In this study, the ultrasonic surface rolling process (USRP) was utilized to enhance the fatigue life of TC17 linear friction welded (LFW) joints. The surface integrity of the joint treated by USRP was measured and characterized. A gradient microstructure, consisting of an amorphous surface layer and a sub-surface nanocrystalline region, was formed on the joint surface. The residual stress on the joint surface reached approximately -800 MPa, gradually decreasing to near zero at a depth of about 800 μm. Meanwhile, the microhardness of the joint surface presented a gradient distribution, and the surface roughness was significantly improved. The USRP-induced changes in joint surface integrity were beneficial for enhancing fatigue crack resistance. The fatigue test at a strain range of 0–0.0093 mm/mm showed that USRP increased the joint life by at least three times, and the fatigue crack initiation site shifted from the joint surface to the subsurface. The fatigue tests, interrupted at 25,000 and 50,000 cycles, revealed that the gradient microstructure, residual stress, microhardness, and surface roughness of the strengthening surface layer in the joint all deteriorated gradually during the fatigue test process, leading to fatigue fracture of the joint.
Song et al. (Wed,) studied this question.