Nanosecond Laser Etching of Surface Drag-Reducing Microgrooves: Advances, Challenges, and Future Directions

Abstract

With the increasing demand for drag reduction, energy consumption reduction, and low weight in civil aircraft, high-precision microgroove preparation technology is being developed internationally to reduce wall friction resistance and save energy. Compared to mechanical processing, chemical etching, roll forming, and ultrafast laser processing, nanosecond lasers offer processing precision, high efficiency, and controllable thermal effects, enabling low-cost and high-quality preparation of microgrooves. However, the impact of nanosecond laser etching on the fatigue performance of substrate materials remains unclear, leading to controversy over whether high-precision shape control and fatigue performance enhancement in microgrooves can be achieved simultaneously. This has become a bottleneck issue that urgently needs to be addressed. This paper focuses on the current research status of nanosecond laser processing quality control for microgrooves and the research status of laser effects on enhancing the fatigue performance of substrate materials. It identifies the main existing issues: (1) how to induce surface residual compressive stress through the thermo-mechanical coupling effect of nanosecond lasers to suppress micro-defects while ensuring high-precision shape control of fixed microgrooves; and (2) how to quantify the regulation of nanosecond laser process parameters on residual stress distribution and fatigue performance in the microgroove area. To address these issues, this paper proposes a collaborative strategy for high-quality shape control and surface strengthening in fixed microgrooves, an analysis of multi-dimensional fatigue regulation mechanisms, and a new method for multi-objective process optimization. The aim is to control the geometric accuracy error of the prepared surface microgrooves within 5% and to enhance the fatigue life of the substrate by more than 20%, breaking through the technical bottleneck of separating “drag reduction design” from “fatigue resistance manufacturing”, and providing theoretical support for the integrated manufacturing of “drag reduction-fatigue resistance” in aircraft skins.

1. Introduction

With the urgent demand for drag reduction, consumption reduction, and low weight in civil aircraft within the aviation industry, drag reduction technology on aircraft skin surfaces has emerged as a key breakthrough [1]. Research indicates that surface microgrooves can alter the turbulence structure in the near-wall region of aircraft, thereby effectively reducing fluid frictional drag [2,3] (as shown in Figure 1). The author’s previous research has verified that approximately V-shaped grooves (44 μm wide, 7 μm deep, and 181 μm spaced) manufactured for civil aircraft cruise conditions achieve a drag reduction rate of up to 9.6% [4]. However, the current technical bottleneck is that traditional microgroove films are prone to aging and detachment. When microgrooves are directly machined on the surface of aircraft skin materials (such as aluminum–lithium alloy, the skin material of the C919 aircraft), improper machining processes may lead to stress concentration caused by microgrooves and their geometric defects (such as microcracks and burrs), resulting in high/low cycle fatigue failure of the skin material, posing a serious threat to flight safety [5,6,7]. There have historically been air disasters caused by aircraft skin fatigue, such as the China Airlines Flight 611 crash in 2002—a Boeing 747 passenger jet disintegrated in mid-air after taking off from Taipei, resulting in 225 deaths. The investigation found that the tail skin had undergone metal fatigue cracks due to improper maintenance in 1980, which gradually expanded and ultimately led to skin tearing and structural failure [8].

In current microgroove processing technologies, mechanical machining introduces residual tensile stress, while chemical etching is environmentally unfriendly and lacks precision. Although ultrafast lasers (femtosecond laser or picosecond laser) have high precision, their low efficiency makes it difficult to meet the large-area processing requirements of aircraft skins. Additionally, roll forming is suitable for the large-area forming and manufacturing of microstructures, but due to the high hardness of skin materials, higher rolling pressure may lead to plastic deformation or rebound of the skin, and the molds wear out quickly, requiring frequent replacement, which increases costs [5]. Nanosecond lasers possess both high efficiency and controllable thermal effects [9]. The recoil pressure generated by their evaporation of metal can induce residual compressive stress (Figure 2). As reported in the literature, nanosecond laser shock can induce residual compressive stress up to −200 MPa on the surface of aluminum alloy, leading to a fatigue-life improvement of over 60% [10,11,12]. Therefore, it may simultaneously achieve high-precision shape control of microgrooves and enhance fatigue performance. However, existing research has primarily focused on the geometric optimization or single-surface enhancement of drag-reducing microgrooves. The impact of high-quality processing of fixed microgrooves on their substrate fatigue performance remains unclear [11,12,13,14]. This uncertainty has led to disputes over whether it is possible to simultaneously achieve high-precision shape control of microgrooves and improve fatigue performance, making it a bottleneck issue that urgently needs to be addressed.

