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Abstract

The aim of this study was the characterization of the microstructure and the mechanical properties of commercially pure titanium (CP Ti) processed by Selective Laser Melting (SLM) in a regulated reactive atmosphere with a slight addition of oxygen (0.2–0.4 vol. %) to enhance the mechanical properties of the material. This work is one of the first extensive studies of the influence of the SLM process on the anisotropic material properties of printed Ti elements. Microstructure and mechanical properties were investigated both in the building platform plane (XY), as well as in the direction of the element’s growth (XZ). The tested sample, fabricated using a power density of only 75 J/mm3, had a density close to the theoretical density of titanium (98.7%) and 0.27-0.50 wt. % oxygen. Observations carried out by light and scanning electron microscopes revealed some micropores typical for laser melting processes. The total porosity was evaluated using X-ray computed microtomography (μ-CT), and was different in the XY and XZ directions. Additional STEM study allowed us to determine the lattice parameters of the dominant martensitic phase (α′). It was shown that the obtained material had a random crystallographic orientation with a texture factor close to 1, due to phase transformation during the manufacturing process. The average roughness Ra parameter was 10.36 μm and 9.11 μm for the top and side surfaces, respectively. The range of the tensile strength of the tested specimens was between 690–830 MPa in the XY plane, and 640–740 MPa in the XZ plane. The maximum elongation at break showed high anisotropy, and was in a range of 16-22% and 8-12% for the XY and XZ planes, respectively. The determined mechanical properties exceed those found in many conventionally obtained titanium alloys due to oxygen solution strengthening.

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... During solid solution strengthening, oxygen and nitrogen alloying elements form solid interstitial solutions within the base metal crystal lattice, leading to increased stress due to cellular lattice deformation and hindering dislocation movement [21][22][23]. Consequently, the material's ultimate tensile strength is enhanced, which was reported in our previous studies on the oxygen strengthening effect during PBF-LB processing of Ti64 [24] and CP Ti [25]. Carbon is less beneficial but is not harmful if carbide formation is avoided, while hydrogen is solely detrimental and should be limited as much as possible [26]. ...
... The voids in the PBF-LB fabricated samples were not uniformly distributed, and their number increased with the sample height. We have observed this phenomenon in many laser-melted powder materials and reported it in our previous study [25]. For samples machined from a reference rod, porosity was usually absent, and where individual voids were present, their location was completely random. ...
... Th voids in the PBF-LB fabricated samples were not uniformly distributed, and their numbe increased with the sample height. We have observed this phenomenon in many lase melted powder materials and reported it in our previous study [25]. For samples ma chined from a reference rod, porosity was usually absent, and where individual void were present, their location was completely random. ...
Article
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Additive manufacturing (AM) technologies have advanced from rapid prototyping to becoming viable manufacturing solutions, offering users both design flexibility and mechanical properties that meet ISO/ASTM standards. Powder bed fusion using a laser beam (PBF-LB), a popular additive manufacturing process (aka 3D printing), is used for the cost-effective production of high-quality products for the medical, aviation, and automotive industries. Despite the growing variety of metallic powder materials available for the PBF-LB process, there is still a need for new materials and procedures to optimize the processing parameters before implementing them into the production stage. In this study, we explored the use of a checkerboard scanning strategy to create samples of various sizes (ranging from 130 mm3 to 8000 mm3 using parameters developed for a small 125 mm3 piece). During the PBF-LB process, all samples were fabricated using Ti grade 2 and were in situ alloyed with a precisely controlled amount of oxygen (0.1–0.4% vol.) to enhance their mechanical properties using a solid solution strengthening mechanism. The samples were fabricated in three sets: I. Different sizes and orientations, II. Different scanning strategies, and III. Rods for high-cycle fatigue (HCF). For the tensile tests, micro samples were cut using WEDM, while for the HCF tests, samples were machined to eliminate the influence of surface roughness on their mechanical performance. The amount of oxygen in the fabricated samples was at least 50% higher than in raw Ti grade 2 powder. The O2-enriched Ti produced in the PBF-LB process exhibited a tensile strength ranging from 399 ± 25 MPa to 752 ± 14 MPa, with outcomes varying based on the size of the object and the laser scanning strategy employed. The fatigue strength of PBF-LB fabricated Ti was 386 MPa, whereas the reference Ti grade 2 rod samples exhibited a fatigue strength of 312 MPa. Our study revealed that PBF-LB parameters optimized for small samples could be adapted to fabricate larger samples using checkerboard (“island”) scanning strategies. However, some additional process parameter changes are needed to reduce porosity.
... Several studies investigated the development of CP-Ti material by the PBF technique considering different process parameters and building conditions [28,30,72,[89][90][91][92][93][94]. They found that two different microstructures can be developed based on the used process parameters, especially laser power and scanning speed. ...
... The formation of α phase (martensite) has a great effect on enhancing the mechanical properties of CP-Ti material printed using the PBF techniques. As reported by several studies [28,30,72,[89][90][91][92][93][94], the PBFed CP-Ti parts showed high mechanical performance even exceeding the values obtained by conventional manufacturing techniques. For [88] instance, Attar et al. [28] obtained CP-Ti samples by PBF technique showing high tensile strength reaching 1136 MPa higher than the values obtained by conventional manufacturing techniques [95]. ...
... Although the enhanced mechanical properties were achieved by using the PBF techniques, the ductility of CP-Ti parts is still a shortcoming. The reduction in ductility mainly comes from the formation of the α martensitic phases due to the rapid cooling rate during the PBF process (quenching effect) [90,93,98]. Zhang et al. [72] optimized the PBF process parameters to accomplish in situ grain refinement during the PBF process, thereby significantly enhancing the ductility ( Fig. 11a-f). ...
Article
In the realm of additive manufacturing, powder bed fusion (PBF) is recognized as an innovative and highly effective technique for manufacturing titanium-based materials. With an understanding of and regulation for the complex microstructural evolution that occurs during the PBF process, several previous studies have been conducted to enhance the reliability, efficiency, and performance of the PBF through investigation and optimization of microstructure evolution in PBF. These studies have examined various aspects, including feedstock materials, process parameters, and post-processing techniques, in order to gain a comprehensive understanding and control over the progression of microstructure in the PBF. Process parameters are widely acknowledged as critical determinants in the PBF process for titanium-based materials, significantly influencing the quality in 3D printed components. Previous studies have provided an in-depth discussion of the effects of process parameters, such as laser power, scanning speed, and hatching space, on the microstructure evolution of Ti-based materials. The primary objective of this review paper is, therefore, to provide a comprehensive and clear explanation of recent efforts, with a particular focus on investigating the complex evolution of microstructures in Ti-based materials during the PBF process. This thorough discussion is devoted to providing a comprehensive understanding of the effects of process parameters on the evolution of microstructures in Ti-based materials.
... To solve these tasks the selective laser melting (SLM) technology can be used, i.e. actively developing additive 3D-printing technology, which makes it possible to create precision products of complex shape [10][11][12]. Despite a large number of studies aimed at improving the mechanical characteristics of SLM-titanium [13][14][15][16][17][18][19][20], the task of a significant increase in the strength of unalloyed titanium was not yet solved and remains very relevant. ...
... In the first approximation, all dependences obtained can be explained by the influence of the volume fraction of pores arising under nonoptimal SLM modes, which correlates with the material density. To explain the presence of two low-density zones in different areas of the SLM parameters space (Fig. 4, a), it is necessary to introduce the concept of volumetric energy density (VED), which is widely used in papers [13,[18][19][20][21][24][25][26][27] in case of solving the problem of optimizing the parameters of SLM-titanium. The volumetric energy densityis the amount of energy released during SLM per unit of volume ...
... The Table contains the values of the volumetric and linear energy obtained in the papers [18,20,21,26,27]. These values are given only for SLM-samples of unalloyed titanium, which had high mechanical characteristics and high relative density of the material (more than 99%). ...
Article
Full-text available
Comprehensive studies of the physical and mechanical properties and structure of VT1-0 titanium samples processed by selective laser melting have been carried out. High strength characteristics (the ultimate tensile strength of 820 MPa, the yield strength of 710 MPa) have been achieved. These exceed by 2 times the values for this material produced using conventional technology. The formation of the martensitic α'-phase, obtained due to the high crystallization rates realized in selective laser melting process, is the reason for the increase in the mechanical characteristics of titanium VT1-0. Mechanical characteristics of titanium VT1-0 subjected to high-temperature annealing demonstrated a monotonous decrease in strength parameters by 15% and an increase in plastic characteristics by 30%. It is shown that the technology of selective laser melting makes it possible to solve the problem of improving the strength characteristics of unalloyed titanium to create a new class of medical devices. Keywords: unalloyed titanium, VT1-0, additive technology, selective laser melting, density, strength, plasticity, elastic modulus, microstructure, implants for surgery.
