Research ArticleClinical Studies
Open Access

Microstructural, Electrochemical, and Mechanical Assessment of Additive Manufactured Titanium Grade 23 for Dental Implants Application

MIRKO TESCHKE, LORENZ GRAFE, SEBASTIAN STAMMKÖTTER, JOCHEN TENKAMP and FRANK WALTHER
In Vivo May 2025, 39 (3) 1751-1766; DOI: https://doi.org/10.21873/invivo.13978

Abstract

Background/Aim: Due to its excellent mechanical properties and biocompatibility, the titanium grade 23 alloy is the material of choice for dental implants. Additive manufacturing enables patient-specific manufacturing and the reduction of stress shielding by using lattice structures instead of solid material. For the simulation and design of such structures, a comprehensive knowledge of the mechanical properties under quasi-static and cyclic loading, the microstructure, and the electrochemical properties is required. In addition, suitable heat treatments must be selected and validated. These properties were determined uniformly and will provide a complete database and benchmark for future applications.

Materials and Methods: The mechanical behavior of the laser powder bed fusion (PBF-LB/M) manufactured alloy Ti6Al4V in the as-built and heat-treated state was characterized in tensile and constant amplitude tests, as well as hardness and microstructure analysis. To characterize the electrochemical properties, electrochemical impedance spectroscopy and potentiodynamic polarization measurements were performed.

Results: For use in medical implants, both conditions fulfilled the mechanical required specification in DIN EN ISO 5832-3, but heat treatment also reduced the high residual stresses caused by the manufacturing process. In the high cycle fatigue range, no significant difference was found between the two material states. The fatigue strength was increased compared to the literature. In electrochemical corrosion investigations, no remarkable differences between the two material states were detected by electrochemical impedance spectroscopy or potentiodynamic polarization measurements, but the high corrosion resistance of PBF-LB/M Ti6Al4V was demonstrated.

Conclusion: The study provides a wide electrochemical and mechanical database for the design of the PBF-LB/M manufactured alloy Titanium grade 21 as an implant material.

Keywords:

Introduction

Due to their high specific strength (1, 2) and excellent biocompatibility (3, 4), titanium alloys are an important material for implant applications, especially dental implants (5, 6). Titanium and titanium alloys, in particular Ti6Al4V, are the medical and technical standard and are the most frequently implanted alloy (5, 7).

There are two grades available of the Ti6Al4V alloy, Grade 5, and Grade 23. Titanium Grade 23 (Ti6Al4V ELI, TAV-ELI, or ASTM B348) has lower contents of oxygen, nitrogen, carbon and iron, which results in better ductility and better fracture toughness compared to normal Ti6Al4V (Grade 5) (8). This is the reason why titanium Grade 23 is often preferred for medical applications (9). Due to the discrepancy between the stiffness of titanium (~110 GPa) and that of human bone (cortical bone: 10-30 GPa), the effect of “stress shielding” can occur (10, 11). This effect can be observed in different types of implants and needs to be investigated and improved.

Additive manufacturing enables the detailed manufacturing of complex geometries that could not be produced using other manufacturing processes. For medical implants, laser powder bed fusion (PBF-LB/M), in which the component is produced layer by layer with a high degree of precision, is particularly important. With the help of PBF-LB/M, special characteristics and microstructure can be reached (11). Especially for patient-specific implants, the process of PBF-LB/M has significant positive effects on the implant but also causes high residual stresses due to the high cooling rates. Due to the excellent near-net shape geometries and precise layer-wise fabrication during PBF-LB/M, fine lattice structures with selected settings and properties can be manufactured. Unique types of lattice structures can be specially used for reducing the stiffness of structures to prevent the effect of stress shielding. Furthermore, the combination of manufacturing process and microstructure is essential for the design and properties of implants related to standard DIN EN ISO 5832-3. To fulfill the requirements of the microstructure given by the standard, additional heat-treatment is necessary as well as for reduction of residual stresses (12). Additional studies (13, 14) showed the effect of different manufacturing processes and heat treatments on the corrosion and mechanical properties of Ti6Al4V.

For the safe design and simulation as well as mechanical and chemical design of PBF-LB/M manufactured medical implants, it is essential to first know the mechanical and electrochemical properties of the solid material. The following investigations are intended to provide a database for future use. Therefore, this study will focus on the complete characterization of the PBF-LB/M manufactured alloy titanium Grade 23. In addition to the as-built condition, the residual stress-free heat-treated condition according to DIN EN ISO 5832-3 will be investigated. Initially, mechanical parameters in the tensile and compressive direction as well as the fatigue behavior in the LCF and HCF range are investigated. The corrosion behavior is then investigated electrochemically in a simulated body fluid. Finally, a comprehensive metallographic analysis of the microstructure and the damage mechanisms is carried out.

Materials and Methods

Materials. The specimens were additively manufactured by laser powder bed fusion (PBF-LB/M) at Laser Zentrum Hannover e.V. (LZH) (Hannover, Germany). The process parameters can be found in Table I. Contour and infill were scanned with different variables. A bidirectional scanning strategy with a rotation of 67° between adjacent layers was used. A post-process heat treatment was conducted at the Surface Engineering Institute (IOT) at RWTH Aachen University. During this heat treatment, the specimens were annealed in a vacuum furnace at 1,050°C for 4 h, followed by furnace cooling. The target microstructure is α-β-microstructure as specified by DIN EN ISO 5832-3 for medical titanium implants. The heat treatment was also intended to reduce residual stresses. For the tensile and fatigue test, the specimen cylinders were manufactured and then machined to the geometry shown in Figure 1. Cylindrical specimens with a diameter of d=5 mm and a height of h=8 mm for the compression tests were directly manufactured and tested without further machining them.

