Electron Beam Welding - Applications of Electron Beam For Welding

Applications of Electron Beam For Welding

Process of penetration

Penetration of electrons

To explain the capability of the electron beam to produce deep and narrow welds, we have to explain the process of "penetration". First of all let us consider the process for a "single" electron.

When electrons of the beam impact the surface of a solid, some of them may be reflected (as "backscattered" electrons), and others penetrate under the surface, where they collide with the particles of the solid. In non-elastic collisions they loose their kinetic energy. It has been proved, both theoretically and experimentally, that they can "travel" only a very small distance under the surface before they transfer all their kinetic energy into heat. This distance is proportional to their initial energy and inversely proportional to the density of the solid. Under conditions usual in welding practice the "travel distance" is of the order hundreds of a millimeter. Just this fact enables, under certain conditions, the fast penetration of the beam.

Penetration of the electron beam The contribution of single electrons to heat is very small, but they can be accelerated by very high voltage, and by increasing their number (the beam current), the power of the beam can be increased to any desired value. By focusing the beam to a small diameter on the surface of a solid object, values of planar power density as high as 104 up to 107 W/mm2 can be reached. Due to the fact that electrons transfer their energy into heat in very thin layer of the solid, as explained above, the power density in this volume can be extremely high. 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, by 108 – 109 K/s.

Resulting effect of 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 may be evaporated totally in a very short time, which can be applied for “machining.