Mechanical properties and bonding mechanism on interfaces containing work-hardening surface layer of roll-bonded 6061 Al/Q235 steel composite plates

The film theory is widely accepted as a bonding mechanism of metallic laminate composites (MLCs). It states that the fresh metal contact after the rupture of the surface film is an important condition for the bonding of MLCs. However, the bonding performance and mechanisms at the interfaces containing surface films have not been sufficiently investigated. In this study, steel–aluminium composite plates were used to study the bonding state, elemental diffusion behaviour, and mechanical properties of the bonding interface between the steel-side work-hardened surface layer (WHSL) and the aluminium matrix. The results indicate that the Al and Mg atoms inside the Al matrix can diffuse to about 100–150 nm into the WHSL (Fe3O4) and undergo selective oxidation, thus generating Fe0.1MgAl2O4 particles during annealing. However, the Fe atoms in the WHSL cannot diffuse into the aluminium matrix, thereby preventing the WHSL–Al interface from generating intermetallic compounds and maintaining beneficial bonding properties after long-term annealing. Additionally, compared to the steel–aluminium composite plate without a WHSL, the steel–aluminium composite plate with a WHSL exhibited a superior bonding performance. It also demonstrated an elongation increase of 28.67%, the mechanism of this increased elongation was also studied. Under the same degree of bending, the composite plates with WHSL maintained a stable bonding state and formed a unique interface deformation mode around the WHSL–Al interface after bending, which are highly desirable bending properties.