Abstract:The externally solidified crystals (ESCs) and divorced eutectics in die castings of magnesium alloy have a great influence on the final performance of castings. Under the cold-chamber high pressure die casting (HPDC) process, samples were produced at different slow shot phase and fast shot phase plunger velocities to investigate the microstructure characteristics of AM60B magnesium alloy. Based on the analysis and statistics of the experimental results obtained by using OM, EBSD, XRD, SEM and EDS, it is concluded that at the slow shot phase of the HPDC process, nucleation and crystals growth occur in the melt in the shot sleeve due to the loss of superheat; the ESCs are migrating to the central region of the castings during the fast shot phase of the melt into the die cavity at high speed, which lead to a higher distribution of the ESCs at the central region than the surface layer of the castings. Besides, the percentage of the ESCs is higher at a lower slow shot phase plunger velocity, and also is higher at the area near the gate than that of the area far from the gate of the castings. Coarse dendrites are observed at the cross section of the castings at a lower fast shot phase plunger velocity. At the last stage of solidification under HPDC process, fully divorced eutectics consisting of α-Mg and β-Mg17Al12 appear at the grain boundary of the primary α-Mg. Islands of β-Mg17Al12 phase are observed at the central region of the castings, while the β-Mg17Al12 phase reveals a more dispersed and granular morphology at the surface layer. The percentage of the eutectics is higher at the central region than that at the surface layer of the castings, whilst the defect band also contains a higher percentage of the eutectics than those at the adjacent regions.