Study on influence of laser processing parameters on thermal effects and surface morphology of GDL

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Femtosecond laser processing is a promising solution capable of achieving precise control over the material structure and improving the quality of the processed material. In this study, femtosecond laser processing technology is used to modify the surface microstructure of gas diffusion layers (GDL) in PEMFC, aiming to enhance the characteristics of gas-liquid two-phase flow and electrochemical performance. In this paper, a novel coupled model based on the coupling of the two-temperature equation, phase transition and thermal stress is proposed. Comparison of the effects of different laser processing parameters on the surface morphology and thermal effects of carbon fibre materials. The impact of repetition rate on the heat-affected zone and pit quality is most significant. When rate increases from 100 kHz to 400 kHz, the heat-affected zone decreases from 42.8% to 29.3%. This process model can provide guidance and prediction for optimizing the laser processing parameters and improving the performance of the microporous structures. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction The surface morphology of the gas diffusion layer has an important influence on the power output of the fuel cells [1]. Due to the extremely short pulse duration and high energy concentration of femtosecond lasers, the material processing with femtosecond lasers results in only a very small heat-affected zone [2,3]. And femtosecond laser processing technology, as a surface structure processing technology, has gained widespread attention due to its advantages such as fast speed, convenience, no need for chemical reagents before and after processing, preparation of uniform structures, and processing area limited only to the material surface [4,5]. Many Scholars generally believe that ultrafast laser drilling technology can produce high-quality micropore structure on its surface [6–8]. Tao and others studied the material removal mechanism and heat affected zone effect in the process of laser drilling [9,10]. By optimizing laser parameters, material selection, and processing strategies, a considerable number of scholars have achieved precise control and customization of various surface structures [11–13]. In the forefront of experiments and simulations of carbon fiber composites, most scholars believe that the actual use of laser to process carbon fiber composites will be more serious than processing the heat affected zone of a single material [14–16]. However, most of the current research focuses on reducing the heat affected zone to improve the microporous structure, ignoring the thermal stress caused by laser heat, which may be the key factor affecting the microporous structure [17–19]. Due to the unique operating mechanism of the femtosecond laser, these optical parameters have different effects on the processing compared to traditional laser processing [20,21]. In order to fully study the microstructure of GDL and explore the internal mechanism of thermal stress changes during ultra-fast laser processing of GDL micropores, this study established a two-temperature equation-phase transformation-thermal stress coupling process model based on ultra-fast laser processing of GDL micropores, studied the influence of ultra-fast laser processing parameters on the micropore structure. This study will explore the influence of femtosecond laser processing parameters on surface morphology and thermodynamic properties of gas diffusion layers from both experimental and simulation perspectives. The simulation part utilizes COMSOL finite element analysis software to perform numerical simulation and analysis on carbon fiber composite materials. Build the 2 D model and combine the phase change model to study the processing process. By setting different laser processing parameters, including power, scanning speed and pulse frequency, the effects on the heat-affected region, thermal stresses and the surface morphology of the material were explored for the gas diffusion layer of carbon fibre paper. 2. Simulation and experimental analysis 2.1 Experimental method The sample of gas diffusion layer selected for this paper was a commercial carbon paper (Sigracet 36 BB, SGL Group), which was hydrophobically treated to have a homogeneous porous thin-layer structure. The carbon paper thickness was 190 ± 20 µm with an average porosity of 0.84. Femtosecond ultrafast laser processing equipment is used to process the microporous structure on the surface of the actual gas diffusion layer. Femtosecond ultrafast laser processing method is used to process micro-holes on GDL surface, and through process experiments, the through holes are explored and processed. Among them, the laser power was 2 W and 5 W for comparison, the scanning speed was 1000 mm/s and 600 mm/s for comparison, and the repetition rate was 50 kHz and 100 kHz for comparison. The processing times were 400 times and the laser pulse width was 270 fs. Table 1 Experimental Table group number Repetition rate power scanning speed Different repetition rates 1 100 kHz 3 W 600 mm/s 2 50 kHz 3 W 600 mm/s Different laser powers 3 100 kHz 3 W 600 mm/s 4 100 kHz 5 W 600 mm/s Different scanning speeds 5 100 kHz 3 W 600 mm/s 6 100 kHz 3 W 1000 mm/s 2.2 Simulation method In this study, COMSOL finite element analysis software is used to simulate and analyze carbon fiber composites, and a two-dimensional model is built to study the processing process combined with phase change model. SGL 36 BB carbon paper is composed of carbon fiber substrate and other polymers, among which epoxy resin accounts for a large proportion, so other polymers are ignored in this study, and a series of changes of materials composed of carbon fiber substrate and epoxy resin under the action of laser are studied emphatically. In this study, the sample thickness is 10 µm. On the other hand, because carbon fiber-reinforced plastics (CFRP) is composed of carbon fiber and epoxy resin, the physical properties of the two materials are quite different. However, the research content of this paper focuses on the macroscopic thermal ablation, so the CFRP can be homogenized, which not only saves the computing resources, but also obtains more accurate results. The model is mainly based on the following assumptions: The intensity distribution of laser is considered as an ideal Gaussian distribution, and the energy of the process is constant. The model is adiabatic, ignoring the effects of convection heat transfer, radiation heat transfer and plasma; Two-temperature model By establishing the two-temperature model proposed by Anisimov in 1974 [22], the model is analyzed: $${C_e}\frac{{\partial {T_e}}}{{\partial t}}{\text{=}}\nabla \left( {{k_e}\nabla {T_e}} \right) - G\left( {{T_e} - {T_l}} \right)+S\left( {x,y} \right)$$ 1 $${C_l}\frac{{\partial {T_l}}}{{\partial t}}=\nabla \left( {{k_l}\nabla {T_l}} \right)+G\left( {{T_e} - {T_l}} \right)$$ 2 Here C is the specific heat capacity, k is conductivity, G is the electron-phonon coupling coefficient, S is the laser intensity, e represents electrons, l represents a lattice, x is the abscissa, y is the ordinate. We usually think that the lattice temperature is the temperature of the model. And the volumetric thermal conductivity k eq is equal to the addition of the thermal conductivity of electrons k e and lattice thermal conductivity k l . For CFRP, the diffusion of free electrons has a very important influence on heat transfer. Therefore, the thermal conductivity of the lattice k l set to 1% of volume thermal conductivity k eq . $${k_l}=0.01{k_{eq}}$$ 3 $${k_e}=0.99{k_{eq}}$$ 4 Laser intensity function Gaussian volumetric heat density is obtained as follows: $$\begin{gathered} S\left( {x,y,t} \right)=\sum\limits_{{i=1}}^{{{N_p}}} {\frac{{0.94J\left( {1 - R} \right)}}{{{t_p}\left( {\delta +{\delta _b}} \right)\left[ {1 - {e^{ - L/\left( {\delta +{\delta _b}} \right)}}} \right]}}} \hfill \\ \exp \left[ {\left( {\frac{{y - \vartriangle y}}{{\delta +{\delta _b}}}} \right) - {{\left( {\frac{{x - vt}}{{{r_s}}}} \right)}^2} - 2.77{{\left( {\frac{{t - \left( {i - 1} \right){t_{sep}} - 2{t_p}}}{{{t_p}}}} \right)}^2}} \right] \hfill \\ \end{gathered}$$ 5 $$J{\text{=}}\frac{P}{{\pi r_{s}^{2}f}}$$ 6 $${t_{sep}}{\text{=}}\frac{1}{f}$$ 7 Among them, R is the surface reflectivity, δ is the optical penetration depth, δ b is the optical ballistic distance, w(y) is the laser spot size,which is a function of the longitudinal depth, t p is that duration of the lase pulse, N p is the number of laser pulses, t sep is the laser pulse interval, J(y) is the function term of the laser influence varying with depth, x(t) is the function of laser spot position changing with time, Δy is the longitudinal displacement of deformed geometry, P is the laser power, f is the pulse repetition rate, L is the model thickness (10 µm in this study), and the influence of thickness is determined by \(\left[1-{\text{e}}^{-\text{L}/\left({\delta }+{{\delta }}_{\text{b}}\right)}\right]\) to decide. Phase change model Assuming that the gas is an ideal gas in a thermal equilibrium state, the vaporization process is described by combining the two-temperature model with Clausius-Clapeyron equation[23]: $$\frac{{dp}}{{d{T_l}}}=\frac{{p{h_v}({T_l})}}{{{R_u}{T_l}^{2}}}$$ 8 $${h_v}({T_l})={h_{v0}}{\left( {1 - {{(\frac{{{T_l}}}{{{T_c}}})}^2}} \right)^{1/2}}$$ 9 $$p={p_0}\exp \left( { - \frac{{\rho {h_v}}}{{{R_u}}}\left( {\frac{1}{{{T_l}}}\sqrt {1 - {{\left( {\frac{{{T_l}}}{{{T_c}}}} \right)}^2}} - \frac{1}{{{T_v}}}\sqrt {1 - {{\left( {\frac{{{T_v}}}{{{T_c}}}} \right)}^2}} } \right) - \frac{{\rho {h_v}}}{{{R_u}{T_c}}}\left( {{{\sin }^{ - 1}}\left( {\frac{{{T_l}}}{{{T_c}}}} \right) - {{\sin }^{ - 1}}\left( {\frac{{{T_v}}}{{{T_c}}}} \right)} \right)} \right)$$ 10 In which latent heat of evaporation is h v0 , p0 is the atmospheric pressure, T c is the critical temperature, R u is the ideal gas constant,and p is the saturation pressure. According to the Hertz-knudsen-Langmuir equation derived from gas dynamics theory[24], the molar evaporation flux is calculated j v : $${j_v}=\frac{{Ap}}{{\sqrt {2\pi M{R_u}{T_l}} }}$$ 11 Here A is the adjustment factor of 0.92 suggested by Xu[25], M is molar mass. The evaporation rate can be calculated from the molar evaporation flux: $${u_v}=\frac{{M{j_v}}}{\rho }=\frac{{AMp}}{{\rho \sqrt {2\pi M{R_u}{T_l}} }}$$ 12 Initial conditions and boundary conditions Assuming that the initial temperature is room temperature (25°C), the initial condition expression is: $${T_e}\left( {x,y,0} \right)={T_l}\left( {x,y,0} \right)={T_0}$$ 13 Because the heat loss on the surface is neglected, the temperature field adopts adiabatic boundary conditions: $$\begin{gathered} {\left. {\frac{{\partial {T_e}}}{{\partial t}}} \right|_{y{\text{=}}0}}{\text{=}}{\left. {\frac{{\partial {T_e}}}{{\partial t}}} \right|_{y=L}}={\left. {\frac{{\partial {T_e}}}{{\partial t}}} \right|_{x{\text{=}} - \frac{1}{2}M}}{\text{=}}{\left. {\frac{{\partial {T_e}}}{{\partial t}}} \right|_{x{\text{=}}\frac{1}{2}M}}{\text{=}}0 \hfill \\ {\left. {\frac{{\partial {T_l}}}{{\partial t}}} \right|_{y{\text{=}}0}}{\text{=}}{\left. {\frac{{\partial {T_l}}}{{\partial t}}} \right|_{y=L}}={\left. {\frac{{\partial {T_l}}}{{\partial t}}} \right|_{x{\text{=}} - \frac{1}{2}M}}{\text{=}}{\left. {\frac{{\partial {T_l}}}{{\partial t}}} \right|_{x{\text{=}}\frac{1}{2}M}}{\text{=}}0 \hfill \\ \end{gathered}$$ 14 Solid mechanics By adding the physical field of solid mechanics to calculate the real-time change of temperature field during laser ablation and the influence of different material properties on the change law of thermal stress field, the corresponding relationship can be expressed as follows: $$\rho \frac{{{\partial ^2}{\text{u}}}}{{\partial {u^2}}}=\nabla \sigma +{F_V}$$ 15 Formula u is the displacement field, ρ is the density of the deformed material, \(\nabla\) is a gradient operator, σ is Cauchy stress tensor, F v is the force per unit deformation volume. Deformation geometry The deformation geometry method is adopted. In this method, the displacement of boundary nodes depends on evaporation speed. This situation can be described by the following function: $${v_{mesh}}= - {u_v}\overrightarrow j$$ 16 This equation is used to describe the change of model surface. The rest of the boundary is under the condition of no slip. 3. Results and discussion At ambient temperature and normal pressure, the diameter of the through hole was measured by scanning electron microscope (model S-3400N, resolution 3.0 nm(30 kV)/10.0 nm(3 kV), magnification 10 ~ 200 K). Table 2 shows the experimental results under different laser process parameters. Figure 1 shows the pit size and morphology under different laser processing parameters. As can be seen from Table 2 , with the decrease of laser repetition rate, the micropore diameter will decrease. This is because the laser repetition rate decreases, which means that the number of laser pulses decreases in the same time, which leads to the decrease of machining aperture, but this trend is not very significant. With the increase of laser power, the pore size of micropores increases significantly. The laser power was from 3 W to 5 W, and the micropore diameter was from 48.41 µm to 65.56 µm, which increased by 26%. This means that for every 1 W increase, the pore size of micropores increases by 5.72 µm. With the increase of laser scanning speed, the number of laser pulses per unit time will decrease, and eventually the aperture will become smaller. This trend is very obvious. It can be seen that when the laser scanning speed increases from 600 mm/s to 1000 mm/s, the micropore diameter decreases from 48.41 µm to 29.88 µm. Therefore, the laser scanning speed has obvious influence on the micropore diameter. Table 2 Experimental results under different laser process parameters group number repetition rate power scanning speed aperture standard deviation Different repetition rates 1 100kHz 3 W 600 mm/s 48.41 µm 5.83 µm 2 50 kHz 3 W 600 mm/s 45.80 µm 4.31 µm Different laser powers 3 100 kHz 3 W 600 mm/s 48.41 µm 5.83 µm 4 100 kHz 5 W 600 mm/s 65.56 µm 5.74 µm Different scanning speeds 5 100 kHz 3 W 600 mm/s 48.41 µm 5.83 µm 6 100 kHz 3 W 1000 mm/s 29.88 µm 6.26 µm The apertures produced by the model simulation laser processing were verified using the above experimental data of femtosecond laser processing (laser power of 3 W and 5 W), see Fig. 2 . For the SGL 36 BB carbon paper used in the experiments, it was homogenized during the simulation process. The processing parameters used during the model simulation process are the same as those used in the experiments of groups 3 and 4 in Table 2 . The results show that the simulation results are in good agreement with the experimental data. The relative error of the simulated aperture over the experimental one is 2.56% when the laser power is at 3W. When the laser power is 5W, the relative error of the simulated aperture over the experiment is 6.59%. The intensity distribution of the laser in the actual laser processing is not Gaussian, and the model cannot be completely adiabatic, which may be a source of error for the idealized assumptions in the simulation. In this section, the influence of laser power is studied by changing different laser power. In each case, the laser power is 2 W, 3 W, 4 W and 5 W, the scanning speed is 600 mm/s, the repetition rate is 100 kHz, and the pulse duration is 270 fs. Figure 3 shows the change of heat affected zone area under different power, and the second, third, fourth, and fifth pulse laser finishing time points are selected for research, namely, 20 µs, 30 µs, 40 µs, and 50 µs. Because the heat affected zone did not produce significantly in the time point of 10 µs, the first pulse was not considered. In this study, the part of the sample whose temperature is higher than 500K is regarded as the heat affected zone. The reason is that epoxy resin will melt and melt at the temperature of 500 K. Once the heat generated by laser exceeds this value, epoxy resin will inevitably melt and carbon fiber will be exposed, which will affect the processing quality. It can be seen that, with the passage of time, because the pulse energy continuously acts on the surface of the workpiece, the heat of the laser propagates around at a rapid speed, and the heat affected zone shows a trend of rapidly increasing at first and then decreasing. This is because the thickness of the workpiece (10 µm) is far less than the length of the workpiece (100 µm), and the laser propagation along the Y axis is basically completed under the action of the first few pulses, and then the laser heat can only propagate along the X axis at the subsequent pulse time. At this time, the effect of increasing the number of pulses on the thermal region is weakening. When the laser power is 2 W, the final heat affected area is 39.9%, while when the laser power is 5W, the final heat affected area is 46.2%. It can be concluded from the Fig. 3 that the increase of laser power has a certain influence on the increase of heat affected zone area in the range of 2W to 5W, but with the increase of laser power, this influence seems to be weakening. This is because with the increase of laser power, the single pulse energy increases significantly, which leads to the increase of heat affected zone area under the same number of pulses, but with the longitudinal heat affected zone of the workpiece being swallowed up, the influence of laser power increase on the increase of heat affected zone area is gradually decreasing. Thermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure 4 shows the evolution of the thermal stress field on the top surface under different power, and selects the time points when the first, second, third, fourth and fifth pulsed lasers respectively end, namely 10 µs, 20 µs, 30 µs, 40 µs and 50 µs. Figure 5 a shows the change of maximum thermal stress under different power, and the time of 30 µs is selected here. It can be seen that the femtosecond laser generates a lot of heat at the same time, and it will also produce the concentration of thermal stress. With the passage of time, the thermal stress distributed around the laser focus quickly disperses around, and the thermal stress gradient decreases. Due to the increase of laser power, the heat generated by laser increases, and then the influence area of thermal stress field mainly depends on laser power. However, the maximum thermal stress value does not depend entirely on the laser power, but the number of laser pulses plays a decisive role in the maximum thermal stress value at lower power. In short, the pit quality increases with the decrease of power and decreases with the increase of pulse number. In order to analyze the relationship between pit depth and laser power under different power, the pit depth under different power is compared, and Fig. 5 b shows the change of pit morphology under different power. It can be seen that with the