Study on the Combined Modification Mechanism of Pand Spand Microstructure Control in Hypereutectic Al–Si Alloys

As traditional hypereutectic Al–Si alloy modifiers, P and Sr can effectively improve the comprehensive mechanical properties of alloys when the alloy is double modified, with the interaction between these composite modifiers affecting the modification effect of the alloy. Therefore, in this study, the transformation of the AlP to Sr3P2 phase during P and Sr composite metamorphism was predicted using first-principles calculations based on density functional theory. In addition, Al–15%Si alloys with P and Sr single and composite metamorphism were prepared by gravity casting, and the metamorphism effects were evaluated by microstructure observations and mechanical property testing. The results showed that the Sr3P2 phase was more stable than the AlP phase. The experimental results demonstrated that the Sr3P2 phase was present in the composite alloy after modification and that the modification effect of primary silicon and eutectic silicon was not ideal. The composite metamorphism improved the comprehensive mechanical properties of the alloy more obviously than single metamorphism, but it did not reach the superposition of single metamorphism mechanical properties, in which the alloy after the addition of Sr and P had the best properties, and the tensile strength, elongation, and hardness reached 301.5 MPa, 0.87%, and 127.8 HV, respectively. Therefore, this method offers an effective strategy for optimizing the modification treatment of hypereutectic Al–Si alloys.