The Challenges and Advances in Recycling/Re-Using Powder for Metal 3D Printing: A Comprehensive Review

: This review explores the critical role of powder quality in metal 3D printing and the importance of effective powder recycling strategies. It covers various metal 3D printing technologies, in particular Selective Laser Melting, Electron Beam Melting, Direct Energy Deposition, and Binder Jetting, and analyzes the impact of powder characteristics on the final part properties. This review highlights key challenges associated with powder recycling, including maintaining consistent particle size and shape, managing contamination, and mitigating degradation effects from repeated use, such as wear, fragmentation, and oxidation. Furthermore, it explores various recycling techniques, such as sieving, blending, plasma spheroidization, and powder conditioning, emphasizing their role in restoring powder quality and enabling reuse.


Introduction
Metal 3D printing, also commonly referred to as additive manufacturing (AM), encompasses various technologies that build parts layer-by-layer, based on a CAD-3D model, using metallic powders.The origins of 3D printing can be traced back to the 1980s with the development of stereolithography (SLA), which used UV lasers to cure photopolymer resins [1].The technology quickly evolved, and in the 1990s, the first patents for metal additive manufacturing were filed [2], leading to the development of techniques such as selective laser sintering (SLS) [3].
These technologies can be broadly categorized into two main groups: those that use a bonding agent to bind the powder and those that melt the material directly using a heat source.Each category faces unique industrial challenges and has distinct advantages and applications (Figure 1).
Binder Jetting [8] involves spreading a thin layer of metal powder onto a build platform, followed by the deposition of a liquid binding agent using an inkjet print head.The binder selectively binds the powder particles together to form a solid layer.This process is repeated layer-by-layer until the entire part is built.Post-processing steps such as curing, debinding, and sintering are required to remove the binder and densify the part.
Binder Jetting is particularly suitable for producing complex geometries, metal prototypes, and small-to-medium batch production.Commonly used materials include stainless steel [9,10], titanium [11], and other alloys [12][13][14][15].Despite its advantages, Binder Jetting Binder Jetting is particularly suitable for producing complex geometries, metal prototypes, and small-to-medium batch production.Commonly used materials include stainless steel [9,10], titanium [11], and other alloys [12][13][14][15].Despite its advantages, Binder Jetting faces industrial challenges such as the need for extensive post-processing to achieve final mechanical properties and managing and recycling unused binder and powder.
Selective Laser Melting (SLM or L-PBF, Laser Powder Bed Fusion) [16] utilizes a highpowered laser to selectively melt and fuse metal powder particles together.A thin layer of powder is spread over the build platform, and the laser traces the cross-sectional geometry of the part, melting the powder.The platform then lowers, and a new layer of powder is spread, repeating the process layer-by-layer.L-PBF is capable of producing fully dense, high-strength metal parts, but it requires support structures to counteract warping due to thermal stresses.
Initially, due to technological limitations in laser power, this process was referred to as selective laser sintering (SLS) [17].Unlike L-PBF, SLS could only achieve sintering, where the metal powder particles are fused together but not fully melted.However, advancements in laser technology have enabled SLM to achieve complete melting of the metal powder, resulting in stronger and more homogeneous parts [18].Despite the dominance of L-PBF in industries requiring fully dense metal parts, SLS still survives in applications where part density and mechanical properties are not critical, such as in prototyping [19].
Electron Beam Melting (EBM or EB-PBF, Electron Beam Powder Bed Fusion) [30] uses an electron beam to melt metal powder in a vacuum environment, reducing oxidation and contamination.A layer of powder is spread, and the electron beam selectively melts the powder.EB-PBF offers higher build rates compared to L-PBF due to the electron beam's Selective Laser Melting (SLM or L-PBF, Laser Powder Bed Fusion) [16] utilizes a highpowered laser to selectively melt and fuse metal powder particles together.A thin layer of powder is spread over the build platform, and the laser traces the cross-sectional geometry of the part, melting the powder.The platform then lowers, and a new layer of powder is spread, repeating the process layer-by-layer.L-PBF is capable of producing fully dense, high-strength metal parts, but it requires support structures to counteract warping due to thermal stresses.
Initially, due to technological limitations in laser power, this process was referred to as selective laser sintering (SLS) [17].Unlike L-PBF, SLS could only achieve sintering, where the metal powder particles are fused together but not fully melted.However, advancements in laser technology have enabled SLM to achieve complete melting of the metal powder, resulting in stronger and more homogeneous parts [18].Despite the dominance of L-PBF in industries requiring fully dense metal parts, SLS still survives in applications where part density and mechanical properties are not critical, such as in prototyping [19].
Electron Beam Melting (EBM or EB-PBF, Electron Beam Powder Bed Fusion) [30] uses an electron beam to melt metal powder in a vacuum environment, reducing oxidation and contamination.A layer of powder is spread, and the electron beam selectively melts the powder.EB-PBF offers higher build rates compared to L-PBF due to the electron beam's high energy density, but it requires vacuum conditions, which limit the size of the build chamber.
EB-PBF is commonly used in aerospace [31] and medical implants [32], particularly orthopedic implants [33].Materials such as titanium and its alloys [34] and cobalt-chrome [35] Metals 2024, 14, 886 3 of 41 are frequently used.Industrial challenges for EB-PBF include the complexity and cost associated with vacuum requirements and managing powder in a vacuum environment.
Direct Energy Deposition (DED) [36] uses focused thermal energy (laser, electron beam, or plasma arc) to fuse metal powder or wire feedstock as it is deposited.The material is directly fed into the heat source, where it is melted and deposited onto the substrate or previously built layers.DED is also suitable for repairing existing components and adding material to existing parts, and it can produce large parts with complex geometries.
All technologies require effective strategies for powder management [43], including recycling and maintaining powder quality standards over time.Direct melting technologies (L-PBF, EB-PBF, and DED) face challenges with residual stresses, necessitating robust thermal management and support strategies.Additionally, Binder Jetting requires extensive post-processing to achieve desired properties and must deal with the issue of residual binder in recycled powders, which can compromise the quality of new builds and requires thorough cleaning and reprocessing [15].
The quality of metal powder used in 3D printing plays a critical role in determining the final properties of printed parts.High-quality powder ensures optimal flowability, which in turn increases the quality of the 3D printing process and the mechanical properties of the final product.Several factors influence powder quality, including particle size and shape, density, roughness, chemical composition, and the presence of impurities [44,45].Understanding and controlling these factors is essential for achieving consistent and reliable results in metal additive manufacturing.
Uniform particle size distribution is vital for achieving a smooth and consistent powder layer during the 3D printing process [46,47].In technologies such as Selective Laser Melting and Electron Beam Melting, keeping a consistent layer thickness is crucial to obtain an even melting and solidification of each layer [48].Powders with a narrow particle size distribution tend to flow better, which improves the packing density and reduces the likelihood of defects such as porosity and incomplete fusion.
Inconsistent particle size can lead to variations in packing density, affecting thermal conductivity and laser absorption during the melting process.This can result in defects such as balling, where un-melted powder forms spherical particles, or lack of fusion between layers.Ensuring a consistent particle size distribution helps in maintaining the integrity of the printed part and achieving desired mechanical properties [49][50][51].
The shape of the powder particles significantly impacts the flowability and packing density of the powder bed [52].Spherical particles are generally preferred in metal 3D printing due to their superior flowability and packing characteristics compared to irregularly shaped particles.Spherical particles reduce friction and allow for a more uniform powder layer, which is essential for high-quality builds [53,54].The method used to produce the powder affects its shape.Gas atomization, a common method for producing metal powders, tends to produce highly spherical particles, while other methods like water atomization may give more irregular results.
The chemical composition of the powder must be carefully controlled to ensure that it meets the specifications required for the intended application.Impurities or variations in the alloy composition can affect the mechanical properties, corrosion resistance, and overall performance of the printed parts [55].For example, the presence of oxygen or other contaminants can lead to the formation of oxides, which can weaken the material [56,57].Maintaining a consistent chemical composition across different batches of powder is essential for achieving repeatable results.
Powder contamination can occur during production, handling, or recycling processes [43,58].Common contaminants include moisture, oils, metal residues from the distribution system, and other foreign particles, which can adversely affect the flowability, Metals 2024, 14, 886 4 of 41 melting behavior, and mechanical properties.Ensuring a clean production environment and using appropriate storage and handling procedures are crucial for maintaining powder purity, in particular considering that, because of their high surface-area-to-volume ratio, particles are more reactive and can easily absorb high amounts of humidity [59].
Contaminants can lead to defects in the printed parts, such as inclusions or voids, which can compromise the mechanical properties and structural integrity of the parts [60,61].For critical applications, such as aerospace and medical implants, maintaining a high powder purity is essential to meet stringent performance and safety standards.
Recent studies have emphasized the importance of monitoring changes in powder characteristics during recycling.Techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), and particle size analysis are used to assess changes in particle morphology, size distribution, and phase composition [62].These analyses help in understanding degradation mechanisms and ensuring that recycled powders maintain their performance characteristics.
Research has shown that repeated recycling can lead to changes in powder properties, such as increased particle size and altered shape due to agglomeration and oxidation.For instance, titanium alloys [63][64][65][66] and nickel-based superalloys [67][68][69] are prone to oxidation, which can affect their flowability and mechanical properties.Studies have investigated the effects of these changes on the final part quality, highlighting the need for strict quality control measures.
Sieving is commonly used to remove oversized particles and contaminants from recycled powder [70].Blending recycled powder with virgin powder is another technique to restore the desired properties.Recent advances have focused on optimizing these processes to maintain a consistent quality of recycled powder.For example, dynamic sieving methods [71] have been developed to improve efficiency and accuracy in separating usable powder from debris.
Plasma spheroidization [72] is an advanced technique that reshapes irregular particles into spherical ones using thermal plasma.This process improves the flowability and packing density of recycled powders, making them suitable for reuse in 3D printing.Research has demonstrated that plasma-spheroidized powders can achieve a comparable performance to virgin powders, thus extending their usable life.
Studies have investigated the mechanical properties of parts printed with recycled powders, focusing on tensile strength, hardness, and fatigue resistance.Results indicate that, with proper recycling and quality control, parts produced from recycled powders can achieve mechanical properties comparable to those made from virgin powders [62].However, the extent of property retention depends on the material and the number of recycling cycles [62].
Research has shown that adjusting process parameters, such as laser power, scanning speed, and layer thickness, can mitigate the effects of powder degradation [73].Optimizing these parameters for recycled powders helps in achieving consistent part quality and performance.For example, increasing the laser power can compensate for the reduced absorption efficiency of oxidized powders.
Recycling powders significantly reduces the material costs associated with metal 3D printing.By extending the usable life of powders, manufacturers can lower their production costs and improve the economic viability of additive manufacturing [73].Research has quantified these cost savings, demonstrating that effective recycling can lead to substantial reductions in overall production expenses [74].
Recycling metal powders contributes to sustainability by reducing waste and minimizing the environmental impact of metal extraction and processing.Studies have highlighted the environmental benefits of powder recycling, including reduced energy consumption and lower carbon emissions [75].These findings support the adoption of recycling practices in industrial applications to promote greener manufacturing processes.
In conducting this review, we gathered and analyzed a wide range of recent studies and publications on the topic of powder recycling in metal 3D printing.Our approach involved an extensive literature search using databases such as Google Scholar, PubMed, and IEEE Xplore to identify peer-reviewed articles, conference papers, and industry reports published within the last ten years.We focused on research that addressed the key aspects of powder quality, recycling techniques, and their impact on mechanical properties and sustainability.
By synthesizing findings from various sources, including experimental studies, theoretical analyses, and case studies, we aimed to provide a comprehensive overview of the current state of knowledge in this field.Additionally, we consulted expert reviews and conducted meta-analyses to identify emerging trends and gaps in the literature, ensuring that our review not only highlights recent advances but also points to future research directions.