increase of laser power, the pit depth gradually increases. When the laser power changes from 2 W to 5 W, the groove depth increases from 1.20 µm to 1.71 µm m Therefore, the laser power has a great influence on the pit depth. In this section, the influence of laser scanning speed is studied by changing different scanning speeds. In each case, the laser power of 200 mm/s, 600 mm/s, 1000 mm/s and 1400 mm/s was used respectively, with a power of 3 W, a repetition rate of 100 kHz and a pulse duration of 270 fs. Figure 6 shows the change of the area of the heat affected zone at different scanning speeds. Because the heat affected zone did not produce significantly within the time point of 10 µs, only the time points of 20 µs, 30 µs, 40 µs and 50 µs were considered. It can be observed that the increase of laser scanning speed on the final heat affected zone size is not obvious. This is because although the higher scanning speed will be beneficial to the heat dissipation, compared with the ultra-short pulse of femtosecond laser, this heat dissipation is negligible. When the scanning speed is 200 mm/s and 600 mm/s, the size of the heat affected zone almost coincides, because the overlap rate of laser spots is similar at these two scanning speeds. The thermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure 7 shows the evolution of thermal stress field on the top surface at different scanning speeds. Figure 8 a shows the change of maximum thermal stress at different scanning speeds, and the time of 30 µs is selected here. It can be seen that the faster scanning speed can not greatly affect the increase of heat affected zone. This is because the pulse width of ultrafast laser is short enough (< 10–12 s), and the increase of laser scanning speed can not have an essential impact on the heat generated by laser. Furthermore, the influence area of thermal stress field has little correlation with laser scanning speed. However, the maximum thermal stress is closely related to the number of pulses, and the lower the scanning speed, the more obvious the thermal stress accumulation, which may lead to material processing damage. In short, the pit quality increases with the increase of scanning speed. In order to analyze the relationship between pit depth and laser scanning speed at different scanning speeds, the pit depth at different scanning speeds is compared, and Fig. 8 b shows the changes of pit morphology at different scanning speeds. It can be seen that with the increase of laser scanning speed, the pit depth gradually decreases. When the laser power changes from 200 mm/s to 1400 mm/s, the groove depth decreases from 1.53 µm to 0.94 µm.Therefore,the laser scanning speed can reduce the pit depth. In this section, the influence of laser repetition rate is studied by changing different repetition rates. In each case, the laser power of 100kHz, 200 kHz, 300 kHz and 400 kHz was used respectively, with the power of 3 W, the scanning speed of 600 mm/s and the pulse duration of 270 fs. Figure 9 shows the change of heat affected zone area under different repetition rates. Because the heat affected zone didn't appear significantly in the first pulse, only the second, third, fourth and fifth pulse end time points were considered. It can be seen that the change of repetition rate is the key factor leading to the final change of heat affected area. A higher repetition rate represents an increase in the number of pulses, but under the condition of constant laser power, the energy carried by each pulse decreases, so it brings a smaller heat affected area. Further, it can be observed that the laser rates of 300kHz and 400kHz have a violent increase in the heat affected zone in the second pulse compared with other laser rates. This is because the higher laser rate brings more pulses, which makes the laser heat concentrated in generate in a short time, which leads to the acceleration of the longitudinal heat propagation speed of the workpiece and the increase of the heat affected zone. When the laser rate is 100 kHz, the final heat affected area is 42.8%, while when the laser rate is 400 kHz, the final heat affected area is 29.3%. It can be concluded from the Fig. 9 that the increase of laser rate can significantly reduce the area of heat affected zone in the range of 100 kHz to 400 kHz. Thermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure 10 shows the evolution of the thermal stress field on the top surface under different repetition rates, and selects the time when the first, second, third, fourth and fifth laser pulses are completed. Figure 11 a shows the change of the maximum thermal stress at different repetition rates, and selects the time when the third laser pulse is completed. It can be seen that the femtosecond laser generates a lot of heat in an instant, and at the same time, it also produces the concentration of thermal stress accordingly. The greater the repetition rate, the smaller the pulse interval between each pulse. Under the same number of pulses, the time of laser action will be shortened, which will lead to the reduction of the regional scope of thermal stress field. The increase of laser repetition rate is the key factor to cause the width of thermal stress field. It can be further known from the Fig. 11 a that the larger repetition rate leads to the obvious diffusion of thermal stress field, which will improve the quality of pits. However, the maximum thermal stress is closely related to the number of pulses, and the lower the repetition rate, the greater the maximum thermal stress.In brief, the pit quality increases with the increase of repetition rate. In order to analyze the relationship between pit depth and laser repetition rate under different power, the pit depth under different repetition rates is compared, and Fig. 11 b shows the changes of pit morphology under different repetition rates. It can be seen that with the increase of repetition rate, the pit depth gradually decreases. When the laser repetition rate changes from 100kHz to 400kHz, the groove depth decreases from 1.45µm to 0.51 µm Therefore, the laser repetition rate has a great influence on the pit depth. 4. Conclusion In this paper, a two-temperature model of ultrafast femtosecond laser drilling is established, and the evaporation rate is calculated by combining the phase change model. The coupling model of ultrafast femtosecond laser drilling for carbon fiber composites is obtained. The effects of laser power, scanning speed and repetition rate on the morphology, heat affected zone and thermal stress during laser drilling were studied. The outstanding conclusions of this study are summarized as follows: (1) The heat-affected area increases as the laser power increases, but the magnitude of the increase in the heat-affected area decreases as the longitudinal heat effect of the workpiece dissipates. The quality of the pit decreases with increasing power. Because the single pulse laser energy increases, the laser power has a significant effect on the hole depth and hole diameter, the pit depth and hole diameter will increase. (2) The quality of the pits increases with an increase in scanning speed, but at the same time, increasing the scanning speed will result in a decrease in the depth of the machining hole. (3) The thermal impact zone and pit quality primarily depend on the repetition rate. Increasing the laser rate can reduce the area of the thermal impact zone, while the quality of the craters increases with the increase in repetition rate. At the same time, as the repetition rate increases, the depth of the craters gradually decreases. (4) Femtosecond laser drilling can be accurately controlled by this model. This research has contributed to the theoretical guidance of laser drilling at present. By precisely adjusting the above parameters, the performance of the fuel cell can be maximally improved to enhance its efficiency and stability. 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Shi, Q., Gu, D., Xia, M., et al., “Effects of laser processing parameters on thermal behavior and melting/solidification mechanism during selective laser melting of TiC/Inconel 718 composites,” Optics & Laser Technology 117, 244–250 (2019). Sugioka, K., & Cheng, Y. “Femtosecond laser processing for optofluidic fabrication,” Lab on a Chip 12(19), 3576–3589 (2012). Chen, J. K., Tzou, D. Y., & Beraun, J. E. “A semiclassical two-temperature model for ultrafast laser heating,” International journal of heat and mass transfer 49(1–2), 307–316 (2006). Velasco, S., Román, F. L., & White, J. A. “On the Clausius–Clapeyron vapor pressure equation,” Journal of Chemical Education 86(1), 106 (2009). Hertel, M., Spille-Kohoff, A., Füssel, U., & Schnick, M. “Numerical simulation of droplet detachment in pulsed gas–metal arc welding including the influence of metal vapour,” Journal of Physics D: Applied Physics, 46(22), 224003 (2013). Xu, C. Y., & Singh, V. P. “Cross comparison of empirical equations for calculating potential evapotranspiration with data from Switzerland,” Water Resources Management, 16, 197–219 (2002). Additional Declarations No competing interests reported. Supplementary Files Supplement1.