Advanced Welding Processes: Technologies and Process Control

158
Advanced welding processes
Filament
supply
Electron
beam
Workpiece
and may be deflected or oscillated magnetically. The complete electron
generator assembly or electron gun is usually mounted either inside or external
to a vacuum chamber which contains the workpiece.
8.4.2 Beam characteristics
Cathode
Anode
8.15 Principle of electron beam welding.
Magnetic
focusing lens
Vacuum
system
Beam deflection
system
Power densities of from 10 10 to 1013 W m –2 are developed at the point of
focus and keyhole welding is the normal operating mode. The forces which
create the keyhole in EBW are:
∑ electron momentum;
∑ vapour pressure;
∑ recoil pressure.
Surface tension and gravitational forces counteract keyhole formation but,
under normal circumstances, the keyhole-forming forces are much higher.
For example, the electron momentum pressure Pa is given by [177]
P a = 2Jm eV/e 2 (8.1)
where J is the current density, V the accelerating voltage and m e is the electronic
mass. For a focused spot 0.3 mm in radius, 100 mA filament current and 100
kV, this force will be around 300 N m –2 . The vapour pressure, which is
temperature dependent, can reach values of 5 ¥ 10 4 N m –2 and the recoil