EXPERIMENTAL AND MODELING STUDIES ON THE STRUCTURE FORMATION OF HIGH PRESSURE DIE CAST MAGNESIUM ALLOY CONSIDERING THE EXTERNALLY SOLIDIFIED CRYSTALS IN THE SHOT SLEEVE

Abstract

The externally solidified crystals (ESCs) in the shot sleeve have a great influence on the final structure of magnesium alloy during cold-chamber high pressure die casting (HPDC) process. In typical HPDC microstructure, a surface layer with uniformly fine grains and a central region containing a mixture of coarse ESCs and fine grains are commonly observed from the cross section of the castings. In the present work, experiments were conducted to investigate the effects of process parameters on the formation of ESCs in the shot sleeve and the final microstructure of magnesium alloy, especially focusing on the grain size, the morphology and distribution of the ESCs. Based on cellular automaton method, a numerical model was developed to simulate the microstructure evolution of magnesium alloy under HPDC process. According to experimental statistics relating the area fraction of the ESCs, a nucleation model was established in which the ESCs in the shot sleeve were taken into account. Simulations were carried out to predict the microstructure of step-shape die castings of AM50 magnesium alloy with different process parameters. It was found that the simulated results were in accordance with the experimental ones.