Objective With developing inertial confinement fusion technology, the damage of optical elements due to pollution has gradually become a critical bottleneck, limiting the efficiency of laser drivers. The pollutants on the surface of optical components of laser drivers comprise three types of particles, namely organic matter, metals, and minerals. The formation modes of particle pollutants primarily include microparticles produced on the surface of optical components irradiated by laser, environmental particles introduced during assembly, and particles produced because of the friction of components. The deposition of micron-sized and submicron-sized particles on the surface of optical components can reduce the transmittance of optical components and damage optical components because of local light intensity enhancement. Therefore, it is critical to realize the clean control of micron and submicron particles. Metal (supporting optical element) wall adsorption and particle splashing induced by laser irradiation are important sources of micron and submicron particle pollutants. Currently, the commonly used control methods include mechanical polishing, micro-arc oxidation, black anodization , hard anodizing, and surface microtexture of the metal tube wall. However, most of these methods have problems and shortcomings. Therefore, an effective method to realize the cleanliness control of metal pipe wall pollutants must be explored. Laser-shock processing (LSP) is a new surface strengthening technology with many advantages and application scenarios. However, few studies have been conducted on cleaning control. Therefore, in this study, 5052 aluminum alloy is taken as an example to study the effect of LSP on generating pollutants induced via nanosecond laser irradiation of aluminum alloy to provide new research methods and useful guidance for controlling and generating pollutants due to laser irradiation of aluminum alloy.
Methods 5052 aluminum alloy was used in this study. First, 5052 aluminum alloy samples were shock-strengthened using a nanosecond laser with different power densities. The hardness and roughness were evaluated, and surface morphology was observed. Then, 5052 aluminum alloy samples after laser-shock strengthening and samples without laser-shock strengthening were irradiated using a nanosecond laser with different energy densities and cleanliness was evaluated. Finally, a comparative analysis of surface hardness, morphology, and roughness of 5052 aluminum alloy before and after LSP and changes in pollutants induced by laser irradiation were conducted. This study revealed the influence of LSP on the generation of pollutants induced by nanosecond laser irradiation of aluminum alloy.
Results and Discussions The surface hardness of 5052 aluminum alloy after laser-shock strengthening under experimental parameters has been considerably improved (14%--20%) (Table 1 and Fig.3), indicating that laser-shock strengthening is effective for improving the surface strength of aluminum alloy. However, the roughness will increase to a certain extent because of the comprehensive macro and micro effects (Fig.3). The number of particles produced by 5052 aluminum alloy increases slowly at first and then increases rapidly, irrespective of LSP (Fig.5). The interaction between the laser and aluminum alloy includes laser cleaning and ablation (Fig.4). When the energy density of laser irradiation is less than 0.65 J/cm 2, the number of particles produced by 5052 aluminum alloy increases slowly as the energy density increases, and laser irradiation only shows the effect of laser cleaning (Fig.6). When the energy density of laser irradiation is less than 0.71 J/cm 2, the number of particles produced by 5052 aluminum alloy increases rapidly as the energy density increases, and laser irradiation shows the effect of laser ablation (Fig.6). Although the roughness of the samples is changed, the roughness of the three samples after strengthening has slight differences and the surface microstructure remains the same (Fig.8), and the roughness has a negligible effect on the laser damage threshold of the materials. Therefore, the number of particles produced in the laser cleaning and ablation stages of the three samples is similar (Fig.7).
Conclusions LSP improves the strength of 5052 aluminum alloy, making the surface of 5052 aluminum alloy produce micro-pit structures, enhancing the adsorption capacity of micro-sized and submicron-sized particle pollutants on the surface. Thus, it reduces the probability of producing pollutants under low energy density laser irradiation. This technology is suitable for low energy laser cleanliness application scenarios, such as the low flux operation of laser drivers and clean environment maintenance of aerospace lasers. To further reduce the probability of aluminum alloy contamination under low energy density laser irradiation, the microstructure and distribution of the aluminum alloy surface can be improved by optimizing the laser impact strengthening parameters (such as power density, spot shape, spot overlap rate, laser impact strengthening times, etc.). Furthermore, the properties of the materials determine the laser damage or laser ablation thresholds of aluminum alloy materials and have nothing to do with laser impact strengthening. Therefore, the cleanliness control of high energy density laser irradiation application scenarios can be realized by replacing other high damage threshold metal or non-metallic materials.
