Welding method ignites thinking in Europe.
Nonvacuum EBW (electron beam welding) is no stranger to US and European manufacturers. In the early 1960s, efforts to use this technology was aimed at accomplishing high volume production of welding tubing at very high speeds. Since its inception, nonvacuum EBW has been used to weld a broad variety of automotive style production components, such as catalytic converters, steering column jackets, and numerous types of powertrain parts in the US and Europe. Recently, Audi adopted a non-vacuum electron beam welding program.
Nonvacuum EBW is performed directly in atmosphere. Although the process eliminates the need to have a vacuum environment around the part, welding in this type of atmosphere can slightly hinder the nonvacuum EB welding (EBW-NV) process from producing exactly the same kind of very high depth-to-width aspect ratio weld that is usually achieved when applying EBW in vacuum. This disparity occurs because the electrons in the beam interact with air molecules, producing a scattering effect resulting in a beam dispersion that causes the beam to gradually increase in size with distance traveled out into the surrounding atmosphere [ILLUSTRATION FOR FIGURE 1 OMITTED]. Although the beam dispersion tends to limit the aspect ratio achievable with the EBW-NV process, the dispersion supplies the process with an enhanced ability for accommodating the typical type of weld joint encountered with today's high volume production joining applications.
In an EBW-NV system, the EB gun's cathode is heated to a temperature that causes the electrons to be randomly "boiled off" its surface; concurrently, this cathode is raised to a high negative DC potential with respect to the electrically grounded gun anode. The cathode (or filament) is housed within an electron gun assembly made up of a cathode, grid cup, and anode. This combination is specifically designed to form a potential field configuration that electrostatically accelerates and shapes the random electrons into a directed beam [ILLUSTRATION FOR FIGURE 2 OMITTED]. The beam is composed of high velocity electrons that pass through a hole in the center of the anode and traverses down the axis of the EBW-NV column with the full energy of high negative voltage being applied to the cathode.
A vacuum environment is required in the gun area where the beam is started. As such, the overall EBW-NV column structure where the EB gun is located consists of a series of individually pumped vacuum stages, each connected to the other with a set of concentrically mounted apertures, i.e. "orifices". Use of this scheme allows the atmosphere (workpiece area) to high vacuum (gun area) pressure gradient to be readily maintained, while simultaneously providing an open pathway that the beam can employ for exiting into the ambient atmospheric pressure region. Electromagnetic focusing and alignment coils, located below the anode, provide the beam focus and alignment capability needed to ensure that the high velocity stream of electrons being produced passes cleanly through the series of in-line orifices it encounters in traversing the succession of progressively increasing pressure stages. The electron beam loses little energy during its passage through the EBW-NV column structure, and eventually, into the atmosphere. When the beam exits the final orifice, the dispersion effect displayed will appear like the one shown in Fig. 1.
The visual beam is formed from ambient gas ionization produced by the scattered electrons. However, this event gives a rough indication on the type of beam dispersion occurring; consequently, the electron beam creating the visual beam glow is actually much smaller than the visible excitation effect it generates. Figure 3 shows where most of the fringe portion of this associated beam glow is obscured by the part being welded. The extent of beam broadening incurred by scattering is reduced by using a helium effluent. The helium is ejected through the beam exit orifice.
Efforts have been made to use light-weight materials such as aluminum to exhibit a weight reduction and high energy absorption capability in new car body designs.
In 1994, a program was started by Audi to evaluate the prospect of replacing steel dashboard cross beam components with an aluminum box beam structure. Nonvacuum EB welding was chosen as the best process for performing this joining task.
The production system for nonvacuum EB welding aluminum dashboard components embodies two CNC X and Y tables, each with dual part-clamping fixtures mounted on a large indexing table married to a room style radiation enclosure. The EBW-NV process proved to be more adaptable to the type of large variations in joint condition (e.g., joint gapping, mismatch between edges, and poor edge quality) that production flange joints tend to display.
Before selecting the EBW-NV process for aluminum dashboard beams, extensive weld development tests were conducted on both coupons simulating actual part joint geometry and on prototype production parts. The purpose of the weld trial program was twofold: first to develop an enhanced EBW-NV beam capacity for reliably welding the aluminum segments to be joined in production; and, secondly, to verify that the new aluminum beam design being produced would have the same stiffness and fatigue characteristics as the original steel beam.
Two halves of the dashboard crossbeam component are placed in the fixture that hydraulically clamps them together to reduce the amount of gapping that exists between the interface surfaces forming the edge joint to be welded. After clamping, the part is transferred into a radiation-tight enclosure and a combination of both part X/Y motion and beam generator Z motion is then used to perform the 3D weld necessary. The joint being welded produces a 3D weld path with respect to the X/Y travel plane, while the electron beam generator's Z motion is totally perpendicular to the plane. The angle formed between the impinging beam and weld joint varies from fully normal to 60 deg off normal. Large lead/lag angles can readily be accommodated by programming the weld speed to decrease from 400 ipm (175 mm/s) to about 300 ipm (127 mm/s) during welding of those particular areas.
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|Publication:||Tooling & Production|
|Date:||Apr 1, 1997|
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