A welding process that produces coalescence with a concentrated beam, composed primarily of high-velocity electrons, impinging on the joint. The process is used without shielding gas and without the application of pressure. See HIGH VACUUM ELECTRON BEAM WELDING, MEDIUM VACUUM ELECTRON BEAM  WELDING, and NONVACUUM ELECTRON BEAM WELDING.

When the high-velocity electrons impinge on a joint, their kinetic energy is converted into heat. The density of energy (or heat) is so great that vaporization of the

metal (or ceramic) usually occurs, creating a cavity called a keyhole. This keyhole allows exceptionally deep penetration, for a relatively narrow width. The vapor cavity is surrounded by a liquid shell which closes behind the beam (in the direction opposite beam travel) to produce a liquid pool by capillary action. The weld and joint are formed on solidification. A vacuum is required to prevent scattering and dispersion of the beam. This vacuum provides shielding to the molten weld pool and surrounding base metal.

Principles of Operation

The heart of the electron beam welding process is the electron beam gudcolumn assembly, a simplified representation of which is shown in Figure E-8. Basically, an electron beam welding gun functions in much the same manner as a TV picture tube. The primary difference is that a TV picture tube uses a low-intensity electron beam to continuously scan the surface of a luminescent screen, and thereby produces a picture.

An electron beam welding gun uses a high-intensity electron beam to continuously bombard a weld joint, which converts that energy to the level of heat input need to make a fusion weld. In both of these cases, the beam of electrons is created in a similar manner.

The electron beam welding gun typically contains some type of thermionic electron emitter (normally referred to as the guncathode” or “filament”), and an anode. Various supplementary devices, such as focus and deflection coils, are also provided to focus and deflect this beam. In EBW, the total beam generating system (gun and electron optics) is called the electron beam gun/column assembly, or simply the electron beam gun column.

There are three basic modes of electron beam welding: high vacuum (EBW-HV), medium vacuum (EBW-MV), and nonvacuum (EBW-NV). The principal difference between these process modes is the ambient pressure at which welding is done.

High vacuum and medium’ vacuum welding are done inside a vacuum chamber. This imposes an evacuation time penalty to create the “high purity” environment. The medium vacuum welding machine retains most of the advantages of high vacuum welding, with shorter chamber evacuation times, resulting in higher production rates.

Nonvacuum EB welding is used to weld workpieces at atmospheric pressure, but a vacuum is still required to produce the electron beam. Although nonvacuum EB welding incurs no pump down time penalty, it is not suitable for all applications because the welds it produces are generally wider and shallower than equal power EB welds produced in a vacuum.

Applications

In general, metals and alloys that can be fusion welded by other welding processes can also be joined by electron beam welding. The weldability of a particular alloy or combination of alloys will depend on the metallurgical characteristics of that alloy or combination, in addition to the part configurations, joint design, process variation, and selection of welding procedure. Considering these variables, the electron beam process can be used to weld steels, stainless steels, aluminum alloys, titanium and zirconium, the refractory metals, and dissimilar metals.

Electron beam welding is primarily used for two distinctly different types of applications: high precision and high production.

High precision requires a high-purity environment (high vacuum) to avoid contamination by oxygen or nitrogen, or both, and with minimum heat effects and maximum reproducibility. These types of applications are mainly in the nuclear, aircraft, aerospace, and electronic industries. Typical products include nuclear fuel elements, special alloy jet engine components, pressure vessels for rocket propulsion systems, and hermetically sealed vacuum devices.

High production applications take advantage of the low heat input and the high reproducibility and reliability of electron beam welding if a high-purity environment is not required. These relaxed conditions permit welding of components in the semifinished or finished condition, using both medium and nonvacuum equipment. Typical examples are gears, frames, steering columns, and transmission and drive-train parts for automobiles; thin-wall tubing; bandsaw and hacksaw blades, and other bimetal strip products.

The major application of nonvacuum electron beam welding is in high-volume production of parts, the size or composition of which preclude effective welding in

a vacuum. The automotive industry employs nonvacuum EB welding for many applications. An example is a torque converter assembly. Manufacturers of welded

tubing also use nonvacuum EB welding. Integrated EB welding machine/tube mill units have been built to weld copper or steel tubing continuously at speeds up to 500 mm/s (100 fdmin).

Advantages

Electron beam welding has unique performance capabilities. The high-quality environment, high power densities, and outstanding control solve a wide range of joining problems. The following are advantages of electron beam welding:

(1) EBW is extremely efficient because it directly converts electrical energy into beam output energy.

(2) Electron beam weldments exhibit a high depth- to-width ratio. This feature allows for single-pass welding of thick joints.

(3) The heat input per unit length for a given depth of penetration can be much lower than with arc welding; the resulting narrow weld zone has low distortion, and fewer deleterious thermal effects.

(4)A high-purity environment (vacuum) for welding minimizes contamination of the metal by oxygen and nitrogen.

(5) The ability to project the beam over a distance of several feet in vacuum often allows welds to be made in otherwise inaccessible locations.

(6) Rapid travel speeds are possible because of the high melting rates associated with this concentrated heat source. This reduces welding time and increases productivity and energy efficiency and increases productivity and energy efficiency.

(7) Reasonably square butt joints in both thick and relatively thin plates can be welded in one pass without the addition of filler metal.

