Lunch Break

The flow properties of a powder are the result of a complex interplay between the characteristics of the grains (size, morphology, water content, crystallinity, etc.) and the interactions between the grains (friction forces, van der Waals interactions, electrostatic forces, capillary forces). In order to control and to optimize processing methods, these powders have to be precisely characterized from different perspectives. In particular, 3D printing applications involving powder require precise, reproducible and appropriate powder characterization methods. In order to obtain thin and homogenous powder layers, a compromise between grain size and flowability has to be found. Unfortunately, when the grain size decreases, the cohesiveness increases and the flowability decreases. Moreover, the powder becomes sensitive to moisture and electrostatic charges. In addition, it is important to reach an optimal density to improve the mechanical properties of the produced part.

We present a methodology [1] based on a set of complementary instruments to perform this characterization: (i) an improved repose angle measurement, (ii) a flowmeter based on the rotating drum geometry, (iii) an automated tapped density measurement, (iv) a laboratory silo and (v) a powder tribo-charger. The classical repose angle measurement method has been revisited to measure powder cohesiveness from the heap irregularities. The rotating drum method is the dynamical version of this cohesiveness measurement to obtain rheological information. The tapped density measurement method has been automated to measure precisely the bulk density, the tapped density and also the compaction dynamics of a powder. The classical laboratory silo handled manually has been automated to obtain Beverloo flow curve. Finally, the tribo-charger measures the ability of a powder to create electrostatic charges (leading to cohesiveness) during a flow in contact with a selected material.

We show how the obtained set of results can be used to solve practical problems encountered in powder-bed based additive manufacturing processes [2].

[1] Measuring the flowing properties of powders and grains, Powder Technology 224, 19–27 (2012)

[2] Rheological behavior of β-Ti and NiTi powders produced by atomization for SLM production of open porous orthopedic implants, Powder Technology 283, 199–209 (2015)