采用不同凝固阶段施加低压交流电脉冲以及在模具中嵌入不同直径金属网限制熔体对流的方法,研究了低压交流电脉冲细化Al-7%Si合金宏观凝固组织的机理。结果表明,低压交流电脉冲处理620 ℃以上的过热熔体以及在α-Al初生相生长后半阶段施加电脉冲均不会细化合金晶粒,晶粒细化主要发生在α-Al初生相形核阶段和生长前半阶段。金属网内外晶粒均得到了细化,但网外晶粒更细小,且网内微观组织为发达的树枝晶,网外为蔷薇状组织。低压交流电脉冲孕育效应及Joule热效应不会对Al-7%Si合金晶粒细化产生明显影响,合金晶粒细化主要是电磁力诱导的熔体流动引起晶核增殖和脉冲电流作用下形核动力学改变而引起形核率增加协同作用的结果。
The grain refinement mechanism of hypoeutectic Al-7%Si alloy under low voltage alternating current pulse (LACP) has been investigated in this work. In which LACP generated by the homemade low voltage modulation pulse generator is imposed in different solidification stages of the alloy and the wire mesh tubes of different diameters which have the effect of limiting the melt convection is embedded in sand mould. The experimental results show that the grains of casting alloy are evenly refined under LACP. The grain refinement will not appear, when LACP is imposed in the stages which are the alloy melt temperature is above 620 ℃ and late stages of crystal growth of primary phase. The grain refinement mainly occurs in nucleation stage and early growth stage of primary phase. The grains of inside and outside of wire mesh tube are refined together under LACP. But the grains of outside of wire mesh tube are much finer. The solidification microstructure of outside of wire mesh tube changes from large dendritic crystal to rose-shape crystal, nevertheless, it is still large dendritic crystal inside of wire mesh tube. Inocu lation effect and Joule heat effect of LACP have little effect on grain refinement of Al-7%Si alloy. The main reasons of grain refinement are the embryos fell off from chilling walls under LACP and the nucleation kinetics of the alloy was changed by LACP, which cause the nucleation rate of alloy increased. In addition, the α-dendrites became fragmentation under the forced melt flow which was induced by electromagnetic force can also lead to the grain refinement during the early growth stage of primary phase.
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