# On the potential of eddy current characterization of the ferritic content of recovered 316L powders after LaserPowder bed fusion fabrication

Romain Saddoud, Kévyn Perlin, Natalia Sergeeva-Chollet, Timothée

Delacroix, Anastasios Skarlatos, Jean-Paul Garandet

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## ► To cite this version:

Romain Saddoud, Kévyn Perlin, Natalia Sergeeva-Chollet, Timothée Delacroix, Anastasios Skarlatos, et al.. On the potential of eddy current characterization of the ferritic content of recovered 316L powders after LaserPowder bed fusion fabrication. Additive Manufacturing Letters, 2024, 9, pp.100207. 10.1016/j.addlet.2024.100207. cea-05439933

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Short Communication

# On the potential of eddy current characterization of the ferritic content of recovered 316L powders after LaserPowder bed fusion fabrication

R. Saddoud<sup>a</sup>, K. Perlin<sup>a</sup>, N. Sergeeva-Chollet<sup>a</sup>, T. Delacroix<sup>b</sup>, A. Skarlatos<sup>a</sup>, J.P. Garandet<sup>c, *</sup>

<figure><img src="image_3.png" /></figure>

ARTICLE INFO

Keywords:

Eddy Current characterization
316 L stainless steel
Laser powder bed fusion
Ferrite content
Oxygen content

A B S T R A C T

An original container was designed to measure the ferritic content of powder batches by the Eddy Current (EC) technique. As opposed to the standard X-Ray Diffraction (XRD) or Electronic BackScatter Diffraction (EBSD) methods, the EC measurements can be implemented on powder batches of significant sizes, say a hundred grams or so. Using a methodology based on the multiple recycling of an initially ferrite-free virgin powder, it was shown that the EC signals are sensitive to the ferritic content of the recovered powder. On the other hand, in the frequency range scanned by the sensor, the EC signals are virtually independent on the oxygen concentration within the tested powder.

1. Introduction

It is now well accepted that the Laser Powder Bed Fusion (L-PBF) technique has the potential to become a real game changer for the fabrication of specific net shape or near net shape parts in various industry sectors. However it is also known that in l-PBF only a fraction of the powder spread is actually melted and solidified, which raises the question of the becoming of the recovered material after fabrication. Reuse without any further treatment would of course be the best option, both from the economical and ecological standpoints. However, exposure to a high temperature environment may lead to a degradation of the powder particles, a fact specially true for one of the most widely used materials in l-PBF practice, namely stainless steel 316 L.

Previous works on the characterization of the powder recovered after fabrication [1–3] have shown that a number of particles indeed undergo surface oxidation, which apart from increasing the oxygen content of the alloy may also impact powder flowability. More relevant for our present purposes is the fact that when powder particles are entrained by the gas flow and melted by the laser beam (the so called hot spatters [4]), they may solidify in ferritic form, likely due to fast cooling. Such small size ferritic particles will not be sieved off after fabrication and may hinder the spreading of the powder bed in future fabrications. This supports the need for characterizing the ferritic content of the recovered powder.

Such a characterization can be performed by methods such as X-Ray

Diffraction (XRD) or Electronic BackScatter Diffraction (EBSD), but the mass of material that can be tested remains very limited, in the gram range at the most. Considering the amount of powder recovered after an l-PBF fabrication, both XRD and EBSD do not appear suitable. As a standard Non Destructive Technique, Eddy Current (EC) testing has been widely used on additively manufactured parts, and has even been successfully implemented in a standard l-PBF printer, allowing the in situ monitoring of defects formation [5]. However, when the EC technique was applied to powder characterization, the results obtained have not been conclusive [6]. This is likely due to the very low electrical conductivity of most metallic powders, which would require very high excitation frequencies. Fortunately, EC testing is sensitive to magnetic materials, so it appeared interesting to test the potential of the technique to assess the ferritic content of recovered 316 L powder.

In this context, the purpose of the present letter is to show that meaningful information on the ferritic content in recovered 316 L powder can be obtained using standard EC equipment. To this end, a specific container adapted to the measurement of EC signals on batches of about 100 gs was designed. EC tests were then performed on a number of powder batches coming from various stages of reuse in successive l-PBF fabrications to show that significant variations in ferritic content can indeed be detected with our measurement technique.

