INTRO: Several hundred aluminium cars with non-fusion seams are operating in Japan, and rail vehicle manufacturers elsewhere are now grasping the economic benefits of a joining technique invented in 1991
BYLINE: John Davenport, Stephan W Kallee and J Graham Wylde
TWI Ltd
A DECADE ago, the joining technique known as friction stir welding barely existed. FSW was invented by TWI Ltd in Great Britain and patented in 1991. Since then, it has been used under licence around the world, particularly for joining aluminium alloys in the shipbuilding, aerospace and automotive industries.
In the rail sector, Japanese manufacturers have been at the forefront in applying FSW to passenger car body structures, although the benefits are increasingly being recognised in Europe. The growing use of double-skin extrusions that require long welded seams has provided one important application, and hundreds of cars with such extrusions joined over the full length by FSW are already operating in Japan.
The welding process
Many advantages of FSW stem from the fact that it is not a fusion welding process. The metal is forced mechanically to flow, as it does during the extrusion process, but its temperature is not raised to the melting point.
Fig 1 shows how a rotating tool consisting of the tool shoulder and profiled pin is forced into the metal at the joint line. As the rotating tool moves along the joint, friction heats the metal which flows plastically to create a solid-state weld.
During welding, the workpieces must be clamped onto a backing bar to avoid movement and bulging of the outer surfaces which would otherwise be created by the forces as the tool moves along the joint line. The workpieces do not have to be in complete contact. Trials have shown that a gap of up to 10% of the workpiece thickness can be accepted before the quality of the weld starts to suffer.
A number of companies are now supplying purpose-built FSW machines to industry. The ESAB machine illustrated (above right) is used by SAPA Group in Sweden to weld extruded aluminium panels used in the shipbuilding, offshore and railway industries.
Because FSW creates a joint without having to melt the metal, the relatively small heat input results in very little distortion, even in the long seam welds often used by rail vehicle fabricators. The advantages may be summarised as:
