Electron beam welding

Electron-beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed under vacuum conditions to prevent dissipation of the electron beam. Electron-beam welding was developed by the German physicist Karl-Heinz Steigerwald in 1949, who was at the time working on various electron-beam applications. Steigerwald conceived and developed the first practical electron-beam welding machine, which began operation in 1958. American inventor James T. Russell has also been credited with designing and building the first electron-beam welder. Electrons are elementary particles possessing a mass m = 9.1 · 10−31 kg and a negative electrical charge e = 1.6 · 10−19 C. They exist either bound to an atomic nucleus, as conduction electrons in the atomic lattice of metals, or as free electrons in vacuum. Free electrons in vacuum can be accelerated, with their paths controlled by electric and magnetic fields. In this way narrow beams of electrons carrying high kinetic energy can be formed, which upon collision with atoms in solids transform their kinetic energy into heat. Electron-beam welding provides excellent welding conditions because it involves: The effectiveness of the electron beam depends on many factors. The most important are the physical properties of the materials to be welded, especially the ease with which they can be melted or vaporize under low-pressure conditions. Electron-beam welding can be so intense that loss of material due to evaporation or boiling during the process must be taken into account when welding. At lower values of surface power density (in the range of about 103 W/mm2) the loss of material by evaporation is negligible for most metals, which is favorable for welding. At higher power density, the material affected by the beam can totally evaporate in a very short time; this is no longer electron-beam welding; it is electron-beam machining. Conduction electrons (those not bound to the nucleus of atoms) move in a crystal lattice of metals with velocities distributed according to Gauss's law and depending on temperature. They cannot leave the metal unless their kinetic energy (in eV) is higher than the potential barrier at the metal surface. The number of electrons fulfilling this condition increases exponentially with increasing temperature of the metal, following Richardson's rule.