Scale Rules for Macrosegregation during Direct-Chill Casting of Aluminum Alloys
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TRODUCTION
MACROSEGREGATION is a widely spread, unrecoverable defect of large-scale billets and ingots produced by a direct-chill (DC) casting route. The fundamental cause of chemical inhomogeneity (macrosegregation) is in the relative movement of solute-rich liquid and solute-lean solid during solidification. There are four well-adopted mechanisms of macrosegregation: thermosolutal convection and transport of solid grains in the slurry and upper mushy zone, and shrinkageinduced flow and deformation of the solid network in the mushy zone. It is clear that the overall macrosegregation pattern is a result of the different contributions of these mechanisms, as summarized in Table I.[1,2] When it comes to practice, it is established that the size of the casting and the casting speed are the main parameters that determine the extent of macrosegregation.[1,3] There is a view that the negative centerline segregation is the sole result of accumulation in this region of solute-lean ‘‘floating’’ crystals.[4] This standpoint is, however, in contrast with (a) experimental observations on negative centerline segregation in billets and ingots without floating crystals[5] and (b) numerical studies that allow one to obtain negative segregation by invoking only the shrinkage-induced flow without the transport of the solid phase.[6,7] The empirical rule says that the casting speed should not exceed A/D in order to minimize the macrosegregation in a round billet while maintaining the required DMITRY G. ESKIN, Senior Scientist, and QIANG DU, Postdoctoral Researcher, are with the Netherlands Institute for Metals Research, 2628CD Delft, The Netherlands. Contact e-mail: [email protected] LAURENS KATGERMAN, Professor, is with the Department of Materials Science and Engineering, Delft University of Technology, 2628CD Delft, The Netherlands. Manuscript submitted June 7, 2007. Article published online March 14, 2008 1206—VOLUME 39A, MAY 2008
castability and acceptable productivity. In this ratio, A is an alloy-dependent constant ranging from 25,000 to 33,300 for aluminum alloys and D is the billet diameter in mm.[8] Hence for a 200-mm billet, the maximum allowable casting speed should be between 125 and 167 mm/min. Livanov et al.[3] studied macrosegregation in a wide range of billet diameters, from 50 to 470 mm, and of casting speeds from 30 to 350 mm/min. They showed that the extent and direction of macrosegregation depends on the average cooling rate in the solidification range. There exists a certain cooling rate for each billet diameter, when the centerline macrosegregation is negligible. For example, this threshold cooling rate is about 2.3 K/s for the 300-mm billet diameter and almost 11 K/s for the 168-mm diameter. Evidently, the cooling rate cannot be a universal criterion for macrosegregation. Livanov et al. then made a remarkable conclusion that the similarity of thermal and strain fields should lead to a similarity in macrosegregation patterns. Hence, they suggested a similarity criterion:[3] K ¼ Vcast D=a
½1
where Vcast is the cast
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