The externally solidified crystals（ESCs）in the shot sleeve have a great influence on the final microstructure and performance of magnesium alloy castings during the cold-chamber high pressure die casting（HPDC）process.The research progress was reviewed on the ESCs in the microstructure of magnesium alloy die castings.Some significant research findings were summarized

which were related with the microstructure characteristics of the ESCs

the validation of the " ESCs" theory by experiments and simulation

the morphology

distribution and content of the ESCs at different locations of the die castings and under different process parameters during the HPDC process

as well as the effects of the ESCs on the mechanical properties of magnesium alloy die castings.Finally

the problems existing in the current study of the ESCs in the microstructure of cold-chamber high pressure die cast magnesium alloy were analyzed

and the research directions of future were forecasted.