Friction stir welding for the fabrication of aluminium rolling stock

Considerations during fabrication of rail vehicles

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 can travel at speeds of up 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 continuously 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.

Friction stir welding of rolling stock

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

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 fumes 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 2.3mm to 6.4mm. The trend is now towards FSW of more than 16mm thick structural components of aluminium rolling stock.

The Scandinavian aluminium extruders Sapa and Marine Aluminium were the first in Europe to commercially apply FSW for 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 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. Hitachi was amongst the first train manufacturers in Japan to recognise the technical and economic 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 Olympic Javelin, to provide domestic services on the new UK Channel Tunnel Rail Link.

Material selection and structural 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 being investigated, 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. Some rolling stock manufacturers claim that foam-filled double-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.

The need for high strength and excellent extrudability limits the choice to those aluminium alloys which soften on heating during welding. It is well recognised that an inherent feature of all fusion welded joints in age hardened aluminium alloys is the considerable strength reduction in the weld region compared with parent material. Unless adequate consideration is given to this point at the design and build stage, fracture may occur during accidents in the vicinity of the weld, which could compromise the crashworthiness of the welded structure. The extent of softening depends on the alloy used and manufacturing process adopted and is therefore an important factor to consider during structural joint design.

Artificially aged 6xxx series aluminium alloys are commonly recommended for use in rail vehicle structures. These alloys have excellent strength and stiffness, but the thermal cycles experienced during welding will lead to a change in grain structure and a reduction in mechanical properties. Weld seams and heat affected zones are often the weakest areas, however good design can overcome potential problems. The use of longitudinal aluminium extrusions with integral stiffeners is attractive for rail car design as the wavelength of buckling can be dictated by the spacing of stiffeners; hence energy absorption during accidents can be very high in such aluminium structures.

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 three examples 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.

Premature failure of welds during these types of accidents of rail vehicles would not be desirable. It was for this reason that the industry wished to understand the root causes of certain aluminium welds ‘unzipping’, a phenomenon reported in a number of previous rail accidents. Modern aluminium trains are therefore 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 2007 while running at 153km/h (95mph). 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 (http://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

Further insight was developed within the EC project ALJOIN, which studied the crashworthiness of joints in aluminium rail vehicles. Within this project Alcan, Bombardier Transportation, DanStir, NewRail (University of Newcastle) and TWI worked together to develop an understanding of the issues and develop new joint designs and welding procedures, to overcome any inherent joint weakness. The initial full-scale tests confirmed once again that welded joints in 6xxx series aluminium alloys will inevitably fracture in the heat affected zone near the weld. The basic cause for such fracture is the strength reduction in welds produced by the currently established fusion and solid-phase welding processes in heat treatable aluminium alloys. It was therefore recommended that the wall thickness in the weld region be increased, to enlarge its load carrying capacity enough for fracture to occur in the parent material.

This has enabled rolling stock manufacturers to develop new joint designs, suited to the specific fabrication process which will be used. The new designs have been subjected to extensive numerical and physical validations. Computer simulations predict that, with the new designs, overload fracture would occur in the parent material rather than along the weld line. The predictions have been verified by extensive full-scale component tests under high-speed impact at Bombardier’s test facility in France. The newly designed FSW and MIG joints meet the stringent requirements for rail vehicle safety.

New international standards for welding of railway vehicles

Regulations covering the fabrication of rail vehicles are changing, and new standards are being developed, such as prEN 12663 on ‘Railway applications – Structural requirements of railway vehicle bodies’ and prEN 15227 on ‘Railway applications – Crashworthiness requirements for railway vehicle bodies’.

The process of welding has always required a high level of skill and control, to ensure that welds are fit for purpose. This recognises the fact that it can be difficult or expensive to check all welds once they are made, while at the same time a poor quality weld could have disastrous implications in terms of safety. Draft European standards such as prEN 15085-2 on ‘Railway applications – Welding of railway vehicles and components’ are therefore adopting an approach which is based on standards already used in the German rail sector. The new approach will require fabricators to demonstrate compliance with ISO 14731 and ISO 3834. Together, these standards cover requirements for the welders and the manufacturing facility. Compliance with these standards must be demonstrated by independent third party assessment. All suppliers to the European market will, in future, be required to engage with an impartial assessor organisation, e.g. under the Welding Fabricator Certification Scheme, with necessary skills and accreditation to do this work (see www.iso3834.org).

Concluding remarks

FSW has been widely recognised for its ability to provide high weld quality and low distortion in a wide variety of aluminium structures. The technical and economic benefits of the FSW process have led to rapid development and international use of the technology in many industrial applications. New standards are being implemented in Europe, and the Welding Fabricator Certification Scheme is designed, to allow welding fabricators to demonstrate compliance with ISO 3834 on quality requirements for fusion welding of metallic materials.