Study On The Effect Of Laser Paint Removal On The Recoating Performance Of Aluminum Alloy Skins

In the aviation industry,for the sake of flight safety,regular maintenance,repair,and major repairs are required for aircraft.The aircraft will undergo multiple repainting and repainting operations throughout its service life.As a new cleaning technology,laser can meet the needs of paint removal efficiency,environmental protection,and other aspects,which has attracted widespread attention from the academic and industrial communities.The current research focus mainly focuses on coupling optimization of laser paint removal parameters,monitoring of paint removal process,and matrix damage control,while there is a lack of research on the repainting performance of new paint layers after flat top laser paint removal.The repainting performance includes the matching between the repainting layer and the substrate and the retained paint layer.Through mechanisms such as mechanical,chemical,wetting,and electrostatic adsorption,the repainting layer forms new interfaces between the substrate or retained paint layers.It is particularly necessary to study the effect of laser cleaning on the surface repainting performance of retained materials based on the practical application requirements of aluminum alloy skin surface repainting.The paper systematically conducts the following research on the different requirements for partial and complete paint removal of aircraft aluminum alloy skin:To investigate the impact of laser cleaning on the repainting performance of retained paint layers in response to the demand for partial repair and partial removal of paint layers.The results show that as the average laser power increases,the removal thickness of the paint layer on the surface of the sample gradually increases;When the flat top laser power is 150 W,the single cleaning thickness is about 5μm.A smaller single cleaning thickness makes it easier to clarify the impact of laser cleaning on the adhesion of the retained paint layer;When the thickness of the retained paint layer is about 12μm,the adhesion between the paint layer and the aluminum alloy substrate decreases sharply and the corrosion protection performance of the paint layer decreases abruptly,which seriously affects the quality of the paint layer recoating,therefore the thickness of the retained paint layer should be strictly controlled when removing paint layer by layer by laser;After cleaning with process parameters such as laser power of 150 W,pulse width of 100 ns,scanning speed of 4000 mm/s,and frequency of 50 k Hz,the surface of the paint layer remains uniform,with a slight increase in surface roughness and a 3%increase in the interface expansion area ratio of the sample.The surface free energy of the paint layer has been increased from 38.11 m N/m to 42.52 m N/m,which preserves the wettability of the paint layer surface and is beneficial for improving the adhesion of the new paint;After recoating with new paint,the bonding performance is good,and the adhesion reaches level 0,achieving controllable laser thickness paint removal,revealing the impact of laser paint removal on the recoating performance of the retained paint layer.In response to the demand for overall removal and repainting of the paint layer on the surface of aluminum alloy skin during aircraft overhaul,a study was conducted on the impact of laser cleaning on the repainting performance of aluminum alloy substrate.The results indicate that an appropriate spot overlap rate can improve the cleaning effect of the laser.Under the experimental conditions of this article,when the scanning spacing is 0.1 mm,the longitudinal overlap rate is 85%,and there are no obvious overlap marks on the macroscopic morphology.The paint layer is largely removed.After determining the scanning interval,the sample was processed by changing the scanning speed using the single factor method.When the scanning speed was 4000 mm/s,the paint layer was completely removed and the substrate was undamaged;After cleaning with laser power of 500 W,scanning spacing of 0.1 mm,frequency of 50 k Hz,pulse width of 100 ns,and scanning speed of 4000 mm/s,the polyurethane paint layer on the surface of aluminum alloy is most effectively removed and the substrate is undamaged.The physical and chemical properties of the substrate surface after laser paint removal are conducive to the recoating of the new paint layer.The surface of the sample is flat and has uniform roughness.After laser paint removal,the porous structure on the surface of the Alodine film is exposed,which can provide mechanical interlocking effect for the coating of the new paint layer.The free energy of the substrate surface after laser paint removal of the aluminum alloy skin is 41.75 m N/m,and the free energy of the surface after laser paint removal is high,indicating that the wetting effect is good,and the bonding ability between the sample and the paint layer is stronger.The new paint layer has strong adhesion after respraying,reaching level 0,which can meet the requirements of re spraying on the surface of the aluminum alloy substrate.Aiming at the influence of environment on the new paint layer on the surface of aluminum alloy substrate,the environmental durability of the new paint layer of the sample in saline immersion,high temperature environment,and low temperature environment was studied.The results show that when immersed in 3.5 wt%Na Cl solution,the adhesion of the paint layer decreases as the immersion time increases.After 12 days of immersion,the adhesion decreases to level 1,and the electrochemical impedance in the low frequency region is 4.3×10~8Ωcm~2,at which time the paint layer still exhibits good impermeability;During high-temperature aging at 150℃,there are no obvious defects on the surface of the paint layer at different high-temperature test stages.After 48 hours of high temperature,the adhesion of the paint layer decreases to level 1,and the impedance in the low frequency region is about 1.9×10~9Ωcm~2,the paint layer can still meet the functional function of coating the aluminum alloy surface-During low-temperature aging at-50℃,at different stages of low temperature,the surface of the paint layer is flat and dense,and the adhesion of the paint layer at low temperature for 8hours does not significantly decrease.The paint layer is tightly bonded to the surface of aluminum alloy,with an electrochemical impedance of 2.02×10~9Ωcm~2 in the low frequency region,showing good low-temperature resistance.The above environmental tests meet the requirements of GJB 385B-2020 Specification for Aliphatic Polyurethane Enamel and Mating Primers for Aircraft Skin.This topic can provide technical support for the application of laser cleaning in the repainting and maintenance of aluminum alloy skin.