1. INTRODUCTION
⌅The high strength/weight ratio, high corrosion resistance and biocompatibility of titanium (Ti) and Ti alloys provide an important role in the aerospace, biomedical, energy, and marine applications (Zadra et al., 2008Zadra, M., Casari, F., Girardini, L, Molinari, A. (2008). Microstructure and mechanical properties of cp-titanium produced by spark plasma sintering. Powder Metall. 51 (1), 59-65. https://doi.org/10.1179/174329008X277000.; Sidambe, 2014Sidambe, A.T. (2014). Biocompatibility of advanced manufactured titanium implants-A review. Materials 7 (12), 8168-8188. https://doi.org/10.3390/ma7128168.; Chen and Thouas, 2015Chen, Q., Thouas, G.A. (2015). Metallic implant biomaterials. Mater. Sci. Eng. R 87, 1-57. https://doi.org/10.1016/j.mser.2014.10.001.; Yadav et al., 2019Yadav, P., Bock, T., Fu, Z., Lorenz, H., Gotman, I., Greil, P., Travitzky, N. (2019). Novel Hybrid Printing of Porous TiC/Ti6Al4V Composites. Adv. Eng. Mater. 1900336, 4-11. https://doi.org/10.1002/adem.201900336.). The density and elastic modulus of Titanium (Ti) and Ti alloys, frequently used in biomedical applications, are different from the bone structure. The difference in elastic modulus results in weak osseointegration and stress concentrations at the attachment points of the implant due to the stress shielding effect (Goharian and Abdullah, 2017Goharian, A., Abdullah, M.R. (2017). Bioinert Metals (Stainless Steel, Titanium, Cobalt Chromium). In Trauma Plating Systems. Biomechanical, Material, Biological, and Clinical Aspects. Elsevier Inc., pp. 115-142. https://doi.org/10.1016/B978-0-12-804634-0.00007-0.; Domínguez-Trujillo et al., 2018Domínguez-Trujillo, C., Ternero, F., Rodríguez-Ortiz, J.A., Heise, S., Boccaccini, A.R., Lebrato, J., Torres, Y. (2018). Bioactive coatings on porous titanium for biomedical applications. Surf. Coat. Tech. 349, 584-592. https://doi.org/10.1016/j.surfcoat.2018.06.037.). Also, high hardness of implant causes bone resorption and shortening of the life of the implant (Frost, 1994Frost, H.M. (1994). Wolff’s Law and bone’s structural adaptations to mechanical usage: an overview for clinicians. Angle Orthod. 64 (3), 175-188. ). These problems can be minimized by producing porous implants with similar mechanical properties to bone tissue. Porous Ti implants with low density can be manufactured by conventional powder metallurgy method and additive manufacturing techniques (Wiria et al., 2010Wiria, F.E., Mian Shyan, J., Lim, P.N., Chung, F., Yeo, J.F., Cao, T. (2010). Printing of Titanium implant prototype. Mater Design 31(Suppl. 1), 101-105. https://doi.org/10.1016/j.matdes.2009.12.050.; Goia et al., 2013Goia, T.S., Violin, K.B., Bressiani, J.C., De Almeida, A.H. (2013). Titanium and Ti-13Nb-13Zr alloy porous implants obtained by space-holder technique with addition of albumin. Key Eng. Mater. 529-530, 574-579. https://doi.org/10.4028/www.scientific.net/KEM.529-530.574.; Tojal et al., 2013Tojal, C., Amigó, V., Calero, J.A. (2013). Fabricación y caracterización de aleaciones porosas de Ti y Ti6Al4V producidas mediante sinterización con espaciador. Rev. Metal. 49 (1), 20-30. https://doi.org/10.3989/revmetalm.1206.; Yang et al., 2018Yang, G., Xu, B., Lei, X., Wan, H., Yang, B., Liu, D., Wang, Z. (2018). Preparation of porous titanium by direct in-situ reduction of titanium sesquioxide. Vacuum 157, 453-457. https://doi.org/10.1016/j.vacuum.2018.09.021.; Yılmaz et al., 2018Yılmaz, E., Gökçe, A., Findik, F., Gulsoy, H.O., Iyibilgin, O. (2018). Mechanical properties and electrochemical behavior of porous Ti-Nb biomaterials. J. Mech. Behav. Biomed. Mater 87, 59-67. https://doi.org/10.1016/j.jmbbm.2018.07.018.; Castillo et al., 2019Castillo, S.M., Muñoz, S., Trueba, P., Díaz, E., Torres, Y. (2019). Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications. Metals 9 (12), 1249. https://doi.org/10.3390/met9121249.).
