Fig. 1 Optical images of FSW welds for pure Cu a, b and Cu-30Zn c, d for overall appearances a, c and stirring veins in SZs b, d. The coordinate system inserted in c denotes the principal directions of welding geometry, welding direction (WD), transverse direction (TD) and normal direction (ND)

Fig. 2 Cross-sectional SEM morphologies of FSW joints for pure Cu a-c and Cu-30Zn d-f in overall cross sections a, d, transition areas between the SZ and the BM b, e, and enlarged images for SZs c, f. AS and RS denote the advancing side and the retreating side of stirring geometry

Fig. 3 EBSD micrographs showing microstructures of FSW joints: a BM of Cu; b SZ of Cu; c BM of Cu-30Zn; d SZ of Cu-30Zn; e grain size (d) distributions in SZs; f distributions of grain boundary misorientations in SZs (d* represents the average grain size. The insets in b, d show the inverse pole figures of the SZs)

Fig. 4 TEM images in SZs of a pure Cu, b Cu-30Zn

Fig. 5 Hardness distributions in welding joints (the indentations were carried out along a line 500 μm far away beneath the friction surface on the cross section)

Fig. 6 a Tensile engineering stress-strain curves of FSW pure Cu, b engineering stress-strain curves of FSW Cu-30Zn, c strain hardening rate (Θ) and true stress (σT) versus true strain for ultrafine-grained SZs

Fig. 7 Macroscopic appearances and DIC-characterized strain fields of transverse samples for pure Cu a1-a3 and Cu-30Zn b1-b3: a1, b1 tensile specimens before and after test. The white frames mark the effective DIC measured areas; a2, b2 strain maps in the stage of nominally uniform plastic deformation; a3, b3 strain maps in the necking stage. ε ydenotes the strain along tensile direction Y, while ε x denotes the strain along transverse direction X. The numbers on the top of each strain subgraph indicate the applied strain; c1, c2 evolution of local strain (locations of A, B and C marked in a2, b2) with the tensile strain