View this table:
Table I.

Process parameters for laser powder bed fusion (PBF-LB/M) process.

Figure 1.
Figure 1.

Specimen geometry for (A) tensile and (B) fatigue tests.

Microstructural analysis and hardness testing. In microstructural analyses, the as-build state without heat treatment (AB) and with heat treatment (HT) were characterized by means of metallographic cross-sections under an optical microscope Zeiss Axio Imager M1 (Zeiss, Oberkochen, Germany). After grinding and polishing, the specimens were etched with potassium hydroxide solution to make the grain boundaries and lamellae visible. The optical images were used to determine the α-lamella thickness and the beta grain size. Colonies of α-lamellae were measured manually at different locations using image processing software. For statistical reliability, a total of 30 α-lamellae per specimen were measured in different areas and then analyzed. The determination of the β-grain size was carried out in accordance with the DIN EN ISO 643, based on the line intersection method.

The macro- and micro-Vickers hardness of the two material states (AB and HT) were determined by hardness measurements on Wolpert Dia-Testor 2 Rc (Instron, Norwood, MA, USA) with a static load of 98.07 N (HV10) and microhardness measurements on HMV-G20S (Shimadzu, Kyoto, Japan) with a static load of 4.903 N (HV0.5). For each condition, 30 hardness measurements were performed according to DIN EN ISO 6507-1 and the mean values as well as the standard deviations were determined.

Quasistatic tensile and compression tests. The tensile and compression tests were performed on the servohydraulic testing system Schenck PC63M with an Instron controller 8800 (Instron UK, High Wycombe, UK) and±63 kN load cell. Both material conditions were tested at room temperature. The test setups for the quasistatic investigations are shown in Figure 2.

Figure 2.
Figure 2.

Test setup for quasistatic characterization. (A) Tensile test and (B) compression test. DIC: Digital image correlation.

Tensile tests were carried out in accordance with the DIN EN ISO 6892-1 standard. The strain was recorded by using an Instron extensometer (Instron UK) with an initial length of l0=12.5 mm as well as the digital image correlation (DIC) System Limess Q400 (Limess, Krefeld, Germany) for tensile tests. The strain rate was 0.00025 s−1. The ultimate tensile strength (UTS), Young’s modulus (YM), yield strength (Rp0.2), and fracture strain (FS) were determined from the stress-strain curves.

The compression tests were performed according to DIN EN ISO 50106. To minimize the friction between the specimens and the pressure plates, the specimen’s contact surfaces were polished and lubricated before installation in the testing machine. The pressure plates were made of Inconel 718 and tungsten carbide to enable strain measurements without deformation of the pressure plates. The tests were carried out stroke-controlled, depending on the specified strain rate of ε·=0.00025 s−1 and the individual specimen height. The strain was recorded using an Instron extensometer attached to the pressure plates. Due to the high Young’s modulus of the tungsten pressure plates, the deformation was negligible, and the measured strains were attributed to the tested specimen. The ultimate compression strength (UCS), Young’s modulus (YM), yield strength (Rp0.2), and the fracture strain (FS) were determined from the stress-strain curves.

Fatigue tests. Fatigue tests were conducted using an Instron 8872 servo-hydraulic fatigue testing machine with ±25 kN load cell (Instron UK). To characterize the fatigue behavior, the low cycle fatigue (LCF) and the high cycle fatigue regime (HCF) were investigated. The setup for the fatigue tests is illustrated in Figure 3. For the measuring of the strain, an Instron 2620-601 extensometer was used (Instron UK). To exclude self-heating of the specimen during the test, the specimen temperature was monitored with thermocouples.

Figure 3.
Figure 3.

Test setup for fatigue characterization.

For constant amplitude tests (CAT), stress-controlled tests with a sinusoidal load-time function were performed at the stress ratio of R=−1 (fully-reversed loading) with a test frequency of f=10 Hz. Run-outs (r-o) were defined as N=2×106 cycles.

For the incremental step test (IST), a similar test setup was used to characterize the LCF regime. The IST was performed strain-controlled with a sinusoidal load-time function at the stress ratio of R=−1 (fully reversed loading) with a test frequency of f=0.05 Hz. To estimate the required strain amplitude and minimize the number of specimens needed, various reference values from the literature were analyzed (15-18). Based on this literature, the IST was conducted with a constant block maximum until reaching a strain amplitude of Δεa,t=1%.

To investigate the fracture surface and failure-initiating defects, scanning electron microscope (SEM) Tescan Mira 3 XMU (Brno, Czech Republic) was used für fractography investigations after the failure of all specimens.

Electrochemical corrosion tests. To analyze the electrochemical properties, potentiodynamic polarization (PDP) measurements, and electrochemical impedance spectroscopy (EIS) measurements were carried out. For both measurements, a standard three-electrode setup was chosen, which can be seen in Figure 4. The tests were performed in Ringer’s solution (B. Braun Melsungen AG, Melsungen, Germany) at 37°C with a pH=5.82.

Figure 4.
Figure 4.

Test setup for potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) measurements.

To estimate the corrosion current densities icorr and Ecorr of both material conditions, PDP measurements were carried out. Relative to the open circuit potential (OCP), a potential area between E=(−1 V)-(+10 V) was applied. The potential was increased with a scan rate of ΔE =1 mV/s after reaching OCP. The analysis was performed using the software Echem Analyst 2 (version 7.10, Gamry Instruments Inc., Warminster, PA, USA) by applying the Tafel extrapolation of the cathodic branch. The resulting corrosion current icorr was divided by the surface area of the gauge length to obtain the corrosion current density.

