The effect of mechanical milling for enhanced recycling Ti6Al4V powder from machining chips | Scientific Reports

Scientific Reports volume 15, Article number: 444 (2025) Cite this article

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This study investigates the optimization of mechanical milling parameters to enhance the recycling of Ti6Al4V machining chips, addressing a significant challenge in sustainable materials processing. The influence of ball-to-powder ratio (BPR) and ball size distribution on powder characteristics, including crystallite size, particle size, and phase composition, was systematically examined. Key findings include a 30% reduction in crystallite size, with the smallest crystallite size of 51.6 nm achieved at a BPR of 10:1, as determined by Rietveld refinement. Dynamic light scattering (DLS) measurements revealed the smallest average particle size of 220.09 nm for a 20:1 BPR with a 25:75 wt% ball size ratio. Energy-dispersive X-ray analysis (EDAX) confirmed the highest Ti content (76.62 wt%) in the 10:1 BPR sample, highlighting the correlation between milling parameters and chemical purity. Electron microscopy showed that ball size distribution significantly influenced particle morphology, with a higher fraction of smaller balls producing a more uniform particle distribution and spherical morphology. Additionally, annealing-induced phase transformations were analyzed, revealing the conversion of TiO into TiO₂ under specific conditions. This study demonstrates that optimized milling parameters can reduce crystallite size and improve particle morphology while achieving high chemical purity, laying the groundwork for practical applications in materials recycling and advanced manufacturing. The findings also show the potential for producing single-phase TiO₂ powders for use in paint and cosmetic products through tailored heat treatment processes.

Ti6Al4V is a titanium alloy commonly used in the biomedical field due to its excellent biocompatibility, corrosion resistance, and mechanical properties1,2,3. Machining this alloy generates a significant amount of machining chips that can be reused to produce nanostructured powders through ball milling4. Identifying a suitable method for recycling and reusing these chips is of significant economic, strategic, and environmental importance5,6. Various methods have been proposed to achieve this, including severe plastic deformation (SPD) and milling methods7,8,9. One of the advantages of the milling method is its potential for industrialization and low cost10,11.

During mechanical alloying, several phenomena occur within the powder particles, the most significant of which include an increase in the density of crystalline defects, particularly dislocations, an increase in strain within the particles, and the interdiffusion of elements.12,13,14. Ali et al.15 utilized a high-energy ball mill with varying milling durations and demonstrated that Ti6Al4V powders subjected to mechanical alloying for extended periods exhibit significantly enhanced properties due to their substantially reduced grain size. Many studies have explored the influence of milling parameters on the characteristics of powders and composites produced using the ball milling method. Varol and Ozsahin16 evaluated the effect of matrix size and milling time on the particle size, apparent density, and specific surface area of flaked Al-Cu-Mg alloy powders. The particle sizes of fine and coarse Al-Cu-Mg alloy powders were 8.6 μm and 61.6 μm, respectively, after 2.5 h of milling. Aydın et al.17 studied recycled Ti6Al4V alloy powder as a reinforcement for polyurethane foam (PUF) core-based sandwich composites with carbon fiber-reinforced polymer (CFRP) face sheets. The Ti6Al4V reinforcement improved the mechanical and modal properties of the composites. Teke et al.18 examined the utilization of Ti6Al4V machining chips as reinforcement particles in epoxy-based composites. Through a pulverization process, they successfully converted Ti6Al4V chips into fine powder form, which was subsequently incorporated as a reinforcing agent. Their findings showed that including Ti6Al4V powder as a filler material significantly improved the tensile and flexural strength properties of the resulting epoxy composites compared to the unmodified epoxy matrix. Wolff et al.19 investigate using recycled Ti6Al4V machining waste as a cost-effective alternative to expensive powder in metal 3D printing, specifically for laser-directed energy deposition (L-DED). Recycled powder was irregular in shape and smaller in size compared to conventional spherical powder. Also demonstrates that recycled titanium waste can be viable for 3D printing, though it produces different material properties than conventional powder. Dhiman et al.20 focused their research on recycling Ti6Al4V machining chips into powder suitable for additive manufacturing. The process involved a multi-stage ball milling technique to break the swarf into fine powder particles. They observed that the recycled powder could be successfully used in direct metal laser sintering to produce high-quality Ti6Al4V components. Varol et al.21 researched the influence of particle size on flake Al2024 matrix powders and the amount of SiC nanoparticles in the particle size distribution using the ball milling method. The particle size of the Al2024 matrix powder was approximately 26 μm after 0.5 h of milling, which decreased to 8 μm when the milling time increased to 2.5 h. Additionally, the presence of SiC nanoparticles increased the hardness of the composite samples. Varol et al.22 used an artificial neural network to evaluate the influence of milling time, milling speed, and initial powder particle size on the thickness of Fe-Al intermetallic coatings. They discovered that increasing the milling time, speed, and size of the milled powders influenced the coating thickness, which increased on the steel substrate with increasing milling time. A research study conducted by Chen et al.23 elucidated the fabrication of Al-Si coatings on Ti6Al4V alloy substrates through the mechanical alloying process utilizing Al-Si powder composites. The investigation examined the impact of various parameters, including the Al-Si compositional ratio, milling time, and rotational speed, on the microstructural characteristics and oxidation resistance of the resulting coatings. Notably, the researchers observed a substantial increase in coating thickness during the preliminary 5-hour period of the milling operation.

