Effects of Process Parameters on the Corrosion Resistance and Biocompatibility of Ti6Al4V Parts Fabricated by Selective Laser Melting.

Excellent biocompatibility and corrosion resistance of implants are essential for Ti6Al4V parts fabricated by selective laser melting (SLM) for biomedical applications. To achieve better corrosion resistance and biocompatibility of Ti6Al4V parts, the effects of SLM processing parameters on the corrosion resistance and the biocompatibility of Ti6Al4V parts are investigated by changing the scanning speeds and laser powers. The detailed influence mechanism of processing parameters on the properties of Ti6Al4V parts is studied from two aspects, including microstructure and defects. It is found that the corrosion resistance and biocompatibility of Ti6Al4V parts can be adjusted by changing the scanning speed and the laser power due to the constituent phase and the number and size of defect holes of Ti6Al4V parts. Compared with the laser power, the scanning speed has a stronger influence on the performance of the part, which can be used as "coarse tuning" based on the performance requirements. At the scanning speed of 1100 mm/s and the laser power of 280 W, Ti6Al4V parts with better corrosion resistance can be obtained. Ti6Al4V parts with better biocompatibility are fabricated at the scanning speed of 1200 mm/s and the laser power of 200 W.

Introduction

Nowadays, artificial implants are extensively applied to replace damaged or diseased parts of human bone tissue, and the global bone repair market has huge potential.[1,2] Metals used as implant materials are usually stainless steel,[3] cobalt–chromium alloy,[4] and titanium alloy.[5] Compared with stainless steel and cobalt–chromium alloys, titanium alloys exhibit a lower elasticity modulus, which avoids stress shielding. Titanium alloys are widely used in the medical field due to their excellent mechanical properties[6,7] and biocompatibility.[8,9] Due to the personalized characteristics of bone tissue engineering, the personalized manufacturing of implants is urgently needed. Due to its layer-by-layer processing principle,[10−12] three-dimensional (3D) printing technology has unique advantages in the personalized manufacturing of implants. Titanium alloy implants are usually fabricated by selective laser melting (SLM), which can effectively shorten the manufacturing cycles, improve material utilization, and fabricate complex personalized parts.[13−16]

Excellent biocompatibility and corrosion resistance of implants are the essential requirements for medical applications. The corrosion resistance and biocompatibility of implants are also vital properties for Ti6Al4V parts fabricated by SLM for biomedical applications. The corrosion of implants caused by the physiological environment results in the precipitation of metallic ions and the destruction of implant surface morphology, leading to not only an inflammatory reaction but also organ damage.[17,18] The biocompatibility of implants is the most basic and important performance after implantation. Hence, it is necessary to study the corrosion resistance and biocompatibility of implants. The corrosion resistance of Ti6Al4V parts is investigated in various solutions.[19−22] Heakal et al. demonstrated that the increase in azide concentration in a solution accelerated the corrosion of Ti6Al4V parts.[19] Sharma et al. reported that better corrosion resistance of Ti6Al4V parts fabricated by SLM could be obtained in NaCl and NaOH than in H2SO4.[20] Corrosion resistance of cast Ti6Al4V parts could be improved due to the formation of a passive film.[21] However, there are relatively fewer studies about the corrosive behavior of Ti6Al4V parts fabricated by SLM in the simulated body fluid. In addition, the processing parameters are also critical for the properties of Ti6Al4V parts fabricated by SLM.[12,23,24] Lu et al. found that Ti6Al4V parts fabricated by SLM had better corrosion resistance (the corrosion voltage of −0.352 V)[23] at the laser power of 200 W. Qian et al. found that the density of Ti6Al4V alloy decreased with the increase in the laser scanning speed, leading to the reduction of corrosion resistance.[12] The corrosion resistance of SLM-fabricated Ti6Al4V parts was anisotropic in different planes and the corrosion resistance of the XY plane was better than that of the XZ plane in the HCl solution.[24] Researchers have also done a lot of research on the influence of SLM processing parameters on the biocompatibility of Ti6Al4V parts. Ni et al. investigated the effects of TiN and TiCrN coating layers deposited on the surface of SLM Ti6Al4V on the mechanical properties and biocompatibility of 3D-printed Ti6Al4V.[25] Cox et al. demonstrated that Ti6Al4V parts processed by SLM changed the surface morphology, resulting in a direct impact on the adhesion of cells and biofilms.[26] The Ti-6Al-4V-6Cu alloy processed by SLM could inhibit the activity of proinflammatory cytokines, regulate angiogenesis and Ni–Cr alloy processed by SLM present a lower human adipose stem cells proliferation and viability compared to Co–Cr.[27,28] Ran et al. evaluated the implications of porosity and pore size of Ti6Al4V scaffolds fabricated by SLM in vivo and in vitro.[29] Pore dimension was also considered for the dental area, which was known to be around 20–25 GPa and proximate to cancellous bone modulus.[14] A dense core associated with peripheral larger pores supports cellular proliferation and mechanical resistance.[30] Ghosh et al. conducted an experimental study to obtain polymer processed by SLM grafted on Ti6Al4V hip prosthesis to offset the surface roughness of untreated titanium. The surface roughness, which is conducive to the absorption of proteins, is conducive to the osseointegration of dental implants but not to hip joint reconstruction.[30] At present, there are few systematic studies on the effect of scanning speed and laser power on the corrosion resistance and biocompatibility of Ti6Al4V parts prepared by SLM, and there are few explanations on the influence mechanism. In this paper, the influence of SLM process parameters on the corrosion resistance and biocompatibility of Ti6Al4V parts is systematically studied, and the influence mechanism of process parameters on the performance of Ti6Al4V parts is studied in detail from the two aspects of microstructure and defects.