EIS measurements were carried out with an effective potential amplitude of E=10 mV over a frequency range from f=20 kHz to 3 mHz. Subsequent data analysis was performed using Gamry Echem Analyst 2 (version 7.10, Gamry Instruments Inc., Warminster, PA, USA) software.

Results and Discussion

Microstructural characterization of the initial conditions. Figure 5 shows the microstructure of the two conditions at the same magnification. The same phases occur in both conditions. The microstructure consists of lamellar α-areas and β-phases. However, there are major differences in the structure. Figure 5a shows columnar grains with martensite needles originating from the columnar grains of the as-built specimen. The microstructures origin lies in the high cooling rate due to the manufacturing process of PBF-LB/M Ti-Grade 23. In contrast to the martensitic needles and columnar grains, the heat-treated specimen shows a more coarse-grained microstructure (Figure 5b). This is an indication of two-phase α-β-microstructure, which forms when Ti-Grade 23 is heat-treated above the transus-temperature of 882°C (19).

Figure 5.
Figure 5.

Light microscopic images of laser powder bed fusion (PBF-LB/M) Ti-Grade 23 alloy. (A) As-built (AB) and (B) heat-treated (HT) conditions.

In accordance with the properties of martensitic materials, a much higher Vickers hardness was found for the AB specimens. They show a hardness of 370±14 HV10 and 373±8 HV0.5, which is lower for the heat-treated samples at 319±17 HV10 and 323±15 HV0.5 (Table II). As Figure 6 shows, there is no significant difference between micro-hardness and macro-hardness within the same condition. The determined microstructural characteristic values for grain size and α-lamella thickness are summarized in Table II. Compared to similar conditions from the literature, the hardness of AB condition is similar, but the hardness of HT condition is higher as in other publications (20-23). The grain size and the α-lamellae thickness differ by a factor of two after heat treatment, i.e., fewer grains and wider α-lamellae are present in the material due to the heat treatment.

View this table:
Table II.

Evaluation of the PBF-LB/M Ti-Grade 23 alloy microstructure and hardness measurements.

Figure 6.
Figure 6.

Comparison of the values of micro- and macro-hardness for as-built (AB) and heat-treated (HT) conditions. HV: Vickers hardness.

Tensile tests. Figure 7a shows a graphical comparison of the YM, UTS, Rp0.2 and FS values of the tensile test for both conditions. In Table III, the individual values are listed. For AB specimens the UTS significantly increased due to the martensitic structure with +35% to 1,172±30 MPa. However, for the HT condition, the UTS of 869±7 MPa is already high enough to fulfill the specifications in DIN EN ISO 5832-3. The YM in tensile direction of 107±1 (AB) and 109±3 GPa (HT) corresponds to the range of 105 to 115 GPa specified in the literature (20, 24, 25). The values for the YM and FS are also within the range of previous studies (20, 25).

Figure 7.
Figure 7.

Results from quasistatic tests: Young’s modulus (YM), ultimate tensile/compression strength (UTS/UCS), yield strength, and fracture strain (FS) in (A) tensile test and (B) compression test for as-built (AB) and heat-treated (HT) conditions.

View this table:
Table III.

Young’s modulus (YM), ultimate tensile strength (UTS/UCS), 0.2%-yield strength (Rp0.2) and fracture strain (FS) of PBF-LB/M Ti-Grade 23 alloy for as-built (AB) and heat-treated (HT) conditions.

The calculation of the Poisson’s ratio ν is carried out in a partial range of the elastic material deformation, in which a constant Poisson’s ratio can be observed. Previous settling effects due to the unevenness of the specimens or the stiffness of the machine are thus not considered and do not influence the results (26). Using this method, a Poisson’s ratio of ν=0.318 was calculated for the AB condition and ν=0.320 for the HT state. These values are comparable with results from other literature (27).

Figure 7b shows a graphical comparison of the YM, UTS/UCS, Rp0.2, and FS values of the compression test for both conditions. In Table III, the individual values are listed. The YM in the compression direction is significantly smaller compared to that determined in the tensile test. Therefore, there is a strong tension-compression asymmetry, which must be considered. This asymmetry has already been observed in the literature (28-30). In addition, differences in the YM in compression direction between the AB and the HT specimens are noticeable. The YM of the AB specimens is almost 28% higher than that of the HT. When evaluating these results, the high standard deviation of up to 21 GPa must be considered. Analogous to the tensile tests, a decrease in UCS and Rp0.2 was observed in the HT condition compared to the AB condition. This can be also attributed to the transformation processes due to heat treatment. The martensitic microstructure in the AB condition is significantly stronger and less ductile than the α-β microstructure in the HT condition.

The requirements for medical implants described in DIN EN ISO 5832-3 are fulfilled by the investigated PBF-LB/M manufactured Ti-Grade 23 for both conditions. However, when used as a dental implant, the implant is mainly subjected to compressive stress, which is not considered in the standard. However, higher strengths and higher fracture strains were measured in the compression test. A comparison of the YM of PBF-LB/M Ti-Grade 23 and human bone (10-20 GPa) (31) shows a very strong discrepancy, which causes the stress-shielding effect described above and can lead to detachment of the implant in the bone. To minimize this effect, lattice structures can be used to reduce the stiffness of the material. Various works already show additively manufactured Ti-Grade 23 specimens with YM of about 10 GPa, which significantly reduces the stress-shielding effect (25, 32).

Since residual stresses in medical implants are undesirable and make simulation very difficult, a condition with reduced residual stresses is preferred. As the residual stresses are high in the AB state due to the high cooling rates in the additive manufacturing process and these are reduced at the selected heat treatment temperature of 1,050°C, the HT state is beneficial here.

