Additive manufacturing
Investigation of Powders Ageing for Additive Manufacturing Process
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.
Introduction
Theoretical Framework
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 note, we will be focused on the GranuFlow, GranuPack and GranuCharge instruments.
GranuFlow
The GranuFlow is an improved laboratory silo compared to the ancient Hall Flow Meter (ASTM B213, ISO4490)
and compared to the “Flow Through An Orifice” method described in the Pharmacopeia (USP1174). It is a straightforward powder flowability measurement device composed of a silo with different apertures associated with a dedicated electronic balance to measure the flowrate. This flowrate is computed automatically from the slope of the mass temporal evolution measured with the balance.
The aperture size is modified quickly and easily with an original rotating system. The measurement and the result analysis are assisted by software. The flowrate is measured for a set of aperture sizes to obtain a flow curve.
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Finally, the whole flow curve is fitted with the well-known Beverloo theoretical model to obtain a flowability index
(Cb, related to the powder flowability) and the minimum aperture size to obtain a flow (Dmin) (for theoretical background, user can refer to Appendix 1). The whole measurement is performed easily, fastly and precisely.
In this paper, we used a complete set of hole diameters: 4, 6, 8, 10, 12 and 14mm. The main purpose of this application note is to provide information about the measurements reproducibility with the GranuFlow and to show some examples about what it is able to offer. In a second part, a comparison between Hall Flowmeter and GranuFlow is presented in order to show the advantage of using GranuFlow.
GranuPack
The bulk density, the tapped density and the Hausner ratio measurement(commonly named “taptap test”)
is very popular for powder characterization because of both the simplicity and the rapidity of the measurement. Moreover, the density and the ability of a powder to increase its density are important parameters for storage, transportation, caking, etc. The recommended procedure is defined in the pharmacopeia.
This simple test has three major drawbacks
First, the result of the measurement depends on the operator. Indeed, the filling method influences the initial powder volume. Secondly, the volume measurements by naked eyes induce strong errors on the results. Finally, with this simple method, we completely miss the compaction dynamics between the initial and the final measurements.
The GranuPack instrument is an automated and improved tapped density measurement method based on recent fundamental research results. The behaviour of the powder submitted to successive taps is analysed with an automatized device. The Hausner ratio Hr, the initial density ρ(0) and the final density after n taps ρ(n) are measured precisely. The tap number is commonly fixed at n=500.
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Moreover, a dynamical parameter n1/2 and an extrapolation of the maximum density ρ(∞) are extracted from compaction curves. Additional indexes can be used but they are not presented in this report.
The powder is placed in a metallic tube with a rigorous automated initialization process. Afterwards, a light hollow cylinder is placed on the top of the powder bed to keep the powder/air interface flat during the compaction process. The tube containing the powder sample rose up to a fixed height of ΔZ and performs free falls. The free fall height is generally fixed to ΔZ = 1mm or ΔZ=3mm. The height h of the powder bed is measured automatically after each tap.
From the height h, the volume V of the pile is computed. As the powder mass m is known, the density ρ is evaluated and plotted after each tap. The density is the ratio between the mass m and the powder bed volume V.
With the GranuPack method, the results are reproducible with a small quantity of powder (typically 35 ml). The Hausner ratio Hr is related to the compaction ratio and is calculated by the equation Hr = ρ(500) / ρ(0), where ρ(0) is the initial bulk density and ρ(500) the tapped density computed obtained after 500 taps.
GranuCharge
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 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.
The GranuCharge instrument automatically and precisely measures the quantity of electrostatic charges created inside a powder during a flow in contact with a selected material.
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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.
GranuFlow analysis
The powder used is made of Stainless-Steel 316L. Two different versions of this powder will be investigated in this paper, the virgin powder and the recycled one.
In this part, the comparison between Hall Flowmeter and the GranuFlow have been studied. Every test was conducted under the same absolute moisture condition (w = 7.5gH20/kgDryAir). Regarding the Hall Flowmeter experimental protocol, tests have been repeated five times with approximately the same powder mass.
Once this step complete, the average values of powders flowrate (for Hall Flowmeter) were selected to be plotted and compared with those obtained by the GranuFlow (error bars are displayed in this Figure, but they are too small to be visible).
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Those results are very interesting, they allow us to see that the GranuFlow and the Hall Flowmeter show similar results (according to Beverloo Law modelling). However, some differences can be highlighted.