... LPBF and EBM are three dimensional (3D) AM processes which use a heat source to melt a pre-alloyed Ti-6Al-4V powder bed in a specified pattern, layer by layer, to obtain the final part [8,14,15]. Due to the high heating and cooling velocities in parts made by LPBF, the resulting microstructure contains acicular martensite (α') with a little portion of phase β [4,16]. On the other hand, the microstructure, as a result of the EBM process, consists of a mixture of α + β type Widmanstätten due to preheating stage during the EBM [17][18][19][20]. ...
... On the other hand, the microstructure, as a result of the EBM process, consists of a mixture of α + β type Widmanstätten due to preheating stage during the EBM [17][18][19][20]. Different types of defects reside in metallic alloys fabricated by additive manufacturing; [16] identified that the number of pores is minor in EBM compared to LPBF, due to the preheating stages used in EBM during the printing process [8]. ...
... Materials 2023,16, 1187 ...
Article
Full-text available
New manufacturing processes for metal parts such as additive manufacturing (AM) provide a technological development for the aeronautical and aerospace industries, since these AM processes are a means to reduce the weight of the parts, which generate cost savings. AM techniques such as Laser Powder Bed Fusions (LPBF) and Electron Beam Fusion (EBM), provided an improvement in mechanical properties, corrosion resistance, and thermal stability at temperatures below 400 °C, in comparison to conventional methods. This research aimed to study the oxidation kinetics of Ti-6Al-4V alloys by conventional and Electron Beam Additive Manufacturing. The thermogravimetric analysis was performed at temperatures of 600 °C, 800 °C, and 900 °C, having a heating rate of 25 °C/min and oxidation time of 24 h. The microstructural analysis was carried out by thermogravimetric analysis. Thickness and morphology of oxide layers were analyzed by field emission scanning electron microscope, phase identification (before and after the oxidation process) was realized by X-ray diffraction at room temperature and hardness measurements were made in cross section. Results indicated that the oxidation kinetics of Ti-6Al-4V alloys fabricated by EBM was similar to conventional processing and obeyed a parabolic or quasi-parabolic kinetics. The samples oxidized at 600 °C for 24 h presented the lowest hardness values (from 350 to 470 HV). At oxidation temperatures of 800 and 900 °C, however, highest hardness values (from 870 close to the alpha-case interface up to 300 HV in base metal) were found on the surface and gradually decreased towards the center of the base alloy. This may be explained by different microstructures presented in the manufacturing processes.
... Для решения этих задач может быть использована технология селективного лазерного сплавления (СЛС) -активно развивающаяся аддитивная тех-нология 3D-печати, позволяющая создавать прецизионные изделия сложной формы [10][11][12]. Несмотря на большое число исследований, направленных на повышение механических характеристик СЛС-титана [13][14][15][16][17][18][19][20], задача значительного повышения прочности нелегированного титана еще не решена и остается весьма актуальной. ...
... Для объяснения наличия двух зон с низкой плотностью в различных областях пространства параметров СЛС (рис. 4, а) необходимо ввести понятие объемной плотности энергии (ОПЭ), которое широко используется в работах [13,[18][19][20][21][24][25][26][27] при решении задачи оптимизации параметров СЛС-титана. Объемная плотность энергии -это величина энергии, выделившейся при СЛС в единице объема материала, обычно представляющаяся выражением ...
... При этом с точки зрения перспектив применения технологии СЛС для производства изделий медицинского назначения, оптимальными следует признать параметры с более высокими скоростями сканирования, так как это существенно сокращает время изготовления изделий. В таблице представлены значения объемной и линейной плотности энергии, полученные в работах [18,20,21,26,27]. Данные значения приведены только для СЛС-образцов нелегированного титана, которые имели высокие механические характеристики и высокую относительную плотность материала (более 99%). ...
Article
Comprehensive studies of the physical and mechanical properties and structure of VT1-0 titanium samples processed by selective laser melting have been carried out. High strength characteristics (the ultimate tensile strength of 820 MPa, the yield strength of 710 MPa) have been achieved. These values exceed by 2 times the values for this material produced using conventional technology. The formation of the martensitic αʹ-phase, obtained due to the high crystallization rates realized in selective laser melting process, is the reason for the increase in the mechanical characteristics of titanium VT1-0. Mechanical characteristics of titanium VT1-0 subjected to high-temperature annealing demonstrated a monotonous decrease in strength parameters by 15% and an increase in plastic characteristics by 30%. It is shown that the technology of selective laser melting makes it possible to solve the problem of improving the strength characteristics of unalloyed titanium to create a new class of medical devices.
... With high O content up to 0.4wt.%, AM bulk titanium modulates a combination of grain refinement and solid solution strengthening, resulting in a high tensile yield strength of ∼ 830 MPa and a large elongation of ∼ 22% [8]. ...
... exhibit high tensile yield strengths of 845-1023 MPa and large elongations of 15-22% (Figure 3(a)). Prominently, they surpass the corresponding ones (Figure 3(c)) of ASTM B381-13 commercial pure Ti and TC4 alloy [24], bulk Ti by AM of HDH [11,12,[15][16][17] and atomized [8,[25][26][27][28][29][30] Ti powders, and bulk TC4 alloy via AM of atomized powders [31][32][33], etc. Therefore, the AM bulk Ti with the intercepted titanium hydrides balances the strength and ductility in bulk Ti materials prepared by various processing methods. ...
Article
Full-text available
We report the interception of two titanium hydrides in bulk Ti via additive manufacturing (AM) of hydride-de-hydride (HDH) Ti powders with high H content: nano-sized, stable face-centered cubic (FCC) δ-TiHx plate within α-Ti grains and ultrafine, metastable face-centered tetragonal (FCT) γ-TiH aciculae crossing adjacent α-Ti grain boundaries. Remarkably, the AM bulk Ti achieves a surprising balance of strength and ductility, surpassing those of AM bulk Ti and TC4 alloys. This balance is attributed to effective work hardening from {111} twinning in the FCC δ-TiHx plate, a hindrance to dislocation motion, and high-density dislocation pile-ups around the FCT γ-TiH aciculae dowel connectors.
... In this last case, small voids with a spherical shape could be observed (Figure 5d-f). This defect morphology could be due to the entrapment of gaseous species in the melting pool during solidification [56,57]. This porosity could be ascribed to moisture and/or internal porosity present in titanium particles used for the additive manufacturing process. ...
... Even though the indentations were performed quite far from the defects visible on the surface, it is very likely that pores randomly distributed below this surface affected the hardness measurement and caused the scattering of the results. The measured microhardness values were consistent (only slightly higher) with those reported in the literature (247-250 ± 19 HV) for Cp Ti processed by L-PBF and not submitted to a post-processing thermal treatment [57]. As a matter of fact, the hardness of the samples processed by L-PBF was higher than that resulting from traditional processing methods owing to the finer microstructure achieved during the L-PBF process, driven by a steep thermal gradient and rapid thermal cycles. ...
Article
Full-text available
This paper focuses on optimizing the process parameters for manufacturing commercially pure titanium grade 2 using Laser Powder Bed Fusion (L-PBF) technology. The most common approach involves trial-and-error builds with varying parameter combinations, followed by characterizing the bulk samples for defects and the microstructure. This method, typically based on Volumetric Energy Density (VED), is time-consuming and overlooks key powder properties. An alternative approach involves the use of efficient Volumetric Energy Density (VEDeff), which represents the energy density effectively available for the L-PBF process, considering both the process parameters and powder properties such as absorptivity and thermal diffusivity. In this study, VEDeff was applied and compared to a work window defined by thermodynamic data, with limits corresponding to the energy needed for titanium melting and evaporation. Forty-two tests were performed with different combinations of laser powers and scanning speeds; the samples were then characterized in terms of porosity, microstructure, and hardness. The findings showed no correlation between VED and the work window while VEDeff aligned with the work window, although the highest relative densities (>99%) and hardness values were achieved in a narrower range. Despite this, the VEDeff approach proved to be a useful starting point for optimizing the process parameters.
... Due to the limitations of the processing stages and the possibility of near net shape production of metallic elements, AM technology is a perfect solution for difficult-to-manufacture materials, such as titanium and titanium alloys [3,4]. A laser beam powder bed fusion (PBF-LB) process has been widely used in recent years for the fabrication of elements made of a commercially pure (CP) Ti Grade 2 [5][6][7][8] or Ti-6Al-4V alloy [3,4]. It involves melting thin metallic powder layers by a laser beam to build 3D parts in a layer-wise manner [9]. ...
... It consisted of columnar prior-β grains with a width of 75 ± 10 µm, which grow along the building direction (Z axis) across many deposited layers as a consequence of directional solidification and thermal gradients during the PBF-LB process. Such columnar architecture is a typical microstructural feature for the additively manufactured materials [8,34,35]. Each columnar grain comprised an acicular martensitic α' microstructure, which is consistent with the other reports in the literature. ...
