ELECTRON BEAM WELDING                                                                                                                                                             [ PRINT ]

Electron beam welding is a phenomena that is thoroughly understood by scientists and engineers that have studied the process in the research lab, but very little information is published for those of us that want to know more about the welding process performed with electron beam energy. We hope the following description may be helpful; High Energy Electron Beam (HEEB) is fundamentally a unique way of delivering large amounts of concentrated thermal energy to materials. There have been many beneficial discoveries as a result of the industrial use of HEEB. Electron Beam Welding Process Fundamentals. In order to get a better understanding of the beam delivery mechanics, it is important to become familiar with the main components of the system. The most common EB systems used in manufacturing today are of the high vacuum design (5x10e-4 torr). Our discussion will primarily focus on this type of system because of their popularity. The other machine types are partial vacuum and non-vacuum equipment that is typically used in mass production where high output is essential. The schematic diagram depicted on this page illustrates the classic triode gun and column assembly. The triode gun design consists of the Cathode (Filament), Bias Cup (Grid) and Anode. Other sub-assembly components that contribute to the triode are: High Voltage Insulator Feed-through, High Voltage Cable and focusing and deflection coils. All of these components are housed in a vacuum vessel called the upper column and is usually kept at pressures of 5x10e-4 torr. The column assembly is held under high vacuum by an isolation valve positioned below the anode assembly.

The vacuum environment provides several benefits:

     1. Provides a controlled environment to protect the gun against welding byproduct.
     2. Provides protection for the incandescent filament against oxidation.
     3. Removes the bulk gas molecules necessary for a stable triode.


Beam Formation: Upper Column
The beam formation begins with emission of electrons from the incandescently heated tungsten filament. During this process the filament is saturated by a predetermined amount of electrical current. Electrons boil off the filament tip as it reaches operating temperature and gather in the grid cup assembly. A negative high voltage potential is applied to the filament cathode assembly, referred to as the accelerating voltage (kV). With the cathode assembly charged at 150 kV the only force preventing the electron beam from propagating is a secondary negatively charged voltage that resides on the grid cup or bias assembly. This voltage, respectively lower than the accelerating voltage, acts as a valve that controls the volume of electron energy that can flow from the cathode emitter to its attracting target. The anode at a positive potential is one of the attracting targets in the triode but its role is more of a beam formation device rather than a collector of electrons. The secondary target is the workpiece which is usually metallic and offers a conductive path to earth to complete the circuit. The electron gun assembly design is a result of many extensive engineering studies as well as experimentation. Some of the early triode designs were mathematically modeled and are still produced today.

Beam Delivery: Lower Column
Other important components of the beam delivery column are the focus and deflection coils and isolation valve. The magnetic focus coil, located beneath the anode assembly, provides the means for squeezing the beam into a tightly focused stream of energy or can be used to widely dispersed energy resource. The deflection coil is another very important component that will contribute to the latter discussion of beam control parameters but for now we will simply say that it is a steering device. The focus coil is circular in design and is concentric with the column. An electrical current is passed through the coil, which produces the resultant magnetic fluxes that act to converge the electron beam. Depth to width ratios of (8:1) or greater are achievable depending on parameters and material conduction. This combination of narrow welds and minimal heat affected zones produced in a vacuum environment results in welds of the highest quality. The deflection coil is configured with four separately wound coils positioned at right angles to the column. The four coils are segmented as sets (x and y) each axis becomes a separate control allowing the energizing of each axis on command, thus steering the beam. Many industrial applications require the precise manipulation of the beam energy so as to provide a pattern for processing. This is usually accomplished by superimposing an AC signal onto the four coils simultaneously therefore creating a specific pattern. The isolation valve serves to isolate the vacuum environment in the upper column from the lower. After the electron beam has passed through the lower column, it enters the chamber cavity. But before discussing the chamber there is an important component that is an integral part of the lower column. The viewing optics are arranged in the lower column in such a manner that when viewing the beam energy through a video camera or magnified optics it gives the view from a parallel plane, giving the viewer the perception of looking down the column.

Beam Interaction in Chamber Cavity
As the beam enters the chamber cavity it is aimed onto a target material placed at a predetermined height representative of the actual workpiece. This procedure is typical in most pre-weld set-up requirements. The welding technician would then follow a process of beam alignment and beam parameter calibration. Unlike laser, the preparation is quite different in the fact that the technician must view the actual beam through the optical system in order to verify the beam alignment and focus. With a laser beam, the technician could not view the beam quality and therefore must rely on instrumentation to profile the beam energy. Once the beam has been tuned and calibrated the equipment is now ready for part processing. The electron beam weld technician is typically a highly skilled individual that has a number of responsibilities. There are several skills that the technician must possess in order to insure the safe operation of the device. These include: the ability to fully understand the equipment systems so they react to a system failure that could result in catastrophic damage; the skill to survey for and measure X rays that are a byproduct of electron collision with target materials (periodic surveys should be performed by the operating personnel) and knowledge of welding fumes and byproduct present in the vented chamber (For example welding of toxic material such as beryllium.) An electron beam technician should have received extensive on the job training or equivalent outside training by industry-recognized specialist.

Electron Beam Interaction in Materials
The focused beam of electrons is directed at a targeted location on the weld joint at which point the kinetic energy of the electrons is converted to thermal energy. The workpiece can either be stationary and the beam energy deflected or the workpiece can be traversed along a desired axis of motion. This motion can be computer controlled such as a CNC table, or a simple rotating mechanism can be employed.

As the beam energy is applied to the moving part several physical transformations take place. The material instantaneously begins to melt at the surface, and then a rapid vaporization occurs followed by the resultant coalescence. Two welding modes can be employed, i.e. a conductance mode or a keyhole mode. In the conductance mode, primarily applicable to thin materials, heating of the weld joint to melting temperature is rapidly generated at or below the materials surface followed by thermal conductance throughout the joint for complete or partial penetration. The resultant weld is very narrow for two reasons; first, it is produced by a focused beam spot with energy densities concentrated into a .010" to.030" area. Secondly, the high energy density allows for rapid travel speeds allowing the weld to occur so fast that the adjacent base metal does not absorb the excess heat therefore giving the E.B. process its distinct minimal heat affected zone.

The keyhole mode is employed when deep penetration is a requirement. This is possible since the concentrated energy and velocity of the electrons of the focused beam are capable of subsurface penetration. The subsurface penetration causes the rapid vaporization of the material thus causing a hole to be drilled through the material. In the hole cavity the rapid vaporization and sputtering causes a pressure to develop thereby suspending the liquidus material against the cavity walls. As the hole is advanced along the weld joint by motion of the workpiece the molten layer flows around the beam energy to fill the hole and coalesce to produce a fusion weld. The hole and trailing solidifying metal resemble the shape of an old fashion keyhole. Both the conductance and keyhole welding modes share physical features such as narrow welds and minimal heat affected zone. The basic difference is that a keyhole weld is a full penetration weld and a conductance weld usually carries a molten puddle and penetrates by virtue of conduction of thermal energy.