A composite laser-based scanning cleaning method for aircraft skin

Abstract

The present invention discloses a composite laser-based scanning cleaning method for aircraft skin, including: S1, constructing a composite laser paint removal device that outputs continuous laser and pulsed laser; S2, using the composite laser paint removal device to perform a scanning cleaning experiment on the aircraft skin, and determining the optimal paint removal parameters at a preset laser moving speed; S3, based on the optimal paint removal parameters, fitting a relationship model between power density and cleaning depth within the optimal power density range of the continuous laser during composite laser paint removal; S4, scanning cleaning of the aircraft skin according to the relationship model at a preset laser moving speed. The method of the present invention comprehensively considers the effects of continuous laser and pulsed laser, determines the optimal paint removal parameters during composite laser cleaning during scanning cleaning, and fits a relationship model between power density and cleaning depth at a certain moment, and the laser parameters can be adjusted based on the relationship model, providing a reference for laser cleaning of aircraft skin.

Description

Technical Field

[0001] The invention belongs to the technical field of laser paint removal, and in particular relates to a composite laser-based aircraft skin scanning cleaning method. Background Art

[0002] Aircraft skin is used to protect the aircraft and is very important for safe flight. Due to aging and damage, it needs to be cleaned and repainted after a certain period of use. Traditional methods for cleaning aircraft skin include chemical methods and mechanical sanding, which have disadvantages such as environmental pollution and low efficiency. Laser cleaning technology has the advantages of being green, pollution-free, non-contact, and highly efficient. It has been extensively studied in the fields of industrial paint removal and rust removal and has been widely used. Research on laser cleaning is mainly divided into laser cleaning mechanism and physical modeling, cleaning parameter control, etc.

[0003] Theoretical research on laser cleaning mainly considers the effects of thermal ablation, thermal stress vibration, and laser plasma. These three have different effects on cleaning. Thermal ablation is easy to leave residues of ablated materials, thermal stress vibration has the best effect, and can achieve stress stripping of paint. The generation of plasma is easy to produce high-pressure shock waves that damage the substrate. Since the parameters of the laser will affect all three effects, the study of the influencing rules has also been carried out. For example, Roberts and Zhang Donghe found that the properties of the cleaning object and the absorption coefficient of the laser will also affect the cleaning effect. Zhu Guodong's team studied Nd:YAG laser energy density of 2-6J / cm 2 The cleaning effect of BMS10-11 coating on the skin of Boeing series aircraft was verified, and the cleaning effect of this parameter on the aircraft skin was confirmed. In fact, the most important laser parameters are the laser operation mode (pulse or continuous), laser energy density, laser movement speed, and laser overlap rate. The precise control of these parameters can ensure the cleaning quality. For example, Siedel's team used an energy density of 0.33J / cm 2 -0.51J / cm 2 Lasers in the range of 100W can remove dirt and paint layers from weathered surfaces with good results; Ji-Eon Kim's team used a Q-switched fiber laser with an average power of 100W to remove primer and oxide layers from the surface of ship steel and found that the use of an energy density of 10.5J / cm 2 The best cleaning effect is achieved when the laser beam overlap is 20%; Brygo found that the laser flux is 1.5 J / cm 2 The 532nm nanosecond pulsed Nd:YAG laser with a repetition rate of 10kHz has the best paint ablation efficiency; Kumar proposed a laser energy density of 7.2J / cm 2The CO2 laser with a laser pulse frequency of 100 Hz has the best effect on removing 40 μm thick epoxy resin paint on the surface of aluminum alloy plate at a scanning speed of 35 mm / s; Shan Teng's team used a nanosecond fiber pulse laser with a maximum power of 30 W to study the removal of 2024 aluminum alloy paint layer and found that when the energy density was 21.23 J / cm 2 Laser cleaning has the best cleanliness and surface integrity. Most of the above studies focus on using a single-mode laser to remove a single paint layer. However, the paint on the surface of the aircraft skin consists of many layers, and the physical and chemical properties of each paint layer are different. It is very difficult to use a single-mode laser to clean each layer of paint.

[0004] The surface of aircraft skin is a multi-layer paint structure, and it is very difficult to clean it using a single-mode laser. For example, Wang Zhenzhou and others used pulsed lasers to conduct layered cleaning of aircraft skins, and obtained the principle of parameter control, achieving perfect cleaning, but pulsed lasers are difficult to control and have low efficiency. In order to improve cleaning efficiency, continuous lasers are often used, but it is easy to cause the paint layer ablation to adhere to the substrate; if pulsed lasers are used alone, although the paint layer can be removed, it is very easy to cause excessive pulsed laser energy, causing irreversible damage to the substrate. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the composite laser-based aircraft skin scanning cleaning method provided by the present invention solves the above-mentioned technical problems.

