Richard Dolby joined TWI, then The Welding Institute, in 1966 and spent 14 years in the Materials Department, where he became head in 1978, and Research Manager in 1980. His major interest centred about the welding metallurgy of ferritic steels, specialising in lamellar tearing and reheat cracking, and in HAZ and weld metal toughness problems. He was appointed Director, Research and Technology, his present post, in 1985.
As technical Chief of the Electron Beam Group, Allan Sanderson is concerned with the design and application of EB welding equipment. He obtained his doctorate on the generation and control of high power beams. This pioneering work led to the dramatic breakthrough in single pass thick section EB welding which has subsequently been applied to a wide range of industrial applications.
Philip Threadgill gained his BSc and PhD in Physical Metallurgy from University College, Swansea. He has worked at TWI since 1976 and is currently R&D Manager for the Friction and Forge Processes group.
Evolutionary developments in electron beam and friction welding are occurring almost by the hour. Richard Dolby, Allan Sanderson and Phil Threadgill take a close look at recent inventions and innovations in both processes. This issue deals with electron beam, part II in March will cover friction welding.
RF excitation techniques, diode guns and switch mode power sources have resulted in compact and more reliable electron beam guns, whilst reduced pressure electron beam welding has opened up new opportunities for large scale construction, with simpler sealing and pumping arrangements, and greater tolerance to leaks.
Friction stir welding has shown amazing growth since its invention in 1990. The process is being used worldwide in the shipbuilding, land transport and aerospace sectors for joining aluminium alloys of varying thicknesses. Excellent progress is being made in the development of procedures for welding steels and titanium alloys. Linear friction welding is now being used in aeroengine manufacture, but more applications will follow when cheaper machines become available.
Here we have two processes which would be regarded as mature welding techniques by many engineers. Friction welding dates back about 50 years while electron beam (EB) welding technology can be said to have started in Germany in 1948, with the first UK patent filings appearing in 1951.
The present authors believe that there has been more invention and innovation in these two processes in the last 10-15 years than most previous periods in their history. The purpose of this work is, therefore, to summarise several new and exciting developments, describing recent process improvements and their applications in the power generation, oil and gas, land transport and aerospace sectors.
The technology changes to be described are contributing not only to reduced manufacturing costs, but also to increased opportunities to manufacture in new ways and to new products as will be shown in the examples given. The inventions and innovations detailed here have arisen out of TWI's core research programme, although the applications were developed with industrial partners.
EB welding technology
Innovations for review
This deals with two main areas of invention and innovation. The first relates to EB gun development, where the main drivers have been the need to extend cathode life, improve beam consistency, and simplify the equipment and its operation. The second area to be covered is the development of reduced pressure (0.1-100 mbar) electron beam welding (RPEBW), which obviates the need for large vacuum chambers with associated sophisticated vacuum sealing and pumping arrangements.
Gun improvements
Directly heated triode guns can be prone to give various problems such as short filament life, beam voltage and current ripple, poor beam reproducibility and a tendency to gun discharge, particularly when welding light alloys.
In the mid 1980s it was recognised that there was a requirement for an indirectly heated diode gun. Sanderson et al re-assessed both the gun and power source approach and designed a unique indirectly heated gun and switch mode power source, without the need for conventional auxiliary power supplies, thus simplifying the system and overcoming the problems noted above.
The heart of the development is the use of RF excitation in the gun cartridge (typically 84MHz). A single turn winding collects the RF power and produces a high current in a ribbon filament. Electrons are drawn from the filament every half cycle producing a beam that then heats the main cathode (see Fig.1). The innovation here requires only one cable connection to the high voltage supply so that a single core flexible cable can be used. TWI uses this technology in its 150kV, 100kW guns and the gun can be housed in a 0.2 x 0.2 x 0.2m cube. With a side entry high voltage cable termination, the overall gun column length is around 750mm, housing two focusing lenses, a high-speed deflection system, a TV camera and coaxial viewing system, and a DC current transformer (see Fig.2).
