Study On Micro-alloying And Microstructure Of Hub Usdealuminum Alloys

A356 is hypoeutectic cast alloy of Al-Si-Mg system. The domestic grade is ZL101. the lightest structural materials with excellent casting fluidity,good air tightness,small shrinkage and hot cracking tendency.it has good mechanical properties,physical properties,corrosion resistance and machinability after metamorphic and heat treatment. So A356 alloys have been certainly used in the fields of automobile manufactory.it becomes the preferred material in lightweight of hub.This thesis based on A356 alloys,Sr and Y were added into A356 alloys and the effect of Sr and Y on microstructure and mechanical properties of A356 alloys was studied. Al-Sr master alloy,Al-Y master alloy, metamorphosed by Al-Sr A356 alloy and compound modificated by Al-Sr and Al-Y A356 alloy were prepared by"mixture method". The as-cast and heat-treated hot-extruded test alloys were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and mechanical property test. The effect of Sr and Y on microstructure and mechanical properties of A356 alloys was studied, the influence of heat treatment on microstructure and mechanical properties of A356 alloys was investigated and the fracture mechanism of the test alloys was analyzed. The results show that:①Al-10Sr master alloy is consist ofα-Al,eutectic compounds,A14Sr,Al2Sr,α-Al is the mainly composition phases. Al-Y master alloy is composed ofα-Al,eutectic compounds,A13Y,Al2Y, A356 alloy is composed ofα-Al, eutectic compounds, a little Fe23Y6,Al4Sr is distributed along the boundary of matrix grain, and Al3Y phase is distributed is dispersed distribution in matrix.②Sr and Y has refining effect on magnesium alloys grains. When Sr content is 0.02wt% and Y content is 0.05wt%, the grains of A356 alloy are the finest. The phenomenon of"poisoning"does not occurred. XRD and EDS shows that with the Y addition the grains of A356 change coarsening.The result of mechanical property test indicates that containing 0.05wt% Y, the as-cast A356 alloy reaches to the best mechanical properties, the tensile strength (reaches to the peak value 241.1MPa). However, with adding too much Y could reduce the mechanical properties of A356 alloys. The elongation of the as-cast test alloys show decreasing trend.③The dynamic recrystallization occurs during the heat treatment.so the grains of heat treatment A356 added with Y alloys are more smaller than as-cast alloys, Al4Sr phase distributes dispersively in the form of block structure or particle structure. Al3Y phase distributes gathering states. The amount and distribution of Al3Y and Al4Sr have great effect on the mechanical properties of the test alloys.④The comparision result of the mechanical properties between as-cast and heat-treated A356 added with Y alloys indicates that: the heat-treated A356 alloys performances good mechanical properties with good elongation and high specific strength. when Y content is 0.05wt%, The tensile strength of heat-treated A356 added with Y alloys and the as-cast alloy reaches its maximum. After heat treatment A356 exhibits good comprehensive mechanical properties⑤The fracture mechanism of as-cast A356 added with Y alloys is cleavage fracture mechanism.but the heat-treated alloys is mixed fracture with dimple fracture and locally cleavage fracture.