Understanding the crater morphology on the surface of a paint layer after a single laser pulse can effectively suppress the superposition effects of multiple laser parameters and the photothermal and photomechanical effects of a pulse overlap. This helps reveal the laser-material interaction mechanism and provides a basis for the optimization of laser parameters. In recent years, many scholars have simulated the morphology of craters on the surface of a paint layer with the help of finite element software after nanosecond pulsed laser action based on the ablation mechanism. The laser parameters are then optimized based on the simulation results. For nanosecond pulsed lasers, the main mechanism of the laser-material interaction varies at different energy densities (the main mechanism is the ablation mechanism at low energy density, and the plasma shock and thermal radiation mechanism at high energy density). The ablation mechanism, plasma shock, and thermal radiation mechanism have different effects on the morphology of the crater. This study aims to establish a model of the damage form and the removal process of the paint layer during a single pulse of a nanosecond laser under different energy densities, to reveal the differences in the influence of the laser-material mechanism on the morphology of craters under different energy densities, and to provide a reference for the precise control and parameter optimization of the paint removal effect at high and low energy densities.
A nanosecond pulsed laser with a wavelength of 1064 nm and beam energy following a Gaussian distribution was applied to the epoxy primer surface. The diameter, depth, and three-dimensional morphology data of the craters on the surface of the paint layer were measured using a 3D optical surface profiler after the laser pulse. A simulation model of crater morphology was established based on the ablation mechanism and the fitting relationship between the depth (d) of the craters and energy densities (F). MATLAB was used to simulate the morphology of the craters in the energy density range of 13.58-27.16 J/cm2, and an experimental verification was carried out. Error analysis of the experimental and simulation results under a high density revealed the influence of the plasma shock and thermal radiation mechanism on the morphology of the crater. The model correction and experimental verification were carried out based on the plasma shock and thermal radiation mechanisms.
The simulation model of crater morphology based on the ablation mechanism has an error of less than 5% for crater depth and diameter at a low energy density (13.58-16.98 J/cm2), less than 5% for crater depth error at a high energy density (20.37-27.16 J/cm2), and up to 40% for diameter error (Fig.5). The error analysis shows that at a high energy density, the plasma shock and thermal radiation mechanisms are the main reason for the diameter error (Fig.7). After the model was corrected based on the above analysis, the diameter and depth errors of the craters under a high energy density were controlled within 5%, which significantly improved the accuracy of the model (Fig.9). The model shows that at a low energy density, the surface of the crater is approximately rotated paraboloid, and the profile of the crater is similar to a parabola; at a high energy density, the surface of the crater can be regarded as a combination of multiple normally-distributed surfaces, and the crater profile as a combination of multiple normal distribution curves.
At different energy densities, differences in the laser-material mechanism are noted; the ablation mechanism at a low energy density and the laser plasma shock and thermal radiation mechanisms at a high energy density are the main interaction mechanisms. Differences in the laser-material interaction mechanisms cause damage to the paint layer. Compared to the ablation mechanism, the plasma shock and thermal radiation mechanisms lead to an increase in the amount of paint removed near the surface of the crater and a wider profile near the crater surface. A simulation model of crater morphology is established for different laser-material mechanisms, thereby effectively improving the model accuracy. The study results provide a reference for the accurate control of the laser paint removal process and the optimization of paint removal parameters under high and low energy densities.
