Research On Composite Modification Of Primary Silicon And Eutectic Silicon In Hypereutectic Al-Si Alloy

The thick irregular primary silicon and coarse needle-like eutectic silicon existing in hypereutectic Al-Si alloy seriously destroy the mechanical properties of the alloy.In order to improve the mechanical properties of the hypereutectic Al-Si alloy, the alloy is used to be modified.In this paper,the content of modifier,the rating of the modifier,and other factors to the effect of the primary silicon and eutectic silicon size,shape,distribution on hypereutectic Al-Si alloy are studied from the perspective of reducing the environmental pollution and bringing the modifier effects into play.The study of modification process parameters and modification mechanism of hypereutectic Al-Si alloys provides some theoretical basis for industrial applications.The results of the experiment show that: To modifier Al-25%Si ally,0.8wt% Al-P alloy addition is at 820℃,and the average grain size of primary silicon phase is 22um.Al-Sr alloy addition is 0.8wt% at 800℃,the coarse needle-like eutectic silicon are subdivided into granular.In P+Sr complex modification,the eutectic silicon is well refined, but the effect is not as good as that of single modification with P as the effect of modification is affected by the join order of modifier.Primary silicon and eutectic silicon can be modified at the same time with RE modifier,but the refinement of primary silicon was not obvious. In Al-P-Sr-RE compound modification, primary silicon and eutectic silicon are well refined and the hardness and wear-resistance are well improved.At the same time,Al-P-Sr-RE compound modification has a good long-term degeneration and re-melting nature. After being insulated for 6h and remelted five times, the average size of the primary silicon is less than 60μm.By spectrum and XRD analysis,the modification of hypereutectic Al-25% Si alloy with Al-P shows that:(1)AlP,which is the primary crystal silicon in the core of heterogeneous nucleation,exists in the core of primary crystal silicon:( 2) a small amount of P distributes on the primary crystal silicon edge,which affects the growth of primary silicon and changes the morphology of primary silicon.Sr and RE by affecting the growth of silicon phase,and the modifier adsorbed on the surface of silicon phase hinder the growth of silicon phase and produce a large number of twins at the same time. Because the role of multiple twins, the growth of silicon phase is isotropic,which makes the silicon phase present granular morphology.During the process of Al-P-Sr compound modification, the interactions of P and Sr generate Sr3P2 phase,thus weaken the P metamorphic effect,but P mainly forms AlP phase and refines the early crystalline silicon;Sr oxidizes easily,so if Sr is added before P,part of the Sr oxidizes,which reduces the contents of Sr in solution. Therefore, more Sr is needed during the process than in the process of adding P before Sr.If P is added firstly,it has sufficient time for the formation of AlP before Sr is added and Sr can interact directly with AlP. However, if Sr is added firstly,the modification effect is enhanced because P reactes with aluminum first and there isn't enough time for the generation of Sr3P2. In order to further understand the morphology characteristics of hypereutectic Al-Si alloy, this paper also studies the five star primary silicon and flake primary silicon and preliminarily reveals their growth process.