The effect of ultrasonic surface rolling process on the gradient microstructure and wear resistance of AZ31 thin sheet

This study investigates the effects of the ultrasonic surface rolling process (USRP) on the surface microstructure, texture, and wear behavior of commercial AZ31 magnesium alloy sheet. The application of USRP induces a depth-dependent gradient microstructure characterized by a gradual transition from fine-grained surface layers to coarser-grained regions. Severe plastic deformation at the surface significantly enhances surface microhardness, with values increasing from 63.8 HV in the untreated specimen to 132.9 HV after USRP-4 passes. The thickness of the plastic deformation layer exhibits process-dependent behavior, growing from 200 µm (two passes) to 250 µm (four passes). Wear test results indicate that the wear resistance of the material is significantly enhanced after USRP treatment. This improvement is primarily attributed to the combined effects of the following factors: surface grain refinement, the role of dislocation-induced twinning, increased hardness, the formation of nanoscale secondary phases, introduction of residual compressive stresses, weakened surface texture, and reduced surface roughness. Notably, both friction coefficients and wear volumes show a direct dependence on the number of rolling passes. This study systematically elucidates the underlying mechanisms linking USRP-induced microstructural evolution to enhanced wear performance, providing critical insights for optimizing surface engineering strategies in magnesium alloys.