A glimpse is given of how electron beam welding has evolved

in the United States over five decades

      Using an electron beam (EB) to perf- orm material processing tasks first be- gan in the early l900's to melt refractory metals in vacuum. Since then, its utili- zation for processing other forms of material has steadily increased as the level of capability of its two main supp- orting technologies, vacuum enginee- ring and electron optics, have continued to evolve and mature with time. Thus, as shown in Table 1, its present day use in the area of industrial material process- ing covers a broad range of applications -- varying in nature from tasks that em- ploy a very low power density beam (very shallow, surface types) to those that employ a very high power density beam (very deep, volumetric types).

     The application of EB to the task of welding (EBW) was initiated in Germany
during the early 1950s, then spread to France by the mid-50s. Shortly there- after, and before the end of that decade, use of the process was introduced into the United States with the purchase of an EBW unit from Germany by Westin- ghouse's Bettis plant. This occurred in response to articles the French Atomic Energy Commission had published out- lining its successful utilization of the EBW process to join  components being used by the nuclear energy industry.

EBW Process Fundamentals

illustrates a triode style (cathode, grid, and anode) gun being used to gene- rate the beam, as well as the electron optical (focusing and deflection) sys- tem utilized for controlling the manner (final beam spot size produced and its positioning) in which this beam is then impinged on the workpiece. In oper- ation, the cathode, an emitter that can be either directly or indirectly heated, provides a source of thermally emitted electrons that are simultaneously acc- elerated and shaped into a collimated beam by the electrostatic field geom- etry produced by the gun grid, anode configuration being employed, and then focused into a highly intense be- am spot on the workpiece.

     Use of a triode style gun allows, at any fixed operating voltage, the magni- tude of electron flow exiting the gun (i.e., the beam operating current, and thus the resulting beam power being delivered to the workpiece) to readily be adjusted as desired. Thus, by simply varying the negative potential differ- ence being applied between cathode and grid, the beam can easily be either instantaneously turned on/off or have its operating level ramped up/down in a highly controlled fashion. In addition to providing the ability to produce either a highly focused or slightly defocused beam spot on the workpiece, the ele- ctron optical system employed also provides the capacity for deflecting the beam spot in either a static (fixed pos- itional change) or dynamic (oscillatory positional change) fashion.

     In contrast to more conventional

conductive means of welding, the main joining advantage EBW provides users is the ability to perform what is comm- only referred to as "keyhole welding", illustrated in Fig. 2. In keyhole welding, the highly intense beam impinged on the workpiece generates a vapor cha- nnel that penetrates into the workp- iece. This allows the beam's energy to be delivered directly to the weld joint faying surfaces deep down inside the workpiece, rather than simply being deposited onto its top surface, as in the case of more conventional welding methods. As this vapor channel is ad- vanced, molten metal being formed at its leading edge continuously flows ar- ound it, solidifying behind its trailing edge to form the final weld joint.

The 1960s thru mid-l980s

     Once introduced into the United Sta- tes, interest in the EBW process quic- kly grew. Numerous EB symposiums, seminars, and conferences were held in the early 1960s, providing attend- ees with a continuous source of infor- mation about the process from both U.S. and foreign users and manufac-turers. Concurrently, various suppliers of the equipment began exhibiting at the welding shows. The combination of these two events strongly contrib- uted to the rapidly expanding interest the U.S. welding community had in both the EBW process and its growing use by industry.

     During the early 1960s, it was esta- blished that the deep penetration res- ults provided by the keyhole form of

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welding achieved with EB depended pri- marily on the magnitude of beam power density being employed rather than the level of operating voltage utilized, and a variety of U.S. based companies bec- ame involved with manufacturing EBW equipment having operating voltages spanning the full range from 25 to 150 kV. The list of these companies included Nuclide, National Research, Hamilton Standard, and Sciaky to name just a few. Before the end of the 1960s, how- ever, many of these companies disco- ntinued this manufacturing practice, leaving Hamilton Standard and Sciaky as the two leading U.S. based prod- ucers of EBW equipment - a position both still hold today, although each has since changed ownership several times.

