Fig. 1 Details of the experiment: a a schematic figure of Mg/Cu dissimilar FSW system, b dimensions of the tool, c specimens for tensile tests

Fig. 2 Macroscopic observations of the Mg/Cu FSW joints with three tool rotating speeds: a 325 r/min, b 625 r/min, c 925 r/min

Fig. 3 SEM images a, e and i and corresponding EDS mapping results b-d, f-h, j-l of the bonding interface of welds obtained with three tool rotating speeds: a-d 325 r/min, e-h 625 r/min, i-l 925 r/min

Fig. 4 Microstructures of Mg/Cu FSW joint with 925 r/min: a SEM image of the joint, b-d magnified SEM images of the joining interface at the upper, middle and lower location of the joint, respectively, e EDS point scanning results

Fig. 5 XRD patterns for the Mg/Cu joints with various tool rotating speeds

Fig. 6 Predicted schematic diagrams of the IMCs formation during Mg/Cu FSW: a contact before welding, b mutual diffusion of Mg and Cu atoms, c formation of IMCs during welding, d IMCs formation and distribution results after welding

Fig. 7 Micro-hardness distribution on the cross-section of the Mg/Cu FSW joints with various tool rotating speeds

Fig. 8 Tensile testing results of the Mg/Cu FSW joints with various tool rotating speeds, the macroscope of fracture surface and SEM images of fracture locations and magnified fracture surface at a 325 r/min, b 625 r/min, c 925 r/min respectively, d the ultimate tensile strength of three tool rating speeds