Frequently Asked Questions
Technologists at TWI have developed a tell-tale system
to reveal any potentially detrimental inconsistencies in electron beam welding of components.
In the last 30 years, electron beam welding has developed from a technique used only in the aerospace and nuclear industries, to a versatile production tool used to join a wide variety of materials, in thicknesses from hundredths of a millimetre to hundreds of millimetres.
The range of industries in which electron beam welding is exploited in similarly impressive, and includes high-volume operations such as the automotive industry. The vast majority of electron beam welds are produced for this type of industry; hardly surprising when many electron beam machines (each capable of producing hundreds of parts per hour) are frequently operated on three shifts.
One of the prime considerations for any welding process is quality assurance. Fortunately electron beam welding is a highly reproducible process owing in part to the quality and durability of the equipment. Many of the hundreds of electron beam machines operating worldwide are at least 10 and sometimes 20 years old, but are still capable of producing excellent results. Most parameters defining the weld are electrical; consequently some electron beam machines can be fitted with automatic systems for monitoring them. Although this does not provide an absolute guarantee, it can still promote a higher degree of weld reproducibility.
Alternatively, some kind of non-destructive examination of the welded joints can be undertaken. However, this is not always either possible or desirable, particularly if the part throughput is high. In production where neither of the above techniques is practised, weld quality rests on the consistency of the components in terms of fit-up, jigging and residual magnetism. A cursory visual inspection of the welded joint is unlikely to reveal any hidden defects. It is even possible for the beam to miss the joint entirely while the weld top bead still lies centrally in the joint preparation. Unfortunately, the first indication that such a component is defective may well be when it fails in service.
The principle of operation of the tell-tale system is simple: In addition to the normal features machined on the components, a witness line is machined on each side of, and equidistant from, the joint. During welding, the line is marked intermittently by the beam itself, which is deflected out of the weld pool to leave a tell-tale mark. The energy required to leave a tell-tale mark is generally a fraction of a Joule. Marking is thus accomplished in a few tens or hundreds of microseconds, i.e. so quickly that the normal welding operation is not affected. A straightforward visual inspection of the component after welding will reveal any variation in weld quality. So long as the tell-tale marks lie over the witness lines, it is certain that the beam has hit the joint accurately. Defects, are easily identified and the component immediately rejected or reworked.
Application of this new technique can result in an end product of consistently higher quality. It may also be used to supplement or replace existing quality assurance procedures. With the potential benefit of reducing production losses due to stoppages, it can help identify problems before defective components are produced.
The tell-tale marking is just one of a number of techniques that can be used to improve the quality and reliability of electron beam welded products. TWI's Electron Beam Group can help with both the expertise and the hardware required to implement any of these techniques.
For further information see Joining Technologies or please contact us.