Research on the Removal Mechanism of Resin-Based Coatings by Water Jet-Guided Quasi-Continuous Laser Cleaning

Abstract

Laser cleaning technology has the advantages of being green and efficient and is expected to become the most promising remanufacturing cleaning technology. However, the quasi-continuous laser can easily cause problems, such as a heat-affected zone and a recast layer on the substrate material, which limits the development of laser cleaning technology. The environmentally friendly water jet-guided quasi-continuous laser proposed in this paper is an innovative laser cleaning technology that can remove the resin-based coating on the metal substrate with high quality. The epoxy resin coating on the 304 stainless steel surface was cleaned by a water jet guided quasi continuous laser, and the surface morphology, surface element content, and surface roughness of the cleaning area were tested and analyzed. The removal mechanism of water jet-guided quasi-continuous laser cleaning technology was revealed, and the influence of process parameters on cleaning surface quality was found. The optimal process parameters were as follows: the energy density of the water jet-guided laser 17.5 J/cm², the cleaning speed 135 mm/s, the cleaning line spacing 0.1 mm, and the laser pulse frequency 900 Hz. Therefore, this study is expected to be an important basis for water jet-guided quasi-continuous laser cleaning technology, and promote the development of water jet-guided laser cleaning technology.

1. Introduction

In aerospace, vehicle transportation, petrochemicals, and other fields, to improve the high temperature resistance, oxidation resistance, wear resistance, and corrosion resistance of metal substrates, the coating is usually coated on the surface of metal substrates. In metal parts, after a long period of service, the surface of the coating will be aging, ruptured, or even peeling, such as in aircraft, which commonly use epoxy coating with an average life of about 4–5 years. To ensure the stability and safety of metal parts during use, it is necessary to completely remove the original coating to facilitate repainting of the metal surface [1,2]. Traditional remanufacturing cleaning technology refers to the method of removing contaminants or coverings from the surface of an object by grinding, sandblasting, and using chemical reagents to restore the object to its original form. However, these methods have significant limitations in the coating cleaning process [3,4,5]. Mechanical grinding cleaning technology can easily damage the surface of the workpiece, and the efficiency is very low and cannot meet the high cleanliness and high precision cleaning requirements. Acid-base chemical cleaning technology can cause corrosion on the metal surface and easily lead to hydrogen embrittlement and other problems, which reduces the performance of the metal. At the same time, waste liquids will cause secondary pollution to the environment, water, and soil. In response to these situations, many new cleaning technologies have been invented and applied, such as molten salt cleaning technology, dry ice cleaning technology, supercritical CO₂ cleaning technology, and laser cleaning technology [6,7,8,9]. Laser cleaning, as an efficient and green cleaning technology, has been widely investigated. The technique is to use continuous or pulsed laser irradiation to clean the surface of the workpiece. Most of the laser energy is instantly absorbed by the coating material, except for a small amount of laser energy that is lost due to thermal convection and thermal radiation. The temperature of the coating material rises rapidly to the vaporization temperature in a very short period of time, and the final removal of the coating from the substrate surface is achieved. Laser cleaning technology has the advantages of high cleaning accuracy, low environmental pollution, and low material damage, which are now widely used in various fields [10,11,12,13,14,15]. In the future, it is expected to partially or completely replace traditional methods and become the most potential remanufacturing cleaning technology in the 21st century.

At present, research on laser cleaning technology mainly focuses on the cleaning mechanism, process law, and influence on the microstructure and properties of the substrate. In terms of cleaning mechanisms, Hu et al. [16] used different parameters for laser cleaning of TiN coatings. By analyzing coating morphology, surface composition, and sample profile, research reveals the morphological change of the TiN coating after cleaning and the laser cleaning mechanism. Zhao et al. [17] studied the mechanism of pulsed laser paint removal from a microscopic perspective using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. Zhang et al. [18] introduced laser cleaning of blue polyurethane paint on an Al-Mg series aluminum alloy substrate using a nanosecond pulsed laser. It was found that ablation, spallation, and vaporization were the main cleaning mechanisms in the process of laser-cleaning paint. In terms of process research, Li et al. [19] studied the effect of laser pulse energy density on paint removal rate, and explored the change process of paint layer from removal to damage with the change of energy density. Brygo et al. [20] studied the effects of laser energy density, repetition frequency, and pulse width on the efficiency of laser rust removal. Jasim et al. [21] used a nanosecond pulsed fiber laser to clean a 20μm thick white polymer paint layer on an aluminum alloy substrate and studied the effect of overlap ratio, paint removal times, and pulse frequency on processing depth. In terms of the effect on the microstructure and properties of the substrate, Zhu et al. [22] studied the residual stress, micro-hardness, and corrosion performance of the aircraft skin surface after laser paint stripping. Compared with the mechanical lapping and laser paint stripping methods, it proves that the laser will not reduce the hardness and corrosion resistance of the aircraft skin. Wang et al. [23] analyzed the effects of different laser energy densities on the surface morphology, surface composition, microhardness, and corrosion resistance of 7075 aluminum alloys. Shi et al. [24] studied the removal of aluminum alloy oxide layers using laser cleaning technology. It was found that the laser cleaning technique effectively enhanced the tensile and bending properties of the aluminum alloy base metal and could expand the application of aluminum alloy in a high-speed rail.

