Spatial mapping of powder layer density for metal additive manufacturing via transmission X-ray imaging

Abstract

Uniform powder spreading is a requisite for creating consistent, high-quality components via powder bed additive manufacturing (AM). 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. Here, we present transmission X-ray imaging as a method for ex situ mapping of the effective depth of model powder layers at process-relevant scale and resolution. Specifically, we study layers of nominal 50–250 m 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 analyzing layer uniformity in the spatial frequency domain. This approach enables quantitative analysis of deposition irregularities, along with 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, 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.