Electron-Beam Technology Makes Tough Metal-Processing Jobs Easy
Melting, refining and crystal growing
The need to melt high melting-point metals, such as tantalum and tungsten, led to the development of an electron-beam (EB) melting method in 1907 by M. von Pirani (U.S. Patent No. 848,600). The invention contained the ideas of self- and work-accelerated guns and of single and multiple gun furnaces. There was no concern regarding the purity of the processed metal at time of the invention. The importance of his invention was not recognized for nearly five decades, and this year marks only the 50th anniversary of the true commercialization of the process by H. Smith. Today, electron-beam melting is the most important electron-beam application.
Two electron-beam melting, metal-purifying processes used today are drip melting and EB cold-hearth refining (EBCHR). In drip melting, the metal is drip melted in a water-cooled ingot mold, which makes it possible to remove entrapped gases and to evaporate all impurities having vapor pressures exceeding that of the metal being processed. Drip melting is the technology used to process refractory metals such as tantalum, niobium and molybdenum and reactive metals such as titanium and zirconium, as well as some platinum metals. Fig. 1 shows a drip-melting facility.
In EBCHR, the molten metal flows along a water-cooled hearth before entering a water-cooled ingot. While flowing along the hearth, high- and low-density inclusions are removed in addition to evaporative purification. High-density inclusions settle and are removed in the scull, while lighter inclusions either are dissolved in the metal or held back by a dam prior to flowing into the ingot. This is the principal technology for processing of reactive-metal scrap. Titanium accounts for 90% of metal so processed.
The commercialization of the process began with the production of small ingots in laboratory scale furnaces. By the 1960s, ingot size approached 150 mm (6 in.) in diameter and weighed several hundred pounds. Transverse guns used in early laboratory type installations have been replaced in today's industrial furnaces by differentially pumped axial guns. The first truly consequential growth in EB melting capacity started in the 1980s. The last important capacity growth in the U.S. was triggered by the introduction of the revolutionary two-chamber design by Timet in 1990 (Fig. 2), materially improving the efficiency of the process by permitting melting in one chamber, while preparing the second one for melting. The furnace is a 5-MW hearth melting installation, powered by six 50-kV, 750-kW electron guns capable of melting at rates approaching 5000 lb/h (2270 kg/h) and producing 35 ton ingots (Fig. 3).
A 600-kW drip melting furnace at the ALTA Group, a Div. of Honeywell and a 5.4-MW hearth melter powered by eight electron guns (the largest EB melting furnace in the U.S.) at International Hearth Melting, Richland, Wash., were commissioned in the 1990s. These are the most modern EB installations ever built; furnaces having a great deal of automation. These furnaces brought U.S. EB melting capacity to nearly 30 MW.
Timet is the largest EB melter (its principal product being processing of titanium scrap) followed by International Hearth Melting. Timet also has been the leading innovator in the U.S. Wah Chang is the leader in refractory-metals processing, with Cabot, H.C. Starck and ALTA sharing this market, and several smaller EB melters rounding up EB melting capacity in the U.S. Titanium scrap processing accounts for bulk of the EB melted tonnage with the balance being refractory metals.
EB melting also is the sole technology that makes it possible to monitor the cleanliness of metals using specially designed button melters. Materials to be evaluated are drip melted into a button mold, floating the inclusions to the surface for subsequent analysis using scanning electron microscopy (SEM) as shown in Fig. 4.
The electron beam also is an integral part of several special reactive- and refractory-metals production processes including making sheet and foil by pouring a stream of metal on chilled rolls (producing a sheet that can be rolled in line to foil), producing pellets by pouring on a chilled shaped roll and dropping the cast pellets on a conveyor belt and producing powder by pouring the melt on a high-speed rotating roll. Capabilities also are at hand to produce directionally solidified turbine blades and a variety of castings, including rods, rings and even hollow shapes; to grow single crystals using Czochralski and Verneuil processes and to carry out floating-zone refining. The 0.125 in. (3 mm) diameter floating zone-refined crystals first produced in 1957 have been superceded by production of large diameter crystals for industrial use with the bulk of the work in the U.S. originating in the former Soviet Union.
