Recycling of Ti-15Mo and Ti-13Nb-13Zr Alloys and their Use in Additive Manufacturing

Recycling of Ti-15Mo and Ti-13Nb-13Zr Alloys and their Use in Additive Manufacturing | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Recycling of Ti-15Mo and Ti-13Nb-13Zr Alloys and their Use in Additive Manufacturing Eliška Galčíková, Jan Krčil, Vladimír Mára, Lucie Pilsová, Jana Sobotová, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8269080/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Mar, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted 5 You are reading this latest preprint version Abstract Recycling of titanium alloys represents a modern and economically attractive processing route, as it enables a significant reduction in production costs associated with the high price of titanium. However, the properties of recycled β-titanium alloys have not yet been sufficiently investigated in the scientific literature, and their potential for processing by additive manufacturing technologies remains largely unexplored. This study focuses on evaluating the recycling of Ti-15Mo and Ti-13Nb-13Zr alloys using the horizontal plasma arc melting (HPAM) method and further examines subsequent processing steps, including hot working to the required shape for atomisation and subsequent powder processing via the LPBF technique. In all technological stages, the microstructure was characterised, chemical analysis was performed, and, where applicable, mechanical properties were evaluated. The results demonstrate that recycling of the investigated alloys is feasible, as is their subsequent processing up to the production of additively manufactured specimens. Nevertheless, strict control of the oxygen content is critical for both alloys, as the current levels exceed the limits specified by the relevant standards. At such elevated oxygen concentrations, the alloys are unsuitable for biomedical applications; however, their use in structural applications remains realistic, particularly considering the production cost reduction achieved through the implementation of recycling. Ti-15Mo Ti-13Nb-13Zr recycling process laser powder bad fusion Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 1 Introduction Titanium and its alloys exhibit a unique combination of chemical, physical and mechanical properties. With a density approximately half that of steel, titanium offers comparable strengths, high-temperature resistance and due to the formation of a stable oxide layer, is also exceptionally resistant to corrosion. Another notable characteristic is its excellent biocompatibility. Despite its exceptional properties and abundant occurrence in the upper earth's crust (approximately 0.64 wt. % in the form of TiO 2 [ 1 ]), the commercial use of titanium is very limited compared to other metals such as aluminium or steel [ 2 ]. The primary reason for this limited use is the high cost of titanium. According to Trading Economics [ 3 ], the price of titanium in October 2025 was approximately $ 46.5 per kg, whereas steel cost only around $ 3.08 per kg. This significant price disparity makes titanium economically unfeasible for most industrial applications. As a result, its use is largely restricted to technologically advanced sectors such as aviation, spacecraft engineering, and biomedical application [ 4 ]. A major factor contributing to the high cost of titanium is its strong affinity for oxygen, which complicates the extraction of titanium from its natural forms and subsequent processing [4; 5]. Despite the existence of several production methods, such as the Hunter process, the Armstrong process or the HAMR process [ 2 ], the Kroll process remains the only method employed on an industrial scale and is economically viable [4; 6]. Nevertheless, the Kroll process is relatively demanding and inefficient, with 361 MJ of energy being consumed to produce 1 kg of titanium, compared to 211 MJ/kg for aluminium and only 23 MJ/kg for steel [ 7 ]. Consequently, novel production methods are being developed, alongside increased efforts to recycle titanium waste [ 4 ]. The recycling of titanium and its alloys has been increasing on a global scale. For example, in the United States, approximately 25 thousand tons of recycled titanium alloys were used in 2004, out of a total consumption of 45 thousand tons. In most cases, the recycled materials consisted of chips, compact pieces, powders, or support structures produced through 3D printing [ 8 ]. In the following years, the consumption of titanium scrap continued to grow steadily, reaching 50–65 thousand tons during the 2017–2019 period [9; 10]. A similar trend has been observed worldwide. Recycling offers two key benefits: it conserves primary raw materials and reduces environmental impact [ 4 ]. As a result, the effective recycling of titanium scrap has garnered increasing attention in recent years (e.g. [ 11 – 17 ]). According to Takeda et al. [ 18 ] a significant demand for recycled titanium is anticipated, even for alloys with relatively high impurity levels. One contributing factor is the potential application of titanium in the automotive industry, which is currently limited primarily by production costs. The demand for secondary titanium alloys is expected to exceed that for ferrotitanium, highlighting the need for effective recycling methodologies. Modern engineering solutions aim to reduce raw material and energy use, ultimately maximizing profit. Additive manufacturing (AM) in the titanium industry offers significant potential to improve material efficiency and streamline production. AM can lower raw material consumption, reduce processing steps, and minimize final machining. A pivotal component of powder-based AM involves the production or procurement of titanium powders that meet strict dimensional and compositional specifications [19; 20]. In the case of titanium alloy powders suitable for use in AM technologies (including the Laser Powder Bed Fusion method), it is relatively difficult to obtain these materials—except for commonly available commercial alloys (especially CP Ti Grade 2 and Ti Grade 5)[19; 21]. This is particularly evident for newly studied and developed titanium alloys (β alloys) containing high-melting-point elements such as Nb, Ta, etc. [ 22 ]. However, these β-titanium alloys appear especially suitable for biomedical applications [23; 24]. AM enables innovative structural designs, such as the printing of porous structures that adjust the mechanical properties of implants to better match those of human bone while also promoting integration with bone tissue [ 25 ]. Although the Ti-6Al-4V ELI alloy (ASTM F3001-14 [ 26 ]) is already used in AM of bone implants, the literature mentions the potential toxicity of aluminium and vanadium in relation to neurodegenerative diseases, such as Alzheimer’s disease [23; 27]. β-titanium alloys are therefore considered a more suitable alternative due to their lower toxicity and reduced elastic modulus (14–85 GPa [ 28 ] which more closely matches the properties of human bone (10–40 GPa [ 29 ]). This article focuses on the recycling of titanium alloys Ti-15Mo and Ti-13Nb-13Zr and their subsequent processing enabling the production of powders suitable for additive manufacturing. These alloys represent modern and promising β-type titanium alloys due to their low elastic modulus. At the same time, they are already used in conventional manufacturing processes and are approved for biomedical applications (ASTM F2066-23 and F1713-08(2021)e1). The utilization of residual material as a feedstock could lead to a reduction in the production costs of these alloys and thus promote their broader application. Even if the same purity and mechanical properties as those of primary alloys are not achieved, the resulting material could still represent a more economically viable option suitable for alternative applications where its use would be desirable from a technical point of view but has so far been limited by the high cost. 2 Methods In this study, two recycled β-titanium alloys, Ti-15Mo and Ti-13Nb-13Zr, were investigated. The aim was to evaluate the material properties at individual stages of technological processing, including recycling, hot forging, ultrasonic atomisation, and additive manufacturing (Fig. 1 ). The choice of analytical techniques was adapted to the geometry and condition of the material at each processing stage. Both alloys were prepared at the Advanced Metal Powders Ltd. (Kravaře, Czech Republic) through the process of recycling. The horizontal plasma arc melting (HPAM) process was employed, resulting in the fabrication of material in the form of irregular cross-section rods. The production involved the utilisation of pure titanium scrap CP Ti GRADE 2, which was doped with pure alloys (Mo flakes or Zr and Nb flakes). The finished remelted bars were controlled for chemical composition using an Handheld ED-XRF spectral analyser DELTA (Olympus, Innov-X Systems, Inc., Newton, MA, USA). These bars were subsequently hot forged without protective atmosphere in multiple steps at 950°C to produce rods with a circular cross-section (8 mm diameter) suitable for atomization process. Prior to further processing, the rods were mechanically cleaned to remove the oxide layer. The next metallurgical step was ultrasonic atomisation using an ATO LAB+ (3D LAB Ltd., Warsaw, Poland) machine. During this process, the molten metal was atomised using ultrasonic vibrations and dispersed into a stream of inert gas. The resulting powders were employed for additive manufacturing by laser powder bed fusion (LPBF) method using a Trumpf TruPrint T1000 (TRUMPF, Ditzingen, Germany) machine in Czech Academy of Sciences, Institute of Physics. For process optimization, cubic specimens (10 mm edge) were printed at varying laser powers (60–200 W) and scanning speeds (300–1900 mm/s). Based on the results, the optimal parameters (Table 1 ) were selected for printing test specimens. Table 1 Selected printing parameters for Ti-15Mo and Ti-13Nb-13Zr alloys Alloy Laser power (W) Scanning speed (mm/s) Ti-15Mo 180 300 Ti-13Nb-13Zr 180 500 Phase 1 - Recycling Microstructural characterization of the recycled rods was performed on both longitudinal and transverse sections. Samples were ground using SiC papers with grit sizes ranging from P280 to P2000 and subsequently polished with a colloidal SiO₂ suspension using H 2 O 2 as a wetting agent (in a 3:1 ratio). The purity and porosity of these sections were evaluated using light microscope (LOM) Neophot 32 (Carl Zeiss, Jena, Germany). The microstructure was revealed by Kroll’s reagent (3 ml HF, 8 ml HNO₃, 100 ml H₂O) and also examined by LOM. For detailed microstructural observation and chemical composition analysis, scanning electron microscopy (SEM) JEOL JSM-7600F (JEOL, Tokyo, Japan) equipped with EDXS detector EDAX Octane Elite (EDAX LLC, Mahwah, NJ, USA) was used. Hardness was measured using the Vickers method with a load of 10 kgf employing an automatic hardness tester Struers Duramin 40AC3 (Struers GmbH, Ballerup, Denmark). To analyse oxygen content, samples of approximately 0.01–0.1 g were taken and analysed using a G8 Galileo (Bruker Elemental GmbH, Kalkar, Germany), inert gas fusion spectrometer. For cytotoxicity evaluation, slices approximately 2 mm thick were prepared and processed according to the same grinding and polishing procedure used for metallographic samples. The tests were carried out in collaboration with UCT Prague, Department of Metallic Materials and Corrosion Engineering, according to ISO 10993-5. The samples were sterilized at 180°C for 2 h and subsequently incubated for 24 h in culture media (extraction ratio 1 ml/1.25 cm²). Extracts were applied to L929 and hFOB 1.19 cell lines cultivated in CO₂ incubators (5% CO₂, 37°C for L929; 34°C for hFOB 1.19). A medium with 0.2% Tween served as a positive control. After 24 h exposure, cell metabolic activity was assessed using the resazurin reduction assay, where viable cells convert resazurin to the fluorescent product resorufin. Fluorescence was measured with a Fluoroskan Ascent FL microplate reader (Thermo Scientific, Waltham, MA, USA). Cytotoxicity of the extracts was expressed as the relative metabolic activity of treated cells compared to negative controls (NC). Phase 2 - Forging In forged stage, the microstructural analyses were conducted using LOM Neophot 32 and digital microscope Olympus DSX1000 (Olympus, Tokyo, Japan) and SEM - JEOL