Bio-Medical Materials and Engineering 24 (2014) 1861–1873 DOI 10.3233/BME-140996 IOS Press

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Particle morphology influence on mechanical and biocompatibility properties of injection molded Ti alloy powder H. Özkan Gülsoy a,∗ , Nagihan Gülsoy b and Rahmi Calı¸sıcı c a

Department of Metallurgy and Material Engineering, Technology Faculty, Marmara University, Göztepe-Istanbul, Turkey b Department of Biology, Faculty of Science and Letter, Marmara University, Göztepe-Istanbul, Turkey c Institute Graduate Studies Pure and Applied Sciences, Marmara University, Göztepe-Istanbul, Turkey Received 24 September 2013 Accepted 7 May 2014 Abstract. Titanium and Titanium alloys exhibits properties that are excellent for various bio-applications. Metal injection molding is a processing route that offers reduction in costs, with the added advantage of near net-shape components. Different physical properties of Titanium alloy powders, shaped and processed via injection molding can achieve high complexity of part geometry with mechanical and bioactivity properties, similar or superior to wrought material. This study describes that the effect of particle morphology on the microstructural, mechanical and biocompatibility properties of injection molded Ti–6Al– 4V (Ti64) alloy powder for biomaterials applications. Ti64 powders irregular and spherical in shape were injection molded with wax based binder. Binder debinding was performed in solvent and thermal method. After debinding the samples were sintered under high vacuum. Metallographic studies were determined to densification and the corresponding microstructural changes. Sintered samples were immersed in a simulated body fluid (SBF) with elemental concentrations that were comparable to those of human blood plasma for a total period of 15 days. Both materials were implanted in fibroblast culture for biocompatibility evaluations were carried out. The results show that spherical and irregular powder could be sintered to a maximum theoretical density. Maximum tensile strength was obtained for spherical shape powder sintered. The tensile strength of the irregular shape powder sintered at the same temperature was lower due to higher porosity. Finally, mechanical tests show that the irregular shape powder has lower mechanical properties than spherical shape powder. The sintered irregular Ti64 powder exhibited better biocompatibility than sintered spherical Ti64 powder. Results of study showed that sintered spherical and irregular Ti64 powders exhibited high mechanical properties and good biocompatibility properties. Keywords: Sintering, powder injection molding, titanium alloy, biocompatibility

1. Introduction The Powder Injection Molding (PIM) process consists of four steps; (1) preparation of feedstock by mixing alloy powder with different binders such as base polymeric and wax binder; (2) injection of feedstock into a mold to make an oversized preform; (3) thermal or solvent debinding to remove the * Address for correspondence: H. Özkan Gülsoy, Department of Metallurgy and Material Engineering, Technology Faculty, Marmara University, 34722, Göztepe-Istanbul, Turkey. Tel.: +90 216 3365770 322; Fax: +90 216 3378987; E-mail: [email protected].

0959-2989/14/$27.50 © 2014 – IOS Press and the authors. All rights reserved

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majority of the binder; and (4) sintering in a controlled atmosphere to density the metal powder. If it is necessary, secondary operations such as heat and surface treatments after sintering can be performed. The process overcomes the shape limitation of traditional powder compaction, the cost of machining, the productivity limits of isostatic pressing and slip casting, and the defect and tolerance limitations of conventional casting [1–3]. Titanium and titanium alloys have a low density, relatively high strength, and excellent corrosion resistance in many media and is known to be biocompatible. Ti64 alloys an excellent choice for applications such as watch parts, medical devices, dental parts and sports goods [2,4,5]. Many Ti64 components can be manufactured as cost-effectively by powder injection molding, a net-shape forming process with an advantage of shape complexity material utilization and high final density [6–10]. Spherical or irregular powder processed via injection molding can achieve high complexity of part geometry with mechanical and corrosion properties. Some studies have shown basic differences between spherical and irregular stainless steel powders, when mixed for powder injection molding [11–14]. Typically, spherical powders are packed to higher density, properties of key importance for injection molding applications [11,13]. However; irregular powders are economical and improve final shape retention owing to the shape characteristics that are generally less spherical and with a more textured surface. Higher densification for spherical powder compared with irregular steel powder attributed to low initial packing of irregular powders [13]. Spherical powders are packed to higher density. On the other hand, irregular powders are improving final shape retention during debinding and sintering as well as it is more economical [11,15,16]. However, an irregular powder comes with a penalty of lower solid loading and sintering density [11] with a corresponding degradation in the mechanical properties [11–13]. Several investigations have used spherical and irregular stainless steel powder to sintering behavior and mechanical properties of PIM process [11,13,17]. These studies showed that the spherical powders have high sintered density than irregular powders. Earlier investigations on PIM Ti64 focused on the effect of powder size, sintering temperature, sintering time, heat treatment, residual carbon content on microstructure, corresponding microstructural characterization, mechanical and biocompatibility [3,6, 7,18–21]. However, microstructural, mechanical properties and bio-activity of spherical and irregular shape Ti64 powder have not been explained and compared. To achieve desirable final material characteristics such as strength, ductility and biocompatibility, the microstructural changes during sintering are very important. The purpose of this study is to evaluate the microstructural evolution, mechanical properties, biocompatibility and compare the densification characteristics in the injection molded spherical and irregular in shape Ti64 powder with wax based binder. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. Tensile and hardness properties of the sintered products were evaluated different condition. Biocompatibility of the all samples in a SBF was investigated. Microstructures, powder morphology, sintered sample fracture surface, surface of after immersion SBF were analyzed under optical microscope (OM), scanning electron microscope (SEM) and Energy-Dispersive X-Ray Spectroscopy (EDS). 2. Experimental procedures 2.1. Test materials and fabrication method In this research, spherical and irregular in shape Ti64 powders were used. The spherical shape Ti64 powder (Ti–5.9Al–3.9V–0.19Fe–0.12O–0.01C–0.01N–0.004H) provided by SOLEA Corp. (France) and