In conventional powder metallurgy method (PM), the end-user products are obtained by sintering the parts that shaped by applying pressure to the powder material filled into the mold. Although this method is fast and economical to produce a large number of small and uncomplicated parts, the cost and time of production are high for producing complex and low quantity of patient-specific implants. In this method, powder material properties, compression pressure, and sintering conditions directly affect the density and mechanical properties of the final products (Esteban et al., 2011Esteban, P.G., Bolzoni, L., Ruiz-Navas, E.M., Gordo, E. (2011). Introducción al procesado pulvimetalúrgico del titanio. Rev. Metal. 47 (2), 169-187. https://doi.org/10.3989/revmetalmadrid.0943.; Castillo et al., 2019Castillo, S.M., Muñoz, S., Trueba, P., Díaz, E., Torres, Y. (2019). Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications. Metals 9 (12), 1249. https://doi.org/10.3390/met9121249.; Cuesta et al., 2019Cuesta, I.I., Martínez-Pañeda, E., Díaz, A., Alegre, J.M. (2019). Cold isostatic pressing to improve the mechanical performance of additively manufactured metallic components. Materials 12 (15), 2495. https://doi.org/10.3390/ma12152495.).
Binder Jetting (BJ) is an additive manufacturing method invented in 1993 at the Massachusetts Institute of Technology (Stevens et al., 2018Stevens, E., Scholder, S., Bono, E., Schmidt, D., Chmielus, M. (2018). Density variation in binder jetting 3D-printed and sintered Ti-6Al-4V. Addit. Manuf. 22, 746-752. https://doi.org/10.1016/j.addma.2018.06.017.). In this method, any type of materials such as polymers, ceramics, metal, and composites can be combined with binders to manufacture parts without using a mold (Karageorgiou and Kaplan, 2005Karageorgiou, V., Kaplan, D. (2005). Porosity of 3D biomaterial scaffolds and osteogenesis. Biomaterials. 26(27), 5474-5491. https://doi.org/10.1016/j.biomaterials.2005.02.002.; Sheydaeian and Toyserkani, 2018Sheydaeian, E., Toyserkani, E. (2018). Additive manufacturing functionally graded titanium structures with selective closed cell layout and controlled morphology. Int. J. Adv. Manuf. Technol. 96 (9-12), 3459-3469. https://doi.org/10.1007/s00170-018-1815-2.; Tran et al., 2019Tran, T.Q., Chinnappan, A., Yoong Lee, J.K., Loc, N.H., Tran, L.T., Wang, G., Kumar, V., Jayathilaka, W., Ji, D., Doddami, M., Ramarkrishna, S. (2019). 3D printing of highly pure copper. Metals 9 (7), 12-20. https://doi.org/10.3390/met9070756.). The product can be fabricated directly from the 3D CAD design file with a 3D printer in the BJ method. Not using molds in manufacturing processes makes the fabrication of complex parts easier, more economical, and faster. The density, microstructure, and mechanical properties of the part produced by the 3D printer vary depending on the powder shape, powder size distribution, layer thickness, and the amount of binder used during combining (Mostafaei et al., 2016Mostafaei, A., Stevens. E.L., Hughes, E., Biery, Sh., Hilla, C., Chmielus, M. (2016). Powder bed binder jet printed alloy 625: Densification, microstructure and mechanical properties. Mater. Design 108, 126-135. https://doi.org/10.1016/j.matdes.2016.06.067.; Bai and Williams, 2018Bai, Y., Williams, C.B. (2018). The effect of inkjetted nanoparticles on metal part properties in binder jetting additive manufacturing. Nanotechnology 29 (39), 395706. https://doi.org/10.1088/1361-6528/aad0bb.; Miyanaji et al., 2019Miyanaji, H., Momenzadeh, N., Yang, L. (2019). Effect of powder characteristics on parts fabricated via binder jetting process. Rapid Prototyping Journal 25 (2), 332-342. https://doi.org/10.1108/RPJ-03-2018-0069.).
