Microstructural evolution mechanism of semi-solid slurry: a study using Phase-Field-Lattice-Boltzmann scheme

The formation mechanism study of a spherical crystal is the theoretical basis for preparing high-quality semisolid slurries with homogeneous spherical microstructure. The interaction between thermal-solute diffusion during solidification and solid-liquid interface migration determines the morphology evolution of microstructure according to the representation theory. Comprehensive research about the effects of local variates on the microstructural evolution of semi-solid slurry in a Representative Elementary Volume (REV) by 2D cases was conducted. A modified Phase-Field-Lattice-Boltzmann scheme using techniques of parallel computing and adaptive mesh refinement (Para-AMR) was adopted. Actual processing parameters were coupled with the numerical scheme. Mesoscopic results were upscaled by averaging them over the simulation domain. Five aspects, growth space, initial undercooling degree, cooling rate, natural convection, and forced convection, were investigated. It is found that the formation of spherical grain requires a specific initial undercooling degree, cooling rate, and nuclei density. Study results indicate that melt convection has little effect on the morphological evolution of grain in the case of rapid isotropic growth, which was verified by experiments using the SEED process. Besides, forced convection has a significant effect on the morphology evolution of grain controlled by solute diffusion, which provides a possibility of an extension of the process window about pouring temperature and alloys with narrow solidification range for preparing semi-solid slurries.