Aiming at the demand of high-energy-absorbing and shock-resistant components for aerospace and transportation, metamaterials with energy absorption have been extensively studied, including truss-lattices, plates-lattices, triply periodic minimal surfaces (TPMS), and bionic metamaterials.
The truss-lattice metamaterials are the spatial structures formed by the multiple connecting rods with lattice or lattice-like arrangement, which possess high mechanical properties and energy absorption. The plate lattice metamaterials are the spatial structures in which the plate vertices replace the lattice nodes and have special plate arrangements. The plates can also generate multiple cavities through specific combinations, thereby achieving the functional effects of sound absorption and noise reduction. The TPMS metamaterials are the spatial structures that possess infinite periodic, continuous, and smooth surfaces in three independent directions. The surfaces have two disjoint regions in space. There are no sharp protrusions and depressions, which can decrease stress concentration. It is the best choice for manufacturing energy-bearing structures. Besides, the researchers have also found that its spatial configuration is similar to the structure of human bone, so it can be used to fabricate bone implants. The bionic metamaterials for energy absorption were first applied in 2000. The structures that exist in living organisms are the result of natural selection and evolution. They are known for their high specific energy absorption efficiency with small mass, which can be used for fabricating energy-absorbing components with impact resistance and energy absorption.
Traditional manufacturing technologies are difficulte to fabricate these metamaterials for their complex structures. The additive manufacturing (AM) technology is based on the principle of discrete slicing and layer-by-layer stacking to rapidly fabricate components, which possesses high manufacturing freedom. The above-mentioned technical characteristics make it an effective way to manufacture energy-absorbing metamaterials with complex structures. Thus, researching and developing metamaterials with energy absorption mean a lot.
The design of truss-lattice metamaterials has first changed from the ordinary regular truss arrangement to the gradient arrangement, and then the solid parts inside the unit cell are reasonably distributed through topological optimization of the computational models to maximize the mechanical properties and energy absorption. However, the design of truss-lattice metamaterials has complex geometric models, diverse mechanical properties, and multidisciplinary. Therefore, the main research direction is to develope efficient and specific mathematical models for the truss-lattice metamaterial design.
The plate-lattice metamaterial evolves based on the truss-lattice metamaterials, whose mechanical properties and energy absorption can be improved through some optimization strategies such as topological optimization. The combined method can realize optimization again, and the effect is remarkable. However, there are process constraints in the AM of plate-lattice metamaterials. The research direction of plate-lattice metamaterials is to optimize the metamaterial and develop a topology suitable for AM for maintaining excellent mechanical properties and energy absorption.
The TPMS metamaterials with heterogeneous and gradient structures are developed. The homogeneous structures is first developed to improve energy absorption, and then the combined TPMS metamaterials to efficiently control the mechanical properties and energy absorption appear. However, diversified TPMS metamaterials have various boundary distributions, so the combination method cannot be simply pieced together. Developing a corresponding mathematical model to achieve the smooth transition of multiple structures needs to be solved urgently.
Compared with the traditional metamaterials, the bionic metamaterials improve the energy absorption, and the bionic metamaterials also change from homogeneous forms to gradient forms to achieve high energy absorption. Later, the design of bionic metamaterials also changes from simply imitating their special macrostructures and microstructures to the combined design of biomimetic and lattice.
At present, with the continuous progress of material design and AM technologies, additive manufacturing of intelligent metamaterials has become a new research direction to ensure that the bionic metamaterials do not break during the process of impact resistance and energy absorption, and the designability and repeatability of bionic metamaterials are greatly improved. Bionic smart metamaterials are developing towards imitating shapes, imitating properties, and imitating functions. However, the development of ultra-high recoverable and deformable smart materials and the design of bionic metamaterials are the current study barriers to the metamaterials with energy absorption, and also the main development direction.
