Transport phenomenon, flow field, and deposition forming of metal powder in the laser direct deposition with designed nozzle

The powder transport characteristics and flow field structure can significantly affect the quality of laser direct metal deposition (LDMD), which are in relation to nozzle structure and powder feeding parameters. Numerical simulation based on the gas-solid two-phase theory is the most effective way to study those. In this work, a double-ring coaxial nozzle was developed, and a 3D numerical model of this nozzle considering particle collision was established to obtain particle space trajectory and powder jet structure. Combining with experiments, the aerodynamics of the nozzle, the transport phenomenon, flow field, and deposition forming of powder particles in the LDMD were studied. Moreover, their internal relation was discussed. Results show that the developed nozzle could transport powder well and prepare excellent deposited layers. The numerical model can accurately predict the structure and transport characteristics of powder jet flow. The powder jet flow is easier to converge, and the focus moves up due to the collision. With the increase of powder feeding rate, the powder jet flow becomes divergent, and the focal points are separated so that the axial concentration distribution presents a “double-peak” characteristic, which is beneficial to reduce the laser attenuation rate on the transmission channel and to ensure the formation of the molten pool by the substrate. The appropriate size of the deposited layer can be obtained at the powder-carrying gas flow rate of 4 L/min and the powder feeding rate of 2.0–3.6 g/min, with the powder utilization ratio up to 54.2% and the microstructure being uniform and dense, meeting the requirement of industrial application.