It’s bond – ultra-thin bond

Researchers have developed a way of joining ultra-thin materials – without using glue or welding filler wire – by adapting an existing process. The new technique, micro-friction stir welding (µFSW), has recently been used to create aluminium-to-aluminium, and aluminium-to-copper, lap and spot welds of 200 microns, and aluminium-to-aluminium butt welds of 300 microns, according to Nee Joo Teh, senior project leader in the microtechnology section at TWI – which specialises in developing new joining technologies. “We are hoping to get this down to around 100 microns and to look at welding metallic alloys,” he says. “We have even done some work on welding wood together.” The team has also used µFSW to bond plastic to plastic, including some high temperature plastics. Welding speeds are 100-500mm per minute. Potential applications include sealing packages within the electronics and photonics industry, and welding aluminium foil for use in the aerospace industry. “We are working on a number of specific applications, which will go into production very soon,” says Teh. “These will be for automotive, aerospace and electronics packaging.” TWI invented friction stir welding in the early 1990s. The process applies pressure to the joint through a rotating tool. The material is softened – although not melted – through frictional heat, and this allows the separate pieces of material to be joined together. Teh says the two techniques appear identical. However, he adds: “The control parameters are very different, because of the thickness of the materials, and the dimensions – or the geometries – of the welding tools that we are using.” The welding tool design is also different for the two techniques – TWI works with clients to develop the best design for an application. “We have a library of design that we have built up over the years,” he says. “For specific applications, we tend to start with something that we think might work for certain materials, then help our clients optimise the design.” TWI is currently developing µFSW equipment for two end users – one in the automotive industry, and one in the electronics industry. “We work with people to integrate the equipment into their existing systems,” Teh explains. “We are not into producing pieces of machine equipment, but we have been involved in developing the design and transferring that to a machine equipment manufacturer.” The company is also starting several internal development programmes that use µFSW, in areas such as hermetic sealing and hermetic packaging for electronics and product packaging. “We are also looking at using the tool as a form of welding contacts – as in interconnections for electrical applications,” he said. He also hopes to use the technique as an alternative to conventional adhesives for bonding plastics. TWI is developing µFSW as an alternative bonding technology, rather than a replacement for conventional techniques. “It is a very low-cost process compared to other conventional welding processes,” states Teh. “And it’s environmentally friendly, in the sense that it doesn’t really have any fumes.” At the same time, it does not require any filler material in the joint. The process uses no glues, chemicals or welding fillers, fluxes or wires and a minimum amount of energy. The strength of the bond is comparable to conventional welding processes, but an advantage is that µFSW allows different materials to be welded together more easily than conventional welding processes. Not having to add a third material to make a joint could also make product design simpler, he adds. The team is also looking into how µFSW could be used to repair materials – using friction mechanics to join them back together. Another key advantage is that it can be used in hazardous environments. “The welding tool doesn’t release any fumes or sparks,” says Teh. “The joint produced by µFSW usually doesn’t require any finishing, or any kind of dressing. It doesn’t require any cleaning either, because it doesn’t leave any residue materials.” Pointers *Researchers have developed a way of joining ultra-thin materials without using glue or welding filler wire *The team has also used µFSW to bond plastic to plastic, including some high temperature plastics. Welding speeds are 100-500mm per minute *TWI is currently developing µFSW equipment for two end users – one in the automotive industry, and one in the electronics industry