Complex Modification Of In Situ Mg₂ Si/AM60B Composite And Microstructural Evolution During Partial Remelting

Si is one of the commonly used alloying elements in the Mg-Al and Mg-Zn based alloys, and Mg-Al-Si alloy is the oldest one of heat-resistant magnesium alloy. The solubility of Si in Mg is only 0.0003% and Si and Mg can generate Mg₂Si phase by in-situ reaction. Mg₂Si has high melting point, low density, high hardness and low coefficient of thermal expansion, and is an ideal reinforcement for magnesium alloys, Simultaneiously, in-situ composites with reinforcing phase generated from reaction have advantages of clean interface, high thermal stability, good compatibility with matrix, low manufacturing cost, is an important development direction of metal matrix composites. But Mg₂Si phase with Chinese characters greatly lacerate the matrix, which reduce the performance of the alloy at room temperature.In this thesis, the modification techqnique of Mg₂Si/AM60B magnesium matrix composite was studied by MgCO₃ and Sr. The effects of the modification on microstructure and mechanical properties was also studied. At the same time, the microstructural evolution of the modified composite was investigated.The results show that the in-situ Mg₂Si/AM60B composite can be produced by adding Al-Si master alloy into AM60B alloy. The number of Mg₂Si increases as the addition amount increases and the dendritic morphology become more developed. The addition of 0.5% Si at 790℃and subsequent hold for 30 min has good modification effect. The morphology of Mg₂Si phase changes from developed dendritic shape or Chinese characters to small polygonal particles. Then treated by 1.2%MgCO₃ at 790℃and held for 10min, the average grain size of matrix is decreased from 260μm to 80μm. Namely, the used complex modification technique has good effect.A semisolid microstructure avaible for thixoforming can be obtained after the modified Mg₂Si/AM60B being partially remelted. During partial melting, the microstructure evolution of the Mg₂Si particles is simple. They evolve from the original polygonal particle to spheroidal particles and at the same time they size also increases. The microstructure evolution of the matrix can be divided into four steps, the initial coarsening due to the dissolution of interdendritic eutectic, structure separation resulted from the melting of the residual eutectic and the penetration of the firstly formed liquid into the recrystallized boundaries, spheroidization due to the partial melting of primary particles and final coarsening attributed to the merging and Ostwald ripening. In addition The more finer the as-cast microstructure is,the more better and rounder the primary particles in the semi-solid microstructure is. Consequently, it is necessary that the as-cast microstructure must be refined to obtain the high-quality semi-solid microstructure.