Previous studies on laser paint removal from aircraft skin mainly focused on the optimization of laser paint removal parameters and the improvement of efficiency. However, to improve the reliability and safety of laser paint removal technology and promote its application in the field of aviation engineering, the potential impact of laser energy absorption on the performance of the substrate during the laser paint removal process must be clarified. According to Fourier’s law of heat conduction and the law of conservation of energy, laser paint removal generally affects the properties of the surface material of the matrix, whereas surface integrity plays an important role in material performance and service life. Therefore, this study aims to investigate the effect of laser paint removal on the surface integrity of aircraft aluminum alloy skin substrates, including the substrate surface morphology and roughness, microhardness, and microstructure.

In this work, paint removal experiments on 2024 aluminum alloy aircraft skins at energy densities of 12.89-25.48 J/cm² were performed using pulsed fiber laser. Then, the qualities of paint removal at different energy densities were analyzed via trinocular continuous zoom stereo microscopy (SM), scanning electron microscopy (SEM), laser confocal microscopy (LSCM), microhardness tester, and X-ray diffractometer (XRD). Subsequently, changes in the surface morphology and roughness of the substrate after removing the coating, as well as the microhardness and microstructure versus energy density, were investigated. Finally, the temperature field distributions at different energy densities were studied using finite element analysis. The effects of the temperature field on paint removal and substrate surface integrity were further discussed. Consequently, the internal relationship between the evolution of the substrate microstructure and hardness change during laser paint removal was revealed.

When the energy density is relatively high (≥22.90 J/cm²), the paint layer is completely removed (Fig. 5). Moreover, the surface roughness (S_a) and peak valley height difference (PVHD) of the substrate gradually increase as the energy density increases (Figs. 6-7). Meanwhile, under high energy density conditions, the refinement of sub-grains on the material surface, an increase in dislocation density (Fig. 9), and precipitation of strengthening phase σ (Al₅Cu₆Mg₂) are observed (Fig. 10). As a result, a small increase in the surface hardness occurs (Fig. 8). At the energy density of 22.90 J/cm², the PVHD is 8.28 μm. Compared with that of the original sample, the microhardness increases by 2.8%, which meets the requirements of the aircraft skin recoating process and application standards. Meanwhile, the calculation results show that the temperature at the junction of the paint layer and the substrate is 415.46 °C. The paint layer is then completely ablated and gasified. Because the temperature of the substrate surface is lower than its melting threshold (500 °C) (Fig. 13), thermoplastic deformation does not occur. The best cleanliness and surface integrity are obtained at 22.90 J/cm².

After laser paint removal, the temperature of the substrate surface increases rapidly with the increase in energy density owing to the thermal effect of the laser. When the energy density increases to 22.90 J/cm², although the roughness of the aluminum alloy substrate surface increases slightly, it still can meet the roughness requirements of the surface coating process. In addition, the surface layer of the substrate hardens owing to the plastic deformation of the material, precipitation of the strengthening phase, and refinement of subgrains. At the energy density of 22.90 J/cm², the hardness of the base material increases by 2.8%, meeting the requirement that the property change of the material after paint removal should not exceed 5% in the aircraft skin material standard.