Selective laser melting (SLM) is a widely popular metal additive manufacturing technique that offers distinct advantages in fabricating bone implants with customized shapes and internal bionic porous structures. In particular, using a very high cooling rate (10³?10⁸ K·s^-1) during the SLM process can inhibit the grain growth of pure Zn and confer good mechanical properties. This study reveals the internal relationship between the microstructure and mechanical anisotropy of SLM-fabricated pure Zn. We also report the influences of the grain characteristics and texture on the anisotropy.

The purity (mass fraction) of Zn powder used in this experiment is 99.9% and the sizes of particles are 7.2?29.7 μm. Pure Zn samples are fabricated using a commercial SLM printing device equipped with a 200 W fiber laser. The density of a pure Zn sample is greater than 99.5% when using optimized forming parameters (laser power P=80 W, and scanning speed V_S=900 mm·s^-1). To investigate the mechanical anisotropy, the fabricated Zn samples with dimensions of 8 mm×8 mm×8 mm are microscopically characterized in the horizontal and vertical directions. After etching with the 4% (volume fraction) nitric acid solution for 5 s, the microstructures on both the horizontal and vertical planes of the Zn samples are characterized using a metallographic optical microscope (OM) and scanning electron microscope (SEM). The grain orientation, grain size, and texture information are analyzed using electronic backscattered diffractometer (EBSD). Moreover, tensile samples with a gauge length of 22.0 mm, width of 3.0 mm, and thickness of 2.8 mm are fabricated for tensile tests.

Significant differences are observed in the microstructures of Zn samples formed by SLM on horizontal and vertical planes. A large number of equiaxed grains are observed on the horizontal plane in the OM and SEM images. In contrast, fish-scale molten pools with depth of 30?50 μm and width of 100?150 μm are found on the vertical plane. Furthermore, most of the grains exhibit preferred orientations along 1ˉ21ˉ0 and 011ˉ0 perpendicular to the building direction (BD) on the horizontal plane. In contrast, on the vertical plane, a majority of the grains display preferred orientations along 0001 (red region) parallel to the BD. Notably, the average grain size (10.21 μm) on the horizontal plane is 40.8% smaller than that (17.24 μm) on the vertical plane. The statistical distribution of grain boundary misorientation angles (Fig.6) and analysis results of the Kernel average misorientation (KAM) (Fig.7) indicate that low-angle grain boundaries (LAGBs) are more prevalent on the horizontal plane and these areas also exhibit a higher dislocation density. The KAM value on the horizontal plane is 0.84°, which is marginally higher than the value of 0.79° observed on the vertical plane. Finally, we report the examination results of the tensile properties of the SLM-fabricated Zn in both the horizontal and vertical orientations (Fig.8). The strain hardening rate of the specimens in the horizontal direction exceeds that of the specimens in the vertical direction. A quantitative analysis of the tensile properties reveals distinct mechanical characteristics for Zn specimens fabricated on different planes. The yield strength, ultimate tensile strength, and elongation of the specimens fabricated on the horizontal plane are 108.0 MPa, 123.5 MPa, and 11.7%, respectively. However, Zn specimens fabricated on the vertical plane exhibit a yield strength of 90.2 MPa, an ultimate tensile strength of 108.0 MPa, and an elongation of 14.1%. Although specimens fabricated on the horizontal plane demonstrate yield and ultimate tensile strengths that are 16.5% and 12.5% greater than their vertical counterparts, respectively, their elongation rate is 17% lower than that of the vertical specimens. The aforementioned results collectively indicate the presence of anisotropy in the mechanical properties of SLM-fabricated Zn.

This study reports the investigation results on the microstructure and mechanical properties of SLM-fabricated Zn in both the horizontal and vertical directions, with a particular focus on the grain morphology and orientation. Furthermore, the relationships between these microstructural aspects and mechanical properties are discussed. The SLM-fabricated Zn exhibits pronounced anisotropy in its tensile strength and ductility. Specimens fabricated on the horizontal plane exhibit a higher yield strength and ultimate tensile strength but a lower elongation rate compared to those fabricated in the vertical direction. The greater strength of horizontally fabricated specimens is primarily attributed to their finer grain size and higher initial dislocation density, which hinder dislocation movement. Conversely, specimens fabricated on the vertical plane demonstrate enhanced ductility because they contain a higher proportion of high-angle grain boundaries, which effectively impede crack propagation and thereby prevent premature fracturing.

Laser additive manufacturing (LAM) is renowned for its exceptional accuracy and the ability to produce complex components with intricate geometries, making it widely used across various industries. The LAM technology primarily encompasses two techniques: laser-directed energy deposition (LP-DED) and laser powder bed fusion (L-PBF). Among these, L-PBF is witnessing rapid advancements and gaining popularity in both scientific research and industrial applications.

Copper and its alloys are pivotal functional materials. They act as crucial strategic reserves for the country, with a significant position within the national economy. Nevertheless, the exceptional thermal conductivity and high NIR reflectivity exhibited by copper and its alloys present notable challenges for LAM in relation to their effective processing and shaping. However, copper and its alloys have excellent electrical and thermal conductivities, along with exceptional mechanical properties. Because of the growing call for intricate functional copper and copper alloy components, the LAM of copper and copper alloy parts has become a research hotspot in recent years.

CuCrZr is a copper-based precipitation hardening alloy. The addition of chromium significantly enhances its mechanical properties when compared to pure copper. Meanwhile, the presence of zirconium effectively hinders the growth of chromium precipitate phases, ensuring a more uniform distribution of precipitates and further strengthening the alloy. Notably, zirconium has minimal impact on the alloy’s electrical conductivity. Firstly, CuCrZr’s remarkable heat resistance and superior strength enable it to maintain its integrity and stability in high-temperature environments, making it an ideal material for manufacturing components exposed to extreme temperatures, such as aerospace engine nozzles and components for ITER. Secondly, this alloy demonstrates excellent resistance to oxidation, corrosion, and erosion caused by high-temperature gases. This exceptional property has facilitated its widespread application in various corrosive environments, including chemical equipment, marine engineering, and the nuclear industry. Thirdly, CuCrZr alloys are renowned for their outstanding electrical and thermal conductivity, making them highly suitable for the production of electrical components and heat sinks. Finally, CuCrZr alloys exhibit favorable machinability and can be shaped using various additive manufacturing methods, including L-PBF and LP-DED. Furthermore, they can be welded to other metals.

This review comprehensively examines the forming behavior, microstructure, and overall performance of CuCrZr alloys across three distinct areas. Firstly, it highlights the need to consider the laser absorption rate in addition to the traditional volumetric energy density when evaluating CuCrZr alloys’ response to laser processing. This is because the absorption of copper and its alloys significantly varies with the laser wavelength, as illustrated in Fig.3 and Table 2. Secondly, the review discusses the densities and process parameters of CuCrZr alloys printed using lasers of various wavelengths, further emphasizing the importance of considering absorption rate (Fig.5 and Table 2). Moreover, the review delves into three types of defects commonly encountered in L-PBF, particularly those that tend to occur during the fabrication of CuCrZr alloy components (Fig.6). It also examines the variations in the alloy's microstructure before and after heat treatment, along with the underlying causes of these changes (Figs.8‒16). This analysis provides valuable insights into the microstructure evolution and its impact on alloy performance. Additionally, the review explores the impact of an enhanced heat treatment routine and process parameters on the mechanical properties of CuCrZr alloys, as presented in Table 3. Furthermore, it investigates the correlation between densification, heat treatment regimen, and both electrical (Fig.19 and Table 4) and thermal (Fig.20 and Table 5) properties.

This review presents an overview of the current status of the research on CuCrZr alloys in relation to their forming behavior, microstructure, and mechanical, thermal, and electrical properties.

