Characterization of the powder stream propagation behavior of a discrete coaxial nozzle for laser metal deposition

Laser metal deposition (LMD) is a blown powder process which can be used for the additive manufacturing of large components or the generation of functional geometries on semifinished parts. In LMD, it is crucial that both the laser intensity and powder mass flow distribution within the process zone are precisely matched for a welding bead of predefined shape and a consistent layer quality. While there are many common tools for the characterization of laser intensity distributions, a deep understanding of powder propagation behavior is still missing. Therefore, the present work thoroughly characterizes the powder stream propagation behavior of a discrete coaxial nozzle with three angle-adjustable powder jets. A line laser is used to selectively illuminate individual layers horizontally to the nozzle, and the intensity of the illuminated powder is recorded with the aid of a CCD camera. An envelope of the powder distribution is then plotted from the individual layers, analogous to a caustic of a laser beam, and, thus, the powder stream is evaluated. A novel method is presented to compensate for the radial asymmetry of a discrete powder nozzle in the evaluation, thus making it comparable with continuous nozzles. The method is validated by characterizing the powder stream propagation behavior of a three-jet discrete nozzle. Influencing factors on the powder stream are the protective gas flow, the powder mass flow, the angle of the powder nozzles, and the interaction of the three powder jets. The investigations are supplemented by a point-particle large-eddy simulation of the particle-laden flow.