An Investigation on Subsurface Microstructural Evolution and Mild to Severe Wear Transition in AZ51 Magnesium Alloy

Abstract

The wear behavior of as-cast AZ51 alloy was investigated using a pin-on-disc configuration within a load range of 20–380 N and a sliding speed range of 0.1–4.0 m/s under dry sliding conditions. The coefficient of friction and wear rate were measured as a function of applied load at various sliding speeds. Morphologies and chemical compositions of worn surfaces were analyzed for determination of wear mechanisms using scanning electron microscope (SEM) and energy-dispersive X-ray spectrometry (EDS). Microstructural evolution, plastic deformation, and hardness of the subsurfaces as well as worn surface hardness were characterized by confocal scanning laser microscopy and hardness testing for the establishment of a corresponding relationship between microstructural evolution and mild to severe wear transition in AZ51 alloy. The results show that there exists a strong relationship between wear behavior and subsurface microstructural evolution; that is, surface oxidation and strain hardening originating from plastic deformation of the microstructure prevail in mild wear regime, whereas thermal softening with dynamic recrystallization (DRX) in subsurface and surface melting dominate in severe wear regime. With increasing load and sliding speed to certain critical states, the friction-induced heat accumulation activates DRX in subsurface and surface melting successively, leading to a rapid rise in wear rate and typical worn surface morphology with extruded edges or multilayered structure edges. The mild to severe wear transition is determined to be controlled by initiation of DRX in the subsurface of AZ51 alloy.