[0006] In order to achieve the above-mentioned purpose of the invention, the technical solution adopted by the present invention is: a composite laser-based aircraft skin scanning cleaning method, comprising the following steps:

[0007] S1. Constructing a composite laser paint removal device that outputs continuous laser and pulsed laser;

[0008] S2. Use a composite laser paint removal device to conduct a scanning cleaning experiment on the aircraft skin to determine the optimal paint removal parameters at a preset laser moving speed;

[0009] S3. Based on the optimal paint removal parameters, a relationship model between power density and cleaning depth within the optimal power density range of continuous laser during composite laser paint removal is fitted;

[0010] S4. At the preset laser moving speed, the aircraft skin is scanned and cleaned according to the relational model.

[0011] Furthermore, in step S1, the composite laser paint removal device includes a continuous laser and a pulse laser, and the continuous laser and pulse laser emitted by the continuous laser and the pulse laser respectively pass through a beam splitter and a reflector and then act on the aircraft skin through a focusing lens.

[0012] Further, in the composite laser paint removal device, the angle between the continuous laser and the pulse laser when incident on the focusing lens is 20°;

[0013] The transmission and reflection energy ratio of the corresponding split laser beams obtained by the continuous laser and the pulse laser through the beam splitter is 8:2;

[0014] The continuous laser includes a flat top beam and a Gaussian beam, and the Gaussian beam covers the center of the flat top beam.

[0015] Furthermore, in the step S2, in the strip scanning cleaning experiment, the ablation effect of the continuous laser plays a major role in cleaning the aircraft skin, and the stress effect of the pulsed laser plays a minor role in cleaning the aircraft skin.

[0016] Furthermore, in step S2, the optimal paint removal parameters determined include the optimal power density range of the continuous laser at a preset laser moving speed, and the optimal energy density and frequency of the pulsed laser;

[0017] Among them, the optimal power density range of the continuous laser is determined based on the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the flat-top beam ablates the aircraft skin, and the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the Gaussian beam ablates the aircraft skin.

[0018] Furthermore, the step S2 is specifically as follows:

[0019] S21, based on the optimal paint removal parameters when the composite laser is used to clean a small circular area of ​​the aircraft skin, and according to which the adjustment range of the power density of the continuous laser is set;

[0020] Among them, the optimal paint removal parameters for cleaning small circular areas include the optimal power density of the flat-top beam and the optimal Gaussian beam power density in the continuous laser, the optimal energy density and frequency of the pulsed laser, and the optimal cleaning time;

[0021] S22, adjusting the power density of the continuous laser within the adjustment range, and using the adjusted power density of the continuous laser, the optimal energy density and frequency for cleaning a small circular area as paint removal parameters at a preset laser moving speed, to perform a scanning cleaning experiment on the aircraft skin;

[0022] S23, continuously adjusting the power density of the continuous laser, analyzing and comparing the corresponding cleaning effects, determining the power density corresponding to partial ablation and damage ablation effects when the continuous laser ablates the aircraft skin, and then determining the optimal power density range of the continuous laser;

[0023] S23. The optimal power density range of the continuous laser, the optimal energy density and frequency when cleaning a small circular area are used as the optimal paint removal parameters for scanning cleaning.

[0024] Furthermore, the step S3 is specifically as follows:

[0025] S31. Based on the equal energy density, a power density conversion equation between scanning cleaning and small circular area cleaning is constructed;

[0026] S32, substituting the experimental data corresponding to the optimal paint removal parameters for cleaning a small circular area into the power density conversion equation to obtain a power density expression for scanning cleaning;

[0027] S33, introducing a heat flux condition to simulate the heat loss caused by the ablation of the aircraft skin paint layer based on the boundary condition, and determining the ablation rate of the paint layer;

[0028] S34, substituting the ablation rate of the paint layer into the power density expression of scanning cleaning, and integrating it with respect to the action time to obtain an expression for the ablation depth during the scanning cleaning process;

[0029] S35, substituting the lower boundary value of the optimal power density range of the continuous laser into the expression of the ablation depth, and fitting it according to the corresponding cleaning experimental data to obtain the relationship between the cleaning time and the ablation depth under the corresponding power density;

[0030] S36, according to the upper boundary value of the optimal power density range, setting the magnitude change ratio of the power density of the continuous laser on the basis of the lower boundary value;

[0031] S37. Substitute the change ratio of the power density of the continuous laser into the relationship between the cleaning time and the ablation depth to obtain a relationship model between the power density and the cleaning depth within the optimal power density range of the continuous laser.

