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Friction stir welding variant for transport applications

Development of the friction stir welding process has now been used to perform right angled inside corner joints.

The process is now available to specialist fabricators using T, L, and V sections, external and internal angle joints and even round sections. The technique uses stationary shoulder friction stir welding technology (SSFSW). It has been adapted to penetrate 6mm into an inside corner joint in an aluminium alloy.

In SSFSW, a pin rotates through a non-rotating shoulder which slides along the joint. This technique originally evolved as a means of welding high temperature low conductivity materials and it became clear during development that it could be adapted to corner welding. During corner welding the shoulder is profiled to match the joint being welded.

Recently Nippon Light Metal approached TWI to apply the SSFSW concept to their particular application of internal corner welding. Nippon Light Metal was granted a patent on the technique in January 2009 in Japan. However TWI continues to hold the rights to use it outside the Far East and can publicise homegrown results with the agreement of NLM.

One of NLM's applications lies in the fabrication of vacuum chambers. From the outside such welding is simple but does not fully penetrate the joint, so an unwelded section exists on the inside. However by welding on the inside, the joint is sealed. The technique has also been used to demonstrate fabricating a tee section from two flat plates by corner welding from each side.

The process could lend itself to joining dissimilar materials and fabricating near net shape structures with tailored properties. Theoretically even joining round sections could be achieved if using a sliding shoe the same shape as a pipe, and weld right through. So as long as it is a regular shape, round or linear, it should work. But the process is much less forgiving than conventional friction stir. With the stationary shoulder technique you can't over plunge the tool, so you can't make up material.

Since the internal angular join is an abrupt one it is by definition a stress concentrator. The next task is to devise a possible way of forming a fillet in the capping bead; a consumable wire is required in the joint.

Applications for the process are many and varied. Says Jonathan Martin 'Aluminium is used extensively in the transport industries and this technique offers the potential to extend the areas where FSW could be applied.'

The macro-sections of SSFSW welds closely mimic those of traditional friction stir welding. It has a nugget, a thermo-mechanical zone and an HAZ, so the properties you would expect from this weld would be very similar to a conventional weld. There's a similar drop-off in hardness in the HAZ to that of a conventional friction stir weld but it is much less than in fusion welding. Nearly all the properties that you would get throughout this weld are considerably better than those achieved in an equivalent fusion welding.

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