Modeling of gas porosity and microstructure formation during dendritic and eutectic solidification of ternary Al-Si-Mg alloys

A two-dimensional (2-D) multi-component and multi-phase cellular automaton (CA) model coupled with the Calphad method and finite difference method (FDM) is proposed to simulate the gas pore formation and microstructures in solidification process of hypoeutectic Al-Si-Mg alloys. In this model, the pore growth, and dendritic and eutectic solidification are simulated using a CA technique. To achieve the equilibrium among multiple phases during ternary Al-based alloy solidification, the phase transition thermodynamics and kinetics are evaluated by adopting the Calphad method. The diffusion equations of hydrogen and two solutes are solved by FDM. The developed CA-FDM coupled model can be used for simulating the evolution of gas microporosity and microstructures, involving dendrites and irregular binary and ternary eutectics, of ternary hypoeutectic Al-Si-Mg alloys. It has the capability of reproducing the interactions between the hydrogen microporosity formation and the growth of dendrites and eutectics, the competitive growth among the growing gas pores of different sizes, together with the time-evolving concentration fields of hydrogen and solutes. The simulated morphology of gas pore and microstructure has a good agreement with the experimental observation. The influences of the initial hydrogen concentration and cooling rate on the microporosity formation are investigated. It is found that the main portion of porosity formation occurs in the eutectic solidification stage through analyzing the profiles of porosity percentage and solid fraction varying with solidification time. The varying features of simulated porosity percentage, the maximum and average pores radii indicate that increasing initial hydrogen concentration promotes the formation of higher final porosity percentage and larger pores, while the size of gas pores will significantly reduce with increasing cooling rate, leading to a lower final porosity percentage.