Influence of Forced/Natural Convection on Segregation During the Directional Solidification of Al-Based Binary Alloys
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s accompanied by a gradual change in composition and is an important and well-known phenomenon during solute-rich alloy solidification, which often results in the deterioration of the material properties. Segregation may occur on various scales in the ingot; microsegregations are linked to composition variations on the scale of the lowest solid structure, whereas macrosegregations concern the scale of the product. For intermediate scales, one particularly striking form is ‘‘channel segregation.’’ Typical dimensions are several centimeters in length with crosssectional dimensions of a few millimeters, so they appear as ‘‘freckles’’ on transverse sections. These freckles have been observed in many materials such as forge ingots, vacuum arc remelted ingots, or directionally solidified turbine blades. Freckles typically have a relatively slow solidification rate (nickel base alloys,[1] high alloy steels,[2,3] and lead-tin alloys[4] or ammonium A. NOEPPEL, A. CIOBANAS, X.D. WANG, K. ZAIDAT, N. MANGELINCK, O. BUDENKOVA, and Y. FAUTRELLE, PhDs, are with the SIMAP/EPM/Grenoble Institute of Technology, 38402 Saint-Martin d’He`res, France. Contact e-mail: yves.fautrelle@ simap.grenoble-inp.fr A. WEISS and G. ZIMMERMANN, PhDs, are with the ACCESS e.V., D-52072 Aachen, Germany. N. MANGELINCK, PhD, is also with L2MP, CNRS-Universite` P. Ce`zanne, Saint Je`r^ ome - Case 142 13397 Marseille Cedex 20, France. Manuscript submitted July 17, 2008. Article published online November 11, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
chloride solutions for laboratory studies[5]). Apart from normal segregation that is linked to the partitioning of a solute at the liquid–solid interface, convection in the mushy zone also can produce such segregations. Convection may originate from thermosolutal buoyancy forces[6–8] as well as from any forced-melt flows (e.g., electromagnetic stirring[9] or volume changes caused by the shrinkage). It has been shown both experimentally and numerically that fluid flow (especially electromagnetically driven flows) can produce macrosegregations[10–13] and change the structure.[13] Furthermore, special design of the flow field can be used to control the segregation pattern[9,10] and even to suppress segregation when stirring is pulsated or periodically reversed.[13,14] Observations also indicate that a forced flow along the columnar front produces a decay of the dendrite spacing.[11] This study analyzes the effects of forced convection on the segregations during solidification of alloys by comparing the results of experiments with numerical modeling. The work is divided into two main parts; each deals with a different type of electromagnetically forced convection. The first type of convection is induced by a rotating magnetic field (RMF), whereas the second is created by a traveling magnetic field (TMF). This study takes into consideration the directional solidification of aluminum-based alloys. Section II provides a brief description of the effects of both RMF and TMF on a liquid metal flow. Section III is devoted to the descri
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