Objective A paint layer can be applied to metals to enhance their surface characteristics. However, in many cases, paint often needs to be removed from the metal surface because of its potential damage to the environment. Paint removal using laser provides several advantages over the conventional techniques such as mechanical or chemical cleaning. Specifically, an accurate removal area, minimal detrimental effects to the substrate, reduction in contaminated waste, and fast cleaning rate are the key favorable factors in paint removal using laser. Several studies have been published in the literature that dealt with the effect of different process parameters for paint removal including the change of the temperature. Other processes that affect the relationship between the laser beam and paint have not been determined. In the present study, we report a novel type of research methods to understand the detailed micro process of paint removal, such as the plasma effect near the paint surface and the microscopic destruction process in the paint. We expect that our basic strategy and findings can help in understanding the characteristics and mechanisms of paint removal.

Methods In this work, 2024 aluminum alloy and polyacrylate resin-based paint were employed. A laser paint-cleaning test was carried out using pulsed laser with a wavelength of 1064nm and a pulse width of 1μs. In the experiment, the focal spot diameter of the Gaussian beam was approximately 78μm. The whole apparatus was completely automatic, that is, a computer controlled the laser power, repetition rate, and scanning speed. The cleaning residues were deposited on a silicon wafer, which was located 17 mm from the surface of the sample, as shown in Fig.1. The effects of scanning speed, pulse frequency, and laser power on the laser-cleaning quality were investigated. According to the morphology and element-valence changes in the cleaned surface and by combining the morphology of the cross section of the paint and particles generated during the cleaning process, the underlying process and mechanisms of the paint removal using pulsed laser were thoroughly investigated. Simultaneously, the temperature and stress-field distributions of the finite-element simulation using COMSOL Multiphysics software were also used for the auxiliary analysis.

Results and Discussions The paint in the experiments could be removed using pulsed laser. The laser-cleaning quality first increased and then decreased (Fig.3, Fig.4) and the surface roughness first decreased and then increased (Table 2, Table 3) with the increase in the scanning speed and pulse frequency. Furthermore, the laser-cleaning quality increased (Fig.5) and the surface roughness first decreased and then increased (Table.4) when the laser power increased. The morphologies and elements of the cleaned-surface study illustrate that the laser plasma and thermal combustion were affected by the absorption of laser energy by the paint during the laser-cleaning process (Fig.6). In addition, the X-ray photoelectron spectroscopy analysis indicates that C—H, C—C, O—H, C=O, C—O, and other covalent bonds in the polymer molecular chain of the paint were broken under the action of the pulsed laser (Fig.7). During the cleaning process, a layered structure was formed in the paint. Obvious cracks appeared that were parallel to the surface of the paint at the fracture section, which extended inside the paint. This result indicates the presence of a mechanical effect perpendicular to the surface of the paint. The cohesion of the lacquer was destroyed, which damaged the paint between the layers, and the paint layer was ejected (Fig.8). The study of the collected particles illustrates that the presence of mechanical mechanisms in the paint-damage process, such as vibration and impact, and the vaporized paint nucleated and grew in the high-energy limited area formed by the pulsed laser, which resulted in the formation of nanoparticles (Fig.9).

Conclusions In the present study, three different process parameters, namely, scanning speed, pulse frequency, and laser power, influence the laser paint-cleaning quality at different levels. The laser-cleaning quality first increases and then decreases with the increase in the scanning speed and pulse frequency and increases as the laser power increases. The laser-cleaning quality is good when the process parameters are as follows: laser power=16.5W, scanning speed=600mm/s, and pulse frequency=30kHz. Under different process parameters, the main mechanism of the laser paint removal is different. With regard to the analysis characterization, we conclude that the effect of the cohesive-failure and crack-propagation-fracture mechanisms is more efficient than the chemical bond-fracture combustion.