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Guidelines for electron-beam welding.

Electron-beam welding (EBW) offers many benefits, including deep, narrow, and almost parallel side welds, plus low total heat input and comparatively small heat-affected zones. As with most good processes, there are ways to make them better. To improve the quality of EBW, we suggest the following:

* Evaluate potential applications before purchasing equipment.

* Follow regular preventive-maintenance programs.

* Use proper fituring materials.

* Control welding parameters closely.

* Control filament peaks.

Train personnel to respect high voltage, clean welding chambers, and study all the variables in this flexible process.

It is important to evaluate potential applications before purchasing EBW equipment. Do you need low-voltage (under 60 kV) or high-voltage (over 60 kV) equipment? Do you need high-vacuum, medium-vacuum, or non-vacuum equipment?

A high-production environment requires equipment that is different from that used in a job shop performing a wide variety of jobs. The range of expected applications must be reviewed for the long term. Where a variety of jobs is expected, for example, high-vacuum, high-voltage equipment probably is best. On the other hand, low-voltage equipment often has a movable gun that offers more flexibility for reaching the work area or welding out of position. Non-vacuum or medium-vacuum equipment is often used for high production applications.

Chamber size is another important consideration. Specify a chamber just large enough to accommodate the size of the expected work, especially for high production, to e evacuation time.

Find out what's available. Determine what you can afford. Used equipment, often priced substantially lower than new equipment with the latest gadgets, can be a cost-effective option when your applications do not require many accessories.


Follow prescribed preventive-maintenance programs on a regular basis. Most importantly, be sure to:

* Keep chambers clean to minimize evacuation time.

* Follow manufacturers' maintenance procedures and use recommended replacement parts.

* Be careful of back-streaming diffusion-pump or mechanical-pump oil into the system; improperly maintained pumps and valves can cause this problem.

* Take advantage of the maintenance training offered by most manufacturers.


Use the proper material for welding fixtures, clamps, supports, etc. Electron beams are attracted or repelled by magnetic charges. Thus, magnetic materials cannot be used in fixturing, because residual magnetism can build up in them, causing the beam to jump. Also, avoid materials that allow static charges to build up. Aluminum and austenitic stainless steel are acceptable, but beware of steel inserts, helicoils, bolts, etc.

Use a gauss meter to measure the amount of magnetism in the tooling before welding begins. The acceptable magnetic field strength will vary depending upon the welding parameters and application, but zero is always acceptable.

High-production environments require fixturing that will last, but frequent inspection is necessary because worn fixturing can allow gaps or misalignment-resulting in poor-quality welds.

Design fixturing or tooling to allow proper accessibility to the welded joints. Also, fixturing must hold parts together well. Wide or uneven gaps give poor-quality welds. Figure 2 shows the results of excessive joint gap.

Design criteria

While EBW can be used to join any relatively weldable material, designers must be wary of specifying material with high gas content or low-melting-point constituents that will vaporize out differentially and produce voids Figure 3). Joint surfaces must be well machined so that the gap is narrower than the beam. Flame-cut surfaces rarely are acceptable for EBW.

Parts are more difficult to weld accurately if the joints are hard to see. Some visible gap, groove, or chamfer is necessary in tight, well-machined joints.

Urge designers to specify self-uping joints, because they are easier to fit up. For example, it's harder to fixture and hold together two cylinders with square butt joints than with stepped joints. One approach to providing alignment in square butt joints is to use a tacking band to hold the parts together. The tacking band has openings for the beam to pass through to make tack welds after the band is removed. The joint can then be welded at higher power, leaving no evidence of the tack welds.

Special applications may require the development of unusual techniques and joint geometries. For instance, engineers have developed a special autogenous multipass welding technique for precise control of penetration and heat input. Figures 4, 5, and 6 show the joint geometries and resulting weld cross sections. Parameter control

Set travel speed accurately. Travel speeds may vary, especially when using manual settings. After setting the dial or potentiometer, use a stopwatch to verify that the travel speed is correct. Time speeds during welding for best results. Also, set accelerating voltage and beam current precisely to get reproducible results. These three parameters control weld-energy input, commonly recorded as Joules/inch.

Power density is perhaps the most critical welding parameter, because it can cause the greatest variations in welds. Power density defines the energy distribution in the weld pool and can be affected by the acceleration voltage, beam current, oscillatory pattern, focus, gun-to-work distance, vacuum level, and method of filament peaking. Unfortunately, measuring power density in EBW is not easy, but methods are available to help control and repeat the important factors.'

Some operators prefer a higher voltage with a lower current, while others prefer a lower voltage with a higher current.

Some prefer to modify power density by changing focus, while others prefer to oscillate the beam. Ultimately, because power density is such a crucial factor, operators will find their own array of welding parameters to arrive at an optimum density based on their own experience. The clue to maintaining reproducibility is careful recording of the settings.

Most reproducibility problems occur because of focus changes. Although focus can be difficult to measure, techniques have been developed to ease the task. The most common method for determining sharp focus is to set a target at the same height as the weld joint; then, with the beam on this target, adjust the focusing-coil current until the brightest or sharpest image is obtained. Using a low beam current (2 mA to 5 mA) to determine sharp focus on a target is a more reproducible approach, but this provides only a reference focus, because the true focus changes as beam current changes.

Filament peaking

Peaking is the technique of applying current to the filament until it reaches the optimum emission temperature for the intended beam current.

Because filaments may vary from batch to batch, you should test beam-parameter settings with every new batch of filaments. One recommended test is to verify that the reference focus setting in the procedure remains the same. This test is also a useful check when filaments from the same batch are used to replace a burned-out filament.

Two problems are associated with the peaking of filaments. One is that filaments peaked too high require more bias voltage to control the beam current. If more bias voltage is used, the voltage difference between the filament and the cathode is larger, and the probability of internal arcing in the electron gun increases.

When internal arcing occurs in the gun, instability or current fluctuations will occur in the electron beam. Such variations in the beam power may not be obvious when welding heavy sections, but usually can be seen in very fine work-such as when welding aluminum at relatively low power.

The other problem occurs when peak filament currents are too low-often the case when a filament has been peaked for low-beam-current welds followed by use for a setup requiring a higher beam current. Even though meters indicate the correct voltage and current caned out in the procedure, the beam power density may not be adequate. The result will be welds with insufficient penetration, caused by low power density.

Measuring and repeating peaks is difficult, even with newer systems that purport to indicate properly peaked filaments. Older equipment is peaked by increasing filament current until the beam current is stabilized-then adjusting meters accordingly.
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:Huber, Richard A.
Publication:Tooling & Production
Date:Jul 1, 1990
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