Spatial Mapping of Powder Layer Density for Metal Additive Manufacturing via X-ray Microscopy

TL;DR

This study introduces transmission X-ray imaging to map powder layer density in metal additive manufacturing, linking layer uniformity to powder properties and boundary conditions, and compares results with DEM simulations.

Contribution

It presents a novel application of X-ray microscopy for direct layer density mapping and integrates spectral analysis and simulations to understand powder spreading dynamics.

Findings

Layer packing fraction correlates with layer thickness and powder flowability.

Spectral density analysis quantifies defect severity and layer uniformity.

DEM simulations reveal the influence of adhesive and gravitational forces on layer density.

Abstract

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.