Effect of ultrasonic peening on fatigue performance of aluminum alloy FSW joints with root defects

Four-point bending fatigue tests were carried out to investigate the effects of ultrasonic impact treatment (UIT), ultrasonic peening (UP) and UIT + UP on the fatigue properties of aluminum alloy FSW joints with root “kissing bonding” defects, and the mechanism of the fatigue performance improvement was explored by analyzing the defect morphologies, microhardness values and residual stress distributions of the samples before and after treatments. Compared with the result that the fatigue strength of FSW joints treated by ultrasonic impact treatment at the root is improved only under 2 × 10⁶ cycles, the results show that UP and UIT + UP can significantly improve the high-cycle fatigue performance, and the fatigue strength is increased by 13% and 40% respectively, corresponding to 2 × 10⁶ cycles. UIT combined with UP cannot only form the compact work hardening layer at the root defects of the FSW joint, increase the microhardness, but also introduce the wider compressive residual stress region near the weld root by UP, eliminate the fold defects probably caused by UIT, significantly improving the fatigue performance.