A novel in-situ gas-phase alloying approach in wire arc additive manufacturing for controlling solidification mode and designing hybrid stainless steels

• Thermodynamic modeling enabled the design of hybrid stainless steel via wire arc additive manufacturing. • The hybrid microstructure uniquely combined high strength and ductility, overcoming duplex stainless steel limits. • Microstructural engineering through shielding gas tailoring offers a new route for developing advanced stainless steels. This study presents a thermodynamically guided in-situ gas-phase alloying approach in wire arc additive manufacturing (WAAM) to enhance duplex stainless steels by shifting the primary solidification mode from δ-ferrite to γ-austenite, producing a nitrogen-enriched alloy with a continuous austenitic matrix that combines duplex-grade strength with superior ductility. Thermodynamic calculations guided nitrogen adjustment in the shielding gas to control solidification and develop high-performance microstructures. Thermodynamic–kinetic modeling predicted nitrogen uptake from the arc plasma, enabling gas composition selection to promote a shift from δ-ferrite to γ-austenite as the primary solidification phase. Nitrogen content analysis and Scheil simulations confirmed a transition to austenite-first solidification at approximately 0.7 wt% nitrogen. Electron Backscatter Diffraction and optical microscopy revealed that nitrogen-enriched (HN) samples exhibited a continuous γ-austenitic matrix with finely dispersed δ-ferrite, whereas nitrogen-lean (LN) samples had a δ-ferritic matrix with isolated γ-austenite islands. HN samples showed greater grain orientation spread, indicating increased internal misorientation. Despite pronounced crystallographic texture, the HN samples demonstrated nearly isotropic tensile behavior along with enhanced yield strength, tensile strength, ∼11 % higher hardness, and improved elongation. These findings demonstrate that melt chemistry control via gas-phase alloying enables phase-engineered microstructures with superior mechanical performance without modifying the filler wire.