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Article

Effect of Powder Recycling on the Surface and Selected Technological Properties of M300 Maraging Steel Produced via the SLM Method

by
Abdesselam Mechali
1,*,
Josef Hlinka
2,
Michal Kresta
2,
Marin Petrovic
3,
Jakub Mesicek
1,
Ibrahim Jahan
4,
Jiri Hajnys
1 and
Jana Petru
1
1
Department of Machining, Assembly and Engineering Metrology, Faculty of Mechanical Engineering, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic
2
Department of Materials Engineering, Faculty of Materials and Technology, VSB-Technical University of Ostrava, 17. listopadu 2172/15, 708 00 Ostrava-Poruba, Czech Republic
3
Faculty of Mechanical Engineering, University of Sarajevo, Vilsonovo setaliste 9, 71000 Sarajevo, Bosnia and Herzegovina
4
Libyan Authority of Science Research, Tripoli P.O. Box 80045, Libya
*
Author to whom correspondence should be addressed.
J. Manuf. Mater. Process. 2024, 8(6), 267; https://doi.org/10.3390/jmmp8060267
Submission received: 14 October 2024 / Revised: 16 November 2024 / Accepted: 19 November 2024 / Published: 27 November 2024
(This article belongs to the Special Issue Advances in Additive Manufacturing and Material Characterization Techniques)
Browse Figures
Versions Notes

Abstract

:
This study delves into selective laser melting (SLM). By using M300 steel in virgin and recycled powder form (after 20 cycles), with the aim of reducing the cost of printing for the practical application of M300 maraging steel, a comprehensive comparison between the two types of powder was evaluated. The powder’s morphology was analyzed using scanning electron microscopy (SEM) and backscattered electrons (BSE). The particles were seen to have a spherical shape, with a notable number of satellites attached to their surfaces. The particle size distribution (PSD) was examined and ranged from 10 to 90 µm for both powders. In addition, the porosity exhibited an average value of 0.07% for the virgin powder and 0.10% for the recycled powder. The microstructure was examined. Additionally, the surface wettability was tested, and it was seen to display wetting behavior for both types of powder, while blackened surfaces showed a higher wetting angle than untreated surfaces (hydrophobic). The 2D roughness measurements showed that the recycled powder had no significant difference from the virgin powder (Ra = 5.33 µm, Rz = 24.17 µm) before blackening and (Ra = 5.48 µm, Rz = 24.07 µm) after blackening. Corrosion tests proved that the recycled powder did not affect the corrosion properties of the material, while blackening caused partial surface corrosion in both types of samples, regardless of the used powder.

1. Introduction

Additive manufacturing, also known as 3D printing, has the ability to produce complex shapes that are impossible with traditional manufacturing methods. This technology has revolutionized the manufacturing industry; it also has the ability to reduce the waste of materials. Additive manufacturing techniques can be applied in many vital and diverse fields, such as aerospace, healthcare, the pharmaceutical and food industries, automotives, and even construction [1,2,3,4,5,6]. The industrial sector, especially the mining sector, has witnessed a huge boom thanks to 3D printing technologies. Selective laser melting (SLM) is considered one of the most important 3D printing technologies due to its great ability to produce very complex geometric shapes directly from digital models, which gives a design characterized by unparalleled flexibility [7,8,9,10]. SLM uses high-powered lasers to fuse and melt powders of materials, using layer-by-layer melting to produce complex metal components [11,12]. In contrast to traditional manufacturing techniques, SLM enables the on-demand production of components to meet specific needs [13]. Consequently, the need for a physical inventory diminishes, leading to enhanced efficiency in inventory management [14,15,16]. The use of SLM for the processing and production of maraging steels has opened up new opportunities for industrial progress [17,18]; this is what distinguishes SLM technology from other 3D printing technologies [19,20].
M300 maraging steel in particular is considered one of the most high-strength steels due to its chemical composition, which contains a large amount of carbon-free steel and nickel. This type of steel has many applications in sensitive fields that require a strong structure. Critical applications such as aerospace, medical, automotive, military, and other demanding applications are suitable for the use of M300 maraging steel [21,22,23,24]. The distinctive characteristics of this steel are predominantly attained through a specialized thermal treatment procedure known as precipitation hardening or age hardening. This process entails the conversion of a martensitic matrix into a thin distribution of intermetallic compounds, which preserves the steel’s ductility while increasing its strength [25,26,27,28].
The efficient recycling of powder materials, especially M300 powders, is a feature of SLM and is achieved while maintaining the basic chemical composition of the metal powder. Recycling leads to a reduction in production costs, which gives SLM significant economic advantages [29,30]. Understanding the benefits of powder recycling is important as it is easier to dispose of powders after printing and reduces the environmental impacts and costs, which represent some of the disadvantages of 3D printing [31,32].
Nonetheless, several issues and obstacles exist in the recycling of powders, since these powders are influenced by the morphologies and dimensions of their particles, as well as the particle distribution. The chemical composition of the powder is influenced by the number of recycling iterations, which subsequently impacts the quality of the printed components [33,34]. Changes that occur during powder recycling negatively affect the powder flow and packing density [35,36]. As a result, this leads to a decline in the mechanical and technological properties of the printed materials [13,31,32].
Despite substantial research on SLM and powder recycling, there are still gaps in our knowledge of how numerous recycling cycles affect essential material qualities. For instance, previous research has primarily concentrated on the mechanical and microstructural characteristics, with less focus on attributes such as corrosion resistance, particularly in surface-treated components. This research fills these gaps by examining the effect of 20 recycling cycles on M300 maraging steel powder and its impact on the surface and technical properties, such as the porosity, wettability, roughness, and corrosion resistance, both with and without blackened surfaces.
This research makes a significant contribution by establishing a connection with continuing attempts to develop manufacturing procedures that are both cost-effective and environmentally friendly, while also maintaining the quality of the technology. In this research, virgin powders and recycled powders are compared in order to assess whether or not powder recycling is feasible for high-performance applications. This is performed in order to strike a balance between economic benefits and environmental sustainability.

