Stirring stuff

Ultra-thin metal foils — used widely in everything fropm secure packaging for electronics to materials for satellites — are so lightweight and fragile that joining them together by lasers or flux-based welding methods is a problem.

In a bid to find a solution, researchers at TWI, the Cambridge joining technology specialist, have adapted a versatile welding method to make what it claims to be some of the smallest welds in the world.

The technique is friction-stir welding, initially developed in the early 1990s for the aerospace industry. Although commonly used for fairly thick sheets of aluminium, copper, steel and titanium, the TWI team has now used it for sheets of aluminium around 300 microns thick.

'At the moment, these materials tend to be joined using phase-change methods — these involve melting the metal or a flux, changing it from a solid to a liquid, which takes quite a lot of energy,' said project leader Nee Joo Teh.

Friction stir welding, by contrast, does not melt the material, and is therefore low energy. 'The process uses no glues, chemicals or welding fillers, fluxes or wires,' he said. 'Environmentally, the process is a major breakthrough.'

A welding tube is rotated at high speed, then brought into contact with the interface between the two sheets to be welded. The spinning tip heats up the interface zone through friction, softening the material on either side.

As the tip moves along the join, the rotation literally stirs the material, leaving a joint in its wake. 'It just uses mechanical and friction forces, softening the material and removing any oxide layer,' said Teh. The team has managed lap-welds and spot-welds in 0.3mm thick aluminium sheets, and butt welds in foils 0.5mm thick, with welding speeds of 100-500mm/min.

Miniaturising the process meant new designs for many of the components, said Teh. 'The key issue here is how to develop a process which is repeatable and highly controllable, and produce joints to meet the strength required for various applications. What goes into the process design is the speed of rotation, the design of the welding tube itself, the clamping of the workpiece on either side, and these are all interrelated.'

Many of the applications are commercially sensitive, said Teh, but one is to make larger foil sheets. 'The automotive and aerospace sectors use these very thin metal sheets, but it isn't cost-effective to make them large. So we're looking at taking metre-square sheets and joining them to make a four sq m-sheet, for larger structures such as satellite systems,' he said. 'We're looking particularly at the mechanical strength of the joints, as they have to be as strong as the sheets themselves. And another of the things we're looking at is the acoustic properties of the join.'

Other applications include sealing high-integrity protective foil packages for the electronics and photonics industries. There are also potential uses in the manufacture of medical devices, and in consumer goods and food packaging.

Most of the development work so far has used aluminium foils, said Teh. 'But we have had success in other materials too,' he said. 'We're working on different metals as well as non-metallic materials.'