Fig. 1 Typical appearance of a FSW joints; and the rotating tool b before and c after FSW. d Schematic illustrations of specimens prepared for tensile test

Fig. 2 Cross-sectional macrostructure of the joints made at different welding speeds: a 40 mm/min; b 60 mm/min; c 80 mm/min; d 100 mm/min; e 120 mm/min

Fig. 3 OM images of the kissing bonds: a 40 mm/min; b 100 mm/min; c 120 mm/min

Fig. 4 a EBSD; b OM images of the BM

Fig. 5 SEM images of the SZ of the joints welded at welding speed of a 40 mm/min; b 80 mm/min; c 120 mm/min

Fig. 6 SEM images of a top; b middle; c bottom region of the joint SZ welded at 40 mm/min

Fig. 7 Microstructures of the HAZ of the joint welded at 40 mm/min. a AS on the HAZ; b enlarged image of square area shown in a

Fig. 8 SEM images of HAZ region of the joints at different welding speeds: a 40 mm/min, b 80 mm/min, and c 120 mm/min in HAZ1; d 40 mm/min, e 80 mm/min, and f 120 mm/min in HAZ2; g 40 mm/min, h 80 mm/min, i 120 mm/min in HAZ3. (B: Bainite, F: Ferrite and M: Martensite)

Fig. 9 Ferrite fraction in different regions of HAZ of the joints at 40 mm/min

Fig. 10 EBSD orientation color maps of SZ center and HAZ2 at different welding speeds: a 40 mm/min, b 80 mm/min, c 120 mm/min in SZ; d 40 mm/min, e 80 mm/min, f 120 mm/min in HAZ2

Fig. 11 Distribution of grain boundary misorientation angle in SZ obtained at different welding speeds of a 40 mm/min; b 80 mm/min; c 120 mm/min

Fig. 12 TEM images of a BM; b-d SZ; e,f HAZ of the sample prepared at 40 mm/min

Fig. 13 Micro-hardness distribution at the mid-thickness of the cross section of the welds at different welding speeds

Fig. 14 a Engineering strain-stress curves of the global tensile specimens; and cross-sectional OM images of fractured specimens welded at b 40 mm/min; c 60 mm/min; d 80 mm/min; e 100 mm/min; f 120 mm/min

Fig. 15 SEM micrographs showing the fracture morphology of the tensile specimen: a, d 40 mm/min; b, e 80 mm/min; c, f 120 mm/min