Modelling and validation of a direct metal deposition nozzle

Laser direct metal deposition (LDMD) will only achieve its full potential as an additive manufacturing process when a connection between the process parameters used, the microstructure created and the properties of a particular metal deposit can be established. This paper addresses the development of a numerical model, using computational fluid dynamics software, to describe an existing LDMD nozzle, in terms of the interaction between the various turbulent gas streams involved and the powder stream. An analytical model was then developed to simulate the thermal interaction between the laser beam and the powder stream. The model predictions have been validated experimentally by mapping the powder flow distribution below the nozzle exit and comparing this to the computed powder flow patterns. The powder particle velocity has been estimated using high speed video imaging and the amount of laser power absorbed by the powder stream was measured experimentally and compared to the model predictions. The model showed the importance of the nozzle gas pressure in relation to the interaction of the laser beam with the powder (in this case Inconel 718) and a microstructural assessment of deposits made under various conditions predicted by the model, made clear the sensitivity of the process to small changes in experimental parameters.