11 May 2017

Electron beam welding

Introduction :

Electron beam welding is a powerful beam welding process. The heat source in electron beam welding for melting joints is a focused beam of high-velocity electrons. The electron beam upon impinging on the workpiece releases the necessary heat by converting its kinetic energy. 
Electron beam welding is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. It is often performed under vacuum conditions to prevent dissipation of the electron beam.

How it works?

The cathode within the electron gun is the source of a stream of electrons. The electrons are accelerated towards the anode because of the large potential difference that exists between them. The potential difference between that are used are of the order of 30 kV to 175 kV. The higher the potential difference, the higher would be the acceleration. The current levels are low, ranging between 50 mA to 1000 mA. Depending on the accelerating voltage, the electrons would travel at the speed of 50000 to 200000 km/s. The depth of penetration of the weld depends on this electron speed which in turn is dependent upon the accelerating voltage.

The electron beam is focused by means of an electron magnetic lens so that the energy is released in a small area. When the high-velocity electron beam strikes the workpiece all the kinetic energy is converted to heat. As these electrons penetrate the metal, the material that is directly in the path is melted and a keyhole is formed melting the metal around the beam. As the beam traverses, the keyhole would also travel along, with the molten metal being pushed back which when solidified and thus forms the joint.

Electron beam welding

Advantages of electron beam welding :

  • The penetration of the beam is high. 
  • The depth-to-width ratios between 10:1 to 30:1 can be easily realized with electron beam welding.
  • It is also possible to closely control this penetration by controlling the accelerating voltage, beam current and beam focus. 
  • The process can be used at higher welding speeds, typically between 125 to 200 mm/s.
  • Filler metal or flux are not needed to be used in this process of welding.
  • The heat liberated is low and also is in a narrow zone. Thus, the heat-affected zone is minimal as well as weld distortion is eliminated.
  • It is also possible to carry out electron beam welding with workpieces in an open atmosphere.
  • The other advantage of using a vacuum is that the weld metal is not contaminated.
Limitations of electron beam welding :
  • EBW is the most costly welding process due to vacuum enclosure.
  • This process requires a vacuum chamber containing a hard vacuum.
  • Only small to medium size items can be welded.
  • Though the welding itself can be done very fast, overall EBW is time-consuming.
  • The equipment is complex and there are quite a few process variables involved.
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