CFD simulation for the modelling of the powder flow on continuous coaxial nozzles in the laser DED process

Laser Directed Energy Deposition (L-DED) is becoming an alternative for the repair and coating of surfaces. Moreover, world-leading machine-tool builders are implementing the L-DED process into hybrid machine tools, to provide the ability to add and subtract material in the same platform. However, the L-DED process is still very complex to set up, since it involves a high number of parameters and physical phenomena. One of the challenges that machine-tool manufacturers face is the characterization of the powder flow through coaxial nozzles. The control of the powder distribution at the focal plane is key to improving the process effiency and to ensuring the quality of the clads. Simulation can be used to increase the understanding of the fluid-dynamic behaviour of complex flows, but it is still far from being considered a robust and closed solution. This work presents a CFD model for the simulation of the powder flow in continuous coaxial nozzles, typically used in the L-DED process. Firstly, a critical comparison of different turbulence algorithms is presented, focusing on their field of application, and supported by a literature review. Secondly, a CFD model for the powder flow simulation of a commercial coaxial nozzle is built and validated with experimental testing. Based on the obtained results, the k-ω transition SST turbulence model is considered the most appropriate for this kind of simulation. Moreover, a study of the influence of the restitution coefficient is presented, being 0.90 the most suitable for the MetcoClad 6 powder.