Today, electron-beam evaporation follows melting in industrial impact. As many as 20,000 transverse electron guns in the 2 to 20 kW power range and having a diversity of features produce thin films for the optical, electronics and aerospace industries.
The first industrial-size market area for EB evaporation is referred to as vacuum web coating, in which axial guns provide the heating. This encompasses coating of polymeric webs up to 60 in. wide and metallic webs up to 35 in. (1,525 and 890 mm) wide using a variety of materials. The evaporant is held either in a water-cooled crucible or in hot crucible (where higher rates of evaporation are needed), as is the coating of metallic webs, which can carry substantial thermal loads brought by the condensing evaporant. Single and multilayer films of metals and oxides can be applied using either direct or plasma-assisted evaporation. Figure 5 shows an aluminum strip coater and Fig. 6 shows a cross section of a polymeric coater powered by two EB guns. Coated products are used in the textile and packaging industries and in the electronics industry producing capacitors, magnetics and semiconductors, special photographic products and multilayer systems for optical variable devices. Custom designed coatings also are stripped from the webs, converted to pigments and used in special paints and inks. Production of transparent barrier coatings for use in diverse applications also is a recent application of this technology.
Overlay coating is an equally important industrial area using electron-beam evaporation. This work began in the late 1960s with industrial-scale coating of jet engine blades with NiCrAlY and CoCrAlY in 1969. The need for higher operating jet-engine temperatures, lower cost and longer life brought the introduction of yttrium-stabilized zirconia coatings in 1988, which still are being used today. Figure 7 shows a recent vintage turbine blade coater.
Electron-beam evaporation can also be used to produce solid bodies of near-net shape and silicon fiber-reinforced titanium metal-matrix composites having very high strength and low density. Two additional areas for evaporative applications are the U.S. Government Uranium Isotope Separation Program, which uses a laser to separate the U235 and U238 from an EB-generated uranium cloud, and the so-called directed vapor deposition (DVD), perhaps the most recent entry in the EB evaporation technology.
In the DVD process, material is thermally evaporated together with vapor entrapped in a carrier gas stream and deposited on a substrate. Plasma activation of the evaporant also can be used. This is a high rate deposition process (10 Km/min over 50 cm2 areas) with the rate of materials use greater than 10 times that of EB PVD. This new technology is in the very early stages of development, but has exciting potential. It is likely that it will take considerable time and effort to bring it to a level where it can have an industrial impact.
Electron-beam welding and machining
EB welding and machining exploits the ability of electron guns to deliver highly columnated beams of electrons capable of delivering power densities in the range of 106 to 109 W/cm2. This technology began in the 1950s, driven by the need to join nuclear fuel elements, and became a production tool with the installation of a Zeiss welding machine in the Westinghouse Bettis plant in 1958. Initially, the process was conducted at a vacuum of 10-4, later becoming a three-mode process: hard-vacuum EB welding (HVEBW) carried out at 10-4 or better, partial soft vacuum EB welding (PVEBW) carried out at 10-2 and nonvacuum EB welding (NVEBW) carried out at atmospheric pressure.
Continuous improvement in the functional capabilities of the process has provided today's EB machines having a much smaller footprint requiring considerably less space than was previously needed. The machines are much more capable of providing end uses for the process having a complete spectrum of operating modes, from manual to semiautomatic to fully automatic. Throughput of conventional part sizes ranging from 300 to 600 parts per hour are readily achievable under fully automatic operating conditions on today's EBW production units. Figure 8 shows a recent nonvacuum EB welder. There are as many as 2,000 EB welding machines in service in the U.S. including machines having vacuum chambers no larger than a cubic foot to those capable of welding the wings of an F16 aircraft. At this time, it appears that the automotive industry is the most active industry acquiring new equipment in the U.S.
