1. Introduction
The Ti6Al4V alloy is a dual-phase titanium alloy, and it is widely used in marine, aerospace and biomedical industries due to its excellent properties, including high strength-to-weight ratio, nonmagnetic, non-toxic, good plasticity and bio-compatibility [
1,
2,
3]. However, the hardness of Ti6Al4V is not high enough to avoid adhesive wear and abrasive wear during the grinding process [
4]. In marine environments, titanium alloys are prone to coming into contact with materials with high corrosion potential such as steel and copper, leading to galvanic corrosion and serious damage [
5,
6]. When used as bio-implant materials, alloying elements of Ti6Al4V are released into the human body slowly, causing local inflammation, neurological disorders, etc. [
7,
8]. Therefore, the surface hardening or preparation of a coating layer is necessary to overcome these problems.
The surface modifications of titanium and its alloys have been carried out in maffle furnaces by many researchers. S. Kumar et al. [
9] treated Ti6Al4V via thermal oxidation at 500, 650 and 800 °C for different hours in air. The results showed a threefold increase in the hardness obtained for the alloy oxidized at 650 °C for 48 h compared to that of the untreated one. A. Shankar et al. [
10] have also reported that both corrosion resistance and hardness of thermal oxidized pure titanium were improved greatly compared with untreated titanium. Z. Wang et al. [
11] treated Ti-15Mo at 1173 K in air for 1–4 h to obtain samples with different endosmic oxygen depths. Due to the oxygen content gradient, these samples exhibited an optimal balance of strength and elongation. The hybrid oxide coatings for medical applications have also been prepared using a two-step procedure, comprising thermal oxidation and subsequent electrochemical oxidation [
12]. The produced coatings showed high corrosion resistance and were not cytotoxic. However, it would take a long time to treat these alloys via thermal oxidation in furnaces. In the meantime, the performance might decrease due to the overall heating. The creation of different compound layers on the Ti6Al4V surface initiated by the use of a laser, e.g., titanium oxides [
13,
14,
15] and titanium nitrides [
16] in different gas atmospheres is an effective method. Titanium oxides or titanium nitrides on the surface can lead to surface hardening and improve wear resistance [
17,
18]. These titanium compounds are non-conductive. As a result, the galvanic corrosion in the marine environment could be inhibited [
5,
19]. Furthermore, the chemical compound layers can keep the implants separate from the body fluid and tissues. And then it is difficult for the toxic alloy elements to diffuse into the human body [
20]. As a result, the biocompatibility and bioactivity of titanium implants are improved [
21].
Many studies on the laser surface processing of titanium alloys have been carried out. The local morphological changes of titanium surface induced by a nanosecond TEA CO
2 laser under different gas atmospheres were investigated by Ciganovic et al. [
22]. The results showed that the roughness of the irradiated area increased in all cases, while the surface features obtained in air and oxygen were smooth, dome-like structures. The dynamic reaction processes between pure titanium and air under continuous-mode laser heating was also investigated by the use of a synchrotron X-ray diffraction test [
23]. Titanium nitrides appear first, while TiO
2 is observed during the cooling period, resulting from the solidification of the liquid titanium oxides. However, the microstructures and physiochemical properties of the coating layer fabricated by using a continuous-mode laser under different gas atmospheres have not been sufficiently examined according to our knowledge.
In the present paper, a continuous-mode laser with a wavelength of 1080 nm was used to irradiate Ti6Al4V in air or pure oxygen, which aimed to prepare protective coatings on the surface. The microstructures and phase compositions of the coatings were investigated via SEM and XRD, respectively. The effects of the atmosphere and scanning speed were studied to highlight the effectiveness of the laser in preparing coatings on the surface. In addition, the physiochemical properties of the coating layers processed under different gas atmospheres, including hardness and corrosion resistance, were tested.
2. Materials and Methods
2.1. Materials and Sample Preparation
Commercial Ti6Al4V plates (80 × 40 × 3 mm
3) were used as the substrate material. The chemical composition is shown in
Table 1. Before the laser processing, the alloy plates were mechanically polished using varying grades of SiC paper (400, 600, 800), rinsed in deionized water and dried using a stream of compressed air.