JSM-7600F. Additionally, hardness measurements were taken using the Struers Duramin 40AC3, and oxygen content analysis was performed using the G8 Galileo. Owing to the appropriate geometry of rods, tensile tests were also performed using an Instron 5582 (Instron, Norwood MA, USA). Small-size test specimens with a round cross-section and a diameter of 8 mm, having a gauge length five times the diameter, were used for the testing. Fractographic analysis was carried out using an Olympus DSX1000 LOM and a JEOL JSM-7600F SEM. Phase 3 - Atomisation In the third – atomised – phase, the morphology and particle size distribution of the powders were evaluated using SEM (JEOL JSM-7600F) and image analysis using software NIS-Elements 5.42.03 Advanced Research (Nikon, NY, USA). The particle size distribution was further verified by laser diffraction analysis using a particle size analyser Mastersizer 3000 (Malvern Panalytical, Malvern, UK). In addition, elemental distribution maps were obtained using an EDXS detector (EDAX Octane Elite). Oxygen content of the powders was also determined (G8 Galileo). Phase 4 – Additive manufacturing In the last phase, specimens for tensile testing were additively manufactured and subsequently machined, as shown in Fig. 2 . Tensile tests were performed on as-built samples using Instron 5582. Cylindrical specimens for compression testing (20mm in height and 20 mm in diameter) were also produced, and compressive tests were conducted on a universal testing machine ZDM 50 (VEB Werkstoffprüfmaschinen, Leipzig, Germany). In addition, cylindrical samples with a diameter of 10 mm and a height of 20 mm were fabricated for metallographic analysis, hardness measurement, and oxygen content determination. Microstructural observations were carried out using LOM (Neophot 32) and SEM (JEOL JSM-7600F) equipped with an EDXS detector. Furthermore, an EBSD detector EDAX Velocity Pro (EDAX LLC, Mahwah, NJ, USA) was employed to analyze the phase composition and grain orientation. Measurement was done at accelerating voltage of 20 kV, tilt angle 70° utilizing 0.2 µm 0.65 µm step size for Ti-15Mo and Ti-13Nb-13Zr respectively. Collected RAW EBSD data were processed and post-processed in EDAX OIM Analysis™ and ATEX analytical software [ 30 ]. Hardness testing (Struers Duramin 40AC3) and oxygen analysis (G8 Galileo) were also performed. 3 Results 3.1 Phase 1 - Recycling The microstructure of both investigated alloys after the recycling step is shown in Fig. 3 . As illustrated in Fig. 3 a, b the Ti-15Mo alloy exhibits equiaxed β grains with sizes ranging approximately from 800 to 3000 µm as a result of slow cooling after the HPAM process. In addition to the grain boundaries, other features are evident in the microstructure, which are most likely associated with the precipitation of the stable α (hexagonal phase) and/or metastable ω phase (hexagonal phase with ellipsoid or plate-like shape)[31; 32]. These features locally align in a manner that resembles subgrain boundaries. Average hardness of the Ti-15Mo alloy is determined as 297 ± 4 HV10. The microstructure of the Ti-13Nb-13Zr alloy after recycling (Fig. 3 c, d) also reveals coarse polyhedral grains comparable in size to the Ti-15Mo alloy. In this particular instance, however, the structure exhibits a typical needle-like morphology, consisting of finely dispersed α martensite in β matrix. The average hardness of this alloy is 269 ± 4 HV10. In both alloys, pores were identified in the microstructure. Based on their morphology and inner surface features (Fig. 3 e), these pores are most likely of gaseous origin and keyholes. The oxygen content is found to be strongly dependent on the sampling location. For Ti-15Mo alloy, the oxygen concentration at the surface reached nearly 1500 ppm, whereas samples taken from the central region contained approximately 230 ppm. The average oxygen concentration is therefore 1055 ± 720 ppm. For the Ti-13Nb-13Zr alloy, the measured oxygen content is 1390 ± 140 ppm, with lower variability due to sampling closer to the surface. The biological response of both materials maintains the relative metabolic activity of cells above the 70% threshold (the cytotoxicity limit), as illustrated in Fig. 4 . No statistically significant reduction in metabolic activity is observed compared to the negative control (NC, culture medium only), confirming the cytocompatibility of the investigated alloys after recycling process. Phase 2 - Forging After hot forging process, the Ti-15Mo alloy (Fig. 5 a, b) exhibits a so-called necklace-type of microstructure[ 33 ], which results from repeated hot working. Despite the significantly altered microstructure, the hardness remains essentially unchanged, with an average value of 297 ± 4 HV10. Tensile testing revealed an ultimate tensile strength of 1005 ± 19 MPa, full set of mechanical property results is summarized in Table 2 . Compared to the previous processing step, the oxygen content in the central part of the specimen increased to 877 ± 171 ppm. The increase in oxygen content is attributed to an improper hot working regime, where the absence of a protective inert gas atmosphere led to the formation of folding defects (Fig. 5 c). The size and severity of the folding defects were variable, and Fig. 6 shows an extreme example of such a defect. EDXS analysis revealed that in the case of prominent folding penetrating deeply into the cross-section predominantly contain molybdenum and elevated levels of oxygen. The microstructure of the Ti-13Nb-13Zr alloy after deformation is shown in Fig. 5 d, e. The microstructure exhibits an acicular morphology; however, the needles appear coarser compared to the previous (recycled) step. Mechanical properties are summarized in Table 2 . The mean hardness values remained very similar, with an average of 279 ± 5 HV compared to 269 ± 4 HV in the recycled condition. The ultimate tensile strength is 839 ± 69 MPa. Table 2 Mechanical properties of Ti-15Mo and Ti-13Nb-13Zr in forged stage Alloy YS (MPa) UTS (MPa) ETF (%) HV10 Ti-15Mo 973 ± 4 1005 ± 19 12,3 ± 0,1 297 ± 4 Ti-13Nb-13Zr 802 ± 30 839 ± 69 3,5 ± 0,2 279 ± 5 Tensile specimens of Ti-13Nb-13Zr fractured in a distinctly brittle manner (Fig. 7 a) and the fracture morphology exhibited a direct correspondence with the microstructure, manifesting as acicular needles. This can be attributed to the high oxygen content detected in the structure (1929 ± 424 ppm). As with the Ti-15Mo alloy, the origin of this problem lies in an improperly adjusted hot working process, which resulted in the formation of folding defects. During heat treatment performed without a protective atmosphere, the surface becomes saturated with oxygen, leading to embrittlement of the outer layer and facilitating crack initiation (Fig. 7 b). The combination of oxygen-induced embrittlement and processing-induced folding defects likely promotes the propagation of brittle fracture, significantly affecting the mechanical behavior. 3.2 Phase 3 - Atomisation In the case of Ti-15Mo alloy powder, the majority of particles exhibit a regular spherical morphology, although some smaller irregular (non-spherical) particles are also present. The surface of certain powders is not entirely uniform and displays noticeable surface irregularities (Fig. 8 ) [ 34 ], which are concomitant with elevated oxygen content. Occasional larger defects, such as satellites and agglomerates, are also present, but only in minor quantities. Furthermore, the analysis indicated a non-uniform distribution of molybdenum, with some particles exhibiting significantly higher concentrations (Fig. 8 ). Furthermore, the oxygen content was found to be 1511 ± 273 ppm. A comparable behavior was observed for the Ti-13Nb-13Zr powder (Fig. 9 ). While the powders predominantly exhibit a spherical morphology, the distribution of niobium and zirconium is not uniform across individual particles, with certain particles showing a pronounced enrichment in these alloying elements. Spot EDXS analysis demonstrated that particle 1, marked at Fig. 9 , contained 9.8 weight % Zr and 10.9 weight % Nb, in contrast to particle 2, which exhibited 25.1 weight % Zr and 20.8 weight % Nb. As was the case with the Ti-15Mo alloy, some particles exhibited distinct surface irregularities where an increased oxygen content was detected. Additionally, combustion elemental analysis showed a pronounced increase in oxygen content, with the powder reaching 5165 ± 462 ppm, representing a substantial rise compared to the previous step. Figure 10 shows the particle size distribution results obtained from image analysis. The powders exhibited relatively high mean particle sizes, namely 62 µm for the Ti-15Mo alloy and 65 µm for the Ti-13Nb-13Zr alloy. The results were further validated by laser diffraction, yielding mean particle sizes of 59.3 µm for Ti-15Mo and 60.2 µm for Ti-13Nb-13Zr. 3.3 Phase 4 - Additive manufacturing (LPBF) Figure 11 a shows the microstructure of the Ti-15Mo alloy in the build direction. Individual melt pools (MP) as well as columnar grain boundaries can be distinguished. The microstructure contains typical defects such as lack-of-fusion (LOF), and occasional gas pores were also identified. In addition, Mo regions in Fig. 11 a that deviate from the characteristic microstructure of additively manufactured alloys were observed, which appear to correspond to a secondary phase. EDXS analysis (Fig. 11 b) confirmed that these regions represent sites of pronounced segregation of Mo. SEM observations (Fig. 11 c) further reveal that the melt pools consist of a cellular substructure, which is characteristic for this alloy, with the cell growth direction being dictated by the local heat flow.[ 35 ] EBSD analysis (Fig. 11 d) revealed that the Ti-15Mo alloy crystallizes completly in the β phase with a body-centred cubic (BCC) lattice. The analysis also confirmed the presence of columnar grains with high-angle grain boundaries (HAGBs in black), which were already evident in the light microscopy observations. Low-angle grain boundaries (LAGBs), highlighted in red, account for 70.5% of all grain boundaries, compared to 29.5% for HAGBs. The high fraction of LAGBs indicates residual internal stresses, as dislocations tend to rearrange and accumulate along these boundaries due to the rapid heating and cooling cycles during processing. This contributes to overall strengthening of the material and primarily affects its mechanical properties such as strength, however it may have a negative impact on ductility. The majority of columnar β grains exhibit a preferred growth orientation along the direction, direction which is parallel to the rolling direction (RD). RD corresponds to the build direction and thus to the direction of the maximum thermal gradient, as confirmed by the inverse pole figure (IPF) triangles shown in Fig. 11 e. The 3D printed Ti-13Nb-13Zr alloy, when observed under a light microscope (Fig. 12 a), exhibits the characteristic microstructure of additively manufactured materials with relatively wide melt pools with low depth. However, the individual melt pools are rather difficult to distinguish, which is most likely related to the magnitude of energy input during additive manufacturing process [ 36 ]. Nevertheless, lack-of-fusion defects were present in the structure, and gas pores were also observed. As illustrated in Fig. 12 d, regions of elemental segregation were identified, similar to the Ti-15Mo alloy. A more detailed SEM analysis (Fig. 12 c) revealed the acicular microstructure typical for this alloy. In this case, however, the needles differ slightly from those observed in the previous processing steps: their edges appear coarser and more ragged. The results of the EBSD analysis are shown in Fig. 12 d, e. The non-indexed areas (represented by the black regions) correspond to regions with a high dislocation density. A high density of line defects leads to distortion and poor visibility of diffraction patterns, making these regions impossible to index. Nevertheless, even with reduced indexing quality, it is still possible to reliably distinguish between the α and β phase regions. The volume fraction of primary β grains (green) is approximately 62%, while the fraction of needle-like α phase (red) is approximately 38%. Majority of α grains are aligned with the crystal direction parallel, or close to parallel to RD, i.e. build direction, as shown in Fig. 12 e and confirmed by IPF triangles Fig. 12 f. The increase in oxygen content in additive