Corrosion and Wear Performance of Aircraft Skin after Laser Cleaning Guodong Zhu a, Shouren Wang a, *, Wei Cheng b, Yuan Ren b, Daosheng Wen a a School of Mechanical Engineering, University of Jinan, Jinan 250022, China b Qilu University of Technology (Shandong Academy of Sciences), Jinan250014, China *Corresponding author at: School of Mechanical Engineering, University of Jinan, Jinan 250022, China. E-mail address: me_wangsr@ujn.edu.cn (S. Wang). Abstract: This paper studies the laser paint stripping technology of the Boeing series of aircraft skins, which provides a reference for improving the efficiency of paint stripping and reducing the maintenance cost of existing aircraft skins. The paint stripping effect of the aircraft skin at a laser energy density of 2-6 J/cm2 was studied. The best cleaning parameter was 5 J/cm2 by SEM and EDS analysis. Aircraft have almost strict needs for safety, so if laser paint stripping technology uses widely in aircraft maintenance, nondestructive cleaning must achieve. The fretting friction and wear performance of aircraft skin rivet holes after laser paint stripping with different energy densities was studied, the friction and wear performance of other parts of the skin were studied and compared with mechanical grinding and laser paint stripping. The results showed that the laser did not reduce the friction and wear performance of any part of the aircraft's skin surface. The residual stress, micro- hardness, and corrosion performance of the aircraft skin surface after laser paint stripping were studied. Compared with the mechanical lapping and laser paint stripping method, it proves the laser will not reduce the hardness and corrosion resistance of the aircraft skin. However, after laser cleaning, the surface of the aircraft skin will produce some plastic deformation, which is the problem that the engineering department needs to pay attention to at present. © 2020 published by Elsevier. This manuscript is made available under the Elsevier user license https://www.elsevier.com/open-access/userlicense/1.0/ Keyword: Laser cleaning; Aircraft skin; Wear resistance; Hardness and residual stress; Corrosion resistance 1. Introduction Nowadays, civil aviation aircraft have become the primary means of transport, which can provide passengers with convenient services [1]. The maintenance of aircraft is essential for the safety of flight [2-4]. Commercial aircraft must be inspected every six years, or after a cumulative trip of 24,000 hours, or after 2,000 takeoffs and landings [5]. During maintaining the aircraft, the paint on the surface of the aircraft needs to remove altogether, and the skin substrate must be inspected for corrosion defects and fatigue cracks to avoid aircraft accidents [6-7]. Therefore, during the paint stripping of the aircraft skin, special attention must be paid to the cleaning method to prevent damage to the substrate. The traditional paint stripping methods mainly include mechanical process, chemical method, and ultrasonic cleaning method [8-10]. Although their technology is mature, there are still many shortages. For example, mechanical cleaning methods easily cause damage to the substrate, chemical cleaning methods cause environmental pollution, and the efficiency of ultrasonic cleaning has limited its large-scale application. Laser cleaning, with its green, efficient, broad applicability, and noncontact [11-13], has been used in recent years to remove paint from aircraft skins. Laser paint 1
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stripping is one of the most promising applications of laser cleaning technology. The process is the interaction between light and matter. When the laser destroys the binding force amid the substrate and the pollutants, the dirt will fall through evaporation, breaking, and vibration [14-15]. The research on laser cleaning technology first started in the 1960s [16]. Since the 1980s, laser cleaning technology has been applied to the restoration and maintenance of artworks and sculptures. In the following decades, laser cleaning technology has been increasingly perfected and gradually applied to various materials. In 1983, Mallets showed the feasibility demonstration and