Based on a literature analysis and preliminary work research, the core challenges of nanosecond laser etching microgroove include the following: (1) How to induce residual compressive stress on the surface of the substrate and suppress micro-defects (such as microcracks) through the thermo-mechanical coupling effect of nanosecond laser, while ensuring high-precision shape control of the fixed microgrooves. Existing research often overlooks the influence of molten pool dynamics during the etching process on microgroove edges’ morphology and residual stress, leading to difficulties in controlling the microgroove morphology and potential micro-defects [15,16,17,18]. (2) How to quantify the regulation of nanosecond laser process parameters on the residual stress distribution and fatigue performance in the microgroove area. Traditional research mainly focuses on regulating the residual stress on the material surface through nanosecond laser shock peening to enhance fatigue performance. However, there is a lack of research on the residual stress distribution in materials after nanosecond laser etching of microgrooves and its impact on fatigue performance (due to a lack of research on the shock peening effect during laser processing). A multi-objective correlation model of “process parameters—microgroove quality—fatigue performance” has not been established [10,11,12,19,20].

Addressing the challenges above, this paper innovatively proposes the following. (1) A synergistic strategy for high-quality shape control and surface strengthening of fixed microgrooves: By establishing a thermo-mechanical coupling model, analyzing the molten pool evolution and stress field during laser processing, and optimizing laser parameters, the geometric accuracy of microgrooves can be ensured. Simultaneously, the recoil pressure is utilized to form a gradient residual compressive stress layer at the bottom and side walls of the microgrooves, which offsets the stress concentration effect caused by geometric discontinuities in the microgrooves. (2) Analysis of multi-dimensional fatigue regulation mechanism: Through high-resolution EBSD (electron backscatter diffraction) and in situ fatigue tests, as well as finite element simulations, the offset effect of residual compressive stress gradient in the microgroove region on the crack initiation location and the blocking mechanism of surface nano-grains on the crack propagation path are revealed. (3) A new method of multi-objective process optimization: The construction of a multi-response model with microgroove machining quality, residual compressive stress, and fatigue-life improvement as optimization objectives. The utilization of the mayfly optimization algorithm to overcome the limitations of traditional single-variable experiments, achieving global optimal matching of nanosecond laser parameters, and enhancing the fatigue performance of skin materials.

In summary, this paper focuses on the high-quality shape control processing of fixed drag-reducing microgrooves and the mechanism for enhancing the fatigue performance of their substrates. It elaborates on the current research status in China and other countries of the forming process of drag-reducing microgrooves, the quality control of nanosecond laser processing of microgrooves, and enhancement of metal fatigue performance by laser action. It analyzes the main problems existing in current research. It looks forward to future research directions, aiming to break through the technical bottleneck of the separation between “drag reduction design” and “anti-fatigue manufacturing” in existing research and provide theoretical support for the integrated manufacturing of “drag reduction-anti-fatigue” for aircraft skins.

2. Forming Process of Drag-Reducing Microgrooves

Various drag reduction technologies have received widespread attention due to the increasing demands for environmental protection and energy conservation. For example, the skin friction drag accounts for 50% of the total drag of a civil aircraft [5]. Every 1% increase in drag on the aircraft surface will result in an annual rise of approximately 15,000 to 100,000 barrels of aviation fuel consumed [21]. Using bionic drag-reducing materials for skin and wing components is one of the methods, besides optimizing the streamlined shape of moving objects, which can significantly reduce fuel consumption, increase air speed, and extend range. As a passive drag-reducing technology, bionic microgroove drag-reducing technology can achieve effective surface drag reduction without additional drag-reducing devices. NASA has listed it as one of the key technologies for aviation vehicles in the 21st century. The difficulty in its application lies in its small scale and high precision. Therefore, the research on drag reduction technology using microgrooves will significantly impact energy conservation and emission reduction and hold vast potential for development in the field of aerospace. Scholars are increasingly studying the processing methods of microgrooves. Normal processing methods include machining, chemical etching, rolling, and laser processing [5].

