Research On Microstructural Evolution And Mild To Severe Wear Transition Of Magnesium Alloys

In recent years, magnesium alloys have been widely used in the electronics,aerospace, and automotive industries due to their attractive properties such as lowdensity, high specific strength, and good electrical and thermal conductivity. However,poor resistance to wear has been a serious impediment against the wider applicationof Mg alloys, preventing them from being used as extensively as Al alloys inautomotive engine components. It is recognized that the microstructure andmechanical properties of magnesium alloys have strong dependence on environmenttemperature, which is an important feature for magnesium alloy differentiating fromsteels or Al alloys. Therefore, development of magnesium alloys with excellentmechanical properties at elevated temperature has always been one of the mostimportant areas in magnesium alloys research. On the contrary,despite the growinginterest in magnesium alloys, there is few studies have been conducted on the frictionand wear behavior of magnesium alloys. And these researches which are lacking ofessential analyses, only focus on the wear behavior of magnesium alloys characterizedby the change of the external parameters, such as sliding velocity, load. Based on suchbackground, we have studied on the correlation between friction-inducedmicrostructural evolution in the subsurface, mechanical properties changes, and wearbehaviors. A novel prediction model of transition loads from mild to severe wear hasbeen established as well.Dry sliding tests were performed onAZ system magnesium alloys (AZ31、AZ51and AZ91) using a pin-on-disc configuration under the similar load ranges and slidingspeeds of0.1-4.0m/s. Friction and wear characteristics of AZ system magnesiumalloys were investigated as a function of the load and sliding speed. Morphologies,compositions and hardness of worn surfaces were characterized by scanning electronmicroscope (SEM), energy dispersive X-ray spectrometer (EDS) and hardness tester.Five wear mechanisms, namely oxidation, abrasion, delamination, thermal softening,and surface melting, have been observed. Oxidation, abrasion and delaminationoperated in mild wear regime, while thermal softening and surface melting dominated in severe wear regime. For the three magnesium alloys, the transition loads from mildto severe wear are all decreased with increasing sliding speed. The wear rate map andthe wear transition map of magnesium alloys are obtained according to the loadranges of wear mechanism operating at different sliding speeds, being of referencesignificance for further study.After the analyses of the equivalent plastic strain distributions beneath the wornsurfaces and microstructural evolution in subsurface of AZ system magnesium alloys,we can conclude that even at a low load and a short sliding distance, a large plasticstrain was induced in the near-surface region. In mild wear regime, with increasingapplied load, deformation zone extended from top parts of surface grains to deepgrains. After the transition from mild to severe wear, a DRX zone occurredimmediately beneath worn surface, followed by a deformation zone. In severe wearregime, when the wear behavior transformed from severe plastic deformation intosurface melting, the subsurface microstructure also transformed from DRX zone+deformation zone into the solidified fine grain zone+DRX zone+deformation zone.Combining the researches of equivalent plastic strain distributions beneath theworn surfaces, microstructural evolutions, mechanical properties and oxidationcharacteristics, correlation between friction-induced microstructural evolution, strainhardening in subsurface and wear properties of AZ system magnesium alloys can beestablished. In mild wear regime, the high hardness values of worn surfaces mayoriginate partially from surface oxidation and partially form strain hardening causedby plastic deformation, and thus leading to a low level or a low slope of wear rate. Insevere wear regime, deformation zone has been replaced by DRX zone. Surfacesoftening originating from DRX or surface melting was dominant factor for the rapidgrowth of wear rate.Based on the analyses of mild to severe wear transition and microstructuralevolution, a criterion for mild to severe wear transition has been proposed accordingto thermal activate dynamic theory. A novel prediction model of transition loads frommild to severe wear has been established as well, of which the effectiveness andapplicability have been verified by comparing the calculated transition load withmeasured transition load of AZ system magnesium alloys and6061Al alloy.Combined with the prediction model of critical loads for surface melting, a map ofmicrostructural evolution in surface layer was obtained for each magnesium alloy.Effect of Al content in AZ system magnesium alloys on the wear characteristics and microstructural evolution has also been studied. Theβ-Mg17Al12phase content andexisting form have been changed with the increasing Al content, and thus leading to asignificant difference of wear behavior and microstructural evolution in subsurface.The achievements we obtained have not only enriched the researches of frictionand wear behaviors in AZ system magnesium alloys, but also established a correlationbetween microstructural evolution, mechanical properties and wear characteristics.We have given a new point of view to describe the friction and wear mechanism, andshown a new direction to research the tribological theories.