The peak power densities were 4.72, 9.43, 14.15, 18.87, and 23.58 MW/mm² for the experimental single pulse energy range of 5 mJ–25 mJ, while the cleaning effect became more and more prominent, as shown in Figure 3 below.

When the single pulse energy reaches 10 mJ, it can be seen that part of the yellow primer has been completely removed, and the white substrate begins to appear, indicating that the cleaning threshold of the paint layer has been reached at this time, which is 9.43 MW/mm²; Upon scaling the single pulse energy to 20 mJ, an entire surface paint layer of the aircraft skin can be effectively and wholly removed, manifesting an optimal cleaning outcome bereft of observable impairments. However, as the laser energy escalates to 25 mJ, the cleaning domain begins to manifest evident instances of pinpoint and streak-based damage. This outcome stems from the peak power density surpassing the threshold for laser cleaning-induced impairment.

A confocal microscope was employed to scrutinize the post-cleaning surface, with the outcomes presented in Figure 4A below. Upon using a single pulse energy of 5 mJ, the cleansed paint layer surface manifested a densely distributed, furrow-like configuration. No accumulation of molten residue was evident, leading to a preliminary inference that the expulsion of the paint layer ensued due to the thermal vibrational effect during laser cleaning. At 10 mJ energy, the thermal vibrational impact intensifies, culminating in a marked augmentation of the paint removal area. Upon reaching a single pulse energy of 20 mJ, the cleansed substrate attains an unblemished state. However, at 25 mJ energy, the substrate’s surface starts to exhibit a pitted and structurally compromised aspect.

The surface roughness of the treated substrates measured 3.10, 1.46, 1.30, 1.19 and 1.99 μm at energy levels ranging from 5 mJ to 25 mJ. It is evident that the surface roughness initially diminishes and subsequently escalates with rising laser energy. Notably, the optimal surface roughness is achieved at an energy of 20 mJ. Upon analysing the points corresponding to 20 mJ and 25 mJ regarding carbon elements beneath the surface, it is observed that the carbon element content increased from 46.74% to 49.52%. This augmentation suggests that excessive energy may lead to substrate impairment, thereby diminishing the efficacy of laser cleaning.

The investigation employed scanning electron microscopy and confocal microscopy to scrutinize the microscopic attributes of the laser-treated aluminum alloy surface. The outcomes are presented in Figure 4B below. Figures a–c are aligned with cleaning angles of 0°, 11° and 21°, sequentially. Upon analyzing the microscopic morphologies, it becomes apparent that with an increment in the cleaning angle, a progressively intricate groove-like structure emerges on the substrate’s surface, while the residual paint fragments decrease incrementally. The three-dimensional morphogram highlights that an elevation in the angle contributes to the escalation of the extreme surface height, surging from 4.978 μm to 6.043 μm. This inclination results in the formation of a “ridge” pattern on the surface, with the corresponding three-dimensional profile crest becoming more comprehensive and expanding in range.

The surface’s elemental distribution was acquired through EDS analysis, as illustrated in Figure 5 below. Figures a–c correspond to cleaning angles of 0°, 11°, and 21°, respectively. The analysis revealed that the carbon element content on the surface exhibited a gradual reduction with an augmentation in the cleaning angle. The mass percentage exhibited an 8.75% decline, while the atomic proportion decreased by 9.79%. Conversely, the content of surface oxygen and aluminum elements witnessed an increment. The mass percentages for oxygen and aluminum increased by 3.92% and 4.84%, respectively, while the atomic ratios augmented by 5.3% and 4.49%, correspondingly.

The surface roughness along with the associated microscopic morphology of the cleaned skin under distinct laser angles is depicted in Figure 6A below. Notably, the surface roughness reaches its zenith at a vertical incidence angle, indicating the attainment of the laser cleaning’s damage threshold. As the angle of incidence progressively elevates, the surface roughness tends to diminish initially and then subsequently expand. The extent of substrate deformation becomes more conspicuous, progressively forming a rippled surface structure. Remarkably, when the cleaning angle attains 11°, the surface roughness reaches a nadir of 1.25 μm.

Throughout the cleaning process, alterations in the extent of defocusing inherently influence the quality of cleaning outcomes. Employing a confocal microscope, the microscopic surface was meticulously examined, and the outcomes are graphically depicted in Figure 6B below. Remarkably, microscopic morphology elucidates that with an increment in the degree of defocusing, the attenuation of laser peak power density undergoes a progressive amplification. Consequently, this engenders an incremental buildup of residual paint layers upon the surface, leading to a deterioration in the efficacy of laser cleaning. Furthermore, at an out-of-focus distance of 30 mm, the surface adopts a planar configuration devoid of groove-like structures, simultaneously achieving a minimal surface roughness of 1.15 μm—a manifestation of optimal laser cleaning efficiency. In stark contrast, as the out-of-focus distance extends to 90 mm, a sudden surge in surface roughness and height disparity is observed, escalating to 3.51 μm and 34.454 μm, respectively. Notably, the emergence of groove-like structures becomes apparent, ultimately culminating in a perceptible degradation of cleaning results.

A distinct overlap region occurs between adjacent laser spots, resulting in multiple cleaning interactions on the mask surface within this zone. This phenomenon significantly influences the overall cleaning outcome. While maintaining consistent experimental parameters, the study examines alterations in substrate surface micro-morphology and roughness when subjected to laser cleaning cycles ranging from 1 to 5 times. The outcomes are illustrated in Figure 8A below.