2.1. Machining of Microgrooves

With the development of micro-machine tools and numerical control technology, surface microstructure processing on a micrometer scale or smaller can be achieved. Denkena et al. [22] have created three types of microstructures with different uses and functional characteristics through diamond fly cutting. Song et al. [23] utilized a micro-milling cutter with a diameter of 0.1 mm to machine corresponding groove structures and square pillar array structures on the surface. Zhao et al. [24] reduced the surface roughness by decreasing the feed rate, achieving a microgroove array with a surface roughness value of 41 nm. However, the cutting tool’s small size and susceptibility to wear remain challenges in current micro-machine tool processing. Traditional grinding involves using irregular wheels to achieve microstructure grinding and shaping to obtain a smooth surface, which exhibits high productivity when machining microgrooves. Denkena et al. [25,26] employed a multi-contour grinding method, as shown in Figure 3a, to grind blades. They used irregular grinding wheels to feed and grind to a specified depth gradually, then axially moved a certain width before continuing grinding. This method offers high processing efficiency and can be used for large-area manufacturing of microgroove structures. However, when the grinding wheel moves, unilateral grinding results in burrs on the microstructure ridges, necessitating further dressing. To further reduce grinding wheel dressing time and enhance machining efficiency, Hockauf et al. [27] employed the “beaver tooth” profiling grinding process based on the aforementioned multi-contour profile grinding wheel. Using a grinding wheel of alternating metal-bonded diamond and pure resin layers, they fabricated microgroove structures with dimensions ranging from 20 to 120 μm. This bio-inspired multi-layer grinding wheel exhibits “self-sharpening” properties, meaning that the soft layers wear out first, highlighting the hard contours. This avoids complex grinding wheel dressing processes, saving up to 95% of grinding wheel dressing time and significantly improving the machining efficiency of the blade surface microstructure. However, further research is needed on the grinding wheel diameter and the number of layers.

2.2. Chemical Etching of Microgrooves

A low-voltage pulsed direct current is applied in the electrolyte using metal materials as anodes and inert electrodes as cathodes. The anode workpiece material is etched away as ions, theoretically achieving micro- to nanometer-scale precision. Rathod et al. [28] successfully fabricated grooves with a depth of 10 μm and a width of 55 μm using electrochemical microfabrication technology, employing a cylindrical microfabrication tool with a diameter of 110 μm and a length of 1050 μm. By adjusting process parameters such as voltage, duty cycle, and processing time, complex microgroove structures such as inverted conical, cylindrical, double-stepped, and spherical shapes were fabricated on the metal surface. Figure 3b shows this process results in a specific taper along the wall. When designing microgroove structures, the electrode size should be adjusted based on the difference in dimensions between the top and bottom. Du et al. [29] proposed a method based on dry film’s chemical etching processing technology, taking 304 stainless steel rotators as the research object. This method aims to reduce the side etching amount of microgrooves by optimizing the etching solution composition and temperature parameters and adding a post-etching polishing process step.

