TWI Knowledge Summary
Electron beam welding - equipment
Electron Beam Welding (EBW) is a fusion process for joining metals which uses a highly focused beam of electrons as a heat source. Usually the electrons are extracted from a hot cathode, accelerated by a high potential - typically 30,000-200,000 volts, and magnetically focused into a spot with a power density of the order of 30,000 W/mm 2 . This causes almost instantaneous local melting and vaporisation of the workpiece material. The electron beam is thus able to establish a 'keyhole' delivering heat, deep into the material being welded. This produces a characteristically narrow, near parallel, fusion zone allowing plain abutting edges to be welded in a single pass for material thicknesses ranging from less than 0.1mm to greater than 200mm.
The process was first developed in Germany and France in the 1950's, but is now well established in all industrial countries. It was initially employed in the nuclear industry for fuel element encapsulations but rapidly spread into the aerospace, automobile and electronics industries. Nowadays it is employed over a very wide variety of applications ranging from, for example, high speed welding of band saw blade materials to the joining of thick section marine components.
EBW is used for joining numerous metallic materials including steels, aluminium, copper, nickel, titanium and magnesium alloys and refractory metals. The process produces high integrity welds, with minimal thermal distortion and freedom from component oxidation.
In most materials, the mechanical properties of EB welds are better than welds made with conventional fusion processes. Generally filler additions and pre-heat are not required. However, in some cases, the addition of filler material or post weld heat treatment may be necessary to develop the full mechanical properties. Material cleaning is essential and for ferritic steels and dissimilar metal combinations it is important to avoid residual magnetism and thermo-electric fields to prevent beam misalignment.
Electron beams for welding are normally generated in a relatively high vacuum (better than 5x10 -5 mbar) but the workpiece can be housed in a chamber maintained at a coarser vacuum level e.g. 5x10 -3 to 10 mbar. It is also possible to project high power electron beams into the atmosphere and produce single pass welds in steel in thicknesses of more that 40mm, but the weld width is typically greater than welds made in vacuum.
Electron beam welding equipment typically comprises of an electron gun, high voltage power source, vacuum chamber or enclosure, pumping equipment and a workpiece or gun manipulator and control system.
EBW equipment can be quite complex but is very versatile and although relatively expensive compared with say arc welding equipment, is capable of economic high volume production.
Currently, the main equipment research areas are concerned with improvements in power supply design, equipment reliability and longevity, beam characterisation, the generation of high intensity beams at low power and the development of systems for use at pressures approaching one atmosphere (1000 mbar).
Further information
Additional content on electron beam welding can be found in the items detailed below:
High voltage power supplies for electron beam welding (July 2000)
A review of electron beam probing systems (August 2000)
Electron beam welding - materials aspects (September 2000)
Mobile electron beam welding - a new on-site process. (Connect July 2000)
Numerous Member reports and Bulletin articles on EB welding are also available to TWI Industrial Members.
You can use the Weldasearch literature database to supplement what you find in JoinIT.
Copyright 2000, TWI Ltd