Fatigue Mechanism and Residual Stress Stabilization Principle of 2024 Aluminum Alloy Subjected to Ultrasonic Surface Rolling Process

Ultrasonic surface rolling process (USRP) represents an innovative technique for deformation-based strengthening. This paper presents a comprehensive analysis of the surface integrity of USRP-treated samples, examines the influence of multiple USRP passes on the fatigue properties of 2024 aluminum alloy, and explores the relaxation behavior of induced compressive residual stresses. Results indicated that USRP treatment produced a superficial layer characterized by reduced roughness, elevated microhardness, significant compressive residual stress, and refined grain structure, collectively enhancing the fatigue performance of the material. The primary factors contributing to fatigue improvement varied with the number of USRP passes: Initial enhancements were attributed to work hardening and microstructural refinement, while with repeated processing, compressive residual stress became the dominant factor. Increasing the number of USRP passes also raised dislocation density at the surface and improved the stability of compressive residual stresses. However, surface microdefects introduced during processing promoted stress relaxation, thereby counteracting the beneficial effects of compressive residual stresses.