This paper aims to provide a better understanding regarding the effects of shot peening (SP), surface mechanical attrition treatment (SMAT), laser shock peening (LSP), surface mechanical rolling treatment (SMRT), and ultrasonic nanocrystal surface modification (UNSM) on the fatigue properties of metals in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The work in this paper finds that SMRT and UNSM generally improve the high-cycle and very-high-cycle fatigue properties of metals, while SP, SMAT, and LSP can have mixed effects. The differences are discussed and analyzed with respect to the aspects of surface finish, microstructure and microhardness, and residual stress. SMRT and UNSM generally produce a smooth surface finish, while SP and SMAT tend to worsen the surface finish on metals, which is harmful to their fatigue properties. In addition to inducing a plastic deformation zone and increasing microhardness, surface treatments can also generate a nanograin layer and gradient microstructure to enhance the fatigue properties of metals. The distribution of treatment-induced residual stress and residual stress relaxation can cause mixed effects on the fatigue properties of metals. Furthermore, increasing residual stress through SP and SMAT can cause further deterioration of the surface finish, which is detrimental to the fatigue properties of metals.

This paper focuses on the twist defects and the control strategy in the process of four-roll bending for aluminum alloy Z-section profiles with large cross-section. A 3D finite element model (3D-FEM) of roll bending process has been developed, on the premise of the curvature radius of the profile, the particularly pronounced twist defects characteristic of 7075-O aluminum alloy Z-section profiles were studied by FE method. The simulation results showed that the effective control of the twist defects of the profile could be realized by adjusting the side roller so that the exit guide roll was higher than the entrance one (the side rolls presented an asymmetric loading mode with respect to the main rolls) and increasing the radius of upper roll. Corresponding experimental tests were carried out to verify the accuracy of the numerical analysis. The experimental results indicated that control strategies based on finite element analysis (FEA) had a significant inhibitory function on twist defects in the actual roll bending process.