Prediction of porosity defect in spheroidal graphite iron castings

Although the volume expansion due to graphite formation in spheroidal graphite (SG) cast iron greatly affects the porosity defects, conventional methods for predicting the defects don't directly consider it. In this paper we propose a new method considering the effect in order to predict more accurately the porosity defects, in particular, the porosity defect caused by casting volume change. In this method, the graphite and austenite volume changes during solidification are calculated under the assumption of one nucleation event using experimental data and quasi-steady state diffusion of carbon through the austenite shell. An equivalent expansion coefficient change is calculated from the volumes of graphite and austenite for each mesh, and used to estimate the deformation and stress distribution. For the stress analysis the voxel method is used to save the computational time and memory. This method suggested that the main mechanism of the porosity formation is the larger expansion in the surface region than in the inner region of casting due to temperature difference, causing tensile stress or negative pressure in the inner region. The comparison of the simulated and experimental results showed that the proposed method is promising for practical estimation of the porosity defect in SG cast iron castings. (C) 2003 Maney Publishing.