The Key Techniques Of Casting Hypereutectic Al-si Alloy Piston

Hypereutectic Al-Si alloy which has small proportion,low coefficient of thermal expansion,good wear resistance and dimensional stability is an ideal piston material. In this paper, the Al-Si alloys piston used in automobile is studied. Measures are taken to refine the microstructure and improve the mechanical properties, such as components optimization,adding alloy elements,P+RE modification,T6 heat treatment and using the end riser feeding process.The research emphasis of this paper are as follows: By adding Ni, Mg, Cu and other trace elements simultaneously, optimizing the composition design and using P+RE complex modification treatment, we changed chemical composition of the Al-Si alloy, improved the organizational structure of the alloy effectively. Using advanced the end riser feeding process, we reduced the shrinkage defect at the top of pistons.The experimental results show that,a large number of five heart valve-like primary silicon existed in the alloy without modification, this growth mechanism of silicon is closely related with the cooling rate and external heterogeneous core; the effect of P+RE complex modification on both primary and eutectic silicon is distinct. The modification makes the coarse block primary silicon be refined to granular and the edges and angles of primary silicon are passivited. The eutectic silicon refined from needle to fibrous ones. Microstructure refined obviously. By properties testing, the addition of alloy components and complex modification make a great improvement in the mechanical properties of the piston, especially in the high-temperature tensile strength and the volume stability, the high-temperature tensile strength reached a maximum at the 18% content of silicon because of excess phase hardening; the coefficient of thermal expansion decreasing with the addition of content of silicon, while the volume stability increasing.Finally, the end riser feeding casting molding process is simulated by ProCAST software. The simulating results indicate that the traditional side riser feeding, can not eliminated Shrinkage of the heavy top of piston ,while the advanced end riser process makes the piston a dense macro-organizational structure, at the same time, this process raises dimensional stability of high temperature and other comprehensive performances of piston effectively, eliminates the shrinkage porosity defects of pin boss and the top of piston,improves the process production rate and reduces the cost of production. The result is consistent with the previous theoretical derivation.