## 4 Results and Discussion
### 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 13: Effect of spot overlap rate of water jet-guided laser on cleaning rate. 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.