TEACHING AND RESEARCH WORK PRESENTATION.

RESEARCH AND TEACHING PRESENTATION

1. Research and Teaching Presentation Dr. Kishore Babu Nagumothu Associate Professor and HOD Dept. of Metallurgical & Materials Engineering National Institute of Technology Warangal - 506 004, India e-mail: [email protected] 1

2. Academic Background and Employment History Academic Qualifications Ph. D, Indian Institute of Technology Madras (Metallurgical & materials Engg .), India. Master of Technology (Metallurgical Engg .), Institute of Technology, Banaras Hindu University, India. Bachelor degree (Metallurgical Engg .), Indian Institute of Metals, Calcutta, India. Diploma (Metallurgical Engg .), Govt. Polytechnic, Vijayawada, India. Employer Name Designation Dates National Institute of Technology Warangal, India   Associate Professor in Metallurgical & Materials Engineering 06-04-2018 to till date Empa (Swiss Federal Laboratories for Materials Science and Technology), Switzerland Scientist 01-07-2014 to 31-12-2017 Singapore Institute of Manufacturing Technology ( SIMTech ), Singapore Scientist 08-03-2010 to 17-06-2014   Nagaoka University of Technology, Nagaoka, Japan Visiting Scientist 08-02-2010 to 28-02-2010 Federal Institute for Materials Research and Testing (BAM), Berlin, Germany Adolf Martens Research Fellowship Award 05-01-2009 to 04-01-2010 (Postdoctoral) Research Interests: Joining of materials – weldability evaluation, microstructural and mechanical property characterization of welds. Physical metallurgy, Physical metallurgy of welding. Phase transformations and microstructural characterization in welded joints. Solid-state joining – friction welding and friction stir welding Nanocomposites Metal additive manufacturing using fusion-based technologies 2

3. Sponsored Research Projects- Ongoing (1.14 Cr) 3 Principal Investigator, Understanding the role of hydrogen in Titanium alloy weldments, ARDB, Rs. 48.23 lakhs, 31-05-2021. Co-Investigator, Development of novel B and C modified fillers for the gas tungsten arc welding of titanium alloys, ARDB, Rs. 48.07 lakhs, 31-05-2021. Mentor, Development of Ti-6Al-4V alloy structure using wire arc additive manufacturing process for aerospace application, DST-TARE, Rs. 18.30 lakhs, 13-12-2021. Completed Sponsored Research Projects : 10, Submitted proposals: 1 GIAN Program : Submitted: 1 Ph.D Guidance: Completed: 4 , Submitted: 1, Ongoing-3 No of Publications : No of Journal Papers: 57 , Conference papers: 17, Book chapter: 1

4. Grain refinement of Ti-15V-3Cr-3Al-3Sn metastable β titanium alloy welds using novel Nickel and Silicon-modified fillers 4 Metastable β- Ti alloys are used in  structural, automotive and aerospace applications  where high specific strengths, low elastic modulus, good fatigue resistance, adequate toughness, excellent corrosion resistance, and good cold formability are required Consumer goods : spectacle frames, cameras, watches, jewelry Sporting goods : golf stick , tennis rackets, bicycle frames, spikes in sprinter’s running shoes Welding Problem : Low strength in as-welded condition due to the formation of coarse columnar β grains in the fusion zone Ti-15V-3Al-3Cr-3Sn (Ti-15-3) β transus line Golf stick driver Landing gears for aircrafts Suspension Spring Coil

5. Modification of Ti-15-3 Filler by Ni and Si Addition Novelty Part I: make GTA welds with cast inserts containing variable Ti-15-3-x wt % Ni and Ti-15-3-x wt % Si (x = 0.15, 0.3 and 0.5) Part II: welding process parameters optimization Part III: microstructure of welds (Optical, SEM, EBSD and TEM) Part IV: evaluate mechanical properties Modified Filler Copper Bar Fig 1: Setup of filler inserted in the plate before welding Fig 2: Welding of the plate Schematic of welding setup Filler rod

6. Fusion zone macrostructure of welds made with Ni and Si modified fillers (As-welded) Autogenous weld Ti-15-3-0.15 Ni Ti-15-3-0.3 Ni Ti-15-3-0.5 Ni Ti-15-3-0.15 Si Ti-15-3-0.3 Si Ti-15-3-0.5 Si Coarse Columnar grains+ equiaxed grains

7. Fusion zone microstructure of welds made with Ni and Si modified fillers (As-welded) Ti-15-3-0.5 Ni Ti-15-3-0.5 Si (e) (f) (g)