manufactured phase was particularly pronounced. In the case of the Ti-13Nb-13Zr alloy, values in the range of 2000 to 12,000 ppm were measured, with the exact concentration depending on the sampling location. In contrast to the Ti-13Nb-13Zr alloy, the increase in oxygen content in the Ti-15Mo alloy was considerably smaller, reaching an average value of 4723 ± 114 ppm. The mechanical properties of both alloys in their as-built state are summarised in Table 3 . A significant increase in hardness was observed for both alloys compared to forged phase, with a more pronounced rise in Ti-13Nb-13Zr. This alloy also exhibited greater variability in the measured values. The increased scatter in hardness values is considered to result from the complex nature of the additive manufacturing process, with the cumulative effects of previous processing steps and the elevated oxygen content within the alloy. Ti-15Mo alloy achieved an ultimate tensile strength of 1211 MPa. In contrast, reliable strength data could not be obtained for the Ti-13Nb-13Zr alloy as the specimens repeatedly failed in the gripping area. The maximum stress reached before fracture was approximately 500 MPa. This premature brittle failure was most likely caused by the alloy's elevated oxygen content. During compression testing, the Ti-15Mo alloy did not exhibit a distinct failure, and the ultimate compressive strength could not be determined, as the specimen continued to deform in a ductile manner. The Ti-13Nb-13Zr alloy achieved a compressive strength of 956 MPa and the specimen failed in a very brittle manner, similar to the behavior observed during tensile testing. As mentioned earlier, this behavior can be attributed to the high oxygen content and possibly to residual stresses introduced during the additive manufacturing process. Table 3 Mechanical properties of additively manufactured Ti-15Mo and Ti-13Nb-13Zr in as-built state Alloy Compression test Tensile test HV10 YS (MPa) UTS (MPa) YS (MPa) UTS (MPa) ETF (%) Ti-15Mo 941 - 1205 ± 8 1211 ± 8 16 ± 1 345 ± 6 Ti-13Nb-13Zr 946 956 - - - 454 ± 15 4 Discussion Phase 1 - Recycling The recycling process using (HPAM) proved to be suitable for both investigated alloys. The measured oxygen contents can be compared with the corresponding standards for these materials. Although these standards apply to alloys in wrought state, such comparison is appropriate for assessing whether the oxygen limits are met. The measured concentrations are in compliance with the applicable ASTM specifications, which define maximum oxygen limits of 2000 ppm for Ti-15Mo (ASTM F2066-23) and 1500 ppm for Ti-13Nb-13Zr (ASTM F1713-08(2021)e1), both in wrought condition. Furthermore, cytotoxicity tests confirmed that the alloys processed in this way comply with ISO 10993-5, indicating their suitability for biomedical applications. Microstructural analysis revealed that the microstructure of the Ti-15Mo alloy corresponds to those reported in the literature for materials produced by remelting techniques [ 37 ], with differences primarily in grain size, which are attributed to variations in solidification rate. The alloy exhibits a β-phase structure, as also supported by the measured hardness of 296 ± 4 HV10, which, according to [ 38 ], is characteristic of a predominantly β structure. The Ti-13Nb-13Zr alloy shows a typical needle-like morphology, consisting of finely dispersed α martensite within a β matrix. According to the literature, the hardness of this alloy varies depending on the prior thermal history and, consequently, on the resulting microstructural morphology. Reported hardness values range from 205 to 323 HV [39; 40], depending on the previous heat treatment, which is consistent with the measured value obtained in this study (269 ± 4 HV10). Phase 2 - Forging During forging, both alloys exhibited the expected microstructural changes, with the effect being more pronounced in the Ti-15Mo alloy. This alloy developed a necklace-type microstructure, which is typical for specific hot-working processes [ 33 ]. In contrast, the Ti-13Nb-13Zr alloy retained its characteristic needle-like microstructure, consistent with data reported in the literature [ 41 ]. Due to the applied forging process, folding defects were observed in both alloys. In general, folding defects contribute to oxygen enrichment, and since oxygen acts as an α-stabilizing element, it affects the phase composition and consequently the mechanical properties. Increase in oxygen content was more significant in the Ti-13Nb-13Zr alloy. The measured oxygen concentration of 1929 ± 424 ppm exceeds the maximum limit of 1500 ppm defined by ASTM F1713-08(2021)e1. The elevated oxygen content had a noticeable effect on the tensile response, resulting in reduced ductility and brittle fracture [42; 43]. The UTS of 839 ± 69 MPa is close to the values specified in ASTM F1713-08(2021)e1 for the alloy in the annealed and aged condition, however, the measured ETF is less than half of the standard specified value. Fractographic examination revealed that the brittle fracture was initiated in the oxygen-saturated surface layer, which was embrittled during hot forging without protective atmosphere. The propagation of the crack was further influenced by the folding defects. For the Ti-15Mo alloy, a lower oxygen content (877 ± 171 ppm) was measured, most likely due to sampling from areas without folding defects, where the local oxygen concentration was reduced, leading to a lower average value. The measured mechanical properties (UTS of 1005 ± 19 MPa, ETF of 12.3 ± 0.1%) are consistent with the values specified in ASTM F2066-23 for this alloy in the α + β annealed condition, also indicating compliance with the oxygen content limit. It can be assumed that a higher oxygen content would lead to a decrease in ductility. According to [ 44 ], total elongation decreases from 61% to 42% as the oxygen content increases from 0.1 wt.% to 0.2 wt.%. A further rise in oxygen concentration continues to reduce ductility while increasing strength. Phase 3 - Atomisation The chemical composition analysis revealed an inhomogeneous distribution of alloying elements within individual powder particles. To the authors’ knowledge, such compositional inhomogeneity has not been previously reported and is most likely associated with defects generated during the deformation process, particularly with the formation of folds that locally promote the accumulation of alloying elements. This processing step also resulted in a further increase in oxygen content. The elevated oxygen content can also be explained by the high surface-to-volume ratio are exposed to atmospheric oxygen for extended periods during handling and transportation, which may lead to surface oxidation [ 45 ]. Only a few larger defects, such as satellites and agglomerates, were detected in the powder. According to the literature, this is a typical feature of additive manufacturing powders; however, their frequency is lower than in powders produced by another conventional techniques such as gas atomization [46; 47]. Phase 4 – Additive manufacturing Additive manufacturing using LPBF process of the prepared powders is feasible, provided that the processing parameters are properly optimized. Even after optimization, the material still exhibits defects typical for additively manufactured components, such as pores and lack-of-fusion regions [ 48 ]. As a potential solution to this issue, the use of hot isostatic pressing (HIP) has been reported in the literature [ 49 ]. HIP has been successfully applied to eliminate porosity in additively manufactured parts; nevertheless, its influence on the microstructure and resulting mechanical properties of the processed alloys requires further investigation. For example, Ref. [ 50 ] reported the preservation of primary β grains after HIP treatment. For the alloys investigated in this study, post-processing by HIP after additive manufacturing has not yet been documented in the available literature. Both alloys show a melt pool microstructure characteristic of 3D-printed materials. Detailed SEM observations of the Ti-15Mo alloy reveal a sub-cellular structure consisting solely of the β phase, which was confirmed by EBSD analysis. Similar findings were reported in studies [ 35 ] [ 51 ], which also identified the presence of the ω phase. In the case of the Ti-13Nb-13Zr alloy, a needle-like morphology can be observed, corresponding to a combination of martensitic α′ and β phases, as reported in phase analysis studies [34; 52; 53]. The volume fraction of these phases is influenced by the thermal cycles experienced during repeated melting and solidification, i.e., by the input energy (printing parameters) and the chosen scanning strategy. For instance, study [ 53 ] have shown that the fraction of martensitic α′ phase may vary from 18.3 to 52.5 vol.% when only the scanning strategy is altered while keeping other parameters constant. Study [ 34 ] even reported up to 99% of the α′ martensitic phase. In the present work, EBSD analysis revealed an α phase content of 41.3 vol.%. In terms of mechanical performance, an overall increase in strength characteristics was observed. The Ti-15Mo alloy exhibited an average hardness of 345 ± 6 HV10, an ultimate tensile strength exceeding 1200 MPa, and an elongation of 16%. The strength value is comparable to those reported for conventionally processed alloys subjected to specific heat treatment regimes, including annealing and subsequent aging [ 32 ]. According to literature [35; 54], the ultimate tensile strength of additive manufactured as-built Ti-15Mo alloys typically ranges from 800 to 1200 MPa, depending on the processing parameters. The relatively high strength achieved in this study may be attributed to the increased oxygen content, which enhances strength but reduces ductility [ 55 ]. Nevertheless, a survey of extant literature reveals an absence of reports on the oxygen content of as-built specimens. It is therefore hypothesised that the values determined in the present study may represent typical levels. The observed mechanical properties may be attributed to the optimised combination of printing parameters, energy density input during the additive manufacturing process, and resulting microstructure. The effect of increased oxygen content is even more pronounced for the Ti-13Nb-13Zr alloy, with values reaching up to 12 000 ppm. The measured hardness reached 454 ± 15 HV10, which is relatively high compared to reported values. For instance, study [ 45 ] reported a hardness of about 255 HV2, whereas study [ 56 ] reported 519 HV. The high oxygen content likely contributed to the increased hardness and strength of the Ti-13Nb-13Zr alloy. However, due to the elevated oxygen content and possibly high internal stresses, tensile testing could not be completed successfully. The specimens fractured within the clamping area rather than in the gauge section, preventing full analysis. This behavior was confirmed during compression testing, where the specimens essentially fragmented upon loading. Nevertheless, it is expected that with a reduced oxygen content, the tensile strength could reach approximately 1000 MPa, as reported in previous studies [52; 56; 57]. 