中文摘要：

为了了解电磁场对组织细化作用的机理，采用低频电磁铸造方法制备直径200mmAl-Zn—Mg—Cu-Zr合金铸锭，并对铸造过程中的温度场进行测量。结果表明：施加磁场产生的强制对流使得温度场均匀且低于液相线约6℃。结晶器中熔体温度场的变化显著改变了熔体从开始浇注到完全凝固的热历史，从而有效促进了异质形核，显著减少晶核的重熔，使更多晶核能够生存下来，并最终促进形成均匀、细小的微观组织。

英文摘要：

In order to understand the mechanism of grain refinement achieved by electromagnetic field, ingots of an Al-Zn-Mg-Cu-Zr alloy with diameter of 200 mm were cast and the temperature field was measured during the casting process. The results show that the electromagnetic field can induce the forced convection, leading to a quite uniform temperature field and a low temperature （6℃ bellow the liquidus）. Therefore, the thermal history of the melt from pouring to being solidified is changed obviously, resulting in an enhanced heterogeneous nucleation and remarkable decrease of the remelting of the formed nuclei and the formation of fine and uniform spherical equiaxed microstructure.