Interfacial characteristics of friction stir welded dissimilar titanium alloys: Timetal-54M and Ti-6242

In the world of joining, riveting and additive manufacturing, weight reduction, and omission of defects (at both macro and micro level) remain of paramount. Therefore, in the wake of ubiquitous fusion welding and widely accepted approach of riveting using Inconel bolts to resist corrosion at higher temperature, friction stir welding (FSW) has emerged as a novice jewel in friction based additive manufacturing industry. With advancements in automation of welding process and tool material, FSW of materials with higher work hardening has also become probable. Process and property relation, especially when heterogeneity (whether atrocious or advantageous) in and around the weld nugget, are inevitable in case of dissimilar titanium alloys, needs to be addressed. Friction stir welded butt joints of α-β titanium alloy (Timetal-54M) and an α titanium alloy (Ti-6242) were produced for four different processing conditions. Material flow and evolving microstructure along with corresponding elements’ distribution was characterized by SEM, and EDS respectively. The necklace shaped macrostructure as observed in the weld nugget consist of untransformed α migrating from the side of Ti-6242, and of basket-weave morphology (prior β grains transforming into α+β, decorated with grain boundary α). Microhardness characterization undoubtedly reveals distinct boundaries between weld nugget and parent material. However, to understand the constitutive behavior of the thermo mechanically affected zone (TMAZ) on the advancing side, or retreating side and of weld nugget, digital image correlation (DIC) technique has been adapted to develop strain maps in transverse tensile specimens. Improved mechanical properties of TMAZ on the advancing side in comparison with retreating side TMAZ are in accordance with hardness values. Occurrence of fracture on the retreating side, morphology of the fractured surface, along with microstructure, and mechanical properties are discussed in that regard.

 Keywords: Friction stir welding; Dissimilar titanium alloys, Zone wise characterization, DIC, Constitutive behavior