Improving the surface quality and its mechanism in ultraprecision machining of Al-Mg-Si alloy by multiple high energy shot peening pretreatment

Achieving high surface quality is vital for improving the reflectivity of aluminum alloy optical components. However, aluminum alloy materials often have large-sized grain structures, and stress accumulates at the grain boundaries during ultraprecision machining processes, resulting in raised grain boundaries and a decrease in surface quality. In this work, we utilized a multiple high energy shot peening (SP) technique to refine the surface grain structure of Al-Mg-Si alloy. The results revealed that a gradient grain structure was formed within the depth range of 0∼150 μm, accompanied by the occurrence of various substructures such as dislocations, stack faults, and low-angle grain boundaries in the severely deformed layer. This led to a significant 30% increase in hardness. Furthermore, the shot peened sample exhibited remarkable ultraprecision machinability. A comparison with the sample that has not undergone SP treatment revealed a decrease in surface roughness from 2.5 nm to 1.4 nm. The improved surface quality can be attributed to the higher deformation uniformity and reduced grain rotation. Our study provided an innovative and cost-effective method for manufacturing large-sized metal optical components.