On the Ultrasonic Atomization of SS316L Parts Manufactured via Laser Powder Bed Fusion for the Closed-Loop Production

Abstract

Sustainable feedstock management remains a major challenge in laser beam powder bed fusion (PBF-LB), where conventional reuse strategies are typically limited to sieving and blending rather than full material regeneration. Ultrasonic atomization (UA) offers a fundamentally different powder production route based on capillary-wave instabilities induced at the surface of a molten metal by high-frequency vibrations. In contrast to turbulence-driven atomization, droplet formation in UA is primarily governed by ultrasonic frequency and intrinsic thermophysical properties of the melt, enabling quasi-deterministic particle formation with high sphericity and reduced satellite formation. In this study, ultrasonic atomization was investigated as a closed-loop route for converting PBF-LB-manufactured 316L stainless steel parts into reusable powder. Printed rods were remelted and atomized under controlled variation of electric current and vibration amplitude. The resulting powders were characterized in terms of morphology, internal microstructure, particle size distribution, chemical composition, and gas impurity content. UA produced highly spherical particles with reduced internal porosity and improved flowability compared to the initial gas-atomized powder, while preserving the principal alloying elements. An increase in oxygen content was observed after recycling, attributed to selective high-temperature oxidation under residual oxygen in nominally inert conditions. The results establish a mechanistic framework for transforming consolidated PBF-LB material into secondary feedstock and identify key parameters governing structural and compositional stability in closed-loop recycling.

1. Introduction

Metallic powders for powder-based technologies and additive manufacturing (AM) are commonly produced by several atomization routes, including gas atomization, centrifugal atomization, plasma atomization, ultrasonic atomization, etc., [1,2,3]. Atomization is a commonly used way to obtain metal powders with spherical particles, which provide high flowability, spreadability, tap density, etc., which are crucial for various manufacturing techniques. Gas atomization is the most widely adopted industrial method due to its high productivity and broad material compatibility, although it typically results in relatively wide particle size distributions and the formation of satellites [4]. Centrifugal atomization techniques, such as rotating disk or electrode processes, provide excellent particle sphericity and high chemical purity but are limited in productivity and achievable size ranges [5]. Plasma atomization enables the production of highly spherical powders with minimal defects and narrow size distributions, albeit at significantly higher operational costs and with restrictions on feedstock form [6].

In contrast to turbulence-driven atomization mechanisms, ultrasonic atomization (UA) is based on capillary-wave instabilities induced at the surface of a molten metal by high-frequency ultrasonic vibrations, typically in the range of 20–60 kHz [7,8,9]. The characteristic wavelength of these capillary waves, and consequently the resulting droplet size, is primarily governed by the ultrasonic frequency and intrinsic thermophysical properties of the melt, such as surface tension and density [10,11,12]. This frequency-controlled breakup mechanism enables quasi-deterministic control of particle size and leads to exceptionally narrow particle size distributions compared to conventional atomization routes. Droplet detachment occurs with minimal aerodynamic disturbance, allowing surface tension-driven spheroidization to dominate and resulting in highly spherical particles with a negligible number of satellites. Although the cooling rates achieved during ultrasonic atomization (on the order of 10³–10⁴ K·s⁻¹) are lower than those typical of plasma-based processes, they remain sufficient to ensure rapid solidification while reducing residual stresses and minimizing process-induced morphological defects [7,13,14].

Among AM techniques, laser beam powder bed fusion (PBF-LB) is prevalent due to a number of advantages, including almost complete freedom in product design, high resolution, and low material waste [15]. For PBF-LB, powder flowability, particle sphericity, and tight control of the particle size distribution are critical parameters governing powder-spreading behavior, layer uniformity, and melt pool stability [16,17]. From this perspective, ultrasonic atomization represents an attractive research-scale route for producing PBF-LB feedstocks with well-defined and reproducible powder characteristics.