Mahboubi et al.24 employed mechanical milling to produce nanocrystalline powder from Ti6Al4V machining chips, utilizing both planetary and shaker milling methods. In the planetary method, five hardened carbon steel balls, each with a diameter of 20 mm, and a ball-to-powder ratio (BPR) of 10:1 were used. Scanning electron microscopy (SEM) and laser particle analysis revealed that after 50 h of milling, 80 μm agglomerated clusters containing 1 μm powder particles were formed. Additionally, microhardness tests showed an increase from 400 HV for the initial chips to 536 HV for the milled powders after 50 h. In a related study, Mahboubi et al.25 investigated the milling behavior of Ti6Al4V alloy machining chips in the presence of 10 wt% Al2O3 nanopowder using a high-energy planetary mill. They conducted analyses via SEM, transmission electron microscopy (TEM), X-ray diffraction (XRD), laser particle sizing, and microhardness measurements. The results demonstrated that the addition of Al2O3 nanoparticles reduced the size of the Ti6Al4V powder from 8 μm (without Al2O3) after 50 h of milling to 4 μm after just 30 h of milling. The study concluded that the presence of Al2O3 nanoparticles enhances work hardening, leading to increased microhardness in both the produced powder and the composite.

Varol et al.26 explored how the concentration of a process control agent (PCA) and the duration of milling affect the morphology, particle size distribution, and crystal structure of Ti6Al4V alloy powders derived from machining chips. Methanol, used as the PCA, was added in concentrations of 0.5%, 1%, and 2% by weight. The milling was carried out with a ball-to-powder ratio (BPR) of 20:1 at a rotational speed of 400 rpm. As the milling time progressed, a consistent evolution in the powder’s morphology was noted for all PCA concentrations. The powders transformed from scrap-like shapes to shell-like structures, followed by irregular and semi-spherical forms, and ultimately, a spherical morphology was achieved.

Numerous studies have focused on various aspects of titanium powder production and its potential applications. Umeda et al.27 studied the production of powder from machined Ti6Al4V alloy chips through heat treatment in a hydrogen atmosphere. Fereiduni et al.28 used Ti6Al4V powder prepared through two different mechanical mixing methods for the additive manufacturing of metal matrix composites (MMCs). Zhou et al.29 developed graphene nanoplates/Ti6Al4V composites by combining high-energy ball milling with spark plasma sintering techniques. Additionally, Ustundag and Varol30 compared commercial Ti6Al4V powder with that obtained from milling alloy machining chips, analyzing the compression behavior of both powders.

A comprehensive review of the literature demonstrates that mechanical milling is an effective method for converting machining chips into metallic powders, making it a promising approach for recycling waste materials such as Ti6Al4V alloy chips. However, the influence of simultaneous milling with different ball sizes at varying weight fractions, as well as the critical effect of the ball-to-powder ratio (BPR) on the properties of powders derived from Ti6Al4V machining chips, remains underexplored. Understanding these factors is crucial, as they directly impact particle size, crystallite size, morphology, and phase composition, which are key determinants of powder quality.

