Powder bed fusion additive manufacturing promotes material efficiency as a unique selling point. Unfused particles are in effect recyclable after processing operation and reused [link]. Powder characteristics however tend to vary following different stages of the AM process and these variations could affect powder behaviour and final part quality. Specifically, feedstock quality must be tightly controlled due to their influence on powder flow and packing density.
Besides in a production context, it is critical to understand the level of feedstock sensitivity on part manufacturing. Granulometry changes, particles morphology variability among other factors, affect powder behaviour variation.
Why should I care?
Powder particles average characteristics
Granulometry (particle size distribution)
Generally, commercial feedstocks for SLM follow a gaussian distribution and existing results have established preliminary powder granulometry requirements suitable for SLM processes: D90/D10 ≤19 and D50/D10≥10 and D90≤ Th_layer [2, 3].
Morphology and internal pores
How particles characteristics impact granular media behaviour
Packing density and surface area
In PBF AM, powder particles are loosely arranged. The bed is randomly organised in a mixture of particle sizes and interstitial pores. This leads to relative packing densities typically in the range of ~40-60% of the parent material. Recoating issues can also disrupt the local packing density of the layer prior to radiation
Compared to bulk solids, a cluster of powder particles has relatively high absorptivity. This is due to the inter-particles voids which promote penetration of the laser source into the powder under a multiple scattering effect [15, 16].
In contrast to thermal absorptivity, thermal conductivity is significantly reduced in powders compared to their bulk forms since porosity limits the number of particles contacts in the powder bed. The effective thermal conductivity of powder particles is mostly constrained by the gaseous medium among the voids.
However, thermal absorptivity values often fluctuate over different points along the powder bed due to the arbitrary dispersion of actual powders particles that generates variation in local particle packing. It was shown  that a higher relative density increases the thermal conductivity of metallic powders.
Recoatability or flowability
The flowability of used powder  may deteriorate with recycling due to possible agglomeration and pre-sintering effects. Decrease in flowability is also tied to the coarsening of the PSD where pre-sintering creates powder clusters of irregular shaped.
Besides particles shape irregularities, a high amount of surface oxides is known to degrade powder flowability . The effect of powder granulometry on oxide contamination is based on the particle size and on the proportion of coarse and fine particles. An increase in surface area is linked with decreasing particles size. This is why oxidation can occur more rapidly in fine powder particles that have higher specific surface values with respect to coarser sized powder grades.
Blending fine particles with coarse powder can help improve powder packing density via effective size mixing and percolation. Yet, the inclusion of fine particles can increase powder cohesion and inter-particles forces. Powder particles sizes below 30um tend to exhibit clustering behaviour . Powder flowability becomes more restricted with decreasing powder particles size .
The addition of fine particles should then be weighted between achieving maximum packing density and flowability to optimise powder performance.
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