TWI Industrial Member Report Summary 719/2001
Currently NVEB welding potentially offers many benefits over and above those of in-vacuum EBW and laser beam welding, notably elimination of the need for a vacuum chamber and single pass thick section welding capability. Unfortunately beam spreading occurs, as the electrons collide with atmospheric gas and particularly with metal ions from the weld pool, causing severe scattering of the beam. This limits not only the weld depth achievable, but also the gun column to workpiece distance. Typically the working distance has to be restricted to 30mm or less, and the maximum penetration achievable in steel is some 50mm without excessive weld width and defect problems.
Plasma control devices such as helium gas jets have been tried, but these appear to have a limited effect compared with CO 2 laser welding experiences. The beam can be cleared of plasma, but only at relatively high gas flow rates which tend to affect metal flow adversely in the weld pool.
In the case of reduced pressure welding operating at above 1mbar, similar plasma and electron scattering effects have been experienced, albeit at longer working distances. Working distance can be extended by reducing the pressure level, but this increases work chamber pump down time and places greater emphasis on chamber seal integrity.
One promising line of research is to pulse the electron beam, so that the plasma and gas pressure in the vicinity of the weld are allowed to decay when the beam is off, but with a sufficiently high duty cycle that the weld pool does not collapse or solidify. From a literature search on laser and EB pulsing it appears that a pulse frequency of 10kHz will be required to gain good control of the plasma.
In order to achieve such high frequencies without compromising the mobility and reliability of EB guns it was considered essential to develop a locally coupled unit. The RF auxiliary supply technology developed at TWI for cathode heating provided a means of achieving this goal.
- To develop a high frequency locally coupled RF pulsed grid supply
- To design and build control electronics to adjust pulse width, amplitude and modulation frequency
- To apply the hardware developed to a reduced pressure/NVEB welding head