Phase-field modelling of gas porosity formation during the solidification of aluminium

Abstract

Abstract The mechanical properties of aluminum cast products depend to a large extend on their specific material microstructure, but can be impaired by defects which appear at the solidification stage, e. g. hydrogen pores. In the present work, we focus on specific issues for the phase-field modelling of microporosity defect formation in aluminium casting connected to hydrogen bubbles. Because of the large differences in the specific volumes of the melt and gas phase, large volume changes have to be handled by the model. In order to take into account different molar volumes, a modification of a multicomponent/multiphase-field model is proposed. It is based upon volume fraction considerations including additional mass transport that allow for the pressure dependent volume expansion of hydrogen bubbles growing in the melt. Furthermore, it also considers the pressure dependency of the hydrogen solubility. A simulation of the microstructure formation for an Al7 % Si0.30 % Mg alloy using the standard phase-field model is shown to be in agreement with experiment. The modified phase-field formulation has been applied to pure aluminium containing hydrogen. The equilibrium state for a gas bubble in liquid aluminium and bubble formation during solidification have been simulated for different pressures.