The automobile industry is a "machine to change the world" and an important pillar industry for promoting national economic development. The automobile industry is heavily invested in high-tech and high-end equipment, which reflects the national manufacturing technological level. Every automobile is a crystallization of modern high technologies. The automobile industry is the largest user of robots, computer numerical control machine tools, and automatic production lines. Modern automobiles also make extensive use of novel materials, processes, equipment, and electronic technologies. The popularity of automobiles has met the people’s enormous demand for travel, so the quality of automobiles has a direct impact on the safety of use. In the automobile manufacturing process, the value of car body accounts for approximately 1/5 of that of the whole car, and the weight of car body accounts for 1/3 of that of the whole care. As a result, the manufacturing quality of car body is directly related to the overall safety and comfort of the vehicle. At the same time, the choice of car body materials is critical in the lightweight development of a car. Stamping, welding, painting, and final assembly are all parts of the automobile body manufacturing process. The welding process, for example, is used to create a body-in-white by welding stamping sheet metal parts together. Poor welding quality can cause deformation and cracking of body sheet metal and abnormal noise, and even endanger the personal safety of passengers.
Steel has been traditionally used as the body material of automobiles. Because the steel plates that make up the body are generally thin, the welding of the body is primarily resistance spot welding, which is widely used in the welding of underbody, side wall, frame, roof, door, and body assembly with as many as 4000-6000 welding spots. In addition, CO2 gas shielded welding, stud welding, arc welding, brazing, and other technological methods are used in the automobile body manufacturing process. Traditional welding technologies for automobile body can essentially meet the quality requirements of an automobile body after welding. As the quantity of produced automobiles grows, the demands for high automobile body manufacturing efficiency increase. Different automobile body parts and welding joint forms have higher requirements for the flexible automobile body manufacturing. Special welding requirements, such as lightweight material welding and dissimilar material welding, have higher requirements for the welding process. It can be seen that high-quality and efficient welding of automobile body is the development trend, and the traditional welding process struggles to meet this demand.
Laser welding technology, as an advanced forming technologyin opto-mechatronics, has many advantages including high energy density, fast welding speed, low welding deformation, and good flexibility. It has become increasingly popular in the welding of automobile body in recent years. Laser welding is a fast and precise welding method that uses a high energy density laser beam as a heat source. The use of laser welding for automobile body has obvious advantages. For example, there is no mechanical contact between the welding device and the weldment, which reduces pollution to the workpiece. Because the heat energy of the laser beam is concentrated, the heat affected zone is small and the thermal deformation and damage are weak. Laser welding produces a beautiful welding seam with excellent mechanical properties. Welding robots and numerical control systems allow for a precise control of energy output, fast welding speed, and high production efficiency. As a result, the laser welding technology can not only improve the precision and efficiency in the car body process, but also improve the rigidity and strength of the car body, and thus the vehicle driving comfort and safety are improved.
The laser welding technology for automobile body is divided into two categories: the laser welding technology and the laser welding intelligent technology. Aiming at the laser welding process of automobile body, the structures of the commonly used materials are summarized (Table 1). The characteristics of the most commonly used laser welding processes for automobile body as well as information on welding parts, welding forms, and welding materials applicable to automobile body are also summarized (Table 2). Then, the commonly used laser deep penetration welding, laser filler welding, laser brazing, and laser-arc hybrid welding processes for automobile body are introduced, with a focus on the principles of these four laser welding processes. In conjunction with the welding characteristics of automobile body, the research progress of various welding processes for automobile body is expounded and summarized. In addition, the new laser welding process is described. These processes primarily include laser spot welding, laser wobble welding, multi-laser beam welding, and remote laser welding. This paper discusses the intelligent laser welding technology for automobile body from two perspectives: welding seam tracking and defect online detection. The welding seam tracking technology employs an advanced vision sensor to identify and track the welding seam of an automobile body before and during welding, and it corrects the movement path of the welding robot in real time to ensure welding stability. For example, the welding seam tracking system developed by Scansonic Company in Germany can achieve a dynamic and accurate identification of welding seams (Fig. 13). The key to detecting weld defects is to create a correlation model between defects and monitoring signals. Ma et al. have developed a correlation model between multivariable signals and porosity defects using visual sensing and a keyhole depth measuring device, and accurately identified the blowhole defects (Fig. 14). Finally, this paper summarizes the current problems in the laser welding process and the intelligent technology, and the future development trend is prospected.
The laser welding technology has become an indispensable and important technology in the development of lightweight automobile body. The laser welding process for lightweight body materials and dissimilar materials still requires extensive research and exploration. At the same time, an intelligent welding system with weld tracking, weld defect detection, and closed-loop control of welding parameters is required.
