Phase Field Model of Semi-solid Slurry Generation and Isothermal Coarsening of Novel Al-15Mg2Si-4.5Si Composite

Indrani Mukherjee, Prosenjit Das
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Abstract

The present study speaks of development of a two-dimensional phase field (PF) model to simulate the cooling slope rheoprocessing of the novel Al-15Mg2Si-4.5Si composite, in view of process optimization and investigation of physics of microstructure formation. In case of cooling slope rheoprocessing, the composite melt starts losing its superheat once it impinges over the slope and transforms into semi-solid slurry during its length of travel over the slope. After experiencing shear flow over the slope, the melt fills an isothermal slurry holding furnace where it undergoes coarsening for a certain length of time. The present PF model simulates how heterogeneous nucleation of solid grains is supposed to happen within the melt, during cooling slope processing, adopting a seed undercooling-based nucleation model. Moreover, the PF model implements a grain coarsening model to simulate the isothermal globularization process of the evolving solid grains of primary Mg2Si and primary Al phases. The interfacial free energy of Al–melt interface is taken from literature, whereas a molecular dynamics (MD) model is employed to estimate the interfacial energy value of the Mg2Si–melt interface. The cooling rate values employed in the present PF model for different melt pouring temperatures are determined experimentally from initial trial experiments, whereas the validation experiments are performed to collect the slurry samples from chosen locations of the melt flow front over the slope and from isothermally kept slurry holding furnace. Micrographs obtained from the above samples confirm the accuracy of the developed 2D PF model to capture microstructural morphology of the composite slurry. Moreover, the model predictions of quantitative parameters such as grain diameter, shape factor/sphericity, and solid fraction are found to be close to the experimental measurements. For example, a representative simulated value of grain size and sphericity of primary Mg2Si grains, after 8 minutes of slurry holding, are as follows: 24.01 and 0.834 μm, whereas the corresponding experimental values are 29.0 and 0.885 μm, respectively.

Graphical Abstract

Abstract Image

新型铝-15Mg2Si-4.5Si 复合材料的半固态浆料生成和等温粗化的相场模型
本研究建立了一个二维相场(PF)模型,用于模拟新型铝-15Mg2Si-4.5Si 复合材料的冷却斜坡流变过程,以优化工艺并研究微观结构形成的物理过程。在冷却斜坡流变过程中,复合材料熔体在撞击斜坡后开始失去过热度,并在斜坡上移动的过程中转变为半固态浆料。在斜坡上经历剪切流之后,熔体进入等温浆料保温炉,在一定时间内发生粗化。本 PF 模型采用基于种子欠冷的成核模型,模拟了在冷却斜坡加工过程中熔体内部固体颗粒的异质成核过程。此外,PF 模型还采用了晶粒粗化模型来模拟原生 Mg2Si 和原生 Al 相固态晶粒的等温球化过程。铝-熔体界面的界面自由能取自文献,而 Mg2Si-熔体界面的界面能值则采用分子动力学(MD)模型估算。本 PF 模型在不同熔体浇注温度下采用的冷却速率值是通过初步试验实验确定的,而验证实验则是从斜坡上熔体流动前沿的选定位置和等温保温浆料炉中采集浆料样品进行的。从上述样品获得的显微照片证实了所开发的二维 PF 模型在捕捉复合浆料微观结构形态方面的准确性。此外,还发现模型对晶粒直径、形状系数/球形度和固体分数等定量参数的预测与实验测量结果接近。例如,在浆料保持 8 分钟后,原生 Mg2Si 晶粒的晶粒大小和球形度的模拟值具有代表性,分别为 24.01 和 0.834:24.01 和 0.834 μm,而相应的实验值分别为 29.0 和 0.885 μm。
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