Corrosion Behavior of NiTi Alloys Fabricate by Selective Laser Melting Subjected to Femtosecond Laser Shock Peening

Abstract

NiTi alloys are commonly used in many fields such as aerospace, mechanical engineering due to their excellent mechanical properties and shape memory effect. In recent years, the emergence of selective laser melting (SLM) technology provides a new method for the preparation of NiTi parts. But the surface corrosion failure of SLM-NiTi is the most common problem. This paper mainly focuses on the research of femtosecond laser shock peening of the surface of SLM-NiTi alloy to improve the corrosion resistance. Selecting different scanning space (1 μm, 3 μm, 5 μm, 10 μm), and analyze the surface morphology of the material through the OM, SEM, EDS and white light interferometer, and investigate the surface nanohardness and corrosion resistance through nanoindentation and electrochemical testing. The research results show that part of the TiO₂ is formed under different scanning spaces, and part of NiO is formed when the scanning space is 1μm. At the same time, it is found that the sample under the condition of 10 μm has the most excellent corrosion resistance and nanohardness. The nanohardness reaches 1303 ± 40 HV and the corrosion current density reaches 1.45 ± 0.1 × 10⁻⁹ A·cm⁻². Proper femtosecond laser treatment can effectively improve the surface strength and corrosion resistance of the NiTi alloys.

1. Introduction

NiTi alloy has excellent biocompatibility, excellent corrosion resistance, unique super-elasticity (SE) and shape memory effects (SME), so it is widely used in many fields such as aerospace and biomedicine [1,2,3,4]. However, poor machinability and work hardening severely limit the application of NiTi alloy, which can only be processed into simple wires, bars and pipes, etc. [5,6,7]. Currently, additive manufacturing (AM) such as selective laser melting (SLM), electron beam melting (EBM), and directed energy deposition (DED) is the best method for for overcoming the problems [4,8,9]. Recently, we optimized the molding quality through orthogonal experiments and obtained the optimal process parameters and scanning strategy [10].

Additive manufacturing NiTi alloys are commonly used materials in aerospace, mechanical engineering, and biomedicine, and structural. However, the material failures caused by surface corrosion are inevitable. Therefore, enhancing surface property is the basis for the long-term use of additive manufacturing of NiTi [11,12,13,14]. Surface treatment is a commonly used method to improve surface corrosion resistance, such as laser cladding [15] and surface film [16]. With the development of nanotechnology, the establishment of surface nanostructures has become a new way to improve surface properties, especially nanosecond laser processing and femtosecond laser processing, commonly used for surface treatment of stainless steel [17], titanium alloy [18], magnesium alloy [19] and aluminum alloy [20] etc.

Trdan et al. [21] studied the effect of femtosecond laser shock peening on the corrosion behavior of AA2024-T3, and it was found that the main reason for femtosecond laser peening to improve corrosion resistance may be the refined structure and the reduction of intergranular corrosion; Kolobov et al. [22] found that TiO₂, Ti₃O₅ and TiO were mainly generated after femtosecond laser treatment of Ti-6Al-4V surface; The nanosecond laser treatment of aluminum alloy surface reported by Boinovich et al. [23] effectively inhibited the occurrence of pitting corrosion. However, Femtosecond laser processing of NiTi surface has not yet been systematically studied. Only Wang et.al analyzed the friction resistance [24] and corrosion resistance [25] of NiTi alloy surface strengthened by femtosecond laser shock peening, and found that the femtosecond laser shock peening can help improve the properties of the NiTi alloy surface. But the effect of femtosecond laser parameters on the surface of NiTi alloy has not been studied, and the current studies on parameters are limited to stainless steel and aluminum alloys. Regarding the additive manufacturing of NiTi, no relevant research has been found either. As we all known, SLM in additive manufacturing technology is widely used in the processing of NiTi alloys [26,27,28]. However, there are still a series of questions on the surface of SLM-NiTi, such as the release of Ni²⁺ [12,29], and the corrosion behavior [30] relatively need to be improved. Therefore, it is feasible and necessary to improve the corrosion resistance of NiTi alloy surface by femtosecond laser surface treatment.

