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针对具有快速凝固特征的压铸工艺, 通过热传导反算法得到较准确的压铸温度场,分析压铸镁合金凝固过程冷却曲线, 建立了形核密度随冷却速率变化的形核模型.采用改进CA方法, 建立了适用于镁合金hcp结构的枝晶生长模型. 模型考虑了溶质扩散、成分过冷、曲率过冷以及界面各向异性等重要因素, 实现了镁合金不同角度枝晶生长, 再现了枝晶二次及三次枝晶臂生长, 定向凝固不同温度梯度及凝固速率下的枝晶竞争生长以及三维枝晶生长等特征. 应用所建立的形核及生长模型模拟了AM50镁合金“阶梯”压铸件不同压铸工艺下的凝固组织,模拟结果与实验结 果相吻合.

As the lightest structural material, magnesium alloy has been widely used in the automotive, aerospace and electronic industries. High pressure die casting (HPDC) process is the dominant process for magnesium alloy products. The microstructure of die cast magnesium alloy has a great influence on the final performance of the castings. Numerical simulation provides a way to predict the solidification structure and the corresponding mechanical properties. However, as one of the most widely used methods in microstructure simulation, the cellular automaton (CA) method has difficulties in simulating the solidification structure of magnesium alloy with hcp crystal structure, though simulations of solidification structure for bcc and fcc metals have been widely reported. Besides, for the microstructure simulation of magnesium alloys by HPDC process, accurate nucleation model has to be considered, and by far little report was found on it. In the present paper, based on the accurate temperature field of die castings obtained by an inverse heat transfer model, analysis of the temperature curves during solidification was made to establish a nucleation model that correlated the cooling rate with the nucleation density of magnesium alloys during solidification of HPDC process. A modified CA model was also developed to simulate the crystal growth of magnesium alloys. It takes account of the solute diffusion,  constitutional undercooling, curvature undercooling, and anisotropy etc. Validations were made to the model, and the results show that the model has the capability to simulate the dendrite growth of magnesium alloy with different growth orientations. Besides, the model can also reveal the dendrite morphology with features of secondary and ternary dendrite branches, the dendrite competition growth under different temperature gradients and solidification rates, and the three dimensional morphology of the dendrite growth. To validate the nucleation and growth model established for magnesium alloy under HPDC process, "step–shape" die castings of AM50 magnesium alloy were produced at different process parameters. The average grain size prediction results are in good agreement with the experimental ones.

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