Metal Powders Used in 3D Printing
The most commonly used metals and alloys in 3D printing include titanium alloys, stainless steels, aluminum alloys, nickel-based superalloys, and cobalt-chrome alloys.In particular, the economic convenience of the use of metal powders in 3D printing is the ability to use, with minimal adaptations, a wide range of high-cost materials to obtain complex shapes that would be impossible to produce by conventional manufacturing methods [76][77][78][79][80].The primary 3D printing methods for metals are Electron Beam Melting, Directed Energy Deposition, Selective Laser Melting, and Binder Jetting.Each of these methods utilizes metal powders with specific characteristics tailored to the process requirements in order to obtain the optimum performances in terms of the mechanical properties or degradation resistance of material.
Aluminum alloys, such as AlSi 10 Mg [89], are valued for their light weight, good thermal conductivity, and mechanical properties.They are commonly used in the aerospace and automotive industries.Other popular aluminum alloys in 3D printing include AlMg1SiCu [90], AlSi12 [91], and AlSi7Mg [92].
Nickel-based superalloys like Inconel 718 [93] and Inconel 625 [94] are known for their high-temperature strength and resistance to oxidation and corrosion.These alloys are essential in the aerospace, power generation, and chemical processing industries.Additional nickel-based superalloys used in 3D printing include Hastelloy X [95], Ni-Cu alloys [96], and Rene 41 [97].
Cobalt-chrome alloys, such as CoCrMo [98], exhibit excellent wear and corrosion resistance and are used extensively in dental and orthopedic implants, as well as in aerospace components.Other cobalt-chrome alloys used include CoCrW [99] and CoCrNiMo [100].
Copper alloys [101,102] are widely used, mainly for their excellent physical properties also at high temperatures.There are several fields where they are used, including as components of steel-making plants, components of nuclear plants, and so on.In the 3D printing field, there are many Cu-based alloys that are specifically designed in order to be produced by laser processes by conditioning their reflectance [103,104].
A list of metal alloys that have found application in 3D printing is presented in Table 1.The quality and performance of metal 3D printing are highly dependent on the properties of the metal powders used (Figure 2).These properties include morphological characteristics such as particle shape, size, distribution, and roughness, as well as chemical composition and other physical properties.Most requirements are universal, meaning that they apply to all 3D printing techniques.In particular, the following properties are considered: Flowability [70]: good flowability is essential for achieving smooth powder spreading and layer consistency, which are critical for the quality and precision of the printed components.Flowability depends on the powder morphological characteristics, such as shape, texture, size, and distribution, on the chemical composition, in particular concerning the surface oxides, and is also influenced by environmental conditions such as moisture content.
Shape [402]: spherical particles are generally preferred across all 3D printing techniques due to their high flowability and packing density, which contribute to consistent layer formation and uniform melting.
The quality and performance of metal 3D printing are highly dependent on the properties of the metal powders used (Figure 2).These properties include morphological characteristics such as particle shape, size, distribution, and roughness, as well as chemical composition and other physical properties.Most requirements are universal, meaning that they apply to all 3D printing techniques.In particular, the following properties are considered: Flowability [70]: good flowability is essential for achieving smooth powder spreading and layer consistency, which are critical for the quality and precision of the printed components.Flowability depends on the powder morphological characteristics, such as shape, texture, size, and distribution, on the chemical composition, in particular concerning the surface oxides, and is also influenced by environmental conditions such as moisture content.
Shape [402]: spherical particles are generally preferred across all 3D printing techniques due to their high flowability and packing density, which contribute to consistent layer formation and uniform melting.
Particle size and distribution [403]: requirements about particle size and distribution tend to vary depending on the technique used.For EB-PBF, powders usually have a Particle size and distribution [403]: requirements about particle size and distribution tend to vary depending on the technique used.For EB-PBF, powders usually have a particle size range of 45-110 µm.A narrow size distribution ensures uniform layer deposition and efficient melting.DED is more forgiving, with a typical particle size range of 50-150 µm, followed by EB-PBF (45-110 µm), then Binder Jetting (20-100 µm), and finally L-PBF (15-45 µm).Apart from DED, which has higher deposition rates but lower resolution compared to the other techniques, the typical powder size range is strictly correlated with the usual layer thickness during printing.
Density [47]: powder density reflects the quality of the powder and the presence of internal voids can usually negatively affect the flowability of the powder due to powder fragmentation during coating and recoating in the powder bed fusion process.Another important parameter to consider is that the powders should have a certain degree of compaction.The packing density indicates how the powders can fill a volume with a low number of voids.This property then reflects the amount of defect associated with the printed component.In fact, the powders, within their PSD, present certain coarse particles that mainly fill the volume and certain fine particles to fill the space between the other particles.
Chemical composition [404]: the chemical composition of powder is crucial to obtain a printed material with certain precise requirements in terms of chemical composition that then have an influence on the 3D-printed part.The chemical composition of the printed component may vary during the printing process due to the evaporation of some elements.
Other physical properties [405]: the physical properties of the powders are crucial for the correct formation of the melt pool according to the chosen process parameters.In fact, melting in the 3D printing process is strongly controlled by the heat transfer process from the energy imparted by the energy source (laser or electron beam).The presence of trace elements in powders or of thin films on the surface can reduce the heat transfer process and thus modify the solidification mechanism of the molten metal.Another important property is related to the optical properties of the powders, which influence the energy adsorption of the printed material.Laser-based 3D printing processes of copper alloys, for example, are complicated by the material's high reflectivity [103].