5 Conclusions This study investigated the evolution of microstructure, oxygen content, and mechanical properties of Ti-15Mo and Ti-13Nb-13Zr alloys processed through recycling using the HPAM method, followed by hot forging, powder atomisation, and additive manufacturing. The results demonstrate that recycling of the investigated alloys via HPAM is feasible, and that powders produced from such recycled materials can be further processed by additive manufacturing. The findings obtained from individual processing stages can be summarised as follows: The recycled Ti-15Mo alloy exhibited a coarse β-grain structure, while Ti-13Nb-13Zr displayed a needle-like α martensite within a β matrix, characteristic of the as-cast condition. Both materials showed hardness values consistent with those reported in the literature. The oxygen content of both alloys remained within the limits specified by ASTM F2066-23 and F1713-08(2021)e1 standards, and no cytotoxic effects were observed, confirming their cytocompatibility after the recycling process. Hot forging led to microstructural refinement, particularly in Ti-15Mo, which developed a necklace-type morphology with high mechanical strength (UTS ≈ 1000 MPa) despite occurrence of folding defects. In Ti-13Nb-13Zr deformation under non-protective atmosphere caused higher increase in oxygen content (≈ 1900 ppm) exceeding the limit set by ASTM F1713-08(2021)e1 resulted in the formation of folding defects, resulting in brittle fracture behavior during tensile testing. It is necessary either to adjust the forming parameters and employ a protective atmosphere or to adopt an alternative dimension-adjustment method prior to atomisation. Gas atomisation produced predominantly spherical powders for both alloys, though minor surface irregularities and compositional inhomogeneities were observed which can be related to elemental segregation occurring during the hot working step. The oxygen content increased moderately, reaching 1511 ± 273 ppm for Ti-15Mo and 5165 ± 462 ppm for Ti-13Nb-13Zr. The particle size distribution was relatively coarse (≈ 60 µm) but within the range suitable for powder-bed additive manufacturing. Additively manufactured specimens exhibited melt pools typical for LPBF with some heterogeneity in elemental distribution and the presence of gas pores and lack-of-fusion defects. The Ti-15Mo alloy achieved an ultimate tensile strength of 1211 MPa and demonstrated ductile behavior under both tension and compression. No distinct failure was observed during compression, and the ultimate compressive strength could not be determined, despite the elevated oxygen level (4723 ± 114 ppm). In contrast, Ti-13Nb-13Zr specimens showed brittle failure during tensile and compression test, probably attributed to excessive oxygen uptake (locally up to 12 000 ppm) and residual stresses generated during the printing process. wing to this extremely brittle behavior, the tensile test for the Ti-13Nb-13Zr alloy could not be completed. The results highlight the key role of oxygen control during high-temperature processing and powder handling. Ti-13Nb-13Zr exhibits higher oxygen sensitivity and limited ductility compared to Ti-15Mo. Ensuring an inert atmosphere during hot working, atomisation, and additive manufacturing is therefore essential for preserving alloy performance. These findings confirm that recycled β titanium alloys, particularly Ti-15Mo, hold strong potential for sustainable additive manufacturing applications, provided that oxygen uptake is effectively mitigated. Declarations Statements and Declarations Funding This project has been financed with the state support of the Technology Agency of the Czech Republic and the Ministry of Industry and Trade of the Czech Republic within the TREND Programme No. FW06010136 and supported by the Grant Agency of the Czech Technical University in Prague, Grant No. SGS23/162/OHK2/3T/12. Competing Interests The authors have no relevant financial or non-financial interests to disclose. 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16:44:29","extension":"png","order_by":28,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":185650,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/ace10a4eb72bde5c797af4b4.png"},{"id":98097985,"identity":"b04b7ea5-fb2f-485d-9b10-98e376589eaf","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"xml","order_by":29,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":147291,"visible":true,"origin":"","legend":"","description":"","filename":"JAMTD25070050structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/663990ad4615d31b0aaa61ef.xml"},{"id":98097984,"identity":"5a55423a-7166-4231-8b60-040bad162ae9","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"html","order_by":30,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":160709,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/cff0f037be2fcde34fadf2ab.html"},{"id":98097955,"identity":"0989ab9a-d3be-4700-a070-9098c1766ff3","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":292525,"visible":true,"origin":"","legend":"\u003cp\u003eMethodology scheme\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/055826ba12038d69f9bf7e59.jpeg"},{"id":98430181,"identity":"ad902342-9705-45f3-878d-879afa22f93b","added_by":"auto","created_at":"2025-12-17 16:44:55","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":120366,"visible":true,"origin":"","legend":"\u003cp\u003eGeometry of additive manufactured sample for tensile test, dimensions \u0026nbsp;\u0026nbsp;in mm\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/10a13a22692c6591c950e565.jpeg"},{"id":98097957,"identity":"c99fdc42-10dc-44f3-9fde-98fbcfd01674","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1207121,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure analysis of recycled β-titanium alloys: (a) LOM of etched Ti-15Mo, (b) SEM of etched Ti1Mo, (c) LOM of etched Ti-13Nb-13Zr, (d) SEM of etched Ti-13Nb-13Zr, (e) SEM observation of gas pore in Ti-13Nb-13Zr\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/ec432b7dafae27407f9b5ada.jpeg"},{"id":98097959,"identity":"5f3b6cd4-c07a-40d9-a3e4-462d81225d89","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":259489,"visible":true,"origin":"","legend":"\u003cp\u003eGraphs of relative metabolic activities of (a) L929 and (b) hFOB 1.19 cells after ~24 h incubation with one-day extracts of Ti-15Mo and Ti-13Nb-13Zr compared to negative control (NC, medium only). Cells incubated in 0.2% Tween solution in medium served as positive control (PC) for cytotoxicity.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/84ac33adfd39dffccf6d43d3.jpeg"},{"id":98429955,"identity":"0da6afc5-6891-42b5-8fde-86fc421ab628","added_by":"auto","created_at":"2025-12-17 16:44:28","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1385501,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure analysis of forged β-titanium alloys: (a) LOM of etched Ti-15Mo, (b) SEM of etched Ti1Mo, (c) LOM of folding defect in etched Ti-15Mo, (d) LOM of etched Ti-13Nb-13Zr, (e) SEM of etched Ti-13Nb-13Zr\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/d2b65705e6742d42f767cb06.jpeg"},{"id":98097963,"identity":"7122a705-9ab4-44ad-b18b-045b11622aa1","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":777627,"visible":true,"origin":"","legend":"\u003cp\u003eAnalysis of folding defects in forged Ti-15Mo (a) LOM observation, (b) linear EDXS analysis\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/29142abee78fe3dda21455f3.jpeg"},{"id":98097961,"identity":"2779469c-9766-4532-b635-7bc812e4fc24","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":1016306,"visible":true,"origin":"","legend":"\u003cp\u003eFractography analysis of Ti-13Nb-13Zr in forged stage after tensile test (a) LOM observation, (b) SEM detail of surface\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/b574ee5f656ae9e4359ee4d9.jpeg"},{"id":98430280,"identity":"84b0448b-f85b-44c7-a2c0-18e1583c3e21","added_by":"auto","created_at":"2025-12-17 16:45:05","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":623868,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of Ti-15Mo powder with EDXS element mapping\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/c812e969b1afdc86fd00fc6d.jpeg"},{"id":98097966,"identity":"d40ef6b4-9997-4786-b103-1b315d84a062","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":677159,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of Ti-13Nb-13Zr powder with EDXS element mapping\u003c/p\u003e","description":"","filename":"floatimage9.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/b509cdbc7cec8d19939e4ae7.jpeg"},{"id":98097968,"identity":"11b61096-b77d-461d-a1c1-512b9b41a95b","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":285658,"visible":true,"origin":"","legend":"\u003cp\u003eParticle size distribution of Ti-15Mo a Ti-13Nb-13Zr powders\u003c/p\u003e","description":"","filename":"floatimage10.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/e4727c504e00d6ab19716efa.jpeg"},{"id":98097970,"identity":"19411597-b183-4cb8-a736-b78639a135a7","added_by":"auto","created_at":"2025-12-12 19:36:51","extension":"jpeg","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":1518371,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure analysis of additively manufactured Ti-15Mo: (a) LOM of etched state (b) SEM image showing molybdenum segregation with EDXS elemental mapping, (c) SEM of etched state, (d) IPF map with low and high angle boundaries showing crystallographic orientation of columnar β grains, (e) Inverse Pole Figures\u003c/p\u003e","description":"","filename":"floatimage11.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/5271fc890ab3f37bdd71bd99.jpeg"},{"id":98430803,"identity":"6ea9064d-b04e-4a08-aff5-593ed2890807","added_by":"auto","created_at":"2025-12-17 16:46:15","extension":"jpeg","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1690890,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure analysis of additively manufactured Ti-13Nb-13Zr: (a) LOM of etched state, (b) SEM image showing niobium and zirconium segregation along with EDXS elemental mapping, (c) SEM of etched state, (d) Phase map (e) IPF-RD map with low and high angle boundaries showing crystallographic orientation of primary α grains and β needles, (f) Inverse Pole Figures\u003c/p\u003e","description":"","filename":"floatimage12.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/f677cdd77c3864e61fdae0b3.jpeg"},{"id":104740445,"identity":"b76227a6-706e-4c68-bafc-37c36160d095","added_by":"auto","created_at":"2026-03-16 16:18:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":10596510,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8269080/v1/36efce2b-d58e-4b5a-8ed5-8990c0b7fead.pdf"}],"financialInterests":"","formattedTitle":"Recycling of Ti-15Mo and Ti-13Nb-13Zr Alloys and their Use in Additive Manufacturing","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eTitanium and its alloys exhibit a unique combination of chemical, physical and mechanical properties. With a density approximately half that of steel, titanium offers comparable strengths, high-temperature resistance and due to the formation of a stable oxide layer, is also exceptionally resistant to corrosion. Another notable characteristic is its excellent biocompatibility. Despite its exceptional properties and abundant occurrence in the upper earth's crust (approximately 0.64 wt. % in the form of TiO\u003csub\u003e2\u003c/sub\u003e [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]), the commercial use of titanium is very limited compared to other metals such as aluminium or steel [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe primary reason for this limited use is the high cost of titanium. According to Trading Economics [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], the price of titanium in October 2025 was approximately \u003cspan\u003e$\u003c/span\u003e46.5 per kg, whereas steel cost only around \u003cspan\u003e$\u003c/span\u003e3.08 per kg. This significant price disparity makes titanium economically unfeasible for most industrial applications. As a result, its use is largely restricted to technologically advanced sectors such as aviation, spacecraft engineering, and biomedical application [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eA major factor contributing to the high cost of titanium is its strong affinity for oxygen, which complicates the extraction of titanium from its natural forms and subsequent processing [4; 5]. Despite the existence of several production methods, such as the Hunter process, the Armstrong process or the HAMR process [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], the Kroll process remains the only method employed on an industrial scale and is economically viable [4; 6]. Nevertheless, the Kroll process is relatively demanding and inefficient, with 361 MJ of energy being consumed to produce 1 kg of titanium, compared to 211 MJ/kg for aluminium and only 23 MJ/kg for steel [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Consequently, novel production methods are being developed, alongside increased efforts to recycle titanium waste [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe recycling of titanium and its alloys has been increasing on a global scale. For example, in the United States, approximately 25 thousand tons of recycled titanium alloys were used in 2004, out of a total consumption of 45 thousand tons. In most cases, the recycled materials consisted of chips, compact pieces, powders, or support structures produced through 3D printing [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In the following years, the consumption of titanium scrap continued to grow steadily, reaching 50\u0026ndash;65 thousand tons during the 2017\u0026ndash;2019 period [9; 10]. A similar trend has been observed worldwide. Recycling offers two key benefits: it conserves primary raw materials and reduces environmental impact [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. As a result, the effective recycling of titanium scrap has garnered increasing attention in recent years (e.g. [\u003cspan additionalcitationids=\"CR12 CR13 CR14 CR15 CR16\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]).