Research On Dry Sliding Wear Property Of AZ51Magnesium Alloy

Magnesium alloys have been considered as a prospective material to lighten theweight of structural components of automobiles and aircrafts because of their low density,high strength-to-weight ratio, good machinability and mechanical properties，However,poor wear resistance has always been a serious drawback to the further application ofmagnesium alloys. The wear property is not only an indispensable indicator for metalforming through great influences on material properties and surface quality but also a vitalreference property for components service process, so research on magnesium wearproperty has important meaning to our industrial society.The present paper presents the results of an experimental study of correlation betweenthe microstructure evolution, hardness change and wear behavior through comparing thedifferent sliding velocity, distance and load of AZ51magnesium dry sliding wear process.Results indicate five main wear mechanisms were observed which are abrasion, oxidation,delamination, thermal softening and surface melting. Abrasion, oxidation and delaminationoperated in mild wear regime, while thermal softening and surface melting dominated insevere wear regime. Mild wear behavior turned to severe wear behavior as sliding velocity,distance and load increasing. Worn surface hardness affected the wear mechanism, thehardness curve of sliding speed at0.1m/s was regarded as the mild to severe transitionhardness criterion line because there was only strain hardening on the worn surface at thisspeed, if worn surface hardness was higher than this criterion line at other sliding speed,mild wear behavior happened, while lower than the criterion line, severe wear behaviorhappened.There was an interesting phenomenon which was the coefficient of friction and wearrate loaded little force at low sliding velocity much higher than loaded the same force atmiddle and high sliding velocity. The reason was micro-cutting realized while slidingslowly. As the force increasing, friction heat on worn surface accumulated stronger which induced material softening, weakened the micro-cutting and decreased coefficient offriction. In severe wear regime, the curve of wear rate increased dramatically after materialsoftening result from dynamic recrystallization (DRX), while this increasing trend had beenbroken by the melting of eutectic Mg17Al12phase which would absorb heat energy toprevent worn surface temperature keeping raising, so this melting could stop wear rateincreasing even decreased it.Wear process was accompanied by material microstructure evolution. In mild wearregime, worn surface experienced deformation, temperature increasing induced by frictionheat accumulation and Mg17Al12phase partial dissolution; force had a changing effect onthe subsurface resulted in deformation decreasing as depth to surface increasing.Deformation could produce strain hardening. The hardness of subsurface would be higherthan cast-die material in near surface and decrease to it as the depth increasing. In severewear regime, worn surface realized DRX and lots of Mg17Al12phase dissolution, as surfacetemperature increasing exceeded material melting temperature, all of the deformed grainson surface translated to fine equiaxial grains, which result in surface hardness decrease.Microstructure evolution in wear process followed a basic rule: strain accumulation at thebeginning, then DRX and last the melt-solid. The evolution would affect hardness and wearbehavior which dominated the changing of wear rate and coefficient of friction.Strain and temperature essentially dominated the whole wear process together. Whenstrain effect more obvious, worn surface microstructure was full of dislocation and twinswhich risen the hardness. So wear resistance improved and mild wear behavior happened.While temperature effect stronger, worn surface microstructure realized DRX and hotextrude morphology appeared because material softening. So wear resistance deterioratedand severe wear behavior happened.