The physical and chemical properties of materials are determined by their element compositions and contents. How to obtain the compositions and content information of materials quickly, accurately, and at low cost has always been the research direction of scholars. The existing methods for the analysis of elements in the materials can be divided into the chemical methods and the instrumental methods. Based on the law of chemical reactions, the chemical methods carry out the qualitative and quantitative systematic analysis on the chemical compositions of samples, including the gravimetric method, the volumetric method, and the colorimetric method. In contrast, the instrumental methods directly obtain the physical and chemical information of the unknown samples through the analytical instruments, such as the inductively coupled plasma mass spectrometry, the Raman spectrum, the near-infrared spectrum, the X-ray spectrometry, and the atomic absorption spectrometry. The above-mentioned methods can obtain the categories and composition information of the samples with high sensitivity and accuracy, but they present complex operation, high cost, and low efficiency. However, with the continuous expansion of application fields, there is a high demand for the analysis technologies. Looking for a newer, faster, and more adaptive detection technology has become a research hotspot.
Laser induced breakdown spectroscopy (LIBS) is an element analysis technology, which uses a laser as the excitation source to ablate the sample and produce plasma. The emission spectrum of the plasma is then detected by a spectrometer to obtain the element category and the content information of the sample to be measured. Compared with other analytical technologies, the LIBS technology has the unique advantages of simultaneous detection of multiple elements, simple structure, fast detection speed, and being not affected by sample morphology. It shows great application prospects in many fields. Based on this, the mechanism, device types, basic research progress, and applications of LIBS are summarized.
The plasma characteristics and self-absorption effect of the LIBS instruments raise the most concerns. By studying the relevant characteristics of plasma, it is helpful to understand the generation mechanism of laser-induced plasma and solve the relevant problems encountered in the LIBS analysis. The self-absorption infulences the linear relationship between the original plasma emission spectral intensity and the concentration of related elements, and thus it reduces the accuracy of a quantitative analysis. To satisfy different analytical requirements, variable types of LIBS instruments are developed, including an LIBS in the laboratory (Fig. 3), a stand-off LIBS (Fig. 4), an on-line LIBS (Fig. 5), and a portable LIBS (Fig. 6). The LIBS in the laboratory has higher sensitivity and reproducibility, which is often used in the study of mechanism and exploratory applications. The stand-off LIBS can realize an in-situ detection of dangerous samples under harsh conditions on the premise of ensuring personnel safety. With the unique advantages of in-situ detection, real-time, fast, and no complex sample pretreatment, the on-line LIBS can quickly process numerous samples on the production line. The portable LIBS has the advantages of small volume, light weight, and convenient use, which has better applicability in industrial fields with harsh conditions.
To improve the analytical performance of the LIBS technology, the signal enhancement methods and the methods for the qualitative and quantitative analysis have become the focus study. The signal enhancement methods mainly contain surface enhancement methods (Fig. 7), inert-gas protection enhancement methods (Fig. 9), confinement enhancement methods (Fig. 10), and double-pulse enhancement methods (Fig. 11). The surface enhancement method ablate the substrate and the sample to be measured at the same time. The high-temperature plasma generated by the substrate heats the sample which can improve the temperature and electron number density of the sample plasma. Using inert gas as ambient gas can prolong the life of luminous atoms in plasma and avoid the light signal from being absorbed by air. Confinement enhancement uses the confinement cavity or magnetic field to affect the external and internal conditions of the plasma and confine the plasma to achieve signal enhancement with the advantages of simplicity, economy, and high feasibility. The double-pulse technology uses the second laser pulse to excite and heat the plasma again, which can greatly increase the temperature of the plasma and enhance the spectral intensity. Various methods are carried out for the qualitative and quantitative analysis, including material identification, element detection, and quantitative analysis.
With the specific advantages of the LIBS technology and the development of above-mentioned methods, the LIBS technology has been successfully used in various fields, including space exploration (Fig. 14), geological prospection (Fig. 15), pollution monitoring (Fig. 16), food safety (Fig. 17), industrial metallurgy (Fig. 18), and biomedicine (Fig. 19). The rapid identification of sample category is the focus of current research, and good analytical results have been obtained. However, due to the change of experimental environments, surface dirt of samples, and the diversity of manufacturing processes and additives, the prediction accuracy of the LIBS technology for real samples is still low. Outlier screening, variable selection, scale transformation, and other spectral preprocessing methods, as well as the improvement and integration of algorithms, are effective ways to solve this problem. Due to the matrix effect, laser energy fluctuation, spectrometer resolution difference, detection environment limitation, and other reasons, the LIBS technology has a large deviation in the prediction of element contents in materials. Optimizing the LIBS instrument platform, studying the signal enhancement methods, and improving the analysis methods are the effective methods to improve the prediction accuracy of the quantitative analysis.