2. Materials and Methods

2.1. Powder Characteristics

The initial powder of M300 maraging steel was provided by the Carpenter Technology Corporation (Renishaw plc, Wotton Under Edge, UK), and the recycled powder was utilized after 20 cycles of printing. The recycled powder was sieved without drying. Drying was not required to prepare the recycled powder. Sieving was accomplished in an argon atmosphere. The nominal chemical composition obtained from the manufacturer was confirmed by dispersive X-ray spectroscopy (EDS). The morphology of the M300 maraging steel was investigated using scanning electron microscopy (SEM) and backscattered electrons (BSE). The particle size distribution (PSD) was analyzed using a MasterSizer 3000 particle size analyzer (Malvern Panalytical Ltd., Malvern, UK). The powder was manufactured in a certain manner to prevent clumping. The dry dispersion unit was charged with the M300 maraging steel powder at an optimized air pressure for dispersion after the activation of the instrument and a baseline measurement to account for background noise. The software created volume and cumulative distributions, producing significant metrics such as D10, D50, and D90. These metrics denote the particle sizes under which 10%, 50%, and 90% of the total sample volume is included. This investigation is essential to evaluate the powder qualities that are critical for SLM processes.

2.2. Three-Dimensional Printing and Printing Parameters

An SLM Renishaw AM500 3D printer (Wotton-under-Edge, Great Britain) was used in order to construct the samples. A laser that has a maximum power rating of 500 W is included in the machinery that constitutes the printer. A purity level of 5.0 was achieved by the inert gas that was used, which was argon. The layer thickness was 50 μm as shown in Table 1. For the purpose of preventing powder oxidation during the setup process, and due to the ineffectiveness of the inert gas in removing metal vapors during the printing process, the chamber was continually purged of air and kept at a level that was lower than 1000 parts per million. The QuantAM software, version 5.0.0.135, was used to set up the system. The samples were printed directly onto the substrate, and there was a need for support materials. The scanning strategy consisted of stripes, as shown in Figure 1b. Two samples were printed to check the porosity, with dimensions of 20 × 20 × 20 mm, and another two samples were printed to check the microstructure, with the same dimensions. Another 4 specimens were printed with dimensions of 50 × 30 × 10 mm; two of these 4 samples were used to check corrosion, and the other two samples were used for surface wettability calculation. The printing was divided into two builds: the first build for the virgin powder and the second build for the recycled powder. Figure 1a illustrates the printing process.

2.3. Microstructure and Metallography Observation

For the evaluation of the corrosive effect and the basic semi-quantitative chemical properties, an analysis of the surface layer was performed using an SEM FEI 450 Quanta FEG (FEI Company, Brno, Czech Republic) equipped with an EDAX EDS detector (AMA-TEK Company, Tilburg, The Netherlands) in the secondary electron mode. Accelerating the voltage to 15 keV enabled us to analyze a wide range of chemical elements from the periodic table. Due to the shape of the analyzed sample, the working distance was 10–15 mm. The samples for metallography and porosity evaluation were mounted in Bakelite resin (Polyfast) with a carbon particle filler supplied by Struers (Roztoky, Czech Republic). This resin safely stabilizes samples during mechanical preparation. The porosity was measured on the samples after mechanical grinding (SiC papers up to #2500) and polishing using equipment and diamond suspensions (up to 0.1 µm particles) produced by Struers (Roztoky, Czech Republic). The microstructure of each sample was further evaluated after chemical etching (22 °C/10 s) in a modified Nital containing 4% HNO3 in ethyl alcohol. The image capture and evaluation were performed using an Olympus IX70 inverted metallographic microscope (Olympus, Prague, Czech Republic).

2.4. Corrosion Test

Corrosion tests were performed on the blackened and untreated surfaces of both samples. There were three solutions used for corrosion testing. The first solution was an acidic environment of hydrochloric acid in purified water with a pH of 3. The second solution was an alkaline mixture of sodium hydroxide in purified water with a pH of 11. The third solution was a 3.5% NaCl water solution. Here, 15 mL of each solution was used for each measurement. The measurement was carried out via a potentiodynamic test, where 0.5 cm2 of the surface was tested. Individual samples were placed in corrosion cells and were connected to the working electrode. Two electrodes were immersed in the solution. The first of them was an auxiliary one consisting of a carbon rod, and the reference electrode was a calomel electrode (SCE) with a potential of 244 mV vs. a standard hydrogen electrode (SHE) against which all listed potentials were further referenced.
Blackening, also known as bronzing or bluing, is a surface treatment after which the steel is partially protected against corrosion. The name is related to the resulting color of the surface. Blackening can be performed cold and hot. Cold blackening takes place by an oxidation reaction at room temperature between the part and the blackening solution. Hydrated forms of magnetite (iron–iron oxide) form on the surface of the component, which protects the surface from further oxidation. Blackening is often used as a surface finish on firearms weapons and mainly cannons. Since the use of the investigated material was considered, among other aspects, in the arms industry, a similar finish was chosen to that which is common in the industry for inking: the CHEM-WELD 9310 cold ink in the form of a solution was used. The solution contained a mixture of selenium dioxide, nickel, and copper sulfate. The samples were decreased and immersed in the aforementioned solution for 60 s. After pulling them out, they were washed first with water and then with alcohol and dried. The final form of the sample after blackening is shown in Figure 2.