Laser cleaning technology mainly includes laser dry cleaning, liquid film-assisted laser cleaning, and laser shock wave cleaning [25]. Laser dry cleaning is the most widely used, but it is easy to produce sputtering risk and dust pollution in the cleaning process, and it will trigger thermally induced defects on the surface of the matrix material, which will manifest as a recast layer and heat-affected zone and have an adverse impact on the mechanical properties of the material. Liquid film-assisted laser cleaning of the liquid film requires more, for the stronger adhesion of contaminants cleaning is not ideal. Laser impact cleaning mainly relies on the role of shock waves in removing contaminants, which are usually applied in particle cleaning. In recent years, water jet-guided laser technology has achieved remarkable results in micro-processing, such as cutting and drilling metals, ceramics, semiconductors, and other materials [26,27,28]. In the area of water jet-guided laser cleaning technology, Madhukar [29] used a fiber laser with a wavelength of 1070nm to conduct paint removal experiments with the aid of water jets. It was found that the paint layer could be completely removed with the assistance of a water jet. Although nanosecond lasers are widely used in water jet-guided laser technology, their application has low cleaning efficiency and high cost, which limits its engineering application.

In this paper, to solve the difficulties of quasi-continuous laser cleaning technology and the bottleneck of water jet-guided laser cleaning technology, a water jet-guided quasi-continuous laser cleaning technology is proposed innovatively. The resin-based coatings on the metal substrates were removed by the combined action of a quasi-continuous laser and a water jet. In this paper, the epoxy resin coating on the surface of 304 stainless steel is used as the cleaning object, the quasi-continuous laser is used to carry out the water jet-guided laser cleaning test, and the surface morphology, surface element content, and surface roughness of the cleaning area are tested and analyzed. The mapping relationship between the water jet-guided quasi-continuous laser cleaning process parameters (energy density of the water jet-guided laser, cleaning speed, cleaning line spacing, and laser frequency) and the surface quality after cleaning (surface morphology and surface roughness) is established. The influence of process parameters on surface quality is analyzed to provide a reference basis for the optimal selection of process parameters for water-guided laser cleaning.

2. Mechanism

Epoxy coating is essentially a high-molecular polymer material. The process of water jet-guided quasi-continuous laser cleaning of metal surface coatings is actually a process of interaction between the water jet-guided laser and the coating. The interaction mechanism of a water jet-guided quasi-continuous laser with resin-based coatings mainly includes the thermal ablation effect, vibration effect, and laser plasma shock effect.

The thermal ablation effect is a main laser cleaning mechanism that requires a relatively low laser energy density and usually occurs with a pulse width of about 1 µs to 1 ms. When the laser is transmitted to the surface of the coating through the water beam fiber, the penetration depth d of the absorbed laser in the coating and the light intensity I follow Beer–Lambert law I = I₀e^−al, where the absorption coefficient of the resin-based coating is a ≈ 10⁶ m⁻¹. The penetration depth of the laser in the coating is only a few microns, and the thermal conductivity of the coating is very low, resulting in a constant build-up of heat on the surface of the coating. The coating absorbs laser energy violently, causing the surface temperature of the coating to rise rapidly above the melting point and boiling point of the coating, which results in vaporization and combustion of the coating, and thus removal [30]. At this time, thermal cracks, melting, and other phenomena may appear on the surface of the substrate.

In the actual water-guided quasi-continuous laser cleaning process, the thermal ablation effect will not appear alone. When a water jet-guided quasi-continuous laser cleans the coating, a liquid film is formed on the surface of the sample. When the liquid film and coating absorb a large amount of laser energy, a large amount of steam accumulates above the sample surface. The accumulated vapor strongly absorbs the laser energy, and the temperature continues to increase to cause ionization. Finally, laser plasma is formed [31]. The outward expansion of the high-temperature and high-pressure plasma generates shock waves [32], and a shock load is applied to the cleaning surface, resulting in a strong shock wave propagating into the coating to effectively remove the resin-based coating on the surface of the metal substrate. In this case, the cleaning speed will be accelerated, but a residual stress layer will be generated on the surface of the substrate, which will affect the material properties to a certain extent.

The thermal stress vibration effect is due to the instantaneous thermal expansion of the irradiated area when the short-pulse laser acts on the surface of the sample. Thermal expansion induces vibration waves at the interface between the coating and the substrate, resulting in strong release stress at the interface. When the release stress exceeds the shear stress of the coating material, the material in the irradiated area will physically fracture, which is interpreted as peeling the coating material from the substrate surface from the point of view of laser cleaning. In the water jet-guided quasi-continuous laser cleaning coating, although the pulse width is long, the erosion and cooling effects of the high-pressure water jet produce a significant temperature gradient on the coating surface. These temperature gradients lead to thermal expansion, resulting in strain and stress waves between the coating and the substrate. Therefore, the thermal stress vibration effect also exists in the water jet-guided quasi-continuous laser cleaning process.

