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Electron beam welding

2007 Schools Wikipedia Selection. Related subjects: Engineering

   Electron beam welding (EBW) is a fusion welding process in which a beam
   of high-velocity electrons are applied to the materials being joined.
   The workpieces melt as the kinetic energy of the electrons is
   transformed into heat upon impact, and the filler metal, if used, also
   melts to form part of the weld. Pressure is not applied, and a
   shielding gas is not used, though the welding is often done in
   conditions of a vacuum to prevent dispersion of the electron beam. The
   process was developed in France and released on November 23, 1957 in
   Paris by J. A. Stohr.

Operation

   As the electrons strike the workpiece, their energy is converted into
   heat, instantly vaporizing the metal under temperatures near 25,000
   °C. The heat penetrates deeply, making it possible to weld much thicker
   workpieces than is possible with most other welding processes. However,
   because the electron beam is tightly focused, the total heat input is
   actually much lower than that of any arc welding process. As a result,
   the effect of welding on the surrounding material is minimal, and the
   heat-affected zone is small. Distortion is slight, and the workpiece
   cools rapidly, and while normally an advantage, this can lead to
   cracking in high-carbon steel. Almost all metals can be welded by the
   process, but the most commonly welded are stainless steels,
   superalloys, and reactive and refractory metals. The process is also
   widely used to perform welds of a variety of dissimilar metals
   combinations. However, attempting to weld plain carbon steel in a
   vacuum causes the metal to emit gases as it melts, so deoxidizers must
   be used to prevent weld porosity.

   The amount of heat input, and thus the penetration, depends on several
   variables, most notably the number and speed of electrons impacting the
   workpiece, the diameter of the electron beam, and the travel speed.
   Greater beam current causes an increase in heat input and penetration,
   while higher travel speed decreases the amount of heat input and
   reduces penetration. The diameter of the beam can be varied by moving
   the focal point with respect to the workpiece—focusing the beam below
   the surface increases the penetration, while placing the focal point
   above the surface increases the width of the weld.

   The three primary methods of EBW are each applied in different welding
   environments. The method first developed requires that the welding
   chamber be at a hard vacuum. As a result, the chamber must be small to
   prevent it from being crushed under atmospheric pressure. Material as
   thick as 15  cm (6  in) can be welded, and the distance between the
   welding gun and workpiece (the stand-off distance) can be as great as
   0.7 m (30 in). While the most efficient of the three modes,
   disadvantages include the amount of time required to properly evacuate
   the chamber and the cost of the entire machine. As electron beam gun
   technology advanced, it became possible to perform EBW in a soft
   vacuum, under pressure of 0.1 torrs. This allows for larger welding
   chambers and reduces the time and equipment required to attain evacuate
   the chamber, but reduces the maximum stand-off distance by half and
   decreases the maximum material thickness to 5 cm (2 in). The third EBW
   mode is called nonvacuum or out-of-vacuum EBW, since it is performed at
   atmospheric pressure. The stand-off distance must be diminished to 4 cm
   (1.5 in), and the maximum material thickness is about 5 cm (2 in).
   However, it allows for workpieces of any size to be welded, since the
   size of the welding chamber is no longer a factor.

Equipment

   The electron beam gun used in EBW both produces the electrons and
   accelerates them, using an emitter made of tungsten that emits
   electrons when heated. The electrons are then attracted to an anode
   inside the tool, where they collect and are then directed with magnetic
   forces resulting from focusing and deflection coils. These components
   are all housed in an electron beam gun column, in which a hard vacuum
   (about 0.00001 torr) is maintained.

   The EBW power supply pulls a low current (usually less than 1  A), but
   provides a voltage as high as 60 kV in low-voltage machines, or 200 kV
   in high-voltage machines. High-voltage machines supply a current as low
   as 40 mA, and can provide a weld depth-to-width ratio of 25:1, whereas
   the ratio with a low-voltage machine is around 12:1. The beam power of
   a power supply is an indicator of its ability to do work, and
   determines the power density (generally 40-4000  kW/cm² or
   100-10,000 kW/in²).

   For the hard vacuum and soft vacuum EBW methods, the welding chamber
   used must be airtight and strong enough to prevent it from being
   crushed by atmospheric pressure. It must have openings so that the
   workpieces can be inserted and removed, and its size must be sufficient
   to hold the workpieces but not significantly larger, as larger chambers
   require more time to evacuate. The chamber must also be equipped with
   pumps capable of evacuating it to the desired pressure. For a hard
   vacuum, a diffusion pump is necessary, while soft vacuums can often be
   obtained by less costly equipment.

   Retrieved from " http://en.wikipedia.org/wiki/Electron_beam_welding"
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   with only minor checks and changes (see www.wikipedia.org for details
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