In this study, the effects of SLM processing parameters (scanning speed and laser power) on the corrosion resistance and the biocompatibility of Ti6Al4V parts are investigated. The detailed influence mechanism of processing parameters on the properties of Ti6Al4V parts is studied from a point of view of microstructure and cavity defects. At the scanning speed of 1100 mm/s, the laser power of 280 W, Ti6Al4V parts with better corrosion resistance. At the scanning speed of 1200 mm/s, the laser power of 200 W, Ti6Al4V parts are with better biocompatibility and the cell proliferation rate is the largest. The corrosion resistance and biocompatibility of Ti6Al4V parts can be regulated by changing the scanning speed and the laser power. This can guide and promote Ti6Al4V parts fabricated by SLM of clinical application.

Materials and Methods

Materials and Sample Preparation

Ti6Al4V alloy spherical powder produced by the gas atomization method (EOS company, Germany) is used in our experiments. The particle size of powder ranges from 25 to 57 μm and the average size is about 38 μm. The chemical composition of the material is shown in Table , and its morphology and particle size distribution are shown in Figure . Different Ti6Al4V parts fabricated by EOS M280 (EOS company, Germany) are obtained by changing the scanning speed and the laser power. The experimental parameters of the scanning speed (v), the laser power (P), the hatch distance (d), and the layer thickness (h) are shown in Table . Ti6Al4V samples (10 mm × 10 mm × 10 mm) are obtained by changing the scanning speed and the laser power. These are then abraded with silicon carbide (SiC) papers (grade from 240 to 2000), immersed in the Keller reagent (95 mL of water, 2.5 mL of HNO3, 1.5 mL of HCl, 1.0 mL of HF), and then ultrasonically cleaned with ethanol and deionized water for 10 min. The surface morphology is obtained by an optical microscope (OM) and s scanning electron microscope (SEM, FEI QUAUTA 200).

Table 1

Composition of the Ti6Al4V Powder (wt %)

Ti6Al4V	Ti	Al	V	O	N	C	H	Fe
wt %	bal.	6.0	4.0	0.09	0.01	0.02	0.02	0.12

Figure 1

Morphology and the particle size distribution of the Ti6Al4V powder.

