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GranuDrum measurement reproducibility has been assessed by NIST!
In powder bed based additive manufacturing processes, the need for a reproducible powder characterization method is very urgent. Learn in this blog article why the GranuDrum secures measurement reproducibility.
Measurement reproducibility is the key to characterize powder
In powder bed based additive manufacturing (AM) processes, the need for a reproducible powder characterization method is very urgent. Different measurement geometries are available: rheometers with a rotating blade inspired by liquid rheometer, the shear cells working at high consolidation, and the rotating drum powder flow tester. In addition, old powder flowmeters like the Hall funnel flowmeter are still commonly used, but they give only limited information. The rotating drum geometry has been studied deeply in the granular material physics community and fits perfectly with AM processes because of the high flow speed, the low consolidation, and the free powder interface. But the assessment of the measurement reproducibility when the same powder is analyzed in different locations by different operators was still an open question. With an independent study, the NIST (National Institute of Standards and Technology) answered this question [1].
NIST studied how to guarantee powder flow measurement reproducibility
To perform the investigation, the NIST selected four powder bed fusion (PBF) 17-4 stainless-steel powders. One common commercial powder and three different powders with customized PSDs (Coarse, Medium and Fine). The samples were measured by GranuDrum with a sequence of 7 rotational velocities ranging from 2 RPM to 30 RPM. The recommendation of using high rotating speeds is one of the main conclusions of the study, in perfect agreement with GranuTools approach.
The powder-air interface behavior can be characterized in many ways. The metrics used in the study are the two main parameters measured by GranuDrum: the dynamic angle of repose (˚) and the cohesive index (unitless). The cohesive index is an original parameter developed by GranuTools to quantify powder cohesion (related to van der Walls, electrostatic and capillary forces) on the flow [2]. Therefore, GranuDrum is much more than an automated angle of repose measurement device. In the framework of AM, the cohesive index is directly linked to the homogeneity of the successive layers [3].
The variability of the measurement has been investigated rigorously. The difference in inter-sample variability and repetitive testing variability is found to be negligible, considering a negligible difference as an overlap between one standard deviation and the average value of the compared results.
The effect of drum cleaning methods and of sample mass variations has been also investigated leading to useful recommendations. The cleaning method has a low impact on the reproducibility. At the opposite, keeping the sample mass constant minimizes drastically the measurement fluctuations from one sample to the other. This straightforward recommendation is easy to implement in the measurement protocol.
The effect of particle size distributions has been analyzed. Coarse, Medium and Fine powders are well discriminated when considering both parameters (flowing angle and cohesive index) on the whole range of rotating speeds because they are showing significantly different rheologies. This differentiation would not be possible with a measurement method considering only one applied stress.
Conclusion: GranuDrum secures measurement reproducibility in powder characterization
In addition to being a measurement method in line with AM applications, GranuDrum is a reproducible measurement method. Moreover, thanks to this deep analysis performed by NIST, we have a set of interesting recommendations to perform precise measurements.
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References
[1] REPEATABILITY AND SENSITIVITY OF A ROTATING DRUM METHOD FOR RHEOLOGICAL CHARACTERIZATION OF STAINLESS STEEL POWDERS USED FOR ADDITIVE MANUFACTURING, J.G. Whiting, V. N. Tondare, S. P. Moylan, Proceedings of the 33rd Annual International Solid Freeform Fabrication Symposium https://www.nist.gov/publications/repeatability-and-sensitivity-rotating-drum-method-rheological-characterization
[2] Measuring powder flow properties in a rotating drum, A. Neveu, F. Francqui, and G. Lumay, Measurement 200, 111548 (2022)
[3] Rheological behavior of β-Ti and NiTi powders produced by atomization for SLM production of open porous orthopedic implants, G. Yablokova, M. Speirs, J. Van Humbeeck, J.-P. Kruth, J. Schrooten, R. Cloots, F. Boschini, G. Lumay, J. Luyten, Powder Technology 283, 199–209 (2015).
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