TWI Industrial Member Report Summary 553/1996
C N Ribton
This report describes developments made to avoid weld defects produced when the accelerating potential in an electron beam welding machine breaks down. The advances made since the report 487/1994 are outlined and in particular the method of high speed voltage recovery following a flashover is described.
Electron beam generation involves the acceleration of electrons in a high strength field between the gun and anode. The gun is pumped to a low pressure to prevent breakdown of the field.
If breakdown does occur (typically this happens when the vacuum is degraded by gases from the weld pool) the beam current will surge (in the case of a triode) or will be interrupted (in the case of a diode), both of which may cause a weld defect. The diode gun is less prone to electric breakdown, and design and analysis (using field modelling software) can reduce the chance of breakdown further. However, some risk is always present and for expensive components, for which repair of weld defects may be difficult or impossible, the problem of discharges has been investigated further.
Previous work (see TWI Report 487/1994) recovered the high voltage in a linear ramp over typically 100msec. This was found to avoid defects. However, such a long recovery time would cause problems for reduced pressure or non- vacuum electron beam welding. In these systems the beam is passed through a series of nozzles to allow a low pressure to be maintained in the upper gun-column, although the work chamber is at atmospheric pressure, or a coarse vacuum.
When the high voltage is recovering following electric breakdown between the gun electrodes, the beam generated is defocused and wider - and will impinge upon the nozzles causing melting, vaporisation and possibly further flashovers.
- To design and construct a device for better detection of gun discharges to improve the power supply response time.
- To determine the best characteristics of the beam feedback circuit in order to allow rapid control of the high voltage during discharges.