In the case of austenitic stainless steel 316L, which is extensively employed in PBF-LB owing to its good weldability and process robustness, ultrasonic atomization provides a distinctive pathway for the recycling of PBF-LB-derived metallic waste, including support structures and failed builds, into secondary powder feedstock suitable for powder bed-based processing [18,19]. Unlike conventional reuse strategies based on repeated sieving and blending [20,21], this approach involves complete remelting of the material, thereby enabling a controlled reforming of particle morphology and size distribution while simultaneously allowing systematic evaluation of changes in chemical composition and impurity levels associated with the recycled feedstock. In this context, ultrasonic atomization offers a well-defined experimental framework to decouple the effects of particle size and morphology from process-related factors and to investigate the influence of atomization parameters on the resulting powder characteristics. Furthermore, this approach enables a detailed assessment of oxygen uptake, elemental composition stability, and phase constitution arising from repeated melting and atomization, as well as their combined impact on powder processability, PBF-LB process stability, and the quality of the fabricated parts. Beyond its immediate relevance for 316L, the development of such a recycling route is motivated by the need to reduce material losses and waste generation in PBF-LB, improve overall material utilization efficiency, and establish scalable processing concepts that can be extended to more expensive and scarce alloys; in this context, 316L serves as a cost-effective and well-characterized model material for systematic methodological development.

The aim of this study is to evaluate ultrasonic atomization as a viable recycling route for PBF-LB-processed 316L stainless steel waste by systematically investigating the effects of atomization parameters and feedstock origin on the morphology, particle size distribution, chemical composition, oxygen content, phase constitution of the produced secondary powders, and suitability for repeated use in the PBF-LB process.

2. Materials and Methods

The experimental work was carried out in accordance with the diagram shown in Figure 1a.

The initial stainless steel powder was used for the first PBF-LB printing to obtain rods, which were then used for ultrasonic atomization. The resulting secondary powder was sieved through a system of screens to obtain a particle size distribution suitable for PBF-LB and was used for the second printing cycle. Thus, a complete cycle of work (Figure 1b) was implemented, simulating recycling of defective builds, supports, and other PBF-LB process waste by ultrasonic atomization using 316L steel as an example, obtaining secondary powder, and verifying its suitability for subsequent 3D printing cycles.

2.1. Raw Material and PBF-LB Process

In this study, virgin 316L stainless steel powder with a fraction of 15–53 μm (supplier Onsint, Moscow, Russia) was used as the starting material. The powder had spherical particles and was intended for various additive applications, including PBF-LB.

The starting material for ultrasonic atomization consisted of rods produced by laser powder bed fusion from virgin 316L stainless steel powder. The PBF-LB process was carried out on an Onsint AM150 (Moscow, Russia) system equipped with an ytterbium fiber laser (IPG Photonics, Marlborough, MA, USA) with a maximum laser power of 500 W, a wavelength of 1070 nm, and a spot size of 80 μm. The laser synthesis process was carried out in an argon atmosphere. The residual oxygen content in the working area did not exceed 0.01%. As a result of the PBF-LB process, 120 rods with a height of 100 mm and a diameter of 5 mm were produced. The main process parameters included laser power of 200 W, scanning speed of 1200 mm/s, scanning step of 0.08 mm, and layer thickness of 0.03 mm.

2.2. Ultrasonic Atomization

The rods were atomized at the ATO Lab Plus US35 ultrasonic metal powder atomization (UA) station (3D LAB sp. z o.o., Warsaw, Poland); a schematic diagram of the equipment is presented in Figure 2. To determine the optimal atomization process conditions that would ensure high powder quality and maximum atomization process productivity, 18 samples were prepared. The preliminary atomization parameters were selected based on the literature data [18,22,23,24]. For each sample, the variable parameters were the electric current and the amplitude of the ultrasonic system vibrations. The values of parameters such as pump power and ultrasonic vibration frequency remained unchanged at 60% and 35 kHz, respectively. The oxygen level in the atomization process did not exceed 100 ppm.

Table 1. Ultrasonic atomization process parameters for the rods obtained via PBF-LB.

Designation	Electrical Current, A	Vibration Amplitude, %	Pump Power, %	Ultrasonic Frequency, kHz
70A60%	70	60	60	35
70A70%	70	70
70A80%	70	80
85A60%	85	60
85A70%	85	70
85A80%	85	80
100A60%	100	60
100A70%	100	70
100A80%	100	80
120A60%	120	60
120A70%	120	70
120A80%	120	80
140A60%	140	60
140A70%	140	70
140A80%	140	80
160A60%	160	60
160A70%	160	70
160A80%	160	80

depicts the process conditions for all samples and their designation for the next sections.

Three rods were atomized for each sample to ensure the reliability of the results and to obtain a sufficient amount of powder for research. It should be noted that increasing the current to 180 A led to instability in the atomization process, as a result of which the spraying process was stopped and the powder obtained was not subjected to further research.

