Recycled Stainless Steel as a Sustainable Feedstock for Direct Metal Laser Sintering: Challenges and Opportunities

Abstract

Direct metal laser sintering (DMLS) is an advanced powder bed fusion (PBF) technology widely utilized in the medical device and aerospace sectors for the production of intricate and high-value components. The powdered metal materials used in the DMLS process can be expensive, and it is uncommon for a single build to exhaust an entire batch of powder. As a result, the un-melted powder characterized by differences in particle size and morphology compared to fresh virgin powder is recommended to be recycled for use in subsequent builds. This comprehensive review delves into the essential role that powder quality plays in the realm of DMLS with a particular focus on effective and sustainable powder recycling strategies. In this study, the effects of recycling stainless steel powder, specifically used in the DMLS process, are rigorously investigated in relation to the quality of the finished components. This paper monitors critical powder material characteristics, including particle size, particle morphology, and the overall bulk chemical composition throughout the recycling workflow. Furthermore, this review brings to light significant challenges associated with the recycling of stainless steel powders, such as the need to maintain consistency in particle size and shape, manage contamination risks, and mitigate the degradation effects that can arise from repeated usage, including wear, fragmentation, and oxidation of the particles. In addition, this paper explores a variety of recycling techniques aimed at rejuvenating powder quality. These techniques, including sieving, blending, and plasma spheroidization, are emphasized for their vital role in restoring the integrity of recycled powders and facilitating their reuse in innovative and efficient manufacturing processes.

1. Introduction

Metal additive manufacturing is an advanced manufacturing technique that constructs functional components by building them up layer by layer, based on a three-dimensional (3D) model created using Computer-Aided Design (CAD) software. This innovative approach allows for the precise fabrication of complex geometries that would be challenging or impossible to achieve with traditional manufacturing methods.

Among the various technologies in metal additive manufacturing that utilize metal powder as raw material, the most widely employed methods are Selective Laser Melting (SLM), Direct Metal Laser Sintering (DMLS) and Directed Laser Melting (DLM)/Direct Energy Deposition (DED).

Selective Laser Melting (SLM) is a sophisticated additive manufacturing technique that employs a high-powered laser to selectively melt and fuse fine metal powder particles. The process initiates with the precise scanning of a focused laser beam across a bed of metal powder, with the laser meticulously melting specific areas of the powder to create solid layers that conform to the specifications outlined in the CAD model. This layer-by-layer construction method continues incrementally until the entire component is fully formed. This capability not only allows for the creation of intricate and complex designs but also results in lightweight structures characterized by exceptional mechanical properties, making SLM a valuable approach in various engineering applications.

Direct Metal Laser Sintering (DMLS) is another prevalent powder metal additive manufacturing technology in use today. This innovative technique allows for the production of a wide array of materials, including aluminum, cobalt–chromium, nickel, titanium-based alloys, as well as refractive metals, stainless steel, and tool steels. DMLS operates on the principle of sintering, which involves the fusion of metal powder particles through high-energy laser beams, all without reaching the melting point of the material [1]. During the sintering process, the metal powders are bonded at the molecular level through diffusion mechanisms, gradually forming a cohesive mass. The underlying working schematic of DMLS bears a resemblance to that of SLM; however, the critical distinction lies in the method of fusion. In DMLS, the metal powders sinter together at a molecular level instead of melting entirely as in SLM, leading to unique properties in the final manufactured parts [2].

On the other hand, Directed Laser Melting (DLM)/Direct Energy Deposition (DED) involves a different approach. In this computer-aided process, a laser is used as a heat source to create a molten pool on a substrate material. Metal powder is then introduced into this melt pool, where it is heated and melted into the pool of molten metal [3]. Following the principles of dispersion and accumulation, the melted powder accumulates layer by layer, forming the desired three-dimensional part. DLM is particularly advantageous for repair applications as it allows for the addition of material to existing components, as well as for the fabrication of complex geometrical shapes.

The DMLS/SLM are cutting-edge manufacturing processes widely employed in aerospace, defense, and repair applications. This technology is particularly effective for working with a variety of advanced materials, including titanium, stainless steel, Inconel, and other high-performance alloys that are essential for meeting the stringent requirements of these industries [4].

The implementation of DMLS technology faces several noteworthy challenges. One significant issue is the inefficiency in the utilization of metal powder during the construction process, where less than 50% of the material ends up being effectively used [5]. This low utilization rate not only complicates efforts to reduce manufacturing costs but also raises concerns about the overall sustainability and energy efficiency of the manufacturing process.

The effectiveness of the DMLS process hinges on several critical factors, including the precise alignment of the laser, the rate and the layer thickness at which powder is spread over the base plate, the intensity of the laser power, and the inherent properties of the materials being used. Ideally, every powder particle exposed under the laser would successfully fuse, directly contributing to the overall build quality and integrity [6].

However, achieving this ideal scenario often involves trade-offs. For instance, enhancing powder melting efficiency might compromise other important characteristics, such as achieving higher geometric resolution in the printed structures or decreasing overall build times [7]. When the process parameters are finely tuned to optimize for these aspects, such as resolution and speed, the efficiency with which powder is melted can significantly decline.

The powder that remains after the deposition process is typically either discarded as waste or repurposed for future use. To increase efficiency and reduce construction costs, it is usually recommended that the leftover powder undergoes a recycling process. This practice is particularly appealing because a significant volume of material can accumulate after each deposition cycle [8]. Recycling not only offers potential cost savings but also supports sustainable manufacturing practices, contributing to a greener engineering landscape [9].

As a result, there is a growing emphasis on thoroughly understanding the intricate relationship between the characteristics of the leftover powder such as particle size distribution, morphology, and chemical composition and the physical and mechanical properties of the components produced from it. Surprisingly, research on the condition and quality of leftover powder specifically in DMLS systems is quite limited. This knowledge gap poses a challenge, as the effects of using recycled powder on the overall build quality, including strength, durability, and precision of the final products, remain largely unknown and warrant further investigation. Exploring these aspects could lead to more informed practices in powder reuse, ultimately enhancing both the efficiency and sustainability of additive manufacturing processes.

The aim of this review article is to provide a comprehensive overview of the recycling of material powder specifically for the DMLS process. This discussion is particularly focused on the reusability of stainless steel, which has seen a significant increase in demand within the manufacturing sector.

This article delves into several critical aspects of stainless steel recycling/reuse, including the impact of alterations in particle morphology on the material’s performance and characteristics. It explores various methods for recycling leftover powder, examining their effectiveness, efficiency, and suitability for different applications. Additionally, this article addresses the quality of the recycled material, discussing how it compares to virgin stainless steel in terms of mechanical properties and overall reliability.