Therefore, the main aim of this study is to investigate the mechanical milling of Ti6Al4V machining waste to produce nanocrystalline powders while systematically analyzing the effects of the BPR and the weight fractions of milling ball sizes. By addressing these knowledge gaps, this research provides valuable insights into optimizing milling parameters for recycling applications. Advanced characterization techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), and X-ray diffraction (XRD), were employed to analyze the resulting powders. This study not only contributes to the growing body of knowledge on sustainable recycling practices but also provides a practical framework for improving material synthesis using waste-derived resources.

Also, our study aimed to develop an economically viable and industrially applicable recycling process for Ti6Al4V alloy machining chips. Utilizing a controlled or inert atmosphere would significantly increase the complexity and cost of the process at an industrial scale. By conducting experiments in air, we deliberately selected conditions that more closely represent realistic industrial implementations. This choice aligns with real-world recycling objectives, where maintaining an inert atmosphere for large-scale operations can pose economic challenges.

Additionally, the focus of our study was to explore parameters such as ball-to-powder ratio (BPR) and the weight fraction of milling balls to establish a framework for optimizing recycling processes. While we acknowledge the formation of oxide phases, such as TiO₂ and Al₂O₃, during air milling, this was an expected outcome under the chosen conditions and does not detract from our overall goals.

Chips produced during the machining of Ti6Al4V medical implants were used in this study. Table 1. lists the chemical compositions of the rods used to produce the chips, which were determined using quantometer analysis. Figure 1 shows the morphology of the obtained chips by SEM, Fig. 1(a), and the XRD pattern of the primary chips, Fig. 1(b). The morphology is similar to that of conventional spring-shaped chips. To remove oil and impurities from the chips, a three-step washing process was employed: (1) The chips were initially washed with hot water. (2) They were then immersed in Aston solution. (3) After rinsing with water, the mixture was placed in an ultrasonic bath at room temperature for 20 min using a solution of 2 wt% hydrochloric acid (HCl), 8 wt% sulfuric acid (H2SO4), and water.

(a) SEM image, and (b) XRD pattern of the primary chips.

Finally, the samples were dried in an oven at 60 °C for 4 h. Once thoroughly cleaned, the chips were carefully chopped into smaller pieces before milling. The milling process was conducted using a planetary ball mill (Amin Asia Instrument Plant) equipped with a high-chrome steel chamber, while the milling balls were made of hardened carbon steel. Ball-to-powder ratios (BPRs) of 10:1, 15:1, and 20:1 were used, with ball sizes of 14 mm and 10 mm in different weight fractions. The milling was carried out at a constant speed of 500 rpm for 10 h. The milling took place at room temperature in an uncontrolled atmosphere, with 30-minute rest intervals during the milling process.

Table 2 lists the experimental conditions used in this study. To investigate the effect of the annealing procedure, one of the 20:1 powder samples was placed in a furnace after 10 h of milling for heat treatment at 420 °C for 3 h. The furnace atmosphere during the annealing process was not specifically controlled and was general in nature. The qualitative XRD (PHILIPS-PW1800 instrument with CuKα radiation, λ = 1.5406Å) analysis with an angular scanning range (10°<2θ < 90°, step time: 0.5s, step size: 0.05°) was conducted to identify the structural and phase changes of the powders during the milling process. Crystallite size, lattice microstrain, and weight% of phases were calculated using the MAUD31 (Materials Analysis Using Diffraction) software. This software utilizes the Rietveld refinement method, which enables precise determination of crystallographic parameters from X-ray diffraction data. SEM and EDAX (Leica Cambridge, Stereoscan S360) were used to determine the surface morphology and chemical composition of the powders. The influence of surface irregularities and powder distribution on the reliability of the EDAX measurements is an important observation. EDAX mapping on the presented images provides a general estimation of the elemental composition; however, localized surface variations due to the non-uniform distribution of powder can affect oxygen detection and quantification. The Clemex image32 analysis software was used to evaluate the particle size distribution. FTIR spectroscopy (TENSOR 27 Analytical Instruments, Bruker Company) was used to determine the presence of the functional groups. Particle size distribution was determined by DLS on the HORIBA LA-950 equipment. As shown in Fig. 1(b), the XRD pattern of the primary chips (as-received sample) exhibits titanium peaks containing alpha and beta phases, which are indexed in Fig. 1(b). Figure 2 shows the schematics of the planetary milling operation used in this study.

