Effects of tool wear on the microstructure evolution and mechanical properties of friction stir welded TA5 alloy

A Co-based tool was employed to weld TA5 alloy to investigate the effects of tool wear on the microstructure evolution and mechanical properties. Results showed that both dense diffusion and loose cracked layers were detected near the surface of as-worn tool, indicating the occurrences of both diffusional wear and abrasive wear. The stir zone (SZ) was divided into the contaminated zone (CZ) and non-contaminated zone (NCZ) according to whether tool constituents were introduced during welding. Continuous and discontinuous recrystallization occurred simultaneously in NCZ, resulting in refined equiaxed α grains. The microstructure in CZ was determined by both thermo-mechanical effects and the content of β-stable tool elements, which reduced α-β transus temperature. As the proportion of tool elements increased, untransformed equiaxed α grains with boundaries rich in foreign elements, transformed acicular α, and retained β emerged successively. The width of transformed needle-like α was decreased to ~ 100 μm, and basket weave structures were obtained when the transition point was reduced to much lower than welding peak temperature. The CZ was provided with extremely high microhardness due to the presences of retained β and needle-like α phases. The SZ tensile specimen was fractured at 824 MPa, 9.1% higher than the BM. Considering periodic tool wear behavior and resultant unique microstructure, the results in this study provide references for the research on material flow and fabrication of high-strength titanium alloy using friction stir processing.