstructures have been widely finding applications in optics, semiconductor, and biomedical industries. Ultraprecision machining with the use of diamond tools has been established as the most preferred method to produce such surfaces. An optical grade surface finish of the order of 10 nm RMS is generally obtained through ultra-precision machining processes.

The key to achieve highly accurate surface profile is to have a machine tool with greater positioning accuracy, precise control over cutting and feed velocities and the stiffness. Above all, in order to achieve high degree of surface characteristics, tooling plays a critical role. Diamond tools are often used in ultraprecision machining because of their unmatched properties, including the highest hardness, quite strong resistance to wear, perfect chemical stability and satisfactory service life, as well as the highest sharpness providing the least resistance.

Figure 1 shows a brief comparison of different levels (operation and surface quality) of machining processes.

Titanium alloys have many excellent properties, for example, the high specific strength, high strength at elevated temperature, outstanding cryogenic property, exceptional resistance to corrosion, good bio-compatibility, and so on. Therefore, titanium alloys are extensively used for aviation, aerospace, biomedical, marine and automotive applications. The unique properties of titanium alloys could provide additional value to optical applications. For example, ultra-precision components like optical mirrors of satellite, work in cryogenic environment.

Titanium alloys exhibit poor machinability. This is due to their high strength and hardness at elevated temperatures, low thermal conductivity, high strain hardening, and high chemical reactivity. Due to low thermal conductivity

Fig. 1 Comparison chart for nano-, micro- and macro-machining processes (based on data in [1]).

Diamond tools are often used in ultraprecision machining because of their unmatched properties, including the highest hardness, quite strong resistance to wear, perfect chemical stability and satisfactory service life, as well as the highest sharpness providing the least resistance.

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of titanium alloys, heat generated during machining is not allowed to dissipate from the tool edge causing tool-tip temperature to be high and hence excess tool deformation and wear. All such factors lead to high cutting temperatures, greater specific cutting force, springback of workpiece material and excessive tool wear and low material removal rate. Titanium is therefore considered as difficult to cut material. Hence, machinability of titanium alloys is always a challenge because of its chemically reactive nature at high temperature. At high temperature, they get welded to the tool metals resulting in chipping off and finally causing failure of the tool. Also, at such higher temperatures, only a few tools are able to survive. For machining titanium alloys, conventional tools used were mainly HSS and carbide tools. Due to problem of high temperature, these tools can be used only at lower cutting speeds, otherwise life of the tools will be very short. In comparison of PCD (polycrystalline diamond), the performance of conventional tools was very poor. Also, PCD tools have the lowest wear rate, higher thermal conductivity, longer tool life at higher cutting speed and produce better quality surface. Due to all these reasons, ultra-precision of titanium alloys became necessary.

Machining of ti alloys using diamond toolsDiamond exhibits the strong covalent bond and the highly compact lattice structure imparting the highest hardness. Such unique properties make the diamond a suitable material for a wide range of engineering applications. Diamond (naturally occurring and synthetic) and the derived composites, such as polycrystalline diamond (PCD) are used in machining of 'difficult-to-machine' materials. However, the diamond tools are not popular in machining of ferrous and nickel based alloys due to rapid graphitization of diamond. The heat during machining ferrous materials initiates chemical reaction, causing carbon in diamond to diffuse into iron material at elevated temperatures during machining. The graphitization leads to a significant tool wear of diamond tools and their machining performance degrades. Despite diamond being used as cutting tool, due to its brittleness, mono-crystal diamonds (MCD) need to be used at correct crystal orientation to prevent fracture and to achieve optimum performance. To incorporate such properties, MCD have been substituted by polycrystalline diamond (PCD). For machining of titanium alloys usually diamond coated tools and PCD tools are used. In machining of titanium alloys, PCD tools found to provide significantly extended tool life over tungsten carbide tools (due to extreme hardness, PCD tools demonstrate almost 500 times wear resistance than WC tools) [2]. However, the presence

of titanium can accentuate the onset of oxidization or graphitization when in contact with diamond at elevated temperatures [3].

Frequent wear occurs in the tools during machining of titanium alloys, caused by a combination of high temperature, high cutting stresses, and the strong chemical reactivity of titanium. The dominant wear mechanisms are listed below:

i. Abrasion wear – It is the primary mode of failure that is produced due to the rubbing action of hard inclusions in the workpiece material and chips.

ii. Adhesion wear – It always occurs due to chemical affinity between workpiece and tool material. Due to the rubbing action of material and tool, adhesive bonds are formed which lead to adhesion wear of PCD tools.

iii. Attrition – Crater wear is also observed during machining of titanium alloys due to high chemical reactivity of titanium alloys which causes chip to weld to the tool. Welded work material on the cutting tool gets compressed and removed in the course of machining. Due to this, cutting tool grains will be plucked, which is called attrition and takes place on both of the crater as well as flank surface.

iv. Diffusion – Diffusion is predominant on rake face. It generates delamination of the coatings and adhesion wear of the tool material. TiC is formed at the rake face of tool due to diffusion of carbon particles from the tool into the titanium layer. This TiC layer is pulled out due to high cutting force resulting in excessive crater wear.

v. Notch wear – It happens when excessive local damage occurs simultaneously at rake face and at flank face.

Ultra-precision Machining of titanium alloysMost of the work on titanium is focused on PCD tools as discussed in the previous section. However, MCD structure offers a number of advantages over PCD (Table 1) as a cutting tool material for extreme precision machining. The geometry of MCD does not change with wear and many more things. Mono-crystalline diamond possesses almost twice the hardness and four times the thermal conductivity over PCD [4]. Butler et al. [5] used MCD tool for micro-milling of Ti-6Al-4V and noted that the performance of MCD tools is sensitive to its crystallographic orientation in the cutting tool. It was

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observed that tools with a rake/flank orientation of {110}/{100} offered a significantly extended tool life over the tools having a rake/flank orientation of {100}/{110}. However, the surface roughness (Ra) was in the range 100 – 270 nm over the cutting length of 25 m.

Colafemina et al. [6] studied the behaviour of Ti–6Al–4V and Ti (CP) in MCD in precision face turning experiments with depth of cut in the range 5-15µm. They noted the least values of surface roughness (Rz) to be 590 nm for Ti (CP) and 416 nm for Ti–6Al–4V under the same machining conditions. Zhang et al. [11] performed turning experiments on Titanium alloy (TC4) to investigate tool wear using diamond tools. The experimentation considered ultrasonic vibration machining method for machining and then effects on the tool wear were analyzed. The study revealed that vibration magnitude and frequency were the most critical factors toward tool life and wear propagation rate. The surface roughness (Ra) was noted to be about 260 nm.

As per ultra-precision machining characteristics, minimum cutting length of 2 km is required to produce a typical high quality optical component.

Table 1: Comparison between mono-crystalline diamonds (MCD) and polycrystalline diamonds (PCD)

MCD

MCD consist of single grains that have sharp edges which ensure clean and efficient cutting action providing high material removal rates.

Brittle

MCD exhibits different properties on different crystallographic planes in different crystallographic directions.

SCD fractures very easily along certain cleavage planes.

Less uniform particle size distribution (PSD)

Less homogeneity and packing density.