Powder Production Methods
As we previously discussed, the use of high-quality metallic powders is fundamental to the success of metal 3D printing.Various methods are employed to produce powders with specific characteristics, and they prevalently fall within three main groups (Figure 3): Metals 2024, 14, 886 11 of 42 density.It is more cost-effective than gas atomization, but it can introduce impurities and lead to the formation of oxides.Plasma atomization uses a plasma torch both to melt metal wire and to disintegrate it into fine droplets.The particles produced by plasma atomization are highly spherical and have excellent purity and consistency, but the technique is more expensive due to the higher energy requirements.An advanced and interesting process for the production of metal powders is the rotating plasma electrode technique [412][413][414].The process consists of a rotating sacrificial electrode installed in a vacuum chamber.The heat source is provided by a counter electrode, usually made of tungsten, which generates a plasma under controlled conditions that is responsible for melting the electrode.The rotation causes the molten particles to be centrifugally expelled, producing small drops of liquid metal which solidify and fall into the collector.This process is a valid method used to produce metal alloys that are reactive to gases.The number of internal defects is usually low compared to conventional processes.
Chemical methods: chemical methods are a group of time-consuming techniques that involve the reduction or decomposition of metal compounds to produce fine powders.These methods can produce highly pure powders with controlled particle sizes and shapes.In chemical reduction [415], metal oxides are mixed with a reducing agent and heated in a controlled environment, where the oxide is reduced to metal powder.The morphology of the powders produced can vary but generally includes a mix of irregular and semi-spherical particles, depending on the reduction conditions.Electrolytic deposition [416] involves the reduction of metal ions from a solution on a cathode to form a powder.This process typically produces dendritic or spongy particles that may require further processing to achieve a desired shape and size.Other chemical methods involve the precipitation of metal from a solution [417], often followed by thermal decomposition.They can produce fine powders with varied shapes, ranging from spherical to irregular, depending on the precipitation conditions.
Each process has its advantages and disadvantages.Mechanical methods like ball milling are suitable for brittle materials but produce irregularly shaped powders.Mechanical methods [406]: ball milling is a mechanical process that involves the grinding of metal powders in a rotating cylindrical chamber with hardened steel or tungsten carbide balls.This method is particularly useful for producing fine and uniform powders from brittle materials.Ball milling is cost-effective, allows for the production of very fine powders, and can be used for a wide range of metals and alloys.The powders produced by ball milling typically have an irregular shape.The mechanical forces involved in the grinding process cause the particles to fracture in a random manner, leading to a non-spherical, angular morphology.This irregular shape can affect the flowability of the powder, making it less ideal for certain 3D printing applications that require a smooth and consistent powder flow [407].It is also a time-consuming process, and the powder produced can be contaminated by the milling media and the chamber [408].
Atomization methods [404]: atomization is one of the most common methods for producing high-quality metal powders.It involves the disintegration of molten metal into fine droplets, which solidify into powder particles.There are three main types of atomization methods, gas [409], water [410], and plasma [411].Gas atomization uses a high-pressure gas stream to break up molten metal into fine droplets, resulting in spherical particles with high flowability and packing density, ideal for 3D printing applications.Water atomization uses high-pressure water jets instead of gas to disintegrate molten metal.The particles produced by water atomization are generally irregular and can have a rough surface texture, which may affect flowability and packing density.It is more cost-effective than gas atomization, but it can introduce impurities and lead to the formation of oxides.Plasma atomization uses a plasma torch both to melt metal wire and to disintegrate it into fine droplets.The particles produced by plasma atomization are highly spherical and have excellent purity and consistency, but the technique is more expensive due to the higher energy requirements.
An advanced and interesting process for the production of metal powders is the rotating plasma electrode technique [412][413][414].The process consists of a rotating sacrificial electrode installed in a vacuum chamber.The heat source is provided by a counter electrode, usually made of tungsten, which generates a plasma under controlled conditions that is responsible for melting the electrode.The rotation causes the molten particles to be centrifugally expelled, producing small drops of liquid metal which solidify and fall into the collector.This process is a valid method used to produce metal alloys that are reactive to gases.The number of internal defects is usually low compared to conventional processes.
Chemical methods: chemical methods are a group of time-consuming techniques that involve the reduction or decomposition of metal compounds to produce fine powders.These methods can produce highly pure powders with controlled particle sizes and shapes.In chemical reduction [415], metal oxides are mixed with a reducing agent and heated in a controlled environment, where the oxide is reduced to metal powder.The morphology of the powders produced can vary but generally includes a mix of irregular and semi-spherical particles, depending on the reduction conditions.Electrolytic deposition [416] involves the reduction of metal ions from a solution on a cathode to form a powder.This process typically produces dendritic or spongy particles that may require further processing to achieve a desired shape and size.Other chemical methods involve the precipitation of metal from a solution [417], often followed by thermal decomposition.They can produce fine powders with varied shapes, ranging from spherical to irregular, depending on the precipitation conditions.
Each process has its advantages and disadvantages.Mechanical methods like ball milling are suitable for brittle materials but produce irregularly shaped powders.Atomization methods (gas, water, and plasma) are favored for producing high-quality spherical powders, though they vary in cost and particle morphology.Chemical methods provide high-purity powders with controlled characteristics, making them suitable for specific applications.