Schematics of the planetary milling operation.

The experimental design, as outlined in Table 2, was developed to enable a comprehensive exploration of two pivotal parameters, namely the BPR (contrasting experiments: 1, 2, and 3) and weight fraction of balls with varying sizes (comparing experiments: 3, 4, and 5). This strategic approach aimed to dissect the impact of these variables on the experimental outcomes.

The alterations in powder morphology throughout the milling process under varying BPRs are shown in Fig. 3. In all the SEM images, a recurring pattern emerged; after 10 h of milling, spherical particles dominated the scene, and they were densely clustered through robust agglomeration. The extent of agglomeration was substantial, rendering a straightforward analysis challenging. Nevertheless, the powders consistently exhibited diminutive dimensions in both scenarios. This pronounced agglomeration phenomenon can be attributed to the amplified nanoscale and high surface-to-weight ratio inherent in these nanomaterials. The 20:1 sample exhibited a heightened degree of agglomeration, potentially attributable to the elevated temperatures engendered by the process. This inference is supported by the abundance of agglomerates in the sample. The particle size evolution during milling depends on the interplay between cold welding and fracturing, which effectively shapes the particles. Broadly, three principal mechanisms govern the morphological transformation of particles during mechanical milling, namely plastic deformation, cold welding, and fracture. The final particle size inside the powder is determined by the complex interactions of these processes.

SEM images for different BPRs, at constant B1 and B2 of 25 and 75 wt%, respectively.

Over the 10-hour milling timeframe, particle deformation persisted, fostering work-hardening and facilitating mechanisms such as fatigue fracture or fragmentation of brittle shells. Consequently, a gradual reduction in particle size was observed, accompanied by an observable shift towards a more spherical powder morphology. As the BPR increased while the milling conditions were maintained constant, the production of finer particles became evident, which is indicative of an increased comminution efficiency under these conditions33,34.

Precise evaluation of particle size was facilitated by the application of Clemex image analysis software and DLS measurements, instrumental techniques for accurate size determination. As shown in Fig. 4(a), a discernible trend emerges where increasing the BPR leads to a reduction in particle size. Furthermore, nuanced surfaces were observed within the 20:1 milled powder sample, with particles exhibiting heightened uniformity. Most particles clustered within the range of 0.1 to 0.3 μm, revealing a pronounced concentration within this specific size spectrum. Furthermore, as shown in Fig. 4(b), analyzed by DLS, at BPR (20:1), the particle size decreases to 220 nm.

(a) Size distribution obtained from SEM results using the Clemex software and (b) DLS measurements, for different BPRs and at constant B1 and B2 of 25 and 75 wt%, respectively.

Additionally, the EDAX analysis of the milled powders provided crucial information regarding the elemental composition and distribution on the powder surface. Figure 5. presents the SEM image along with the elemental distribution mapping of the milled powder particles. The EDAX analysis, depicted in Fig. 6, revealed that discernible Fe impurities were evident in the milled powder; furthermore, alongside the primary material peaks encompassing Ti, Al, V, and Fe, only oxygen presence was detected. A significant aspect was the presence of oxygen contamination in the raw powder (as observed via EDAX analysis), which can be due to the presence of surface oxides on the particles. Augmentation of the BPR resulted in an increase in Fe impurities, a trend that is conspicuously highlighted within these observations.

SEM image and Elemental distribution mapping of the milled powder particles at a BPR of 20:1(50–50) wt%.

EDAX results for different BPRs and at constant B1 and B2 of 25 and 75 wt%, respectively.

