Research On Phase Equilibrium Of The Al-Si-P Ternary System And Melt-treatment Of Hypereutectic Al-Si Alloy

Hypereutectic Al-Si alloy has been widely used because of its good mechanical properties. Primary Si in the casting structure could improve the wearing resistance properties because of its higher hardness. However, the polygonal primary Si would cause stress concentration, which will lead to the decline of mechanical properties of the alloy. Modification and melt-treatment are generally used to improve mechanical properties of hypereutectic Al-Si alloy. So far, the best modifier is Al-P and Al-Si-P master alloy. However, the explanation about the mechanism of modifying primary Si with P by Al-Si-P ternary phase diagram are not found. On melt-treatment of hypereutectic Al-Si alloy, the optimal parameters on over-heating temperature, holding time and cooling velocity have not been reported.In order to obtain the phase equilibrium information of Al-Si-P ternary system, equilibrating alloy method, SEM-EDS analysis technology and X-Ray analysis technology were used. The phase equilibrium of Al-Si-P ternary system at 550℃and 700℃was determined. The results show that the three-phase region named Si+SiP+AlP exists at both 550℃and 700℃isothermal sections, andα-Al+Si+AlP at 550℃and L+Si+AlP at 700℃, respectively. What's more, a two-phase region named L+AlP was discovered at 700℃.For the purpose of finding out the effect of different melt-treatment process on solidification microstructure of hypereutectic Al-Si alloy, the microstructure of the alloy prepared by casting at different temperature, water-quenching at room temperature, rapidly cooling at copper mould and dry quenching at room temperature respectively were studied. Following conclusions have been obtained:1) The particle size of primary Si in Al-21wt.%Si alloy gradually decreases with the increase of casting temperature in the range of 660 ~ 780℃. The finest as-cast microstructure was obtained when casting temperature was at 780℃.2) The solution rate of primary Si in Al-21wt.%Si alloy melt increases with the increasing of overheating temperature. When the overheating temperature is high enough and the cooling rate is rapid enough, the primary Si and eutectic Si could be refined effectively and the morphology of the primary Si changed gradually to the globular one. The structural heredity could be eliminated by overheating the melt at high temperature and then cooling it at rapid velocity. However, when the overheating temperature is low or the cooling rate is slow, the structural heredity of Al-21wt.%Si could not be eliminated even the holding time at high temperature was considerably long.