Recrystallization behavior and strengthening mechanism of friction stir welded T-joint of Ti80 titanium alloy

In this study, Ti80 titanium alloy T-joints were produced by friction stir welding (FSW), and the joint temperature and residual stress distribution of the joints were analyzed by numerical simulation based on the ABAQUS platform, and the correlation between microstructure and performance of FSWed T-joint was investigated. The results show that the SZ is lamellar α grains due to the fact that the peak temperature of all joints exceeds β phase transus. The deformation and dynamic recrystallization behavior take place in the β phase region. During the subsequent cooling stage, the acicular α/α' phase is precipitated from refined prior β phase. The non-uniform heating and cooling results in the significant microstructure differences in the weld thickness direction. The top and middle of the SZ are composed of basketweave structure, while the bottom of the SZ shows a bimodal structure. The microhardness distribution along the skin shows a higher value in the weld area, and the weakest area is located in the HAZ due to recovery, resulting in a significant reduction in dislocation density of the weld. The tensile specimen breaks in the HAZ when it is stretched along the skin direction, while it breaks in a direction parallel to the skin when it is stretched along the stringer. The fracture mechanism of the joint changes from a hybrid mode of ductile and brittle fracture to ductile fracture as the rotation speed increases from 600 rpm to 750 rpm.