Electron beam melting (EBM), invented nearly 100 years ago, is a two-mode process. The original drip-melting mode relies on selective vacuum purification, removing impurities with vapor pressures higher than the metal being processed. In cold-hearth refining, or EBCHR, inclusions are removed as the molten metal flows through the hearth by flotation and sedimentation in addition to the evaporative purification. Those floating on the surface break down on exposure to the beam and either evaporate or dissolve in the metal. The heavy inclusions sink to the bottom of the hearth and are removed with the skull. Any light inclusions that remain floating are prevented from flowing into the ingot by a dam.
Materials processed
Metals and alloys processed using the drip melting mode include tantalum and its alloys with Mo and W; niobium and its alloys with Zr and Sn, zirconium and hafnium and their alloys; platinum and irridium and their alloys; silicon; vanadium; Ur and alloys and steel (only in Russia).
EBCHR is the principal process for Ti scrap remelting, often with sponge added. The process accounts for almost 90% of Ti produced in the U.S. today. A good part of this is CP titanium and specifically CPI, CP4, CP7 and CP12 alloys. The process also produces some high purity titanium for use in electronics.
In combination with vacuum arc remelting, EBCHR becomes the principal process for the production of premium quality Ti alloys for aircraft rotating components with over 40-million pounds having been so produced by the year 2000 [1]. The more important of these are Ti-6-4, Ti-6-4 ELI, Ti-17. This is also the technology used to produce clean super alloys, and in a few instances, special ceramics. The materials mentioned above are processed in single- and multigun (as many as eight) furnaces of various designs, ranging in power from 30 kW to 6 MW. Today there are some 30 EB furnaces in the U.S. having a total capacity exceeding 28 MW of EB melting power-the world leader.
Titanium and its alloys are used in airframe components and jet engine parts, chemical plants, desalinating plants, power stations, sputter targets, hip joints, dental implants, golf clubs and eye glass frames. A steady growth in capacity is expected as growth in demand for EBCHR-processed materials increases. An exciting growth above and beyond this gradual growth could take place if a notable reduction in the cost of extracting Ti from its ore is achieved, and this is independent of recently referred alternate process and corresponding cost reduction [2]. The lower priced Ti sponge together with EB-melted Ti scrap will permit penetration in the automotive market. In 1997, it was estimated that the price of Ti mill product to enable penetration into the automotive market would have to be $2.00/lb [3].
Tantalum and its alloys are used in chemical process equipment, sputter targets, prosthetic devices, capacitors and high temperature parts, and niobium and its alloys are used in superconductors, chemical process equipment, sputter targets, special implants and shape memory alloys.
An EB melting test is also the key to determination and measurement of alloy cleanliness [4]. The sample material to be tested is melted under controlled conditions and the button so produced is evaluated (Fig. 1).
Current status
As in any field of technology, there are leaders in EBMR technology. On the equipment end, the pioneering organization has been von Ardenne Anlagentechnik, an electron-gun producer, which alone or with partners has built bulk of the EB melting furnaces in the U.S. This company has maintained uninterrupted R&D in electron guns; guns rated at 100 to 300 kW of power were perfected by 1991. At that time, effort was initiated to bring comparable performance for guns rated at 500 to 700 kW. This effort led to the development of the EH800V gun [5], which is shown in Fig. 2. Table 1 compares the details of this gun with the last of the 750 kW. Leybold (now ALD) has been the principal competitor to von Ardenne over the years.
On the process side, the leadership of two individuals must be recognized: Professor Alex Mitchell of the University of British Columbia for his efforts and contributions to the understanding of the science of the EB melting and refining process, and Howard Harker, the leader of the Timet (Morgantown, Pa.) team. In addition to effecting practical process advances, Harker designed and led the construction of the pace-setting twin chamber furnaces [6].
In attempting to improve the furnace performance and develop trouble-free melting operations, the Timet team concentrated on the guns, the key to trouble-free furnace operations. The requirements for uninterrupted production present many challenges to personnel operating the guns that power the furnaces. According to Timet's Mark Pauster, an expert system was selected in an effort to aid furnace operators in the diagnosis and correction of electron beam gun problems. The current expert system in the Morgantown facility is in its early stages of implementation and covers operational causes related to high gun operating temperatures. This initiative is in alignment with the company's extensive continuous improvement program aimed not only at improving operational efficiencies of the furnaces, but also developing titanium alloys and products efficiently through electron beam melting-one of Timet's core competencies. This system is related to a family of software systems that help to minimize manufacturing equipment downtime due to both scheduled preventative maintenance and unscheduled repairs.
