Simplified computation of macrosegregation in multicomponent aluminum alloys

Abstract

An approximate method for calculating the macrosegregation in a multicomponent aluminum alloy is proposed. This method is based on the use of a predefined solidification path (i.e., relation between the solute concentration in the liquid phase and the solid fraction) instead of addressing the fully coupled micro-macrosegregation problem. In determining the solidification path, it is assumed that the total solute concentration is constant, and that the solidification history is the same everywhere in the casting. In this manner it becomes quite easy to take into account how the macrosegregation development is affected by the solute diffusion in the dendrites and the precipitation of secondary phases, provided that such effects are accounted for in the model used for determining the solidification path. In order to demonstrate the approximate method, the inverse segregation formation at a chill surface of an Al-4 pct Mg-0.2 pct Fe-0.15 pct Si-0.3 pct Mn (AA5182) alloy is calculated. In this case study, the solidification path is determined prior to the macrosegregation computation by a microsegregation model discussed elsewhere, and the solid and liquid densities are related to the concentrations of the different alloying elements by a simple mixture law without distinguishing between the different solid phases that are formed. The accuracy of the approximate method is discussed by considering a binary alloy. It turns out that the macrosegregation formation at a chill surface of an Al-4 pct Mg alloy is fairly close to that resulting from a modeling in which the variation of the total solute concentration is taken into account. Furthermore, the mixture law is compared to a more elaborate treatment of the densities involving both primary and eutectic solid phases. This comparison is carried out for an Al-4.5 pct Cu alloy for which literature data exist. The mixture law is found to give a reasonable accuracy in the calculated macrosegregation.