The friction stir welding (FSW) process has emerged as a viable welding process for aluminum alloys, after the predecessor “friction welding” process (a rubbing process) was found to be restricted. In FSW, metal is physically moved from one side of the interface to the other through shear displacements induced in the workpiece by the rotating tool. Incremental advances in improving the material of construction of the conventional FSW apparatus and in improving complex procedures continue to emerge, but it is becoming increasingly evident that the current processes and apparatus for FSW are less than optimum in various ways. In particular, concerns exist in applying FSW to higher melting point metals because of excessive heating of the apparatus, chemical reactions, wear, and simply fracture of the expensive apparatus.
Researchers at the University of Michigan have developed a welding device that addresses some of the limitations of current FSW technology. In particular, welding device focuses the high plastic shear rate near the interface region of the workpiece undergoing joining within a narrow zone. This is accomplished by using two or multiple rotating pins, and by defining that zone by the distance between the rotating pins of the two-pin head or a multi-pin head, which is intentionally controlled to be only a millimeter or few millimeters wide by controlling the spacing between the pins. The shear strain distributions superpose to enhance the local deformation rate in the region where material transfer occurs, producing a rapid friction-driven transfer of material across interfaces to be joined. A relative motion between workpiece and apparatus is introduced such that the material on the interface. Repeatedly varying the shearing axis direction of travel causes cyclic weakening of the material due to alternating stresses imposed in the transverse direction of overall travel of the weld head. This process takes advantage of the unique material softening property together with the higher shear rate between the pins. The contribution of these two effects causes breakdown of the grain structure of the metals and produces a refined grain structure, which can lower deformation resistance.
Applications and Advantages
- Friction stir welding of metals with higher melting point
- Produces refined grain structure that can lower deformation resistance