\u003c/p\u003e\u003cp\u003eAccording to Takeda et al. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] a significant demand for recycled titanium is anticipated, even for alloys with relatively high impurity levels. One contributing factor is the potential application of titanium in the automotive industry, which is currently limited primarily by production costs. The demand for secondary titanium alloys is expected to exceed that for ferrotitanium, highlighting the need for effective recycling methodologies.\u003c/p\u003e\u003cp\u003eModern engineering solutions aim to reduce raw material and energy use, ultimately maximizing profit. Additive manufacturing (AM) in the titanium industry offers significant potential to improve material efficiency and streamline production. AM can lower raw material consumption, reduce processing steps, and minimize final machining. A pivotal component of powder-based AM involves the production or procurement of titanium powders that meet strict dimensional and compositional specifications [19; 20].\u003c/p\u003e\u003cp\u003eIn the case of titanium alloy powders suitable for use in AM technologies (including the Laser Powder Bed Fusion method), it is relatively difficult to obtain these materials\u0026mdash;except for commonly available commercial alloys (especially CP Ti Grade 2 and Ti Grade 5)[19; 21]. This is particularly evident for newly studied and developed titanium alloys (β alloys) containing high-melting-point elements such as Nb, Ta, etc. [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. However, these β-titanium alloys appear especially suitable for biomedical applications [23; 24]. AM enables innovative structural designs, such as the printing of porous structures that adjust the mechanical properties of implants to better match those of human bone while also promoting integration with bone tissue [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAlthough the Ti-6Al-4V ELI alloy (ASTM F3001-14 [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]) is already used in AM of bone implants, the literature mentions the potential toxicity of aluminium and vanadium in relation to neurodegenerative diseases, such as Alzheimer\u0026rsquo;s disease [23; 27]. β-titanium alloys are therefore considered a more suitable alternative due to their lower toxicity and reduced elastic modulus (14\u0026ndash;85 GPa [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e] which more closely matches the properties of human bone (10\u0026ndash;40 GPa [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]).\u003c/p\u003e\u003cp\u003eThis article focuses on the recycling of titanium alloys Ti-15Mo and Ti-13Nb-13Zr and their subsequent processing enabling the production of powders suitable for additive manufacturing. These alloys represent modern and promising β-type titanium alloys due to their low elastic modulus. At the same time, they are already used in conventional manufacturing processes and are approved for biomedical applications (ASTM F2066-23 and F1713-08(2021)e1). The utilization of residual material as a feedstock could lead to a reduction in the production costs of these alloys and thus promote their broader application. Even if the same purity and mechanical properties as those of primary alloys are not achieved, the resulting material could still represent a more economically viable option suitable for alternative applications where its use would be desirable from a technical point of view but has so far been limited by the high cost.\u003c/p\u003e"},{"header":"2 Methods","content":"\u003cp\u003eIn this study, two recycled β-titanium alloys, Ti-15Mo and Ti-13Nb-13Zr, were investigated. The aim was to evaluate the material properties at individual stages of technological processing, including recycling, hot forging, ultrasonic atomisation, and additive manufacturing (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The choice of analytical techniques was adapted to the geometry and condition of the material at each processing stage.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eBoth alloys were prepared at the Advanced Metal Powders Ltd. (Kravaře, Czech Republic) through the process of recycling. The horizontal plasma arc melting (HPAM) process was employed, resulting in the fabrication of material in the form of irregular cross-section rods. The production involved the utilisation of pure titanium scrap CP Ti GRADE 2, which was doped with pure alloys (Mo flakes or Zr and Nb flakes). The finished remelted bars were controlled for chemical composition using an Handheld ED-XRF spectral analyser DELTA (Olympus, Innov-X Systems, Inc., Newton, MA, USA). These bars were subsequently hot forged without protective atmosphere in multiple steps at 950\u0026deg;C to produce rods with a circular cross-section (8 mm diameter) suitable for atomization process. Prior to further processing, the rods were mechanically cleaned to remove the oxide layer. The next metallurgical step was ultrasonic atomisation using an ATO LAB+ (3D LAB Ltd., Warsaw, Poland) machine. During this process, the molten metal was atomised using ultrasonic vibrations and dispersed into a stream of inert gas. The resulting powders were employed for additive manufacturing by laser powder bed fusion (LPBF) method using a Trumpf TruPrint T1000 (TRUMPF, Ditzingen, Germany) machine in Czech Academy of Sciences, Institute of Physics. For process optimization, cubic specimens (10 mm edge) were printed at varying laser powers (60\u0026ndash;200 W) and scanning speeds (300\u0026ndash;1900 mm/s). Based on the results, the optimal parameters (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) were selected for printing test specimens.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSelected printing parameters for Ti-15Mo and Ti-13Nb-13Zr alloys\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAlloy\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eLaser power (W)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eScanning speed (mm/s)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-15Mo\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e180\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-13Nb-13Zr\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e180\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 1 - Recycling\u003c/b\u003e\u003c/p\u003e\u003cp\u003eMicrostructural characterization of the recycled rods was performed on both longitudinal and transverse sections. Samples were ground using SiC papers with grit sizes ranging from P280 to P2000 and subsequently polished with a colloidal SiO₂ suspension using H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e as a wetting agent (in a 3:1 ratio). The purity and porosity of these sections were evaluated using light microscope (LOM) Neophot 32 (Carl Zeiss, Jena, Germany). The microstructure was revealed by Kroll\u0026rsquo;s reagent (3 ml HF, 8 ml HNO₃, 100 ml H₂O) and also examined by LOM. For detailed microstructural observation and chemical composition analysis, scanning electron microscopy (SEM) JEOL JSM-7600F (JEOL, Tokyo, Japan) equipped with EDXS detector EDAX Octane Elite (EDAX LLC, Mahwah, NJ, USA) was used. Hardness was measured using the Vickers method with a load of 10 kgf employing an automatic hardness tester Struers Duramin 40AC3 (Struers GmbH, Ballerup, Denmark). To analyse oxygen content, samples of approximately 0.01\u0026ndash;0.1 g were taken and analysed using a G8 Galileo (Bruker Elemental GmbH, Kalkar, Germany), inert gas fusion spectrometer. For cytotoxicity evaluation, slices approximately 2 mm thick were prepared and processed according to the same grinding and polishing procedure used for metallographic samples. The tests were carried out in collaboration with UCT Prague, Department of Metallic Materials and Corrosion Engineering, according to ISO 10993-5. The samples were sterilized at 180\u0026deg;C for 2 h and subsequently incubated for 24 h in culture media (extraction ratio 1 ml/1.25 cm\u0026sup2;). Extracts were applied to L929 and hFOB 1.19 cell lines cultivated in CO₂ incubators (5% CO₂, 37\u0026deg;C for L929; 34\u0026deg;C for hFOB 1.19). A medium with 0.2% Tween served as a positive control. After 24 h exposure, cell metabolic activity was assessed using the resazurin reduction assay, where viable cells convert resazurin to the fluorescent product resorufin. Fluorescence was measured with a Fluoroskan Ascent FL microplate reader (Thermo Scientific, Waltham, MA, USA). Cytotoxicity of the extracts was expressed as the relative metabolic activity of treated cells compared to negative controls (NC).\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 2 - Forging\u003c/b\u003e\u003c/p\u003e\u003cp\u003eIn forged stage, the microstructural analyses were conducted using LOM Neophot 32 and digital microscope Olympus DSX1000 (Olympus, Tokyo, Japan) and SEM - JEOL JSM-7600F. Additionally, hardness measurements were taken using the Struers Duramin 40AC3, and oxygen content analysis was performed using the G8 Galileo. Owing to the appropriate geometry of rods, tensile tests were also performed using an Instron 5582 (Instron, Norwood MA, USA). Small-size test specimens with a round cross-section and a diameter of 8 mm, having a gauge length five times the diameter, were used for the testing. Fractographic analysis was carried out using an Olympus DSX1000 LOM and a JEOL JSM-7600F SEM.\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 3 - Atomisation\u003c/b\u003e\u003c/p\u003e\u003cp\u003eIn the third \u0026ndash; atomised \u0026ndash; phase, the morphology and particle size distribution of the powders were evaluated using SEM (JEOL JSM-7600F) and image analysis using software NIS-Elements 5.42.03 Advanced Research (Nikon, NY, USA). The particle size distribution was further verified by laser diffraction analysis using a particle size analyser Mastersizer 3000 (Malvern Panalytical, Malvern, UK). In addition, elemental distribution maps were obtained using an EDXS detector (EDAX Octane Elite). Oxygen content of the powders was also determined (G8 Galileo).\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 4 \u0026ndash; Additive manufacturing\u003c/b\u003e\u003c/p\u003e\u003cp\u003eIn the last phase, specimens for tensile testing were additively manufactured and subsequently machined, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Tensile tests were performed on as-built samples using Instron 5582. Cylindrical specimens for compression testing (20mm in height and 20 mm in diameter) were also produced, and compressive tests were conducted on a universal testing machine ZDM 50 (VEB Werkstoffpr\u0026uuml;fmaschinen, Leipzig, Germany). In addition, cylindrical samples with a diameter of 10 mm and a height of 20 mm were fabricated for metallographic analysis, hardness measurement, and oxygen content determination. Microstructural observations were carried out using LOM (Neophot 32) and SEM (JEOL JSM-7600F) equipped with an EDXS detector. Furthermore, an EBSD detector EDAX Velocity Pro (EDAX LLC, Mahwah, NJ, USA) was employed to analyze the phase composition and grain orientation. Measurement was done at accelerating voltage of 20 kV, tilt angle 70\u0026deg; utilizing 0.2 \u0026micro;m 0.65 \u0026micro;m step size for Ti-15Mo and Ti-13Nb-13Zr respectively. Collected RAW EBSD data were processed and post-processed in EDAX OIM Analysis\u0026trade; and ATEX analytical software [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Hardness testing (Struers Duramin 40AC3) and oxygen analysis (G8 Galileo) were also performed.