## 4 Results and Discussion
### Effect of Laser Spot Overlap Rate
(**a**) 150 mm/s\({}_{\text{S}}\); (**b**) 150 mm/s\({}_{\text{S}}\); (**c**) 300 mm/s\({}_{\text{S}}\); (**d**) 300 mm/s; (**e**) 450 mm/s; (**f**) 450 mm/s; (**g**) 600 mm/s; (**h**) 600 mm/s. 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 \(17.5\,\mathrm{J/cm^{2}}\), the transverse spot overlap rate \(\gamma_{1}\) and the longitudinal spot overlap rate \(\gamma_{2}\) 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 \(\gamma_{1}\) is 70% and the longitudinal laser spot overlap rate \(\gamma_{2}\) 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 \(\gamma_{1}\) and the longitudinal laser spot overlap rate \(\gamma_{2}\) 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 \(\gamma_{2}\) 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.