Advancements In Electron Beam Melting and Refining
Alec Mitchell (University of British Columbia, Vancouver, B.C., Canada), a pioneer in electron-beam (EB) technology, noted that EB melting of reactive, refractory and special alloys is maturing due to better understanding of gun design and vacuum technology and the development and insight of the principles underlying control of the melting process and the metallurgical structure of the product. Process modeling is difficult because many of the critical parameters, such as fluid flow, are strongly influenced by local, rapid events associated with beam movement and instabilities. Im-provements still are needed in the control of product chemical composition and the structure of cast ingots.
Valentina Arjakova (All-Russia Scientific Institute, Russia) discussed EB melting using drip- and hearth-melting furnaces and casting in Russia. Furnaces are powered by EB guns (rated from 100 to 3,000 kW) from von Ardenne (Dresden, Germany) and the Paton Institute. Cast ingots sizes are up to 10 t for steel, 2 to 5 t for titanium, up to 2 t for zirconium and 5 t for niobium, tantalum and molybdenum. Also discussed was an electron beam skull furnace (ELGP facility) developed in collaboration with Ukrainian Institute of Casting Problems, which can cast 200-kg (440 lb) zirconium ingots. Electron beam processing also is used for ingot surface refining.
EB hearth melting and vacuum arc remelting (VAR) have been used for more than a decade to produce premium quality titanium alloys for aircraft engine rotating components. Cliff Shamblen (General Electric Aircraft Engines, Cincinnati, Ohio, USA) noted that more than 40-million lb (18 million t) have been produced using EB and plasma for hearth remelting. This approach has significantly reduced the incidence of melt-related defects such as hard alpha and high-density inclusions compared with the triple VAR process (Fig. 1).
Jean Sommeria (Techmeta, Annecy, France) discussed two very large EB welders used in special applications. An EB welding Hypermachine system powered by two 20-kW, 70-kV EB guns and including an integrated x-ray inspection station, is used to weld 3 m diameter by 11 m high (10 by 36 ft) high-strength steel rocket boosters. Another system used to continuously weld reinforced superconductor material is powered by two 20-kW, 60-kV EB guns.
H. Padamsee (Cornell University, N.Y., USA) reported on new projects calling for niobium (Nb) cavity components for which the accellerator community is seeking higher RRR Nb from the Nb-producing industry. Tables 1-4 show Nb cavities for the completed projects and those approved or contemplated for the future.
Roger Lachenbruch (Poly-cold Systems Inc., San Rafael, Calif., USA) discussed water-vapor cryopumping in vacuum furnaces. The author noted that the use of closed-loop refrigeration provides the advantage of high-speed water-vapor cryopumping to achieve process quality and throughput in vacuum process systems.
Gene Ligman (Edwards High Vacuum, Milpitas, Calif., USA) presented the benefits of dry running vacuum pumps in titanium refining applications. Benefits include stable pumping performance and predictable roughing, good dust handling using existing filters, noise reduction, oil-free exhaust fumes, elimination of oil reclaim and recirculation, increased up time and maintenance cost savings.
C.A. Blue (Oak Ridge National Laboratory, Oak Ridge, Tenn., USA) discussed plasma arc lamp processing of materials using a new heat source called radiant plasma arc (Fig. 2). The lamp is capable of producing power densities up to 3.5 kW/cm2 over surfaces as large as 3.175 cm by 35 cm (1.25 by 14 in.). The author claims this is the most powerful plasma arc source in the world, and that further development could triple the power density. Current applications include fusing coatings, heat treating, direct sheet fabrication and ceramics processing. Melting has not been investigated, but the heat source could be of interest to the electron-beam melting and refining industry.
Steady-state air leaks and transient leaks can be potential sources of titanium-nitride and hard alpha defects caused by nitriding and oxidation of ingot crown material or furnace fall-in material. J. van den Avyle (Sandia National Laboratories, Albuquerque, N.M., USA) noted that residual gas analyzers (RGA) can monitor furnace atmosphere, but the technique needs further development and calibration for industrial use. Leak-rate measurement sensitivities vary with leak location and RGA sampling location and are strongly affected by titanium gettering during melting.
C. Deus (von Ardenne Anlagentechnik GmbH, Dresden, Germany) discussed computer simulation in modern EB-gun design. High-power electron guns contain components that contribute to the formation of an electron beam tailored to application requirements. High-power beam specifics require incorporation of beam-generated plasma effects into the simulation regime to make correct predictions on beam propagation through the entire gun. Simulation software is aimed at improving performance and reliability of existing guns and evaluating new concepts within shorter development cycles. Designs based on simulation software work as predicted (Fig. 3).
A presentation by Robert McKoon (Lawrence Livermore National Laboratory, Livermore, Calif., USA) entitled "Design and Applications of Electron Beam Guns," briefly addressed the principles of electron gun design, and discussed the advantages and limitations of various gun designs as they evolved over the past several decades.
J. McDonald (Sandia National Laboratories, Albuquerque, N.M., USA) reviewed the automated fast response power control operation of two 600-kW von Ardenne EH600S guns at Sandia's Plasma Materials Test Facility. A high-power tetrode vacuum tube regulator circuit allows fast switching of the guns. The system produces 1,200 kW continuously over several hours or pulsed operation as short as 5 ms. The system is used to study the thermal response of medium-sized, high heat flux test samples and components for use in tokomak fusion reactors and other high-energy sources.