Through parameter design in the early stage, SLM-NiTi with excellent surface quality and excellent mechanical properties was obtained (Figure 1). In this paper, we selected different femtosecond laser scanning spaces for exploration (

Table 1. Femtosecond laser parameter design.

Sample	Powder (w)	Scanning Speed (mm/s)	Frequency (kHz)	Scanning Space (μm)
1 μm	8	60	20	1
3 μm	8	60	20	3
5 μm	8	60	20	5
10 μm	8	60	20	10

) and analyzed the trends of sample surface composition and corrosion resistance.

2. Fabrication and Experimental Methods

2.1. Femtosecond Laser Shock Peening Experiments

The SLM-NiTi alloy prepared by the previous research was selected in this study. Before the experiment, the surface of each sample was polished with abrasive papers (using SiC paper 180# to 2000#), ultrasonic cleaning and alcohol wipe were performed. The femtosecond laser system (RAPID 10 Industrial Picosecond Laser System) was produced by the USA with wavelength 1064nm, pulse width 20 ± 5 ps, beam diameter 3 mm and direct spot 32–50 μm. All samples are selected with a power of 8 W, a scanning speed of 60 mm/s, and a frequency of 20 kHz, and different scanning distances, 10 μm, 5 μm, 3 μm, and 1 μm are selected respectively. And the processing schematic diagram is shown in Figure 2.

2.2. Test Methods

The optical microscope (OM, Zeiss AXIO, Oberkochen, Germany) and scanning electron microscope (SEM, ZEISS EV018, Oberkochen, Germany) equipped with energy dispersive spectroscopy (EDS, Oxford Instruments, Abingdon, UK) were used to the surface morphologies of the different samples. The white light interferometer was used to test the roughness and 3D images of sample surface. An X-Ray diffractometer (XRD, SHIMADZU XRD-7000, Shimadzu Scientific Instruments, Tokyo, Japan) with Cu Kα radiation operated at 40 kV and 30 mA was used to determine the phase composition. And the nanoindenter (Anton Paar Step-NHT3, Graz, Austria) was used to measure the nano-hardness, the indentation adopted a load of 20 mN, and pause 5 s, a frequency of 10.0 Hz.

The electrochemical polarization curves and electrochemical impedance spectroscopy (EIS) curves of HP and LP were carried out in 3.5 wt % NaCl at 25 ± 0.5 °C (RT) on the electrochemical station (VersaSTAT3, Princeton Applied Research, Oak Ridge, TN, USA). In the application of the three-electrode system and the electrochemical test, the reference electrode was a saturated calomel electrode (SCE), the counter-electrode is a platinum sheet with an area of 1 cm × 1 cm, and the working electrode was the tested sample. The sample was immersed in 3.5 wt% NaCl for 1 h before the experiment to ensure the stability of the open circuit potential (OCP). The EIS measurement was conducted at the OCP by applying a sinusoidal penitential of 10 mV in the frequency range from 10⁵ to 10⁻² Hz. And ZsimpWin (Version 3.60) software was used for fitting analysis. The potentiodynamic polarization curves were obtained from −1 to 1 V VS OCP with a scanning rate of 1 mV/s. The Tafel extrapolation method was used to obtain the values of corrosion current density (I_corr) and corrosion potential (E_corr). All the electrochemical tests were carried out three times for each condition.

3. Results

3.1. XRD Analysis

Figure 3 depicted the XRD pattern of SLM-NiTi alloys after femtosecond laser shock peening. It can be seen from that after the femtosecond laser, the TiO₂ was formed on the surface. This was mainly because after shock peening in the air, Ti combines with O to form TiO₂. Secondly, we found that the volume fraction of B19′NiTi increased significantly. This may be due to the fact that the melting point of Ni is lower than that of Ti, dissipating prior to Ti, and combining with O. This was why the 1μm sample (large remelting area and high surface temperature) could be seen more obvious NiO phase.

