After an aircraft is in service for a certain period, various factors, such as external forces, light, and humidity, affect the paint layer on its skin surface. This leads to aging, cracking, peeling, and other phenomena. Hence, removing the original coating on the metal material surface becomes a necessary step before repainting it. Laser cleaning offers advantages, including non-contact, environmental friendliness, precision, and no secondary pollution. It can replace traditional mechanical polishing and chemical paint removal methods, enhancing the cleanliness of aircraft surface paint layers. Due to the strict safety requirements of aircraft, it is crucial to understand whether the use of lasers for removing surface paint layers impacts the fatigue properties of the aircraft. In this study, a nanosecond pulse laser is used to clean aviation aluminum alloys coated with a fluid-resistant epoxy primer coating. Subsequently, the effects of the laser cleaning process on the microstructure and fatigue properties of the aluminum alloys are examined.
A pulsed laser was used to remove the surface coating of the aviation aluminum alloys. The effects of the laser cleaning process on the microstructure, mechanical properties, and fatigue properties of the aviation aluminum alloys were examined via appearance inspection, optical microscope (OM), scanning electron microscope (SEM), in situ temperature detection, mechanical property detection, and high cycle fatigue test.
The results show that when the laser power and pulse frequency are 80 W and 100 kHz, the epoxy primer coating on the surface of the aluminum alloy can be removed. However, some residual paint layer remains on the surface of the sample. When the laser power and pulse frequency are 500 W and 500 kHz, the maximum surface temperature does not exceed 115 ℃, and no obvious heat affected zone is observed. Laser cleaning increases the surface roughness, with partial ablation and melting occurring within the depth range of 10 μm. After laser cleaning, the hardness of the material increases. As the laser power, frequency, and energy density increase, the rate of hardness increase decelerates. The tensile property results indicate that the tensile strength, yield strength, and elongation of the sample after laser cleaning are slightly lower than those of the blank sample. Through high-cycle fatigue testing, when compared to those of the blank sample, the fatigue properties of the painted sample after laser cleaning decrease by 11.76%. This mainly stems from the increased surface roughness caused by laser cleaning. However, after anodizing and painting treatments, laser cleaning does not further exacerbate the fatigue damage caused by anodizing.
Analyses are conducted on the appearance, microstructure, roughness, in-situ temperature, mechanical properties, and fatigue performance of the samples after laser cleaning. With a laser power and pulse frequency of 80 W and 100 kHz, a residual paint layer remains on the sample surface. However, at elevated levels of 500 W and 500 kHz, oxidation might appear on the substrate surface. The process of laser cleaning tends to increase surface roughness, causing partial ablation and melting within the depth range of 10 μm. The surface temperature during this procedure increases in tandem with the increase in laser power and pulse frequency. But even at peaks of 500 W and 500 kHz, the maximum surface temperature stays below 115 ℃. After cleaning, the material hardness increases. However, as the laser power, frequency, and energy density increase, the increase in hardness decelerates. There is a minor reduction in the sample tensile strength, yield strength, and elongation. When compared to the untreated samples, those cleaned by laser but not anodized or painted show a reduction in fatigue properties by 9.34%. In comparison, samples that undergo anodizing and painting processes after cleaning experience a reduction in fatigue properties by 13.84%. Specifically, after painting, laser cleaning results in a decrease in fatigue properties by 11.76%. Notably, laser cleaning does not further increase the fatigue damage due to anodizing.