Article
Full-text available
The aim of this study was to show the effect of manufacturing defects in a commercially pure Ti Grade 2 produced by a laser beam powder bed fusion (PBF-LB) process on its high-cycle fatigue life. For this purpose, the high-cycle fatigue performance of PBF-LB Ti Grade 2 was compared to its ultrafine-grained (UFG) counterpart processed by hydrostatic extrusion exhibiting a similar mechanical properties under static tensile. The yield strength of the PBF-LB and UFG Ti Grade 2 was 740 and 783 MPa, respectively. The PBF-LB Ti Grade 2 consisted of a typical columnar of prior β grains with an acicular martensite α’ microstructure, while UFG Ti Grade 2 was mainly composed of fine, equiaxed α phase grains/subgrains with a size of 50–150 nm. A residual porosity of 0.21% was observed in the PBF-LB Ti Grade 2 by X-ray computed tomography, and, despite similar yield strength, a significantly higher endurance fatigue limit was noticed for UFG Ti Grade 2 (420 MPa) compared to PBF-LB Ti Grade 2 (330 MPa). Fatigue striation analysis showed that the fatigue crack propagation rate was not affected by manufacturing technology. In turn, the high-cycle fatigue life was drastically reduced as the size of manufacturing defects (such as pores or lack of fusion zones) increased.
... Another factor that drastically affects the mechanical strength of Ti alloys is the oxygen content. Controlling the addition of oxygen, which induces the solution-strengthening phenomenon in the manufacturing process, can improve the mechanical properties of additively manufactured parts [38][39][40]. In additional to differences in machinal properties, the surface roughness, Ra, of PBF-LB manufactured parts (typically 5-40 µm [41]) also differs compared to their machined counterparts (typically 0.4-6.3 ...
... A high surface roughness can induce stress concentration on the surface and adversely affect the fatigue properties [86]. In general, the surface roughness of additively manufactured Ti alloys is in the range of 10 to 70 µm (Ra), which is significantly larger than that achieved by machining [39,[87][88][89]. These differences can also influence the machinability of additively manufactured parts by affecting the cutting force, surface integrity, and tool wear. ...
Article
Full-text available
Titanium alloys are extensively used in various industries due to their excellent corrosion resistance and outstanding mechanical properties. However, titanium alloys are difficult to machine due to their low thermal conductivity and high chemical reactivity with tool materials. In recent years, there has been increasing interest in the use of titanium components produced by additive manufacturing (AM) for a range of high-value applications in aerospace, biomedical, and automotive industries. The machining of additively manufactured titanium alloys presents additional machining challenges as the alloys exhibit unique properties compared to their wrought counterparts, including increased anisotropy, strength, and hardness. The associated higher cutting forces, higher temperatures, accelerated tool wear, and decreased machinability lead to an expensive and unsustainable machining process. The challenges in machining additively manufactured titanium alloys are not comprehensively documented in the literature, and this paper aims to address this limitation. A review is presented on the machining characteristics of titanium alloys produced by different AM techniques, focusing on the effects of anisotropy, porosity, and post-processing treatment of additively manufactured Ti-6Al-4V, the most commonly used AM titanium alloy. The mechanisms resulting in different machining performance and quality are analysed, including the influence of a hybrid manufacturing approach combining AM with conventional methods. Based on the review of the latest developments, a future outlook for machining additively manufactured titanium alloys is presented.
... Nonetheless, it has been shown to be effective in evaluating mechanical properties such as relative density, fatigue life, and hardness of the product [26][27][28][29][30]. More importantly, due to the large crystal structure difference between the hightemperature stable phase and the low-temperature stable phase of titanium alloys [31], as well as the technical characteristics of the PBF technology, the microstructure of the finished product exists to influence mechanical properties [32]. However, the means of characterisation in this field currently require significant time to improve accuracy, making it difficult to propose new ideas. ...
Article
Full-text available
Metal products fabricated by additive manufacturing (AM), face challenges in mechanical property evaluation due to the complex thermal history of the process. This study used Ti-6Al-4V alloy, widely used in AM to examine the applicability of the Gibson-Ashby model for samples with porosities lower than 5%. Observations from electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) demonstrated that the Gibson-Ashby model's accuracy is limited due to the changes in the topology of pores and the phase proportions from different processing parameters. However, adaptive neuro fuzzy inference systems (ANFIS) reduced prediction errors on Young's modulus to 0.66 GPa and quantified the combined influence of microstructure variations. The proposed deviation factor addressed the model's neglect of microstructural changes. This laid the foundation for the establishment of a database to precisely control the mechanical properties of the products, thus promoting the optimisation of AM-produced titanium alloy for further practical applications.
... While analyzing the mechanical properties it is important to take the size effect into consideration. This matter has been explored extensively in various research papers [36,37]. The research emphasized that understanding the theoretical distinctions in the mechanical properties of normal and mini-samples requires considering a combination of internal and external factors. ...
Article
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The present research analyzes the impact of heat treatment atmosphere followed by finishing surface machining of small elements of Inconel 939 fabricated through laser powder bed fusion (LPBF). The analysis involved annealing in two gas mediums, solution treatment, and aging to achieve the desired microstructure and mechanical properties. The finishing surface was performed using various variants of abrasive machining. A more than fivefold reduction in the average roughness height Ra from 5.6 µm to 1.15 µm was achieved using metal balls as an abrasive, which was required for further processing. Residual stress tests have shown that due to heat and abrasive treatment, tensile stresses change into compressive ones. After printing, samples are characterized by tensile residual stresses on the surface (+ 428 MPa), while after heat treatment, compressive stresses occur (− 179 MPa). Abrasive machining with metal balls increases the value of compressive stresses to − 464 MPa. In addition, the impact of post-processing on the microstructure of Inconel 939 was discussed in terms of mechanical properties. The yield strength of 1184 MPa and elongation values of 19.3% were obtained for samples after HT in an argon atmosphere and abrasive machining with a ceramics roller. These studies provide valuable new information on the effective heat treatment and optimization of the finishing machining of Inconel 939, especially in achieving the desired surface roughness, microstructure, and mechanical properties for aerospace applications.
... Generally, four major factors are helpful to determine the performance of AM products: material properties [5], environment/process conditions [6], AM machine selection [7], and machine process parameters [8]. Prashanth K.G. [5], [9] summarized the aspects of materials that are extensively used in the SLM process for fabricating 3D parts, such as aluminum [10], iron [11], cobalt [12], copper [13], nickel [14], titanium [15], [16], and steel [11], [17]- [20] -based alloys, as well as their benefits and applications in various industries. Furthermore, major inadequacies such as brittle failure, process improvements, material-structure-property relationships, computational analysis, etc. were addressed. ...
Article
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Laser Powder Bed Fusion (LPBF) is a revolutionizing additive manufacturing (AM) that melts the powder particles to create innovative products, but it faces substantial challenges in monitoring and predicting the quality of the printed samples. Traditional density measurement methods, like the Archimedes technique, have been employed to determine the density of maraging steel samples produced under varying machine settings (MACH-S) process parameters, including laser power and scan speed. However, these machine-dependent parameters alone are insufficient for accurately predicting part density. To address this issue, an optical coaxial pyrometer in the AconityMINI system was utilized to capture thermal emissivity data, from which six statistical pyrometer-driven (PYRO-D) features, such as mean, median, and standard deviation, were extracted. The proposed feature-driven model then interprets machine settings and pyrometer data into physics-based (PHYS-B) features, including laser energy density, radiation pressure, intensity, temperature, and wavelength. With these three sets of input features, the study investigates the effectiveness of four supervised machine learning (ML) regression models: Random Forest (RF), K-Nearest Neighbor (KNN), Support Vector Regression (SVR), and Multi-Layer Perceptron (MLP), for predicting the density of printed samples. The performance of these models was enhanced through three hyperparameter optimization (HPO) techniques: Random Search (RS), Grid Search (GS), and Bayes Search (BS), alongside a feature selection (FS) method to refine the input feature dimensions. The findings indicate that RS is the most effective HPO technique across all models and highlight the significance of MACH-S and PHYS-B features in improving model predictions over PYRO-D features, achieving an R 2 score of 0.948 and a mean-squared error (MSE) of 0.007. Finally, the feature-driven model incorporating optimal machine, pyrometer, and physical features was utilized for density prediction through ML. Future research will aim to extend this approach to predict layer-wise part quality by incorporating a broader range of process parameters, focusing on understanding influencing factors such as porosity and surface roughness in AM printed parts.
... The fast cooling conditions associated to plasma-atomization and LPBF suggest however that martensitic '− is the most likely phase. These results are in a good agreement with the previously reported phase identifications by Wisocki et al. 2017 and Attar et al. 2017 [35,36]. In Figure 4, the MMCs patterns can be compared with those from Cp-Ti and TiC powders, used as references. ...
