Application of Thermal Plasma Arc Melting for Critical Alloy Ingot Preparation

Volatile foreign dependence, cyclic availability, and restricted critical material stockpiles have focused considerable attention on the feasibility of new and more efficient processing/ recycling schemes. Over the past several years United Technologies Research Center (UTRC) has been investigating plasma processing techniques for the reprocessing and upgrading of critical materials and alloys. As part of this effort, a laboratory scale dc thermal plasma reactor operating in the transferred mode was developed and successfully demonstrated. The reactor was originally developed for recycling MCrAlY and Ni alloy materials and more recently modified for processing titanium (Ti) alloys. The reactor demonstrated the ability to produce homogeneous void-free 5-cm-diameter ingots in an inert atmospheric pressure environment without loss of volatile alloy constituents and with no significant contamination introduction. Exploratory experiments were also conducted to investigate the effectiveness of this type of plasma reactor for reducing the surface oxygen contamination of the Ti alloy feed material, as generated during high-speed machining operations. The process is based on high-reactivity atomic hydrogen species generated in the plasma region for reducing the oxygenated titanium surface compounds (measured to be predominantly TiO2). Only modest oxygen reductions were achieved. Aspects of this research and development will be described in this paper.