Preliminary trials were conducted to determine the operational stability window of the UA process. It was established that currents above 170–180 A led to arc instability and excessive melt fluidity, while currents below 70–85 A promoted the formation of oversized molten droplets that were difficult to atomize in a controlled manner. In the latter case, a breakup occurred in an irregular and unstable regime. Therefore, the systematic investigation was limited to a current range of 70–160 A, where stable droplet formation and reproducible atomization conditions were achieved.

In order to remove remnants of the original rods and large powder particles unsuitable for the PBF-LB process, the atomized powder was sieved on an ED-6700 electrodynamic vibrating table (Ekros, Saint Petersburg, Russia) through a sieve with a mesh size of 53 μm. The efficiency of the atomization process was evaluated by the yield of usable powder, i.e., the percentage of powder with a fraction of less than 53 μm from the total mass of the rods. The mass of the rods and atomized powder was determined on ZMD-2 laboratory scales (Fangrui, Shenzhen, China).

2.3. Powder Analysis

The particle morphology was studied using a Tescan Vega 3 SBH scanning electron microscope (Tescan, Brno, Czech Republic) equipped with secondary electron (SE) and backscattered electron (BSE) detectors. Additionally, SEM images were processed in ImageJ ver. 2.16.0/1.54p software to evaluate the sphericity of the powder particles. The sphericity coefficient was calculated as follows:

$$C_s={\displaystyle \frac{4\pi S}{P^2}}$$

(1)

where C_s is the sphericity coefficient, S is the particle projection area, μm², and P is the perimeter of the particle projection, μm.

Local determination of chemical composition was performed using energy-dispersive X-ray spectroscopy (EDS). The chemical composition of the powder was studied using an inductively coupled plasma optical emission spectrometer, PlasmaQuant 9100 Elite (Analytik Jena, Jena, Germany). The measurement results were processed using Aspect PQ ver. 1.4 software with algorithms for automatic baseline and spectral noise correction. For analysis, two 0.3 g samples were selected from each powder sample atomized under regimes 1–18. To study elements such as Al, Cr, Ni, Mn, Mo, Cu, V, and Co, one of the powder samples was dissolved in a solution consisting of 20 mL of distilled water, 5 mL of sulfuric acid, and 7 mL of nitric acid. Nitric acid was added to the heated solution. The solution was boiled for 2 min and then cooled. The cooled solution was transferred to a 500 mL volumetric flask and brought to the mark with distilled water. For Si analysis, 20 mL of distilled water and 5 mL of sulfuric acid were added to the powder sample. The resulting solution was heated, and then 7 mL of nitric acid and 2 mL of hydrofluoric acid were added. The Si content was analyzed in a cooled solution that had been aged for 24 h. At the same time, a chemical blank sample was prepared by repeating the dissolution and dilution procedure without adding the powder sample. This blank sample was used to account for the background signal from acids and distilled water. Ready-made calibration solutions were used for calibration. The concentration of each element directly obtained from the device in mg/L (ppm) according to the calibration graph was converted into a mass fraction in the powder.

For metallographic examination, powder particles were pressed into conductive epoxy resin. Grinding and polishing were performed on a Polylab P12Mk machine (Kemika, Moscow, Russia). To reveal the microstructure, the powder samples were kept in a solution consisting of 30 mL of distilled water, 20 mL of hydrochloric acid, and 10 g of trivalent iron chloride for 30 min.

The particle size distribution of the powder was studied using laser diffraction with an Analysette 22 particle size analyzer (Fritsch, Idar-Oberstein, Germany). The width of the particle distribution was estimated by the span that was calculated by the following expression:

$$Span={\displaystyle \frac{D_{90}-D_{10}}{D_{50}}}$$

(2)

The mass fraction of oxygen was measured by reduction melting in an inert gas stream on an ONH-100 analyzer (Melytec, Moscow, Russia).

The flow rate of powders was measured using a Hall funnel in accordance with ISO 4490 [25].

3. Results and Discussion

3.1. Powder Morphology

The first stage of this work was to study the powders obtained by ultrasonic atomization of printed rods under various conditions. For this purpose, a comparative analysis was carried out with the original virgin 316L stainless steel powder. Figure 3 shows SEM micrographs demonstrating the morphology of 316L stainless steel powder samples. Figure 3A,B show virgin 316L powder; Figure 3C,D show 316L UA powder 70A60%; Figure 3E,F show UA powder 160A80%.