Preprint
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This study outlines a 3-step production process for Ti-TiC Metal Matrix Composite (MMC) employing powder Mechanical Blending, Laser Powder Bed Fusion (LPBF), and a heat treatment. A TiC fraction of more than 20 vol% was formed in-situ through the reaction of titanium with carbon during the LPBF process. The manufactured MMC pieces showed a record stiffness and fracture strain combination. The LPBF energy density impacted strongly porosity type. The as-built microstructure displayed an homogeneous distribution of sub-stoichiometric TiC dendrites which were fully converted into equiaxed TiC grains during the heat treatment. The TiC C/Ti ratio was found to be close to 0.5, resulting in an effective reinforcement content nearly twice the one expected for stoichiometric TiC, leading to stronger reinforcement. The mechanical analysis revealed a Young’s modulus of up to 149 GPa and an ultimate tensile strength of up to 770 MPa, indicating a 27% and 34% improvement, respectively, compared to commercially pure Titanium produced by LPBF and heat treated in the same conditions. A fracture strain of up to 2.8% was achieved. The combination of in-situ formation of defect-free TiC reinforcement and subsequent heat treatment enables to reach an exceptional ductility, considering the 20 vol% reinforcement content.
... XRD patterns of the as-built LPBF MMC samples and feedstock powders, with selected 2-θ angles between 33° and 44°, are shown in [34,35]. Both as-built LPBF TiC23 MMC and TiC45 MMC samples display α-Ti peaks as well as TiC peaks. ...
Preprint
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Titanium-based metal-matrix composites manufactured by additive manufacturing offer tremendous lightweighting opportunities. However, processing high reinforcement content remains challenging. This study reports an improved manufacturing process for Ti-TiC enabling high reinforcement content and significant fracture strain concurrently: mechanical blending, followed by laser powder bed fusion and a single heat treatment. As-built microstructure shows both un-melted TiC particles and sub-stoichiometric TiC dendrites resulting from a partial dissolution of TiC particles. The heat treatment is shown to fully convert TiC dendrites into equiaxed TiC grains. Reduction of the C/Ti ratio in TiC during the process results in an increase in the reinforcement content, from a nominal 12 vol% to an effective 21.5 vol%. Ti-TiC tensile samples reached fracture strains of up to 1.7%, Young’s moduli of up to 149 GPa and ultimate tensile strengths of up to 827 MPa. Lower TiC initial powder size distributions displayed the best mechanical performance.
... Using a lower laser power for the support structure helped make the support structure easier to remove by hand. The scaffolds were created in an inert argon atmosphere, and the oxygen level ranged between 0.2 and 0.4 vol% to enhance the material's strength through a solid solution strengthening mechanism [41,42]. The detailed parameters and architecture of support structures were described in our previous study [40]. ...
Article
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Researchers agree that the ideal scaffold for tissue engineering should possess a 3D and highly porous structure, biocompatibility to encourage cell/tissue growth, suitable surface chemistry for cell attachment and differentiation, and mechanical properties that match those of the surrounding tissues. However, there is no consensus on the optimal pore distribution. In this study, we investigated the effect of pore distribution on corrosion resistance and performance of human mesenchymal stem cells (hMSC) using titanium scaffolds fabricated by laser beam powder bed fusion (PBF-LB). We designed two scaffold architectures with the same porosities (i.e., 75 %) but different distribution of pores of three sizes (200, 500, and 700 μm). The pores were either grouped in three zones (graded, GRAD) or distributed randomly (random, RAND). Microfocus X-ray computed tomography revealed that the chemically polished scaffolds had the porosity of 69 ± 4 % (GRAD) and 71 ± 4 % (RAND), and that the GRAD architecture had the higher surface area (1580 ± 101 vs 991 ± 62 mm2) and the thinner struts (221 ± 37 vs 286 ± 14 μm). The electrochemical measurements demonstrated that the apparent corrosion rate of chemically polished GRAD scaffold decreased with the immersion time extension, while that for polished RAND was increased. The RAND architecture outperformed the GRAD one with respect to hMSC proliferation (over two times higher although the GRAD scaffolds had 85 % higher initial cell retention) and migration from a monolayer. Our findings demonstrate that the pore distribution affects the biological properties of the titanium scaffolds for bone tissue engineering.
... sample. It could be observed that an acicular martensitic microstructure was formed in the N 2 layer due to a higher cooling rate during the LPBF process [48]. In contrast, the LSTi sample showed a coarse block or lath α/α ′ -Ti microstructure in the LPBF forming Ti in Ar, as shown in figure 11(c). ...
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Highlights TiN was in-situ synthesized by laser powder bed fusion under different N 2 + Ar atmosphere. TiN/Ti gradient layered structure composites show high strength and ductility. Gradient layered structure Ti composite exhibits periodic changes in hardness of Ar and N 2 region. Hetero-deformation induced strengthening enhances performance of layered structure Ti composites.
... It can be seen from the gure that the spherical particles of 316L stainless steel powder were fewer, and that most of the particles were irregular in shape, the surface was angular, the particle size was uneven, whereby a few of the particles were relatively large while the ne particles agglomerated, which was a result of the particle surface energy. [26][27][28] In this experiment, three types of 316L stainless steel powders (AISI316L) were selected for SLM forming and were mainly investigated based on the following two points: (1) the inuence of different powder properties on the spheroidization of SLM forming; (2) for the same type of powder, the inuence of different forming processes on the spheroidization of the powder forms. The properties of the three powders selected for this experiment are shown in Table 2. ...
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Selective laser melting (SLM) additive manufacturing technology with different oxygen contents leads to the appearance of spherical solids of different sizes on the surface of the part, which affects the mechanical properties of the part, surface roughness, etc. In this study, the SLM molding technique was applied using three different 316L metal powders with different oxygen contents. The spheroidization properties and morphology of the samples were observed using a Quanta 200 environmental scanning electron microscope (ESEM), and the samples were observed microscopically and subjected to EDX spectroscopy using metallographic microscopy, and the mechanical properties were investigated. The results of the study showed that when using gas atomized powders, no spheroidization occurred when the oxygen content of the powders was 5.44 ± 0.01% in all cases, whereas using water atomized powders produced spherical structures with larger dimensions. This observation was closely related to the shape and particle size of the powder. When 316L metal powder with an oxygen content of 4.52 ± 0.01% was used for molding, small spherical structures appeared on the surface of the samples. When metal powder with an oxygen content of 5.44 ± 0.01% was used for the molding, larger spherical structures appeared on the surface of the samples. When the powder with an oxygen content of 5.90 ± 0.01% was used for the molding, more small spherical structures and some large spherical structures appeared on the surface of the samples. This suggests that higher oxygen levels may inhibit the occurrence of spheroidization. EDX spectroscopic analysis revealed that the white matter on the surface of the samples without spheroidization was mainly composed of Fe and Cr, whereas the white matter on the surface of the large-sized spherical structures was mainly composed of Si and Mn, which may be related to the oxygenophilicity of the various substances.
... Alloys with a lamellar microstructure are characterized by a high fracture toughness and lower strength and ductility, an equiaxed microstructure has a higher strength and ductility, and a lower fracture toughness. The grain size is also a factor, coarse grains are more ductile compared with fine grains(Kang & Yang, 2019;Wysocki et al., 2017). The differences could also be explained by the composition of the materials, the MI group was made of CP Ti grade 4 and the AM groups of Ti grade 23 (TiAl6V4 ELI). ...
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Objectives The objective of this in vitro study was to evaluate the shear bond strength between the ceramic veneer and additively manufactured titanium with different surface treatments, and to compare with milled titanium. Also, to characterize the surface and the presence of an α‐case layer of additively manufactured and milled titanium. Material and Methods Sixty additively manufactured titanium grade 23, and 20 milled titanium grade 4 cylindrical specimens were divided into four groups based on surface treatments, air‐particle abrasion and grinding. After ceramic veneering half of each group were thermocycled. The bond strength was analyzed using a shear bond strength test. The surfaces were analyzed using interferometry and scanning electron microscopy. Results The grinding procedure and air‐particle abrading pressure had no significant effect on the shear bond strength ( p = .264 and p = .344). Thermocycling showed a tendency towards an effect but not significant ( p = .052). The group with the highest air‐abrading pressure showed the highest surface roughness. No presence of an α‐case layer was detected in any of the groups. Conclusion Additively manufactured titanium grade 23 may be veneered with ceramics without prior grinding of the surfaces.
... Phase composition can also be affected by the printing processes of Ti alloys accompanied by optional precipitation of intermetallics [56]. At high cooling rates, depending on the class of a Ti alloy, a structure from a quasi-equilibrium Widmanstett structure consisting of α-phase plate packets [57] to a non-equilibrium fine-dispersed acicular martensite structure [58] with a high density of dislocations and twins [59] can be formed out of β-phase grains. The formation of such non-equilibrium structures leads to a significant increase in the strength and a loss in ductility of printed Ti alloys compared to those produced by traditional methods of metal forming [60,61]. ...
