Study On Microstructure And Mechanical Properties Of High Vacuum Die-casting Al-7Si-Mg-Mn Alloys And Its Automobile Shock Absorber Tower

The development of high strength and ductility die-casting alloys has always been a research hotspot in the automotive field,among which the Al-10Si-Mg-Mn alloy is the most widely used aluminum alloy in the die-casting industry.However,due to the great unstablility in the performance of die castings,the high silicon content and general plasticity of the alloy itself,its application in higher toughness and ductility situations is limited.In recent years,research on high-strength and low-silicon aluminum alloys has gradually increased.The proportion of eutectic regions in low-silicon Al-Si-Mg alloys has decreased and the number of?-Al matrixes has increased.The alloy itself has good ductility.Then microalloying is used to control a variety of nano-precipitated phases in the aluminum matrix,which can hinder the movement of dislocations to increase the strength of the alloy,and ultimately obtain high strength and ductility aluminum alloys.In this paper,Al-7Si-Mg-Mn alloys with different Mg content were prepared by high vacuum die-casting,and the aging peak time of the alloys with different Mg content was studied through the aging hardening experiment,and the heat treatment process of the alloys with different Mg content was determined.The effect of Mg content on the microstructure was studied by scanning electron microscopy and transmission electron microscopy.Quantitative statistics are carried out on the ageing precipitation phase?",the corresponding relationship between the Mg content and the precipitation phase?"is summarized,and the influence law of the precipitation relative to the mechanical properties of the alloy is summarized.The results show that the Mg content has no effect on the microstructure of the alloy.The Mg content will prolong the peak aging time of the alloy.The Mg content in the alloy is positively correlated with the number density of the precipitated phase?".The precipitated phase?"is the main reason for the alloy strength improvement after heat treatment.Based on the optimal properties of Al-7Si-Mg-Mn alloy,different contents of Cr element were added.Firstly,the phase diagram software Thermo-Calc was used to calculate the multi-phase diagram and phase transformation process curve of Al-7Si-Mg-Mn-xCr alloy,and the effect of Cr content on the microstructure of the alloy was predicted.Al-7Si-Mg-Mn-Cr alloys with different Cr content were prepared by high vacuum die-casting,and the effects of Cr content on the microstructure and mechanical properties of the alloys were investigated.The effects of Cr content on the area fraction and length-width ratio of the iron-rich phase in the alloy were summarized by quantitative statistical analysis of several backscattering photos with different Cr content.Transmission electron microscopy(TEM)was used to analyze the characteristics of nano-sized precipitates containing Cr,determine the formation time,and analyze and summarize the influence of precipitates containing Cr on the strength of the alloy.The results show that a new phase Al₁₃(Fe,Mn,Cr)₄Si₄ will be formed by adding Cr in Al-7Si-Mg-Mn alloy,and the increase of Cr content will lead to the increase of the number of iron-rich phase and reunite phenomenon.The nano-sized precipitates containing Cr are formed in the solid solution stage,and the size and number of precipitates containing Cr remain stable in the subsequent aging treatment stage.However,it can inhibit the precipitation of?"phase to some extent.The best performance Al-7Si-Mg-Mn-(Cr)alloy high-vacuum die-casting is used to prepare automobile shock absorber tower.The microstructure and mechanical properties of different parts of the shock absorber tower were analyzed in the as-cast and heat-treated state.The reasons for the differences in the microstructure and properties of different sampling parts were summarized,and the corresponding improvement measures were proposed.