Objective Studying the nonlinear relationship between laser-induced damage through-hole threshold and material properties, dimensions, and laser spot size is of great significance for evaluating the ablation effects of high-energy lasers on materials. Although many scholars have conducted extensive research on the effects of laser ablation, studies on the influence of spot size generally remain at the level of temperature rise and qualitative analysis. Research on the more practically valuable through-hole characteristics is relatively scarce, posing significant challenges to the accurate assessment of through-hole threshold. At the same time, laser ablation effects are closely related to the material’s physical properties, making it essential to correlate spot size effects with material characteristics. Therefore, investigating the influence of spot size on laser ablation in typical materials and establishing the corresponding relationships among spot size, material properties, and through-hole threshold is of great value for guiding engineering applications.

Methods The transient thermal analysis model based on the ANSYS APDL program was developed to study the influence of spot size on laser ablation behavior. The ablation effect was simulated by the element death and alive method. The temperature-dependent material properties were taken into account. A high-power laser-spot experimental platform was established to validate the simulation results.

Results and Discussions The results indicated that under the competing effects of laser energy input and material heat transfer capabilities, the influence of spot size on short-time laser ablation of aluminum alloy and carbon fiber reinforced polymer manifested at different scales (Fig.5, Fig.6, Fig.8-Fig.9). The critical spot size range for significant change in through-hole threshold was 10-60 mm for aluminum alloy and 1-20 mm for carbon fiber reinforced polymer. The critical laser spot size was roughly twice the characteristic length of thermal diffusion. The through-hole power density threshold of aluminum alloy decreased from 5000 W/cm² (10 mm spot diameter) to 500 W/cm² (60 mm spot diameter). For carbon fiber reinforced polymer, the threshold dropped from 5000 W/cm² (1 mm spot diameter) to 1000 W/cm² (20 mm spot diameter). A clear correlation was observed between the through-hole power density threshold and the spot size (Fig.7, Fig.10). The simulation results showed good agreement with experimental data (Fig.11-Fig.12, Tab.3).

Conclusions This paper establishes a finite element simulation model for transient thermal analysis of laser ablation based on the APDL parametric design language, investigating the influence of spot size on aluminum alloy and CFRP. The results demonstrated that the through-hole power density threshold in aluminum alloy varied nearly 10-fold within the 10-60 mm spot diameter range, while that of CFRP exhibited an approximately 5-fold variation within the 1-20 mm range under short-time conditions. The critical laser spot size was approximately twice the characteristic length of thermal diffusion. In the laser ablation process of 3 mm-thick aluminum alloy, a power density of 5000 W/cm² was required to achieve rapid perforation at a small spot diameter of 10 mm, while this threshold rapidly decreased to 800-2000 W/cm² for medium spot diameters of 20-40 mm and further dropped to 300-500 W/cm² for large spot diameters of 60-100 mm. In the laser ablation process of 1.5 mm-thick CFRP, the short-time through-hole power density decreased from 5000 to 1000 W/cm² as the spot diameter increased from 1 mm to 20 mm. When the spot diameter exceeded 20 mm, the influence of spot size became essentially negligible with the through-hole power density threshold stabilizing at 800-1000 W/cm². The key conclusions provide significant guidance for laser spot size selection and ablation capability evaluation in laser ablation applications.