1) The majority of E_A values obtained using near-infrared lasers are lower than those obtained using green lasers. This difference can be explained by the absorption rate of the CuCrZr alloy, which is significantly higher for green lasers compared to near-infrared lasers. Notably, the absorption rate of the CuCrZr alloy decreases monotonically as the laser wavelength increases. A particularly sharp decrease is observed when the laser wavelength exceeds 550 nm. When comparing the CuCrZr alloy processed with green lasers to that processed with near-infrared lasers, it is evident that the former exhibits a narrower range of density fluctuations. However, the overall density is lower, indicating the potential for further optimizing the green laser process parameters. Common and challenging defects encountered during the LAM of CuCrZr alloys include pores, cracks, and unfused powder.

2) ST modifies the melt pool boundary and grain morphology of the alloy, whereas DAH generates precipitates that bolster its mechanical properties. The L-PBF method yields a CuCrZr alloy rich in supersaturated Cr and Zr atoms within the matrix, owing to the solid solution. Subsequently, DAH triggers precipitation within this matrix, primarily forming Cr, Cu_xZr_y, and Cr₂Zr precipitates. These micro- and nano-sized precipitates significantly enhance the alloy’s mechanical properties. However, as the treatment temperature rises, the precipitate distribution transitions from uniform to partially concentrated. Specifically, aging treatments at 500 ℃ for 2 hours or 550 ℃ for 1 hour attenuate the {111} and {200} crystal plane orientations while strengthening the {220} crystal plane orientation in the CuCrZr alloy.

3) In as-built state, the CuCrZr alloy demonstrates relatively weak mechanical properties, with yield strengths ranging from 175.2 MPa to 400.0 MPa, ultimate tensile strengths between 254.6 MPa and 447.0 MPa, and elongations varying from 10.0% to 49.4%. However, it is noteworthy that by utilizing green lasers, it is possible to fabricate an as-built CuCrZr alloy with superior mechanical properties. Among various methods, direct aging treatment stands out as the most effective means to enhance these mechanical properties. This treatment achieves optimal results when conducted at approximately 500 ℃. Subsequently, the CuCrZr alloy undergoes significant improvements in its mechanical properties after this aging treatment. Specifically, yield strengths increase to range from 361.0 MPa to 527.0 MPa, ultimate tensile strengths improve to fall between 466.0 MPa and 612.0 MPa, and elongations enhance to vary from 12.3% to 21.8%. These remarkable improvements in the mechanical properties of the CuCrZr alloy can be attributed to the formation of precipitates during direct aging treatment, along with a reduction in both dislocation density and thermal residual stress.

4) The CuCrZr alloy, in its as-built state, demonstrates electrical conductivity ranging from 21% IACS to 30.0% IACS and thermal conductivity varying between 100.0 W/(m·K) and 307.0 W/(m·K). This range of conductivities is primarily attributed to the presence of numerous oversaturated Cr and Zr atoms within the alloy matrix. These atoms cause lattice distortion in the grains, which enhances the scattering effect on free electrons. Consequently, the alloy exhibits lower electrical and thermal conductivities. To enhance both electrical and thermal conductivities, the most effective heat treatment process is SAAH. This process, performed at temperatures ranging from 950 ℃ to 1000 ℃ followed by an additional 500 ℃, significantly improves the conductivity values. Specifically, it elevates the electrical conductivity to a range of 84.0% IACS to 88.1% IACS and boosts the thermal conductivity to levels between 297.0 W/(m·K) and 350.0 W/(m·K). Additionally, the combination of solution annealing and age hardening treatment effectively reduces dislocation density and residual stresses within the alloy. This treatment also produces precipitates, which collectively contribute to further enhancing the electrical and thermal conductivities of the CuCrZr alloy.

The following are anticipated to be the future research prospects and development directions for CuCrZr alloys.

1) Optimization of the process parameters of green laser processing and conducting a comparative analysis of the microstructure and properties achieved through green laser processing and near-infrared laser processing. Fabrication of CuCrZr through the implementation of hybrid laser systems (blue/green laser + NIR laser).

2) The traditional volumetric energy density used in the optimization study of the process parameters still has large limitations because it does not take into account the characteristics of the material. There is an urgent need for a method that can comprehensively consider the material properties and LAM process parameters.

3) Currently, the ideal equilibrium between tensile strength and ductility has yet to be determined, and the amalgamated thermal and electrical characteristics remain unclear. Moreover, the mechanical, electrical, and thermal features have not been sufficiently and comprehensively explored.

4) The EB-PBF method has been used to produce an equiaxed copper alloy containing nickel, aluminum, and bronze (C63000). This alloy boasts isotropic mechanical properties and high levels of strength and elongation. There is potential in the future to produce CuCrZr alloys with an equiaxial grain structure.

Currently, the recognized optimal forming angle for selective laser melting (SLM) is 45°. When the forming angle of a sample is less than 45°, support structures are required for assisting the formation. Although the addition of support structures can effectively assist in the formation of parts, drawbacks, such as increased printing time, increased material consumption, increased difficulty in post-processing, and increased roughness of the support surfaces, are often observed. In some cases, for parts with internal cavities or complex channels, the inability to remove internal support can render SLM unsuitable for fabrication. Therefore, it is important to explore a forming method for samples with low forming angles in SLM to reduce printing costs and expand the applicability range.

First, the treatment process of a horizontal suspension sample is discussed. Different process combinations are used to form single- and double-layer overhanging structures to obtain a stable overhang process. Accordingly, an adaptive method of lower surface process area division based on the machining layer angle is proposed to adjust the size of the lower surface process area applied to the overhanging sample. The feasibility of the low angle forming method was verified by forming a 30° overhanging sample with different downward comparison layer numbers. Samples with different widths and thicknesses were formed using the same forming process, and the applicability of the forming method was verified.

No obvious overheating was observed on the surface of the single-layer overhang formed by the fusible process, and only a few micropores were present. The formation of a double-layer overhang effectively ensured bonding between the layers. When the layer comparison interval was opened, the sample was forced to interrupt printing at a 6 mm overhang length when T=1, 10, and 20, whereas the sample without layer comparison could be formed up to a 10 mm overhang length. When the layer interval T was further increased to 30, the effective forming length of the sample reached 12 mm. Finally, when T was greater than 40, stable formation of the designed height of the sample was achieved. When printing overhang samples of different sizes, it was found that increasing the thickness exacerbated the deformation of the sample but did not affect the realization of the designed height. This indicates that this method has a certain applicability in low-angle forming.

(1) In the horizontal overhang formation process, a high energy density caused serious spatter accumulation on the surface of the overhanging sample and destroyed the interlayer bonding under the action of the scraper. However, an energy density that is very low will lead to an insufficient weld lap or even failure to form. Using a fusible process, the sample can achieve an effective multilayer lap. (2) Under suitable conditions for the overhang process, the division of the forming region on the inner surface was a key factor affecting the formation of low-angle samples. For 30° forming, the overhanging area of the sample can resist the thermal stress deformation caused by the high-energy inner surface laser only when the layer comparison interval T is greater than or equal to 40, thus achieving low-angle printing. (3) The quality of the lower surface was mainly affected by powder bonding and sinking of the molten pool. In addition to powder sticking, the upper surface was mainly influenced by the staircase effect and the gap between the laser contour and the boundary of the melt pool. Additionally, the difference in the contact area with the powder was the main reason for the difference in powder sticking between the upper and lower surfaces. (4) When forming the low-angle samples, as the thickness of the sample increased, the deformation of the sample gradually intensified, whereas an increase in the width had almost no effect on the deformation of the low-angle samples.

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology that utilizes high-power lasers to melt and stack powdered materials for rapid prototyping. However, metals used in current lightweight and high-strength LPBF processes are extremely limited. In particular, in terms of aluminum alloys, LPBF-formed aluminum alloys are mainly focused on Al-Si alloys. The inherent un-weldability of 7050 aluminum alloy and the high thermal stress induced by rapid cooling during the additive manufacturing process cause cracking or deformation during LPBF, limiting the application of LPBF technology in the preparation of this type of alloys. This study investigates the LPBF formation of TiB₂/7050 composite and subsequent heat treatment (solution + aging treatment) process, exploring the effects of different process parameters and heat treatment conditions on the microstructures and room temperature tensile properties of TiB₂/7050 composite.