model optimization of laser paint stripping [17]. In 1996, Tsunemi [18] used TEA CO2 laser to ablate and clean the paint on metal surfaces, and the cleaning effect was excellent. In 2010, Chen [19] used a CO2 laser to clean the paint on the surface of ships and marine engineering steels and came up with the best cleaning method. In 2013, Madhukar [20] used Yb: fiber lasers to study the behavior of continuous wave and repetitive pulse lasers to remove paint. In 2015, Deliang [21] studied morphology and elemental composition of the particles emitted by nanosecond Nd: YAG pulsed laser radiation, and studied the laser paint stripping. In 2018, Li [22] studied the laser incident energy density and paint removal and got the ideal parameters for the complete removal of the paint layer. In recent years, with the rapid development of laser technology, laser cleaning has become more automated, more productive, and less expensive. It has been widely used in paint removal and rust removal [23], tire mold cleaning [24], cultural relics protection [25], nuclear purification [26]. This article studies the cleaning effect of Nd: YAG laser on the coating of Boeing series aircraft skin BMS10-11. The best parameters of paint layer removal explored by changing different laser energy densities and the mechanism of paint removal analyzed by SEM and EDS. The most crucial point of cleaning is not to damage the substrate, which is more important for aircraft skin. Therefore, the friction and wear performance of laser-washed aircraft skins and traditionally cleaned aircraft skins was studied. The typical fretting friction and wear existing around the skin's riveted holes [27], so we also performed the fretting friction and wear performance studies to prove nondestructive cleaning. Besides, we also perform electrochemical corrosion experiments, residual stress tests, and micro- hardness tests on laser-washed and traditionally-washed skins, to provide a reference for large-scale applications of laser cleaning in aircraft skins. 2. Experimental Procedure 2.1 Sample Material Preparation The experimental materials were Boeing aircraft skins (Shandong Taikoo Aircraft Engineering Co., Ltd., Jinan, China). The skin divides into three layers (as shown in Fig. 1), the first layer is a BMS10-11 primer, the second layer is an aluminum-clad layer (pure aluminum), and the third layer is a substrate (2024 aluminum alloy). The skin material was cut into 20×20 mm and φ24 mm samples for laser cleaning experiments and friction and wear tests. 2.2 Laser cleaning experiment The experimental equipment was a medium-power high-energy laser diode pumped-pulsed solid-state laser cleaning equipment (SC200W-350KW, Laser Institute, Shandong Academy of Sciences, Jinan, China). The schematic diagram of laser cleaning is shown in Fig. 2, and the main parameters of the experimental equipment are shown in Table 1. The laser medium is placed between two parallel mirrors (one of which is a total-reflection mirror and one is a semi-reflection mirror) to form an optical resonant cavity. The axially spreading monochromatic light propagates back and forth 2
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in the cavity. After the monochromatic light is enhanced into a laser in the resonant cavity, the laser can be emitted from the output-mirror with its high permeability to become a continuous laser. Through laser Q-switching, the continuous laser can be converted into a high peak power pulse laser. In the experiment, the laser cleaning speed was 5mm/s, the light spot diameter was 0.5 mm, the spot overlap rate was 0.3, the field lens focal length was 100 mm, and the repetition frequency was 10 kHz. The laser with different energy density is controlled by changing the loading current of the laser. Finally, five groups of energy densities were selected: 2J/cm2, 3J/cm2, 4J/cm2, 5J/cm2, 6J/cm2. We used SEM and EDS (JSM-7610F, JEOL, Tokyo, Japan) to analyze the samples after laser cleaning to study the cleaning effect. 