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"3 Results","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Phase 1 - Recycling\u003c/h2\u003e\u003cp\u003eThe microstructure of both investigated alloys after the recycling step is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, b the Ti-15Mo alloy exhibits equiaxed β grains with sizes ranging approximately from 800 to 3000 \u0026micro;m as a result of slow cooling after the HPAM process. In addition to the grain boundaries, other features are evident in the microstructure, which are most likely associated with the precipitation of the stable α (hexagonal phase) and/or metastable ω phase (hexagonal phase with ellipsoid or plate-like shape)[31; 32]. These features locally align in a manner that resembles subgrain boundaries. Average hardness of the Ti-15Mo alloy is determined as 297\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV10.\u003c/p\u003e\u003cp\u003eThe microstructure of the Ti-13Nb-13Zr alloy after recycling (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, d) also reveals coarse polyhedral grains comparable in size to the Ti-15Mo alloy. In this particular instance, however, the structure exhibits a typical needle-like morphology, consisting of finely dispersed α martensite in β matrix. The average hardness of this alloy is 269\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV10.\u003c/p\u003e\u003cp\u003eIn both alloys, pores were identified in the microstructure. Based on their morphology and inner surface features (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee), these pores are most likely of gaseous origin and keyholes.\u003c/p\u003e\u003cp\u003eThe oxygen content is found to be strongly dependent on the sampling location. For Ti-15Mo alloy, the oxygen concentration at the surface reached nearly 1500 ppm, whereas samples taken from the central region contained approximately 230 ppm. The average oxygen concentration is therefore 1055\u0026thinsp;\u0026plusmn;\u0026thinsp;720 ppm. For the Ti-13Nb-13Zr alloy, the measured oxygen content is 1390\u0026thinsp;\u0026plusmn;\u0026thinsp;140 ppm, with lower variability due to sampling closer to the surface.\u003c/p\u003e\u003cp\u003eThe biological response of both materials maintains the relative metabolic activity of cells above the 70% threshold (the cytotoxicity limit), as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. No statistically significant reduction in metabolic activity is observed compared to the negative control (NC, culture medium only), confirming the cytocompatibility of the investigated alloys after recycling process.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003e\u003cb\u003ePhase 2 - Forging\u003c/b\u003e\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e\u003cp\u003eAfter hot forging process, the Ti-15Mo alloy (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, b) exhibits a so-called necklace-type of microstructure[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e], which results from repeated hot working. Despite the significantly altered microstructure, the hardness remains essentially unchanged, with an average value of 297\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV10. Tensile testing revealed an ultimate tensile strength of 1005\u0026thinsp;\u0026plusmn;\u0026thinsp;19 MPa, full set of mechanical property results is summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Compared to the previous processing step, the oxygen content in the central part of the specimen increased to 877\u0026thinsp;\u0026plusmn;\u0026thinsp;171 ppm.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe increase in oxygen content is attributed to an improper hot working regime, where the absence of a protective inert gas atmosphere led to the formation of folding defects (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). The size and severity of the folding defects were variable, and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows an extreme example of such a defect. EDXS analysis revealed that in the case of prominent folding penetrating deeply into the cross-section predominantly contain molybdenum and elevated levels of oxygen.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe microstructure of the Ti-13Nb-13Zr alloy after deformation is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed, e. The microstructure exhibits an acicular morphology; however, the needles appear coarser compared to the previous (recycled) step. Mechanical properties are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The mean hardness values remained very similar, with an average of 279\u0026thinsp;\u0026plusmn;\u0026thinsp;5 HV compared to 269\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV in the recycled condition. The ultimate tensile strength is 839\u0026thinsp;\u0026plusmn;\u0026thinsp;69 MPa.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eMechanical properties of Ti-15Mo and Ti-13Nb-13Zr in forged stage\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAlloy\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eYS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eUTS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eETF (%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eHV10\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-15Mo\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e973\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e1005\u0026thinsp;\u0026plusmn;\u0026thinsp;19\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e12,3\u0026thinsp;\u0026plusmn;\u0026thinsp;0,1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e297\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-13Nb-13Zr\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e802\u0026thinsp;\u0026plusmn;\u0026thinsp;30\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e839\u0026thinsp;\u0026plusmn;\u0026thinsp;69\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e3,5\u0026thinsp;\u0026plusmn;\u0026thinsp;0,2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e279\u0026thinsp;\u0026plusmn;\u0026thinsp;5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTensile specimens of Ti-13Nb-13Zr fractured in a distinctly brittle manner (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003ea) and the fracture morphology exhibited a direct correspondence with the microstructure, manifesting as acicular needles. This can be attributed to the high oxygen content detected in the structure (1929\u0026thinsp;\u0026plusmn;\u0026thinsp;424 ppm). As with the Ti-15Mo alloy, the origin of this problem lies in an improperly adjusted hot working process, which resulted in the formation of folding defects. During heat treatment performed without a protective atmosphere, the surface becomes saturated with oxygen, leading to embrittlement of the outer layer and facilitating crack initiation (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003eb). The combination of oxygen-induced embrittlement and processing-induced folding defects likely promotes the propagation of brittle fracture, significantly affecting the mechanical behavior.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Phase 3 - Atomisation\u003c/h2\u003e\u003cp\u003eIn the case of Ti-15Mo alloy powder, the majority of particles exhibit a regular spherical morphology, although some smaller irregular (non-spherical) particles are also present. The surface of certain powders is not entirely uniform and displays noticeable surface irregularities (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e) [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], which are concomitant with elevated oxygen content. Occasional larger defects, such as satellites and agglomerates, are also present, but only in minor quantities.\u003c/p\u003e\u003cp\u003eFurthermore, the analysis indicated a non-uniform distribution of molybdenum, with some particles exhibiting significantly higher concentrations (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). Furthermore, the oxygen content was found to be 1511\u0026thinsp;\u0026plusmn;\u0026thinsp;273 ppm.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eA comparable behavior was observed for the Ti-13Nb-13Zr powder (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e). While the powders predominantly exhibit a spherical morphology, the distribution of niobium and zirconium is not uniform across individual particles, with certain particles showing a pronounced enrichment in these alloying elements. Spot EDXS analysis demonstrated that particle 1, marked at Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e, contained 9.8 weight % Zr and 10.9 weight % Nb, in contrast to particle 2, which exhibited 25.1 weight % Zr and 20.8 weight % Nb. As was the case with the Ti-15Mo alloy, some particles exhibited distinct surface irregularities where an increased oxygen content was detected. Additionally, combustion elemental analysis showed a pronounced increase in oxygen content, with the powder reaching 5165\u0026thinsp;\u0026plusmn;\u0026thinsp;462 ppm, representing a substantial rise compared to the previous step.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the particle size distribution results obtained from image analysis. The powders exhibited relatively high mean particle sizes, namely 62 \u0026micro;m for the Ti-15Mo alloy and 65 \u0026micro;m for the Ti-13Nb-13Zr alloy. The results were further validated by laser diffraction, yielding mean particle sizes of 59.3 \u0026micro;m for Ti-15Mo and 60.2 \u0026micro;m for Ti-13Nb-13Zr.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Phase 4 - Additive manufacturing (LPBF)\u003c/h2\u003e\u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea shows the microstructure of the Ti-15Mo alloy in the build direction. Individual melt pools (MP) as well as columnar grain boundaries can be distinguished. The microstructure contains typical defects such as lack-of-fusion (LOF), and occasional gas pores were also identified. In addition, Mo regions in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea that deviate from the characteristic microstructure of additively manufactured alloys were observed, which appear to correspond to a secondary phase. EDXS analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb) confirmed that these regions represent sites of pronounced segregation of Mo.\u003c/p\u003e\u003cp\u003eSEM observations (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ec) further reveal that the melt pools consist of a cellular substructure, which is characteristic for this alloy, with the cell growth direction being dictated by the local heat flow.[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]\u003c/p\u003e\u003cp\u003eEBSD analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ed) revealed that the Ti-15Mo alloy crystallizes completly in the β phase with a body-centred cubic (BCC) lattice. The analysis also confirmed the presence of columnar grains with high-angle grain boundaries (HAGBs in black), which were already evident in the light microscopy observations. Low-angle grain boundaries (LAGBs), highlighted in red, account for 70.5% of all grain boundaries, compared to 29.5% for HAGBs. The high fraction of LAGBs indicates residual internal stresses, as dislocations tend to rearrange and accumulate along these boundaries due to the rapid heating and cooling cycles during processing. This contributes to overall strengthening of the material and primarily affects its mechanical properties such as strength, however it may have a negative impact on ductility. The majority of columnar β grains exhibit a preferred growth orientation along the \u0026lt;\u0026thinsp;001\u0026thinsp;\u0026gt;\u0026thinsp;direction, direction which is parallel to the rolling direction (RD). RD corresponds to the build direction and thus to the direction of the maximum thermal gradient, as confirmed by the inverse pole figure (IPF) triangles shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ee.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe 3D printed Ti-13Nb-13Zr alloy, when observed under a light microscope (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ea), exhibits the characteristic microstructure of additively manufactured materials with relatively wide melt pools with low depth. However, the individual melt pools are rather difficult to distinguish, which is most likely related to the magnitude of energy input during additive manufacturing process [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Nevertheless, lack-of-fusion defects were present in the structure, and gas pores were also observed. As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed, regions of elemental segregation were identified, similar to the Ti-15Mo alloy.