高硅含量过共晶铝硅合金半固态重熔组织演变

摘要

The microstructure evolution of casting Al-25%Si alloy during semi-solid remelting was studied by the water quenching experiment, and the quantitative statistics and characterization of the size, shape and volume fraction of the primary Si phase were conducted. The results show that the microstructure evolution includes three stages within the process of heating insulation above the eutectic temperature, i.e., the eutectic Si phases getting granulated and dissolving, the primary Si phases breaking and the sharp corner passivating and the shape tending to be round. The size of primary Si phases is coarsened through the Oswald ripening, its shape factor increases and the volume fraction reduces. The alloy heating to 590-600 ℃ and holding for 30-50 min has a low size coarsening rate (K) of primary Si phase with round shape and controllable volume fraction, which satisfies the requirement of the semi-solid processing. The size coarsening rate constant K depends on the solidification velocity and heating temperature. The alloy solidified in the steel mold has high K value of 23.83-38.88μm3/s. Moreover, the K value decreases with increasing temperature. While the K value of the alloy solidified in the copper mold is lower and equal to 10.91-19.87 μm3/s, which increases with increasing temperature.%采用水淬实验研究熔铸法制备的Al-25%Si合金半固态重熔组织演变，对初生硅相的尺寸、形态和体积分数进行定量统计和表征。结果显示：在共晶温度以上保温过程中，合金组织经历了共晶硅的粒状化和溶解、初生硅相熔断和尖角钝化以及形态圆整化3个阶段。初生硅相在重熔过程中发生“Oswald”熟化粗化，尺寸增加，同时形状因子增大，体积分数减小。合金在590~600℃保温30~50 min，初生硅尺寸粗化速率缓慢，形态圆整并且体积分数可控，可以满足半固态加工要求。尺寸粗化速率常数K与合金的初始凝固冷却速度和加热温度有关。其中，在钢模中凝固的合金的K值为23.83~38.88μm3/s ，且随加热温度升高， K值减小；铜模中凝固的合金K值为10.91~19.87μm3/s，温度升高，K值增大。