Modeling of macrosegregation in direct-chill casting of aluminum alloys: Estimating the influence of casting parameters

The influences of the most important process parameters on macrosegregation in Al–5.25 wt.% Cu direct-chill cast billets of 218 and 282 mm diameter were studied by performing numerical simulations. Solidification and transport of heat, momentum, and species were modeled using the Bennon–Incropera one-phase continuum mixture model. A rigid, coalesced solid phase was assumed in the mushy zone, which was modeled as a porous medium. The model was solved numerically, using the finite volume method in axisymmetric geometry. Five cases, encompassing variations of billet diameter, casting velocity, casting temperature, and mold cooling type were simulated. A detailed analysis of the interplay of transport of species by flows induced by thermal and solutal natural convection as well as by solidification shrinkage and their effect on the final macrosegregation pattern is given. It was shown that the process parameters affect macrosegregation through their direct impact on the thermosolutal flow in the liquid pool and the mushy zone. The main factors were shown to be the depth of the liquid pool, and the temperature difference that drives the thermal natural convection in the liquid pool.