Friction stir welding (FSW) has been widely applied to metals with moderate melting points, primarily aluminum alloys. Until recently, however, there were no tool materials that would stand up to the high stresses and temperatures necessary for FSW of materials with higher melting points, such as steels, stainless steels, and nickel-based alloys. Over the past five years, tungsten alloys and polycrystalline cubic boron nitride (PCBN) have been used to create FSW tools for use in steel, stainless steel, titanium, and nickel-based alloys. Properties of the resultant welds have been shown to be outstanding. Although some issues remain (primarily limited tool life with tungsten-based tools), FSW has been demonstrated as a technically and economically feasible process in high-temperature materials.
Friction Stir Welding (FSW) is a solid-state joining process invented by The Welding Institute (TWI) of Cambridge, England (Thomas, Nicholas, Needham, Murch, Temple smith, and Dawes, 1991). In the FSW process, a rotating tool containing a pin and a shoulder is plunged into the joint between two work pieces, generating heat by friction. Once the heat has built up to the desired level, the tool is translated along the joint. Plasticized base material passes around the tool where it is consolidated due to force applied by the shoulder of the tool (Fig. 1). Initially, FSW was applied primarily to aluminum alloys, which could be easily welded due to the relatively low softening temperatures of these alloys. Other relatively soft metals, such as copper, lead, zinc, and magnesium, have also been welded. In contrast, for a number of years it was difficult to weld ferrous alloys and other high-softening temperature metals due to the lack of suitable tool materials. Beginning in 1998, tool materials and welding processes capable of welding steels, stainless steels, nickel-based alloys, and titanium alloys have been developed.
