EBW struggles to break out of the box.
That is the key to the potential for electron beam welding (EBW). If you can arc-weld it, you can EB weld it, and the high power density (far higher than lasers can presently produce) and the extremely small (lateral) intrinsic penetration of electrons into the workpiece yield an almost instantaneous local melting and vaporization--welding rates are not limited by thermal conduction. Welds are much deeper and narrower than arc welds, yet with lower heat input to distort the part. With focusing tricks, you can even get a weld profile that is wider at the bottom than it is at the topside entry point.
Another selling point the EBW people are quick to point out is energy efficiency. They cite wall-socket-to-workpiece efficiencies ranging from 30 percent (using a high-power machine for low-power welds) to an impressive 75 percent, which is significantly higher than laser-weld efficiencies.
There are disadvantages, of course (and plenty of competitors around to point them out). Equipment costs are high. The best welds require high vacuums (in the 1 X 10.sup.-.4 Torr range), but compromises can be made to get satisfactory production rates. EB welding in nonvacuums will speed the process considerably, at the expense of a diffused beam--much wider welds, narrower depths, much shorter gun-to-part distances, and shielding problems from the X-ray effects of the beam cutting air.
In automotive applications, EBW makers compromise on a partial vacuum to gain acceptable production rates. The parts are usually mounted on rotary indexing tables with chambers that closely conform to the part shape. The chambers are open on top for loading and slipping under the vacuum gun, where the chamber is quickly sealed, evacuated, EB welded, and then rotated back to the load/unload station. Total cycle time per part is typically 8 to 10 sec. Ring gears have been welded to flywheels in this manner for a number of years.
Another high-volume, very successful application is welding of bimetal bandsaw blades. EBW produces a very precise, narrow weld that is as strong as either material. When the saw teeth are later cut, the teeth are entirely high-strength cutting material and the band all highly flexible carrier material.
In nonvacuum applications, the beam is greatly diffused by air and the benefits from EBW become difficult to distinguish from more conventional welding methods. Speed is excellent, reaching 12 m/min, and the gun can become much more mobile than high vacuum systems where the gun is fixed and you must move the part.
Yet, probably the biggest single problem EBW has today is education. There are relatively few users outside of automotive and aerospace, yet the acceptance of EBW in these strikingly diverse fields implies that there is great potential for EBW in many other application areas, once its characteristic strengths and limitations are more fully understood.
EBW came out of the lab and into practical applications in the early and mid-60s. Aerospace, nuclear, and other high-technology industries discovered EBW could satisfy their demands for strong, deep welds in titanium, stainless steel, high nickel, and aluminum alloys, and for parts exposed to extreme cold and vibration. Then, in the late 60s, introduction of partial and nonvacuum EBW systems allowed them to begin penetrating the high-volume applications. Today, EB welded parts range from heavy-duty transmission gears to tiny biomedical implants.
In the EBW gun, a heated tungsten filament emits electrons that are accelerated and focused into a beam electrostatically, and concentrated and/or deflected by an electromagnetic coil.
Once the beam leaves the gun, its ability to stay concentrated is a function of the vacuum in the welding chamber. If the welding chamber is at the same level of vacuum (1 X 10.sup.-.4 Torr), the beam is not diffused by collisions with air molecules. When it contacts the workpiece, it melts a hole of uniform diameter through it to a depth controlled by the dwell time or rate the workpiece is being moved under the beam. As the beam moves along the workpiece, the melted materials fuse behind it, forming a bond as strong as the original material. There is normally no need for filler material.
This type of weld is ideal for applications requiring minimal fusing energy, precise dimensional control, minimal heat-affected zone, and high weld purity. There is very low thermal distortion and minimal alteration in metallurgical structure of the base material. Welds can be formed in close proximity to heat-sensitive materials.
Basically, the EB weld is deeper, narrower, and produced with less heat input than arc welding. There is no need for multiple passes; single-pass welds to 12" depths have been produced at rates of over 5 ipm. Weld depth-to-width ratios of 20:1 are possible.
The process is very flexible. Accelerating voltage can be varied to change the energy content of each electron, or beam current can be manipulated to control the total number of electrons in the beam. This is no more complicated than the grid controls on a cathode-ray tube. The focus, focal length, and alignment of the beam can be easily varied by electromagnetic coils, and a variety of beam patterns can be created--for example, zig-zag patterns for creating special area effects for surface heat treating.
Difficult-to-weld materials, such as titanium, benefit from the extremely pure environment of the EB vacuum chamber. Highly toxic beryllium-copper welding byproducts are easily controlled. Copper, aluminum, and other highly reflective materials have no adverse effect on beam power.
The best example of what can be done today with EBW is provided by a system recently built by Leybold-Heraeus Vacuum Systems Inc, Enfield, CT. It was built for a major (unnamed) aerospace manufacturer and shipped this month. It is one of the largest EBW systems built in recent years and sold for approximately $2.5 million.
Key features are the room-size high-vacuum welding chamber, cryogenic vacuum pumping, automatic weld-path adjustment in real time, and air-levitated pallet-shuttle system for manual part loading/unloading.
The system is designed for large rotary parts from 30" to 108" dia, weighing up to 10 tons. It can join an inner hub to an outer ring with two continuous weld paths with weld depths from 2.5" to 4.5". Maximum weld depth is 12". The bottom line is a big boost in productivity: in 3 hours it will produce a part that now requires up to a week of submerged-arc welding.
The electron gun is rated at 60 kW, continuous duty, about twice that of other high-voltage units operating in the US today. Accelerating voltage range is 0 to 175 kV, beam current range is 0 to 400 mA, focal-length range is 2" to 72", and gun tilt range is [plus-or-minus]35 degrees from the vertical with an accuracy of [plus-or-minus]0.01 degrees. A dual lens is the basis for the 6-ft focal range; the Z axis when you think of the system in machine-tool terms.
The welding chamber is 192" X 132" X 84" high, and constructed of structurally reinforced 1"-thick stainless-steel plate. The chamber base has precision guide rails that provide [plus-or-minus]0.001" positioning accuracy for the stacked X-axis and rotary-axis tables. A Y-axis table is also provided, but not normally used. See photos.
One of the most important features is the system's real-time seam-tracking capability. The welding cycle is programmed so that periodically, the electron beam drops to a lower power level, scans ahead of the weld spot to locate the seam surfaces, and quickly returns to the weld in a matter of milliseconds. This produces surface reactions on the seam edges that are detected optically and sent to the CNC to reposition the part for any deviation caused by thermal expansion or part runout.
The vacuum pumping system uses cryogenic pumps for the critical hard-vacuum portion of the vacuum pump-down cycle, instead of the more conventional diffusion pumps. This supercold approach eliminates part contamination from oil backstreaming, reduces pumping time during periods of high-shop humidity, and reduces energy consumption for pumping. Total evacuation time for the huge chamber is 20 min.
All tooling motions are CNC controlled by an Allen-Bradley 7320 controller, including gun-column tilt, vacuum-pump sequences, and all positioning.
For more information on EBW systems from Leybold-Heraeus, circle E3.
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|Title Annotation:||electron beam welding|
|Author:||Sprow, Eugene E.|
|Publication:||Tooling & Production|
|Date:||Mar 1, 1985|
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