Aluminum alloy has excellent properties such as low density, high strength, corrosion resistance, good thermal conductivity, etc. It can be used to manufacture machine parts and components that bear large loads, and is therefore widely used in key parts of machinery, automobiles, electronics, aviation, and aerospace industries [1–4]. However, due to its active chemical property, aluminum alloys are prone to natural oxidation in the atmosphere [5, 6]. Traditional means of removing oxide film include physical and chemical methods, which are prone to causing physical damage such as scratches on the surface of aluminum alloy, polluting the environment, and not meeting the requirements of green cleaning. However, laser cleaning technology mainly uses the energy of a laser beam to act on the oxide film, and through a series of chemical and physical reactions, the oxide film is stripped off from the material surface. Laser cleaning has the advantages of less matrix damage, no pollution, simple process, wide adaptability high efficiency, etc., which is more in line with the current needs for environmental protection and socially sustainable development [7, 8]. In recent decades, laser cleaning technology has been increasingly improved and gradually applied to various metal materials [9, 10]. Kumar et al. [11] used lasers of different wavelengths to clean thin oxides on a tungsten band. The results indicate that at low energy densities with wavelengths of 1,064 nm and 532 nm, the removal mechanism is mainly spallation, and the higher laser fluence leads to the sublimation of the oxide layer. Chen et al. [12] analyzed a method of laser stripping film with theory, and revealed the stripping mechanism in the process of laser cleaning aluminum alloy, put forward two kinds of mechanisms (phase explosion and ablation) in the process of laser stripping of aluminum alloy, and explored the cleaning threshold for the ablation mechanism. Tong et al. [13] experimented with laser cleaning of aluminum alloy. Through simulation, it was determined that the main mechanism was vaporization. It was found that the secondary oxidation of the aluminum alloy occurred when the laser energy was greater than 25 W. However, due to the great physical and chemical differences between the aluminum layer and the oxide layer, research on laser cleaning of the oxide layer is very limited. Wang et al. [14] studied the mechanism of laser cleaning of 7075 aluminum alloy oxide film based on shock waves generated by gasification and explosion, which can remove a large amount of oxide film without damaging the substrate. Zhu et al. [15] investigated the use of a Q-switched diode-pumped solid-state (DPSS) laser to clean 5A12 aluminum alloy. After laser cleaning, the surface cleanliness and strength of the aluminum alloy were significantly improved.

In this paper, the cleaning of oxide film on the surface of 7A04 aluminum alloy was studied by changing the laser fluence. In the experiment, by observing the effect of laser cleaning on the surface morphology of aluminum alloy, the corresponding threshold range of laser cleaning was obtained, and its mechanism was analyzed. These results provide practical theoretical data and process options for the laser cleaning of aluminum alloy oxide film on the surface of high-speed train bodies and parts, which are of great significance for engineering applications.

As Fig. 1 shows, the experimental measurement system is schematic. The laser emitted by a Vlite-300 nanosecond laser (wavelength: 1,064 nm, pulse width: 12 ns, and beam intensity exhibits an approximately Tophat beam distribution in space; Beamtech Optronics, Beijing, China) was focused on the target by going through a focusing lens. The high-speed photograph and laser shadowgraph method were used in the plasma expansion measurement system to record the morphology and evolution of plasma. The system included a 532 nm continuous green laser with single-frequency operation, a beam expander, focusing lens, and high-speed camera (Fastcam SA-Z; Photron, Tokyo, Japan). The shooting speed of the camera was 10,000 fps, the resolution was 640 × 280, and the exposure time was 1/119149 s. The specification of the beam splitter was 80:20 (transmittance:reflectivity), with 80% of the laser light shining on the target material and 20% of the laser light shining on the probe of the energy meter. An Ophir energy meter recorded the laser energy in real time, and a Kleiber KMGA 740 infrared thermometer (KLEIBER Infrared GmbH, Unterwellenborn, Germany) was used to record and monitor the process of temperature change at a point on the measurement target surface. The testing equipment to characterize the effect of laser cleaning aluminum alloy oxide film was a Leica CTR6000 metallographic microscope (Leica Biosystems, WA, USA). The diameter of the laser spot was controlled by changing the distance between the focusing lens and the target material. The laser spot radius was 0.50 mm, 1.05 mm, and 1.40 mm, respectively, for experiments. The output voltage of the nanosecond laser was changed to control the output laser energy. The test target was 7A04 aluminum alloy with an area of 25 mm × 25 mm and a thickness of 1.5 mm, and the thickness of the oxide film was about 5 μm. Before laser cleaning, the samples were subjected to acetone cleaning to remove surface oils and allow for air-drying at room temperature. The samples were subsequently stored for approximately two months under controlled conditions of constant temperature (25 ℃) and humidity (35% RH), during which time natural oxidation occurred. The experiment was conducted at room temperature and under normal atmospheric pressure, with each nanosecond laser fluence condition repeated five times. The data analyzed in this study represent the average of five experimental repetitions.

