Investigation Of Microstructure And Properties Of Al-Mg-Si Alloy With High Content Of Mg

The materials application in aerospace and automobile industries has been stressed on thedevelopment in saving weight, increasing properties, lowering economic costs, environmentalcontamination and energy consumption, etc. Considering the requirement of saving weight, itis very significant for vehicles and automobiles to use materials with low specific-weight.Therefore the development of the aluminum alloy with high specific strength and highspecific stiffness in aerospace and automobile industry has extensive prospect.Al-Mg-Si alloys belong to the6×××series. They are widely used in industries because oftheir excellent corrosion resistance, formability, weldability etc. In this study, we tried toincrease the amount of Mg greatly to lower the density of the Al-Mg-Si alloy. In order tocontrol the existence state of Mg, Si was added with certain Mg/Si ratio. And most of Mgforms Mg2Si phase. Mg2Si phase is a kind of brittle phase. It distributes in the matrix withChinese-script shape and polygon shape. Furthermore, it would cut off the matrix and affectthe properties of the alloy. In order to improve the microstructure and properties of Al-Mg-Sialloy with high contentration of Mg, we studied the size, the shape and the distribution ofMg2Si phase by adding Sr elements as well as the microstructures and properties.Two series of alloys with15%Mg and18%Mg, respectively were prepared in thisexperiment. The microstructures of cast alloys were investigated by means of metallographobservation, XRD, SEM. And the mechanism of microstructures modification through Sraddition was discussed. The representative alloys were choosed to be hot extruded. Themicrostructure development of these alloys was analysised. And the effects of heat treatmentof annealing and aging on the microstructures and mechanical properties were studied.XRD results show that the main phases are α-Al matrix and Mg2Si phase in the castingalloys and hot extruded alloys. Moreover, the alloys with15%Mg contained a small quantityof AlMg phase, the alloys15%Mg contains Al₁₂Mg₁₇phase, and the new Al4Sr phase formedafter adding Sr.The microstructure analysis shows that the Mg2Si phase and Al₁₂Mg₁₇phase are modified by Sr addition. The size of primary bulk Mg2Si phase decreases and the eutectic Mg2Si phaseis smashed. Bar-shaped Al₁₂Mg₁₇phase is broken into rod-shaped particles. The modifiedeffect is best when the quantity of Sr is0.05%. However, the Mg2Si phase begins to clusterwith the increasement of Sr addition.The microstructures of the alloys are obviously refined after extruding. The dynamicrecrystallization happens in part of the alloys. The distribution of the Mg2Si phase becomesdispersed with rod-shaped or fined particles, but the large primary Mg2Si phase still exists. Itis showed that the above alloys have good plasticity. Solid solution and aging treatment canimprove the tensile strength of the alloy, but reduce the plasticity. The textures of extrudedalloys are weakened when Sr is added, and annealing makes the intensive textures weakened.