1. Introduction
In recent times, metal additive manufacturing processes have become popular beyond rapid prototyping. The powder-bed fusion group of processes, including laser beam (PBF-LB) and electron beam (PBF-EB), are widely used for the fabrication of industrial goods requiring high design complexity and low-volume batches. Most of the applications for the PBF-EB process require high purity, repeatability, and defect-free parts. For example, the aerospace industry, for structural and rotating parts, demands low content of interstitials, defects and contamination that could compromise the performance in operating conditions. Inconel 718, being one of the early adopted materials in PBF processes, is one of the most demanded, given its high strength, corrosion resistance, welding processability and wide operating temperatures (up to 650 °C) [
1,
2].
The PBF-EB process consolidates powder particles into 3D objects obtained from a computer-aided-design (CAD) file [
3]. In PBF-EB, the electron beam serves as an energy source that preheats and melts the powder particles under a high vacuum to protect powders from oxidation [
4]. The PBF-EB process starts with raking the powder particles onto a substrate layer by layer, forming the powder bed. Homogeneous spreadability of the powder is necessary for good powder packing. This is followed by a preheating step, in which the deposited layer is scanned with a low-power density electron beam. Due to the high pre-heating temperature (close to 1000 °C for Inconel 718), the parts are to be extracted from a partially sintered powder ‘cake’. In fact, powder sintering relies on high thermal and electrical conductivity between metal particles. The electrons, in the case of poor electrical conductivity of the powder cake, can accumulate on the powder surface and grow electrostatic repulsion forces, which can lead to the smoking effect, a term used when particles are ejected from the build envelope by powder–beam interaction [
5]. This effect does not occur often when melting if not observed previously during the pre-heating stage. The smoking effect must be avoided since it is usually a source of build failure.
After melting with a focused electron beam and cooling down the build chamber, the built part can be extracted. Since most of the powder used that surrounds the part has not been melted, it can be recovered for subsequent build cycles. This recovery process makes use of grit blasting to separate agglomerates of powder particles and to mix the heat-affected powder with powder from the hoppers that were not exposed to the process. The recovered powder can be loaded into the hoppers to restart the PBF-EB process. This results in a mix of powders with different thermal history. Given the high temperatures at which the PBF-EB process takes place, the surface chemistry of the powder particles can vary with an increasing number of reuse cycles and, consequently, the thickness of the oxide layer, as described by Hryha et al. [
6] in a different study. Surface chemistry is one of the key properties linked to charging, since the oxide layer that surrounds the powder particle acts as a dielectric layer and, hence, determines the electrical properties of the powder bed; this effect was simulated by Chiba et al. [
7].
Ensuring part repeatability, even at high reuse cycles, is key for process cost effectiveness and sustainability. Reusability studies of Inconel 718 in PBF-EB are limited in comparison with other materials (e.g., Ti-6Al-4V) and AM techniques (e.g., PBF-LB). Powder reuse in PBF-LB has reported pick up of oxygen and improved values in flowability of Inconel 718 powder, with no detrimental effect in the mechanical properties [
8,
9]. However, the oxidation mechanisms and process conditions in PBF-EB are different than other metal AM processes. In PBF-EB, the high vacuum protects the powder from oxidation at high temperatures, although upon reuse of the powder, oxidation will take place, generating Al-rich oxides, as studied by Gruber et al. [
10]. As observed, as well, by Raza et al. [
11], in the PBF-EB working conditions, the sublimation pressure of the oxides for the elements Ni, Cr and Al are lower than of pure metals, in which Al has the highest equilibrium sublimation pressure, followed by Cr. In a following study by Gruber et al. [
12], surface oxidation was correlated to defect formation, in which the Al-rich oxides deposited on the reused powder particles will tend to cluster in the liquid metal, solidifying as inclusions. The effect of the impact using Charpy V-notch (CVN) test samples of virgin and reused Inconel 718 powder processed by PBF-EB was studied by Gruber et al. [
13]. The conclusion is a large heterogeneity of fracture features in both variations of powder (virgin and reused), with dominated by Al-rich oxides films and non-metallic inclusions.
