After serving in the RAF as a radar technician for two years, Terry Layzell took a degree in electrical engineering at City University and later worked as a design engineer on several major national projects, including Instrument Landing Systems (ILS), TXE4 telephone exchanges and System X.
He joined TWI in 1990 to maintain and develop high voltage switch mode power supplies for the Electron beam department. His current research into imaging techniques using backscattered electrons is due to end in 1994.
Unreliable and extremely fallible optical techniques for viewing electron beam in-chamber work could be a thing of the past if Terry Layzell's work on imaging using backscattered electrons is taken up ...
Most of us are fortunate enough to take vision for granted, but good quality viewing of the weld pool and joint line inside a vacuum chamber whilst electron beam (EB) welding takes place requires a number of special optical arrangements to make it possible.
A lead glass window, a very bright light inside the chamber, a telescope or a closed circuit TV camera are the usual means of viewing EB welding. Unfortunately these all suffer from the effects of condensed metal vapour which quickly coats glass and prevents the passage of light. Also, the intense brightness of the weld pool can make it difficult to see the surrounding surface clearly, and impossible to see the weld pool itself unless special filters are used.
Viewing systems
A number of techniques are used to overcome these problems, including movable windows and regular cleaning of the optical surfaces. Replacement lamp glasses and closed vapour shutters, which are only opened briefly when viewing, are essential. Opaque discs mounted in the centre of the camera lens are also used to obscure the brilliance of the weld pool and reveal more of the joint line and its surroundings. Special filters can be installed to make it possible to see the weld pool, but when they are used the surrounding surfaces and joint line are too dim to be visible. Nevertheless, these techniques are at least partially successful and have been used for a number of years.
One might consider it reasonable to assume that all EB welding beams are, or could be, steered by CNC, so that viewing is unnecessary. This is probably true in most cases, but suppose the joint line moves because of distortion of the workpiece, or perhaps the CNC fails at a crucial moment. Suppose the gun was arcing excessively or, for any reason, the joint is not forming properly in the correct position. In these cases it is useful, or even essential, for the operator to know what is happening to the weld so that corrective action can be taken immediately. A viewing system is essential for this and it is also used for aligning the tracer beam to the joint line before welding begins.
One of the difficulties for an imaging system to cope with is seeing small surface details at the bottom of a relatively deep, narrow slot or hole. If the light source is not coaxial with the welding beam, it casts shadows which obscure the area of interest. A telescope or a TV camera can be designed with an optical system having an axial hole through which the welding beam can pass, and which has an almost coaxial light source, and these viewing systems would be able to see down to the bottom of the deep hole. But they would still suffer from metal vapour and excessive weld pool brilliance, as previously mentioned, as well as the narrow viewing angle and depth of focus problems sometimes associated with this sort of equipment.
So that is the present situation. We have gone some way towards finding solutions to the problems, but what is really needed is a quantum leap towards a new system which has all the advantages of the present viewing equipments and none of their disadvantages. It must be durable, easily fitted or retrofitted to any type of electron gun, must provide high resolution images, must be immune to weld pool glare and metal vapour deposits. It must also require very little or no maintenance, must be a truly coaxial system, relatively inexpensive, capable of a wide viewing angle, with a large depth of focus and able to see weld pool, joint line and surrounding surfaces as well as detail at the bottom of deep holes and slots.
Backscattered electrons
Such a system now exists. During a research project at TWI funded by Industrial Members and the UK Department of Trade and Industry, a method which uses the welding beam itself, was designed and developed to form electronic images of the surface by catching electrons which are unavoidably backscattered when the beam impinges on the surface ( Fig.1). The spatial distribution of these electrons represents angular changes at the surface as the beam is made to scan across it. Instant by instant changes in backscattered electron current represent corresponding instant by instant changes in the surface angles at the point where the beam strikes it. The current therefore represents the entire area of the scan as a video signal and the surface can be displayed on various types of TV monitor for viewing by the operator ( Fig.2).
Because the illuminating source for the pictures is the welding electron beam itself, the system is inherently coaxial, and can resolve detail down to approximately one half of the beam diameter at any point on the surface or down a deep, narrow slot. It is immune to weld pool optical brightness because it works with electrons, not photons.
