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

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.