Study on the Laser-Resistant Coating Protection Effect of 7075 Aluminum Alloy

1. Introduction

With the rapid development of laser weapon technology, the demand for laser irradiation protection of aerospace materials is increasingly urgent [1,2,3,4,5]. Studies have shown that under laser irradiation, metallic materials will experience temperature rise, melting, vaporization, spattering and other phenomena; while non-metallic materials may suffer from sublimation, chemical reactions, coking, exfoliation and other problems, eventually leading to structural failure [6,7]. At present, the main protection method to solve this problem is to coat functional coatings on the target surface to improve the laser-induced damage resistance. In terms of functional characteristics, existing coatings can be divided into four categories: reflective, ablative, thermal insulating and composite [8]. In recent years, high-reflectivity materials have attracted much attention due to their excellent laser protection performance. Such materials effectively reflect laser energy, making the energy acting on the substrate lower than the material damage threshold, thus achieving effective protection of the substrate and becoming a research focus of current laser protection materials [9,10]. Numerous studies have demonstrated that high-reflectivity coatings play a vital role in laser-induced damage resistance: Zhu et al. investigated the laser ablation behavior of plasma-sprayed perovskite coatings and found that the temperature rise rate was significantly inversely proportional to the coating reflectivity, confirming the key role of reflectivity in laser protection [11]. Zheng et al. further studied the ablation characteristics of plasma-sprayed Ta₂O₅ coatings, indicating that coatings with low initial reflectivity could achieve a significant increase in reflectivity after laser irradiation [12]. In the development of new high-reflectivity coatings, Yang et al. prepared a novel laser protection coating using phenolic resin and zirconium dioxide as raw materials. When irradiated for 8 seconds at a laser power density of 1503 W/cm², the reflectivity increment of the coated surface reached the maximum (75.7%) [13]. Experiments showed that the excellent heat absorption and thermal conductivity of the coating enabled it to effectively resist high-intensity continuous laser irradiation. In addition, Yang et al. developed a laser ablation-resistant composite coating by coating a SiC whisker and ZrB₂ particle-reinforced ZrO₂ sol-gel coating on an aluminum alloy substrate. After irradiation for 10 seconds at a laser power density of 6.3 kW/cm², the backside temperature of the substrate was only 150 ℃ [14].

Given that the study of ablation effect is an important part of evaluating the performance of protective coatings [15], this paper takes 7075 aluminum alloy, a typical aerospace material, as the substrate, coats a high-reflectivity laser-resistant coating on its surface, systematically studies its damage characteristics under continuous laser irradiation, and comprehensively analyzes its protective performance by combining numerical simulation and experimental verification.

2. Numerical Simulation Model and Parameter Settings

2.1. Software Introduction

The numerical simulation in this paper adopts COMSOL Multiphysics software, a powerful multi-physics field simulation software widely used in thermal, electromagnetic, structural, acoustic, fluid and other fields, with significant advantages especially in dealing with complex coupling problems.

In laser simulation, COMSOL supports the accurate construction of Gaussian heat source models and can define the heat flux distribution towards the target surface according to laser power, spot radius and spatial distribution. Compared with traditional software, COMSOL can not only conveniently express laser input in the form of Gaussian function, but also conduct detailed modeling of laser energy changes with time, space or material response through custom expressions, which is especially suitable for the analysis of laser transient heating, periodic pulse loading and multi-pulse cumulative effects.

2.2. Thermal Model

The thermal model describes the heat conduction process inside the material after laser energy input, which is the basic part of the entire thermo-mechanical coupling analysis. Considering the temporal variation and spatial inhomogeneity during laser irradiation, a transient heat conduction model combined with Gaussian heat source input is used to construct the real thermal loading boundary.

The heat conduction process follows Fourier’s law, and its governing equation is:

$$\rho C_P{\displaystyle \frac{\partial T}{\partial t}}=\mathsf{\nabla }\cdot \left(k\mathsf{\nabla }T\right)+Q$$

(1)

In the equation, $\mathsf{\rho }$ is the material density, ${\mathrm{C}}_{\mathrm{P}}$ is the specific heat capacity, $\mathrm{k}$ is the thermal conductivity, $\mathrm{T}$ is the temperature, $\mathrm{t}$ is the time, and $\mathrm{Q}$ is the heat source term representing the energy input per unit volume. Under laser loading, $\mathrm{Q}$ is converted from the heat flux on the target surface and applied to the material surface in the form of boundary heat flux.

To accurately reflect the spatial distribution characteristics of the laser beam, the heat source term is defined by a two-dimensional Gaussian distribution function, expressed on the target surface as:

$$q\left(r\right)={\displaystyle \frac{2P}{\pi R^2}}exp(-2{\displaystyle \frac{r^2}{R^2}})$$

(2)

where $P$ is the total laser power, $R$ is the laser spot radius, and $r$ is the radial distance from the laser incident center. This heat flux is applied to the upper surface of the material and acts continuously with time to simulate the continuous laser irradiation process.

