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Electron Beam Welding ( EBW )
Electron beam welding (EBW) is a process that melts and joins metals by heating them with an electron beam. As shown in lateral figure (a), the cathode of the electron beam gun is a negatively charged filament .When heated up to its thermionic emission temperature, this filament emits electrons.
These electrons are accelerated by the electric field between a negatively charged bias electrode (located slightly below the cathode) and the anode. They pass through the hole in the anode and are focused by an electromagnetic coil to a point at the workpiece surface.
The beam currents and the accelerating voltages employed for typical EBW vary over the ranges of 50–1000mA and 30–175kV, respectively. An electron beam of very high intensity can vaporize the metal and form a vapor hole during welding, that is, a keyhole, as depicted in last figure (b).
The beam diameter decreases with decreasing ambient pressure . Electrons are scattered when they hit air molecules, and the lower the ambient pressure, the less they are scattered. This is the main reason for EBW in a vacuum chamber.
The electron beam can be focused to diameters in the range of 0.3–0.8mm and the resulting power density can be as high as 1010W/m2 .The very high power density makes it possible to vaporize the material and produce a deep penetrating keyhole and hence weld. A single-pass electron beam weld and a dual-pass gas–tungsten arc weld in a 13-mm-thick (0.5-in.) 2219 aluminum, the former being much narrower . The energy required per unit length of the weld is much lower in the electron beam weld (1.5 kJ/cm, or 3.8 kJ/in.) than in the gas–tungsten arc weld (22.7 kJ/cm, or 57.6 kJ/in.).
Electron beam welding is not intended for incompletely degassed materials such as rimmed steels. Under high welding speeds gas bubbles that do not have enough time to leave deep weld pools result in weld porosity. Materials containing high-vapor-pressure constituents, such as Mg alloys and Pb containing alloys, are not recommended for EBW because evaporation of these elements tends to foul the pumps or contaminate the vacuum system.
Figure: Electron beam welding:
(a) process (b) keyhole.
Advantages and Disadvantages
With a very high power density in EBW, full-penetration keyholing is possible even in thick workpieces. Joints that require multiple-pass arc welding can be welded in a single pass at a high welding speed. Consequently, the total heat input per unit length of the weld is much lower than that in arc welding, resulting in a very narrow heat-affected zone and little distortion. Reactive and refractory metals can be welded in vacuum where there is no air to cause contamination. Some dissimilar metals can also be welded because the very rapid cooling in EBW can prevent the formation of coarse, brittle intermetallic compounds.When welding parts varying greatly in mass and size, the ability of the electron beam to precisely locate the weld and form a favorably shaped fusion zone helps prevent excessive melting of the smaller part. However, the equipment cost for EBW is very high. The requirement of high vacuum (10-3–10-6 torr) and x-ray shielding is inconvenient and time consuming. For this reason, medium-vacuum (10-3–25torr) EBW and nonvacuum (1 atm) EBW have also been developed. The fine beam size requires precise fit-up of the joint and alignment of the joint with the gun. Residual and dissimilar metal magnetism can cause beam deflection and result in missed joints .
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