Figure 1. The experimental facility for laser cleaning of aluminum alloy oxide film.

When laser light is incident on a target, the oxide film absorbs the laser light and converts it into thermal energy for the system, which manifests itself by causing temperature change. When the laser fluence reaches a high enough temperature to exceed its melting and boiling points, combustion, decomposition, or vaporization occurs, resulting in removal from the surface of the adsorbed substrate.

In industrial production, the surface area of the object being cleaned is much larger compared to its thickness. Therefore, the process of laser cleaning can be considered as heating an infinitely thin plate using a laser and can be described using a one-dimensional heat conduction equation. The assumptions for this description are: (1) The oxide layer and substrate material are uniform and isotropic. (2) The target material is infinite. (3) The temperature at infinity along the radial and z-directions is room temperature. (4) The convective and radiative heat losses on the surface of the target material can be ignored. We established a Cartesian coordinate system, and the heat conduction equation can be described as:

where ρ is the material density, C is the specific heat capacity, K is the thermal conductivity, T is the instantaneous temperature, t is the laser action time, and Q is the heat generation per unit volume of material per unit time. The surface heat flux density at Z = 0 follows a uniform distribution. The heat generation of a nanosecond laser is given as:

where α(T) is the light absorption coefficient, and f (x, y)g(t) is the spatiotemporal distribution of the laser beam. Solving the above equation gives the temperature distribution.

The surface evaporation rate of the target material is

where L_v is the latent heat of the substance, c is the specific heat capacity, and T₀ is room temperature. Therefore, the gasification depth is

where t_m is the melting point.

To understand the influence of laser characteristics on cleaning efficiency, the finite element simulations of different laser energy fluence were carried out. The simulation of laser irradiation was conducted using COMSOL Multi-physics field coupled finite element simulation software. The settings of relevant parameters during laser cleaning of aluminum alloy oxide film are shown in Table 1.

TABLE 1. Thermal and physical properties parameters of substrate and oxide film.

Properties	Oxidation Film	Substrate
Thermal Conductivity (W/m·K)	10	164
Specific Heat Capacity (J/kg·K)	750	896
Density (kg/m3)	3,970	2,700
Thermal Expansion Coefficient (K−1)	8.6 × 10−6	23.5 × 10−6
Poisson’s Ratio (%)	0.22	0.33
Vaporization Temperature (K)	3,253	2,793
Latent Heat of Matter (J/kg)	8.373 × 106	9.462 × 106

Figure 2 shows the curve of temperature variation with time for the surface of the oxide film irradiated with different power densities for a laser radius of 1.4 mm. When the laser irradiates the material surface, energy accumulates on the material surface, the temperature rises rapidly to the boiling point of the oxide film, and the oxide layer evaporates. When the laser irradiation ends, the energy accumulation effect diminishes, the heat continues to diffuse, and the temperature drops rapidly. As the temperature decreases, it briefly remains near the gasification temperature of the oxide film due to the presence of latent heat effects, when the energy fluence is 1.41 J/cm², thus creating a temperature plateau period. The plateau period occurs only at higher energies. In addition, the higher the maximum power density, the higher the temperature at the end of single-pulse loading.

Figure 2. Change of temperature under different energy fluence when the spot radius is 1.4 mm.

Figure 3 shows the variation curves of oxide film cleaning depth with different laser radii and different laser fluences, as well as a comparison with the profile of the incident laser pulse. When the laser spot radius is 0.5 mm and the laser fluence is 0.33 J/cm², the cleaning depth is 0.07 μm. At this time, the oxide film has just been cleaned. When the laser fluence is 1.09 J/cm², the cleaning depth is 4.96 μm. The axial radius of the melt crater is also close to the radius of the corresponding irradiated laser spot.

Figure 3. Change curves of oxide film cleaning depth and profile of incident laser pulse. The laser spot radius varied as (a) 0.5 mm, (b) 1.05 mm, and (c) 1.4 mm, respectively.

Figure 4 shows the temperature distribution obtained using an infrared spot thermometer during the cleaning experiment when the laser radius is 1.4 mm. It can be seen that the temperature trend is similar to that of the simulation, and the maximum temperature values reached are similar.

Figure 4. Temperature experimental results under different energy fluences when the spot radius is 1.4 mm.