Figure 7 effectively juxtaposes the XRD patterns arising from a 10-hour milling session of chips with varying BPRs (10:1, 15:1, and 20:1) while preserving a constant weight fraction of the milling ball sizes (25 and 75 wt% of 14 and 10 mm balls, respectively). The results revealed that the milling process induced shifts in the peak positions. In just 10 h of milling, titanium monoxide (TiO) peaks emerged (JCPDS Card No. 01–086-2352), XRD revealed peaks at 36.21 o (111), 42.06 o (200), 60.99 o (220), 73.04 o (311), and 76.86 o (222). Additionally, a peak at 44.35 o (011) indicated the presence of a minor iron (Fe) related peak (JCPDS Card No. 96–720-4905). The presence of iron impurities can be attributed to the wear experienced by the steel balls and chambers. The intensity of the iron-related peak increases in tandem with augmented BPR, as evidenced by the 2θ angle of 45 degrees. Noteworthy trends were observed when scrutinizing the variations in peak intensity and width across different BPR values. This intriguing correlation could potentially be related to the alterations in the crystallite size and lattice strain stemming from the varying BPR conditions. To learn more about the intricate crystallographic transformations that occur during milling, a detailed Rietveld analysis was used to facilitate a more nuanced understanding of the underlying changes.

XRD pattern of the powders milled at different BPRs while keeping the weight fractions of milling ball sizes constant (25 and 75 wt% of 14 and 10 mm balls, respectively).

The lattice microstrain, crystallite size, and weight percentages of TiO and Fe were determined using Rietveld analysis, and the comprehensive findings are presented in Table 3, which focuses on the discernible effects of varying the BPR. An analysis of the results revealed that the samples exhibited marginal disparities in terms of phase alterations. The Rietveld analysis pointed towards a comparatively higher impurity content in the 20:1 sample, registering at 0.72 wt%, which is significantly different from the 0.45 wt% impurity content of the 10:1 sample. This disparity can be attributed to the higher milling energy experienced by the sample at an elevated BPR and increased frequency of collisions. This phenomenon is further substantiated by the increased intensity of the iron impurity peak in the 20:1 sample.

In essence, the strong correlation between the increased milling energy and enhanced ball impact introduces more contaminants into the system. Furthermore, intriguing trends become evident when examining the intensity and width of the peaks. In the 20:1 sample, an increased peak intensity and a subtle reduction in the peak width were observed. Also, Table 3 shows the crystallite size and microstrain values for BPR 20:1 and 10:1 are very similar, and that the largest crystallite size and smallest microstrain are observed at BPR 15:1. The data in Table 3 indicate that at BPR 15:1, the conditions are optimal for grain growth, resulting in the largest crystallite size (64.66 nm) and the smallest microstrain (0.0044%). This suggests that the intermediate BPR provides a balance between energy input and mechanical impact, facilitating efficient grain refinement and defect generation. Other researchers have reported similar findings, lending robust support for these intriguing observations35.

The detection of the TiO phase rather than the TiO₂ phase after ball milling in air can be attributed to several factors inherent to the mechanical milling process and the experimental conditions:

During mechanical milling, repeated collisions and deformation of particles create fresh, highly reactive surfaces. However, these surfaces may not remain exposed to oxygen long enough for complete oxidation to TiO₂. The high-energy environment of the milling process frequently disrupts the oxide layer growth, preventing it from advancing fully to the more oxidized TiO₂ phase.

Although milling was conducted in air, the actual oxygen content available inside the milling chamber may have been restricted due to the high volume of powder and energetic collisions. The repeated impacts and fragmentation of particles reduce the effective exposure of the powder to atmospheric oxygen, thereby limiting the extent of oxidation to TiO₂.

The composition of the Ti6Al4V alloy plays a significant role in its oxidation behavior. The presence of alloying elements such as aluminum and vanadium can influence oxidation mechanisms, potentially moderating the oxidation process and stabilizing substoichiometric oxides like TiO.

TiO is a substoichiometric oxide of titanium that forms under conditions where oxygen availability or diffusion is limited. The high-energy impacts during milling generate localized heating and surface reactions, favoring the formation and stabilization of the TiO phase. The dynamic milling environment disrupts the growth of a fully oxidized TiO₂ layer, maintaining the predominance of the less oxidized TiO phase.

