Fig. 1 Schematic illustration of RFSSW processes: a pre-friction stage, b penetration stage, c refill stage, d pull-out stage

Fig. 2 Joint obtained at sleeve plunge depth of 2.5 mm: a surface appearance, b cross section, c PrimB, d SecB, e hook

Fig. 3 Interface characteristics of joints with different sleeve plunge depths: a 1.75 mm, b 2.0 mm, c 2.25 mm, d 2.5 mm

Fig. 4 Hook morphologies of joints with different sleeve plunge depths: a1.75 mm, b 2.0 mm, c 2.25 mm, d 2.5 mm

Fig. 5 Grain structures in different microstructural zones of typical joint: a BM, b HAZ, c TMAZ, d SZ edge, e SZ center

Fig. 6 Grain structures in joint with different sleeve plunge depths: a TMAZ formed at 1.75 mm b TMAZ formed at 2.5 mm c SZ formed at 1.75, d SZ formed at 2.5 mm

Fig. 7 Hardness distribution at mid-thickness of upper sheet of the joint under different sleeve plunge depths

Fig. 8 Correlation of sleeve plunge depth with TFSL

Fig. 9 OM images for shear fracture: a macroscopic fracture, b cross section

Fig. 10 SEM images of: a shear fracture surface, b-d magnified views of the regions B-D marked in a

Fig. 11 OM images for plug type facture: a macroscopic fracture, b cross section

Fig. 12 SEM images of: a plug type fracture, b-d magnified views of the regions B-D marked in a

Fig. 13 OM images for tensile shear facture: a macroscopic fracture, b cross section

Fig. 14 SEM images of: a tensile shear fracture, b, c magnified views of the regions B and C marked in a