2.3 Friction and Wear Test The RTEC friction and wear tester (MFT-50, San-Jose, CA, USA) was used to study the friction and wear performance of the aircraft skin after laser cleaning. The diagram of friction and wear test is shown in Fig. 3. The friction was the dry reciprocating friction between the ball and the plate. A GCr15 bearing steel ball (Ra <0.1μm) with a diameter of 6.35mm and hardness of HV750 was selected as the counter-wearing part. The load force was 10N, the wear time was 30 minutes, the displacement amplitude was 4.5mm, and the running frequency was 2Hz. The RTEC fretting friction and wear tester (MFT-2000, San-Jose, CA, USA) was used to study the fretting friction and wear performance around the rivet holes of the aircraft skin after laser cleaning. The diagram of fretting friction and wear test is shown in Fig. 3, and the fretting wear test area is shown in Fig. 4. Similarly, the friction method was the dry reciprocating friction at room temperature. A GCr15 bearing steel ball (Ra <0.1μm) with a diameter of 9.525mm and hardness of HV750 was selected as the counter-wearing part. The load force was 10N, the wear time was 30 minutes, the displacement amplitude was 50μm, and the running frequency was 2Hz. After performing the friction and wear experiment, a white light interferometer (MFT-4000, Lanzhou Huahui Instrument Technology Co., Ltd, Lanzhou, China) was used to collect the wear morphology and measure the wear volume and friction coefficient. 2.4 Hardness and Residual Stress Test A micro-Vickers' hardness tester (402-MVD, Wilson, Norwood, USA) was used to test the micro-hardness of aircraft skin surface after laser cleaning. The test method was 4 points measurement, the load was 200 g, measuring the same sample for three times and take the average value. An X-ray residual stress analyzer (iXRDCOMBO, Proro, Canada) was used to perform a residual stress test on the aircraft skin surface after laser cleaning. The radiation type was Cr_K-Alpha, the diffraction Bragg angle was 139.0 degrees, and the wavelength was 2.291 Å. Analyze and characterize the strength and tensile bending performance of the aircraft skin surface after laser cleaning. 2.5 Electrochemical experiment An electrochemical workstation (CHI604E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) was used to study the electrochemical characteristics of aircraft skins after laser cleaning. The schematic diagram of the electrochemical workstation is shown in Fig. 5. The electrolyte was a 3.5% NaCl solution. The experiment used a classic three-electrode, with a saturated calomel electrode as the reference electrode, a platinum plate as the auxiliary electrode, and the aircraft skin sample as the working electrode. Before the experiment, the non-working surface was wrapped with Kraft silica gel and then immersed in the solution for 30 minutes to stabilize the open circuit potential. To obtain a 3
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complete potentiodynamic polarization curve, the initial potential was set to - 1.2 V in combination with the open-circuit potential, and the final potential was - 0.6 V. The potentiodynamic polarization curve was measured at a scanning speed of 10 mV/s. The experimental data were recorded with analysis software (CHI604E Electrochemical Analyzer, version 15.03, 3700 Tennision Hill Drive Austin, Austin, TX, USA). After fitting the polarization curve, the anodic polarization curve slope (Ba), the cathodic polarization curve slope (Bc), corrosion potential (Ecorr), and corrosion current density (Icorr) can be obtained. 3. Results and Discussion 3.1 Analysis of Laser Cleaning Effect 3.1.1 Surface morphology characterization of aircraft skin after laser cleaning The macromorphology of the aircraft skin under the laser energy density cleaning of 2J/ cm2- 6J/ cm2 is shown in Fig. 6, and the micromorphology is shown in Fig. 7. Combined with Fig. 6(a) and Fig. 7(a), it can be concluded that the surface of the aircraft skin without laser cleaning was tightly attached to the green BMS10-11 primer and mixed with a little stain. When the laser energy density was 2 J/cm2, although the paint layer on the upper sheet of the skin surface had begun to crack (Fig. 6(b)), the BMS10-11 primer on the lower layer of the skin surface still completely covered the skin surface (Fig. 7(b)). When the laser energy density was 3 J/cm2, the primer on the surface of the skin had