Figure 8 shows the infrared (IR) absorption spectra of the subject-milled powders. The band at approximately 1400 cm−1 was definitively linked to Ti-O-Ti interactions. In the realm of solids, absorption bands within the 100–1000 cm−1 range typically signify the vibrations of ions residing within the crystal lattice. Moreover, the absorption bands at 1430, 1600, and 870 cm−1, while present, have been ascribed to adsorbed water and hydrocarbon impurities inherent to the powders. Furthermore, steps have been taken to mitigate their effects. We analyzed the influence of the BPR on the IR absorption spectra. An increase in the BPR resulted in an increase in the observable peaks associated with Fe impurities, which is a significant correlation that aligns with the findings from the earlier XRD analysis36.

FTIR spectra of 10 h ball-milled powders with BPRs of 10:1 and 20:1, at constant B1 and B2 of 25 and 75 wt%, respectively.

Figure 9 shows the SEM images of the powders milled at a constant BPR of 20:1 but at varying weight fractions of different-sized milling balls (B1 (14 mm)-B2 (10 mm) wt%). An examination of the SEM images revealed that the incorporation of larger balls did not translate into the commensurate refinement of the powders. This visual narrative underscores that a greater presence of smaller balls within the system contributes to a heightened uniformity in powder distribution. This effect was not isolated; rather, the inclusion of smaller balls imparted a more spherical morphology to the powder.

SEM of the powders milled at a constant BPR of 20:1 but at varying weight fractions of different sized milling balls (B1 (14 mm)-B2 (10 mm) wt%).

In essence, these observations offer insights into the nuanced dynamics governing milling processes, underscoring the intricate interplay between the ball size, distribution uniformity, and resultant particle morphology.

A precise particle size assessment was meticulously carried out using Clemex image analysis software and DLS measurements. As shown in Fig. 10(a), a substantial majority of the particles clustered within the size range of 0.12 to 0.23 μm. Distinctive outcomes were observed for the powder sample milled with 25 wt% of B1 and 75 wt% of B2, where the resultant particles exhibited a significantly uniform dispersion. As can be seen in Fig. 10(b), a strategic increase in the proportion of smaller balls corresponds to a detectable decrease in particle size. These insights underscore the intricate relationship between the weight fraction of the milling ball sizes, particle dimensions, and their resultant distribution, a phenomenon governed by the collision dynamics between the particles and milling media. Despite the general concordance between the two methods, the particle size measurements obtained via DLS exhibit subtle variations when compared to those derived from SEM. This divergence can be attributed to the inherent limitations of DLS in discriminating between individual nanoparticles and their agglomerates during the measurement process.The propensity of DLS to detect agglomerates with greater frequency than discrete nanoparticles stems from its inability to differentiate between these two entities. Consequently, in scenarios where the precise determination of nanoparticle dimensions is crucial, particularly in samples characterized by a specific degree of agglomeration, it is advisable to augment DLS analysis with SEM observations. This complementary approach facilitates a more comprehensive and accurate assessment of nanoparticle size distribution within the sample37.

(a) Size distribution obtained from SEM results using Clemex software and (b) DLS measurements, for constant BPR and varying weight fractions of different sized milling balls (B1 (14 mm)-B2 (10 mm) wt%).

Figure 11 compares the XRD patterns resulting from a 10-hour milling session of chips at a consistent BPR of 20:1, albeit with varying weight fractions of milling ball sizes (B1 = 14 mm and B2 = 10 mm). In these samples, the BPR remained constant; however, the composition and arrangement of the small and large balls in the mixture, which initially had a 3% difference in weight, underwent variations. This is evident from the significant alterations in the observed peaks in the XRD pattern of the raw chips that underwent a complete transformation owing to milling. The pronounced alterations in these peaks prominently indicate the presence of a TiO phase across the peaks, with no secondary phases detected apart from the iron impurity.

XRD pattern of the powders milled at a constant BPR of 20:1 but with different weight fractions of milling balls of varying sizes (B1: 14 mm and B2: 10 mm).