They produce random scratches and micro-cracking.

Low wear and sharp cutting edge.

PCD

PCD consists of micron-sized synthetic diamond powders bonded together by sintering at high pressures and temperatures.

Tough

PCD is isotropic i.e. exhibiting uniform properties in all directions.

Crystallites are oriented in random direction in PCD passing from one crystallite to another tends to held up. Hence, they do not fracture easily.

More uniform PSD hence crack

More homogeneity and packing density

They do not produce random scratches and abrade a workpiece uniformly.

High wear.

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FEATURED ARTICLE

Sakamato et al [7] used small depth of cut (1 �m) for ultra-precision cutting with diamond tool to finish beta titanium alloy. The surface finish obtained was below 100 nm (Rz) at feed rates below 50 �m/rev (Table 2).

issues in Ultra Precision Machining of ti alloysSome of the important parameters which are concerned during ultra-precision machining of titanium alloys are discussed below:

i. Graphitization - The major concern during machining of titanium is poor tool life due to high cutting temperature and high pressure at the tool-chip interface which initiates the chemical interaction between the titanium and the diamond resulting in the graphitization of the diamond tool.

ii. Excessive tool wear - Zareena et al. [8] used a protective barrier in the form of a thin Perfluoropolyether (PFPE) coating for diamond toolsin order to reduce the tool wear. The PFPE coating aids in minimizing the initial negative chemical interaction between the cutting tool and the workpiece and thus delaying the onset of graphitization of the tool and thus the tool retains its sharp edge. The surface roughness (Ra) obtained was of nanometre level sustainable over a cutting length of about 3 km. Such lubrication will reduce friction, further reducing cutting force and power and hence the cutting temperature. In this way, functional life of the tool and machinability of Ti alloy is increased. But the thickness of lubricant layer should be comparable to molecular layer. This is required to retain the sharpness of the cutting edge, which is acrucial parameter in UPM to maintain the surface quality and precision of the part machined.

iii. High specific cutting energy –As the size of chip decreases, specific cutting energy increases substantially. This is known as “size effect” and happens because, load is very high and the contact

Table 2: Effect of cutting length over surface finish of Ti-22V-4Al (based on data in ref. [7])

Cutting Length (m)

100

6,000

21,000

Rz (nm)

79

97

204

Ra (nm)

16

20

30

Some of the important parameters which are concerned during ultra-precision machining of titanium alloys are graphitization, excessive tool wear, high specific cutting energy and delamination.

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area is very small. Such high stresses will again lead to graphitization and reduce the cutting edge strength.

iv. Delamination - During UPM of Ti alloys, a strain gradient in workpiece is observed, as compressive stresses and tension are generated in the surface layer and in the subsurface layer of the material, respectively. The strain gradient causes delamination in the form of the grains of the surface getting detached itself from the sub surface grains [12].

Table 3 shows summary of ultraprecision machining of different titanium alloys.

Conclusions and recommendations• Titanium alloys have considerable potential in high

precision optical components due to their excellent strength to weight ratio and corrosion resistance properties.

• Difficulties in machining of Ti alloys originate mainly from high cutting temperature, chemical reaction with tools and low elastic modulus.

• Low thermal conductivity, high temperature and pressure at the tool-chip interface results in graphitization of diamond tool which makes it very difficult to be machined.

• The smallest values of roughness for Ti alloys made UPM to be a good option for finishing.

• For ultra-precision machining, MCD tools are proving

Table 3: Ultraprecision machining of various titanium alloys

BrittleTi Alloy

Ti (CP)

Ti-22V-4Al

Ti-22V-4Al

Ti-6Al-4V

Ti-6Al-4V

Ti-6Al-4V

Process

SPDT

SPDT

SPDT

SPDT

Micro Milling

Ultrasonic vibration assisted turning

Surface Roughness (Ra )

7 nm [8]

100 nm [9]

20 nm [7]

8 nm [8]

60 nm [10]

250 nm [11]

Tool Material

MCD with PFPE coating

MCD

MCD

MCD with PFPE coating

MCD

MCD

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FEATURED ARTICLE

to be ideal choice tool material. PCD tools are given wide consideration in machining of titanium alloys at macro level.

• For UPM of Ti alloys, tool life has to be functionally doubled to get a high quality optical component.

• Graphitization of diamond causing excessive tool wear is one of the major concerns and is being reduced by having coated tool (e.g. PFPE coatings).

• However, ultra-precision machining of these alloys with unfriendly machining characteristics require further understanding of the mechanisms behind the tool wear rates.

• With these insights, strategies need to be developed in order to produce high quality surfaces and components.

rEFErENCES

[1] E. Brinksmeier and W. Preuss, “Micro-machining.,” Philos. Trans. A.

Math. Phys. Eng. Sci., vol. 370, no. 1973, pp. 3973–92, Aug. 2012.

[2] J. P. Davim, Ed., Machining of Titanium Alloys, Materials Forming,

Machining and Tribology. Springer Verlag, Berlin, Germany, 2014.

[3] W. Konig and N. Neises, “Turning Ti-6Al-4V with PCD,” Ind. Diam.

Rev., vol. 93, no. 2, pp. 85–88, 1993.

[4] E. Uhlmann, E. Wörner, and M. Brücher, “More efficient cutting

processes due to the heat-spreading effect of CVD diamond.,”

Ind. Diam. Rev., vol. 03, no. 1, pp. 25–29, 2003.

[5] P. W. Butler-Smith, D. A. Axinte, and V. Limvachirakom, “Preliminary

study of the effects of crystal orientation of a CVD monocrystalline

diamond in micromilling of Ti–6Al–4V,” Proc. Inst. Mech. Eng. Part B

J. Eng. Manuf., vol. 224, no. 8, pp. 1305–1312, Aug. 2010.

[6] J. P. Colafemina, R. G. Jasinevicius, and J. G. Duduch, “Surface

integrity of ultra-precision diamond turned Ti (commercially pure)

and Ti alloy (Ti-6Al-4V),” Proc. Inst. Mech. Eng. Part B J. Eng. Manuf.,

vol. 221, no. 6, pp. 999–1006, Jun. 2007.

[7] S. Sakamoto, A. Shinozaki, and H. Yasui, “Possibility of ultra-

precision cutting of titanium alloy with diamond tool,”

2005. [Online]. Available: http://www.aspe.net:16080/publications/

Annual_2005/Posters/6Process/2Mach/1780.PDF. [Accessed: 20-

Nov-2014].

[8] A. R. Zareena and S. C. Veldhuis, “Tool wear mechanisms and tool

life enhancement in ultra-precision machining of titanium,” J.

Mater. Process. Technol., vol. 212, no. 3, pp. 560–570, Mar. 2012.

[9] H. Yasui, A. Shinozaki, and A. Toyama, “Effect of low cutting speed

on ultra-precision cutting of titanium alloy with coated-cemented-

carbide tool,” 2006. [Online]. Available: http://aspe.net/

publications/Annual_2006/Posters/5Process/2Mach/2030.PDF.

[Accessed: 20-Nov-2014].

