Study on Vortex Flow-Based Friction Stir Welding of Medium-Thick Titanium Alloy Plates

Vortex flow-based friction stir welding (VFSW) is performed on 6 mm thick TC4 titanium alloy plates using the inner sleeve made of W-25%Re alloy,the lower sleeve made of WC-15%Co alloy and the 16 mm diameter TC4 titanium alloy stirring rod as stirring tools. The weld formation, microstructure, tool debris residue within the weld, and mechanical properties are investigated. The results indicate that under the rolling speed of 700 r/min and welding speed of 30 mm/min, complete penetration of the 6 mm thick TC4 plate is achieved in a single pass, and the weld surface is good. The stir zone primarily consists of coarse lamellar structures. Along the plate thickness direction, the size of the prior β grains decreases gradually. The transition zone exhibits characteristics of a bimodal microstructure. Significant grain coarsening is observed in the heat-affected zone compared to the base material. Tool debris residue is predominantly distributed on the weld top surface. The advancing side residue primarily consists of W-Re alloy debris, while the retreating side contains a mixture of W-Re alloy and WC-Co alloy. The presence of debris can enhance the weld hardness. However, the debris accumulation region on the retreating side becomes the weakest area of the joint, where cracks preferentially initiates and propagates towards the weld root. The maximum tensile strength of the joint reaches 1 044.5 MPa, equivalent to 87.8% of that of the base material, but the elongation after fracture decreases compared to the base material.