Formation Mechanism Of Solidification Microstructure Without Eutectic Si In Al-Si Alloy

There are two fundamental reasons for the lack of progress in improving the ductility of hypereutectic Al-Si alloy.First,the primary silicon is not fine enough.Second,the number of?Al phases between the primary silicon is small,and the number of lamellar eutectic silicon phases is very large.Therefore,the method refining primary silicon while simultaneously obtaining a single?_All matrix is effective to improve the ductility of hypereutectic Al-Si alloy.This paper took Al-20%Si alloy,Al-10.5%Si alloy and Al-5%Si alloy as the research object.The method by intense stirring the melt during Cu-P alloy modification and controling the cooling rate during solidification was used.The change in the silicon phase size with cooling rate and modified treatment parameters was determined by examining the change in the solidification microstructures.Finally,the formation mechanism of the solidification microstructure without eutectic Si in hypereutectic Al-Si alloy was analyzed.It was found that P entering the melt of Al-Si alloy not only had modified effect on primary silicon,but also could change the morphology and size of eutectic silicon under certain conditions,and even inhibited the formation of eutectic silicon.The results show that intense stirring the melt during Cu-P alloy modification when the temperature is 973K facilitates forming of fine AlP that provide strong conditions for nucleation of Si phase.Three conditions under which eutectic Si disappears and fine primary silicon is uniformly distributed in the?Al matrix in hypereutectic Al-Si alloy were determined.Those are faster cooling rate,strong nucleation ability of silicon during the solidification and the silicon concentration is more than 10%in Al-Si alloy.Solidification microstructures of Al-20% Si alloy and Al-10.5%Si in which eutectic Si disappears and fine primary silicon is uniformly distributed in an?_All matrix were obtained after intense stirring the melt during Cu-P alloy modification when the cooling rate was 16.3K/s.In Al-20%Si alloy,the modification effect is the best when the modification temperature is 973K,and the refining rate of primary silicon and eutectic silicon can reach up to 53.7%and 100%.In Al-20%Si alloy,when the cooling rate is slow,the refining effect of primary silicon is higher than that of eutectic silicon;when the cooling rate is fast,the refining effect of eutectic silicon is higher than that of primary silicon;the difference between the two is related to the modification process,and the difference is larger when the modification temperature is 973K.In Al-10.5%Si alloy,the influence of stirring speed on silicon phase size is greater than that of stirring time.