From Machining Chips to Raw Material for Powder Metallurgy—A Review

Chips are obtained by subtractive processes such as machining workpieces and until recently considered as waste. However, in recent years they are shown to have great potential as sustainable raw materials for powder technologies. Powder production from metal chips, through the application of solid-state processes, seems to be an alternative to conventional atomization from liquid cooled with different fluids. However, chip material and processing have an essential role in the characteristics of powder particles, such as particle size, shape, size distribution and structure (4S’s), which are essential parameters that must be considered having in mind the powder process and the metallurgy applications. Moreover, different approaches refereed in the application of this new “powder process” are highlighted. The goal is to show how the actual research has been transforming subtractive processes from a contributor of wastes to clean technologies.


Introduction
The evolution of technology and increase in population have directed human beings to consumerism, facing exponential industrial growth with environmental impacts, such as consumption of minerals, generation of energy and production of wastes, highly seen in the steel industry. Thus, circular economy is highlighted regarding recycling or adding value to chips through the definition of new applications [1]. Hence, different studies evaluated the reuse of metal chips in various applications, e.g., the replacement of sands by metal chips in concrete production [2], the production of porous structures by sintering powders mixed with metal chips [3] or the use of steel chips as reinforcement in metal matrices [4,5]. However, the success of these applications depends on the state of interactions of chips with the matrix, e.g., the oxidation of chips in concrete may occur; the bonding between matrix and chips or between chips-chips is essential for the strength of densified product. Regarding the success of using metal chips as new resources, one study highlighted the effect of extrusion temperature and rate of extruding on the densification of product, though authors reported that the mechanical properties of the extruded product were poorer than that of the commercial material, that shortcoming was attributed to the porosity formed in the extruded chips [6].
Other application for metal chips can involve powder production by milling. This approach seems noticeable since the use of these materials appears sustainable [7,8]. Moreover, metal chips are available as major wastes in the machining industry and the use of

Powder Production
Milling techniques can include: (1) attritor ball milling that can be categorized as dry grinding or wet grinding, using regular speed (to 400 rpm) or high speed (400-1800 rpm) attritors [10,16]; (2) roll milling [17]; (3) planetary type [7,13]; (4) disc milling [10,18,19]. Powder production mostly involves fracturing cleaned and dried chips/swarf; meanwhile, cold welding between fragments may occur; however, this joining depends on the energy and conditions of milling. Chips face plastic deformation and consequently fracture. The fragmentation occurs by compaction or impaction accompanied or not with friction depending on the milling technique and procedure. Moreover, particle fragments can be associated with grain refinement, possible to reach nanocrystallinity through high energy milling [20].
Regarding mechanical milling processes, such as the planetary technique, it is important to consider the processing parameters and conditions. Milling energy is influenced by milling time and revolutions per minute (rpm), even ball to powder ratio (BPR). Other conditions, such as the milling atmosphere, the use of process control agent (PCA), and the temperature of milling, are significant too. These conditions affect the evolution of the 4S's of the milled product [21]. Moreover, the characteristics of the milling, such as the material and size of the milling jar and that of the balls, seem to be effective; one study showed that coarser particles were the result of balls with 20 mm diameter than balls of 6 mm [22].
Though the application of cryogenic ambient during milling seems to be effective for powder fragmentation, influencing ductile to brittle fracture temperature transition, some authors reported that disc milling was more productive than ball milling or cryogenic milling/grinding system, taking into account the time of milling for transferring all chips into powders [18].
The jet milling technique was also applied for the powder production from bronze chips [23]. This technique implemented a jet flux for impacting chips on a hard target by which impaction and attrition, rather than wear, were responsible for fragmentation. The impact angle and the distance between the nozzle and target were effective parameters. It revealed that fragmentation by ball milling required larger time in comparison with jet milling by which a faster fragmentation and a greater efficiency were achieved; however, the effect of jet milling was pronounced in the first cycle attributed to the presence of cracks and defects caused by the machining process [23].
Fragmentation is greatly influenced by the type of milling process applied on chips, a higher efficiency of powder production was highlighted for the mill shaker technique than the planetary milling, achieving a faster size refinement was attributed to the milling energy induced by the shaking process [24]. Moreover, the collision energy also depends on the milling technique. Some authors attributed the efficiency of a high fragmentation to impact energy implemented by disc milling in comparison with cryogenic mill and ball miller [18].
In addition, the application of PCA is important; it is usually used to avoid adhesion between particles to balls or to prevent cold welding between particles in order to dominant fracturing; PCA can define the distribution size and morphology of the milled particles. Methanol and stearic acid (SA) are common PCAs. One study presented the production of a d 50 of 100 µm to 325 µm; however, the finest particles (d 50 of~140 µm) were produced by introducing 0.5 wt.% SA to the chips before milling [8].
Regarding the milling atmosphere, the most common is Argon gas, which is very useful to avoid oxidation of metal particles, as contaminations, during milling [20]. However, other strategies can be selected for that purpose, such as adding PCA, e.g., a toluene environment [25].
Regarding the effect of BPR, the application of more balls increases collisions with particles increasing fragmentation. Therefore, the application of a higher BPR can increase the efficiency of milling [25]. Thus, it would be interesting to evaluate the effect of BPR on fragmentation vs. the influence of milling time for future objectives as a solution for reducing energy consumption.