3. Materials and Methods

3.1. Specimen Preparation

In the experiment, 304 stainless steel is used as the base material. The steel is cut into specimen blocks with dimensions of 50 mm × 50 mm × 3 mm. As shown in Figure 1, after the surface of the substrate is subjected to pretreatment such as grinding, degreasing, and cleaning, a black epoxy resin coating with a thickness of 140 μm ~ 150 μm is applied to the surface. The thermal conductivity of epoxy resin powder coating is 0.14 W/(m·K), and the thermal expansion coefficient is 62.4 × 10⁻⁶ K⁻¹, and the density is 1.21 × 10³ kg/cm³, and the specific heat capacity is 1884 J/(kg·K). The chemical composition of the resin-based coating mainly includes C, H, and O elements. The main composition and content of 304 stainless steel are shown in

Table 1. Chemical composition and content of 304 stainless steel.

Element	C	Mn	P	S	Si	Cr	Ni
Mass fraction/%	≤0.08	≤2.0	≤0.045	≤0.03	≤1.00	18.0–20.0	8.0–11.0

.

3.2. Equipment

The water jet-guided quasi-continuous laser cleaning equipment used in the experiment is shown in Figure 2. The equipment is mainly composed of a quasi-continuous fiber laser (YLR-150/1500-QCW-MM-AC, IPG), beam shaping device, water-optic coupling device, high-pressure pure water device, three-axis precision mobile platform, and computer control system. The maximum average power of the fiber laser is 150 W, the pulse frequency is adjustable in the range of 10~50 kHz, and the minimum pulse width is 20 µs. The diameter of the water beam fiber is about 0.5 mm, and the water jet remains stable in the pressure range of 0.8~1.6 MPa. The design and manufacture of water jet-guided laser cleaning equipment conforms to international standards, relevant safety requirements, and environmental standards of China. The geometric accuracy, working accuracy, and tolerance of the water jet-guided quasi-continuous laser cleaning equipment conform to relevant standards. A national standard is adopted for the measurement units of all parts and various instruments.

In the process of water jet-guided quasi-continuous laser cleaning, the pulsed parallel beam emitted by the laser is converted into a Gaussian-focused beam through a beam shaping system. The focused spot is coupled with the steady-state water jet in the water-laser coupling device to form a water beam fiber with a uniform laser energy distribution. Then, the water beam fiber irradiates the part to be cleaned of the sample so that the attached coating, paint, rust, and other pollutants absorb the laser energy and undergo a series of complex physical and chemical reactions (including vibration, melting, combustion, and gasification). Eventually, the contaminants are removed from the sample surface. The sample is placed on a three-axis precision mobile platform, and a water jet-guided laser cleaning experiment can be realized by controlling the movement of the mobile platform in the X and Y directions.

After the water jet-guided laser cleaning experiment is completed, the surface morphology of the cleaned area is observed by a 3D digital microscope (DSX1000, OLYMPUS), and the surface roughness is measured by an optical profiler (Micro-XAM100, KLA Tencor). Field emission scanning electron microscopy (Apreo, FEI) is used to analyze the microscopic morphology of the cleaned area, and an X-ray energy dispersive spectrometer (EDS) attached to the SEM was used to detect the element content of the cleaned area.

3.3. Experimental Design

The effects of the energy density of water jet-guided laser I, cleaning speed v, cleaning line spacing L₂ and laser pulse frequency f on the cleaning effect of water jet-guided quasi-continuous laser were studied by single-factor experiments. To achieve the purpose of efficient cleaning, a single cleaning process was adopted in this study. Figure 3 is a schematic diagram of the water jet-guided laser cleaning path. The surface cleaning area of the sample is a square of 10 mm × 10 mm, and the laser spot overlap is according to the “S” path. The transverse overlap rate between the water jet-guided laser spots is set to γ₁, and the longitudinal overlap rate is set to γ₂. The transverse spot overlap rate is mainly determined by the laser pulse frequency and spot moving speed, while the longitudinal overlap rate is determined by the cleaning line spacing set by the computer control program.

The transverse spot overlap rate can be expressed as [33]:

$${\gamma }_1=\left(1-\frac{v}{f\times D}\right)\times 100\%$$

(1)

The longitudinal spot overlap rate can be expressed as follows:

$${\gamma }_2=\left(1-\frac{L_2}{D}\right)\times 100\%$$

(2)

The energy density of a water jet-guided laser is closely related to the laser power and laser pulse frequency. The laser energy density I can be expressed as follows:

$$I=\frac{E}{S}=\frac{4E}{\pi \times D^2}=\frac{4\times P}{\pi \times D^2\times f}$$

(3)

where E is the energy of a single pulse, S is the cross-sectional area of the water beam fiber, P is the average cleaning power of the water jet-guided laser, and D is the diameter of the water beam fiber.

To quantitatively assess the surface quality of the water jet-guided laser cleaning area, the collected pictures of the cleaned area are binarized. Figure 4a,b show photos of the sample surface after water jet-guided laser cleaning and its binarized image, respectively. In addition to most of the silver-white areas that are clean, there are also some areas that appear yellow, brown, and black, namely residual resin-based coating and over-cleaning ablation marks. The cleanliness η was introduced as an important indicator to assess the quality of water jet-guided laser cleaning, which is expressed as follows:

$$\eta =\frac{N_c}{N}$$

(4)

where N is the total number of pixel points for calculating the binarized processed image, and N_c is the number of pixel points for calculating the white area (original substrate surface).

