Electroslag Refining as a Clean Liquid Metal Source for Atomization and Spray Forming of Superalloys

Abstract

Ceramic Inclusions - A Problem A pilot plant has been constructed to demonstrate the concept of using a combination of electroslag refining (ESR) and an induction-heated, segmented, water-cooled copper guide tube (CIG) to melt, refine, and deliver a stream of liquid metal to a spray forming process. The basic ESR system consists of a consumable electrode of the alloy to be melted, a liquid slag, and a watercooled copper crucible. The liquid slag is heated by passing an ac-electric current from the electrode through the slag to the crucible. The liquid slag is maintained at a temperature high enough to melt the end of the electrode. As the electrode melts, a refining action takes place-oxide inclusions are exposed to the slag and are dissolved. Droplets of molten metal fall through the slag and are collected in a liquid metal pool contained in the crucible below. By the addition of the induction-heated, segmented, water-cooled copper guide tube (CIG) to the bottom of the crucible, a liquid metal stream can be extracted from the liquid metal pool. This stream makes an ideal liquid metal source for atomization and spray forming. The pilot plant has been operated at a melt rate of 15 to 25 kg/min with the Ni-base superalloys Alloy 718, Rene’ 9.5 and Rene’ 88. Process optimization and cleanliness evaluation studies are in progress. Ceramic inclusions can reduce low-cycle-fatigue (LCF) life in both the powder metallurgy approach and the spray forming approach to the preparation of superalloys [5,6]. Mechanistically, a large ceramic inclusion will crack early in life and act as a crack starter for the surrounding superalloy. Direct observation of this event is shown in Figure I. The aluminum oxide particle cracked very early in life and the crack rapidly extended into the surrounding superalloy.