Fatigue properties of Ti-6Al-4V alloy friction stir welding joint obtained under rapid cooling condition

Friction stir welding joints of Ti-6Al-4V titanium alloy were obtained under air and rapid cooling conditions. After post welding heat treatment, the low-cycle fatigue properties of the joints were manly investigated. The results show that the application of liquid nitrogen on the joint top surface decreases the peak welding temperature and the temperature gradient along the joint thickness, which results in a finer and more homogeneous microstructure. Consequently, the improved tensile properties and microhardness are obtained. Fatigue tests show that the joints obtained under rapid cooling condition possess an increased fatigue life. The W-Re alloy contaminations resulting from the tool wear play a significant role in the joint fatigue performance. The fatigue specimens fracture in the SZ due to the existence of contaminations and these contaminations with different sizes have different influences on the fatigue crack initiation and propagation.