Orthogonal methods are employed to select laser process parameters, including laser power levels of 210, 225, 240, and 255 W. The scanning speed levels are set at 400, 450, 500, 550, and 600 mm/s, whereas hatch spacings are set at 75 μm and 90 μm. The scanning strategy involves a layer-by-layer rotation of 90° using the stripe scanning method. In total, there are 40 sets of process parameters. The heat treatment is conducted via solution treatment in an muffle furnace. After the samples are solubilized at 475 °C for 1 h, they are cooled in room temperature water and then aged at 120 °C for 12 h in a tubular resistance furnace before final cooling in ambient air. Tensile experiments are performed using a room temperature tensile device at a strain rate of 10^-4 s^-1. Tensile specimens with appropriate dimensions are prepared, and three samples in equivalent conditions are tested to obtain the average values. X-ray diffractometer is employed to analyze the phase composition of the samples. The scan angle range is set between 20° and 120° with a scan rate of 2 (°)/min. Microstructural characterization is performed using a scanning electron microscope (SEM), and the material surface elemental compositions are analyzed using an energy-dispersive spectrometer (EDS).

Under the following four sets of process parameters (laser power + scanning speed + scanning spacing), the relative densities of the printed sample are higher: 240 W+450 mm/s+75 μm (No.1), 240 W+500 mm/s+75 μm (No.2), 240 W+450 mm/s+90 μm (No.3), and 240 W+500 mm/s+90 μm (No.4). The obtained relative densities of TiB₂/7050 composite samples are approximately 98.3%, 98.7%, 98.5%, and 99.1%, respectively (Fig.2). The tensile experiments under these four sets of parameters are conducted at room temperature on the as-built and heat-treated samples. In the as-built state, the TiB₂/7050 composite exhibits the highest strength under the condition 240 W+450 mm/s+75 μm (No.1), with tensile strength, yield strength, and elongation being 286 MPa, 250 MPa, and 2.3%, respectively (Fig.4). After heat treatment, both the strength and plasticity of the material significantly improve for all four sets of parameters. In particular, under the as-built condition of 240 W+450 mm/s+75 μm (No.1), the material achieves tensile strength, yield strength, and elongation values of 351 MPa, 294 MPa, and 4.2%, respectively (Fig.4). Additionally, in the as-built state, the primary microstructure of the composite consists of α-Al phases and TiB₂ particles. After heat treatment, a significant number of secondary phases precipitate from the matrix, and SEM results show the presence of abundant precipitates in the forms of both particles and elongated phases (Fig.6).

The addition of TiB₂ particles significantly suppresses the cracking of 7050 alloys through the LPBF process and exhibits favorable formability. The optimal process parameters in the as-built state are 240 W+450 mm/s+75 μm, which result in the highest strength of the TiB₂/7050 composite. The tensile strength, yield strength, and elongation are 286 MPa, 250 MPa, and 2.3%, respectively. The microstructure of the TiB₂/7050 composite after heat treatment consists of fine-sized equiaxed grains. The grain size is concentrated in the range of 1?3 μm, with TiB₂ particles evenly dispersed in the grain boundaries and within the equiaxed grain structure, effectively promoting the formation of heterogeneously nucleated refined grains. After heat treatment, the tensile strength, yield strength, and elongation of the specimens of TiB₂/7050 composite are 351 MPa, 294 MPa, and 4.2%, respectively.

Ti-6Al-4V is a benchmark Ti alloy. Laser wire additive manufacturing (LWAM) offers advanced manufacturing capability to the alloy for applications possibly including exploration of outer space. As a typical multiple-variable process, LWAM is complex, which, however, can be analyzed, predicated or even optimized by artificial intelligence (AI) methods such as machine learning (ML). In this study, printing parameters of the Ti-6Al-4V is firstly optimized using single-track-single-layer experiments, and then single-track-multiple-layer samples are printed, whose properties in terms of hardness and compressive strength are analyzed subsequently by both experiments and ML. The two ML approaches, artificial neural network (ANN) and support vector machine (SVM), are employed to predict the experimental results, whose coefficients of determination R² show good values. Further optimized properties are realized by adopting genetic algorithm (GA) and simulated annealing (SA) approaches, which contribute to high mechanical properties achieved, for instance, an engineering compressive strength of about 1694 MPa. The results here indicate that important mechanical properties of the LWAM-prepared Ti alloys can be well predicted and enhanced using suitable ML approaches.

With the development of aerospace devices with a larger size, higher load bearing capacity, and longer life, lightweight structures with high stiffness have become basic requirements in the design and manufacture of aircraft, rockets, satellites, and other aerospace products. Carbon-fiber-reinforced composites combine the advantages of a carbon-fiber-reinforced phase and polymer-matrix phase, including a light weight, high specific strength, corrosion resistance, strong designability, and other outstanding advantages, which give them a wide range of application prospects in the aerospace field, showing great potential for development. However, in the processing and manufacture of aerospace equipment, a molding process is used to form carbon-fiber-reinforced polymer (CFRP) parts as a single piece, which makes it difficult to meet the part assembly needs. Thus, a large number of cutting, drilling, trimming, and other processes are required before the actual final assembly. At present, the most commonly used processing methods for carbon-fiber-reinforced composite materials include traditional machining, ultrasonic-vibration-assisted machining, water-jet machining, electric-discharge machining, and laser processing. However, the characteristics of CFRP, such as its heterogeneity, anisotropy, and lamination structure, make it prone to processing defects such as delamination, burrs, tearing, and heat-affected zones (HAZs) during processing, which greatly affect the load-bearing performances of CFRP parts and typically make CFRP a difficult material to process.

Laser processing is a flexible and controllable manufacturing method that eliminates the problems of tool wear and mechanical stress. It is expected to become an effective means of processing CFRP with little damage and high efficiency. However, CFRP components have different thermodynamic properties, and it is easy to produce a large HAZ during laser processing. It is necessary to comprehensively consider the change in the energy absorption of the material with temperature. Otherwise, the high-quality and high-precision processing of CFRP is difficult.

At present, research on the laser processing of CFRP at home and abroad focuses on the thermal damage problem represented by HAZs and the processing quality problem represented by the slit depth and width. The common processing methods can be divided into traditional laser processing, ultrafast laser processing, and water-guided laser processing. A traditional laser relies on the thermal effect to complete the ablation, melting, and removal of materials, which usually produces a 100-μm wide HAZ (Fig. 5). An ultrafast laser has the characteristics of an ultra-short pulse width, an ultra-high instantaneous power density, and nonlinear processing, which can effectively control the HAZ and improve the processing accuracy (Fig. 6). A water-guided laser uses a water jet to homogenize the laser light field and remove the debris, which can improve the processing quality and efficiency of CFRP processing. The increase in the processing requirements in the field of aerospace strategic planning has revealed the many advantages of laser processing, including its use in drilling/cutting, surface treatment, and welding. However, the interaction mechanism between the laser and CFRP material is complex. Researchers mainly analyze the relationship between the energy transfer and temperature rise response of different components of the material during the interaction between a continuous or traditional pulsed laser and the CFRP at the macro level, or qualitatively analyze the phase of the interaction between the laser and material based on experimental phenomena (Fig. 11). The application status of carbon-fiber-reinforced composites in the aerospace field is reviewed, with a focus on CFRP laser-processing technology.

This study reviews the research progress of various CFRP processing methods, compares and analyzes their advantages and disadvantages; introduces the current research status of CFRP laser processing from the perspectives of methods, processes, and mechanisms; summarizes the application of CFRP in the aerospace field; analyzes and discusses the remaining challenges facing CFRP laser processing; and provides the corresponding prospects. Compared with other processing methods, laser processing, especially ultrafine laser processing, can achieve non-contact "cold" processing, reduce heat accumulation, improve the processing accuracy, and is expected to become an effective means to improve the quality of CFRP processing. However, the mechanism of the thermal damage produced by CFRP laser processing is still unclear, and the nonlinear, unbalanced, and heterogeneous energy transmission process during CFRP laser processing is not well understood. In order to further improve the processing quality and efficiency of CFRP laser processing, more in-depth exploration and research are required for ultrafast laser processing technology and water-guided laser processing technology. In view of the higher micro-machining requirements of aerospace CFRP components, further research is needed on the micro-mechanism of the interaction between the laser and heterogeneous CFRP materials, and new methods and new processes need to be further developed. These efforts are expected to further improve the processing quality, accuracy, and efficiency of CFRP laser processing.

