  Laser powder bed fusion (LPBF) is an additive manufacturing (AM) technology.
To achieve high product quality, the powder is best spread as a uniform, dense
layer. The challenge for LPBF manufacturers is to develop a spreading process
that can produce a consistent layer quality for the many powders used, which
show considerable differences in spreadability. Therefore, we investigate the
influence of material properties, process parameters and the type of spreading
tool on the layer quality. The discrete particle method is used to simulate the
spreading process and to define metrics to evaluate the powder layer
characteristics. We found that particle shape and surface roughness in terms of
rolling resistance and interparticle sliding friction as well as particle
cohesion all have a major (sometimes surprising) influence on the powder layer
quality: more irregular shaped particles, rougher particle surfaces and/or
higher interfacial cohesion usually, but not always, lead to worse
spreadability. Our findings illustrate that there is a trade-off between
material properties and process parameters. Increasing the spreading speed
decreases layer quality for non- and weakly cohesive powders, but improves it
for strongly cohesive ones. Using a counter-clockwise rotating roller as a
spreading tool improves the powder layer quality compared to spreading with a
blade. Finally, for both geometries, a unique correlation between the quality
criteria uniformity and mass fraction is reported and some of the findings are
related to size-segregation during spreading.
