Application of plasma arc melting technology to processing of reactive metals

Abstract

The requirements and limitations of furnaces and plasma torches for cold hearth remelting processes are discussed and existing transferred arc plasma technology critically reviewed, placing particular emphasis on identifying which commercial systems are applicable to these processes. Areas requiring further research are identified. Titanium, Ti alloys, and superalloys are used extensively in the aerospace industry. Waste material must be recycled, owing to its high value, and refined to remove impurities. Compositional specifications must also be maintained. Plasma melting processes have been demonstrated to be a viable alternative to electron beam remelting and offer certain metallurgical advantages. An introduction to plasma melting technology is given in which the underlying concepts of cold hearth melting — electric arcs, plasma torches, and power supplies — are described; followed by a detailed review of the current state of the art of commercial plasma arc technology and processes; discussions of the process and plasma torch criteria pertinent to the development of plasma cold hearth melting processes; and conclusions and recommendations. It is concluded that while plasma cold hearth melting processes have been developed these are currently confined to the recycling of materials for the aerospace industry before vacuum arc remelting. Only hollow electrode type torches are acceptable in this application. The use of Ti and superalloy materials will expand into other, less critical, areas in the future and more and/or larger furnaces will be required. Further work is required in the areas of process, furnace, and torch design and development, specifically the influence of process conditions on product quality, and studies of torch performance – electrode erosion, operating power, and arc-melt interactions.