begun to peel off significantly, and the metal color began to appear; that is, the aluminum-clad layer is exposed (Fig. 6(c) and Fig. 7(c)). When the laser energy density reached 4 J/cm2, the paint layer on the surface of the skin was almost removed (Fig. 6(d)), but the microscopic morphology showed there were a lot of waves on the surface (Fig. 7(d)). The preliminary guess was the aluminum- clad layer had just leaked, and the thermal oxidation occurred. When the laser energy density reached 5 J/cm2, it can be seen from the macromorphology and micromorphology the cleaning effect was the best now. The surface paint layer was removed entirely, and the surface morphology after cleaning was flat (Fig. 6(e) and Fig. 7(e)). When the energy density reached 6 J/cm2, waves occurred again on the surface of the skin after cleaning (Fig. 6(f) and Fig. 7(f)). We guessed the high-energy laser penetrated the aluminum-clad layer, so the aircraft skin exposed the substrate itself, that is, the 2024 aluminum alloy. It would be demonstrated in later EDS experiment. 3.1.2 Mechanism of laser paint stripping During the cleaning process, with the increase of laser energy density from 2 J/cm2 to 5 J/cm2, the primer gradually fell off completely, and there was no visible metal vapor on the aircraft skin surface. We collected the paint layer fragments that were desorbed by laser cleaning, and performed SEM and EDS analysis, as shown in Fig. 8. In combination with energy spectrum 1-3 and Fig. 1, it can be seen all elements of the BMS10-11 primer were contained in the debris falling off the aircraft skin surface. The fragments of the paint layer under the SEM were lumpy, and EDS analysis showed that most oxygen was present in some lumps. We can infer the laser paint stripping is mainly because of absorbing laser energy by the skin surface paint layer, which is then thermally oxidized and detached from the skin surface by vibration. 3.1.3 Process of laser paint stripping EDS analysis was performed on all cleaned samples, and the results are shown in Fig. 9. At 2 4
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J/cm2, the BMS10-11 primer layer absorbed the laser energy and began to crack, and the content of oxygen rose rapidly. Some such as Si, Ca, W, Cl, which is easily oxidized at high temperatures, formed oxides and detach from the surface of the skin. At 3 J/cm2, the elements in the paint layers such as Si, Ca, W, Re, Mg, and Fe had disappeared. Most of the paint layers had been removed, the content of Al increased rapidly from 0.4% to 76.9%, and the skin began to expose the aluminum-clad layer (Fig. 6(c)). At this time, the oxygen on the skin surface was in a low state. With the increase of laser energy density, at 4 J/cm2, the Al content continued to rise to 79.3%, and the paint layer on the surface of the skin was almost removed. The aluminum-clad layer began to be thermally oxidized, resulting in waves on the surface of the skin. At 5 J/cm2, the Al content increased to 79.5% of the highest point. The aluminum-clad layer continued to be thermally oxidized, and the parts that had been oxidized before were remelted, making the skin surface appear relatively flat, which can be regarded as the best cleaning state. When the laser energy density reaches 6 J/cm2, the high-energy laser ionized and vaporized the Al in the aluminum-clad layer, the Al element content decreased, and the metal vapor was visible on the surface. EDS showed that Cu appeared on the surface of the skin. It is combining with Fig. 6(f), the 2024 aluminum alloy substrate began to appear on the skin surface. The principle of aircraft skin cleaning is not to damage the substrate. The skin loses the protection of the aluminum coating and loses its high corrosion resistance, which is not conducive to the repainting of the paint layer in the next step. We can divide the whole cleaning into two stages: 5 J/cm2 is the best cleaning parameter; Before 5 J/cm2 is regarded as the first stage, which is the under-cleaning stage; After 5 J/cm2, it is the over-cleaning, which is the second stage. 