Effect of the Particle Size Distribution and Morphology on Powder Processability in Laser Powder Bed Fusion

Abstract

The raw material of the Laser Powder Bed Fusion (LPBF) process is the powder. It strongly influences the part quality. Despite its significance, there is a lack of quantified and broadly accepted powder requirements. Therefore, the influence of two powder properties, particle size distribution (PSD) and morphology, were investigated and quantified, and recommendations for those two powder properties were derived in this thesis. This was done with the part density as the primary quality metric for part quality. The results show that part density is impeded by too coarse powder, too fine powder, or too low particle circularity. The form of the PSD did not exhibit an impact on part density. Three thresholds were identified for the low end of the PSD, high end of the PSD, and powder morphology. The lower threshold for particle morphology is a circularity of 0.8, as no improvement of part density was found for powders with a higher circularity. The lower threshold for the PSD is caused by too low powder flowability. Three metrics for this threshold were derived: A D10 of 10 µm, an apparent density of 50 %, or a Hausner ratio of 1.3. All three metrics predicted flowability problems reliably for all 39 powders of this thesis as long as the powder circularity was greater than 0.8. The upper threshold for the PSD is caused by the interaction of the powder with the melt pool. Too coarse powder particles increase melt pool fluctuation and decrease melt pool size, which in turn causes porosity. This threshold depends on the processing parameters and cannot be universally stated. For some parameters, this threshold was exceeded for powder with a D50 of only 30 µm. For some, more robust, parameters this threshold was not even reached for the coarsest powder with a D50 of 60 µm. As long as none of the three thresholds are exceeded, the part density is unaffected by the powder PSD and morphology. The existence of this insensitive region facilitates the qualification of powders for LPBF according to the three identified thresholds stated above.