The early 2 to 6-kW power systems have evolved to systems having beam power up to 100 kW and greater. Ongoing enhancement in controls has produced machines having control capabilities beyond the fondest dreams of those working in the early days of the technology. Switch mode-style high-voltage power supplies; variable frequency, programmable pattern-type deflection-power supplies, on-line beam diagnostic devices and PLC/CNC methods of overall system control are but a few of the highly advanced process control methods, which are readily available on most today's systems.
Increasing the power density delivery capability of a welding gun to 106 to 109 W/cm2 gives the equipment the capability to drill holes. The idea originated with Manfred von Ardenne in 1938, but it wasn't until 1960 that the drilling process became a practical realization. Steigerwald, under the Zeiss umbrella, developed and produced micromachining systems, identified as BFM100W. The second generation of these machines could drill at rates of up to 150 holes/second; holes from 0.1 to 0.8 mm diameter and 0.5 mm deep (0.004 to 0.03 in. and 0.02 in.) in production conditions and 0.05 mm diameter and 10 mm deep (0.02 in. and 0.4 in.) under laboratory conditions. Spinners for glass-fiber manufacture are one of the principal parts produced having 30,000 drilled holes/unit, 0.6 to 0.8 mm in diameter by 5 mm deep (0.02 to 0.03 in. and 0.2 in.). Centrifugal extractor baskets also are produced having 500,000 drilled holes/ unit, 0.2 mm diameter and 3 mm deep (0.008 in. and 0.12 in.).
A third important product is perforated titanium sheet used in fuel management systems for satellites, which have hole densities about 20 x 106 holes/m2, 0.07 mm diameter and 0.25 mm deep (0.003 in. and 0.01 in.). In the 1960s, guns similar to those used for drilling were used to produce engravings. An even more complex gun equipped with three static and three dynamic lenses having characteristics needed for engraving was developed in 1982. The system demonstrated a high evaporating rate of over 100,000 cells/sec. The technology has not been accepted although it has interesting potential; more than half of all of these systems remain in Germany. While the capabilities of these systems are exciting, the production costs involved are beyond justification for wide market acceptance. The existing systems were built almost two decades ago, and there is a low probability new ones will be built.
Perhaps the last of the EB welding technology-related applications is in the area of surface modification. This can be effected either in the solid state or through melting the surface. In the first case, the treated surface can be hardened or annealed. When the surface is liquified, the remelting produces a new, more desirable structure.
The process is a result of the conversion of the beam to heat. The acceleration voltage and power of the impinging beam is converted to heat in a surface layer in the range of 0.01 to 0.02 mm (0.0004 to 0.0008 in.) in thickness depending on the atomic number of the material. A wide range of thermal treatments can be used to produce various effects. Applications include treating tooling surfaces, surface hardening guides, fusion treating gray cast iron cam shafts before machining and fusion treating aluminum cylinders to improve wear resistance and glazing Ti-alloy aircraft components to improve corrosion resistance. The bulk of activities in this technology area, both in development and commercial applications, were carried out in the former East Germany. The technology is firmly established, but there is only limited growth.
Electron-beam technology has a bright future and will grow and continue to make notable contributions in the U.S. industry. In melting, there will be new capacity growth beginning in 2002. This is driven by the anticipated needs for niobium, and several furnaces are in the study or planning stages with the first of these, a 900-kW unit, to go on stream in the refractory metals business later this year. EB-cleaned metals must be vacuum arc remelted for structural refinement. Efforts that began some time ago will continue in the quest to produce clean metals of desired structures in the purifying melt. While automation and computer controls have had a major impact on productivity of EB melting installations, there will be continued efforts to further reduce operations manpower.
Web coating and overlay coating operations will continue to grow with new installations to serve both areas. The web-coating field is quite diversified and growth in the number of systems, while difficult to estimate accurately, is certain to continue because of the high productivity of these systems.
By contrast, it is possible to estimate activities in the overlay coating field for 2003 or 2004. It appears that at least two overlay coating systems are in the discussion stage and likely to be ordered in 2003.
Growth in use of electron beam welding has slowed down considerably. However, in a number of applications where this technology is the only one that can do the job, or where it offers substantial quality and economic advantages, we will continue to see new machines placed in service.