TWI Industrial Member Report Summary 557/1996
C N Ribton and T B Layzell
This work reports on monitoring equipment for high power electron beams. Measuring the beam prior to welding offers the higher quality control levels and potentially can lead to significant cost savings in reduced scrap rates.
Electron beam (EB) welding machines allow the complete welding job to be defined in terms of electrical parameters. In modern machines, a CNC system, or similar control processor, is used to set up each welding task. Variability in the electrical supplies is usually detected by a parameter monitoring system. However, variability in gun performance may also lead to significant changes to welding performance. These changes may be detected by carrying out token welds prior to the main workpiece. It is more satisfactory to examine the beam intensity distribution to allow variations to be monitored, and thereby detect when unsatisfactory welding performance is most likely to be produced.
Welding performance of EB machines is highly dependent upon the beam intensity and position of focus. Beams rarely have a regular intensity profile and often guns produce oval shaped beams containing higher intensity 'hot spots'. Operators of production welding machines are generally more concerned about the reproducibility of welding performance rather than the irregular nature of the beam intensity distribution. Unfortunately, very small changes in the gun or machine parameters (especially depeaking of the filament) can lead to large changes in the beam profile and this can lead to erratic welding performance. Lack of reproducibility is one of the most common problems reported to TWI from production industry.
The aim of this project was to tackle these problems by developing a system which could measure directly the beam intensity distribution. Previously, rotating probes and slit probes have been used which provide an indication of beam intensity but cannot readily be used to give an intensity map over a cross-section of the beam. Devices are commercially available which will produce intensity maps of a beam cross section. However, they have a number of disadvantages: the beam is sampled progressively over a long period of time, so it is not possible to obtain a 'snap-shot' of the beam at any particular instant; and the screen update time is long, typically 30sec to display a histogram of the beam intensity distribution.
It was proposed to address these problems by designing and building a probe which could sample the beam within a short period of time and display the information as a histogram or intensity map within a few seconds.
- To produce high speed beam analysis equipment to allow recording and display of the beam intensity distribution.