The intensity of the peaks corresponds to the sample with B1 = 25 wt% and B2 = 75 wt% appearing diminished compared to the other samples. Conversely, the peak intensity reached its zenith at the sample with B1 = 75 wt% and B2 = 25 wt%. This trend aligns with our understanding that elevated peak intensity correlates with increased crystallinity and larger crystallite sizes. Conversely, a decrease in strain was also observed. Regarding the peak position, a subtle shift became apparent as the lattice parameters were altered. An increase in the proportion of smaller balls within the milling chamber led to a reduction in the crystallite size and Fe impurities in the resulting powder. This effect was further corroborated by the decrease in the peak intensity and concurrent broadening of the peak width in the XRD pattern. This interplay stems from the difference in wear between the smaller and larger balls within the milling chamber, which significantly influences the observed patterns.

Through rigorous Rietveld analysis, the lattice microstrain, crystallite size, and weight percentages of TiO and Fe were meticulously determined, revealing an intriguing interplay between the weight fractions of the ball sizes. These findings are illustrated in Table 4, which provides a comprehensive overview of the observed effects.

The smallest crystal size was observed in the sample with 25 and 75 wt% B1 and B2, respectively, a correspondence mirrored by the results obtained from the XRD pattern (distinctly lower peak intensity). Subsequently, the crystallite size trajectory of the sample with 50 wt% of both B1 and B2 increased, culminating in the maximal crystallite size of 60.78 nm for the sample with 75 wt% of B1 and 25 wt% of B2. Moreover, previous studies38 supported these observations, indicating a correlation between larger milling chamber balls and the resulting crystallite sizes of the obtained powders. This phenomenon is due to the fundamental interplay between the ball size and collision frequency during the powder-ball amalgamation. When finer balls were introduced into the mix, the collision frequency increased, which invariably amplified the effective surface area of the produced powders. A significant contrast emerged when comparing the sample with 50 wt% of both B1 and B2 and that with 75 wt% of B1 and 25 wt% of B2. The sample with 50 wt% of both B1 and B2 had a smaller crystallite size due to the higher proportion of smaller balls. The inclusion of larger balls, while not fostering size reduction, exacerbated the prevalence of Fe impurities within the powders. This augmentation can be attributed to the heightened friction, inter-ball interaction, and increased likelihood of wear, which collectively contributed to the elevated Fe impurity content within the mix. These intriguing findings are consistent with the analogous outcomes reported by other researchers39.

Supplementary insights into the elemental composition residing on the surface of the milled powders, contingent on variations in the weight fraction of different ball sizes, were effectively revealed through an EDAX analysis. The discerning outcomes of this analysis, visually depicted in Fig. 12, provide valuable insights into the powder composition.

EDAX results for powders milled at a constant BPR of 20:1 but at varying weight fractions of different sized milling balls (B1 (14 mm)-B2 (10 mm) wt%).

The EDAX analysis revealed the presence of fewer Fe impurities within the milled powder in the sample with 25 wt% of B1 and 75 wt% of B2 compared to that within the other two samples, indicating that if the percentage of small balls is higher, the ball wear will be lower, and consequently less Fe impurity will enter the system. Oxygen was registered as the sole additional constituent beyond the primary materials, such as Ti, Al, V, and Fe. However, some degree of oxygen contamination, as observed in the EDAX analysis of the raw powders, can be attributed to the surface oxide layers enveloping the particles. The EDAX analysis findings harmoniously converged with those obtained by the Rietveld analysis, underscoring the robustness of these outcomes.

The increase in the prevalence of larger balls within the milling chamber aligns directly with the increase in the Fe impurity content. This nuanced relationship resonates with the intricate interplay among particle collision, wear, and surface interaction dynamics, which ultimately affects the composition of the resultant powders.

Figure 13 shows the diffraction patterns of the Ti6Al4V samples subjected to planetary ball milling before and after undergoing a designated heat treatment. The aim here is to discern and juxtapose the annealing response of the milled powder with the experimental conditions for milling and heat treatment thoughtfully aligned to mirror each other (comprising 10 h of milling followed by a 3-hour exposure to a temperature of 420 °C).

XRD patterns of a Ti6Al4V alloy powder sample after 10 h of milling and heat treatment at 420 °C for 3 h, with BPR = 20:1; (a) 50 wt% of B1 (14 mm), and 50 wt% of B2 (10 mm), (b) 75 wt% of B1 (14 mm), and 25 wt% of B2 (10 mm). .