2.3. Rolling of Microgrooves

A smooth roller and a mold plate with microgroove structures on its surface are used. The microgroove structures on the mold plate are transferred to the workpiece through the pressure applied by the flat roller, as shown in Figure 3c. This method has advantages such as high forming efficiency and good consistency of grooves, making it suitable for rapidly forming large areas with micro-fine surface features. Gao et al. [30] analyzed the influence of grain size on the dimensional effect during rolling by micro-rolling copper plates with three different grain sizes. They proposed that as the grain size increases, the grain boundary strengthening effect weakens, the flow stress decreases, and the rolling load decreases, which improves the material’s filling rate. Although mold plates with holes are easier to process, the larger contact area during processing requires greater rolling force, making it difficult to process materials with high yield strength. Moreover, due to the limitation of mold plate size, continuous rolling processing over a large area cannot be carried out. Hu et al. [31] and Wang [32] utilized precision cutting technology to process microgroove structures on the work roll and employed roll forming to produce micron-scale groove structures. Lu et al. [33] first used electric discharge machining (EDM) to process microgrooves on the roll surface and then performed overall rolling on annealed Al and Cu plates to continuously produce microgrooves with a width of 250 μm and an aspect ratio greater than 2. Single-pass roll forming requires high manufacturing standards for the rollers, and the residual stress in the material after single-sided rolling is relatively large. Uneven stress distribution can lead to bending and deformation in the workpiece [31]. Zhou et al. [34] processed microgroove structures on the roll surface using EDM, and then rolled micron-scale grooves on the surface of aluminum alloy plates. They analyzed issues such as surface flatness, uniformity of groove depth, and accumulation during the forming process of textured plates, and found that when the relative linear velocity of the two rolls was between 0 and 2 mm/s, it could significantly improve the flatness and groove profile of the plate. Romans et al. [35] manufactured work rolls by tightly winding tough thin steel wires on the roll surface. By changing the wire diameter, different semi-circular microgroove structures could be obtained on the plate. This processing method avoided the difficulty in processing work rolls. The surface quality of the plates formed by this process mainly depends on factors such as the surface quality, hardness, and wear resistance of the steel wire. The wire-wound rolling process requires a large rolling pressure to achieve a better hole filling rate, but during the rolling process, due to the wear and pulling of the thin steel wire, loosening and misalignment fractures are prone to occur, affecting the quality of the roll forming. Limited by the shape of the steel wire, only semi-circular groove structures can be processed. Shimoyama et al. [36] proposed a periodic strain rolling process, using two sets of different rolls to perform two passes of rolling on the plate. This two-pass rolling process can change the local strain distribution of the plate and has great potential for controlling microstructure and texture evolution. Klocke et al. [37,38,39] proposed a new incremental roll forming process. Based on the traditional roll forming of microgroove structures, a small rolling device was designed, using a hydraulic device to provide constant pressure when the rolling gap changes. A pair of tiny rollers were machined on high-speed steel material using ultra-fine diamond cutting tools to process U-shaped grooves on the blade surface. The rolling equipment was used to roll out a 70 μm wide microgroove structure on the compressor blade, achieving a drag reduction of 6%. The reported rolling process can obtain microgroove structures on softer flat plates, and the incremental rolling method can produce microgroove structures on the surface of blade materials. However, the shape differs significantly from the designed semi-circular groove morphology. For blade-shaped or near-blade-shaped structures with better drag reduction capabilities, they have not been applied due to stability and processing difficulties. More effective forming processes still require in-depth research [5].

2.4. Laser Etching of Microgrooves

Laser etching is a method of processing that utilizes the photothermal effect generated by the high energy density achieved at the focal point after focusing the energy of light. Kaakkunen et al. [40,41] employed nanosecond pulsed lasers to ablate the NACA 0024 airfoil, achieving a trapezoidal structure on the micrometer scale. Wind tunnel test results indicated that, despite the slightly inferior quality of the grooves produced by this method, it still reduced wall shear stress, exerting a certain drag-reducing effect in turbomachinery. Due to the long pulse width and low laser intensity, the material melted and continuously evaporated, resulting in significant thermal shock to the material and a certain degree of heat-affected zone, limiting the processing accuracy. Only by reducing the heat impact can the processing quality be improved. Laser processing is fast and can be applied to practical drag-reducing surface industrial applications. The author has accumulated certain experience in the design of drag-reducing microgrooves on the walls of civil aircraft and the nanosecond laser etching process of aviation alloys. Based on the strategy of “multiple scanning with small laser energy and planning groove spacing based on the radial thermal influence zone”, a nanosecond laser was used to fabricate approximately V-shaped grooves (width 44 μm, depth 7 μm, spacing 181 μm) with high machining accuracy (dimensional error < 2%) on the surface of aviation titanium alloy, achieving a drag reduction rate of up to 9.6% [4,9,15,16]. Siegel et al. [42] processed grooves with a depth of 20 μm on NACA 6510 compressor blades using a picosecond laser. Femtosecond laser use can avoid thermal damage to the top of the grooves caused by illumination. Wind tunnel experiments were conducted on the processed compressor blades, resulting in a drag reduction of approximately 7.2%. Zemaitisa et al. [43] used an ultraviolet picosecond laser to ablate polytetrafluoroethylene (PTFE). By controlling process parameters such as preheating temperature, heating PTFE can increase laser ablation efficiency by 30%. A near-edge microstructure with a spacing s = 250 ± 5 μm, height h = 127 ± 3 μm, and thickness t = 25 ± 1 μm was obtained, as shown in Figure 3d. Wind tunnel experiments were conducted to study the drag reduction effect on the PTFE surface. The results showed that a drag reduction of 6% could be achieved when the dimensionless rib spacing was between 14 and 20.

Figure 3. Forming methods of drag-reducing microgrooves. (a) Machining of microgrooves [25]; (b) Chemical etching of microgrooves [28]; (c) Rolling of microgrooves [30]; (d) Laser etching of microgrooves [43].