Particle Size
Particle size analysis determines the progress of fragmentation during mechanical milling chips for powder production. This analysis can be performed by several techniques: sieving that is reported in wt.%, image analysis acquired by transmitting optical microscope (OM) or stereoscopic macroscope (SM) or scanning electron microscope (SEM). Other techniques include static light scattering (SLS), laser particle size analyzer (LPSA) or laser diffraction (LD) particle size analyzer (PSA).
The evolution of particle size is governed by the fragmentation of metal chips and is dependent on the materials characteristics and processing conditions as well. Regarding the influence of material, some authors showed the production of particles in dissimilar sizes from similar milling of different chips; that difference was attributed to the primary conditions of the chips used for milling [17]. Table 1 shows the function of the material chips, the different studies performed during the last decennia concerning particle size and the milling process.  Sieving PSA [33] Some authors revealed the success of applying a pre-heat treatment on a Ti alloy, heating in an H 2 -Ar mixture atmosphere, on decreasing particle size, the production of fine particles was attributed to the formation of brittle TiH 2 compound during the pre-heat treatment step [26]. Another study evaluated the effect of chemical composition on fragmentation, the application of similar milling conditions on two steels, an extra low carbon steel and a low carbon type, the former type faced with agglomeration formation caused by cold welding. Moreover, same authors revealed that the larger the BPR ratio, the finer the particle size.
Regarding the achievement of smaller particle size, some studies performed showing that the fragmentation can increase in the presence of a harder phase mixed with the matrix during milling; some authors proved this concept by milling Ti 6 Al 4 V chips mixed with 10 wt.% of alumina particles, showing a sharp reduction of particle size [27].
Milling time also influences fragmentation and particle size formation, e.g., an increase from 3 h to 5 h milling increased yield fragmentation [25]. Fragmentation continues by prolonging milling time though agglomeration of fine particles and formation of clusters can appear, e.g., particles of~1 µm, in clusters, formed within 50 h ball milling of Ti 6 Al 4 V particles [24]. Figure 1 illustrates the particle size of recycled powders in different alloys influenced by processing methodology and conditions.

Particle Size Distribution
The particle size distribution (PSD) can be affected by the presence of additives, as PCA and even reinforcements (e.g., niobium carbide (NbC), vanadium carbide ( silicon carbide (SiC) or Titanium carbide (TiC)), milling time or by the technique ch for milling. Regarding the use of SA as a PCA, the procedure of addition is very impor Table 2 shows the function of the material chips and the different studies perfor during the last decennia concerning particle size distribution function of mi conditions (Table 1). Some authors revealed the production of finer particles, d50 of alm 100 µ m, by the addition of SA in the beginning of the milling process [8]. As regard fragmentation in the presence of reinforcement, an addition of a 3% NbC resulte narrowing particle size distribution [28].
A comparative study between jet milling and ball milling revealed a larger p broadening for the PSD of particles obtained by ball milling attributed to the harde of particles that occurred during the ball milling process [23].  Table 1.

