7 th International Aachen Welding Conference
by R E Dolby, A Sanderson and P L Threadgill, TWI, UK
3 rd/4 th May 2001
Recent TWI inventions and innovations in electron beam welding and friction welding are described. 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 Al alloys of varying thickness. Excellent progress is being made in the development of procedures for welding steels and titanium alloys. Linear friction welding is now being employed 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 commenced in Germany in 1948 
, with the first UK patent filings appearing in 1951. [3,4]
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 paper 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 in sections 2 and 3 of this paper have arisen out of TWI's core research programme, although the applications were developed with industrial partners.
2. EB Welding technology
2.1 Innovations for review
This paper will deal 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.
2.2 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, and Sanderson et al 
reassessed 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. This is shown in Fig.1. The innovation here requires only one cable connection to the high voltage supply so that a single core flexible cable can be employed. 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 3 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. The gun is shown in Fig.2.
Fig.1. Circuit diagram and principles of operation of an RF excited filament and indirectly heated cathode
Fig.2. RF excited 150kV, 100kW in-chamber mobile gun - covers removed
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.
2.3 Reduced pressure welding
Out of vacuum EB welding is a well-recognised technology and has been employed 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 5 mbar ( 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.
Fig.3. 200kV, 300mA electron beam in helium atmosphere at 5mbar pressure
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 (i) the use of RPEBW for steel pipeline girth welds, and (ii) 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  describe the work done on optimisation of vacuum conditions, weld procedure optimisation, and the associated weld quality and mechanical properties.
Fig.4. Laboratory prototype Reduced Pressure EB pipe welding machine showing 28" diameter pipe being loaded for welding with internal clamp in place
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  and Fig.5 shows the canister design and the EB weld position. Figure 6 shows the prototype head welding assembly in Sweden.
Fig.5. Spent nuclear fuel element containment canister showing position of EB weld ( courtesy of SKB)
Fig.6. Reduced Pressure gun column, canister and remote lid placing mechanism in SKB's vacuum chamber
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.
3. Friction welding
3.1 Innovations for review
The two innovations to be described are developments of rotary friction welding where exciting new machine concepts have emerged within the last 15 years. Friction technology has expanded at an amazing pace in this period and we have seen an extraordinary period of invention and innovation. Both friction stir and linear friction welding chosen for review have made a strong impact commercially in a very short period, and enabled existing products to be made at greatly reduced cost and totally new products to be manufactured for the first time.
3.2 Friction stir welding
This process (FSW) was invented in 1990 and patented by TWI in 1991 
. The concept is simple and now well known. A rotating tool with a central probe is passed into the components to be welded and transversed along the joint line. The joint created is a solid phase weld, with no melting involved. The initial work was done on Al alloy sheet and plate, and this is where there has been rapid commercial exploitation. However, other metal alloy systems, including Pb, Zn, Mg, Cu, Ti and steel can be welded by FSW, although industrial use of FSW for these alloys is still under development. Since the invention, there have been strong industrial drivers for the wider use of FSW in the welding of Al and its alloys, for example, how to weld faster, weld thicker, weld very thin material, and weld different alloys. In addition, there is the increasing requirement to weld other alloy systems because of the information emerging from Al alloy FSW manufacture and the associated cost savings and new product opportunities.
The ability to weld faster at given thicknesses relates crucially to the FSW tool design and at TWI, a new development reported by Dawes et al  has been the introduction of a scroll profile on the tool shoulder as shown in Fig.7(a). The scroll channel captures most of the material extruded while plunging the tool pin into the workpiece, and when the tool pin is travelling along the joint, a radially inward mechanical advantage is provided by the rotation of the scroll, increasing the compression about the upper threads of the tool pin.
Fig.7. Various FSW tools (a) Scroll shouldered FSW tool;
(b) Whorl TM type with frustum shaped probe and coarse auger thread;
This development was tested on a 5083 Al alloy of 6mm thickness and high quality welds were achieved at double the speed made using FSW tools with no machined scroll. This development has also allowed tools to be used in the 0° tilt position, eliminating the normal 1-3° backward tilt. This simplifies the set up and, in principle, facilitates welding in the x and y directions.
In the initial development phase with industrial clients, Al alloy thicknesses were in the range 1.5mm - 12mm and the tools used contained threaded surfaces on the pin. Heat is generated on the surface by friction between the rotating shoulder and the workpiece surface, and this is the main source of heat for the welding of thin sheets. More heat must be supplied as the sheet/plate thickness increases and, in addition, there are requirements for the probe to create sufficient working of material around the joint line and efficient flow of the material around the tool as the weld proceeds. Using these principles, Thomas and Gittos  report the development of two new types of tool known as Whorl TM and Triflute TM. These are shown in Figs.7(b) and 7(c) and involve a frustum-shaped probe with auger type threads, with and without the presence of helical flutes.
xperiments with these new tools have shown that for Al alloy plates of 25mm or greater, one pass welds can be made with excellent quality as shown in Fig.8(a). For example, with 6082 alloy, 25mm thick welds have been made at up to 250mm/min and 7075 alloy, 25mm thick welds at up to 60mm/min. In contrast, when using the same materials and thicknesses along with conventional threaded tools, welds were of very poor appearance and quality.
