Friction Stir Welding for Railway Rolling Stock

 

Paper published in Welding Journal, October 2002

Abstract

Friction stir welded structures are now revolutionising the way in which trains and trams are built. In Europe, suppliers to the railway rolling stock industry are exploiting the process for the prefabrication of large panels, which are made from aluminium extrusions. In Japan, complete trains are being assembled from hollow extrusions using the innovative process.

Introduction

Modern railway carriages 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 crash worthiness of vehicles because of the absence of transverse welds and the high buckling strength of the panels under longitudinal compression. ^[1]

Friction stir welding (FSW) was invented and patented ^[2,3] in 1991 at TWI in Cambridge (United Kingdom), and currently 76 organisations hold licences to use the process. Since the initial invention, there has been much activity throughout the world in the development of tool designs, applications and friction stir welding machines, which has resulted in more than 550 patent applications. ^[4]

An important aspect, which generates increasing interest in FSW, is its potential to contribute to the crash worthiness of aluminium vehicles that can otherwise fail in the heat affected zone along weld seams. This has been observed in European accidents, notably in Eschede in Germany in June 1998 and Ladbroke Grove in Britain in October 1999. In the report on the latter it was recommended that consideration should be given 'to the use of alternatives to fusion welding' and 'the use of improved grades of aluminium which are less susceptible to fusion weld weakening'. ^[5]

Advantages of friction stir welding:

• 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

Present limitations of friction stir welding:

• Welding speeds are slower than those of some fusion welding processes
• Workpieces must be rigidly clamped
• Backing bar required
• Keyhole at the end of each weld

Friction stir welded aluminium panels for rolling stock in the European railway industry

The Scandinavian aluminium extruders Sapa and Hydro Marine Aluminium were the first in Europe to commercially apply the friction stir welding process for the manufacture of single-wall aluminium roof panels for rolling stock applications. Since 1997, Alstom LHB in Salzgitter (Germany) have purchased these prefabricated panels for Copenhagen suburban trains ( Fig.1 and 2). Since early 2001 they have used friction stir welded aluminium side walls and since 2002 FSW floor panels for Munich suburban trains. These panels are made by Sapa.

In March 1999, Alstom LHB engineers considered friction stir welding hollow aluminium profiles for making floor and side panels, but calculated that a three-shift operation would be necessary to achieving return on investment in an acceptable time span. They estimated that the most significant technical and economic benefits could be achieved by applying FSW to aluminium joints of more than 12mm thickness. This would replace mechanised MIG welding which necessitates the associated activities of pre-heating and grinding of intermediate beads. Additionally, it would lead to improved quality of the welds. Therefore, successful FSW experiments were conducted in up to 23mm thick aluminium plates, to demonstrate how MIG welds could be replaced in the underframe area of rolling stock.

Bombardier in Derby (United Kingdom) has carried out FSW experiments for butt and lap welds and has conducted fatigue tests at TWI. ^[6] They have stated that one of the major advantages ofFSW was the ability to weld larger joints with reduced distortion. However, they concluded that investment in large purpose built FSW machines was currently difficult to justify - partly due to insufficient volume of work. Using a subcontractor or job shop is now being considered.

Up to 16m long SuperStir ^TM machines have been designed, built, and commissioned by ESAB in Laxå (Sweden). One of them has been installed at Sapa and is used for the production of large panels and heavy profiles with a welding length of up to 14.5m and a maximum width of 3m ( Fig.3). This machine has three welding heads, which means that it is possible to weld from two sides of the panel at the same time, or to use two welding heads (positioned on the same side of the panel) starting at the centre of the workpiece and welding in opposite directions. ESAB's newest gantry machine has now been installed at DanStir in Copenhagen, Denmark (15 x 3 x 1m,Fig.4).

The EuroStir ^® project on accelerating the industrialisation of friction stir welding

A number of European companies requested the provision of job shop services and low cost feasibility studies, to share the cost for capital investment, licences and R&D efforts. Some of them proposed teaming-up in a collaborative project, which has the overall objective to accelerate the use of friction stir welding in Europe. This EuroStir ^® project was launched in December 2000 and will last for five years. It is part-funded by EUREKA, which is a pan-European initiative for promoting collaborative research in advanced technology.

The Research and Development Phase of the € 6.8M project will take 2½ years. Deliverables will be proven welding procedures for test pieces and prototypes incomprehensive detail. Equally important will be the detailed comparison between types of equipment ( Fig.5 and 6), which will enable potential users to make informed investment choices. In a EuroStir ^® case study funded by Railway Safety (United Kingdom) and two rolling stock leasing companies (ROSCOs) it is planned to compare the quality of friction stir and MIG welding in appropriate aluminium alloys. It has been proposed to undertake small-scale static and dynamic tests, i.e. high strain rate tensile tests and drop weight tests.

The Dissemination Phase of the EuroStir ^® project will be mainly funded by industry and is planned to take 2½ years. It will involve seminars, workshops and demonstrations. Manufacturing economics will feature strongly in this Phase. A vital project achievement will be the establishment of at least 25 user organisations across Europe within 5 years. The project currently has 34 collaborators and is open for further participants from EUREKA countries.

Friction stir welding in Japan

Hitachi in Japan uses the double-skin design of the car, which is constructed from friction stir welded aluminium extrusions. One of the reasons for this is the exceptionally low distortion of the FSW process. This contrasts quite markedly with the distortion that can occur when arc welding thin gauge aluminium and eliminates the need for straightening and filling. To date, Hitachi has delivered a range of vehicles for both commuter and express use ( Fig.7and 8), and this 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 Industries ^[7] for the floor panels of the new Shinkansen ( Fig.9 and 10). Some of these trains operate 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 3km of welds had been produced. The weld quality was confirmed to be excellent based on microstructural, X-ray and tensile test results.

Conclusions

• Friction stir welding has found commercial application to aluminium rolling stock around the world and several machine manufacturers can provide suitable welding machines.
• A total of 34 companies has teamed up in the EuroStir ^® project, to get friction stir welding out of the laboratories and into the industrial manufacturing workshops. This activity will generate additional applications for FSW within the rail industry.

References

1. Davenport J, Kallee S W and Wylde J G: 'Europe follows Japan into friction stir welding.' Railway Gazette International, November 2001, pp.777-780.
2. Thomas W M, Nicholas E D, Needham J C, Murch M G, Temple-Smith P and Dawes C J (TWI): 'Improvements relating to friction welding'. European Patent Specification EP 0 615 480 B1. l2.espacenet.com/dips/viewer?PN=EP0615480
3. Midling O T, Morley E J, Sandvik A (Norsk Hydro, rights transferred to TWI): 'Friction stir welding'. European Patent Specification EP 0 752 926 B1.
4. patents is available.
5. www.hse.gov.uk/comah/bpgrange/append/i-o.htm see Chapter 13.9, page 206 of the *.pdf file.
6. Roe M: 'Friction stir welding - progress so far'. First Railway Industry FSW Meeting, Great Abington (UK), 11 April 2002, www.eurostir.co.uk/eurostir_railway11042002_05.pdf
7. Kumagai M and Tanaka S (Sumitomo Light Metal Industries): 'Properties of aluminium wide panels by friction stir welding'. First International Symposium on FSW, 14-16 June 1999, Thousand Oaks, California, USA.

 

Stephan Kallee, John Davenport and Dave Nicholas work at TWI Ltd in Cambridge, UK (Tel: +44 1223 899000, friction@twi.co.uk). The authors wish to thank the companies mentioned for permission to publish photographs and information on their use of friction stir welding.