The tribological behavior of a surface-nanocrystallized magnesium alloy AZ31 sheet after ultrasonic shot peening treatment

A magnesium alloy AZ31 sheet was processed by ultrasonic shot peening treatment to fabricate a surface nanocrystalline, and a ball-on-disk dry sliding wear test was performed to evaluate the tribological behavior after treatment. The microstructure observation indicated a gradient nanocrystalline structure was formed after USSP treatment. The microhardness at the top surface was improved from 60 HV to 145 HV after treatment. The formed nanocrystalline resulted in an easy formation of MgO patches on the surface and reduced the coefficient of friction. Moreover, the formed nanocrystalline leaded to a retard of delamination with increasing the sliding speed and applied load, which was due to its stronger sub-surface. Under high sliding speed (0.5 m/s) and high applied load (50 N), it was firstly found that the formed nanocrystalline prevented the happening of thermal softening and melting. The possible reasons accounting for the prevention of thermal softening and melting were discussed accordingly.