3.2. White Light Interference and Microstructure

Figure 4 showed the OM image of different samples. It could be found that the surface undergoes a femtosecond laser strengthening treatment with obvious discoloration reaction, which indicated the formation of new phase, and the products generated on the surface of different samples are not the same, especially the 1 μm sample, this is mainly caused by the TiO₂ or NiO produced. Figure 5 showed the SEM images and EDS analysis under different parameters. It can be found that the laser ablation spots on the surface after femtosecond laser processing were arranged in an orderly manner with a small number of unmelted particles which showed that the ablation layer is formed on the surface of NiTi alloy. This is the result of the interaction between the femtosecond pulsed laser beam and the NiTi alloy. At the same time, we found that the samples with scan spacing of 1 μm and 3 μm had cracks, which mainly because the scan spacing was small led to thermal stress concentration and cracking. EDS analysis was further carried out to analyze the surface composition. The content of Ni on the surface of the substrate was significantly reduced, and the content of Ti and O increased. This might be due to the formation of TiO₂ on the surface of NiTi alloy. Especially when the scanning distance is reduced to 1 μm, the Ni content increased significantly, which might be caused by the formation of NiO on the surface of the sample, which is consistent with the XRD analysis results. The surface roughness of processed samples was measured by white light interference (Figure 6). It was illustrated that femtosecond laser processing may increase surface roughness [17] which is mainly due to laser ablation and slight plastic deformation. And increasing the scanning distance would decrease the remelting area, leading to a decrease in surface roughness.

3.3. Nanoindentation Hardness Analysis

Figure 7 showed the nano-hardness curve of the samples. Under the condition of 1 μm, due to the small scanning spacing, the surface of the sample was overburned and softened, which reduced the surface nano-hardness of the sample (206 ± 20 HV), while the surface grains of other samples were refined and the hardness increased due to femtosecond laser processing. Simultaneously, the 10 μm sample exhibited the highest surface nano-hardness (1303 ± 40 HV), this is in line with the Hall-Petch formula [31], i.e., ${\mathsf{\delta }}_{\mathrm{s}}={\mathsf{\delta }}_{\mathrm{i}}+{\mathrm{k}}_{\mathrm{s}}{\mathrm{d}}^{-1}$. Where d is the crystal grain diameter, δ_s is the yield strength of the material, δ_i is the movement resistance of the dislocation in the base metal, and k_s is the crystal structure constant.

3.4. Electrochemical Corrosion Behavior

The potentiodynamic polarization curves of different samples were shown in Figure 8.

Table 2. Corrosion parameters extracted from the Polarization curves.

Samples	Ba/mV	Bc/mV	Ecorr/V	Icorr/A·cm−2
As-fabricated	279.46 ± 2.5	169.68 ± 4.1	−0.32 ± 0.02	1.00 ± 0.2 × 10−8
1 μm	56.86 ± 2.1	78.77 ± 2.5	−0.29 ± 0.04	4.95 ± 0.3 × 10−8
3 μm	61.43 ± 1.3	74.15 ± 3.3	−0.31 ± 0.08	4.08 ± 1.1 × 10−7
5 μm	66.19 ± 2.2	81.98 ± 2.5	−0.24 ± 0.03	1.97 ± 0.3 × 10−9
10 μm	74.71 ± 2.3	71.53 ± 3.4	−0.27 ± 0.05	1.67 ± 0.1 × 10−9

showed that the corrosion current density (I_corr) and corrosion potential (E_corr) obtained in the polarization curve. The I_corr always determined the corrosion resistance of the samples and showd the corrosion rate of the material in a corrosive medium. It can be found that the I_corr was the lowest under the condition of 10 μm, which is mainly because the formation of TiO₂ protects the substrate. But the sample surface of 1 μm and 3 μm is higher than the as-fabricated. This might be related to the shock peening effect caused by the scanning spacing, and the excessively large remelting area leads to the overburning of the sample surface (the overburning holes produced in Figure 4d). Simultaneously, the cracks produced by overburning at 1 μm and 3 μm in Figure 5 are also important factors affecting corrosion behavior.