Particle Size Distribution
The particle size distribution (PSD) can be affected by the presence of additives, such as PCA and even reinforcements (e.g., niobium carbide (NbC), vanadium carbide (VC), silicon carbide (SiC) or Titanium carbide (TiC)), milling time or by the technique chosen for milling. Regarding the use of SA as a PCA, the procedure of addition is very important. Table 2 shows the function of the material chips and the different studies performed during the last decennia concerning particle size distribution function of milling conditions (Table 1). Some authors revealed the production of finer particles, d 50 of almost 100 µm, by the addition of SA in the beginning of the milling process [8]. As regards the fragmentation in the presence of reinforcement, an addition of a 3% NbC resulted in narrowing particle size distribution [28]. A comparative study between jet milling and ball milling revealed a larger peak broadening for the PSD of particles obtained by ball milling attributed to the hardening of particles that occurred during the ball milling process [23]. Table 3 and Figure 2 summarize the different research works about the particle shape function of material and the milling process ( Table 1). The morphology of the milled particles is strongly influenced by the milling time, i.e., during the initial steps a flake-like shape is obtained, this shape changes to spherical particles by prolonging milling time by which particles are repeatedly cold welded and fractured [24]. Some authors reported the production of H13 particles with metallic appearance of an aspect ratio close to one by prolonging milling time [13]. Tin bronze alloy Jet milling powder: irregular shape Ball milling powder: rounded with smooth surface SEM [23] Moreover, the powder shape is influenced by the milling technique, the production of angular and agglomerate-free particles was remarked by the performance of shaker milling that was independent on milling time [24]. The diameter of the balls used in mechanical milling also has an influence on the morphology of the powder particles. Applying a similar BPR, large balls (diameter = 20 mm) efficiently break up chips to coarse powder particles while small balls (diameter = 6 mm) effectively modify the powder morphology to nearspherical [22]. Some authors highlighted the effectiveness of ball size on the morphology of milled particles, i.e., a two-stage ball milling starting by large balls and ended with small balls, ϕ = 6 mm and 20 mm, resulted in the production of near-spherical particles [22,34]. Spring-like scraps turn into flaky powder form SEM [29] Tin bronze alloy Jet milling powder: irregular shape Ball milling powder: rounded with smooth surface SEM [23] Figure 2. Illustration of particle shape mentioned in Table 3 (diamond inserts in this graph represent studies discussed in this review article, as cited in Table 3). Table 4 shows the structure of milled chips for different materials. Some researchers illustrated the presence of sub-micrometric to nanometric grain sizes as well as a martensitic structure H13, (AISI) tool steel, chips produced by a machining process in ai [35]. A recent study based on SEM, X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Electron Backscatter Diffraction (EBSD) obtained by TEM (t EBSD), revealed that the microstructure inside the adiabatic shear band, consisting o severe deformation, is composed of ultrafine and nanocrystalline grains, adjacent areas included thin martensite laths with high dislocation density and nanocrystalline grains as well [36]. The target was to highlight the effectiveness of milling metal chips for powde production. Nevertheless, the creation of nanostructured powders was also desired. Some authors also reported the production of nanometric martensitic grains through the  Table 3 (diamond inserts in this graph represent studies discussed in this review article, as cited in Table 3).