According to SEM micrographs (Figure 3), particles of virgin 316L stainless steel powder are predominantly spherical in shape. A large number of surface defects, such as satellites and shells, are observed. Shell formation occurs when recrystallized particles collide with a drop of melt. Along with this, both agglomerates and particles with a shape close to spherical are found in the powder. The sphericity coefficient of virgin powder averages 0.56.

The morphology of particles obtained by ultrasonic atomization is more spherical compared to the particles of virgin 316L powder. The sphericity coefficient values for powder samples obtained at different atomization parameters vary in the range of 0.84–0.90. The surface of the particles is smooth and has a minimal number of satellites. The production of highly spherical particles with a negligible number of satellites is a direct consequence of the ultrasonic atomization mechanism: droplet breakup under the action of capillary waves occurs in isolation with minimal interaction between the forming particles, which does not create conditions for the formation of satellites and shells.

A detailed analysis of particle morphology at higher magnification revealed a relationship between powder particle size and surface quality (Figure 4A,B): the larger the particle size, the more pronounced the surface relief becomes. The pronounced surface relief observed on ultrasonically atomized 316L particles can be attributed to the capillary-wave-driven breakup mechanism inherent to ultrasonic atomization, combined with the rapid solidification of detached droplets. During atomization, standing capillary waves develop on the melt surface, imparting a periodic modulation that is partially inherited by the emerging droplets. For sufficiently rapid solidification, these surface perturbations become kinetically frozen, resulting in a characteristic micro-relief. The increasing roughness with particle size can be rationalized by the quadratic scaling of droplet cooling time with diameter, which allows larger droplets to sustain liquid surface instabilities for longer periods prior to complete solidification. As a result, nonlinear capillary-wave evolution, thermocapillary convection, and secondary melt redistribution can develop more fully, leading to enhanced surface structuring. Similar size-dependent roughening effects have been reported in rapidly solidified metallic droplets produced by ultrasonic and centrifugal atomization, supporting the proposed mechanism.

The obtained element distribution maps show that there are areas with increased aluminum and silicon on the surface of atomized particles smaller than 5 μm (Figure 4A,B). Along with this, in powder samples 70A80% and 100A80%, there are particles on the surface of which areas with a high content of manganese, silicon, and oxygen were found (Figure 4C).

Localized surface regions enriched in Mn, Si, and O were detected on a fraction of particles produced under atomization conditions 100A60% and 70A80%, with higher amplitude leading to more intensive cavitation and capillary waves causing the disintegration of the surface liquid film [26]. This compositional signature indicates the formation of manganese–silicate oxide phases, which can be rationalized by the preferential oxidation of Mn and Si during droplet flight and rapid solidification. Owing to their high oxygen affinity, both elements tend to segregate toward the melt surface, where they readily react with residual oxygen present in the atomization atmosphere. Elevated melt temperatures and prolonged liquid lifetimes associated with these processing conditions further promote elemental diffusion, evaporation–condensation phenomena, and thermocapillary convection, thereby enhancing localized oxide formation. The preferential occurrence of such features under specific regimes suggests a strong coupling between atomization intensity, droplet thermal history, and surface oxidation kinetics, which is particularly relevant in the context of recycling-based powder production.

Thus, ultrasonic atomization significantly improves the morphology of the powder compared to gas atomization. The reduction in the number of satellites and agglomerates and the increase in sphericity are factors that directly affect the rheological properties of the powder, so reduced interparticle friction, better flowability, and denser packing are expected during the formation of the powder layer in the PBF-LB process, which will be discussed below.

In addition to particle morphology, an important characteristic of the powder is the particle size distribution, which directly influences flowability and packing behavior. The influence of ultrasonic atomization parameters on the particle size distribution of 316L powder is shown in Figure 5. Within the investigated range, electric current and vibration amplitude have a limited effect on cumulative particle size distribution. The characteristic values remain relatively stable: D10 varies between 13.5 and 15.0 μm, D50 between 31.0 and 34.0 μm, and D90 is approximately 49 μm. It should be noted that particle size measurements were performed after sieving the atomized powder. Therefore, large differences in the upper particle size range were not expected, particularly for D90, since the classification procedure itself constrains the distribution.