As it has been presented in this literature survey, many different methods were performed to produce porous CP-Ti samples. Binder jetting and uniaxial pressing have its own advantages and disadvantages. Comparing the microstructural and mechanical properties of CP-Ti parts produced by binder jetting and axial pressing allows us to know more about these two production methods. In this study, the microstructure and mechanical properties of porous Ti parts fabricated by conventional powder metallurgy and binder jetting methods are compared and the effects of the manufacturing method on the microstructure and the mechanical properties were investigated. For this purpose, Ti powders were shaped using these methods and then sintered at 1200 ºC in an atmosphere-controlled furnace.
2. MATERIALS AND METHOD
⌅2.1. Materials
⌅In this study, commercial -325 mesh titanium powders with 99.5% purity (Alfa Aesar, 42624) were used to manufacture Ti specimens. The size distribution of titanium powders was analyzed using Microtrac s3500 laser diffraction particle analysis system. The sizes of Ti powders were measured d10 = 7.11 µm, d50 = 17.75 µm and d90 = 32.35 µm. The powder size distribution as well as a SEM image of the Ti powders are shown in Fig. 1.
The flow characteristic of powder material can be measured with many methods such as static repose angle, Hausner ratio, mass flow through an orifice, and powder internal cohesion (Amidon, Meyer and Mudie, 2017Amidon, G.E., Meyer, P.J., Mudie, D.M. (2017). Particle, powder, and compact characterization. In Developing Solid Oral Dosage Forms: Pharmaceutical Theory and Practice. Chapter 10, Second Edition. Elsevier Inc., pp. 271-293. https://doi.org/10.1016/B978-0-12-802447-8.00010-8.; Ziaee and Crane, 2019Ziaee, M., Crane, N.B. (2019). Binder jetting: A review of process, materials, and methods. Additive Manufacturing 28, 781-801. https://doi.org/10.1016/j.addma.2019.05.031.). In this study, flow characteristics of Ti powders were determined as Hausner ratio (tap density over apparent density). Powder materials with a Hausner ratio of less than 1.4 have sufficient flow characteristics (Amidon, Meyer and Mudie, 2017Amidon, G.E., Meyer, P.J., Mudie, D.M. (2017). Particle, powder, and compact characterization. In Developing Solid Oral Dosage Forms: Pharmaceutical Theory and Practice. Chapter 10, Second Edition. Elsevier Inc., pp. 271-293. https://doi.org/10.1016/B978-0-12-802447-8.00010-8.). Table 1 shows that the Hausner ratio of Ti powders was 1.21 which means that Ti powders had acceptable flowability and were suitable for Binder Jetting.
| Bulk volumen (mm3) | Tapped volume (mm3) | Hausner ratio | d10/d50/d90 (µm) |
|---|---|---|---|
| 35 | 29 | 1.21 | 7.11/17.75/32.35 |
2.2. Manufacturing of test specimens
⌅Ti test specimens were shaped with a uniaxial pressing (MSE, uniaxial) and a customized Binder jetting 3D printer. Figure 2 shows a schematic of the Binder Jetting process setup. In the uniaxial press method, Ti powders were filled into a mold with a diameter of 10 mm and compacted with a pressure of 25, 100 and, 200 MPa to produce Ti specimens with varied density.
In the Binder Jetting method, cylindrical specimens of diameter 10 mm and height 10 mm were fabricated in a 3D printer with processing parameters as follows: binder saturation of 100%, a layer thickness of 100 µm, binder temperature of 45 ºC. HP liquid binder was used in the fabrication of test specimens. During Binder Jetting manufacturing process, the powder material was spread on the powder bed as a thin layer and the binder adhesive was deposited on the top of the powder layer where required. This process was repeated for the second and other layers until the fabrication was completed. After fabrication, the unbound powder materials surrounding the object in the pool were cleaned to obtain green parts.
