Effect of Particle size of monomodal 316L powder on powder layer density in powder bed fusion

Abstract

Powder layer density is an important measure for understanding the effect of powder on part quality in powder bed fusion. The density of thin layers, as they are deposited in powder bed fusion, differs from the density of powder in large containers. This study investigates this difference. Therefore, six monomodal powders with different particle size distributions, from coarse to fine, are spread in an 84.5 µm deep cavity to determine their powder layer densities for a single layer. A linear dependence of powder layer density on the D50 of powder is discovered for monomodal powders with good flowability. This dependence can be explained by the wall effect. Fine powders with low flowability show an increase in the standard deviation of the powder layer density. These findings suggest the existence of a particle size distribution that is sufficiently small to minimize the wall effect in a thin layer while still being sufficiently large to guarantee a good flowability of the powder.

The Influence of Particle Shape, Powder Flowability, and Powder Layer Density on Part Density in Laser Powder Bed Fusion

“…The aspect ratio was lower than the circularity for all powders. The absolute values of the circularity and aspect ratio were within the range of what has been reported in the literature for the respective production processes [11,[25][26][27][28][29]. Furthermore, the results showed a strong correlation between powder circularity and aspect ratio with a Pearson correlation coefficient (PCC) of r (12) = 0.91, p < 0.001.…”

“…For all powders, the PLD in the 84.5 µm cavity was lower than in the 141.4 µm cavity, and both PLDs were lower than the apparent density of the powders. This was due to the wall effect, which is the reduction of packing density through vacant sites in a packed powder in the presence of a wall, as discussed in [29,30]. For a solid material layer with a height of 30 µm, recent studies indicate that 141.1 µm is close to the realistic effective powder layer thickness for the LPBF process; Wischeropp et al [6] measured it to be between 130 and 165 µm.…”

Powder Surface Roughness as Proxy for Bed Density in Powder Bed Fusion of Polymers

“…The profiler features a repeatability in the Z and X axes of 0.4 and 0.5 μm respectively, and a XY pixel size of 10 μm (set by the axis encoder) over a scanning length in the X direction of about 16 mm. Using a steel blade within the test bench, about 12 cubic centimeters of powder were deposited into the metal cavity with a circular shape, a nominal depth of 140 μm and a calculated volume of 177.71 mm 3 using the procedure developed by Haferkamp et al [ 15 ]. The goal of the cavity is to mimic the actual layer thickness of a powder bed fusion process, and in this specific case, an intuitive correlation between powder bed in a PBF machine and layer density in the test bench can be assumed.…”

“…Powder layer density is one of the metrics used in the literature [ 14 ] to study powder flowability and the associated influence on powder bed fusion processes. Haferkamp et al [ 15 ] demonstrated a lack of correlation between layer density and part quality with a wide range of 316L stainless steel powders characterized by different particle size, however the dynamics of the material-melt pool interactions in metal laser powder bed fusion is drastically different than when using polymer powder. Since preheating is applied in the case of polymer feedstock, the laser-matter interaction happens close to the powder melting point and hence, a reduced thermal shock for the feedstock leads to a more stable process.…”