Tue, 12 March, 2024
Originally invented at TWI in 1991, friction stir welding (FSW) has found use in a range of applications across industry, including for the rail industry.
With modern high-speed trains reaching speeds of up to 357 mph on metal rails and 361 mph of magnetic levitation tracks, there is a continuing demand for the methods used to fabricate trains to ensure passenger safety.
In addition, there are other concerns relating to safety, cost and the environment as well as manufacturer goals to reduce weight, create end-of-life material reuse and maintain the aesthetic appeal of vehicles.
FSW has become a popular welding technique for rail carriages, offering the potential for good weld performance and cost effective production. Without the need for filler wire, shielding gas and producing no fumes or ultraviolet rays, FSW also avoids thermally triggered component distortion since the process occurs below the material melting point.
Aluminium panels for rolling stock have proven to be good recipients of the FSW process with several manufacturers across the world opting for the technique. Aluminium has become an increasingly popular material choice for railcars, with artificially-aged 6xxx series aluminium being widely used due to their excellent strength and stiffness. However, welding methods can cause a change in grain structure and a reduction in mechanical properties, with heat affected zones and weld seams being particularly prone to failure. These challenges can be mitigated against with design considerations to ensure fracture does not occur in the area of the weld, which could undermine the crashworthiness of the structure.
In the event of a crash, aluminium carriages incorporate a crumple zone that absorbs much of the crash energy. However, it is vital that the parts of the vehicle containing passengers remains intact and the premature failure of welds could prove disastrous in such instances.
TWI has previously investigated the impact performance of components joined using MIG welding and FSW. It was found that even the narrowest heat affected zone (HAZ) of MIG welds was wider than that found with the friction stir welds being tested. The FSW specimens also showed higher proof and ultimate stress values than with the MIG welds. Although there is still a requirement for large-scale testing, these investigations showed that all fractures occurred in the heat softened regions around the weld.
TWI has also worked alongside project partners to develop new joint designs and procedures that can overcome joint weaknesses in 6xxx series aluminium alloys. An increase in the wall thickness was recommended to prevent the fractures that inevitably occurred first in the HAZ near the weld. This increase in wall thickness enlarges the structure’s load carrying capacity and allowed for new joint designs for rolling stock manufacturers so that any overload fractures happen in the parent material rather than at the weld line.
In summary, FSW has been recognised as a good solution for welding aluminium railcar structures, providing technical, economic and environmental benefits while maintaining the important safety considerations for passengers.
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