Modeling of hydrogen porosity formation during solidification of dendrites and irregular eutectics in Al-Si alloys

A two-dimensional multi-phase cellular automaton-finite difference method (CA-FDM) model is proposed for the simulation of hydrogen porosity formation during solidification of dendrites and irregular eutectics in aluminum alloys. The present model encompasses a multi-phase system of liquid, gas porosity, dendrites and eutectic phases. The growth of gas pores and solid phases is simulated using a CA technique. The diffusion of hydrogen and silicon is calculated using the FDM. The proposed model is applied to simulate the temporal evolution of casting gas porosity in an Al-7 wt% Si alloy. It allows visualizing the interactions between the formation of microporosity and the growth of solid phases, as well as the evolving concentration fields of hydrogen and silicon. The competitive growth mode between the gas pores with different sizes is analyzed in detail by comparing the local hydrogen concentration and saturation at the gas/liquid interface. The pores with relative larger sizes are found to grow preferentially. The simulations are also performed to investigate the influences of cooling rate and initial hydrogen concentration on microporosity formation. The simulated time-dependent variations of porosity percentage and solid fraction indicate that the main portion of porosity is formed in the stage of eutectic solidification. When applying a higher cooling rate, the final pore size is smaller and more uniform, and the porosity percentage is lower, while the porosity density is higher. A decreased initial hydrogen concentration evidently reduces the porosity percentage. The simulation results compare reasonably well with the experimental data reported in the literature.