Most reusability studies covering different materials are still based on traditional flowability testing methods used in powder metallurgy, such as Hall and Carney flowmeters, which are only suitable for a free-flowing powder subjected to gravitational forces. However, these methods have been questioned repeatedly in the PBF process, given the thin-layer application requirements [
14,
15]. Alternatively, more advanced rheological equipment has been used to characterize metal powder, in particular, Freeman FT4, and rotation drum analyzers offer quantifiable measurement capabilities [
16,
17,
18]. Spreading devices to mimic the powder-bed application have been also used to characterize powder in various conditions [
19,
20]. While the effect of reuse on the surface chemistry of Inconel 718 powder has been investigated before, the impact of this on the rheological behavior and charging of the powder bed have not been addressed to the best of our knowledge. In this work, we use X-ray photoelectron spectroscopy (XPS) to study the surface oxide chemistry of virgin and reused powder particles, which can have a strong influence not only on the charging behavior of the powder in the process but also its spreadability. Rheological properties of the powders under different regimes and speed conditions using the rotation drum principle are studied and linked to the spreading speed of the raking device in PBF-EB.
2. Materials and Methods
The Inconel 718 atomized powder was supplied by Arcam AB, Mölndal, Sweden, with composition as listed in
Table 1 and particles between 45 and 115 µm (
Figure 1a,b). Two variations of the same powder were studied, in virgin and reused state. The virgin powder refers to the material that is freshly atomized and has not been processed yet, while the reused powder has been part of the process for multiple build cycles and has been discarded due to smoking during PBF-EB processing and is, hence, unsuitable for reuse.
The particle surface morphology of the samples was studied by high-resolution scanning electron microscopy (HR-SEM) using a Carl ZEISS-LEO Gemini 1550 SEM microscope (ZEISS Microscopy, Jena, Germany), equipped with a field emission gun. The bulk oxygen and nitrogen content of the virgin and reused powder samples were measured by a hot fusion analysis using an ON836 instrument (LECO, St. Joseph, MI, USA). The particle size distribution was measured by laser diffraction using an HELOS (Sympatec, Clausthal-Zellerfeld, Germany) with an RODOS dry-disperser analyzer. To measure the moisture content on the particle surface, thermogravimetric analysis was performed using a thermal analyzer STA449 (NETZSCH Thermal Analysis GmbH, Bayern, Germany). A calibration testing curve using only alumina filler was subtracted from the measurement to eliminate the effect of the gas flow and improve measurement accuracy. The samples were heated from room temperature to 300 °C with a heating rate of 10 K/min followed by an isothermal for 60 min under scientific purity (99.9999%) argon gas atmosphere.
The rheological behavior of virgin and reused Inconel 718 powder was studied by the rotation drum principle using a Revolution Powder Analyzer (RPA) by Mercury Scientific Inc (Newtown, CT, USA). The test is carried on a rotating and transparent drum filled with powder. A camera in front of a light source records pictures of the powder surface area on movement. An image analysis software converts the image in different parameters associated with powder flowability. Given the rotation of the drum, the powder is carried up the side of the drum causing it to collapse or avalanche by its own weight. The test is designed to characterize the avalanche angle which flows in similar manner as powder spreading during layer-based application in PBF processes, and, hence, allows one to evaluate particle-to-particle and particle-to-base plate interactions. Equal sample size of 110 g of virgin and reused material was freely loaded in the drum. The rheological and charging tests were performed at atmospheric conditions of 26 °C and 43 RH%. Sample drying was performed ex situ at 100 °C for 2 h. For the flowability measurements a constant speed of 0.6 RPM was selected in which 150 powder avalanches were analyzed. Additionally, a separate experiment varying speed from 1 to 70 RPM was set in which 75 avalanches were analyzed for each angular speed. The selection of the speed range was based on the recoater linear speed of about 250 mm/s which is equivalent to an angular speed of 48 RPM as per Equation (1).
where
is the angular speed,
is the linear speed of the recoater and
R the radius of the revolution powder analyzer.
The tap density was obtained by the packing test which consists of vibrating the rotation drum repeatedly for ten cycles,
is the average of ten measurements. This is divided by
, in this case 8.2 g/cm
3, to obtain the relative density as in Equation (2).
Using the ION Charge Module integrated in the RPA, the powder-charging properties are studied. Measuring the charging properties of PBF-EB powder is important, especially upon reuse, given potential changes in the oxide layer. The measurements were taken both during rotation at a constant speed of 10 RPM and during decay in static mode. Before the measurements, the drum is thoroughly cleaned with a solution of 90% Isopropanol and 10% water and gently dried with a paper towel. This is followed by deionizing of all the parts of the drum including both sides of the polycarbonate transparent lids and metal cylinder for 30 s. This procedure is carried out to improve accuracy since electrostatic charge measurements are very sensitive to handling.
For surface analysis, X-ray photoelectron spectroscopy (XPS) was conducted using PHI 5500 (Physical Electronics, Chanhassen, MN, USA) equipped with a monochromator Al Kα source. The samples for the measurement were prepared on a 3M carbon tape substrate. To obtain statistically authentic data, the analysis area for measurement was 300 μm × 300 μm. The pass energy used for the survey and the narrow elemental scans was 226 eV and 69 eV, respectively. For the depth profile analysis, Ar + ions were used to etch the sample surface. The etch depth was calibrated on Ta2O5 foil, and all the information about etching depth is as per that standard. The obtained data were analyzed using the MultiPak 9.7.0.1 software provided by PHI.
