Improved tool design causes a stir

Friction Stir Welding (FSW) has come a long way since its invention in 1991. The Welding Institute (TWI), the inventor and patent holder for the process, based in Cambridge, UK is currently working on industry projects to improve tool efficiency to allow even higher welding speeds in aluminium alloys. Original tool designs allowed welding speeds of about 100 mm/min but modern tool design has increased that to over 430 mm/min in 6mm thick seawater resistant aluminium sheets. Chris Dawes, TWI?s principal consultant for friction stir welding, told The Motor Ship that he expects this to increase even further in the future. The speed increase, coupled with improved weld quality, is making this process even more attractive to aluminium shipbuilders. Dawes says that “tensile strengths which are equal to parent metal in its H321 condition can be achieved,” and this “? increasingly applies to other alloys as well.” He explains TWI is currently testing the technology on butt joints in 16mm thick plate ? welding in one pass. At the other end of the scale, he says ,TWI is testing the new tool technology in 2mm butt joints and for making lap joints in 3mm sheets. The thing about FSW is it isn?t really a welding process as the metal does not actually melt ? the secret behind the strength of the ?weld?. And the secret of getting a good weld using the process is to “crush, transport and forge at the lowest temperature possible,” says Dawes, which is where the new tool technology comes in. The aim is: “? to develop next generation tools that will weld considerably faster and also, if possible, reduce the annealing effect in work-hardened or heat-treated alloys,” says Dawes. He explains it is very easy to eliminate the effects of mechanical work hardening on 5083 alloys during welding and the heat from any welding processes is enough to bring a 6082 ? T6 (solution heat treated and artificially aged) alloy back to the O (annealed) condition. With its new tool design, Dawes says the increased speed will reduce the amount of heat inputted per unit length of weld so that the hardness in the heat affected zone will not be reduced. TWI is also investigating joint design to “develop joint designs that simplify structures enabled by FSW technology,” says Dawes. Portable machine developed High-speed ferry builders are already using panels made by FSW. The panels are made by joining extrusions that can be produced in standard sizes to produce panels that have low rates of distortion and thermal stresses. Many of these have been produced by Hydro Marine Aluminium in Haugesund, Norway. As they are only stiff in the longitudinal direction, they can be rolled up for transport and shipped stacked on top of each other. A portable FSW machine has also been developed by the University of Adelaide and tested in the construction of an ocean viewer vessel in Cairns, Australia. The Japanese have also been strong in developing the process and have used the process to produce honeycomb panels and seawater resistant panels up to 1,250 x 5,000mm. ESAB in Laxa, Sweden, has recently delivered a new FSW machine that it calls SuperStir to TWI, which gives a welding area of up to 5 x 8m. The gantry type machine is equipped with two heads; one for high speed welding of thinner section and one for welding sections up to 25mm in a single pass. When The Motor Ship visited TWI in June, the research institute was completing modifications for easier operation. SuperStir machines have also been recently supplied to DanStir ApS in Denmark and EADS/Institute de Soudure in France. The company has previously supplied ten sets to the automotive, shipbuilding and aerospace industries. Large Euro project The end of last year saw the start of what is probably the largest FSW project to date. Called EuroStir, it is a five-year project worth nearly Ä7 million involving 31 main participants from six European countries. The project?s aim, which Stephan Kallee, collaborative project manager for friction and forge welding processes at TWI, describes as “small in concept but large in reality,” is to get FSW out of the laboratory and adopted by industry. Countries involved in the project include the UK, Germany, Denmark, France, Poland and Sweden in an effort to achieve a “critical mass of expertise to catch up and overtake competitors from abroad,” says the project outline. Although the process was developed in the UK and was first used commercially in Sweden, subsequent development has been largely carried out in the US and Japan where huge funds have been put into projects for the railway and aerospace sectors. The project outline says that “the success in Japan and the US makes it clear that FSW will not only be dominant in the automated joining of all relatively soft materials such as aluminium, magnesium and copper but, due to the very superior joint properties and low cost, will also be applied to other materials and applications where these features are valuable.” The project does not currently involve any shipbuilders, probably because of the downturn in the fortunes of fast ferry builders at present. However, Kallee told The Motor Ship that any shipbuilders interested in joining the project group and taking advantage of cutting edge developments in FSW should contact him. n