\u003c/p\u003e\u003cp\u003eA more detailed SEM analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ec) revealed the acicular microstructure typical for this alloy. In this case, however, the needles differ slightly from those observed in the previous processing steps: their edges appear coarser and more ragged.\u003c/p\u003e\u003cp\u003eThe results of the EBSD analysis are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ed, e. The non-indexed areas (represented by the black regions) correspond to regions with a high dislocation density. A high density of line defects leads to distortion and poor visibility of diffraction patterns, making these regions impossible to index. Nevertheless, even with reduced indexing quality, it is still possible to reliably distinguish between the α and β phase regions. The volume fraction of primary β grains (green) is approximately 62%, while the fraction of needle-like α phase (red) is approximately 38%. Majority of α grains are aligned with the \u0026lt;\u0026thinsp;001\u0026thinsp;\u0026gt;\u0026thinsp;crystal direction parallel, or close to parallel to RD, i.e. build direction, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ee and confirmed by IPF triangles Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003ef.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe increase in oxygen content in additive manufactured phase was particularly pronounced. In the case of the Ti-13Nb-13Zr alloy, values in the range of 2000 to 12,000 ppm were measured, with the exact concentration depending on the sampling location. In contrast to the Ti-13Nb-13Zr alloy, the increase in oxygen content in the Ti-15Mo alloy was considerably smaller, reaching an average value of 4723\u0026thinsp;\u0026plusmn;\u0026thinsp;114 ppm.\u003c/p\u003e\u003cp\u003eThe mechanical properties of both alloys in their as-built state are summarised in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. A significant increase in hardness was observed for both alloys compared to forged phase, with a more pronounced rise in Ti-13Nb-13Zr. This alloy also exhibited greater variability in the measured values. The increased scatter in hardness values is considered to result from the complex nature of the additive manufacturing process, with the cumulative effects of previous processing steps and the elevated oxygen content within the alloy.\u003c/p\u003e\u003cp\u003eTi-15Mo alloy achieved an ultimate tensile strength of 1211 MPa. In contrast, reliable strength data could not be obtained for the Ti-13Nb-13Zr alloy as the specimens repeatedly failed in the gripping area. The maximum stress reached before fracture was approximately 500 MPa. This premature brittle failure was most likely caused by the alloy's elevated oxygen content.\u003c/p\u003e\u003cp\u003eDuring compression testing, the Ti-15Mo alloy did not exhibit a distinct failure, and the ultimate compressive strength could not be determined, as the specimen continued to deform in a ductile manner. The Ti-13Nb-13Zr alloy achieved a compressive strength of 956 MPa and the specimen failed in a very brittle manner, similar to the behavior observed during tensile testing. As mentioned earlier, this behavior can be attributed to the high oxygen content and possibly to residual stresses introduced during the additive manufacturing process.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eMechanical properties of additively manufactured Ti-15Mo and Ti-13Nb-13Zr in as-built state\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"7\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eAlloy\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eCompression test\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c6\" namest=\"c4\"\u003e\u003cp\u003eTensile test\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eHV10\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eYS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eUTS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eYS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eUTS (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eETF (%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-15Mo\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e941\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e1205\u0026thinsp;\u0026plusmn;\u0026thinsp;8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e1211\u0026thinsp;\u0026plusmn;\u0026thinsp;8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e16\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e\u003cp\u003e345\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTi-13Nb-13Zr\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e946\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e956\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c6\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c7\"\u003e\u003cp\u003e454\u0026thinsp;\u0026plusmn;\u0026thinsp;15\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e"},{"header":"4 Discussion","content":"\u003cp\u003e\u003cb\u003ePhase 1 - Recycling\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe recycling process using (HPAM) proved to be suitable for both investigated alloys. The measured oxygen contents can be compared with the corresponding standards for these materials. Although these standards apply to alloys in wrought state, such comparison is appropriate for assessing whether the oxygen limits are met. The measured concentrations are in compliance with the applicable ASTM specifications, which define maximum oxygen limits of 2000 ppm for Ti-15Mo (ASTM F2066-23) and 1500 ppm for Ti-13Nb-13Zr (ASTM F1713-08(2021)e1), both in wrought condition. Furthermore, cytotoxicity tests confirmed that the alloys processed in this way comply with ISO 10993-5, indicating their suitability for biomedical applications.\u003c/p\u003e\u003cp\u003eMicrostructural analysis revealed that the microstructure of the Ti-15Mo alloy corresponds to those reported in the literature for materials produced by remelting techniques [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e], with differences primarily in grain size, which are attributed to variations in solidification rate. The alloy exhibits a β-phase structure, as also supported by the measured hardness of 296\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV10, which, according to [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e], is characteristic of a predominantly β structure.\u003c/p\u003e\u003cp\u003eThe Ti-13Nb-13Zr alloy shows a typical needle-like morphology, consisting of finely dispersed α martensite within a β matrix. According to the literature, the hardness of this alloy varies depending on the prior thermal history and, consequently, on the resulting microstructural morphology. Reported hardness values range from 205 to 323 HV [39; 40], depending on the previous heat treatment, which is consistent with the measured value obtained in this study (269\u0026thinsp;\u0026plusmn;\u0026thinsp;4 HV10).\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 2 - Forging\u003c/b\u003e\u003c/p\u003e\u003cp\u003eDuring forging, both alloys exhibited the expected microstructural changes, with the effect being more pronounced in the Ti-15Mo alloy. This alloy developed a necklace-type microstructure, which is typical for specific hot-working processes [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. In contrast, the Ti-13Nb-13Zr alloy retained its characteristic needle-like microstructure, consistent with data reported in the literature [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eDue to the applied forging process, folding defects were observed in both alloys. In general, folding defects contribute to oxygen enrichment, and since oxygen acts as an α-stabilizing element, it affects the phase composition and consequently the mechanical properties. Increase in oxygen content was more significant in the Ti-13Nb-13Zr alloy. The measured oxygen concentration of 1929\u0026thinsp;\u0026plusmn;\u0026thinsp;424 ppm exceeds the maximum limit of 1500 ppm defined by ASTM F1713-08(2021)e1. The elevated oxygen content had a noticeable effect on the tensile response, resulting in reduced ductility and brittle fracture [42; 43]. The UTS of 839\u0026thinsp;\u0026plusmn;\u0026thinsp;69 MPa is close to the values specified in ASTM F1713-08(2021)e1 for the alloy in the annealed and aged condition, however, the measured ETF is less than half of the standard specified value. Fractographic examination revealed that the brittle fracture was initiated in the oxygen-saturated surface layer, which was embrittled during hot forging without protective atmosphere. The propagation of the crack was further influenced by the folding defects.\u003c/p\u003e\u003cp\u003eFor the Ti-15Mo alloy, a lower oxygen content (877\u0026thinsp;\u0026plusmn;\u0026thinsp;171 ppm) was measured, most likely due to sampling from areas without folding defects, where the local oxygen concentration was reduced, leading to a lower average value. The measured mechanical properties (UTS of 1005\u0026thinsp;\u0026plusmn;\u0026thinsp;19 MPa, ETF of 12.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1%) are consistent with the values specified in ASTM F2066-23 for this alloy in the α\u0026thinsp;+\u0026thinsp;β annealed condition, also indicating compliance with the oxygen content limit. It can be assumed that a higher oxygen content would lead to a decrease in ductility. According to [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e], total elongation decreases from 61% to 42% as the oxygen content increases from 0.1 wt.% to 0.2 wt.%. A further rise in oxygen concentration continues to reduce ductility while increasing strength.\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 3 - Atomisation\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe chemical composition analysis revealed an inhomogeneous distribution of alloying elements within individual powder particles. To the authors\u0026rsquo; knowledge, such compositional inhomogeneity has not been previously reported and is most likely associated with defects generated during the deformation process, particularly with the formation of folds that locally promote the accumulation of alloying elements.\u003c/p\u003e\u003cp\u003eThis processing step also resulted in a further increase in oxygen content. The elevated oxygen content can also be explained by the high surface-to-volume ratio are exposed to atmospheric oxygen for extended periods during handling and transportation, which may lead to surface oxidation [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eOnly a few larger defects, such as satellites and agglomerates, were detected in the powder. According to the literature, this is a typical feature of additive manufacturing powders; however, their frequency is lower than in powders produced by another conventional techniques such as gas atomization [46; 47].\u003c/p\u003e\u003cp\u003e\u003cb\u003ePhase 4 \u0026ndash; Additive manufacturing\u003c/b\u003e\u003c/p\u003e\u003cp\u003eAdditive manufacturing using LPBF process of the prepared powders is feasible, provided that the processing parameters are properly optimized. Even after optimization, the material still exhibits defects typical for additively manufactured components, such as pores and lack-of-fusion regions [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. As a potential solution to this issue, the use of hot isostatic pressing (HIP) has been reported in the literature [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. HIP has been successfully applied to eliminate porosity in additively manufactured parts; nevertheless, its influence on the microstructure and resulting mechanical properties of the processed alloys requires further investigation. For example, Ref. [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e] reported the preservation of primary β grains after HIP treatment. For the alloys investigated in this study, post-processing by HIP after additive manufacturing has not yet been documented in the available literature.