Nitrogen-regulated δ→γ phase transformation behavior in wire arc additively manufactured duplex stainless steel with in situ acoustic emission monitoring

Duplex stainless steel, DSS, is prone to δ/γ phase imbalance during wire arc additive manufacturing, WAAM, which affects mechanical and corrosion performance. In this work, 2209 DSS walls were deposited under Ar, Ar + 2%N₂ and Ar + 10%N₂ shielding gases to regulate the δ/γ phase balance, while contact acoustic emission, AE, signals were synchronously acquired to evaluate acoustic responses associated with microstructural evolution. Increasing the N₂ fraction promoted γ-austenite formation and shifted the deposits from a near-balanced duplex structure to an austenite-rich state. Scheil-Gulliver calculations, XRD and EBSD indicated primary δ-ferrite solidification followed by enhanced δ→γ transformation during cooling and interlayer reheating. AE features, including RMS, characteristic frequency and spectral entropy, varied systematically with γ-austenite fraction. PCA further distinguished the acoustic responses under different shielding gases, and trend-level association linked AE features with phase constitution and tensile behaviour. These results demonstrate that AE can serve as an auxiliary process-level indicator for phase-balance regulation and process optimisation in WAAM-fabricated DSS.