The matrix effect is the most critical problem that limits the wide application of the LIBS technology. With the continuous development of the LIBS instruments and the corresponding components, this problem can be effectively solved, but it will take a long time. The improvement of the analytical chemistry method will be an effective way to improve the application performance of the LIBS technology. In order to realize the rapid and sensitive detection of massive materials, the development of an on-line LIBS device will be the development trend in the future.
Gas turbine is the most promising device for power generation and ship power in this century due to its high efficiency and low emission. The hollow turbine blade with a complex structure is the key part of a gas turbine. The working temperature of the turbine blade is very high, which requires high metal quality and elaborate structures of turbine blades. High gas temperature means high working efficiency. With the further increase of industrial demand, the gas temperature of a turbine reaches more than 1700 ℃. Therefore, the turbine blades with more delicate and complex hollow structures should be fabricated to improve the cooling efficiency. The ceramic cores and shells are important components for casting of superalloy turbine blades due to their high temperature capabilities and chemical inertness. The traditional methods to fabricate ceramic shells and cores are the investment casting method which is time-consuming, high cost, low yield, and not sufficient for the update-requirement of turbine blades. In order to overcome these problems, additive manufacturing (AM) has been gradually developed. The AM technology has been widely used in different fields such as medicine, engineering, and aerospace to fabricate delicate parts without any molds. It can save materials, accelerate the research of new products, and meet customized needs, thus greatly reducing fabrication costs. In the past few decades, dozens of AM technologies have been developed and each of them has its unique application fields. As for the fabrication of ceramic cores and shells, the most suitable laser-based AM technologies are selective laser sintering (SLS) and stereolithography apparatus (SLA).
Different from other AM technologies, the most significant advantage of SLS is that it doesn’t need any support during the fabrication process. Large-scale ceramic parts with precise structures can be fabricated by SLS. The preparation of ceramic cores and shells by the SLS process is shown in Fig. 3. First, the green bodies of ceramics are first built by the SLS equipment, and then the green bodies are infiltrated with silica sol or other materials which could fill the pores of ceramics to improve the density of green bodies. The organic binder in the green bodies can be removed through thermal decomposition at 600 ℃ and the green bodies are sintered at 1200-1600 ℃ to obtain the densified ceramic cores and shells. SLA has been known for several years as an AM technology to fabricate the polymer-based parts. This technology utilizes liquid photo-curable resin which can be cured under ultraviolet light or laser irradiation. At present, there are two main processes for preparing ceramic cores and shells by the SLA technology (Fig. 6). The first process combines SLA and gel-casting. A resin mold is first made by SLA and then the ceramic slurry is injected into the mold cavity to get the green body of ceramic core and shell. In the other process, the ceramic slurry and photo-curable resin are mixed and the ceramic green bodies are directly formed by SLA. Both of ceramic green bodies made by these two technologies have enough density so that the infiltration process is no longer required and the rest of the post-processing process including debinding and sintering are the same as that of SLS. Characteristics of different laser-based AM processes used for ceramic core and shell manufacturing are shown in Table 2. It can be concluded that both of two AM technologies have their own strengths and weaknesses. SLS has an advantage in manufacturing large size ceramic cores and shells, while the parts made by SLA have higher resolution.
In this review, the development of these two AM processes in the fabrication of ceramic cores and shells are introduced in terms of material preparation, green body fabrication, and post-processing. The advantages, weaknesses and future development of both two methods have been discussed.
Nanomaterials with small features and large surface-to-volume ratios have drawn tremendous research attention in various fields including energy devices, microelectronics, and biomedicine. By far, researchers have realized high-quality fabrication of various nanomaterials through solid-phase, liquid-phase, or vapor-phase method. However, the fabrication of nanomaterial-based functional devices usually requires subsequent material transfer and assembly processes. Therefore, to effectively realize the integration of nanomaterials and make full use of their unique properties, the transfer-free growth of patterned nanomaterials is very important.
