中文摘要：

在AZ91D镁合金真空压铸过程中使用2种分段慢压射速度（两段和三段）。在充型之前型腔内真空压力变化情况通过真空传感器进行监测。压铸件组织通过光学显微镜和图像分析软件进行分析。结果表明，采用三段慢压射速度时充型时刻型腔真空压力明显降低，导致所得压铸件中气孔含量很低。通过恰当选用分段慢压射速度，在充型之前金属液在压室中的停留时间可缩短，在慢压射过程后期阶段金属液在压室中的流动也将受到抑制，这些因素可导致所得压铸件中压室预结晶组织含量较低。

英文摘要：

Two multi-step （two-step and three-step） slow shot speeds were used in the vacuum die casting process of AZ91D magnesium alloy. The vacuum pressure variation in the die cavity before mold filling was monitored by using a pressure sensor. The microstructures of the produced castings were analyzed with optical microscope and image analysis software. The experimental results demonstrate that, the vacuum pressure in the die cavity at the beginning of mold filling is significantly reduced by using three-step slow shot speed, resulting in a low gas porosity level in the produced castings. At an appropriate multi-step slow shot speed, the dwell time of the liquid metal in the shot sleeve before mold filling can be reduced and the flow of the liquid metal in the shot sleeve at the later stage of the slow shot process can be restrained, which cause a low externally solidified crystal content in the produced castings.