Vacuum consumable arc remelting electrode gap control strategies based on drop short properties
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V A C U U M consumable arc remelting is a process used to control the solidification of segregation sensitive alloys. This control is accomplished by manipulation of the system thermodynamics in a manner such that the advancing solidification region has an abundant supply of liquid metal while simultaneously minimizing local solidification time. Energy is added to the system by means of a vacuum arc and the internal distribution of this energy within the system influences melt rate, fluid flow within the molten pool, and the volume of the molten pool. Fluid flow is particularly affected by transient behavior of this energy distribution. Hence, the remlting operation must be controlled to give a steady melt rate. One of the more obvious variables affecting this internal distribution is electrode gap (arc length). For example, as electrode gap increases, arc energy which could be used for melting is radiated directly to the crucible wall and lost to the cooling water. In situ adjustment of electrode gap based on feedback from arc events would keep the melt rate within narrow limits and improve ingot quality. The importance of electrode gap control is widely recognized by metals producers as is the difficulty of finding and implementing a suitable control strategy. From previous work 1,2it was shown that metal transfer occurs as drops which momentarily short out the arc (drop shorts) are transferred from the cathode (electrode) to the anode. The goal of this work is to evaluate electrode gap strategies based on analysis of the electrical waveforms associated with these drop shorts. Three control strategies were investigated: 1) Period 2) Resistance 3) Energy-resistance Only the drop short period strategy was successful. Since a great deal of the terminology used in this paper is developed from recent work the sequence of reporting starts with a background section. This section includes a brief discussion of the metal transfer process, F. J. ZANNER is a Member of the Technical Staff, Sandia National Laboratories, Division 5836, Albuquerque, NM 87185. Manuscript submitted October 27, 1980. METALLURGICAL TRANSACTIONS B
a definition of electrode gap, and the importance of electrode gap as related to energy distribution within the system. An experimental procedures section follows wherein the experimental apparatus and techniques used to make the measurements are described. Also included in this section is a description of the data reduction techniques. Next the experimental results are presented with emphasis on the electrode gap period models followed by drop short resistance analysis and energy-resistance relationships for drop shorts. A discussion section is then presented with particular emphasis on application of the models and data to industrial arc remelting situations. Lastly, the conclusions are presented. BACKGROUND In previous work t,2 on the vacuum consumable arc remelt process, the author has shown that at short electrode gaps metal is transferred by the formation and subsequent rupture of molten metal columns
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