Research On Sliding Wear Behavior Of AS31Alloy

Magnesium alloys have excellent performance at room temperature, but thecreep strength declines dramatically and the high temperature performance is poorwhen the temperature is over120℃. AS system magnesium alloys are one of theearliest used heat-resistant magnesium alloys in automobiles due to their attractiveproperties such as good heat resistant performance and low cost. In recent years, theyhave been used in automotive engine and air cooling engine crankcase, etc. However,there are few studies have been conducted on the friction and wear behavior of ASsystem magnesium alloys. Obviously, it is a disadvantage for AS system magnesiumalloys to be used as high temperature parts of automobiles. In such background, thistest is researched on dry sliding wear behavior of AS31alloy.Dry sliding wear tests are performed on AS31alloy using a pin-on-disc testingmachine under25℃. Variations in coefficient of friction and wear rate with load forAS31alloy are presented under sliding speeds of0.1-4.0m/s and loads range of20-360N. Combining with the wear rate-load curves, the SEM images of wornsurfaces are observed. Five wear mechanisms were identified. Oxidation wear,abrasion wear and delamination wear are the main wear mechanisms in mild wearregime, while thermal softening and surface melting are the dominant wearmechanisms in severe wear regime. The curve of the transition loads from mild tosevere wear at different sliding speeds and the wear map (wear rate map and wearmechanisms transition) are obtained. Mild wear prevails at the sliding speed of0.1m/sand0.5m/s. However, there is a mild-severe transition when the slidingspeed≥0.785m/s, and the transition loads from mild to severe wear decrease withincreasing the sliding speed.The plastic deformation and microstructural evolution beneath the worn surfaceand the variations in hardness are analyzed in detail to study the correlation betweenthem and the transition from mild to severe wear. It will provide good criterion for thecharacteristics under different wear mechanism and mild-severe wear transition.In mild wear regime, with increasing applied load, the depth of deformation zonebeneath worn surface increases, while the plastic deformation causes some twins.After the transition from mild to severe wear, a DRX zone occurs beneath wornsurface, following by a deformation zone, while the recrystallization grains are replaced by the deformed microstructure. When the wear behavior transforms fromsevere plastic deformation to surface melting, accordingly, the subsurfacemicrostructure transforms into the solidified fine grain zone, DRX zone anddeformation zone.In mild wear regime, the increase of hardness originates from the strainhardening caused by plastic deformation, and thus leading to no wear rate increasesdramatically. In the severe wear regime, softening originating from DRX and surfacemelting offsets the strain hardening, so that the hardness decreases and the wear rategreatly increases. Surface softening is the key for the transition from mild to severewear. Surface hardness in severe wear regime is still higher than the hardness ofas-received alloy because of refinement of subsurface grains formed in the DRX andsolidification.Dry sliding tests are performed on AS31alloy at0.785m/s and140N withdifferent sliding distance, in order to study the plastic deformation and microstructureevolution during the sliding wear process. We can find that even a short slidingdistance (18.84m), a large plastic deformation occurred beneath the worn surface. Thedegree of plastic deformation is reinforced with the increase of distance, consequentlythe hardness increases there.