The cleaning rate U_s is introduced as an important indicator for evaluating the cleaning efficiency of the water jet-guided laser, and its expression is [34]:

$$U_s=\frac{S_c}{t}=\frac{v^2}{f{N}_t}$$

(5)

where S_c is the surface area of the sample cleaned by the water jet-guided laser, t is the cleaning time, and N_t is the minimum number of cleanings when cleanliness η reaches 90%.

4. Results and Discussion

4.1. Effect of Energy Density of Water Jet-Guided Laser

The effect of different energy densities of water jet-guided lasers on the surface morphology of the cleaned area when the spot overlap is 70% is shown in Figure 5. When the energy density is lower than 7.5 J/cm², the metal substrate surface is almost completely covered by the resin-based coating, but some craters formed after the decomposition of the coating can also be seen. With an increase in energy density, the coating on the surface decreases gradually, and the exposed base metal area increases gradually. When the energy density reached 12.5 J/cm², the black coating on the surface of the substrate is basically removed, and the complete cleaning threshold of the coating is reached at this time. When the energy density continues to increase to 17.5 J/cm², the coating is completely removed, and the substrate surface stripe texture is revealed with no obvious thermal damage, so the surface finish of the cleaning area is better. With the energy density reaching 20 J/cm², the cleaned substrate surface has thermal damage caused by laser irradiation. When the energy density exceeds 25 J/cm², the degree of laser ablation on the substrate surface increases, and honeycomb melting craters are formed. The comparison between the melting craters and the substrate is more obvious, and the degree of damage is greater.

Combined with the microscopic morphology of the substrate surface after water jet-guided laser cleaning in Figure 6, the effect of energy density on the cleaning effect can be divided into three stages. As shown in Figure 6a, during the first stage, the resin-based coating is not completely removed, and residual coating particles are attached to the substrate surface. The reason for this phenomenon is that the coating material absorbs laser energy in a short time, resulting in melting, and releases energy in a short time, resulting in solidification. The phenomenon in which coating materials absorb and release heat rapidly under laser action is called laser fused. After laser irradiating the coating surface, the coating absorbs laser energy and causes the temperature to rise rapidly. When the temperature in this area exceeds the gasification threshold of the coating, material removal is achieved. The temperature rise model generated by a pulsed laser in pulse width time can be expressed as [35]:

$$\Delta T=\frac{2F\gamma }{\kappa }\sqrt{\frac{\alpha \tau }{\pi }}$$

(6)

where F is the power of the water jet-guided laser, and γ is the absorption rate of the material, and κ is the thermal conductivity coefficient of material, and α is the thermal diffusivity of the material, and τ is the laser pulse width. Because the thermal conductivity of the resin-based coating is very low, if the energy of the single-pulse laser is not enough, the cleaning depth of the water jet-guided laser will not reach the surface of the substrate, resulting in a layer of the substrate surface left after the ablation of the coating residue. From Equations (3) and (6), it can be seen that the temperature rise of the coating surface is linearly related to the laser energy. As the energy density increases, the surface temperature of the coating also increases, which increases the depth of the cleaning crater and the peeling volume of the coating. Based on the ablation traces and fracture traces of the coating, it is indicated that the main mechanisms of the water jet-guided quasi-continuous laser cleaning at this time are the thermal ablation effect and the thermal stress vibration effect.

As shown in Figure 6b, during the second stage, a better configuration is obtained between the energy density of the water jet-guided laser and other process parameters. The heat generated by laser irradiation can remove the coating on the substrate surface, and there are no obvious thermodynamic damage characteristics (micropores, microcracks, etc.) on the substrate surface. In the process of water jet-guided laser cleaning resin-based coatings, there is a critical energy density. When the energy density is lower than the critical value, the coating cannot be completely removed. This critical value is called the complete cleaning threshold of a water jet-guided laser. There is also a critical value for the metal substrate. When the energy density is higher than this value, it may cause damage to the metal substrate. This critical value is called the substrate damage threshold. At this stage, the energy density of the water jet-guided laser is between the complete cleaning threshold of the coating and the damage threshold of the substrate. The cleaning process is affected by the combined effects of thermal ablation and thermal stress vibration.

As shown in Figure 6c, during the third stage, the energy density of the water jet-guided laser exceeds the damage threshold of the substrate, and a large number of craters are formed on the surface of the metal substrate. The crater depth increases with the increase of the energy density. The analysis shows that there are two main reasons for the formation of craters on the substrate surface: first, the formation of molten craters on the substrate surface due to the thermal accumulation effect under laser irradiation, and second, the formation of craters on the substrate surface due to plastic deformation under the impact effect. At the same time, the formation of craters implies the sublimation of solid metals under the action of a strong laser, which provides the conditions for the formation of laser plasma. When the energy density reaches the thermal damage threshold of the metal substrate, the surface of the substrate absorbs laser energy and melts. However, the molten layer formed is shallow, the molten slurry has almost no fluidity, and the surface of the molten pool is not easily deformed. At this time, the accompanying shock wave pressure is very small and has little effect on the molten pool. The craters are mainly formed by the melting of the substrate surface. With the increase in energy density, the metal substrate forms a flowing molten pool under laser irradiation, and the molten slurry is heated to form a steam mass. The eruption of metal vapor produces a huge recoil pressure effect on the surface molten pool, and the fluid moves along the solid–liquid interface toward the edge of the pool, gradually forming a crater. When the energy density is high, the substrate surface absorbs a large amount of laser energy and forms a deeper and more fluid-molten layer. Under the action of laser pulse load and plasma blast impact, the molten metal slurry is discharged to the edge, increasing the crater depth and forming molten bead-like spatters at the edge. At this stage, the cleaning mechanism of a water jet-guided quasi-continuous laser is the coexistence of the thermal ablation effect and laser plasma shock effect.

