Investigation on the formation of microporosity in aluminum alloys

A mathematical model on the microporosity of aluminum castings is proposed, considering the pressure increasing induced by dissolved hydrogen, solidification shrinkage and interface energy. Two types of feeding mode during solidification has been considered, which corresponds to two different microstructures, i.e. columnar dendrite and equiaxed dendrite. For columnar dendritic solidification, Darcy's law was adopted to calculate the pressure drop induced by shrinkage. For equiaxed dendritic solidification, Hagen-Poiseuille equation was adopted before the mass flow stopped. The critical solid fraction f_sc was obtained by the mathematical model with the consideration of dissolved hydrogen, SDAS (secondary dendrite arm spacing), grain size and the mass flow passage. The calculation examples are presented and compared with experimental data and other models of former researchers.