Powder production routes, actual AM process and recycling methods all influence particles characteristics. In powder bed fusion, these properties affect the homogeneity and density of the powder layers spread across the build platform and, in turn, the process repeatability and the parts quality. Quantifying a combination of factors defining a ‘good’, process-able powder is still required for AM. Yet little has been studied to link traditional powder measurements to its flowability and to its AM process-specific spreading abilities. In this post, let’s discuss suitable parameters and values to qualify flowability of metal powders for selective laser melting (SLM).
Much time and efforts focus on developing processing maps for commercial metal powders. In addition to laser parameters as well as physical and chemical properties of the raw material, powder layer properties directly affect the processing window: by changing laser absorption properties (with density); by affecting the melt pool dynamics and influencing pores formation; etc
In that respect, it is intuitive to assume that “good” natural powder flowability is required to facilitate layer recoating. To describe flowability for SLM, it is necessary to define meaningful parameters and to establish their acceptable range of values.
Various different techniques are used for general powder flow measurement [4-7]. Few of them are meaningful in the context of AM: the Hausner ratio HR, defined as the ratio between tapped and bulk density  is widely used and described in ASTM D7481-09 . The angle of repose defined in ISO-4490  /ASTM B213  is described as the slope angle of the cone formed by powder particles after they’ve flown freely through a funnel onto a plate. It is generally considered as a measure for powder flowability and recommended by ASTM as the characterisation method for metal powders for AM .
In the context of AM technology, two main methods are typically used to assess powder flowability :
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.
Qualitative method: optical assessments
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.
Quantitative method: avalanche angle with powder revolution analyser
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 .
Before testing the flowability, a fluidisation cycle is commonly used to normalise the powder condition in the drum: this gets rid of gravity (the powder ‘settles’) and handling effects.
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.
The aim is to derive a quantitative assessment of the flowability in order to evaluate the usability of powders in powder bed fusion AM. This is done by evaluating the results of optical assessment, Hausner ratios HR, volume expansion ratio, avalanche angle and surface fractal.
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.
To sum up
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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