By summarizing current practices, challenges, and advancements in this field, this article aims to contribute valuable insights for manufacturers looking to optimize their material usage and improve sustainability in the production process.

2. Types of Stainless Steel Alloys

The field of additive manufacturing (AM) has seen a significant rise in the use of various metals and alloys, each chosen for their unique properties and applications. Among the most prevalent materials employed in metallic AM are titanium alloys, stainless steels, aluminum alloys, nickel-based superalloys, and cobalt–chrome alloys [10]. One of the key advantages of using metal powders in this technology is the economic convenience it offers, as it allows manufacturers to work with a diverse array of traditionally high-cost materials [11]. This versatility facilitates the production of intricate shapes and complex geometries that would otherwise be unattainable through conventional manufacturing techniques [12].

The main methods for AM printing metals include Electron Beam Melting (EBM), Directed Energy Deposition (DED)/Direct Energy Deposition (DED), Selective Laser Melting (SLM), Direct Metal Laser Sintering (DMLS) and Binder Jetting. Each of these processes has been optimized for specific types of metal powders that possess tailored characteristics to meet different operational requirements [13]. This ensures the final products exhibit optimal performance concerning mechanical strength, durability, and resistance to degradation, which are critical in demanding applications.

Specifically, stainless steels such as 316L and 17-4PH are widely recognized for their exceptional corrosion resistance, impressive mechanical properties, and overall cost-effectiveness. These alloys find extensive use in various industries, particularly in the manufacturing of tools, medical devices, and structural components, where reliability and performance are paramount. Other frequently utilized stainless steel grades include 304L, 15-5PH, and 410, each selected for its specific attributes that contribute to the effectiveness of the end product in various applications. Table 1 presents a list of common types of steel alloys used in AM.

Table 1. Stainless steel alloys used in AM printing.

No.	Steel Alloys	Reference	Process
1	316	[14]	SLM
		[15]	EB-PBF
		[16]	DED
		[17]	DMLS
2	316L	[18]	SLM
		[19]	EB-PBF
		[20]	DED
		[21]	DMLS
3	17-4 PH	[22]	SLM
		[23]	DED
		[24]	DMLS
4	15-5 PH	[25]	SLM
		[26]	L-PBF
		[27]	DMLS
5	310S	[28]	SLM
6	410	[29]	SLM
7	321	[30]	SLM
		[31]	EB-PBF
8	420	[32]	SLM
		[33]	EB-PBF
		[34]	DMLS
9	430	[35]	SLM
10	303	[36]	DED
11	304	[37]	SLM
		[38]	EB-PBF
		[39]	DED
12	304L	[40]	SLM
		[41]	EB-PBF
		[42]	DMLS
13	321	[43]	SLM
		[31]	EB-PBF
14	904L	[44]	SLM
15	2507	[45]	SLM
16	2205	[46]	SLM
17	4140	[47]	SLM

3. Recycling Methods for Stainless Steel

The practice of powder recycling has emerged as an increasingly vital component of DMLS, it enables the effective reuse of excess or spent powders. This not only helps in minimizing material waste but also significantly reduces production costs, making the process more sustainable and economical. A variety of recycling techniques have been developed to support this initiative, encompassing a wide range of approaches [48]. These methods include more straightforward mechanical recycling techniques, which typically involve sieving or granulating the used powders, as well as advanced technologies such as plasma or laser post-treatment. These cutting-edge recycling processes not only help to restore the quality of the recycled powder but also ensure that it retains the necessary properties for high-performance DMLS applications. Ultimately, the focus on both powder quality and recycling techniques is paramount in pushing the boundaries of what is achievable in DMLS.

3.1. Batch Recycling Strategy

The first strategy employed in powder management is known as the single batch strategy. This approach involves utilizing a specific quantity of virgin powder, sourced from one or multiple traceable powder lots, throughout several consecutive build cycles. After each build cycle, the remaining powder undergoes a filtering process to eliminate any sintered agglomerates or deformed angular particles that may have formed during the process [49]. This allows for the retention of suitable powder while ensuring that any compromised particles are effectively removed. The same batch of powder can be reused until it shows signs of degradation or falls out of specification (OOS), or until there is not enough powder left to complete the next build cycle.

The second strategy, known as the frequent refreshing or Top Up strategy, involves rejuvenating the used powder by mixing it with a specified quantity of virgin powder after a predetermined number of build cycles, or even between each cycle [50]. This approach offers significant advantages for additive manufacturing (AM) users, allowing them to reduce powder costs and extend the usability of the powder over a greater number of build cycles compared to the traditional single batch method [51,52]. However, it is important to note that while the frequent refreshing strategy helps in reducing waste, it only partially mitigates the issue of powder inhomogeneity unless thorough blending is executed. A critical drawback of this method is that some of the reused powder can suffer from a high degree of degradation, potentially affecting the quality of the final products.

Another batch recycling strategy is the batch aging strategy. The batch aging strategy in powder reuse involves storing unused powders from different builds separately and mixing them only when they reach similar aging profiles based on reuse cycles or time [50,53]. This reduces variation in powder properties and ensures consistent build quality. It is ideal for high throughput AM plants but sacrifices traceability and demands extensive storage and batch management. While it improves powder utilization, it requires careful inventory control and frequent quality checks.

3.2. Mechanical Recycling Methods

Mechanical recycling methods serve as effective means for purging large particles and contaminants from metal powders, contributing significantly to their quality and usability. Within the realm of powder recycling such methods are:

Sieving: This technique involves the utilization of a mesh screen through which used metal powders are passed. As the powders are sifted, particles of varying sizes are separated based on their ability to pass through the screen. Sieving is the most prevalent method employed in additive manufacturing due to its simplicity and efficiency [54]. It allows for the rapid identification and removal of larger, undesirable particles, ensuring that only uniform-sized powders remain for further processing.

Centrifugal Separation: This method operates on the principle of centrifugal force to segregate powders according to their density. As the material is subjected to rapid rotation within a separator, heavier particles move outward while lighter ones remain closer to the center [55]. This method is particularly useful in industrial settings where processing large volumes of material is necessary. However, it falls short in effectively removing fine contaminants, which can pose challenges for ensuring the purity of the recycled powder.

In addition to these methods, ball milling represents another mechanical process utilized in powder recycling. In this technique, high-dimensional metal powders are subjected to mechanical forces within a rotating cylinder, where they collide with hardened steel balls that act as grinding media [56]. This process results in the reduction in particle size, converting coarse powders into finer granules suitable for subsequent applications. However, ball milling carries inherent risks, as the intense mechanical forces involved can lead to structural damage in powder particles, such as surface oxidation or contamination from the milling media itself.