Particle Structure
In addition, other authors showed that the use of PCA, such as SA, influenced the particle shape, depending on the milling time in the presence of SA [8].
Regarding particle shape applied in powder metallurgy, ISO 3252:2019 provides complete information, defining acicular particles to spheroidal type, defined by microscopic observations. However, microscopic images can be evaluated quantitatively, defining aspect ratio (proportion of the longest diameter to the shortest aspect of one particle). Nonetheless, a recent study applied the plasma spheroidization technique on milled particles in order to accomplish the aspect ratio of one so that spherical shaped particles could be achieved; however, the shape factor was dependent on the powder feeding rate during the spheroidization process [16]. Table 4 shows the structure of milled chips for different materials. Some researchers illustrated the presence of sub-micrometric to nanometric grain sizes as well as a martensitic structure H13, (AISI) tool steel, chips produced by a machining process in air [35]. A recent study based on SEM, X-ray diffraction (XRD), Transmission Electron Microscopy (TEM) and Electron Backscatter Diffraction (EBSD) obtained by TEM (t-EBSD), revealed that the microstructure inside the adiabatic shear band, consisting of severe deformation, is composed of ultrafine and nanocrystalline grains, adjacent areas included thin martensite laths with high dislocation density and nanocrystalline grains as well [36]. The target was to highlight the effectiveness of milling metal chips for powder production. Nevertheless, the creation of nanostructured powders was also desired. Some authors also reported the production of nanometric martensitic grains through the introduction of extremely high strain rate into austenitic stainless steel chips [7]. Moreover, other researchers showed that the application of shaker milling, that is a higher energetic process than ball milling, revealed a faster rate to achieve nanocrystallinity-10 h of shaking was equivalent to 40 h of mill in a planetary milling [24]. Some authors mentioned the effectiveness of milling speed on attaining particle size, a non-uniform particle size distribution (211 µm produced at 1200 rpm for 24 min) without any change in the structure, the crystallite size decreased by an increase in milling speed and not milling time [29]. Considering the milling time, some authors attributed the increase in grain size to the heat effect caused by long milling time [8]. Nonetheless, the crystallite analyses in these studies were performed by X-ray diffraction (XRD), considering the width of peaks, EBSD or TEM. Magnetic analysis can confirm if austenite microstructure has transformed into martensite; some authors showed that annealing at 700 • C, for 1 h, reduced the strain but the microstructure still remained with a significant proportion of martensite [7]. Regarding the influence of the milling process, for instance in TiAlV alloy, some authors showed that the peak broadening in XRD was higher in powders resulting from shaker milling than ball milling [24]. Ti -Grain size of 10-20 nm [31] The efficiency of shake or jet millings can exceed ball milling due to a higher energy inherent to the technique. Moreover, disc milling shows a similar trend. Regarding milling conditions, an increase in milling time leads rise yield fragmentation. However, agglomeration of fine particles and formation of clusters can happen, caused by a cold welding effect. Similar behavior can be achieved by the application of the highest ball number. In what concerns temperature, a cryogenic ambient during milling also means high powder fragmentation for materials that promote a ductile to brittle fracture. The use of controlling agents such as stearic acid or toluene also influences the fragmentation. The use of a controlled atmosphere such as argon can avoid oxidation and contamination of milled particles. Moreover, the presence of additives or milling time affect powder characteristics, in particular particle size distribution. Regarding particle shape, the diameter of the balls appears to have the strongest effect on reaching near spherical powder particles. Moreover, an increment in milling time leads to a modification of shape factor to high values. Nevertheless, shaker milling lets angular shapes and non-agglomerate structures, independently of time. Concerning particle structure, shaker milling reveals a fast rate to achieve nanocrystallinity over planetary milling. However, crystallite size can decrease by increasing the milling speed and not milling time. The presence of a hard phase mixed with the matrix as reinforcement increases the chips fragmentation, promoting particle size reduction. Moreover, the addition of hard reinforcement encourages the reduction of powder crystallite size. The phase transformation from austenite to martensite phase can occur, particularly in austenitic stainless steel, due to intensive plastic deformation.