The corrosion behavior of the SLM-NiTi and different scanning space samples in the 3.5 wt % NaCl solution was further studied via EIS measurements (as shown in Figure 9). It was worth noting that the 10 μm sample showed the largest loop than the others, had the highest value of low-frequency impedance modulus i.e., |Z| value, and the phase angles at the medium frequency range of 10 μm was the highest, which means that best corrosion protection performance [32,33].

To more accurately explain the results of EIS in detail, the equivalent electrical circuits are shown in Figure 10. The fitting results were shown in

Table 3. Fitting results of the electrochemical parameters from EIS plots.

Samples	Rs (Ω·cm2)	CPE-film	Rp (Ω·cm2)	n
As-fabricated	57.11 ± 2.5	(2.168 ± 0.3) × 10−5	(1.873 ± 0.4) × 105	0.8412 ± 0.04
1 μm	56.23 ± 3.5	(2.302 ± 0.2) × 10−5	(1.285 ± 0.3) × 105	0.8008 ± 0.03
3 μm	51.17 ± 4.8	(2.779 ± 0.5) × 10−5	(1.685 ± 0.2) × 104	0.7187 ± 0.01
5 μm	48.24 ± 2.7	(1.681 ± 0.4) × 10−5	(2.439 ± 0.5) × 105	0.8596 ± 0.03
10 μm	53.18 ± 2.3	(1.365 ± 0.3) × 10−5	(3.439 ± 0.4) × 105	0.8865 ± 0.05

, in which R_s represents the solution resistance; CPE-film is used instead of a pure capacitive, which is defined by admittance (Y₀) and power index number (n), and can be calculated by the formula (Y_CPE(ω) = 1/Z_CPE = Y₀(jω)ⁿ) [34]. R_p represents charge transfer resistance. Under the condition of 10μm, it had the smallest CPE-film value and a largest R_p value, where the CPE-film value was related to the corrosion area [35]. At the same time, the value of R_p was positively correlated with corrosion resistance [32], indicating that it had the best corrosion resistance under the condition of 10 μm, which was consistent with the results obtained by the polarization curve and EIS. Refinement of grain size might be the main reason for improving corrosion resistance, the crystal grains were remarkably refined, enabling the passivation film to be easily formed at the grain boundary by reducing the Cl- diffusion in the solution. However, the difference in surface roughness also affected the corrosion behavior of the material surface. Rough sample surfaces are more likely to form corrosion sites, which will cause corrosion. The increase of the scanning distance leads to a smaller surface roughness, which further improves the corrosion resistance.

4. Conclusions

This paper mainly studied the effect of femtosecond laser processing on the corrosion resistance of SLM-NiTi, mainly focusing on the effect of scanning space on the performance, and the conclusions are as follow:

(1) The scanning distance will affect the phase composition and surface roughness at the same time. After the femtosecond laser, the TiO₂ was mainly generated. When the scanning space was too small (1 μm), part of NiO was generated. The reduction of the scanning distance will result in an increase in surface roughness.

(2) Femtosecond laser shock peening could effectively increase the nanohardness of the material, and the nanohardness of the 10 μm sample was the highest, which can reach 1303 ± 40 HV. However, if the scanning distance is too small (1 μm), the hardness will be significantly reduced.

(3) The sample under the condition of scanning distance of 10 μm exhibited the most excellent corrosion resistance, and the I_corr could reach 1.45 ± 0.1 × 10⁻⁹ A·cm⁻². The grain refinement after femtosecond laser processing may be the main reason that affects the hardness and corrosion resistance.