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Fig. 1. Cumulative particle size distributions for the spherical and irregular powder. Table 1 Powder characteristics of Ti64 powders Item Vendor Shape Particle size (µm) D10 D50 D90

Spherical Ti64 SOLEA Corp. Irregular

Irregular Ti64 Phelly materials Spherical

10.32 24.61 45.61

15.92 31.17 56.13

irregular shape Ti64 powder (hydride–dehydride, commercial purity-CP) provided by Phelly Materials. Particle size distributions were determined on Malvern Mastersizer equipment and given in Fig. 1; indicate similar median particle sizes for two type powders. The physical characteristics of spherical and irregular Ti64 powders are given Table 1. Morphology of the powders, observed using scanning electron microscopy are given Fig. 2(a–b). A multiple component binder system consisting of paraffin wax, polypropylene, carnauba wax and stearic acid was used. The critical powder loading for injection molding was 62.5 and 50 vol.% for spherical and irregular powders, respectively. Feedstocks were injected using a 12.5 MPa specially made injection molding machine to produce tensile test specimens and biomaterials samples. Debinding was conducted in a two-step solvent/thermal operation. Green parts were debinded at 70◦ C for 8 h in heptane, followed by thermal debinding step at 600◦ C for 1 h and pre-sintered at 5◦ C/min to 900◦ C for 1 h in pure Argon. The samples were sintered in an atmosphere controlled vertical recrystallised alumina tube furnace at 1300◦ C for 10 h under high vacuum (10−3 Pa). 2.2. Physical and mechanical tests The densities of the sintered samples were measured by means of the Archimedes water-immersion method. All tensile tests were performed using Zwick 2010 mechanical tester. The hardness tests were

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Fig. 2. Particle morphologies of Ti64 powder (a) spherical and (b) irregular in shape.

performed using an Instron-Wolpert Dia Testor 7551 at HRB scale. The samples were cut from the tensile bars polished as standard metallographic procedures. A Kroll reagent (3 ml HF, 6 ml HNO3 in 100 ml H2 O) was used to etch the samples for optical metallography. The fracture surfaces were examined using a scanning electron microscope (Jeol-JSM 6335F). Five samples were tested under the same conditions to guarantee the reliability of the results. 2.3. In vitro biocompatibility tests The biomaterials samples with a size of ∅15 mm × 5 mm were prepared. The samples as prepared were polished (Ra = 0.800–0.830 µm) by rough sand paper and rinsed ultrasonically in acetone, absolute alcohol and deionized water in turn for five times. Finally, they were dried in an oven at 70◦ C for 24 h. The samples treated for investigation in SBF were obtained. The preparation process of SBF was as Table 2. The solution was prepared by dissolving reagent grade sodium chloride (NaCl), potassium chloride (KCl), calcium chloride dihydrate (CaCl2 · 2H2 O), magnesium chloride hexahydrate (MgCl2 · 6H2 O), sodium hydrogen carbonate (NaHCO3 ), dipotassium hydrogen phosphate trihydrate (K2 HPO4 ·3H2 O), sodium sulphate (Na2 SO4 ) in deionized water. Then the solution above was buffered to physiological pH 7.4 at 37.5◦ C by both hydrochloric acid (HCl) and tris (hydroxymethyl)-aminomethane ((CH2 OH)3 CNH2 ) [22]. The pH value of SBF as obtained was similar to that of human blood plasma as shown in Table 2. The ion concentration of the SBF is shown in Table 3. The sintered samples were immersed in sealed test tubes containing 20 ml of SBF for 2, 5, 10 and 15 days. The experiment was performed in a laboratory water bath that was maintained at a constant temperature of 37.5◦ C and subjected to a continuous vibrating motion to help maintain uniform ion concentration. After immersion in SBF for various periods, the sintered samples were retrieved, gently rinsed with distilled water, and dried at 45◦ C for 1 day. Three samples were used for each immersion periods. After drying the samples were sputter coated with Au. The surface of the samples