Powder Degradation Mechanisms
The powders used in the 3D printing process can degrade due to the interaction of the powders with the printing environment and in particular with the energy source (laser and electron beam), the molten metal, the chemical compound in the printing chamber, and the contact with mechanical components present in the printing chamber (re-coater) [73,418] (Figure 4).Degradation mainly occurs due to thermal effects, chemical effects, and/or mechanical effects.Collectively, these degradation processes can alter the above properties and therefore the performance of the powder when reused for other printing cycles.By detailing the source of degradation, a careful analysis of the printing process should be carried out, particularly for powder bed fusion processes [419].Atomization methods (gas, water, and plasma) are favored for producing high-quality spherical powders, though they vary in cost and particle morphology.Chemical methods provide high-purity powders with controlled characteristics, making them suitable for specific applications.

Powder Degradation Mechanisms
The powders used in the 3D printing process can degrade due to the interaction of the powders with the printing environment and in particular with the energy source (laser and electron beam), the molten metal, the chemical compound in the printing chamber, and the contact with mechanical components present in the printing chamber (re-coater) [73,418] (Figure 4).Degradation mainly occurs due to thermal effects, chemical effects, and/or mechanical effects.Collectively, these degradation processes can alter the above properties and therefore the performance of the powder when reused for other printing cycles.By detailing the source of degradation, a careful analysis of the printing process should be carried out, particularly for powder bed fusion processes [419].Some mechanisms of powder degradation are common across all the different 3D printing techniques, particularly those involving mechanical damages such as wear, fragmentation, and deformation.However, there are specific mechanisms that are unique to certain techniques or require particular conditions.For instance, processes such as dealloying, sintering, and oxidation necessitate an intense source of heat, which is absent Some mechanisms of powder degradation are common across all the different 3D printing techniques, particularly those involving mechanical damages such as wear, fragmentation, and deformation.However, there are specific mechanisms that are unique to certain techniques or require particular conditions.For instance, processes such as dealloying, sintering, and oxidation necessitate an intense source of heat, which is absent in the printing phase of Binder Jetting.
Within beam-based techniques, Electron Beam Melting operates in a more controlled atmosphere, which significantly reduces the risk of oxidation compared to other methods.The various techniques also differ in their susceptibility to contamination.Binder Jetting is more prone to contamination due to binder residuals on the powder, while the EB-PBF controlled atmosphere also reduces the contamination risks.Directed Energy Deposition (DED) techniques face variable contamination risks depending on the location and size of the equipment, particularly if used for on-site builds, which can increase the likelihood of contamination.
The review of powder degradation mechanisms is mainly based on the alloys listed in Table 1.
The first source of powder degradation is the interaction of the powder with the molten pool [420][421][422][423].In fact, the pool can be a source of molten metal projection due to the flow of molten metal and the resulting spread of molten droplets to the surrounding part.This metal projection can produce the following: metal jets, droplet spatter, and powder spatter.In the case of droplet spatter, molten metal droplets are spread from the molten pool to form a powder-like solidified material that rarely resembles the original powder in shape, dimension, or chemical composition.More or less the same observation is made for metal jets, but in this case the molten metal is usually overheated with respect to the previous droplets.In the case of powder spatter, the molten metal drops usually fall on the surrounding powder, causing morphological changes in the particulate.
Another important source of degradation is related to the movement of powders during the printing process; in this case, the particles can be moved from the original process site due to fluodynamic interaction with the molten pool or with the printing environment.In this case, the affected powders can interact with the molten bath, heating particles surrounding the molten pool, or with the energy source.In both cases, the powder can be altered and thus degraded from its original properties.
An overview of the eight main powder degradation mechanisms is presented in Figure 5. part.This metal projection can produce the following: metal jets, droplet spatter, and powder spatter.In the case of droplet spatter, molten metal droplets are spread from the molten pool to form a powder-like solidified material that rarely resembles the original powder in shape, dimension, or chemical composition.More or less the same observation is made for metal jets, but in this case the molten metal is usually overheated with respect to the previous droplets.In the case of powder spatter, the molten metal drops usually fall on the surrounding powder, causing morphological changes in the particulate.Another important source of degradation is related to the movement of powders during the printing process; in this case, the particles can be moved from the original process site due to fluodynamic interaction with the molten pool or with the printing environment.In this case, the affected powders can interact with the molten bath, heating particles surrounding the molten pool, or with the energy source.In both cases, the powder can be altered and thus degraded from its original properties.
An overview of the eight main powder degradation mechanisms is presented in Figure 5.

Thermal Effects
Thermal degradation mechanisms in metal 3D printing are primarily associated with the extreme temperatures involved in the printing process, including pre-heating, heating during melting and solidification, and subsequent thermal cycling during the build and post-processing stages; this is not to forget the powder/beam interaction.
By analyzing the effect of thermal degradation due to the interaction of the energy Metals 2024, 14, 886 14 of 41

