Published in Railway Gazette International, November 2001
The railway vehicle industry, particularly in Japan, has been quick to exploit a novel joining technique that had barely been invented a decade ago. The technique, friction stir welding (FSW), was invented by TWI Ltd in the UK and patented in 1991. Since then, it has seen commercial use, under licence, in a range of applications around the world, particularly for joining aluminium alloys. To date, FSW has been used in the shipbuilding, aerospace, automotive and rail industries. TWI, one of Europe's largest independent research and technology organisations, continues to work with companies throughout the world to develop new applications.
The Friction Stir Welding process
The many advantages of FSW stem from the fact that it is not a fusion welding process. Figure 1
shows how a rotating tool consisting of the tool shoulder and profiled pin is inserted at the joint line between the two pieces of metal to be joined. As the rotating tool moves along the joint the friction heats the metal which flows plastically to create a solid state weld.
During welding the workpieces are clamped onto a backing bar to avoid the movement which would otherwise be created by the forces as the tool moves along the joint line. The workpieces do not have to be in complete contact. Trials have shown that a gap of up to 10% of the workpiece thickness can be accepted before the quality of the weld starts to suffer.
A number of companies are now supplying purpose built FSW machines to industry. For example, figure 2 shows an ESAB machine at SAPA used to weld aluminium extrusions for panels used in the shipbuilding, offshore and railway industries.
The FSW process creates a joint without having to melt the metal. The relatively small heat input results in very little distortion, even in the long seam welds often used in the rail vehicle industry. The process advantages are:
- Low distortion
- Excellent mechanical properties
- No porosity in the welds
- No weld spatter; the surface and root of the joint are clean and their cosmetic appearance so good that they need not even be painted
- No fume, increasingly important as health and safety standards tighten
- The process can operate in any orientation, because gravity has no influence
- Energy efficient
In the early days of FSW one limitation was seen to be the welding speed. Recent intensive research has made significant increases in welding speed and new tool designs are now in use.
An important aspect which generates increasing interest in FSW is its potential to contribute to the crashworthiness of aluminium rail vehicles. Table 1 includes some of the recommendations of the Ladbroke Grove Rail Enquiry. One recommendation is that consideration is given, in the case of new vehicles constructed of aluminium, to the use of alternatives to fusion welding.
Table 1: Recommendations on crashworthiness
The Part 1 Report of the Ladbroke Grove Rail Enquiry has now been published by the Rt Hon Lord Cullen PC. It states the following:
'The experts on crashworthiness were agreed in recommending that consideration should be given, in the case of new vehicles constructed of aluminium, to the following:
(i) the use of alternatives to fusion welding;
(ii) the use of improved grades of aluminium which are less susceptible to fusion weld weakening; and
(iii) the further development of analytical techniques to increase confidence in the crashworthiness of rail vehicle structures, particularly those constructed of aluminium.'
Ref: The report may be found at: www.hse.gov.uk/comah/bpgrange/append/i-o.htm - PDF file of 298 pages. Background details are available at http://www.lgri.org.uk/
Many rail car manufacturers have moved some of their production from fusion welding to FSW. While they report a reduction in total cost, the details of their analyses are understandably confidential. Some of the factors which contribute to FSW being a low cost route are:
- No filler wire
- No gas shielding for welding aluminium
- No grinding, brushing or pickling required in production
- The use of a non-consumable tool. The tool can typically be used for over one kilometre of welding in 6000 series aluminium alloys
- Very little distortion means that the often substantial costs involved in straightening and filling prior to painting are not required
- As there is no porosity there is never any need for rectification work to correct this condition
Industrial applications of FSW
The very first commercial application of FSW was in the manufacture of hollow aluminium panels for the freezing of fish on fishing boats. These panels are made from friction stir welded aluminium extrusions at SAPA Group, Sweden. The minimal distortion and high reproducibility make FSW both technically and economically a very attractive method to produce these stiff panels.
Since these early days, extensive use has been made of FSW in the shipbuilding and marine industries. Pre-fabricated wide aluminium panels for high-speed ferry boats can be produced by FSW and are commercially available. The panels are made by joining extrusions, which can be produced in standard size extrusion presses. Compared with fusion welding, the heat input is very low and this results in low distortion and reduced thermal stresses. Annually, more than 70km of defect free friction stir welds have been produced over the last three years at Hydro Marine Aluminium in Haugesund (Norway) mainly for shipbuilding panels. Panels for the rail industry are also produced here.
Friction stir welding is also finding application in the aerospace industry, where some of the aluminium alloys in use are difficult to join by fusion welding. Already, spacecraft fuel tanks joined by FSW have been used in space and many other aerospace applications are under development.
