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, t hat accounts for fluid flow ill the melt and mushy zone, as well as in a sl urry 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 d istribution of macrosegregation is consistent with trends observed in DC ca st ingots. These trends include the development of negative segregation at the centerline, subsurface solute enriched and depleted regions, and positi ve segregation at the billet surface. Negative segregation at the centerlin e increased with an increase in the packing fraction at which free-floating dendrites are presumed to coalesce into a rigid dendritic structure. Likew ise, 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 r eserved.