Mechanisms of carrier-gas flow rate effects on particle dynamics and deposition quality in laser-directed energy deposition governed by the lift-to-drag ratio

• Carrier-gas flow rate drives a multi-scale cause-effect chain from jet momentum to particle impacts and finally to single-track build quality, revealing why deposition outcomes can change sharply within a narrow operating range. • Increasing gas flow amplifies particle pushing forces faster than particle lifting forces, producing a clear saturation behavior that limits lateral spreading while intensifying normal impingement. • A critical inertial transition is identified where particle rebound and redistribution strengthen abruptly, locking the powder layer into a low-angle, flattened spreading state that promotes smoother tracks. • An optimal carrier-gas window is established where powder capture and surface smoothness are maximized, whereas excessive gas flow causes powder sweeping and powder-deficient deposition despite higher jet speed. The influence of carrier-gas flow rate on powder transport and deposition quality remains a key yet insufficiently quantified issue in laser-directed energy deposition (L-DED). This study combines numerical modelling with experiments to reveal how carrier-gas flow rate controls gas-flow structures, particle motion, collision behavior, and single-track morphology. Flow rates from 8 to 14 L min⁻ 1 are investigated to establish a mechanistic transmission pathway from aerodynamic momentum to macroscopic track quality. The results show that drag increases more rapidly with flow rate than lift, causing saturation in their relative contribution to particle lateral migration. Once a critical inertial condition is exceeded, particle rebound and redistribution intensify, the normal-to-tangential impact-force ratio increases markedly, and the powder repose angle stabilizes at a low level. This shift transforms the powder layer from heap-like accumulation to flattened spreading. Both simulations and measurements identify an optimal carrier-gas flow window of 10–12 L min⁻ 1 , where powder capture efficiency and surface smoothness are maximized, while higher flow rates induce powder sweeping and reduced deposition efficiency. These findings provide a transferable criterion for selecting carrier-gas flow conditions in L-DED.