Figure 3. Forming methods of drag-reducing microgrooves. (a) Machining of microgrooves [25]; (b) Chemical etching of microgrooves [28]; (c) Rolling of microgrooves [30]; (d) Laser etching of microgrooves [43].

In summary, the use of rapidly developing machining technologies such as precision cutting, precision milling, and precision grinding can achieve high-precision surface microgrooves (as shown in Table 1). However, wear in cutting tools or grinding wheels can occur. Additionally, due to the size effect, the machined microgrooves tend to exhibit high surface roughness and precision deviations. Furthermore, these machining technologies always involve varying degrees of material loss and can produce residual tensile stress on the surface, which adversely affects the fatigue life of the material. Electrochemical micro-machining has limitations in tool wear when fabricating high-aspect-ratio microgrooves on metal surfaces. Due to the taper formed along the vertical walls, it is difficult to maintain a uniform cross-section of the groove at higher depths, resulting in low machining accuracy. Moreover, there is the problem of chemical reagent pollution in the environment. Roll forming generally requires high rolling pressure, making it difficult to process materials with high yield strength. Additionally, due to the limitation of mold plate size, continuous rolling processing over a large area cannot be carried out. Furthermore, high rolling pressure may lead to plastic deformation or rebound of the skin, and the molds wear out quickly, requiring frequent replacement, which increases costs. Although ultrafast laser processing technologies such as femtosecond lasers and picosecond lasers have high machining accuracy, the equipment usage cost is high (approximately 1000 yuan/hour according to research), thus increasing the processing cost of workpieces. Moreover, the removal of material is low, and the machining efficiency is not high. Nanosecond lasers combine advantages such as high machining efficiency and controllable thermal effects, enabling high-precision and high-efficiency machining of drag-reducing microgrooves. Moreover, the recoil pressure generated by the nanosecond laser evaporation of metal can induce residual compressive stress on the surface of the skin material, thus holding promise for simultaneously improving the fatigue performance of the skin material. Therefore, in the following, we will provide a detailed review of the current research status in China and other countries on the quality control of nanosecond laser machining of microgrooves and the study of laser effects on enhancing metal fatigue performance. We will analyze the main problems existing in current research and ultimately look forward to future development directions, providing theoretical support for the integrated manufacturing of “drag reduction-fatigue resistance” for aircraft skin materials.

Table 1. Processing technologies of microgrooves.

Processing Methods	Existing Problems	References
Mechanical machining	Introducing residual tensile stress.	[22,23,24,25,26,27]
Chemical etching	Being environmentally unfriendly and lacking precision.	[28,29]
Roll forming	High rolling pressure may lead to plastic deformation or rebound of the skin, and the molds wear out quickly, requiring frequent replacement, which increases costs.	[30,31,32,33,34,35,36,37,38,39]
Nanosecond laser processing	The impact of high-quality processing of fixed microgrooves on their substrate fatigue performance remains unclear.	[4,9,15,16,40,41]
Ultrafast laser processing	Low efficiency makes it difficult to meet the large-area processing requirements of aircraft skins.	[42,43]

Table 1. Processing technologies of microgrooves.

Processing Methods	Existing Problems	References
Mechanical machining	Introducing residual tensile stress.	[22,23,24,25,26,27]
Chemical etching	Being environmentally unfriendly and lacking precision.	[28,29]
Roll forming	High rolling pressure may lead to plastic deformation or rebound of the skin, and the molds wear out quickly, requiring frequent replacement, which increases costs.	[30,31,32,33,34,35,36,37,38,39]
Nanosecond laser processing	The impact of high-quality processing of fixed microgrooves on their substrate fatigue performance remains unclear.	[4,9,15,16,40,41]
Ultrafast laser processing	Low efficiency makes it difficult to meet the large-area processing requirements of aircraft skins.	[42,43]

3. Quality Control of Nanosecond Laser Machining of Microgrooves

Compared to traditional machining techniques such as chemical etching and ultrashort pulse laser processing, nanosecond lasers combine high efficiency and controllable thermal effects, enabling high-quality microgroove processing [44,45,46]. Historically, scholars both domestically and internationally have primarily focused on in-depth research on the fundamental theories and processing techniques of laser machining to regulate the quality of nanosecond laser processing of microgrooves, laying a theoretical and technological foundation for research on the shape control manufacturing of microgrooves with fixed geometric parameters.

