Electron beam welding (EBW) is widely applied to high value components eg aeroengine assemblies. Just prior to welding, the operator must be sure that the equipment will reproduce the best quality weld. TWI has developed a beam diagnosis system that offers this assurance.
Many electron beam welding systems are used for precision work in which the reproducibility of the beam is paramount. Various techniques exist for ensuring beam (and thus weld) quality, from welding of testpieces (either a standard type, or one representative of a particular joint) to direct measurement of the beam itself.
Whilst testpiece welding can confirm good beam quality, the process is often time-consuming, and the results, if not acceptable, are difficult to interpret in terms of the machine set-up changes that might be required. Beam probing techniques potentially offer both an assurance of beam quality, and information that could be more readily interpreted in terms of machine set-up, or cathode condition.
The slit probe unit developed at TWI consists of a refractory metal slit through which a small portion of the beam current passes as the beam is deflected electrically (at ~2.5km per second) over it.
As the beam crosses the slit probe unit, the electrons passing through the slit are captured by a Faraday cup, and a signal is obtained. Resolution is not limited by the design of the probe in the same way as it is with other systems.
The detector element itself is fully enclosed, and only 'sees' a tiny portion of the beam power. This means that the system is resistant to electrical noise pickup, and signal degradation due to ion emission, secondary electron emission, and electron backscatter.
Typically a 0.1mm slit width is used, giving a significantly improved resolution when compared to many other systems. The probe is a rugged device that can be left in the chamber permanently or moved from one machine to another to allow comparison.
The TWI slit probing system, which consists of three main elements as shown in the illustration is capable of a wide variety of beam diagnostic tasks including:
- beam asymmetry detection
- comparison of probe traces with stored reference traces
- beam current estimation
- beam diameter estimation
- beam power density distribution estimation
- focus estimation
An industrial prototype, using two slit probe units (X and Y axis), has been supplied by TWI to an aerospace company.
For further information contact Olivier Nello, e-mail: firstname.lastname@example.org or Bruce Dance, e-mail: email@example.com of the Electron Beam Group at TWI.