3.2 Analysis of friction and wear performance after laser cleaning 3.2.1 Friction and wear performance analysis Friction and wear experiments were performed on the aircraft skin samples cleaned under different laser energy densities and compared with traditional mechanical lapping. The morphology of the wear scar on the surface of each sample is shown in Fig.10. The friction coefficient curve of skin under different energy density laser cleaning is shown in Fig. 11, and the wear amount and average friction coefficient are shown in Fig. 12. Next, the friction and wear properties of the skin surface are analyzed by the friction coefficient and wear amount. The average friction coefficient of the aircraft skin surface after mechanical lapping was 0.3919, and the wear amount was 0.3694 mm3. The average friction coefficients of the aircraft skins cleaned under the laser energy density of 2 J/cm2-6 J/cm2 were 0.4084, 0.4021, 0.4255, 0.3887, 0.4283, and their wear amounts were 0.3048 mm3, 0.4476 mm3, 0.5332 mm3, 0.3472 mm3, 0.5472 mm3. When the laser energy density was 2 J/cm2, the paint layer on the surface of the skin had just started to peel off, and most of the primer was still attached to the surface of the skin. It can be seen from Fig. 11 that the friction coefficient of skin was in a low state at the early stage of wear, and then increased rapidly, and finally maintained at about 0.4084. At this time, the wear amount was relatively low. According to the wear scar morphology in Fig. 10(b), it is inferred that the main wear mechanism of the skin at this time is adhesive wear [28], which is different from abrasive wear under other laser parameters (Fig.10(a、c-f)). When the laser energy density was 3 J/cm2, most of the paint on the skin surface had been removed, and the aluminum-clad layer was exposed. As can be seen from Figure 11, without the protection of the paint layer, the friction coefficient value of skin at the early stage of wear was relatively high, and then slowly reduced to a stable value. At this time, the average friction coefficient of the skin began to increase to 0.4201, and the wear amount also began to increase. When the energy density is 4 J/cm2, the paint layer on the aircraft's skin surface was almost removed, and the 5
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aluminum coating began to oxidize, which makes the skin surface fluctuate. The average friction coefficient of the skin was further increased to 0.4255, and the wear amount reached the maximum value. When the energy density reached 5 J/cm2, with the further thermal oxidation and remelting of the oxidized part of the aluminum-clad layer, a compact and flat oxide film were formed on the surface of the aluminum-clad layer. The average friction coefficient of the aircraft skin surface decreased to 0.3887, and the amount of wear decreased significantly, which was better than that of the skin surface cleaned by mechanical lapping. When the energy density reached 6 J/cm2, the aluminum-clad layer on the skin surface began to crack, exposing the 2024 aluminum alloy substrate, and the surface began to fluctuate again. The friction coefficient of the skin surface began to increase to 0.4283, and the wear amount also began to increase. 3.2.2 Fretting Friction and Wear Performance Analysis The aircraft skin's rivet holes cleaned by laser with different energy density were tested for fretting friction and wear and compared with the traditional mechanical cleaning. The morphology of the wear scar on the surface of each sample is shown in Fig. 13, and the fretting behavior area is shown in Fig. 14. It can be seen from Fig. 13(a-f) that the contact pressure on the skin surface caused plastic deformation and adhesion of the skin, and the wear debris was not easy to be removed, which accelerated the fretting wear process. When the vibration was large enough, fretting wear would become the core of fatigue crack, leading to fatigue fracture. In Fig. 14(a-f), the fretting behavior curve is a parallelogram, indicating that there was a slip zone in the whole fretting wear. In the inclined part of the behavior curve, the static friction between the friction pair and the skin surface occurred local sliding. With the increase of slip, the friction was greater than the static friction, and the relative motion tended to be stable. The