Ian Sheppard/CAMBRIDGE

The UK Welding Institute (TWI) has developed a means by which previously "unweldable" aluminium alloys, such as the 2000 and 7000 series, can be joined.

It had previously been impossible to weld harder aluminium alloys because heating would cause the heat-treated metal to revert to its previous, softer, characteristics, or to become brittle.

"Friction-stir" welding is a solid-phase process, involving the generation of sufÌcient heat to fuse two pieces of metal together through friction between the parts and a rotating tool, the details of which remain a closely guarded secret.

Sue Dunkerton, head of advanced materials and processes at TWI, says that the advance was identiÌed through the UK's Technology Foresight initiative, which aims to increase productivity in manufacturing, and pinpointed the need to supplant the widespread use of rivets.

According to Chris Dawes, principal research engineer at the Cambridge-based centre for work into advanced joining technologies, the process "plasticises" the metal instead of melting it, crushing the surface oxide and enabling molecular bonding to occur. A rotating metal "consumable" leaves a deposit fused between the two surfaces. The tool is "remarkably simple", says Dawes, who adds that there are "no fumes, shielding gases or arc-lights and no unwanted deposits".

The institute's sister organisation in the USA, the Edison Welding Institute in Columbus, Ohio, and industrial partners - including British Aerospace, Boeing and Lockheed Martin - participated in development. In four years, the process has gone "from laboratory curiosity to an industrially viable technique".

In initial tests in what is the Ìrst aerospace application to reach the prototype stage, TWI fabricated three 2000-series aluminium spacecraft fuel-tanks for Boeing, using the friction-stir process, and tested them to destruction by pressurising them until they "barrelled", says Dawes. The result "-almost matched the metal in its annealed condition, which surprised us", he adds. Tests also showed that failure was not at the weld, but at the "slightly heat-affected zone".

"Excellent" fatigue tolerance has also been demonstrated, says Dawes, even with the unÌnished weld which already has a surface Ìnish good enough to "-dramatically cut Ìnishing costs-pre- and post-weld preparation is far reduced". TWI has proven the technique on materials with thicknesses varying between 1.5mm and 18mm. Dawes believes that the theoretical maximum thickness of 37.5mm is feasible.

There is great interest in the process for 7000-series aluminium, says Dawes, particularly for wing spars, and he predicts weight savings of around 20% for load-bearing ßoors, with the Ìrst commercial application in about Ìve years.

Dunkerton says that,with interest growing in the metal scandium, the only mine for which is in Uzbekistan (Flight International, 27 August-2 September), even higher-strength alloys will be available and friction-stir welding will be the only conceivable way to avoid even greater use of riveting.o

Source: Flight International