Friction-stir welding

Friction-stir welding was invented and experimentally proven by Wayne Thomas and a team of his colleagues at the TWI Welding Institute, U. K., in December 1991. TWI holds a patent for the process.

In FSW, a cylindrical-shouldered tool, with a profiled threaded / unthreaded probe (nib) is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces of sheet or plate material, which are butted together. The parts have to be clamped rigidly onto a backing bar in a manner that prevents the abutting joint faces from being forced apart. Frictional heat is generated between the wear resistant welding tool shoulder and nib, and the material of the work-pieces. This heat, along with the heat generated by the mechanical mixing process and the adiabatic heat within the material, cause the stirred materials to soften without reaching the melting point (hence cited a solid-state process), allowing the traversing of the tool along the weld line. The welding of the material is facilitated by severe plastic deformation in the solid state involving dynamic recrystallization of the base material.

Simple schematic diagrams of the FSW process (as shown below): (A) Two dissimilar metal workpieces butted together, along with the tool (with a probe); (B) The progress of the tool through the joint, also showing the weld zone and the region affected by the tool shoulder.