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There is an increased demand for larger and better-quality vacuum arc melted superalloy ingots. However, as the ingot size increases, the tendency for segregation increases. The segregation can be minimized either by decreasing the melt rate or by improving the heat transfer from the ingot. The former technique results in decreased productivity and increased energy consumption during melting. The heat transfer from the ingot to the cooling water in VAR is mainly controlled by the heat transfer coefficient between the ingot and crucible. This heat transfer coefficient can be increased by introducing a gas of high thermal conductivity, like helium, in the shrinkage gap between the ingot and crucible. Even though this technique is being used by superalloys manufacturers, no systematic study has been carried out to quantify the effect of helium gas pressure. Hence, the present experimental work was undertaken to study the effect of helium gas cooling on the heat extraction rate, molten metal pool depth, mushy zone size and segregation during vacuum arc remelting of alloy 718. The experiments were carried out in a laboratory vacuum arc furnace capable of making 210 mm diameter ingots weighing up to 150 kgs. In this paper, the effect of helium gas pressure on the solidification behavior of alloy 718, like molten metal pool depth, surface quality of the ingot, dendrite arm spacing, mushy zone size and Laves phase, have been discussed. The increase in helium gas pressure resulted in a decreased molten metal pool depth. For example, with 60 mm of helium gas pressure, the pool depth decreased from 200 mm to 127 mm, i.e., about 36% decrease. Measurements on dendrite arm spacing from edge to center of the ingot in the radial direction showed that, with the helium gas cooling, the dendrite arm spacing decreased at the edge as well as at the center of the ingot. The calculated mushy zone sizes using the DAS measurements showed a decrease in mushy zone size with the helium gas cooling. The amount of Laves phase formed decreased with helium gas cooling. The helium gas cooling between the ingot and the crucible has shown to improve the solidification characteristics and minimize segregation in alloy 718. Superalloy 718-Metallurgy and Applications Edited by EA. Loria The Minerals, Metals & Materials Society, 1989

Solidification of Alloy 718 During Vacuum Arc Remelting with Helium Gas Cooling Between Ingot and Crucible