Microstructure and Mechanical Properties of TC4 Titanium Alloy Joint Fabricated By Vortex Flow-Based Friction Stir Welding

Abstract Tool wear is a critical challenge in titanium alloy friction stir welding (FSW). Vortex flow-based FSW (VFSW) is a potential solution for this issue because the VFSW process uses a similar material to the base metal as the tool. In this study, TC4 titanium alloy was welded by VFSW for the first time. Parameter optimization of the vortex flow formation is first done, and then the weld formation and the joints’ microstructure and mechanical performance are investigated at different traversing speeds. The results prove that the plunging velocity is the dominant factor in the vortex depth. A lower plunging speed is beneficial for the formation of a deeper vortex. Full penetration is achieved at traversing speeds of 50–120 mm/min at 300 r/min. At welding speed above 130 mm/min, insufficient penetration defects occur. In the heat affected zone, grain coarsening and β-phase volume fraction increase arise, whereas, in the weld nugget, an α+β lamellar structure emerges. Under the optimized parameters, the joint tensile strength is nearly 98% of the base material, but the elongation decreases significantly. The oxides drawn into the weld by the vortex at the junction between the vortex and the base material are the main reason for the small elongation. This study proves that the VFSW process is feasible for titanium alloys.