[10] S. I. Jaffery, N. Driver, and P. T. Mativenga, “Analysis of process

parameters in the micromachining of Ti-6Al-4V alloy,” in

Proceedings of the 36th International MATADOR Conference, 2010,

pp. 239–242.

[11] Y. Zhang, Z. Zhou, J. Wang, and X. Li, “Diamond tool wear in

precision turning of titanium alloy,” Mater. Manuf. Process., vol. 28,

no. 10, pp. 1061–1064, Oct. 2013.

[12] Colafemina, J., Jasinevicius, R. and Duduch, J. 2007. Surface

integrity of ultra-precision diamond turned Ti (commercially pure)

and Ti alloy (Ti-6Al-4V). Proceedings of the Institution of

Mechanical Engineers, Part B: Journal of Engineering Manufacture.

221, 6 (2007), 999–1006.

For ultra-precision machining, MCD tools are proving to be an ideal choice tool material. PCD tools are given wide consideration in machining of titanium alloys at macro level.

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R & D UPDATES

Fabric Permeability Computation with Varying Fiber Distribution and Packaging

introductionPermeability of a fibrous medium is a vital parameter determining the flow behaviour of a fluid in the medium. Fabrics are made of fiber tows stitched or weaved together. The tows are bundles of thousands of untwisted continuous fiber filaments. The result of this type of fabric architecture is that the fabric behaves as a dual scale porous media exhibiting both macro and micro level permeability.

Dual scale permeability characterizes the fluid flow front in the fabric – both inter tow and intra tow [1]. The global resistance to the fluid flow is characterized by the bulk or macro level permeability, which is largely determined by the arrangement of tows. The fluid flow within the tow depends on the tow level or micro level permeability, which is determined by fiber arrangement, size and packaging within the tows.

Ongoing and further research aims at developing models which can compute accurately both macro and micro scale fabric permeability. Such models are important tools for composite process engineers to optimize resin infusion processes.

This article is an effort in the direction of permeability modelling. It attempts to highlight the variation in longitudinal (along fiber direction) saturated permeability at micro level produced with change in fiber arrangement, size and distribution within the tows.

Modeling MethodologyassumptionsTo model permeability a homogenous resin porous medium saturated with resin is assumed. Then a representative volume element (RVE) is considered for different fiber arrangements. It is assumed that the fluid follows a steady state laminar flow inside the RVE.

Geometry• Three different fiber arrangements are considered for the computation

of permeability: Square, Hexagonal and Random, units cells of which are shown in Fig. 1-4.

• Interspacing between the fibers is varied to vary the volume fraction.

• Simulations are run for two different fiber diameters.

Permeability of a fibrous medium is a vital parameter

determining the flow behaviour of a fluid in the medium.

Fabrics are made of fiber tows stitched or weaved together.

The result of this type of fabric architecture is that the fabric

behaves as a dual scale porous media exhibiting both macro and micro level permeability.

KASHYAP MOHAN

Research Assistant, NCAIR

SAJAN JOSHI

M.Tech 2nd Year, NCAIR

14.

Equations and Boundary ConditionsSymmetric conditions are applied at the RVE boundary surface that is there is no velocity gradient in direction normal to boundary surface. At the fiber surface, no slip condition exist which indicates that the net velocity is zero (Fig. 4-5). [2]

Flow behaviour inside the RVE is modelled using Stokes flow equation. The flow boundary conditions, interface conditions and consistency conditions (conservation of mass and momentum) are used to solve for velocity and pressure distribution.

Permeability can be calculated as [3]:

(1)Ki = saturated permeability in i directionQ = volumetric flow rateA = cross section areaΔP = pressure difference across the RVEL = length of RVE

1

4

2

5

3

Fig. 4 Fibers arranged in square pattern. Fig 5 RVE for square arrangement illustrating

the boundary conditions.

Fig. 1-3 Schematic showing RVE’s of square, hexagon and random fiber arrangement.


15.

Meshing and Simulation3-D RVE’s are modelled in COMSOL multi-physics software package.

Meshing is done using the inbuilt meshing parameters for “FINER” mesh size. Extruded triangular elements are used to mesh the RVE as shown in Fig. 6. Pressure difference of 20000 Pa is applied across the RVE. Resin viscosity is taken to be 1 Pas.

The models are simulated as per the applied boundary conditions to obtain velocity and pressure distribution in the RVE. (Fig. 7-8). The velocity distribution is not uniform throughout the RVE. Therefore velocity average over the outlet surface is calculated. The computed surface average velocity is substituted in Eq (1) to get the permeability value.

results and discussionPacking density is represented by the volume fraction of fibers within the tows.

In Fig. 9, the variation of longitudinal permeability with fiber volume fraction is plotted for 15 µm diameter fibers arranged in hexagonal and square arrangements.

From the graph it is obtained that longitudinal permeability decreases with an increase in the fiber volume fraction for both ordered arrangements.

In Fig. 10, the variation of longitudinal permeability with fiber volume fraction is plotted for two different fiber sizes i.e. diameter 10 µm and 20 µm arranged in square pattern.

87

Fig. 6a,b,c Extruded triangular mesh for square arrangement. 6a 6b 6c

Fig. 7 Velocity distribution after simulation for square arrangement

Fig 8 Pressure distribution after simulation for square arrangement.


16.


At a given fiber volume fraction, an increase in longitudinal permeability is observed with an increase in fiber diameter. A similar trend is observed for hexagonal arrangement.

In Fig.11, longitudinal permeability is plotted for different fiber arrangements on the same graph.

It is observed that for a given volume fraction and fiber diameter, random arrangement gives the maximum permeability and hexagonal arrangement gives the least permeability.

The difference in longitudinal permeability of different fiber arrangements is more for lower values of fiber volume fractions.

Fig. 9 Variation in longitudinal permeability with fiber volume fraction for square and

hexagonal arrangements.

Fig. 10 Variation in longitudinal permeability with fiber fraction for different fiber diameters in

square arrangement

17.

Conclusion and Future WorkThis work simulates and analyses the variation in the longitudinal permeability of a fabric at micro level due to changes in fiber arrangement, size and distribution. It is observed that micro level longitudinal permeability decreases with an increase in the volume fraction of fiber. Longitudinal permeability shows an increase with an increase in diameter of fiber. Also, random arrangement of fibers shows greater permeability as compared to ordered arrangements.

It can be concluded that the micro level permeability also depends on many factors other than volume fraction of fibers. Fabrics can be tailor made considering the above observations so as to improve resin saturation within the tows.

The future work entails• Development of analytical functions to determine the variation micro

level permeability due to fiber arrangement and size related parameters.

• Integration of effects of resin viscosity in the simulation and analytical models for more accurate results.

rEFErENCES

[1] Fuping Zhou, Nina Kuentzer, Pavel Simacek, Suresh G. Advani, Shawn Walsh: Analytic

characterization of the permeability of dual-scale fibrous porous media, Composites Science

and Technology 66 (2006) 2795-2803

[2] Alexander l. Berdichevsky and ZhongCai: Preform permeability predictions by Self-

Consistent Method and Finite Element Simulation, Polymer Composites, April 1993, Vol. 14,

No. 2

[3] N.K. Naik, M. Sirisha, A. Inani : Permeability Characterization Of Polymer Matrix Composites

By RTM/VARTM, Progress in Aerospace Sciences 65 (2014) 22-40

Fig. 11 Comparison in longitudinal permeability of different fiber arrangements.

