By establishing a finite element model of laser cleaning of 2024 aluminum alloy oxide film, the temperature change law of the aluminum alloy surface during laser cleaning of the oxide film was obtained; the effects of different laser energy density and laser cleaning speed on its surface temperature change were investigated respectively. Among them, the greater the energy density, the higher the temperature rise rate of the aluminum alloy surface oxide film, and the surface temperature of the aluminum alloy oxide film slowly decreases with the increase of the cleaning speed. Subsequently, nanosecond pulse laser cleaning of 2024 aluminum alloy oxide film was carried out to explore the effects of laser power, pulse frequency and cleaning speed on the surface morphology before and after cleaning. The research results show that when the laser power is 400 W, the laser frequency is 4.5kHz, and the cleaning speed is 5 mm/s, the surface oxide film can be completely removed without damaging the aluminum alloy substrate; under this laser cleaning parameter, the microstructure of the aluminum alloy substrate after cleaning is intact, and the mass fraction of oxygen is 10.03%.
In recent years, the changes and progress of science and technology have promoted the research and exploration of human application of aerospace technology, and the use standards of aluminum alloy structural materials have become increasingly stringent. 2024 aluminum alloy materials are widely used in aerospace and other industrial fields, such as aircraft structural parts, rocket structural parts and shells of space vehicles such as the Long March series, and missile structural parts. Although 2024 aluminum alloy has many advantages, it also has problems such as low surface hardness and poor wear resistance. In harsh use environments, aluminum alloy components riveted and pinned on the flight fuselage are prone to wear and loosening, which in turn causes accidents, so it is necessary to remove this layer of oxide film on the surface.
Traditional methods of cleaning and removing oxide film (mechanical grinding, chemical cleaning and shot peening, etc.) take a long time, and the surface quality of the aluminum alloy after cleaning is difficult to control, the cleaning efficiency is low, and it may even cause a certain range of damage to the aluminum alloy matrix. Laser cleaning oxide film technology has the advantages of high cleaning efficiency, good controllability and no damage to the aluminum alloy matrix. The application of laser cleaning in many industrial fields is increasing. The cleaning principle is mainly: through high-energy laser radiation on the substrate surface, the surface oxide film is broken and disintegrated in a very short time to achieve the effect of removing the surface oxide.
The process parameters selected for laser cleaning directly affect the laser cleaning effect. Reasonable parameter configuration is an important guarantee for the laser to completely remove the oxide film and improve the surface quality after cleaning. Meja et al. studied the laser removal of aluminum alloy oxide layer under different laser wavelengths to obtain the laser cleaning threshold. Jasim et al. used 250 ns pulse laser to remove 20 μm thick transparent polymer on the surface of 5005A aluminum alloy, and explored the effect of laser power and frequency on the cleaning effect. Dimogerontakis et al. used nanosecond pulses to perform Nd:YAG laser cleaning on anodized aluminum-magnesium alloys, and found that the ablation mechanism was a thermal oxidation mechanism, and determined the cleaning threshold of laser cleaning of aluminum alloy surface oxides.
The finite element simulation software is used to establish a nanosecond pulse laser cleaning model of 2024 aluminum alloy surface oxide film, and the model is used to analyze the temperature field. Subsequently, the laser cleaning equipment was used to carry out nanosecond pulse laser cleaning experiments on the anodic oxide film on the surface of 2024 aluminum alloy to explore the effects of laser pulse frequency, laser power and laser cleaning speed on laser removal of oxide film.
1 Numerical simulation of laser cleaning
1.1 Establishment of finite element model
The finite element model of laser cleaning of 2024 aluminum alloy oxide film is shown in Figure 1. The model size is 100 mm×10 mm×10 mm 2024 aluminum alloy substrate and 0.1 mm thick oxide film.
1.2 Effect of energy density on temperature distribution
It can be clearly seen from Figure 2 that the larger energy density causes the maximum temperature of the oxide film surface to rise continuously. When q=30.85 J/cm’2, the maximum temperature of the oxide film surface reaches nearly 950 ℃. As time increases, the maximum temperature of the oxide film surface tends to stabilize. This phenomenon shows that the energy density is positively correlated with the degree of heating of the oxide film.
1.3 Effect of laser cleaning speed on temperature distribution
Figure 3 shows the surface temperature distribution of aluminum alloy oxide film at different cleaning speeds. It can be clearly seen from Figure 3 that the surface temperature of the oxide film decreases with the increase of cleaning speed. In the process of increasing the cleaning speed from v=5 mm/s to v=8 mm/s, the overall temperature of the oxide film on the surface of the aluminum alloy slowly decreases with the increase of cleaning speed.
2 Experimental materials
The substrate used is a 2 mm thick 2024 aluminum alloy plate. After the surface of the substrate is anodized, a layer of aluminum oxide film is formed on the surface of the substrate. Many fine micropores appear on the surface of the anodized sample.
3 Analysis of cleaning effect under different parameter combinations
After conducting orthogonal tests on three key laser process parameters such as laser power, laser pulse frequency and cleaning speed, a set of experimental parameters that can completely remove the 2024 aluminum alloy oxide film is found. The laser power, pulse frequency and cleaning speed of this set of laser parameters are 400W, 4.5 kHz and 5 mm/s respectively. The two-dimensional morphology of the 2024 aluminum alloy oxide film after laser cleaning is shown in Figure 5. It can be clearly seen that the surface morphology after cleaning with different combination parameters is different. In Figure 5 (a), there are many surface pits and a large number of crack and broken marks. The bright 2024 aluminum alloy oxide film is irregularly distributed, but the upper oxide film is significantly less than the lower oxide film. In Figure 5 (b), there are more crack and broken marks in the surface pits. Compared with Figure 5 (a), it can be observed that a large number of bright white oxide films are removed, and the breaking effect is more obvious. In Figure 5 (c), there are few bright oxide film fragments, and a large area of dark color is aluminum alloy-based. At this time, the oxide film is basically completely removed.
4 Conclusions
By establishing the finite element model of laser cleaning of 2024 aluminum alloy oxide film and the numerical model of laser light source, the temperature variation law of the aluminum alloy surface during laser cleaning of oxide film was obtained. Design
A laser cleaning experiment of 2024 aluminum alloy oxide film was conducted. The two-dimensional morphology of the aluminum alloy surface under different cleaning process parameters after the experiment was observed. The following conclusions were drawn: 1) During the movement of the rectangular laser light source, its action range is only the oxide film part on the surface of the 2024 aluminum alloy. The heat flux density on the aluminum alloy substrate is extremely small. As the rectangular laser spot continues to move, the heat flux density influence range also moves. The part with the largest influence range is the center line part of the rectangular laser spot. Compared with other parts, a larger temperature gradient appears; 2) In the process of increasing the laser cleaning speed from v=5 mm/s to v=8 mm/s, the overall temperature of the aluminum alloy oxide film slowly decreases with the increase of the cleaning speed. The higher laser cleaning speed does not match the parameters such as laser power and pulse frequency, resulting in a poor cleaning effect; 3) Through the laser cleaning experiment of the oxide film on the surface of 2024 aluminum alloy, a set of laser cleaning parameters with relatively ideal cleaning effect are obtained: laser power of 400 W, pulse frequency of 4.5 kHz, and cleaning speed of 5 mm/s.
