Preparation And Study On Microstructure And Properties Of Al-30%Si Alloy

Hypereutectic Al-Si alloy with high Si content has the characteristics of low thermal expansion coefficient,good thermal conductivity and good mechanical properties.The preparation of Al-Si alloy with high Si content by melting-casting method has the advantages of simple process,low cost and high material density.However,with the increase of Si content,a large number of coarse primary silicon and slender acicular eutectic silicon appear in the alloy,which severely cracks the matrix and reduces the properties of the alloy.Therefore,it is necessary to take effective measures to improve the microstructures of the alloys in order to improve the mechanical and physical properties of the alloys.In this paper,Al-30%Si alloys were prepared by melting-casting method.The growth pattern of primary and eutectic silicon in as-cast state and the effects of overheat treatment,melt miscibility treatment,modification treatment and solid solution treatment on the microstructures of Al-30%Si alloys were studied.The effects of different treatments on the bending strength,thermal conductivity and thermal expansion coefficient of Al-30%Si alloy were studied.The results and conclusions are as follows:(1)In the solidification process of Al-30%Si alloy,the growth mechanism of TPRE(twin concave angle mechanism)is observed in both plate and five-petal star primary silicon in the microstructure.,and the growth and morphology of Si phase are influenced by its growth environment(such as temperature,concentration and impurities).(2)The effect of superheat treatment from 200 to 500 ? on solidification structure of Al-30%Si alloy was studied.The results show that with the increase of superheat,the average size of primary silicon decreases.But the shape of primary silicon is still plate-like and five-petal star-like,while the size and morphology of eutectic silicon remain basically unchanged.(3)The effect of melt miscibility at different temperatures on solidification structure of Al-30%Si alloy was studied.The temperature of high-temperature melt was1350 ?,and the temperature range of low-temperature melt was from 740 ? to 820 ?.The results show that melt miscibility treatment at high and low temperatures has limited effect on improving the structure of the alloy.The effect is better when the melt temperature is 760 ?.The average size of primary silicon decreases,and the five-petal star-shaped primary silicon disappears basically,but the size and morphology of eutectic silicon remain unchanged.(4)Al-3%P master alloy was used to modify Al-30%Si alloy.The results show that P element has significant refining effect on the primary silicon of Al-30%Si alloy.When P content is 0.15%,the refining effect is the best.The average size of primary silicon is reduced to 33.2 ?m.The morphology of eutectic silicon is changed from plate and five-petal star to polygonal,and the average size of eutectic silicon is also reduced relatively,but the morphology is still needle-like.Moreover,after modification with0.15% P,the bending strength of the alloy increases from 125.76 MPa to 207.53 MPa,and the thermal conductivity increases from 121.67 W/(m k)to 175.21 W/(m k),but the change of thermal expansion coefficient is small.(5)The effect of solid solution treatment on the microstructure of Al-30%Si alloy after P modification was studied.The primary silicon and eutectic silicon were heated and kept warm by single-step solution treatment with only one heating and two-step solution treatment with twice heating respectively.The results showed that the morphology of primary silicon and eutectic silicon could be improved to a certain extent by using single-step solution treatment at 530 ? for 6 h,and the spine angle of primary silicon was basically the same.The morphology of eutectic silicon tends to be rounded,and the eutectic silicon changes from fine needle-like to short rod-like.However,the morphology of primary silicon can be changed from polygonal block to nearly spherical by two-step solution treatment at 570 ? for 2h+530 ? for 2 h.Moreover,the morphology and distribution of eutectic silicon are further improved,and the eutectic silicon is transformed into fine spherical particles,which are evenly dispersed in Al matrix.At the same time,the bending strength of the modified alloy can be further improved by solution treatment.The bending strength of the modified alloy can be increased to 218.31 MPa after single-step solution treatment(530 ? for 6 h),and232.45 MPa after two-step solution treatment(570 ? for 2h+530 ? for 2 h).