Cavity growth - or lack thereof - in imperfectly roll bonded sheet metal

Roll bonding is a well-established solid-state joining method that has also found commercial use. In current industrial practice, a near-perfectly bonded interface is commonly aimed for, achieved by optimizing surface preparation and rolling parameters. On the other hand, an intriguing recent study has shown that roll bonding scrap metal has the potential to provide significant environmental benefits while not jeopardizing performance, despite the resulting imperfect interface with a distribution of cavities. This present work focuses on the mechanical aspects of this observation, aiming to provide a deeper understanding of the influence (or lack thereof) of the cavity content on the mechanical behavior of a roll bonded stainless steel, and a roll bonded mild steel. Imperfectly bonded samples of both steels are studied using mechanical tests and microstructure characterization, which both revealed that the interface cavities exhibit limited growth and no crack nucleation or propagation into the interface. A numerical finite element model of the imperfectly bonded sheet confirmed a tendency for low cavity growth by showing that opening stresses on the interface cavity remain low for various geometries and loading conditions, in line with linear elastic fracture mechanics calculations. Overall, this experimental-numerical study provides insights regarding the unexpected formability (and thus the wide applicability) of imperfectly roll bonded sheets.