A presentation by E. Maedler (von Ardenne Anlagen-technik GmbH, Dresden, Germany) addressed the improved performance of electron guns obtained with a new midfrequency-power supply. The principal benefit of this power supply is the elimination of the deleterious effect of gun arcing in both melting and coating applications.
E. Maedler also discussed a new electron beam deflection control system, which allows automatic adaptation of deflection sequence parameters according to measured process data. It allows tracking of moving parts using an electron beam to control the electron beam heating pattern for evaporation rates, and allows adjustment of melting parameters to feed rates. User-programmed medium- and long-term deflection sequence variation provides mechanisms for long-term stabilization of continuous EB processes. All EB processes can thus be optimized, reducing the frequency of operator interaction, which improves process stability and reproducibility. Figure 4 shows typical beam patterns generated using dynamic algorithm realizing smooth radial power-density variation.
C. Chinnis and M. Mede (Retech, Ukiah, Calif., USA) noted that titanium plasma capacity continues to expand. There are about 10 plasma furnaces in operation today, with the largest to be installed in Russia-a 5,100 kW, 4,000 t/yr plasma cold hearth furnace, which will be powered by 5 plasma torches. Plasma melting also is being used to produce magnetic materials, high-purity coating targets and shape-memory alloys.
EB melting is used to purify silicon for photovoltaics as reported by T. Ikeda (Tokyo University Institute of Industrial Science, Japan). Unidirectional solidification of silicon is achieved using a water-cooled copper model as a crystallizer. Antimony is easily removed by evaporation and segregation during continuous ingot casting. Resistivity is increased from 0.001 to 100 _.
T. Ikeda also discussed nitrogen transfer in molten steel during EB melting. Results from a study of the rate under vacuum (0.01 Pa) in molten steel indicate a strong O2 contact dependence. The rate of nitrogen removal is measured as a function of impurity content using a quench chemical analysis technique in a small scale EB furnace.
J.M. Vilca (Engelhard-CLAL LP, Carteret, N.J.) reported that the significant increase in both the process of the platinum group metals and the lease rates has placed an emphasis on reducing refining times. Cross-contamination of the platinum group metals and alloys occurs either in manufacturing or industrial use. The significant difference in the vapor pressure of platinum group metals versus temperature offers the possibility to remove cross contamination using electron-beam melting in selected situations. Table 5 shows the results of EB refining Pt-10 Rh and iridium.
Elena Koleva (Bulgarian Academy of Sciences, Sofia, Bulgaria) discussed statistical analysis of EB melting and refining computer simulation. Statistical modeling allows estimating the temperature distribution and thermal flow in a cast ingot, as well as the solidification rate.
Superalloy-waste refining using combined induction and EB melting in a ceramic crucible was discussed by S. Ladokhin (Institute for Casting Problems, Ukraine). The technique provides a high level of refinement at a rather low specific electric energy consumption. This is a promising technique to obtain different cast parts superalloy waste. The basic parameters of the installation for combined induction EB melting are:
- Melting-chamber volume = 2.4 m3
- Mold-chamber volume = 1.2 m3
- Induction-heating power = 250 kW
- Frequency of induction heating = 2.4 kHz
- Electron-beam heating power = 150 kW
- Electron-beam heating voltage = 30 kV
- Electron-beam gun type = axial
- Melting crucible volume = 2-6 liter
- Mold temperature = 900-1,000?C
- Melt-chamber pressure = 0.13 Pa
Tables 6 and 7 show data on the effects of the hybrid melting processes on long-term stress-rupture properties.
M. Pauster (Timet North American Operations, Morgantown, Pa., USA) discussed the latest developments with the von Ardenne electron gun, which improves reliability. The improvement in heating characteristics using a new cathode unit is shown in Fig. 5.
G. Mattausch (FEP, Dresden, Germany) discussed EB high-rate evaporation of metals and compounds in reactive and nonreactive ways. For directed vapor deposition (DVD), high plasma density is needed to provide a high degree of ionization in the very dense vapor and jet gas atmosphere. The plasma source must be viable in a pressure range from 1 to 100 Pa, and must be localized to match the spread of the vapor gas jet. The charge carrier loss by jet stream must not disable the plasma function. Figure 6 shows the principle plant set up. A special plasma is needed for DVD.
J.F. Groves (University of Virginia, Charlottesville, VA) discussed directed vapor deposition technology, a process designed to enhance the creation of high-performance thick and thin film coatings on small surface areas of 100 cm2 or less. Development of DVD tech-nology has been driven by desire to combine four processing capabilities into one industrially appealing system. Specific processing capabilities are: 1) very high deposition rate (5 _m/min and higher over 100 cm2 (15.5 in.2); 2) very high material-use efficiencies on 100 cm2 (efficiencies should be at least three times those of other coating technologies; 3) process control of the atomic structure of the growing film; and 4) a highly flexible definition of growing film atomic composition.
For more information: Robert Bakish, Bakish Materials Corp., 171 Sherwood Place, PO Box 148, Englewood, NJ 07631; tel: 201-567-5873; fax: 201-567-6684; E-mail: email@example.com