Fig. 1 A representative of bobbin FSW tools [13]

Fig. 2 Two kinds of fixed-gap bobbin tools [16]: a fixed bobbin; b floating bobbin

Fig. 3 Pinless floating-bobbin tool [24]

Fig. 4 a Cylindrical threaded pin and its weld cross section; b tapered pin with grooves and its weld cross section [26]

Fig. 5 Features of cylindrical pins [19]: a threaded with 3 flats pin; b threaded pin; c with grooves and 1.5 mm pitch; d with grooves and 2 mm pitch; e Left Hand and Right Hand threaded pin; f 4 flats pin

Fig. 6 Adjustable-gap bobbin tool [20]

Fig. 7 a Details of the DBT; b schematic of DBT-FSW [21]

Fig. 8 Counter-rotating bobbin tool [22]

Fig. 9 Semi-stationary shoulder bobbin tool [25]

Fig. 10 Temperature history of different feature points [32]

Fig. 11 a Temperature profile across the thickness [36]; b velocity profile in the fluid region [37]

Fig. 12 Calculated velocity field in BT-FSW: a vector map; b contour map [18]

Fig. 13 Comparison between predicted shear layer and weld microstructure [42]: a microstructure at RS; b velocity profile; c combined

Fig. 14 X-ray pictures of weld cross sections, tracer material placed horizontally in the a AS, b RS [43]

Fig. 15 a A macrograph showing a typical BT-FSW joint overlaid with the probe profile; EBSD micrographs showing the grain morphologies of b the BM; c the TMAZ on the AS; d the SZ; e the TMAZ on the RS [15]

Fig. 16 a Weld cross sections of BT-FSW joints; grain morphologies of b ellipse-shaped region c triangle-shaped region [53]

Fig. 17 a Weld cross-sectional macrograph of SSUBT-FSW; material flow and defect locations in b BT-FSW c SSUBT-FSW welds [31]

Fig. 18 a Negative optical macrographs of joints produced at different rotation speeds, after being etched by a 2% NaOH solution; b a backscattered electron micrograph showing the detail of the JLR region, without etching; c a schematic diagram showing the influence of the rotation speed on the position and morphology of the JLR. [15]

Fig. 19 Typical fracture locations of joints at different rotation speeds (rpm): a 400; b 600; c, d 800; e, f 1000 [15]

Fig. 20 Hardness distributions of the BT-FSW joints at different welding speeds (mm/min) [53]: a 50; b 100; c 150

Fig. 21 Robotic Weld Tool used for welding of Constellation Ares I Propellant Tanks [66]

Fig. 22 LC-FSW machine and welded panel [68]

Fig. 23 a “Fuxing” high-speed train; b its flat roof produced using BT-FSW [70]