Linking powder electrostatic & flowing behaviour to its C / O / S & N content

Introduction

Generalities

Granular materials and fine powders are widely used in industrial applications.
To control and to optimize processing methods, these materials have to be precisely characterized. The characterization methods are related either to the properties of the grains (granulometry, morphology, chemical composition, …) and to the behaviour of the bulk powder (flowability, density, blend stability, electrostatic properties, …). However, concerning the physical behaviour of bulk powder, most of the techniques used in R&D or quality control laboratories are based on old measurement techniques.

During the last decade, we have updated these techniques to meet the present requirements of R&D laboratories and production departments. In particular, the measurement processes have been automatized and rigorous initialization methods have been developed to obtain reproducible and interpretable results. Moreover, the use of image analysis techniques improves the measurements precision. A range of measurement methods has been developed to cover all the needs of industries processing powders and granular materials.

However, in this application only the GranuDrum and GranuCharge instruments will be presented:

• The GranuDrum to measure flowing properties (Flowing angle, dynamic cohesive index, first avalanche angle and powder aeration)
• The GranuCharge to measure powder electrostatic properties.

GranuDrum

The GranuDrum instrument is an automated powder flowability measurement method based on the rotating drum principle.

A horizontal cylinder with transparent sidewalls called drum is half filled with the sample of powder. The drum rotates around its axis at an angular velocity ranging from 2 rpm to 60 rpm. A CCD camera takes snapshots (30 to 100 images separated by 1s) for each angular velocity. The air/powder interface is detected on each snapshot with an edge detection algorithm.

Afterwards, the average interface position and the fluctuations around this average position are computed. Then, for each rotating speed, the flowing angle (also known in the literature as ‘dynamic angle of repose’) αf is computed from the average interface position and the dynamic cohesive index σf is measured from the interface fluctuations.

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In general, a low value of the flowing angle αf corresponds to a good flowability. The flowing angle is influenced by a wide set of parameters: the friction between the grains, the shape of the grains, the cohesive forces (van der Waals, electrostatic and capillary forces) between the grains. The dynamic cohesive index σf is only related to the cohesive forces between the grains. A cohesive powder leads to an intermitted flow while a non-cohesive powder leads to a regular flow. Therefore, a dynamic cohesive index closes to zero corresponds to a non-cohesive powder.

When the powder cohesiveness increases, the cohesive index increases accordingly.

In addition to the measurement of both the cohesive index σf and the flowing angle αf as a function of the rotating speed, the GranuDrum allows to measure the first avalanche angle and the powder aeration during the flow.

GranuCharge

Electrostatic charges are created inside a powder during a flow. This apparition of electric charges is due to the tribo-electric effect, which is a charge exchange at the contact between two solids.

During the flow of a powder inside a device (mixer, silo, conveyor, …), the tribo-electric effect takes place at the contact between the grains and at the contact between the grains and the device. Therefore, the characteristics of the powder and the nature of the material used to build the device are important parameters.

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The GranuCharge instrument measures automatically and precisely the quantity of electrostatic charges created inside a powder during a flow in contact with a selected material. The powder sample flows inside a vibrating V-tube and fall in a Faraday cup connected to an electrometer. The electrometer measures the charge acquired by the powder during the flow inside the V-tube.

In order to obtain reproducible results, a rotating or a vibrating device is used to feed the V-tube regularly.

Selected Powders

A Stainless-Steel powder provided by Höganäs company was used during this study:

	d10 (µm)
	27.46
	d50 (µm)
	43.03
	d90 (µm)
	62.44

Two different version of this powder are available: the original one (as provided by the supplier) and a recycled one, obtained after a Selective Laser Melting (SLM) process in a 3D printer. However, these powders were stocked in our laboratory during several months. Therefore, the history of the samples is unknown and their properties could not correspond to the specifications of the producers for fresh powders.

GranuDrum Analysis

Experimental Protocol

For an experiment with the GranuDrum, powders were poured inside the measuring cell just after box opening. The quantity of powder used was approximately 50ml. Every powder was analysed under standard conditions (23%RH and 21°C). Twenty GranuDrum velocities were investigated (from 2 to 60rpm) and for each velocity, 50 pictures were taken to increase the accuracy/repeatability of measurement.

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Experimental Results

All presented measurements were performed by increasing the Drum speed and then by decreasing it (the main purpose of this step is to highlight a granulation phenomenon, i.e., a thixotropic behaviour). The dynamic Angle of Repose quantify powder flowability (it considers the three cohesive forces: Van der Waals, Electrostatic and capillary, but also the contact forces between grains). The Cohesive Index is linked to the fluctuations of the interface (powder/air) position, and it only represents the three contact forces. Thus, the Dynamic Angle of repose characterize powder flow-ability, while Cohesive Index quantify powder spreadability.