The Golden Years

     The 1960s were unquestionably the
golden years for EB welding in the Uni- ted States. During this ten-year period alone some 500 EBW machines, more than a third of the total number of units installed over the past 50-plus years, were installed in the United States. In addition, while the EBW units installed during the early 1960s mainly had rel- atively small high-vacuum chambers with very limited workpiece motion and beam power output capability, those being supplied by the mid-1960s con- sisted of units having fairly large high- vacuum chambers with the capacity to provide both multiaxis workpiece and/or gun motion, as well as beam power out- puts ranging up to 45 kW. At about the same time, units with the ability to

apply the process under both partial vacuum and nonvacuum conditions, as well as high vacuum, were introd- uced concurrent with units that addit- ionally employed both CNC-operated workpiece motion and beam param- eter control.

EBW Shows lts Versatility

During the 60s, EBW became a multi- mode welding process viewed as hav- ing potential applications for virtually any type of joining task imaginable, in- cluding a metal dress produced from aluminum foil and mesh materials to highlight the joining versatility of the pr- ocess. This resulted in the developm- ent of a number of fairly innovative me- ans and methods for expanding the EBW process' scope of application du- ring that decade.
A few examples follow:
     1) The Perforated Wall Hollow Cath- ode. A cold cathode device for genera- ting electron beams that could be con- figured into annular, cylindrical, and spherical shapes one form of which provided a fairly simple device for use in joining tubular components in a bell-jar style of environment.
     2) Hand-Held EB Gun. A device dev- eloped for NASA astronauts to use for both fabrication and repair type weld- ing in space.
     3) Portable EBW Pipe Welding Mac- hine. A system produced for the AGA that employed a crane-supported, C- clamp style high-vacuum chamber containing a trolley-driven, right-angle EB gun for use in providing 360-deg peripheral welds on pipe segments in

the field.
    4) Clam-Shell Style EBW System. A
unit delivered to Grumman Aerospace for welding the large moving structure components used in the manufacture of the Grumman F-14 TOMCAT fighter aircraft.
     Other products of the 1960s includ- ed the adaptation of nonvacuum EB to tube mills, which provided the capacity to perform high-speed out-of-vacuum tube welding that was continuously



Fig. 1 - Schematic representation of an EB gun and column assembly.

Fig. 2 - Schematic of the keyhole welding method.

Fig. 3 - Graph of annual EBW machine installations in the U.S. during the first 25-year period.

Fig 4 - One of the first EBW units supplied to the auto industry.

Fig. 5 Graphic illustration of EB job shop welding facility locations in the United States

being formed directly from strip material. Anot- her was the employment of individually pum- ped pre-high-vacuum and post-high-vacuum pressure stages, separated by apertures that allowed the constant passage of the dual strip material being joined in high vacuum. This pr- ovided the ability to weld the bimetal product being utilized by the saw blade industry in a continuous air-to-air fashion.

A Shift in the 1970s

     Annual EBW machine installations in the United States, which had reached a peak of about 100 units per year before the end of the 1960s, began a noticeable decline by the start of the 1970s. This resulted from a saturation of EBW equipment in the primary markets of that era, the aerospace, medical, aeronautical, and AEC-DOE industries. However, during the 1960s, the U.S. automotive industry became quite interested in the various joining advant- ages EBW might offer it if the production wel- ding capacity of the process could be greatly increased. Batch loading methods were init- ially employed to help increase the number of parts capable of being processed each time the weld zone environment had to be evacu- ated to a high-vacuum level. Then, as the par- tial vacuum and nonvacuum modes of EBW application matured with use during the 1960s, much greater production rate capac- ities were made available since both these modes of EBW application either appreciably reduced or entirely eliminated any production time that would be lost loading parts into a high vacuum environment.
     Thus, with a growing number of EBW mac- hine sales being made to the auto industry starting in the late 1960s and extending out through the 1970s, annual EBW machine inst- allations in the U.S. again began approaching the 100 level. However, a series of events ori- ginating toward the close of the 1970s (prob- lems in the auto industry, a general sagging economy, and the advent of laser beam wel- ding [LBW] as a competitor) once again contr- ibuted to a decrease in this number by the start of the 1980s - Fig. 3.
     The makeup of suppliers of EBW equip- ment to the U.S. market also began changing during the 1970s. Foreign manufacturers of the equipment (Steigerwald, TorrVac, and We- ntgate) started entering the marketplace by licensing various U.S.-based companies orig- inally involved with manufacturing EBW equ- ipment, but that had discontinued that practice and started up EBW job shops to act as their sales representatives. Several new U.S. bas- ed suppliers (Union Carbide and Westingh- ouse) also entered the market at about the same time, thereby providing Sciacky and

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Fig. 6 - Examples of EBW units. A -  (left) Moderately priced, scaled-down machine; B - (right) specialized, higher-cost system. 

