Electron beam welding was developed in the late 1950s. It was quickly embraced by high-tech industries, such as aerospace, for the precision and strength of its resultant welds. An electron beam can be very accurately placed, and the weld can retain up to 97% of the original strength of the material. It is not an exaggeration to state that EB welding, in terms of the quality of the weld, is unbeatable.
EB welding is simple to explain. A tungsten filament is heated and power is applied to the point that the filament gives off electrons. These electrons are accelerated and focused using electrical fields and magnetic “lenses.” This invisible stream of fast-moving electrons has tremendous kinetic energy. When these electrons strike a metal part, the kinetic energy is transferred to the molecular lattice of the material, heating it almost instantaneously.
The power delivered by an electron beam can be massive—up to 10,000 kW/mm3. In fact, an electron beam welding system can throw enough power to simply vaporize metal (a process called electron beam machining). EB welding machines generally come in two power classifications: low voltage (60 kV); or high voltage (150 kV). A typical high-voltage machine rated to 7,500 watts can produce a weld in steel 2 in. deep with a width of approximately 10 percent of the penetration depth.
The logistics of operating an EB welding system aren’t simple, however. The process has to happen in a vacuum, otherwise, air/gas particles scatter and diffuse the electrons. A vacuum requires a vacuum chamber, so the size of a part to be welded is limited by the size of the chamber. Vacuum chambers can be small or large, but the larger the chamber, the longer it will take to establish the proper vacuum level, which is at a minimum 1.0 x 10-3 torr. The use of a vacuum, as well as the presence of X-radiation (a byproduct of the beam), precludes human handling, so the entire process has to be externally controlled, generally using CNC tables.
EB welding has been fully automated for decades. The collusion of all this technology—high voltage, vacuum, and high-tech automation—means that EB welding requires well-trained operators and very competent maintenance, and that the setup and running of an EB welding system can be expensive.
EB welding requires a precise fit between the parts being welded, as use of a filler material is generally eschewed. Proper fixturing also minimizes the effects of shrinkage and warping during welding. The parts are generally securely fixed to a motion-controlled table to precisely move the areas to be welded into contact with the electron beam. Most EB welding machines utilize a fixed beam with the part being manipulated under it via CNC.
The electron beam has to be carefully calibrated and focused and timed with the CNC motion to deliver a consistent weld with uniform penetration and minimal porosity. Each welding cycle involves loading the welding chamber, pumping down the vacuum, welding the part, and then venting the vacuum.
The chokepoint in the electron beam welding process is the pumping up and down of the vacuum chamber and the loading/unloading of the parts. Hence, it is imperative that the
engineers and technicians involved maximize the number of parts to be welded each cycle and optimize the movement of the CNC table. When this is all done correctly, electron beam welding achieves very high quality with high cost-effectiveness.
Electron beam welding systems can weld all weldable metals and some metals that are not typically welded. EB welds are incredibly strong and pure. Impurities in the weld are vaporized, and welding in a vacuum means there are no gases or air to react and cause oxides.
EB welding can also join dissimilar materials that would otherwise be unweldable due to differences in melting points, which result in intermetallic compounds that cause brittleness. The precise nature of the electron beam and tight heat-affected area allow EB welding to basically melt the lower-temperature material onto the unmelted, higher-temperature material, resulting in a compact, vacuum-tight weld.
EB welding has some aspects of it that are cumbersome, but the products of EB welding are first-class in all respects.