7. Effect of Heat Treatment on Friction Stir Welded Dissimilar Titanium Alloys

A significant drop in the hardness values have been achieved in the friction stir welded dissimilar titanium alloys after post weld heat treatment. A closer look at the microstructural evolution suggests that the heat treatment at 933 °C changes the microstructure in the weld nugget from globular blocky grains to distinct microstructure with β flakes distributed uniformly in the α phase. With Ti-6242 FG (β transus = 996 °C), and Ti-54 M (β transus = 966 °C) being on the retreating and advancing side respectively, a series of microstructure evolves from retreating to advancing side. With a difference in the composition of the alloys, difference in the diffusion of elements is also responsible for the corresponding microstructure. Diffraction patterns were measured by X-ray diffraction method. Diffraction peaks for ADV side were slightly shifted towards left in comparison with retreating, and center of the weld along with significant difference in the preferred crystal orientation between as welded and post weld heat-treated conditions.