2.3. Geometric Model

The geometric model is constructed for two types of targets: 7075 aluminum alloy target and 7075 aluminum alloy target coated with laser-resistant coating, to compare their thermo-mechanical response differences. To simplify the simulation calculation, the model adopts a two-dimensional structure taken from the central section of the circular target, and the corresponding geometry is built in the software with boundary conditions applied.

The 7075 aluminum alloy target model is a single-material circular target with a diameter of $\mathrm{\varnothing }$110 mm and a thickness of 3 mm. The 7075 aluminum alloy target model coated with laser-resistant coating is based on a 7075 aluminum alloy substrate with a diameter of $\mathrm{\varnothing }$110 mm and a thickness of 2.95 mm, coated with a 50 μm-thick laser-resistant coating, keeping the overall geometric dimensions consistent with the 7075 aluminum alloy target model. The contact condition between the laser-resistant coating and the substrate is set as perfect contact.

2.4. Boundary Setting Method and Basis

In the thermo-mechanical coupling simulation model, for thermal boundary conditions, the laser heat source is applied to the top surface of the geometric model through Gaussian distribution heat flux, whose spatial distribution follows the mathematical description of Gaussian function, consistent with the energy distribution characteristics of the actual laser beam. Considering the transient characteristics of laser action and the dominant effect of high-energy input, the influence of convection and radiation is ignored, and all boundaries except the laser loading surface are set as adiabatic conditions.

For mechanical boundaries, a free displacement constraint strategy is adopted, that is, no external fixed constraint is applied to the entire target, aiming to eliminate the interference of boundary conditions on the local stress field and purely present the inherent thermal stress distribution caused by temperature gradient. The stress field is generated through the thermal expansion constitutive relation.

2.5. Software Parameter Settings

In the numerical simulation, the total laser power is set to 8 kW, the laser spot radius is 1.5 cm, and the laser action time is 30 s. Given that this simulation focuses on the thermo-mechanical response under instantaneous laser action with a short laser duration, constant physical parameters of each material at room temperature are adopted in the simulation to improve the stability and simplicity of model calculation, ignoring the influence of temperature on the thermophysical and mechanical properties of materials. The relevant material property parameters involved are shown in Table 1.

For solution parameter settings, considering the short laser loading time and drastic temperature field change, the simulation adopts transient multi-physics field coupling solution, using the software’s built-in automatic time step control and BDF implicit time integration method to ensure stable solution in the nonlinear region. The overall time interval is set to 30 s, and the calculation step is set to 0.2 s.

3. Test Conditions and Methods

3.1. Test Equipment and Working Conditions

The equipment used for laser irradiation test mainly includes fiber laser, beam expander system, laser power meter, high-speed camera, supplementary light source, thermocouple and temperature dynamic acquisition system matched with thermocouple.

To better compare the laser-induced damage resistance of 7075 aluminum alloy before and after coating with laser-resistant coating, on the one hand, variables are strictly controlled to ensure the consistency of laser action conditions and target geometric dimensions; on the other hand, three repeated tests are carried out for both test materials to reduce random errors.The test optical path and equipment layout are shown in Figure 1.

To prevent laser reflected light from damaging the laser, the laser incident angle is set to 5°.

3.2. Test Method

In the test, the damage process of the target during laser irradiation, the temperature change at the center of the target back surface and the breakdown time of the target were obtained respectively.

Since the center of the target shows a state of high brightness and molten material spattering at the moment of breakdown, to more accurately obtain the target breakdown time, a combination of high-speed camera images, thermocouple measured temperature and power meter measured values is used to judge the target breakdown time. The judgment method of thermocouple measured temperature for target breakdown time is: when the target is broken down, the thermocouple is directly irradiated by laser, and the temperature instantly exceeds the thermocouple range, resulting in an abnormal jump in the measured temperature. The time when this phenomenon occurs is taken as the target breakdown time. The judgment method of power meter measured value for target breakdown time is: start the power meter and collect data at the same time as the laser emits laser. When the target is broken down, the power meter will receive laser and show a measured reading. The time when the power meter starts to show a reading is taken as the target breakdown time.

4. Result Analysis

4.1. Numerical Simulation Results

The numerical simulation results of 7075 aluminum alloy target are shown in Figure 2.

The results show that the 7075 aluminum alloy target is broken down after 5.8 s of laser irradiation, and the breakdown aperture increases with time until it approaches the laser beam diameter.

The numerical simulation results of the target coated with laser-resistant coating are shown in Figure 3.

The results show that after 5.8 s of laser irradiation, the maximum temperature of the 7075 aluminum alloy target coated with laser-resistant coating is about 426℃, and after 30 s of irradiation, the maximum temperature of the target only reaches about 656℃, close to the melting point of 7075 aluminum alloy, without breakdown, indicating that the laser-resistant coating has good protective effect.

