Achieving excellent strength-ductility synergy of wire-arc additive manufactured Mg-Gd-Y-Zr alloy via friction stir processing

• FSP refines oxide and reduces porosity in the WAAM-GW92K alloy. • WAAM-GW92K precipitates eutectic phases, transforming to nano-phase via FSP. • FSP's high heat input/low strain rate yields ultrafine equiaxed grains. • Grain refinement is the main strengthening mechanism for FSPed GW92K alloy. Friction stir processing (FSP) was applied to wire-arc additively manufactured (WAAM) Mg-9.54Gd-1.82Y-0.44Zr (GW92K) alloy to address coarse microstructure and porosity defects inherent to layer-by-layer deposition. FSP induced complete dissolution of the coarse Mg 5 (Gd,Y) eutectic network (initial size: 3.3 ± 0.5 µm) and triggered dynamic recrystallization, achieving a 69.5% grain refinement from 16.4 µm (WAAMed) to 5.0 µm (FSPed). This microstructural transformation enhanced ultimate tensile strength (UTS) by 32% (217 ± 3 MPa → 286 ± 2 MPa), yield strength (YS) by 46% (124 ± 2 MPa → 182 ± 7 MPa), and elongation (EL) by 35% (9.7 ± 1.1% → 13.1 ± 1.4%). Quantitative analysis via Hall-Petch relationship confirmed that grain refinement contributed ∼50 MPa (79%) of the total YS increment, while nano-precipitation (β′′/β′ phases <20 nm) effects accounted for the remaining ∼13 MPa. The simultaneous strength-ductility enhancement originates from FSP-induced defect elimination (porosity reduction: 1.75% → 0.18%) and dual-phase grain boundary pinning by Zr particles and β-Mg 5 (Gd,Y) precipitates. These findings establish FSP as a viable post-treatment for overcoming WAAM limitations in high-performance Mg-RE alloy fabrication.