The cleaning results were observed by metallographic microscopy, as shown in Fig. 5. The initial surface of the untreated aluminum alloy had a large amount of gray-black oxide adhering to the substrate with natural form defects, including scratches and surface damage. After absorbing the laser energy, the aluminum was rapidly evaporated or heated to high temperatures in a very short period of time, and the pressure became high due to thermal coupling, making the molten substrate and oxide layer splash. There were obvious traces of solidification at the extreme edges, which may be due to the ripples formed within the fluid as the metal melts and then re-solidifies. When the laser spot radius was small, it may be due to the shielding effect of the plasma, which affected the radiation effect of the laser energy, and the spot appeared unevenly round.

Figure 5. Micrographs of laser cleaning at different laser energies and different laser spot radii. The energy fluence change of nanosecond laser varied as follows: (a) 0.39 J/cm², (b) 0.77 J/cm², (c) 1.09 J/cm², (d) 1.88 J/cm², (e) 0.41 J/cm², (f) 0.72 J/cm², (g) 0.93 J/cm², (h) 1.21 J/cm², (i) 0.42 J/cm², (j) 0.81 J/cm², (k) 1.18 J/cm², and (l) 1.45 J/cm².

As shown in Fig. 6, the plasma motion process when the target surface was cleaned by different laser energy densities was captured by an ultra-high-speed camera. Nanosecond laser-induced plasma was rapidly generated and expanded, generating a shock wave in the opposite direction to the laser, and quickly left the target surface. The high temperature of the plasma core produced a high evaporation pressure, so that when the molten oxide, substrate, and thermal coupling effect combined, the cleaning process may have spatter particles generated. The higher the laser fluence of the laser, the farther the longitudinal expansion distance and the transverse expansion distance of the plasma. The loss of radiant energy due to heat conduction and the loss of laser energy due to gas expansion, both heat losses, can have a large impact on the plasma shockwave.

Figure 6. Plasma evolution during cleaning process taken by high-speed camera. The energy fluence change of the nanosecond laser varied as follows: (a) 0.33 J/cm², (b) 0.66 J/cm², and (c) 1.09 J/cm².

Finally, we discuss the mechanism of single-point cleaning of aluminum alloy oxide films with different nanosecond laser energy densities, as shown in Fig. 7. According to the [16], selective vaporization usually occurs when the absorption rate of laser light by surface contaminants is significantly higher than the absorption rate of laser light by the substrate. According to the physical properties, it is known that the absorption rate of aluminum alloy oxide film to laser light is much greater than the absorption rate of aluminum alloy to laser light, so ablation vaporization is the main cleaning mechanism. When the laser energy is relatively small and radiated to the target surface, the oxide film surface absorbs the laser energy and converts it into internal energy, causing the surface temperature to rise rapidly above the vaporization temperature of the material, thus causing the oxide film to be removed from the material surface in the form of vaporization. With the increase in laser energy, the temperature reaches the point at which the plasma is ignited, ionization produces plasma, and plasma shock wave pressure on the oxide layer is caused by sputtering, which can be used as an auxiliary mechanism of laser cleaning. The laser cleaning mechanism should consider both the nature of the contamination layer and the substrate layer.

Figure 7. The mechanism of laser cleaning aluminum alloy oxide film.

In this work, we conducted an experimental study on the nanosecond laser single-point cleaning of aluminum alloy oxide film to obtain the cleaning threshold for different laser spot radii. We investigated the effect of laser fluence on the cleaning efficiency of the aluminum alloy oxide film and verified the simulation results. Additionally, we analyzed the mechanism of laser cleaning of aluminum alloy oxide film. The results showed that when the laser fluence was less than 0.33 J/cm², the nanosecond laser single-point cleaning of aluminum alloy oxide film had almost no cleaning effect. As the laser fluence increased, the cleaning phenomenon and cleaning efficiency on the substrate surface improved.

There are two different mechanisms of cleaning in the cleaning process. The first is the ablation effect, where laser irradiation of the oxide film on the surface of the substrate causes the oxide film to absorb the energy of the laser and convert it into heat. As a result, the temperature of the material surface rises above the melting point of the oxide film, and it is removed from the adsorbed substrate surface. The second cleaning mechanism is that the laser reaches the metal substrate surface through the oxide film, creating a plasma between the substrate and the oxide film. When the laser energy reaches a certain value, it triggers plasma expansion and explosion, producing a plasma shock wave, which causes the oxide film layer to be removed. In future research, a combination of two laser systems can be considered for cleaning aluminum alloy oxide films to further expand related applications.

We are grateful to the Jilin Key Laboratory of Solid-State Laser Technology and Application for supporting the equipment in the experiments.

National Natural Science Foundation of China (Grant No. U19A2077).

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

The data that support the findings of this study are available from the corresponding author upon reasonable request.