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3996929","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":275806512,"identity":"0e860956-2522-432b-bdab-39b73fb9c9e6","order_by":0,"name":"Xuan Xie","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA0UlEQVRIiWNgGAWjYDACZijN2MB84MCHH6RpYUs8OLOHNPt4jA9zsBGhzuA48zFpnpo7ic39Zz4cZuBhkOcXO4Bfi2QzW7Ixz7FniY0zcjccLrBgMJw5OwG/Fn5mHsPHPGyHgVp4NxyewcOQYHCbgBY2Zv4Ph3n+AbX0n3lwmIeNCC1AWxgf87YBtTTkMBCnBegXY8O5fYeNG2ekGQADWYKwXwzOH34m8ebbYdmN/Ycff/jww0aeX5qAFjgwbABTEkQqBwF5EtSOglEwCkbBCAMAUZRF+oKSaJYAAAAASUVORK5CYII=","orcid":"","institution":"Jiangsu University","correspondingAuthor":true,"prefix":"","firstName":"Xuan","middleName":"","lastName":"Xie","suffix":""},{"id":275806513,"identity":"992f37d2-0314-499d-a834-9f4104ea1007","order_by":1,"name":"Changwu Tang","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Changwu","middleName":"","lastName":"Tang","suffix":""},{"id":275806514,"identity":"5896e591-eeba-4c7c-a687-d36d7a5ff845","order_by":2,"name":"Changguo Wang","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Changguo","middleName":"","lastName":"Wang","suffix":""},{"id":275806515,"identity":"154bc670-9622-4ace-a26d-f3885e528f40","order_by":3,"name":"Sheng Xu","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Sheng","middleName":"","lastName":"Xu","suffix":""},{"id":275806516,"identity":"88812cab-24d7-4bc2-a837-d3146d4d5e6c","order_by":4,"name":"Bifeng Yin","email":"","orcid":"","institution":"Jiangsu University","correspondingAuthor":false,"prefix":"","firstName":"Bifeng","middleName":"","lastName":"Yin","suffix":""}],"badges":[],"createdAt":"2024-02-28 14:35:42","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3996929/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3996929/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":52019454,"identity":"065fe92b-5399-4320-8a37-0ebb73255ce7","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":428290,"visible":true,"origin":"","legend":"\u003cp\u003ePit size and morphology under different laser process parameters.\u003c/p\u003e","description":"","filename":"Fig.1.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/3c49cf8132ca6d0065b470c3.png"},{"id":52019452,"identity":"e6bb4a61-79a7-4526-aacd-f4e6f752c341","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":10054,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of the apertures produced by laser processing of carbon paper at powers of 3W and 5W in the simulated results with the experimental data from femtosecond laser processing.\u003c/p\u003e","description":"","filename":"Fig.2.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/65109cdbb9733ab978ecc3a0.png"},{"id":52019449,"identity":"baaab14a-9e5e-4642-b213-4c9ed3ded23e","added_by":"auto","created_at":"2024-03-05 13:36:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":18089,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the size of the heat-affected area at different power levels\u003c/p\u003e","description":"","filename":"Fig.3.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/f2ee9c84aec518750a928f5d.png"},{"id":52019457,"identity":"73cc0aaf-5078-467b-ace0-e74bf854ef69","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":81299,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of thermal stress on top surface under different power (pulse action completed)\u003c/p\u003e","description":"","filename":"Fig.4.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/d664573d89d78e6184e6ca2e.png"},{"id":52019453,"identity":"76b87a3f-0bc1-41d7-8eb1-7e790a882131","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":30274,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of maximum thermal stress under different power (completed in 30 μs) (a), Changes of pit morphology under different power (b).\u003c/p\u003e","description":"","filename":"Fig.5.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/3674be0867c5dd87cf30fdb2.png"},{"id":52019447,"identity":"5eee0dc4-f19b-4c9b-97eb-9de0695241b3","added_by":"auto","created_at":"2024-03-05 13:36:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":26106,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the area of heat-affected regions at different scanning speeds\u003c/p\u003e","description":"","filename":"Fig.6.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/274415356e256be7fa364349.png"},{"id":52019458,"identity":"f81533a1-0ca5-42d1-b62e-9b5156c799b2","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":79910,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of thermal stress on the top surface at different scanning speeds (completion of pulse action)\u003c/p\u003e","description":"","filename":"Fig.7.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/f29f76d599e886b1fa837864.png"},{"id":52019677,"identity":"7b63a29d-9efe-465d-9631-c5b5b76df52a","added_by":"auto","created_at":"2024-03-05 13:44:23","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":40894,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of maximum thermal stress at different scanning speeds (pulse action completed) (a), Changes of pit morphology at different scanning speeds (b).\u003c/p\u003e","description":"","filename":"Fig.8.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/5e378ef3514ab6467c8d8f46.png"},{"id":52019456,"identity":"7ddc637b-b9bb-4173-bafb-4dba917b2aa4","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":22403,"visible":true,"origin":"","legend":"\u003cp\u003eChanges in the size of the heat affected area at different repetition rates\u003c/p\u003e","description":"","filename":"Fig.9.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/d23b77cedaf90247d68d8769.png"},{"id":52019459,"identity":"563d4246-e6bb-459f-8f36-4835f633952c","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":102711,"visible":true,"origin":"","legend":"\u003cp\u003eEvolution of thermal stress on the top surface at different repetition rates (completion of pulse action)\u003c/p\u003e","description":"","filename":"Fig.10.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/ad3bbfded8e3fe120a80ca02.png"},{"id":52019450,"identity":"a725be9d-837a-4047-a12f-4bb678a52c3d","added_by":"auto","created_at":"2024-03-05 13:36:22","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":40471,"visible":true,"origin":"","legend":"\u003cp\u003eVariation of maximum thermal stress under different repetition rates (the third pulse is completed) (a), Changes of pit morphology under different repetition rates (b).\u003c/p\u003e","description":"","filename":"Fig.11.png","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/f334395449b9627debc3013d.png"},{"id":56312690,"identity":"55b57e5f-273b-4ca2-a8fe-b33e2ec4dbb3","added_by":"auto","created_at":"2024-05-11 16:29:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1311550,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/403417fd-018b-4fca-9337-03e9a47ec768.pdf"},{"id":52019455,"identity":"ac0ff904-834f-4293-bff5-a8a2b0be157f","added_by":"auto","created_at":"2024-03-05 13:36:23","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1361177,"visible":true,"origin":"","legend":"","description":"","filename":"Supplement1.docx","url":"https://assets-eu.researchsquare.com/files/rs-3996929/v1/3a8546d741b02f8969a72bc6.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Study on influence of laser processing parameters on thermal effects and surface morphology of GDL","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe surface morphology of the gas diffusion layer has an important influence on the power output of the fuel cells [1]. Due to the extremely short pulse duration and high energy concentration of femtosecond lasers, the material processing with femtosecond lasers results in only a very small heat-affected zone [2,3]. And femtosecond laser processing technology, as a surface structure processing technology, has gained widespread attention due to its advantages such as fast speed, convenience, no need for chemical reagents before and after processing, preparation of uniform structures, and processing area limited only to the material surface [4,5]. Many Scholars generally believe that ultrafast laser drilling technology can produce high-quality micropore structure on its surface [6\u0026ndash;8]. Tao and others studied the material removal mechanism and heat affected zone effect in the process of laser drilling [9,10]. By optimizing laser parameters, material selection, and processing strategies, a considerable number of scholars have achieved precise control and customization of various surface structures [11\u0026ndash;13]. In the forefront of experiments and simulations of carbon fiber composites, most scholars believe that the actual use of laser to process carbon fiber composites will be more serious than processing the heat affected zone of a single material [14\u0026ndash;16]. However, most of the current research focuses on reducing the heat affected zone to improve the microporous structure, ignoring the thermal stress caused by laser heat, which may be the key factor affecting the microporous structure [17\u0026ndash;19].\u003c/p\u003e \u003cp\u003eDue to the unique operating mechanism of the femtosecond laser, these optical parameters have different effects on the processing compared to traditional laser processing [20,21]. In order to fully study the microstructure of GDL and explore the internal mechanism of thermal stress changes during ultra-fast laser processing of GDL micropores, this study established a two-temperature equation-phase transformation-thermal stress coupling process model based on ultra-fast laser processing of GDL micropores, studied the influence of ultra-fast laser processing parameters on the micropore structure.