Investigation of thermal-fluid dynamics in directed energy deposition of 316 L stainless steel with laser beam oscillation

The introduction of laser beam oscillation in directed energy deposition (DED-LBO) significantly influences the thermal-fluid behavior and molten pool formation during the process. This study presents a high-fidelity CFD model, integrated with a ray-tracing algorithm, to investigate the laser-material interaction and molten pool behaviors under linear and circular oscillation mode during the DED-LBO process of 316 L stainless steel. The results show that both the average interaction angle between the laser rays and the molten pool surface, as well as the laser absorptivity, vary periodically over time due to the periodic movement of the oscillating laser. This periodic heat input condition induces fluctuations in both temperature and fluid velocity within the molten pool. A higher oscillation frequency leads to the reduced temperature and fluid velocity. Compared to the circular oscillation mode, the fluid velocity is larger under the linear oscillation mode, primarily due to the larger temperature gradient. However, the surface area of the molten pool is larger under the circular oscillation mode, resulting in the capture of more powder particles. Moreover, the calculated Peclet number and Marangoni number are both larger than unit, indicating that thermal convection is the dominant heat transfer mechanism and Marangoni force is the primary driving force during the DED-LBO process. A good agreement is achieved between the simulated and experimental dimensions of the deposited tracks, with a relative error of <11.2 %. This study could enhance the understanding of thermal-fluid transport behavior of the molten pool during the DED-LBO process and provide insights for optimizing process parameters.