The development of new energy vehicles is the effective way for China to transition from a large automobile country to an automobile power; this is also a strategic initiative to address climate change and promote green development. As important materials for manufacturing new energy vehicles, the excellent mechanical properties of automotive aluminum alloys have brought new challenges during the subsequent welding processes. The original fine grains and nanoprecipitates of the automotive aluminum alloy are destroyed by the welding heat source, producing noticeable softening in the weld seam and heat-affected zone. Consequently, the mechanical properties of traditional arc-welded joints are inferior to the requirements of industrialization. High-quality and efficient welding of automobile aluminum alloys is a development trend, and traditional welding processes struggle to satisfy this demand.

The high-power-density fusion welding process, that is, the laser welding process with low heat input, is used to shorten the softening zone and reduce the adverse effects of the heat source on the substrate plates. Moreover, it also significantly improves welding efficiency because of its high power density. However, as a high inversion material, the laser absorption of aluminum alloys is less than 5%, leading to enormous energy wastage and danger. Meanwhile, the fine spot (diameter of 0.2‒0.6 mm) indicates the poor bridging ability of the laser welding process, and the burning loss produces a weld seam with a poor forming quality and high softening degree. Thus, a laser-arc hybrid welding process is used to weld the aluminum alloy. In the laser-arc hybrid welding process, the laser beam and arc interact in a common weld pool, and their synergic effect increases laser absorption and bridging ability. Moreover, wire filling compensates for the burning loss, thereby improving the forming quality and mechanical properties of the weld seam. At the beginning of the 21st century, a laser-arc hybrid welding process was applied to car structure manufacturing. Since then, researchers from China, Italy, and Canada have focused on regulating the microstructure and mechanical properties of laser-arc hybrid welded joints. Owing to the substantial reduction in the price of laser machines, laser-arc hybrid welding of aluminum alloys has received more attention in the past three years.

Automobile aluminum alloys are divided into two categories: heat-treatable aluminum alloys [Al-Mg-Si(Cu) and Al-Zn-Mg(-Cu)] and non-heat-treatable aluminum alloys [Al-Mg(-Mn)]. Considering their characteristics, typical welding defects such as softening, pores, and hot cracking are summarized in Fig. 4, and the softening mechanisms of the heat-affected zone and weld seam are analyzed and summarized in Figs. 5 and 6. The regulatory mechanisms of welded joints using the laser-cold metal transfer (CMT) hybrid welding process and welding seam alloying (solute elements, refined grain elements, and nanometallic intermetallic compounds with high melting points) are summarized in Figs. 8 and 9. The formation mechanism of typical pores in laser-arc hybrid weld is summarized in Fig. 12. To suppress keyhole-induced pores, a scanning laser-arc hybrid welding process is used to improve keyhole stability. The suppression mechanism is illustrated in Fig. 14. Other methods for suppressing keyhole collapse including process parameter optimization, droplet transfer behavior regulation, and external field assistance are also analyzed in this study. Finally, this study summarizes the current problems in the laser-arc hybrid welding process of aluminum alloys and proposes future development trends.

The laser-arc hybrid welding process has many advantages in suppressing welding defects and improving the welding coefficient of automobile aluminum alloys. Based on the softening mechanism, the softening degree of the welded joints is effectively decreased by innovation in the welding process and optimization of the alloying elements. However, commercial welding wires cannot be used for fabricating weld seams with excellent mechanical properties. Enhancing weld seams by welding seam alloying requires extensive research and exploration. Meanwhile, flux-cored welding wires that can be flexibly composition-designed and short-produced are more suitable for the development of new ground filling materials. Recently, new hybrid laser sources such as hybrid diode-fiber lasers, hybrid blue-light diode-fiber lasers, and core-diameter ring light spots have been used to improve the weld formation of automobile aluminum alloys. Thus, the effect of new laser technology on welding defects, particularly pores in laser-arc weld seams, still requires extensive research and exploration.

Laser manufacturing technology is an efficient manufacturing approach with high precision, high efficiency, low energy consumption, and low cost. The sustained and rapid development of laser manufacturing technology has provided significant opportunities for the industry. To improve the manufacturing quality, laser hybrid manufacturing technology has received significant attention. Among these, ultrasound-assisted laser manufacturing has gradually become a research hotspot worldwide.

The ultrasonic energy field has both volume and surface effects, and can achieve stress superposition, shock waves, and acoustic softening in solid materials to optimize and control their mechanical properties. Ultrasonic vibration can also affect the molten pool flow and solidification behavior of semisolid/liquid materials through cavitation and flow effects and promote a uniform distribution of elements and grain refinement. The mechanical effect of ultrasound promotes the slag emission and reduces the shielding of the laser beam, leading to improvement the quality and efficiency of laser ablation. Therefore, ultrasonic vibrations play a significant role in laser manufacturing processes.

Among various hybrid processes, three different ultrasonic application modes were adopted: fixed-contact, mobile-contact, and non-contact modes (Fig.1). Applying ultrasonic vibration in fixed-contact mode leads to continuous and stable transmission of ultrasound with less energy dissipation but has significant limitations on workpiece shape and size. In the mobile-contact mode, the acoustic energy can be transmitted well at the interface and is less shape-restricted; however, there may be a disconnected contact phenomenon between the ultrasonic head and the workpiece. In the noncontact mode, the process is completely unaffected by the workpiece shape; however, there is significant energy attenuation when the acoustic wave is transmitted in a gas or liquid medium.

In this study, the mechanisms and effects of ultrasonic vibration on laser processing are reviewed based on a summary of the latest research progress. Ultrasonic-assisted laser manufacturing technology with various ultrasonic application modes was comprehensively discussed for laser additive manufacturing, laser formative manufacturing, and laser subtractive manufacturing. The principles and technical characteristics of each hybrid manufacturing technology are discussed, and the influence of ultrasonic vibration on the laser manufacturing process is summarized (Table 1).

In laser additive manufacturing, laser energy deposition technology, synchronously assisted by ultrasonic vibration, is widely used for surface modification, additive repair, and coating preparation. The application of ultrasound inhibits the generation of columnar crystals, resulting in a reduction in the microstructural anisotropy, pores, inclusions, and microcracks. In addition, laser powder bed melting synchronously assisted by ultrasonic vibration has been used in the rapid manufacturing of complex components. The influence of ultrasound on the melting process improved the comprehensive mechanical properties of the parts and reduced the anisotropy in laser powder bed melting. Laser additive manufacturing combined with ultrasonic impact peening can improve the properties of additive manufactured parts by conducting a post-treatment of the ultrasonic impact on the surface of the parts after laser additive manufacturing. The combined effects of grain refinement strengthening and dislocation strengthening result in deep strengthening, defect suppression, and shape and performance control.

For laser formative manufacturing, ultrasonic-assisted laser welding inhibits the defects caused by sudden heating and cooling by applying ultrasonic vibration to the welding pool and then regulates the welding microstructures to achieve high-quality welding. Laser impact combined with ultrasonic impact peening triggers a high-frequency impact on the surface of a material, which has the advantages of both ultrasonic and laser impacts. The microstructure and surface residual stress can be effectively regulated, and the surface accuracy and mechanical properties of the metallic materials can be improved. Laser quenching with ultrasonic impact can significantly improve the mechanical properties of the reinforced layer owing to the multicycle characteristics and excellent control ability of the surface structure and strain state.