The change in sample surface morphology will significantly affect roughness. Line roughness is based on the line profile method of assessing roughness and is used to characterize the roughness of a one-dimensional profile of a surface. Surface roughness is based on area morphology and is used to characterize the roughness of an object’s two-dimensional surface morphology. The surface roughness is more suitable for characterizing the change in material surface roughness after water jet-guided laser cleaning. Figure 7 shows the effect of the energy density of the water jet-guided laser on the surface roughness of the sample. To ensure the accuracy of the measurement results, each sample is tested three times and then averaged. It can be seen from the diagram that with the increase in energy density of the water jet-guided laser, the roughness of the cleaning surface is the “N” type change trend.

Combining Figure 7 and Figure 8, the variation process of sample surface roughness with energy density can be divided into five stages. When the energy density of the water jet-guided laser is small, as shown in Figure 8a, the temperature rise effect on the coating surface is not significant, and the crater diameter d and depth h produced by the laser on the coating surface are small. The adjacent craters do not overlap, so the coating integrity is maintained better, and the Ra parameter of the roughness is only 1.736 μm. As the laser energy increases, as shown in Figure 8b, the diameter and depth of the craters produced by the water jet-guided laser on the coating surface increase accordingly. The adjacent craters lap each other; most of the coating surface is covered by craters, and part of the metal matrix is exposed. At this time, the Ra parameter increases to 3.251 μm. When the energy density of the water jet-guided laser is 15 J/cm² to 20 J/cm², as shown in Figure 8c, the resin-based coating continuously absorbs energy and is removed. As the energy density continues to increase, the cleaning depth increases, the remaining coating thickness decreases, and the smooth surface of the metal substrate is gradually revealed. The surface flatness of the sample also increases, and the Ra parameter of roughness reaches the minimum value (0.762 μm). When the energy density of the water jet-guided laser reaches 25 J/cm², as shown in Figure 8d, the Ra parameter of the roughness increases substantially to 3.825 μm. Due to the increase in energy density and the heat accumulation effect of spot overlap, the thermal ablation depth of the water jet-guided laser continues to increase. After the surface of the metal substrate is cleaned by a water jet-guided laser, a layer of honeycomb-like craters is formed, and the damaged substrate improves the surface roughness of the sample. When the energy density of the water jet-guided laser exceeds 30 J/cm², as shown in Figure 8e, the Ra parameter of the sample exceeds 4.689 μm. At this time, the metal substrate absorbs a lot of heat and forms a deep, molten layer. This leads to further accumulation of melt and further increases the surface roughness of the sample.

4.2. Effect of Laser Spot Overlap Rate

The macroscopic process of water jet-guided laser cleaning is that a high-speed moving water beam fiber guides the laser to irradiate the surface of the sample, and removes the coating within a certain range in the form of overlapping laser spots. Figure 9 shows the effect of different water jet-guided laser cleaning speeds on the surface morphology of the sample during single-pass water jet-guided laser cleaning (the line spacing is 0). When the energy density is 17.5 J/cm², a single-pulse laser can remove epoxy resin coatings with a thickness of 150 µm. Figure 9a,b shows that when the cleaning speed v is 150 mm/s and the laser pulse frequency is 900 Hz, the spot overlap rate γ₁ of the water jet-guided laser is obtained from Equation (1) as 67%. Multiple pulsed lasers are superimposed to form a strip-like distribution; the coating in the cleaning area is basically removed, and only a small amount of coating adheres to the surface of the substrate. Figure 9c,d shows that when the cleaning speed v is 300 mm/s, the spot overlap rate γ₁ is 33%, and the cleaning craters just overlap, with incomplete peeling of the coating at the crater overlap. When the cleaning speed v is 450 mm/s, the cleaning craters form by the water jet-guided laser should be tangent in theory. However, Figure 9e,f shows that there is an obvious coating residue between the craters. Figure 9g,h shows that the cleaning craters are completely separated when the cleaning speed v is 600 mm/s. By comparing the surface morphology of the samples under different cleaning speeds, it is found that when the laser pulse frequency is constant, the smaller the cleaning speed, the greater the spot overlap rate, the more significant the heat accumulation effect, and the better the cleaning effect. As the cleaning speed increases, the spot overlap rate gradually decreases until it reaches zero. At this time, the coating between the laser spots is not irradiated by the laser energy, and the heat accumulation effect almost disappears. The heat-affected zone is small so that the coating cannot be completely removed by a single cleaning.

