Investigation of Macrosegregation Formation in Aluminium DC Casting for Different Alloy Systems
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THE redistribution of solute at the scale of the cast product due to the relative motion between the solid and liquid phase is referred to as macrosegregation. This relative motion is driven by shrinkage-induced flow, natural convection due to thermal and solutal gradients, movement of the equiaxed grains and thermally induced deformations of the mushy zone. A rather comprehensive description of these mechanisms can be found in the literature.[1] Due to the severity of this casting defect, significant effort has been dedicated to the understanding and modeling of macrosegregation formation in DC casting. A study on the effect of shrinkage-induced flow and thermal-solutal convection in DC casting, based on the volume-averaging method proposed by Ni and Beckermann,[3] was made by Reddy and Beckermann.[2] Reddy and Beckermann studied Al-Cu billet and controlled natural convection intensity with mushy zone permeability. For a moderately permeable mush, they
AKASH PAKANATI is with the Department of Materials Technology, NTNU, 7491 Trondheim, Norway. Contact e-mail: [email protected] MOHAMMED M’HAMDI is with the Department of Materials Technology, NTNU and also with SINTEF Materials and Chemistry, 0314 Oslo, Norway. HERVE´ COMBEAU and MIHA ZALOZˇNIK are with the Institut Jean Lamour, CNRS – Universite´ de Lorraine, Campus Artem, 2 alle´e Andre´ Guinier, 54000 Nancy, France and also with the Laboratory of Excellence on Design of Alloy Metals for low-mAss Structures (’DAMAS’), Universite´ de Lorraine, Lorraine, France. Manuscript submitted December 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
observed positive segregation at the center and negative segregation close to the surface. Significant improvements in modeling have been achieved over the years, especially pertaining to grain motion. Wang and Beckermann[4,5] proposed the first model to numerically simulate equiaxed dendritic solidification in the presence of natural convection. Vreeman and Incropera[6,7] conducted a study on DC-cast billets with Al-Mg and Al-Cu. Their model accounted for grain motion and thermal-solutal convection. Several recent advances were made in the modeling of solidification[8–14] and DC-casting process.[15–23] Zalozˇnik and Combeau[12] proposed an operator splitting scheme to couple macroscopic transport and grain growth in a two-phase multiscale solidification model. The model was further extended to include inoculant motion.[16,24] Zalozˇnik et al.[16] conducted a systematic study of the influence of various transport mechanisms contributing to macrosegregation in an Al-Zn system. For a case with only thermal-solutal convection as the driving force, they also observed positive segregation at the center and negative segregation at the surface, respectively. This pattern was attributed to both copper and zinc being heavier than aluminum, resulting in the contribution of thermal and solutal convection. In contrast, Jalanti[25] and Bedel[18] both independently concluded that the thermal-solutal convection in DC casting of Al-Mg contribute
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