Local deviations observed at a vibration amplitude of 70% most likely correspond to a transition between atomization conditions rather than a systematic parameter effect. In ultrasonic atomization, small changes in operating conditions can temporarily disturb the capillary-wave breakup and lead to fluctuations in particle size distribution.

Comparison with the initial gas-atomized powder is presented in Figure 6 and shows that ultrasonic atomization produces slightly coarser particles. The median particle size increases by approximately 30%. Nevertheless, a D50 value around 31 μm remains within the optimal range for PBF-LB processing and is therefore not expected to adversely affect build stability.

At the same time, the width of the particle size decreases with increasing current and vibration amplitude, from 1.15 to 1.00 (

Table 2. Characteristics of atomized 316L powders.

Current, A	Vibration Amplitude, %	d10, μm	d50, μm	d90, μm	Flow Rate, s/50 g	Span
70	60	14	31	49	5.2	1.15
70	70	14	30	49	4.3	1.17
70	80	14	26	48	4.5	1.31
85	60	14	30	48	4.4	1.13
85	70	15	31	49	4.2	1.10
85	80	15	32	49	5.1	1.06
100	60	14	32	49	4.6	1.09
100	70	12	24	47	4.2	1.46
100	80	15	32	49	4.3	1.06
120	60	13	30	48	4.4	1.17
120	70	12	23	45	4.7	1.43
120	80	15	32	49	3.9	1.06
140	60	12	29	47	4.4	1.21
140	70	13	28	47	5.1	1.21
140	80	13	29	48	3.6	1.21
160	60	14	32	49	4.1	1.09
160	70	15	29	48	3.9	1.14
160	80	15	34	49	3.7	1.00

). A narrower distribution improves powder flowability, reflected in the reduction in flow time from 5.2 s to 3.66 s. Such a distribution is generally considered beneficial for PBF-LB due to more uniform layer packing and stable powder spreading.

The narrow particle size distribution affects the flowability of the atomized powder; namely, it reduces the flow time from 5.2 to 3.7 s. It should be noted that the narrow powder particle distribution (span~1.00) enhances the density of the powder layer after the recoating process. Dense packaging of powder influences the number of metallurgical pores during the consolidation via the PBF-LB process. The main mechanism for metallurgical pore formation is connected with trapped gas (argon in the present study) inside the powder media. During laser melting, gas escapes from the melt pool as bubbles due to buoyancy force and Marangoni convection. However, the probability of trapped gas is present for all materials. Moreover, no clear dependence of metallurgical pores’ quantity on PBF-LB process conditions was found; i.e., metallurgical pores can be found in the whole range of process parameters. Additionally, lower density of powder will increase the fraction of gas inside the powder layer; therefore, the probability of pore appearance will be increased for powders with higher span values. Thus, recycling of the powder by ultrasonic atomization can elevate the relative density of final products.

3.2. Powder Microstructure

Representative cross-sectional SEM images of particles obtained by gas atomization and ultrasonic atomization are shown in Figure 7. Both powders exhibit microstructures typical of rapidly solidified austenitic stainless steels, consisting of a cellular–dendritic network formed during rapid solidification of individual droplets. For the virgin gas-atomized powder (Figure 7A), the internal structure appears relatively uniform across the particle cross-section without a pronounced radial gradient at the investigated particle size scale. No clear core–shell morphology is observed, indicating that heat extraction sufficiently rapidly occurred to suppress strong internal thermal gradients within the droplets. Occasional small rounded pores are observed within some particles, which can be attributed to entrapped gas or shrinkage during the final stage of solidification. Particles produced by ultrasonic atomization (Figure 7B) exhibit a noticeably finer and more homogeneous cellular–dendritic morphology. The overall structure is more uniform compared to the gas-atomized reference powder. In contrast to the gas-atomized reference powder, the number of internal pores is lower, and no distinct central shrinkage cavities are observed within the investigated particles.

The observed differences can be explained by the distinct droplet formation mechanisms. During gas atomization, droplets are exposed to turbulent gas flow, which can promote gas entrapment and internal melt circulation prior to solidification, leading to occasional pore formation. In ultrasonic atomization, droplets individually detach due to capillary-wave instability and solidify under more isolated conditions, reducing the probability of gas capture and allowing more uniform solidification front propagation. The combination of finer cellular structure and reduced internal porosity suggests a shorter liquid lifetime and more stable solidification conditions in ultrasonically atomized particles. Such characteristics are expected to be beneficial for PBF-LB processing, as internal particle porosity may act as a source of lack-of-fusion defects or gas pores during remelting.

