Correlation between friction-induced microstructural evolution, strain hardening in subsurface and tribological properties of AZ31 magnesium alloy

Dry sliding tests were performed on as-cast AZ31 magnesium alloy using a pin-on-disc configuration. Coefficients of friction and wear rates were measured within a load range of 5-360 N and a sliding velocity range of 0.1-4.0 m/s. Morphologies, compositions and hardness of worn surfaces were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) and hardness tester. Microstructural evolution, strain hardening and dynamic crystallization (DRX) generated in subsurfaces of AZ31 alloy during sliding were found to correlate with the tribological properties obtained. The subsurface microstructures beneath the contact surface were subjected to large plastic strains, and experienced strain hardening, DRX and melting as a result. The roles of surface hardening and thermal softening on the mild to severe wear transition were investigated in detail. It was shown that the transition occurred when the surface layer softened with DRX. Surface oxidation and strain hardening played an important role in maintaining the mild wear, and thermal softening originating from DRX in subsurface and surface melting were responsible for the severe wear. A transition load model, which can be used predict the critical load for transition from mild to severe wear, has been developed using the method of DRX kinetics. A map has been constructed for presenting microstructural evolution and hardness change in surface layer based on the calculated mild to severe wear transition loads and critical loads for surface melting.