2.5. Surface Wettability

The wettability of the specimens was assessed using the sessile drop technique. The surface contact angle was determined using the SEE system, while the free surface energy was estimated using the Advex Device software. The examined surface had 2 µL droplets of double pure water attached to it. The contact angle θ was obtained by the tangent to the profile at the point where the three phases (liquid, solid, gas) made contact with the plane of the sample surface. The free surface energy of the solid sample was determined with Young’s Equation (1), where YS, YSL, and YL represent the interfacial tension per unit length of the solid–vapor, solid–liquid, and liquid–vapor contact line, respectively [37].
Ysv − Ysl = Ylv cos θ

2.6. Two-Dimensional Surface Roughness

An Alicona InfiniteFocus 5G optical microscope (IF MeasureSuite, Alicona Imaging GmbH, Raaba/Graz, Austria) was used to assess the 2D roughness of the surface. Each surface of the three printed samples was measured three times, and we took the mean value from each measurement with its standard derivation. Each sample had a scanned area of 7.3 mm × 7.3 mm. On the scanned sample, three areas measuring 2.5 mm by 2.5 mm were evaluated. For the 2D roughness, the measured length was ln = 13 mm, and there were 3 measurements per sample. A cut-off filter was applied in accordance with the ISO 4288 standard for surface texture; for Ra 0.1 to 2 µm, a 0.8 mm filter was used. For Ra values of 2–10 µm, a 2.5 mm filter was used.

3. Results

3.1. Morphology of the Powder

Particles of the powders were examined using scanning electron microscopy (SEM) and backscattered electrons (BSE). Figure 3 displays photographs of the M300 virgin and recycled powders, revealing particles having a spherical shape. Additionally, a notable portion of the particles contained smaller particles, known as satellites, attached to their surfaces. The analysis of the virgin and used powders indicates that the gas atomization process used to produce the powder resulted in the formation of non-spherical particles. Additionally, satellites are seen as a flaw in the manufacturing process. Satellites are generated when powders are produced using strategies that involve varying the cooling rate, adjusting the surface tension, or using improper atomization settings. However, in this particular scenario, the satellites were formed due to gas atomization. Additionally, the satellites were present in the recycled powder; this occurs when a metal powder is reused and recycled in selective laser melting (SLM). This recycling process can change the surface properties, causing smaller particles to stick to larger ones and form satellites.
Upon conducting a thorough analysis, we discovered that, for both powders, the particles did not necessarily exhibit an isometric form. In addition to the spherical particles seen in Figure 4, the powders also exhibited irregular, elongated, and agglomerated shapes. This behavior was mostly attributed to the unique melting and solidification process that is characteristic of additive manufacturing.
Manufacturing smooth and spherical powders may be challenging, even though they are often applied for many uses in additive manufacturing. The outcome often has a mostly spherical shape. The thermal conductivity of the molten metal may influence the shape of the particles by regulating the pace of cooling during particle production and subsequent solidification.
Figure 4b reveals that the recycled M300 maraging steel powder also contained a small number of foreign particles (dark grey). The results of the EDX analysis in four zones, as shown in Figure 4a, showed that the particles of the virgin powder were different in shape, but their chemical composition was constant as shown in Table 2. There were dark aluminum and silicon particles in the recycled powder, as seen in Figure 4b; these particles were found to be the reason for the SLM printing errors. These particles were left in the press chamber as excess powder after the previous print. According to the composition, it can be estimated that they were derived from Ti-6Al-4V (titanium alloy) powder, which is a rich aluminum–vanadium alloy. However, the normal particles in the recycled powder, as shown in Table 3, had almost the same chemical composition as the virgin powder.
Based on the testing results, the particle sizes of the virgin powder were determined to be within the acceptable range of 10 to 90 µm. In contrast, the particle sizes of the used powder ranged from 15 to 90 µm, as shown in Figure 5. The powder that had undergone recycling had a higher volume density. Table 4 presents a concise overview of the data regarding the distribution of the particle sizes. The values of Dv (10), Dv (50), and Dv (90) for the virgin powder were measured to be 18.4 μm, 30.6 μm, and 50.0 μm, respectively. For the recycled powder, the corresponding values were 22.6 μm, 34.3 μm, and 51.1 μm. This distribution was considered suitable for the given purpose.

