Published on 21 January 2013
Technologists at TWI have developed a telltale system to reveal any potentially detrimental inconsistencies in electron beam welding of components.
Over several decades, electron beam welding (EBW) 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 EBW is exploited is similarly impressive, and includes high volume operations such as the automotive industry. The vast majority of EB welds are produced for this type of industry; hardly surprising when many EB 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, EB welding is a highly reproducible process owing in part to the quality and durability of the equipment. Many of the hundreds of EB sets operating worldwide are at least ten and sometimes 20 years old but are still capable of producing excellent results. Most parameters defining the weld are electrical; consequently some EB 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 are practised weld quality rests on the consistency of the set parameters of the EB machine and 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 telltale 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 telltale mark. The energy required to leave a telltale 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 telltale 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.
The telltale system cannot, however, ensure that the beam is not angled in relation to the joint interface, which might itself leave a missed joint defect, particularly in thicker sections. Angling can be caused by simple mechanical misalignment or certain magnetic effects. Good welding practice avoids the former pitfall; the latter is caused by unwanted magnetic fields which do not remain constant over the length of the joint. In this case, the telltale deviated, indicating a potentially poor weld
The telltale marks melted into the surface serve as a permanent record of the welding process. As the number and distribution of the marks is relatively unimportant, they can be used to identify particular component batches or components welded on a particular machine.
The markings also contain beam quality information; any change in the size, shape or depth of the marks could indicate a change in the beam current, focus, accelerating voltage or, perhaps the condition of the cathode. Similarly, a change in the spacing of the markings could be used as an indication that the calibration of the EB machine needs checking or adjusting.
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 telltale marking is just one of a number of techniques that can be used to improve the quality and reliability of EB-welded products TWI's EB Department can help with both the expertise and the hardware required to implement any of these techniques.
For information about TWI's capabilities please contact us.