Results and Discussions The paint-stripping effect improves gradually as laser energy density, single pulse energy, and laser cleaning intensity increase. However, the surface of the sample will have brownish yellow striped topography if the cleaning density is too high, causing ablation damage to the titanium-alloy surface (Figs. 3 and 4). As the laser cleaning speed decreases, heat accumulates on the surface of the paint layer and the paint removal effect improves incrementally (Figs. 5 and 6). When the energy density is 2.22-4.00 J/cm², the surface roughness increases and then decreases with increasing energy density. However, when the energy density exceeds 4.44 J/cm², the excessive laser-pulse energy damages the substrate, and the surface roughness increases again (Table 1). The surface roughness increases and then decreases as the laser cleaning speed decreases from 9 mm/s to 3 mm/s (Table 2). The findings from the phase analysis indicate that the surface composition of the sample after laser cleaning comprises only Ti and Ti₆O, without CaCO₃ and TiO₂, which indicates that the paint layer is cleaned when the laser energy density is 4.00 J/cm² and the cleaning speed is 3 mm/s (Fig. 7). The average Vickers hardness of the cleaned titanium-alloy surface is 368.74 HV, which is approximately 7.4% greater than that of the original substrate [Fig. 8(b)].

Typically, a protective coating is sprayed onto the surface of materials to increase their service life, reduce the cost of use, and improve the aesthetics of the application. In complex environments and long-term application conditions, however, the paint layer will deteriorate, necessitating its removal in order to routinely inspect the substrate surface for detects. Traditional paint-stripping techniques have their own drawbacks, and laser cleaning technology has garnered more attention due to its benefits, including high efficiency and environmental friendliness. However, there are few reports on the laser cleaning technology of titanium-alloy surface-paint layers and the surface properties of the cleaned material. Moreover, the quality of laser cleaning appears to be highly dependent on the choice of process parameters. Herein, the effects of laser energy density and laser cleaning speed on the cleaning effect are investigated, and the optimal laser-paint removal process parameters are determined using phase analysis. In addition, the Vickers hardness of the substrate cleaned by laser is evaluated.

The epoxy zinc yellow-paint coated layer on the surface of TC4 titanium alloy was experimentally investigated using a pulsed fiber laser with a flat 1.5-mm² top spot. The pulsed laser had a pulse width of 70 ns, pulse frequency of 10 kHz, a galvanometer scanning speed of 3000 mm/s, a laser energy density of 2.22-4.44 J/cm², and a laser cleaning speed of 3-9 mm/s; accordingly, the single cleaning was performed in the experiment. The surface morphologies and roughnesses of the samples were measured by a laser confocal microscope, and the effects of laser energy density and cleaning speed on the removal of the paint layer were investigated. In addition, the laser energy density was determined by changing the laser power; an X-ray diffraction (XRD) was performed using a diffractometer to characterize the phase transition of the laser-cleaned paint layer to analyze the cleaning quality; Vickers hardnesses before and after paint removal were measured using a Vickers hardness tester, and the effect of laser cleaning on the surface properties of the substrate was investigated.

With an increase in laser energy density or a decrease in cleaning speed, the cleaning effect is enhanced gradually. However, if the laser energy density is too high, excessive cleaning occurs. Accordingly, when the laser energy density is 4.44 J/cm² and the cleaning speed is 3 mm/s, the substrate is damaged. The damage is observed as the surface of the substrate becomes brownish yellow and the surface roughness increases. Thus, the laser energy density and cleaning speed have a significant effect on the surface roughness of the cleaned sample. In addition, the surface roughness increases and then decreases as the laser energy density increases or cleaning speed decreases. Accordingly, at an energy density of 4.00 J/cm², the surface roughness of the cleaned sample is maximum (S_a = 24.956 μm) and minimum (S_a = 2.082 μm) for cleaning speeds of 6 mm/s and 3 mm/s, respectively, which is comparable to the roughness of the original substrate surface. Additionally, when the energy density is 4.00 J/cm² and the cleaning speed is 3 mm/s, CaCO₃ component is not observed in the phase analysis of the cleaned surface, indicating that the paint layer has been entirely removed. Moreover, the average Vickers hardness of the cleaned titanium-alloy surface is greater than that of the original substrate, suggesting that the surface hardness of the TC4 titanium alloy can be enhanced by removing the paint layer with laser.