Contribution of forced centreline convection during direct chill casting of round billets to macrosegregation and structure of binary Al–Cu aluminium alloy

Abstract

Effects of upward or downward forced flow in the centre of the sump of an Al alloy billet on structure and macrosegregation were studied. The billet was cast at a pilot vertical direct chill caster and had dimensions of 195 mm diameter and 1400 mm length. Introduction of forced flow in the liquid part of the sump resulted in some changes in the structure and dramatic changes in macrosegregation. Upward flow coarsened the structure and increased negative centreline segregation. Downward flow made the structure finer and more uniform across the billet, and suppressed the macrosegregation. Strong downward flow produced positive macrosegregation in the billet. The experimental results are analysed using computer simulations of heat flow and solute transport, and discussed in terms of macrosegregation mechanisms.