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We overview recent findings achieved in the field of model-driven development of additively manufactured porous materials for the development of a new generation of bioactive implants for orthopedic applications. Porous structures produced from biocompatible titanium alloys using selective laser melting can present a promising material to design scaffolds with regulated mechanical properties and with the capacity to be loaded with pharmaceutical products. Adjusting pore geometry, one could control elastic modulus and strength/fatigue properties of the engineered structures to be compatible with bone tissues, thus preventing the stress shield effect when replacing a diseased bone fragment. Adsorption of medicals by internal spaces would make it possible to emit the antibiotic and anti-tumor agents into surrounding tissues. The developed internal porosity and surface roughness can provide the desired vascularization and osteointegration. We critically analyze the recent advances in the field featuring model design approaches, virtual testing of the designed structures, capabilities of additive printing of porous structures, biomedical issues of the engineered scaffolds, and so on. Special attention is paid to highlighting the actual problems in the field and the ways of their solutions.
... The mechanical properties of the 3D printed material differ, depending on the orientation of the sample in relation to the building platform. An important feature of 3D printing is that the porosity of an element and can be controlled by parameters such as the atmosphere, additional heating, and the size of the powder used for printing [8]. According to the available literature, the DMLS method makes it possible to manufacture a wide range of materials, from biomaterials [9] to aerospace engine parts [10]. ...
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The development of powder metallurgy methods in recent years has caused traditional casting methods to be replaced in many industrial applications. Using such methods, it is possible to obtain parts having the required geometry after a process that saves both manufacturing costs and time. However, there are many material issues that decrease the functionality of these methods, including mechanical properties anisotropy and greater susceptibility to cracking due to chemical segregation. The main aim of the current article is to analyze these issues in depth for two powder metallurgy manufacturing processes: laser powder bed fusion (LPBF) and hot-pressing (HP) methods-selected for the experiment because they are in widespread use. Microstructure and mechanical tests were performed in the main manufacturing directions, X and Z. The results show that in both powder metallurgy methods, anisotropy was an issue, although it seems that the problem was more significant for the samples produced via LPBF SLM technique, which displayed only half the elongation in the building direction (18%) compared with the perpendicular direction (almost 38%). However, it should be noted that the fracture toughness of LPBF shows high values in the main directions, higher even than those of the HP and wrought samples. Additionally, the highest level of homogeneity even in comparison with wrought sample, was observed for the HP sintered samples with equiaxed grains with visible twin boundaries. The tensile properties, mainly strength and elongation, were the highest for HP material. Overall, from a practical standpoint, the results showed that HP sintering is the best method in terms of homogeneity based on microstructural and mechanical properties.
... In recent years, sintered titanium-based alloys have been widely used for aerospace and biomedical applications because of their appropriate properties such as low density, high strength, resilience, high toughness, low Young modulus, excellent corrosion resistance, and biocompatibility [1][2][3][4][5]. Therefore, researchers have developed alternative PM methodologies with the purpose to fabricate titanium alloys with a significant costaffordable reduction, as compared to existing technologies, by decreasing the consumption of CP titanium without degradation of their properties [6][7][8][9][10][11]. ...
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The structure changes, microstructure evolution, and mechanical properties during Powder Metallurgy (PM) through High Vacuum Sintering of a Ti-TiH2 matrix reinforced with Titanium Diboride (TiB2) particles were investigated. Composites were fabricated at 850, 1100, and 1300 °C. The strategy for the fabrication process was to use the PM route employing titanium hydride (TiH2) to reduce the consumption of Commercially Pure Titanium (CP-Ti). The structure of the composites was analyzed using X-Ray Diffraction (XRD), while Optical Microscopy (OM), and Field-Emission Scanning Electron Microscopy (FE-SEM) analysis were used to study the microstructure. Vickers microhardness and nanoindentation were performed to evaluate the elastoplastic and mechanical properties. According to the results, the unreinforced Ti-TiH2 sample presented higher sinter-ability, attaining relative density values of 93% with the higher sintering temperature. Composite samples showed TiB and TiB2 phases without the presence of any TiH2 residual phase. The highest mechanical properties were measured for reinforced samples with 30 vol.% of TiB2, sintered at 1300 °C, showing values of 509.29 HV and 4.94 GPa for microindentation Vickers and nanoindentation essays, respectively, which resulted in 8.5% higher than the values for the unreinforced sample. In addition, their H/Er and H3/Er2 ratios are higher than those of CP-Ti suggesting a better wear resistance of the Ti-TiH2 matrix-reinforced samples, combined with its mechanical properties makes it more suitable than CP-Ti for its potential in biomedical applications.
... On the other hand, the number of studies on the process parameter development for Cp-Ti with the L-PBF process is quite limited in the literature (see Table 1). Almost Full [11] Cp-Ti -50 25 333 80 75 98.7% [12] Cp-Ti (Grade 1) 50 50-250 30 ---Over 99% [8] Cp-Ti (Grade 1) -100 30 385 120 72 Almost Full [13] Cp-Ti (Grade 1) 120 120-440 30 1000 120 33-122 Almost Full [14] Cp-Ti (Grade 2) 80 165 100 138 100 120 99.5% [15] Cp-Ti (Grade 2) 70 90 50 100-400 -90 99.5% [16] Cp-Ti -210 30 1000 120 58 99.5% [17] Cp-Ti (Grade 1) 60-70 250-340 50 700-800 100-120 52-97 98.2% ...
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Laser powder bed fusion (L-PBF) process parameters can be changeable depending on the part geometry due to thermal conductivity differences. The number of studies on the process parameter development for commercial pure titanium (Cp-Ti) with the L-PBF process is also quite limited in the literature. The aim of this study is to present a comprehensive process development for the production of Cp-Ti bulk and thin structures with the L-PBF technology. In the first phase, the right process parameters, including scan speed, laser power, hatch distance, and layer thickness, were identified with prismatic specimens with thin walls so that the obtained parameters could be used for both bulky sections and thin features such as lattice structures. The process parameters were varied to change the volumetric energy density from 19 to 208 J/mm3 among 80 different parameter sets. Parameter sets having a Volumetric Energy Density (VED) value between 32 J/mm3 and 47 J/mm3 gave almost fully dense Cp-Ti parts while the laser power was set to 200–250 W and the scan speed was used as 1000–1400 mm/s. Finally, Vickers hardness and tensile tests were applied to highly dense Cp-Ti parts. This study involving investigating the effect of process parameters on a wide range demonstrated that L-PBF is a favorable manufacturing technology for Cp-Ti parts with almost full density and good mechanical properties as well as good dimensional accuracy even on thin geometries. Moreover, the results show that combining parameters into a single one, i.e., VED, is not a proper way to optimize the process parameters since increasing laser power or decreasing the scan speed may alter the results, although VED is increased in both manners.
Article
Purpose Additive manufacturing became the most popular method as it enables the production of light-weight and high-density parts in effective way. Selective laser melting (SLM) is preferred by means of producing a component with good surface quality and near-net shape even if it has complex form. Titanium alloys have been extensively used in engineering covering a variety of sectors such as aeronautical, chemical, automotive and defense industry with its unique material properties. Therefore, the purpose of this review is to study the tribological behavior and surface integrity that reflects the thermal and mechanical performances of the fabricated parts. Design/methodology/approach This paper is focused on the tribological and surface integrity aspects of SLM-produced titanium alloy components. It is aimed to outline the effect of SLM process parameters on tribology and surface integrity first. Then, thermal, thermal heat, thermomechanical and postprocessing surface treatments such as peening, surface modification and coatings are highlighted in the light of literature review. Findings This work studied the effects of particle characteristics (e.g. size, shape, distributions, flowability and morphology) on tribological performance according to an extensive literature survey. Originality/value This study addresses this blind spot in existing industrial-academic knowledge and goals to determine the impact of SLM process parameters, posttreatments (especially peening operations) and particle characteristics on the SLMed Ti-based alloys, which are increasingly used in biomedical applications as well as other many applications ranging from automobile, aero, aviation, maritime, etc. This review paper is created with the intention of providing deep investigation on the important material characteristics of titanium alloy-based components, which can be useful for the several engineering sectors.
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Over the past two decades, additive manufacturing (AM) has gained attention for its numerous benefits. Despite the growing interest in AM processes for as build Ti6Al4V alloys, there is a lack of comprehensive benchmark research on their impacts. This study addresses this gap by conducting a thorough standard comparison encompassing microstructure, cutting speed, chip analysis, surface roughness, and hardness. Additionally, a conventionally processed Ti6Al4V (grade 23) alloy is a reference for thorough comparison. The research outcomes contribute to a better understanding of AM techniques’ implications on building selective laser melted (SLM) Ti6Al4V alloys. Specifically, SLM Ti6Al4V grade 23’s investigated material, demonstrates lower cutting forces. This observation is attributed to inherent characteristics, including porosity and the presence of unmelted particles resulting from the SLM process. Particularly, remarkable is the substantially higher hardness observed in SLM Ti6Al4V compared to its conventionally processed counterpart. This insight not only underscores the distinctive attributes of the material but also provides valuable knowledge for optimizing the application of AM Ti6Al4V alloy.