Table 2

Experimental Parameters of the Scanning Speed and the Laser Power

number	v (mm/s)	P (W)	d (mm)	h (mm)
1	1000	280	0.14	0.03
2	1100	280	0.14	0.03
3	1200	280	0.14	0.03
4	1300	280	0.14	0.03
5	1400	280	0.14	0.03
6	1200	200	0.14	0.03
7	1200	240	0.14	0.03
8	1200	320	0.14	0.03
9	1200	360	0.14	0.03

Corrosion Behavior

The electrochemical test is used to evaluate the corrosion resistance of Ti6Al4V parts fabricated by SLM. A CHI660D electrochemical workstation and 0.9% sodium chloride solution as an electrolyte are used in our experiments. Potentiodynamic polarization is tested in an electrochemical cell with the three-electrode system, which consists of a Ti6Al4V sample working electrode (10 mm × 10 mm × 10 mm), a platinum counter electrode, and a saturated calomel electrode with a Luggin capillary bridge. The experiment is performed at room temperature, and the distance between the Luggin capillary and the surface of the working electrode is fixed at 2 mm. According to the ISO10271-2011 standard, the potentiodynamic current/potential curves are recorded by a C view software with the scanning speed of 10 mV/s from −1.6 to 1.5 V. When the electrochemical reaction is in equilibrium, the sample is in a self-corrosion state, and the net current is not accumulated. When the system is out of the equilibrium state, the amount of material released from the cathode is proportional to the current strength and conduction time. Hence, the corrosion rate is assessed using the corrosion current density. The potential–current density curve (as a logarithm of current in the form of Tafel graph), open-circuit potential, and corrosion current density are obtained.

Biocompatibility

In vitro cytotoxicity is used to characterize the biological properties of Ti6Al4V parts. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method is used to assess the cytotoxicity of the material. A microplate reader is used to obtain the absorption value of MTT dissolved in dimethyl sulfide. The absorbance value can reflect the number of surviving cells and the strength of cell metabolic activity. The relative growth rate (RGR) of the cells is calculated based on the absorbance value. The samples are extracted in a cell culture medium containing 10% calf serum for 24 h at the ratio of 1.25 cm2:1 mL (surface area of parts: extraction medium) at 37 °C. Mouse fibroblasts L929 are used to subculture the vigorously growing cells for 48–72 h in the experiment. The prepared 1 × 105/mL cell suspension is inoculated on a 96-hole plate, and blank control, negative control, positive control, and material group are set up. Each group is equipped with at least six holes, and 100 μL of the cell suspension is inoculated for each hole. After being cultured in a 5% CO2 incubator for 24 h, the original culture medium is discarded. The blank control group is added with a fresh cell culture medium. The negative control group was added with high-density polyethylene extract. The positive control group is added with 5% dimethylsulfoxide (DMSO). The material group is added with the Ti6Al4V part extract (divided into two groups, 100 and 50% extract), and 100 μL of the Ti6Al4V part extract is added in each hole, and then put in a 5% CO2 incubator for 24 h. After discarding the culture medium in the net hole, 50 μL of the MTT solution with a mass concentration of 1 g/L is added to each hole. After 2 h of continuous culture, the liquid in the net hole is discarded. One hundred microliters of isopropanol is then added and mixed evenly. Finally, the absorbance at 570 and 650 nm wavelength of the enzyme standard instrument is obtained and the relative value-added rate is calculated according to the formulawhere RGR is the relative growth rate (%), A is the absorbance of the test group (negative and positive groups), and AB is the absorbance of the blank control group.

Results and Discussion

Effects of the Scanning Speed

In this experiment, Ti6Al4V parts are fabricated by SLM parameters as follows: the scanning speed v = 1000, 1100, 1200, 1300, and 1400 mm/s, the laser power P = 280 W, the hatch distance h = 0.14 mm, and the layer thickness d = 0.03 mm. The corrosion behavior of Ti6Al4V parts fabricated by SLM is evaluated by an electrochemical test. Figure shows the self-corroding Tafel potentiodynamic polarization curves of five groups of Ti6Al4V parts. The self-corrosion potential is a stable potential when the system is not subjected to external polarization. The higher the self-corrosion potential, the smaller the corrosion tendency. At the scanning speed of 1100 mm/s, the self-corrosion potential of the Ti6Al4V part is about −146.2 mV, showing the lowest corrosion tendency. At a scanning speed of 1300 mm/s, the self-corrosion potential is −116.2 mV, and the related corrosion tendency is the greatest. The self-corrosion potential only reflects the stability of the system.

Figure 2

Tafel potentiodynamic polarization curves of Ti6Al4V samples fabricated at different scanning speeds against corrosion.

