Impact of crystal sedimentation and viscoplastic semi-solid dynamics on macrosegregation

Abstract

A numerical investigation of melt convection with solid-phase transport during equiaxed solidification is performed to examine the structural and compositional evolution of an Al-Si alloy. Nucleation and growth kinetic laws are coupled with the general conservation equations employed in fluid dynamics to predict the development of the solidification structure. Besides crystal sedimentation due to buoyancy-induced forces in the low solid fraction range, the model covers also the viscoplastic semi-solid dynamics at high solid fractions. A test case is devised such that two viscoplastic structures grow from the vertical walls towards the centre of the domain and eventually merge at an arbitrary distance from the bottom of a simple cavity setup. This introduces difficulties for the solidification-driven feeding flow which has been associated with pressure drops across the domain. Dilatation of the mush is also predicted in the lower part of the solidifying viscoplastic mush, which means that additional melt is drawn to this area. Interestingly, besides liquid feeding, also a significant amount of solid feeding has been detected to compensate for solidification shrinkage. Such synergetic behaviour between the phases is analysed in terms of macrosegregation.