The effect of free-floating dendrites and convection on macrosegregation in direct chill cast aluminum alloys Part II: predictions for Al-Cu and Al-Mg alloys

In Part I, a binary mixture model of the DC casting process was proposed, that accounts for fluid flow ill the melt and mushy zone, as well as in a slurry zone characterized by the transport of solute-depleted, free-floating dendrites. In this paper, the model is applied to the DC casting of Al-4.5 wt% Cu and Al-6.0 wt% Mg billets, and the predicted surface-to-centerline distribution of macrosegregation is consistent with trends observed in DC cast ingots. These trends include the development of negative segregation at the centerline, subsurface solute enriched and depleted regions, and positive segregation at the billet surface. Negative segregation at the centerline increased with an increase in the packing fraction at which free-floating dendrites are presumed to coalesce into a rigid dendritic structure. Likewise, negative segregation at the centerline and positive segregation in the enriched region increased with an increase in the characteristic diameter of the free-floating dendrites. (C) 1999 Elsevier Science Ltd. All rights reserved.