Full Size Measurement and Simple Prediction on Macro Segregation of Aluminum Alloys Elements in Industrial Direct Chill Casting Slab

Abstract

In direct chill casting of aluminum alloys, negative macro segregation of eutectic elements and positive macro segregation of peritectic elements can occur at the center of the slab. It is very important to investigate the mechanism of these macro segregations because they influence the mechanical and chemical properties of the wrought products. The authors have performed a two-dimensional measurement of the macro segregation by optical emission spectroscopy, and important characteristics of this phenomenon were observed. They are explained qualitatively and quantitatively by our proposed hypothesis, the partially swept solute model. The model assumes that the sump flow penetrating into the mushy zone will sweep out the enriched or diluted liquid solute until the critical solid fraction $f_s^{\ast}$ is reached, when the dendrite arms begin to sufficiently entangle with each other, making it difficult for the sump flow to penetrate into the mushy zone. Then, the ordinary solidification process continues. In case of severe segregation, the microstructure at the center of the slab becomes a granular crystal because the enriched or diluted solute in front of the solidification interface is swept out by the penetration flow and dendrite growth is inhibited.