AM Powders Flowability and the Influence of Particle Size Distribution and Shape

Fine powders and granular materials are extensively employed in industrial applications. These materials have to be properly characterized to control and to improve the processing techniques. The characterization techniques are related to the grains’ properties such as granulometry, chemical composition, morphology, etc., or to the bulk powder behavior such as density, flowability, electrostatic properties, blend stability, and so on.

Conversely, with regards to the physical behavior of bulk powder, the majority of the methods employed in quality control and research and development laboratories are based on conventional measurement methods. Over the last 10 years, these methods have been updated to fulfill the current requirements of production departments and research and development laboratories.

The measurement processes, in particular, have been automatized and stringent initialization techniques have been devised to achieve interpretable and reproducible results. Furthermore, the precision of the measurements is improved using image analysis methods.

An array of measurement techniques has been developed to meet all the requirements of industries processing granular materials and powders. Conversely, only the GranuCharge and GranuDrum instruments will be described in this article:

• GranuCharge for measuring the electrostatic properties of powder
• GranuDrum for measuring flowing properties like dynamic cohesive index, flowing angle, powder aeration, and first avalanche angle

GranuDrum

GranuDrum instrument serves as an automated powder flowability measurement technique predicated on the principle of the rotating drum. A horizontal cylinder featuring transparent sidewalls known as the drum is half filled with the powder sample. The drum spins around its axis at an angular velocity ranging between 2 and 60 rpm. Snapshots (30–100 images separated by 1 second) for each angular velocity are taken by a CCD camera. An edge detection algorithm is used to detect the air/powder interface on each individual snapshot, followed by calculating the average interface position and the variations around this average position.

Subsequently, the flowing angle α_f — also called “dynamic angle of repose” in the literature — for each rotating speed is calculated from the average interface position, and the dynamic cohesive index σ_f is determined from the interface fluctuations.

Generally, a low value of the flowing angle α_f relates to a good flowability. A wide set of parameters — such as the shape of the grains, the friction between the grains, and the cohesive forces (capillary, electrostatic and van der Waals forces) between the grains — affect the flowing angle. The dynamic cohesive index σ_f is only associated with the cohesive forces existing between the grains. Moreover, a cohesive powder and a non-cohesive powder lead to an intermitted flow and a regular flow, respectively. As a result, a dynamic cohesive index close to zero relates to a non-cohesive powder. The increase in the powder cohesiveness is directly proportional to the increase in the cohesive index.

Besides the measurement of both the flowing angle α_f and the cohesive index σ_f as a function of the rotating speed, the GranuDrum instrument enables measuring the powder aeration and the first avalanche angle at the time of the flow.

GranuPack

For powder characterization, parameters like the tapped density, the bulk density, and the Hausner ratio measurement (often called “taptap test”) are quite popular because the measurement is both simple and fast. In addition, the density and the potential of powder to boost its density are equally significant parameters for transportation, storage, caking, and so on. The pharmacopeia has defined the proposed procedure.

Although this test is simple, it has three main disadvantages. Firstly, the measurement result is dependent on the operator. The filling technique indeed affects the initial powder volume. Secondly, measuring the volume through the naked eye introduces major errors in the results. Finally, the compaction dynamics between the initial and the final measurements are entirely missed with this simple technique.

The GranuPack instrument is an improved and automated tapped density measurement technique predicated on the latest basic research results. An automatized device is used to analyze the behavior of the powder submitted to successive taps. The initial density ρ(0), the final density after n taps ρ(n), and the Hausner ratio Hr are precisely determined. The tap number is generally fixed at n = 500. An extrapolation of the maximum density ρ(∞) and a dynamical parameter n_1/2 are also derived from compaction curves. While additional indexes can be used, they are not described in this article.