<footnote><sup>a</sup> Université Paris-Saclay, CEA, LIST/DIN, F-91190, Palaiseau, France</footnote>

<footnote><sup>b</sup> Université Paris-Saclay, CEA, ISAS/DPC, F-91190, Palaiseau, France</footnote>

<footnote><sup>c</sup> Université Grenoble Alpes, CEA, LITEN/DTNM, F-38000, Grenoble, France</footnote>

<footnote>* Corresponding author.
E-mail address: jean-paul.garandet@cea.fr (J.P. Garandet).</footnote>
## 2. Materials and methods

In order to carry out EC measurements on a representative powder batches, an original container made of plastic materials (metals have obviously to be avoided) has been designed. Special attention was paid to the issues of powder density and trapped air. More specifically, as shown in detail in Fig. 1, the container consists in:

- A housing chamber in PolyMethyl MethAcrylate (PMMA) with an internal diameter of 45 mm (1), allowing to accommodate a relevant amount of the powder (2) to be tested, that rests on a supporting pad, also made of PMMA.

- A PolyVinylChloride (PVC) film (3) of typical thickness 50 µm, for the containment of the powder within the housing chamber and the procurement of a reference surface for the positioning of the EC probe, with the additional advantage of protecting the operator from the powder particles.

- This PVC film is maintained over the powder heap with a PolyOxy-Methylene (POM) sealing lid linked to the housing chamber thanks to a set of 8 PolyTetraFluoroEthylene (PTFE) screws (4).

- A POM internal moving piston, able to control the volume of the powder and to ensure its contact with the protection film. The motion of the piston is implemented using 4 $M10 \times 35$ stainless steel screws (5). The choice of stainless steel is justified by the relatively large mechanical stresses necessary to move the piston, and their location at the rear of the container ensures no impact on the EC measurement.

- A gas evacuation (6) device, consisting of an acrylic tube connected to a syringe, to avoid pressure increases within the housing chamber and PVC film deformation in relation to the motion of the piston.

- A lateral paper filter able to contain the powder within the housing chamber but loose enough to allow gas evacuation

The EC sensor used in the present work consists in a single copper coil, which ensures both signal excitation and reception, in a double function operation mode. The principle of the technique is to measure the impedance of the electric circuit consisting of the coil itself, the surrounding air and the powder to be tested. Both real (resistance) and imaginary (reactance) components of the signal are recorded. When the sensor operates far away from any electrically conducting material, e.g. in air, its reactance is purely inductive. When it senses the powder within the container, its impedance is modified. Such a modification is enhanced when the tested powder contains ferritic particles, further changes are expected on both the resistance and the reactance components. It is the measurement of those changes that is the object of the present work.

More specifically, the coil used in the present work has an height of

<figure><img src="image_4.png" /><figcaption>Fig. 1. Schematic of the container.</figcaption></figure>

1.5 mm and internal / external diameters of respectively 8.5 and 12 mm, see Fig. 2(a). Its number of loops is 48, resulting in a coil resonance frequency of 7.5 MHz. This is an important issue when selecting the measurement frequency range, as it is well known from standard EC practice that in order to obtain reliable data the measurement range should remain significantly below the resonance frequency. As can be seen in Fig. 2(a), the coil comes with an additively manufactured support made of polylactic acid for an easier handling.

In addition to the above described coil, the experimental system features a commercial Keysight E4990A device to carry out the impedance measurements, see Fig. 2(b). The data reported in the present work was acquired in the 1 to 1.2 MHz range. More precisely the scanning was made at steps with steps of 50 kHz. With this choice of experimental conditions, a whole acquisition requires only a few seconds.

The powder batches tested in the present work come from a previous study dedicated to the characterization of recovered powder and parts after numerous l-PBF fabrications [3]. More specifically, a gas atomized batch powder from Oerlikon Metco Europe GmbH with a particle size distribution of 20–45 µm, was successively run through 15 l-PBF fabrications to follow the evolution of the powder upon reuse. At each stage, a number of characterizations were carried out, the interested reader being referred to the original publication [3] for more details on the procedure. For our present purposes, the description and the characteristics of the tested powder batches samples are summarized in Table 1.

