HIGH ENERGY DENSITY WELDING PROCESSES
2.5.1 Electron Beam welding
• Electron beam welding uses the kinetic energy of a dense focussed beam of high velocity electrons as a heat source for fusion. In the equipment for this process, electrons are emitted by a cathode, accelerated by a ring-shaped anode, focussed by means of an electromagnetic field and finally impinge on the workpiece as shown schematically in Fig. 2.20. The operation takes place in a vacuum of about 10-3 mm of mercury. Accelerating voltages are in the range of 20-200 kV and welding currents are a few milliamperes, the total power is of the same order of magnitude as in SMAW, except that in this process power concentrations of 1-100 kW/mm2 are routinely achieved and upto 10 MW/mm2 can be obtained.
• As the accelerating voltage is increased, the intensity of the X-rays emitted from anode increases. In high voltage equipment means are used to limit X-ray emission within permissible limits.
• Focussing coils can concentrate the beam on a spot of a few micron in diameter. With such a concentrated spot there is a threshold voltage above which the beam penetrates
the metal and when the work is traversed relative to the beam a weld bead of exceedingly narrow width relative to the plate thickness is formed.
Fig. 2.20 Principle of electron beam welding
• This type of weld could be used for welding dissimilar materials and it is used when the effect of welding heat is to be minimized (distortion is minimum).
• The beam may be defocussed and could be used for pre-heating or post-welding heat treatment. Periodic defocussing could be useful for metals having high vapour pressure at the melting point. The process is applicable to metals that do not excessively vaporize or emit gas when melted. Can weld metals sensitive to interstitial embrittlement.
• The process is specially suitable for welding dissimiiar metals and reactive metals (super alloys (previously impossible to weld)) and for joints requiring accurate control of weld profile and penetration and for joining turbine and aircraft engine parts where distortion is unacceptable. Its major limitation is the need for a vacuum chamber. It can join plate thicknesses from thin foils to 50 mm thick plates. The gun is placed in a vacuum chamber, it may be raised lowered or moved horizontally. It can be positioned while the chamber is evacuated prior to welding. The circuit is energised and directed to the desired spot. Usually the beam is stationary and the job moves at a desired speed.
• Temperatures attained can vaporise any known metal (even tungsten). There are three commercial versions of the EBW process, depending upon the degree of vacuum used as given in the following table:
Table 2.1 Commercial versions of EBW process
• Deep penetration, with depth-to-width ratio of 20 : 1, is a unique characteristic of this process. It is mainly due to high power densities achievable with electron beams, which cause instantaneous volatilization of metal. A needle like metal vapour filled cavity or keyhole is produced through the metal plate thickness. As the welding proceeds this key-hole moves forward alongwith the beam and gravity and surface tension act to cause molten metal to flow into the cavities just behind. The limited ability of the beam to traverse the metal thickness is a unique property that ensures full penetration through the metal thickness.
• The process can be adapted to numerical control and can be performed in air or under a blanket of CO2 but the welds suffer from contamination.