Thermal Effects
Thermal degradation mechanisms in metal 3D printing are primarily associated with the extreme temperatures involved in the printing process, including pre-heating, heating during melting and solidification, and subsequent thermal cycling during the build and post-processing stages; this is not to forget the powder/beam interaction.
By analyzing the effect of thermal degradation due to the interaction of the energy beam with powders (Figure 4), one can see that the effects are mainly related to the presence of molten metal drops that undergo a very fast solidification when expelled from the energy source [424,425].In this case, there are multiple phenomena affecting powder composition and size/geometry.Indeed, a molten metal drop can be directly ejected from the molten pool and in this case there is an independent solidification that can produce, as a function of the overheating temperature reached by the metal, either a new crystal or a phase change.In stainless steels, for example, when powders are overheated the microstructure can transform from austenite to ferrite [426].Usually, the phase transformation occurs when there is a strong beam interaction with the molten metal drop.Other effects due to the melting process can be related to slag projection, which can produce a new single-oxide powder [427].
Another important thermal degradation mechanism is related to the presence of lowmelting-point elements that can evaporate/sublimate [428].In some cases, such as with Ti alloys or aluminum alloys, low-melting-point elements can vaporize during the powder/beam interaction, producing element depletion in the printed component [429].Part of the produced vapor re-condenses when the elements are driven out of the energy source and are cooled.The re-condensed particles usually present a non-conformal morphology (not-spherical shape) and a chemical composition not in agreement with the chemical composition of the not-affected powders.
Another significant thermal degradation mechanism is powder oxidation, wherein exposure to elevated temperatures in environments with certain partial pressures of oxygen (even when small) can lead to the formation of oxide layers on the surface of metal powders [430,431].These oxide layers can negatively impact powder flowability, heat transfer, and the mechanical properties of printed parts.In this case, some materials are more prone to O adsorption, for example Ti alloys, than other materials (e.g., Ni-based alloys).The amount of O adsorption depends on many factors, for example powder age/reuse cycles, material chemical affinity, and atmosphere control in the printing chamber.The effect of gaseous element adsorption is to reduce the impact on both the physical properties of the powders as well as the solidification behavior of the molten metal.As a consequence, the modification of the chemical composition of the powders can lead to a change in powder behavior during the printing process or during the solidification of the molten pool, which then has an influence on the microstructure of the printed material.The chemical composition change, and in particular the adsorption of gaseous elements, usually has an impact on both the inclusion content of the printed material and also on the defect amount of the printed material (pores).It is also possible that a splat of metal oxides (slags) can coat the surrounding powders.
An important thermal effect is related to the fact that the powders can interact with the molten metal by producing a heat-affected zone (HAZ) on the surrounding powders [432,433].These effects can produce a partial melting or a melting at a low temperature of the powders, usually producing elongated powders in a non-spherical shape.These particles usually have an effect on the flowability and also on the apparent density of the powders.If the powder is not molten but heated at temperatures below the melting temperature of the metal, some microstructural changes in the powder can occur.
Partial sintering can also be considered a thermal degradation mechanism in metal 3D printing [434].Partial sintering occurs when metal powders are subjected to temperatures below their melting points but high enough to induce partial bonding and consolidation of powder particles.This phenomenon can occur during the preheating and initial stages of the printing process, where the temperature of the powder bed is elevated to facilitate subsequent fusion and solidification, or simply in the regions surrounding the molten metal, where the temperature progressively decreases with the distance.While controlled partial sintering can be actually used to improve powder flowability, packing density, and layer adhesion, excessive or uncontrolled partial sintering can lead to issues such as poor powder flowability, increased porosity, reduced surface roughness, and dimensional inaccuracies in printed parts [434][435][436].Therefore, it is essential to carefully control and optimize the thermal conditions to minimize the negative effects of partial sintering and ensure the quality and integrity of printed parts.An indirect thermal-effect-related phenomena is the formation of particles coated with metal spatters that can produce an external overlayer of metal which can modify the geometry and the microstructure of the powder.

Mechanical Effects
Mechanical degradation mechanisms in metal 3D printing encompass a broad range of physical forces and stresses experienced by metal powders throughout the printing process.These mechanisms include powder handling, spreading, compaction, and fusion, as well as frictional interactions between the powder and various components of the printing system.
One common mechanical degradation mechanism is powder attrition, wherein repeated handling and mechanical agitation during powder storage, transportation, and processing can cause abrasion and fragmentation of powder particles [437,438].This can lead to changes in particle size distribution, morphology, and surface roughness, affecting powder flowability and packing density.Additionally, frictional forces between the powder and components such as powder distribution systems, re-coaters, and metal blades can further contribute to powder degradation by causing wear and erosion of both the powder particles and the machine components themselves.These frictional interactions can result in the generation of fine particles, powder contamination, and increased levels of airborne debris, all of which can impact the quality and consistency of printed parts.
Furthermore, mechanical degradation can occur during the printing process itself, particularly in techniques involving powder bed fusion, where powders are subjected to mechanical compaction, layer-by-layer deposition, and energy inputs from lasers or electron beams [439][440][441].The mechanical forces exerted on the powder bed during these processes can induce powder densification, deformation, and consolidation, affecting the microstructure and properties of printed parts.Moreover, inadequate powder handling practices, such as improper powder storage conditions or excessive vibrations during printing, can exacerbate mechanical degradation mechanisms, leading to defects and inconsistencies in printed parts.Another important parameter that can influence mechanical degradation is related to the presence of internal voids/porosities that enhance the powder degradation by means of mechanical failure of the powder.In addition, the microstructural alteration of the powders due to heat transfer can decrease the mechanical properties of the powder particles and thus the mechanical resistance of the powder.

Chemical Effects
Chemical degradation mechanisms in metal 3D printing are interactions between metal powders and various chemical agents present in the printing environment.These mechanisms include reactions with reactive gases, moisture, contaminants, low-meltingpoint element evaporation/sublimation, and the binder used in certain printing processes.
One chemical degradation mechanism in metal 3D printing is powder contamination [58,70,[442][443][444], wherein metal powders can become contaminated by airborne particles, residual processing materials, or surface contaminants introduced during handling and storage.Contaminants such as oils, greases, dust, and other airborne pollutants can adhere to powder surfaces, leading to defects in printed parts and affecting their properties.
Another important chemical phenomenon that can occur, as previously discussed under the "thermal effects" sub-chapter, is the sublimation of low-melting-point elements during the 3D printing process, which also results in alterations in the chemical composition [428].In particular, evaporation [445][446][447][448] can deplete the powders of alloying elements that change the properties of the printed component, if the powders are used to produce a component.Usually, the evaporated elements precipitate in the powder bed due to the possible condensation when the volatile elements are cooled in the cold areas of the printing chambers.Ti, Zn, Mg, and Al alloys are most sensitive to this phenomenon.
Another significant consideration for powder recycling in metal 3D printing involves the removal of residual binder from the printed parts.In Binder Jetting processes, where metal powders are selectively bonded together using a liquid binder, small amounts of residual binder may remain within the printed parts after the printing process is complete [449].This residual binder can interfere with subsequent printing runs and affect the properties of recycled powders.
To address this, special attention must be given to the removal of residual binder during the powder recycling process [450].Various techniques, such as solvent extraction, thermal treatments, and chemical baths, can be employed to effectively remove the binder from the printed parts.Solvent extraction involves immersing the printed parts in a solvent that dissolves the binder, leaving behind clean metal powders.Thermal treatments, such as sintering or pyrolysis, can also be used to burn off the binder at elevated temperatures, leaving behind pure metal powders.Additionally, chemical baths containing specific reagents can be used to chemically dissolve the binder without affecting the metal powders.

