Monitoring of powder flow focus in laser metal deposition: experimental and numerical analysis of gas-powder interaction mechanisms

The powder flow focus is critical in determining the quality of Powder-based Laser Metal Deposition (LMD-p). However, systematic studies on the monitoring of powder focus remain limited. To address this, the influence of in-process parameters and the underlying physical mechanisms was comprehensively investigated by combining the optical measurement experiments and CFD-DEM two-way coupling simulations in this study. Four quantitative descriptors were introduced to describe the spatial distribution of powder focus: Powder Focus Length (PFL), Relative Focus Density (RFD), Half Max-Density Focus Diameter (HMFD), and Half Max-Density Distance (HMD). The results demonstrate that the powder focus is synergistically influenced by the Shieldgas, Carriergas and powder feeding rate. Adjusting the coaxial shielding gas can effectively regulate the focal position by about 21.4 % (2.5 mm) without affecting focus distribution. Increasing the Carriergas enhances the convergence of the powder flow and improves focus position stability. A higher powder feeding rate results in a larger HMFD and RFD of powder focus, which also provides a greater HMD for the stand-off distance from the nozzle. The influencing mechanisms of gas on powder flow dynamics were further analyzed, and the initial velocity as well as gas flow field were found to be the key factors that influence the powder flow dynamics.