High speed imaging of powder incorporation into the melt pool in LAM

Abstract

Laser additive manufacturing, LAM, involves complex physical mechanisms, such as melt flow, powder incorporation and smoothening by surface tension. Observation of the process by a high speed imaging camera provides information on the melt flow mechanisms at the surface. LAM can be categorized into Selective Laser Melting, SLM, and Direct Metal Deposition, DMD. During SLM powder grains frequently gather to larger drops that subsequently are incorporated into the very long melt pool. The transformation of the spacious powder bed to a dense, solid track causes a density increase that drags powder particles towards the molten pool. Carbon particles as part of the powder in the melt pool started flashing in phase with pulsed laser irradiation, which can be useful for detection purposes. During DMD, the travelling powder leaving the nozzle can experience gathering ahead of the melt pool before incorporation. Depending on the location of the interaction of a powder particle with the melt pool, time can become too short for melting. Then the particle sticks to the surface. Particles can even be reflected from the melt pool. Moreover, under certain conditions the particles travelling through the laser beam not only can experience melting but even boiling. The ablation pressure generated by the latter rapidly accelerates the particle in direction of the laser beam. Since most phenomena in LAM take place at the surface, high speed imaging provides highly valuable information to understand and optimize the process. In particular, for the blown powder process two camera views, from the top and side, are of complementary benefit. Statistical analysis can provide additional quantitative information.