It is observed that micro level longitudinal permeability

decreases with an increase in the volume fraction of fiber.

Longitudinal permeability shows an increase with an

increase in diameter of fiber. Also, random arrangement

of fibers shows greater permeability as compared to ordered arrangements.


18.

R & D UPDATES

JIGAR P. GODA

Research assistant, NCAIR, IIT Bombay

Comparison of Liquid Composite Moulding (LCM) processes

introductionLiquid Composite Moulding is a closed mould polymeric composite process in which the matrix impregnate the dry fabric under the influence of external pressure. Polymeric composites can be made in many ways, including hand lay-up and spray-up, prepreg vacuum bagging and autoclave curing, filament winding, compression moulding of Sheet Moulding Compound (SMC) and Bulk Moulding Compound (BMC), and Liquid Composite Moulding (LCM). More industries are converting to Liquid Composite Moulding (LCM) processes due to growing demand in composite industry for quality, economy, productivity, etc. With the vast range of processes and the huge amount of variables to consider, the selection of perfect process is very complicated. This article gives information on various LCM processes and helps on selection of the suitable process as per application

There is no single nomenclature for grouping, thus the processes are differentiated into three types on the basis of the moulds used, type of clamping as well as pressure maintained. The processes are classified into these types as represented in the Table 1.

Pressure maintained

Vacuum Pressure

only

(0-1 bar abs)

Gauge + vacuum

pressure

(0-8 bar abs)

Gauge + vacuum

pressure

(0-15 bar abs)

PARAMETERS

Mould used

Single sided rigid

mould with flexible

covering

Rigid or semi-rigid

both side tooling

Both sided rigid

mould

Type of clamping

Vacuum clamping

only

Vacuum + mechanical

clamping

Clamping under

presses

PROCESSES

• Vacuum Assisted Resin

Transfer Moulding (VARTM)

• Seemann Composite Resin

Infusion Moulding Process

(SCRIMP)

• Resin Infusion under Flexible

Tooling (RIFT)

• Flex Moulding

• Vacuum RTM (VRTM)

• Resin Transfer Moulding

Light (RTM Light)

• Resin Transfer Moulding Eco

(RTM Eco)

• Resin Transfer Moulding

(RTM)

• High Pressure Resin Transfer

Moulding (HPRTM).

TYPES

Type 1

Type 2

Type 3

Table 1. Classification of Liquid Composite Moulding processes

Industries today are increasingly converting to Liquid Composite

Moulding (LCM) processes due to growing demand in composite industry for quality, economy,

productivity, etc. With the vast range of processes and the

huge amount of variables to consider, the selection of perfect

process is very complicated.

19.


Process descriptiontype 1Polymer composite parts are made by placing dry fibre reinforcing fabrics into a single-part open mould, enclosing the mould with a vacuum bag and drawing a vacuum in order to ensure a complete preform infiltration with liquid resin. There is no external pumping pressure applied to the resin. Vacuum bag with other consumables such as peel ply, flow mesh is utilized in VARTM / SCRIMP processes. These consumables can be used only once, thus leading to lot of wastage which is prevented in RIFT/ Flex Moulding by using reusable silicon bag preformed in the shape of mould. This modification also helps increase production time by deducing the setup time.

type 2These processes utilize both moulds, the base mould being rigid with the counter mould can be semi rigid to save on cost. Both these moulds are clamped by high vacuum pressure generated at the periphery of the mould. Liquid resins impregnate the dry fabric under vacuum. To accelerate the process resin injection pump may be used.

type 3Mechanical clamping is used (hydraulic press, nuts and bolts, toggle clamps etc.) to hold two rigid moulds in place during resin infusion. Dry reinforcement is stacked between these moulds and liquid resin is injected under positive hydraulic pressure with help of injection machine. Vacuum is maintained inside the mould to improve the part quality and prevent voids. HPRTM is scaled up version of RTM which is used to manufacture big parts. Here, the resin is infused at very high pressure (>12 bar) to reduce production time.

Advantages and Limitations of all the processes are summarised in the Table 2.

20.


Table 2. Advantages and Limitation of processes

TYPE 1

TYPE 2

TYPE 3

ADVANTAGES

- Moderate investment due to reduction in tooling cost.

- Void free parts can be manufactured.

- Sandwich construction.

- Large possibility of shapes.

- Variation in part thickness can be achieved from the same mould.

- Easy visual monitoring of resin flow.

- High productivity.

- Reduced labour costs.

- Cosmetic surfaces on both “A” and “B” side of part.

- Enhanced dimensional stability.

- Sandwich-construction.

- Improved process control.

- Almost negligible consumables required.

- Long mould life.

- No VOC emissions.

- Very high rate of production.

- Ideal for parts of high production rate.

- Ideal for small and medium size parts.

- Structural parts can be manufactured with consistency and dimensional stability.

- Almost negligible consumables required.

LIMITATIONS

- Skilled labour is required.

- Risk of leakage leads to voids formation.

- Loss of consumables and issue of recycling.

- Setup time is comparatively high.

- Only one cosmetic surface is possible.

- Vacuum pump and injection machine are required.

- Both male and female moulds are required.

- Skilled labor required.

- Investment cost is high compared to VARTM process.

- Hydraulic press and lifting facilities are required

- High pressure injection machine and both side rigid metal moulds are a must.

- High investment of the equipment.

- High precision of moulds required.

- Visual monitoring of resin flow is not possible.

21.

Selection of a processIt is dependent on following parameters:

• Part size and layup thickness variability.

• Total number of parts and production rate.

• Integrated specification (strength to weight ratio, cosmetic surfaces, chemical, fire and thermal resistance, sandwich construction, etc.)

• Investment and Production costs.

All these parameters are compared for all types of processes in the Table 3.

SummaryAs Liquid Composite Moulding encompasses a wide variety of composite manufacturing methods, the sub-processes should be tailor-made for every part considering the advantages and limitations they offer. The specifications from a part should be first found and then the correct process should be moulded accordingly.

TYPE 1

VARTM / SCRIMP / RIFT /

Flex Moulding

Small, Medium and Large

Can be varied as required

500-1000

200-500

Single side only

Possible

High

Depends on matrix

used.

Low

High

TYPE 2

VRTM / RTM Light / RTM

Eco

Small and medium

Cannot be varied

1000-5000

500-2000

Both side

Possible

Medium

Depends on matrix

used.

Medium

Medium

TYPE 3

RTM / HPRTM

Small and medium

Cannot be varied

2000-10000

2000-5000

Both side

Not Possible

High

Depends on matrix

used.

High

Medium

Specification

Part Size

Layup thickness variability

Number of Parts

Production rate / year

Cosmetic Surfaces

Sandwich composite

Strength to weight ratio

Chemical, Fire and Thermal

resistance

Investment Cost and

maintenance cost

Production Cost

Table 3. Comparison of processes for various parameters


22.