3.2. Porosity

The porosity of the material was determined using the thresholding technique. The aggregate ratio of black dots representing pores was calculated after the conversion of the photographs into black and white [37]. This was accomplished after the conversion of the pictures. This method enables adherence to the benchmark set by ASTM E1245. A decision was made to use modest magnification in order to mitigate the possible effects caused by the varying pore concentrations at distinct sample locations. These photos were acquired for each sample. The photos were captured at 100× magnification, and none of them exhibited any signs of etching. Figure 6 illustrates the standard images composed of samples collected from each batch. The porosity was computed five times and averaged at five different depths. The measurements of the porosity are shown in Table 5. The samples exhibited a range of porosity, with values of 0.07% for sections perpendicular to the printing plane in the case of the virgin powder, while it was 0.09% for the used powder. For sections parallel to the printing plane, the average porosity for each powder was 0.07% and 0.10%, respectively. The average porosity for each section was 0.07%. Upon comparing our findings on the porosity of M300 maraging steel to the data presented in the publication [38] by Ronda et al., it was found that they reported a porosity range of 0.12% to 1.12%. A recycled powder was investigated by Sun et al. in [39] and provided values of 1.0% at the horizontal cross-section and 1.2% at the vertical cross-section.
Individual pores may act as stress concentrators; hence, reducing the porosity can improve the mechanical and fatigue characteristics. The pores were identified as sealed chambers; these do not hold powder particles. Spherical holes were the consequence of gas being trapped during the melting of the metal, while irregular pores were generated owing to insufficient melting between the fillets or layers caused by incorrect processing settings [40,41,42,43].

3.3. Microstructure

Typically, the structure of the sample that was created matched that of the maraging steel after being 3D-printed using selective laser melting (SLM) [42,43]. The melt pools in Figure 7a,c were created by the fast fusion of the powder, which was then rapidly cooled. When seen at a greater level of magnification (Figure 8a,c), the fine-grained microstructure of the individual melt pools becomes apparent. The melting borders of the deposited particles’ separate layers are clearly evident in this detailed structure. The metallographic samples clearly show the relief of the separate melt pools that have been welded together in the longitudinal direction, i.e., the direction of the build. The grains are randomly arranged, either inside discrete pools of melted material or across the borders between these pools. The observed phenomenon is a result of heterogenous nucleation, which occurs when a liquid metal solidifies at the solid edges of the melt pool. During this process, the atoms of the solidifying metal partially adopt the orientation of the surrounding grains and grow in a direction that aligns with a negative temperature gradient [44]. Figure 7b shows that a layered structure could be observed on both samples, which was the result of the step-by-step printing technology by melting the powder layer. In Figure 8b, we can observe elongated grains of powder particles.
In comparison to the virgin powder samples, the largest magnifications from Figure 8d to Figure 8c show individual grains (powder particles) in the perpendicular direction. This is why the grains appear equiaxed, while, in both powder samples, they are elongated.

3.4. Surface Wettability and 2D Roughness Measurements

In order to guarantee that the results were not impacted by any roughness or surface defects, the contact angle measurements were carried out on surfaces that were very precise [37]. An ultrasonic bath that contained acetone was used to clean each component of the sample separately before the test was performed. The surfaces that were being tested were arranged such that they faced upwards in order to avoid any possible scratching. There was the deliberate placement of the droplets outside of the pores that were visible, which may have had an effect on their shape [37]. During this test, a sample that was produced from the virgin powder was compared to a sample was constructed from the recycled powder. Table 6 offers a presentation of the average findings of this test, together with their respective standard deviations. Images that are representative of droplets on the surfaces that were examined are shown in Figure 9a (virgin powder sample) and Figure 9b (used powder sample). The virgin powder sample’s surface wettability was lower than that of the used power sample, which had the same surface wettability behavior (wetting behavior). This led to the creation of a unique surface energy value, characterized by a rise in the surface energy and a fall in the contact angle. Both the virgin and used powder samples exhibited wetting behavior when the contact angle was less than 90 degrees, indicating a hydraulic role for both. As shown in Figure 9c,d, the blackened surface behaved more hydrophobically, and the wetting angle was higher than 125°. The influence of the used powder on the wettability was negligible due to the measurement deviation. This effect caused a reduction in the surface energy for both the virgin and recycled powder specimens.
The 2D surface roughness data are shown in Table 6, depicting the median roughness levels of both the virgin and used powder samples. The analysis revealed that the as-constructed sample had a mean value of the arithmetical mean height Ra = 4.68 µm, which is less than the range of roughness values seen in 3D-printed samples (ranging from 5 µm to 20 µm), as indicated in reference [44]. However, in the case of the used powder, it was Ra = 5.33 µm, which was 13% greater than in the case of the virgin powder. A similar observation was made in the case of Rz = 21.72 µm for the virgin powder and Rz = 24.17 µm for the used powder. This indicates an 11% higher value for the case of the used powder. Blackening caused an increase in the arithmetical mean height Ra for the virgin powder of 16% (from 4.68 µm to 5.62 µm) and of 15% in the value of Rz (from 21.72 µm to 25.31 µm); however, in the case of the recycled powder sample, these values were observed to be almost the same value for Ra and Rz.

