Lap joining of Ti6Al4V titanium alloy by vortex flow-based friction stir welding

The current study uses a novel vortex flow-based friction stir lap welding (VFSLW) process to weld 1.4 mm thick Ti6Al4V sheets, trying to replace the diffusion bonding in the superplastic forming/diffusion bonding process. The mechanical properties and joining mechanism of the VFSLW joint were investigated. Good weld formation was obtained at 300–350 rpm and 80 mm/min. No hook defect was formed in the lap interface. The lap joint fractured across the stir zone (SZ) in the top plate under the optimal parameters. The cracks initiated from the oxidation defect, where the oxides on the workpiece surface were involved in the SZ by the vortex during the welding. It suggests that a good argon shield is very important for the VFSLW of titanium alloy. The highest tensile strength reaches ∼890 MPa, up to 95 % of the base material. The formation of an α + β lamellar structure in the SZ is due to the peak welding temperature exceeding the β-transus temperature. In the bonded zone (BZ), a very thin layer with ultrafine α grains was formed by dynamic recrystallization in the α phase field, although its two sides are α + β lamellar structures. This is because of the oxidation on the workpiece surface during the welding process and the O element is a strong α stabilizer.