Another facet of the development has been the change from a triode to a diode gun. Diodes do not surge in current if flashovers occur but instead the beam current is reduced. Thus they are to be preferred for reducing the risk of defects in high value added components. In addition, EB flashovers are best controlled by the use of switch mode power supplies. TWI pioneered their use in 1986 for high power EB welding (>100kW) and they have a number of advantages over alternative methods. In particular, the associated control equipment is independent of accelerating voltage and beam power, and the gun column is free of high voltage supply components.
In summary, RF excitation, diode guns and switch mode power supplies with flashover control, have been very significant advances in the last decade, enabling more compact and more reliable guns to be developed, delivering greatly enhanced beam quality.
Reduced pressure welding
Out-of-vacuum EB welding is a well-recognised technology and has been used widely in the automotive sector for thin component manufacture in the USA for several decades. In 1992, TWI demonstrated that very narrow satisfactory electron beams could be produced at 5mbar ( Fig.3) and that it was difficult to distinguish welds made in this pressure regime from those produced at 5 x 10 -3 mbar. Welds in C-Mn steel of 100mm thickness were soon being made reliably at pressures of ~1 mbar.
These achievements were made possible by the development of a 200kV, 100kW EB system which could operate over the pressure range 1000 mbar to 0.01 mbar, using differentially pumped stages in the beam transfer column. At the extremity of the gun column, an over-pressure stage was added through which helium was bled, which helped to minimise beam scattering and reduced the risk of metal vapour entering the gun housing, held typically at 10 -6 mbar.
It was quickly seen that this development could do away with large vacuum chambers and the worry of leaks and seals. With the new system, pressures of ~1 mbar could be achieved with simple mechanical pumps and crude local seals. It was also shown that the system was very tolerant to fluctuations in working vacuum pressure, gun to work distance and workpiece cleanliness. These advantages led to the development of systems for two large scale applications; first the use of RPEBW for steel pipeline girth welds, and second, the use of RPEBW for sealing of copper canisters to contain high level nuclear waste.
In the case of steel pipelines, a prototype machine has been built for Saipem for girth welding 28 inch diameter pipes to API 5L-X70, with wall thicknesses of ~40mm. The system shown in Fig.4 comprised an external EB chamber sealed to the pipe by flexible seals, an internal clamp providing a local vacuum, and a mobile EB gun mounted on the outer chamber which can rotate around the pipe. A CNC system controls and maintains EB parameters, vacuum system and gun movement, and a real time seam tracker detects the joint line using backscattered electrons. Punshon et al [1] describe the work done on optimisation of vacuum conditions, weld procedure optimisation, and the associated weld quality and mechanical properties. The work has shown that the RPEBW process is capable of producing single pass welds with satisfactory quality in modern pipeline steels. The ability to weld at 1 mbar allows rapid cycle times, and permits the use of crude local seals and simple vacuum engineering.
The development of RPEBW for copper canister fabrication has taken place over the last decade for the Swedish Nuclear Fuel and Waste Management Company (SKB). The principle of using copper as the main corrosion barrier is accepted in Sweden, and EB was seen as one process which could weld thick section copper satisfactorily and use remote handling methods. The RPEBW variant was considered the best route for controlling weld defects and dealing with outgassing problems associated with double skinned cylinders.
The equipment and procedures are described by Nightingale et al [2] and Fig.5 shows the canister design and the EB weld position. Figure 6 shows the prototype head welding assembly in Sweden.
In summary, the development of reduced pressure technology has opened up new possibilities for large-scale manufacture. The equipment does not require complicated sealing arrangements, leaks can be tolerated, pumping systems are simpler, and yet weld quality is similar to high vacuum EB welding.
References
| N° | Author | Title | |
| 1 | C S Punshon, A Sanderson, A Belloni: | 'Reduced pressure EB welding for steel pipelines'. 6th Int. Conf. on 'Welding and melting by electron and laser beams'. CISFFEL 6, Toulon, June 1998, L'Institute de Soudure. | Return to text |
| 2 | K R Nightingale, A Sanderson, C Punshon, L O Werne: | 'Advances in EB Technology for the fabrication and sealing of large scale copper canisters for high level nuclear waste burial'. 6th Int. Conf. on 'Welding and melting by electron and laser beams'. CISFFEL 6, Toulon, June 1998, L'Institute de Soudure. | Return to text |
Part 2 on Friction welding developments
