With the development of high-end equipment manufacturing, a series of high-performance thin-walled cavity parts requiring precision hole drilling, such as automobile fuel injector nozzles and turbine blades, have emerged. The overall performance of such parts depends on the hole-drilling technology. Laser drilling, as a non-contact processing method, has the advantages of high efficiency, high flexibility, low thermal influence, and insensitivity to the processed material, and it has been widely used in the manufacturing of high-performance parts. However, in the laser drilling process for thin-walled cavity parts, it is difficult for the focused laser beam to immediately desist after breaking through the front wall. This causes a back strike injury on the back wall, as shown in Fig. 4. Back strike protection methods can be divided into two main categories: passive protection methods, which involve the utilization of protective materials to fill cavities, and active protection methods, which use sensing signals for the real-time control of the drilling process. Passive protection methods are characterized by their simplicity, high stability, and suitability for mass production. Therefore, they are widely used in processing. Currently, research on back strike protective materials has preliminarily explored the influence of composition and proportion on protective performance, but few studies have been undertaken on factors such as particle size, shape, and binder. In addition, to quickly respond to the drilling requirements of different apertures, a method that can reduce the number of trial-and-error tests and efficiently develop back strike protective materials must be explored.

To overcome the problem of the low protection ability of small particles and simultaneously prevent the protection failure caused by large particles generating filling gaps, this work simulated the blocking rate of particles to the drilling area at different particle diameters via random sampling using a Monte Carlo method. Figure 1 illustrates the calculation space. The optimal protective particle size was selected by analyzing the statistical characteristics of the blocking rate distribution. The 6061 aluminum alloy is used as the cover plate for the test pieces to simulate the front wall, and the GH3536 superalloy is used as the bottom plate to simulate the back wall, as shown in Figs. 4(b) and (c). The protective abilities of synthetic graphite particles, green silicon carbide (SiC), and zirconium oxide (ZrO₂) were tested using a laser scanning process system [Fig. 4(a)]. Polyvinyl alcohol (PVA) was used as the binder in the protective material. Using a design of experiments (DOE) methodology according to the factor level settings in Table 5, the significance of the shape of the particle and binder were tested, and the depth of the back strike pit was measured via confocal laser scanning microscopy. Breakthrough timing experiments of the protective materials were carried out by using a video camera to analyze the protective abilities of different material compositions, as shown in Fig. 7.

The distribution of the blocking rate obtained from the Monte Carlo simulation is plotted on a violin diagram, as shown in Fig. 3. With an increase in the particle diameter, the distribution area of the blocking rate is gradually elongated and transformed from a multi-peak distribution to a single-peak distribution, and the peak value is constantly shifted in the positive direction with an increase in particle diameter. At a particle diameter of 0.7 mm, the upper quartile of the blocking rate distribution reaches 100%. Simultaneously, the distribution appears to extend in the negative direction and gradually broadens to 0% as the particle diameter increases. From the ANOVA and significance test for the model of the depth of the back strike pit in Table 7, it is determined that the shape of the protective material (spherical or polygonal) has a significant effect on the protective ability of the material, whereas the addition of a binder has no significant effect on the protective ability. Polygonal SiC particles offer better protection than spherical ZrO₂ particles because of the lower porosity of the polygonal particles, which reduces multiple reflective absorption within the protective material (Fig. 6). The addition of the binder slightly reduces the depth of the back strike pit (Fig. 6). Breakthrough experiments indicate that SiC particles exhibit the longest breakthrough time (Fig. 9). Based on the nature of the materials, we posit that the better protection of SiC particles compared with that of graphite particles, which are polygonal, is attributable to their intra-substrate transmittance and wide forbidden band properties. In addition, although spherical ZrO₂ has the widest forbidden band and the highest laser ablation threshold among the three protective materials, it exhibits the poorest protective ability (Fig. 9). We assume that the shape of the material plays a more important role in backstrike protection than do material properties.

Based on the analysis of the Monte Carlo simulation of the blocking rate distribution, it is found that the optimal filler particle size for 0.3 mm hole laser drilling is 20 mesh (≈0.8 mm). The results of the screening DOE and breakthrough experiments show that the shape of the particles has a significant effect on protective performance and that irregular polygonal particles have better protective performance than spherical particles. However, the effect of the binder is not significant. Meanwhile, the protective material filled with SiC particles has better protective performance than do other compositions, with a breakthrough time of 37.494 s. Based on this study, a rapid selection and optimization strategy for the particle-filled back strike protective material is realized through a combination of Monte Carlo simulations and protection ability verification experiments.