Discussion

First, no thixotropic behaviour can be highlighted (by increasing and decreasing the drum speed). Therefore, these samples are not sensitive to agglomeration. Due to its original algorithm images treatment, i.e. the Cohesive Index, powders spreadability can be achieved with great accuracy (1.8%). Indeed, for high velocities (above 45rpm, i.e. 200mm/s, see Appendix 1 for speed conversion), we can see that the virgin powder Cohesive Index is lower than the Used sample, therefore, it is better to use the original powder for a recoater/SLM process than the used one (G. Yablokova et al., 2015). Moreover, Figure 2 allows to make process speed optimisation. Indeed, if one wants to achieve a good spreadability at high speed, we can deduce that a recoater speed around 170mm/s is the perfect choice, because the Cohesive Index is acceptable and the process speed is high enough.

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Figure 3 informs us about powders flow-ability. First, bellow 20 rpm it is difficult to make a differentiation between one powder to another, this observation may be due to the fact that these powders have similar flowing behaviour. Secondly, above 20 rpm, it is possible to deduce that the flow-ability of the virgin powder is better than the used one. Indeed, at 50rpm, the virgin powder Dynamic Angle of Repose is close to 60°, while it is equal to 65° for the used sample. Finally, one possible explanation for the flow-ability / spreadability differences between the virgin and used powder, may be the tribo-electric effect. Indeed, it is possible that the used powder is more sensitive to electrical charges, especially at high speed. Thus, to confirm this hypothesis, several tests were carried out with the GranuCharge instrument.

Carbon, Sulphur, Oxygen and Nitrogen content analysis

Materials and methods

The Carbon, Sulphur, Oxygen and Nitrogen content of the samples were measured by the LECO center (LECO European Application and Technology Center, 10589 Berlin, Germany) using the CS744 and ONH836 Elemental Analysers. All the measurements were repeated three times (n = 3). The typically sample mass was approximately 1g.

Experimental Results

The following figures represent the Carbon / Sulphur and Oxygen / Nitrogen content measured for the virgin and recycled Stainless-Steel powders. The standard deviation is also displayed:

Discussion

The LECO results can be summarised by the following table:

The Carbon and Sulphur analysis is highly interesting. Indeed, it highlights that the Selective Laser Melting process slightly modify the composition of the powders. We can see that the carbon and sulphur content is more important for the used powder (C = 0.017% and S = 0.00697%) compared to the virgin one (C = 0.014% and S = 0.00565%). Moreover, the Oxygen and Nitrogen content of the used powder (O = 0.0767% and N = 0.0709%) is also more important than the virgin one (O = 0.0629% and N = 0.0563%). These observations are interesting, indeed, despite the fact that the SLM is conducted under inert atmosphere, it is impossible to avoid powders oxidation to happen.

GranuCharge Analysis

Experimental Protocol

The tribo-electric effect of the powders was investigated with the help of the GranuCharge instrument. For each experiment with the GranuCharge, Aluminium / Stainless-Steel 316L pipes and rotating feeder were used (cf. Figure 10):

The quantity of powder used for each measurement was 50mL and the powder was not reused after a measurement. Tests have been repeated four times and the average value was plotted. All powders were analysed under standard conditions (38%RH and 26°C).

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At the beginning of the test, the initial powder charge density (qi, in nC/g) is measured by introducing powder inside the Faraday cup. Once this step is completed, the powder is poured inside the rotating feeder, and then the experiment starts. The final charge density is measured at the end of experiment (qf, nC/g). Table 2 summarizes all the results obtained with the GranuCharge instrument. Each charge density value corresponds to the average value calculated between the four tests (in μC/kg and σ is related to the standard deviation obtained for every tests):

Results and Interpretation

Figure 11 is interesting, because it shows that with the GranuCharge instrument, differentiations between powders can be made with great accuracy (best accuracy is 3.4%). Even with the initial charge densities are found to be significantly different from one powder to another. Indeed, for the virgin powder, initial charge density is the highest (-0.017nC/g), while for the recycled sample, is the lowest with a charge density equal to -0.011nC/g. Moreover, the samples charges densities are negative, therefore, they are anionic powders.

After a flow in contact with aluminium and Stainless-Steel 316L pipes, the electrical charges build up by the powders are also negatives. However, whatever the pipes material used, the electrical charge builds up is higher for the virgin powder than the used one. Thus, the virgin sample is more sensitive to the triboelectric effect. One possible explanation for these observations may be provided by the Carbone analysis. Indeed, the carbon content is higher for the used powder compared to the virgin one. Moreover, carbon is known to be an anti-static agent. Therefore, an increase in the powder carbon content may lead, not only, to a decrease in the initial electrical charge, but also, to a lower charge density build up after a flow. However, we need to be careful with the previous conclusion.

Indeed, the recycling process after a SLM operation may lead to several complex modifications in the powder physico-chemical properties.
Changes such has Particles Size Distribution / shape and surface properties may also occur and making results interpretation difficult to achieve.

