Manufacturable bioinspired interlocking interface for toughened Al/steel hybrid joints

The development of Al/steel hybrid joints is important for lightweight structures, but their reliability is limited by brittle FeAl intermetallic compounds and unfavorable stress states at flat interfaces. Herein, a manufacturable bioinspired crest-trough interlocking interface was designed for arc joining of Al to steel, coupling geometric interlocking with interfacial metallurgical regulation. The influence of interlocking continuity on interfacial lattice damage, fracture path, and joint strength was investigated using multiscale characterization, mechanical testing, density functional theory (DFT) calculations, and molecular dynamics (MD) simulations. The optimized Bio-2 joint reached a tensile strength of 247.9 MPa, about 72.9% higher than that of the untreated joint, and its fracture path shifted from the steel-side η-Fe₂(Al, Zn)₅ intermetallic layer to the Al-side Zn diffusion zone. The crest-trough geometry prolonged the local high-temperature residence, promoting thermally assisted lattice relaxation in η-Fe₂(Al, Zn)₅. Accordingly, the dislocation density decreased from 5.7 × 10⁹ to 2.6 × 10⁹ mm⁻², and the fracture toughness increased by 41.6%. A geometry induced mixed mode strengthening model further shows that crown regions govern crack initiation, while shoulder regions promote traction redistribution and crack propagation resistance under mixed Mode I and II loading. The DFT and MD results suggest that lattice relaxation enhances electronic delocalization, charge redistribution, and dislocation-mediated stress delocalization in the η-Fe₂Al₅-type lattice. This work provides a process-compatible interface engineering strategy for robust Al/steel hybrid joints.