Integrating interfacial bonding and coordinated deformation in steel/Al bilayers via a pin-less friction stir-assisted synchronous joining-forming approach

The potentials of steel/Al alloy laminated structures for lightweighting in the automotive industry are increasingly recognized, particularly for reducing energy consumption. However, optimal formability and high-temperature stability in these hybrid structures remain a persistent challenge during the current manufacturing process. This challenge arises from the intricate control required over the Fe-Al intermetallic compounds (IMCs) layer at the dissimilar interface. In this work, steel/Al alloy bilayer laminated structures were fabricated by a thermomechanical joining-forming approach involving pin-less friction stir-assisted incremental sheet forming with synchronous bonding technology. These structures exhibit desirable shear bonding strength of 129 MPa (equivalent to 93.5 % of the shear strength of the AA5052-H32 base material). Mechanisms of local plastic flow, viscoplastic flow, and fast inter-atomic diffusion are evaluated using quasi-static elements. A phenomenological bond fraction model is proposed to describe the collapse of interfacial ellipsoidal microvoids. Cohesive numerical simulation reveals that the interfacial plastic strain is dispersedly distributed with slight cohesive damage due to stable heat-force conditions. Experimental results with confirmed models demonstrate the stability of the programmable process for mechanical-metallurgical bonding and coordinated deformation. Furthermore, a cyclic thermomechanical loading mode is proven effective in tuning interfacial diffusion and inhibiting the growth of the IMC layer. This integrated joining-forming strategy is also applicable to other challenging metallic pairs that are prone to brittle IMCs.