Scholars often predict the processing morphology in fundamental theoretical laser processing research by establishing finite element models. Stein et al. [47], at the Polytechnic University of Madrid in Spain, pioneered the establishment of a two-dimensional finite element model for the pulsed-laser processing of photovoltaic materials. Although the instantaneous removal of materials was not considered, the laser processing profile was approximately predicted by predicting the region beyond the gasification temperature. Furthermore, Vasantgadkar et al. [48], of the Indian Institute of Technology Mumbai Branch, developed this model, adding consideration of the temperature-dependence characteristics of the target, the plasma shielding effect, and absorptivity, which significantly improved the accuracy of the model in predicting ablation depth. On this basis, significant progress has been made in the research of Wang et al. at the University of Florida in the United States, considering the immediate removal effect of materials during the ablation process, providing a new perspective for understanding the ablation depth under high laser energy density. Zhang et al. [49,50] at the Harbin Institute of Technology in China established a finite element model for nanosecond laser-etched microgrooves. They developed ABAQUS based on Fortran language to quickly remove materials that have reached the vaporization temperature, achieving the simulation of microgroove morphology. Furthermore, the authors [4,16] proposed a high-precision and efficient prediction method for microgroove morphology etched using a nanosecond laser. This method is based on the principle of energy conservation, which involves performing periodic continuous equivalence tests on pulsed lasers to significantly improve computational efficiency. The response surface methodology is used to modify the finite element model under the action of an equivalent laser to ensure computational accuracy. Later, Zhao et al. [51], at Donghua University, established a complex, comprehensive three-dimensional finite element model that couples heat transfer and molten metal flow during laser etching, revealing titanium alloys’ nanosecond laser etching mechanism. However, to balance computational efficiency and accuracy, the above studies simplified the model, often ignoring the influence of the dynamic behavior of the melt pool on the edge morphology and residual stress of the microgrooves during the etching process, resulting in difficulty in controlling the microgroove morphology and possible micro defects.

In laser processing technology research, scholars have explored the technological rules of laser etching microgrooves through single-factor experiments, adopted multi-objective optimization methods, and developed new processes to achieve high-quality microgroove preparation (the general experimental systems and test conditions are shown in

Table 2. Experimental systems and test conditions.

Systems	Parameters	Conditions	References
Nanosecond laser system	Pulse duration: 10–100 ns	Material: aluminum alloy, titanium alloy et al.	[9,15]
Auxiliary gas system	Pressure: Variable	-	[45]
Scanning system	Scanning speed: Variable	Environment: air, liquid	[53,58]

). Takayama et al. [52] at Keio University in Japan conducted a series of nanosecond laser etching experiments on the surface of the single-crystal diamond, exploring the influence of processing times on the morphology of microgrooves and discovering that the morphology of microgrooves exhibits specific changes with the increase in processing times. Charee et al. [53] at the Rajamangala University of Technology in Thailand studied the impact of laser etching on surface quality from both laser parameters and processing environment perspectives, proposing methods to optimize processing parameters to reduce microgroove damage. Sahu et al. [45], at the Indian Institute of Technology, investigated the influence of laser parameters and auxiliary gas pressure on the sidewall taper angle of microgrooves through experiments. They established a regression model, discovering the influence pattern of process parameter interactions on the sidewall taper angle. Okamoto et al. [46], at Okayama University in Japan, proposed a two-step scanning method to prepare large-sized high-quality microgrooves on single-crystal diamond. Zhang et al. [54], at the Beijing University of Technology, processed SiCp/AA2024 composite materials using high-energy nanosecond lasers under high-pressure auxiliary gas, discovering the influence pattern of pulse width on microgroove quality based on single-factor experiments. Similarly, Zhang et al. [55] at the Nanjing University of Aeronautics and Astronautics explored the influence of single factors on the etching morphology of microgrooves, discovering the material removal behavior of single-crystal diamond during multiple infrared nanosecond laser ablations. Furthermore, Xing et al. [56,57] at Southeast University investigated the influence of various factors on the microgroove processing results on the surface of the microcrystalline diamond, discovering the influence pattern of laser parameters on surface quality, microgroove size, and other responses through single-factor and multi-objective optimization experiments, achieving high-quality preparation of microgrooves. Subsequently, the authors [9,15] proposed a multi-objective optimization method based on response surface methodology and genetic algorithms during their research on the nanosecond laser etching of microgrooves on titanium alloy surfaces, achieving high-quality processing of microgrooves using nanosecond lasers. Additionally, Wang et al. [58] from the Chinese Academy of Sciences studied the nanosecond laser processing of microgrooves on the surface of TC4 titanium alloy in both air and liquid environments, discovering that processing in a static liquid environment had minimal improvement on the recast layer. The studies above have achieved significant results in researching the influence of process parameters on the geometric dimensions or surface quality of microgrooves. However, there is less research on the high-precision shape control processing of microgrooves with fixed geometric parameters. The design unit provides the size parameters of general drag-reducing microgrooves. Thus, further research is needed on high-quality shape control machining methods for fixed microgrooves.

