Fig. 1 Schematic illustration of refill friction stir spot welding stages [7]

Fig. 2 Cross section of a typical joint [7]

Fig. 3 Macroscopic appearance of a FSpW joint with refilled probe hole [16]

Fig. 4 Cross sections of FSpW joints welded at tool rotation speeds of a 1250, b 2500 rpm (“U” line with local melting) [17]

Fig. 5 Cross section microstructure of an AZ31 friction spot joint: a cavities resulting from local melting; b hooking region [12]

Fig. 6 Quenched sample a and the welded sample c; and a micrograph taken from the weld center, as marked in a and c, for b and d, respectively [13]

Fig. 7 Representative macro- and microstructure of Al/Cu joints: a overview of the joint with rotation speed of 1200 rpm, plunge depth of 1.6 mm, b and c magnified views of locations b, c; d overview of the joint with rotation speed of 2000 rpm, plunge depth of 2 mm, e and f magnified views of locations e, f [23]

Fig. 8 SEM micrographs of joint interface produced with a tool RS of 3000 rpm a under the pin (weld center); b under the sleeve [25]

Fig. 9 Microstructure of a weld produced by FSpW. Cross-section image a, base materials b, c; higher-magnification images of selected areas d-i [30]

Fig. 10 Joint cross-sectional view of aluminum alloy to galvanized steel sheets [31]

Fig. 11 Typical microhardness profile measured in the mid-thickness of the upper sheet of FSpW joint [5]

Fig. 12 Two different fatigue failure modes for AA5754/Ti6Al4V dissimilar joints under high and low cycle conditions [33]

Fig. 13 Typical tool used to produce the friction spot welding welds [7, 35]

Fig. 14 Geometrical feature of the standard sleeve a and the modified sleeve b; typical cross section of the weld fabricated by the modified tool c [37]

Fig. 15 Failed sleeve and pin tool in the welding of Al 2099-T83 alloy [46]

Fig. 16 Application samples of FSpW in aerospace for a rocket propellant tank parts; b Al-Li alloy spacecraft cabin

Fig. 17 FSpW connection of fuselage structure with sealant interlayer [47]