In laser subtractive manufacturing, ultrasound increases the plastic flow capacity of a material in ultrasonic-assisted laser ablation technology. The surface melt discharge and surface evaporation of the material are considerably promoted, resulting in an improvement in the surface quality of the removal area and the quality of the hole. In ultrasound-assisted laser ablation for nanoparticle preparation, the ultrasound in the liquid causes cavitation bubbles to form and collapse repeatedly. Additional ablation of the nanoparticles induced by ultrasound enhances the density of the nanoparticles in the liquid and improves the synthesis rate. In ultrasound-assisted laser polishing technology, ultrasound can reduce the bonding tendency between particles and the material surface, leading to a reduction in surface oxidation, an increase in the material removal rate, and a reduction in surface polishing roughness. In ultrasonic-assisted laser cleaning, ultrasonic vibration not only makes the surface easier to clean, but also suppresses the defects induced by high temperatures, significantly improving the processing efficiency and surface cleaning quality.

Ultrasonic-assisted laser manufacturing has gradually become a popular approach for fabricating various structures. A developmental trend in the ultrasonic-assisted laser manufacturing technology is expected. Further fundamental studies on hybrid mechanisms will be conducted to understand complex hybrid manufacturing processes. Broadening the diversity of materials and process applicability will expand their application areas. An innovative design for the ultrasonic application mode and equipment will be developed to improve the integration. In addition, creative laser manufacturing technologies can generate new hybrid manufacturing processes through the development of new light sources, thereby providing significant support for manufacturing innovation and application expansion.

In recent years, notable progress has been made in the development of equipment components aimed at precision and miniaturization. These miniature components typically exhibit complex geometries. They are composed of diverse materials. Further miniaturization of these components has led to an increased demand for precision welding. Consequently, the assembly of small parts and the packaging of devices require increasingly high levels of connection accuracy and quality control. High-quality micro-welding technologies for metallic materials have important applications in aerospace, power batteries, biomedicine, and other fields. For instance, micro electro mechanical systems (MEMS), characterized by feature sizes ranging from 1 μm to 1 mm, are commonly packaged using micro-welding technology. Moreover, power battery electrode foils, with thicknesses as low as 6?12 μm, require precise connections for current export. Furthermore, the assembly of components and metal shell sealing in implantable biomedical devices rely heavily on micro-welding technology.

Common micro-welding techniques include resistance micro spot welding, ultrasonic micro-welding, micro tungsten inert gas(TIG) welding, and laser micro-welding. Compared with conventional micro-welding methods, laser micro-welding offers several advantages, including a small focusing spot size, precise heat input control capability, high welding speed, and compatibility with various weldable materials.

This study investigates the laser micro-welding technology of metal materials, providing a comprehensive analysis of its significance, microscale effects, welding modes, laser selection, and defect and quality control measures. It is difficult to reach a consensus on a precise definition of laser micro-welding. The connotations of laser micro-welding are comprehensively summarized based on previously reported studies. Strictly speaking, laser micro-welding pertains to a laser welding process where at least one feature size of the connected material or weld is less than 100 μm. Laser micro-welding involves two welding modes: conduction and penetration welding. In laser micro-welding, oxidation promotes fluctuations in the penetration-welding process, resulting in a transient phase. Subsequently, the influence of microscale effects is introduced. When workpiece dimensions are reduced to the micron scale, typical microscale effects occur. The physical characteristics observed during laser micro-welding, such as heat transfer and molten pool flow, differ from those observed during macro-welding (Fig.2). Based on microscale effects, the defects and quality control measures in laser micro-welding are summarized according to the process parameters. Welding defects such as lack of penetration, burn-through, spatter, humping, porosity, and cracking can occur during the laser micro-welding process, and optimization of the welding process parameters is an important means of controlling weld formation and welding defects. These parameters include the laser wavelength, laser power, spot diameter of the laser, pulse laser parameters, welding speed, and scanning path.

Furthermore, the applications of laser micro-welding to both similar and dissimilar metal materials are reviewed. Laser micro-welding is used to join precision components in the electronics, automotive, aerospace, and medical industries (Fig.12). Notable applications include pressure sensors, bipolar plates for fuel cells, aerospace engine blades, electronic component pins, copper-printed circuit boards, satellite collimator components, cardiac pacemakers, and lithium-ion battery tabs.

Finally, the challenges and future development directions of laser micro-welding technology for metallic materials are summarized, including the welding mechanism of metal and non-metallic materials, new process technology, and laser micro-welding systems.

The characteristics of laser micro-welding are complex owing to microscale effects. Although laser micro-welding has been widely used for connecting metal materials, some challenges remain. First, there is a burgeoning demand for the joining of dissimilar materials, including the micro-welding of dissimilar metals and metal/non-metallic materials. Dissimilar materials with different physical properties pose significant challenges in welding. Second, increasing the welding speed is important for improving the production rate. However, humping occurs at high welding speeds. To address this, process innovation and the recombination of multiple energy fields are required to further increase the critical speed of humping by controlling the flow characteristics of the molten pool and the solidification process during micro-welding. This is essential for improving the production rates and ensuring the product yield in high-speed welding. Finally, the development of intelligent laser micro-welding systems is a key future trend. The use of an intelligent laser micro-welding system has the potential to improve weld quality and welding efficiency.

The primary objective of this study is to transform the status quo of laser-welding defect detection. By developing an online deep learning system, this study aims to enable the identification and measurement of surface defects in laser-welded aluminum-alloy sheets with high precision and efficiency. The specific focus is on two prevalent defects: undercuts, characterized by the insufficient melting of the base material at the weld toe, and sagging, which is the undesirable downward displacement of the material along the weld seam. The use of high-density point cloud data is key to overcoming the limitations of traditional defect detection methods and enhancing the adaptability of the system to diverse welding conditions.

A binocular-structured light sensor capable of capturing detailed point cloud data of defects in laser-welded samples is used. This sensor is strategically positioned to cover the entire welding area,which ensures the collection of comprehensive defect data. The acquired point cloud data undergo meticulous preprocessing to eliminate noise and artifacts, resulting in a clean and informative dataset. The dataset serves as the foundation for training the faster region-based convolutional neural network (Faster R-CNN) model, a deep learning architecture renowned for its object detection capabilities. The Faster R-CNN model is augmented with an area recommendation network, a critical addition to improve defect localization precision. The training process involves subjecting the model to various defect scenarios to ensure its adaptability to various welding conditions and defect types.

The trained Faster R-CNN model exhibits an outstanding recognition precision rate of 93% when is tested on high-density point cloud data. This significant improvement compared to that of the model trained on images from a traditional two-dimensional vision sensor demonstrates the efficiency of leveraging point cloud data in defect detection. The ability of the Faster R-CNN model to recognize and locate defect positions is essential for swift, accurate, real-time online detection during laser welding. A noteworthy finding of the study is the significant increase in the maximum welding speed allowed by the developed inspection system for online detection. The system demonstrates a maximum speed of 316.87 mm/s, a considerable advancement beyond typical laser-welding speeds. This achievement not only showcases the potential for high-speed online detection without compromising precision but also underscores the transformative impact of the developed system on industrial practices. The discussions extend beyond the principal results, exploring the implications of the system performance in various laser welding scenarios. Variations in the material thickness, welding parameters, and defect types are systematically analyzed to assess the robustness of the proposed model. The results show the adaptability of the model to different welding conditions, highlighting its versatility in practical applications. The robustness test also provides insights into potential optimizations and improvements, setting the stage for future developments in laser-welding defect detection. The study emphasizes the significance of defect localization in achieving precise measurements. The integration of an area recommendation network with the Faster R-CNN model significantly contributes to improved defect localization, a critical factor for enhancing defect measurement accuracy. This aspect of the model design is examined in detail, clarifying the mechanisms that contribute to its superior performance in defect detection.

The developed online detection system, powered by the Faster R-CNN model and high-density point cloud data, achieves a recognition precision rate of 93%. This demonstrates a substantial advancement in defect detection. By effectively addressing the challenges of classifying and measuring surface defects in laser welding, the system is established as a transformative technology with far-reaching implications in the manufacturing industry. The integration of high-density point cloud data provides rich information that enhances the efficiency and accuracy of defect detection. This breakthrough not only mitigates the limitations of traditional two-dimensional vision sensors but also positions the system as a pioneering solution for high-speed online detection in laser-welding processes. The study opens new avenues for research and development in smart manufacturing, paving the way for the integration of advanced technologies in industrial applications.