Using a stringent automated initialization process, the powder is first placed in a metallic tube and then a light hollow cylinder is mounted on top of the powder bed to keep the air/powder interface flat at the time of the compaction process. The tube filled with the powder sample rises up to a fixed height of ΔZ and carries out free falls. The height of the free fall is usually fixed to ΔZ = 1 mm or ΔZ = 3 mm. After each tap, the height h of the powder bed is automatically determined. The volume V of the pile is calculated from the height h of the powder.

Since the powder mass m is known, the density ρ is assessed and plotted following each tap. Here, the density refers to the ratio between the powder bed volume V and the mass m. In the case of the GranuPack method, reproducible results are obtained with a small amount of powder (usually 35 mL). The Hausner ratio Hr is associated with the compaction ratio and is computed by the equation Hr = ρ(500)/ρ(0), where ρ(0) refers to the initial bulk density and ρ(500) refers to the computed tapped density achieved after 500 taps.

GranuCharge

During a flow, electrostatic charges are generated inside a powder. This apparition of electric charges is attributed to the triboelectric effect, in which charge exchange takes place at the contact between two solids. When a powder flows inside a device (such as a conveyor, silo, mixer, and so on), the triboelectric effect occurs at the contact between the device and the grains and at the contact between the grains. Therefore, the properties of the powder, as well as the nature of the material used for developing the device, are considered as vital parameters.

Figure 1. GranuCharge

The GranuCharge instrument precisely and automatically determines the quantity of electrostatic charges that are generated within a powder during a flow in contact with a chosen material.

After flowing inside a vibrating V-tube, the powder sample reaches a Faraday cup linked to an electrometer, which, in turn, determines the charge obtained by the powder at the time of the flow within the V-tube. To achieve reproducible results, a vibrating or a rotating device is used for feeding the V-tube on a regular basis.

The triboelectric effect is essentially a result of one object acquiring electrons on its surface, and thus turning out to be negatively charged, and another object losing electrons, thereby turning out to be positively charged.

The relative tendencies of the materials involved to lose or gain electrons decide which material becomes positive and which becomes negative. Certain materials have a greater tendency to gain electrons compared to others, in the same way that others have a tendency to lose electrons more easily. The triboelectric series was developed (see Table 1) to represent these trends. It lists materials that have a tendency to charge positively and others that have a tendency to charge negatively. Materials that do not have a tendency to behave either way are listed in the middle of the table. Yet, this table only provides information related to materials charging behavior tendency. For this reason, the GranuCharge instrument was developed to provide exact numerical values related to the powders’ charging behavior.

Table 1. The triboelectric series

Selected Powders

Powders Description

For this study, two bronze powders — called sample A and B — were chosen, which were supplied by the Retsch company.

Figure 2. Sample A

Figure 3. Sample B

All powders are brown/orange in color; however, sample A is darker compared to sample B. No bag of silica gel was found inside the powders boxes to avoid humidity effect.

Particles Size Distribution and Shape Analysis

Analysis and characterization of the particles size distribution (PSD) and shape were performed by the Retsch company through the CAMSIZER X2 instrument — a robust and highly versatile particle analyzer with a broad measuring range combining next-generation camera technology with flexible dispersion options. The CAMSIZER X2 instrument, based on the principle of dynamic image analysis, offers precise information about particle size and shape of suspensions, granules, and powders in a measuring range between 0.8 μm and 8 mm.

Figure 4. CAMSIZER X2

Figure 5. Particles shape analysis (sphericity)

The previous figure enables reaching a conclusion related to particles sphericity. Obviously, Sample B, with 30% of particles with sphericity above 0.95, is mostly composed of spherical particles, while the shape of the sample A particles is more “chaotic” with 95% of particles with sphericity below 0.95.

Figure 6. Particle Size Distribution

This particle size distribution is considered as interesting since it helps in concluding that sample B (with a d50 close to 48 μm) is coarser compared to sample A (d50 close to 38 μm).

It was emphasized by Retsch preliminary observations that the flowabilities of the samples in quasi-static conditions are almost the same.To prove these assumptions, three GranuTools instruments will be used during this study: the GranuPack (tapped density), the GranuDrum (rheometer for powder) and the GranuCharge (triboelectric effect measurements).

