A crashworthy solution...

TWI Bulletin, September - October 2008

friction stir welding in aluminium rolling stock

Since the invention of friction stir welding at TWI in 1991, companies from all parts of the world have implemented the process, predominantly in the fabrication of aluminium components and panels. As Stephan Kallee and Calvin Blignault report friction stir welded structures are now revolutionising the way in which trains, metro cars and trams are built (Fig.1).

Fig.1. Modern railcar design - Mock-up of the Hitachi Class 395 at Ebbsfleet International railway station

Considerations during fabrication

The first steam locomotive, named The Rocket, could travel at a constant speed of 39km/h (24mph). Since then, manufacturing technology has rapidly progressed, until today, where high-speed trains travel at up to 574km/h (357mph) on metal rails and 581km/h (361mph) on magnetic-levitation tracks. However, the history of rail accidents and fatalities due to high-speed collisions has placed tremendous demand on the methods used for fabrication of rail vehicles and the procedures that ensure passenger safety. The demands on railcars today are becoming more and more challenging with the need to satisfy diverse requirements such as improved safety, comfort, cost effectiveness and environmental considerations. Environmental requirements include noise emissions, energy efficiency, carbon footprint and recycling. Manufacturers are continually trying to improve initial and through-life cost, weight, aesthetics, crashworthiness and end-of-life reuse of materials. These factors are addressed by innovative designs of the railcar structure and the selection of appropriate joining technologies and materials (Fig.2).

Fig.2. Friction stir welded six-car Hitachi Class 395 known as Olympic Javelin or Bullet Train at the Ashford Depot in Kent

Friction stir welding of rolling stock

Joining techniques commonly used for the construction of rail vehicles are metal inert gas welding (MIG), friction stir welding (FSW), resistance spot welding, bolting and riveting. Increasing interest is also being shown inadhesives, hybrid laser-arc welding and friction stir spot welding (FSSW).

Friction stir welding (FSW) was adopted by several rolling stock manufacturers as an alternative welding technique for rail carriage structures. If used correctly, the FSW process has significant potential to reduce the width of the heat affected zone (HAZ) and the degree of thermal softening experienced in the weld region. Another driver for the uptake of this process is its combination of cost effectiveness and good weld performance.

The process is energy efficient and also environmentally friendly, because it requires no filler wire or shielding gas and creates no fume or ultraviolet rays. A further benefit is that the heat input during the FSW process is relatively low compared to MIG welding, therefore reducing the overall level of component distortion. This stems from the fact that the FSW process operates below the melting point of the material to be joined.

Up to 22m long FSW machines have been designed, built, and commissioned by a number of international machine manufacturers. Several of them are installed at aluminium extruders and are used for the production of large panels and profiles with typical wall thicknesses from 2.3 to 6.4mm. The trend is now towards FSW of more than 16mm thick structural components of aluminium rolling stock (Fig.3).

Fig.3. Heavy-duty friction stir welding machine at TWI for welding up to 75mm thick aluminium parts

The Scandinavian aluminium extruders Sapa and Hydro Marine Aluminium were the first in Europe to apply FSW commercially to the manufacture of aluminium panels for rolling stock. Alstom LHB in Germany makes use of prefabricated FSW panels to manufacture Copenhagen and Munich suburban trains. Bombardier Transportation in Derby, UK, uses FSW panels for their Electrostar vehicles (Fig.4-6) and for replacement stock for the Victoria Line of the London Underground network. In Japan, Nippon Sharyo obtains FSW floor panels from Sumitomo Light Metal Industries for the production of Shinkansen trains. Nippon Light Metals makes use of FSW for the fabrication of subway rolling stock, and Kawasaki Heavy Industries uses friction stir spot welding (FSSW) to attach stringers to roof panels for the prototype Fastech 360Z train.