Topological photonics has been a rapidly growing field over a decade. Photonic topological insulators (PTIs) have wave-vector space topologies that lead to unique surface states of light. PTI edge states are immune to structural defects and disorders, and thus, can be used for the robust manipulation of light and topological lasing. Topological photonics also provides a powerful platform for experiments with topological concepts developed for condensed matter phenomena, such as the quantum Hall effect and quantum spin Hall effect. Because of fundamental differences between electrons and photons, the field has developed new fundamental topological ideas for diverse photonic platforms, such as photonic crystals, coupled ring resonators, photorefractive crystals, and ultrafast-laser direct-writing (ULDW) waveguides. ULDW enables the three-dimensional (3D) fabrication of integrated optical circuit chips for a wide range of applications, such as optical communications, data storage, sensing, topological physics, and quantum computing. Ultrafast lasers enable nonlinear absorption and material structure changes, which induce permanent refractive index changes inside transparent materials, such as glass. Compared with two-dimensional photonic crystal fabrication via planar lithography, continuous ULDW can form an arbitrary 3D waveguide geometry with high precision and speed.

The paraxial propagation equation in ULDW waveguide arrays is analogous to the Schr?dinger equation. In the past decade, various topological phenomena, including chiral edge states, higher-order topological insulators, anomalous Floquet photonic insulators, non-Hermitian topology, nonlinear topology, and nonabelian topology, have been demonstrated in ULDW waveguide arrays. ULDW-enabled topological photonic devices have applications in intrachip optical networks, optical computing, and quantum information processing, and have the potential to outperform their electronic counterparts in communication, energy consumption, and computation speed bottlenecks. Conventional photonic circuits are prone to fabrication errors, which limit their performance. Quantum computing is highly sensitive to system noise and errors. Topological optical chips fabricated using ULDW have proven to be robust against device defects and can maintain quantum entanglement.

We review photonic topological insulators engineered using ULDW in recent years and their underlying topological phases. First, we briefly discuss the background and mechanism of the ULDW waveguide and several techniques to improve the waveguiding performance, such as insertion loss and propagation loss (Fig.1). Next, we introduce the paradigmatic Su-Schrieffer-Heeger (SSH) model (Fig.2) and topologically invariant Zak phases to distinguish between the nontrivial and trivial topological phases of the SSH model. We discuss the experimental implementation of the SSH model and other various one-dimensional static topological insulators that exhibit topological edge localization (Fig.3). An adiabatic quantized Thouless pump can be achieved by slow deformation of the off-diagonal Aubry-André-Harper (AAH) model and the lopsided Rice-Mele model (Fig.4). Different experimental observations have shown that the edge of the topological edge mode in a honeycomb waveguide lattice is influenced by edge type, strained deformation, and transverse momentum (Fig.5). We summarize different high-order topological insulators (HOTIs), such as the two-dimensional SSH model, Kagome model, honeycomb with Kekulé distortion, and disclination array, demonstrating their topological corner states and disclination states (Fig.6). Floquet topological insulators can break the time-reversal symmetry via periodic z-axis (effective time axis) modulation of the two-dimensional geometry to induce a nonzero Chern number or winding number of the Floquet energy band and enable topological chiral edge modes on the geometry surface (Fig.7). Several schemes have been proposed, including helical waveguides, curved waveguides, and index modulation. Mukherjee and Maczewsky discovered an anomalous Floquet topological insulator with a zero Chern number but a nonzero winding number. Non-Hermitian (Fig.8) and nonlinear (Fig.9) topologies are beyond conventional topological concepts. Parity-time symmetry breaking phenomena in different non-Hermitian systems have been investigated using waveguide wiggling, inserted scatter points, and breaking points. Cerjan demonstrated a Weyl exceptional ring using a bipartite non-Hermitian optical helical waveguide array. Researchers have experimentally achieved nonlinearity-induced and tunable topological solitons in HOTIs, disclination-defect states, and off-diagonal AAH arrays. Jürgensen observed a fractional Thouless pump in off-diagonal AAH arrays via nonlinear tuning. Several research groups have experimentally investigated the ability of ULDW PTIs, including SSH arrays, off-diagonal AAH arrays, HOTIs, and fractal anomalous photonic insulators, to topologically protect photonic quantum entanglement via Hong-Ou-Mandel interference and quantum cross-correlation measurements (Fig.10). Nonabelian braiding, as a promising quantum computing tool, has been proposed and achieved using two-mode braiding (utilizing the geometry phases of arrays) and a Thouless pump (Fig.11).

ULDW has advantages, such as robustness, high nanofabrication precision, rapid prototyping, and 3D fabrication capabilities. The ULDW has proven to be a versatile platform for realizing various types of novel photonic topological insulators and exploring emergent topological phenomenon, such as high-dimensional, non-Hermitian, nonlinear, and nonabelian topologies. Those beyond the conventional PTIs are unclear and require further experimental and theoretical investigations. Compared with other materials, glass waveguides have disadvantages in terms of their electro-optic (EO) modulation capabilities. Glass waveguides can be thermally modulated, which is slow and power-consuming. Laser-directed wiring in other materials with improved electro-optic properties, such as lithium niobate, or integrating glass waveguides with EO modulators, may be potential solutions. More innovative ULDW PTIs designs are required to further reduce the influence of noise and protect the quantum states in quantum information applications.

Femtosecond (fs) laser is an emerging technology with immense potential for precision processing, addressing limitations that constrain conventional laser-based techniques, such as low spatial resolution, uncontrollable thermal effects, and induced mechanical stress. Femtosecond lasers enable the precise processing of micro-nanostructures, including the drilling of micro-holes, fabrication of photonic crystals, construction of nano-/micro-devices, and applications in biomedicine. Laser ablation in liquids operates in intricate environments, with the liquid phase playing multiple crucial roles, including stress mitigation, enhancement of manufacturing precision, material removal, and prevention of material redeposition. A liquid environment also results in the creation of a transient high-pressure microenvironment inside gas bubbles formed by ultrafast, high-intensity laser pulses, facilitating the production of metastable material phases such as diamond, which often requires a physicochemical environment deviating from thermodynamic equilibrium.

In the context of the rapidly emerging nanotechnology society, laser-based techniques are gaining momentum in various industrial sectors, including electronics, drug delivery, and energy storage. Conventional wet chemistry methods have limitations, such as contamination, material deactivation, and difficulties in the fabrication of metastable phases. Laser-based synthesis and material processing offer a flexible and powerful approach to micro/nanofabrication, addressing challenges that limit the applicability of conventional techniques in manufacturing and contributing to the growth of nanotechnology in various applications.

Despite the immense potential of femtosecond laser ablation in liquids for nanomaterial synthesis, the intricacies of the process, intertwined with the physical and chemical reactions taking place hand-in-hand, present challenges in mechanistic understanding and controllable fabrication. A comprehensive understanding of the advantages and complexities of femtosecond laser-liquid media interactions requires advanced high spatial and temporal microscopy/spectroscopy. This review offers a critical overview of ultrafast physicochemical phenomena in laser-induced liquid-phase ablation, where the established reaction pathways and mechanisms governing the formation of micro/nanostructures are outlined and cataloged. Meanwhile, by laying out the current status of femtosecond laser liquid ablation, the limitations and future directions of the field are also discussed, leading to insights into promising future directions and significance for a broader scientific community.

Femtosecond laser ablation in a liquid covers a wide range of temporal and spatial scales and involves complex physical and chemical events. Figure 1 qualitatively illustrates the distribution of various processes, including laser propagation, focusing, and the generation of nanostructures in cavitation bubbles, where the timescales range from milliseconds to femtoseconds. Various techniques, such as time-resolved spectroscopy, shadow imaging, interference methods, and holographic detection, are used to capture ultrafast events occurring upon light-matter interactions (Table 1). To better elucidate the mechanistic details, the femtosecond laser ablation probe has shifted towards higher temporal and spatial resolutions, multi-angle observations, and continuous/single-shot probing techniques (Fig. 2). These advancements have enabled a deeper understanding of the micro/nano fabrication process, leading to improved controllability and large-scale production.

