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

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.

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