fretting friction coefficient curve of the skin at the rivet holes under laser cleaning with different energy densities is shown in Fig. 15, and the wear amount and average friction coefficient are shown in Fig. 16. After mechanical lapping and cleaning, the average friction coefficient of the skin was 0.6524, and the wear amount was 0.007515 mm3. The average friction coefficient of the skins after laser cleaning of 2 J/cm2-6 J/cm2 energy density were 0.4022, 0.6736, 0.8026, 0.6483 and 0.7381, and the wear amount were 0.002133 mm3, 0.01914 mm3, 0.02691 mm3, 0.01024 mm3 and 0.02012 mm3. It can be seen when the laser energy density was 2J /cm2, the paint layer on the skin surface just began to fall off. Due to the existence of primer, the friction coefficient and wear amount of the skin surface was the minimum. With the increase of laser energy density, the paint layer on the skin surface was gradually removed, thus exposing the aluminum-clad layer. The average friction coefficient of the skin surface without primer protection increased gradually, reaching the maximum value of 0.8026 at 4 J/cm2. When the energy density reached 5 J/cm2, combined with Fig. 7, it can be concluded that thermal oxidation occurred on the skin surface, forming a dense and flat oxide film on the surface of the aluminum-clad layer. At this time, the average friction coefficient of the skin surface was reduced to 0.6463, which was superior to 0.6524 of the skin surface cleaned by mechanical lapping, and the wear amount was 0.01024 mm3, which was higher than 0.007515mm3 of the skin surface cleaned by mechanical lapping. In combination with Fig. 13 (a) and (b-f), we can see that the abrasion scar on the surface of the skin after mechanical lapping was smaller than that after laser cleaning because the mechanical grinding improved the hardness of the skin surface and made the abrasion scar shallower. When the energy density was 6 J/cm2, the aluminum-clad layer on the surface of the skin continued to be thermally oxidized and vaporized by high-energy laser irradiation, which caused the aluminum-clad layer to crack. The friction coefficient of the skin surface rose to 0.7381 again, and the wear amount also increased significantly. 6
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Through analysis, we can infer that compared with the traditional cleaning method, when the laser energy density is 5 J/cm2, laser cleaning will not reduce the friction and wear performance of the skin surface and quicken the fretting fatigue wear on the rivets, which guarantees the safety of aircraft. 3.3 Analysis of surface hardness and residual stress after laser cleaning The surface micro-hardness values of aircraft skins under laser cleaning with different energy densities are shown in Fig. 17(a), and the surface residual stress values are shown in Fig. 17(b). As shown in Fig. 17(a), the micro-hardness of the skin surface after laser cleaning was generally increased, indicating that the pulse laser made the aircraft skin surface form a hardening layer. It can be seen from Fig. 17(b) that there was significant residual tensile stress on the aircraft skin surface after laser cleaning, and it increased with the increase of laser energy density, which indicates that the pulse laser caused plastic deformation on the skin surface. Fig. 18 is a schematic diagram of the aircraft skin surface change during the laser cleaning process. When the pulsed laser was irradiated to the aircraft skin surface, the primer layer and the aluminum-clad layer on the skin surface absorbed laser energy to vibrate and rupture and evaporate. The rupture of BMS10-11 coating caused the shock wave generated to act on the skin surface, forming dense and stable dislocation structure on the skin surface, thus making the skin surface hardened [29]. At the same time, the residual stress on the skin surface was released and redistributed, which leads to plastic deformation. In summary, laser cleaning can improve the hardness of aircraft skin surface and generate residual tensile stress that affects the stability of components, which is currently a problem that the engineering department needs to solve. 