Effects of microscale phenomena and density in modelling macrosegregation

Segregation phenomena in binary alloys occur at two distinct length scales; macrosegregation, driven by flow, at the scale of the process and microsegregation, driven by mass diffusion, at the scale of the dendrite arms. In this paper a dual scale model of segregation, which accounts for and couples both scales, is presented and applied in the prediction of the shrinkage driven, inverse, chill face segregation which occurs in the uni-directional casting of Al-Cu alloys. The central focus of the work is to access the impact on inverse segregation of (i) the microscale treatment (in particular the handling of the solid sate mass diffusion) and (ii) the choice of density models. The major conclusions are that (i) a limiting Scheil treatment is often sufficient to account for microsegregation effects and (ii) density models, previously presented in the literature, lead to dramatically different predictions in the levels of inverse segregation.