[0032] Furthermore, in step S31, the power density conversion equation is:

[0033]

[0034] Where P represents the power density of the continuous laser during scanning cleaning, P d represents the optimal power density of continuous laser when cleaning a small circular area, where v represents the scanning speed during scanning cleaning, t represents the scanning time during scanning cleaning, r represents the spot radius, and t d Indicates the optimal cleaning time of continuous laser when cleaning small circular areas;

[0035] In step S32, the power density expression of scanning cleaning is:

[0036]

[0037] In step S33, the expression of the ablation speed of the paint layer is:

[0038] v n =q n / ρ n H s

[0039] In the formula, v n Indicates the ablation rate of the paint layer, q n Represents the energy absorbed by the paint layer per unit area, ρ n Indicates the density of the paint layer, H s Indicates the sublimation heat of the paint layer;

[0040] In step S34, the power density expression of scanning cleaning is substituted into the ablation rate of the paint layer to obtain:

[0041]

[0042] Then the expression of ablation depth in scanning cleaning process is obtained as follows:

[0043]

[0044] In the formula, c represents the definite integral parameter.

[0045] The beneficial effects of the present invention are:

[0046] (1) The present invention uses a composite laser formed by a combination of continuous laser and pulsed laser to clean the aircraft skin, which effectively solves the problems of difficulty and low efficiency in cleaning the aircraft skin using a single-mode laser, and the ablation products produced by continuous laser ablation of the paint layer adhere to the substrate and are difficult to remove, causing irreversible damage.

[0047] (2) The method of the present invention comprehensively considers the effects of continuous laser and pulsed laser, determines the optimal paint removal parameters during scanning cleaning and composite laser cleaning, and fits the relationship model between power density and cleaning depth at a certain moment. The laser parameters can be adjusted based on the relationship model, providing a reference for laser cleaning of aircraft skin. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 A flow chart of a composite laser-based aircraft skin scanning cleaning method provided in an embodiment of the present invention.

[0049] Figure 2 A schematic structural diagram of a composite laser paint removal device provided in an embodiment of the present invention.

[0050] Figure 3 A schematic diagram of an aircraft skin sample provided in an embodiment of the present invention.

[0051] Figure 4 The scanning cleaning experiment results provided by the embodiment of the present invention.

[0052] Figure 5 The electron microscope morphology under different powers provided in the embodiment of the present invention.

[0053] Figure 6 A schematic diagram of a cleaning depth versus time curve provided in an embodiment of the present invention.

[0054] Figure 7 The cleaning depth variation curve obtained by theoretical analysis provided in the embodiment of the present invention is shown as follows: DETAILED DESCRIPTION

[0055] The specific implementation modes of the present invention are described below so that those skilled in the art can understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific implementation modes. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations utilizing the concept of the present invention are protected.

[0056] The embodiment of the present invention provides a composite laser-based aircraft skin scanning cleaning method, such as Figure 1 As shown, the following steps are included:

[0057] S1. Constructing a composite laser paint removal device that outputs continuous laser and pulsed laser;

[0058] S2. Use a composite laser paint removal device to conduct a scanning cleaning experiment on the aircraft skin to determine the optimal paint removal parameters at a preset laser moving speed;

[0059] S3. Based on the optimal paint removal parameters, a relationship model between power density and cleaning depth within the optimal power density range of continuous laser during composite laser paint removal is fitted;

[0060] S4. At the preset laser moving speed, the aircraft skin is scanned and cleaned according to the relational model.

[0061] In step S1 of the embodiment of the present invention, the composite laser paint removal device is constructed as follows Figure 2 As shown, it includes a continuous laser and a pulse laser, the continuous laser and the pulse laser emitted by the continuous laser and the pulse laser respectively pass through a beam splitter and a reflector and then act on the aircraft skin through a focusing lens; wherein the continuous laser includes a flat-top beam and a Gaussian beam, and the Gaussian beam covers the center of the flat-top beam.

[0062] In the composite laser paint removal device of this embodiment, the angle between the continuous laser and the pulsed laser when incident on the focusing lens is 20°. Specifically, the angle can be adjusted to ensure that the continuous laser and the pulsed laser cover the same area.

[0063] In this embodiment, the energy ratio of the transmission and reflection of the corresponding split laser beams obtained by the continuous laser and the pulsed laser through the beam splitter is 8: 2. Specifically, the ratio can be adjusted according to the requirements of cleaning the paint layer and power calculation.

[0064] Specifically, in this embodiment, the output beam of the continuous laser can be flat-top and Gaussian, with an output energy between 50W and 2000W, a minimum irradiation time of 30ms, and an output spot diameter of 700μm to 1100μm; the pulse laser energy range of the pulse laser output is 0.2J to 2J, the output frequency range is 1Hz to 20Hz, and the output spot diameter is 2mm.

