Friction stir welding (FSW) was invented by TWI in 1991 [1] and substantial development has been conducted subsequently. It allows metals, including aluminium, [2±12] lead, [13] magnesium, [14] steel, [15] titanium, [16] zinc, copper, [17] and metal matrix composites [18,19] to be welded continuously. Many alloys, which are regarded as difficult to weld by fusion processes, may be welded by FSW. The basic principle of the FSW process is shown in Figure 1.A non-consumable rotating tool is employed of various designs, which is manufactured from materials with superior high temperature properties to those of the materials to be joined. Essentially, the probe of the tool is applied to the abutting faces of the workpieces and rotated, thereby generating frictional heat, which creates a softened plasticized region (a third-body) around the immersed probe and at the interface between the shoulder of the tool and the workpiece. The shoulder provides additional frictional treatment to the workpiece, as well as preventing plasticized material from being expelled from the weld. The strength of the metal at the interface between the rotating tool and the workpiece falls to below the applied shear stress as the temperature rises, so that plasticized material is extruded from the leading side to the trailing side of the tool. The tool is then steadily moved along the joint line giving a continuous weld.Although incipient melting during welding has been reported for some materials, FSW can be regarded as a solid state, autogenous keyhole joining technique. The weld metal is thus free from defects typically found when fusion welding, e.g., porosity. Furthermore, and unlike fusion welding, no consumable filler material or profiled edge preparation is normally necessary.The process has already made a significant impact on the aluminium-producing and user industries worldwide and FSW is now a practical technique for welding aluminium rolled and extruded products, of thickness ranging from 0.5 to 75 mm. The present paper describes recent developments in FSW tool design, as this is the key to the successful application of the process.Tools and Techniques: Conventional Rotary Welds: Although FSW consistently gives high quality welds, proper use of the process and control of a number of parameters is needed to achieve this. A key factor in ensuring weld quality is the use of an appropriate tool and welding motion.The importance of the tool is illustrated in the following recent example involving the lap welding of 6 mm 5083-O, aluminium alloy wrought sheet. In preliminary trials a conventional cylindrical threaded pin probe tool was used which gave a good as-welded appearance. A typical pin type probe is shown in Figure 2.However, bend testing showed the weld to be weak due to excessive thinning of the top sheet and thickening of the bottom sheet caused by a pressure differential during welding (see Fig. 3).The failure followed the original interfacial surface oxide layers, which in 5083-O condition aluminium alloy, are known to be pa...