4.2. Test Results

From the images captured by the high-speed camera in the test, shown in Figure 4 to Figure 6, under the action of laser irradiation, the surface of the 7075 aluminum alloy target gradually melts and bulges with the increase of temperature, then the bulge ruptures, molten material spatters, and the target is broken down. It can be seen from the images that the initial melting time of the target surface is 1.2 s, 1.2 s and 1.1 s respectively, and the breakdown time is 4.0 s, 4.3 s and 4.1 s respectively.

Figure 4. High-speed camera shooting results of the first target of 7075 aluminum alloy

Figure 4. High-speed camera shooting results of the first target of 7075 aluminum alloy

Figure 5. High-speed camera shooting results of the second target of 7075 aluminum alloy

Figure 5. High-speed camera shooting results of the second target of 7075 aluminum alloy

Figure 6. High-speed camera shooting results of the third target of 7075 aluminum alloy

Figure 6. High-speed camera shooting results of the third target of 7075 aluminum alloy

As shown in Figure 7 to Figure 9, after coating with laser-resistant coating, the target surface shows no reaction for a long time under laser irradiation. With the deposition of laser energy, the coating material on the target surface begins to melt and glow, followed by molten material spattering and target breakdown. The test results show that the initial melting time of the target surface coated with laser-resistant coating is 25.0 s, 25.1 s and 23.1 s respectively, and the breakdown time is 27.2 s, 27.2 s and 25.1 s respectively.

Figure 7. High-speed camera shooting results of the first target of 7075 aluminum alloy with coating

Figure 7. High-speed camera shooting results of the first target of 7075 aluminum alloy with coating

Figure 8. High-speed camera shooting results of the second target of 7075 aluminum alloy with coating

Figure 8. High-speed camera shooting results of the second target of 7075 aluminum alloy with coating

Figure 9. High-speed camera shooting results of the third target of 7075 aluminum alloy with coating

Figure 9. High-speed camera shooting results of the third target of 7075 aluminum alloy with coating

The temperature rise curve at the center of the target back surface measured by thermocouple sensor is shown in Figure 10. According to the measurement results, the front surface temperature of the 7075 aluminum alloy target is transmitted to the back surface in about 2 s, and the back surface temperature rises rapidly, reaching the melting point of 7075 aluminum alloy in about 3.5 s, and an abnormal jump in temperature begins to occur in about 4.5 s. For the target coated with laser-resistant coating, the back surface also begins to receive the temperature transmitted from the front surface in about 2 s of laser action. Over time, the back surface temperature rises slowly and remains below 400℃ for a long time. Until about 27 s, an abnormal jump in the back surface temperature of the target occurs.

The power meter measurement results are shown in Figure 11. The results show that in the three tests of 7075 aluminum alloy target, the power meter collects laser power at about 4 s; in the three tests of target coated with laser-resistant coating, the power meter collects laser power at about 27 s in the first two tests and at about 25 s in the third test.

4.3. Comparative Analysis of Results

Combined with the aforementioned numerical simulation and test results, the breakdown time of 7075 aluminum alloy target in numerical simulation is slightly later than that in actual test, and the laser-resistant coating target in numerical simulation does not break down after 30 s of laser irradiation, while the actual test target breaks down about 25 s, showing a deviation between numerical simulation and actual test. Preliminary analysis may be caused by the following two reasons: on the one hand, the material parameter settings in numerical simulation tend to be idealized, and the surface condition, local coating composition and reflectivity of the target used in actual test are different from those in numerical simulation, resulting in result differences; on the other hand, the microstructure and mechanical properties of 7075 aluminum alloy will change after reaching the phase transition temperature, which is ignored in numerical simulation, resulting in result differences.To better analyze the test results, the target breakdown times obtained by different methods in the test are further sorted out in Table 2.

From the average value of target breakdown time obtained by three different measurement methods, there are slight deviations among the three methods, with the maximum deviation of only 0.67 s, all of which can effectively judge the target breakdown time. Taking the high-speed camera shooting results as the main reference, the average breakdown time of 7075 aluminum alloy target is 4.13 s, while that of the target coated with laser-resistant coating is 26.5 s, prolonged by 541.6%. Based on this, it can be concluded that the laser-resistant coating has good protective effect.

5. Conclusions

Numerical simulation and test results show that the laser-resistant coating prepared in this paper exhibits significant performance on the surface of 7075 aluminum alloy, prolonging the breakdown time of the material under the same laser action conditions by 541.6%, effectively enhancing the laser-induced damage resistance of the material, and has good application potential and development value. In the future, the laser protection performance of the laser-resistant coating in complex environments will be further improved and the application scenarios will be expanded through continuous optimization of the preparation process and material formula, providing more comprehensive technical solutions for different fields.