3.4 Analysis of corrosion performance after laser cleaning The potentiodynamic polarization curves of aircraft skins after laser cleaning with different energy densities are shown in Fig. 19, and the electrochemical parameters of the tested samples are shown in Table 2. The corrosion potential (Ecorr) characterizes the thermodynamic stability of the test sample under electrochemical corrosion conditions [30]. The corrosion current density (Icorr) means the corrosion rate and breakdown potential are the lowest potential values at which pitting occurs [31]. From Table 2 and Fig. 15, it can be got that corrosion current density of the skin after mechanical lapping was 2.414×10-4 A.cm-2, and the corrosion potential was -0.755V. When the laser energy density was 2 J/cm2, the paint layer was still densely attached to the skin surface. The corrosion current density of the skin was 1.585×10-4 A.cm-2, and the corrosion resistance was the best at this moment. With the further increase of the laser energy density, the paint layer on the surface of the aircraft's skin was gradually removed. The exposure of the aluminum-clad layer made the corrosion current density of the skin increase obviously, which made the corrosion resistance of the skin decrease. At 4 J/cm2, the corrosion current density of the skin reached the maximum value of 2.907 × 10-4 A.cm-2 and the corrosion resistance of the skin was the worst. When the energy density reached 5 J/cm2, the aluminum-clad layer oxidized and formed a dense oxide film on the skin surface, which made the corrosion resistance of the skin significantly improved. At this time, the corrosion current density of the skin decreased to 1.601 × 10-4 A.cm-2 and the anti-corrosion performance was better than that of the mechanically cleaned skin. When the energy density reached 6 J/cm2, the high-energy laser caused the oxide film on the surface of the aluminum-clad layer to crack again. The corrosion current density of the skin rose to 1.996 × 10-4 A.cm-2, and the corrosion resistance of the skin fall again. Compared with the traditional mechanical cleaning method, when the laser energy density is 2 J/cm2, 5 J/cm2, 6 J/cm2, laser cleaning will not reduce the corrosion resistance of the aircraft's skin surface, and the corrosion performance is the best when the laser energy density is 5 J/cm2. 7
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Conclusion In this work, the medium-power high-energy laser diode-pumped pulsed solid-state laser cleaning equipment was used to remove the BMS10-11 primer on the surface of the Boeing series aircraft skin. The cleaning effect under different energy density was studied, and the surface morphology, friction and wear properties, micro-hardness, and residual stress were tested and analyzed. The main results are as follows, which can provide a reference for the large-scale application of laser cleaning in aircraft maintenance. (1) The Nd: YAG laser can effectively remove the BMS10-11 paint layer on the surface of the Boeing series aircraft skins. The cleaning effect is best when the laser energy is 5 J/cm2. Excessive cleaning will penetrate the aluminum-clad layer of the skin surface and damage the substrate. (2) Under the condition of 5 J/cm2 laser cleaning, the friction and wear properties of the aircraft skin surface and rivet hole will not be reduced. Compared with the traditional cleaning method, the fretting fatigue wear of rivet can be reduced. (3) Laser cleaning can make the surface of the aircraft skin hardened and slightly strengthen the surface; the surface of the skin after laser cleaning has plastic deformation, which increases the residual tensile stress. (4) Compared with traditional cleaning methods, when the laser energy density is 2 J/cm2, 5 J/cm2, 6 J/cm2, laser cleaning will not reduce the corrosion resistance of the aircraft's skin surface, and the corrosion performance is the best when the laser energy density is 5 J/cm2. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51872122), Shandong Key Research and Development Plan (No. 2017GGX30140, 2016JMRH0218), Shandong Major Science and Technology Innovation Projects (No. 2018CXGC0808) and Taishan Scholar Engineering Special Funding (No. ts201511040). References [1] C. Zhenli, M. Zhang, C. Yingchun, S. Weimin, T. Zhaoguang, L. Dong, B. Zhang. Assessment on critical technologies for conceptual design of blended-wing-body civil aircraft, J. Chinese Journal of Aeronautics. https://doi.org/10.1016/j.cja.2019.06.006. 