GranuPack Analysis

Experimental Protocol

For every experiment performed with the GranuPack instrument, 1000 taps were applied to the sample with 1 Hz tap frequency, with the measurement cell free-fall being 1 mm (∝ tap energy). Hygrometry and air temperature are recorded prior to an experiment. Certain samples were investigated twice. The objective of this experiment is to demonstrate the high precision of the GranuPack instrument as well as to emphasize the aging of powders and how it affects their flowability.

Experimental Data

Before each experiment, the powder mass is recorded and the sample is subsequently poured within the measurement cell by following the software instructions (that is, without user dependency). Under the same moisture conditions (21 °C and 45% RH), the bulk densities of powders were analyzed.

The full compaction curves, shown in Figure 7, are the bulk density (ρ(n)) vs. the number of taps. However, the error bars shown are too small to be perceptible (bulk density error is around 0.4%).

Figure 7. Bulk density versus tap number for polyamide 2200 powders

Table 2 shows the complete results, in which ρ(500) is the bulk density after 500 taps (g/mL). ρ(0) is the initial bulk density (in g/mL) and ρ(∞) is the optimal bulk density (in g/mL), which is computed by a model provided in the GranuPack software and predicts the minimum density that can be achieved by the tapping test.

Cr and Hr are the Carr and Hausner ratios, and τ and n_1/2 are two parameters connected to the compaction kinetic (cf. Appendix 1).

Table 2. GranuPack results: comparison between all powders

Results Interpretation

With regards to the initial bulk densities, these samples are seen to be extremely different. Obviously, sample A bulk density is close to 4.638 g/mL, while it is equivalent to 5.360 g/mL in the case of sample B. The trends are the same for the tapped densities (after 1000 taps), that is, sample A tapped density (5.144 g/mL) is lower compared to sample B (5.844 g/mL).

For the compaction dynamic (n_1/2 parameter), the compaction can be deduced to be faster for sample B (average n_1/2 = 58.25) compared to sample A (average n_1/2 =79.47). Conversely, the Retsch observations in relation to the flowability are established with the Hausner ratio, and with regards to the measurement accuracy (0.8% on the Hausner ratio), the products flowabilities are found to be similar (Hr close to 1.100).

GranuDrum Analysis

Experimental Protocol

For an experiment performed with the GranuDrum instrument, powders were dispensed inside the measuring cell soon after opening the box. The amount of powder used was roughly 50 mL. Standard conditions (45% RH and 22 °C) were used to investigate every powder. Next, 20 GranuDrum velocities were analyzed (from 2 to 60 rpm) and for every velocity, 40 pictures were obtained to boost the measurement’s accuracy and repeatability.

Experimental Results

The cohesive index and the dynamic angle of repose as a function of the GranuDrum rotating speed are shown in Figure 8. All the presented measurements were carried out by increasing the speed of the drum and then by decreasing it (the chief purpose of this step is to emphasize a granulation phenomenon, that is, a thixotropic behavior).

Figure 8. Cohesive index versus rotating drum speed

The cohesive index — associated with the variations of the interface (air/powder) position — only indicates the three contact forces. Therefore, the cohesive index measures powder spreadability.

Results Interpretation

To begin with, when considering a low velocity (2 rpm), the classification of the powders flowability is rather difficult to obtain. This observation proves the GranuPack conclusions (with the Hausner ratio). Yet, when the speed of the drum increases, the products can be easily characterized.

A shear-thickening behavior is shown by sample B, which means its spreadability decreases when the applied shear stress becomes increasingly significant. Perhaps, this fact can be attributed to its spherical shape. The maximum measured cohesive index, however, is close to 8 at 60 rpm, and therefore, the product spreadability is excellent across the entire velocity range. Considering that the particle size distribution is inversely proportional to flowability, observations like these are in good agreement with the characterization of particles described before.

