Large plastic deformation treatment on surface and fatigue strengthening mechanism of friction stir welding joints of 7075 aluminum alloy

The gradient grain structure was prepared on the surface of friction stir welding (FSW) joints by means of surface mechanical rolling treatment (SMRT). Electron backscatter diffraction (EBSD) results show that a gradient grain layer with a thickness of about 550 μm is formed on the surface of the sample after SMRT, and the surface finish is significantly improved. A compressive residual stress layer with a thickness of about 170 µm is formed due to the action of large plastic deformation. Uniaxial fatigue test at a stress ratio of 0.1 reveals that the S-N curves of welded joints exhibit linear descendance with the reduction of stress, and fatigue cracks exclusively initiate at specimen surfaces, whereas the S-N curves of FSW joints after SMRT demonstrate bilinear characteristics. Scanning electron microscope (SEM) results demonstrate that the cracks in the first stage of the S-N curve initiate at the specimen surface, while those in the second stage initiate inside the specimen, accompanied by fish-eye morphology characteristics. Compared with welded joint specimens, the fatigue strength after SMRT is significantly improved, especially when a fatigue crack is initiated internally, the fatigue strength is comparable to that of the base metal. The increase in fatigue strength can be attributed to the gradient structure formed by SMRT, the formation of a thick compressive residual stress layer, the improvement of surface finish, and the very high cycle fatigue (VHCF) characteristics.