Impact on Powder Properties
All types of degradation processes, such as contamination, thermal decomposition, and partial sintering, can lead to significant changes in the properties of metal powders used in 3D printing.
The most important powder technological properties, such as flowability, packing density [49,50], and energy adsorption, are a function of the powder size distribution, the morphology of the powders, and the presence of contaminants.The powder size distribution has the greatest influence on packing density [451].Indeed, recycled powders usually have, for materials that have not experienced element sublimation, a broader distribution that moves towards coarser particles [73] (Figure 6).In this case, the packing density is usually reduced and, as a result, the thermal energy is transferred to the powders via fewer contact points during the printing process.To address this, special attention must be given to the removal of residual binder during the powder recycling process [450].Various techniques, such as solvent extraction, thermal treatments, and chemical baths, can be employed to effectively remove the binder from the printed parts.Solvent extraction involves immersing the printed parts in a solvent that dissolves the binder, leaving behind clean metal powders.Thermal treatments, such as sintering or pyrolysis, can also be used to burn off the binder at elevated temperatures, leaving behind pure metal powders.Additionally, chemical baths containing specific reagents can be used to chemically dissolve the binder without affecting the metal powders.

Impact on Powder Properties
All types of degradation processes, such as contamination, thermal decomposition, and partial sintering, can lead to significant changes in the properties of metal powders used in 3D printing.
The most important powder technological properties, such as flowability, packing density [49,50], and energy adsorption, are a function of the powder size distribution, the morphology of the powders, and the presence of contaminants.The powder size distribution has the greatest influence on packing density [451].Indeed, recycled powders usually have, for materials that have not experienced element sublimation, a broader distribution that moves towards coarser particles [73] (Figure 6).In this case, the packing density is usually reduced and, as a result, the thermal energy is transferred to the powders via fewer contact points during the printing process.On the other hand, in materials with a high propensity for alloy depletion, the distribution becomes broader and the average particle size can also be reduced at a high number of re-using cycles [73].In this case, the powders during the printing process adsorb high amounts of energy.These effects are also related to the powder morphology; indeed, a not-spherical powder grain has an influence on the PSD and thus on the aforementioned properties.
In addition, the flowability is reduced due to the fact that the not-spherical powders do not flow well during the re-coater action [452,453].The effect of the powder impurities, instead, is to alter the physical properties of the powders, in particular by altering the heat transfer properties or the melting point of the powder itself.By increasing the impurity On the other hand, in materials with a high propensity for alloy depletion, the distribution becomes broader and the average particle size can also be reduced at a high number of re-using cycles [73].In this case, the powders during the printing process adsorb high amounts of energy.These effects are also related to the powder morphology; indeed, a not-spherical powder grain has an influence on the PSD and thus on the aforementioned properties.
In addition, the flowability is reduced due to the fact that the not-spherical powders do not flow well during the re-coater action [452,453].The effect of the powder impurities, instead, is to alter the physical properties of the powders, in particular by altering the heat transfer properties or the melting point of the powder itself.By increasing the impurity amount, which usually is observed during the recycling cycle increase, a decrease in thermal conductivity and an increase in the melting point is observed.In particular, the oxides that are present on the external surface reduce the heat conductivity of the powders, while the internal impurities usually change the physical properties of the molten pool and thus the solidified microstructures.These property changes can then have an influence on the printed component as described in the next paragraph.Generally speaking, the property that is mainly influenced by the decay of the powders due to reuse is flowability.

Effect of Recycled Powders on 3D-Printed Components
As discussed in the previous paragraph, reused powders have different properties compared to the virgin powder due to the interaction of these powders with the energy beam and the surrounding environment.In particular, it has been discussed how the reused powder influences the packing density, flowability, and physical properties, which in turn affects the outcome of the printing process itself.
A simplified schematic of the seven main categories of printing defects that can be caused by recycled powders is presented in Figure 7.
Metals 2024, 14, 886 17 of 42 powders, while the internal impurities usually change the physical properties of the molten pool and thus the solidified microstructures.These property changes can then have an influence on the printed component as described in the next paragraph.Generally speaking, the property that is mainly influenced by the decay of the powders due to reuse is flowability.

