Structure formation and macrosegregation under different process conditions during DC casting

Billets of an Al–4.5 wt.% Cu alloy direct-chill cast under different process conditions were systematically examined. Effects of casting speed and water-flow rate on structure and macrosegregation profiles are studied and correlated to computer simulated solidification patterns. The results show that grain size, dendritic arm spacing, amount and size of pores tend to coarsen towards the center of a billet and with decreasing casting speed and water-flow rate. Both decreasing casting speed and water-flow rate produce less macrosegregation, although the effect of casting speed is much larger. Computer simulation was used to determine the geometry of the billet sump, temperature and melt flow patterns. Main parameters that influence the structure formation and macrosegregation during direct-chill casting are shown to be the depths of the liquid pool and the sump, and the flow pattern in the slurry region. The appearance of coarse grains in the central part of the billet is explained from the transport of solid phase within the slurry region.