[0065] When the composite laser paint removal device is working, the continuous laser and the pulsed laser are divided into two beams by a beam splitter, and 80% of the laser energy of the continuous laser and the pulsed laser are applied to the aircraft skin at an angle of 20° after passing through a 1064 reflector and a focusing lens (focal length of 20 cm), and the remaining 20% ​​of the laser energy is used for energy monitoring. In this embodiment, the aircraft skin is placed on a computer-controlled 3D platform for precise adjustment of laser cleaning.

[0066] In step S2 of the embodiment of the present invention, in the strip scanning cleaning experiment, the ablation effect of the continuous laser plays a major role in cleaning the aircraft skin, and the stress effect of the pulsed laser plays a minor role in cleaning the aircraft skin.

[0067] In step S2 of the embodiment of the present invention, the determined optimal paint removal parameters include the optimal power density range of the continuous laser at a preset laser moving speed, and the optimal energy density and frequency of the pulsed laser;

[0068] Among them, the optimal power density range of the continuous laser is determined based on the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the flat-top beam ablates the aircraft skin, and the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the Gaussian beam ablates the aircraft skin.

[0069] Step S2 of the embodiment of the present invention is specifically as follows:

[0070] S21, based on the optimal paint removal parameters when the composite laser is used to clean a small circular area of ​​the aircraft skin, and according to which the adjustment range of the power density of the continuous laser is set;

[0071] Among them, the optimal paint removal parameters for cleaning small circular areas include the optimal power density of the flat-top beam and the optimal Gaussian beam power density in the continuous laser, the optimal energy density and frequency of the pulsed laser, and the optimal cleaning time;

[0072] S22, adjusting the power density of the continuous laser within the adjustment range, and using the adjusted power density of the continuous laser, the optimal energy density and frequency for cleaning a small circular area as paint removal parameters at a preset laser moving speed, to perform a scanning cleaning experiment on the aircraft skin;

[0073] S23, continuously adjusting the power density of the continuous laser, analyzing and comparing the corresponding cleaning effects, determining the power density corresponding to partial ablation and damage ablation effects when the continuous laser ablates the aircraft skin, and then determining the optimal power density range of the continuous laser;

[0074] S23. The optimal power density range of the continuous laser, the optimal energy density and frequency when cleaning a small circular area are used as the optimal paint removal parameters for scanning cleaning.

[0075] In this embodiment, Figure 3 The Boeing 717 model aircraft skin sample shown in the figure is used as an example for the strip scanning cleaning experiment. The sample has a 1 mm bright silver aluminum alloy metal substrate. The aluminum alloy surface has three layers of materials: red polyurethane topcoat (thickness of about 79 μm), gray epoxy primer and yellow-green chemical conversion coating (thickness of about 130 μm).

[0076] In step S21 of this embodiment, the following optimal paint removal parameters are taken as an example when the composite laser is used to clean a small circular area of ​​the aircraft skin; specifically, the flat-top beam power density of the continuous laser part is about 52.64W / mm 2 , the Gaussian beam power density is about 56.62W / mm 2 , and the energy density of the pulse laser part is about 0.42J / mm 2 , the frequency is about 10Hz.

[0077] In steps S22-S23 of this embodiment, based on the optimal paint removal parameters for cleaning a small circular area, the continuous laser power was fluctuated by 40% as a control group, and the laser speed was set to 1 cm / s. Figure 4 A series of experiments are shown.

[0078] During the experiment, the strip scanning cleaning effect of composite lasers with different energy parameters on the paint removal of aircraft skin surface was studied. Figure 4 As shown, when the flat-top beam power density is 31.58W / mm 2 , Gaussian beam power density is 33.97W / mm2 The continuous laser and energy density are 0.42J / mm 2 When the composite laser composed of a pulse laser with a frequency of 10 Hz acts on the skin surface at a moving speed of 1 cm / s, the skin surface remains basically intact ( Figure 4 As shown in (a), there is an obvious white area only where the pulse laser acts. Figure 4 (b) is the enlarged morphology of the center of the skin surface after cleaning, and there are obvious ablation marks on the surface. At the same time, in the area where the pulse laser is applied, such as Figure 4 (c) The upper boundary shown has a mostly white surface, but not the silvery bright color of the base, indicating that a large amount of white primer remains here and has not been cleaned to the metal base. Figure 4 (d) is basically light red, indicating that a large amount of topcoat remains here. From the cleaning effect of the composite laser at this power density, it can be seen that the cleaning effect of the composite laser at this power density is not ideal. It only slightly ablates the surface of the paint layer, and a large amount of topcoat remains. However, the pulsed laser also has a good removal effect on the slightly ablated paint layer.