2.3. Sintering
⌅The sintering process was carried out in a High Temperature Tube Furnace (Protherm). Schematic presentation of the sintering cycle for Ti is shown in Fig. 3. Before the sintering process, the binder and space holder were burnout at 250 ºC. Then the furnace was heated at a rate of 5 °C·min-1 to the sintering temperature and held isothermally for 2 h. The specimens fabricated by PM, and 3D printer were sintered at 1200 ºC under argon atmosphere.
The relative density of the sintered test specimens was measured by the Archimedes water immersion principle using a balance with 0.0001 g precision. The porosity ratio of the test specimens was calculated using Eq. (1) (P: Porosity ratio, ρ: Measured density of Ti, ρs: Calculated theoretical density of Ti).
Microstructural analyses of specimens were made after the standard metallographic procedure, which is a grinding sequence of 180, 400, 800, and 1200 grit silicon carbide paper and later polishing on a 1 mm alumina suspension. Microstructures and pore morphologies of Ti test specimens were analyzed using a scanning electron microscope (JEOL brand JSM 6060) and SEM-coupled Energy Distributor Spectrometry (EDS), respectively. SEM images were taken from polished Ti samples. EDS analyses were taken from the area for specimens produced with the PM and 3D printer. In addition, EDS analysis was performed for points 1 and 2 of the BJ-produced samples shown in Fig. 6d. The phase contents of sintered specimens were characterized by XRD (Rigaku D/MAX 2000 X-ray generator and diffractometer) with Cu Kα radiation (1.54059 A°) at a scanning rate of 2 °/min.
Compression and microhardness tests were performed to determine the mechanical properties of the specimens. The microhardness (represented by HV) values of the alloys were measured using a hardness testing apparatus (Wilson 402 MVS, USA) using a load of 50 g for 15 s. A total of 5 measurements were made to determine the hardness of specimens for each test specimens. Compressive tests were performed at room temperature with a ZwickRoell testing machine at 30 MPa/s loading rate. The compression strengths of the specimens were determined by an average of 3 specimen’s compressive tests.
3. RESULT AND DISCUSSION
⌅3.1. Density and porosity
⌅The relative densities of the sintered test specimens are shown in Table 2. During compacting, the pressure applied to the powder material results in the rearrangement of the particles, deformation, and the hardening of the material to form a solid model (Lemoisson and Froyen, 2005Lemoisson, F., Froyen, L. (2005). Understanding and improving powder metallurgical processes. In Fundamentals of Metallurgy. Woodhead Publishing Series, pp. 471-502. https://doi.org/10.1533/9781845690946.2.471.; Križan et al., 2016Križan, P., Matúš, M., Beniak, J. (2016). Relationship between compacting pressure and conditions in pressing chamber during biomass pressing. Acta Polytech. 56 (1), 33-40. https://doi.org/10.14311/APP.2016.56.0033.). In the compacting curve of ceramics and metals, there is a steep increase at first in density and then a slower increase followed by a plateau in density (Francis, 2016Francis, L.F. (2016). Powder Processes. In Materials Processing. A Unified Approach to Processing of Metals, Ceramics and Polymers. pp. 343-414. https://doi.org/10.1016/B978-0-12-385132-1.00005-7.). While the relative density of the sample shaped with 25 MPa compacting pressure is 94.32%, the relative density of specimens shaped with 200 MPa compacting pressure increases to 98.19%. A relative density value of 92.63% was obtained in specimens fabricated with a 3D printer without applying pressure. In this study, the density of samples pressed with low compacting pressure is closer to the samples fabricated by 3D printer. In uniaxial pressing, as the compression pressure increased from 25 MPa to 200 MPa, a continuous increase in densities of test samples was observed due to the inter-particle spaces decreased and the particles moved closer to each other.
