Theoretical and experimental studies on pore formation during casting solidification

Elimination of microporosity in castings is important to assure casting quality. Of particular concern are the long freezing range alloys, in which the mechanism of pore formation has not been clearly understood so far. The present work describes a theoretical model to describe the feeding in the two phase mushy region during casting solidification. Considering a one-dimensional two-phase flow problem with moving reference frame, the equations for the conservation of mass and momentum are solved in the mushy region. The extent of liquid feeding is limited by the resistance offered by the shape, size and number of the evolving solid particles or dendrites. The feeding analysis is carried out for mushy regions consisting of two typical casting macrostructures: the columnar and equiaxed dendrites. Based on the analysis, a new porosity criterion called the Feeding Resistance Number, a dimensionless function comprising of various solidification variables, is suggested separately for columnar and equiaxed dendritic regions. Results obtained indicated that columnar dendritic regions are generally less prone to pore formation than equiaxed dendritic regions. Theoretical results are compared with experimental observations in cylindrical castings of Al-4.5%Cu alloy, a typical long freezing range alloy. The Feeding Resistance Number is applied to an industrial scale castings of aluminum alloy A356 and correlations are obtained with actual casting porosity. When compared to conventional criteria functions, the correlations obtained with the Feeding Resistance Number show significantly superior results.