Effect of Recycled Powders on 3D-Printed Components
As discussed in the previous paragraph, reused powders have different properties compared to the virgin powder due to the interaction of these powders with the energy beam and the surrounding environment.In particular, it has been discussed how the reused powder influences the packing density, flowability, and physical properties, which in turn affects the outcome of the printing process itself.
A simplified schematic of the seven main categories of printing defects that can be caused by recycled powders is presented in Figure 7.The presence of defects in the powders can result in internal defects, surface roughness, microstructure, mechanical properties, and degradation (corrosion) resistance.Usually, the strategy of using reused powders is to continuously screen the powders after each printing job until the powders are finished, or to add a quantity of used powders to new powders.Details of the powder reuse strategy will be discussed in a separate paragraph.The overview of the impact of powder reuse on 3D printed components is based on the most common metal alloys used in additive manufacturing processes, which are listed in Table 1.
In terms of internal defects, many authors have observed an increase in void density and dimension in printed components produced with recycled powders [62,454].This has been observed by CT scan measurements and also by light microscopy observations.In particular, the effect of using recycled powders is to increase the number and size of discontinuities.The morphology is also changed to more elongated voids, similar to the The presence of defects in the powders can result in internal defects, surface roughness, microstructure, mechanical properties, and degradation (corrosion) resistance.Usually, the strategy of using reused powders is to continuously screen the powders after each printing job until the powders are finished, or to add a quantity of used powders to new powders.Details of the powder reuse strategy will be discussed in a separate paragraph.The overview of the impact of powder reuse on 3D printed components is based on the most common metal alloys used in additive manufacturing processes, which are listed in Table 1.
In terms of internal defects, many authors have observed an increase in void density and dimension in printed components produced with recycled powders [62,454].This has been observed by CT scan measurements and also by light microscopy observations.In particular, the effect of using recycled powders is to increase the number and size of discontinuities.The morphology is also changed to more elongated voids, similar to the absence of fusion discontinuities.The increase in the number of elongated voids is probably related to the reduced flowability of the powders, which increases the tendency of the material to produce a lack of fusion, while the increase in the number of voids in general could be attributed to the variation in packing density compared to virgin powders.Regarding the chemical composition of the printed material with the reused powders, an increase in the oxygen content in the printed part has been observed for materials sensitive to O adsorption.This usually leads to the formation of non-metallic inclusions [62], usually of nanometric size, or to O enrichment in the metal matrix [455].The surface texture is also affected by the use of recycled powders.In fact, a reduction in surface quality has been observed and this is usually attributed to the presence of non-spherical powders, impurities in the powders, and reduced flowability.All printed surfaces are affected by a reduction in surface quality [443,456].By detailing the discussion of the most common alloys used for the 3D printing process, a general increase in defect amount and size is observed in austenitic stainless steel [457] and Ti alloys.On the other hand, the inclusion content is strongly influenced by the O content of the powders and usually the dimension of these particles is in the sub-nanometric range.It seems that these particles are responsible for the mechanical property increase due to the fact that they act as precipitates.A general increase in both O and inclusion content has been observed in all the most used alloys systems (Ni, Ti, stainless steel, and Al).
Regarding the effect of recycled powders on microstructure, many studies have been carried out on the effect of using both pure recycled powders and recycled powders mixed with virgin powders [443,[457][458][459].In the latter case, no microstructural differences have been observed when using virgin or virgin/blended recycled powders [460].However, some differences have been observed in components printed with recycled powders.In this case, the microstructure is refined and the content of some secondary phases is observed.In particular, the grain refinement can be attributed to both the presence of a higher amount of oxides and a change in the melting/cooling properties of the molten metal.This was observed in Ni [461], Ti, and stainless steels [462].The refinement was detected through EBSD analysis and the elongated columnar grains are usually thinner in components produced with recycled powders.In Ti alloys, on the other hand, a decrease in the beta phase was observed, probably due to the increase in the O content, which is an alpha stabilizer phase [459,463].This element probably inhibited the formation of the beta phase.Some studies on stainless steels have also shown that the presence of powders with an undesirable phase (delta ferrite phase) promotes the formation of these phases in the printed material [62].To avoid this, the authors suggested magnetic separation of delta ferrite powders from the batch.
Obviously, the effect of powder reuse has an impact on the mechanical properties, as 3D-printed parts with reused powders show a higher number of defects, sometimes microstructural changes and altered chemistry [458].In terms of hardness tests, no significant differences were observed between components printed with reused powders and those printed with recycled powders [62].The differences are all within the experimental error for all the most common alloys systems.It is likely that the hardness is strongly linked to the tensile strength of material, which is slightly affected by re-using the powders.In this case, the measurement method seems not to be sensitive for detecting the small variations in the printed material.Indeed, tensile properties are slightly affected by the reuse process.In general, an increase in tensile strength (TS) is observed, accompanied by a decrease in ductility, especially in materials that tend to adsorb trace elements (O) from the printing environment.This behavior was observed in Ti and Al [464,465] alloys.On the other hand, Ni [461] and stainless steel did not show any appreciable variation of TS and YS (Yield Strength).A general decrease in ductility has been observed in all the most common metal systems and this is related to both an increase in inclusion content and increase in porosity size and distribution.
On the other hand, the fatigue properties are affected by the use of recycled powders [466,467].It has been observed that the fatigue limit usually decreases as a function of the number of recycling cycles.This is due to the fact that the components printed with reused powders have a higher number of internal defects, which are also coarser with respect to the components printed with virgin powders.Some studies have been carried out by analyzing the statistical distribution of failures for samples tested at the same stress level.The results showed that the specimens produced with reused powders had failures that were anticipated with respect to the specimens produced with virgin powders.A different behavior was observed in 17-4 PH steel [468], where the fatigue resistance increased as a function of reuse cycles and this was related to the reduction in agglomerates in powders.
Although the effect of reused powders on the corrosion properties of printed alloys has not been deeply investigated, some studies performed on Al alloys [459] have evidenced that the specimens made with reused powders have a higher activity compared to the same specimens made with new powders.This effect is related to the lower internal quality of the material produced by 3D printing processes.
A qualitative summary of the effect of recycled powder use on the microstructural and mechanical properties of 3D-printed parts is shown in Figure 8. the components printed with virgin powders.Some studies have been carried out by analyzing the statistical distribution of failures for samples tested at the same stress level.
The results showed that the specimens produced with reused powders had failures that were anticipated with respect to the specimens produced with virgin powders.A different behavior was observed in 17-4 PH steel [468], where the fatigue resistance increased as a function of reuse cycles and this was related to the reduction in agglomerates in powders.
Although the effect of reused powders on the corrosion properties of printed alloys has not been deeply investigated, some studies performed on Al alloys [459] have evidenced that the specimens made with reused powders have a higher activity compared to the same specimens made with new powders.This effect is related to the lower internal quality of the material produced by 3D printing processes.
A qualitative summary of the effect of recycled powder use on the microstructural and mechanical properties of 3D-printed parts is shown in Figure 8.

Powder Reuse Strategies
Powder reuse strategies are some methods set up to optimize the reuse of powders in order to obtain a 3D part with an acceptable quality by wasting the least amount of powder [73].Each strategy has its own advantages and disadvantages depending on how the method is set up.For industrial production, one of the most important parameters is the traceability of the powder batch, also considering that the strategy should be as simple as possible and at the same time have the minimum waste of powder.The most used strategies in both industry and academia are as follows: the single-batch method, the

Powder Reuse Strategies
Powder reuse strategies are some methods set up to optimize the reuse of powders in order to obtain a 3D part with an acceptable quality by wasting the least amount of powder [73].Each strategy has its own advantages and disadvantages depending on how the method is set up.For industrial production, one of the most important parameters is the traceability of the powder batch, also considering that the strategy should be as simple as possible and at the same time have the minimum waste of powder.The most used strategies in both industry and academia are as follows: the single-batch method, the collective aging method and the replenishment method.
Researchers often recycle powders when a batch becomes depleted (insufficient for another component) in the single-batch method [469].This approach is valued for its simplicity (requiring only continuous post-printing sieving) and high traceability.However, despite ongoing process improvements, it still generates significant powder waste [470].
More complex recycling processes, like the "collective ageing method", can reduce the amount of wasted powder [471,472].This method consists of aging many different batches separately, and then mixing them with powders of the same ageing time.This method is more suitable for high-productivity plants, but traceability is sacrificed.Another important disadvantage is the need to manage many batches and large quantities of stored powder [473,474].
The replenishment method can be considered a hybrid process [73,405,469].After each printing process, the powders are first replenished with new powders up to a certain number of printing cycles.After these cycles, the replenished powders are a mixture of virgin powders and reused powders with low working cycles (lower than the original powders).The main disadvantage of this process is its complexity, as the powders have to be managed in terms of continuous replenishment with both reused and virgin powders, and it requires accurate management of the reused powders that are then used to replenish the print batch.Traceability is lost after the first print batch, similar to the replenishment method.A major advantage of this strategy is the significant reduction in powder waste.
The above strategies are some of the most common ones found in industrial and academic fields, where 3D printing is constantly evolving as far as recycling strategies are concerned.

Advances in Powder Recycling Techniques
As powder recycling has become an essential aspect of metal 3D printing, enabling the reuse of excess or spent powders and reducing material waste and production costs, various recycling techniques have been developed.These recycling methods range from simple mechanical approaches to more advanced technologies like plasma and laser post-treatments.

Mechanical Recycling Methods
Mechanical recycling methods are the simple and cost-effective way to eliminate large particles and contaminants.There are two main mechanical powder recycling methods available: Sieving [475,476], which involves passing used metal powders through a mesh screen to separate particles of different sizes.It is the most common method used in additive manufacturing, often combined with other techniques to further improve the outcome.
Centrifugal separation [477], which uses centrifugal force to separate powders based on their density, but is less effective in removing fine contaminants.Despite being the industrial standard in many production fields, centrifugal separators are not usually applied in current 3D printing systems, as they are more effective on larger volumes.
Mechanical recycling methods can damage the particles because of wear and mechanical stress, resulting in changes in morphology and contaminations from the sieving equipment.