\u003c/p\u003e \u003cp\u003eThis study will explore the influence of femtosecond laser processing parameters on surface morphology and thermodynamic properties of gas diffusion layers from both experimental and simulation perspectives. The simulation part utilizes COMSOL finite element analysis software to perform numerical simulation and analysis on carbon fiber composite materials. Build the 2 D model and combine the phase change model to study the processing process. By setting different laser processing parameters, including power, scanning speed and pulse frequency, the effects on the heat-affected region, thermal stresses and the surface morphology of the material were explored for the gas diffusion layer of carbon fibre paper.\u003c/p\u003e"},{"header":"2. Simulation and experimental analysis","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Experimental method\u003c/h2\u003e \u003cp\u003eThe sample of gas diffusion layer selected for this paper was a commercial carbon paper (Sigracet 36 BB, SGL Group), which was hydrophobically treated to have a homogeneous porous thin-layer structure. The carbon paper thickness was 190\u0026thinsp;\u0026plusmn;\u0026thinsp;20 \u0026micro;m with an average porosity of 0.84. Femtosecond ultrafast laser processing equipment is used to process the microporous structure on the surface of the actual gas diffusion layer.\u003c/p\u003e \u003cp\u003eFemtosecond ultrafast laser processing method is used to process micro-holes on GDL surface, and through process experiments, the through holes are explored and processed. Among them, the laser power was 2 W and 5 W for comparison, the scanning speed was 1000 mm/s and 600 mm/s for comparison, and the repetition rate was 50 kHz and 100 kHz for comparison. The processing times were 400 times and the laser pulse width was 270 fs.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperimental Table\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003egroup number\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRepetition rate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003epower\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003escanning speed\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent repetition rates\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent laser powers\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent scanning speeds\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1000 mm/s\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Simulation method\u003c/h2\u003e \u003cp\u003eIn this study, COMSOL finite element analysis software is used to simulate and analyze carbon fiber composites, and a two-dimensional model is built to study the processing process combined with phase change model. SGL 36 BB carbon paper is composed of carbon fiber substrate and other polymers, among which epoxy resin accounts for a large proportion, so other polymers are ignored in this study, and a series of changes of materials composed of carbon fiber substrate and epoxy resin under the action of laser are studied emphatically. In this study, the sample thickness is 10 \u0026micro;m. On the other hand, because carbon fiber-reinforced plastics (CFRP) is composed of carbon fiber and epoxy resin, the physical properties of the two materials are quite different. However, the research content of this paper focuses on the macroscopic thermal ablation, so the CFRP can be homogenized, which not only saves the computing resources, but also obtains more accurate results.\u003c/p\u003e \u003cp\u003eThe model is mainly based on the following assumptions:\u003c/p\u003e \u003cp\u003eThe intensity distribution of laser is considered as an ideal Gaussian distribution, and the energy of the process is constant.\u003c/p\u003e \u003cp\u003eThe model is adiabatic, ignoring the effects of convection heat transfer, radiation heat transfer and plasma;\u003c/p\u003e \u003cp\u003eTwo-temperature model\u003c/p\u003e \u003cp\u003eBy establishing the two-temperature model proposed by Anisimov in 1974 [22], the model is analyzed:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${C_e}\\frac{{\\partial {T_e}}}{{\\partial t}}{\\text{=}}\\nabla \\left( {{k_e}\\nabla {T_e}} \\right) - G\\left( {{T_e} - {T_l}} \\right)+S\\left( {x,y} \\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${C_l}\\frac{{\\partial {T_l}}}{{\\partial t}}=\\nabla \\left( {{k_l}\\nabla {T_l}} \\right)+G\\left( {{T_e} - {T_l}} \\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere \u003cb\u003eC\u003c/b\u003e is the specific heat capacity, \u003cb\u003ek\u003c/b\u003e is conductivity, \u003cb\u003eG\u003c/b\u003e is the electron-phonon coupling coefficient, \u003cb\u003eS\u003c/b\u003e is the laser intensity, \u003cb\u003ee\u003c/b\u003e represents electrons, \u003cb\u003el\u003c/b\u003e represents a lattice, \u003cb\u003ex\u003c/b\u003e is the abscissa, \u003cb\u003ey\u003c/b\u003e is the ordinate. We usually think that the lattice temperature is the temperature of the model.\u003c/p\u003e \u003cp\u003eAnd the volumetric thermal conductivity \u003cb\u003ek\u003c/b\u003e\u003csub\u003e\u003cb\u003eeq\u003c/b\u003e\u003c/sub\u003e is equal to the addition of the thermal conductivity of electrons \u003cb\u003ek\u003c/b\u003e\u003csub\u003e\u003cb\u003ee\u003c/b\u003e\u003c/sub\u003e and lattice thermal conductivity \u003cb\u003ek\u003c/b\u003e\u003csub\u003e\u003cb\u003el\u003c/b\u003e\u003c/sub\u003e. For CFRP, the diffusion of free electrons has a very important influence on heat transfer. Therefore, the thermal conductivity of the lattice \u003cb\u003ek\u003c/b\u003e\u003csub\u003e\u003cb\u003el\u003c/b\u003e\u003c/sub\u003e set to 1% of volume thermal conductivity \u003cb\u003ek\u003c/b\u003e\u003csub\u003e\u003cb\u003eeq\u003c/b\u003e\u003c/sub\u003e.\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${k_l}=0.01{k_{eq}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$${k_e}=0.99{k_{eq}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eLaser intensity function\u003c/p\u003e \u003cp\u003eGaussian volumetric heat density is obtained as follows:\u003cdiv id=\"Equ5\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ5\" name=\"EquationSource\"\u003e\n$$\\begin{gathered} S\\left( {x,y,t} \\right)=\\sum\\limits_{{i=1}}^{{{N_p}}} {\\frac{{0.94J\\left( {1 - R} \\right)}}{{{t_p}\\left( {\\delta +{\\delta _b}} \\right)\\left[ {1 - {e^{ - L/\\left( {\\delta +{\\delta _b}} \\right)}}} \\right]}}} \\hfill \\\\ \\exp \\left[ {\\left( {\\frac{{y - \\vartriangle y}}{{\\delta +{\\delta _b}}}} \\right) - {{\\left( {\\frac{{x - vt}}{{{r_s}}}} \\right)}^2} - 2.77{{\\left( {\\frac{{t - \\left( {i - 1} \\right){t_{sep}} - 2{t_p}}}{{{t_p}}}} \\right)}^2}} \\right] \\hfill \\\\ \\end{gathered}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ6\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ6\" name=\"EquationSource\"\u003e\n$$J{\\text{=}}\\frac{P}{{\\pi r_{s}^{2}f}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e6\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ7\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ7\" name=\"EquationSource\"\u003e\n$${t_{sep}}{\\text{=}}\\frac{1}{f}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e7\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAmong them, \u003cb\u003eR\u003c/b\u003e is the surface reflectivity, \u003cb\u003eδ\u003c/b\u003e is the optical penetration depth, \u003cb\u003eδ\u003c/b\u003e\u003csub\u003e\u003cb\u003eb\u003c/b\u003e\u003c/sub\u003e is the optical ballistic distance, \u003cb\u003ew(y)\u003c/b\u003e is the laser spot size,which is a function of the longitudinal depth, \u003cb\u003et\u003c/b\u003e\u003csub\u003e\u003cb\u003ep\u003c/b\u003e\u003c/sub\u003e is that duration of the lase pulse, \u003cb\u003eN\u003c/b\u003e\u003csub\u003e\u003cb\u003ep\u003c/b\u003e\u003c/sub\u003e is the number of laser pulses, \u003cb\u003et\u003c/b\u003e\u003csub\u003e\u003cb\u003esep\u003c/b\u003e\u003c/sub\u003e is the laser pulse interval, \u003cb\u003eJ(y)\u003c/b\u003e is the function term of the laser influence varying with depth, \u003cb\u003ex(t)\u003c/b\u003e is the function of laser spot position changing with time, \u003cb\u003eΔy\u003c/b\u003e is the longitudinal displacement of deformed geometry, \u003cb\u003eP\u003c/b\u003e is the laser power, \u003cb\u003ef\u003c/b\u003e is the pulse repetition rate, \u003cb\u003eL\u003c/b\u003e is the model thickness (10 \u0026micro;m in this study), and the influence of thickness is determined by \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\left[1-{\\text{e}}^{-\\text{L}/\\left({\\delta }+{{\\delta }}_{\\text{b}}\\right)}\\right]\\)\u003c/span\u003e\u003c/span\u003e to decide.\u003c/p\u003e \u003cp\u003ePhase change model\u003c/p\u003e \u003cp\u003eAssuming that the gas is an ideal gas in a thermal equilibrium state, the vaporization process is described by combining the two-temperature model with Clausius-Clapeyron equation[23]:\u003cdiv id=\"Equ8\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ8\" name=\"EquationSource\"\u003e\n$$\\frac{{dp}}{{d{T_l}}}=\\frac{{p{h_v}({T_l})}}{{{R_u}{T_l}^{2}}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e8\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ9\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ9\" name=\"EquationSource\"\u003e\n$${h_v}({T_l})={h_{v0}}{\\left( {1 - {{(\\frac{{{T_l}}}{{{T_c}}})}^2}} \\right)^{1/2}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e9\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equ10\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ10\" name=\"EquationSource\"\u003e\n$$p={p_0}\\exp \\left( { - \\frac{{\\rho {h_v}}}{{{R_u}}}\\left( {\\frac{1}{{{T_l}}}\\sqrt {1 - {{\\left( {\\frac{{{T_l}}}{{{T_c}}}} \\right)}^2}} - \\frac{1}{{{T_v}}}\\sqrt {1 - {{\\left( {\\frac{{{T_v}}}{{{T_c}}}} \\right)}^2}} } \\right) - \\frac{{\\rho {h_v}}}{{{R_u}{T_c}}}\\left( {{{\\sin }^{ - 1}}\\left( {\\frac{{{T_l}}}{{{T_c}}}} \\right) - {{\\sin }^{ - 1}}\\left( {\\frac{{{T_v}}}{{{T_c}}}} \\right)} \\right)} \\right)$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e10\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIn which latent heat of evaporation is \u003cb\u003eh\u003c/b\u003e\u003csub\u003e\u003cb\u003ev0\u003c/b\u003e\u003c/sub\u003e, \u003cb\u003ep0\u003c/b\u003e is the atmospheric pressure, \u003cb\u003eT\u003c/b\u003e\u003csub\u003e\u003cb\u003ec\u003c/b\u003e\u003c/sub\u003e is the critical temperature, \u003cb\u003eR\u003c/b\u003e\u003csub\u003e\u003cb\u003eu\u003c/b\u003e\u003c/sub\u003e is the ideal gas constant,and \u003cb\u003ep\u003c/b\u003e is the saturation pressure.