High-strength Mg–Gd–Zn–Zr alloy with intragranular nano-precipitates mediated by hot-wire arc directed energy deposition

Abstract

To optimize the microstructure and the mechanical property, this work proposes a hot-wire arc directed energy deposition (HWA-DED) strategy for Mg–15Gd–1Zn–0.4Zr alloy (wt.%) by coordinating wire preheating with arc melting to reduce effective arc heat input. Compared with conventional wire-arc DED, HWA-DED yields refined grains (average size reduced from 9.0 ± 0.3 µm to 7.1 ± 0.2 µm), a higher fraction of high-angle grain boundaries, weaker texture, and a more uniform precipitation architecture. Three-dimensional X-ray microtomography and TEM reveal that HWA-DED produces finer and denser (Mg,Zn)₃Gd precipitates with increased volume fraction (3.82% vs. 2.73%), along with lamellar LPSO structures and nanoscale zones that elevate dislocation density. Consequently, the as-deposited HWA-DED components exhibit higher hardness (113 HV vs. 86 HV) and significantly improved tensile strength while maintaining comparable ductility. For HWA-DED, the ultimate tensile strength (UTS), yield strength (YS), and elongation are 300 ± 5 MPa, 210 ± 4 MPa, and 7.0 ± 0.5% in the horizontal direction (H), and 302 ± 6 MPa, 214 ± 6 MPa, and 7.3 ± 0.4% in the vertical direction (V), respectively. For conventional WA-DED, these values are 260 ± 4 MPa, 192 ± 4 MPa, and 7.2 ± 0.1% (H), and 262 ± 6 MPa, 196 ± 6 MPa, and 7.5 ± 0.3% (V), respectively. Furthermore, fractography demonstrates a transition from quasi-cleavage in WA-DED to transgranular ductile tearing with deeper dimples in HWA-DED, which aligns well with the redistributed precipitates and the suppression of boundary-controlled damage. Ultimately, without requiring any post-process heat treatment, the HWA-DED strategy achieves a superior strength-ductility synergy for Mg–Gd–Zn–Zr alloys.