Fig. 1 Engineering application fields of heterogeneous structures

Fig. 2 Composition structures: a molecular chains in polymer, b atoms in metal

Fig. 3 FSW with controlling shape and performance: a welding tool, b surface [38]

Fig. 4 Radar chart of the microstructural characteristics versus mechanical properties [38]

Fig. 5 Schematic of different types of metal surface irregularites [52]

Fig. 6 Categories of surface pre-treatment methods

Fig. 7 Typical TEM and selected area diffraction (SAD) of Cu-CFRTP joint interface showing Cu2O thin transition layer [24]

Fig. 8 Parts of C=O groups developed into C-O-Al bonds at joining interface: a as-received nylon, b Nylon with Al2O3 coating [59]

Fig. 9 a Isosurface extracted from a volumetric Gaussian density map and atomistic views of the annealed Al-PP structure, b Amount of formed bonds as a function of simulation time, c Stress-strain curve, von Mises stress distribution at d 8% of strain for the Al-PP structure, e 18% of strain showing the detachment of PP from the Al interface [22]

Fig. 10 Schematics: a FSpJ welding tools, b welding processes [63]

Fig. 11 a Illustration of the primary bonding mechanisms, b a fracture surface [63]

Fig. 12 Schematics of F-ICJ: a positioning of the polymer stud into the metallic cavity, b rotating of the welding tool, c plunge and friction steps that can plasticize the polymer stud, d forging step when rotation velocity is reduced and an additional axial pressure is exerted, e consolidation of the plasticized polymer stud, forming a stake; f a F-ICJ joint [66]

Fig. 13 Structural design of the studs: a No-radius design, b surface-radius design, c recessed-radius design [69]

Fig. 14 Friction staking: a tool system, b joining sequence [70]

Fig. 15 FFSJ process: a schematic, b joint formation, c macrostructure in cross-section, d comparisons of joint strength [71]

Fig. 16 Schematic of the different steps for the joining process [44]