The performance of corrosion dynamics is characterized by corrosion current density. Table shows the measured values of the corrosion current density and self-corrosion potential of the five groups of Ti6Al4V parts. With the increase in the scanning speed, the corrosion resistance first improves and then degrades gradually. At the scanning speeds of 1000, 1300, and 1400 mm/s, the corrosion current density is relatively larger, 3.486 × 10–5, 3.477 × 10–5, and 3.439 × 10–5 A/cm2, respectively. At the scanning speeds of 1100 and 1200 mm/s, the corrosion current densities of the prepared parts are about 2.507 × 10–5 and 2.701 × 10–5 A/cm2, respectively. The larger corrosion current density means a larger corrosive quantity. When the scanning speed increases from 1000 to 1100 mm/s, the corrosion current density reaches the minimum value (2.507 × 10–5 A/cm2). When the scanning speed exceeds 1100 mm/s, the corrosion current density increases. Hence, from the perspective of corrosion potential and corrosion current density, Ti6Al4V parts fabricated at 1100 mm/s scanning speed have better corrosion resistance. Hence, the corrosion resistance of Ti6Al4V parts can be adjusted by changing the scanning speed.

Table 3

Tafel Curve Measurement Result

parameter	corrosion rate (mV/s)	self-corrosion potential (mV)	corrosion current density (A/cm2)
1 (1000 mm/s)	10	–121.2	3.486 × 10–5
2 (1100 mm/s)	10	–146.2	2.507 × 10–5
3 (1200 mm/s)	10	–124.3	2.701 × 10–5
4 (1300 mm/s)	10	–116.2	3.477 × 10–5
5 (1400 mm/s)	10	–126.9	3.439 × 10–5

To evaluate the biological performance of Ti6Al4V parts, cell culture and proliferation are also investigated in this study. Figure depicts the morphology of L929 cells cultured in different extracts for 24 h: (a) the blank control group, (b) the negative control group, (c) the positive control group, (d) 1000 mm/s, (e) 1100 mm/s, (f) 1200 mm/s, (g) 1300 mm/s, and (h) 1400 mm/s. Cell morphologies shown in Figure a,b are similar, while the cells shown in Figure c have a round shape. The validity of the experiment is substantiated by cell morphologies shown in Figure a–c. When the scanning speed increases from 1000 to 1400 mm/s, the cell round shrinkage rate is about 13, 15, 8, 5, and 23%. Hence, the scanning speed had a great influence on the morphology of L929 cells. The normal rate of cell morphology is relatively higher at the scanning speed of 1300 mm/s. As shown in Figure , when the scanning speed is 1300 mm/s, the cell proliferation rate is the largest (74.4%), and when the scanning speed is 1400 mm/s, the cell proliferation rate is the smallest (65.2%). Hence, the biological performance of Ti6Al4V parts can be adjusted by changing the scanning speed.

Figure 3

Effect of scanning speed on cell morphology: (a) blank control group, (b) negative control group, (c) positive control group, and (d) 1000 mm/s at (e) 1100 mm/s, (f) 1200 mm/s, (g) 1300 mm/s, and (h) 1400 mm/s.

Figure 4

Relative growth rate (RGR) of cells grown for 24 h at different scanning speeds.

To understand the detailed influence mechanism of scanning speed on the corrosion resistance and biological performance, the SEM images of Ti6Al4V parts fabricated at different scanning speeds are shown in Figure . The above experimental phenomena are attributed to the constituent phase and the number and size of defect holes of Ti6Al4V parts. As shown in Figure , at the scanning speeds of 1000 mm/s, more acicular α′-Ti phase and a small number of defect holes with the size of about 6 μm are observed on the surface of Ti6Al4V parts. It is known that the acicular α′-Ti phase is easily dissolved and favorable for cell growth and attachment.[31] In addition, the large defects hole not only increase the contact area between the etching solution and Ti6Al4V parts but also hinder the proliferation of cells and release toxic ions, resulting in a low relative growth rate and a high round shrinkage rate of cells on the surface of the part. At the scanning speed of 1100 mm/s, the β-Ti phase is most commonly observed and defects holes are relatively rare. It is known that the β-Ti phase plays an important role in resisting dissolution.[31] However, there are few α′-Ti phases conducive to cell growth and attachment. Hence, Ti6Al4V parts fabricated at 1100 mm/s scanning speed have better corrosion resistance but poor biocompatibility. At the scanning speed of 1200 mm/s, relatively more β-Ti phase and a small amount of α′-Ti phase are observed and a certain number of defect holes with larger size are also obtained on the surface of Ti6Al4V parts. Due to the existing big holes and α′-Ti phase, the corrosion resistance of Ti6Al4V parts is reduced, compared with the situation of 1100 mm/s. On the other hand, the α′-Ti phase leads to the acceleration of cell proliferation and an increase in the relative growth rate. At the scanning speed of 1300 mm/s, more acicular α′-Ti phase and tiny holes are observed, which is conducive to the adhesion and proliferation of the cells and increasing the relative growth rate of the cells. At the scanning speed of 1400 mm/s, more acicular α′-Ti phase and a lot of defect holes (both large holes and tiny holes) are observed. The Ti6Al4V parts fabricated at the scanning speed of 1400 mm/s are with the worst corrosion resistance and biocompatibility.

