Macrosegregation Modelling of DC-Casting Including Grain Motion and Surface Exudation

The as cast chemical composition of a DC-cast billet or slab involves mechanisms as movement of solid grains, that are lean on alloying elements, as well as movement of interdendritic liquid, that are enriched on alloying elements. These phenomena can result in the composition being out of the specification at the surface area for billets or in the centre for large slabs. In this study macrosegregation including thermal and solutal convection, solidification shrinkage, grain motion and surface exudation is simulated on a billet cast at the Alcoa Lista plant. The model includes the air-gap formation against the mould during DCcasting and the flow of liquid through the semi-solid surface that is created under such conditions. Modelling results are compared with measurements of chemical composition through the billet from centre to surface and on the exuded layer itself as well as a characterization of the thickness of the exuded layer.