Effect of sand dilation on core expansion during steel casting

The thermo-mechanical behavior of the bonded sand used for molds and cores has a strong effect on dimensions of steel castings. Experiments are conducted in which a thick-walled hollow carbon steel cylinder is cast using a silica sand core. The temporal evolution of the inner diameter of the cylinder is measured in-situ during solidification and cooling by utilizing quartz rods connected to LVDTs (Linear Variable Differential Transformers). It is found that the inner diameter increases significantly during the initial stages of solidification when the steel offers little restraint to core expansion. Without accurately modeling this initial core expansion, the final cylinder dimensions at room temperature cannot be predicted. Preliminary simulations using the measured linear thermal expansion coefficient of the core considerably under-predict the measurements, which suggests that shear induced sand dilation also contributes to core expansion. The Drucker-Prager Cap model, which can predict dilative behavior, is used to simulate the mechanical behavior of the core. Utilizing this model in conjunction with an elasto-visco-plastic constitutive law for the steel, the stress simulations successfully predict the observed dimensional changes in the casting during solidification.