\u003c/p\u003e \u003cp\u003eAccording to the Hertz-knudsen-Langmuir equation derived from gas dynamics theory[24], the molar evaporation flux is calculated \u003cb\u003ej\u003c/b\u003e\u003csub\u003e\u003cb\u003ev\u003c/b\u003e\u003c/sub\u003e:\u003cdiv id=\"Equ11\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ11\" name=\"EquationSource\"\u003e\n$${j_v}=\\frac{{Ap}}{{\\sqrt {2\\pi M{R_u}{T_l}} }}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e11\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eHere \u003cb\u003eA\u003c/b\u003e is the adjustment factor of 0.92 suggested by Xu[25], \u003cb\u003eM\u003c/b\u003e is molar mass. The evaporation rate can be calculated from the molar evaporation flux:\u003cdiv id=\"Equ12\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ12\" name=\"EquationSource\"\u003e\n$${u_v}=\\frac{{M{j_v}}}{\\rho }=\\frac{{AMp}}{{\\rho \\sqrt {2\\pi M{R_u}{T_l}} }}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e12\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eInitial conditions and boundary conditions\u003c/p\u003e \u003cp\u003eAssuming that the initial temperature is room temperature (25\u0026deg;C), the initial condition expression is:\u003cdiv id=\"Equ13\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ13\" name=\"EquationSource\"\u003e\n$${T_e}\\left( {x,y,0} \\right)={T_l}\\left( {x,y,0} \\right)={T_0}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e13\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eBecause the heat loss on the surface is neglected, the temperature field adopts adiabatic boundary conditions:\u003cdiv id=\"Equ14\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ14\" name=\"EquationSource\"\u003e\n$$\\begin{gathered} {\\left. {\\frac{{\\partial {T_e}}}{{\\partial t}}} \\right|_{y{\\text{=}}0}}{\\text{=}}{\\left. {\\frac{{\\partial {T_e}}}{{\\partial t}}} \\right|_{y=L}}={\\left. {\\frac{{\\partial {T_e}}}{{\\partial t}}} \\right|_{x{\\text{=}} - \\frac{1}{2}M}}{\\text{=}}{\\left. {\\frac{{\\partial {T_e}}}{{\\partial t}}} \\right|_{x{\\text{=}}\\frac{1}{2}M}}{\\text{=}}0 \\hfill \\\\ {\\left. {\\frac{{\\partial {T_l}}}{{\\partial t}}} \\right|_{y{\\text{=}}0}}{\\text{=}}{\\left. {\\frac{{\\partial {T_l}}}{{\\partial t}}} \\right|_{y=L}}={\\left. {\\frac{{\\partial {T_l}}}{{\\partial t}}} \\right|_{x{\\text{=}} - \\frac{1}{2}M}}{\\text{=}}{\\left. {\\frac{{\\partial {T_l}}}{{\\partial t}}} \\right|_{x{\\text{=}}\\frac{1}{2}M}}{\\text{=}}0 \\hfill \\\\ \\end{gathered}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e14\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eSolid mechanics\u003c/p\u003e \u003cp\u003eBy adding the physical field of solid mechanics to calculate the real-time change of temperature field during laser ablation and the influence of different material properties on the change law of thermal stress field, the corresponding relationship can be expressed as follows:\u003cdiv id=\"Equ15\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ15\" name=\"EquationSource\"\u003e\n$$\\rho \\frac{{{\\partial ^2}{\\text{u}}}}{{\\partial {u^2}}}=\\nabla \\sigma +{F_V}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e15\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eFormula \u003cb\u003eu\u003c/b\u003e is the displacement field, \u003cb\u003eρ\u003c/b\u003e is the density of the deformed material, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\nabla\\)\u003c/span\u003e\u003c/span\u003e is a gradient operator, \u003cb\u003eσ\u003c/b\u003e is Cauchy stress tensor, \u003cb\u003eF\u003c/b\u003e\u003csub\u003e\u003cb\u003ev\u003c/b\u003e\u003c/sub\u003e is the force per unit deformation volume.\u003c/p\u003e \u003cp\u003eDeformation geometry\u003c/p\u003e \u003cp\u003eThe deformation geometry method is adopted. In this method, the displacement of boundary nodes depends on evaporation speed. This situation can be described by the following function:\u003cdiv id=\"Equ16\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ16\" name=\"EquationSource\"\u003e\n$${v_{mesh}}= - {u_v}\\overrightarrow j$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e16\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThis equation is used to describe the change of model surface. The rest of the boundary is under the condition of no slip.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eAt ambient temperature and normal pressure, the diameter of the through hole was measured by scanning electron microscope (model S-3400N, resolution 3.0 nm(30 kV)/10.0 nm(3 kV), magnification 10\u0026thinsp;~\u0026thinsp;200 K). Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the experimental results under different laser process parameters. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the pit size and morphology under different laser processing parameters.\u003c/p\u003e \u003cp\u003eAs can be seen from Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, with the decrease of laser repetition rate, the micropore diameter will decrease. This is because the laser repetition rate decreases, which means that the number of laser pulses decreases in the same time, which leads to the decrease of machining aperture, but this trend is not very significant. With the increase of laser power, the pore size of micropores increases significantly. The laser power was from 3 W to 5 W, and the micropore diameter was from 48.41 \u0026micro;m to 65.56 \u0026micro;m, which increased by 26%. This means that for every 1 W increase, the pore size of micropores increases by 5.72 \u0026micro;m. With the increase of laser scanning speed, the number of laser pulses per unit time will decrease, and eventually the aperture will become smaller. This trend is very obvious. It can be seen that when the laser scanning speed increases from 600 mm/s to 1000 mm/s, the micropore diameter decreases from 48.41 \u0026micro;m to 29.88 \u0026micro;m. Therefore, the laser scanning speed has obvious influence on the micropore diameter.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eExperimental results under different laser process parameters\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003egroup number\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003erepetition rate\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003epower\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003escanning speed\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eaperture\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003estandard deviation\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent repetition rates\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e48.41 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.83 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e50 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e45.80 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e4.31 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent laser powers\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e48.41 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.83 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e5 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e65.56 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.74 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eDifferent scanning speeds\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e600 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e48.41 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e5.83 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e100 kHz\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3 W\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1000 mm/s\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e29.88 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e6.26 \u0026micro;m\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe apertures produced by the model simulation laser processing were verified using the above experimental data of femtosecond laser processing (laser power of 3 W and 5 W), see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. For the SGL 36 BB carbon paper used in the experiments, it was homogenized during the simulation process. The processing parameters used during the model simulation process are the same as those used in the experiments of groups 3 and 4 in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The results show that the simulation results are in good agreement with the experimental data. The relative error of the simulated aperture over the experimental one is 2.56% when the laser power is at 3W. When the laser power is 5W, the relative error of the simulated aperture over the experiment is 6.59%. The intensity distribution of the laser in the actual laser processing is not Gaussian, and the model cannot be completely adiabatic, which may be a source of error for the idealized assumptions in the simulation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn this section, the influence of laser power is studied by changing different laser power. In each case, the laser power is 2 W, 3 W, 4 W and 5 W, the scanning speed is 600 mm/s, the repetition rate is 100 kHz, and the pulse duration is 270 fs.