Fatigue tests. In Figure 8, the stress amplitude and the number of cycles to failure Nf, are plotted for both material states in a Woehler (S-N) diagram. For both tested states, a rather small influence of the heat treatment on the HCF behavior could be determined. In the AB state, the estimated fatigue strength (2×106) is 342 MPa whereas in the HT state the estimated fatigue strength (2×106) is 365 MPa. Figure 8 also shows a Basquin-fit of each material condition. Despite the good correlation, with an R2=0.81 (AB) and R2=0.96 (HT), individual outliers can be identified especially for the AB condition. Compared with the fatigue behavior of the comparable material analyzed by Leuders et al. (17), a better fatigue behavior in AB and HT conditions can be observed. Specimens by Leuders et al. show fatigue strength of around 300 MPa (AB) or even lower (HT), whereas the specimens in this paper reach higher fatigue strength.

Figure 8.
Figure 8.

Woehler (S-N) curves for as-built (AB) and heat-treated (HT) conditions.

The IST allowed the LCF behavior of the material to be analyzed in addition to the HCF behavior. From the plot of hysteresis loops at 0.6%, 0.8%, 0.9%, and 1.0% strain amplitude plotted in a stress-strain diagram, the masing behavior of both material states can be seen (Figure 9). Masing behavior describes the property of some materials, where the doubled initial stress curve and the stress branches of the hysteresis develop almost identically (33). This material behavior occurs more clearly in the heat-treated specimen than in the as-built specimen. Based on Christ and Mughrabi (34), the cyclic stress-strain curve was determined from saturated hysteresis loops, in addition to the quasistatic stress-strain curve if the material shows masing behavior. By Morrow’s approach, the determined CSS curve can be represented by a potency approach:

Figure 9.
Figure 9.

Masing behavior for (A) as-built (AB) and (B) heat-treated (HT) conditions.

Embedded Image Eq. 1

Here, two coefficients K′ (cyclic strength coefficient) and n′ (cyclic strain hardening coefficient) are determined approximately via linear regression in the double logarithmic representation of the CSS curve (35). The beginning of the plastic region of the CSS curve was calculated according to Ramberg et al. (35, 36) via the 0.2% equivalent yield strength R′p0.2. The determined cyclic coefficients are presented in Table IV.

View this table:
Table IV.

Cyclic strength coefficient (K′), cyclic strain hardening coefficient (n′), cyclic 0.2% yield strength (R′p0.2) and damage parameter (PHL) of PBF-LB/M Ti-Grade 23 alloy for as-built (AB) and heat-treated (HT) conditions.

Figure 10 compares the quasistatic stress-strain-curve (QSS) and the cyclic stress-strain-curve (CSS) for both conditions. Both graphs of the AB condition behave very similarly, which indicates a high strain tolerance. The HT condition material will harden under cyclic loading. This is also shown by the number of blocks the material withstood. In AB condition, the specimen was under cyclic strain for 34 blocks before the fracture occurred. In the HT condition, the specimen withstood the cyclic loading for only 7 blocks. Compared to the results from quasistatic tests, this material behavior of both conditions matches the results of a high strain resistance in AB condition and a lower strain resistance in HT condition.

Figure 10.
Figure 10.

Comparison of quasistatic stress-strain-curve (QSS) and cyclic stress-strain-curve (CSS) for (A) as-built (AB) and (B) heat-treated (HT).

By considering all, overlaid hysteresis loops at εt=1% of the respective material state, the crack opening and closing behavior of both materials could be additionally investigated. The AB material showed no behavior according to any of the damage parameters described by Haibach (37). In the HT state, however, a material behavior could be observed which leads to the determination of the damage parameter according to Haibach and Lehrke and is shown in Table IV (Figure 11) (37).

Figure 11.
Figure 11.

Crack opening and closing behavior of the specimens shortly before failure of the heat-treated (HT) condition with crack opening drawn in.

Fractography. In general, the fracture-initiating defect can be determined by tracing the so-called river lines back to their origin (Figure 12). River lines occurred in all specimens considered and simplify the determination of the fracture-inducing defect. Sustained, cyclic loading produces first microcracks and then macrocracks over time. These propagate in a controlled manner until the fatigue fracture occurs, which can be visually distinguished from the preceding crack growth even at low resolution.

Figure 12.
Figure 12.

Fracture mechanics due to cyclic loading of laser powder bed fusion (PBF-LB/M) Ti-Grade 23 alloy for as-built (AB) and heat-treated (HT) conditions. The continuous green lines divide the fracture surface into different zones.

SEM was used to identify the critical defects which lead to failure for each specimen. Different failure mechanisms were identified, which can be divided into three defect types (Figure 13). The first type is a gas pore which can form during the manufacturing process in PBF-LB/M process (Figure 13A). In addition, edge defects formed by intrusion and extrusion could be detected on specimens (Figure 13B). Figure 13C shows the third observed fracture mechanism: A combination of near-surface gas pores and edge defects. Looking at the occurrence probability of the three different fracture mechanisms, the main cause of specimen failure is the surface defect caused by intrusion and extrusion. One sample broke internally by a pore and two others by near-surface pores (Figure 14). The only specimen that failed through a pore had less than 106 cycles to failure despite a stress amplitude below the estimated fatigue strength (AB). The stress concentration at the pores significantly reduced the lifetime. Teschke et al. (38) and Tenkamp et al. (39, 40) were able to show this in model-based investigations.

Figure 13.
Figure 13.

Exemplary representation of the three failure mechanisms of laser powder bed fusion (PBF-LB/M) Ti-Grade 23 alloy after constant amplitude testing. Scanning electron microscope (SEM) images of the different defect types: (A) a gas pore which can form during the manufacturing process, (B) edge defects formed by intrusion and extrusion, and (C) a combination of near-surface gas pores and edge defects.

