触变成形AZ91D镁合金的组织与二次凝固行为

Microstructural characteristic and secondary solidification behavior of AZ91D alloy prepared by thixoforming

采用光学显微镜和扫描电镜对AZ91D镁合金组织进行分析,研究AZ91D镁合金在半固态触变成形过程中的组织特征与二次凝固行为。结果表明,成形件的充型过程是按先底部后上部的顺序填充。触变成形件中心线部位的组织中初生相含量比边缘部位的高,产生宏观上的液相偏析。沿高度方向上,中心线处的初生相颗粒尺寸从上部到底部,先减小后增大。二次凝固过程包括3个阶段:第1阶段是初生α-Mg相生长;第2阶段是液相内部重新形核并生长成细小的等轴晶;第3阶段是剩余的液相发生共晶反应形成共晶体。共晶反应按离异方式生长,共晶组织中的Mg17Al12在初生相颗粒周围形成"晕圈"。

Microstructural characteristic and secondary solidification behavior of the remaining liquid in semi-solid AZ91D alloy slurry in the thixoforming process were investigated by the experiments and microstructure analysis using optical microscope （OM） and scanning electron microscope （SEM）. The results show that the mould-filling process of the product is from the bottom to the top orderly. There is macro-segregation of liquid between the center and edge of the products whose primary phase fracture at the center is higher than that at the edge. The size of primary phase particles in the formed product decreases firstly, and then increases along the axial （central） line from header to bottom. The secondary solidification occurs for the remaining liquid inside the die cavity involving： the secondary primary dendrites first directly developed from the surface of the primary particles without re-nucleation; then the residual liquid solidified by re-nucleation/growth, and finally eutectic reaction. The eutectic reaction is carried out with the way of divorced growth model so that a halo of Mg17Al12 forms around the primary phase particle.