The effect of lasers on mechanical properties, control of coloring, and exploration of new coloring processes has been explored in recent years. However, limited research has been conducted on the effect of fluctuations in the process parameters on the stability of titanium alloy surface coloring. In this study, a 355 nm wavelength ultraviolet nanosecond laser is used to investigate the effect of laser process parameter fluctuation on the stability of the titanium surface coloring, to determine the reasons for the formation of different colors from the perspective of thin-film interference, and to explore the effect of different laser process parameters on the surface morphology and elemental composition of the colored samples.
The matrix method was adopted to design different laser process parameter ranges, determine the parameter range of stable color formation, and prepare stable colors such as orange, golden, blue, green, purple, and other colors on the sample. The CIE1976 standard chromaticity system of the International Commission on Illumination was used to calibrate the color samples based on the stable colors formed by laser process parameters, and the variation intervals of the scanning speed, laser power, and hatching distance were set according to gradients of equal value or equal ratio, respectively. The color stability of each sample was tested under different process parameter changes using a spectrophotometer. Laser confocal microscopy was used to observe the microscopic topography of samples of different colors, and the coloring stability of each color sample under different process parameters was examined. The samples were analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and the change characteristics of the samples were further explained based on the microstructure and elemental composition of the samples. X-ray diffraction analysis was performed on each colored sample and the titanium alloy substrate to analyze whether the laser action formed a diffraction peak on the surface of the sample that was different from that of the titanium alloy substrate.
Various color samples, such as orange, golden, blue, green, and purple, are prepared on the titanium alloy using 355 nm wavelength ultraviolet nanosecond laser processing equipment. The stable colors of the titanium alloy are achieved under a relatively low accumulated fluence, as shown in Table 2. This paper quantitatively analyzes the three laser-induced process parameters through a color difference experiment, examining the effect of fluctuations in the power, scanning speed, and hatching distance within a small range on the color of the titanium alloy surface. The change in hatching distance has the greatest impact on coloring stability, as shown in Fig 6. Microstructural observation and elemental composition analysis of the prepared samples of different colors are carried out. From the perspective of the microstructure, a comparison of the structure and thickness of the oxide film and the proportion of the element content of each color sample are analyzed to illustrate the changing characteristics of the color.
By laser induction of the titanium alloy using 355 nm wavelength ultraviolet nanosecond processing equipment, two different microscopic morphologies are observed under a laser confocal microscope after the laser acts on the surface of the titanium alloy. When the cross-section is analyzed and tested by SEM, a two-layer structure with noticeable structural differences is found. In the XRD analysis, the color samples form diffraction peaks different from those of the substrate, confirming that the superstructure observed under SEM analysis is a titanium oxide layer. It is further confirmed that the newly formed diffraction peak has the structure of TiO/TiO2 by searching for relevant information. In the EDS elemental composition analysis test, the samples formed at low speed, low power, and high defocus amounts, such as purple and green, are compared with samples showing orange, blue, and golden colors formed at high speed, high power, and low defocus amount. The former samples form an oxide layer that is thicker, higher content in oxygen, and lower content in carbon. Under the effect of a nanosecond laser, the different samples formed on the surface of the titanium alloy are directly related to the thickness of the oxide film. The thickness of the oxide film results from the combined effect of material heating and surface oxidation under the thermal effects of different laser process parameters.