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Laser powder bed fusion (LPBF) is an emerging technology in the field of manufacturing due to its promising features such as complexity of design, dimensional accuracy and minimum material wastage, etc. However, enhancement of its mechanical properties by optimizing the process parameters such as laser power, scan speed, and hatch spacing is a current trend of research. The present study firstly reveals the effect of laser power and scan speed for a fixed hatch spacing on the relative density and secondly, the role of hatch spacing on the relative density, surface roughness and mechanical properties of LPBF commercially pure Ti metal (CP-Ti). The samples were printed by varying the laser power in the range of 220 to 340 W, scan speed of 600 to 1500 mm/s, and hatch spacing of 0.1 to 0.22 mm. Results showed that the highest relative density of 99.4% was achieved for the sample built with a laser power of 220 W, scan speed of 900 mm/s, and hatch spacing of 0.13 mm. The microstructure of different hatch spacing LPBF CP-Ti revealed the formation of the martensite phase with change in its flake length due to rapid cooling. The average hardness of ~ 273.63 HV, tensile strength of ~ 771.2 MPa, elongation of ~ 15.49%, and impact toughness of ~ 67 J and surface roughness of ~ 9.4 µm was achieved for the LPBF CP-Ti samples processed with the optimised parameters.
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Adding appropriate amount of copper to titanium alloy can improve the wear resistance, biological corrosion resistance and antimicrobial properties of titanium-based human implants. In this work, the effect of laser powder bed fusion (LPBF) process parameters (volumetric energy density, VED) on the forming quality and tribo-corrosion of Ti–4wt%Cu (Ti–4Cu) alloy was systematically investigated. The results revealed that the relative density and untreated surface roughness of the Ti–4Cu alloy fabricated by LPBF can reach 99.9% and 11.5 μm, respectively. The microstructure of Ti–4Cu at different volumetric energy densities alloy was mainly composed of acicular α-Ti phase and a small amount of Ti2Cu. In addition, the tribo-corrosion results confirmed that friction and corrosion are mutually promoting processes. Under the impact of the corrosion medium, Ti–4Cu alloy formed a loose and rough oxide film, which could be easily removed in the sliding friction process, thus accelerating the wear process, and the constant exposure of fresh surface caused by wear also accelerated the corrosion process. The volume wear rates under pure friction and tribo-corrosion conditions of the Ti–4Cu samples fabricated at VED of 66.67 J/mm3 were 1.03 * 10−6 mm2/N and 1.25 * 10−6 mm2/N, respectively. This work highlights the importance of VED on the microstructure and tribo-corrosion properties of LPBF-fabricated Ti–4Cu alloys, which is of great significance for broadening the medical applications of Ti–Cu alloys.
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The paper concerns the numerical design of novel three-dimensional titanium scaffolds with complex openporous structures and desired mechanical properties for the Powder Bed Fusion using Laser Beam (PBF-LB). The 60 structures with a broad range of porosity (38–78%), strut diameters (0.70–1.15 mm), and coefficients of pore volume variation, CV(Vp), 0.35–5.35, were designed using the Laguerre-Voronoi tessellations (LVT). Their Young’s moduli and Poisson’s ratios were calculated using Finite Element Model (FEM) simulations. The experimental verification was performed on the representative designs additively manufactured (AM) from commercially pure titanium (CP Ti) which, after chemical polishing, were subjected to uniaxial compression tests. Scanning Electron Microscopy (SEM) observations and microtomography (μ-CT) confirmed the removal of the support structures and unmelted powder particles. PBF-LB structures after chemical polishing were in close agreement with the CAD models’ dimensions having 4–12% more volume. The computational and experimental results show that elastic properties were predicted in very close agreement for the low CV(Vp), and with even 30–40% discrepancies for CV(Vp) higher than 4.0, mainly due to PBF-LB scaffold architecture drawbacks rather than CAD inaccuracy. Our research demonstrates the possibility of designing the open-porous scaffolds with pore volume diversity and tuning their elastic properties for biomedical applications.
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Laser powder bed fusion (L-PBF) has attracted significant attention since its inception, providing unprecedented advantages to fabricate metallic components with complex geometry. The quality and performance of as-printed alloys is an intricate function consisting of numerous factors linking the feedstock powders, manufacturing, and post-treatment. As the starting materials, powders play a critical role in influencing the printing consistency, total fabrication cost, and mechanical properties. In consideration of its importance for L-PBF, the present review aims to review the recent progress on metallic powders for L-PBF focusing on powder characterization, powder fabrication, and powder reuse. The methods of powder characterization and fabrication were presented in the beginning by analyzing the principles and corresponding advantages and limitations. Subsequently, the effect of powder reuse on the powder characteristics and mechanical performance of L-PBF parts is analyzed focusing on steels, nickel-based superalloys, Ti and Ti alloys, and Al alloys. The evolution trend of powders and as-printed parts varies for different alloy systems based on the existing studies, which makes the proposal of a unified reuse protocol infeasible. Finally, perspectives are presented to cater to the increasing applications of AM technologies for future investigations. The present state-of-the-art work can pave the way for the broad industrial applications of L-PBF by enhancing printing consistency and reducing the total cost from the perspective of powders.
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Laser powder bed fusion (LPBF) of titanium or titanium alloys allows fabrication of geometrically more complex and, possibly, individualized implants or osteosynthesis products and could thus improve the outcome of medical treatments considerably. However, insufficient LPBF process parameters can result in substantial porosity, decreasing mechanical properties and requiring post-treatment. Furthermore, texturized parts with anisotropic properties are usually obtained after LPBF processing, limiting their usage in medical applications. The present study addresses both: first, a design of experiments is used in order to establish a set of optimized process parameters and a process window for LPBF printing of small commercially pure (CP) titanium parts with minimized volume porosity. Afterward, the first results on the development of a biocompatible titanium alloy designed for LPBF processing of medical implants with improved solidification and more isotropic properties are presented on the basis of conventionally melted alloys. This development was performed on the basis of Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au, a near-α alloy presented by the authors for medical applications and conventional manufacturing, with yttrium and boron additions as additional growth restriction solutes. In terms of LPBF processing of CP titanium grade 1 powder, a high relative density of approximately 99.9% was obtained in the as-printed state of the volume of a small cubical sample by using optimized laser power, scanning speed, and hatch distance in combination with a rotating scanning pattern. Moreover, tensile specimens processed with these volume settings and tested in the as-printed milled state exhibited a high average yield and ultimate tensile strength of approximately 663 and 747 N/mm², respectively, combined with a high average ductility of approximately 24%. X-ray diffraction results suggest anisotropic mechanical properties, which are, however, less pronounced in terms of the tested specimens. Regarding alloy development, the results show that yttrium additions lead to a considerable microstructure refinement but have to be limited due to the occurrence of a large amount of precipitations and a supposed higher propensity for the formation of long columnar prior β-grains. However, phase/texture and microstructure analyses indicate that Ti-0.44O-0.5Fe-0.08C-0.4Si-0.1Au-0.1B-0.1Y is a promising candidate to achieve lower anisotropy during LPBF processing, but further investigations on LPBF printing and Y2O3 formation are necessary.
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Titanium (Ti) and its alloy implants with porous structures manufactured by selective laser melting (SLM) can match the elastic modulus of human bone to reduce the stress‐shielding effect and satisfy the personalized requirement in orthopedic surgery. Compared with conventional casting and forging Ti and its alloy implants, SLM implants possess unique microstructural features and excellent comprehensive mechanical properties. However, the unmelted powder particles inevitably adhere to the surfaces of SLM implants, which may result in excessive surface roughness and potential health risks. Moreover, there are significant issues encountered, such as bioactivity, toxicity, antibacterial activity, corrosion, and wear resistance. Consequently, surface modification methods are essential to remove the unmelted powder particles and improve biological and mechanical properties of SLM implants. Herein, the research efforts focus exclusively on chemical (acid treatment, alkali treatment, sol–gel, chemical vapor deposition, and atomic layer deposition) and electrochemical methods (anodization and microarc oxidation) for SLM Ti and its alloy implants, especially for porous structures. Particularly, the characteristics of these methods are summarized, and their commonly used pre‐ and post‐treatment methods are introduced. In addition, the development trends and challenges in surface modification of SLM Ti and its alloy implants are discussed.
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While the role of boron (B) has been thoroughly clarified in titanium (Ti) castings, the microstructural changes triggered in additive manufacturing (AM) are still the subject of debate in the literature. Many contributions have confirmed the B-induced microstructural refinement in Ti-based AM parts. The formation of TiB in titanium matrix composites (TMCs) may increase strength. In some cases, B may also promote the columnar-to-equiaxed transition, thus mitigating the anisotropic effects associated with the strong epitaxial growth of unidirectional columnar grains typical of AM. However, as critically discussed in this review, some pitfalls remain. Due to fast cooling, the microstructural evolution in AM may deviate from equilibrium, leading to a shift of the Ti-B eutectic point and to the formation of out-of-equilibrium phases. Additionally, the growth of TiB may undermine the ductility and the crack propagation resistance of AM parts, which calls for appropriate remediation strategies.
