What is the effect of paint removal by laser cleaning on the mechanical and microstructural characteristics of aluminum alloy, especially on aircraft skins?
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The Importance of Cleaning Rusted Surfaces
The skin of an airplane is made of aluminum, aluminum alloys, and other metals and is directly exposed to a low-temperature, high-pressure environment. The purpose of the skin is to keep the shape of the craft intact, maximize sunlight reflection, and improve aerodynamics. Regular aircraft maintenance is necessary to prevent paint from cracking, aging, peeling, and damaging fuselage parts due to mechanical scratches, electromagnetic radiation, and complicated service environments with temperature and humidity variations. The coatings must be thoroughly stripped during an aircraft overhaul to inspect the substrate for flaws.
Due to the complex environment during flight, the surface paint layer of the aircraft skin would become damaged over a lengthy period of operation. The surface paint must be removed as part of routine maintenance to prevent fatigue damage to the skin. This procedure is used to precisely detect problems such as pits and fatigue fractures on the skin's surface, in addition to altering the appearance of the aircraft.
This is just one pertinent example of why surface cleaning is essential.
Common Surface Cleaning Methods
Currently, mechanical treatment and chemical procedures, both expensive, inefficient, and seriously harmful to the environment, are used to strip paint from aircraft skin. They also cause some damage to the foundation and other elements of the skin. These methods also run the danger of physically harming the workers. A substitute cleaning method that is efficient, non-contact, non-destructive, highly motivated, and environmentally friendly is required.
Laser cleaning removes particular attachments from the substrate with little to no damage to the substrate by utilizing the materials' interaction mechanism with the laser. During the cleaning process, the attachments on the surface that need to be cleaned absorb the laser energy, breaking the bond that attaches the removed object to the substrate. Laser cleaning is non-contact, selective, efficient, possibly highly automated, and high precision.
Additionally, studies have demonstrated that laser cleaning produces finer microstructures and less surface roughness than other conventional cleaning processes, which improves the adherence of new coatings applied to metal substrates. The laser cleaning procedure is flexible and can be used on various contaminants and substrates. Recently, laser cleaning has been thought of as a pre-treatment method for coating, welding, and joining multiple materials, as well as a post-treatment method to remove stains and discoloration after welding.
Effect of Paint Removal by Laser Cleaning on Aluminum Alloy
Paint removal is a critical application of laser cleaning. It has been discovered to be an effective method for getting paint off a surface. Long-reported and studied, pulsed laser paint removal from aircraft and workpieces has recently attracted increased interest.
The use of lasers to remove paint from metal surfaces has been extensively researched. Several of them have explored the workings of laser cleaning and the ideal process variables. Also, several researchers have looked into a few characteristics of the metal substrate following laser cleaning. The ability of laser cleaning technology to remove paint from metal surfaces has recently been demonstrated. Also, after laser cleaning, various metal substrate properties were reported to be enhanced. There are, however, few publications on the mechanisms underlying the modifications in the characteristics of metal substrates following laser cleaning.
Recent Developments
In a study recently published in the journal Vacuum, researchers investigated the mechanical properties and microstructural traits of the 2024-T351 aluminum alloy specimens that had paint removed by laser cleaning. Before and after laser cleaning, the samples' tensile characteristics and surface microhardness were measured. The microstructural features were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM).
The experiment's findings showed that the specimen treated by laser cleaning had an increase in surface microhardness and tensile strength of 10.6% and 8.4%, respectively, compared to the untreated sample.
In conclusion, the study reported that the surface paint layer on the 2024-T351 aluminum alloy substrate could be successfully removed by laser cleaning under optimized experimental conditions. The substrate's surface roughness remained relatively the same. Moreover, the 2024-T351 aluminum alloy's mechanical characteristics were enhanced by laser cleaning, and surface microhardness, ultimate tensile strength (UTS), and yield strength (YS) increased by 9.7%, 6.6%, and 13.2%, respectively. The 2024-T351 aluminum alloy surfaces underwent dislocation proliferation and grain refinement during laser cleaning, enhancing the material's mechanical characteristics.
In another recent study published by the Photonics journal, authors observed that the specific multi-layer paint on the aluminum alloy aircraft skin surface was removed by modifying the laser parameters. After laser cleaning, multi-layer paint produced a significantly more complicated surface than standard single-layer paint. Analyzing the surface morphology, chemical compositions, and surface functional groups of the Boeing 737 aircraft skin samples, the successful paint removal parameters—laser energy density of 5.09 J/cm2 and scanning speed of 700 mm/s—were discovered with no damage to the aluminum substrate.
After laser cleaning, there was a 3.587% improvement in surface nanoindentation hardness compared to the typical mechanical lapping sample, with the paint removed and unharmed. The microstructure observed using electron backscatter diffraction (EBSD), in which plastic deformation resulted in strain hardening of the substrate surface, also supported the increase in nanoindentation hardness.
To summarize, as a non-contact and non-destructive technology, laser cleaning provides an alternative method for the paint stripping of aircraft skins. Also, the laser cleaning process' induction of grain refinement and dislocation propagation significantly contributed to improving the microhardness and tensile characteristics of the aluminum alloy. Moreover, heat breakdown, evaporation, and spallation are the mechanisms of laser paint removal from aluminum alloy aircraft skin. Laser cleaning is anticipated to be the future of large-scale, highly effective, and low-pollution removal of more intricate paint layers from airplane surfaces.
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References and Further Reading
Sun, Q., et al. (2023). Mechanical properties and microstructure characteristics of 2024-T351 aluminum alloy specimen subjected to paint removal by laser cleaning. Vacuum, 211, 111927. https://doi.org/10.1016/j.vacuum.2023.111927
Li, W., et al. (2023). Removal Mechanisms and Microstructure Characteristics of Laser Paint Stripping on Aircraft Skin Surface. Photonics, 10(1), 96. https://doi.org/10.3390/photonics10010096
Zou, W., et al. (2021). Characteristics of the audible acoustic signal in the process of laser cleaning of paint on the metal surface. Optics and Laser Technology, 144, 107388. https://doi.org/10.1016/j.optlastec.2021.107388
Lu, Y., et al. (2020). Ultraviolet laser cleaning and surface characterization of AH36 steel for rust removal. Journal of Laser Applications, 32, 032023.
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