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Manufacturing crashworthy rail vehicles

Connect, no. 144, September - October 2006, p.6

Bombardier Transportation manufactures rail vehicles in many countries around the world. At its plant in Derby, England, it fabricates the car bodies from aluminium extrusions using mainly MIG welding. Increasingly, it is finding opportunities to use friction stir welding (FSW) with its potential for reduced cost. The fact that no welding fume is generated and little distortion occurs are added advantages.

Whatever fabrication technology is used, it is important that the rail vehicles have an acceptable level of crashworthiness. Bombardier and TWI were key partners in the Framework 6 EC project ALJOIN - Crashworthiness of Joints in Aluminium Rail Vehicles. Other partners were DanStir, Newrail, D'Appolonia and Alcan. ALJOIN and other projects have provided the insight to allow Bombardier to develop appropriate joint designs, each suited to the specific fabrication process which will be used.

An inherent feature of all welded joints in solution treated and artificially aged aluminium alloys is the considerable strength reduction in the weld region compared with parent material. Consequently fracture may occur along the weld line due to over-loading, which could compromise the crashworthiness of the welded structure. As a result of Bombardier working with TWI and other companies, new joint designs and MIG and FSW procedures, have been developed to overcome any inherent joint weakness. The new designs have been subjected to extensive numerical and physical validations. Computer simulations predict that, with the new designs, overload fracture occurs 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 joint design leading to fracture in the parent material away from the weld a) Computer simulation result
New joint design leading to fracture in the parent material away from the weld a) Computer simulation result
b) Specimen after high speed impact test
b) Specimen after high speed impact test

Bombardier's approach is to have a full understanding of a range of fabrication options, so that the most appropriate may be selected for any specific application. It anticipates that MIG welding will continue to be used for many joints, but already FSW is being used for all lower bodyside panels for Electrostar vehicles, achieving significant cost savings. Bombardier continues to work with TWI to develop further innovative joining techniques to maintain their edge in the competitive world of rail vehicle manufacture.

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