(8) Hermetic closures can be welded with the high or medium vacuum modes of operation while retaining a vacuum inside the component.

(9) The beam of electrons can be magnetically deflected to produce various shaped welds, to improve weld quality, or increase penetration.

(10) The focused beam of electrons has a relatively long depth of focus, which will accommodate a broad range of work distances.

(11) Full penetration, single-pass welds can be produced with nearly parallel sides, and exhibiting nearly symmetrical shrinkage.

(12) Dissimilar metals and metals with high thermal conductivity, such as copper, can be welded.

Limitations

Some of the limitations of electron beam welding are:

(1) Capital costs are substantially higher than those of arc welding equipment. However, depending on the volume of parts to be produced, the final per-piece cost attainable with EBW can be highly competitive.

(2) Preparation for welds with high depth-to-width ratio requires precision machining of the joint edges, exacting joint alignment, and good fit-up. In addition,

the joint gap must be minimized to take advantage of the small size of the electron beam. However, these precise part preparation requirements are not mandatory if high depth-to-width ratio welds are not needed.

(3) The rapid solidification rates achieved can cause cracking in highly constrained, low ferrite stainless steel.

(4) For high and medium vacuum welding, work chamber size must be large enough to accommodate the assembly operation. The time needed to evacuate the chamber will influence production costs.

(5) Partial penetration welds with high depth-to- width ratios are susceptible to root voids and porosity.

(6) Because the electron beam is deflected by magnetic fields, nonmagnetic or properly degaussed metals should be used for tooling and fixturing close to the beam path.

(7) With the nonvacuum mode of electron beam welding, the restriction on work distance from the bottom of the electron beam gun column to the work will limit the product design in areas directly adjacent to the weld joint.

(8) With all modes of EBW, radiation shielding must be maintained to ensure that there is no exposure of personnel to the x-radiation generated by EB welding.

(9) Adequate ventilation is required with nonvacuum EBW, to ensure proper removal of ozone and other noxious gases formed during this mode of EB welding.

Equipment

High vacuum, medium vacuum, and nonvacuum EBW equipment employs an electron beam gun/column assembly, one or more vacuum pumping systems, and a power supply. High and medium vacuum equipment operates with the work in an evacuated welding chamber. Although nonvacuum work does not need to be placed in a chamber, a vacuum environment is necessary for the electron beam gun column. All three basic modes can be performed using so-called high-voltage equipment, i.e., equipment using gun columns with beam accelerating voltages greater than 60 kV. Nonvacuum electron beam welding performed directly in air requires beam accelerating voltages greater than

150 kV. High vacuum and medium vacuum welding can also be performed with so-called low-voltage equipment (ie., equipment with gun columns that employ beam accelerating voltages of 60 kV and lower). Because high-voltage gun columns are generally fairly large, they are usually mounted on the exterior of the welding chamber, and are either fixed in position or provided with a limited amount of tilting or translational motion, or both. Low-voltage gun columns are usually small. Some units are “fixed” externally. Others are internally mounted “mobile” units capable of being moved about, with up to five axes of combined translational motion.

Electron Beam Guns. An electron beam gun generates, accelerates, and collimates the electrons into a directed beam. The gun components can logically be divided into two categories: (1) elements that generate free electrons (the emitter portion), and (2) a rodor

disc-type filament indirectly heated by an auxiliary source, such as electron bombardment or induction heating. The specific emitter design chosen will affect the characteristics of the final beam spot produced on the work.

Power Supplies. The electron gun power source used for an electron beam welding machine is an assembly of at least one main power supply and one or more auxiliary power supplies. It produces high voltage power for the gun arid auxiliary power for the emitter and beam control.

Vacuum Pumping Systems. Vacuum pumping systems are required to evacuate the electron beam gun chamber, the work chamber for high and medium vacuum modes, and the orifice assembly used on the beam exit portion of the gun/column assemblies for medium vacuum and nonvacuum welding. Two basic types of vacuum pumps are used: one is a mechanical piston or vane-type, and the other is an oil-diffusion-type pump used to reduce the: pressure.

Work Chambers. Work chambers of low-voltage systems are usually made of carbon steel plate. The thickness of the plate is designed to provide adequate x-ray protection and the structural strength necessary to withstand atmospheric pressure. Lead shielding

may be required in certain areas to ensure total radiation tightness of the system.

Safety

Since electron beam welding machines employ a high-energy beam of electrons, the process requires users to observe several safety precautions not normally necessary with other types of fusion welding equipment. The four primary potential dangers associated with electron beam equipment are electric shock, x-radiation, fumes and gases, and damaging visible radiation. In addition to the potential dangers associated with welding specific materials, such as beryllium, there may also be a potential danger associated with collateral materials (solvents, greases and others) used in operating the equipment. Precautionary measures should be taken to assure that all required safety

procedures are strictly observed. ANSUAWS F2.1, Recommended Safe Practices for Electron Beam Welding and Cutting, and ANSIIASC 249.1, Safety in Welding and Cutting (latest editions) give the general safety precautions that must be taken.

For information on fundamentals of electron beam welding, process variations, equipment, weld characteristics, welding procedures, fixturing, filler metal additions, selection of welding variables, weldability of metals, weld quality, safety precautions, and bibliography, see American Welding Society, Welding Handbook, Vol. 2, 8th Edition. Miami, Florida: American Welding Society, 1991.

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