Recycling Chips for Powder Metallurgy Applications
The recycled powder particles obtained from metal chips recovery are noteworthy for powder metallurgy applications, conventional and advanced processes, such as pressing Materials 2021, 14, 5432 9 of 13 and sintering, hot isostatic pressing (HIP) or additive manufacturing (AM), respectively, including direct energy deposition (DED) or selective laser melting (SLM) Table 5 shows the densification processes applied on powder milled from different materials. Tin bronze alloy -Pressing [23] Some researchers studied compacted disks of Al powder using recycled chips, chemically cleaned prior to milling to reduce oxide on surfaces, attaining a green density of 80%; these authors attributed the lack of strength of sintered powders to the weak densification [10]. Other study applied hot isostatic pressing (HIP) to increase the density of compacted and sintered Ti 6 Al 4 V powder, but the densification of commercial powders was still higher than powders produced by milling [32]. With regards to the densification of recycled powders through conventional techniques, some authors suggested the production of porous bearings or even high-density P/M structural components, using recycled tin bronze powder [23].
Metal chips were applied also for producing metal matrix powder composites. Taking into account the milling time, increase in particle size by extending milling time caused by cold welding, some authors overcame that shortcoming in the production of a duplex stainless steel composite powder mixed with vanadium carbide (VC) particles [15]. This study also presented that: the increase in the reinforcement concentration results in the reduction of average particle size. However, a similar study shows that the hardness of the sintered composite was smaller than the as-received alloy, and the failure was attributed to the porosity of sintered composite [37]. Other researchers also exhibited a significant effect on the fragmentation of particles in the presence of reinforcements, d = 80 µm in Ti 6 Al 4 V powder and 4 µm in Ti 6 Al 4 V-10 wt.% nanoalumina [27]. The same study revealed a great reduction in the crystallite size of nanocomposite powder in comparison with nonreinforced, respectively, 15 nm and 90 nm. The hardness of the nanocomposite increased 168%. Thus, grain coarsening did not occur in the annealed nanocomposite powders, at 600 • C for 1 h, whereas the milled-annealed Ti 6 Al 4 V powders showed significant grain coarsening [27]. Other researchers evaluated the recycling of a duplex stainless steel in the presence of 3% of NbC by milling, the transformation of austenite to martensite occurred, induced by severe plastic deformation though that transformation reduced in the presence of NbC [28]. Some researchers produced in situ TiC reinforced Ti composite powder by adding graphite to Ti chips; these authors showed that the graphite behaved as an inhibitor for fragmentation as well as oxidation [31].
Additive manufacturing, despite the high costs involved, has been acquiring significant importance due to its sustainability and the reduction of waste [34], obtained by skipping subtractive machining processes to achieve the final product. Other advantages from this technique are the possibility to make complex geometries, high production and low cost of transport and storage. In this process, mechanical milling is an alternative for gas atomization (GA) [34], the most common technique to produce powder with a shape factor close to 1 and controlled particle size distribution. GA is not the best option since it consumes a lot of energy, resulting in high costs and limited alloys composition for production by atomization [34,38]. Thus, powders for AM should have appropriate 4S's, depending on technique, e.g., d 90 of 40-140 µm for direct laser deposition (DLD) or d 90 of 20-50 µm for selective laser melting (SLM) and d 90 of 10 µm for fused filament fabrication (FFF) [39,40]. Some authors developed an optimized procedure to produce powders appropriate for AM, a two-stage milling approach, i.e., balls with a diameter of 20 mm were used to fracture the steel chips and then balls with a diameter of 6 mm allowed the powder to acquire a shape factor close to 1 with a smoother surface feature. Moreover, as the milling time increased, less flattened are the particles [22]. However, a recent study performed atomization of milled particles to produce spherical shaped powders suitable for AM processing, such as DED and SLM technologies, reaching relative densities of 99% [16].

Mechanical Properties
The ball milling can induce plastic deformation as well as accumulation of internal strain, leading to the grain refinement of powder structures. This effect can induce strengthening, e.g., some authors reported the increase in hardness (850 HV) for stainless steel recycled powder (100 h of milling), the initial chips a hardness of 374 HV. This influence is attributed to the formation of a nanosized martensite phase [7]. A similar trend was observed in other metals, affected by milling time as well [8,9,11,[22][23][24]32].
Regarding the mechanical properties of materials after densification, Table 6 revealed that mechanical properties, such as hardness or strength, depend on material composition and processing, appearing additive manufacturing is more promising. Tin bronze alloy Pressing Green strength Powder from jet milling (~12 MPa) had higher strength then powder from ball milling (~5 MPa). [23] The reduction in mechanical properties, such as strength and hardness, of recycled powders is attributed to efficiency of densification [8,10,23,32].

Conclusions and Future Perspectives
From machining work parts is feasible for a significant number of metallic material chips (aluminum alloys, carbon steels, stainless steels, tool steels, Ti-Al alloys, superalloys) to produce powder through milling techniques. In general, these new "raw materials" take the advantage of severe plastic deformation to proceed with the fragmentation of chips. Moreover, the addition of other materials (i.e., nanoceramics) could also promote the fragmentation of ductile metallic alloys.
Depending on the pristine material, powder milling of chips have a main role in the final powder characteristics (4S's). The new powder particles resulting from the milling of metallic chips can be applied mainly to powder metallurgy technology approaches, from conventional processes (subtractive and replicative) to additive manufacturing. This review highlights the role of metallic chips in manufacturing, and it enhances that the production of wastes is not a prerogative of subtractive process, it could be essential to additive manufacturing to guarantee its sustainability.
Regarding future perspectives, it can involve the transformation of chips in outstanding powder raw material with characteristics not available in atomized powder particles. This target will be attained by a detailed study of microstructures function of waste material and processing, milling conditions and the nanoreinforced additions. Particular attention will be dedicated to the technologies selected for 3D object production.