Modification Evolution Of Eutectic Silicon In AlSi7Mg Alloy And Micro-Mechanism

Modification of eutectic silicon in hypoeutectic Al-Si alloy is the key method to improve the castability and mechanical properties of such alloys. Modification of eutectic silicon is mainly affected by the nucleation and growth process of eutectic silicon during solidification. Furthermore, the nucleation and growth process of eutectic silicon is closely related to the solidification process. The morphologies of eutectic silicon with different modifying agents and at different cooling rates were investigated, and the interactions of modifying agent and cooling rate on modification of eutectic silicon were studied. The solidification was analyzed by thermal analysis and quenching analysis. The nucleation mechanism and growth mechanism of modification were studied and discussed.The morphology of eutectic silicon results showed that, at low cooling rate conditions, the unmodified eutectic silicon exhibits a coarse plate-like morphology, and Sr modified eutectic silicon is a typical fibrous morphology, while the Sb modified eutectic silicon exhibits a typical refined lamellar structure and along with clusters of rutectic silicon. The modification behavior of rare earth elements(Y and Yb) is similar to that of Sb, results in a typical refined lamellar morphology. At low cooling rate, the optimal dosage of Sb, Sr, Y and Yb modifying agent in AlSi7Mg alloy is 0.2wt.%, 0.04wt.%, 0.3wt.% and 0.3wt.%, respectively.The research on the modification of eutectic silicon at low cooloing rate indicated that the mixed enthalpy and atomic radius ratio of mofifying agent is not the sole basis of the modification. The cooling rate during solidification and compound forming ability of mofifying agent has significant effect on the modification of eutectic silicon. Meanwhile, the little influence on the nucleation of primaryα(Al) and the significant restriction of the nucleation of eutectic silicon, resulting in the increase of solidification range, as a result, the grain size increases after modification. In addition, modification of eutectic silicon promotes the transition of primaryα(Al) dendrites to the columnar grain, and the reduce of interfacial energy results in the decrese of the secondary arm spacing.The results of cooling rate and modifying agents on the modification of eutectic silicon showed that the modification of eutectic silicon is significantly influenced by the cooling rate. For the unmodified eutectic silicon, the large cooling rate can promote the transition of eutectic silicon from a coarse plate to a fibrous structure, resulting in modification. The morphology transition of the Sb modified eutectic silicon with different cooling rates is similar to that of the unmodified eutectic silicon, but the critical cooling rate is smaller compared to that of the unmodified eutectic silicon. The effect of cooling rate on the Sr modified eutectic silicon is less, the morphology of eutectic silicon after Sr modification at low cooling rate exhibits a fibrous one, and the structure of eutectic silicon transforms to a refined spike-like one at the high cooling rate. However, the Y or Yb modification of eutectic silicon is significantly influenced by the cooling rate. At low cooling rate, the modification of Y or Yb results in a typical refined lamellar structure, instead, the morphology of eutectic silicon exhibits a fibrous or spike-like structure at high coolong rate, indicating that the modification of these two modifying agents are strongly dependent on the cooling rate during solidification. Based on the above results, the interaction of cooling rate and modifying agents on multiple parameters selection polt of eutectic silicon is established.The thermal analysis and quenching analysis results showed that, besides Sb, all the modification of Sr, Y and Yb significantly depress the eutectic temperature, however, the drop of eutectic temperature is not directly related to the modification of eutectic silicon. The drop of the nucleation and growth temperature in the rare earth element(Y or Yb) modified eutectic silicon has played a not decisive role in the morphology of eutectic silicon.Modifying agent itself and the AlSb, Al2Si₂Sr, Al2Si₂Y and Al2Si₂Yb phases formed in the modification can not be the heterogeneous nuclear particles of eutectic silicon. Modification of eutectic silicon can remove or poison the heterogeneous nuclear particles of eutectic silicon, and the growth of eutectic silicon under larger undercooling results in a refined lamellar structure. Whereas, the calculation results showed that the increased interface velocity caused by reduced nucleation is not sufficient to cause a plate-fibrous transition. The transition of eutectic silicon morphology occurs mainly in the growth stage of eutectic silicon, and the solidification is the key link of control during modification.The research on the growth of eutectic silicon with and without modification revealed that, the growth mechanism of the unmodified eutectic silicon is the intrinsic steps in the solid-liquid interface, and the eutectic silicon exhibits obvious growth edges. With the increase of cooling rate, the eutectic silicon branches in-plane firstly, and a second stage involves the initiation of out-of-plane rod growth and finally results in a fibrous structure. The growth behavior of the Sb modified eutectic silicon is similar to that of the unmodified eutectic silicon, and the refinement of the Sb modified eutectic silicon is mainly achieved by the hindrance of AlSb phase on the lateral growth of eutectic silicon. The growth mechanism of eutectic silicon is significantly affected by Sr modification, and the mechanism of intrinsic steps in the solid-liquid interface has given way to the impurity induced twinning mechanism. The absorbtion of Sr atoms onto the solid-liquid interface induce the change of Si atoms stacking order, and a high density twinning changes the growth mechanism of eutectic silicon. At low cooling rate, the rare earth element (Y or Yb) is very prone to form rare earth bearing compound, and the growth mechanism of eutectic silicon is mainly the intrinsic steps in the solid-liquid interface, resulting in a refined lamellar structure. When the cooling rate reaches the critical cooling rate, the growth mechanism of eutectic silicon changes to the impurity induced twinning mechanism.