3.3. Thermal Recycling Methods

Thermal recycling processes are primarily concerned with the chemical properties of the metal powders, specifically addressing the presence of absorbed gases and impurities that can compromise their quality. In industry, there are two predominant thermal recycling methods that are employed:

Vacuum Degassing: This technique involves subjecting the used powder to elevated temperatures within a vacuum environment. The primary goal is to effectively eliminate any absorbed gases and contaminants that have settled within the powder [51]. While this method can achieve significant purification, it is also notably energy-intensive and has the potential to alter the intrinsic properties of the powders, which can impact their performance in subsequent applications.

Re-sintering: This method entails heating the used powder to a temperature sufficient to fuse together the small particles, thereby removing contaminants and enhancing the overall properties of the powder [57]. However, it is important to note that re-sintering technology has not yet gained widespread acceptance in the field of additive manufacturing. Currently, it remains in the early stages of development for metal powder recycling applications.

In addition to these methods, conventional remelting can also be recognized as a form of thermal recycling. In this process, end-of-life powders are repurposed as scrap material for further melting. However, it is crucial to understand that these powders often fall short of the necessary chemical composition standards. This is typically due to the presence of impurities exceeding acceptable levels, along with a deficit of essential elements in their composition.

3.4. Chemical Recycling Methods

Acid etching is a valuable technique used to enhance the rheological properties of metallic powders, which can significantly influence their behavior during processes such as AM or other additive manufacturing methods [58]. The acid etching helps to reduce the oxide layer on metallic powders, making it especially beneficial for metals such as aluminum and copper, which are prone to oxidation [58]. This method can be applied to both virgin powders those that are newly produced and have not been previously used as well as recycled powders, allowing for improved material performance and efficiency in resource utilization.

In contrast, electrochemical etching serves a similar purpose but employs an electrochemical process to achieve surface modification [59]. This technique is particularly advantageous for treating materials that may resist conventional acid treatments, thereby broadening the range of metals that can benefit from enhanced surface properties. By improving these surface characteristics, electrochemical etching can further optimize the performance and quality of metallic powders in various industrial applications.

3.5. Re-Melting Powder

In this process, waste powder generated during the manufacturing of metal components is transformed into finer powder droplets suitable for reuse [60]. Two prominent techniques for producing metal powders are gas atomization (GA) and ultrasonic atomization (UA), each with unique characteristics and advantages.

Gas atomization is a widely used method that involves the disintegration of molten metal into fine droplets through high-velocity jets of inert gas. This powerful technique leads to the rapid solidification of the droplets, resulting in spherical powder particles with typical dimensions ranging from 10 to 150 microns [61]. The quality of the powders produced by GA is often high, making it a preferred choice for various applications

On the other hand, ultrasonic atomization employs ultrasonic vibrations to generate fine droplets from molten metal. This method typically operates at lower pressures and temperatures, which translates to significantly reduced energy consumption compared to gas atomization [62]. UA not only requires less energy input but also boasts high conversion rates, achieving over 95% of raw material transformed into usable powder. The efficiency and lower operational costs make ultrasonic atomization an appealing alternative, especially for applications where high-quality metal powders are essential.

In summary, while gas atomization excels in producing high-quality powders, it comes with higher energy and operational costs. In contrast, ultrasonic atomization offers a more energy-efficient solution with an impressive conversion rate, creating a compelling case for its use in various metal powder production scenarios.

4. Challenges in Using Recycled Stainless Steel

The quality of metal powder used in the DMLS process is paramount in determining the characteristics and performance of the final printed components. High-quality metal powders offer enhanced flowability, which is crucial for a smooth and efficient DMLS operation. This improved flowability directly contributes to the overall quality of the printing process and the mechanical properties of the end product, ensuring that it meets the desired specifications and standards.

Several key factors significantly influence the quality of the metal powder. These include particle size and shape, which affect how well the powder can flow and pack together during printing. The density of the powder is also critical, as it impacts the strength and durability of the printed parts. Additionally, the roughness of the powder particles can influence surface finish and adhesion between layers.

Moreover, the chemical composition of the powder is essential, as it determines the material properties of the resulting components, such as corrosion resistance and thermal conductivity. The presence of impurities can adversely affect these properties, leading to compromised performance in the final product.

Therefore, a comprehensive understanding and careful control of these various factors are vital for achieving consistent, high-quality results in metal additive manufacturing. By ensuring that the metal powder is of the highest quality, manufacturers can enhance the reliability and effectiveness of their DMLS processes. All these factors are explained in the subsequent subsections.

4.1. Powder Morphology

Powder morphology plays a pivotal role in determining the quality and consistency of structures produced through DMLS and other AM techniques. Key morphological attributes include particle shape, size, size distribution, and surface texture [63]. These characteristics directly affect powder flowability, packing density, and energy absorption during melting, all of which are critical for achieving uniform layer formation and defect-free builds [64].

Metal particle properties play a critical role in governing powder behavior and process performance, particularly in applications such as additive manufacturing. These properties can be broadly classified into morphology, chemistry, microstructure, surface, thermal, and bulk characteristics. Morphological attributes include particle size and size distribution, shape, and surface roughness, which together influence circularity, aspect ratio, and specific surface area, thereby affecting flowability and packing density. Chemical properties encompass both bulk and surface composition, where surface chemistry often dictates oxidation behavior and inter-particle interactions. Microstructural features such as internal porosity, crystal structure, grain size, oxide layer thickness, and phase constitution (single or polycrystalline, gas-filled or not) directly impact mechanical integrity and thermal response. Additional intrinsic properties include true density, hardness, magnetic behavior, and surface charge, the latter being governed by electrostatic potential and tribocharging effects. Thermal stability, characterized by melting temperature, specific heat capacity, and optical properties such as reflectivity and absorptivity, determines energy absorption during processing. Bulk powder behavior is further influenced by tap density, flowability (quantified by basic flow energy), hygroscopicity, surface energy, and wettability, which collectively control powder spreading, layer uniformity, and process reliability [64,65].

Spherical particles with smooth surfaces are generally preferred across all AM processes due to their superior flowability and high packing density [66]. These traits facilitate consistent powder delivery, uniform spreading, and predictable melting behavior. In contrast, irregularly shaped or rough-surfaced particles tend to agglomerate, exhibit poor flow, and cause uneven energy absorption, leading to defects such as porosity, lack of fusion, and surface roughness in the final part [67].

