EFFECT OF MOLD PROPERTIES ON THERMOELASTIC INSTABILITY IN UNIDIRECTIONAL PLANAR SOLIDIFICATION

Abstract

The residual stress and the finished shape of a casting depend upon the thermal history of the solidification process and, hence, on the thermal boundary conditions at the mold/casting interface. However, the contact resistance at this interface will depend upon the local contact pressure, which itself is affected by thermal distortion of the solidified casting. The thermal and mechanical problems are therefore coupled and must be investigated as a system. This coupled process has the potential for instability, resulting in both surface and internal cracking of the cast product through nonuniform thermal distortions. In this article, the effects of distortivity and the thickness of the mold are investigated under the simplifying assumption that the casting and the mold materials have zero thermal capacity. This extends earlier works that are restricted to the case of rigid mold. The results document the effect of mold distortivity on the growth of a perturbation in a nominally plane solidification front due to thermomechanical coupling associated with a pressure-dependent thermal contact resistance. The perturbation in a solidification front tends to a maximum value and then decays to zero at larger values of time. The magnitude of this perturbation, in general, increases with the distortivity of the mold. The effect on the contact pressure, however, depends on the thickness of the mold. For small thickness, the mold is flexible and not much pressure is needed to straighten it out. For larger thickness, the stiffness is increased, but the perturbation in contact pressure associated with a given perturbation in heat flux at the mold surface is reduced. These two effects counter each other for highly distortive molds. However, the second overcomes the first, and therefore perturbation grows more slowly for less distortive molds as solidification proceeds.