Research in transient observation of femtosecond laser ablation is crucial for understanding and controlling rapid physical and chemical dynamics during manufacturing. These studies reveal how materials interact and evolve over different time scales, offering insights into the optimization of femtosecond laser ablation products. However, such research in liquid environments is often tool-driven, with challenges posed by the liquid phase environment for high temporal and spatial resolution measurements and the characterization of complex physicochemical processes. To provide an in-depth analysis using transient observation techniques, the current study divides femtosecond laser liquid ablation into four stages based on time scales: generation and evolution of laser filament (Fig. 3), generation and evolution of solvated electrons (Fig. 4), generation and evolution of plasma (Fig. 5), and generation and evolution of cavitation bubbles (Fig. 6). These stages are underpinned by recent ultrafast studies that reveal the optical, physical, and chemical mechanisms underlying femtosecond laser ablation. Note that these processes are not mutually exclusive, with interactions and transformations occurring across both time and space alongside other concurrent physical and chemical events. Research on ultrafast laser ablation in liquid offers richer information regarding the physicochemical details and propels the controllability and progress of precision manufacturing in micro/nano science.

While femtosecond laser liquid ablation technology shows great promise in micro/nano-manufacturing, photonics, and biomedicine, understanding its intricate mechanisms is crucial for its wide application and mass production. For example, a knowledge gap persists between the early nanoparticle generation and cavitation bubble stages in the theoretical study of femtosecond-laser-induced nanostructure fabrication. Owing to the complexity of the system involved, quantitative models predicting the outcome of laser ablation are scarce, and the exploration of microstructural manufacturing mechanisms remains limited. Advanced time-resolved characterization techniques with the following capabilities are indispensable to track the evolution of physicochemical properties during femtosecond laser liquid ablation and represent future trends.

Because of the ultrafast nature of femtosecond laser liquid ablation at the micro/nanoscale, characterization techniques must offer high temporal and spatial resolutions with reasonable signal-to-noise ratios. An enhanced time resolution can reveal fundamental aspects within femtoseconds of laser excitation, whereas an improved spatial resolution can provide more accurate surface information. Owing to the limited time and space requirements of the probe, a high signal-to-noise ratio is essential for the effective capture of transient events.

Traditional techniques, which are largely pump-probe-based, presuppose consistent sample attributes before and after detection. However, in liquid-phase ablation, the fluid dynamics and external factors can disrupt data collection. Innovations such as ultrafast continuous imaging address these challenges and collect data across all delays from one laser pulse. Such progress, previously observed in femtosecond laser ablation in air, has now been incorporated into the study of liquid-phase ablation, paving the way for the real-time monitoring of femtosecond laser fabrication processes.

In the realm of multidimensional information extraction, current methods largely rely on femtosecond laser pump detection. This photon input-output system typically yields data in spectroscopy or imaging formats, contingent on materials exhibiting an optical response. However, emerging characterization tools, including photon-, X-ray-, and electron-based instruments, are unlocking the potential of techniques such as time-resolved X-rays and energy spectroscopy. These results provide real-time insights into the atomic details, valence states, and configurations during femtosecond-laser-driven liquid-phase ablation.

This review delineates the principal stages of femtosecond laser ablation in liquids and presents a comprehensive model framework. However, it is imperative to recognize persistent ambiguities within this domain. Attention is now directed towards the promise of transient observation techniques for forthcoming developments in femtosecond laser ablation in liquids. These methodologies offer profound insights that can drive future progress in this field.

With the increasing demands of internet-of-things and big data, flexible electronics have become a key technology. Among them, flexible micro-nano sensors, as an important part of flexible electronics, have revolutionized the physical form of conventional rigid devices. It has significantly facilitated the interconnections among human beings, machines, and environment, serving as a vital role for the advancement of intelligent electronics. For instance, embedding flexible micro-nano sensors in smart skins renders sensitive detection of external pressure and deformation, making them applicable in robotics and bionic hands. In biosensing applications, these sensors facilitate real-time monitoring of the internal micro-environment, drug release, and cellular activities. When applied in hand motion tracking and tactile feedback, they contribute to enhancing virtual reality interactions. It is envisioned that the emerging development of flexible micro-nano sensors will enable a new era of transformation within the industry.

Currently, a variety of commercial micro-nano manufacturing methods have been widely applied in micro-nano devices, including physical/chemical vapor deposition, photolithography, and nano-imprinting. However, the fabrication of multifunctional flexible micro-nano sensors often involves the combination of multiple manufacturing methods to achieve various tasks like sensitive material deposition, patterning, as well as generation of micro-nano structures. With the evolution of flexible micro-nano sensors towards miniaturization, integration, intelligence, and customization, the higher technical requirements are posed for the efficient multifunctional preparation of sensitive materials and the controllable fabrication of micro-nano structures.

Among various technologies available, hybrid laser fabrication based on laser additive, formative, and subtractive manufacturing, along with their combined processing modes, meets the heterogeneous requirements of multifunctional flexible micro-nano sensors in terms of multiple scales, dimensions, and materials. Leveraging its rich reaction mechanisms, flexible and controllable regulation, high-precision processing, and multi-material compatibility, it breaks through the limitations of traditional manufacturing technologies in multitasking, multithreading, and multifunctional combined processing. Based on laser and matter interactions, hybrid laser fabrication realizes cross-scale shape control and property control, which opens up a new path towards the integrated structure-material-function manufacturing of various flexible micro-nano sensors.

Several published review articles on “laser fabrication of flexible micro-nano sensors” are available, which mainly elaborate on laser synthesis of micro-nano materials and laser processing of micro-nano structures. This paper focuses on hybrid laser fabrication and discusses its application strategies in the realization of flexible micro-nano sensors from a global perspective. It sequentially introduces three laser-based manufacturing methods, including additive, formative, and subtractive fabrication. The processing mechanisms and typical target materials involved are discussed. The paper highlights the technical advantages and applications of hybrid laser fabrication in flexible micro-nano sensors (Fig.1).

In particular, based on different processing strategies of hybrid laser fabrication: (1) Laser additive manufacturing utilizes laser as a localized energy source to heat and melt nano-precursors, which accumulates layer by layer after sintering to form functional structures. Examples include laser reduction sintering of metal or metal oxide nano-inks such as copper, silver, and nickel (Fig.2). (2) Laser formative manufacturing refers to laser-induced interfacial reactions used to change the physical or chemical properties of materials, while almost maintaining their initial volume. This is typically used to regulate the conductivity or functionalize the properties of polymers (Fig.3). (3) Laser subtractive manufacturing mainly achieves pyrolysis, ablation, patterning, and micro-nano texturing of materials. For example, it is applied to enhance the performance of pressure sensors and fabricate high-resolution interconnect circuits (Fig.4). Rationally combining these three laser processing strategies makes it possible to deposit nano-materials, physicochemically modify sensitive media, and pattern and precisely form micro-nano structures in multifunctional devices. This makes it feasible for the multifunctional integration and versatile manufacturing of flexible micro-nano sensors, making it a potential alternative to traditional manufacturing methods.

Subsequently, this paper discusses some typical applications of hybrid laser fabrication in flexible physical, chemical, and electrophysiological sensors in recent years, as well as flexible multi-modal sensor systems. It comprehensively demonstrates a wide range of multifunctional applications in the fields of wearable healthcare, human-machine interactions, and environmental monitoring.