Condensed metal vapour deposits ( Fig.3) on the collector have no effect on its ability to respond to the backscattered electron current and it therefore continues working regardless of how long it has been in use, or how much vapour has been generated. The ultimate limit to its life is set by a very gradual build up of conductive deposits across the insulators of the detector plates, but the life of these has been extended enormously by surrounding the insulators with gapped, and therefore non-conductive, vapour shields. It is estimated that the insulators would require cleaning only once in 8000 hours' use and cleaning them takes only a few minutes.
The viewing angle is easily altered by turning a manual control on the beam deflection unit to alter the offset and the area scanned. Similarly, the picture magnification can be altered from a value less than unity to about 20 times, when it produces images reminiscent of a scanning electron micrograph although not, of course, at great magnification.
The maximum limit to magnification is determined by timing jitter caused by electrical noise coming from the EB generation equipment, but for most applications magnifications of unity to about three or four are adequate.
The inherent resolution in images generated by an EB of some 150keV energy is much greater than that obtained using photons because the wavelength of these electrons is much shorter than that of photons. In practice, however, the diameter of a low power beam is not expected to be less than about 0.2mm, giving a resolution of about 0.1mm. For a beam of tens of kW the beam diameter is likely to be of the order of 0.6mm, giving a resolution of 0.3mm. For welding work requiring, say, a 6kW beam, a resolution of 0.3mm gives adequate picture quality, especially in view of all the other beneficial picture characteristics previously mentioned.
Equipment development
The TWI imaging equipment was first developed with a line scan period of 65 µsec and 512 picture lines per frame and this is perfectly adequate for beam power up to about 6kW. For higher beam powers it is necessary to scan in a much shorter time to prevent overlapping scans from melting the surface. Therefore another equipment was made to have a line scan time of 5 µsec. At working distances of some 150mm using this new equipment the beam moves along the workpiece surface at up to 5 km/sec and melting cannot occur, even at high power, so long as there are not too many overlapping scans. For aluminium, up to 20 overlapping scans are possible using a 100kW beam of 1mm diameter. This is better than required in most instances.
The monitor receives five refreshes per second and to prevent picture flicker the information is continuously fed to it from an electronic framestore, similar to the type used for TV frame freeze.
Pictures can be stored on computer disk for record purposes and for quality control of the welding process. With the 65µsec scan equipment, pictures can be improved using any of a number of signal processing methods such as morphology, background subtraction, edge enhancement and false colour and using these techniques it is hoped that an early indication of faults in the weld may be possible. This is the subject of ongoing research work.
The projected cost of the equipment depends very much on how it is packaged, what degree of software functions are needed, how much investment is to be put into its further development and what quantities are to be produced. The original prototype cost something in the order of £100 000, but it is believed that with care and given reasonable production quantities, a version could be produced for a small percentage of this figure.
Signal noise
One of the problems encountered progressively as the beam power is increased is signal noise because of positively charged ions which are formed as the incident beam passes through metal vapour. Because they are generated at random intervals and carry a positive electric charge which momentarily cancels the negative charge of signal carrying electrons, they cause noise which degrades the monitor image. To overcome this problem a special form of detector was developed in which the ions are filtered from the electrons by the process of charge separation. [1]
The detector plates are not placed in direct line of sight of the charge carriers but behind a negatively charged plate. The only way for electrons to reach the detector plates is to bounce off a specially shaped metal mirror placed behind the detectors and screening plate.
The ions are attracted to the negative screen whilst electrons are repelled by it. As backscattered electrons have energy which is virtually the same as those in the incident beam the negative plate only deflects them slightly.
The pictures in Fig.4 were obtained using this detector, but for other special applications TWI has designed other forms of detector which could prove more suitable.
The future
It may be possible in future to build an imaging system which, given a sufficiently fine tracer beam from the electron gun, could act as an electron microscope for studying fine surface details and at the flick of a switch become a welder and surface imager. One equipment would be able both to perform welding and to provide for surface studies at a cost only slightly greater than that of EB welding equipment.
TWI has received a number of enquiries from users of EB welding equipment for its imaging systems. The applications include quality control, reliability and down-time studies when compared with optical systems, imaging down a deep, narrow slot and imaging of surface cladding by electron beams.
Reference
| N° | Author | Title |
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| 1 | Sanderson A and Ribton C N: | 'Improvements relating to charged particle collectors'. European patent specification No. 97302431.9 | Return to text |