Figure 14.
Figure 14.

Identified critical defects and their origin.

Corrosion resistance. The corrosion tests aimed to investigate the corrosion behavior and the changes resulting from the heat treatment of PBF-LB/M Ti-Grade 23. Figure 15 shows exemplary results from the PDP measurement of PBF-LB/M manufactured Ti-Grade 23 in AB and HT states. In the presented Tafel diagram, a very similar course of both material conditions can be seen. Only the characteristic values determined (Table V) show a slightly reduced corrosion current density icorr in the HT state, which would indicate an increased corrosion resistance of the HT specimen. However, both the high standard deviation of the measurement and the logarithmic scaling of the current density should be considered here. Taking these points into account, the PDP measurement does not seem to show any influence of the heat treatment on the corrosive properties of the material.

Figure 15.
Figure 15.

Tafel curves obtained in Ringers solution at 37°C for as-built (AB) and heat-treated (HT) conditions.

View this table:
Table V.

Results from potentiodynamic polarization (PDP) measurements of PBF-LB/M Ti-Grade 23 alloy for as-built (AB) and heat-treated (HT) conditions.

To be able to make conclusions about the corrosion behavior of the overall system by means of EIS measurements, the equivalent circuit shown in Figure 16C was first created. It consists of a resistor RU, which represents the resistance of the electrolyte solution, a so-called constant phase element CPE, which represents the behavior of the oxide layer, and a further resistor RP connected in parallel, which reflects the electron exchange at the electrode. Using this equivalent circuit to provide the most accurate fit to the measured data, the variables of the individual components and their meaning for the corrosion behavior can be analyzed. As Table VI shows, there are only minor differences between the two material states. Only the polarization resistance RP is lower for HT samples. However, a high standard deviation is present here, which must be considered. Overall, the heat treatment seems to have only a minor influence on the corrosion behavior of the material here as well.

Figure 16.
Figure 16.

Electrochemical impedance spectroscopy (EIS) measurement with Bode-diagrams of (A) impedance and (B) phase angle, and (C) used equivalent electric circuit.

View this table:
Table VI.

Results from electrochemical impedance spectroscopy (EIS) measurements of PBF-LB/M Ti-Grade 23 alloy for as-built (AB) and heat-treated (HT) conditions.

The higher polarization resistance of the AB samples suggests better corrosion properties compared to the HT condition. However, the results from the PDP measurement, which determined the corrosion current density icorr, should also be considered. Here, the HT samples exhibited a 26.5% lower corrosion current density. According to Zhang et al. (41), the behavior of polarization resistance and corrosion current density is inversely proportional to each other, which makes the PDP measurement contradictory to the EIS measurement, suggesting better corrosion properties for the HT samples. Considering the previously mentioned high standard deviation, it seems more plausible that the corrosion properties of the two material conditions do not differ as significantly as initially assumed. In principle, both states exhibit very high corrosion resistance, which makes Ti-Grade 23 an excellent material for biomedical applications (14, 42-44).

The determined properties of the investigated materials and material states strongly depend on the manufacturing process and post-treatment. Changes in the manufacturing process (e.g., process parameters, machine, powder) or post-treatment (e.g., heat treatment temperature, cooling rate, shielding gas) can change the microstructure and thus the properties of the material. Therefore, in the case of different production routes, it must be considered whether the properties determined in this study are transferable. This means that the investigations using the PBF-EB/M-manufactured titanium grade 21 alloy cannot be transferred without restriction to other manufacturing processes and heat treatments.

Summary and Conclusion

Additive manufacturing enables patient-specific implants and the reduction of stress shielding by using lattice structures instead of solid material. For the safe use and simulation-based design of additively manufactured implants made of titanium grade 23, an in-depth understanding of the mechanical damage behavior, the microstructure, and the electrochemical properties is required. The aim of this study was to characterize the PBF-LB/M manufactured alloy Ti6Al4V. As-built specimens and heat-treated specimens annealed for 4 hours at 1,050°C in a vacuum atmosphere were examined.

The heat treatment transformed the martensitic microstructure of the as-built state into a (α+β) mixed microstructure. As a result, the hardness as well as the tensile and compressive strength decreased. In addition, an increased fracture strain was observed in the heat-treated specimens. The Young’s modulus remained almost constant, and a pronounced tension-compression asymmetry of all mechanical properties was observed, regardless of the heat treatment. Both conditions meet the requirements of DIN EN ISO 5832-3 for use in medical implants. However, heat treatment effectively reduced the high residual stress resulting from the manufacturing process. In terms of low cycle fatigue, the as-built condition showed improved fatigue behavior, although both conditions exhibited hardening characteristics. No significant differences were observed between the two material conditions in the high cycle fatigue range. In particular, the fatigue strength at 2×106 cycles was 365 MPa, exceeding values reported in the existing literature. Electrochemical corrosion tests, including electrochemical impedance spectroscopy and potentiodynamic polarization measurements, showed no significant differences between the two material states, but confirmed the excellent corrosion resistance of PBF-LB/M Ti6Al4V. The mechanical and electrochemical properties of PBF-EB/M-processed alloy titanium grade 23 fulfill all mechanical and electrochemical requirements for use as an implant material. The data obtained in this study can be used in the future for the design and calculation of implants and represent a benchmark for other manufacturing routes.

Acknowledgements

The authors gratefully acknowledge to the Laser Center Hannover (LZH, Hannover, Germany), especially Anne Jahn and Joerg Hermsdorf, for the additive manufacturing of the specimens in the framework of an excellent scientific collaboration.

Footnotes

  • Authors’ Contributions

    All tests were performed by M.T. and L. G.; the figures were prepared by M.T. and L. G.; the original draft was written by M.T., L. G. and S. S.; F.W. and J. T. supervised the project and reviewed the manuscript. All Authors have read and agreed to the published version of the manuscript.