[0079] When the flat-top beam power density is expanded to 52.64W / mm 2 , Gaussian beam power density expanded to 56.62W / mm 2 When the skin surface is severely ablated, the paint layer on the surface is ablated and bonded into blocks. Figure 4 (e) as shown. Figure 4 (f) shows the enlarged morphology of the center of the skin surface after cleaning. It is almost gray-black with a little red, indicating that the paint layer on the surface has been severely ablated. In addition, the pulsed laser acts on the lower boundary, causing the entire piece of paint ablation to warp, revealing the silver-colored metal base underneath. Figure 4 As shown in (g), there is no pulsed laser on the upper boundary, and the paint layer ablation product does not warp, but only changes color due to ablation, indicating that the stress effect of the pulsed laser is sufficient to remove the paint layer ablation adhesive, as shown in Figure 4 (h) shown.

[0080] When the flat-top beam power density is further expanded to 73.7W / mm 2 , the Gaussian beam power density is further expanded to 79.26W / mm 2 , the paint layer on the surface of the skin falls off directly, revealing the silver-colored base underneath. Figure 4 (i) However, after zooming in, it can be clearly seen that there is a small amount of black residue on its surface, as shown in the figure below. Figure 4 (j), and at the upper boundary ( Figure 4 (k)) and the lower boundary ( Figure 4(l)), there are black residues. Combined with the experimental conclusion of the cleaning experiment of the small circular area mentioned above, it is sufficient to show that the overall power density of the continuous laser is too high, causing damage to the substrate.

[0081] contrast Figure 4 (e) and Figure 4 (i) It was found that the paint layer could be successfully removed at both powers. Figure 4 (e) The laser power is low, the paint layer is not completely ablated, and under the impact of the pulsed laser, the paint layer warps but does not fall off completely; Figure 4 In (i), the laser power is high and the substrate is exposed. The morphology of the two is observed using a SEM electron microscope. Figure 5 shown.

[0082] When the flat-top beam power density is 52.64W / mm 2 , Gaussian beam power density is 56.62W / mm 2 When cleaning, the paint layer remains as Figure 5 As shown in (b), there are a lot of fine ablation residues on the surface, but the overall paint layer is intact and covers the substrate. The morphology of the substrate obtained by peeling off the paint layer is as follows Figure 5 As shown in (c), the substrate is clean and flat, with only scratches on the aluminum substrate. This indicates that there is almost no paint ablation, substrate ablation oxide, or substrate damage on the surface. It also indicates that the composite laser with this parameter setting has basically achieved a non-destructive effect on the skin cleaning. When the flat-top beam power density is expanded to 73.7W / mm 2 , Gaussian beam power density expanded to 79.26W / mm 2 When the base morphology is Figure 5 As shown in (e), there is a large amount of spattered paint on its surface after ablation, which adheres to the surface of the substrate and damages the substrate. Figure 5 (f) as shown.

[0083] According to the different ablation mechanisms studied above, different ablation effects will be formed on paint-free and primer, which will affect the corresponding ablation depth. The following is a comparative study on the changes in ablation depth under the action of different composite lasers, such as Figure 6 shown.

[0084] from Figure 6 It can be seen that when the continuous laser power density is 31.58W / mm 2When the continuous laser power density increases to 52.64W / mm, the skin surface temperature gradually rises, but the temperature does not reach the melting point of the topcoat, and the ablation depth is shallow; (2) from 65ms to 100ms, the skin surface temperature reaches the melting point of the topcoat at 65m, but the temperature rise is small afterwards, so the ablation degree of the paint is small and the cleaning depth is less than 30μm. 2 The ablation process is relatively complex and can be roughly divided into five stages: (1) From 0ms to 35ms, the temperature gradually rises but is lower than the melting point of the topcoat, so it is only slightly ablated, and the cleaning depth slowly deepens; (2) From 35ms to 65ms, the temperature has reached the melting point of the topcoat, 573K, the topcoat is violently ablated, and the cleaning depth deepens sharply; (3) From 65ms to 76ms, the topcoat is completely ablated and the primer begins to be ablated. Since the melting point of the primer is lower than that of the topcoat, the ablation is farther and the depth increases rapidly, reaching about 172μm after about 10ms; (4) From 76ms to 80ms, the topcoat and primer are completely ablated, and the chemical conversion layer on the surface of the substrate is ablated. The action time is about 4ms, and the cleaning depth reaches about 220μm, that is, the position of the metal substrate; (5) From 80ms to 100ms, it is the ablation process of the substrate when the skin is completely ablated. In this stage, the phase change caused by skin ablation and its interference with the incident laser will cause the substrate temperature to fluctuate, but generally speaking, the temperature is relatively stable and will not cause ablation damage to the substrate. 2 There are also five regions, and the meaning of each region is the same as the previous parameter setting, except that the temperature rise rate is faster and the time distribution is shifted forward. However, the maximum temperature becomes larger and the duration increases (50ms), which will aggravate the ablation damage effect of the substrate.