BiBlioGraPHY / rEFErENCES

[1] P. K. Mallick, “FIBRE REINFORCED COMPOSITES, Materials, Manufacturing, and Design “

[2] W.D. Brouwer*, E.C.F.C. van Herpt, M. Labordus, “Vacuum injection moulding for large

structural applications,” Composites: Part A 34 (2003) 551–558

[3] http://www.tech.plym.ac.uk/sme/fpcm//fpcm09%5Cpdf/IA2/3.pdf last retrieved on

November, 20, 2014

[4] http://www.compositesworld.com/knowledgecenter/closed-moulding/Closed-Mould-

Process/Resin-Transfer-Moulding last retrieved on November, 21, 2014

[5] http://www.composite-integration.co.uk/about-rtm.php last retrieved on November, 16,

2014


23.

R & D UPDATES

Modeling and simulation of process induced deformations in fiber reinforced polymer (FRP) matrix compositesVINAYAK KHANDARE

Research Associate, NCAIR, IIT Bombay

introductionFiber reinforced polymer (FRP) matrix composites, have proved themselves to be high in performance applications like aerospace application, marine etc. There are numerous techniques to manufacture such composites. They are autoclave process, resin transfer moulding, compression moulding, bag moulding etc, some of which are simple, while some are complex. For manufacturing large shapes and high performance components, autoclave is the most suitable process, despite, its higher cost. In this technique, the prepreg i.e. the pre-cured resin fiber layups are cut, stacked and laid on an open mould. This is then placed in the autoclave, where it is subjected to cure cycle (heating and cooling). The autoclave process generates large residual stresses which is a major disadvantage leading to part distortion. These stresses are generated mainly due to anisotropic thermal expansion, shrinkage of resin at cool-down and tool part interaction. These distortions are named as process induced deformations (PIDS) namely spring-in, warpage and shrinkage. Spring-in, is the increase in enclosed angle between two flanges whereas, warpage is the deviation of the flat surface. The deformations can be minimized by compensating tool at design level or by controlling the factors or mechanisms that are responsible for causing such deformations. FEA simulations were performed to understand the effect of different parameters on PIDS by using ABAQUS/COMPRO. The constitutive behaviour of resin is taken into account.

BackgroundSeveral researchers have focused on the parameters that contribute to the PIDS. An overview of some of their works is shown in Table 1. Table 1, suggests that the tool part CTE, thickness of the component, layup variation and tool radius has an impact on PIDS. This article illustrates the effect of these parameters and is further verified with the help of FEA simulations.

Modeling approach Composites process modeling, usually employs an 'integrated sub-model' approach. A composites process modeling program COMPRO was developed on this approach, which is basically a plug-in to 3rd party finite element based simulation software. It consists of three analysis modules namely Thermo-Chemical module, Flow-Compaction module and Stress-Deformation module, which simulates the impact of the evolving material properties on the thermal and cure history of the part, resin flow within a fully saturated part, and residual stress and part deformations of the part after tool removal. In this

Fiber reinforced polymer (FRP) matrix composites, have proved

themselves to be high in performance applications like aerospace application, marine

etc. There are numerous techniques to manufacture such

composites. They are autoclave process, resin transfer moulding,

compression moulding, bag moulding etc, some of which are simple, while some are complex.

24.

study however, the analyses have been performed using only the thermo-chemical and stress-deformation modules. The results obtained from the Thermo-chemical module are directly used in the analysis performed using the Stress-Deformation module.

thermo-Chemical Module The Thermo-Chemical module analyzes essentially the heat transfer from the surrounding air to or from the tool and composite part layup assembly. Further, the heat generated from within the part due the exothermic chemical reaction that takes place in the curing resin is also considered. Following is the governing equation for the thermo-chemical analysis:

where,

The expression 𝜕𝛼𝜕𝑡 is given by the Scholz equation for cure kinetics as follows:

and

(1)

(2)

(3)

Modeling and simulation of process induced deformations in fiber reinforced polymer (FRP) matrix composites

RESEARCH

Carolyne Albert and

Fernlund G. (2002),

Fernlund G., Anoush

Poursartip (2003)

Cann MT, Adams DO

(2001)

Chensong dong (2010)

E. Kappel, D. Stefaniak,

C. Huhne (2013)

REMARKS To minimize spring-in

• Select tooling material having the coefficient of thermal expansion

(CTE) as close as possible to the part CTE

• C-shaped geometry gives more spring-in than L-shaped parts

• Defined the effect of different cure cycle (single & dual hold) along

with release agent

• The experimental study concluded to find effect of layup on

deformations. By placing a 90° ply at the tool-part interface, warpage

is virtually eliminated

• FEA study concluded that spring-in increases with the radius–

thickness ratio

• The specimen flange length should be considered as an essential

parameter when investigating spring-in of L-profiles

• The effect of different tool materials (Al & Invar) and layup variation is

taken into account

Sr. No.

1

2

3

4

5

Table 1. Summary of literature review

25.

The values for the various terms in the above equation are given in Table 2.

Stress-deformation ModuleThe stress-deformation module analyzes the residual stresses and strains in the composite part during the cure cycle. As described earlier, the residual stresses and PID are caused due to the difference in the thermo-mechanical properties of the resin and the fibers, volumetric shrinkage of the resin during cure, and tool-part interaction. In COMPRO, the constitutive behaviour of the resin is modelled as that of a fully visco-elastic material.

The governing equation for a stress-deformation analysis is expressed as a function for the potential energy for a system as given below:

where,

Parameter

Initial fiber volume fraction

Resin nominal density

Resin heat of reaction

Activation energy

Pre-exponential cure rate coefficient

First exponential constant

Second exponential constant

Diffusion constant

Critical degree of cure at T=0 K

Constant accounting for increase in critical

resin degree of cure with temperature

Values

Vf = 0.574

ρr = 1.3 × 103 kg/m3

HR = 5.4 × 105 J/Kg

∆E = 66.5KJ/gmole

A = 1.53 × 105s–1

m = 0.813

n = 2.740

C = 43.1

αC0 = –1.684

αCT = 5.475 × 103 K–1

Table 2. Equation parameters and their values

(1)

(2)

U InternalworkΩ Workdonebyexternalforcesε Straintensorσ StresstensorD Materialstiffnessmatrix

ε0 Free/thermalstresstensorσ0 Initialstresstensoru DisplacementvectorX Bodyforcesqs Surfaceforces


26.

Present work The parametric study is done on L-section (two flanges and a 900 corner) and engine compartment geometry. The L-section geometry is shown in figure 1. The pre-preg material used in all cases is HEXCEL AS4-8552.

Effect of tool radius In the present study, an attempt has been made to study effect of tool radius on spring-in for L-section component as shown in Figure 1. The tool radius of component varied from 3 to 6 mm in step of 1 mm, and at each radius spring-in, simulation has been performed by keeping all other parameters constant, and the spring-in is obtained.

The other parameters include; Composite thickness is 4 mm (20 plies) and Layup orientation is [0, 90] s, Tool material is aluminium, a dual-hold cure cycle is employed, with a peak hold temperature of 180°C, and a cure cycle duration of 18300 s as shown in Figure 2.

