13-05-2013, 03:14 PM
ELECTRON BEAM WELDING
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DEFINATION
A metal joining process that uses a narrow stream of electrons to produce a highly concentrated heat source.
Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed in conditions of vaccum in order to prevent dissipation of the electron beam.
INRODUCTION TO EBM
High energy density beam welding refers to electron or laser processes where a beam of electrons or photons, respectively, can be focused to power densities high enough to melt and vaporize the metals being joined. The high power densities and associated metal vaporization can be used to produce welds with high depth-to-width aspect ratios, small heat-affected zones, and reduced distortion. In certain applications, high energy density beam welds can offer both high quality and cost-effectiveness when compared to other welding methods. This article provides a brief history of electron and laser beam welding, discusses the properties of electrons and photons used for welding, and contrasts electron welding by way of an introduction to the sections that follow, where each process is described separately. In addition, a section is included on the growing field of microjoining with electron and laser beams, which is becoming more important as miniaturization of components continues in the areas of microelectronics, implantable medical devices, sensors, optoelectronics, and micromechanical systems. Although many of the principles are the same between macro- and microjoining, when weld dimensions become less than a millimeter, spot size, beam control, and part manipulation become more challenging, joint preparation and cleanliness become more important, and not all of the material properties scale at the same rates. These factors bring new challenges to both the technological and metallurgical aspects of microjoints made by electron and laser beams.
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.
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 equipment 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 equipment 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
HISTORY
Electron beam welding was developed by German physicist Karl-Heinz Steigerwald of Stuttgart, West Germany before 1958. Steigerwald had been conducting experiments on their electron microscope to increase power of the scope and found out to their surprise, that the specimen which was being examined would disappear. Later, Steigerwald determined that by regulating the power settings, the specimen would melt and resolidify. Thus the electron beam welder was born.
In 1959 through agreements with the Carl Zeiss Co., United Technologies Corporation (then called United Aircraft) and its division, Hamilton Standard, sold "Zeiss" electron beam welders in the United States. Hamilton Standard then sold E.B. machines with a label "Hamilton - Zeiss" and finally produced machines labeled "Hamilton Standard".
In 1976 Hamilton Standard sold its interest in the electron beam welding business to Leybold - Heraeus Vacuum Systems Inc. In 1989 the entire operation was taken over by PTR - Precision Technologies Inc., Enfield, CT. To date, over 800 electron beam welders have been installed around the world through the combined efforts of Zeiss Co./ Hamilton Standard/ Leybold-Heraeus/ PTR Precision Technologies Inc.
The various electron beam systems available are: high vacuum, partial vacuum, and non-vacuum and are used accordingly depending upon the application.
Today, electron beam welding is used in many diversified industries throughout the world such as: nuclear, aerospace, aircraft, automotive, instrumentation, medical, electronics, commercial and job shops