FRICTION WELDING
In this, friction is employed to generate heat b/w two sliding or rotating metal surfaces. This process is usually carried out by placing the pieces to be welded in chucks on a common horizontal axis. First part is rotated and seconds remains stationary. Pressure is applied to generate enough heat to reach a bonding temperature within a few seconds. At that point,rotation is stopped suddenly and pressure is maintained or increased till welding is complete. Friction welder helps in achieving consistently high quality of joints each within a few seconds in various similar and dissimilar metal combinations. This is widely employed for joining of tool steel to carbon steel for tooling purposes, joining of aluminium to copper for end termination of electrical connections, etc.

Friction Stir Welding
Friction Stir Welding (FSW) has been invented, patented and developed for industrial applications by TWI, UK. In this, a cylindrical tool with a profiled pin is rotated and slowly plunged into the joining area b/w two pieces of sheet or plate material, which are butted together. The parts have to be clamped into a backing bar in  such a manner that prevents the abutting joint faces from being forced apartly. Frictional heat b/w  the wear resistant welding tool & the work piece 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 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 also be used to join aluminium sheets and plates without filler wire or shielding gas. Material thickness  ranges from 1.2 mm to 75 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 include a variety of aluminium alloys, copper alloys and AI-Li alloys.