Fig. 1. Schematic illustration of friction-stir welding process (after R. Nandan et al. [4]).

Fig. 2. Typical example of a welding tool (after L. Zhou et al. [17]).

Fig. 3. Typical examples of (a) tool wear (after L. Zhou et al. [21]) and (b) tool debris in stir zone (after S. Yoon et al. [53]).

Fig. 4. (a) Effect of processing variables on temperature profile (after B. Li et al. [12]), and (b) evolution of tool temperature with welding time (after A.L. Pilchak et al. [24]). Note: The labels in the upper right corner of (b) indicate tool rotational rate (i.e. 120, 150 and 200 rpm) and locations of the temperature measurements (i.e. pin or tool shoulder).

Fig. 5. Examples of processing windows: (a) after J. Su et al. [42], (a) after P. Edwards et al. [44], (a) after P. Edwards et al. [47].

Fig. 6. Typical welding defects: (a) kissing bond (or lack of penetration), and (b) root cavitations (after P.D. Edwards et al. [63]).

Fig. 7. Schematic illustration of stationary-shoulder FSW (Courtesy of TWI Ltd).

Fig. 8. Examples of residual stress distributions: (a) after S. Pasta et al. [38], (a) after A. Steuwer et al. [45]. Note: Longitudinal stresses are shown in (a).

Fig. 9. Typical low-magnification overview of stir zones produced in welds of different thickness (after P.D. Edwards et al. [63]).

Fig. 10. Typical backscattered scanning-electron-microscopic images of mill-annealed material (after J. Su et al. [42]).

Fig. 11. Scanning-electron-microscopic micrographs illustrating microstructures in different locations of heat-affected zone: near base material (a, d), in the central part (b, e), and near stir zone (c, f). After J. Su et al. [42].

Fig. 12. Scanning-electron-microscopic micrographs illustrating inhomogeneous microstructure distribution throughout the stir zone thickness: (a) the microstructure near the upper surface of the stir zone, (b) the microstructure in the stir zone mid-thickness, (c) the microstructure in the bottom part of the stir zone, and (d) the microstructure at the weld root (after S. Yoon et al. [9]).

Fig. 13. Typical microstructures which may be observed in stir zone depending on peak temperature: (a) globular (after A.L. Pilchak et al. [34]), (b) bimodal (after A.L. Pilchak et al. [35]), (c) β transformed (after A.L. Pilchak et al. [24]).

Fig. 14. Typical 0001 pole figures measured in (a) globular microstructure, and (b) β transformed microstructure (after S. Yoon et al. [9]). In the pole figures, ND and TD are normal direction and transverse direction, respectively.

Fig. 15. Examples of microhardness profiles measured in friction-stirred material: (a) after Y. Zhang et al. [58], (b) after A.R. Nasresfahani et al. [29], (c) after L. Zhou et al. [17].

Fig. 16. Microhardness maps showing microhardness variation within stir zone as well as with thickness of the stirred material, after P.D. Edwards et al. [63].

Fig. 17. (a) Typical deformation diagrams illustrating longitudinal tensile behavior of stir zone material (after J. Su et al. [42]), and (b) effect of spindle speed on strength of the stir zone material (after Y. Zhang et al. [58]).

Fig. 18. Preferential strain localization in heat-affected zone during transverse tensile tests of friction-stir joints (after M. Ramulu et al. [74]).

Fig. 19. Typical effect of spindle speed on strength (a) and ductility (b) of friction-stir joints. (after Y. Zhang et al. [58]).

Fig. 20. Typical fracture locations observed in friction-stir joints: (a) heat-affected zone, and (b) stir zone. (after S. Ji et al. [19]). Note: In (b), the weld was produced by using the backing-plate-heating technique [19].

Fig. 21. (a) Typical example of fatigue behavior of friction-stir joints (after P. Edwards et al. [44]), and (b) variation of fatigue crack propagation rate in different microstructural regions of friction-stir weld (after S. Pasta et al. [38]).

Fig. 22. Effect of microstructure morphology in stir zone on fracture toughness (after K. Kitamura et al. [8]).

Fig. 23. (a) Inhomogeneous superplastic performance of friction-stir joints (after P.D. Edwards et al. [80]), (b) deformation diagrams illustrating superplastic behavior of β transformed structure in stir zone (after L.H. Wu et al. [31]).

Fig. 24. Backscattered scanning-electron-microscopic images of (a) β transformed microstructure and (b) globular microstructure in stir zone after 50 h immersion in 20% HCl (after M. Atapour et al. [39]).

Fig. 25. Superplastically formed 4 m diameter Ti-6Al-4V engine inlet (after D.G. Sanders et al. [71]).

Fig. 26. A prototype combat vehicle hull fabricated by using FSW (Courtesy of EWI).