Modelling issues in macrosegregation predictions in direct chill castings

Abstract

Macrosegregation is a major concern in direct chill (DC) casting because it can lead to non-uniform mechanical properties which affects the behaviour of the metal during downstream forming and heat treatments. The main mechanism behind macrosegregation is well understood: the transport of segregated alloying elements at the scale of the casting by the relative movement of liquid and solid phase in the mushy zone. While previous numerical simulations of DC-casting investigated the effects of one or more of the mechanisms leading to macrosegregation, one modelling issue is relatively unexplored, i.e. the effect of discretisation errors, i.e. numerical diffusion and dispersion, on the predictions. This paper addresses the performance of several discretisation schemes in five typical fluid flow benchmarks which all have some similarities with the flow in the liquid sump of a DC-cast ingot. A selection of the studied discretisation schemes is then used to simulate macrosegregation in an Al–4.5wt.%Cu alloy in two particular cases: a small diameter ingot with thermal buoyancy only, and a large diameter ingot in which both thermosolutal buoyancy and solidification shrinkage is taken into account. Overall best scheme was found to be the STOIC scheme, both for the momentum equations as well as for the species transport equation. Simulations with the small diameter ingot showed that false segregation caused by numerical diffusion is found with the poorly performing schemes. In the case of the large diameter ingot the choice of the discretisation scheme has shown to affect the sign of centreline segregation. Predictions with the STOIC scheme are qualitatively in agreement with experimental data.