Powder bed surface relief formation and denudation in selective laser melting

Recent studies have revealed the importance of powder transport by gas-phase flows in the laser-interaction zone in laser powder bed additive manufacturing. The understanding of such a mass transfer mechanism is necessary for developing and optimizing laser-assisted processes of additive manufacturing. Powder bed surface relief around the remelted track is experimentally characterized by metallography and laser scanning profilometry in single-track experiments with powders of various materials and various particle sizes. Denudation zones with sharp irregular boundaries containing particle agglomerates are observed for finer powders with smaller particles. Denudation zones without well-defined boundaries containing single particles are observed for coarser powders. The balance of forces applied to a particle is theoretically analyzed to understand powder rearrangement in the laser-interaction zone. The drag force is estimated by a similarity point-source model of the entrainment flow with a correction for the finite size of the evaporation spot. The adhesion force appears to be greater than the gravity one for the fine powders and lower than the gravity for the coarse powders, thus explaining the observed difference in the denudation zone morphology. The measured variation of the denudation width with the material properties and the particle size is consistent with the theoretical predictions.