MFD Electron Beam WeldingStaff
A Brief Introduction to E Beam WeldingPlease take few minutes to familiarize yourself with some of the capabilities and requirements of MFD's E Beam Welder.
IntroductionThe Sciaky electron beam welding machine, located in the rear of building 31’s clean room, was originally purchased to fabricate copper vacuum chambers for the PEP-II high-energy ring. Since the completion of PEP-II, the machine has seen usage fabricating a variety of copper chambers for SSRL and BaBar. The machine’s primary designed utility was developed to insure that the clean assembly of long intricate chamber shapes could take place in a timely, controlled, and repetitive manner. To comply with the rigid construction requirements of the PEP-II project, the machine was constructed of stainless steel and uses no hydrocarbon lubricants. A dry rough-pump and cryo-pumps produce the required system vacuum. All welding motions and control parameters are CNC controlled. Though the electron beam machine is primarily used by SLAC to join copper structures, it remains capable of welding most metals that are weldable by other standard processes within specific limitations. Tooling design, material selection, machine capacity, and joint design are fundamental considerations that require specific planning and will weigh heavily on project success. We therefore highly recommend you seek consultation by the Mechanical Fabrication Department in the very early stages of project planning. Machine CapacityInherent to the electron beam welding process is the necessity that the procedure takes place in a closed vacuum system at approximately 1x10-4 Torr. This closed system and initial machine design characteristic introduces physical size restrictions on parts that can be processed. The maximum rectangular envelope that can be accommodated is 20ft in length by 23 inches in width by 15 inches in height. Here's a simple illustration, not to scale, demonstrating the working envelope of the E Beam Welder:
Machine ConfigurationMechanicalThe machine is configured with a standard large central work chamber and two antechambers on either side. The antechambers serve as loading and docking areas for the tooling table that transports the work piece into and through (“U” axis motion) the central chamber where the welding takes place. The load chamber, located on the right-hand side, can be valved off for rapid processing of small parts. The weld head that produces the electron beam has 4 degrees of movement. Three axes are CNC controlled: “Z” – up and down, “Y” – front to back, and “B” – gun rotational from front to back. The gun has the additional capability of being mechanically manipulated to point (length-wise downstream) at either antechamber. The welding machine also can accommodate rotary axis (“R” axis) motion for the welding of cylindrical or round shapes. It is important to note that all axes and gun parameters are under simultaneous control and can be synchronized to produce highly sophisticated routines. Once the preliminary weld development and programming routines are completed, cost effective processing of any batch size can take place. ElectricalSimply put, the electron beam is produced by heating a tantalum filament, applying a 60kv plate voltage, and passing the stream of electrons through a focusing/deflection coil, which sharply impinges these electrons onto a moving work piece. The combination of accelerating voltage, beam current, focus, deflection, and work piece feed rate produces the desired recast penetration. The maximum power output of our machine is 500kW. The maximum recast depth that this power could achieve is approximately 2 inches in copper. The electron beam process is termed a rapid solidification process, which enables deep penetrations to be achieved while minimally affecting the state of the surrounding material. The welding head is steered by collecting data points that represent a desired path and then recalling the data for welding. The head is also equipped with real-time television and seam tracking. The joint data is entered and stored by the reception of reflected electron or visual spotting. Tooling and Joint considerationsInherent to all joining processes is the consideration of part distortion due to shrinkage encountered during the solidification of molten metal. Though rapid solidification processes such as electron beam or laser produce minimal distortion, they are nevertheless present and must be considered. Joint design and restraint fixtures are invaluable aids at countering these forces. Some standard joint configurations used by other processes can also be used for election beam welding, but will require modification to take full advantage of the unique character associated with electron beam welds. The addition of filler wire is not standard practice. The natural shaping of the recast due to focused electron beam bombardment is a narrow interface that has a spiked (pointed) formation. Therefore, maximum joint gaps should not exceed .010 inch and the use of self-locating backup steps is desirable. If filler material is required, its incorporation can be made possible by machining performed at strategic locations on the part’s surface prior to welding. Given that the preponderance of electron beam welding performed at SLAC are copper components, the most commonly employed joint configurations are lipped-butt (top or bottom), lap, and butt. Because each application has unique requirements, it is contingent upon the designer or engineer to obtain direction from the qualified engineering staff at MFD. We will assist you in the selection of best joint configuration, joint tolerances, material selection, and the design of restraint fixturing. Due to the reaction of electrons to magnetism and contamination restrictions, selection of fixturing materials is limited to nonferrous materials such as stainless steel or aluminum. CleaningUnder normal conditions, vapor degreasing is sufficient preparation for processing components prior to introduction to the welding chamber. Cleaning requirement above and beyond vapor degreasing of both fixtures and parts for ultra-high vacuum use is a consideration. Refer to SLAC specification: FP-202-631-14-RX. |
To provide sufficient space for the gun to move inside the welder, assemblies must be no larger than fifteen inches in height and twenty-three inches in width. The welder can accommodate pieces up to twenty feet in length.