Friction stir welding is a fully penetrating solid phase process, which can be used to join metal sheets – at present mainly from aluminium – without reaching their melting point.

Friction stir welding (FSW) has been invented, patented and developed for its industrial applications by TWI (The Welding Institute) in Cambridge, UK. In friction stir welding, a cylindrical shouldered tool with a profiled pin is rotated and slowly plunged into the joining area between two pieces of sheet or plate material, which are butted together. The parts have to be clamped onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. Frictional heat between the wear resistant welding tool and the workpieces causes the latter to soften without reaching the melting point and allows traversing of the tool along the weld line. The plasticised material is transferred to the trailing edge of the tool pin and is forged by the intimate contact of the tool shoulder and the pin profile. On cooling down, it leaves a solid phase bond between the two pieces.

Friction stir welding can be used to join aluminium sheets and plates without filler wire or shielding gas. Material thicknesses from 1.6 to 30 mm can be welded at full penetration and without porosity or internal voids. High integrity welds with low distortion can be achieved in many aluminium alloys, even those considered difficult to weld by conventional fusion welding techniques. Materials that have been successfully friction stir welded to date include a variety of aluminium alloys (2xxx, 5xxx, 6xxx, 7xxx and 8xxx series) and Al-Li alloys More recently, friction stir welding has also been demonstrated for the joining of lead, copper, magnesium and even titanium alloys.