Figure 5

SEM images of Ti6Al4V parts fabricated at different scanning speeds: (a) 1000 mm/s, (b) 1100 mm/s, (c) 1200 mm/s, (d) 1300 mm/s, and (e) 1400 mm/s.

Effects of Laser Power

The Ti6Al4V parts are fabricated by SLM parameters as follows: the laser power P = 200, 240, 280, 320, and 360 W; the scanning speed v = 1200 mm/s; the hatch distance h = 0.14 mm; and the layer thickness d = 0.03 mm. The corrosion behavior is evaluated by an electrochemical test. Figure shows the self-corroding Tafel potentiodynamic polarization curves of five groups of Ti6Al4V samples. The self-corrosion potential of Ti6Al4V parts sharply increases and then rapidly decreases with the increase in the laser power. The self-corrosion potential is −264.9 mV at the laser power of 320 W, which indicates the smallest corrosion tendency. At the laser power of 360 W, the self-corrosion potential is −102.5 mV, exhibiting the highest corrosion tendency.

Figure 6

Tafel potentiodynamic polarization curves of Ti6Al4V samples fabricated at different laser powers.

Table shows the measured values of the corrosion current density and self-corrosion potential of five groups of Ti6Al4V parts. As the laser power increases, the corrosion current density shows oscillating behavior: decreasing sharply, then increasing quickly, and finally rapidly decreasing. At the laser power of 200, 240, and 320 W, the corrosion current density is relatively larger (3.003 × 10–5, 3.171 × 10–5, and 3.356 × 10–5 A/cm2) and the corrosion resistance is relatively lower. At the laser power of 360 W, the relatively smaller corrosion current density (2.801 × 10–5 A/cm2) indicates better corrosion resistance. The corrosion current density reaches the minimum (2.701 × 10–5 A/cm2) at 280 W laser power, and the Ti6Al4V parts exhibit the best corrosion resistance. Hence, the corrosion resistance of Ti6Al4V parts can be adjusted by changing the laser power.

Table 4

Tafel Curve Measurements

parameter	corrosion rate (mV/s)	self-corrosion potential (mV)	corrosion current density (A/cm2)
1 (200 W)	10	–124.0	3.003 × 10–5
2 (240 W)	10	–192.7	3.171 × 10–5
3 (280 W)	10	–250.7	2.701 × 10–5
4 (320 W)	10	–264.9	3.356 × 10–5
5 (360 W)	10	–102.5	2.801 × 10–5

Figure depicts the cell morphology of L929 cells cultured in different extracts for 24 h. Figure a shows the cell morphology of the blank control. As shown in Figure b, the cell morphology of the negative control group is normal, which is similar to the cell morphology given in Figure a. While the morphology of the cells of the positive control is rounded, as shown in Figure c. The cell morphology shown in Figure a–c proves that the experiment is effective. Figure d–h shows the difference in cell morphology on the surface of Ti6Al4V parts fabricated under different laser powers. When the laser power is increased from 200 to 360 W, the cell round shrinkage rates are about 11, 10, 8, 7, and 10%. The changes in cell proliferation rate in five groups of different laser powers are investigated in this study, as shown in Figure . It is easily found that the changing trend of cell proliferation rate is consistent with that of cell morphology. With the increase in laser power, the cell proliferation rate first slowly decreases and then slowly increases. When the laser power is 280 W, the minimum cell proliferation rate is obtained (70.8%). When the laser power is 200 W, the maximum cell proliferation rate is 75.5%. Hence, the biological performance of Ti6Al4V parts can be adjusted by changing the laser power.