3.5. Corrosion

The test findings obtained in an acidic pH 3 environment can be found in Table 7, while the polarization curves are depicted in Figure 10. The corrosion potential ranged from −454 mV to 493 mV vs. the SCE for the samples. The untreated surface of the recycled sample powder exhibited the lowest corrosion potential. Typically, metals with lower corrosion potential are less prone to galvanic corrosion. From this perspective, the charred specimen should have exhibited the highest level of resistance to the recycled powder. The corrosion current was consistently lower on untreated surfaces. Blackened surfaces exhibited a greater corrosion current compared to untreated surfaces, with the new powder showing a corrosion current three times higher than that of the recycled powder. An elevated current is typically associated with reduced polarization resistance and indicates a rise in the corrosion rate. Corrosion occurred at a faster pace in the acidic environment on surfaces that had been blackened compared to surfaces that had not been treated. Nevertheless, the findings did not indicate a discernible pattern suggesting that the recycled powder had any impact on the corrosion characteristics. The pristine surface of the unused powder exhibited marginally superior resistance to corrosion compared to the recycled powder. In contrast, the blackened surface of the unused powder exhibited a corrosion current and, consequently, a corrosion rate that was three times higher than that of the recycled powder.
The impact of an alkaline environment with a pH of 11 is demonstrated by the findings shown in Table 8 and the polarization curves depicted in Figure 11. The corrosion potentials exhibited significant differences; however, unexpectedly, equivalent outcomes were observed for mismatched pairs of samples, such as the untreated virgin and recycled blackened materials and vice versa. The untreated surface of the virgin sample exhibited the lowest corrosion potential, while the blackened surface of the recycled sample showed corrosion potential that was only 4 mV higher. The corrosion current exhibited significant fluctuations, ranging from tenths to tens of microamperes per square centimeter, spanning three orders of magnitude. The process of blackening had an adverse impact on the rate of corrosion, even in an alkaline environment. Blackened surfaces exhibited a greater corrosion rate. There was no discernible distinction in the rate of change in the case of the virgin powder, at only 7.70 μm/year. The corrosion rate of the recycled powder, after undergoing blackening treatment, was found to be two orders of magnitude higher than that of the untreated surface. Specifically, it measured 202.4 μm/year, compared to 9.70 μm/year.
The corrosion test findings obtained in a 3.5% NaCl solution can be seen in Table 9, while the corresponding polarization curves are depicted in Figure 12. The corrosion potentials of the untreated surfaces were greater than those of the black surfaces. Regarding the corrosion potential, the observed trend indicates that blackening leads to a decrease in the corrosion potential and hence enhances the corrosion resistance. The corrosion current was greater for samples manufactured from the recycled powder. The polarization resistance was comparable for both surfaces of the recycled sample. However, for the virgin sample, it was higher on the untreated surface. The corrosion rates of recycled samples were greater than those of virgin samples, but they did not exhibit a consistent pattern. The use of recycled blackening powder resulted in an enhancement in the corrosion resistance. Conversely, in the case of the virgin powder, the improvement was nearly twice as large.
An electron microscope was also used to examine the surfaces of the samples. Figure 13 depicts the surface of the sample in three different scenarios: (a,b) prior to blackening and prior to corrosion; (c,d) subsequent to blackening from corrosion; (e,f) subsequent to blackening and subsequent to corrosion.
Figure 13 clearly illustrates that the blackening resulted in the partial corrosion of the untreated powder particles. Prior to undergoing the blackening process, the particles exhibited a smooth surface. However, after the blackening process, imperfections in the form of pits became evident on the particles, as shown in Figure 13. The existence of these dimples indicates the onset and progression of corrosion processes, which were already active locally during the blackening phase. As a result of the attack, the surfaces of these areas remain active and vulnerable to further corrosion. This is because the thin layer of oxide that typically forms on the surface during SLM processing and provides some protection is absent. This assertion aligns with the findings provided in this paper, which indicate that blackened surfaces exhibit much faster corrosion rates in comparison to untreated surfaces. Additionally, blackened surfaces demonstrate lower corrosion potential and polarization resistance values. Corrosion can be observed on the surface as the presence of corrosion products, which appear as small, bright patches on the frames; see Figure 13e,f.
EDX analysis was also performed on the blackened surface before and after the corrosion tests. The locations of analysis and the obtained spectra can be seen in Figure 14. The results of the EDX analysis are recorded in Table 10 and Table 11. The results of the EDX analysis showed that copper and selenium, which were present in the used ink, were found on the surface of the sample after blackening. Corrosion formed on the surface due to oxygen- and iron-rich products from the metal matrix.