[0085] In summary, when the composite laser performs strip scanning cleaning on the aircraft skin with a moving speed of 1 cm / s, the optimal paint removal parameters of the continuous laser are set to "the power density of the flat-top beam in the continuous laser part is 52.64 W / mm 2 ~73.7W / mm 2 , the power density of the Gaussian beam is 56.62W / mm 2 ~79.26W / mm 2 "; Based on this, in step S24 of this embodiment, when the preset laser moving speed is 1 cm / s, the optimal paint removal parameters are: the power density of the continuous laser part should be in the range of 52.64 W / mm 2 ~73.7W / mm 2 The energy density of the pulse laser is fixed at 0.42 J / mm 2 , the frequency is fixed at 10Hz.

[0086] In the embodiment of the present invention, the composite laser cleans the aircraft skin by relying on the ablation effect of the continuous laser on the topcoat and primer, so that the paint layer becomes an ablation that is easy to separate from the skin surface, and the pulsed laser part produces a great stress effect on the ablation to achieve thorough cleaning of the paint layer. For the ablation effect of the continuous laser, the ablation of the skin is through the topcoat, primer, metal conversion layer to the substrate. When the power is too low, only partial ablation will occur, which cannot provide a basis for pulsed laser cleaning; and when the power density is too high, although the ablation is relatively thorough, it will also cause damage to the metal substrate, and the skin and the substrate will merge to form oxides that penetrate into the substrate, which are difficult to remove even with pulsed lasers; and the pulsed laser mainly cleans the ablation by superimposing stress on the high-temperature ablation; from the simulation results, the high temperature of the continuous laser is used to induce a higher temperature rise and generate a higher stress, thereby producing a good cleaning effect, which is also the advantage of the composite laser effect; but too low or too high laser pulse energy has caused incomplete cleaning of the ablation or caused damage to the substrate. It can be seen that when the composite laser is acting, the continuous laser and the pulsed laser act together, neither of which is indispensable, and all parameters need to be controlled.

[0087] Based on the above analysis, the ablation effect is mainly related to the parameter setting of the continuous laser, and the basis for judging non-destructive cleaning is the cleaning depth. Therefore, this embodiment mainly explores the relationship between the cleaning depth and the power density of the continuous laser.

[0088] Step S3 of the embodiment of the present invention is specifically:

[0089] S31. Based on the equal energy density, a power density conversion equation between scanning cleaning and small circular area cleaning is constructed;

[0090] S32, substituting the experimental data corresponding to the optimal paint removal parameters for cleaning a small circular area into the power density conversion equation to obtain a power density expression for scanning cleaning;

[0091] S33, introducing a heat flux condition to simulate the heat loss caused by the ablation of the aircraft skin paint layer based on the boundary condition, and determining the ablation rate of the paint layer;

[0092] S34, substituting the ablation rate of the paint layer into the power density expression of scanning cleaning, and integrating it with respect to the action time to obtain an expression for the ablation depth during the scanning cleaning process;

[0093] S35, substituting the lower boundary value of the optimal power density range of the continuous laser into the expression of the ablation depth, and fitting it according to the corresponding cleaning experimental data to obtain the relationship between the cleaning time and the ablation depth under the corresponding power density;

[0094] S36, according to the upper boundary value of the optimal power density range, setting the magnitude change ratio of the power density of the continuous laser on the basis of the lower boundary value;

[0095] S37. Substitute the change ratio of the power density of the continuous laser into the relationship between the cleaning time and the ablation depth to obtain a relationship model between the power density and the cleaning depth within the optimal power density range of the continuous laser.

[0096] In step S31 of this embodiment, the power density conversion equation is:

[0097]

[0098] Where P represents the power density of the continuous laser during scanning cleaning, P d represents the optimal power density of continuous laser when cleaning a small circular area, where v represents the scanning speed during scanning cleaning, t represents the scanning time during scanning cleaning, r represents the spot radius, and t d It represents the optimal cleaning time of continuous laser when cleaning a small circular area. In the above formula, the acceleration process of the three-dimensional platform is ignored and it is regarded as a uniform motion.

