Distributions of grains and precipitates in gradient lamellae Al–Zn–Mg–Cu alloy by ultrasonic surface rolling processing

Improving surface properties can effectively expand the industrial application of aluminum alloys. In this study, a gradient nanostructured surface layer was successfully fabricated in an aged Al–Zn–Mg–Cu alloy by ultrasonic surface rolling processing (USRP). The microstructure and microhardness of the gradient nanostructured surface layer were investigated. The results show that a perfect gradient lamellae structure was fabricated in the surface layer. The average structural size in the topmost surface layer is approximately 41 nm, and the depth of nanostructure lamellae layer is up to 50 μm. After the USRP and two years of natural aging, the gradient distribution of precipitates was formed in gradient lamellae structure. A large number of discontinuous coarse η phase are formed on the lamellae boundary throughout the lamellae structure. However, the precipitates inside the lamellae have gradient distribution. The volume fractions of precipitates increase gradually from nanostructure to ultrafine-grained structure, and the main precipitates are GPII zones. The microhardness in the topmost surface is increased by 40% and gradually decreases from the surface to the matrix. The increase in microhardness is attributed to the fact that the enhancement of the hardness by the grain refinement is greater than the decrease due to the reduction of the precipitates inside the gradient lamellae structure from the surface to the matrix. This study provides technical method and theoretical basis for improving the surface hardness of Al–Zn–Mg–Cu alloy.