Invented in 1991 at The Welding Institute (UK), Friction Stir Welding (FSW) is a solid-state joining process wherein severe plastic deformation in a confined region results in intimate contact between material on either side of the joint line. This intimate contact results in a strong, metallurgical bond. Heating is produced by relative motion between a non-consumable, rotating tool and the two plates to be joined. The shape of the tool promotes high hydrostatic pressure along the joint line, causing consolidation of the material softened by the
combination of frictional and deformation heating. The process may be described as a combination of in situ extrusion and forging. The microstructure of a friction stir weld is unlike that of a fusion weld in that no solidification products are present and the grains in the weld region are equiaxed and highly refined. Indeed, the FSW microstructure is that of a wrought rather than a cast product.
Although FSW has been put to use in full-scale production of ferry boats and space launch components made from aluminum, there is still a great deal to learn about the basic mechanisms and the details of the process. In addition, a great deal of work must be performed to extend the benefits of FSW from aluminum alloys to other, more refractory materials. Our work at USC encompasses both process modeling (with accompanying experimental verification) and joint performance analysis. The overall goal of our research is to develop a complete understanding of the interrelationship between weld processing parameters, base metal properties, and weld performance so that optimum welding conditions can be specified with a minimum of trial and error. Friction stir welding work at USC is being or has been supported by NASA, AFRL/ML, ONR, DOE (Savannah River Site), and Lockheed-Martin Corp.