Fig.4. Assembly of Class 377 Electrostar electric multiple units at Bombardier Transportation in Derby (UK) using prefabricated friction stir welded side skirt panels made by Sapa in Finspong (Sweden)

Fig.5. Friction stir welded side skirt panels are made by Sapa from double-skinned extrusions and used by Bombardier for commuter and underground trains

Fig.6. Friction stir welded aluminium panels have an excellent surface finish after painting due to the low amount of distortion caused by the low-temperature FSW process

Hitachi was among the first train manufacturer in Japan to recognise the benefits of FSW. They have delivered FSW vehicles for both commuter and express trains for use in Japan and overseas, such as the Class 395, known as OlympicJavelin, to provide domestic services on the new UK Channel Tunnel Rail Link (Fig.2).

Material selection and joint design

Steel and aluminium rail cars both have their own advantages and limitations, but an increasing number of vehicles are now made from aluminium. Alternative grades of aluminium alloys and joint designs are constantly beinginvestigated, and the selection of appropriate materials is governed by the joining method and structural strength requirements (both static and dynamic).

In principle, there are many aluminium alloys to choose from; however, modern aluminium railcars are commonly made from complex double-skinned extrusions (Fig 5, 6). Some rolling stock manufacturers claim that foam-filleddouble-skin structures provide far better sound insulation characteristics than single-skin aluminium or steel structures. The double skin-structure is practically uniform, and the entire mass acts effectively to insulate sound.

Crashworthiness of aluminium rolling stock

Rail accidents such as the high-speed ICE train disaster in Eschede, Germany, in June 1998, the Ladbroke Grove accident in Britain in October 1999 and the Amagasaki commuter train crash near Osaka, Japan, in April 2005 are threeexamples of accidents, which have highlighted the need to further improve crashworthiness of aluminium rail vehicles. In the event of impact the carriage crumple zone absorbs a significant amount of the crash energy, but it is important that the part of the rail vehicle containing passengers remains substantially intact.

Modern aluminium trains are designed to have well defined crash properties, as was impressively demonstrated by the Virgin Pendolino train that derailed due to a defective set of points near Grayrigg in Cumbria, UK, in February 2007while running at 153km/h (95mph). (Fig.7).

 Fig.7. The crash-site of the Virgin Pendolino train

Photo: Courtesy of RAIB

According to a progress report on an ongoing investigation by the Rail Accident Investigation Branch (RAIB, 3 October 2007, www.raib.gov.uk), the Pendolino train exhibited overall a good standard of crashworthiness and this helped to minimise the number of casualties and the extent of their injuries in the high-speed derailment. In a derailment such as at Grayrigg, the behaviour of the rolling stock structures and the performance of the vehicle interiors have a major effect on the number of casualties. The vehicle structures assisted in minimising injuries, given the speed of the derailment and the presence of a high steep embankment down which the vehicles ran after derailing.

Experimental work, managed by TWI under the EuroStir^® project, investigated impact performance of FSW and MIG welded components. This industrialisation study was funded by the Rail Safety and Standards Board (RSSB), Angel Trains Ltd and HSBC Rail (UK) Ltd. The aluminium alloys used for this evaluation were 3mm thick 6005-T6 and 6082-T6 rolled sheets, extruded strips or extruded box sections. A special test was developed and validated for small-scale tests that successfully forced the weld region into tension, as it is thought occurred in the Ladbroke Grove accident.

The idea was to try and simulate the welds 'unzipping'. It was however recommended that large-scale tests be performed to validate the results of this preliminary work. The results of this study are available in the TWI report entitled: 'Comparison of friction stir and MIG welding - Preliminary small scale and dynamic tests', and can be found on the RSSB website (www.rssb.co.uk/pdf/reports/research/T035_rpt_final.pdf). The main conclusions drawn from this work are summarised below:

• Friction stir welds have a narrow ductile heat affected zone (HAZ) surrounding the weld, whilst MIG welds are surrounded by a wider, softer region. The narrowest MIG weld HAZ was wider than those of any of the friction stir welds tested.
• FSW specimens tested in tension had higher proof and ultimate stress values than comparative MIG welds. All fractures occurred in the heat softened regions around the weld.
• Full-scale or large-scale testing is required, to establish a true comparison between MIG and FSW joints.