Civil aircrafts operate under severe conditions, leading to issues such as peeling and cracking in the aircraft paint layer. This makes localized or comprehensive paint removal and surface maintenance pivotal during C-check or D-check procedures. Current paint removal methods, such as manual grinding and chemical stripping, are widely used. However, they have significant shortcomings. Manual grinding often lacks precision and can damage the aircraft surface, while chemical stripping is complicated and environmentally detrimental. These challenges hinder sustainable and efficient advancements in the civil aviation industry. Laser paint removal has emerged as a promising solution, offering high precision, reduced pollution, and automation possibilities. It is rapidly gaining global attention. However, a knowledge gap exists in understanding the exact mechanism of paint removal during laser ablation, particularly regarding the impacts of single-pulse thermal stress and plasma-induced removal. This study bridges this gap by first determining the vaporization point and strength limit of the paint layer. Then, single-pulse laser ablation simulations are compared with experimental results to better understand thermal stress paint removal during laser cleaning. This research sheds light on paint removal mechanisms and evaluates the impacts of varying scanning speeds on laser paint removal efficiency. Hence, the aim is to offer valuable insights and references for enhancing the use and development of laser paint removal techniques on civil aircraft skin.
In this study, the vaporization point and strength limit of the paint layer are first determined via thermogravimetry and stress-strain experiments. Subsequently, a finite element analysis of the single-pulse laser ablation-thermal stress paint removal process is conducted using the COMSOL software. Experiments on laser paint removal, both single-pulse and multi-pulse, at varying scanning speeds (ν=1000, 900, 800, and 700 mm/s), are performed on the composite paint system of the LY12 aluminum alloy substrate using nanosecond pulsed fiber lasers. Then, a white light interferometer (WLI) is employed to inspect and analyze the crater profile created by the single-pulse laser. An optical microscope (OM) is used to characterize the resulting surface and cross-sectional morphology from the multi-pulse line scanning laser paint removal. Additionally, a scanning electron microscope (SEM) and an energy-dispersive X-ray spectroscope (EDS) are utilized to analyze the microstructure and compositional changes on the cleaned surface after the paint is removed.
After the completion of single-pulse laser irradiation (t=200 ns), the paint ablation process does not cease. The accumulated heat causes the surface temperature of the residual paint layer to further increase until t=750 ns. When the temperature falls below its vaporization point, the ablation paint removal process concludes (Figs. 6 and 7). At this moment, the action depth of thermal stress generated by the coupling of the temperature field on the surface of the residual paint layer gradually increases from the bottom to the edge of the crater, while the corresponding values progressively decrease (Fig. 9). The maximum value (Fig. 9, σmax=2.7×107 Pa) approaches the strength limit of the paint layer [Fig. 11(a), σ=2.68×107 Pa], leading to a U-shaped stress-damage zone on the surface of the residual paint layer (Fig. 12). This results in physical damage, such as delamination and fragmentation of the residual paint layer (Fig. 13). During the laser ablation process, both plasma impact and shielding effects coexist. The plasma impact causes the actual width of the ablated crater to be larger than the simulated results, while the shielding effect results in the experimental ablation depth of the crater being smaller than the simulated results (Fig. 14). In the process of single-pulse laser paint removal, the ablation and plasma paint removal effects gradually intensify before the laser irradiation ends and then gradually weaken after the irradiation completes. However, the thermal stress paint removal effect remains unchanged (Fig. 15). In the multi-pulse surface scanning laser paint removal experiments, the actual spot overlap rate is higher than the theoretical value (Fig. 17), resulting in the removal depth of the paint layer being greater than that of the single-pulse results. As the scanning speed gradually reduces, the removal effect of the paint layer gradually enhances due to ablation, the plasma effect gradually weakens, and the depth of paint removal and the deposition amount of β-type copper phthalocyanine along with the functional oxidized particles gradually increase [Figs. 16(a), (c), (e), and (g), and Table 10].
In this study, the single-pulse nanosecond laser thermo-mechanical coupling paint removal transient process is simulated using the COMSOL software. The results show that the behavior of ablative removal of paint layers does not finish at the end of one pulse cycle, and the heat accumulated on the cleaning surface extends the ablation process by 550 ns. The simulated thermal stress value is slightly larger than the experimentally determined tensile limit of the paint layer. This causes the residual paint layer on the cleaning surface to produce a delamination and cracking zone ranging from 0.6 μm to 2.8 μm. The laser ablation paint removal process triggers plasma impact and shielding. This makes the experimental crater paint removal width larger and the depth smaller than the simulation results. In the single pulse laser paint removal process, the ablation and plasma paint removal effects gradually intensify before the laser irradiation ends and then gradually weaken after the irradiation ends, but the thermal stress paint removal effect remains unchanged. As the scanning speed gradually decreases, the ablative paint removal effect strengthens, the plasma effect weakens, the thickness of the paint layer removal increases, and the amount of functionally oxidized particles deposited on the paint layer also increases. When ν=1000 mm/s, the topcoat is partially removed and the primer is slightly damaged. The topcoat is removed cleanly, and the primer is partially removed when ν=900 mm/s. However, for ν=800 mm/s and 700 mm/s, the area and depth of the residual primer continue to decrease with the reduction in the scanning speed, and the oxidized film is exposed.
