Effect of ultrasonic rolling on the fatigue performance of laser-welded TC4 titanium alloy joints

To improve the fatigue performance of laser-welded TC4 titanium alloy joints, ultrasonic rolling processing (USRP) is employed herein. Multiple passes of USRP (viz., one, three, and five) are conducted using an ultrasonic rolling device. Results reveal that USRP considerably improves the fatigue limit and life of the welded joints. At room temperature, the fatigue strength of the weldment increases by 2.04–4.58% and the corrosion fatigue life increases by 1.72–2.88 times. In addition, to reveal its strengthening mechanism, the effects of USRP on the surface morphology, microstructure, surface residual stress, and microhardness of the laser-welded TC4 titanium alloy joints are investigated. USRP leads to a shift in the crack initiation point to the subsurface and formation of a hardened layer with high residual stress on the surface via the application of considerable static pressure input and multiple passes. Consequently, fatigue striations become narrower and denser. Compared to the traditional weld surface treatment, USRP substantially improves the surface quality and fatigue performance of laser-welded TC4 titanium alloy joints.