The Effect Of Surface Mechanical Attrition Treatment On The Sliding Wear Behavior Of AZ31 Wrought Magnesium Alloy

Magnesium alloys are of great interest and practical application for the automotive and aerospace industries, mainly due to its low density and high specific strength, excellent damping capacity, good recycling capacity, etc. However, the poor wear resistance of these alloys strongly limits their application. Surface mechanical attrition treatment(SMAT), as a kind of simple and effective way to form nanocrystalline layer on the surface of metal material, has been proven to be an effective approach to improve the wear resistance of copper, steel, nickel and aluminum material. However, few literatures are available to date describing the effect of SMAT on the wear behavior of magnesium alloys, especially not reported in AZ31 alloy. So in this paper, the rolled AZ31 wrought magnesium alloy was chosen as the object, focusing on the effect of SMAT on the dry sliding wear behavior of magnesium alloy.In this paper, as-received AZ31 wrought magnesium alloy was treated by SMAT for 3 min. The microstructure, phase composition and texture of AZ31 alloy before and after SMAT was observed by optical metallographic microscope and XRD respectively. The microhardness and tensile mechanical properties of AZ31 alloy before and after SMAT were tested. The effect of SMAT on the dry sliding wear behavior of AZ31 magnesium alloy under different velocity and load was analyzed. The friction coefficient and wear rate were calculated. The composition and morphology of worn surface and debris were analyzed by SEM and EDS. The wear mechanisms of AZ31 before and after SMAT under different condition were discussed.The microstructures of AZ31 magnesium before and after SMAT showed that a gradient nano structure was formed in the surface of AZ31 alloy after SMAT. The grain size of AZ31 alloy after SMAT in surface layer was refined. The thickness of ultra-fined grain(UFG) layer formed on the surface of AZ31 alloy after SMAT was 85 μm. Among the ultra-fined grain layer, the nanocrystalline layer was in the top surface and the grain size was 42.7 nm obtained from the XRD analysis. The surface rolling base texture was weakened obviously and random crystallographic orientations was obtained after SMAT.The results of microhardness and mechanical properties of AZ31 alloy before and after SMAT showed that SMAT can significantly improve the hardness and strength of the alloy. After SMAT, the hardness of top surface is 130.8 Hv, increased by 52.6 % than as-received alloy. As a gradient variation along the thickness direction, the area with hardness increased is about 400 microns, and the hardness of the area above 400 is almost as same as the as-received one. Meanwhile, the tensile strength and yield strength of AZ31 alloy after SMAT was increased by 16.8 % and 26.2 %, respectively. But the elongation was reduced by 47.4 %. The improvement of strength and hardness and reduce of ductility were mainly attributed to the effect of grain refinement after SMAT, improving the resistance to plastic deformation. The refined grains resulted in the increase of grain boundaries, which restrain the dislocation movement during tensile, so the ductility was reduced.The experimental results of dry sliding friction and wear of AZ31 alloy before and after SMAT showed that at the low speed of 0.05 m/s, the wear resistance of the alloy after SMAT has been improved in the load within the range of 10~30 N, but decreased when the load exceeded 30 N. This is mainly attributed to the grain refinement in the surface after SMAT, which resulted in the increase of the hardness, strength and better load bearing capacity. So the plowing and cutting was decreased and the wear resistance was improved. However, if the load exceeds the toughness limit, the speed of crack propagation would increase faster with a high level of fracture, resulting in decrease of wear resistance. In high speed of 0.5 m/s, the friction coefficient and wear of the alloy after SMAT were lower than that of the as-received coarse grained alloy and the worn surface was also smoother, which means that the wear resistance was improved after SMAT. It was mainly attributed to that the nanostructured layer generated by SMAT with the large number of grain boundaries which could act as diffusion channels to absorb much more external oxygen atoms and form continuous in-situ oxide film more easy during the wearing. The effective diffusion coefficient was increased with deceasing grain size. The in-situ oxide film in SMATed alloy was combined with matrix closer than oxide debris film in as-received alloy.Compared with the low speed of 0.05 m/s, the wear rates of AZ31 alloy before and after SMAT at high speed of 0.5 m/s were much lower, which means the significant increase of wear resistance at high speed. It was mainly because of the transition of the wear mechanism under different speed. At low speed of 0.05 m/s, the main wear mechanism of AZ31 alloy before and after SMAT was abrasive wear. When the sliding speed increased to 0.5 m/s, the main wear mechanism was transformed into oxidative wear. The oxide film was formed in the worn surface due to the increase of friction heat at high speed. Meanwhile, the oxide film has an excellent friction-reducing property and can prevent the direct contact between the metal and ball and hinder further oxidation effectively, resulting in the decrease of wear rate.This study has an important theoretical and practical significance in friction and wear behavior of magnesium alloys and lays a foundation for the further research and application of magnesium alloys.