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Modeling and simulation on the mold filling, solidification and microstructure evolution of high pressure die casting process of magnesium alloys are of great importance for optimizing the die design, controling the microstructure formation and improving the quality of the castings.In this paper, a 3D mathematical model was proposed to simulate mold filling in high-pressure die casting (HPDC) process to improve accuracy and consider surface tension.A theoretical liquid-particle flow model was created to simulate the movement of external solidified crystals (ESCs) during the mold filling process.A thermal boundary-condition model at the metal/die interface of HPDC process was developed based on two important correlations, the first is the relationship between the maximum interracial heat transfer coefficient and the initial die surface temperature, and the other is between the interfacial heat transfer coefficient and the casting solid fraction.2-D numerical models based on the cellular automaton (CA) method were developed to simulate the microstructure evolution of magnesium alloys during HPDC process.Special attention was paid to establish a nucleation model considering both of the nucleation of ESCs in the shot sleeve and the massive nucleation in the die cavity.Meanwhile, simulation of the formation of fully divorced eutectic was also taken into account in the CA model.In addition, the 3-D morphology of α-Mg dendrites of Mg-Sn and Mg-Gd alloys was investigated and characterised using the synchrotron X-ray tomography technique.Results show that the α-Mg dendrites have eighteen primary branches with six growing along (11(2)0) directions in the {0001 }basal plane and twelve along (11(2)3) directions in non-basal planes.A 3D phase field model was developed to model the 3D dendrite morphology of magnesium alloys and the simulation results reproduced the experimental results on various key growth aspects including dendrite morphology and side-branching patterns.