1. A qualitative optical evaluation, mostly based on experience;
2. A quantitative evaluation based on the avalanche angle and the surface fractal of the powders. measured using a revolution powder analyzer.
Let’s assume a few batches of powders with particles varying in size between 4-75um. Their usability in SLM machines equipped with recoating blades and powder reservoirs is limited to 2.5 to ensure homogeneous layers. This limit may vary with different recoating configurations.
Measuring the avalanche angle only requires a simple set up. A rotating, transparent drum is filled with a known amount of powder. A camera records backlit binary images of the powder free surface and the cross-sectional area of powder inside the drum.
Picture analysis extrapolates different avalanche angles that vary with powder flowability.
This technique can be used to quantify powders based on the avalanche flow index (AFI) and cohesive interaction index (CoI). It has been used successfully to characterise different Ti6Al4V powders for SLM  and plastic powders for SLS .
The avalanche test also allows a statistical analysis of many different parameters such avalanche angle, surface fractal, volume expansion rate, etc:
• Avalanche angle: angle between the horizontal reference line and the linear fit of the free powder surface just before an avalanche starts;
• Avalanche surface fractal: measure of the fractal dimension (=’jaggedness’) of the surface profile . This parameter fits the perception of a powder level of cohesiveness and is an indirect measure of inter-particle forces.
• Volume expansion ratio : ratio between the volume measured inside the drum and the volume occupied by the powder in the preparation container.
Differences between experience-based evaluation and Hausner ratio HR indicate that HR is not sufficiently representative of flowability in the case of fine metal powders.
Practically, measuring avalanche angle and fractal surface seem more meaningful phenomenological parameters. They naturally encompass inter-particles forces, morphology impact, size distribution, etc…
That may be why the direct comparison of the optical evaluation with the measured avalanche angle and surface fractal values shows a clear correlation.
For powders exhibiting natural ‘good’ flowability, avalanche angle and surface fractal have low values and narrow standard deviations. It is assumed the presence of particles clusters would trigger more irregular occurrence of avalanches and a more jagged powder surface.
Using a revolution powder analyser shows it is possible to quantify powders flowability for AM. The results correlate with optical qualitative, experience-based evaluations.
Yet, this doesn’t take into consideration machine-specificities such as spreading mechanism, or required flowability limits... Besides, additional layer properties, including density and physical and optical layer properties, need to be defined and quantified. Measuring correlation between flowability, powder density, particle size distribution and particle shape would present great interest to assess their impact on SLMed part quality.
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