Ultrasonic-driven grain uniformity of commercially pure nickel in continuous heat input directed energy deposition.

Wire arc-based metal additive manufacturing (AM) commonly results columnar grain structures aligned with the build direction, causing anisotropic mechanical properties and performance degradation. To mitigate the formation of columnar grain structures, ultrasonic vibration (UV) at 20 kHz was applied during the continuous heat input directed energy deposition (DED) process, targeting microstrucrual refinement in high-purity Ni alloy. The application of UV disrupted thermal gradients and facilitated dynamic recrystallization, transitioning columnar grains into equiaxed grains near the substrate. Electron backscatter diffraction (EBSD) analysis revealed a marked reduction in crystallographic texture, showing randomized grain orientations in UV-treated samples, contrasting with the strong anisotropy in untreated samples. High-speed imaging revealed localized vibrations at the interface between the metal bead and molten metal, forming band-like structures that progressively refined dendrites during solidification. These findings demonstrate the potential of UV to refine grain structures, enhance isotropy, and improve the mechanical properties of AM-produced materials.