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FRICTION STIR WELDING
provide far better sound insulation Artificially aged 6xxx series Amagasaki commuter train crash near
characteristics than single-skin aluminium alloys are commonly Osaka, Japan, in April 2005, are three
aluminium or steel structures. The recommended for use in rail vehicle examples of accidents which have
double skin-structure is practically structures. These alloys have excellent highlighted the need to further improve
uniform, and the entire mass acts strength and stiffness, but the thermal crashworthiness of aluminium rail
effectively to insulate sound. cycles experienced during welding will vehicles. In the event of impact the
The need for high strength and lead to a change in grain structure and a carriage crumple zone absorbs a
excellent extrudability limits the choice to reduction in mechanical properties. Weld 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
Assembly of Class 377 Electrostar multiple units at Bombardier Transportation in Derby, UK, using
prefabricated friction stir welded side skirt panels made by Sapa in Finspong, Sweden
performance of the vehicle interiors have a
major effect on the number of casualties.
those aluminium alloys which soften on seams and heat affected zones are often The vehicle structures assisted in
heating during welding. It is well the weakest areas, however good design minimising injuries, given the speed of the
recognised that an inherent feature of all can overcome potential problems. derailment and the presence of a high steep
fusion welded joints in age hardened The use of longitudinal aluminium embankment down which the vehicles ran
aluminium alloys is the considerable extrusions with integral stiffeners is after derailing.
strength reduction in the weld region attractive for rail car design as the Experimental work, managed by TWI
compared with parent material. Unless wavelength of buckling can be dictated by under the EuroStir
®
project, investigated
adequate consideration is given to this the spacing of stiffeners; hence energy impact performance of FSW and MIG
point at the design and build stage, absorption during accidents can be very welded components. This industrialisation
fracture may occur during accidents in the high in such aluminium structures. study was funded by the Rail Safety and
vicinity of the weld, which could Standards Board (RSSB), Angel Trains Ltd
compromise the crashworthiness of the Crashworthiness of aluminium and HSBC Rail (UK) Ltd. The aluminium
welded structure. The extent of softening rolling stock alloys used for this evaluation were 3mm
depends on the alloy used and Rail accidents such as the high-speed ICE thick 6005-T6 and 6082-T6 rolled sheets,
manufacturing process adopted and is train disaster in Eschede, Germany, in extruded strips or extruded box sections.
therefore an important factor to consider June 1998, the Ladbroke Grove accident A special test was developed and validated
during structural joint design. in Britain in October 1999 and the for small-scale tests that successfully
102 EUROPEAN RAILWAY REVIEW
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ISSUE 3
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2008
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