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THE metallurgical structure of a Vacuum Arc Remelting (VAR) ingot depends critically on the temperature distribution during solidification, and because advection is the dominant mechanism for transporting heat in the metal pool, the correct characterization of the liquid metal behavior is very important. There are two competing sources of liquid metal motion in the pool. One of them is buoyancy, which arises from temperaturedependent density differences within the pool; and the other is the distributed Lorentz force, which results from the flow of direct current through the pool from the arc. It is well known that a reversal in flow direction gives rise to a change in the pool shape and influences the ingot structure.[1] The surface of the pool is heated by the arc and probably has a temperature 50 C to 150 C above the alloy liquidus;[1,2] the temperature difference between this heated top surface and the area at the edge of the pool, which is cooled by contact with the crucible, drives a buoyancy flow. Metal travels radially outward across the pool top surface to the pool edge where it is cooled and flows down past the dendrite tips at the solidification front, finally rising up the pool axis to repeat the D.M. SHEVCHENKO and R.M. WARD, Research Associates, are with the Metallurgy and Materials Department, University of Birmingham, Birmingham, B15 2TT, England. Contact e-mail: [email protected] This article is based on a presentation given at the International Symposium on Liquid Metal Processing and Casting (LMPC 2007), which occurred in September 2007 in Nancy, France. Article published online December 24, 2008. METALLURGICAL AND MATERIALS TRANSACTIONS B

circulation. At the same time, electric current, introduced to the pool surface by the arc, flows through the pool to the ingot and then to the crucible wall. The current density, J, with its associated magnetic field, B, gives rise to the Lorentz force F = J 9 B. Because the current density is not uniform in the pool, the Lorentz force is higher in the top of the pool and lower at the bottom. Therefore, in the absence of buoyancy, the Lorentz force drives the fluid flow radially inward near the top of the pool and then down the centerline of the arc current. So, the balance between these two forces controls the flow in the pool. Previous modeling of the pool behavior (assuming that the arc distribution was axisymmetrical)[3–5] has indicated that, under the usual melting conditions for 508-mm-diameter nickel superalloy ingots, the pool was mostly controlled by the buoyancy force. However, analysis of the arc behavior during an experimental melt of a 440-mm INCONEL* 718 electrode into a 508-mm *INCONEL is a trademark of Inco Alloys International, Huntington, WV.

ingot under nominally diffuse conditions suggested that, most of the time, the electrical center of the arc was located at a radial distance of around 100 mm from the ingot centerline and, superimposed upon the normal high-speed random motion typical of diffuse arc VAR, rotated in a time-averaged sense clockwi