4. Discussion

Two types of powders were used for printing. The first, labeled the virgin powder, was an unused powder supplied by the manufacturer. After the objects were printed, the excess powder from the construction chamber was used again for printing and therefore was referred to as recycled. The powders used were examined on a scanning electron microscope. Both the virgin and recycled powders did not have entirely spherical particles, but many of them had satellites on the surface, which had a slightly different chemical composition, but not to the extent that they were particles of a different material. The unchanged morphologies of the powders and negligible variations in their chemical compositions correspond to the results of other studies dealing with the issue of powder recycling in additive manufacturing. The work of Mohd et al. [45] focused on powders of steel 316L and aluminum alloy AlSi10Mg. In the article by Cordova et al. [46], the four most used powders for SLM technology are compared, namely Inconel 718, Ti6Al4V, AlSi10Mg, and the Scalmalloy alloy composed of aluminum, magnesium, and scandium. In all cases, powder recycling did not affect the shape or chemical composition of the powders. In addition to the particles of maraging steel, the recycled powder also contained particles that consisted only of aluminum and silicon. These particles probably entered into the powder from the printing chamber, in which another material, probably silumin (AlSi alloy), was printed before the maraging steel was printed. The contamination of the chamber with another powder should be avoided when the powder is reused. Products printed with SLM technology have a characteristic structure that is formed by elongated structures in the direction of printing, resembling teardrops in the transverse direction, and forms a layered structure. The recycled powder had no effect on the structure of the material. The work of Sun et al. [39] confirmed that maraging steel samples printed from both recycled and unused powders have the same martensitic-type structure. Regarding the micropurity, both samples contained defects and impurities. Pores and cavities of various sizes were present. Some of them were formed as remnants of gas bubbles present in the material during printing. The incomplete melting of the powder created drop-shaped cavities called keyholes; microcracks also occurred. The inclusions present were subjected to EDX analysis. Oxygen- and titanium-rich inclusions were present in the virgin sample, and the elongated inclusions were also iron-rich. Inclusions containing a large amount of aluminum were found in the recycled sample. From the high oxygen content, it can be estimated that the inclusions were probably complex oxides of these elements. Apart from the presence of aluminum in the recycled sample, the inclusions and defects were comparable in both samples. The presence of complex oxides of aluminum and titanium was confirmed for both types of powders in the work of Sun [39]. The reason given here is that, although the printing was carried out in a protective atmosphere, part of the oxygen remained in the chamber and reacted preferentially with these elements. The porosity of the printed samples was comparable for both powders. This statement is supported by the work of Sun [39] for maraging steel of a different grade, but also by the work of Giganto et al. [47], in which stainless steel 17-4 PH was used. Thus, the reuse of the powders did not affect the resulting porosity of the prints. In the case of our material, in order to make the results more accurate, it would be necessary to perform measurements on a recycled sample not contaminated with aluminum. The surface properties were measured on untreated samples and on samples subjected to cold blackening. By measuring the roughness, it was found that blackening and the type of powder had no effect on these parameters. The values of the parameters Ra and Rz were very similar. To refine the results and verify the influence of the recycled powder, it would be necessary to perform more measurements on more samples. From the work of Rayan et al. [48] and Sukal et al. [49] for MAR 300 steel, however, it can be said that the recycled powder has no effect on the roughness parameters. The wettability measurements showed that the blackened surface behaved more hydrophobically, and the wetting angles for both types of powders were higher than 125°, as compared to the untreated surfaces, where they reached 83°. The recycled powder showed, on average, lower wetting angle values than the virgin powder, but the deviations in the measurements were quite high, preventing us from drawing conclusions about whether the type of powder would affect the wettability of the surface. The corrosion properties were also evaluated on the untreated and blackened surfaces. All samples were subjected to a potentiodynamic corrosion test in three-year-thick solutions. The first solution represented an acidic environment with pH 3; the second, on the contrary, was alkaline, with pH 11; and the last solution was a 3.5% NaCl solution. In the acidic environment, the corrosion rate was measured to be higher for the blackened surfaces than for the untreated surfaces. However, the results did not show a trend in which the recycled powder had an effect on the corrosion properties. The untreated surface of the virgin powder had slightly better corrosion resistance than the recycled powder. Conversely, the blackened surface of the virgin powder had a corrosion rate that was three times higher than that of the recycled powder. The results for the alkaline environment again show that blackening did not help to increase the corrosion resistance. The untreated surfaces of both samples had lower corrosion rates than the blackened samples. The effect of powder recycling on the corrosion properties could not be assessed again. On the untreated surface of the recycled powder, the corrosion rate was five times lower than on the virgin powder, while, on the blackened surfaces, the opposite was observed. The difference in the corrosion rate between the surfaces of the recycled powder was more than twenty times. When exposed to a 3.5% NaCl solution, higher corrosion rates were measured on the recycled powder samples than on the virgin powder samples. The results are again incongruous, where blackening improved the anti-corrosion properties of the recycled powder but, on the other hand, led to their deterioration. From the results of any of the corrosion tests, it cannot be said with certainty that the recycled powder would worsen or improve the corrosion properties of the material. The measured data were so inconsistent that further measurements would be necessary to determine the effect. In the work of Barile et al. [50], dealing with the effect of powder recycling on the corrosion properties of AlSi10Mg alloy prints, it was concluded that recycled powder does not significantly affect the corrosion properties of 3D prints.

5. Conclusions

The aim of this work was to determine whether the recycling of powder for 3D printing via the SLM method has an effect on the surface and selected technological properties of M300 steel. The results can be summarized in the following points.
  • The chemical composition of the recycled powder is comparable to the unused powder, if any, and the printing chamber is free of all powder residues from previous prints. The same applies also to the chemical composition of the printed objects. In the case of this work, the powder was contaminated with aluminum and silicon particles. These elements subsequently influenced the chemical compositions of the prints from the recycled powder.
  • The particles for both the virgin and used powders were discovered to have mostly a spherical shape, with a notable proportion of satellites sticking to the surfaces of the particles. The analysis of both powders indicates that the gas atomization process results in a non-ideal spherical form for minor amounts of particles.
  • The particle size distribution (PSD) showed that the size of the particles for the virgin powders varied from 10 to 90 µm, and it ranged from 15 to 90 µm in the case of the recycled powder. This was considered to be the most suitable distribution for the given application.
  • The type of powder used does not affect the resulting microstructure of the material.
  • Reusing the powder does not affect the resulting porosity of the objects. The virgin powder samples exhibited a mean value of 0.07% for both parallel and perpendicular sections, while values of 0.09% and 0.10% were found in the case of the used powder.
  • The measurement of the roughness on untreated and blackened surfaces of both types of samples showed that the type of powder or surface treatment by blackening does not affect the surface roughness parameters.
  • The corrosion resistance showed variability and was influenced by the surface treatments. Blackened surfaces exhibited partial corrosion, independently of whether the virgin or recycled powder was used. This suggests that a recycled powder may not consistently maintain its corrosion resistance under all conditions.
This study indicates that recycled M300 maraging steel exhibited many properties that were similar to those of the virgin powder, but it did not eliminate the existence of some differences, which require further testing. Further research and expansion are necessary, particularly in the practical applications of recycled powders, given the observed differences in the corrosion resistance and contamination risks.