The formed craters after removing part of the coating by the water jet-guided laser show that the size and shape of the cleaning craters are not exactly the same and that the diameter of the cleaning craters is slightly smaller than the diameter of the water beam fiber. The cleaning effect at the bottom of the crater is not uniform, and the boundary line of the crater is irregular. The inner wall of the crater has both a smooth transition “slope” shape and a near vertical transition “cliff” shape. The main reasons for these phenomena are as follows:

(1)

After the laser is coaxially coupled with the water jet, the quasi-continuous laser is transmitted in the form of a multi-mode laser in the water beam fiber. The laser energy distribution of the cross-section of the water beam fiber is no longer Gaussian but is transformed into a flat-top distribution, so the inner wall of the pit presents a “cliff” shape.

(2)

Due to the turbulent effect of the water jet and the unstable transition caused by the water jet hitting the wall, both affect the roundness of the cross-section of the water beam fiber, resulting in uneven distribution of laser energy on the cross-section of the water beam fiber, resulting in different shapes of cleaning craters.

(3)

Under the action of the water jet-guided laser, a Knudsen layer is formed at the gas–liquid interface of the crater [36]. The Knudsen layer has a strong shielding effect on heat transfer. The thickness of the Knudsen layer is about several times the mean free path of a particle, and this phenomenon results in discontinuous temperature, density, and pressure across this thin layer. Therefore, the shape of the cleaning craters is affected.

Figure 10 shows the effect of the spot overlap rate of the water jet-guided laser on the surface morphology of the sample. When the energy density of the water jet-guided laser is 17.5 J/cm², the transverse spot overlap rate γ₁ and the longitudinal spot overlap rate γ₂ are 70%. The surface morphology of the water jet-guided laser cleaning area is shown in Figure 10a,b. It is found that there are obvious differences in the surface morphology of the water jet-guided quasi-continuous laser after transverse overlap and longitudinal overlap. The coating removal effect in the overlapping area of the transverse laser spot is good, but there is a residual coating in the overlapping area of the longitudinal laser spot. The difference shows that heat transfer and heat accumulation occur in the laser-irradiated area and its adjacent area. In the process of transverse laser spot overlapping, due to the short time interval between two adjacent spots, the area irradiated by the previous pulse laser has not cooled when the current pulse laser acts on the coating surface. The two adjacent laser spots lead to the superposition of heat in the overlapping area and its adjacent area, and the heating area of the resin-based coating increases so as to realize the complete removal of the coating. In the process of longitudinal laser spot overlap, due to the long time interval between the two adjacent spots overlap, there is no energy accumulation effect, or the accumulation effect is weak, resulting in some coatings remaining on the substrate surface. When the transverse laser spot overlap rate γ₁ is 70% and the longitudinal laser spot overlap rate γ₂ is 80%, the surface morphology of the water jet-guided laser cleaning area is shown in Figure 10c,d. The coating is basically removed, showing a dense metal matrix and smooth surface. When the transverse laser spot overlap rate γ₁ and the longitudinal laser spot overlap rate γ₂ are both 80%, the surface morphology of the water jet-guided laser cleaning area is shown in Figure 10e,f. Due to the high overlapping rate of transverse spots, the action time of the laser thermal ablation effect is prolonged, resulting in the formation of molten craters in the laser irradiation area and the formation of molten deposits around the craters. At the same time, due to the weak heat accumulation effect when the longitudinal spots overlap, rows of unablated substrate surfaces can be clearly seen.

Figure 11 shows the effect of different longitudinal spot overlap rates on surface roughness when the transverse spot overlap rate of the water jet-guided laser is 70%. It can be seen from the figure that the surface roughness of the sample first decreased and then increased. This is related to the overlap of adjacent craters in the water jet-guided laser cleaning process. Combined with Figure 12, the effect of the longitudinal spot overlap rate of the water jet-guided laser on the surface roughness of the sample can be divided into four stages. When the longitudinal spot overlap rate γ₂ is 0, as shown in Figure 12a. After the transverse spot overlaps, a continuous groove is formed on the surface of the sample. The coating in the groove has a good cleaning effect and reveals the metal substrate. The large spacing of adjacent longitudinal light spots makes the grooves on the coating surface unable to overlap. The number of grooves per unit area is small, and the local area is still flat. At this time, the Ra parameter of the roughness is only 2.491 μm. When the longitudinal spot overlap rate γ₂ is lower than 40%, as shown in Figure 12b, the adjacent grooves achieve a smaller degree of overlap, the number of grooves per unit area increases, and the Ra parameter increases to 6.915 μm. With the further increase in the longitudinal spot overlap rate, the coating on the substrate surface is further removed, and the surface roughness is subsequently reduced, as shown in Figure 12c. When the longitudinal spot overlap rate γ₂ reaches 80%, the coating removal effect is good, the substrate surface is relatively flat, there are no obvious traces of ablation craters, and the surface roughness is reduced to the minimum (Ra = 1.057 μm). When the longitudinal spot overlap rate γ₂ exceeds 90%, as shown in Figure 12d, the crater morphology with regular arrangement can be clearly observed, the degree of undulation increases, and the Ra parameter increases to 2.826 μm at this time. Therefore, it can be obtained that the longitudinal spot overlap rate is 80%; that is, when the cleaning line spacing is 0.1 mm, the water jet-guided laser cleaning effect is the best.

