在不同凝固阶段合金的半固态组织和脉冲电流热效应的实验研究基础上,探讨了低压脉冲电流下AZ91D镁合金半固态组织的形成机制。结果表明:在合金的液相线以下开始脉冲放电,二次枝晶臂分离需要较长的时间,一次枝晶臂难以球化;从合金的液相线以上开始脉冲放电,不会析出粗大的枝晶,二次枝晶臂分离比较容易。脉冲电流降低了半固态合金的冷却速度,同时造成了合金熔体强烈的温度起伏;随着脉冲电压的增加,半固态合金的冷却速度逐渐减小,熔体的瞬时平均温升逐渐增加。初始形核率的提高和二次枝晶臂的根部熔断是低压脉冲电流下球状或颗粒状初生α-Mg形成的重要机制。 Based on the experimental study of semi-solid microstructure and pulse current thermal effect in different solidification stages, the formation mechanism of semi-solid AZ91D magnesium alloy under low-voltage pulse current was discussed. The results show that the pulsed discharge begins below the liquidus of the alloy and the secondary dendrite arm takes a long time to separate. Once the dendrite arm is hard to be spheroidized, the pulse discharge starts from above the liquidus of the alloy and no coarse Dendrites, secondary dendrite arm separation easier. The pulse current reduced the cooling rate of the semi-solid alloy and caused the strong temperature fluctuation of the alloy melt. With the increase of the pulse voltage, the cooling rate of the semi-solid alloy decreased gradually and the instantaneous average temperature rise of the melt gradually increased. The increase of initial nucleation rate and the root blown secondary dendrite arm are important mechanisms for the formation of spherical α-Mg or spherical primary α-Mg under low-voltage pulse current.