Author Contributions
Conceptualization, L.C. and A.R.; methodology, L.C.; formal analysis, L.C. and A.R.; investigation, L.C. and A.R.; writing—original draft preparation, L.C. and A.R.; writing—review and editing, E.H.; visualization, L.C.; supervision, E.H.; funding acquisition, E.H. All authors have read and agreed to the published version of the manuscript.
Funding
The work was performed in the framework of the Centre for Additive Manufacturing-Metal (CAM2) hosted by Chalmers University of Technology, supported by VINNOVA (grant number: 2016-05175).
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
Not applicable.
Acknowledgments
The authors thank Höganäs AB for the particle size distribution and bulk composition analysis of the powder samples and Arcam AB for providing the powder materials.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Inconel 718 powder (a) morphology of the virgin powder and (b) particle size distribution of virgin and reused powders.
Figure 2.
SEM micrographs of surface of virgin powder at 500, 2000 and magnifications, respectively, (a1–a3) and reused powder (b1–b3). Inserts in (a3,b3) at 100 K show presence of particulate oxide sizing up to 100 nm in case of reused powder and clean surface in case of virgin powder.
Figure 3.
Oxidation of powder in the sintered cake: (a) agglomerate of particles found in reused powder, (b) interparticle necks formed during sintering and (c) Al-rich oxides observed at the interparticle necks.
Figure 4.
XPS survey spectra of virgin and PBF-EB reused Alloy 718 powder.
Figure 5.
Comparison of depth profile of Al2s, Cr2p and Ti2p spectra for (a) virgin and (b) reused powder.
Figure 6.
Comparison of (a) variation in normalized oxygen intensity with etched depth and (b) average oxide particulate/layer thickness for virgin and reused powder calculated by using O1s spectra. Depth is measured relative to the etch rate of Ta2O5, which was used to calibrate the etch rate.
Figure 7.
(a) Flowability metrics (avalanche energy, break energy and cohesion) and (b) avalanche angle using a constant speed of 0.6 RPM. Multi-flow test metrics (c) avalanche energy, (d) break energy, (e) cohesion and (f) avalanche angle. (g) Avalanche morphology at 1 RPM. All measurements show the behavior of virgin and reused powder in as-received (AR) and dried (D) condition.
Figure 8.
Electrostatic charge measurement of virgin and reused powder in as-received (AR) and dried (D) condition.
Figure 9.
Schematic of powder deposition and interaction between particles of virgin (a) and reused (b) powder in PBF-EB.
Table 1.
Composition of Inconel 718 powder.
Elements | Ni | Co | Cr | Mo | Ti | Mn | Nb | Ta | Al | Fe | Si | C |
---|
wt.% | 54.1 | 0.04 | 19.0 | 2.99 | 1.02 | 0.12 | 4.97 | <0.01 | 0.52 | 17.12 | 0.06 | 0.03 |
at% | 53.38 | 0.04 | 21.16 | 1.80 | 1.23 | 0.13 | 3.10 | <0.01 | 1.12 | 17.75 | 0.12 | 0.14 |
Table 2.
Nitrogen and oxygen content determined by inert gas fusion and moisture measured by thermogravimetry analysis.
Material | O (%) | H2O (%) | D10 (µm) | D50 (µm) | D90 (µm) |
---|
virgin | 0.0153 | 0.0255 | 51.73 | 78.32 | 113.90 |
reused | 0.0184 | 0.0229 | 47.30 | 71.45 | 108.98 |
Table 3.
Apparent density, tap density measured by the RPA and relative density calculated using Equation (2).
Material | Apparent Density (g/cm3) | Loose Volume Fraction (%) | Tap Density (g/cm3) | Packed Volume Fraction (%) |
---|
Virgin AR | 4.65 ± 0.03 | 56.69 ± 0.31 | 4.96 ± 0.05 | 60.50 ± 0.63 |
Virgin D | 4.60 ± 0.02 | 56.08 ± 0.27 | 4.90 ± 0.07 | 59.73 ± 0.87 |
Reused AR | 4.57 ± 0.02 | 55.71 ± 0.19 | 4.67 ± 0.02 | 57.02 ± 0.29 |
Reused D | 4.55 ± 0.03 | 55.54 ± 0.42 | 4.82 ± 0.05 | 58.81 ± 0.63 |
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