Enhancement of mechanical properties in TIG-welded Mg-Gd-Nd-Zn-Zr alloy via multi-step heat treatment: Regulation of grain size and precipitation behavior

Mg-Nd-Gd series magnesium alloys exhibit promising weldability and mechanical performance, demonstrating significant potential for aerospace applications. This study investigates the effects of post-weld heat treatments (PWHT) on the microstructure and mechanical properties of tungsten inert gas (TIG) welded joints of a novel Mg-3Nd-3Gd-0.2Zn-0.5Zr (EV33) alloy. Results indicate that the fusion zone (FZ) exhibits poor microstructural thermal stability and is prone to abnormal grain coarsening (AGC) during post-weld solution (T4) treatment. Increasing the T4 temperature from 480°C to 540°C leads to a dramatic increase in grain size from 12.3 μm to 512.7 μm, while the content of residual second phases decreases from 3.42% to 0.43%. The application of a novel multi-step T4 treatment (480°C × 6 h + 520°C × 2 h + 540°C × 1 h) effectively mitigates AGC while achieving near-complete dissolution of eutectic phases. A grain growth model during T4 treatment is established based on the Sellars equation, and the reason for AGC has been clarified. By optimizing the PWHT process, the joint achieves desirable comprehensive mechanical properties (UTS = 319 ± 10 MPa, YS = 187 ± 7 MPa, EL = 6.1 ± 0.4%). A physical model depicting the failure mode related to the corresponding strengthening mechanisms is proposed.