The optimal particle size range varies depending on the AM technique. For instance, Laser SLM/DMLS typically requires fine powders in the range of 15–50 µm to ensure precise layer resolution and controlled melting. The DLM, on the other hand, accommodates larger particles ranging from 50 to 150 µm due to its continuous powder feed and larger melt pool dynamics [68].

Heiden et al. [65] observed that thermal cycling during melting and solidification can alter the surface structure of particles. Specifically, the intense localized heating may shatter rough surface features. Their study compared virgin powders characterized by clean, spherical shapes with reused powders, some of which exhibited visible signs of surface porosity, microcracks, and internal voids. Interestingly, Heiden et al. also reported a reduction in both surface and internal porosity in certain reused particles, suggesting that repeated laser exposure may increase porous structures and weaken the particles.

A similar trend was observed by Nima et al. [69], who studied SS 316L powder and noted its potential for reuse across multiple cycles without significant changes in particle morphology. They discovered that the overall powder size distribution remained relatively stable for several cycles. However, it was also noted that the population of finer particles measuring less than 10 micrometers decreased over time, while the quantity of larger particles increased with each reuse cycle. In Shihua [70], evidence is also provided of an increase in the number of larger particles and a decrease in the finer particles, as shown in Table 2. Additionally, the recycled powder exhibited distinctive characteristics such as numerous bonded particles, fused formations, and clusters, along with the presence of more satellite particles and some instances of particles exhibiting deformations in shape [71].

Table 2. Average particle size and D_10, D_50 and D_90 values of virgin and recycled powder [70].

Powder Type	$\mathbf{Mean} \mathbf{Diameter} \mathsf{\mu }\mathbf{m}$	${\mathit{D}}_{\mathbf{10}}$$\_\mathbf{10}, \mathsf{\mu }\mathbf{m}$	${\mathit{D}}_{\mathbf{50}}$$\mathsf{\mu }\mathbf{m}$	${\mathit{D}}_{\mathbf{90}}$$, \mathsf{\mu }\mathbf{m}$
Virgin	91.19	56.03	85.38	138.1
Recycled primary	95.46	58.72	90.01	143.3
Recycled secondary	100.3	57.11	93.22	157.6

Another essential factor in evaluating the quality of recycled powder is its density, as the presence of voids can significantly impair the flowability of the powder. This issue arises because voids, created during the fragmentation of particles, disrupt the melting process of the powder particles [20,23,24]. Moreover, it is crucial to consider the degree of compaction achieved within the powders. Packing density serves as an important indicator, demonstrating how effectively the powders can occupy a given volume while minimizing the presence of voids. This characteristic directly correlates with the likelihood of defects in the final printed component [72].

In the context of particle size distribution (PSD), the powders exhibit a mix of particle sizes: coarser particles primarily contribute to filling the overall volume, whereas finer particles play a vital role in occupying the spaces between the larger particles. This interplay between particle sizes is key to ensuring optimal packing and overall performance of the powder during printing processes.

4.2. Oxidation and Contamination

Oxidation plays a crucial role in determining the quality of recycled materials used in additive manufacturing. When oxidation occurs, it modifies the chemical composition of the powder’s surface by creating various metal oxides, such as FeO, Fe₂O₃, Cr₂O₃, and CoO [54]. These oxides can significantly influence the reactivity of the powder, as well as its bonding and sintering characteristics throughout the manufacturing process.

Additionally, the oxidation process can induce partial sintering or even melting of the powder particles that are located near the boundaries of the built part. This can result in the undesirable formation of multi-particle aggregates or clusters, which may hinder the overall quality of the final component [73]. Moreover, these transformations can disrupt the powder’s flowability and packing behavior, both of which are essential for achieving consistent and uniform layers during the additive manufacturing process. The interplay of these factors underscores the importance of managing oxidation to ensure high-quality recycled materials in additive manufacturing.

Slotwinski et al. [68] conducted an in-depth comparative analysis of virgin and recycled 17-4 stainless steel, revealing some intriguing findings. Their study indicated that there were no significant differences in elemental concentrations between the virgin and recycled powders after undergoing a sieving process. However, when examining the sieve residual, marked alterations were noted in comparison to the virgin powder. Specifically, the residual demonstrated the absence of metallic iron on its surface, a clear indication of complete oxidation caused by the elevated temperatures of the laser during processing. Furthermore, the analysis reported reduced levels of essential elements such as copper, chromium, and carbon in the residual, while iron and silicon concentrations were noticeably heightened compared to those of the virgin powder.

In a complementary study, Nima et al. [69] provided additional support for these findings by focusing on stainless steel 316L. They meticulously characterized both virgin and recycled powders, analyzing aspects such as microstructure, surface and bulk composition, and hardness. Their results revealed that the recycled powder exhibited greater oxidation on its surface, along with a heightened concentration of metallic oxides. Furthermore, the microstructural evaluation highlighted the presence of more surface satellites, cracks, porosity, bonded particles, and clusters in the recycled powder when compared to the virgin counterpart. Galicki et al. [74] and Heiden et al. [65] both observed a notable increase in oxygen uptake in recycled 316L stainless steel (SS) powder. This uptick can be attributed to two primary factors: the oxidation that occurs during the manufacturing process and the presence of residual oxygen in the build chamber. In a similar vein, Gorji et al. [75] reported an increase in the metal oxide content within the recycled 316L SS powder. They noted a significant morphological transformation where the initially spherical particles evolved into a more irregular shape, accompanied by the formation of agglomerates and spatter. Thus any increase in oxygen absorption and subsequent oxide formation must be carefully considered prior to reusing recycled powders, as these factors can negatively impact the performance and reliability of the final built parts during their service life [76].

4.3. Microstructural Change

The use of recycled stainless steel powder in DMLS introduces several microstructural challenges that can compromise the mechanical performance and reliability of fabricated components. These defects stem from powder degradation, inconsistent melting behavior, and repeated thermal cycling during deposition. As DMLS relies on continuous powder feeding and localized melting, any variation in powder quality especially from reused sources can significantly influence the resulting microstructure. Svozilova et al. [60] revealed a notable alteration in the microstructure of the recycled powder compared to its counterparts. Both the virgin and reused powders exhibited a microstructure predominantly consisting of martensite, characterized by its hardness and strength. In contrast, the recycled powder displayed a different microstructure, featuring a presence of austenite grains along with precipitates located at the grain boundaries [60]. This unique occurrence of austenite, which is inherently softer than martensite, has the potential to significantly influence the mechanical properties of the recycled powders, potentially leading to variations in performance and durability.