[0099] In step S32 of this embodiment, the power density expression of scanning cleaning is:

[0100]

[0101] In step S33 of this embodiment, the expression of the ablation speed of the paint layer is:

[0102] v n =q n / ρ n H s

[0103] In the formula, v n Indicates the ablation rate of the paint layer, q n Represents the energy absorbed by the paint layer per unit area, ρ n Indicates the density of the paint layer, H s Indicates the sublimation heat of the paint layer;

[0104] Specifically, in the ablation process of the continuous laser in the composite laser, the heat loss caused by the paint layer is simulated by adding boundary adjustment in this embodiment. When the paint layer reaches its ablation temperature, it becomes gaseous and disappears from the modeling domain. Therefore, it is necessary to limit the fixed material temperature to be lower than the paint layer ablation temperature. When the solid material temperature reaches the ignition point of the paint layer, the paint layer burns and loses a certain amount of mass and heat, which depends on the material density and sublimation heat. Based on this, a heat flux condition is introduced in this embodiment to simulate this situation, namely:

[0105] qa = h n (T - T n )

[0106] where q n represents the energy absorbed by the paint layer surface, h n represents the heat transfer coefficient related to the surface temperature. When T n > T, it means that ablation does not occur and the heat transfer coefficient is 0. When T n < T, it means that ablation has occurred and the heat transfer coefficient increases linearly. To avoid the model temperature exceeding the paint layer ablation temperature, the slope of the heat transfer coefficient is relatively large, and the material ablation rate is obtained as v n = q n / ρ n H s .

[0107] In step S34 of this embodiment, substituting the power density expression of the scanning cleaning into the ablation rate of the paint layer, we get:

[0108]

[0109] Furthermore, the expression for the ablation depth in the scanning cleaning process is obtained as:

[0110]

[0111] In the formula, c represents the definite integral parameter.

[0112] In step S35 of this embodiment, when the flat-top beam power density in the continuous laser of the composite laser is 52.64 W / mm 2 , and the Gaussian beam power density is 56.62 W / mm 2 , the relationship between the cleaning time and the ablation depth is obtained by fitting as:

[0113] l = 0.404×(0.2564t) 2 + 0.4991

[0114] In step S36 of this embodiment, based on the flat-top beam power density of 52.64 W / mm 2 and the Gaussian beam power density of 56.62 W / mm 2 , when the power densities of the two beams change, let the proportion of the change in the flat-top beam power density be p1, and the proportion of the change in the Gaussian beam power density be p2. Specifically:

[0115]

[0116] where α1 is the flat-top beam power density magnitude, and α2 is the Gaussian beam power density magnitude.

[0117] Based on the above formula, the relationship model between power density and cleaning depth in step S37 of this embodiment is obtained as follows:

[0118] l=0.404[1.6×(p1-0.2)+2×(p2-0.3)](0.2564t) 2 +0.4991

[0119] In step S4 of this embodiment, when cleaning the aircraft skin, the aircraft skin is scanned and cleaned based on the cleaning depth requirement, according to the above relationship model, and the pulse energy density and frequency obtained when determining the optimal paint removal parameters.

[0120] In this embodiment, in order to trust the above relationship model, several composite laser parameters are substituted to obtain the following: Figure 7 The change curve shown, and the specific values ​​of the cleaning depth of 50ms and 80ms are shown in Table 1;

[0121] Table 1: Theoretical cleaning depth table

[0122]

[0123] Table 1 shows that when α1 is 52.64 W / mm 2 , α2 is 56.62W / mm 2 The theoretical calculated cleaning depth at 80ms is 219.7536μm, which is basically consistent with the cleaning depth at 80ms in the experiment. In addition, when α1 is 105.28W / mm 2 ,α2 is 113.23W / mm 2 And α1 is 157.92W / mm 2 , α2 is 113.23W / mm 2 The cleaning depth calculated by theoretical calculation is greater than 220μm at 50ms, which is greater than the sum of the thickness of the topcoat, primer and composite coating. This shows that the cleaning has reached the metal substrate and caused damage to the substrate at 50ms, and at 100ms, the depth calculated by theoretical calculation is far more than 220μm, indicating that the damage caused is greater; this is consistent with the experimental results, and also verifies the objectivity and feasibility of the relationship model between power density and cleaning depth at the above-mentioned moment derived from this theory, and also shows that it is indeed of guiding value for actual laser paint removal.

[0124] Those skilled in the art will appreciate that the embodiments described herein are intended to help readers understand the principles of the present invention, and should be understood that the protection scope of the present invention is not limited to such specific statements and embodiments. Those skilled in the art can make various other specific variations and combinations that do not deviate from the essence of the present invention based on the technical revelations disclosed by the present invention, and these variations and combinations are still within the protection scope of the present invention.

