Formation mechanisms on amorphous-enhanced interfaces of Al/steel bimetallic freeform components via wire-based friction stir additive manufacturing

Al/steel bimetallic components suffer from deteriorated performance due to the presence of brittle intermetallic compounds (IMCs) at the interface. Wire-based friction stir additive manufacturing, a solid-state technique characterized by high strain rates, low heat input, and excellent shape flexibility, was utilized to fabricate Al/steel bimetallic components with controllable interfacial layers. The integrated pre-plasticization process and eccentric pin design promote the accumulation of plastic strain and the generation of high strain rates at the Al/steel interface, triggering interfacial amorphization and suppressing the nucleation and growth of detrimental crystalline IMCs. This solid-state process further preserves a continuous amorphous interfacial layer (200 nm thick), replacing the conventional thick brittle IMCs. The amorphous interlayer consists of an O-enriched sublayer and an Al-Fe-Si sublayer, further suppressing atomic diffusion and thus achieving an ultra-thin interfacial structure. The joint exhibited an ultimate tensile strength of 196 ± 4 MPa and fractured on the aluminum alloy side, verifying the superior interfacial bonding performance enabled by the amorphous layer.