Re-healing of separated steel substrate and work-hardened surface layer in roll-bonded steel–aluminum laminates

Roll bonding is ideal for preparing metallic composite laminates (MCLs). When MCLs are prepared by roll bonding, a work-hardened surface layer (WHSL) is typically prepared on the plate surface to promote bonding. However, differences in the mechanical properties of the WHSL and substrate lead to the WHSL separated from steel substrate during roll bonding, resulting in low bonding strengths of the roll-bonded laminates. This study focused on steel–Al laminates to successfully heal the cracks between the WHSL and the steel substrates via heat treatment. The crack-healing mechanism, element diffusion behavior, bonding property, and fracture morphology of the laminates were investigated. The results show that after roll bonding, the WHSL of the steel surface was separated from the steel substrate, and the steel–Al laminates exhibited a lower bonding strength. After annealing the laminates with cracks between WHSL and steel substrate at 450 °C for 60 min, a 426.8 nm Fe2O3 layer was generated between the WHSL and the steel substrate, healing the cracks. Additionally, good connectivity was observed between the WHSL and steel substrate. The shear strength of the steel–Al laminates increased by 1.7 times after the formation of the Fe2O3 layer. Analysis of the fracture morphology showed that shear fracture primarily occurs at the interface between the WHSL and steel substrate before the formation of the Fe2O3 layer; after the Fe2O3 layer is formed, shear fracture primarily occurs in the aluminum substrate. This study provides a new strategy for improving the bonding strength of roll-bonded steel–Al laminates.