Regarding the ferritic content of the powders, the proposed values were assessed through XRD and EBSD on small powder batches for purposes of calibration. This allowed to check that the as-received powder was essentially free from magnetic particles, but that the amount of ferrite phase increased significantly through the reuse cycles. Fig. 3(a) shows a comparison of XRD diffractograms for the virgin powder (R0) and the 15-times reused powder. Differences in peaks intensities with the (110) plane, characteristic of the ferrite phase, allowed to perform Rietveld refinement and to assess a ferritic content around 4 wt.% for the R15 sample.

Fig. 3(b) displays an example of complementary EBSD analyses, with powders recovered from the sieve residues (SR). Phase maps allow to identify fully ferritic particles, and image analyses with phase contrast enable the ferritic content to be deduced from area measurements, amounting to circa 10% in the case of SR. On all the data reported in Table 1, the relative uncertainty can be taken as +/- 10%.

It should be stated that austenitic particles are expected to be covered by a protective chromia based film, whose thickness depends on their thermal history. The oxide content of the various recycled powder batches was characterized [3]. As the thickness of the oxide layer on the surface of the metallic powder particles could a priori impact the EC measurements, a batch of furnace oxidized virgin powder, was also tested. As such, the powder did not experience a melting process, meaning that it could be assumed to be devoid of ferritic particles, just as the unoxidized virgin powder, and thus allow to check the effect of the oxygen independently of the increase of the ferritic content.

<figure><img src="image_5.png" /><figcaption>Fig. 2. Double function EC sensor with its polymer support (a) and Keysight E4990A impedancemeter (b).</figcaption></figure>
<table><caption>**Table 1**

Characteristics of the tested powder batches.</caption><thead><tr><th>Batch denomination</th><th>Description</th><th>Ferritic content (wt.%)</th><th>Oxygen content (wppm)</th></tr></thead><tbody><tr><td>R0</td><td>Virgin powder</td><td>~ 0</td><td>474</td></tr><tr><td>R6</td><td>Six times reused powder</td><td>1</td><td>585</td></tr><tr><td>R15</td><td>Fifteen times reused powder</td><td>4</td><td>665</td></tr><tr><td>SR</td><td>Sieved residues</td><td>10</td><td>1600</td></tr><tr><td>OF</td><td>Furnace oxidized virgin powder</td><td>~ 0</td><td>970</td></tr></tbody></table>

With some simplifying assumptions, e.g. considering an assembly of spherical monodisperse particles, the overall amount of oxygen can be translated in a chromia oxide layer thickness [7]. Using the average particle radius of the powder (r = 16.0 µm) determined by laser granulometry, the oxide thickness ranges from 12 nm for the virgin powder to 24 nm for the furnace oxidized powder, values that appear in the same range as published data [8–9]. Let us notice that for sieve residues such an approach would not make sense in view of the perturbed nature of the particles shapes.

Regarding the experimental procedure, the powder to be tested is first filled within the housing chamber, and the PVC film and sealing lead are screwed to the housing chamber. After manual shaking, the piston is moved forward to compress the powder. In some cases, the powder equipment is set on a vibrating platform to enhance powder density prior to piston motion. The EC sensor is then set in contact with the PVC film to ensure a constant lift-off, and the EC measurements are repeated ten times to estimate the dispersion of the measurements.

### 3. Results

The raw resistance and reactance data are shown in Fig. 4. A first significant result is that an increase of the level of both the resistance and reactance signals with the ferritic content of the powders can be unambiguously observed. When the measurements are repeated ten times, the relative dispersion (standard deviation/mean) is very low, significantly below 0.001 in all cases, except for the sieve residues where it can reach 0.003. This means that the observed variations on the results of Fig. 4 are significantly larger than the measurements dispersion, which confirms the potential of EC measurements for the detection of the ferrite content in recycled powders.

A detailed understanding of the quantitative relationship between

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) XRD diffractograms of virgin (R0) and 15-times reused (R15) powders; (b) EBSD phase maps and inverse pole figure map of sieve residues (SR).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Results of resistance (a) and reactance (b) measurements on the various powder.</figcaption></figure>
the ferritic content of the powders and the amplitude of the resistance and reactance signals is beyond the scope of the present work, but some qualitative arguments can nevertheless be proposed. From a physical standpoint, the powder to be tested consists in a mixture of austenitic and ferritic individual metallic particles, covered in all cases by an oxide film of varying thickness.