3.3. Chemical Composition of the Powder

The results of determining the chemical composition of the powder, shown in

Table 3. Chemical composition of 316L powder samples [wt. %].

Element	Standard Composition of 316L [27]	Virgin Powder	Atomized Powder (70A60%)	Atomized Powder (160A80%)
Cr	15.5–17.5	15.54 ± 0.716	15.75 ± 0.264	15.92 ± 0.078
Ni	10–14	11.72 ± 0.824	11.34 ± 0.120	11.44 ± 0.180
Mo	2–3	2.03 ± 0.085	2.27 ± 0.027	2.32 ± 0.028
Mn	≤2.0	0.88 ± 0.094	0.27 ± 0.004	0.28 ± 0.005
Si	≤1.0	1.20 ± 0.032	1.33 ± 0.069	1.37 ± 0.064
Cu		0.10 ± 0.018	0.12 ± 0.001	0.12 ± 0.005
Al		0.10 ± 0.002	0.13 ± 0.001	0.12 ± 0.002
V		0.07 ± 0.018	0.09 ± 0.001	0.09 ± 0.012
Co		0.26 ± 0.030	0.23 ± 0.003	0.23 ± 0.002
Fe	Bal.	Bal.	Bal.	Bal.

, allow us to compare the chemical composition of 316L powder samples obtained by gas and ultrasonic atomization methods, as well as to evaluate the effect of ultrasonic atomization on the chemical stability of the material.

The mass fraction of silicon (Si) was found to be 0.2% higher than specified, which may be due to the heterogeneity of the source material, as well as the presence of trace amounts of aluminum (Al), copper (Cu), cobalt (Co), and vanadium (V). The content of the main alloying elements (chromium (Cr), nickel (Ni), and molybdenum (Mo)) is within the standard range.

In order to study the effect of ultrasonic atomization on the change in the chemical composition of 316L powder, powder samples were obtained at the minimum and maximum atomization parameters, namely, a current of 70 A and a vibration amplitude of 60% and a current of 160 A and a vibration amplitude of 80%. Analysis of the results revealed the following patterns:

• In atomized powders, there is a significant decrease in the mass fraction of manganese (Mn) by more than three times compared to the initial powder (from 0.88% to 0.27–0.28%). This change in chemical composition is due to the evaporation of Mn, which has the highest equilibrium vapor pressure among all elements in the powder composition. Mn evaporation most likely occurred during the PBF-LB process due to the high temperatures reached in the melt bath. Along with this, this process is characteristic of the atomization process of 316L stainless steel [18,22].

The introduction of Mn into 316L stainless steel as a technological additive is necessary for deoxidizing the steel and converting sulfur into refractory manganese sulfide, which prevents the formation of silicon sulfide at the grain boundaries, causing red brittleness and hot brittleness of the steel. Mn also stabilizes the austenitic phase [28]. The addition of Mn increases the hardness and tensile strength of steel [28,29].

Due to the nature of the atomization process, the exposure time to the electric arc and the existence of the melt during atomization are significantly longer than in the PBF-LB process. In this regard, it can be assumed that there is greater evaporation of Mn directly in the ultrasonic spraying process compared to PBF-LB. This conclusion is consistent with the data presented in the study [30].

In addition, excessive Mn evaporation during PBF-LB can contribute to laser beam instability due to the formation of condensed vapors and the appearance of side effects such as keyhole pores and molten splashes ejected from the melt bath. To minimize the impact of Mn evaporation on the quality and properties of products obtained in the PBF-LB process, the following strategies should be used: parametric optimization of the PBF-LB process to ensure a compromise between sufficient melting and an acceptable level of Mn evaporation and establishing laminar protective gas flow at a sufficient speed above the surface of the powder layer to remove evaporation products. In connection with the above, the Mn evaporation process described above requires control during cyclic atomization and the PBF-LB process.

During the PBF-LB process, due to its high affinity for oxygen, Mn segregation along grain boundaries is expected, with the formation of nano-oxides that block dislocation movement [31,32]. Mn evaporation may contribute to a decrease in the number of corresponding particles, reducing the creep of 316L steel, but this assumption requires further study.

2.

The measured content of Cr, Ni, and Mo in atomized powders is within the range typical for this material, confirming the chemical stability of the process with respect to the main alloying elements of the material.