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Table 2 Receipt of the simulated body fluid Reagent NaCl NaHCO3 KCl K2 HPO4 · 3H2 O MgCl2 · 6H2 O 1 kmol/m3 HCl CaCl2 Na2 SO4 (CH2 OH)3 CNH2 1 kmol/m3 HCl

Vendor Assay min. 99.5% Assay (after drying) min. 99.5–100.3% Assay min. 99.5% Assay min. 99.0% Assay min. 98.0% 87.28 ml of 35.4% HCl is diluted to 1000 ml with volumetric flask Assay min. 95.0%. Use after drying at 120o C for more than 12 h Assay min. 99.0% Assay (after drying) min. 99.9% See above

Amount 7.996 g 0.350 g 0.224 g 0.228 g 0.305 g 40 cm3 0.278 g 0.071 g 6.057 g Appropriate amount for adjusting pH

Table 3 Ion concentrations of the SBF and human blood plasma (mmol/l) Sample Blood plasma SBF

Na+ 142.0 142.0

K+ 5.0 5.0

Ca2+ 2.5 2.5

Mg2+ 1.5 1.5

HCO3 − 27.0 4.2

Cl− 103.0 147.8

HPO4 2− 1.0 1.0

SO4 2− 0.5 0.5

was finally examined with secondary electrons (SE) mode by SEM (Jeol-6510LV) under a voltage of 20 kV. 2.4. Cell culture tests For the cell culture tests, the 3T3 rat fibroblast cell line was provided by American Type Culture Collection (Rockville, MD, USA) and was grown in a monolayer culture in Dulbecco’s Modified Eagle’s Medium-F12 (DMEM-F12; Biological Industries, Israel) supplemented with 10% heat-inactivated foetal calf serum (Sigma Chemical Co., St Louis, Missouri). Following trypan blue exclusion assay, 3T3 cells were plated in six-well culture plates containing 5 ml DMEM-F12 medium at a concentration of 1 × 105 cells/well. After attachment, two samples (spherical and irregular) were placed in the wells. Cells were harvested at 72 h, and the total cell number was determined by using an automated cell counter (Nucleocounter, Denmark). The apoptotic index was determined with a flow cytometric Annexin-Vfluorescein isothiocyanate/propidium iodide (Annexin-V-FITC/PI) staining kit (BD Pharmingen, San Diego, CA, USA). 3T3 cells were seeded on microslides for control group and seeded on the samples. They are fixed with 2.5% glutaraldehyde, post-fixed in 1% osmium tetraoxide, dehydrated in a graded acetone series and incubated in amyl acetate. The microslides and samples were then critical-point dried [23], sputter coated with gold-palladium and subjected to scanning electron microscopy (Jeol-6510LV). Statistical Packages for the Social Sciences (SPSS) 17.0 statistical software (SPSS, Inc., Chicago, IL, USA) was used for the statistical analysis. All of the results were analyzed using Student’s t-tests. The data are represented as the means ± standard error mean. The criterion for statistical significance was p < 0.05.