In summary, as shown in Figure 4, domestically and internationally, scholars are inclined to explore the influence of process parameters on the geometric dimensions and surface quality of microgrooves, and have achieved high-quality processing of microgrooves using finite element analysis methods, process parameter optimization, and new processes. However, there is less research on the shape and property control of microgrooves with fixed geometric parameters, such as the regulation of laser parameters on surface residual stress during the nanosecond laser etching of microgrooves and the impact of processing microgrooves on the fatigue performance of the substrate material. This limits the application progress of nanosecond laser etching microgroove technology in the field of drag reduction on aircraft skins, which needs further research and resolution.

4. Laser Action on Enhancing Metal Fatigue Performance

Research on enhancing metal fatigue performance through laser action primarily focuses on the mechanisms and applications of laser shock peening (LSP) technology. In recent years, scholars, both in China and internationally, have significantly improved metal fatigue performance through LSP.

In the research on the LSP mechanism, scholars have primarily focused on regulating residual stress, surface quality, and microstructure by LSP to enhance fatigue performance. Dwivedi et al. [59], at the Indian Institute of Technology, India, evaluated the effects of LSP and ultrasonic peening (USP) on the surface integrity and ratcheting fatigue performance of high-strength steel alloy. They found that while LSP significantly improved surface hardness and introduced deep residual compressive stress, the fatigue strength of samples treated with USP decreased significantly, mainly due to increased surface roughness, lower residual compressive stress, and weakened texture strength. Bae et al. [60] at the Kwangju Institute of Science and Technology, South Korea, investigated the effectiveness of LSP on Inconel 738 low-carbon materials at room temperature and high temperatures. They found that LSP significantly improved surface hardness and residual compressive stress and increased fatigue life by approximately 2.4 times at room temperature. However, the effect was insignificant at 850 °C, possibly due to thermal relaxation. Khanigi et al. [61] at Tabriz University, Iran, proposed the influence of post-treatment methods without absorptive coatings on the residual stress field in Rene 80 nickel-based superalloys through three-dimensional finite element simulations and experimental verification. They found that a single round of LSP significantly increased residual compressive stress and improved yield strength and microhardness. Finally, Digonta et al. [62] at the University of Memphis, USA, reviewed the impact of LSP as a post-treatment technique for improving fatigue performance and simulated the residual compressive stress in fatigue performance modeling using an average stress model. Bai et al. [63], at Chongqing University, China, analyzed the impact of LSP on the fatigue life of steel–concrete composite beams through numerical simulations. They found that LSP could uniformly distribute residual compressive stress on the material surface, inhibit fatigue crack propagation, and thus enhance fatigue life. Pan et al. [10], at Air Force Engineering University, observed the microstructure and modulated residual stress to discover the impact of LSP on the fatigue performance of 7075 aluminum alloy, significantly improving fatigue strength. Finally, Sun et al. [64], at Shenyang University of Technology, studied the evolution of residual stress in 8Cr4Mo4V steel and its impact on fatigue performance through experiments and numerical simulations. They found that LSP could generate significant residual compressive stress on the surface layer and significantly improve the material’s surface hardness and fatigue strength. However, the studies above primarily focused on the impact of individual factors such as residual stress, surface quality, and microstructure on fatigue performance, with limited research on the enhancement mechanism of fatigue performance under the synergistic effect of multiple factors.