The behavior of Sample A is indeed more complicated between 2 and 16 rpm, highlighting a shear-thickening trend, and the powder spreadability increases between 16 and 40 rpm. Above 40 rpm, another shear-thickening behavior is finally determined, leading to a decrease in the powder spreadability. Such a complicated behavior could be linked to the particles shape, which is further away from a sphere. Finally, this product highlights a lower flowability/spreadability owing to its lower particle shape distribution (compared to sample B).

Such highly interesting results, particularly for sample A, indeed shows that the GranuDrum measurements help in defining an optimal recoater, that is, to acquire the optimal spreadability at the optimal process speed. It was seen that this value is close to 40 rpm, and thus to 170 mm/second (see Appendix 2). Moreover, observations made on powders cohesive index at low speed (that is 2 rpm) are in good compliance with Retsch thoughts and GranuPack data interpretations.

GranuCharge Analysis

Experimental Protocol

The powders’ triboelectric effect was analyzed using the GranuCharge instrument. For every experiment performed with the GranuCharge instrument, vibrating feeder and aluminum/stainless-steel 316L pipes were used. For each measurement, 25 mL of powder was used and the same was not reused following a measurement. All powders were examined under standard conditions (21 °C and 50% RH).

Figure 9. Photography of the vibrating/rotating feeder and stainless-steel pipes

At the start of the test, the initial powder charge density (q_i, in nC/g) is determined by pouring the powder within the Faraday cup. After completing this step, the powder is introduced inside the rotating feeder, and the experiment is then started. Toward the end of the experiment, the final charge density is determined (q_f, nC/g).

The GranuCharge instrument was used to obtain all the results summarized in Table 3. Δq = qf − q₀, in nC/g and %lost is the percentage of particles entrapped in the GranuCharge pipes.

Table 3. Synthesis of the results obtained with the GranuCharge instrument

Figure 10. Histogram of the comparison between the initial and final charge densities for every powder.

Results Interpretation

With regards to initial charge measurements, sample B (q₀ = 0.136 nC/g) is seen to be more charged compared to sample A (q₀ = 0.042 nC/g). In addition, the trends are the same after a flow in contact with the stainless-steel 316L pipes, which means that the build-up of electrical charges is higher for sample B (Δq = −0.099 nC/g) compared to sample A (Δq = −0.063 nC/g).

Furthermore, all the powders are also cationic (initially positively charged); however, they obtain a negative amount of charges upon flowing inside the stainless-steel 316L pipes.

Particle size distribution could be responsible for the build-up of a higher amount of charges for sample B (compared to sample A). Obviously, a lower particle size distribution translates to a higher specific area, and as a result, more friction between the pipes and particles surface at the time of the flow.

Conclusions

Two samples were chosen during this analysis. Both were bronze powders — one with huge particle size and spherical shape, and the other with small particles and a non-spherical shape. Yet, in spite of these variations, a conclusion related to powder flowability in quasi-static conditions could not be reached. Such observations were validated through the GranuPack and GranuDrum instruments because the products underline comparable Hausner ratio and cohesive index (at 2 rpm) values. Nevertheless, a number of differences were seen when operating at higher speeds:

1. Smallest particle size distribution demonstrates the lower cohesive index, and thus the best spreadability. Moreover, a shear-thickening behavior is emphasized due to the spherical shape.
2. The lowest spreadability is exhibited by the biggest particle size distribution (because of its highest cohesive index), which also exhibits an intricate rheological behavior as a result of its non-spherical shape.

Appendix

Appendix 1: GranuPack Theoretical Background

The dynamical parameter n_1/2 corresponds to the number of taps needed to reach one-half of the compaction curve.

The compaction curve is fitted by a theoretical model to obtain the characteristic tap number τ.

Appendix 2: Relation Between Drum Rotating Speed and Process Speed (in mm/second)

Figure 11. Relation between drum rotating speed and process speed (in mm/second).

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This information has been sourced, reviewed and adapted from materials provided by Granutools.

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