Physics of Electron Beam Heating
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, as conduction 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 and magnetic 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 – 107 W/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”.
Read more about this topic: Electron Beam Welding
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