8. The influence of a solute on the grain size can be described by the growth restriction factor, Q given by Maxwell & Hellawell (1975):     Higher Q results in smaller grain size values in the solidified alloy . (Anis et al, 2016; Arif et al, 2017). Q = m l ( k -1) C m l is the slope of the liquidus k is the solute partition coefficient (k = C s /C l ; C s and C l are the compositions of solid and liquid at the S/L interface) C is the initial melt composition . Growth restriction factor Table: Segregating power of some solute elements in Titanium ( Berminham et al, 2009) 8 Growth rate α 1/Q Higher the GRF, lower the growth rate and finer the grain size

9. FZ EBSD of welds made with Ni and Si modified filler compared with autogenous weld Autogenous weld Weld prepared with Ti-15-3-0.5 Si filler Weld prepared with Ti-15-3-0.5 Ni filler Avg FZ grain size - 335 ± 12 μ m Avg FZ grain size - 122 ± 12 μ m Avg FZ grain size - 135 ± 6 μ m 9

10. FZ TEM of PWHT welds made with Ni and Si modified filler compared with autogenous welds Autogenous The average size of α precipitates formed in all conditions measured approximately 35 ± 5 nm. Addition of Ni and Si to the fillers resulted in a reduction in grain size, thereby increasing the grain boundary area The availability of a high volume fraction of metastable β and high grain boundary area leads to the higher density of α precipitates. 10

11. Autogenous Tensile plot of Ti-15-3 welds with Ni/Si modified fillers The welds prepared using Ti-15-3-0.5Ni and Ti-15-3-0.5Si fillers exhibited equiaxed grains while solidification, primarily due to higher constitutional supercooling caused by the low solubility of these solutes in Ti . In contrast, the autogenous welds and welds prepared displayed columnar and equiaxed structure. The welds produced using Ti-15-3-0.5 Ni filler and Ti-15-3-0.5 Si filler showed higher strength compared to autogenous weld. The higher strength at welds prepared with Ti-15-3-0.5 Ni filler and Ti-15-3-0.5 Si filler can be attributed to the lower columnar width of β grains and the formation of equiaxed grains in FZ. The welds subjected to PWHT showed higher YS, UTS and lower ductility values than as-welded samples because of the uniform precipitation of α-phase in a matrix of β grains. The precipitation of the α phase in the PWHT samples has been confirmed through TEM analysis. 11

12. Research Plan For Next Five Years 12 Development of new titanium fillers for GTA welds Friction of welding of dissimilar titanium allo ys Wire arc additive manufacturing of Titanium alloys, Ni-base alloys and Stainless steels Friction stir welding of nanocomposites Welding of Niobium alloys Grain refinement of fusion zone using ultrasonic vibration techniques

13. Google Scholar 13

14. Teaching Presentation on Solidification Cracking 14

15. Solidification Cracking (Hot Cracking) Typical welding defect occurring during solidification Base metal having a wider solidification temperature range (Impurities involved in BM) Incorrect filler wire/parent metal combination Residual stresses (welding under restraint conditions) Occurs in terminal stages of solidification Many alloys (Al & Mg, Stainless steels, Ni-base) are susceptible 308 stainless steel weld The fracture surface often reveals the dendritic morphology of the solidifying weld metal Gross or microscopic cracking Intergranular Governs weldability Occurs in castings, ingots and welds Al-2024 Solidification cracking at weld centrline 15 Intergranular cracking 308-SS

16. Solidification Cracking (Hot Cracking) Solidification cracking in welds has long been attributed to strain accumulation in the mushy zone trailing the weld pool, arising from both solidification shrinkage and thermal contraction. Low melting point liquid, formed as a result of solute partitioning, is concentrated at grain boundaries where it is subjected to these combined shrinkage and thermal strains. 16 Severity of tensile stresses increases with both the degree of constraint and thickness of the workpiece

17. Solidification Cracking Factors considered during solidification cracking Separation of grains due to tensile stresses to cause cracking If the grains grow faster, tensile stresses cannot separate the grains Liquid feeding comes from the weld pool. If the liquid feeding is good, it helps for bonding of the grains. If the liquid feeding is slow, there is going to be high crack susceptibility. Because tension is going to separate the grains. 17

18. Effect of composition on crack sensitivity of some aluminum alloys solidification cracking susceptibility is strongly affected by the composition of the weld metal Al-Cu Alloys crack susceptibility Increases to a maximum value at about 3% Cu and then decreases with any further increase in the copper content Small quantity of eutectic -cracks abundance eutectic -heals If a sufficient amount of liquid metal is present near the cracks, it can backfill and heal the incipient cracks Effect of Composition on Solidification Cracking 18

19. Very little addition of S and P results in increase in solidification temperature range The wider the solidification range, the more grain boundary area in the weld metal remains liquid during welding and susceptible to solidification cracking 19