First, although both instruments results can be correlated, the Hall Flowmeter only gave us data for one diameter, while the GranuFlow allow to fully characterize the powder with a lot different holes diameter. Moreover, the Hall Flowmeter can make no difference between those two samples, while the GranuFlow shows (both with Cb parameter and curves analysis) that SS 316L virgin powder has a slightly better flowability than the recycled one.
GranuPack Analysis
In this part, powders bulk densities were investigated under the same moisture conditions (39% RH and 25°C).
The main purpose is to prove the high accuracy of the GranuPack instrument and to highlight the ageing of powders.
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Raw Results are shown by next chart
| Sample | p(0) (g/mL) | p(500) (g/mL) | n1/2 | Hr | Cr | p(∞) (g/mL) | τ |
|---|---|---|---|---|---|---|---|
| SS 316L Virgin Powder | 4.450 | 4.834 | 43.7 | 1.086 | 7.9 | 5.020 | 73.2 |
| SS 316L Recycled Powder | 4.324 | 4.793 | 31.7 | 1.108 | 9.8 | 4.998 | 56.7 |
Table 1: GranuPack Results: Comparison Between Virgin And Recycled SS 316L Powder
Were ρ(0) is the initial bulk density (in g/mL), ρ(500) is the bulk density after 500 taps (g/mL) and ρ(∞) is the optimal bulk density (in g/mL, calculated by a model available in the GranuPack software) which the minimum density the tapping test can achieved. Hr and Cr are respectively, the Hausner and Carr ratios. N1/2 and τ are two parameters linked to the compaction kinetic.
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Figure 2 and Table 1 show that the GranuPack instrument can make slight differentiations between the virgin and recycled powder. Indeed, by the look of n1/2 and τ, the compaction kinetic is faster for the recycled powder than the virgin one. Even the initial bulk density is different. However, the classical Hausner ratio is unable to highlight high flowability differences.
GranuCharge analysis
The triboelectric effect of the SS 316L powders were investigated with the help of the GranuCharge instrument. Each measurement was conducted at 26°C and 38% RH. 4 kind of pipes materials were analysed: Stainless-Steel 316L, Aluminium, PVC, and PE. Powders initial charges were measured 16 times to get an average initial charge density value.
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For the final charge density values, tests have been repeated many times in order to show GranuCharge accuracy. Circles points represent Virgin powder, while squares ones represent Recycled powder.
Table 2 summarize all the results obtained with the GranuCharge instrument. Each charge density value corresponds to the average value calculated with all reproducibility tests.
| Powders Name | Average Initial Charge Density qi (µC/kg) |
|---|---|
| SS 316L Virgin | -1.6810-2 |
| SS 316L Recycled | -1.0810-2 |
| Powders Name | Pipes Type | Average Initial Charge Density qi (µC/kg) |
|---|---|---|
| SS 316L Virgin | Stainless-Steel | -1.27 10-1 |
| Aluminium | -6.92 10-2 | |
| PVC | 1.26 | |
| Polyethylene | 9.39 10-1 | |
| SS 316L Recycled | Stainless-Steel | -1.05 10-1 |
| Aluminium | -4.97 10-2 | |
| PVC | 8.58 10-1 | |
| Polyethylene | 5.20 10-1 |
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This table is interesting, because it shows that with the GranuCharge instrument, differentiations between powders can be made with great accuracy. Even with initial charge densities, which are highly different (close to 56%). Each powder (virgin and recycled) is charged positively on polymer and negatively on metal pipes at the end of experiment. On metal and polymer pipes, virgin powder acquires more electrical charges than recycled one.
Conclusion
✓ Ageing of powders is difficult to highlight with classical methods
✓ Hopefully, with their high accuracy measurements, the GranuFlow and the GranuPack, can show small differences between the virgin and recycled version of the same powder (when the classical Hall Flowmeter is unable to do so). However, if one wants to see a higher difference between powders, he needs to investigate the tribo-electric effect, which is measured by the GranuCharge instrument.
✓ Indeed, this analyser is highly sensitive to particles surface state, such as oxidation, pollution, rugosity, etc.
✓ Tests have shown that the GranuCharge can measure, with a great accuracy, the initial powder charge density (highlighting a difference between virgin and recycled SS 316L powder of 56%). Moreover, for the same pipe material, the final charge density between virgin and recycled powder is also shown.
✓ Ageing of powders is followed by the GranuCharge instrument with a great accuracy (cf. reproducibility tests) and it is a good way to show the ageing of powders.
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