低频电磁铸造Al-4.5%Cu合金微观偏析研究

利用直接水冷半连续(常规DC)铸造工艺和低频电磁铸造(LFEC)工艺分别制备了Al-4.5%Cu(质量分数)合金铸锭.测量了常规DC铸造和LFEC过程中的温度曲线, 研究了低频电磁场条件下的铸锭微观组织变化和Cu元素的微观偏析. 通过金相观察发现, 在低频电磁场作用下, α-Al和$theta$相的共晶组织变得细小, 其面积分数明显减小. 利用电子探针测量结果绘制Cu元素成分曲线, 发现在凝固的最初过渡区, Cu元素的成分曲线在低频电磁场作用下升高, α-Al中Cu元素含量增加. 通过计算得到Cu元素的有效分配系数k_e, 发现在最初过渡区k_e线性增大到1. LFEC工艺使得Cu元素的k_e变大, 并且随着电流强度的增强这一趋势愈发明显. 由于低频电磁场加速了凝固前沿的冷却速度, 使更多的Cu原子固溶在 α-Al中, Cu元素在铸锭中的微观偏析得到了改善.

INVESTIGATION ON MICROSEGREGATION OF Al-4.5%Cu ALLOY PRODUCED BY LOW FREQUENCY ELECTROMAGNETIC CASTING

Microsegregation caused by the redistribution of alloying elements means the inhomogeneity of the chemical composition on the scale of a single grain. To decrease the degree of microsegregation is beneficial to avoiding ingots hot crack and shortening the homogenizing treatment time. In this study, the microsegregation of Cu element in the Al-4.5%Cu (mass fraction) alloy was investigated. Al-4.5%Cu ingots with diameter of 200 mm were prepared by the processes of conventional direct chill (DC) casting and low frequency electromagnetic casting (LFEC), respectively. The temperatures were measured during solidification processes. The effects of low frequency electromagnetic field on the microstructures and microsegregation of Cu element were investigated from eutectic analysis and electro probe microanalysis (EPMA). By the OM observations, it was found that the area fraction and dimensions of eutectic decreased markedly. The concentration profiles of Cu element were obtained with EPMA results, which showed that the profiles increased (i.e. the content of Cu element in α-Al phase increased) in the initial transit region due to the effect of low frequency electromagnetic field. The effective distribution coefficient k_e of Cu element was calculated. In the initial transit region, k_e increased to unit linearly. At the same time, the low frequency electromagnetic field made k_e a bigger value, and the value increased with the increasing of the current intensity. Because the cooling rate at the solidification front was promoted by the low frequency electromagnetic field, and there was more Cu element in α-Al, the microsegregation of Cu element in the ingot was alleviated by the LFEC process.