| Manufacturing Method | Compaction Pressure (MPa) | Relative Density (%) |
|---|---|---|
| Binder Jetting | 0 | 92.63 ± 5.6 |
| Conventional Powder Metallurgy | 25 | 94.32 ± 2.03 |
| 100 | 96.86 ± 1.5 | |
| 200 | 98.19 ± 1.2 |
The SEM images of the Ti specimens fabricated using uniaxial press and 3D printer are shown in Fig. 4. It is observed that pores of the specimens produced with uniaxial press method have a spherical morphology and shrink with the increase of compression pressure. The compression pressure applied to the powder material enables the small irregular pores to combine and hence lead to a reduction of the large-sized pores (Amidon et al., 2017Amidon, G.E., Meyer, P.J., Mudie, D.M. (2017). Particle, powder, and compact characterization. In Developing Solid Oral Dosage Forms: Pharmaceutical Theory and Practice. Chapter 10, Second Edition. Elsevier Inc., pp. 271-293. https://doi.org/10.1016/B978-0-12-802447-8.00010-8.; Castillo et al., 2019Castillo, S.M., Muñoz, S., Trueba, P., Díaz, E., Torres, Y. (2019). Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications. Metals 9 (12), 1249. https://doi.org/10.3390/met9121249.). It is seen that the pores of specimens fabricated by 3D printer are non-uniform and exist in different sizes. In the Binder Jetting method, pores are non-uniform due to the lack of compression pressure, the droplet effect of the liquid binder and, the accumulation of binder between particles (Kunchala and Kappagantula, 2018Kunchala, P., Kappagantula, K. (2018). 3D printing high density ceramics using binder jetting with nanoparticle densifiers. Mater. Design 155, 443-450. https://doi.org/10.1016/j.matdes.2018.06.009.; Stevens et al., 2018Stevens, E., Scholder, S., Bono, E., Schmidt, D., Chmielus, M. (2018). Density variation in binder jetting 3D-printed and sintered Ti-6Al-4V. Addit. Manuf. 22, 746-752. https://doi.org/10.1016/j.addma.2018.06.017.; Castillo et al., 2019Castillo, S.M., Muñoz, S., Trueba, P., Díaz, E., Torres, Y. (2019). Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications. Metals 9 (12), 1249. https://doi.org/10.3390/met9121249.).
The XRD pattern of Ti- α phase and Ti specimens fabricated by uniaxial press (25 MPa) and BJ are shown in Fig. 5. The diffraction peaks corresponding to Ti-α phase were observed in all the specimens (Balbinotti et al., 2011Balbinotti, P., Gemelli, E., Buerger, G., Amin de Lima, S., De Jesus, J., Almeida, N.H., Rodrigues Enriques, V.A., Almeida Soares, G. (2011). Microstructure development on sintered Ti/HA biocomposites produced by powder metallurgy. Mat. Res. 14 (3), 384-393. https://doi.org/10.1590/S1516-14392011005000044.). EDS analysis graph of the Ti specimens were shown in Fig. 6. XRD and EDS analyzes showed that Ti-α phase is predominant which indicate that there is no a significant amount of impurities in titanium structure.
3.2. Microhardness
⌅Microhardness values of test specimens are shown in Fig. 7. It is clear that the shaping method directly affects the microhardness of the structure. The hardness of specimens fabricated with the conventional powder metallurgy increases as the compaction pressure increase. While the microhardness value measured in test specimens produced with 25 MPa compression pressure is 317±10 HV0.05, the microhardness value when the compaction pressure increases to 200 MPa is 347±15 HV0.05. This increase in microhardness indicates that the microhardness depends on the density and pore sizes (Gagg et al., 2013Gagg, G., Ghassemieh, E., Wiria, F.E. (2013). Effects of sintering temperature on morphology and mechanical characteristics of 3D printed porous titanium used as dental implant. Mater. Sci. Eng. C 33 (7), 3858-3864. https://doi.org/10.1016/j.msec.2013.05.021.; Zhao et al., 2019Zhao, D., Han, C., Li, Y., Li, J., Zhou, K., Wei, Q, Liu, J., Shi, Y. (2019). Improvement on mechanical properties and corrosion resistance of titanium-tantalum alloys in-situ fabricated via selective laser melting. J. Alloys Compd. 