A Multiscale 3D Model of the Vacuum Arc Remelting Process
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(VAR) is one of the few approved methods for producing high quality, segregation-free ingots of aerospace alloys for rotating components. The Ni-based alloys produced must exhibit excellent cleanliness, homogeneity, and be defect free. However, with the drive for new alloys, and larger components, models of the process are required to both aid understanding of the physics involved and predict final product quality. VAR is a metallurgical refining process where a long cylindrical electrode is melted in a vacuum furnace fitted with a water-cooled copper crucible by application of a Direct Current (DC). The DC current forms an electrical arc between the electrode and ingot, which melts the electrode, and transfers mass into a melt pool to form the ingot (Figure 1). The processing conditions in the ingot must be closely controlled to obtain the fine microstructure and compositional homogeneity required to achieve aerospace grade material. The final microstructure—and therefore the likelihood of defects—depends not only on local temperature gradients in the mushy zone during solid-
KOULIS PERICLEOUS, Professor, and GEORGI DJAMBAZOV, Senior Research Fellow, are with the Centre for Numerical Modelling and Process Analysis, University of Greenwich, London, U.K. Contact e-mail: [email protected] MARK WARD, Research Fellow, is with the School of Metallurgy and Materials, University of Birmingham, Birmingham, U.K. PETER D. LEE, Professor, and LANG YUAN, Research Fellow, are with the Manchester X-ray Imaging Facility, School of Materials, The University of Manchester, Manchester, U.K. Manuscript submitted August 10, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
ification but also on the fluid dynamics within the liquid metal pool. Pool behavior is governed by the competing influences of thermal plus solutal buoyancy and the electromagnetic Lorentz force generated by the arc current.[1,2] In terms of modeling, these influences can be evaluated using a macro-model of the process. Since the VAR furnace is cylindrical, prior models have assumed a two-dimensional (2D) axisymmetric mesh with a central, Gaussian distributed arc.[2–5] However, recent observations by some of the authors have shown that the arc does not remain in the center, but fluctuates in location as measured optically and via the extraneous magnetic fields appearing outside the outer jacket.[6,7] Further, for many conditions, when time averaging over a few seconds, the arc ensemble can be approximated as rotating at a particular rate, circumferentially almost half a radius off center, with the direction of rotation often reversing (illustrated by Figure 2). So far, there is no explanation as to the cause of this rotation. However, as a result of this arc motion, the axisymmetric assumption becomes invalid and a three-dimensional (3D) time-dependent model becomes necessary.[8,9] In addition to the main arc, observations show the appearance of radial arcs, striking the furnace wall. Due to these effects, a 3D transient, multiscale approach is followed in this study. One
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