## 4 Results and Discussion
### Effect of Laser Pulse Frequency
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  
Figure 14: Effect of laser pulse frequency on the surface morphology of the samples. (**a**) Surface morphology at 300 Hz; (**b**) SEM morphology at 300 Hz; (**c**) Surface morphology at 600 Hz; (**d**) SEM morphology at 600 Hz; (**e**) Surface morphology at 900 Hz; (**f**) SEM morphology at 900 Hz; (**g**) Surface morphology at 1200 Hz; (**h**) SEM morphology at 1200 Hz. 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 \(\upmu\)m. 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.