804, 288-298. https://doi.org/10.1016/j.jallcom.2019.06.307.). The hardness values of the specimens produced with 3D printing method without applying pressing were measured as 238±8 HV0.05. Although the relative density of the test samples produced with the Binder Jetting method and 25 MPa compression pressure is close to each other, the hardness values of BJ test samples are lower. Previous studies reported similar results that the compacting pressure lead to the increase in densification, hardness and compressive strength (Ryan et al., 2008Ryan, G.E., Pandit, A.S., Apatsidis, D.P. (2008). Porous titanium scaffolds fabricated using a rapid prototyping and powder metallurgy technique. Biomaterials 29 (27), 3625-3635. https://doi.org/10.1016/j.biomaterials.2008.05.032.; Castillo et al., 2019Castillo, S.M., Muñoz, S., Trueba, P., Díaz, E., Torres, Y. (2019). Influence of the Compaction Pressure and Sintering Temperature on the Mechanical Properties of Porous Titanium for Biomedical Applications. Metals 9 (12), 1249. https://doi.org/10.3390/met9121249.). The hardness of Ti samples, has 97% relative density, fabricated by spark plasma sintering were found as 341.71 HV (Mahundla et al., 2021Mahundla, M.R., Matizamhuka, W.R., Shongwe, M.B. (2021). The effect of densification on hardness of Ti, Ti-6Al-4V, Ti-34Nb-25Zr alloy produced by spark plasma sintering. Mater. Today 38 (Part 2), 605-608. https://doi.org/10.1016/j.matpr.2020.03.468.). In another study, Ti samples spark plasma sintered at 1200 ºC had hardness of 391 HV (Shahedi Asl et al., 2018Shahedi Asl, M., Namini, A.S., Motallebzadeh, A., Azadbeh, M. (2018). Effects of sintering temperature on microstructure and mechanical properties of spark plasma sintered titanium. Mater- Chem. Phys. 203, 266-273. https://doi.org/10.1016/j.matchemphys.2017.09.069.). The hardness of the Ti samples produced by the 3D printer method and with lower density was found to be 182.8 HV (Xion et al., 2012Xiong, Y., Qian, C., Sun, J. (2012). Fabrication of porous titanium implants by three-dimensional printing and sintering at different temperatures. Dent. Mater. J. 31 (5), 815-820. https://doi.org/10.4012/dmj.2012-065.). These results show that the production method and density directly affect the hardness of Ti samples.
3.3. Compressive strength
⌅The compressive (yield) strengths of Ti test specimens are shown in Fig. 8. In previous study the compressive strength of pure Ti produced with 3D binder jetting method was found between 167 to 455 MPa (Wiria et al., 2010Wiria, F.E., Mian Shyan, J., Lim, P.N., Chung, F., Yeo, J.F., Cao, T. (2010). Printing of Titanium implant prototype. Mater Design 31(Suppl. 1), 101-105. https://doi.org/10.1016/j.matdes.2009.12.050.). The compressive strength of pure Ti fabricated by selective laser melting (SLM) varies between 235±52 and 1136±15 MPa depending on the porosity ratio (Attar et al., 2015Attar, H., Löber, L., Funk, A., Calin, M., Zhang, L.C., Prashanth, K.G., Scudino, S., Zhang, Y., Eckert, J. (2015). Mechanical behavior of porous commercially pure Ti and Ti-TiB composite materials manufactured by selective laser melting. Mater. Sci. Eng. A 625, 350-356. https://doi.org/10.1016/j.msea.2014.12.036.). In conventional powder metallurgy method, when the compression pressure is increased from 25 MPa to 200 MPa, the compressive strength increases from 928±21 MPa to 1154±32 MPa. The increase in compaction pressure causes the decrease of the amount of porosity and pore interconnectivity, resulting in a part with higher compression strength (Križan et al., 2016Križan, P., Matúš, M., Beniak, J. (2016). Relationship between compacting pressure and conditions in pressing chamber during biomass pressing. Acta Polytech. 