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows the change of heat affected zone area under different power, and the second, third, fourth, and fifth pulse laser finishing time points are selected for research, namely, 20 \u0026micro;s, 30 \u0026micro;s, 40 \u0026micro;s, and 50 \u0026micro;s. Because the heat affected zone did not produce significantly in the time point of 10 \u0026micro;s, the first pulse was not considered. In this study, the part of the sample whose temperature is higher than 500K is regarded as the heat affected zone. The reason is that epoxy resin will melt and melt at the temperature of 500 K. Once the heat generated by laser exceeds this value, epoxy resin will inevitably melt and carbon fiber will be exposed, which will affect the processing quality.\u003c/p\u003e \u003cp\u003eIt can be seen that, with the passage of time, because the pulse energy continuously acts on the surface of the workpiece, the heat of the laser propagates around at a rapid speed, and the heat affected zone shows a trend of rapidly increasing at first and then decreasing. This is because the thickness of the workpiece (10 \u0026micro;m) is far less than the length of the workpiece (100 \u0026micro;m), and the laser propagation along the Y axis is basically completed under the action of the first few pulses, and then the laser heat can only propagate along the X axis at the subsequent pulse time. At this time, the effect of increasing the number of pulses on the thermal region is weakening.\u003c/p\u003e \u003cp\u003eWhen the laser power is 2 W, the final heat affected area is 39.9%, while when the laser power is 5W, the final heat affected area is 46.2%. It can be concluded from the Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e that the increase of laser power has a certain influence on the increase of heat affected zone area in the range of 2W to 5W, but with the increase of laser power, this influence seems to be weakening. This is because with the increase of laser power, the single pulse energy increases significantly, which leads to the increase of heat affected zone area under the same number of pulses, but with the longitudinal heat affected zone of the workpiece being swallowed up, the influence of laser power increase on the increase of heat affected zone area is gradually decreasing.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the evolution of the thermal stress field on the top surface under different power, and selects the time points when the first, second, third, fourth and fifth pulsed lasers respectively end, namely 10 \u0026micro;s, 20 \u0026micro;s, 30 \u0026micro;s, 40 \u0026micro;s and 50 \u0026micro;s. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea shows the change of maximum thermal stress under different power, and the time of 30 \u0026micro;s is selected here. It can be seen that the femtosecond laser generates a lot of heat at the same time, and it will also produce the concentration of thermal stress. With the passage of time, the thermal stress distributed around the laser focus quickly disperses around, and the thermal stress gradient decreases. Due to the increase of laser power, the heat generated by laser increases, and then the influence area of thermal stress field mainly depends on laser power.\u003c/p\u003e \u003cp\u003eHowever, the maximum thermal stress value does not depend entirely on the laser power, but the number of laser pulses plays a decisive role in the maximum thermal stress value at lower power. In short, the pit quality increases with the decrease of power and decreases with the increase of pulse number.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn order to analyze the relationship between pit depth and laser power under different power, the pit depth under different power is compared, and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb shows the change of pit morphology under different power. It can be seen that with the increase of laser power, the pit depth gradually increases. When the laser power changes from 2 W to 5 W, the groove depth increases from 1.20 \u0026micro;m to 1.71 \u0026micro;m m Therefore, the laser power has a great influence on the pit depth.\u003c/p\u003e \u003cp\u003eIn this section, the influence of laser scanning speed is studied by changing different scanning speeds. In each case, the laser power of 200 mm/s, 600 mm/s, 1000 mm/s and 1400 mm/s was used respectively, with a power of 3 W, a repetition rate of 100 kHz and a pulse duration of 270 fs.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the change of the area of the heat affected zone at different scanning speeds. Because the heat affected zone did not produce significantly within the time point of 10 \u0026micro;s, only the time points of 20 \u0026micro;s, 30 \u0026micro;s, 40 \u0026micro;s and 50 \u0026micro;s were considered. It can be observed that the increase of laser scanning speed on the final heat affected zone size is not obvious. This is because although the higher scanning speed will be beneficial to the heat dissipation, compared with the ultra-short pulse of femtosecond laser, this heat dissipation is negligible. When the scanning speed is 200 mm/s and 600 mm/s, the size of the heat affected zone almost coincides, because the overlap rate of laser spots is similar at these two scanning speeds.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe thermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the evolution of thermal stress field on the top surface at different scanning speeds. Figure\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003ea shows the change of maximum thermal stress at different scanning speeds, and the time of 30 \u0026micro;s is selected here. It can be seen that the faster scanning speed can not greatly affect the increase of heat affected zone. This is because the pulse width of ultrafast laser is short enough (\u0026lt;\u0026thinsp;10\u0026ndash;12 s), and the increase of laser scanning speed can not have an essential impact on the heat generated by laser. Furthermore, the influence area of thermal stress field has little correlation with laser scanning speed. However, the maximum thermal stress is closely related to the number of pulses, and the lower the scanning speed, the more obvious the thermal stress accumulation, which may lead to material processing damage. In short, the pit quality increases with the increase of scanning speed.\u003c/p\u003e \u003cp\u003eIn order to analyze the relationship between pit depth and laser scanning speed at different scanning speeds, the pit depth at different scanning speeds is compared, and Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003eb shows the changes of pit morphology at different scanning speeds. It can be seen that with the increase of laser scanning speed, the pit depth gradually decreases. When the laser power changes from 200 mm/s to 1400 mm/s, the groove depth decreases from 1.53 \u0026micro;m to 0.94 \u0026micro;m.Therefore,the laser scanning speed can reduce the pit depth.\u003c/p\u003e \u003cp\u003eIn this section, the influence of laser repetition rate is studied by changing different repetition rates. In each case, the laser power of 100kHz, 200 kHz, 300 kHz and 400 kHz was used respectively, with the power of 3 W, the scanning speed of 600 mm/s and the pulse duration of 270 fs.\u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows the change of heat affected zone area under different repetition rates. Because the heat affected zone didn't appear significantly in the first pulse, only the second, third, fourth and fifth pulse end time points were considered. It can be seen that the change of repetition rate is the key factor leading to the final change of heat affected area. A higher repetition rate represents an increase in the number of pulses, but under the condition of constant laser power, the energy carried by each pulse decreases, so it brings a smaller heat affected area. Further, it can be observed that the laser rates of 300kHz and 400kHz have a violent increase in the heat affected zone in the second pulse compared with other laser rates. This is because the higher laser rate brings more pulses, which makes the laser heat concentrated in generate in a short time, which leads to the acceleration of the longitudinal heat propagation speed of the workpiece and the increase of the heat affected zone.\u003c/p\u003e \u003cp\u003eWhen the laser rate is 100 kHz, the final heat affected area is 42.8%, while when the laser rate is 400 kHz, the final heat affected area is 29.3%. It can be concluded from the Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e that the increase of laser rate can significantly reduce the area of heat affected zone in the range of 100 kHz to 400 kHz.