Author Contributions

Conceptualization, J.H. (Josef Hlinka), M.K. and A.M.; methodology, A.M.; software, J.H. (Jiri Hajnys), J.H. (Josef Hlinka). and A.M.; formal analysis, A.M.; investigation, A.M.; data curation, J.H. (Josef Hlinka); writing—original draft preparation, J.M., J.P., M.K., I.J., M.P., J.H. (Josef Hlinka), I.J. and A.M.; project administration, J.P.; funding acquisition, J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This article was co-funded by the European Union under the REFRESH—Research Excellence For Region Sustainability and High-Tech Industries, project number CZ.10.03.01/00/22_003/0000048, via the Operational Programme Just Transition and has been conducted in connection with the Students Grant Competition SP2024/087, “Specific Research of Sustainable Manufacturing Technologies”, financed by the Ministry of Education, Youth and Sports and Faculty of Mechanical Engineering, VŠB-TUO.

Data Availability Statement

The datasets presented in this study are available on request from the corresponding author; they are not publicly available as they form part of ongoing research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Jmmp 08 00267 g001
Figure 1. Schematic diagram showing the building of samples by SLM: (a) SLM build, (b) scanning strategy.
Figure 1. Schematic diagram showing the building of samples by SLM: (a) SLM build, (b) scanning strategy.
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Figure 2. Specimen with marked area of blackening.
Figure 2. Specimen with marked area of blackening.
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Figure 3. M300 maraging steel powder morphology of the particles (SEM) with 500× magnification: (a) virgin powder; (b) recycled powder.
Figure 3. M300 maraging steel powder morphology of the particles (SEM) with 500× magnification: (a) virgin powder; (b) recycled powder.
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Figure 4. M300 maraging steel powder morphology (BSE) with 2000× magnification: (a) virgin powder; (b) used powder.
Figure 4. M300 maraging steel powder morphology (BSE) with 2000× magnification: (a) virgin powder; (b) used powder.
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Figure 5. Particle size distribution for virgin and recycled M300 maraging steel powders.
Figure 5. Particle size distribution for virgin and recycled M300 maraging steel powders.
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Figure 6. Micro-images of surfaces of samples used for porosity determination with 100× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
Figure 6. Micro-images of surfaces of samples used for porosity determination with 100× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
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Figure 7. Microstructure with 100× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
Figure 7. Microstructure with 100× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
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Figure 8. Macrostructure with 500× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
Figure 8. Macrostructure with 500× magnification: (a) virgin powder section perpendicular to the printing plane; (b) recycled powder section perpendicular to the printing plane; (c) virgin powder section cut parallel to the printing plane; (d) recycled powder section cut parallel to the printing plane.
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Figure 9. Contact angles: (a) virgin specimen; (b) recycled specimen; (c) virgin specimen after blackening; (d) recycled specimen after blackening.
Figure 9. Contact angles: (a) virgin specimen; (b) recycled specimen; (c) virgin specimen after blackening; (d) recycled specimen after blackening.
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Figure 10. Polarization curves, pH 3 solution.
Figure 10. Polarization curves, pH 3 solution.
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Figure 11. Polarization curves, pH 11 solution.
Figure 11. Polarization curves, pH 11 solution.
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Figure 12. Polarization curves, 3.5% NaCl solution.
Figure 12. Polarization curves, 3.5% NaCl solution.
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Figure 13. Images of the sample surface in secondary electrons: (a,b) unblackened, pre-corrosion; (c,d) blackened, before corrosion; (e,f) blackened by corrosion.
Figure 13. Images of the sample surface in secondary electrons: (a,b) unblackened, pre-corrosion; (c,d) blackened, before corrosion; (e,f) blackened by corrosion.
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Figure 14. EDX analysis areas of the surface: (a) before corrosion (b) after corrosion.
Figure 14. EDX analysis areas of the surface: (a) before corrosion (b) after corrosion.
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Table 1. Fabrication parameters.
Table 1. Fabrication parameters.
ParameterValue
Laser power250 W
Printing strategyStripe
Hatch spacing0.11 mm
Scan speed650 mm/s
Preheat temperatureAmbient
Layer thickness 50 μm
Table 2. The virgin M300 maraging steel’s chemical composition (wt.%).
