Direct Chill (DC) Casting of Aluminium involves alloys employing different solute elements. In this paper a qualitative analysis and comparison of macrosegregation formation is presented for three different alloy systems: Al-Mg, Al-Zn and Al-Cu. For this purpose, a multiphase, multiscale solidification model based on volume averaging method accounting for shrinkage induced flow, thermal-solutal convection and grain motion is used and applied to an industrial scale DC Cast ingot. The primary difference between these alloys is thermal-solutal convection with Al-Mg having a competing thermal and solutal convection whereas the other two systems have a cooperating thermal and solutal convection. In the study, the combined effect of the macrosegregation mechanisms is analyzed for each alloy in order to assess the role of the alloy system on the final macrosegregation.
Macrosegregation is a result of the interplay of various transport mechanisms, including natural convection, solidification shrinkage, and grain motion. Experimental observations also indicate the impact of grain morphology, ranging from dendritic to globular, on macrosegregation formation. To avoid the complexity arising due to modelling of an equiaxed dendritic grain, we present the development of a simplified three-phase, multiscale equiaxed dendritic solidification model based on the volume averaging method, that accounts for the above-mentioned transport phenomena. The validity of the model is assessed by comparing it to the full three phase model without simplifications. It is then applied to qualitatively analyze the impact of grain morphology on macrosegregation formation in an industrial scale direct chill (DC) cast aluminium alloy ingot.
Macrosegregation is a severe defect present in direct-chill (DC) cast aluminium ingots and billets. In the recent years, experimental studies were conducted to modify and to an extent optimize macrosegregation formation by modifying the inlet melt flow. Due to several limitations, the grain settling behavior and corresponding liquid flow pattern is difficult to analyze using experiments. Simulations on the other hand can provide this insight. However, conducting 2D sheet ingot simulations, as has been previously done, provides an incomplete description of flow pattern. To avoid this and as a first qualitative study, full scale 3D sheet ingot simulation results with two different inlets are presented in this paper. A simplified three-phase multiscale solidification model accounting for solidification shrinkage, natural convection and equiaxed grain growth and transport is used to conduct this study. We show that modification of inlet flow results in modification of grain settling and eventually leading to modification of macrosegregation. The impact of grain morphology is also additionally analyzed.
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