The findings reveal that after a single instance of laser cleaning, the skin’s surface remains free from apparent structural damage, signifying optimal cleaning efficiency. Furthermore, with escalating cleaning repetitions, the diameter of the damaged structures expands, exacerbating the degree of impairment. Across the range of 1–5 cleaning cycles, the associated surface roughness values are 1.81, 1.97, 3.18, 3.20, and 3.83 μm, respectively. This trend underscores that heightened cleaning frequency not only amplifies surface roughness but also intensifies the extent of surface damage.

As depicted in Figure 8B, when the laser energy is set at 470 mJ, a singular laser cleaning operation results in an undisturbed surface with no indications of melted damage; the surface exhibits a sleek appearance. Nonetheless, with a progressive increment in pulse count, the surface’s disparity in height augments from 33.607 μm to 47.001 μm. Notably, following five consecutive cleaning cycles, a heightened presence of melt-induced damage becomes apparent on the skin’s surface, coupled with an emerging trend of substrate detachment.

Laser cleaning was performed for 1, 3, and 5 cycles, with the corresponding surface elemental distribution depicted in Figure 9. It is evident that the carbon element content on the surface exhibits an initial increase followed by a subsequent decline with varying pulse numbers. Notably, in comparison to a single laser cleaning session, the atomic percentage of carbon element reduces by 2.81%, oxygen element by 3.76%, while the atomic percentage of aluminum element increases by 6.59% following five successive cleaning cycles. These outcomes underscore that augmenting the frequency of cleaning sessions can effectively enhance the overall cleaning efficiency.

The microscopic morphology of the cleaned surface was examined at single pulse energies of 270, 370, 470, 570, and 670 mJ, as depicted in Figure 10A below.

The investigation revealed that a pulse energy of 270 mJ initiated damage to the skin substrate, manifested as densely distributed, molten-sputtering, crater-like structures. As energy escalated to 570 mJ and 670 mJ, surface damage experienced substantial augmentation, rendering the system chaotic and challenging to discern, with the surface cloaked in black residue. The 3D morphology diagram unveiled a progressive emergence of skin surface damage structures as energy levels rose, concomitant with a gradual increment in surface roughness. Notably, the minimum surface roughness was attained at 270 mJ, measuring 3.07 μm.

In addition, at laser energy levels of 270, 470, and 670 mJ, the surface height differences are 29.912, 33.438, and 52.283 μm, respectively. These results indicate that surface damage experiences a sudden increase once the energy surpasses a specific threshold value, as illustrated in Figure 10B below.

The elemental distribution on the skin’s surface is depicted in Figure 11 below. The observations reveal a slight initial rise followed by a significant decline in surface carbon content as energy levels increase. Simultaneously, the content of elemental oxygen exhibits a gradual reduction, while the content of elemental aluminum experiences a noteworthy escalation. In terms of atomic percentages, a collective decrease of 1.58% and 0.89% is observed for carbon and oxygen elements, respectively, alongside a rise of 2.46% for aluminum elements. These outcomes suggest that elevated energy levels might induce the disruption of the alumina film, thereby exposing the substrate and engendering alterations in surface elemental composition.

A comparison of the effects of different cleaning times and energies on the microscopic morphology of the skin surface post-cleaning is presented in Figure 12 below. Keeping other variables constant, graphs (a), (b), and (c) correspond to laser energies of 270, 470, and 670 mJ, respectively, while graphs (d), (e), and (f) represent cleaning times of 1, 3, and 5 times, respectively. The findings indicate that as energy gradually increases, the substrate surface damage structure remains relatively unchanged, yet the heat-affected area due to laser cleaning significantly expands. Therefore, it can be concluded that an increase in the number of cleaning cycles does not notably influence changes in the laser cleaning area, but it does lead to a significant increase in surface damage.

Laser cleaning experiments were conducted on aircraft skins of different colors using energies ranging from 270 mJ to 670 mJ. The results obtained through confocal microscope observation are presented in Figure 13 below.

Within the energy range of 270 mJ–470 mJ, complete removal of the surface paint layer from the green aircraft skin was not achieved. Significant paint residues were visible on the cleaned skin’s surface. Thorough cleaning was achieved at an energy level of 570 mJ, while substrate damage occurred at 670 mJ. In contrast, complete surface cleaning was achieved at a laser energy of 270 mJ when cleaning the blue skin. With increasing energy, surface damage gradually escalated. Across laser energies of 270 mJ–670 mJ, the associated surface roughness for green skin cleaning stood at 4.2, 3.65, 3.30, 2.02, and 2.66 μm, respectively. For blue skin cleaning, the corresponding surface roughness values were 3.07, 3.18, 3.14, 3.28, and 3.31 μm. To further comprehend the influence of paint color on cleaning outcomes, subsequent experiments were undertaken.

An infrared grating spectrometer was employed to measure the reflected light. Stronger received reflected light indicates lower absorption capacity of the paint layer towards the laser, while weaker received reflected light signifies stronger absorption capability of the paint layer towards the laser. To ensure the efficiency and precision of the infrared spectrometer acquisition, the laser’s repetition frequency for this experiment was set to 200 Hz, ensuring the detection probe remained consistently positioned for accurate readings.

Twenty laser cleaning experiments were conducted on both green and blue skins, respectively, with the spectrometer probe collecting reflected light waves at the same position. Finally, the spectra depicting absorbed light intensity versus laser wavelength were plotted in Figure 14 below. During the laser cleaning experiment on the green aircraft skin, the reflected laser light intensity reached 10,720 cd. Conversely, in the case of the blue aircraft skin, the reflected light power was only 6,193 cd, highlighting a nearly twofold difference between the two scenarios.