Thermal Recycling Methods
Thermal recycling processes are more focused on the chemical composition of the powders, in particular trapped gases and impurities.There are two main thermal recycling methods that are found industrial applications: Vacuum degassing [478,479]: this involves heating the used powder in a vacuum to remove absorbed gases and contaminants, but is energy-intensive and can alter the properties of the powders.
Re-sintering [480,481]: this involves heating the used powder to fuse small particles together, removing contaminants and improving powder properties.This technology is rarely used in additive manufacturing, and is still in its infancy for metal powder recycling.
Conventional remelting can also be considered a thermal recycling method [469].In this case, the end-of-life powders are used as scrap for further melting.It is important to point out that the end-of-life powders usually do not fulfil the requirements in terms of chemical composition due to both the presence of impurities above the acceptability level and the lack of elements in their chemical composition.To solve this quality problem, an addition of fresh metal with appropriate composition is added and, to reduce the gas in metal issues, a vacuum process is performed.

Chemical Recycling Methods
Acid etching [482,483] can be utilized to improve the rheological properties, decrease the laser reflectance, and reduce the oxide layer on metallic powders, in particular for metals like aluminum or copper.This technology can be used either on virgin or recycled powders.
Electrochemical etching, like acid etching, can improve the surface properties of metallic powders, in particular for materials that are resistant to conventional acid treatments [484].

Emerging Technologies and Future Perspectives
Additive manufacturing has been an environmentally friendly process since its inception [485].In fact, its peculiarity of producing components by adding material instead of subtracting material has considerably revolutionized the approach in the industrial field by increasing the sustainability of the production process.However, it has to be taken into account that this process also generates small amounts of waste, mainly consisting of end-of-life powders, i.e., powders that do not meet the quality requirements to be used.As previously indicated in this work, powder reuse strategies should be better implemented to reduce the complexity of powder management and automation; in high-productivity plants, this can be a solution to simplify the management issues and thus reduce the human factor.In process and powder management, care should also be taken in powder handling to avoid contamination, which can reduce powder quality and thus the reuse life of the powder.Re-use strategies should also be optimized to increase powder re-use life.
Scrapped powders can be conditioned during the production process to avoid remelting.In this case, a huge energy saving can be expected and the life of the powders can be extended by reducing the amount of scrapped powders.This strategy can also be applied before the end of the life of the powders, with the main aim of prolonging the time until they are scrapped.The emerging technologies may be suitable for this purpose in the near future [485].
Plasma cleaning [486] involves using a high-temperature ionized gas (plasma) to remove contaminants like moisture and trapped gases from the powder particles, or to etch surface oxide layers.Apart from cleaning, plasma can also be used for powder spheroidization.
Plasma spheroidization [72,487] is a relatively novel technique that utilizes thermal plasma to reshape irregular powder particles into near-perfect spheres.This significantly improves the flowability and packing density of recycled powders, making them more suitable for reuse in 3D printing processes.Research has shown that plasma-spheroidized powders can achieve comparable performance to virgin powders, extending their usable life and minimizing waste.

Conclusions
Powder recycling offers a technological solution for reducing costs and environmental impact in metal 3D printing technologies.However, achieving high-quality parts from recycled powders presents significant challenges.Recycled powders undergo modifications in their chemical composition, morphology, microstructure, and size distribution compared to virgin powders.These changes can negatively impact the flowability and density of the powder, which in turn affect the surface quality and mechanical properties of the printed parts.A decrease in surface quality and an increase in internal defects are commonly observed, particularly affecting ductility and dynamic mechanical properties.
Despite these limitations, several strategies hold promise for improving the quality of parts printed from recycled powders.Pre-treatment techniques like plasma or laser cleaning can remove contaminants and improve powder morphology, leading to better printability.Optimizing the 3D printing process parameters for recycled powders can also compensate for some of the potential flowability or density issues, but blending controlled amounts of virgin powder with recycled powder is so far more effective in improving the overall quality of the feedstock.
Traceability is also a crucial issue for ensuring consistent quality and managing potential safety concerns associated with recycled powders.A robust system should be implemented to track the origin, processing history, and properties of the powder throughout its life cycle.Furthermore, minimizing powder waste during the printing process is essential to maximize the efficiency and sustainability of powder recycling in 3D printing.
Looking ahead, further research is needed to fully unlock the potential of metal powder recycling.Advanced powder characterization techniques can help predict the printability and quality of recycled powders, allowing for tailored pre-treatment strategies for different metal powders and contaminants.Exploring novel powder blending strategies can optimize the properties of the feedstock.Finally, developing closed-loop powder recycling systems that minimize waste generation would significantly enhance the sustainability of this approach.
Metal powder recycling has the potential to become a reliable and environmentally friendly method for producing high-quality metal parts via 3D printing, once these challenges are finally solved.

Figure 2 .
Figure 2. Five major powder characteristics that affect flowability and that are altered in recycled powders: powder distribution, powder average size, surface roughness, particle shape, and presence of contaminants.

Figure 2 .
Figure 2. Five major powder characteristics that affect flowability and that are altered in recycled powders: powder distribution, powder average size, surface roughness, particle shape, and presence of contaminants.

Figure 3 .
Figure 3. Scheme of the most common powder production processes.

Figure 3 .
Figure 3. Scheme of the most common powder production processes.

Figure 4 .
Figure 4. Simplified diagram of the effects of the interaction between a high-energy beam and metallic powders.The green region represents the heat-affected zone (HAZ), while red is used for the melted material.

Figure 4 .
Figure 4. Simplified diagram of the effects of the interaction between a high-energy beam and metallic powders.The green region represents the heat-affected zone (HAZ), while red is used for the melted material.

Figure 5 .
Figure 5. Simplified models for the eight main mechanisms of degradation occurring on recycled powders: deformation, contamination, oxide deposits, fragmentation, wear, sintering, dealloying, and oxidation.

Figure 5 .
Figure 5. Simplified models for the eight main mechanisms of degradation occurring on recycled powders: deformation, contamination, oxide deposits, fragmentation, wear, sintering, dealloying, and oxidation.

Figure 6 .
Figure 6.Qualitative comparison of recycled powder size distribution as a function of number of cycles.The arrows indicate the increase in reuse cycles.A brighter color indicates a more recycled powder.

Figure 6 .
Figure 6.Qualitative comparison of recycled powder size distribution as a function of number of cycles.The arrows indicate the increase in reuse cycles.A brighter color indicates a more recycled powder.

Figure 7 .
Figure 7. Seven categories of defects that can be observed on 3D-printed components and can sometimes be caused by the use of recycled powders (circled ones): distortions, roughening, cracks, inclusions, delamination, staining, and porosities.The circled images correspond to the effects most observed due to powder recycling.

Figure 7 .
Figure 7. Seven categories of defects that can be observed on 3D-printed components and can sometimes be caused by the use of recycled powders (circled ones): distortions, roughening, cracks, inclusions, delamination, staining, and porosities.The circled images correspond to the effects most observed due to powder recycling.

Figure 8 .
Figure 8. Qualitative summary of property variation as a function of recycling cycles.Continuous line corresponds to metals more sensitive to impurity pick up, while dotted lines correspond to metals less prone to impurity pick up.

Figure 8 .
Figure 8. Qualitative summary of property variation as a function of recycling cycles.Continuous line corresponds to metals more sensitive to impurity pick up, while dotted lines correspond to metals less prone to impurity pick up.

Table 1 .
List of the most common alloys used in 3D printing, with relevant literature references.