Effect of ultrasonic surface rolling process on the fatigue performance of the 7075 aluminum alloy

The impact of the gradient nanostructures on the fatigue properties of aluminum alloys remains limited. The ultrasonic surface rolling process (USRP) was utilized in this study to generate the gradient nanostructure on the surface of 7075 aluminum alloy, and the high fatigue properties with the stress ratio R =  – 1 were following tested. The findings indicated that the fatigue limits of 3- and 6-passes-treated samples were found to reach 225 MPa (125%) and 200 MPa (100%), respectively, surpassing those of untreated sample. The characterizations of scanning electron microscope (SEM), laser confocal scanning microscope (LCSM), and X-ray diffractometer (XRD) showed a positive correlation between the number of rolling passes and the enhancement of the gradient hardening layer and residual compressive stress, contributing to the improvement in fatigue limit. Meanwhile, the SEM analysis of the fracture indicated that the fatigue crack initiation site was altered as a result of surface modification, and the crack initiation point of the 3-passes-treated sample was located further from the surface. Additionally, finite-element simulation was employed to analyze the stress distribution across the cross-section, and the fatigue risk coefficient R_f was used to quantify the impact of residual stress distribution and surface hardening on the crack initiation site. The results demonstrated that USRP not only altered the surface condition of the aluminum alloy but also changed its stress distribution in the cross-section. The combined effect of the two controlled the crack initiation site and the fatigue life of the 7075 aluminum alloy.