The Effects Of Phosphorus Modification On The Freezing Interface Temperature And Solidification Structure Of Hypereutectic Al-16Si Alloy In DC Casting

Hypereutectic Al-Si alloys have been widely used in automotive applications, especially in the piston industry, because of their excellent abrasion and corrosion resistance, low coefficient of thermal expansion and high strength-to-weight ratio. To improve the structures of hypereutectic Al-Si alloys, there were many methods having been used successfully, such as rapid cooling, low temperature casting and chemical modification and so on, especially phosphorus modification. However, the effects of phosphorus modification on the freezing interface temperature have not been studied.In this work, in-situ heat measurements were performed on the Al-16Si alloy during slow cooling and direct chill (DC) casting. Cooling curves and first-order differential cooling curves of Al-16Si alloy were obtained in different casting conditions. Analyses on the cooling curves, first-order differential curves and solidification structure in different casting conditions were carried out to find the effects of phosphorus modification on the freezing interface temperature and solidification structure of Al-Si alloys. The primary conclusions that may be derived from this work are as follows:(1) In the slow cooling, after modification, the equilibrium solidification of primary silicon decreases4.05℃, the nucleation and growth temperature of eutectic has no changes, and the size of primary silicon reduces significantly, but the eutectic has a slight coarse.(2) In the DC casting, after modification, the nucleation temperature of primary silicon in the same position decreases3.05～9.39℃, the growth temperature of eutectic almost maintains between570and573℃, the temperature gradient of the freezing interface goes up. Besides, there are shallow solid-liquid interface, narrow and uniform two-phase region and bad ingot surface after modification.(3) About the changes of DC casting microstructures, after modification, we would find that the size of primary Si particles decreases greatly, the number of primary Si particles increases greatly, the distribution of primary Si particles become more uniform, and the a-Al dendritic is finer, and the difference of the two sides microstructures and the center microstructures decreases to a large extent.(4) Phosphorus modification can increase the thermal gradient of the freezing interface during DC casting, and decrease the solidification temperature of primary Si. The solidification ending temperature, i.e. the eutectic temperature does not change.