Effect Of Alloying Elements On Microstructure And Properties Of Hypoeutectic Al-Si Alloy With Fe Contain

As a typical aluminum alloy,Al-Si alloy is widely used in automobile,aerospace and other fields.Fe is considered to be an important harmful element in Al-Si alloy.The properties of long needle-like Fe phase will cause serious damage to Al-Si alloy.However,there is no clear conclusion on how to weaken the harmful effect of Fe on Al-Si alloy.Although Fe element is harmful to Al-Si alloy,adding appropriate amount of Fe element into Al-Si alloy can significantly increase the demoulding ability of castings,especially for castings with complex structure,which can effectively avoid the occurrence of die sticking.Therefore,in actual industrial production,a certain amount of Fe element is often added to the alloy artificially.Therefore,it is necessary to find a way to ensure that the casting can be demoulded smoothly after molding,and to weaken the adverse effects of long needle-like Fe phase.The most common way is to add master alloy elements,and different alloy elements have different mechanisms.Therefore,it is necessary to explore the influence of alloy elements on the microstructure and properties of alloy,so as to select different methods according to different requirements in actual production.In this paper,based on Al-8Si-0.1Cu-0.6Mg-0.7Fe alloy,the effects of Mn,Ce,Mn and Ce compound addition and V element on the microstructure and properties of the alloy were discussed,and the influence rule and mechanism of different elements were revealed.On this basis,the effect of Mn/V substituting Fe on the alloy was studied to ensure the total amount of alloy elements unchanged.Finally,the alloy with high performance was selected for T6 heat treatment to study the improvement degree of heat treatment on the properties of the alloy.The main findings are as follows:（1）Mn element can transform the long needle-likeβ-Al₅Fe Si phase into Chinese-character-likeα-Al₁₅（Fe Mn）₃Si₂ phase,and improve the Fe phase obviously.When Mn/Fe=0.8,the mechanical properties of the alloy reach the best,especially for the strength,its yield strength and tensile strength increase by 33.6%and 20.2%.However,the thermal conductivity of the alloy decreases greatly with the addition of Mn first,and then increases and then decreases with the increase of Mn/Fe,but the increase range is small.As for Ce element,it can cut off theβ-Fe phase in the alloy,but it will not change theβ-Fe phase intoα-Fe phase.When the content of Ce is 0.4 wt.%,the average length of Fe phase can be reduced to 7.19μm.when the content of Ce is 0.7 wt.%or more,massive Al-Ce compound will appear.The mechanical properties of the alloy first increase and then decrease with the increase of Ce content,and reach the best when Ce content is 0.4 wt.%.The thermal conductivity of the alloy decreases continuously with the increase of Ce content,but the decrease range is small,and the maximum decrease is only 5.10%,which is far less than the effect of Mn on the thermal conductivity of the alloy.Although,the addition of Mn and Ce can also refine theβ-Fe phase in the alloy,it is better than that of the addition of Ce alone and weaker than that of Mn alone,and the Ce content of the appearance of Al-Ce compound is lower than that of Ce alone.The thermal conductivity of the alloy is almost the same as that of the alloy with Mn Addition.（2）The addition of V can improve the Fe phase in Al-8Si-0.1Cu-0.6Mg-0.7Fe alloy obviously.Different from Mn,V can transform the long needle likeβ-Fe phase intoα-Al₈Fe₂Si.When the content of V is 0.6 wt.%,the improvement effect on Fe phase is the best,and the mechanical properties of the alloy are the best,especially for the elongation of the alloy,the increase rate can reach 73.1%.However,the thermal conductivity of the alloy decreases greatly with the addition of V element,after that,there will be a small increase with the increase of V content,but the increase is limited.（3）With the decrease of Fe and the increase of Mn,the amount of Fe phase decreases gradually,and the shape of it changes from Chinese character shape to short rod shape,and finally to granular shape.When the content of Fe and Mn is 0.25 wt.%and 0.45 wt.%respectively,the mechanical properties of the alloy reach the maximum,its yield strength,tensile strength and elongation are 122.4MPa,242.6MPa and 9.44%respectively However,the thermal conductivity continues to decrease,and the speed is faster and faster.And with the decrease of Fe and the increase of V in the alloy,the amount of Fe phase decreases obviously,and the shape of Fe phase changes from plum blossom shape to block shape,and finally to granular shape.When the content of Fe and V is 0.6 wt.%and 0.1 wt.%,the strength of the alloy reaches the maximum,its yield strength,tensile strength and elongation are 133 MPa,249.3 MPa and 10%,When the content of Fe and V is 0.25 wt.%and 0.45wt.%,its yield strength,tensile strength and elongation are 120.2MPa,238.7MPa and 12.4%.The thermal conductivity of the alloy shows a continuous decreasing trend,and the decreasing speed is first fast,then slow,and finally tends to be stable.（4）After T6 heat treatment,the Fe phase of the alloy will not change obviously,but the eutectic silicon in the alloy will spheroidize obviously.Heat treatment can significantly improve the strength,the yield strength increases in 100-150MPa,the tensile strength increases in 60-100MPa,but the elongation decreases by 15%-45%.And the thermal conductivity will increase after heat treatment because of the"purification"effect of aging treatment on the alloy matrix.From the results of heat treatment,Al-8Si-0.1Cu-0.6Mg-0.6Fe-0.1V has the best comprehensive properties.Its yield strength,tensile strength,elongation and thermal conductivity are 241.5MPa,321.2MPa,8.94%and158.9W/（m·K）.