Electron Beam Welding - Electron Beam Welding Equipment

Electron Beam Welding Equipment

Any electron beam equipment comprises:

1 - : electron gun, generating the electron beam,

2 - : working chamber, mostly evacuated to "low" or "high" vacuum,

3 - : work-piece manipulator (positioning mechanism),

4 - : supply and control/monitoring electronics.

Electron gun

In the electron gun, the free electrons are gained by thermo-emission from a hot metal strap (or wire), which are then accelerated and formed into a narrow convergent beam by electric field produced by three electrodes: the electron emitting strap, the cathode, connected to the -pole of the high (accelerating) voltage power supply (30 - 200 kV) and the +H.V. electrode, the anode. There is a third electrode charged negatively with respect to the cathode, called Wehnelt or control electrode. Its negative potential controls the portion of emitted electrons entering into the accelerating field, i.e. the electron beam current.

After passing the anode opening the electrons move with constant speed in a slightly divergent cone. For the technological applications the divergent beam has to be focused, which is realized by the magnetic field of a coil, the "magnetic focusing lens.

For the proper function of the electron gun, it is necessary that the beam is perfectly adjusted with respect to the optical axes of the accelerating electrical lens and the magnetic focusing lens. This can be done by applying magnetic field of some specific radial direction and strength, perpendicular to the optical axis before the focusing lens. This is usually realized by a simple correction system consisting of two pairs of coils. By adjusting the currents in these coils any required correcting field can be produced.

After passing the focusing lens the beam can be applied for welding directly or after being deflected by the deflection system. This consists of two pairs of coils, each pair for one of the X and Y directions. These can be used for "static" or "dynamic" deflection. The static deflection is useful for exact positioning of the beam by welding. The dynamic deflection is realized by supplying the deflection coils by currents which can be controlled by the computer. This opens new possibilities of electron beam applications, like e.g. surface hardening or annealing, exact beam positioning, etc.

The fast deflection system can also be applied (if provided with appropriate electronics) for imaging and engraving. In this case the equipment is operated similarly as a scanning electron microscope, with the resolution of about 0,1 mm (limited by the beam diameter). In a similar mode the fine computer controlled beam can "write" or "draw" a picture on the metal surface by melting a thin surface layer.

Working chamber

Since the publication of the first electron beam welding machines at the end of 1950s, the application of electron beam welding spread rapidly into industry and research in all highly developed countries. Up to nowadays uncountable number of various types of electron beam equipments have been designed and realized. In most of them the welding takes place in the working vacuum chamber in high or low vacuum environment.

The vacuum working chamber may have any desired volume from a few liters up to hundreds of cubic meters. They can be provided with electron guns supplying electron beam with any required power up to 100 kW, or even more if needed. In micro-electron beam devices the components in the tenths of a millimeter dimension range can be precisely welded. In welders disposing with high enough power electron beams, welds up to 300 mm deep can be realized.

There are also welding machines in which the electron beam is brought out of vacuum into the atmosphere. With such equipments very large objects can be welded without huge working chambers.

Work-piece manipulators

The electron beam welding can never be "hand-manipulated", even if not realized in vacuum, as there is always strong X-radiation. The relative motion of the beam and the work-piece is most often rotation or linear travel of the work-piece. In some cases the welding is realized by the beam being moved by the computer controlled deflection system. The work-piece manipulators are mostly designed individually to meet specific requirements of the welding equipment.

Power supply and control/monitoring electronics

Any electron beam equipment must be provided with appropriate supply of power for the beam generator. The accelerating voltage may be chosen between 30 and 200 kV. Usually it is about 60 or 150 kV, depending on various conditions. With rising voltage the technical problems and the price of the equipment are rising rapidly, hence, whenever it is possible the lower voltage about 60 kV is to be chosen. The maximum power of the H.V. supply depends on the maximum depth of weld required.

The high voltage equipment must also supply the low voltage above 5 V for the cathode heating, and negative voltage up to about 1000 V for the control electrode.

The electron gun also needs low voltage supplies for the correction system, the focusing lens, and the deflection system. The last one may be very complex when it should provide the computer controlled imaging, engraving and similar applications of the beam.

Complex electronics may also be needed for the control of the work-piece manipulator.