A coupled thermomechanical, thermal transport and segregation analysis of the solidification of aluminum alloys on molds of uneven topographies

A coupled thermomechanical, thermal transport and segregation analysis of aluminum alloys solidifying on uneven surfaces is presented here. Uneven surfaces are modelled as sinusoids with different wavelengths and amplitudes. Effects of various coupling mechanisms between the solid-shell deformation, air-gap formation, heat transfer, fluid flow and segregation, near the mold-metal interface, are observed for different mold topographies during the early stages of solidification of an aluminum alloy. The role of inverse segregation, arising from shrinkage driven flow in the melt, melt superheat and varying mold surface topography on nucleation of air-gaps and evolution of stresses in the solidifying shell is examined. The numerical model consists of a volume-averaged solidification model coupled with a small-deformation model combining elasto-viscoplastic deformation in the solidifying shell with air-gap nucleation and imperfect contact at the metal/mold interface. Heat transfer at the mold-metal interface is either contact pressure or air-gap dependent and is modelled using the actual contact pressure or air-gap size obtained from the contact sub-problem at the metal-mold interface. Variation in heat transfer leads to variations in fluid flow, segregation and stresses developing in the solid and mushy-zone, which in turn affect the morphology of the growing solid-shell. A wavelength range that leads to a reduction in equivalent stresses, segregation and growth front morphology unevenness, in the evolving solid-shell, is obtained for varying solute concentrations. One of the main objectives of the current analysis is to seek optimal mold surface topographies that minimize surface defects leading to desired cast surface morphologies.