Recent Advances in the Development of Thermomechanical Models for Growth Instability in Solifification of Metals

During solidification of metals, solidification front often exhibit non-uniformities known as cellular undulations. These undulations, which are macroscopic (in contrast to the microscopic dendrite morphology), represent a growth instability where certain regions of the freezing front grow preferentially over others. These undulations are generally not observed in most industrial casting operations unless the casting process is interrupted and the residual fluid decanted. Nevertheless, freezing front growth instability can lead to catastrophic failure of a casting operation known as breakout, in which the lateral strength of localized regions of the metal shell does not sufficiently resist pressure from the residual molten fluid, and can enhance both surface and internal cracking of the cast product through non-uniform thermal distortions. Study of thermal stress and deformation development in the early stages of solidification can be an important tool for understanding the formation of cracks, liquation or other defects in the ingot surface. Since the main driving force of thermal stress development in a solidifying body is the temperature reduction process, control of the heat extraction through the mold-shell interface during the early stages of solidification is one of the most challenging and important problems associated with solidification of metals. In current practices surface defects are removed through expensive surface milling and scalping processes. Understanding the effect of mold topography on the heat extraction process and the resulting shell growth may allow certain control of cast surface morphologies and thus lead to a reduction of unnecessary post-casting operations needed to remove surface defects. We will here briefly review the theoretical and experimental studies as well as discuss some directions for future research in growth instability in solidification in response to the thermal boundary conditions at the mold-shell interface.