Porosity is one of the most common defects identified in builds created from recycled powder. This issue can stem from various factors, including trapped gases within the material, incomplete fusion during the melting process, or surface oxidation that occurs over time [77]. For instance, Revilla et al. [78] noted that the reuse of 316L stainless steel powders in directed energy deposition (DED) processes often resulted in an increase in porosity. They identified that surface oxidation and a decrease in flowability were significant contributors to this porosity, ultimately hindering the complete melting and fusion of the powder during deposition. Saboori t et al. [79] noted that the porosity of the built part increases when using recycled powder. The study shows that the sample produced with recycled powder contains approximately 2.4% spherical inclusions. This increased porosity has significant implications, as it can greatly reduce the material’s fatigue strength and compromise its corrosion resistance.

Cracking presents another major challenge, especially in alloys such as 17-4 PH stainless steel and 316L stainless steel. The rapid heating and cooling cycles characteristic of DMLS processes generate thermal stresses that can form microcracks. This problem is exacerbated when recycled powders contain inclusions or exhibit irregular morphologies [80]. In a recent study, Morales et al. [81] documented the occurrence of microcracking in components fabricated from 17 to 4 PH stainless steel using AM techniques. They highlighted that the reuse of powder aggravated this issue by causing uneven deposition and creating stress concentrations at the boundaries of the grain structures.

Moreover, grain coarsening and variations in microstructure are frequently observed in builds utilizing recycled powders [82]. Inconsistent dynamics within the melt pool often resulting from disparities in particle sizes and shapes can lead to non-uniform solidification [83]. This inconsistency results in elongated or cellular grain structures. Zhi et al. [84] investigated 316L stainless steel and discovered a mix of grain morphologies and misorientation distributions. They found that these microstructural characteristics were largely influenced by the condition of the recycled powder and its reuse history. Such variations in microstructure can significantly impact the mechanical properties of the material, including its ductility and toughness, which are essential for ensuring reliable performance in demanding applications [72].

4.4. Process Parameter Sensitivity

Recycled powders used in DMLS exhibit unique characteristics that set them apart from their virgin counterparts, even when their chemical composition appears unchanged. The process of reusing these powders introduces subtle yet significant alterations in particle morphology, an increase in surface oxidation, and changes in flowability [60]. Each of these factors plays a crucial role in determining how the material interacts with the laser during processing and ultimately impacts melt pool dynamics.

One of the prominent differences between virgin and recycled powders lies in their physical attributes. Virgin powders typically consist of clean, uniform, spherical particles that possess predictable absorption characteristics. This allows for the optimization of processing parameters, like laser power, scan speed, hatch spacing, and layer thickness, to reliably yield dense, homogeneous microstructures [69]. In stark contrast, recycled powders tend to accumulate surface oxides and exhibit slightly irregular shapes alongside broader size distributions. These deviations can significantly alter laser absorptivity and the stability of the melt pool.

As a result, the direct application of parameters fine-tuned for virgin powders to recycled feedstock can lead to undesirable outcomes. For instance, a scan speed optimized for virgin powder may be too rapid for recycled powders, which often demonstrate diminished flowability or increased reflectivity, leading to lack-of-fusion defects [85]. Similarly, hatch spacing that works perfectly with virgin powders may not account for the reduced packing density in recycled powders, resulting in inadequate overlap and void formation. Layer thickness also becomes a critical variable, as recycled powders with compromised flowability may not spread evenly, causing discrepancies in layer height that disrupt melt pool continuity. Collectively, these factors can diminish the final part’s density, compromise surface quality, and introduce inconsistencies within the microstructure [86].

Despite these challenges, research indicates that some degree of parameter transfer can still be achieved, particularly when the recycled powders are only lightly reused or when hybrid feedstocks combinations of virgin and recycled materials are utilized [87]. In such scenarios, minor adjustments such as slightly increasing laser power, decreasing scan speed, or tightening hatch spacing could compensate for the altered behavior of the recycled powders. However, as these powders undergo more cycles of reuse, the differences in their properties amplify, necessitating a full re-optimization of the parameters to maintain the desired mechanical performance. This underscores the importance of developing adaptive parameter strategies alongside in situ monitoring systems capable of detecting signs of melt pool instability and adjusting energy input in real time [60].

In particular, repeated recycling markedly alters certain powder characteristics, making certain DMLS processing parameters much more sensitive to these changes than others. Among all the parameters, laser power and scan speed are particularly affected by powder reuse; the accumulated surface oxidation and roughened particle morphology diminish absorptivity, resulting in the need for increased energy input or slower scanning speeds to ensure stability within the melt pool [20]. Hatch spacing is similarly influenced; as recycled powders exhibit reduced flowability and poorer packing density, they require tighter overlaps to avert void formation.

Layer thickness becomes critical with repeated recycling, as the physical properties of the powders degrade [40]. Moreover, preheat temperature is shown to be also sensitive to variations in particle morphology, flowability, and absorptivity. Recycled powders, having accrued surface oxides, might develop rougher particle surfaces and lose their spherical shape, all of which profoundly alters their thermal conductivity and interaction with the laser. These changes enhance the influence of preheat temperature, as recycled powders tend to react less predictably to thermal inputs, thereby escalating the risk of melt-pool instability or incomplete fusion [88].

Thus, the complexities introduced by the degradation of recycled powders indicate that, while a degree of parameter transfer might be feasible during the early stages of reuse, the cumulative effects necessitate a systematic approach to re-optimization in order to safeguard density, enhance surface finish, and maintain mechanical performance in the final parts produced.

4.5. Mechanical Properties

The mechanical performance of components produced through DMLS is significantly affected by the quality of the powder used, particularly when recycled feedstock is involved. Recycled stainless steel powders, most commonly 316L or 17-4 PH, experience various morphological and chemical transformations over multiple reuse cycles. These transformations can have notable effects on key properties such as tensile strength, hardness, ductility, and fatigue resistance.

Research indicates that the tensile strength and hardness of components fabricated with recycled powders often remain on par with those produced from virgin powders, especially after one or two cycles of reuse [64]. This durability can be attributed to the inherent strength of stainless steel alloys and the high energy input characteristic of the AM process, which ensures adequate melting and fusion of the material. However, when the powder is reused multiple times, there is a decreasing trend in the mechanical properties of the built part. A comprehensive analysis of mechanical properties is presented in Table 3. Also, when it comes to ductility and elongation at break, a decline is commonly observed with increased reuse of powder. This deterioration is likely linked to rising levels of porosity, the development of microcracking, and grain coarsening phenomena within the material.

Table 3. A summary of the mechanical properties and performance of recycled and virgin powder.