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3. Results and discussion 3.1. Microstructures and mechanical properties Figure 3 shows photographs of the sintered samples for tensile tests and SBF tests. There are no defects evident such as blistering or slumping during molding, debinding, or sintering stages. All samples prepared from Ti64 powder were sintered at 1300◦ C for 10 h. Theoretical densities of both spherical and irregular in shape Ti64 samples was increased with increasing sintering temperatures and times. As expected the higher green density spherical Ti64 samples have more density than lower green density irregular Ti64 samples in all sintering temperatures and times [11,13,18]. Irregular shape powders exhibit poor packing characteristics and low green density. Spherical shape powders exhibit good packing. Consequently, irregular shape samples give low theoretical density at sintering temperature. The results showed that spherical shape Ti64 powder could be sintered to a maximum 99.3% of theoretical density and irregular shape powder could be sintered to maximum 97.9% of theoretical density. Figure 4 shows the microstructural evolution of the spherical and irregular powders in the centre area of the samples. As microstructural, irregular shape particles give rise to low density values for all temperatures depending on sintering temperatures and times. When the sintering temperatures reached 1300◦ C and 10 h porosities, which present at the contact area of all particles, are closed by α and β phases [3,10,18]. However, most of the inner-particles porosities are still present for irregular shape Ti64 samples. With the increasing sintering temperature the amount porosity rapidly decreases for both types of samples. The amount of porosity is higher for irregular shape Ti64 particles than for spherical shape particles for all sintering temperatures and times. The morphologies of fracture surface of the sintered spherical and irregular Ti64 powder at 1300◦ C for 10 h is shown in Fig. 5(a–b). Fracture surface of irregular Ti64 samples exhibit dimpled features and porosities as shown in Fig. 5(b). The morphology of features surface of spherical Ti64 powder is shown in Fig. 5(a). This sample exhibit quasi cleavage fracture and low porosity. In added samples, changes the fracture mode from dimpled to intergranular mode, thereby resulting in lowering of ductility [3,6–9,13]. The mechanical properties of sintered Ti64 are determined by microstructures, which consist mainly of α phase, β phase and porosity. In general, mechanical properties increased with increase β phase fraction and decrease porosity [4,5,7,13]. The porosity and α/β fraction play an important role in the mechanical properties. Irregular powder samples have more closed-porosity in microstructure. With increasing sintering temperature, the porosity continuously decreases, resulting in mechanical properties

Fig. 3. Photographs of sintered Ti64 samples. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/ BME-140996.)

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Fig. 4. Microstructures in central area of spherical and irregular sintered samples at 1300◦ C for 10 h, (a) spherical and (b) irregular powder.

Fig. 5. Fractographs of sintered samples, (a) spherical and (b) irregular powder.

be improved. Tensile strength of irregular powder samples is low, because of more porosity. Table 4 gives the overall mechanical properties of samples produced from spherical and irregular Ti64 powders. 3.2. In vitro apatite analysis SEM morphologies of the sample surfaces after immersion in the SBF for various times are shown in Figs 6 and 7. It can be seen that after 2 days immersion, many globular apatite particles have grown

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H.Ö. Gülsoy et al. / Particle morphology influence on mechanical and biocompatibility properties Table 4 Mechanical properties of sintered spherical and irregular Ti64 powder Sample 3

Theoretical density, g/cm Ultimate tensile strength, MPa Elongation, % Hardness, HRC

Spherical

Irregular

99.3 711 7.12 38

97.9 692 6.03 36.1

Fig. 6. SEM morphologies of the surfaces of sintered spherical samples, (a) 2 days immersion, (b) 15 days immersion in SBF.

Fig. 7. SEM morphologies of the surfaces of sintered irregular samples, (a) 2 days immersion, (b) 15 days immersion in SBF.

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on the surface of the samples. The nucleation of the apatite prefers to occur in pores or scratch, where the size of the apatite particles is obviously much larger than that on the smooth part of the surface. With the increase of immersion time, the quantity and size of the apatite particles increase gradually. After 15 days immersion, the surface of the samples can be more covered with apatite [24–27]. In the immersed parts, the topological surfaces showed the formation of sufficiently large neck regions between the particles, and the interconnected pore structure was maintained. The occurrence of these phenomena was believed to be due to the enhanced bioactivity coupled with diffusion reactions between the part surfaces and SBF. After 2 days of immersion for all samples, intensive dissolution was seen as the surface was progressively being dissolved away, and the calcium phosphate was converted into fine needlelike structures. After 15 days of immersion for all samples, the precipitation process seemed to have been completed with the formation of large amounts of calcium phosphate crystals on the surfaces [27]. The EDS spectrum in Fig. 8 and element concentrations in Table 5 show that the dominating elements on the surfaces of sample are Na, Mg, P, Cl, K and Ca after immersion in SBF for 15 days. It indicates that with the increase of immersion time, Na, Mg, P, Cl, K and Ca concentration increases obviously. The content of Ti64 is scarcely observed on the surface and only Ca and P can be examined on the biomaterial surfaces after immersion for 15 days. It reveals that the apatite areas containing rich Ca and P on the biomaterials surfaces are formed. However, the apatite particles as formed on the biomaterial surfaces is different from the HA of biomaterials [22,25–28]. Accordingly, spherical sample and irregular sample

Fig. 8. EDS spectra of the surfaces of the (a) spherical and (b) irregular powder sample after 15 days immersion time. (Colors are visible in the online version of the article; http://dx.doi.org/10.3233/BME-140996.)