Although methods have been developed to realize the in-situ transfer-free patterned growth of nanomaterials, such as ultraviolet lithography, electron beam lithography, solution-based direct-patterning technique, and continuous wave/long pulsed laser selective induction, it is still difficult to meet the demands of customized patterning, precise processing, and in-situ heterogeneous integration of nanomaterials on thermal-sensitive, flexible, and curved substrates. The UV lithography and electron beam lithography techniques are cumbersome, time-consuming, and usually need a vacuum chamber. Besides, they are difficult to apply to curved substrates. The solution-based direct-patterning technique requires the subsequent high-temperature annealing process, which is difficult to apply to thermal sensitive substrates. The CW/long pulsed laser selective induction method is difficult to achieve high precision and highly localized growth due to the diffraction limit effect and the sizeable heat-affected zone.
Due to these drawbacks of the existing methods, researchers have attempted to use a femtosecond laser to realize the direct patterned growth of nanomaterials. As a "cold processing" method with a high peak power, the femtosecond laser direct writing is a promising tool to achieve the direct patterned growth of nanomaterials. The focus of a femtosecond laser can be regarded as a flexible, controllable and highly localized micro-reactor, which can realize the fixed-point growth of nanomaterials. At the same time, according to the pre-designed patterns, the laser focus position can be changed by the galvanometer, displacement stage or other equipment to realize the transfer-free patterned growth of nanomaterials. Compared with the current commonly used CW or long pulsed laser, a femtosecond laser has unique advantages in the transfer-free patterned growth of nanomaterials. First, due to its small heat-affected zone, it can be applied to thermal-sensitive substrates. Second, the ultra-high energy density of a femtosecond laser can induce nonlinear multi-photon absorption of precursors, which can realize the direct absorption of laser energy. Therefore, the femtosecond laser induced direct patterned growth of nanomaterials can be applied to transparent substrates without heat-absorbing layers. Third, the threshold effect of nonlinear absorption and the small heat-affected zone of a femtosecond laser can realize the high-precision growth of nanomaterials. Thus, the femtosecond laser induced patterned growth of nanomaterials has unique advantages and excellent prospects.
In this review, we first summarize the commonly used patterned synthesis methods of nanomaterials and their problems, including UV/electron beam lithography, solution-based direct patterning, and CW/long pulsed laser induced growth of nanomaterials. Then we discuss the unique advantages of the femtosecond laser-induced patterned growth method of nanomaterials, including high precision, highly localized growth, and high processing compatibility with thermal sensitive and transparent substrates. Next, the recent progress of the femtosecond laser induced direct patterned growth of nanomaterials and their applications are reviewed, including metal, metal oxide, metal sulfide, and carbon-based nanomaterials. For metal materials, researchers realized silver and gold patterned micro-nano structures with high conductivity [Fig.4(a)], which are comparable to the bulk materials. To grow more high-precision products, researchers realized silver nanostructures with a minimum feature size of only 180 nm with the help of surfactant [Fig.4(c)]. Researchers realized a stable 3D connection between two pairs of metal electrodes. As for metal oxides, researchers realized the patterned SnO2structure with the line width of about 150 nm through femtosecond laser direct writing (FLDW) and subsequent annealing process (Fig. 5). Our group realized the patterned growth of ZnO and SnO2 through femtosecond laser direct writing without subsequent annealing (Figs. 8 and 9). The minimum linewidth is about 800 nm. For metal sulfide, our group realized the patterned growth of MoS2 through femtosecond laser induced photochemical reaction (Fig. 10). For carbon-based nanomaterials, researchers realized the patterned growth of graphene through femtosecond laser induced reduction of graphene oxide [Fig.12(b)]. Researchers realized the patterned growth of graphene through FLDW of co-sputtering Ni/C films. The sheet resistivity of the products is about 205 Ω/sq [Fig.12(a)].