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Titanium alloys are widely used in various technological fields due to their excellent performance. Since the early stages of the 3D printing concept, these alloys have been intensively used as materials for these processes. In this work, the evolution of the performance of the 3D printing process has been studied by analysing the microstructure and the mechanical properties, fatigue and tensile, of the Ti gr. 23 alloy produced by two different models of Concept Laser M2 Cusing machines (an old model and a more recent one). The process parameters recommended by the manufacturer were adopted for each machine. Both microstructural and surface texture characterisations were carried out to better correlate the differences with the production process technique. For the same purpose, tensile tests and microhardness profiles were obtained, while the dynamic mechanical properties were evaluated by means of fatigue tests aimed at determining the fatigue limit of the material using a staircase approach. The mechanical tests were carried out on specimens with three different orientations with respect to the building platform, using two different SLM techniques. The fatigue behaviour was then analysed by evaluating the fracture surfaces and, in particular, the crack nucleation sites. By comparing the calculated fatigue values with the results of local fatigue calculations, an estimate of the residual stresses near the crack nucleation site was obtained. The results showed that the specimens produced on a newer machine had lower roughness (about 10%), slightly higher ductility, and a higher fatigue limit (10–20 MPa) compared to the specimens produced with the same material but on older equipment.
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The distinct deposition method of selective laser melting (SLM) enables the near‐net‐shaping of high speed steel tools with complex geometries and integrated functions, such as adapted cooling channels, which offers economic advantages and has thus attracted considerable attention. However, SLM‐processed high speed steels are prone to cracking due to their high carbon and alloying element contents and high internal stresses. Varying the steel composition to stabilize austenite can help to reduce the residual stresses. This work investigates the effect of nitrogen atmosphere on the densification behavior, microstructural evolution and mechanical properties of high speed steel in the SLM process. Results demonstrate that nitrogen can dissolve in the steel. In combination with suitable SLM parameters, this leads to a fully austenitic microstructure with no cracks. After heat treatment, the microstructure of steel transforms into tempered martensite, along with the precipitation of tiny V‐rich M(C,N) carbonitride and V‐rich MC carbide. The hardness and bending strength of the tempered sample reach the highest of 61.3 HRC and 3659 MPa, respectively. Its abrasive resistance is also improved. This study provides an alloy design idea that is based on the reactive atmosphere of high speed steel and other materials for processing by SLM. This article is protected by copyright. All rights reserved.
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Nowadays, post-surgical or post-accidental bone loss can be substituted by custom-made scaffolds fabricated by additive manufacturing (AM) methods from metallic powders. However, the partially melted powder particles must be removed in a post-process chemical treatment. The aim of this study was to investigate the effect of the chemical polishing with various acid baths on novel scaffolds’ morphology, porosity and mechanical properties. In the first stage, Magics software (Materialise NV, Leuven, Belgium) was used to design a porous scaffolds with pore size equal to (A) 200 µm, (B) 500 µm and (C) 200 + 500 µm, and diamond cell structure. The scaffolds were fabricated from commercially pure titanium powder (CP Ti) using a SLM50 3D printing machine (Realizer GmbH, Borchen, Germany). The selective laser melting (SLM) process was optimized and the laser beam energy density in range of 91–151 J/mm3 was applied to receive 3D structures with fully dense struts. To remove not fully melted titanium particles the scaffolds were chemically polished using various HF and HF-HNO3 acid solutions. Based on scaffolds mass loss and scanning electron (SEM) observations, baths which provided most uniform surface cleaning were proposed for each porosity. The pore and strut size after chemical treatments was calculated based on the micro-computed tomography (µ-CT) and SEM images. The mechanical tests showed that the treated scaffolds had Young’s modulus close to that of compact bone. Additionally, the effect of pore size of chemically polished scaffolds on cell retention, proliferation and differentiation was studied using human mesenchymal stem cells. Small pores yielded higher cell retention within the scaffolds, which then affected their growth. This shows that in vitro cell performance can be controlled to certain extent by varying pore sizes.
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The solid solution strengthening of a-Ti was investigated in respect of dislocation nucleation and dissociation in all four active glide modes. A series of TiþX alloys (X ¼ Al, Sn, V, Zr and O) was selected to analyze the impact of solute valence structure (Al, Sn e p type elements, V, Zr e d type elements) and lattice site (interstitial O) on the mechanisms responsible for variation of mechanical properties. The computational procedure relied on the generalized stacking fault energy (GSFE) concept combined with the nudged elastic band method that enables full atomic relaxation and determination of the true, minimum energy GSFE path. Additionally, various concentrations of solutes and their distance to the glide plane were considered as well. Our study revealed a strong, nonlinear influence of X position on GSFE and migration of O atoms during the crystal slip. These new phenomena allowed one to determine three solution strengthening mechanisms: (I) hindrance of prismatic dislocation emission and reconfiguration of 1/3 <1120> screw dislocation cores (p type solutes), (II) hindrance of prismatic dislocation emission (V) and SFE reduction in other modes (both d type solutes) and (III) suppression of dislocation nucleation in all modes caused by O. We found that the stacking faults formed by the single partial dislocations have a thickness of few atomic layers and exhibit a highly non-uniform structure. Their ability to accommodate the lattice deformation introduced by solute elements greatly affects the stacking fault energies of the a-Ti alloys.
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Selective Laser Melting (SLM) is an additive manufacturing technology used to directly produce metallic parts from thin powder layers. To evaluate the anisotropic mechanical properties, tensile test specimens of the Ni-base alloy Hastelloy X were built with the loading direction oriented either parallel (z-specimens) or perpendicular to the build-up direction (xy- specimens). Specimens were investigated in the “as-built” condition and after high temperature heat treatment. Tensile tests at room temperature and at 850°C of “as-built” material have shown different mechanical properties for z- and xy-specimens. The anisotropy is reflected in the Young's modulus, with lower values measured parallel to the build-up direction. It is shown that the anisotropy is significantly reduced by a subsequent recrystallization heat treatment. The characterization of microstructural and textural anisotropy was done by Electron Back Scatter Diffraction (EBSD) analysis. Predictions of Young's modulus calculated from the measured textures compare well with the data from tensile tests.
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Titanium (Ti) and its alloys may be processed via advanced powder manufacturing routes such as additive layer manufacturing (or 3D printing) or metal injection moulding. This field is receiving increased attention from various manufacturing sectors including the medical devices sector. It is possible that advanced manufacturing techniques could replace the machining or casting of metal alloys in the manufacture of devices because of associated advantages that include design flexibility, reduced processing costs, reduced waste, and the opportunity to more easily manufacture complex or custom-shaped implants. The emerging advanced manufacturing approaches of metal injection moulding and additive layer manufacturing are receiving particular attention from the implant fabrication industry because they could overcome some of the difficulties associated with traditional implant fabrication techniques such as titanium casting. Using advanced manufacturing, it is also possible to produce more complex porous structures with improved mechanical performance, potentially matching the modulus of elasticity of local bone. While the economic and engineering potential of advanced manufacturing for the manufacture of musculo-skeletal implants is therefore clear, the impact on the biocompatibility of the materials has been less investigated. In this review, the capabilities of advanced powder manufacturing routes in producing components that are suitable for biomedical implant applications are assessed with emphasis placed on surface finishes and porous structures. Given that biocompatibility and host bone response are critical determinants of clinical performance, published studies of in vitro and in vivo research have been considered carefully. The review concludes with a future outlook on advanced Ti production for biomedical implants using powder metallurgy.
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Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of α/β titanium alloys. The relationship between the SLM process parameters and the microstructure of titanium alloys has been the object of several works, but the texture formation during the SLM process has yet to be understood. In the present study, the texture formation of Ti-6Al-4V components was investigated in order to clarify which microstructural features can be tailored during the SLM process. The microstructural characterization of the as-built components was carried out using various microscopy techniques. Phase and texture analysis were carried out using backscattered electron imaging and diffraction. It was found that as-built components consist exclusively of α′ martensitic phase precipitated from prior β columnar grains. The texture of the prior β phase was reconstructed and discussed in relation to the used SLM process parameters. It was found that the β grain solidification is influenced by the laser scan strategy and that the β phase has a strong 〈100〉 texture along its grain growth direction. The α′ martensitic laths that originate from the parent β grains precipitate according to the Burgers orientation relationship. It was observed that α′ laths clusters from the same β grain have a specific misorientation that minimizes the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms.
