The Friction Stir Processing And Fabricating Surface Composites Of Thixoformed AZ91D Mg Alloy

Magnesium alloys are the most important structural metal materials in the industry, which will be used more and more widely in the future. However, they are faced with many difficulties in industrial applications because of the limitation of their some characteristics, such as large grain size, inhomogeneous microstructure, relatively poor friction and wear properties and poor corrosion resistance.In this thesis, in order to improve the surface properties of thixoformed AZ91D magnesium alloy, to change the surface microstructure and fabricate surface composites(Al. SiC. Al and SiC) were studied by using a new material processing technology- friction stir processing (FSP). The results were also compared with those of the permanent mould cast alloy.The results indicated that the surface microstructures of both the thixoformed alloy and permanent mould cast alloy became very fine (grain size of 4um after 4 processing times). A large amount 8 phase dissolved into the a phase grains and the microstructures became more homogeneous. The hardness of the resulting microstructures also increased. The grain size decreased with the increase of processing time and did not changed when processing time exceeded 4 times. However, the microstructure of the thermal mechanical affected zone could be further refined and homogenize. This effect was more obvious for the thixoformed alloy. The grains of the both alloys all grew at elevated temperatures. But the growth of the permanent mould cast alloy was more significant.The Al particles could be uniformly distributed in the surface layers of both the AZ91D alloys by FSP. By comparison, the distribution in the thixoformed alloy was more uniform. The Al particles partially dissolved into the a phase, partially became into 6 phase and the residual was still remained in the form of pure Al. The corrosion resistance of the both alloys was improved because the Al with good corrosion resistance was composed into their surfaces. But the corrosion resistance of the thixoformed alloy was relatively higher because the distribution of Al particles was more uniform and the solubility of Al in the a phase was higher. After solid solution treatment, the residual pure Al gradually transformed into β phase, which also coarsened and agglomerated, so the corrosion resistance decreased.The SiC particles and the mixture of SiC particles and Al particles could also be uniformly distributed in the surface layers of both the AZ91D alloys by FSP. Thematrix microstructure became finer and more homogeneous compared with those of the alloys without SiC particles and their wear resistance was improved. The friction and wear properties of the thixoformed alloy were better than those of the permanent mould cast alloy. Because of the blocking of SiC particles for the moving of grain boundaries, the microstructure was more stable at elevated temperatures. The distribution of the mixture of SiC and Al particles was more uniform in the thixoformed alloy, so its wear resistance was better than that of the permanent mould cast alloy.