The automotive industry is also adopting FSW. For example, in Japan Showa Denko are using FSW in the manufacture of suspension arms. One benefit is that the rubber of the suspension arm can be vulcanised before joining due to the low heat input of the new assembly method.
Applications in the railway rolling stock industry
Modern passenger rail cars are increasingly produced from longitudinal aluminium extrusions with integrated stiffeners. Using this concept the whole body shell can be made from either single wall or hollow double skin extrusions.
This design approach can enhance the crashworthiness of vehicles because of the absence of transverse welds and the high buckling strength of the panels under longitudinal compression. Large aluminium extrusions with complicated shapes are available and are being used in the manufacture of single and double deck trains.
Because of its suitability for long seams and the other advantages of FSW already mentioned, the aluminium rail car construction industry has recognised the benefits of the process. A number of rail car fabricators are using the process themselves or are incorporating friction stir welded panels into their vehicles. Although there is some activity in Europe, the Japanese rail car manufacturers are leading the world in this application.
One rail car manufacturer, which has been very quick to realise the benefits of FSW, is Hitachi. Hitachi now makes a feature of the fact that they use FSW for the main longitudinal seams of their A-train concept rail cars in their publicity material. One of the reasons for this is the exceptionally low distortion of the process. This contrasts quite markedly with the distortion which can occur when arc welding thin gauge aluminium and thus eliminates the need for straightening and filling.
Hitachi uses both the single wall and the double skin design of the car, which is constructed from aluminium extrusion. To date they have delivered a range of vehicles for both commuter and express use, Table 2. Two of these, the Commuter EMU Series 20000 for Seibu Railways and the Express EMU Series 885 for JR-Kyushu are shown in figures 3 and 4.
Table 2: Hitachi applications of FSW
|Vehicle type||Railway||Number of vehicles||Application|
|Commuter EMU Series 815
||Full length of double skin side panels
|Commuter EMU Series 20000
||Full length of double skin side and roof panels
|Express EMU Series 885
||Full length of double skin side and roof panels
|Express EMU Series 683
||Full length of double skin side and roof panels
Hitachi's work on the application of friction stir welding to railway vehicles has been recognised in Japan by the award of the prestigious Okouchi Award jointly to Hitachi and TWI.
Nippon Sharyo and one other Japanese company have been using friction stir welded panels produced by Sumitomo Light Metal Co for the floor panels of the new 700 series Shinkansen ( figure 5) operating on the Tokaido Line (Tokyo to Osaka) and Sanyo Line (Osaka to Hakata). Some of these trains operate on the Nozomi service at speeds up to 285km/hour.
Nippon Light Metals have also made use of friction stir welding for subway rolling stock. By 1998 they reported that over 3 km of welds had been produced. The weld quality was confirmed to be excellent based on microstructural, X-ray and tensile test results.
Finally, another Japanese rail car manufacturer is known to have produced many hundreds of aluminium rail cars of single skin construction using FSW. In these cases FSW has been used in varying degrees for the body construction. These cars are operating on commuter, express and subway services throughout Japan.
In Europe less use is being made of FSW, but this looks set to change as the benefits of the process are increasingly recognised. Already Alstom have a number of uses for friction stir welding. Extruded aluminium profiles have been friction stir welded for the roof and sidewall panels of suburban trains in Copenhagen and elsewhere. Friction stir welded skirt panels will also be used on Virgin's West Coast Main Line vehicles supplied by Alstom.
Industrialisation of FSW in Europe has been stimulated by the formation in December 2000 of the €7M Eureka project called EuroStir ®
. This project, the purpose of which is to focus on industrialisation of friction stir welding, has already attracted over 30 collaborators. The railway industry is taking advantage of this project and more rail industry companies are expected to join. At the time of writing the collaborators include Alstom Transport in France and DanStir in Denmark, who are currently conducting industrialisation studies on railcar components for the approval by rail authorities. TWI has recently installed a state of art friction welding machine that will be used for manufacturing welded prototypes up to 5 x 8 x 1m ( Figure 6
Early application of FSW in the rail sector has concentrated on joining extruded sections less than 10mm thick. However, the application of FSW is not limited to thin sections. Figure 7 shows a double sided friction stir weld produced in 50mm 6000 series aluminium plate.
Many more commercial uses on rail vehicles are in the pipeline around the world. Friction stir welding is well placed to create a stir in the industry for some time to come.
The authors wish to thank the rail vehicle manufacturers mentioned in the text for permission to publish information on their usage of friction stir welding.