Compared with traditional methods, the femtosecond laser induced direct patterned growth technique has many unique advantages. Due to the extremely small heat-affected zone and the nonlinear multi-photon absorption effect of a femtosecond laser, the femtosecond laser induced direct patterned growth technique can realize the high precision, highly localized patterned growth of nanomaterials and has high processing compatibility with thermal sensitive and transparent substrates. Besides, the femtosecond laser induced direct patterned growth technique does not need a vacuum chamber or the high-temperature annealing process. Thus, it has drawn tremendous research attention around the world. Although the femtosecond laser induced direct patterned growth technique has made some progresses, several problems remain to be resolved. First, the products need to be expanded and the precursor needs to be optimized to reduce the required laser energy and take full use of the advantages of a femtosecond laser. Second, in term of the processing system, a Gaussian beam can be converted into a flat-top beam by beam shaping, thereby improving the uniformity of the products. The processing efficiency can be improved by employing scanning devices with high scanning frequency or adopting parallel processing strategies including multi-point scanning, line scanning , and plane projection. Finally, the application of this method needs to be explored, such as MEMS, soft electronics, metasurfaces, energy and catalytic devices.
The maximum-hardened layer depth of 6.3 mm can be obtained from the laser-induction hybrid quenched process on 42CrMo steel. The results show that the specimen with the hardened layer depth of 6.3 mm has the lowest surface damage, shallowest surface wear scar depth, shortest cross-sectional crack length, and best rolling contact fatigue performance among three specimens. After laser-induction hybrid quenching, a certain residual compressive stress exists on the specimen surface, which can improve the rolling contact fatigue performance of the material. As the depth of the hardened layer increases, the fatigue failure type of laser-induction hybrid quenched 42CrMo steel under heavy load conditions changes from external cracking to internal cracking and spalling. The angle between the cracks and surface decreases from 90° to 15°. While the crack propagation angle reduces, the crack extension becomes smoother and the rolling wear and fatigue damage of the specimen are alleviated.
In the study, we examine the influence of welding speed on bead appearance and low-temperature impact toughness of 440 MPa grade high-strength marine steel when laser-arc welding was used. Susceptibility to root humping increases with welding speed, and severe boot humping occurs at welding speed of 1.8 m/min. At the scanning speed of 1.2 m/min, the weld metal exhibits exceptional low-temperature impact toughness. At the low welding speed of 0.8-1.0 m/min, -40 ℃ sharp impact absorbed energy of the weld metal is low, owing to a lot of porosity in the weld metal. When the welding speed increases from 1.2 to 1.8 m/min, the number of acicular ferrite decreases while the amount of lath bainite increases, resulting in a steady decrease in the impact toughness.
The deposited microstructure of SLM TC4 constitutes coarse β columnar crystals with martensite α′ and α″ as the intragranular substructures. The microstructure of SLM TC4 after solution aging heat treatment is bimodal, and a secondary α phase is generated during the aging process. During the cyclic annealing process, parts of the grain boundary α phase and lath α phase are broken to form an equiaxed α phase. After the cyclic spheroidizing annealing and solution aging treatment, the microstructure of the sample is mainly composed of the lath α phase, equiaxed α phase, and secondary α phase; the volume fraction of the equiaxed α phase is relatively high. The standard deviation of the mechanical property data of SLM TC4 is generally large, which is significantly reduced after the heat treatment. The strengths of the samples treated either solely by solution aging or a combination of cyclic spheroidizing annealing and solution aging are roughly equivalent. The sample plasticity obtained by the combined cyclic spheroidizing annealing and solution aging is better than that obtained by solution aging alone. After the heat treatment methods, the mechanical properties of the samples exceed the national forging standard. The plastic anisotropy of SLM TC4 depositions is high. However, the three heat treatment processes significantly reduce the plastic anisotropy. Among these processes, the solution aging treatment provides the deposited TC4 with the lowest anisotropy of the mechanical properties. SLM TC4 depositions have mixed fracture morphology, while the deposited TC4 after heat treatment shows a typical ductile fracture.
This study employed a level set method to establish a two-dimensional finite element model of nanosecond laser to simulate the forming process of titanium alloy micro-pit structure covering heat transfer, fluid flow, surface tension, and Marangoni effect. In a single pulse period, the Marangoni effect caused by the surface tension gradient significantly affects the formation of the crater structure during melting, vaporization, liquid phase migration, and solidification of the material. The formation of sputtered droplets arises from the combined effect of recoil pressure, surface tension, and thermal capillary force on liquid phase migration and sputtering. In the multipulse period, the differential surface tension of the ablation edge in the horizontal and vertical directions causes the height of the crater to increase significantly with the number of ablation; however, the width of the crater does not change significantly. Because the influence of plasma shielding on the incident laser was not considered in the model, it was only suitable for predicting the geometric size of the micro-pits when the number of ablation was less than 15 times.