  • Funding

    The authors gratefully acknowledge the funding by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for the subproject 3 (“Mechanism-based characterization of the fatigue and corrosion fatigue properties of additively manufactured TPMS lattice structures under physiological conditions”) within the Research Unit 5250 “Mechanism-based characterization and modeling of permanent and bioresorbable implants with tailored functionality based on innovative in vivo, in vitro and in silico methods” (project no. 449916462).

  • Conflicts of Interest

    The Authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  • Received August 22, 2024.
  • Revision received October 28, 2024.
  • Accepted November 6, 2024.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) 4.0 international license (https://creativecommons.org/licenses/by-nc-nd/4.0).

References

  1. ↵
    1. Shibo G,
    2. Xuanhui Q,
    3. Xinbo H,
    4. Ting Z,
    5. Bohua D
    : Powder injection molding of Ti–6Al–4V alloy. J Mater Process Technol 173(3): 310-314, 2006. DOI: 10.1016/j.jmatprotec.2005.12.001
    OpenUrlCrossRef
  2. ↵
    1. Niinomi M
    : Mechanical properties of biomedical titanium alloys. Mater Sci Eng A 243(1-2): 231-236, 1998. DOI: 10.1016/S0921-5093(97)00806-X
    OpenUrlCrossRef
  3. ↵
    1. Sidambe A,
    2. Figueroa I,
    3. Hamilton H,
    4. Todd I
    : Metal injection moulding of CP-Ti components for biomedical applications. J Mater Process Technol 212(7): 1591-1597, 2012. DOI: 10.1016/j.jmatprotec.2012.03.001
    OpenUrlCrossRef
  4. ↵
    1. Markhoff J,
    2. Krogull M,
    3. Schulze C,
    4. Rotsch C,
    5. Hunger S,
    6. Bader R
    : Biocompatibility and inflammatory potential of titanium alloys cultivated with human osteoblasts, fibroblasts and macrophages. Materials (Basel) 10(1): 52, 2017. DOI: 10.3390/ma10010052
    OpenUrlCrossRefPubMed
  5. ↵
    1. Sidambe AT
    : Biocompatibility of advanced manufactured titanium implants-a review. Materials (Basel) 7(12): 8168-8188, 2014. DOI: 10.3390/ma7128168
    OpenUrlCrossRefPubMed
  6. ↵
    1. Nicholson JW
    : Titanium alloys for dental implants: a review. Prosthesis 2(2): 100-116, 2020. DOI: 10.3390/prosthesis2020011
    OpenUrlCrossRef
  7. ↵
    1. Niinomi M,
    2. Nakai M,
    3. Hieda J
    : Development of new metallic alloys for biomedical applications. Acta Biomater 8(11): 3888-3903, 2012. DOI: 10.1016/j.actbio.2012.06.037
    OpenUrlCrossRefPubMed
  8. ↵
    1. Ahmed Y,
    2. Sahari K,
    3. Ishak M,
    4. Khidhir B
    : Titanium and its alloy. IJSR 3(10): 1351-1361, 2014.
    OpenUrl
  9. ↵
    1. Nikiel P,
    2. Wróbel M,
    3. Szczepanik S,
    4. Stępień M,
    5. Wierzbanowski K,
    6. Baczmański A
    : Microstructure and mechanical properties of Titanium grade 23 produced by selective laser melting. Archiv Civ Mech Eng 21(4): 152, 2021. DOI: 10.1007/s43452-021-00304-5
    OpenUrlCrossRef
  10. ↵
    1. Hanada S,
    2. Masahashi N,
    3. Semboshi S,
    4. Jung T
    : Low Young’s modulus of cold groove-rolled β Ti–Nb–Sn alloys for orthopedic applications. Mater Sci Eng A 802: 140645, 2021. DOI: 10.1016/j.msea.2020.140645
    OpenUrlCrossRef
  11. ↵
    1. Koju N,
    2. Niraula S,
    3. Fotovvati B
    : Additively manufactured porous Ti6Al4V for bone implants: a review. Metals 12(4): 687, 2022. DOI: 10.3390/met12040687
    OpenUrlCrossRef
  12. ↵
    1. Vrancken B,
    2. Thijs L,
    3. Kruth JP,
    4. van Humbeeck J
    : Heat treatment of Ti6Al4V produced by Selective Laser Melting: Microstructure and mechanical properties. J Alloy Comp 541: 177-185, 2012. DOI: 10.1016/j.jallcom.2012.07.022
    OpenUrlCrossRef
  13. ↵
    1. Sui A,
    2. Liu B,
    3. Liu C,
    4. Wang D,
    5. Wang E
    : Effect of heat treatment on the corrosion resistance and mechanical properties of selective laser melting Ti6Al4V alloy. Materialwiss Werkstofftech 52(12): 1319-1327, 2021. DOI: 10.1002/mawe.202100201
    OpenUrlCrossRef
  14. ↵
    1. Tamayo JA,
    2. Riascos M,
    3. Vargas CA,
    4. Baena LM
    : Additive manufacturing of Ti6Al4V alloy via electron beam melting for the development of implants for the biomedical industry. Heliyon 7(5): e06892, 2021. DOI: 10.1016/j.heliyon.2021.e06892
    OpenUrlCrossRefPubMed
  15. ↵
    1. Xi J,
    2. Hu Y,
    3. Xing H,
    4. Han Y,
    5. Zhang H,
    6. Jiang J,
    7. Nikbin K
