Study On The Damage Behavior Of Anodic Oxide Layer Of Aluminum Alloy By Short Pulse Laser Cleaning

During aircraft service,the composite coatings on aluminum alloy skin are susceptible to damage such as cracking,detachment,and aging due to various environmental factors such as radiation,temperature,and pressure.Therefore,regular cleaning and re-coating of the surface coatings of the skin are necessary.Currently,the most commonly used methods for this are mechanical polishing,high-pressure methods,sandblasting,and chemical solvents,but they have limitations such as easy damage to the substrate,poor environmental friendliness,and low automation.Short-pulse laser cleaning technology has the advantages of being environmentally friendly,efficient,convenient,and controllable.It has been widely used in the heritage restorationand and rail transportation industries.However,research and application in the field of aircraft coating removal that requires non-destructive substrate treatment are incomplete.The main reason is that the interaction between laser and coating is complex,making it difficult to accurately predict and control the heat generated by the laser.Additionally,the real-time damage signals generated by the substrate are poor,and recognition is difficult.As a result,there is no effective judgment basis for non-destructive substrate control,which can easily lead to damage to the substrate of the aircraft skin.This paper focuses on the step-by-step removal method proposed in the National Natural Science Foundation of China（No.52065041）for laser non-destructive cleaning of aluminum alloy skin coatings under open-loop conditions.The first step involves removing the coating to the critical thickness,which is the minimum thickness of the coating where the laser does not directly affect the anodic oxide layer and substrate.The second step involves the secondary removal of the coating within the threshold of the anodic oxide layer’s maximum damage state,which represents the absence of damage to the substrate.Based on the first step,the second step of the step-by-step approach is investigated in this paper.Specifically,this study investigates the damage behavior and maximum damage state energy and temperature thresholds of the aluminum alloy anodic oxide layer.Using 2024 aluminum alloy as the test material,laser cleaning experiments are conducted on the anodic oxide layer,the anodic oxide layer with a residual smooth paint layer,and the anodic oxide layer with a residual rough paint layer.The samples are then examined using scanning electron microscopy（SEM）,energy dispersive spectroscopy（EDS）,and laser confocal microscopy（LCSM）to explore the damage behavior mechanism of the laser on the anodic oxide layer,study the influence of laser process parameters on the degree of damage,establish a thermal absorption model for the anodic oxide layer under laser energy,and determine the maximum damage energy and temperature thresholds for different surface states.The study provides a theoretical and experimental basis for achieving laser non-destructive cleaning technology with temperature testing as feedback signal in the future.The specific research content and results are presented below:（1）In terms of laser cleaning process,the influence of laser cleaning parameters on the damage degree of anodic oxide layer was investigated.It was found that as the laser power increased,the damage degree of anodic oxide layer also increased.As the pulse frequency increased,the damage degree of anodic oxide layer decreased first and then increased.And as the scanning speed increased,the damage degree of anodic oxide layer decreased.（2）Regarding the behavior of damage to the anodic oxide layer,two modes were identified:ablation damage and cracking damage.The influence of different process parameters on the behavior of damage to the anodic oxide layer was investigated,and it was found that ablation damage and cracking damage correspond to different threshold ranges and boundary conditions.The lowest energy density for ablation damage of the anodic oxide layer was found to be 0.075 J/cm²,while the lowest energy density for cracking damage was 0.087 J/cm².The energy density for complete damage of the anodic oxide layer was determined to be 0.126 J/cm².（3）In terms of the thermal absorption of laser by anodized aluminum oxide layer,through analysis of the absorption process and establishment of a thermal absorption model,it was found that different types of damage in the anodized aluminum oxide layer corresponded to different energy and temperature thresholds.The initial energy threshold range for the erosion damage in the anodized aluminum oxide layer was8.40x10^-5J with an initial temperature threshold of 3290 K.The initial energy threshold range for the cracking damage in the anodized aluminum oxide layer was 9.60×10^-5J with an initial temperature threshold of 3515 K.The maximum energy damage threshold for the anodized aluminum oxide layer was 1.40×10^-4J with a maximum temperature threshold of 4240 K.（4）In terms of the effect of residual paint layer on the absorption of anodic oxide layer,an equivalent model of rough surface and a calculation model of absorption rate at different inclinationsθwere established.By modifying the mathematical relationship between process parameters and absorbed energy in（3）,the changes in absorbed energy and temperature of anodic oxide layer under the rough surface of the paint layer were obtained.When the energy absorbed by the anodic oxide layer under the flat surface is 1.64×10^-4J and the energy absorbed by the anodic oxide layer under the rough surface is 1.67×10^-4J,the corresponding temperatures reach 4587 K and 4628K,respectively,at which time the anodic oxide layer has been completely damaged.