Figure 7

Effect of laser power on cell morphology: (a) blank control group, (b) negative control group, (c) positive control group, (d) 200 W, (e) 240 W, (f) 280 W, (g) 320 W, and (h) 360 W.

Figure 8

Relative growth rate (RGR) of cells grown for 24 h in the test article extracts at different laser powers.

As shown in Figure , Ti6Al4V parts fabricated by SLM at different laser powers have different microstructures and defects, causing different corrosion resistance and biological properties of Ti6Al4V parts. As shown in Figure , at the laser power of 200, 240, and 320 W, relatively more α′-Ti phase and more hole defects appear on the surface of Ti6Al4V parts, which leads to the reduction of corrosion resistance. At the laser power of 280 W, relatively more β-Ti phase and relatively fewer defects are observed, Ti6Al4V parts are with better corrosion resistance. Due to the β-Ti phase, α′-Ti phase, and hole defects, the corrosion resistance of Ti6Al4V parts is relatively weakened at 360 W laser power, compared with the situation of 280 W. For the biological properties of Ti6Al4V parts, at the laser power of 200, 320, and 360 W, α′-Ti phase and tiny defect holes are observed, which is conducive to the adhesion and proliferation of cells and increase in the relative growth rate. Hence, Ti6Al4V parts have relatively a higher relative growth rate. However, defect holes of Ti6Al4V parts cause the release of toxic ions. At the laser power of 240 W, relatively fewer α′-Ti phase and relatively more β-Ti phase are obtained on the surface of Ti6Al4V parts, which leads to a low relative growth rate of the surface cells. Due to the existing big holes, the cell round shrinkage rate is relatively higher. At the laser power of 280 W, relatively fewer α′-Ti phase is observed and there are almost no hole defects.

Figure 9

SEM images of Ti6Al4V parts at (a) 200 W, (b) 240 W, (c) 280 W, (d) 320 W, and (e) 360 W.

Conclusions

Selective laser melting (SLM), as one of the typical additive manufacturing technologies, has been widely used in the medical field, especially for implant applications. The corrosion resistance and the biocompatibility of implants are vital properties for Ti6Al4V parts fabricated by SLM for biomedical applications. To achieve better corrosion resistance and biocompatibility of Ti6Al4V parts, the effects of SLM processing parameters on the corrosion resistance and the biocompatibility of Ti6Al4V parts are investigated by changing the scanning speeds (1000, 1100, 1200, 1300, and 1400 mm/s) and the laser powers (200, 240, 280, 320, and 360 W). The experimental results show that (1) the corrosion resistance and biocompatibility of Ti6Al4V parts can be regulated by changing the scanning speed and the laser power due to the constituent phase and the number and size of defect holes of Ti6Al4V parts; (2) the large number of defect holes leads to a relatively lower growth rate and a high round shrinkage rate of cells on the surface of the part, due to the increasing of the contact area (between the etching solution and Ti6Al4V parts) and the release of the toxic ions; (3) tiny holes are conducive to the adhesion and proliferation of the cells and the increase in the relative growth rate of the cells; (4) compared with laser power, the scanning speed has a stronger influence on the performance of the part; (5) at the scanning speed of 1100 mm/s, the laser power of 280 W, the hatch distance of 0.14 mm, and the layer thickness of 0.03 mm, Ti6Al4V parts show better corrosion resistance; (6) Ti6Al4V parts with better biocompatibility are fabricated at the scanning speed of 1200 mm/s, the laser power of 200 W, the hatch distance of 0.14 mm, and the layer thickness of 0.03 mm, and the cell proliferation rate is the largest (75.5%). The proposed research is applied to improve the biological activity of titanium alloy implants, which can increase the surgical success rate of implants and promote the clinical application of implants. However, the more detailed influence mechanism of scanning speed on the properties of Ti6Al4V parts is not investigated in this study, as it is still being investigated.