Results and Discussions As the pulsed laser scanning speed is increased, the surface oxide film and oil stain are gradually removed, and the original dark-gray surface of the TC4 titanium alloy becomes golden-yellow, then light-yellow, and finally silvery-white (Fig. 2). The microscopic morphology of the TC4 surface changes with increasing scanning speed. When the scanning speed is low, ridge structures and cracks form on the surface, along with severe thermal oxidation. The surface becomes smoother and the thermal oxidation decreases with increasing scanning speed, but the effect of removing the oxide film is reduced at a large scanning speed (Fig. 3). The scanning speed affects the number of pulses in the unit spot area, the cumulative input energy, and the overlap rate of the spot; therefore, pulsed laser at different scanning speeds produces different surface morphologies of the titanium alloy substrate (Fig. 4). With an increase in scanning speed, the O content on the TC4 surface first increases then decreases, and then increases, while the Ti content shows the opposite trend. The main component of the oxide film on the TC4 surface is TiO2. When the scanning speed is 500 mm/s, TiO forms on the surface by thermal oxidation. When the scanning speed is increased to 9000 mm/s, the surface oxygen content is the lowest, the substrate material is exposed, and the cleaning quality is the best (Figs. 5, 6, and 7). The roughness of the TC4 surface first increases and then decreases with increasing scanning speed, which is closely related to the changes in surface morphology (Fig. 8). Due to the formation of a remelted layer with grain refinement and acicular martensite α′ and laser shock strengthening effect on the surface, the hardness of the TC4 surface increases after laser cleaning, and then gradually decreases with increasing scanning speed (Figs. 9 and 10). Compared to the original, the laser-cleaned TC4 surface has increased corrosion resistance due to the remelted layer, reduced roughness, and the formation of a dense oxide film by slight oxidation (Figs.11 and 12).
TC4 titanium alloys have excellent overall performance and are widely used in the manufacture of key components for aerospace and military hardware. However, the oxide film on the surface can adversely affect its formation and performance, which results in porosity in the weld, reduced mechanical properties, reduced electrical conductivity, and weakened bonding of the plating or coating with the substrate. Therefore, the oxide film must be removed from the titanium alloy. Traditional methods for cleaning the oxide film on the surface of titanium alloys have many limitations. However, laser cleaning technology compensates for the shortcomings of traditional cleaning approaches with features such as being green, being efficient, and causing low damage. Several studies have discussed the influence of laser energy density on the cleaning effect. In this study, the effects of scanning speed on the cleaning effect of the oxide film on TC4 titanium alloy surfaces are studied. The results can help optimize the laser cleaning process and improve the cleaning quality and efficiency of titanium alloys.
In this study, a nanosecond pulsed fiber laser is used to remove oxide film and oil stain from a TC4 titanium alloy surface at different scanning speeds. The surface morphology of the cleaned TC4 titanium alloy is observed by scanning electron microscopy (SEM). The effect of different scanning speeds on the surface chemical composition and chemical bonds of the sample surface is analyzed by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The surface roughness is measured using a laser confocal microscope and the hardness of the surface is measured using a Vickers hardness tester with a load of 200 g. Finally, the corrosion resistances of TC4 titanium alloy before and after laser cleaning are measured on an electrochemical workstation.
The scanning speed has a significant effect on the cleaning of oxide film and oil stain on the surface of TC4 titanium alloy. When the scanning speed is 500 mm/s, the surface becomes golden-yellow due to thermal oxidation, and significant remelting and cracking occur. The O content (mass fraction) reaches 24.86%, and the oxide TiO forms. With an increase in scanning speed from 3000 mm/s to 9000 mm/s, the original dark-gray surface of TC4 becomes silvery-white, and the amount of thermal oxidation is gradually reduced. When the scanning speed is 9000 mm/s, the O content(mass fraction) reaches its lowest value of 4.54%, and the Ti content (mass fraction) reaches its highest value of 84.2%. The oxide film and oil stain are removed, and the substrate material is exposed. The surface roughness of TC4 titanium alloy gradually decreases with increasing scanning speed. Laser cleaning technology can improve the hardness and corrosion resistance of a TC4 surface, and the enhancement effect is affected by the scanning speed.
