Modeling of the Coupling of Microstructure and Macrosegregation in a Direct Chill Cast Al-Cu Billet
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ll (DC) casting is the principal process for the production of wrought aluminum alloys. It is used to produce cylindrical billets for extrusion of profiles and rectangular ingots for rolling of plates, strip, and foil. Macrosegregation is a common defect in DC casting and can lead to non-uniform material properties. Since it is generally aggravated by higher casting speeds and casting size, it also limits productivity. Macrosegregation is characterized by variations of the chemical composition on a scale larger than the crystal grains, which cannot be mitigated during the further downstream processing. The redistribution of the chemical species (solutes) at the product scale during solidification is caused by the relative motion of liquid and solid with different compositions. It is generally
LAURENT HEYVAERT, MIHA ZALOZ˘NIK, and HERVE´ COMBEAU are with the Department SI2M, Institut Jean Lamour, CNRS Universite´ de Lorraine, Parc de Saurupt CS 50840, 54011 Nancy CEDEX, France. Contact e-mail: [email protected] MARIE BEDEL is with the Department SI2M, Institut Jean Lamour, CNRS Universite´ de Lorraine, and also with the E´cole Nationale Supe´rieure d’Arts et Me´tiers, Campus d’Aix-en-Provence, 13617 Aix-enProvence, France. Manuscript submitted February 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
admitted that the macrosegregation in DC casting is a consequence of an intricate interplay of the melt flow, induced by thermosolutal natural convection, shrinkage, and pouring, and of the transport of solute-lean free-floating grains.[1–3] Melt flow due to stress-induced deformation can also play a role.[4] The importance of the individual mechanisms varies depending on the alloy, casting shape and size and process parameters.[3] It is however difficult to construct a simplified general image of the macrosegregation formation in DC casting, because all these strongly coupled transport phenomena have a significant impact. A thorough understanding of the involved mechanisms and of their coupling is required to be able to develop robust solutions that would mitigate the macrosegregation.[5] The interplay of the transport mechanisms that cause macrosegregation is closely linked to the microstructure of the solidifying grains. The grain size and the grain morphology (dendritic or globular) determine the dynamics of settling of the free-floating equiaxed grains, the compactness of the packed grain layer, and its hydrodynamic permeability. They thus affect the macrosegregation induced by the settling of solute-lean grains and the expulsion of solute-rich liquid—the prime cause of the negative centerline segregation. Furthermore, the grain size and morphology influence the macrosegregation formed by permeation of solute-rich liquid through the packed layer. These effects have been clearly
demonstrated in steel ingot casting, using a combination of industrial-scale experiments and multiscale numerical modeling.[6–10] It was shown that globular free-floating grains lead to much more pronounced settling-induced macrosegregat
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