Coupled Modeling of Gas and Shrinkage Microporosity with a Gas-Liquid-Solid Multi-Phase-Field Model

One of the most important types of defects that form during solidification is microporosity. There are two types of microporosity, gas porosity and shrinkage porosity. Gas porosity is caused by the gas element supersaturation originating from the difference in solubility between liquid and solid while shrinkage porosity is due to the volume difference between liquid and solid combined with restricted feeding during the last stage of solidification. Due to the difference in formation mechanism, there is a convention that gas and shrinkage porosities are examined separately. However, the two types of porosity can be coupled, i.e., both gas supersaturation and shrinkage flow can contribute simultaneously to the formation of porosity. As a model to incorporate both mechanisms is lacking, we developed a gas-liquid-solid multi-phase-field model to fill the gap. This model contains two core components: a quantitative multi-phase-field model describing the motion of different interfaces, which can reduce to the sharp interface model in the thin-interface limit; and a diffuse interface model of two-phase flows to introduce the shrinkage flow and the motion and deformation of the gas bubbles. A lattice Boltzmann method is used to solve the phase-field model and calculate the shrinkage flow. It is expected our model can provide a unified approach to study gas and shrinkage porosity and shed a new sight on the porosity formation.