  Uniform powder spreading is a requisite for creating consistent, high-quality
components via powder bed additive manufacturing (AM), wherein layer density
and uniformity are complex functions of powder characteristics, spreading
kinematics, and mechanical boundary conditions. High spatial variation in
particle packing density, driven by the stochastic nature of the spreading
process, impedes optical interrogation of these layer attributes. Thus, we
present transmission X-ray imaging as a method for directly mapping the
effective depth of powder layers at process-relevant scale and resolution.
Specifically, we study layers of nominal 50-250 micrometer thickness, created
by spreading a selection of commercially obtained Ti-6Al-4V, 316 SS, and
Al-10Si-Mg powders into precision-depth templates. We find that powder layer
packing fraction may be predicted from a combination of the relative thickness
of the layer as compared to mean particle size, and flowability assessed by
macroscale powder angle of repose. Power spectral density analysis is
introduced as a tool for quantification of defect severity as a function of
morphology, and enables separate consideration of layer uniformity and
sparsity. Finally, spreading is studied using multi-layer templates, providing
insight into how particles interact with both previously deposited material and
abrupt changes in boundary condition. Experimental results are additionally
compared to a purpose-built discrete element method (DEM) powder spreading
simulation framework, clarifying the competing role of adhesive and
gravitational forces in layer uniformity and density, as well as particle
motion within the powder bed during spreading.
