Robust interfacial bonding achieved via phase separation induced by enhanced Al diffusion during AZ31/high-entropy alloy friction stir welding

• Dissimilar welding between HEA with sluggish diffusion kinetics and Mg alloy is successfully achieved for the first time. • Engineering interfacial HEA into nanocrystalline-structure is effective in triggering diffusion-controlled interfacial metallurgical reaction. • A ∼500 nm duplex-phased interfacial layer developed by phase separation, acting as a bridge to construct metallurgical bonding. • Interfacial layer is identified to be composed of alternating BCC AlNi-type and tetragonal FeCr-type intermetallics. • Exceptional tensile lap-shear strength of HEA/AZ31 joints was achieved with fracture largely occurred within AZ31 nearby interface rather than right along the interface. Welding high-entropy alloy (HEA) to Mg alloy has gained increasing attention for multi-metal structure design, while intrinsic sluggish diffusion kinetics of HEA confines diffusion-controlled interfacial reactions and makes it challenging to establish robust metallurgical bonding. This study investigated welding of FeCoCrNiMn HEA to commercial AZ31 as a model combination to pioneer this field. Interfacial phase separation phenomenon was observed, with the diffusion accelerated by in-situ engineering a submicron-scale thick (∼400–500 nm) HEA nearby the interface into nanocrystalline-structure during friction stir welding. Abundant grain boundaries generated in this nanocrystalline-interlayer serve as diffusion short-circuits and energetically preferred nucleation-sites, which promoted Al in AZ31 to diffuse into HEA and triggered quick separation into body-centered cubic AlNi-type and tetragonal FeCr-type intermetallics. HEA and AZ31 were thus metallurgically bonded by these interfacial intermetallics. The joint shows exceptional strength in tensile lap-shear testing with fracture largely occurred within AZ31 rather than right along interface as commonly reported previously for dissimilar joints.