\u003c/p\u003e\u003cp\u003eBoth alloys show a melt pool microstructure characteristic of 3D-printed materials. Detailed SEM observations of the Ti-15Mo alloy reveal a sub-cellular structure consisting solely of the β phase, which was confirmed by EBSD analysis. Similar findings were reported in studies [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e] [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e], which also identified the presence of the ω phase. In the case of the Ti-13Nb-13Zr alloy, a needle-like morphology can be observed, corresponding to a combination of martensitic α\u0026prime; and β phases, as reported in phase analysis studies [34; 52; 53]. The volume fraction of these phases is influenced by the thermal cycles experienced during repeated melting and solidification, i.e., by the input energy (printing parameters) and the chosen scanning strategy. For instance, study [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e] have shown that the fraction of martensitic α\u0026prime; phase may vary from 18.3 to 52.5 vol.% when only the scanning strategy is altered while keeping other parameters constant. Study [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e] even reported up to 99% of the α\u0026prime; martensitic phase. In the present work, EBSD analysis revealed an α phase content of 41.3 vol.%.\u003c/p\u003e\u003cp\u003eIn terms of mechanical performance, an overall increase in strength characteristics was observed. The Ti-15Mo alloy exhibited an average hardness of 345\u0026thinsp;\u0026plusmn;\u0026thinsp;6 HV10, an ultimate tensile strength exceeding 1200 MPa, and an elongation of 16%. The strength value is comparable to those reported for conventionally processed alloys subjected to specific heat treatment regimes, including annealing and subsequent aging [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. According to literature [35; 54], the ultimate tensile strength of additive manufactured as-built Ti-15Mo alloys typically ranges from 800 to 1200 MPa, depending on the processing parameters. The relatively high strength achieved in this study may be attributed to the increased oxygen content, which enhances strength but reduces ductility [\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e]. Nevertheless, a survey of extant literature reveals an absence of reports on the oxygen content of as-built specimens. It is therefore hypothesised that the values determined in the present study may represent typical levels. The observed mechanical properties may be attributed to the optimised combination of printing parameters, energy density input during the additive manufacturing process, and resulting microstructure.\u003c/p\u003e\u003cp\u003eThe effect of increased oxygen content is even more pronounced for the Ti-13Nb-13Zr alloy, with values reaching up to 12 000 ppm. The measured hardness reached 454\u0026thinsp;\u0026plusmn;\u0026thinsp;15 HV10, which is relatively high compared to reported values. For instance, study [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e] reported a hardness of about 255 HV2, whereas study [\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e] reported 519 HV. The high oxygen content likely contributed to the increased hardness and strength of the Ti-13Nb-13Zr alloy. However, due to the elevated oxygen content and possibly high internal stresses, tensile testing could not be completed successfully. The specimens fractured within the clamping area rather than in the gauge section, preventing full analysis. This behavior was confirmed during compression testing, where the specimens essentially fragmented upon loading. Nevertheless, it is expected that with a reduced oxygen content, the tensile strength could reach approximately 1000 MPa, as reported in previous studies [52; 56; 57].\u003c/p\u003e"},{"header":"5 Conclusions","content":"\u003cp\u003eThis study investigated the evolution of microstructure, oxygen content, and mechanical properties of Ti-15Mo and Ti-13Nb-13Zr alloys processed through recycling using the HPAM method, followed by hot forging, powder atomisation, and additive manufacturing. The results demonstrate that recycling of the investigated alloys via HPAM is feasible, and that powders produced from such recycled materials can be further processed by additive manufacturing.\u003c/p\u003e\u003cp\u003eThe findings obtained from individual processing stages can be summarised as follows:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eThe recycled Ti-15Mo alloy exhibited a coarse β-grain structure, while Ti-13Nb-13Zr displayed a needle-like α martensite within a β matrix, characteristic of the as-cast condition. Both materials showed hardness values consistent with those reported in the literature. The oxygen content of both alloys remained within the limits specified by ASTM F2066-23 and F1713-08(2021)e1 standards, and no cytotoxic effects were observed, confirming their cytocompatibility after the recycling process.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eHot forging led to microstructural refinement, particularly in Ti-15Mo, which developed a necklace-type morphology with high mechanical strength (UTS\u0026thinsp;\u0026asymp;\u0026thinsp;1000 MPa) despite occurrence of folding defects. In Ti-13Nb-13Zr deformation under non-protective atmosphere caused higher increase in oxygen content (\u0026asymp;\u0026thinsp;1900 ppm) exceeding the limit set by ASTM F1713-08(2021)e1 resulted in the formation of folding defects, resulting in brittle fracture behavior during tensile testing. It is necessary either to adjust the forming parameters and employ a protective atmosphere or to adopt an alternative dimension-adjustment method prior to atomisation.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eGas atomisation produced predominantly spherical powders for both alloys, though minor surface irregularities and compositional inhomogeneities were observed which can be related to elemental segregation occurring during the hot working step. The oxygen content increased moderately, reaching 1511\u0026thinsp;\u0026plusmn;\u0026thinsp;273 ppm for Ti-15Mo and 5165\u0026thinsp;\u0026plusmn;\u0026thinsp;462 ppm for Ti-13Nb-13Zr. The particle size distribution was relatively coarse (\u0026asymp;\u0026thinsp;60 \u0026micro;m) but within the range suitable for powder-bed additive manufacturing.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eAdditively manufactured specimens exhibited melt pools typical for LPBF with some heterogeneity in elemental distribution and the presence of gas pores and lack-of-fusion defects. The Ti-15Mo alloy achieved an ultimate tensile strength of 1211 MPa and demonstrated ductile behavior under both tension and compression. No distinct failure was observed during compression, and the ultimate compressive strength could not be determined, despite the elevated oxygen level (4723\u0026thinsp;\u0026plusmn;\u0026thinsp;114 ppm). In contrast, Ti-13Nb-13Zr specimens showed brittle failure during tensile and compression test, probably attributed to excessive oxygen uptake (locally up to 12 000 ppm) and residual stresses generated during the printing process. wing to this extremely brittle behavior, the tensile test for the Ti-13Nb-13Zr alloy could not be completed.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eThe results highlight the key role of oxygen control during high-temperature processing and powder handling. Ti-13Nb-13Zr exhibits higher oxygen sensitivity and limited ductility compared to Ti-15Mo. Ensuring an inert atmosphere during hot working, atomisation, and additive manufacturing is therefore essential for preserving alloy performance. These findings confirm that recycled β titanium alloys, particularly Ti-15Mo, hold strong potential for sustainable additive manufacturing applications, provided that oxygen uptake is effectively mitigated.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eStatements and Declarations\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis project has been financed with the state support of the Technology Agency of the Czech Republic and the Ministry of Industry and Trade of the Czech Republic within the TREND Programme No. FW06010136 and supported by the Grant Agency of the Czech Technical University in Prague, Grant No. SGS23/162/OHK2/3T/12.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEliška Galčíková:\u0026nbsp;\u003c/strong\u003eConceptualization, Methodology, Investigation, Visualisation, Writing – original draft; \u003cstrong\u003eJan Krčil:\u003c/strong\u003e Conceptualization, Methodology, Investigation, Writing - review and editing; \u003cstrong\u003eVladimír Mára:\u003c/strong\u003e Methodology, Writing - review and editing; \u003cstrong\u003eLucie Pilsová:\u003c/strong\u003e Visualization, Writing - review and editing; \u003cstrong\u003eJana Sobotová:\u003c/strong\u003e Supervision, Writing - review and editing;\u0026nbsp;\u003cstrong\u003eZdeněk Míchal:\u0026nbsp;\u003c/strong\u003eMethodology,\u003cstrong\u003e\u0026nbsp;Dalibor Vojtěch:\u0026nbsp;\u003c/strong\u003eSupervision,\u003cstrong\u003e\u0026nbsp;Jiří Režnar:\u0026nbsp;\u003c/strong\u003eResources\u003c/p\u003e\n\u003cp\u003eAll authors have read and agreed to the published version of the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eRudnick RL, Gao S (2003) Composition of the Continental Crust. 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J Alloys Compd 762:289\u0026ndash;300. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.jallcom.2018.05.179\u003c/span\u003e\u003cspan address=\"10.1016/j.jallcom.2018.05.179\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Ti-15Mo, Ti-13Nb-13Zr, recycling process, laser powder bad fusion","lastPublishedDoi":"10.21203/rs.3.rs-8269080/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8269080/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRecycling of titanium alloys represents a modern and economically attractive processing route, as it enables a significant reduction in production costs associated with the high price of titanium. However, the properties of recycled β-titanium alloys have not yet been sufficiently investigated in the scientific literature, and their potential for processing by additive manufacturing technologies remains largely unexplored. This study focuses on evaluating the recycling of Ti-15Mo and Ti-13Nb-13Zr alloys using the horizontal plasma arc melting (HPAM) method and further examines subsequent processing steps, including hot working to the required shape for atomisation and subsequent powder processing via the LPBF technique. In all technological stages, the microstructure was characterised, chemical analysis was performed, and, where applicable, mechanical properties were evaluated. The results demonstrate that recycling of the investigated alloys is feasible, as is their subsequent processing up to the production of additively manufactured specimens. Nevertheless, strict control of the oxygen content is critical for both alloys, as the current levels exceed the limits specified by the relevant standards. At such elevated oxygen concentrations, the alloys are unsuitable for biomedical applications; however, their use in structural applications remains realistic, particularly considering the production cost reduction achieved through the implementation of recycling.\u003c/p\u003e","manuscriptTitle":"Recycling of Ti-15Mo and Ti-13Nb-13Zr Alloys and their Use in Additive Manufacturing","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-12 19:36:46","doi":"10.21203/rs.3.rs-8269080/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2026-01-02T14:20:28+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-12-07T05:08:53+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-06T22:42:11+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-05T04:19:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"The International Journal of Advanced Manufacturing Technology","date":"2025-12-03T05:31:19+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"554f54c7-c276-4b78-ba4b-4cab033c1fc8","owner":[],"postedDate":"December 12th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-03-16T16:14:43+00:00","versionOfRecord":{"articleIdentity":"rs-8269080","link":"https://doi.org/10.1007/s00170-026-17801-7","journal":{"identity":"the-international-journal-of-advanced-manufacturing-technology","isVorOnly":false,"title":"The International Journal of Advanced Manufacturing Technology"},"publishedOn":"2026-03-14 15:58:28","publishedOnDateReadable":"March 14th, 2026"},"versionCreatedAt":"2025-12-12 19:36:46","video":"","vorDoi":"10.1007/s00170-026-17801-7","vorDoiUrl":"https://doi.org/10.1007/s00170-026-17801-7","workflowStages":[]},"version":"v1","identity":"rs-8269080","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8269080","identity":"rs-8269080","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}