A novel in-situ gas-phase alloying approach in wire arc additive manufacturing for controlling solidification mode and designing hybrid 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 S-ferrite to gamma-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 S-ferrite to gamma-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 gamma-austenitic matrix with finely dispersed S-ferrite, whereas nitrogen-lean (LN) samples had a S-ferritic matrix with isolated gamma-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, similar to 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.