04-12-2012, 02:14 PM
Electron beam welding (EBW)
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Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is 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. The welding is often done in conditions of a vacuum to prevent dispersion of the electron beam. German physicist Karl-Heinz Steigerwald, who was at the time working on various electron beam applications, perceived and developed the first practical electron beam welding machine which began operation in 1958.[1]
Physics of electron beam heating
It is well known that electrons are elementary particles possessing the mass m = 9.1E10-31 kg and negative electrical charge e = 1.6E10-19 C. They exist either bound to an atomic nucleus, asconduction electrons in the atomic lattice of metals, or free electrons in vacuum.
The free electrons in vacuum can be accelerated and their orbits controlled by electric andmagnetic fields. In this way we can form narrow beams of electrons carrying high kinetic energy, which at collisions with atoms in solids transform their kinetic energy into heat. Thanks to some specific conditions, this way of heating gives us exceptional possibilities. These conditions are:
Strong electric field can accelerate electrons to a very high speed, i.e. the electron beam can carry high power, equal to the product of beam current and accelerating voltage. Increasing the beam current and the accelerating voltage, the beam power can be increased to any practically desirable value.
Using magnetic lenses the beam can be shaped into a narrow cone and focused to a very small diameter with a very high power density in the plane of impingement of the beam on the surface of some solid. Values of power density in crossover (focus) of the beam as high as 10^4 – 10^6 W/mm^2 can be achieved.
The depth of penetration of electrons into solids, as will be shown later, is in the order of hundredths of a millimeter. The volume density of power in the small volume in which the kinetic energy of electrons is transformed into heat, can reach values of the order 105 – 107W/mm3. Consecutively, the temperature in this volume increases extremely rapidly, 108 – 1010 K/s.
Resulting effect of the electron beam under such circumstances depends on conditions; -first of all on physical properties of the material. Any material in very short time can be melted, or even evaporated. Depending on conditions, the intensity of evaporation may vary, - from negligible to essential. At lower values of surface power density (in the range of about 103 W/mm2) the loss of material by evaporation for most metals is negligible, which is favorable for welding. In the upper region of the power density the material affected by the beam may be evaporated totally in a very short time, which can be applied for “machining”.
]Beam formation
Cathode - the source of free electrons
Tungsten cathodes: strap - wire
Conduction electrons (that are not bound to the nucleus of atoms) move in crystal lattice of metals with velocities distributed according to Gauss law, depending on temperature. They can not leave the metal unless their kinetic energy (in eV) is higher than the potential barrier at the metal surface. Number of electrons fulfilling this condition increases with increasing temperature of the metal exponentially, according to Richardson's rule.
As a source of electrons for electron beam welders, the material must fulfil more requirements:
to achieve high power density in the beam, the emission current density [A/mm2], hence the working temperature, should be as high as possible,
to keep evaporation in vacuum low, the material must have low enough vapour pressure at working temperature.
The emitter must be mechanically stable, chemically not sensitive to gases present in vacuum atmosphere (like oxygen and water vapour), easily available, etc.
These and some other conditions limit the choice of material for the emitter to metals with high melting points, - practically only two of them, tantalum and tungsten. With tungsten cathodes, emission current densities about 100 mA/mm2can be achieved, but only a small portion of emitted electrons takes part in beam formation, depending on the electric field produced by anode and control electrode voltages. The type of cathode most frequently used in electron beam welders is made of tungsten strip, about 0.05 mm thick, shaped as shown in Fig. 1a. The appropriate width of the strip depends on the highest required value of emission current. For the lower range of beam power, up to about 2 kW, the width w=0.5 mm is appropriate.