The expert system was developed with an easy-to-use rule editor, and is deployed on a commercially available runtime Decision Support System (DSS) platform (developed by Eric W. Stein, ews3@psu.edu). The system runs on a standard PC running Windows 95, NT 2000, or later. These PCs are located in the control pulpits of the three furnaces (one per furnace), which are networked to the company's Intranet. The system implemented includes a knowledge-based design to aid operators in diagnosing operational problems with electron beam guns.
The knowledge base was constructed from information acquired from two major sources: text-based and human-based knowledge. It comprises many modules that deal with different conditions and furnaces. This interactive program begins by asking the pulpit operator a set of general questions about the nature of the failure and drills down to more specialized questions as the session continues, self-pruning as it goes. The system is anticipated to produce a significant reduction in the learning curve associated with the operation of an electron beam gun [7].
The justification of the system is in hard cost savings, improved product consistency and quality. This is how Timet, the leader in this technology, effects improvements in its operations; an approach that has been quite productive. It is the belief of the author that all other electron-beam melting furnace operators would be inclined to develop or acquire similar systems to improve their operations. While Timet has presented the information on the expert system in general terms [8], system details are proprietary. The use of such control technology will lead to substantial improvement in EB melting operations across the board, as well as to improved melting operation economics.
EB melting technology has seen tremendous progress in the past two decades. This period was also the most consequential in terms of capacity growth in the U.S. It witnessed the construction of the most powerful and most advanced EB installation in the world. Two particularly noteworthy installations are Timet's second twin chamber furnace built in 1997 and powered by six 825-kW guns [8] and International Hearth Melting's (Richland, Wash.) 5.4-MW, eight-gun powered furnace also built in 1997 [9]. Timet's twin chamber furnace is capable of casting 50,000-lb Ti slabs at melting rates exceeding 5,000 lb/h. The International Hearth Melting furnace is the largest U.S. EB melting installation, capable of casting 50,000 lb slabs at casting rates to 8,000 lb/hr.
Looking ahead
A basic understanding of the process that occurs during EB melting has lagged behind the commercial use of the technique, and problems remain. Solutions to these problems are certain to further improve process economics and are likely to lead to even wider use of the technology.
While real-time composition control has seen considerable advances, there is still much to be accomplished in this area with the aim of improving ingot homogeneity.
The peculiarities of the EB melting process permit a degree of control over the ingot solidification structure not possible in other processes. Yet, the capability to control the solidification of the ingot has not been thoroughly explored. It is expected that in the future we will be capable of evaluating the real contribution that this aspect of electron-beam melting can make. Further studies in this area should lead to procedures to improve ingot structures. The ultimate objective is to produce structures required by the aircraft industry without the need for additional vacuum arc remelting.
References
- C. E. Shamblen, A. P. Woodfield, A Decade of Titanium Alloy Hearth Melting, Proc., Conf. Electron Beam Melting and Refining - State of the Art 2000, R. Bakish (Ed), Bakish Materials Corp., p 27-34
- G. Crawley, How to Extract Low Cost Titanium, Adv. Matls & Proc., Vol. 161, No. 11, Nov. 2003, p 25-27
- F.H. Froes, O.N. Senkov, Ti Today And Tomorrow, Proc., Conf. Electron Beam Melting and Refining-State of the Art 1997, R. Bakish (Ed), Bakish Materials Corp., p 2-27
- W.H. Sutton, D. Clark, Development Of An EB Melting Test To Evaluate The Cleanliness of Superalloys, Proc., Conf. Electron Beam Melting and Refining - State of the Art 1983, R. Bakish (Ed.), Bakish Materials Corp., p 36-60
- M. Neumann, C. Deus, B. Wenzel, EH800V - The Completion of von Ardenne's New EB Gun Series, E-Beam 2002, S. Schultz (Ed), Surface Solutions, p 9-1 - 9-6
- S.T. Tilmont, H.R. Harker, Maximelt - Axel Johnson Metals' New Electron Beam Furnace, Proc., Conf. Electron Beam Melting and Refining - State of the Art 1990, R. Bakish (Ed), Bakish Materials Corp., p 135-145
- D. May, M.C. Pouster, E. Stein, The Application Of An Expert System In Troubleshooting EB Gun Operational Problems, E-Beam 2002, S. Schultz (Ed), Surface Solutions p 17-1 - 17-4
- M.C. Pauster, H.R. Harker, Maximelt II - Titanium Hearth Technologies' New Electron Beam Furnace, Proc., Conf. Electron Beam Melting and Refining - State of the Art 1997, R. Bakish (Ed), Bakish Materials Corp., p 262-267
- C. Aguirre, S. Richman, I.A. Grosse, Design Construction And Commissioning Of Top Gun, Proc., Conf. Electron Beam Melting and Refining - State of the Art 1998, R. Bakish (Ed), Bakish Materials Corp., p 41-43
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