| Cast Al-Si-Mg alloy is widely used in automobile industry field and etc.because of the good fluidity,low density,low coefficient of thermal expansion and high specific strength of the alloy.The microstructure of untreated cast Al-Si-Mg alloy contains thick ?(Al)dendritic crystal and bulky or acicular eutectic Si phase,which is very easy to split the matrix of the alloy,damaging the mechanical properties of the alloy.The melt of cast Al-Si-Mg alloy is usually treated by adding grain refiners such as Al-Ti or Al-Ti-B and etc.and modifiers such as Na and Sr and etc.to optimize the microstructure of the alloy in industry.But this kind of treatment process of double addition is relatively complex,and the treatment effect is also unstable because of many influential factors.Recent studies have shown that Sc has both refinement and modification effects for cast Al-Si-Mg alloy,and adding a small amount of Zr can improve the acting effect of Sc.However,the relative study has been very sparse yet.Based on the standard composition of A356 alloy,12 kinds of cast Al-Si-Mg alloys with different Sc or Sc,Zr contents were prepared by adding Sc or Sc and Zr.The effects of Sc and Zr addition on the structure and properties of cast Al-Si-Mg alloy were investigated.The main research contents and results are as follows:The effect of Sc addition on the structure of the cast Al-Si-Mg alloy was studied,and the quantitative analysis of the size of ? phase,secondary dendritic arm(Al)spacing(SADS)and the aspect ratio of eutectic Si phase in the alloy were carried out.The results show that with the increase of Sc content from 0 wt.% to 0.58 wt.%,the macrostructure of the alloys is significantly refined,and the higher the Sc content is,the more significant the refining effect is.In the microstructure of the alloy,?(Al)phase is transformed from thick dendritic crystal to the smaller dendritic crystal,the average grain size decreases from 4334?m to 1076?m,and SADS decreases from 52?m to 28.5?m.The morphology of the eutectic Si phase also changes from bulky or acicular shape to short rod or vermicular shape,and the aspect ratio of eutectic Si phase decreases from 6.7 to 4.0.However,when the Sc content is increased to 0.58 wt.%,the aspect ratio of the eutectic Si phase slightly increases,indicating that the excessive Sc content cannot strengthen the modification effect of Sc on the eutectic Si phase.In addition,the morphology of the ?(Al)in the heat-treated alloy changes little compared with that in the as-cast alloy.But the aspect ratio of the eutectic Si phase decreases,which is the result of the Si phase nodulizing in the process in heat treatment.The effect of combination addition of Sc and Zr on the structure of cast Al-Si-Mg alloy was studied.The results indicate that adding a small amount of Zr to the alloy with Sc content of 0.30 wt.% or 0.45 wt.% can greatly improve the optimization effect of Sc on the structure of the alloy.The addition content of 0.12 wt.%Zr can make the macrostructure of the alloys with two Sc contents be more α(Al)significantly refined.Moreover,the size of phase and SDAS in the alloys also all decrease further,and the morphology of α(Al)the phase is mainly equiaxed dendritic crystal.As the Zr content continues to be increased,the size of α(Al)the phase and SDAS of the alloys with two Sc contents do not obviously change and basically remain the same.For the eutectic Si phase,adding Zr can further improve the morphology of the eutectic Si phase of the alloys with two Sc contents,and make it show a finer vermicular or granular shape.And,with the increase of Zr content,the aspect ratio of the eutectic Si phase shows a trend of decreasing first and then increasing.When the Zr content is 0.21 wt.%,the aspect ratio of the eutectic Si phase of 0.30 wt.%Sc alloy is the minimum,while that of the eutectic Si phase of 0.45 wt.%Sc alloy is the minimum when the Zr content is 0.12 wt.%.Taken together,of Sc and to cast Al-Si-Mg alloythe combination addition Zr can enhance the grain ?(Al)phase and eutectic Si phase in refinement and modification effects of Sc on the the alloy.When the total addition content of Sc and Zr is of 0.50 wt.%-0.60 wt.%,the refinement and modification effects are the best.It make it possible to reduce the addition content of Sc by increasing the Zr content of the alloy appropriately.Similar the alloy with the alone addition of Sc,the effect of heat treatment on the ?(Al)to phase in the alloy with the combination addition of Sc and Zr is not big,but the aspect ratio of the eutectic Si phase decreases.The eutectic Si phase in the cast Al-Si-Mg alloy with the combination addition of Sc and Zr was observed and analyzed by transmission electron microscopy.It was found that there were a large number of twin crystals in the eutectic Si phase,which indicates that the modification of Sc and Zr elements to eutectic Si phase is an impurity induced twinning mechanism.The mechanical properties of the cast Al-Si-Mg alloy with the addition of Sc and Zr were investigated by tensile testing and hardness testing.The results show that,for the alloy with the alone addition of Sc,the hardness,yield strength,tensile strength and elongation of the as-cast and heat-treated alloys all increase first and then decrease with the increase of Sc content,and reach the highest when Sc content is 0.45 wt.%.For the alloy with the combination addition of Sc and Zr,with the increase of Zr content,the hardness,tensile strength,yield strength and elongation of the two kinds of the alloys containing 0.30 wt.%Sc and 0.45 wt.%Sc show a trend of increasing first and then decreasing.When the Zr content is of 0.21 wt.%,the comprehensive mechanical properties of the alloy containing 0.30wt%Sc are the best,while,the comprehensive mechanical properties of the alloy containing 0.45 wt.%Sc are the best when the Zr content is of 0.12 wt.%.These results are basically consistent with the evolution rule of the microstructure of the two kinds of the alloys with the change of Zr content. |
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