3.

A slight increase in Si content was observed in atomized powders compared to the original powder. These changes in chemical composition may be associated with local heterogeneity in the distribution of Si in the powder particles.

A comparison of the chemical composition of powders obtained under different atomization samples (70A60% and 160A80%) shows that changes in current strength and vibration amplitude in the studied range do not lead to significant changes in the chemical composition of 316L stainless steel.

3.4. Oxygen and Nitrogen Contents After Recycling Atomization

The oxygen and nitrogen contents measured in the initial gas-atomized powder and in powders obtained by ultrasonic atomization are presented in Figure 8. The recycled powders exhibit a consistently higher oxygen concentration compared to the reference material, whereas the nitrogen content remains nearly unchanged within experimental scatter.

The analysis was performed by inert gas fusion, which determines the total oxygen content irrespective of whether oxygen is present in dissolved form or bound in oxide phases. Therefore, the observed increase reflects a real change in the oxygen balance of the material. The absence of a corresponding rise in nitrogen indicates that additional oxygen was not introduced during handling in air (e.g., moisture adsorption or surface contamination at ambient temperature), since such exposure would typically affect both gases. Instead, the change is associated with processes occurring at elevated temperatures.

The most probable origin of the increased oxygen content is additional oxidation during high-temperature stages of the processing route, i.e., during PBF-LB melting and/or subsequent remelting and atomization. Even in nominally inert atmospheres, a residual oxygen partial pressure is always present. At the temperatures involved in melt pool formation and atomization, molten steel exhibits a strong chemical affinity for oxygen, allowing selective oxidation to occur without noticeable nitrogen uptake. This interpretation is consistent with the observed dependence on atomization parameters: higher current, corresponding to higher melt temperature, results in higher oxygen levels, while larger vibration amplitude tends to reduce the oxygen content, likely due to shorter interaction time between the liquid metal and the surrounding atmosphere. The presence of Mn-Si-O enriched regions on some particle surfaces further supports oxygen fixation during solidification after high-temperature exposure.

Overall, the results indicate that the increase in oxygen content originates from selective high-temperature oxidation under non-ideal inert conditions. Even when processing is carried out in controlled atmospheres, a finite oxygen partial pressure remains and becomes thermodynamically significant at melting temperatures, leading to gradual oxygen accumulation in the recycled material. This implies that in practical recycling routes, the oxygen level should be considered a process-dependent parameter rather than a constant material property. With repeated recycling cycles, the oxygen content is therefore expected to increase at a diminishing rate and approach a quasi-steady level governed by the balance between oxidation reactions and alloy deoxidation capacity. From an application perspective, this behavior suggests that recycled powder does not necessarily need to be used as a stand-alone feedstock: blending the secondary powder with lower-oxygen virgin powder may provide a practical means of maintaining the oxygen content within specification limits for demanding components while still enabling material reuse.

The increase in oxygen content after one recycling cycle also has implications for further re-atomization. From a thermodynamic perspective, continuous linear oxygen accumulation is unlikely, since oxidation during high-temperature processing is governed by residual oxygen partial pressure and the alloy’s deoxidation capacity. After initial oxidation and stabilization of oxide phases, the system is expected to approach a quasi-equilibrium condition in which oxygen uptake is limited by process atmosphere and melt chemistry. Consequently, subsequent recycling cycles would more likely lead to a gradual stabilization of oxygen content rather than unlimited growth. This consideration is important for evaluating the long-term viability of closed-loop powder recycling strategies.

3.5. Efficiency of Ultrasonic Atomization

The efficiency of ultrasonic atomization was evaluated based on the yield of usable powder as a function of two processing parameters: electric current and vibration amplitude. As shown in Figure 9, the yield of usable powder ranges from 47% to 63%. The highest yield (63%) was obtained at the minimum current of 70 A and the lowest vibration amplitude of 60%. At amplitudes of 70% and 80%, the average yield gradually decreased. For example, at 60% amplitude, the average yield for all samples was 59%; when at the amplitudes of 70% and 80%, the average yields were 53% and 51%, respectively. As far as the yield dependence on current is concerned, higher yield values tend to be achieved with increased current.

