Exceptional metallurgical bonding mechanism via Nano-thick layer of interfacial solid solution in dissimilar welding

In dissimilar welding, interfacial metallurgical bonding typically involves the formation of brittle intermetallic compounds (IMCs). Effective bonding without interfacial IMCs was reported only in a few combinations with high mutual solubility, such as Ag/Au, or in limited specific cases involving Al/steel welding, where interfacial amorphous phases formed. This study reveals a new metallurgical bonding mechanism in friction stir lap welding between immiscible AZ31 Mg alloy and DC01 steel, using high-resolution transmission electron microscopy. Rather than IMCs, AZ31 and DC01 were bonded via a nanometer-thick interfacial layer of Fe(Al) solid solution formed by Al atoms diffusing across the interface from AZ31. This mechanism was found highly effective, as evidenced by the lap shear strength of AZ31/DC01 joint, reaching 101 MPa, which is 78 % of the shear strength of AZ31 base material and 89 % higher than that of pure Mg/DC01 joint. Fracture predominantly occurred in AZ31 stir zone rather than right along interface.