In LSP application research, scholars have discovered the influence patterns of LSP on material fatigue behavior, fatigue strength, and fatigue life. Rodríguez et al. [65] at the University of Hidalgo San Nicolás in Mexico investigated the effects of LSP on the fatigue crack propagation and fracture toughness of Inconel 718 samples in different aging states, finding that aging time affects the precipitation of precipitated phases, thereby influencing the hardness and plastic flow behavior of the samples. Maleki et al. [66], at the Polytechnic University of Milan in Italy, studied the effects of various post-treatment techniques (such as LSP and USP) combined with stress relief on the tensile properties and fatigue behavior of laser powder bed fusion stainless steel 316 L samples, significantly improving fatigue behavior. Flores-García et al. [67] at the Mexican Industrial Engineering and Development Center studied the effects of LSP on the fatigue life of 304 stainless steel flat samples, finding that LSP significantly increases fatigue life, and explained the fatigue failure mechanism through scanning electron microscopy images. Liu et al. [68], at Xi’an Jiaotong University in China, explored the effects of laser shock. Through experiments, they recorded the effect of peening composite strengthening on the detail fatigue rating cutoff (DFR_cutoff) of TC4 titanium alloy, laying the foundation for subsequent research. Subsequently, Sun et al. [11], at the China Aviation Manufacturing Technology Research Institute, designed an orthogonal experiment to optimize the laser shock parameters of 2050 aluminum–lithium alloy, finding that under optimal conditions (laser power density of 5.30 GW/cm², overlap rate of 50%, and two shocks), the fatigue life is increased by 22% and 63% at residual compressive stress levels of −260 MPa and −200 MPa, respectively. Lu et al. [69] at Beihang University discovered the influence patterns of LSP on the fatigue strength of the leading edge of blades damaged by external objects, verifying the potential application of LSP in aero-engines. Ye et al. [20], at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, proposed a discontinuous zero overlap rate composite path strategy, significantly improving the DFR_cutoff value of the TC4 titanium alloy. Jin et al. [70], at the Shenyang University of Chemical Technology, studied the effects of LSP on the fatigue life of FV520B steel through a combination of experiments and simulations, finding that this technology can effectively inhibit crack initiation and propagation, extending fatigue life. However, the studies above did not focus on the impact-strengthening effect during the laser etching of microgrooves, leading to ongoing controversy over whether high-quality microgroove processing can be achieved simultaneously with the improvement of fatigue performance in the base material.

As shown in Figure 5, the research on laser shock peening technology in enhancing metal fatigue performance has progressed from basic exploration to method optimization and engineering application. This enriches the theoretical foundation and provides significant technical support for practical industrial applications. However, current research primarily focuses on the impact of individual factors (such as residual stress, microstructure, surface quality, etc.) on fatigue performance, lacking research on the shock peening effect during the nanosecond laser etching of microgrooves and the mechanism of multi-factor synergistic impact on the fatigue performance of substrates. Especially for high-strength and high-toughness materials, optimizing laser parameters to maximize fatigue performance while ensuring microgroove processing quality remains a research challenge.

5. Conclusions

This review analyzes the research status of the nanosecond laser etching of drag-reducing microgrooves, addressing two critical bottlenecks in current research: the separation of geometric control from fatigue performance enhancement, and the lack of multi-dimensional mechanistic understanding. While traditional methods (e.g., mechanical machining, chemical etching) introduce residual tensile stress or environmental concerns, nanosecond lasers uniquely enable high-precision microgroove fabrication while inducing residual compressive stress through recoil pressure—a dual advantage critical for aviation safety. The key findings include the following:

(1)

Synergistic processing-property control: The thermo-mechanical coupling effect of nanosecond lasers allows for the simultaneous optimization of microgroove geometry and substrate strengthening. However, the interplay between molten pool behavior, stress field evolution, and defect suppression requires deeper exploration through advanced modeling and in situ characterization.

(2)

Multi-dimensional fatigue mechanisms: Residual compressive stress gradients, nanocrystalline surface layers, and geometric discontinuity mitigation collectively enhance fatigue life. Yet, quantitative relationships between laser parameters, stress distribution, and crack initiation/propagation remain underexplored.

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Industrial translation challenges: Current studies predominantly focus on laboratory-scale validation. Scaling nanosecond laser processing for large-area aircraft skin applications demands innovations in process efficiency, cost-effective parameter optimization, and fatigue performance standardization.

Future research should prioritize the following:

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The development of multi-scale models integrating thermal ablation dynamics, phase transformations, and fatigue crack evolution.

(2)

In situ experimental platforms to correlate real-time process monitoring with post-processing microstructure and fatigue behavior.

(3)

Intelligent manufacturing frameworks leveraging machine learning to optimize laser parameters for site-specific microgroove geometries and fatigue resistance.

This review lays the groundwork for next-generation aircraft skin technologies that harmonize aerodynamic efficiency with structural reliability by unifying drag reduction design with fatigue-resistant manufacturing principles.