In addition, the stability of molten droplet formation prior to ultrasonic breakup plays an important role in determining process efficiency. During preliminary observations, at the lower boundary of the investigated current range, formation of relatively large molten droplets was detected. Such droplets require sufficient vibrational energy to undergo stable capillary-wave disintegration; otherwise, breakup may proceed in a less controlled mode, reducing the fraction of usable powder. At elevated current values, increasing melt fluidity may approach the limits of arc stability, which can also affect atomization behavior. Therefore, the effective yield is governed by the existence of a stable operating window in which droplet formation and capillary-wave breakup remain controlled.

Nonlinear dependence of yield on process conditions (current and amplitude) reflects the sensitivity of the process to secondary factors. Rather than being solely governed by applied power, the atomization efficiency is controlled by the stability of the liquid layer formed on the vibrating surface. The electric current primarily affects the melt temperature and therefore viscosity and surface tension of the liquid metal. At low current, the melt remains relatively viscous, suppressing capillary-wave formation and leading to separation of larger droplets and reduced yield of the target fraction. Increasing current decreases viscosity and promotes the formation of stable capillary waves, which results in more regular droplet detachment and a higher fraction of spherical particles. The vibration amplitude mainly controls droplet detachment kinetics. Higher amplitude accelerates liquid displacement and reduces the time required for droplet formation. While this can facilitate breakup, excessively rapid detachment destabilizes the capillary behavior and promotes splashing and the formation of off-size particles, thereby lowering the yield of the usable fraction. This explains the observed decrease in yield when amplitude increases from 60% to 80% at most current levels. The non-monotonic behavior at intermediate amplitudes indicates a transition between stable capillary-wave atomization and a more chaotic breakup.

Thus, the efficiency of ultrasonic atomization is determined by the balance between thermal input, which enables the formation of a stable liquid film, and detachment kinetics, which governs the breakup mechanism. The optimal conditions correspond to a stable capillary atomization mode achieved at high current and moderate vibration amplitude (60%), where droplet formation proceeds regularly and the yield of usable powder is maximized.

It should be noted that in practical applications, support structures and defective PBF-LB parts may exhibit complex geometries. However, since the recycling route involves complete remelting prior to ultrasonic atomization, the original shape of the component does not influence droplet formation. Complex-shaped parts would require preliminary mechanical size reduction or remelting into intermediate feedstock, which represents a standard metallurgical preprocessing step and does not limit the feasibility of the proposed approach.

4. Conclusions

Ultrasonic atomization was investigated as a recycling route for consolidated 316L stainless steel parts via laser beam powder bed fusion (PBF-LB). The process included remelting manufactured rods and the production of secondary powder suitable for further powder bed processing. The following conclusions can be made regarding the reported results:

• Ultrasonic atomization produces highly spherical powder particles with a significantly reduced number of satellites and agglomerates compared to the virgin gas-atomized powder. Particle morphology is governed by capillary-wave-droplet detachment and surface-tension-driven spheroidization, resulting in improved geometric uniformity.
• The internal microstructure of ultrasonically atomized particles is finer and more homogeneous, with reduced internal porosity relative to the reference powder. This indicates more stable solidification conditions and reduced melt circulation during droplet formation.
• The primary alloying elements (Cr, Ni, Mo) remain within the compositional range of 316L after recycling atomization. A noticeable decrease in Mn content was observed due to evaporation during high-temperature processing, while Si shows a slight increase. These changes do not strongly depend on the atomization process conditions within the investigated parameter window.
• The oxygen content of the recycled powder increases, whereas nitrogen remains nearly constant. This behavior suggests selective high-temperature oxidation under non-ideal inert conditions rather than simple atmospheric contamination. The oxygen level therefore becomes a process-dependent parameter in recycling routes and may approach a steady state during repeated cycles.
• The efficiency of ultrasonic atomization is controlled by the stability of the liquid film on the vibrating surface. Increasing thermal input promotes stable capillary breakup and increases yield, while excessive vibration amplitude destabilizes droplet formation. The optimal window of process parameters in this study corresponds to higher current and lower vibration amplitude.
• The particle size distribution after sieving remains relatively stable across processing parameters (D50 ≈ 31 μm), while its width decreases, improving powder flowability. The resulting distribution is suitable for further PBF-LB processing.

Overall, ultrasonic atomization demonstrates the capability to convert PBF-LB waste into reusable powder feedstock while maintaining chemical stability and favorable powder characteristics. Further research will focus on the analysis of material properties after PBF-LB consolidation of secondary atomized powder.