(155f) Novel Tool for Evaluating Powder Feedstock Suitability for AM Spreading Processes

In Additive Manufacturing (AM) Powder Bed Fusion (PBF) processes, powders are spread into thin layers on top of each other. Between layer depositions, particles at selected points are fused or melted together to form part of the structure being built. The thickness of the subsequent layers sets the depth-resolution of the product, but also, together with the powder layer density, the thickness affects the laser or electron beam power necessary to fuse powder over the whole layer depth correctly. If the powder layer thickness and density are not reliably uniform, the final product may show defects such as porosity, lack of fusion and residual stresses, amongst others. PBF processes require a calculated balance between layer characteristics and particle fusing methods, so being able to reliably predict the characteristics of the layers formed by particular feedstock in a specific process is critical to the quality of the printed product.

Small-particle powder mixes are often used because they allow for thinner layers and therefore finer build resolutions, but they are also often related to higher inter-particle cohesive forces that hamper the formation of uniform layers. However, the performance of a fine powder in a recoating process is also influenced by particle shape and size distribution, as well as by bulk and dynamic powder behavioural properties such as flowability, density and permeability, and external factors such as recoater type and speed, entrained moisture and process temperature. It is extremely difficult to deduce a universal model for the prediction of spreading behaviour from all these variables, for all materials, and for the wide variety of process parameters used by the whole industry.

In this work we consider the specifications of a typical spreading/recoating operation (recoater type and speed, gap height, and spreading surface), and the challenges associated with assessing powders for suitability (from technology limitations to the usefulness of measurable metrics). We propose an ex-situ method for realistically replicating the spreading process, both on a flat surface, and on a previously coated powder layer, using only small sample volumes. Then, we explain a technique for quantifying the spreading performance using technology that applies an appropriate trade-off between measurement resolution and range, and realistic usefulness of results. We describe how the instrument can obtain relevant quantitative measurements proven to be effective for the assessment of powder layer uniformity and consistency, for any type of AM powder.

We present quantitative assessments of powder layer quality and feedstock suitability for several powder samples that are varied in material (two different metals and a polymer with different treatments) and particle sizes (overall range 16 – 150 µm), but all samples were produced specifically for AM processes. The bulk properties of these powders were also evaluated using an FT4 Powder Rheometer^® (Freeman Technology Ltd, UK) to investigate the general relationship between flow behaviour and spreading performance metrics across such a wide variety of materials.