	Ultimate Tensile Strength (UTS)		Elongation		Yield Strength (YS)		Number of Times Recycled	Reference
	Virgin	Recycled	Virgin	Recycled	Virgin	Recycled
1	628.2 ± 6.6	651.6 ± 43.9	31.2 ± 2.2	16.2 ± 2.1	469.6 ± 3.4	458.4 ± 29.9	10	[79]
2	602 ± 2	612 ± 2	55 ± 7	44 ± 2	425 ± 6	445 ± 4	9	[5]
3	677.39	679.03	19.75	17.5			1	[70]
4	677.39	670.95	19.75	17.58			2	[70]
5	1334.04	1332.35	18.07	17.07	1513.95	1517.24	4	[89]
6	1334.04	1304.11	18.07	16.31	1513.95	1508.55	9	[89]

In the study conducted by Contaldi et al. [89], the impact of reusing powder on the mechanical properties of two distinct types of precipitation hardening stainless steel was thoroughly examined. These two materials are characterized by their differing primary microstructures: one being martensitic, identified as EOS Stainless Steel PH1, and the other being austenitic, known as EOS Stainless Steel GH1. The researchers focused on assessing how the characteristics of the powder changed during reuse, while also comparing both the static and fatigue behavior of components manufactured with either virgin or reused powder.

The findings revealed the mechanical properties of PH1 displayed a remarkable resilience to the effects of powder reuse, with only a slight influence observed on the elongation at break. In contrast, the properties of GP1 were significantly impacted, particularly regarding yield strength and elongation at break, highlighting the differing responses of the two materials to the reuse of powder.

Fatigue strength emerges as a critical attribute that is particularly sensitive to the degradation of the powder. Defects on the surface, foreign inclusions, and internal pores that are introduced during the recycling process act as stress concentrators, impairing the material’s capacity to endure cyclic loading [90].

5. Opportunities and Advantages

The integration of recycled stainless steel powders into DMLS presents several opportunities that align with both industrial and environmental objectives. While challenges exist in maintaining powder quality, the potential advantages of powder reuse and recycling are substantial. These benefits extend across various spheres, encompassing environmental protection, economic advantages, and application-specific improvements. By utilizing recycled powders, the additive manufacturing industry can reduce waste, conserve natural resources, and lower production costs. The following subsections will delve deeper into these compelling advantages and elaborate on how they contribute to a more sustainable future in manufacturing.

5.1. Environmental Condition

DMLS is gaining recognition as a more environmentally friendly alternative to conventional manufacturing processes. The sustainability of DMLS presents a significant hurdle that needs to be addressed for wider adoption. The ecological impact of AM processes is heavily influenced by the ability to recycle and reuse materials efficiently [91]. As such, researchers have been exploring effective methods for recycling unused material powder generated during the DMLS process. Studies suggest that more than 40% of potential waste material can be eliminated through the implementation of additive manufacturing techniques, and an impressive 95% of the unused material can be repurposed for future use [92]. However, it is important to note that these assertions have not yet been validated across all types of materials.

The environmental impact of the sustainability of AM can be considered in three categories, including resource consumption, waste management and pollution control. The main environmental issue today is resource consumption, particularly energy consumption. However, other factors also need to be considered, such as resource use, pollution, effects on human health, and social impact. To accurately evaluate the sustainability of the AM process and its environmental effects, it is essential to consider all of these aspects equally.

In a significant study, Fabe et al. [93] explored the cumulative energy demand (CED) associated with the metal powder used for fabricating components through AM. By integrating life cycle material flow analysis with detailed process energy intensities, their model provides a comprehensive overview of the energy consumption involved in creating metal AM parts. It also focuses on the recyclability of the metal powder, examining how effective recycling practices can mitigate the environmental impact associated with AM. Overall, their findings underscore the importance of improving sustainability measures within the DLM process to facilitate its adoption and minimize ecological consequences. In another study, Jon et al. [94] highlighted the importance of promoting the reuse and recycling of metal powder to minimize environmental impact. This recycling effort supports corporate sustainability initiatives and ensures compliance with environmental, social, and governance (ESG) frameworks, which are becoming increasingly significant in the aerospace, biomedical, and automotive sectors.

The AM sector is a strong proponent of Ecodesign, also known as Design for Environment, which is recognized in Europe and the United States, respectively. This innovative approach to product development emphasizes the consideration of environmental factors alongside traditional criteria such as functionality, durability, cost-effectiveness, speed to market, esthetics, ergonomics, and overall quality [95]. As consumer demand for various products and goods continues to grow daily, new paradigms have emerged in response to the urgent need for waste reduction and the conservation of natural resources [96].

One significant concept that has gained traction is the circular economy. The European Commission defines this model as a sustainable production system designed to retain the value of products, materials, and resources for as long as possible while minimizing waste generation. The circular economy promotes the creation of high-value material cycles that go beyond conventional recycling practices [97].

An analysis conducted by Sauerwein et al. [98] on various design projects reveals that AM offers unique opportunities to implement circular design strategies such as product upgrades and repairs, which can significantly extend a product’s lifespan, even in cases where these considerations were not part of the initial design phase. However, despite these promising opportunities, fully realizing the potential of AM in supporting design for a circular economy presents several challenges. Key among these is the urgent need to develop methods for high-value reuse and recycling.

5.2. Economic Benefits

One of the most immediate and significant advantages of recycling stainless steel powders in the process of Direct Metal Laser Sintering (DMLS) is the substantial reduction in material costs [99]. The production of virgin stainless steel powders, which occurs through a method known as gas atomization, is both energy-intensive and costly [61]. In fact, the expenses associated with acquiring these virgin powders can constitute as much as 40–60% of the total cost of producing parts in metal additive manufacturing. Consequently, the ability to reuse powder emerges as a crucial lever for enhancing economic viability within this industry [100]. By extending the life of metal powders through multiple reuse cycles or by reconditioning them, manufacturers have the potential to significantly lower their procurement costs, particularly advantageous when producing prototypes or non-critical components [48].

Numerous studies underscore the benefit of blending virgin and recycled powders, showing that this approach can effectively maintain acceptable mechanical properties while simultaneously reducing material costs. For instance, research conducted by Gorji et al. [69] illustrates that recycled 316L stainless steel powders can be utilized multiple times without a notable deterioration in mechanical performance. This not only curtails the waste produced from metallic powders but also helps lower overall printing expenses. Similarly, a study by Pereira et al. [90] highlighted that reused stainless steel powders retained adequate mechanical integrity for tooling applications, yielding cost savings of approximately 20–30% in comparison to utilizing virgin feedstock.

