Laser micromachining technology refers to the machining of microstructures with certain sizes, shapes, and arrangements on the surface of workpieces, which can endow the material surface with characteristics such as superhydrophobicity, superhydrophilicity, wear resistance, and friction reduction. Laser micromachining has enormous application prospects and value in fields such as aerospace, automobiles, energy, and materials. In general, the application environment of microstructures is extremely harsh, with relatively high speeds, wide temperature ranges, and severe vibrations. Therefore, the requirements for processing quality are extremely stringent. Laser micromachining technology has excellent characteristics, such as high precision, no contact or pollution, high energy density, wide adaptability to various materials, and a small heat-affected zone, thus gradually becoming the mainstream in fine machining technology research. In this study, a nanosecond pulse laser is used for micromodeling experimental research on the surface of stainless steel. By adjusting different process parameters such as scanning speed, laser power, repetition frequency, and pulse width, the material removal effects under different laser processing parameters as well as the change laws of surface morphology and wettability are analyzed. The optimal parameter group is obtained through process optimization. This provides a technical reference and a theoretical basis for research on the surface states and wettability-related aspects of laser-processed materials.

The material selected for the experiment is 304 stainless steel. The sample dimensions are 50 mm×50 mm×1 mm. The nanosecond pulsed laser processing equipment used in the experiment is equipped with a field lens. The incident spot radius is calculated to be approximately 20 μm. The microtexture morphology of the surface of the 304 stainless steel is observed using a laser confocal microscope. Wettability is measured using a contact angle tester. Roughness is measured using a contact roughness meter, and the surface profile is fitted using DataView TIME3200 software. In this study, the processing time is set to 30. By changing parameters such as the laser processing power, scanning speed, repetition frequency, and pulse width, a 7 mm×7 mm rectangular texture is processed to study the changes in microtexture depth, morphology state, and wettability.

In analyzing the experimental results, when the processing parameters are laser power of 6 W, scanning speed of 800 mm/s, repetition frequency of 30 kHz, and pulse width of 20 ns, a micro-texture depth of 18?22 μm is obtained, and the microstructure surface is uniform. Therefore, the influence of the filling direction on the processed surface morphology is studied using these parameters. Unidirectional, cross, four-way, and six-way fillings are selected to process the microtextures and explore the improvement effect of the filling direction on the surface morphology state (Fig. 17). As shown in the figure, the surface morphology of the one-way filling has a furrow-like morphology formed by the successive microchannels, whereas cross-filling produces a grid-like microstructure. Increasing the filling direction further gradually flattens the surface morphology. This is because as the number of changes in the scanning direction increases, the energy absorbed by the material surface becomes more uniform, reducing the influence of the characteristics of the Gaussian distribution of the laser energy. The surface roughness after six-way filling is the lowest obtained by a contact roughness meter (Fig. 18), reaching about 0.301 μm. In unidirectional filling, the roughness is 0.380 μm. The roughness in cross filling is the largest, reaching 1.151 μm, and that in four-way filling reaches 0.601 μm. Microchannels are observed to have a lower impact on surface roughness. The generation of the microchannel morphology is mainly due to the line spacing of laser scanning. In this experiment, the scanning line spacing is uniformly 10 μm. Therefore, the distance between the pulse centers of two adjacent channels is less than the spot radius; thus, the height difference of the generated microchannel structure is not obvious. For cross filling, owing to the interaction of energy, the ablation effect is not obvious on the area with energy deficiency, resulting in the accumulation of molten matter, which has a very serious impact on surface morphology. As the filling direction continues to increase, the surface ablation coverage effect increases; the area of the energy-deficient regions is significantly reduced; and the surface state is improved. The microtexture obtained by the cross filling method has the worst hydrophilicity (Fig. 19), and the contact angle of the left droplet is 80.74°. In the above figure, when cross filling is used, the internal structure height of the texture is observed to be relatively high and evenly distributed. There are more gaps at the bottom to accommodate the droplet, and more microstructures significantly hinder the spreading of the droplet. With other filling methods, the low surface roughness reduces the internal height undulations of the texture. Therefore, the droplet spreading on the textured surface is less hindered, and the droplets exhibit better hydrophilicity. In particular, when processing microtextures by six-way filling, because of the large number of filling directions and more uniform pulse distribution, the texture surface is relatively flat, and the obtained droplet has the smallest contact angle of 56.2°.

The surface state is mainly regulated by the pulse overlap rate and energy density. When the pulse overlap rate and energy density are relatively high, the surface appears charred black, and a recasting effect is evident. As the pulse overlap rate and energy density continue to decrease, the color of the microgroove surface changes from deep to light; the surface gradually becomes uneven; and pulse ablation traces become obvious. When the pulse overlap rate and energy density are low, the Gaussian distribution of the laser energy is manifested as volcanic morphology on the microgroove surface. Wettability is mainly affected by the surface micromorphology and surface energy of the microtextures. This paper focuses on discussing the influence of micromorphology changes on wettability under different laser parameters. The wettability gradually increases with an increase in processing power and gradually weakens with an increase in scanning speed. However, the repetition frequency and pulse width do not directly affect the energy distribution of the laser pulses; therefore, the change in wettability is not significant. In experimental research and analysis of the filling direction, the more the filling directions, the more uniform the surface energy distribution of the material, which improves surface morphology and wettability of the microgroove. Considering the requirements of processing efficiency, processing quality, and processing depth, when the processing time reaches a certain value, the influence on the depth gradually weakens. Therefore, in the experiment, a laser power of 6 W, scanning speed of 800 mm/s, pulse width of 20 ns, repetition frequency of 30 kHz, six-way filling method, and processing times of 30 are selected, thereby obtaining a microgroove with a depth of 18?22 μm and good wettability.