Application Note using:
The influence of particle size distribution on metallic powders spreadability and triboelectricity
Granular materials and fine metallic powders are widely used in many industries (3D Printing, sintering, alloys, …). To control and to optimize processing methods, these materials must be precisely characterized. The characterization methods are related either to the properties of the grains (size, chemical composition, …) and to the behaviour of the bulk powder (flowability, shape, density, electrostatic properties, …).
During some processes, powders need to be conveyed inside a pipe, however during this step, electrostatic charges may appear due to the triboelectric effect and lead to the formation of agglomerates and/or to the powder sticking on the pipe surface. In this paper, a new instrument able to measure with great accuracy (± 0.5nC) the electrical charges acquired by powders during a flow in contact with various pipe materials is presented. The GranuDrum device will be used in order to asses powder spreadability.
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. 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, GranuDrum allows to gmeasure the first avalanche angle and the powder aeration during the flow.
Electrostatic charges are created inside a powder during a flow. This apparition of electric charges is due to the triboelectric 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 triboelectric 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.
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.
The triboelectric effect is a result in one object gaining electrons on its surface, and therefore becoming negatively charged, and another object losing electrons, thus, becoming positively charged.
Which material becomes negative and which becomes positive depend on the relative tendencies of the materials involved to gain or lose electrons. Some materials have a greater tendency to gain electrons than others, in the same manner that others tend to lose electrons easier. To represent these trends, the triboelectric series was developed (Table 1). It lists materials with tendency to charge positively and others with tendency to charge negatively. In the middle of the table are listed materials that do not show tendency to behave either way.
However, this table only gives information about materials charging behaviour tendency. This is for this reason that the GranuCharge was developed: to give precise numerical values about powders charging behaviour.
Three different samples were selected. All these samples come from the same powder (AlSi10Mg) but from different batches with various particle sizes and shapes as shown in Table 2.
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 ambient conditions (43%RH and 22°C). At least 16 GranuDrum velocities were investigated (from 2 to 40-50rpm) and for each velocity, 50 pictures were taken to increase the accuracy/repeatability of measurement.
Over the whole speed range, sample C has a lower Cohesive index than other samples and powder A has the highest one. This observation allows to say that powder C will be easiest to spread with a recoater than all other samples. Thus, sample A will be the worst candidate for a recoater process.
Electrostatic Charges Measurements
For each experiment with the GranuCharge, the rotating feeder was used. Stainless-steel 316L pipes were selected. Every powder was analysed under standard conditions (40% Relative Humidity, RH and 22°C).
Measurements have been repeated three times and the average value was plotted. The quantity of product used for each measure was approximately 40ml and the powder was not recycled after a measurement.
Before the experiment, powder mass (mp, in g), is recorded. At the beginning of the test, the initial powder charge density (q0, in µC/kg) is measured by introducing powder inside the Faraday cup. Final charge density is also measured at the end of experiment (qf, µC/kg) and charge density variation (∆q) is then calculated. σ, represents the standard deviation calculated with the three repeatability tests. The following histogram summarizes all experimental results obtained with the GranuCharge instrument:
These results are highly interesting, because they allow powder to be classified. For every sample, the initial charge (q0, in nC/g) is positive, while, electrical charges after a flow (qf – q0, in nC/g) in contact with Stainless-Steel 316L pipes are negatives.
The comparison between a powder’s initial charges shows that before any flow occurs, Sample A is the most charged sample with a charge density close to 0.12nC/g. Sample B comes in second position (q0 = 0.9nC/g), and finally Sample C is the less charged powder with a charge density of 0.05nC/g. These differences may be due to the particle size distributions. Indeed, according to Coulomb law; the lower the particle diameter, the higher the electrical charges. Moreover, powder grain size is lower for Sample A than Sample C.
However, after a flow in contact with SS 316L tubes, the electrical charges build up by Sample A is the lowest, while it is similar (considering error bars) for Samples B and C. To explain these differences, two phenomena must be considered; powder flowability and specific area. Indeed, the higher the flowability, the strongest the friction between particles and pipes surface and also between two particles, which will lead to a higher charge build up. Moreover, particles with higher specific area are more in contact with pipes, and thus acquire more charge.
Table 2 shows that particles size distribution is higher for Sample C than Sample B and thus, specific area is lower for Sample C, while Figure 3: Cohesive index versus drum rotating speed for each sample. indicate that the spreadability for Sample C is better than Sample B. Finally, a combination of those two factors may explain why the electrical charges between Samples B and C are similar.
In this paper we have shown that the developed triboelectric effect measurement instrument (called GranuCharge) is able to make powder classification with great accuracy (0.5nC). With this instrument it is possible to calculate a powder’s initial charge and electrical charge build up in contact with various pipe materials. This measurement will be a good way to investigate and optimise powder ageing, especially in the additive manufacturing industries after a SLM processes.
The authors would like to thank IMR Metal Powder Technologies GmbH for providing the powders.
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