Arc voltage distribution properties as a function of melting current, electrode gap, and CO pressure during vacuum arc r
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INTRODUCTION
VACUUM arc remelting (VAR) is a process widely used throughout the specialty metals industry to produce segregation-sensitive metal alloys. The standard ‘‘straight polarity’’ process involves striking an electrical arc between an electrode (cathode) consisting of the material to be cast and the bottom of a cylindrically shaped, water cooled, copper crucible (anode). Heat from the arc causes material at the electrode tip to melt and drip into the crucible, forming a molten pool. As the process continues, a semidirectionally solidified ingot forms with a molten pool situated on top. Because the electrode is necessarily of a smaller diameter than the crucible, it must be translated downward to maintain a constant spacing between the electrode tip and the molten pool surface as the ingot grows. This spacing is known as the electrode gap (G) and is only approximately defined, since both surfaces are molten and in constant motion. When processing segregation-sensitive materials, it is important that the solidification front reach a steady-state condition and not be significantly perturbed during the remelting operation. This assures that the product will be free of solidification defects which could be initiated by instabilities in the liquid-solid interface region (the mushy zone). The underlying assumption of modern VAR control practice is that steady-state melting conditions are met by holding the melting current (I), electrode gap, ingot wall cooling gas flow rate, and ambient furnace pressure (P) constant. Of these process variables, the most difficult to control is RODNEY L. WILLIAMSON, Senior Member of Technical Staff, and FRANK J. ZANNER, Distinguished Member of Technical Staff, are with the Materials Processing Department, Sandia National Laboratories, Albuquerque, NM 87185-1134. S.M. GROSE, Superintendent-Melting, is with INCO Alloys International, Inc., Huntington, WV 25705. Manuscript submitted March 7, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
the electrode gap. Modern VAR control systems use either mean arc voltage (VM) or drip-short frequency (fDS) as an electrode gap indicator; however, we have observed that neither of these responses are immune to process perturbations that affect the electrode tip temperature distribution. Common perturbations of this type observed in industrial practice are glow (either ‘‘dirt’’ or gas induced), anomalous local heating due to lateral electrode cracks or voids, and deviations in electrode tip geometry (such as rounding). The latter condition is particularly difficult to deal with from a control standpoint because it can lead to a relatively large increase in electrode gap with no concomitant change in fDS and virtually none in VM. Adequate control of the VAR process requires finding and monitoring responses that give unambiguous information about the magnitude of the electrode gap. Though the mean of the arc voltage distribution has been used for years as an indicator of electrode gap, the other common properties of this distribution, namely, t
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