Upon scrutinizing the diffraction pattern images, a discernible transformation was observed in both samples, which can be attributed to the annealing process occurring in an accessible oxygen environment. This transformation is discernible through the emergence of new peaks in the XRD pattern of the annealed sample, indicating the conversion of the titanium monoxide (TiO) phase into titania (TiO₂) peaks (JCPDS Card No. 01–088-1175). A tetragonal rutile structure was formed after 3 h.

Although the diffraction patterns of the annealed samples exhibited remarkable similarity, nuanced distinctions existed on the surface of the two. In particular, the intensity of the peaks revealed that one of the samples underwent a higher degree of titanium monoxide phase conversion into titania. This signifies a greater extent of TiO interacting with oxygen, subsequently leading to a substantial decline in the intensity of its characteristic peaks, which reverberate through the diffraction pattern.

This study presents novel insights into optimizing mechanical milling parameters for recycling Ti6Al4V machining chips, advancing our understanding of sustainable materials processing. Key factors such as the ball-to-powder ratio (BPR) and the weight fraction of different ball sizes were thoroughly explored, providing valuable insights. Through detailed experimental analyses, the study advances understanding of the complex phenomena involved.

Crystallite size variations were found to strongly correlate with the parameters under investigation. Increasing the BPR consistently led to a notable reduction in crystallite size, confirmed by the decreased intensity of XRD peaks, which indicates lower crystallinity. The smallest crystallite size of 51.6 nm was recorded in the 10:1 BPR sample, as determined through Rietveld analysis, highlighting the importance of BPR in controlling collision frequency and energy input during milling.

The weight fraction of different ball sizes also had a significant effect on particle size and distribution. A higher proportion of smaller balls promoted more uniform particle distribution and a more spherical morphology. The smallest average particle size, determined by DLS, was 220.09 nm for BPR 20:1 (25–75 wt%). Conversely, the use of larger balls resulted in less effective size reduction and increased Fe contamination due to heightened friction and wear. The highest Ti content, 76.62 wt%, was observed in the 10:1 sample, as shown by EDAX analysis.

The impact of annealing on powder phases was also explored, revealing the transformation of the TiO phase into TiO2, as indicated by new peaks in the XRD patterns. This transformation was not complete in all conditions, highlighting the complexities of phase changes under varying heat treatments. By increasing the BPR from 10:1 to 20:1, crystallite size was reduced by up to 30%. When smaller balls dominated the milling media, particle sizes predominantly ranged between 0.12 and 0.23 μm. In contrast, a mixture of 75 wt% larger balls and 25 wt% smaller balls produced the largest crystallite size of 60.78 nm.

This study, through rigorous analysis and careful attention to key parameters, provides a comprehensive framework for employing mechanical milling to tailor powder properties effectively. By elucidating the interplay between various factors and their impacts, this research contributes to scientific understanding and establishes a foundation for optimized milling strategies in applications such as material synthesis and recycling. Post-heat treatment analysis revealed that most of the powder had converted to TiO₂. By modifying heat treatment conditions, such as increasing the duration and temperature, a complete single-phase TiO₂ can be achieved, which holds potential for applications in paint production and cosmetic products.

Data will be made available on request by sending an email to corresponding author.

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Department of Materials Science and Engineering, Faculty of Engineering, Urmia University, Urmia, 5756151818, Iran

Seyyed Amir Reza Alavizadeh, Mehrdad Shahbaz & Majid Kavanlouei

Department of Materials Science and Engineering, Inha University, Incheon, 22212, Korea

Sang Sub Kim

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Author Contributions: Conceptualization, M.SH.; investigation, S.A.A.,M.SH., M.K.; original draft preparation, S.A.A., M.K.; writing—review and editing, M.SH., M.K., S.S.K. All authors have read and agreed to the published version of the manuscript.

Correspondence to Mehrdad Shahbaz.

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Alavizadeh, S.A.R., Shahbaz, M., Kavanlouei, M. et al. The effect of mechanical milling for enhanced recycling Ti6Al4V powder from machining chips. Sci Rep 15, 444 (2025). https://doi.org/10.1038/s41598-024-84913-z

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Received: 26 September 2024

Accepted: 30 December 2024

Published: 02 January 2025

DOI: https://doi.org/10.1038/s41598-024-84913-z

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