Study on surface characteristics of 7050-T7451 aluminum alloy by ultrasonic surface rolling process

Ultrasonic surface rolling process (USRP) is an effective method to improve material surface quality, such as surface finish, microstructure, and stress state. Previously, USRP is usually used in comparably hard materials (e.g., instance steel and titanium alloys). In this paper, attention is focused on low hardness aluminum alloy which is widely used in the aviation industry. Aluminum alloy 7050-T7451 is used to investigate its surface characteristics in the ultrasonic rolling experiment. With the aid of surface optical profiler, X-ray stress analyzer, scanning electronic microscope (SEM), and energy-dispersive spectrometer (EDS), the differences of surface characteristics are explored in the USRP-treated area and that of the turning area. In addition, the influence of feed rate on surface integrity is also investigated. The results show that surface integrity is improved by USRP, and the best quality is obtained with the feed speed of 0.10 mm/r. Under the optimal experimental condition, surface roughness (Ra) is reduced to 0.059 μm, axial and tangential surface compressive residual stress is increased to −130.6 and −330.8 MPa, respectively, and surface microhardness is increased by 41.3 %. Metal flow traces, fusion of surface grain boundary, and the phenomenon of impurity phase diffusion are observed in the cross section of the treated specimen. The internal strengthening mechanism of the USRP-treated surface is probed.