56 (1), 33-40. https://doi.org/10.14311/APP.2016.56.0033.; Mostafaei et al., 2017Mostafaei, A., Toman, J., Stevens, E.L., Hughes, E.T., Krimer, Y.L., Chmielus, M. (2017). Microstructural evolution and mechanical properties of differently heat-treated binder jet printed samples from gas- and water-atomized alloy 625 powders. Acta Mater. 124, 280-289. https://doi.org/10.1016/j.actamat.2016.11.021.; Yegyan Kumar et al., 2019Yegyan Kumar, A., Wang, J., Bai, Y., Huxtable, S.T., Willams, C.B. (2019). Impacts of process-induced porosity on material properties of copper made by binder jetting additive manufacturing. Mater. Design 182, 108001. https://doi.org/10.1016/j.matdes.2019.108001.). The compressive strength of the Ti pieces produced by the 3D printing method (342 ±14 MPa) resulted in lower values. The main factor in the mechanical properties of the porous materials is the total porosity (Veljović et al., 2011Veljović, Dj., Jančić-Hajneman, R., Balać, I., Jokić, B., Putić, S., Petrović, R., Janaćković, Dj. (2011). The effect of the shape and size of the pores on the mechanical properties of porous HAP-based bioceramics. Ceramics International 37 (2), 471-479. https://doi.org/10.1016/j.ceramint.2010.09.014.). However, in this study, the specimens with similar total porosity ratios are compared. The more spherical pores and strong spherical agglomerates in the samples produced by axial pressing resulted in better mechanical properties. The irregular pore structures of the specimens produced by BJ method cause lower strength (Križan et al., 2016Križan, P., Matúš, M., Beniak, J. (2016). Relationship between compacting pressure and conditions in pressing chamber during biomass pressing. Acta Polytech. 56 (1), 33-40. https://doi.org/10.14311/APP.2016.56.0033.; Yan et al., 2017Yan, L., Wu, J., Zhang, L., Liu, X., Zhou, K., Su, B. (2017). Pore structures and mechanical properties of porous titanium scaffolds by bidirectional freeze casting. Mater. Sci. Eng. C 75, 335-340. https://doi.org/10.1016/j.msec.2016.12.044.). It is seen that the pore morphology affects the mechanical properties. Although the parts produced by 3D printing have lower strength, they are still higher than the strength of the cortical bone (compressive strength 114-195 MPa) (Torres-Sanchez et al., 2017Torres-Sanchez, C., Al Mushref, F.R.A., Norrito, M., Yendall, K., Liu, Y., Conway, P. (2017). The effect of pore size and porosity on mechanical properties and biological response of porous titanium scaffolds. Mater. Sci. Eng. C 77, 219-228. https://doi.org/10.1016/j.msec.2017.03.249.; Yılmaz et al., 2018Yılmaz, E., Gökçe, A., Findik, F., Gulsoy, H.O., Iyibilgin, O. (2018). Mechanical properties and electrochemical behavior of porous Ti-Nb biomaterials. J. Mech. Behav. Biomed. Mater 87, 59-67. https://doi.org/10.1016/j.jmbbm.2018.07.018.). Also, with additive manufacturing techniques, more successful results might be obtained in patient-specific biomedical applications (Harun et al., 2018Harun, W.S.W., Manan, N.S., Kamariak, M.S.I., Sharif, S., Zulkifly, A.H., Ahmad, I., Miura, H. (2018). A review of powdered additive manufacturing techniques for Ti-6al-4v biomedical applications. Powder Technol. 331, 74-97. https://doi.org/10.1016/j.powtec.2018.03.010.). Compared to many conventional manufacturing methods, additive manufacturing methods provide significant advantages in high-tech applications, orthopedic and dental applications (Avila et al., 2018Avila, J.D., Bose, S., Bandyopadhyay, A. (2018). Additive manufacturing of titanium and titanium alloys for biomedical applications. In Titanium in Medical and Dental Applications. Elsevier Inc., pp. 325-343. https://doi.org/10.1016/B978-0-12-812456-7.00015-9.).
4. CONCLUSION
⌅Test specimens were fabricated and sintered by conventional powder metallurgy and 3D printing using commercial CP-Titanium powder. Better mechanical properties are obtained in specimens fabricated by compaction pressure. Although the mechanical strength of the specimens fabricated with a 3D printer are lower, they provide sufficient strength for biomedical applications.