    : The low-cycle fatigue behavior, failure mechanism and prediction of SLM Ti-6Al-4V alloy with different heat treatment methods. Materials (Basel) 14(21): 6276, 2021. DOI: 10.3390/ma14216276
    OpenUrlCrossRefPubMed
    1. Zhang P,
    2. He AN,
    3. Liu F,
    4. Zhang K,
    5. Jiang J,
    6. Zhang DZ
    : Evaluation of low cycle fatigue performance of selective laser melted titanium alloy Ti–6Al–4V. Metals 9(10): 1041, 2019. DOI: 10.3390/met9101041
    OpenUrlCrossRef
  17. ↵
    1. Leuders S,
    2. Lieneke T,
    3. Lammers S,
    4. Tröster T,
    5. Niendorf T
    : On the fatigue properties of metals manufactured by selective laser melting – The role of ductility. J Mater Res 29(17): 1911-1919, 2014. DOI: 10.1557/jmr.2014.157
    OpenUrlCrossRef
  18. ↵
    1. Gupta A,
    2. Bennett CJ,
    3. Sun W
    : Fatigue property-performance relationship of additively manufactured Ti-6Al-4V bracket for aero-engine application: An experimental study. Procedia Struct Integr 38: 40-49, 2022. DOI: 10.1016/j.prostr.2022.03.005
    OpenUrlCrossRef
  19. ↵
    1. Okabe T,
    2. Hero H
    : The use of titanium in dentistry. Cells Materials 5: 211-230, 1995.
    OpenUrl
  20. ↵
    1. Longhitano GA,
    2. Larosa MA,
    3. Jardini AL,
    4. Zavaglia CADC,
    5. Ierardi MCF
    : Correlation between microstructures and mechanical properties under tensile and compression tests of heat-treated Ti-6Al–4 V ELI alloy produced by additive manufacturing for biomedical applications. J Mater Process Technol 252: 202-210, 2018. DOI: 10.1016/j.jmatprotec.2017.09.022
    OpenUrlCrossRef
    1. Emminghaus N,
    2. Bernhard R,
    3. Hermsdorf J,
    4. Kaierle S
    : Residual oxygen content and powder recycling: effects on microstructure and mechanical properties of additively manufactured Ti-6Al-4V parts. Int J Adv Manuf Technol 121(5-6): 3685-3701, 2022. DOI: 10.1007/s00170-022-09503-7
    OpenUrlCrossRef
    1. Song B,
    2. Dong S,
    3. Zhang B,
    4. Liao H,
    5. Coddet C
    : Effects of processing parameters on microstructure and mechanical property of selective laser melted Ti6Al4V. Materials Design 35: 120-125, 2012. DOI: 10.1016/j.matdes.2011.09.051
    OpenUrlCrossRef
  23. ↵
    1. Khorasani A,
    2. Gibson I,
    3. Awan US,
    4. Ghaderi A
    : The effect of SLM process parameters on density, hardness, tensile strength and surface quality of Ti-6Al-4V. Addit Manuf 25: 176-186, 2019. DOI: 10.1016/j.addma.2018.09.002
    OpenUrlCrossRef
  24. ↵
    1. Wang Y,
    2. Martinsen K,
    3. Yu T,
    4. Wang K
    1. Nes EV,
    2. Hovig EW,
    3. Feitosa L,
    4. Sørby K
    : Effect of building orientation in mechanical properties of Ti6Al4V produced with laser powder bed fusion. In: Advanced manufacturing and automation XI. Wang Y, Martinsen K, Yu T, Wang K (eds.). Singapore, Springer Singapore, pp. 239–245, 2022. DOI: 10.1007/978-981-19-0572-8_30
    OpenUrlCrossRef
  25. ↵
    1. Hudák R,
    2. Schnitzer M,
    3. Králová ZO,
    4. Gorejová R,
    5. Mitrík L,
    6. Rajťúková V,
    7. Tóth T,
    8. Kovačević M,
    9. Riznič M,
    10. Oriňaková R,
    11. Živčák J
    : Additive manufacturing of porous Ti6Al4V alloy: Geometry analysis and mechanical properties testing. Appl Sci 11(6): 2611, 2021. DOI: 10.3390/app11062611
    OpenUrlCrossRef
  26. ↵
    1. Singh P,
    2. Lewandowski J
    : The effects of reinforcement additions and heat treatment on the evolution of the Poisson Ratio during straining of discontinuously reinforced aluminum alloys. Metal Mater Trans 26A: 2911-2921, 1995.
    OpenUrl
  27. ↵
    1. MatWeb.com
    : Titanium Ti-6Al-4V (Grade 5), Annealed Bar - Datasheet. Available at: https://www.matweb.com/search/DataSheet.aspx?MatGUID=10d463eb3d3d4ff48fc57e0ad1037434 [Last accessed on November 5, 2024]
  28. ↵
    1. Raghavendra S,
    2. Molinari A,
    3. Fontanari V,
    4. Luchin V,
    5. Zappini G,
    6. Benedetti M,
    7. Johansson F,
    8. Klarin J
    : Tensile and compression properties of variously arranged porous Ti-6Al-4V additively manufactured structures via SLM. Procedia Struct Integr 13: 149-154, 2018. DOI: 10.1016/j.prostr.2018.12.025
    OpenUrlCrossRef
    1. Khan AS,
    2. Yu S,
    3. Liu H
    : Deformation induced anisotropic responses of Ti–6Al–4V alloy Part II: A strain rate and temperature dependent anisotropic yield criterion. Int J Plast 38: 14-26, 2012. DOI: 10.1016/j.ijplas.2012.03.013
    OpenUrlCrossRef
  30. ↵
    1. Odenberger E,
    2. Hertzman J,
    3. Thilderkvist P,
    4. Merklein M,
    5. Kuppert A,
