Development of functionally graded austenitic lightweight steel through electrically assisted pressure solid-state joining

Functionally graded materials synergistically combine dissimilar components and can be engineered to exhibit gradual controlled variations in composition, structure, or properties, thus featuring advantageous mechanical properties and finding numerous practical applications. However, lightweight functionally graded materials such as those based on lightweight steels (LWSs) remain underexplored, albeit their advancement has significant merit in reducing the overall weight of a component or structure. To address, this study investigates the effective application of austenitic Fe–Mn–Al–C lightweight steels via the fabrication of a functionally graded material, enabling a synergistic combination of their dissimilar properties. Focusing on the mechanical properties of austenitic LWS that can be controlled through κ-carbide precipitation, we propose a novel functionally graded material developed by joining Mo-doped LWS and Si-doped LWS, which exhibit different κ-carbide precipitation behaviors. Electrically assisted pressure joining, an effective solid-state joining technique capable of enhancing atomic diffusion, was employed to strongly bond two dissimilar LWSs with improved joint integrity while preserving a homogeneous austenite matrix at the joint. Mechanical and microstructural characterization demonstrated that a high-quality and reliable solid-state joint was achieved within a short timeframe of a few minutes without elemental segregations and phase transformations in the metal matrix. The opposing tendencies of Mo to retard the κ-carbide kinetics and Si to enhance it resulted in two divided regions: a Mo-doped low hardness zone and a Si-doped high hardness zone in the joined LWS. Furthermore, by exploiting carbon diffusion driven by the chemical potential gradient, we successfully attained remarkable gradients in the amount of κ-carbide precipitate and hardness, from the joint interface to the Si-doped high hardness region. These findings manifest the applicability of the suggested technique in the meticulous design of functionally graded LWS joint materials.