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H.Ö. Gülsoy et al. / Particle morphology influence on mechanical and biocompatibility properties Table 5 Element concentrations of sintered spherical and irregular powder in SBF at 15 days Elements (wt%) 2 days

Spherical powder Irregular powder

Na 3.41 5.12

Mg 0.1 0.1

P 1.23 2.36

Cl 14.02 18.54

15 days K 0.12 0.33

Ca 1.23 1.98

Na 9.53 13.8

Mg 0.88 0.94

P 3.20 4.50

Cl 17.94 24.79

K 0.35 0.65

Ca 3.90 6.06

biomaterials is those which possess benign biocompatibility and favorable biocompatibility and accord with the requirement of biocompatibility and biocompatibility for biomedical materials. 3.3. In vitro biocompatibility and cell culture Our cell culture data indicate that sintered spherical and irregular Ti64 powders are biocompatible. The results of the cytotoxicity tests are presented in Fig. 9 in graphs of cell number (X104 ). Figure 9(a) shows the number of cell represented at the end of 72 h (p < 0.001) in all experimental groups. When compared to cell proliferation in metal containing groups, the irregular Ti64 sample was much higher than spherical Ti64 samples. Both groups were significantly less (p < 0.001) than the results of control group. From the cell viability test (Fig. 9(b)) the group of fibroblast exposed to the pure culture medium (control group) group showed 98% cell viability. In the metal containing groups, cell viability was maintained at 87% and 90% for sintered spherical and irregular Ti64 powders, respectively. Compared to the control samples, all metallic groups show low vitality. Especially, spherical samples have lowest value of vitality. Apoptosis rate of fibroblasts were quantified using flowcytometry in Fig. 9(c). When compared to the control group and metal containing groups, sintered spherical and irregular Ti64 powders was much higher apoptosis rate than the control [28–30]. Because of that the control group has not toxic substance, the apoptosis rate was determined very low. Whereas the metallic materials exhibits toxicity for cell and they increases apoptosis ratio as seen our data [31]. Figure 10 shows the morphologies of the cells on samples after 72 h of culture. Cells were flattened and well spread across the sintered spherical and irregular Ti64 powders surfaces after 72 h, as shown in Fig. 10(a–b). On both of sample surfaces, cells were seen to adhere to surface (e.g. filopodia), and were connected to the substrate in addition to neighboring cells. Moreover, on the irregular Ti64 sample, as shown in Fig. 10(b), cells were observed to grow on surface. In contrast, on the spherical Ti64 sample surface cell growth was less pronounced, with limited establishment of cell spreading being seen after 72 h days of culture (Fig. 10(a)). On the irregular Ti64 sample cell numbers were increased, and the entire sample’s surface was covered with cells having numerous filopodia extensions attached to the surface. However, on the spherical Ti64 sample surface, relatively fewer cells were observed. Higher amount of porosity in the irregular Ti64 samples may lead to this situation [23,30]. 4. Conclusions In conclusion, the spherical shape Ti64 powder (62.5% solid loading) could be sintered to 99.3% of theoretical density while the irregular shape Ti64 powder (50% solid loading) powder could be sintered to 97.9% of theoretical density at 1300◦ C for 10 h under vacuum. Amount of porosity decrease both spherical and irregular shape Ti64 powders with increasing sintering temperature and times. When the

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Fig. 9. Effect of particle morphologies on amount of (a) cell number, (b) cell vitality and (c) cell apoptosis for Ti64 samples.

mechanical properties of spherical and irregular shape Ti64 samples compared, spherical shape samples have higher mechanical properties. Mechanical and SBF immersion tests and results show that the Ca-deficient apatite is bone-like apatite with excellent biocompatibility. In contrast, the spherical Ti64 samples not presented better cell viability. The PIM-Ti64 (spherical or irregular powders) materials are high strength and biocompatibility materials and possess promising applications as biomedical materials.

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Fig. 10. SEM micrographs of cell morphology after 72 h of culture on (a) spherical and (b) irregular sample surfaces.

Acknowledgements This work was supported by the Scientific Research Project Program of Marmara University (Project No: FEN-A-130213-0043). The authors wish to express their thanks to Prof. Dr. Ayhan Bilir, Prof. Dr. Esin Aktas and Asist. Prof. Dr. Mine Ergüven for performing cell culture tests in this work.

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Particle morphology influence on mechanical and biocompatibility properties of injection molded Ti alloy powder.

Titanium and Titanium alloys exhibits properties that are excellent for various bio-applications. Metal injection molding is a processing route that o...
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