Interactions between solidification and fluid flow. Effects on cast structures and segregations

Abstract
Natural or forced fluid flow of the bulk melt can induce several changes in the solidification history of a casting : it may change the heat transfer conditions, the as-cast grain distribution, and the segregations. On the other hand, solidification can result in intense fluid flow due to natural solutal convection and in severe local segregations. The two following practical cases illustrate the variety and complexity of interactions between the fluid flow and the solidification of alloys : - the effects of the stirring of the bulk liquid on the formation of the equiaxed zone during the continuous casting of steels, - the formation of freckles and related segregations during the directional solidification of nickel-base alloys. Informations are given about, either new experimental observations, or up-dated physical and numerical models related to each case chosen here as examples. In the case of the influence of stirring on the formation of the equiaxed zone, emphasis is put on the importance of the interaction of the fluid flow with the dendrites in the columnar zone and in the stirred region itself. In the case of the freckles, evidence is brought forward that the dendritic nature and structure of the mushy zone are not the only causes of the dependence of the freckling on the heat transfer conditions during directional solidification.