In order to study the effect of variation in process parameters on the final ingot of INCONEL 718 during vacuum arc remelting, a 508-mm-diameter vacuum arc remelting (VAR) ingot of INCONEL 718 was produced under conditions of variation in electrode diameter and arc gap. It was sectioned and analyzed in the as-cast state. Freckle was found in the midradius and center regions when the electrode diameter and arc gap changed. The experimental results were discussed in relation to composition, current distribution, and fluid flow. Freckle formation was found to be associated with large changes in melting conditions probably resulting in significant perturbation of the fluid flow in the melt pool and mushy zone. Instability in the solidification process may contribute to the initiation and formation of freckle defects in INCONEL 718 ingot.

The purpose of this paper is to show the results of some work at the IRC in Materials for High Performance Applications at the University of Birmingham into the effect of processing and process parameters on the microstructure, macrostructure and chemistry of 100 mm diameter single plasma-melted ingots of a gamma-based titanium aluminide (Ti-48at%Al-2at%Mn-2at%Nb). The microstructure of the aqmelted bars is almost completely lamellar and consists of a chill layer of fine prior alpha grains at the surface and larger columnar grains growing into the centre of the bar. These microstructural features show little variation with processing conditions. The orientations of the alpha grains have been used to determine the effect of plasma brch current and ingot withdrawal rate on melt pool shape under a variety of operating conditions and it has been established that, at typical operating conditions, the ingot withdrawal rate has a more significant effect on melt pool depth than the plasma torch current, especially at faster withdrawal rates. Chemical analyses has shown that there is negligible net loss of any of the major alloying elements, although the degree of as-cast chemical homogeneity needs to be increased in the light of the extreme microstructural sensitivity. Further investigation has shown that one major cause of inhomogeneity is macrosegregation indued by short-term variations in melt pool shape. m he-implications of these results for the processing of titanium aluminides are then discussed.

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