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This work presents a comprehensive study of the densification behavior, phase and microstructure development, hardness and wear performance of commercially pure Ti parts processed by selective laser melting (SLM). An in-depth relationship between SLM process, microstructures, properties, and metallurgical mechanisms has been established. A combination of a low scan speed and attendant high laser energy density resulted in the formation of microscopic balling phenomenon and interlayer thermal microcracks, caused by a low liquid viscosity, a long liquid lifetime, and resultant elevated thermal stress. In contrast, using a high scan speed produced the disorderly liquid solidification front and considerably large balling, due to an elevated instability of the liquid induced by Marangoni convection. A narrow, feasible process window was accordingly determined to eliminate process defects and result in full densification. The phase constitutions and microstructural characteristics of SLM-processed Ti parts experienced a successive change on increasing the applied scan speeds: relatively coarsened lath-shaped α → refined acicular-shaped martensitic α′ → further refined zigzag-structured martensitic α′, due to the elevated thermal and kinetic undercooling and attendant solidification rate. The optimally prepared fully dense Ti parts had a very high hardness of 3.89 GPa, a reduced coefficient of friction of 0.98 and wear rate of 8.43 × 10−4 mm3 N−1 m−1 in dry sliding wear tests. The formation of an adherent, plastically smeared tribolayer on the worn surface contributed to the enhancement of wear performance.
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The applications of Ti and its alloys are limited to high-performance products because of the expensive material cost and poor plastic formability. In order to develop a cost-effective processing route for pure Ti and its alloys, pure Ti powder was used as raw material and consolidated by different powder metallurgy routes in this study. Warm compaction and cold compaction were employed to consolidate Ti powder and spark plasma sintering (SPS) was used as a reference method. The obtained compacts were hot extruded subsequently. The microstructures and mechanical properties of the hot-extruded pure Ti were evaluated. It was found that the samples prepared by warm compaction showed a higher ultimate tensile strength of 973.6 MPa, a better elongation of 26% and a higher Vickers hardness of 389.8 Hv compared with the other two methods. Effects of grain orientation, grain refinement and solid solution strengthening on mechanical properties were discussed. It was found that the main strengthening mechanism for the sample prepared by warm compaction was oxygen solid solution strengthening resulting from in-process in this study. The strengthening effect of oxygen solid solution was calculated as 769.8 MPa/mass% [O].
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In this study, a selective laser melting experiment was carried out with Ti6Al4V alloy powders. To produce samples with maximum density, selective laser melting parameters of laser power, scanning speed, powder thickness, hatching space and scanning strategy were carefully selected. As a statistical design of experimental technique, the Taguchi method was used to optimize the selected parameters. The results were analyzed using analyses of variance (ANOVA) and the signal-to-noise (S/N) ratios by design-expert software for the optimal parameters, and a regression model was established. The regression equation revealed a linear relationship among the density, laser power, scanning speed, powder thickness and scanning strategy. From the experiments, sample with density higher than 95% was obtained. The microstructure of obtained sample was mainly composed of acicular martensite, α phase and β phase. The micro-hardness was 492 HV0.2.
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Titanium and its alloys are attractive materials due to their unique high strength-weight ratio that is maintained at elevated temperatures and their exceptional corrosion resistance. The major application of titanium has been in the aerospace industry. However, the focus shift of market trends from military to commercial and aerospace to industry has also been reported. On the Other hand, titanium and its alloys are notorious for their poor thermal properties and are classified as difficult-to-machine materials. These properties limit the use of these materials especially in the commercial markets where cost is much more of a factor than in aerospace. Machining is an important manufacturing process because it is almost always involved if precision is required and is the most cost effective process for small volume production. This paper reviews the machining of titanium and its alloys and proposes potential research issues.
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Purpose This paper seeks to investigate the possibility of producing medical or dental parts by selective laser melting (SLM). Rapid Manufacturing could be very suitable for these applications due to their complex geometry, low volume and strong individualization. Design/methodology/approach The SLM‐process has been optimized and fully characterized for two biocompatible metal alloys: Ti‐6Al‐4V and Co‐Cr‐Mo. Mechanical and chemical properties were tested and geometrical feasibility, including process accuracy and surface roughness, was discussed by benchmark studies. By developing a procedure to fabricate frameworks for complex dental prostheses, the potential of SLM as a medical manufacturing technique has been proved. Findings Optimized SLM parameters lead to part densities up to 99.98 percent for titanium. Strength and stiffness, corrosion behavior, and process accuracy fulfil requirements for medical or dental parts. Surface roughness analyses show some limitations of the SLM process. Dental frameworks can be produced efficiently and with high precision. Originality/value This study presents the state‐of‐the‐art in SLM of biocompatible metals by thoroughly testing material and part properties. It shows opportunities for using SLM for medical or dental applications.
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Commercially Pure Porous Titanium (CPPTi) can be used for surgical implants to avoid the stress shielding effect due to the mismatch between the mechanical properties of titanium and bone. Most researchers in this area deal with randomly distributed pores or simple architectures in titanium alloys. The control of porosity, pore size and distribution is necessary to obtain implants with mechanical properties close to those of bone and to ensure their osseointegration. The aim of the present work was therefore to develop and characterize such a specific porous structure. First of all, the properties of titanium made by Selective Laser Melting (SLM) were characterized through experimental testing on bulk specimens. An elementary pattern of the porous structure was then designed to mimic the orthotropic properties of the human bone following several mechanical and geometrical criteria. Finite Element Analysis (FEA) was used to optimize the pattern. A porosity of 53% and pore sizes in the range of 860 to 1500 μm were finally adopted. Tensile tests on porous samples were then carried out to validate the properties obtained numerically and identify the failure modes of the samples. Finally, FE elastoplastic analyses were performed on the porous samples in order to propose a failure criterion for the design of porous substitutes.
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The influence of porosity on the deformation and fracture behavior of two alloys, powder-fabricated Ti and Ti-6Al-4V, with differing levels of matrix strain hardening has been examined both experimentally and analytically. A large strain elastoplastic finite element model based on a regular array of equal-sized spherical voids is used to predict bulk porosity effects; the analysis is in good agreement with the experimentally observed rates of void growth but underestimates the degradation of strength with increasing porosity. The effects of porosity on a local scale, especially as regards fracture, are examined by a model of a porous continuum which contains imperfections whose magnitude depends upon the maximum porosity path within the continuum. At critical values of strain these imperfections cause localization of plastic flow. The predicted values for the strains at localization are in good agreement with measured fracture strains. The analysis thus explicitly recognizes that a primary effect of pores on fracture is to localize deformation into narrow regions of high porosity ("imperfections") which are present even in random distributions of pre-existing pores and which are the sites of macrofracture initiation.
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This paper presents in-time motion adjustment in laser cladding manufacturing process as a means to improve dimensional accuracy and surface finish of the built part. Defects occurring during laser cladding degrade the part quality such as dimensional accuracy and surface finish. In this paper, in-time motion adjustment strategy was presented to remedy and eliminate defects occurring during laser cladding to improve the dimensional accuracy and surface finish. Based on the relationship between the motion of laser head relative to the growing part and other parameters in effects on clad profile, the laser traverse speed, stand-off distance and laser approach orientation to the existing clad layer were adjusted by instructions from a close-loop control system in real time to remedy and eliminate defects. The results of the experiments verified the effects of in-time motion adjustment on dimensional accuracy and surface finish.
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Miniaturization of sheet metal working processes causes changes in the relative contribution of relevant process parameters. As a result, so-called second-order size effects occur. The present paper should give a survey of different kinds of second-order size effects that can be expected in tensile testing, air bending and punching of sheet metal. These size effects show to be commensurable when the process conditions are similar to a certain amount, as it is the case for tensile testing and air bending. However, they change or even diminish when the process conditions are very different, as for example in punching.
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The effects of various metallic ions using various metallic powders on the relative growth ratio of fibroblasts L929 and osteoblasts MC3T3-E1 cells were carried out. Ti, Zr, Sn, Nb and Ta had evidently no effect on the relative growth ratios of cells. Otherwise, Al and V ions exhibit cytotoxicity from a concentration of > or = 0.2 ppm. This Al effect on cells tend to be stronger in medium containing small quantity of V ions (< or = 0.03 ppm). The new Ti-15%Zr-4%Nb-4%Ta-0.2%Pd alloy exhibited a higher corrosion resistance in physiological saline solution. The addition of 0.02%O and 0.05%N to Ti-Zr alloy improved the mechanical properties at room temperature and corrosion fatigue strength. The relative growth ratios for the new Ti alloy plate and the alloy block extraction were unity. Further, the relative growth ratios were almost unity for the new Ti alloy against apatite ceramic pins up to 10(5) wear cycles in Eagle's MEM solution. However, there was a sharp decrease for Ti-6%Al-4%V ELI alloy from 3 x 10(4) wear cycles as V ion was released during wear into the wear test solution since the pH of the Eagle's MEM increases with increasing wear cycles.
  • X Yang
  • Richard Liu
Yang, X., Richard Liu, C., 1999. MACHINING TITANIUM AND ITS ALLOYS. Machining Science and Technology 3, 107-139.