Laser Melting Treatment on NiTi Alloy and Its Biological Corrosion Properties

NiTi alloy with equal atomic ratio has been widely used in biomedical field because of its excellent mechanical properties and shape memory effect. However, NiTi alloy can be corroded and released harmful Ni2+ ions in human body. In this paper, laser melting was conducted on the surface of the NiTi alloy to enhance the corrosion resistance in simulated body fluid (SBF). The microstructure, phase structure of NiTi alloy modified by laser melting treatment with various process parameters, as well as the biological corrosion properties of the melted layer in SBF solution were studied. The surface physical properties of the molten layer, such as contact angle and hardness were also tested. The NiTi alloy specimens were cut into the size of 10 mm×10 mm×2 mm by wire electrical discharge machining, polished by sandpaper, and then cleaned in deionization water. Laser melting was performed on the surface of the NiTi alloy specimens by Diode laser pumped ultraviolet laser (MP101). Microstructure and corrosion morphology were observed by using optical microscope (OM, AxioImager A2) and scanning electron microscope (SEM, FEI Quanta 250) with an Oxford Instrument INCA EDS analyser capability. The EDS analysis has been performed with an acceleration voltage of 10 keV and Cu calibration. The water contact was tested using an angle measuring instrument (DSA20). The surface hardness of the specimens was measured by a Vickers micro-hardness tester (HV1000). The phase structure of the specimens was analyzed by X-ray diffractometer (XRD, D8 Discover). The specimens were sealed with epoxy resin, exposing a surface area of 1 cm2 and immersed in SBF solution at 37 ℃ for 168 h for the corrosion test. The concentration of Ni2+ ions in SBF solution was measured by inductively coupled plasma emission spectrometer (ICP, Optima 7000 DV) with 0.1 mg/L Ni standard solution. The electrochemical experiment was carried out using electrochemical workstation (CHI 660D) in SBF solution at 37 ℃. A three-electrode system was adopted, in which the specimen was the working electrode, the platinum electrode was the auxiliary electrode, and the reference electrode was the saturated calomel electrode. The results show that a melted layer with thickness of about 90-150 μm can be formed on the surface of NiTi alloy after laser melting treatment. The melted layer is mainly composed of TiO2, β phase and tiny amounts of TiO; the substrate is mainly composed of acicular and flake-like β phase and a small amount of striped R phase. The average surface micro-hardness of the alloy was increased by 153-279HV compared with that of the substrate; the alloy surface changed from hydrophilic to hydrophobic. Compared with the untreated specimen, the corrosion potential of the specimens after laser melting in SBF solution were positively shifted 435 and 413 mV, respectively; and the corrosion current density were decreased by 83% and 62%, respectively; the concentration of Ni 2+ ions in SBF solution was decreased by about 1/3 after 168 h immersion. The pitting corrosion in SBF solution could be impeded effectively by the oxide film and the melted layer on the surface of the NiTi alloy which were modified by laser melting treatment. © 2023, Chongqing Wujiu Periodicals Press. All rights reserved.