Microstructure Formation in Dissimilar Metal Welds: Electron Beam Welding of Ti/Ni
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FUSION welding of dissimilar metals introduces a set of challenging problems that do not arise while joining similar metals. One primary difference is that in dissimilar metal welding (DMW), different thermo-physical properties (e.g., melting point, thermal conductivity, specific heat, density) of the base metals influence both macroscopic transport phenomena and microstructure formation. This has been studied using computational modeling as well as experiments for Cu/ Ni binary couples.[1,2] These show that thermal diffusivity of the base metals is the major parameter in determining melting ratio and consequently the average composition of welds. Similarly, experiments on Fe/Ni and Fe/Cu systems highlighted the importance of metallurgical phenomena (e.g., nucleation, phase selection in heterogeneous melts) during DMW.[3,4] Understanding phase selection and solidification in a dissimilar setting is very important for joining alloys with significantly different Ti and Ni contents: Ti to stainless steel (SS),[5–8] Ti3Al-alloy to Ni-alloy,[9,10] NiTi to Ti6Al4V,[11] Ti6Al4V to SS,[12–15] Ti6Al4V to Inconel 718,[16] NiTi to SS,[17–24] and NiTi to Ni-alloy,[25] for example. The binary Ti/Ni couple provides a model for
SUBHRADEEP CHATTERJEE, Assistant Professor, is with the Indian Institute of Technology Hyderabad, Kandi, Sangareddy 502285, India. Contact e-mail: [email protected] T.A. ABINANDANAN, Professor, and KAMANIO CHATTOPADHYAY, Honorary Professor, are with the Indian Institute of Science, Bangalore 560012, India. G. MADHUSUDHAN REDDY, Scientist G, is with the Defence Metallurgical Research Laboratory, Hyderabad 500058. Manuscript submitted July 13, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
all these systems to study the fundamental aspects of microstructure formation. Some of earliest efforts to join Ti/Ni using electron beam welding (EBW) reported formation of intermetallic phases which rendered these joints brittle.[26,27] Our previous laser welding experiments on this couple[28,29] explored the close link between heat transfer (influenced primarily by the different thermal diffusivities of Ti and Ni) on one hand and specific metallurgical features of Ti-Ni system on the other; the results showed the average weld composition to be Ti-rich (attributed to slower heat transfer through Ti), which in turn influenced phase selection in the weld. In the present study, we report observations from EBW of a Ti/Ni dissimilar couple. A critical comparison between these results and those from the earlier laser welding studies[28,29] on the same dissimilar couple gives new insights on microstructure development in these welds.
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EXPERIMENTAL DETAILS
EBW experiments were carried out in a low-voltage machine of Techmeta (France) make. Base metal plates for welding were cut from commercially pure (more than 99.5 pct) Ni and Ti plates of thickness 3 mm. Standard machining, pickling, and cleaning procedures were employed for the joint preparation. Plates were clamped on a fixture with minimum restraint in a butt welding geom
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