Macro-segregation in aluminium alloy sprayformed billets

It is generally assumed that during sprayforming the amount of micro- and m acro-segregation in the sprayforms will be limited or absent because of low liquid fractions, small grain size and high cooling rate. This paper shows that macro-segregation can occur in sprayforms of Al, even when small liqu id fractions are present during growth of the sprayform. This segregation o ccurs by an inverse mechanism, with solute-rich liquid sucked back through a solid network by solidification shrinkage. Liquid dow outwards is further enhanced by the centrifugal force produced by sprayform rotation. The liqu id sucked outwards is enriched in solute (with partition coefficients <1), resulting in solute depletion at the centre and enrichment at the surface. Elements that form dispersoid particles in the spray do not segregate becau se they are locked into the solid network. The segregation profile can be m odelled using equations similar to those used to describe inverse segregati on. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.