On Macrosegregation
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ING solidification of alloys, the solid usually has a lower composition compared to the liquid from which it crystallizes. The corresponding solute redistribution leads to an accumulation of solute in the melt at the solid/liquid interface and thus to a constantly changing composition of the solid layers that form. The solute enrichment of the remaining liquid continues until the occurrence of a low melting point phase finally completes the solidification process. The resulting inhomogeneity of the solute elements is termed as microsegregation as it occurs on the scale of the growing crystals, the scale of the microstructure that forms.[1] On the other hand, the solute composition may also vary on the scale of the whole casting. Such macroscopic inhomogeneities of solute elements are termed as macrosegregation.[2] The difference between microsegregation and macrosegregation is that macrosegregation cannot be removed by heat treatment as diffusion in solids is slow even for elevated temperatures and large diffusion distances might result in impracticably long holding times for homogenization. A review of the history of publications on macrosegregation (started as early as 1540 A.D.) can be found in Reference 3. From the pioneering work of Kirkaldy
ANDREAS LUDWIG, Full Professor, is with the Chair of Simulation and Modeling of Metallurgical Processes, Montanuniversitaet Leoben, Leoben, Austria. Contact e-mail: ludwig@unileoben. ac.at MENGHUAI WU, Associate Professor, and ABDELLAH KHARICHA, Senior Scientist, are with the Chair of Simulation and Modeling of Metallurgical Processes, Montanuniversitaet Leoben, and also with the Christian-Doppler Laboratory for Advanced Process Simulation of Solidification and Melting, Montanuniversitaet Leoben. Manuscript submitted September 20, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
mentioned in Reference 4 and Flemings et al. in the latter half of the 1960s,[5–7] it is well known that macrosegregation may form when the (micro)segregated melt adjacent to some solid is swept away by a relative motion between the solid and its surrounding liquid. The following six major phenomena have been identified as the reasons for such a relative motion[8]: forced flows due to pouring, gas purging, mechanical
and electromagnetic stirring, etc.; buoyancy-induced flows due to thermal and solutal
gradients in the liquid; flow that feeds the solidification shrinkage and the
contractions of the liquid and solid during cooling; movement of free (equiaxed) grains or solid frag-
ments; deformation of the solid network due to thermal
stresses, metallostatic head, or external forces on the solid shell; motion of gas bubbles that develop during solidification. As melt flows, it is not easy to quantify the motion of crystals and deformation of a solid skeleton, and thus reports on macrosegregation formation often limit themselves to one of the following four cases: (i) DCcasting of copper-based alloys, (ii) DC-casting of aluminum-based alloys, (iii) continuous casting of steel, and (iv) ingot castin
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