Design-Based Assessment of Continuous Coaxial DED Nozzles Considering Substrate-Induced Gas Flow Effects

Abstract

Continuous coaxial nozzles are widely used in directed energy deposition (DED) processes; however, many numerical studies rely on substrate-free free-jet models that fail to capture critical gas–surface interactions. This study presents a design-oriented investigation of continuous coaxial DED nozzles by examining the influence of nozzle geometry and gas flow interactions under realistic deposition conditions. A three-dimensional CFD framework incorporating particle tracing was applied, in which a physical substrate was explicitly included at the intended standoff distance to account for stagnation pressure formation, gas rebound, and near-wall flow effects. Two nozzle geometries, a narrow nozzle (β = 24°) and a wide nozzle (β = 35°), were evaluated under varying carrier and shaping gas velocities relative to a fixed optical shielding gas velocity. Powder convergence behavior was quantified using a layered particle counting strategy and analyzed through number density distributions in the focal region. The results show that nozzle geometry strongly governs powder focusing behavior. The narrow nozzle consistently produced a compact and axisymmetric powder stream with higher catchment efficiency, whereas the wide nozzle exhibited diffuse flow patterns and reduced focal coherence. Excessive shaping gas velocities were found to degrade convergence by expanding the substrate-induced stagnation pressure zone. These findings provide a physics-based rationale for geometry-dependent nozzle selection in continuous coaxial DED applications.