After water jet-guided laser cleaning with different spot overlap rates, the variation pattern of cleanliness η obtained by the image processing method with the number of cleanings is shown in Figure 13. When the spot overlap rate of the water jet-guided laser is 70%, the cleanliness η reaches 91.07% after cleaning twice. When the laser spot overlap rate is 60%, the cleanliness η reaches 92.26% after four times cleaning. When the laser spot overlap rate is 50%, it is necessary to clean six times to reach 91.84%. When the cleaning times continue to increase, the η value first increases and then decreases. The minimum number of cleaning times obtained are substituted into Equation (5), and the corresponding water jet-guided laser cleaning rate can be calculated as 10.13 mm²/s, 9 mm²/s and 9.38 mm²/s, respectively. Therefore, the 70% laser spot overlap rate with the highest cleaning rate is selected as the best spot overlap rate, and the cleaning speed can be obtained as 135 mm/s.

4.3. Effect of Laser Pulse Frequency

In the process of water jet-guided laser cleaning, set the average cleaning power of the water jet-guided laser as 25 W, and adjust the cleaning speed to keep the laser spot overlap rate consistent under different laser pulse frequencies. It can be seen from Figure 14a,b that when the laser pulse frequency is lower than 300 Hz, the surface of the metal substrate is obviously ablated. The laser-irradiated area is covered by a molten layer, and the molten material formed due to laser ablation and the traces of water jet-guided laser cleaning can be clearly observed. Equation (4) shows that the energy density is inversely proportional to the laser pulse frequency when the water jet-guided laser cleaning power and cleaning speed are unchanged. A smaller laser pulse frequency will cause the energy density of the water jet-guided laser to exceed the damage threshold of the metal substrate. The coating on the substrate surface absorbs energy, vaporizes rapidly, and forms a large amount of plasma. The plasma explosion produces large pressure and absorption waves, causing the molten metal pool to burst out in the form of metal splashes and vapor droplets and forming craters on the substrate surface. Part of the molten material is cooled and solidified at the edge of the crater before it can be gasified, forming a dendritic mastoid structure. At this time, the cleaning mechanism of a water jet-guided quasi-continuous laser is mainly a thermal ablation effect and laser plasma shock effect.

Figure 14c,d shows that when the laser pulse frequency is 600 Hz, there is no residual coating material on the sample surface after water jet-guided laser cleaning. The molten craters formed by ablation appear in the laser-irradiated area. The depth and size of the craters are small, and there is no obvious splashing melt at the edge of the crater. The reason is that with the increase in laser pulse frequency, the energy density of the water jet-guided laser decreases, and the heat accumulation effect on the surface of the metal substrate and the impact effect of laser plasma decrease. The melting degree of the metal substrate is reduced, and the molten matter accumulated at the edge of the crater is reduced. At this time, the cleaning process of a water jet-guided quasi-continuous laser is the joint action of the thermal ablation effect and impact effect.

Figure 14e,f shows that when the laser pulse frequency reaches 900 Hz, the sample surface after water jet-guided laser cleaning shows metallic luster, and the substrate surface has no obvious residual coating and traces of over-cleaning; the cleaning effect is better. The reason is that the coating on the metal substrate surface absorbs a large amount of laser energy in a short time under the irradiation of a high pulse frequency laser, the temperature rises rapidly, and heat is transmitted to the substrate surface through the coating according to Fourier’s law. The coating material melts or vaporizes due to the high temperature, and the coating is removed by the erosion effect of the high-pressure water jet. Due to the different thermal expansion coefficients of the epoxy resin coating and metal substrate, with the increase in temperature difference at the bond, the coating expands under the action of thermal stress, and the melted coating is vibrated and bounced off the substrate. Because the temperature close to the substrate surface has exceeded the complete cleaning threshold of the coating and has not reached the damage threshold of the substrate, the substrate surface will not melt after the coating is removed. At this time, the cleaning mechanism of a water jet-guided quasi-continuous laser includes the thermal ablation effect and the thermal stress vibration effect.

Figure 14g,h shows that when the laser pulse frequency exceeds 1200 Hz, most of the surface cleaned by the water jet-guided laser is still covered by the coating. A large number of craters appear on the surface of the coating; there is a thick coating on the edge of the crater, and the interior of the crater shows the characteristics of ablation. The reason is that the laser pulse frequency is high, the energy density of the water jet-guided laser is low, and the thermal conductivity of the coating is poor. Under the cooling action of a high-pressure water jet, the heat generated on the surface irradiated by the laser cannot be completely transferred to the whole coating, resulting in the non-molten coating remaining on the surface of the metal substrate. At this time, the cleaning mechanism of the water jet-guided quasi-continuous laser is mainly the ablation effect.

