Az91d Magnesium Alloy Friction And Wear Behavior And Mechanism Studies

AZ91D alloy is one of the most commercial magnesium alloys because of its good foundry properties and mechanical properties. However, porosity can be easily formed in the castings due to its wide solidification range. The investigations on its wear behavior are very scarce, which needs to be intensively studied. The novel thixoforming technique can significantly reduce or eliminate porosity. Therefore, in this thesis, the wear characteristics of thixoformed (ThF) AZ91D alloy were investigated.The friction and wear characteristics of thixoformed AZ91D alloy were studied under dry friction pin-on-disk conditions, as well as those of the solid solution treated and aged AZ91D alloys. For contrast, the wear behavior of conventionally casting (CC) AZ91D alloy was also studied. According to the stability of friction coefficient and wear resistance, the optimal thixoforming parameters were confirmed. The wear mechanisms were also discussed by analyzing the morphologies of worn surfaces and wear debris, the changes in wear rate and friction coefficient. The results showed that optimal thixoforming parameters were: heating temperature of 585°C, heating time of 60min and mould preheating temperature of 300°C. The wear mechanism of CC alloy gradually changed in turn from oxidation and abrasive wear to abrasive wear, adhesive wear and melt wear with the increase of loading. But for the ThF alloy, the mechanism changed from oxidation wear to mild delamination wear, mild adhesive wear and severe adhesive wear. That is to say, the wear resistance of the ThF alloy is better than that of the CC alloy, and the thixoforming improved the wear resistance. The solid solution treatment and age treatment all improved the stability of average friction coefficient. The influence of solid solution treatment on wear rate was very small while the effect of aging was quite complicated.At the same time, the friction and wear behaviors of the ThF AZ91D alloy under dry friction reciprocating sliding motion were also studied. The results indicated that the sliding wear mechanism of the alloy included two kinds, mild wear and sever wear. The mild wear comprised oxidation and abrasive wear and abrasive and mild delamination wear while the severe wear included severe plastic deformation wear and melt wear. The loading and sliding velocity corresponding to the wear mechanism change were obtained from the values of loading and sliding velocity corresponding to a sharp change in wear rate. The wear mechanism change was further confirmed through analyzing the changes in wear surface morphology, wear debris morphology and its constituents and wear surface temperature. For each wear mechanism (except the melt wear), the wear surface temperature obeyed a function with the expression of F^1/4V^1/2. The result showed that the wear mechanism changed from the mild wear to the severe wear at the surface temperature of 75°C±5°C. The wear mechanism map of the thixoformed AZ91D alloy was drawn according to the above analysis.