Conclusion

• ✓ Ageing of metallic powders for AM is difficult to highlight with classical methods (Hall flowmeter, Densi-tap) due to their bad reproducibility and user dependency.
• ✓ However, with the GranuTools and LECO instruments, it is possible to follow with great accuracy the recycling after a SLM process.
• ✓ Indeed, with the GranuDrum instrument it is possible to conclude about the flowability and spreadability of a virgin and used SS 316L powder (accuracy close to 2%). We have shown that the flowability and spreadability of the virgin powder is better than the used one.
• ✓ With the GranuCharge analysis we have seen that the virgin powder is more sensitive to electrical charge (the best achieved accuracy was 3.4%). However, using aluminium as a process material can decrease the electrical charges build up, especially, with a recoater as a way to spread the powder.
• ✓ The GranuCharge results seems to be confirmed by the LECO analysis with the Carbon, Sulphur, Oxygen and Nitrogen contents (CS 744 and ONH 836 instruments). These measurements allow us to say that the Carbon, Sulphur, Oxygen and Nitrogen contents are higher for the recycled powder than the virgin one. Since Carbon is an anti-static agent, it can be a possible explanation about the GranuCharge analysis.

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References

Cascade of granular flows for characterizing segregation, G. Lumay, F. Boschin, R. Cloots, N. Vandewalle, Powder Technology 234, 32-36 (2013)

Combined effect of moisture and electrostatic charges on powder flow, A. Rescaglio, J. Schockmel, N. Vandewalle and G. Lumay, EPJ Web of Conferences 140, 13009 (2017)

Compaction dynamics of a magnetized powder, G. Lumay, S. Dorbolo and N. Vandewalle, Physical Review E 80, 041302 (2009)

Compaction of anisotropic granular materials: Experiments and simulations, G. Lumay and N. Vandewalle, Physical Review E 70, 051314 (2004)

Compaction Dynamics of Wet Granular Assemblies, J. E. Fiscina, G. Lumay, F. Ludewig and N. Vandewalle, Physical Review Letters 105, 048001 (2010)

Effect of an electric field on an intermittent granular flow, E. Mersch, G. Lumay, F. Boschini, and N. Vandewalle, Physical Review E 81, 041309 (2010)

Effect of relative air humidity on the flowability of lactose powders, G. Lumay, K. Traina, F. Boschini, V. Delaval, A. Rescaglio, R. Cloots and N. Vandewalle, Journal of Drug Delivery Science and Technology 35, 207-212 (2016)

Experimental Study of Granular Compaction Dynamics at Different Scales: Grain Mobility, Hexagonal Domains, and Packing Fraction, G. Lumay and N. Vandewalle, Physical Review Letters 95, 028002 (2005)

Flow abilities of powders and granular materials evidenced from dynamical tap density measurement, K. Traina, R. Cloots, S. Bontempi, G. Lumay, N. Vandewalle and F. Boschini, Powder Technology, 235, 842-852 (2013)

Flow of magnetized grains in a rotating drum, G. Lumay and N. Vandewalle, Physical Review E 82, 040301(R) (2010)

How tribo-electric charges modify powder flow-ability, A. Rescaglio, J. Schockmel, F. Francqui, N. Vandewalle, and G. Lumay, Annual Transactions of The Nordic Rheology Society 25, 17-21 (2016)

Influence of cohesives forces on the macroscopic properties of granular assemblies, G. Lumay, J. Fiscina, F. Ludewig and N. Vandewalle, AIP Conference Proceedings 1542, 995 (2013)

Linking compaction dynamics to the flow properties of powders, G. Lumay, N. Vandewalle, C. Bodson, L. Delattre and O. Gerasimov, Applied Physics Letters 89, 093505 (2006)

Linking flowability and granulometry of lactose powders, F. Boschini, V. Delaval, K. Traina, N. Vandewalle, and G. Lumay, International Journal of Pharmaceutics 494, 312–320 (2015)

Measuring the flowing properties of powders and grains, G. Lumay, F. Boschini, K. Traina, S. Bontempi, J.-C. Remy, R. Cloots, and N. Vandewalle, Powder Technology 224, 19-27 (2012)

Motion of carbon nanotubes in a rotating drum: The dynamic angle of repose and a bed behavior diagram, S. L. Pirard, G. Lumay, N. Vandewalle, J-P. Pirard, Chemical Engineering Journal 146, 143-147 (2009)

Mullite coatings on ceramic substrates: Stabilisation of Al2O3–SiO2 suspensions for spray drying of composite granules suitable for reactive plasma spraying, A. Schrijnemakers, S. André, G. Lumay, N. Vandewalle, F. Boschini, R. Cloots and B. Vertruyen, Journal of the European Ceramic Society 29, 2169–2175 (2009)

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)

The flow rate of granular materials through an orifice, C. Mankoc, A. Janda, R. Arévalo, J. M. Pastor, I. Zuriguel, A. Garcimartín and D. Maza, Granular Matter 9, p407–414 (2007)

The influence of grain shape, friction and cohesion on granular compaction dynamics, N. Vandewalle, G. Lumay, O. Gerasimov and F. Ludewig, The European Physical Journal E (2007)

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