The laser treatments were performed using a continuous-mode fiber laser with a 1080 nm wavelength. The laser spot with a diameter of about 4 mm and a Gaussian energy distribution was moved over the substrate surface to form an oxidation region. The laser power was fixed at 500 W. The scanning speed ranged from 10 mm/s to 20 mm/s. The processing atmosphere included two kinds: air and compressed oxygen. The compressed oxygen was supplied coaxially, and the gas pressure was maintained at 0.2 MPa. The parameters of the laser processing are summarized in
Table 2.
2.2. Characterization of the Microstructure and Phase Composition
The surface morphologies of the Ti6Al4V substrates processed by using a laser were observed using an industrial camera equipped with a 32-megapixel CCD (charge-coupled device) sensor. The frame rate could reach 60 frames per second. The zoom lens of the camera had a variable magnification from 0.75 to 4.5. The cross sections of the substrates were exposed via wire electric discharge machining. Upon mounting with acrylic resin, the samples were ground successively with SiC grinding papers (400, 600, 800, 1000 grit), polished with a polycrystalline diamond suspension (2.5 μm) and ultrasonically rinsed in ethanol for 10 min. Subsequently, the polished samples were etched with the Kroll’s Reagent for 15 s, then cleaned using deionized water to wash off the etching solution. The cross-sectional microstructures were examined via scanning electron microscopy (SEM, FEI Quanta 250, Thermo Scientific, Waltham, MA, USA) equipped with energy dispersive spectroscopy (EDS). The typical regions with different metallographic structures in the cross sections were selected for EDS testing. The phase composition of the oxidization layers was analyzed via X-ray diffraction (XRD, Bruker D8 Advance, Billerica, MA, USA).
2.3. Hardness and Corrosion Test
After the laser processing, Vickers microhardness measurements with a load of 19.61 N for 15 s were performed on the surface of the Ti6Al4V substrate to evaluate the surface hardening. The tester was 200HVS-5 provided by Huayin company (Weinan, China). Five measurements were averaged for each sample.
The corrosion properties of the untreated and laser-treated samples were evaluated by means of potentiodynamic polarization curves. The electrolyte solution was prepared using the salt solution (0.62 M NaCl solution). The electrochemical experiments were carried out using an electrochemical measurement system (IVIUM Vertex. One, Eindhoven, The Netherlands). The corrosion cell, which contained 50 mL of electrolyte, was combined with a typical three-electrode configuration. The silver chloride electrode (SCE, R0302) was employed as the reference electrode and the counter electrode (CE) was a platinum plate with an area of 1 cm2. The samples were used as the working electrode (WE). The sample for the corrosion test was scanned once by the laser, and the scanning line for the test was 1 cm in length. Potentiodynamic polarization tests were carried out at a scan rate of 10 mV/s from −2 V to +1 V vs. SCE with respect to the open circuit potential (OCP). The corrosion potential and corrosion current density were determined from the polarization curves using the Tafel extrapolation method.
4. Conclusions
In this study, the Ti6Al4V alloy was irradiated using a continuous laser with a wavelength of 1080 nm in air or compressed oxygen to fabricate anti-corrosion hardening coatings. The laser scanning speed was varied to adjust the heat input. When the alloy was treated in air, a whole coating composed of TiO2 and TiN was prepared on the surface. The thickness of the coatings under different scanning speeds was similar and was approximately 1 μm. However, with the scanning speed decreasing from 20 mm/s to 10 mm/s, the width of the melting area increased from 0 mm to around 1.8 mm. And a rippled surface was formed. When prepared under compressed oxygen with a speed of 10 mm/s, a coating with a thickness of about 60 μm was prepared. When the scanning speed was 10 mm/s, an intermedia layer containing N was formed at the bottom of the coating. The outside of the coating was mainly composed of Ti and O. With the increased scanning speed, the thickness of the coating decreased. Combined with the XRD results, the coating prepared under compressed oxygen consisted of TiO2, TiN and Ti. The testing results show that the hardnesses of samples Ti-A10 and Ti-O10 increased by around 160% and 140% over that of untreated samples, respectively. The anti-corrosion performance of the samples treated via laser scanning was also improved.