20. 20

21. 21 Thank you for your attention

22. 22

23. The wider the solidification range, the more grain boundary area in the weld metal remains liquid during welding and susceptible to solidification cracking 23

24. Harmful impurities such as sulfur and phosphorus are more soluble in delta-ferrite than in austenite, the concentration of these impurities at the austenite grain boundaries, and thus their damaging effect on solidification cracking, can be reduced if delta-ferrite is present in significant amounts when delta-ferrite is the primary solidification phase, the substantial boundary area between delta-ferrite and austenite acts as a sink for sulfur and phosphorus. This decreases the concentration of such impurities at the austenite grain boundaries and, therefore, reduces solidification cracking 24

25. Primary solidification phase For austenitic stainless steels the susceptibility to solidification cracking is much lower when the primary solidification phase is -ferrite rather than austenite. As the ratio of the Cr equivalent to the Ni equivalent increases, the primary solidification phase changes from austenite to  -ferrite, and cracking is reduced. Figure (a) for arc welding, this change occurs at Creq/Nieq = 1.5. Figure (b) for pulsed laser welding, this change occurs in the Creq/Nieq range of 1.6–1.7. In both cases Cr eq = Cr + 1.37Mo + 1.5Si + 2Nb +3Ti and Ni eq = Ni + 0.3Mn + 22C + 14.2N + Cu. under high cooling rates in laser or electron beam welding, a weld metal that normally solidifies as primary ferrite because of undercooling, and this is consistent with the change occurring at a higher Cr eq – Ni eq ratio in pulsed laser welding 25

26. Primary solidification phase when the carbon content is greater than 0.53, austenite becomes the primary solidification phase and solidification cracking becomes more likely. In fact, the wider solidification temperature range at a higher carbon content further increases the potential for solidification cracking. 26

27. 27

28. The higher the surface tension of the grain boundary liquid, the larger its dihedral angle alloy 1100, which is essentially pure aluminum and thus not susceptible to cracking, the susceptibility decreases with increasing dihedral angle of the grain boundary liquid. 28

29. 29

30. Grain structure steep angle of abutment between columnar grains growing from opposite sides of the weld pool, welds made with a teardrop-shaped weld pool tend to be more susceptible to centreline solidification cracking than welds made with an elliptical-shaped weld pool. A steep angle seems to favor the head-on impingement of columnar grains growing from opposite sides of the weld pool and the formation of the continuous liquid film of low-melting-point segregates at the weld centerline . 30

31. Grain Refinement Al-Li alloy 2090 GTA welds (with 2319 filler) No inoculation With Ti Fine, equiaxed structures are more resistant to hot cracking Problem film formation less likely due to increased GB area Popular techniques: Pulsing, MAO, and Inoculation 31

32.                           a) 0% and b) 16% 4043 filler dilution Figure 1: Comparison of gas-tungsten arc welds (top view) made on aluminium 6060   In many instances, alumimium weldability can be improved by diluting the weld pool with an appropriate filler metal, shifting the alloy to a less susceptible composition . For example, when using a 4043 filler alloy, 6xxx alloys become readily weldable. Control of solidification structure 32

33. Control of solidification structure columnar grains that reverse their orientation at regular intervals force the crack to change its direction periodically, thus making crack propagation difficult 33

34. , Thermal contraction solidification cracking occurred in the second (left-hand-side) weld of the inverse “T” joint due to the fact that the degree of restraint increased significantly after the first (right-hand-side) weld was made. 34

35. Favorable welding conditions Reducing strain: preheat, less strain, and low heat input Favorable weld geometry (low restraint weld design) Control of weld geometry Control of grain structure Grain refining ( Ti , V, Zr ) Controlling the weld metal chemical composition Filler metal/base metal Dilution Control of Solidification cracking 35

36. Use of favorable welding conditions High-energy density processes reduces distortion and thermally induced strains Preheat Low restraint Avoid teardrop-shaped weld pools Control weld geometry 36

37. Control of weld geometry When a concave single-pass fillet weld cools and shrinks, the outer surface is stressed in tension. The outer surface can be considered as being pulled toward the toes and the root. However, by making the outer surface convex, pulling toward the root actually compresses the outer surface and offsets the tension caused by pulling toward the toes. Consequently, the tensile stresses along the outer surface are reduced, and the tendency for solidification cracking to initiate from the outer surface is lowered. 37

38. Control of weld geometry deep narrow welds with a low width-to-depth ratio can be susceptible to weld centreline cracking. This is because of the steep angle of the abutment between columnar grains growing from opposite sides of the weld pool Avoid narrow, deep welds 38