The microstructure characteristics of the defect band in magnesium alloy castings during the cold-chamber high pressure die casting（HPDC）process was investigated

and the corresponding relationship between the defect band and the externally solidified crystals（ESCs）

porosity and process parameters was established

based on which the formation and evolution mechanism of the defect band was discribed.The results reveal that the microstructure at the cross section of magnesium alloy die castings can be divided by the defect band into three regions with obvious differences in microstructure characteristics compared with each other.With the increase of the slow shot plunger velocity

the area fraction of the ECSs in the microstructure of die cast magnesium alloy is decreased gradually

and the defect band becomes more clear and its location is closer to the center of die castings.The width of the defect band is reduced

and the porosity inside the defect band presents more concentration.With the increase of the fast shot plunger velocity

the ESCs become more dispersed

and the defect band is closer to the central region of die castings.When there is absence of fast shot

double defect band can be observed.The morphology and distribution of the defect band in the microstructure of die cast magnesium alloy is determined by the metal flow pattern during the filling process.Under the intensive flush and intensification pressure of melt flow

the grains in the contour of the metal liquid would be fragmented and rotated

resulting in a gap whose volume is larger than the volume of remaining liquid among grains.During the solidification process

the defect band with aggregated porosity can form following the contour of melt flow.