Figure 15 shows the EDS results for the sample surface after water jet-guided laser cleaning with different laser pulse frequencies. Region A is the molten substance at the edge of the crater, region B is the center of the micro-ablation crater, region C is the smooth metal substrate surface, and region D is the surface residual coating. Region A, Region B, Region C, and Region D are shown in Figure 14. It can be seen from the EDS data that the residual coating contains not only a large number of C and O elements, but also Ca, Al, Si, and Ba elements, which are mainly derived from additives such as silicate and calcium carbonate in epoxy resin coating. The Pt element in the figure is derived from the metal film plated on the surface of the workpiece during the preparation of the SEM sample. The cleaned substrate surface mainly contains C, O, Fe, Cr, and other elements, while the disappearance of the peak values of Ca, Al, Si, and other elements indicates that the resin-based coating has been well removed. The peak values of carbon and oxygen at A are significantly higher than those at B and C because the molten metal at the edge of the cleaning pit is oxidized by combining with oxygen in the air before solidification, and the oxygen content is greatly increased. Moreover, the liquid or gaseous molten materials have a large surface area and adsorb more carbon elements in the air. With the increase in laser pulse frequency, the effect of thermal ablation and laser plasma impact on the cleaned surface is reduced, and the oxidation or ablation reaction of the cleaned surface is weakened, the content of oxygen element is reduced, and the probability of carbon element integrating into the molten substrate is reduced.

The effects of different laser pulse frequencies on the surface roughness of the sample after water jet-guided laser cleaning are shown in Figure 16. With an increase in laser pulse frequency, the surface roughness of the sample first decreases and then increases. Figure 17a shows that when the laser pulse frequency is low, the energy density of the water jet-guided laser is high, and a large amount of energy is absorbed on the surface of the molten substrate. The molten layer is deep, and the fluidity is large. At the same time, the impact force produced by the laser plasma explosion is also large. Under the combined action of laser plasma explosion, the molten slurry gathers around, increasing the diameter d and depth h of the craters on the substrate surface, and the Ra parameter of roughness is 4.927 μm. Figure 17b shows that as the frequency of laser pulses increases, the molten layer formed on the surface of the substrate becomes shallower, and the fluidity of the melt decreases. Moreover, the impact force of laser plasma decreases, which makes the fluidity of the melt layer well balanced with the impact pressure received, and the diameter d and depth h of the craters on the base surface also decrease. The Ra parameter decreased to 2.154 μm. Figure 17c shows that when the laser pulse frequency is 900 Hz, the coating on the surface of the sample has a good removal effect, and no residual coating can be observed. The surface finish of the metal substrate is high, which is close to the original substrate morphology. The Ra parameter of roughness reaches the minimum value (0.832 μm). Figure 17d shows that when the laser pulse frequency continues to increase, the energy density of the water jet-guided laser is low, and mottled craters are formed on the coating. The stereoscopic effect of the coating surface is obvious, and the Ra parameter of roughness increases to 3.638 μm.

In conclusion, a water jet-guided quasi-continuous laser cleaning coating is a complex result of the coupling of multiple mechanisms. It includes not only the thermal ablation effect of energy absorption but also the thermal stress vibration effect of energy conversion and the laser plasma impact effect. At the same time, it is also affected by the erosion effect of the high-pressure water jet.

5. Conclusions

In this paper, the epoxy resin coating on a 304 stainless steel surface was cleaned using water jet-guided quasi-continuous laser cleaning technology. First, the cleaning mechanism of the water jet-guided quasi-continuous laser was analyzed, and then the effects of energy density, spot overlap rate, and laser pulse frequency of the water jet-guided laser on the cleaning effect were studied. The conclusions are as follows:

(1)

The energy density and spot overlap rate of the water jet-guided laser have an obvious influence on the surface integrity of the substrate after cleaning. When the spot overlap rate is 70%, the complete cleaning threshold of the resin-based coating is between 12.5 J/cm² and 17.5 J/cm² of the energy density of the water jet-guided laser, and reaches the damage threshold of the metal substrate when the energy density is 20 J/cm². With an increase in the spot overlap rate, the thermal accumulation effect caused by laser irradiation will reduce the thermal damage threshold of the metal substrate.

(2)

After the coating is removed by the action of the water jet-guided laser, the surface of the substrate appears as a large number of craters with a depressed center and raised surroundings. The formation of craters is mainly the result of the combined effects of surface melting and shock waves. As the laser energy absorbed by the metal substrate increases, the crater size gradually increases, and the oxygen content and surface roughness of the substrate surface also increase.

(3)

The relationship between the peeling amount of epoxy resin coating and process parameters (energy density of water jet-guided laser, spot overlap rate, and laser pulse frequency) shows that the removal rate of the coating increases with the increase of energy density and spot overlap rate of water jet-guided laser and the decrease of laser pulse frequency. When the energy density of the water jet-guided laser is 17.5 J/cm², the transverse cleaning speed is 135 mm/s, the longitudinal cleaning line spacing is 0.1 mm, and the laser pulse frequency is 900 Hz, the coating is better removed.

(4)

The mechanism of water jet-guided quasi-continuous laser cleaning of epoxy resin coating on metal substrate is a multi-effect coupling mechanism, mainly including the thermal ablation effect, laser plasma shock effect, thermal stress vibration effect, and water jet erosion effect. These mechanisms interact with and influence cleaning quality in the cleaning process. Under conditions of low energy density and low spot overlap rate, the cleaning mechanism is mainly a thermal ablation effect. Under the conditions of high energy density and high spot overlap rate, the cleaning mechanism is mainly the combined effect of the thermal ablation effect and the plasma shock effect.