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermal stress coupling analysis can reflect the interaction process between thermal temperature field and stress field. Figure\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e shows the evolution of the thermal stress field on the top surface under different repetition rates, and selects the time when the first, second, third, fourth and fifth laser pulses are completed. Figure\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea shows the change of the maximum thermal stress at different repetition rates, and selects the time when the third laser pulse is completed. It can be seen that the femtosecond laser generates a lot of heat in an instant, and at the same time, it also produces the concentration of thermal stress accordingly. The greater the repetition rate, the smaller the pulse interval between each pulse. Under the same number of pulses, the time of laser action will be shortened, which will lead to the reduction of the regional scope of thermal stress field. The increase of laser repetition rate is the key factor to cause the width of thermal stress field. It can be further known from the Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003ea that the larger repetition rate leads to the obvious diffusion of thermal stress field, which will improve the quality of pits. However, the maximum thermal stress is closely related to the number of pulses, and the lower the repetition rate, the greater the maximum thermal stress.In brief, the pit quality increases with the increase of repetition rate.\u003c/p\u003e \u003cp\u003eIn order to analyze the relationship between pit depth and laser repetition rate under different power, the pit depth under different repetition rates is compared, and Fig.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e11\u003c/span\u003eb shows the changes of pit morphology under different repetition rates. It can be seen that with the increase of repetition rate, the pit depth gradually decreases. When the laser repetition rate changes from 100kHz to 400kHz, the groove depth decreases from 1.45\u0026micro;m to 0.51 \u0026micro;m Therefore, the laser repetition rate has a great influence on the pit depth.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this paper, a two-temperature model of ultrafast femtosecond laser drilling is established, and the evaporation rate is calculated by combining the phase change model. The coupling model of ultrafast femtosecond laser drilling for carbon fiber composites is obtained. The effects of laser power, scanning speed and repetition rate on the morphology, heat affected zone and thermal stress during laser drilling were studied. The outstanding conclusions of this study are summarized as follows:\u003c/p\u003e \u003cp\u003e(1) The heat-affected area increases as the laser power increases, but the magnitude of the increase in the heat-affected area decreases as the longitudinal heat effect of the workpiece dissipates. The quality of the pit decreases with increasing power. Because the single pulse laser energy increases, the laser power has a significant effect on the hole depth and hole diameter, the pit depth and hole diameter will increase.\u003c/p\u003e \u003cp\u003e(2) The quality of the pits increases with an increase in scanning speed, but at the same time, increasing the scanning speed will result in a decrease in the depth of the machining hole.\u003c/p\u003e \u003cp\u003e(3) The thermal impact zone and pit quality primarily depend on the repetition rate. Increasing the laser rate can reduce the area of the thermal impact zone, while the quality of the craters increases with the increase in repetition rate. At the same time, as the repetition rate increases, the depth of the craters gradually decreases.\u003c/p\u003e \u003cp\u003e(4) Femtosecond laser drilling can be accurately controlled by this model. This research has contributed to the theoretical guidance of laser drilling at present.\u003c/p\u003e \u003cp\u003eBy precisely adjusting the above parameters, the performance of the fuel cell can be maximally improved to enhance its efficiency and stability. Step-by-step optimisation in experiments and comprehensive consideration of material properties and process requirements are needed to obtain the best combination of femtosecond laser processing parameters.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003cstrong\u003eConflict of interest\u003c/strong\u003e \u003cp\u003eThe authors declare no conficts of interest\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding.\u003c/h2\u003e \u003cp\u003eNational Science Foundation of Jiangsu Province (Nos. BK20230534); China Postdoctoral Science Foundation (Nos. 2023M730757); Open Fund for National Engineering Laboratory of High Mobility Riot Control Vehicle Technology( Nos.EM20240006).\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eXuan Xie prepared the main manuscript of the paper, Changwu Tang , Changguo Wang , Sheng Xu , Bifeng Yin prepared all the fgures in the manuscript, and all the authors reviewed the manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eCindrella, L., Kannan, A. M., Lin, J. F., et al., \u0026ldquo;Gas diffusion layer for proton exchange membrane fuel cells\u0026mdash;A review,\u0026rdquo; Journal of Power Sources 194(1), 146\u0026ndash;160 (2009).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePhillips, K. C., Gandhi, H. 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A., et al., \u0026ldquo;Single-shot selective femtosecond laser ablation of multi-layered Ti/Al and Ni/Ti films: \u0026ldquo;Cascaded\u0026rdquo; heat conduction and interfacial thermal effects,\u0026rdquo; Applied Physics Letters 112(2) (2018).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eQiu, Z., Jiang, L., Hu, J., et al., \u0026ldquo;High-quality micropore drilling by using orthogonally polarized femtosecond double-pulse bursts,\u0026rdquo; Applied Surface Science 613, 156033 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShi, Q., Gu, D., Xia, M., et al., \u0026ldquo;Effects of laser processing parameters on thermal behavior and melting/solidification mechanism during selective laser melting of TiC/Inconel 718 composites,\u0026rdquo; Optics \u0026amp; Laser Technology 117, 244\u0026ndash;250 (2019).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSugioka, K., \u0026amp; Cheng, Y. \u0026ldquo;Femtosecond laser processing for optofluidic fabrication,\u0026rdquo; Lab on a Chip 12(19), 3576\u0026ndash;3589 (2012).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eChen, J. K., Tzou, D. Y., \u0026amp; Beraun, J. E. \u0026ldquo;A semiclassical two-temperature model for ultrafast laser heating,\u0026rdquo; International journal of heat and mass transfer 49(1\u0026ndash;2), 307\u0026ndash;316 (2006).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVelasco, S., Rom\u0026aacute;n, F. L., \u0026amp; White, J. A. \u0026ldquo;On the Clausius\u0026ndash;Clapeyron vapor pressure equation,\u0026rdquo; Journal of Chemical Education 86(1), 106 (2009).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHertel, M., Spille-Kohoff, A., F\u0026uuml;ssel, U., \u0026amp; Schnick, M. \u0026ldquo;Numerical simulation of droplet detachment in pulsed gas\u0026ndash;metal arc welding including the influence of metal vapour,\u0026rdquo; Journal of Physics D: Applied Physics, 46(22), 224003 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eXu, C. Y., \u0026amp; Singh, V. P. \u0026ldquo;Cross comparison of empirical equations for calculating potential evapotranspiration with data from Switzerland,\u0026rdquo; Water Resources Management, 16, 197\u0026ndash;219 (2002).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3996929/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3996929/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Traditional macroscopic structural design for proton exchange membrane fuel cells (PEMFC) has gradually become insufficient to meet the demands for improving fuel cell performance. Femtosecond laser processing is a promising solution capable of achieving precise control over the material structure and improving the quality of the processed material. In this study, femtosecond laser processing technology is used to modify the surface microstructure of gas diffusion layers (GDL) in PEMFC, aiming to enhance the characteristics of gas-liquid two-phase flow and electrochemical performance. In this paper, a novel coupled model based on the coupling of the two-temperature equation, phase transition and thermal stress is proposed. Comparison of the effects of different laser processing parameters on the surface morphology and thermal effects of carbon fibre materials. The impact of repetition rate on the heat-affected zone and pit quality is most significant. When rate increases from 100 kHz to 400 kHz, the heat-affected zone decreases from 42.8% to 29.3%. This process model can provide guidance and prediction for optimizing the laser processing parameters and improving the performance of the microporous structures.","manuscriptTitle":"Study on influence of laser processing parameters on thermal effects and surface morphology of GDL","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-03-05 13:36:17","doi":"10.21203/rs.3.rs-3996929/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"0c67cc42-7df4-4097-bba4-0d2bbb32e8c0","owner":[],"postedDate":"March 5th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-05-11T16:21:13+00:00","versionOfRecord":[],"versionCreatedAt":"2024-03-05 13:36:17","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3996929","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3996929","identity":"rs-3996929","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}