## 4 Results and Discussion  
### 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/cm\({}^{2}\), 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/cm\({}^{2}\), 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/cm\({}^{2}\), 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/cm\({}^{2}\), the cleaned substrate surface has thermal damage caused by laser irradiation. When the energy density exceeds 25 J/cm\({}^{2}\), 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  
Figure 4: Water jet-guided laser cleaning area and its binarization comparison chart. (**a**) Color photo; (**b**) Binarized photo. is achieved. The temperature rise model generated by a pulsed laser in pulse width time can be expressed as [35]:  
\[\Delta T=\frac{2F\gamma}{\kappa}\sqrt{\frac{\pi\tau}{\pi}} \tag{6}\]  
where \(F\) is the power of the water jet-guided laser, and \(\gamma\) is the absorption rate of the material, and \(\kappa\) is the thermal conductivity coefficient of material, and \(\alpha\) is the thermal diffusivity of the material, and \(\tau\) 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.