Fig. 1 Schematic illustration of BT-FSW and the sample locations for fatigue tests

Fig. 2 Schematic illustrations of the sample locations for EBSD and TEM observations

Fig. 3 Cross-sectional morphologies of BT-FSW 2219-T87 Al alloy joints under welding speeds of a 200 mm/min, b 400 mm/min

Fig. 4 Microstructures of the BT-FSW joints of 300-200 sample: a NZ, b TMAZ, c HAZ; 300-400 sample: d NZ, e TMAZ, f HAZ

Fig. 5 Precipitate distribution of a BM and sample 300-200: b NZ, and c TMAZ

Fig. 6 a Microhardness distribution of transverse cross section along the center lines of BT-FSW joints and 2D hardness contour map under welding speeds of b 200 mm/min, c 400 mm/min

Fig. 7 Tensile fracture locations of BT-FSW joints under welding speeds of a 200 mm/min, b 400 mm/min

Fig. 8 Log-log form stress-fatigue life (S-N) curves for BM and BT-FSW joints

Fig. 9 Fatigue strength and tensile strength of various 2xxx series Al alloy joints [2,50,52,53]

Fig. 10 Cross section fracture morphologies of BT-FSW joints under different maximum stress: a 300-200-120 MPa, b 300-200-240 MPa, c 300-400-120 MPa, d 300-400-240 Mpa

Fig. 11 Fracture surface morphologies of BM at a maximum stress of 200 MPa: a low magnification overview, b crack initiation zone, c crack propagation zone, d final fracture zone

Fig. 12 Fracture surface morphology of sample 300-200 at a maximum stress of 120 MPa: a low magnification overview, b crack initiation zone, c crack propagation zone, d final fracture zone

Fig. 13 Fracture surface morphology of sample 300-400 at a maximum stress of 120 MPa: a low magnification overview, b crack initiation zone, c crack propagation zone, d final fracture zone