熔体过热处理对AZ91D镁合金组织与性能的影响

Influences of Melt Superheating Treatment on Microstructure and Properties of AZ91D Magnesium Alloy

采用自制的电阻炉研究熔体过热温度对AZ91D镁合金凝固组织和力学性能的影响规律。结果表明:AZ91D镁合金的铸态组织随着熔体过热温度的提高由树枝晶形态向等轴晶转变,晶粒尺寸逐渐减小,超过850℃后晶粒尺寸变化不大。AZ91D镁合金的抗拉强度、条件屈服强度和伸长率均随熔体过热温度的提高呈先增大后减小的变化趋势,力学性能在850℃时达到最大值。DSC分析表明,提高熔体温度使凝固开始点温度降低,凝固区间缩小,临界晶核半径减小,增加了熔体中的过冷度,提高了熔体中非均匀形核率,是镁合金晶粒细化的主要原因。熔体过热温度的提高导致共晶温度提高,使共晶相粗大。

The influences of the melt superheating temperature on the solidification structure and mechanical properties of AZ91 D magnesium were studied by a home-made electrical resistance furnace. The results show that with the increase of melt superheating temperature, the as-cast structure of AZ91 D alloy changes from dendrite morphology into equiaxed grain morphology, the grain size gradually decreases, while the change of the grain size is not obvious when the melt superheating temperature exceeds 850 °C. With the enhancement of melt superheating temperature, the tensile strength, offset yield strength and specific elongation increase firstly and then decrease. The mechanical properties of AZ91 D reach the maximum at 850 °C. The analysis of DSC shows that increasing the melt superheating temperature reduces the beginning solidification temperature, shrinks the freezing range, decreases the critical nucleus radius, increases the degree of supercooling of the melt and enhances the heterogeneous nucleation rate, which are the primary reason of grain refinement in magnesium alloy. The higher superheating temperature increases the eutectic transformation temperature, and hence the eutectic phase becomes coarse.