Breaking the anisotropic strength-ductility barrier in WA-DED AZ31 Mg alloy via spiral oscillation-enabled microstructure control: a combined experimental and simulation study

In this paper, a spiral oscillation (SO) strategy has been applied to fundamentally alters solidification behaviors to fabricate AZ31 Mg components with unprecedented isotropic and enhanced properties. The deposition current, deposition speed, oscillation amplitude, and oscillation interval for the SO strategy are 110 A, 5 mm/s, 4 mm, and 3 mm, respectively. A multi-scale finite element-sharp interface model is established and employed to simulate the wire-arc directed energy deposition (WA-DED) process for the first time. The effects of SO strategy and underlying mechanisms have been quantitatively revealed through a combination of electron backscatter diffraction (EBSD), scanning electron microscope (SEM), transmission electron microscope (TEM) and simulation. The results reveal that the SO strategy transforms the flow pattern from symmetric Marangoni vortices (0.1–1.1 m/s) to complex asymmetric ones (0.2–1.2 m/s), generating intense fluid vortices that induce dendrite fragmentation. It also produces a distinctive thermal field characterized by a low temperature gradient (∼0.03 K/µm) and high cooling rate (∼537.5 K/s), where thermal undercooling dominates nucleation. These synergistic effects promote extensive heterogeneous nucleation and facilitate a robust columnar-to-equiaxed transition (CET). Consequently, the SO-fabricated component exhibits exceptional isotropy and a synergistic improvement in strength and ductility, achieving an ultimate tensile strength of 231 MPa and elongation of 16.1%, corresponding to increases of 38% and 151% over the SL strategy. Furthermore, fundamental insights into the evolution of thermal-fluid dynamics have been established, encompassing the generation of asymmetric flow fields that induce dendrite fragmentation and thermal‑undercooling‑driven nucleation of equiaxed grains. Collectively, these findings provide a robust theoretical foundation for advancing WA‑DED of Mg alloys.