Figure 3 shows the increase in spring-in with respect to increase in tool radius. The reason for spring-in is the huge amount of stress generation in

Fig. 1 L-section geometry Fig. 2 Cure cycle Fig 1 Fig 2

Fig. 3 Tool radiuses vs. Spring-in


27.

the corner region. Hence, with an increase in tool radius the corner region gets increased thus with an increase in corner region, stresses also get increased causing part to distort more to relieve stresses.

Effect of thickness VariationThe effect of variation of thickness on spring-in during composite processing has been studied on L-section geometry shown in Figure 1. The thickness of component varied as 1.6 mm (8 plies), 4.0 mm (20 plies) and 8.0 mm (40 plies) and at each component thickness spring-in simulation has been performed by keeping all other parameters constant and spring-in is obtained. The other parameters includes; Tool radius is 5 mm and Layup orientation is [0, 90] s, Tool material is aluminium, The cure cycle duration is 18300 s as shown in Figure 2. It is observed from Figure 4 that the thickness also affects the spring-in. It is seen that the spring-in decreases as component thickness increases. Therefore, the results obtained attribute to the fact that the thin parts relieve stresses better than thicker components, since deformation obtained in case of thinner part is more. This trend is corroborated by the observations of the other researchers from the literature.

Effect of toolingIn the present study, an attempt has also been made to study effect of tool material on spring-in, to find out how composite part distorts, when coefficient of thermal expansion is taken into account.

a) l-Section geometryTooling material varied as aluminium and invar (CTE Invar < CTE Aluminium) and separate simulation is performed for each tooling material. The tooling material is varied in by keeping other parameters constant; Composite thickness = 4mm (20 plies) and Layup orientation = [0, 90] s, Tool radius = 5 mm, the cure cycle is dwell hold as shown in the Figure 2. Spring-in obtained for L-section geometry is shown with the help of Figure 6. It is observed from the Figure 5 that aluminium gives higher spring-in than Invar. The reason for this is, the CTE of invar is less than that of aluminium

Fig. 4 Thicknesses vs. Spring-in


28.

hence the tool and part CTE difference in case of aluminium is higher than invar. Therefore, difference in expansion rates of tool and part in case of aluminium is higher, which causes the more stretching of bottom ply (ply in contact with tool) than invar. Hence, more residual stresses are generated in the composite part to be produced, which results into more spring-in. The reduction in spring-in by using invar is 8.5%.

b) Engine - Compartment geometryThe engine compartment geometry with specifications is shown in Figure 7. The engine compartment is expanding in two directions, thus having two different radii namely, front and back where the back radius is greater in magnitude than that of the front radius. The composite part is a 4 mm (20 plies) and ply layup considered for this geometry is [0, 90]s.

Tooling material varied as aluminium and composite tooling (CTE Composite < CTE Aluminium ) and separate simulations are performed for each tooling material. The spring-in obtained by finite element simulation are recorded and is as shown in the Figure 8.

It is observed that the spring-in obtained for aluminium tooling for single

Fig. 5 Tool materials vs. Spring-in (L-section) Fig. 6 L-section with spring-in

Fig 5 Fig 6

Fig. 7 Engine Compartment geometry


29.

flange on front side is 2.65°, whereas on the backside of compartment spring-in is 3.8°. Therefore, the total spring-in is the sum of two flanges spring-in. Hence, it is concluded that as one goes from front-side to back-side, the radius increases, therefore, the spring-in in the same direction, also increases.

It is also observed from the graph that, the spring-in obtained for front and back portion of geometry using composite tooling is very less compared to aluminium tooling (i.e. by using composite tooling the reduction in spring-in for front portion is 93.5 % and reduction in spring-in for back portion is 57.36 %). Hence, the effect of tooling as well as radii is seen in this case of geometry. Therefore, from above result, it is concluded that by using tool material with CTE as close as to part to be made, less spring-in can be ensured. These findings also corroborate the trends in the literature.

tool CompensationFurther in this work, the effect of tool-compensation is studied for the L-Section. In the case of L-section, the spring-in for one flange is 0.49°. In order to get the desired dimensions, the tool compensation is required. Therefore, to compensate the tool, angle of the tool in the corner region is increased from 90° to 90.49° and new analysis is then performed with the modified tool geometry, and the composite spring-in in that case is found to be 0.50°. Therefore the difference between the desired section surface and the actual surface is 0.01°.

This is a significant change over the previous value of 0.49°. Similarly, the tool compensation is done on all the geometries to achieve close tolerance components.

ConclusionsBased on this work the following conclusions are derived.• The effect of radius variation shows that, with an increase in tool radius

there is an increase in spring-in. This is because the corner portion of the part is more sensitive to the spring-in.

Fig. 8 Tool materials vs. Spring-in (Engine Compartment)


30.

• The thickness variation shows that, thicker parts undergo less spring-in than thinner parts. This can be attributed to the fact that the thinner sections can relive residual stresses better than the thicker sections, as they are less stiff in the corner regions.

• Using tooling CTE (coefficient of thermal expansion) as close as possible to parts CTE, lesser spring-in can be ensured.

• By compensating tool, it is possible to achieve good dimensional accuracy to avoid fit-up problem during assembling of an aircraft structure.

rEFErENCES

[1] Crolyne A., & Fernlund G. (2002), Spring-in and Warpage of Angled Composite Laminates.

Composites Science and Technology, vol. 62 , 1895-1912.

[2] G. Fernlund & Poursarti A. (2003), Residual Stress, Spring-in and Warpage in Autoclaved

Parts, ICCM 14 Conference Proceedings, San Diego.

[3] M.T. Cann & DO Adams (2001), Effect of part-tool interaction on cure distortion of flat

composite laminates, SAMPE Technical conference, Long Beach.

[4] Dong C. (2010), A parametric study on the process-induced deformation of composite

T-stiffener structures, Composites: Part A 41, 515–520.

[5] Kappel E., Sefniak D., & Huhne C. (2013), Process distortions in prepreg manufacturing of an

experimental study on CFRP L-profiles, Composite Structures 106 , 615-625.

[6] Johnston A. (1997), An Integrated Model of the Development of Process-Induced

Deformation in Autoclave Processing of Composite Structure, University of British

Columbia, Vancouver, Ph.D. Thesis.

[7] G. Fernlund, Rahman, N. R.Courdji, Bresslauer M., Poursartip A., Willden K., et al. (2002),

Experimental and numerical study of the effect of cure cycle, tool surface, geometry, and

lay-up on dimensional fidelity of autoclave-processed composite parts, Composites part-

A 33 , 341-351.


31.

IR. BEY VRANCKEN

Dept. of Materials Engineering (MTM),

KU Leuven, Belgium

PROF. DR IR. JAN VAN HUMBEECK


KU Leuven, Belgium

DR. IR. AKHILESH K. SWARNAKAR,


KU Leuven, Belgium


introductionAdditive Manufacturing (AM) is receiving widespread attention as a key part of the third technological revolution. While the first revolution revolved around mechanization of labor and the second revolution brought about mass production, the so-called third industrial revolution encompasses the digitization of manufacturing and society as a whole. For example, starting from a virtual CAD model, AM allows the production of this model in a near-net-shape way by adding material where needed rather than subtracting from the unnecessary. According to the report published in 2014 by Wohlers Associates, Inc., the AM industry was worth 3.07 billion US dollars in 2013 and has recorded an increase of 34.9% compared to 2012. This number is expected to exceed $7 billion by 2016, with a double digit growth percentage for many years to come[1]. However, the metal AM market comprises only a small percentage of all AM revenues, but is expected to grow faster than the AM industry average.