Table 2. The virgin M300 maraging steel’s chemical composition (wt.%).
ElementAlNiCoMoTiVSiFe
Manufacturer-17–197–104.50–5.200.30–1.20-0.08Bal
Zone 10.7117.209.044.130.780.270.2267.65
Zone 20.7316.869.323.850.88-0.3568.01
Zone 30.7217.409.433.500.800.130.2267.82
Zone 40.8312.3711.331.430.76--73.27
Table 3. The used M300 maraging steel’s chemical composition (wt.%).
Table 3. The used M300 maraging steel’s chemical composition (wt.%).
ElementAlNiCoMoTiVSiFe
Manufacturer-17–197–104.50–5.200.30–1.20-0.08Bal
Zone 11.3917.909.124.550.800.100.2465.92
Zone 289.25-----10.75-
Zone 389.80-----10.20-
Zone 41.0115.8610.213.25---69.67
Table 4. Particle size distribution results.
Table 4. Particle size distribution results.
ParameterVirgin PowderUsed Powder
Laser obscuration (%)0.691.34
Dv (10) μm18.4 22.6
Dv (50) μm30.6 34.3
Dv (90) μm50.0 51.1
Table 5. Average porosity values for each sample.
Table 5. Average porosity values for each sample.
SampleAverage Porosity (%)
Virgin section perpendicular0.07 ± 0.02
Virgin section parallel0.07 ± 0.03
Used section perpendicular0.09 ± 0.04
Used section parallel0.10 ± 0.05
Table 6. Values of contact angle and calculated surface energy for REF sample and wrought sample.
Table 6. Values of contact angle and calculated surface energy for REF sample and wrought sample.
SampleContact Angle (◦)Surface Energy (mJ·m−2)RaRzWetting Behavior
Virgin untreated76.58 ± 10.9437.644.68 ± 0.5121.72 ± 1.99Wetting
Virgin blackened83.70 ± 9.8933.155.33 ± 0.1624.17 ± 0.99Wetting
Recycled untreated127.72 ± 7.757.865.62 ± 1.4225.31 ± 0.61No wetting
Recycled blackened131.60 ± 4.006.215.48 ± 0.4724.07 ± 1.86No wetting
Table 7. Results of the potentiodynamic corrosion test in an acidic environment, pH 3.
Table 7. Results of the potentiodynamic corrosion test in an acidic environment, pH 3.
pH 3E (i = 0)Corrosion CurrentRpCorrosion Rate
mVμA/cm2kOhm·cm2μm/Year
Virgin untreated−478.6012.303.46144.20
Virgin blackened−475.2067.270.84788.90
Recycled untreated−493.2016.973.04198.90
Recycled blackened−454.6021.672.32254.10
Table 8. Results of the potentiodynamic corrosion test in an alkaline environment, pH 11.
Table 8. Results of the potentiodynamic corrosion test in an alkaline environment, pH 11.
pH 11E (i = 0)Corrosion CurrentRpCorrosion Rate
mVμA/cm2kOhm·cm2μm/Year
Virgin untreated−440.504.378.6051.20
Virgin blackened−259.705.029.6058.90
Recycled untreated−242.100.8344.509.70
Recycled blackened−436.9017.262.80202.40
Table 9. Results of the potentiodynamic corrosion test in a 3.5% NaCl solution.
Table 9. Results of the potentiodynamic corrosion test in a 3.5% NaCl solution.
3.5% NaClE (i = 0)Corrosion CurrentRpCorrosion Rate
mVμA/cm2kOhm·cm2μm/Year
Virgin untreated−435.5021.31423.40249.90
Virgin blackened−392.3039.13312.30458.80
Recycled untreated−414.0061.40273.90720.00
Recycled blackened−391.1045.90277.30537.90
Table 10. Results of EDX analysis of the sample after blackening, before corrosion. All proportions of elements are given in wt.%.
Table 10. Results of EDX analysis of the sample after blackening, before corrosion. All proportions of elements are given in wt.%.
ElementOSeALSiWMoTiVFeNiCu
Zone 17.776.942.870.511.083.943.760.0949.9912.332.32
Zone 211.0312.63-0.37-4.130.77-49.4512.332.32
Table 11. Results of EDX analysis of the sample after blackening and after corrosion. All proportions of elements are given in wt.%.
Table 11. Results of EDX analysis of the sample after blackening and after corrosion. All proportions of elements are given in wt.%.
ElementOSeNaErMoClTiFeCoNiCu
Zone 115.22-1.1713.812.880.72.6741.996.1212.143.39
Zone 231.811.754.1-1.550.90.8945.895.557.55-
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Mechali, A.; Hlinka, J.; Kresta, M.; Petrovic, M.; Mesicek, J.; Jahan, I.; Hajnys, J.; Petru, J. Effect of Powder Recycling on the Surface and Selected Technological Properties of M300 Maraging Steel Produced via the SLM Method. J. Manuf. Mater. Process. 2024, 8, 267. https://doi.org/10.3390/jmmp8060267

AMA Style

Mechali A, Hlinka J, Kresta M, Petrovic M, Mesicek J, Jahan I, Hajnys J, Petru J. Effect of Powder Recycling on the Surface and Selected Technological Properties of M300 Maraging Steel Produced via the SLM Method. Journal of Manufacturing and Materials Processing. 2024; 8(6):267. https://doi.org/10.3390/jmmp8060267

Chicago/Turabian Style

Mechali, Abdesselam, Josef Hlinka, Michal Kresta, Marin Petrovic, Jakub Mesicek, Ibrahim Jahan, Jiri Hajnys, and Jana Petru. 2024. "Effect of Powder Recycling on the Surface and Selected Technological Properties of M300 Maraging Steel Produced via the SLM Method" Journal of Manufacturing and Materials Processing 8, no. 6: 267. https://doi.org/10.3390/jmmp8060267

APA Style

Mechali, A., Hlinka, J., Kresta, M., Petrovic, M., Mesicek, J., Jahan, I., Hajnys, J., & Petru, J. (2024). Effect of Powder Recycling on the Surface and Selected Technological Properties of M300 Maraging Steel Produced via the SLM Method. Journal of Manufacturing and Materials Processing, 8(6), 267. https://doi.org/10.3390/jmmp8060267

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