Experimental evaluation of the metal powder particle flow on the melt pool during directed energy deposition

The experimental evaluation of melt surface flow in blown laser cladding and additive manufacturing usually employs tracer particles added to the powder feed. This paper presents details of a high-speed imaging (HSI) and image processing technique, which can directly monitor the flow of standard (nontracer) particles on the surface of the melt. This technique should improve the accuracy of flow observations as no foreign bodies are added to the melt. To verify the technique, 316 Stainless steel powder was laser clad onto a substrate of the same material, and an HSI frame rate of 40 000 frames/s was employed with a specific illumination system. The images were subsequently image processed and the trajectories of the powder feed particles on the melt surface are analyzed and discussed here. As would be expected in this case, the observed melt surface flow was in agreement with what would be expected of a melt with a negative surface tension gradient as a function of temperature.