This article gives a brief overview of the main advantages and the most important parameters that highlight the resulting mechanical properties and applications in Additive Manufacturing.

Process aspectsSelective Laser Melting (SLM) is an AM technique by which complex shaped, fully functional metal parts can be produced. A high power laser locally melts successive layers of powder according to 2D slices extracted from a 3D CAD model. The highly localized heat input allows full melting of any metal, even tungsten or tantalum.

SLM and AM in general possess many advantages compared to the traditional manufacturing techniques such as casting, milling or forging. The layer wise build up enables designer freedom of shapes and parts. Geometrically complex designs such as scaffold structures, conformal cooling channels and internal cavities can lower the weight of products or increase performance, both of which are factors of major importance in aerospace. Moreover, the lead time is greatly reduced. While the actual manufacturing of a part is not particularly fast, the preparation time is reduced by the digital nature of the SLM process. Lastly, contrary to subtractive techniques where the eventual part may only comprise of 10% of the material of the block it was made from, SLM has material use efficiency. Thus any unused powder is sieved and recycled with minimal amounts of waste, leading to material usage of 90% and more in SLM technique.

The SLM process is schematically represented in Fig. 1 The feed container rises to expose fresh powder to the scraper, and the build platform is lowered

TECHNOLOGY UPDATES

32.

by one layer thickness, typically between 20 and 100 µm thick. A layer of powder is deposited by a roller or scraper and the excess powder is caught in an overflow container. Using a set of mirrors, the laser is then deflected onto the powder bed surface. An f-θ lens ensures that the focal point of the laser is always located on the powder surface. Parts are always attached to a base plate to fix them during the build process and to minimize any warping caused by residual stresses.

Several parameters, both adjustable and fixed, affect the quality and properties of the produced parts. These parameters are summarized in Table 1.

The first goal in successfully processing a material via SLM is to obtain near full density, above 99.5% of the theoretical density. This is typically achieved by varying the four main process parameters: laser power [W], scan speed [mm/s], scan spacing [µm] and the layer thickness [µm].

Fig. 1 Schematic representation of the SLM process

Table 1: Important parameters that can be varied to optimize the quality of SLM parts

Parameter

Laser Power [W]

Scan Speed [mm/s]

Scan Spacing [µm]

Layer Thickness [µm]

Scan Strategy

Symbol

P

v

h

t

Typical value

50-300

100-1500

50-100

20-100

Zigzag, Island Scanning

Comment

Can/Should be rotated between layers.

Main adjustable parameters to optimize the density. These parameters cannot be adjusted independently without affecting the quality of the part.

Description

The pattern that the laser follows to scan one layer.


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These parameters are self-explanatory except for the scan spacing, which is the distance between adjacent scan vectors in one layer (refer Fig. 2). This distance should be smaller than the width of the melt pool to ensure good overlap between the tracks and to avoid aligned porosity.

The laser power and scan speed determine the energy input per length unit [J/mm]. This energy input should be large enough to create a melt pool of sufficient depth, larger than the layer thickness, to fuse the layers together. SLM machines usually feature a laser of up to 400W, but due to new developments in laser technology the current trend is to introduce higher power lasers, up to 1kW. These lasers will facilitate processing materials that do not absorb the laser power well and/or have a high thermal conductivity, such as Al alloys and Cu.

Apart from high power lasers, another aspect of recent and future SLM machines is the increasing amount of quality control. (Infra-red) camera systems that monitor the melt pool dimensions, temperature and the powder layer are currently being used to implement online feedback control.

Due to the localized heat input, thermal gradients and cooling rates can be as high as 106 K/m and 106 K/s. These large gradients lead to thermal stresses, which in turn create either micro - macro cracks or cause deformation of the part. Problems with residual stresses, along with availability of suitable powder and laser absorption issues, are one of the major factors for the limited amount of materials that are processable via SLM.

PropertiesThe thermal gradients create unique microstructures unlike any found after casting or forging or any other manufacturing technique. In Fig. 3a, the equiaxed microstructure of hot rolled Ti-6Al-4V consists of globular, light α grains, with about 10% darker β phase at the triple points. This microstructure is desirable for its adequate strength and ductility and other mechanical properties. However, the microstructure of Ti6Al4V material produced by SLM consists of columnar prior β grains that grew more or less parallel to the building direction (vertical), as shown in Fig.3b. These grains grow across multiple layers, up to several millimeters long, and have a width more or less equal to the hatch spacing. The insert shows that the β phase, stable at high temperature, has fully transformed to a fine plate like martensite called α’.

Fig. 2 Different scan strategies. Top: zig-zag scanning. Bottom: Island scanning,

in which the area is divided into smaller subsections which are scanned subsequently

Fig. 3 TMicrostructure of Ti6Al4V. a) Rolled and mill annealed, equiaxed

microstructure, b) SLM produced, martensitic microstructure


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As the microstructure is very fine, the SLM produced material is stronger than the conventional material. The trade-off for the high strength is a lower ductility, as is evident from the stress strain curves in Fig.4. Other materials produced by SLM are at least equally strong and equally ductile as their conventional counterparts, and can be made superior through the use of proper heat treatments. For Ti-6Al-4V, a heat treatment is needed to increase the ductility. To achieve optimal results, the heat treatment needs to be adjusted because the microstructure of SLM produced material differs from the conventional microstructure. For example, by applying a solution heat treatment to AlSi10Mg produced via SLM, the fine microstructure and high hardness is lost, which cannot be recuperated by aging.

Additionally, other properties such as fatigue, fatigue crack growth rate and fracture toughness of SLM produced Ti-6Al-4V are still a topic of research. Results thus far have indicated that these properties are either equal or below those of conventional material. However, applying a HIP treatment will increase the fatigue limit substantially up to the point where the SLM material is as good as conventional material.

applications Several industrial sectors have shown great interest in AM, for various reasons. The medical industry seeks to take advantage of the geometrical freedom to produce patient specific implants. This type of mass customization is already used for nearly 100% of cochlear implants, and is increasingly used for dental applications or even complicated hip replacements. In the latter case, intricate porous structures at the surface of the part can enhance growth of bones and lead to a better fixation of the implant. Materials of interest are Ti-6Al-4V, CoCr and various polymers. The oil and gas sector can also benefit by being able to produce intricate channels inside a functional part made with a Ni-based super alloy.

Weight reduction is the main reason for the increased interest from the aerospace and automotive racing industry. By optimizing the design, often

Fig. 4 Stress strain curves for Ti-6Al-4V produced via SLM compared to curves for conventional

material [2]


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done by using topology optimization software, the weight of parts can be drastically reduced while leaving the structural performance unchanged. The weight reduction can be achieved for numerous components of an aircraft. Once of major step towards using the AM technology in aerospace sector is replacing the non-critical, polymer parts by AM equivalents. In this context, Airbus, the major aircraft manufacture in Europe has recently announced that the new A350 XWB will feature more than 1000 different 3D printed parts. The next step is to replace semi-critical components that have to bear some structural loads. Hinge brackets are an example of these kinds of parts. A study by EADS demonstrated that by replacing some of the hinge brackets in an Airbus A320, a weight reduction of approximately 10 kg could be achieved per aircraft [3].