The economic advantages of recycling stainless steel powders reach beyond merely direct material savings [101]. By adopting powder recycling practices, companies can also alleviate waste management and disposal costs since unused or partially sintered powders no longer need to be discarded as scrap [102]. This aspect is especially pertinent in industries such as aerospace and biomedical manufacturing, where stringent quality standards often lead to the generation of large volumes of unutilized powder. By establishing effective powder recycling protocols, organizations can significantly reduce disposal expenses while simultaneously enhancing their overall resource efficiency [103].

Moreover, the implementation of powder recycling supports greater scalability and accessibility within the realm of additive manufacturing. Small and medium enterprises (SMEs), which frequently contend with the high entry barriers posed by the costs of virgin powders, can take advantage of recycled powders to enter the market at a more manageable expense [104]. This democratization of access not only fosters innovation but also accelerates the adoption of additive manufacturing technologies across a variety of sectors.

Lastly, the economic benefits associated with recycling are amplified when integrated with circular manufacturing models. Closed-loop systems, wherein waste powder is systematically collected, processed, and reintegrated into production, not only help to minimize costs but also bolster the resilience of the supply chain [105]. In periods marked by raw material shortages or price fluctuations, recycled powders offer a stable and cost-effective alternative, ensuring uninterrupted production flow and contributing to the sustainability of manufacturing operations.

5.3. Application-Specific Tailoring

Recycled stainless steel powders present a wealth of functional applications that capitalize on their unique characteristics, even when they exhibit slight reductions in mechanical performance when compared to virgin powders. In the biomedical sector, for example, the use of recycled or blended powders can result in a desirable decrease in stiffness, which is particularly advantageous for diminishing the phenomenon known as stress shielding that often occurs in orthopedic implants and scaffolds. The research conducted by Alam et al. [106] provided that recycled 316L stainless steel powders not only maintained biocompatibility essential for any materials used in medical applications but also displayed impressive corrosion resistance. These properties render them particularly suitable for non-load-bearing implants, representing a significant breakthrough in situations where cost considerations and accessibility to medical materials are critical. By using recycled powders, manufacturers can create patient-specific implants more affordably, overcoming one of the major obstacles related to the high price of virgin feedstock [107].

Additionally, recycled powders have proven to be invaluable within the aerospace and automotive industries, especially for the development of prototypes and the construction of jigs and fixtures [108]. Such applications are typically less demanding in terms of mechanical performance compared to components that are vital for flight safety or structural integrity, which allows manufacturers the flexibility to allocate virgin powders for those high-stakes parts. In a notable study by Pereira et al. [90], it was found that reused stainless steel powders retained adequate mechanical integrity for tooling applications. This finding underscores the potential for cost-effective production that simultaneously minimizes material waste. Such a strategic approach not only fosters rapid prototyping cycles but also significantly reduces overall production costs, making recycled powders especially attractive for industries that frequently engage in iterative design improvements [109].

The advantages of recycled powders extend further into the realm of lightweight structural components. A comprehensive study by Gorji et al. [69] demonstrated that 316L stainless steel powders could be reused multiple times without substantial loss of mechanical integrity, making them ideal for producing lattice structures and lightweight supports. In these applications, the emphasis is placed on achieving reduced density and enhanced material efficiency rather than sheer ultimate strength. This capability is particularly important for sustainable design in sectors such as automotive and consumer products, where weight reduction can lead to improved energy efficiency and performance [110].

Another promising avenue is the blending of virgin and recycled powders to develop hybrid feedstocks. Research by Ferro et al. [111] highlighted that such hybrid mixtures of materials, specifically IN718 and 316L stainless steel, could maintain acceptable mechanical properties while significantly diminishing overall powder consumption. This innovative strategy allows manufacturers to fine-tune the powder composition to meet the diverse requirements of specific applications, creating a harmonious balance between sustainability and high-performance material qualities [112].

The recycled powders are well-suited for applications involving tooling and industrial equipment, where attributes such as wear resistance and dimensional accuracy are often more critical than achieving the highest ultimate tensile strength [113]. A similar analysis was conducted by Singh et al. [114] by hybrid mixing sieved SS17-4 and Ti6Al4V powders. The results show good mechanical properties but significantly less than the virgin powders. With carefully optimized processing parameters in place, recycled powders can reach near-full density and deliver acceptable surface finishes, making them a practical choice for producing molds, dies, and fixtures [115]. This application not only maximizes the financial and environmental benefits associated with powder reuse but also ensures that the functional durability of these tools is maintained [116]. Collectively, these multifaceted examples illustrate the broad potential of recycled powders across a diverse array of sectors, including biomedical [117], aerospace [118], automotive [119], and industrial applications, effectively contributing to both sustainability and cost-efficiency in modern manufacturing practices.

An additional dimension of sustainability that deserves emphasis is the potential for alternative reuse of recycled powders across different additive manufacturing platforms. Powders that no longer satisfy the stringent particle size distribution, flowability, or morphology requirements for DMLS can still retain significant value in other powder-based processes with more tolerant feedstock specifications. For example, binder jetting accommodates broader particle size distributions and irregular morphologies because the powder is not melted during processing; several studies have shown that partially degraded stainless steel powders can still achieve high packing density and acceptable sintering behavior in binder jet systems [120,121]. Similarly, directed energy deposition (DED) is also less sensitive to powder sphericity and fine-particle content, making it an ideal secondary pathway for powders that have undergone multiple DMLS cycles. Pereira et al. [90] demonstrated that reused 316L powders with reduced flowability still performed effectively in DED, producing dense builds with only minor adjustments to energy input. Highlighting these cross-platform reuse opportunities reinforces the broader sustainability narrative by showing how powders can be repurposed rather than discarded, enabling a cascading reuse strategy in which feedstock is progressively redirected to processes with increasingly flexible requirements.

Beyond binder jetting and directed energy deposition, powder-based cold spray additive manufacturing represents a highly promising pathway, as it relies on high-velocity particle impact rather than melting, making it tolerant to irregular or partially oxidized particles [122]. Similarly, powder extrusion and metal paste extrusion systems can incorporate fine or irregular recycled powders into a binder-rich feedstock, enabling the fabrication of near-net-shape components followed by debinding and sintering.

6. Recent Advances and Future Direction

Powder recycling presents an innovative and sustainable solution aimed at reducing both costs and environmental impacts associated with metal AM technologies [105]. However, producing high-quality parts using recycled powders poses several significant challenges. When recycled, powders experience alterations in their chemical composition, morphology, microstructure, and particle size distribution compared to their virgin counterparts [71]. Despite these inherent challenges, a variety of strategies show promising quality of parts fabricated from recycled powders.