The oxide layer at the surface of the particles acts as a barrier to the eddy currents induced in the interior of the particles, which results in a confinement of the latter inside the particle volume. Such a galvanic isolation between the particles results in very small effective conductivity values (that is the conductivity seen by the sensing coil) for the powder material with respect to the bulk material. Such a conductivity reduction can be of more than three order of magnitudes, which can make the EC measurement on non-magnetic materials very difficult [6].

On the other hand, an increase in permeability (associated in our case to an increase of the ferritic content) will allow measurements to be carried out at effective values as low as $100 \Omega^{-1}m^{-1}$, that is circa four order of magnitudes below the electrical conductivity of the bulk 316 L material. Preliminary calculations using the Civa software [10] show that in an effective ferromagnetic medium of such a low effective conductivity, both real and imaginary part of the measured signals are expected to take values in the experimentally observed range.

The second major result is that the curves for batches R0 and OF appear virtually superimposed, with only a slight difference on the resistance data that remains nevertheless within the measurement uncertainty. This means that a doubling of the thickness of the oxide film on top of the austenitic particles (from 12 nm in the R0 batch to 24 nm in the OF batch) has no significant impact on the signals, which can be qualitatively explained by the fact that the current confinement is very weakly affected by the thickness of the oxide layer at least in the studied frequency range.

A rigorous procedure to explain the effect of both ferritic matter and oxide layers would require first the solution of the eddy-current problem at the particle level and then to account for the interaction between particles, which is however a complicated problem and lies well beyond the scope of this work. In any case, our data show that in our experimental conditions the ferritic and oxide effects can be uncorrelated in practice.

To address the issue of the robustness of the techniques, measurements were also carried out after submitting the powder within the containers to shaking on a vibrating table, in other words with a different geometrical arrangement of the particles. This did not significantly change the observed resistance and reactance absolute values, but the dispersion was found to be significantly higher. Nevertheless, it remained below 1 %, and we therefore considered that the relative uncertainty of the measurements can be considered to be of the order of 1 %, or in dimensional terms at the most of circa 1.5 Ω on the reactance signal on sieve residues.

In any case, the observed variations on the results of Fig. 4 are significantly larger than the measurements accuracy, which confirms the potential of EC measurements for the detection of the ferrite content in recycled powders.

## 4. Conclusions and perspectives

The objective of the present work was to propose a proof of concept of the potential of the EC technique to characterize the ferritic content of 316 L powders after an I-PBF fabrication. To this end, a specific container was designed to allow the acquisition of EC signals on relatively large (say in the hundred of grams range) batches containing various amounts of ferrite. In the frequency range scanned by the EC device, the results show that the EC signals are only marginally sensitive to the oxygen concentration within the powders but depend clearly on their ferritic content.

A first obvious perspective would be to further analyze the signals on the basis of a physical model in order to deduce the conductivity and

capacity of the effective medium through an inverse procedure, with the ultimate objective of correlating signal variations and ferritic contents as mentioned above. Such a model would also be useful in assessing whether an oxide layer may have a capacitive effect in other frequency ranges, which could also be tested experimentally.

Still on the experimental standpoint, another perspective would be to adapt the proposed methodology to other materials such as Inconel superalloys, where the recovered powder after I-PBF fabrication may also contain magnetic particles [11].

### CRediT authorship contribution statement

**R. Saddoud:** Writing – original draft, Software, Methodology, Investigation, Data curation, Conceptualization. **K. Perlin:** Methodology, Investigation, Data curation, Conceptualization. **N. Sergeeva-Chollet:** Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization. **T. Delacroix:** Resources, Methodology, Investigation. **A. Skarlatos:** Conceptualization, Writing – review & editing. **J.P. Garandet:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

SADDOUD, PERLIN, SERGEEVA-CHOLLET and GARANDET have patent #FR 2307173 pending to CEA. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data Availability

Data will be made available on request.

### References

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[2] F.C. Pinto, I.R. Souza Filho, M.J.R. Sandim, H.R.Z. Sandim, Defects in parts manufactured by selective laser melting caused by δ-ferrite in reused 316 L steel powder feedstock, Addit. Manufa. 31 (Jan. 2020) 100979, https://doi.org/10.1016/j.addma.2019.100979.

[3] T. Delacroix, F. Lomello, F. Schuster, H. Maskrot, J.-P. Garandet, Influence of powder recycling on 316L stainless steel feedstocks and printed parts in laser powder bed fusion, Addit. Manuf. 50 (2022) 102553. doi.org/10.1016/j.addma.2021.102553.

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