A third and final class of parts for which production through AM is beneficial are complex, critically loaded parts. Not only would design optimization lead to a reduction in weight, but also an improved efficiency. One such example of a satellite combustion chamber design is shown in Fig. 5, measuring 220 mm in height and 200 mm diameter at the base. The chamber is normally made with thick walls to accommodate the stresses during launch, but is of no further use during operation. Using a porous mesh structure (only 12% density compared to solid material), the structural loads can be absorbed at a greatly reduced weight. The prototype part shown in Fig. 5 is made out of Ti-6Al-4V, but the actual part would be made out of a refractory metal such as tantalum or tungsten to withstand the heat during combustion.

Due to the strict safety demands in the aerospace sector, research is still ongoing to validate the use of AM for this last class of critical parts. Ultimately, turbine blades are an ideal candidate of a part to be produced via additive manufacturing. The intricate shape and internal channels are easily produced via AM. Moreover, one-off replacement parts could be produced instead of having to order a whole new set of blades, lowering maintenance cost and time. An example of a relatively critical part in a jet engine is the fuel nozzle of the new LEAP engine by GE. Roughly 30.000 nozzles will be produced by a whole factory of metal AM machines. The AM fuel nozzle not only weighs 25% less, but also increases fuel use efficiency and is five times more durable [5]. As another example of a part out of the second class with semi-critical loading, Airbus has decided to use AM to produce titanium brackets in the future A350 airplane because of the weight reduction but also the possibility to quickly produce spare parts on demand [3].

Apart from the multitude of components, through which benefits are achieved by weight savings and better performance, perhaps the greatest added value of AM for aviation and aerospace is the reduced lead time for component, up to 80%. Moreover, the number of parts that need to be kept in inventory can also be reduced. Lastly, Electron Beam Melting is an AM technique similar to SLM but using an electron beam rather than a laser to melt the powder. Using EBM, low pressure turbine blades made of γ-titanium aluminide (TiAl) can be manufactured. However, this is still in the development phase[6].

Fig. 5 A case of typical combustion chamber (prototype) fabricated by SLM of a compact in-space satellite (220 mm in height and 200 mm

diameter at the base). Image courtesy of 3D Systems –Layer Wise [4]


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ConclusionSelective Laser Melting is part of a rapidly expanding industry, offering more design freedom, shorter lead times and a reduced need for tooling because of the near net shape production. Several process parameters affect the final quality of the part, with properties at least equal to those of cast material, and in some cases surpassing those of wrought parts. The possibility to optimize designs and associated increased efficiency and reduced weight are beneficial for many industrial sectors including aerospace.

rEFErENCES

[1] Wohlers Associates, Inc., Wohlers Report. 2014.

[2] B. Vrancken, L. Thijs, J.P. Kruth, J. Van Humbeeck, Heat treatment of Ti6Al4V produced

by Selective Laser Melting: Microstructure and mechanical properties. Journal of Alloys and

Compounds, 2012. 541(0): p. 177-185.

[3] Press Release - Printing the future: Airbus expands its applications of the revolutionary

additive layer manufacturing process. 2014; Available from: http://www.airbus.com/.

[4] 3D Systems - LayerWise. Available from: http://www.layewise.com.

[5] GE Additive Manufacturing( http://www.ge.com/stories/advanced-manufacturing).

[6] ARCAM, EBM in Aerospace (http://www.arcam.com/solutions/aerospace-ebm/).


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NASA's 10-engine electric plane completes flight test

In one of the recent developments, NASA has developed the Greased Lightning or GL-10 prototype which is a battery –powered plane with 10 engines. This aeroplane can both take off and land like a helicopter as well as fly efficiently like an aircraft. This plane could be used for delivery of small packages or vertical takeoff and landing, long endurance surveillance for agriculture, mapping and other applications. Research is ongoing for a scaled up version which would be a good option for one to four- seater person size personal air vehicle. The GL-10 is currently in the design and testing phase. http://www.brahmand.com/news/NASAs-10engine-electric-plane-

completes-flight-test/13916/1/18.html

A huge breakthrough in nuclear fusion

A technological breakthrough in alternate energy has come about in developing a power source based on nuclear fusion, and the first reactors, small enough to fit on the back of a truck, could be ready for use in some years from now. This technology uses the energy released during nuclear fusion when atoms combine into more stable forms. The energy created through fusion is 3-4 times more powerful than the energy released by fission.

This potential energy source is a part of the comprehensive approach to solving global energy and climate change problems. The use of this energy would produce far less waste than coal-powered plants.http://www.businessinsider.com/andrea-shalal-lockheed-nuclear-

fusion-breakthrough-2014-10

Carbon-fiber epoxy honeycombs mimic the material performance of balsa wood

Balsa wood has been used for centuries to have lighter and stiffer material for producing sandwich panels in larger structures. However, it is expensive and natural variation in the grain structure could be impediment to the required performance. Recently, material scientists at Harvard and Wyss universities have developed cellular composite material with superior properties. It mimics

AEROSPACE NEWS BRIEFS

the balsa wood structure where most of the space is empty and only walls carry the load. Using 3D printing technology and fiber-reinforced epoxy based ink, they were able to achieve strength, twice that of the best printed polymer composite. Key factor in the structure is the ability to control fiber orientation which allows better component design and material efficiency.http://www.seas.harvard.edu/news/2014/06/carbon-fiber-epoxy-

honeycombs-mimic-material-performance-of-balsa-wood

Composite plane life cycle assessment shows lighter planes are the future

Composite plane life cycle assessment has been carried out for the first time by the Universities of Sheffield, Cambridge and University College London and extrapolated the results to the global fleet. This research was later published in the International Journal of Life Cycle Assessment and estimated that by the year 2050, composite planes could reduce emissions from the global fleet by 14-15 % relative to a fleet that maintains its existing aluminium based configuration. Composite planes will also help resolve the industry target to halve carbon dioxide emissions for all aircrafts by 2020. http://www.sciencedaily.com/releases/2014/12/141216100517.htm

Unidirectional carbon fiber prepreg tapes just 15 gsm

A group of researchers at North Thin Ply Technology (NTPT), Penthalaz-Cossonay, Switzerland has developed possibly the thinnest prepreg tape in the market. This is an unidirectional prepreg tape with an areal weight of 15 gsm. NTPT achieved this by spreading an intermediate modulus carbon fiber and adding the high Tg epoxy resin ThinPreg 120EPHTg – 402. This prepreg can be used to create lightweight sandwich panels to be used for ultralight aircrafts, UAVs, other aerospace applications and rigid wings for racing yachts.http://www.compositesworld.com/products/unidirectional-carbon-

fiber-prepreg-tapes-just-15-gsm

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ELEVONS are aircraft control surfaces that combine the functions of the elevator (used for pitch control) and the aileron (used for roll control), hence the name.

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