U.S. market employed some form of digital-style controls.

The mid-1980s to Present

Since EBW equipment being made available to the U.S. mar- ket by the mid-1980s had been appreciably enhanced over that which had been available during the 1960s and 1970s, and with most all suppliers of EBW units by then providing systems with PLC and/or CNC system controls, solid-state control modules, improved beam current, and deflection pa- ttern control capabilities, as well as optionally offering joint tracking, data acquisition, and beam diagnostic packages, the cost of purchasing an EBW unit had increased signif- icantly. Thus, with the state of the economy at that time and the tighter restrictions being placed on the purchase of capital equipment as a result of these poor economic cond- itions, the decrease in annual new machine sales that had begun near the start of the 1980s continued on throughout that entire decade.
     However, with an installed base of better than 800 EBW units already in place in the United States by the beginning of the 1980s, the market was somewhat bolstered by an incr- ease in the sales of customer support functions (field serv- ice, spare parts, and accessories), as well as by a new ave- nue for equipment sales that had begun opening up by then. This was the sale of "upgrade packages" (i.e., retrofit, refurb- ishment, and modernization kits) for existing equipment, an area that provided a good potential for sales since more than half the EBW units in the existing installed machine base were 1960s vintage units. Upgrade packages being offered consisted of such major items as PLC or CNC machine con- trol changeovers and beam generation system changeovers, the major type of machine makeovers that could generally be funded through an existing maintenance budget, rather than requiring authorization for a capital equipment expenditure. This market for upgrades continues to provide an area of good sales opportunity even today, as present day estimates

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indicate that some 900 to 1000 of the roughly 1400 to 1500 EBW units insta- lled in the United States during the past 50-year period (~75%) continue to rem- ain in use today, either at the original purchaser's facility or at the facility of the second or third (and even subsequent) own- er of the unit. Several hundred of these units still in use today (~30%) are either 1960 or 1970 vintage systems.

The Growth of Job Shop EBW

     Another area of business initiated by the increased cost involved with purch- asing a new EBW system by the 1980s was that of performing contract, job sh- op EB welding. This was due to the fact that this form of EBW business offered potential users of the process, those generally interested in applying it to their product but are unable to achieve the level of machine time utilization nec- essary to justify the outright purchase of an EBW system, the opportunity to eco- nomically outsource EB welding tasks to an EB job shop. This job shop EB welding business has continued to ex- pand with time, and, as indicated by Fig.5, there are presently more than 50 EB job shops in operation across the United States. The majority of these sh- ops are located on the East and West coasts, primarily in California and New England. These facilities range in size from small ones having as few as one or two EBW units in operation up to large ones with as many as five to ten units in operation. Included in this are several EB job shop welding busines- ses being operated by both the leading U.S.-based EBW equipment providers, PTR (formerly Hamilton/Leybold) and Sciacky.

New Offerings

     In the 1990s, EBW equipment sup- pliers began to offer more moderately priced, scaled-down systems (Fig. 6A) as an alternative to the highly specia-lized, high-expense systems (Fig. 6B). These more compact and cost effec- tive units were generally of the low- voltage variety with a beam power out- put capacity of 15kW, or under. Most, however, continued to provide their us- ers with all the same equipment enha- ncements present on the larger and more expensive units. Purchasers of EBW equipment since the early 1990s were thereby provided with the choice of a full range of high- to low-voltage, high-vacuum, partial vacuum, and non- vacuum high to low power units that are high and moderately priced. The use of EBW equipment was also expanded during the 1980s and 1990s to include tasks other than welding such as surface treatment (harden- ing, glazing, and texturing), brazing, and free form style fabrication (near-net-shape and additive manufactur- ing). Concurrently, the equipment was also shown to have the optional capa- city of supplying users with computer enhanced beam diagnostic images and the ability to perform "simulated" multi-beam processing tasks.

Summary

     During more than a 50-year period, EB welding in the United States has gradually evolved from being a highly unique laboratory tool initially consid- ered of interest primarily to the nuclear and aerospace industries, to a highly sophisticated production tool that is

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