Objective Aircraft must be regularly overhauled during service. To repaint and obtain a new and beautiful coating and detect internal defects in the aircraft body or other key structural components, the original paint layer must be removed from the fuselage. The removal of paint from the aircraft fuselage plays an important role in maintenance. As a green and highly efficient technique, laser depainting has garnered considerable attention in recent years. The process and mechanism of laser depainting both require further investigation. The most commonly used lasers for depainting include CO₂ and nanosecond lasers, and they have been discussed and analyzed with respect to cleaning efficiency and removal mechanism. In this study, the effect of laser at different time scales on the cleaning quality of laser paint removal from Al alloy surfaces was explored. The research results lay the foundation for the laser depainting of homemade large C919 aircraft body.

Methods Millisecond and nanosecond pulsed lasers were used to clean the epoxy paint coating on the 2024 Al alloy surface. The millisecond laser has a wavelength, maximum pulsed frequency, and maximum laser power of 10.6 μm, 5 kHz, and 2 kW, respectively. The nanosecond laser has a wavelength, pulsed frequency, pulse width, and maximum single pulse energy of 1064 nm, 2--50 kHz, 30--100 ns, and 100 mJ, respectively. During the cleaning process, the laser cleaning physical phenomenon was monitored online using a high-speed camera and a secondary light source with a wavelength of 808 nm. After the laser cleaning experiment, the macroscopic and microscopic morphologies of the sample were observed and investigated using the camera and scanning electron microscopy. Lastly, the physical mechanism of two types of laser paint removal was discussed by combining the thermoelastic vibration and heat conduction models.

Results and Discussions Both lasers can effectively remove the coating and obtain a clean substrate surface with appropriate process parameters. However, the paint removal characteristics of the two lasers were quite different. Nanosecond laser paint removal showed considerably higher energy efficiency than millisecond laser paint removal. A layer of charcoal ash remained on the sample surface after millisecond CO₂ laser cleaning, which was a combustion product of paint coating. The substrate surface did not melt during the cleaning process. In the case of millisecond laser depainting, a significant amount of black smoke was generated and the paint layer was melted during the cleaning process. During the nanosecond pulse laser cleaning process, strong plasma was observed and large pieces of paint were stripped from the substrate surface. After cleaning a paint layer using millisecond laser depainting, the substrate maintained a state of unmelted surface. However, some defect micropores and microcracks appeared on the surface, which were induced by the thermal stress of the substrate. In the case of nanosecond laser depainting, the substrate surface was completely remelted when the laser power was higher than 250 W. The microstructure formation on the surface was related to a fast cooling process. Under the action of the nanosecond laser, the temperature of the substrate increased to the melting point, thus yielding a thin melting metal. Because of the large cooling velocity, the existence time of the molten pool was short. Hence, the molten pool instantaneously solidified and did not have sufficient time to spread, finally forming the microstructure. The main mechanism of millisecond laser paint removal is vaporization and combustion reaction, while that of nanosecond laser paint removal is the thermoelastic vibration effect.

Conclusions To clean a 50-μm-thick paint layer using the millisecond laser, the optimized process parameters include pulse frequency, pulse duration, and laser power of 500 Hz, 0.6 ms, and 500 W, respectively. The optimized process parameters of the nanosecond laser for cleaning a 50-μm-thick paint layer include pulse frequency, pulse duration, and laser power of 5 kHz, 60 ns, and 200 W, respectively. Compared with the nanosecond laser, the millisecond laser induces lesser remelting owing to the deeper thermal diffusion after cleaning of the paint layer. The main mechanism of millisecond laser paint removal is vaporization and combustion reaction, while that of nanosecond laser paint removal is the thermoelastic vibration effect. The energy efficiency of nanosecond and millisecond laser depainting is 300 and 5 J/mm ³, respectively. The difference in the energy efficiency is attributed to thermal diffusion depths and cleaning mechanisms of the two lasers.