Fig.8. Cross sections of various friction stir welds (a) 7075 Al alloy, 25mm thick;
(b) Type 316 stainless steel, 5mm thick
One issue common to all FSW techniques at present is the requirement to react the downforce against the back of the weld. Fixed backing bars are the normal method of supplying the reaction force, but this limits the flexibility of the process. In the original patent, a bobbin tool was conceived which eliminates the need for backing bars. The tool, which is shown in Fig.7(d), is a conventional tool with an extended pin and an opposing tool shoulder. This second shoulder is the integral backing bar. In recent experiments  the bobbin tool design was used with scroll shoulders and a threaded profile which changed direction at the mid-point. This approach produced good welds in 6mm 6081-T6 Al alloy and appears promising.
The use of FSW for 'difficult to weld' Al alloys and alloy systems other than Al is under intense development. It has already been demonstrated that 2000 and 7000 series, including Al-Li alloys can be welded and Pb, Zn and Mg alloys are also readily weldable by the process. Dissimilar metal joints, e.g. aluminium to magnesium alloy combinations are feasible and the greatest challenge now is to weld titanium alloys, nickel base alloys and steel. Here the tool material is crucial and the search is on for high melting point materials of adequate hot strength from which to manufacture appropriate FSW tools. After only a few years work it is now feasible to weld metre lengths of Ti-6Al-4V alloy, and also Type 304L and Type 316L stainless steel in thicknesses up to 6mm, Fig.8(b). Ferritic steels such as 0.03C-12Cr alloy and C-Mn steels have also been satisfactorily welded at greater thicknesses, e.g. 12mm.
In summary, exciting progress has been made in FSW tool design. Welding speeds have been more than doubled compared to the originally developed tools. One pass welds can now be made in up to 50mm thickness in 6000 series Al alloys using the new tool technology and welds in thin Al alloy sheet can approach 3 metres/min. In addition, a range of Pb, Zn, Mg, stainless steels and ferritic steels can be welded. Industrial exploitation is gathering pace and current applications for Al alloys are numerous in high speed ships (Al superstructures, bulkheads, floors), railways (Shinkansen and other trains in Japan), aerospace (Delta II rocket fuel tanks and various automotive components. Figure 9 shows two examples.
Fig.9. Recent FSW applications (a) Decks in a high speed aluminium ferry;
(b) Fuel tanks in recent Delta 2 rocket launched by NASA
3.3 Linear friction welding
Whilst the idea of linear motion was patented in 1969  the development of suitable machines for welding engineering size components only started just over a decade ago. A consortium of four British companies was formed in mid '80s led by TWI which designed and built prototype machines with a linear reciprocating mechanism. Based on an electromechanical system, the machines provided a reciprocating frequency of 5-75 Hz and a reciprocating amplitude of 0-±3mm, with a maximum axial welding force of 150kN. The process opens up new design and manufacturing possibilities for metals, and is capable of welding square and rectangular components in one shot with accurate alignment of parts. With appropriate tooling it can be used for more 'irregular' components such as turbine blades.
Early trials commenced with Ti-6Al-4V alloy Type 304 stainless steel, 6063 Al alloy and C-Mn steel and were successful in producing sound joints. Since then tooling has improved and section sizes of 20mm x 100mm are now possible. The process seems very well suited to the joining of intermetallics such as Ti aluminides  and successful welds can be made provided care is taken to control cooling rates.
To date, only the aircraft engine industry has used the process, e.g. for fan blade assemblies, including blisks, due mainly to the very high costs involved in machining and tooling. Figure 10 shows a linear friction welded blisk produced by MTU München for Eurofighter, while Fig.11 shows cross sections of a linear friction weld in a single crystal Nickel based cast alloy, where the technique is under development in the USA. However, there has always been strong interest in the process from other sectors, in particular mass production industries such as automotive components, and it is expected that applications will increase as the equipment cost drops. TWI is involved with a consortium of European SMEs to design and build a low cost machine, and this is now being assembled. This machine uses hydraulic actuation, allowing the use of novel stored energy approaches, and it is expected that the cost will be less than half of an electromechanical machine of similar capacity.
Fig.10. Linear friction welded blisk produced by MTU München (a) as welded;
Fig.11. Linear friction weld in a single crystal cast nickel based alloy
4. Concluding remarks
This paper has dealt with electron beam and friction technologies only, but the innovation involved in the last decade or so has been surprising. It shows that given a concentration of effort and funding, engineers, metallurgists and physicists are capable of making leaps forward in joining technology which can have major impact on the creation of wealth, both in reducing manufacturing costs and facilitating the fabrication of totally new products. Creativity in the scientists and engineers involved must be encouraged and fostered, because the opportunities for further significant developments over the complete range of joining processes, including arcs, lasers, fastening, resistance and adhesives, as well as electron beam and friction, are immense.
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