    6. Stöhr T,
    7. Lechler J,
    8. Oldenburg M
    : Thermo-mechanical sheet metal forming of aero engine components in Ti-6Al-4V – PART 1: Material characterisation. Int J Mater Form 6(3): 391-402, 2013. DOI: 10.1007/s12289-012-1093-8
    OpenUrlCrossRef
  31. ↵
    1. Keller TS,
    2. Mao Z,
    3. Spengler DM
    : Young’s modulus, bending strength, and tissue physical properties of human compact bone. J Orthop Res 8(4): 592-603, 1990. DOI: 10.1002/jor.1100080416
    OpenUrlCrossRefPubMed
  32. ↵
    1. Ramirez D,
    2. Murr L,
    3. Li S,
    4. Tian Y,
    5. Martinez E,
    6. Martinez J,
    7. Machado B,
    8. Gaytan S,
    9. Medina F,
    10. Wicker R
    : Open-cellular copper structures fabricated by additive manufacturing using electron beam melting. Mater Sci Eng A 528(16-17): 5379-5386, 2011. DOI: 10.1016/j.msea.2011.03.053
    OpenUrlCrossRef
  33. ↵
    1. Yadav SS,
    2. Roy SC,
    3. Goyal S
    : A comprehensive review and analysis of Masing/non-Masing behavior of materials under fatigue. Fatigue Fract Eng Mater Struct 46(3): 759-783, 2023. DOI: 10.1111/ffe.13906
    OpenUrlCrossRef
  34. ↵
    1. Christ HJ,
    2. Mughrabi H
    : Cyclic stress-strain response and microstructure under variable amplitude loading. Fatigue Fract Eng Mat Struct 19(2-3): 335-348, 1996. DOI: 10.1111/j.1460-2695.1996.tb00971.x
    OpenUrlCrossRef
  35. ↵
    1. Ramberg W,
    2. Osgood W
    : Description of stress-strain curves by three parameters. Technical Note No. 902, National Advisory Committee for Aeronautics, Washington DC, 1943.
  36. ↵
    1. Kühne D,
    2. Spak B,
    3. Kästner M,
    4. Brosius A,
    5. Fiedler M
    : Consideration of cyclic hardening and residual stresses in fatigue life calculations with the local strain approach. Arch Appl Mech 91(8): 3693-3707, 2021. DOI: 10.1007/s00419-021-01950-0
    OpenUrlCrossRef
  37. ↵
    1. Haibach E
    : The influence of cyclic material properties on fatigue life prediction by amplitude transformation. Int J Fatigue 1(1): 7-16, 1979. DOI: 10.1016/0142-1123(79)90039-2
    OpenUrlCrossRef
  38. ↵
    1. Teschke M,
    2. Moritz J,
    3. Tenkamp J,
    4. Marquardt A,
    5. Leyens C,
    6. Walther F
    : Defect-based characterization of the fatigue behavior of additively manufactured titanium aluminides. Int J Fatigue 163: 107047, 2022. DOI: 10.1016/j.ijfatigue.2022.107047
    OpenUrlCrossRef
  39. ↵
    1. Tenkamp J,
    2. Stern F,
    3. Walther F
    : Uniform fatigue damage tolerance assessment for additively manufactured and cast Al-Si alloys: An elastic-plastic fracture mechanical approach. Addit Manuf Lett 3: 100054, 2022. DOI: 10.1016/j.addlet.2022.100054
    OpenUrlCrossRef
  40. ↵
    1. Tenkamp J,
    2. Stammkötter S,
    3. Walther F
    : Qualification of uniform fatigue damage tolerance law for additively manufactured and cast Al-Si alloys. Procedia Struct Integr 42: 328-335, 2022. DOI: 10.1016/j.prostr.2022.12.040
    OpenUrlCrossRef
  41. ↵
    1. Zhang X,
    2. Jiang Z,
    3. Yao Z,
    4. Song Y,
    5. Wu Z
    : Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density. Corros Sci 51(3): 581-587, 2009. DOI: 10.1016/j.corsci.2008.12.005
    OpenUrlCrossRef
  42. ↵
    1. Mah D,
    2. Pelletier MH,
    3. Lovric V,
    4. Walsh WR
    : Corrosion of 3D-printed orthopaedic implant materials. Ann Biomed Eng 47(1): 162-173, 2019. DOI: 10.1007/s10439-018-02111-1
    OpenUrlCrossRefPubMed
    1. Alves AC,
    2. Sendão I,
    3. Ariza E,
    4. Toptan F,
    5. Ponthiaux P,
    6. Pinto AMP
    : Corrosion behaviour of porous Ti intended for biomedical applications. J Porous Mater 23(5): 1261-1268, 2016. DOI: 10.1007/s10934-016-0185-0
    OpenUrlCrossRef
  44. ↵
    1. Dabrowski B,
    2. Kaminski J,
    3. Swieszkowski W,
    4. Kurzydlowski KJ
    : Porous titanium scaffolds for biomedical applications: corrosion resistance and structure investigation. Mater Sci Forum 674: 41-46, 2011. DOI: 10.4028/www.scientific.net/MSF.674.41
    OpenUrlCrossRef
Back to top

In this issue

Print
Download PDF
Article Alerts
Email Article
Citation Tools
Reprints and Permissions
Microstructural, Electrochemical, and Mechanical Assessment of Additive Manufactured Titanium Grade 23 for Dental Implants Application
MIRKO TESCHKE, LORENZ GRAFE, SEBASTIAN STAMMKÖTTER, JOCHEN TENKAMP, FRANK WALTHER
In Vivo May 2025, 39 (3) 1751-1766; DOI: 10.21873/invivo.13978
Twitter logo Facebook logo Mendeley logo

Jump to section

Keywords