Microsegregation during the solidification of an Al-Mg-Si alloy in the presence of back diffusion and macrosegregation

A model for one-dimensional, transient multicomponent microsegregation in ternary alloys subject to changes in overall composition and back diffusion in the primary phase during solidification is presented. A commercially available thermodynamic package (JMATPRO) is used to determine ternary phase diagrams and alloy properties. The models are suitable to be subprograms in macroscale transport models for alloy solidification processes (such as direct chill and strip casting), and calculations were performed for a commercially interesting aluminum alloy (Al-1 wt.% Mg-1.15 wt.% Si). During cooling at a constant rate, the composition was varied positively or negatively to induce dissolution or accelerated freezing of the solid. Composition profiles in the primary Al phase, temperature histories, and liquidus paths were calculated. As expected, the limited diffusion in this model increased the amount of primary solid formed. In multicomponent systems, the back diffusion has this effect not only because of the accelerated freezing rate, but also because it alters the path down the liquidus surface.