Claims

1. A composite laser-based aircraft skin scanning cleaning method, characterized in that: The following steps are involved: S1. Constructing a composite laser paint removal device that outputs continuous laser and pulsed laser; S2. Use a composite laser paint removal device to conduct a scanning cleaning experiment on the aircraft skin to determine the optimal paint removal parameters at a preset laser moving speed; In the step S2, in the scanning cleaning experiment, the ablation effect of the continuous laser plays a major role in cleaning the aircraft skin, and the stress effect of the pulsed laser plays a minor role in cleaning the aircraft skin; The step S2 is specifically as follows: S21, based on the optimal paint removal parameters when the composite laser is used to clean a small circular area of ​​the aircraft skin, and according to which the adjustment range of the power density of the continuous laser is set; Among them, the optimal paint removal parameters for cleaning small circular areas include the optimal power density of the flat-top beam and the optimal Gaussian beam power density in the continuous laser, the optimal energy density and frequency of the pulsed laser, and the optimal cleaning time; S22, adjusting the power density of the continuous laser within the adjustment range, and using the adjusted power density of the continuous laser, the optimal energy density and frequency for cleaning a small circular area as paint removal parameters at a preset laser moving speed, to perform a scanning cleaning experiment on the aircraft skin; S23, continuously adjusting the power density of the continuous laser, analyzing and comparing the corresponding cleaning effects, determining the power density corresponding to partial ablation and damage ablation effects when the continuous laser ablates the aircraft skin, and then determining the optimal power density range of the continuous laser; S23, using the optimal power density range of continuous laser, the optimal energy density and frequency when cleaning a small circular area as the optimal paint removal parameters for scanning cleaning; S3. Based on the optimal paint removal parameters, a relationship model between power density and cleaning depth within the optimal power density range of continuous laser during composite laser paint removal is fitted; S4, performing scanning cleaning on the aircraft skin according to the relational model at a preset laser moving speed; The step S3 is specifically as follows: S31. Based on the equal energy density, a power density conversion equation between scanning cleaning and small circular area cleaning is constructed; S32, substituting the experimental data corresponding to the optimal paint removal parameters for cleaning a small circular area into the power density conversion equation to obtain a power density expression for scanning cleaning; S33, introducing a heat flux condition to simulate the heat loss caused by the ablation of the aircraft skin paint layer based on the boundary condition, and determining the ablation rate of the paint layer; S34, substituting the power density expression of scanning cleaning into the ablation rate of the paint layer, and integrating it with respect to the action time, to obtain an expression of the ablation depth during the scanning cleaning process; S35, substituting the lower boundary value of the optimal power density range of the continuous laser into the expression of the ablation depth, and fitting it according to the corresponding cleaning experimental data to obtain the relationship between the cleaning time and the ablation depth under the corresponding power density; S36, according to the upper boundary value of the optimal power density range, setting the magnitude change ratio of the power density of the continuous laser on the basis of the lower boundary value; S37. Substitute the change ratio of the power density of the continuous laser into the relationship between the cleaning time and the ablation depth to obtain a relationship model between the power density and the cleaning depth within the optimal power density range of the continuous laser.

2. The composite laser-based aircraft skin scanning cleaning method according to claim 1 is characterized in that: In step S1, the composite laser paint removal device includes a continuous laser and a pulse laser. The continuous laser and the pulse laser emitted by the continuous laser and the pulse laser respectively pass through a beam splitter and a reflector and then act on the aircraft skin through a focusing lens.

3. The composite laser-based aircraft skin scanning cleaning method according to claim 2 is characterized in that: In the composite laser paint removal device, the angle between the continuous laser and the pulsed laser when incident on the focusing lens is 20°; The transmission and reflection energy ratio of the corresponding split laser beams obtained by the continuous laser and the pulse laser through the beam splitter is 8:2; The continuous laser includes a flat top beam and a Gaussian beam, and the Gaussian beam covers the center of the flat top beam.

4. The composite laser-based aircraft skin scanning cleaning method according to claim 1, characterized in that: In step S2, the optimal paint removal parameters determined include the optimal power density range of the continuous laser at a preset laser moving speed, and the optimal energy density and frequency of the pulsed laser; Among them, the optimal power density range of the continuous laser is determined based on the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the flat-top beam ablates the aircraft skin, and the power density boundary values ​​corresponding to partial ablation and damage ablation effects achieved when the Gaussian beam ablates the aircraft skin.

5. The composite laser-based aircraft skin scanning cleaning method according to claim 1, characterized in that: In step S31, the power density conversion equation is: In the formula, Indicates the power density of continuous laser during scanning cleaning. represents the optimal power density of continuous laser when cleaning a small circular area, where Indicates the scanning speed during scanning cleaning. Indicates the scanning time during scanning cleaning. represents the spot radius, Indicates the optimal cleaning time of continuous laser when cleaning small circular areas; In step S32, the power density expression of scanning cleaning is: In step S33, the expression of the ablation speed of the paint layer is: In the formula, Indicates the ablation rate of the paint layer, Indicates the energy absorbed by the paint layer per unit area. Indicates the density of the paint layer, Indicates the sublimation heat of the paint layer; In step S34, the power density expression of scanning cleaning is substituted into the expression of the ablation speed of the paint layer to obtain: Then the expression of ablation depth in scanning cleaning process is obtained as follows: In the formula, Represents the definite integral parameter.

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