A study on the tribological behavior of AZ31 magnesium alloy sheets processed by temperature-assisted ultrasonic shot peening

Mg alloy, as a promising lightweight structural material, finds applications in various fields. However, poor surface properties like wear resistance greatly limit its applications. As a surface modification technology, ultrasonic shot peening (USP) has gained significant popularity. In this study, temperature-assisted USP (TA-USP) was firstly applied to AZ31 Mg alloy. The influence of peening duration and temperature on microstructural evolution and mechanical properties was investigated in details. The ball-on-disk dry sliding wear test was performed to examine the tribological behavior. The results show that a gradient microstructure and surface nanocrystallization are introduced by TA-USP. The gradient becomes significantly pronounced as the peening duration extends. The surface hardness increases from the as-annealed 60.9 ± 1.8 HV to 159.0 ± 6.1 HV after processed at room temperature with 800 s, and a depth of about 600 μm for the affected region is achieved. When the peening temperature is raised, the gradient weakens, even disappears at 300 °C, and the hardness decreases. Notably, the gradient microstructure and enhanced hardness exhibit significant thermal stability below 200 °C. The wear curves and morphology analysis suggest that TA-USP obviously improves wear resistance. The surface nanocrystalline results in an easy formation of MgO patches and reduces the coefficient of friction. Moreover, the presence of gradient microstructure also retards the delamination phenomenon. Increasing peening duration or temperature to a moderate level will lead to an enhanced wear resistance, owing to its dependence on both hardness and toughness. A simplified wear transition map is roughly established to predict optimum wear resistance.