32(8) (2019) 1797-1827. [2] Q. Deng, B. F. Santos, R. Curran. A practical dynamic programming based methodology for aircraft maintenance check scheduling optimization, J. European Journal of Operational Research. 281(2) (2020) 256-273. https://doi.org/10.1016/j.ejor.2019.08.025. [3] J. Sheng, D. Prescott. A coloured Petri net framework for modelling aircraft fleet maintenance, J. Reliability Engineering & System Safety. 189 (2019) 67-88. https://doi.org/10.1016/j.ress.2019.04.004. [4] Y. Qin, Z. X. Wang, F. T. S. Chan, S. H. Chung, T. Qu. A mathematical model and algorithms for the aircraft hangar maintenance scheduling problem, J. Applied Mathematical Modelling. 67 (2019) 491-509. https://doi.org/10.1016/j.apm.2018.11.008. [5] C. Sriram, A. Haghani. An optimization model for aircraft maintenance scheduling and re- 8
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Output Spot Size D 0.5-2.5 mm Scan Line Width 1 01/10/24 cm Producer Laser Institute, Shandong Academy of Sciences, China Table 2. Electrochemical parameters of the aircraft skin surface after laser cleaning Energy density Ecorr Icorr Ba Bc (J/cm2) (V vs. Ag/Ag Cl) (A.cm-2) (mVdec-1) (mVdec-1) Mechanical lapping -0.755 ± 0.06 2.414 × 10-4 9.47 5.12 2 -0.698 ± 0.06 1.585 × 10-4 3.997 7.01 3 -1.022 ± 0.06 2.859 × 10-4 0.405 9.111 4 -0.981 ± 0.06 2.907 × 10-4 0.531 5.666 5 -0.758 ± 0.06 1.601 × 10-4 8.670 6.577 6 -0.905 ± 0.06 1.996 × 10-4 5.336 4.768 List of figure captions Fig. 1. Schematic diagram of aircraft skin structure. Fig. 2. Schematic diagram of laser cleaning. Fig. 3. Schematic diagram of friction and wear test. Fig. 4. The fretting wear test area. Fig. 5. Schematic diagram of the electrochemical workstation. Fig. 6. Macromorphology of aircraft skin surface under different laser energy density cleaning: (a) not cleaned; (b) 2 J/cm2; (c) 3 J/cm2; (d) 4 J/cm2; (e) 5 J/cm2; (f) 6 J/cm2. Fig. 7. Micromorphology of aircraft skin surface under different laser energy density cleaning: (a) not cleaned; (b) 2 J/cm2; (c) 3 J/cm2; (d) 4 J/cm2; (e) 5 J/cm2; (f) 6 J/cm2. Fig. 8. Schematic diagram of laser paint stripping. Fig. 9. Laser cleaning effect under different energy densities. Fig. 10. The morphology of the wear scar on aircraft skin surface under different laser energy density cleaning: (a) mechanical cleaning; (b) 2 J/cm2; (c) 3 J/cm2; (d) 4 J/cm2; (e) 5 J/cm2; (f) 6 J/cm2. Fig. 11. The friction coefficient curve of skin under different energy density laser cleaning. Fig. 12. (a) Average friction coefficient of the aircraft skin surface under different laser energy density cleaning; (b) Friction wear amount of the aircraft skin surface under different laser energy density cleaning. Fig. 13. The morphology of the fretting wear scar on aircraft skin surface under different laser energy density cleaning: (a) mechanical cleaning; (b) 2 J/cm2; (c) 3 J/cm2; (d) 4 J/cm2; (e) 5 J/cm2; (f) 6 J/cm2. Fig. 14. Fretting behavior area of aircraft skin surface cleaned by different laser energy density: (a) mechanical cleaning; (b) 2 J/cm2; (c) 3 J/cm2; (d) 4 J/cm2; (e) 5 J/cm2; (f) 6 J/cm2. 11
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Fig. 15. The fretting friction coefficient of skin under different laser energy density cleaning. Fig. 16. (a) The average fretting friction coefficient of the aircraft skin surface under different laser energy density cleaning; (b) Fretting wear amount of the aircraft skin surface under different laser energy density cleaning. Fig. 17. (a) The surface micro-hardness values of aircraft skins under laser cleaning with different energy densities; (b) The surface residual stress value of the aircraft skins under laser cleaning with different energy densities. Fig. 18. Schematic diagram of the aircraft skin surface change during the laser cleaning process. Fig. 19. The potentiodynamic polarization curves of aircraft skins after laser cleaning with different energy densities. Fig. 1. 12
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