One effective approach involves pre-treatment techniques such as plasma or laser cleaning, which serve to eliminate contaminants and refine the morphology of the powder [57]. This process leads to improved printability and the overall quality of the printed components. Additionally, meticulously optimizing the DMLS process parameters specifically for recycled powders may help mitigate issues related to flowability and density [123]. Nevertheless, current evidence suggests that blending controlled amounts of virgin powder with recycled powder remains the most effective strategy for boosting the overall quality of the feedstock.

Recent advancements in the recycling of stainless steel powders for additive manufacturing have focused on elevating powder quality, prolonging the reuse cycles of these materials, and seamlessly integrating sustainability practices into industrial workflows [124]. A significant milestone in this area has been the enhancement of powder characterization techniques. Tools such as advanced electron microscopy, X-ray photoelectron spectroscopy (XPS) [68], and inductively coupled plasma optical emission spectroscopy (ICP-OES) [125] empower researchers to detect subtle alterations in chemical composition, surface oxidation levels, and particle morphology that occur after multiple reuse cycles.

Moreover, the development of powder reconditioning technologies represents another crucial advancement in this field. Techniques such as plasma spheroidization [124], ultrasonic sieving [126], and mechanical re-melting have been created to restore the particle morphology and flowability of recycled powders. These methods not only reduce agglomeration but also enhance the packing density of the powders, thereby improving the quality of the prints produced. Notably, research conducted by Powell et al. [51] indicated that reused stainless steel powders achieved mechanical properties comparable to those of virgin powders, underscoring the potential of reconditioning technologies as a viable, cost-effective, and sustainable solution for the future of metal printing.

The integration of machine learning with in situ monitoring systems heralds a new era in additive manufacturing, marked by its transformative potential [127]. By harnessing real-time data collected from sensors embedded in these advanced manufacturing systems, machine learning algorithms can accurately predict the degradation of metal powders [128]. This capability allows for the dynamic adjustment of process parameters, ensuring that the quality of the parts produced remains consistent, even when utilizing recycled powders [129].

In the realm of practical applications, hybrid feedstock strategies are increasingly gaining popularity. This method involves blending virgin powders with recycled ones, enabling manufacturers to strike a delicate balance between sustainability and performance [130]. By tailoring the composition of the powder to meet the specific needs of various applications, manufacturers are able to achieve more efficient and eco-friendly outcomes. For instance, Ferro et al. [111] showcased that hybrid mixtures of IN718 and 316L stainless steel not only maintained acceptable mechanical properties but also led to a noticeable reduction in overall powder consumption. This strategy is particularly advantageous for multi-material builds, where precise compositional control is vital to the success of the final product.

Looking towards the future, the establishment of standardized certification pathways for recycled powders is crucial. Regulatory bodies in industries such as aerospace and biomedicine are beginning to explore guidelines for the reuse of these materials. However, comprehensive standards that ensure quality and safety remain elusive. Developing robust protocols for the management of powder lifecycles including aspects like storage, handling, and reconditioning will be instrumental in promoting industrial adoption and maintaining consistent quality across the board. Furthermore, coupling powder recycling initiatives with life cycle assessment (LCA) frameworks will generate quantitative evidence showcasing the environmental benefits of these practices. This, in turn, will strengthen the argument for the adoption of circular manufacturing models, fostering a more sustainable future in manufacturing.

7. Conclusions

Powder recycling represents a promising technological approach to lowering costs and reducing the environmental footprint associated with metal Direct Metal Laser Sintering (DMLS) technology. However, the path to producing high-quality parts from recycled powders is fraught with challenges. When powders are recycled, they undergo significant changes in their chemical composition, morphology, microstructure, and size distribution compared to their virgin counterparts. These alterations can adversely affect the flowability and density of the powder, subsequently impacting the surface finish and mechanical properties of the manufactured components. As a result, a decline in surface quality and an increase in internal defects are often encountered, which can severely compromise the ductility and dynamic mechanical properties of the final products.

The subject of recycling stainless steel powder for multiple applications in DMLS is attracting growing interest among professionals in the additive manufacturing industry. While certain applications may strongly discourage the reuse of powder altogether due to concerns about quality and performance, others are actively investigating the feasibility and benefits of utilizing the same material for several iterations. Research focusing on the quality of manufactured parts has revealed a noteworthy trend: as the number of times the powder is reused increases, so does the maximum deviation of part dimensions from their intended specifications. This phenomenon suggests that repeated usage can lead to variations in precision. Additionally, the evaluation of surface characteristics has shown that the roughness of overhanging features tends to worsen with each cycle of powder reuse. This deterioration in surface quality may be attributed to the accumulation of larger particles within the powder, which significantly impacts the smoothness of the finished parts.

Currently, there is a lack of standard protocols or widely accepted guidelines surrounding these practices. This document presents an initial effort to establish standardized methods for recycling and reusing stainless steel powder within the DMLS process. Preliminary analyses indicate that recycled powders exhibit more oxidation on their surfaces and a higher concentration of metallic oxides. Furthermore, the microstructure of recycled powders reveals a greater presence of surface satellites, cracks, porosity, bonded particles, and clusters when compared to virgin powders. Although both categories of powder display similar phases, the recycled powder shows a slight decrease in microhardness, YS and UTS properties.

Despite these challenges, various strategies are emerging that hold the potential to enhance the quality of parts produced from recycled powders. For instance, pre-treatment techniques such as plasma or laser cleaning can effectively remove contaminants and improve powder morphology, leading to enhanced printability. Additionally, optimizing DMLS process parameters tailored specifically for recycled powders may help mitigate some issues related to flowability and density. However, blending controlled amounts of virgin powder with recycled powder has proven to be a more effective strategy for improving the overall quality of the feedstock.

Another critical aspect of powder recycling is traceability, which is essential for ensuring consistent quality and addressing potential safety concerns that may arise with recycled materials. Implementing a robust tracking system to monitor the origin, processing history, and properties of the powder throughout its life cycle is vital. Furthermore, reducing powder waste during the printing process is crucial for maximizing both the efficiency and sustainability of powder recycling initiatives in DMLS. Nevertheless, components produced from recycled and reused powders have demonstrated the ability to meet production standards, highlighting that stainless steel powder possesses economic recyclability and that recycled powder holds potential commercial value.

Looking to the future, additional research is necessary to fully realize the potential benefits of metal powder recycling. Advanced techniques for powder characterization can facilitate predictions regarding the printability and quality of powders, thus enabling the development of tailored pre-treatment strategies for various metal powders and contaminants. Exploring innovative powder blending strategies can help optimize the properties of the feedstock. Ultimately, the creation of closed-loop powder recycling systems that minimize waste generation could significantly enhance the sustainability of this manufacturing approach.