Mechanisms of porosity formation during solidification: A theoretical analysis

Abstract

The formation of porosity during solidification is of great commercial importance and scientific interest. This is particularly so for the question of the “feeding length” of a riser. In this work, a number of theoretical models are derived and their predictions are compared to experimental observations. The comparisons show that in directional solidification, a “thermodynamic” model is useful in predicting when porosity may form. The amount of porosity predicted is too high, however, since it ignores the nucleation of the pore and growth by diffusion of dissolved gas to growing pores. A surprising conclusion of this study is that Darcy’s law does not appear to be a controlling factor in porosity distribution or formation. In particular, Darcy’s law cannot explain feeding length measurements made in steel castings. A simple “geometrical” criterion is presented instead to describe when shrinkage porosity will occur. This new model suggests a number of interesting experiments, which are proposed in discussion.