Hybrid laser fabrication involves multiple disciplines such as optical engineering, materials science, and mechanical manufacturing. Utilizing multi-pulse and multi-wavelength laser manufacturing systems, it judiciously integrates the characteristics of additive, formative, and subtractive processing, allowing efficient deposition of sensitive materials, modification of material properties, and precise preparation of micro-nano structures. This integration provides a robust solution for realizing high-performance flexible micro-nano sensors, overcoming the technical challenges of traditional methods. Nevertheless, there are still a couple of challenges to address for practical applications. First, the further improvement of precision is restricted by the optical diffraction limit, which hinders the manufacturing of high-density and highly integrated devices. Second, traditional laser processing mainly works through the point scanning mode, and the preparation of complex structures usually takes several hours or even longer, which greatly reduces the yield and cannot meet the needs of large-area or mass-processing. In addition, the diversity of target materials has posed a challenge for hybrid laser fabrication in relation to multimodal processing. Finally, in terms of device applications, overcoming signal crosstalk among different sensing units is a key issue in the design and fabrication of devices, given the potential of hybrid laser fabrication for the integrated processing of multifunctional sensors. In conclusion, hybrid laser fabrication is envisioned to accelerate the innovation of flexible micro-nano sensors and expand the application scenarios of laser processing.

Surface-enhanced Raman spectroscopy (SERS) harnesses metallic nanostructures combined with optical fields to create localized surface plasmon resonance (LSPR), yielding significant Raman scattering enhancement. However, 'top-down' manufacturing methods for SERS substrates are often costly due to complex fabrication processes. Photochemical reduction synthesis, known for its high chemical purity and good process controllability, has gained attention but typically requires a high-power laser for rapid preparation. With advancements in high-power femtosecond-pulsed lasers, laser direct-writing has become viable for single-step SERS substrate fabrication. Nevertheless, the small focal laser spot limits efficiency in large-area fabrication of patterned micro-nanostructures. Dielectric microspheres, with their ability to focus incident lasers at their bottom beyond the diffraction limit, offer a solution for parallel nanomanufacturing. This study developed a one-step photochemical reduction technique for hierarchical silver micro-nanostructures using a dielectric microsphere array, demonstrating its ultra-sensitive Raman detection capability.

A polydimethylsiloxane (PDMS) film was prepared by mixing PDMS with a curing agent and then spin-cured. Barium titanate microspheres, with high refractive indices (1.9), were pressed into a monolayer close-packed array on the PDMS film via mechanical grinding. An uncured PDMS film was placed onto this array, transferring and semi-embedding the microspheres into PDMS (PDMS/MS), followed by curing. The PDMS/MS film was then covered with a silver nitrate and trisodium citrate solution. A 532 nm line CW laser, with power ranging from 49?168 μW, focused by PDMS/MS into the solution, induced the reduction reaction. Consequently, Ag⁺ was reduced under focused laser irradiation, forming a hierarchical silver micro-nanostructure (AgNPs/AgMRs) at the bottom of the microspheres. The surface morphology of the Ag micro-nanostructures was examined using SEM. The influence of microsphere diameter and photoreduction parameters on the morphology was both theoretically and experimentally investigated. Raman spectra of various analytes at different concentrations were acquired to optimize the hierarchical AgNP/AgMR/MS structure, with COMSOL simulations revealing the Raman enhancement mechanisms.

The study explored how microsphere diameters affect the hierarchical Ag micro-nanostructure. With a diameter increase to 21 μm, AgMRs with three concentric circles were formed. Increasing the diameter further to 39 μm resulted in the focused light energy at the microsphere bottom falling below the photochemical reduction threshold, leading to the formation of only a small amount of AgNPs (Fig. 2). Optimal photochemical reduction parameters were experimentally determined: a 1∶4 molar concentration ratio of silver nitrate to trisodium citrate, a 98 μW laser power, and an 80 s irradiation time produced clear AgMRs with high-density AgNPs at the microsphere bottom (Fig. 3). This configuration achieved a detection limit of 10^-14 mol/L for methylene blue solution and an enhancement factor (ζ) of up to 9.50×10⁹. The SERS structure exhibited good reproducibility and compatibility for practical applications, as shown in Fig. 4. Furthermore, an enhancement factor of 9.53×10⁹ for the hierarchical AgNP/AgMR/MS structure was obtained through numerical simulation (Fig. 6), aligning well with experimental results. The Raman enhancement channels were attributed to electromagnetic enhancement from microsphere nanofocusing, localized surface plasmon resonances in AgNPs/AgMRs, and the directional antenna effect of the AgNPs/AgMRs/MS hybrid structure.

This study proposes a new technique for fabricating hierarchical metal micro-nanostructures through optical field modulation of dielectric microspheres. Rapid photochemical reduction of the hierarchical Ag micro-nanostructure was achieved using the unique focusing properties of dielectric microspheres. The impact of precursor molar concentration ratio, microsphere diameter, laser power, and irradiation time on the morphology of the hierarchical Ag micro-nanostructures was thoroughly examined. An optimal Raman-enhancing hierarchical AgNP/AgMR/MS structure was fabricated using 21 μm diameter barium titanate microspheres, a 1∶4 silver nitrate to trisodium citrate molar concentration ratio, a 98 μW laser power, and an 80 s irradiation time. Experiments and numerical simulations indicated that the Raman enhancement channels of the hierarchical AgNP/AgMR/MS structure stemmed from microsphere nanofocusing, localized surface plasmon resonance of the hierarchical Ag micro-nanostructure, and directional emission from the hybrid structure. The hierarchical AgNP/AgMR/MS hybrid structure demonstrated an enhancement factor of up to 9.50×10⁹ and a detection limit of 10^-14 mol/L for trace detection. This study provides a new strategy for creating ultra-sensitive dielectric/metal hybrid SERS substrates with low cost and high performance for practical applications.

Infrared light, which is also known as infrared radiation, is located between the visible and microwave bands, with a wavelength range of 0.76?1000.00 μm. Unlike visible light, which can be directly perceived by the human eye, infrared radiation is located outside the range of human visual perception, with a wavelength range that is approximately 2500 times wider than that of visible light. The special physical properties of infrared radiation give it a wide range of applications in the aviation, aerospace, biomedical, industrial, military, and scientific research fields, along with others. The infrared-radiation propagation process includes strong absorption and scattering, along with a wide range of wavelengths. General optical devices cannot detect and utilize infrared radiation, and there are different application requirements for different infrared wavelengths. It is also necessary to choose appropriate infrared materials when preparing infrared optical devices. With the development of theoretical research, and science and technology, future opto-mechanical systems need to integrate as many functions as possible in as small a range as possible, which leads to new requirements for the miniaturization and integration of micro-optical components. Compared with traditional infrared optical devices, infrared micro-optical devices have advantages that include a small size, light weight, good stability, flexible manufacturing methods, low cost, and easy integration. Therefore, they have very bright application prospects in fields that include infrared sensing and imaging, infrared detectors, and infrared window penetration enhancement. At the same time, hard and brittle materials can withstand high temperature and high pressure in various extreme environments because of their good optical properties and physicochemical stability, and have irreplaceable roles in certain demanding military fields. However, their excellent physicochemical stability also produces greater challenges when preparing infrared micro-optical devices made of various hard and brittle materials.

In response to this demand, various high-precision preparation methods have been proposed, such as diamond turning, photolithography, and nanoimprinting. All of these methods have their own advantages, but they are not well suited for the preparation of complex three-dimensional micro-optical devices on the surfaces of hard and brittle materials. Femtosecond-laser processing is also an emerging high-precision micro-nano manufacturing technology. Because of the extremely short pulse width and high peak power of a femtosecond laser, high-precision true three-dimensional micro-nano structures can be prepared on the surfaces of various hard and brittle materials, as well as inside of them, which leads to a new method for solving the problem of preparing infrared micro-optical devices using hard and brittle materials. Femtosecond-laser processing technology has been rapidly developing in recent years, and new femtosecond-laser composite processing technologies have also been continuously derived. Therefore, this paper summarizes the preparation and application of infrared micro-optical devices using hard and brittle materials based on femtosecond-laser processing in recent years, with the goal of promoting the development and application of the technology in this field.

Starting with femtosecond-laser processing technology and commonly used hard and brittle infrared materials, this paper surveys the developments in this field in recent years. First, femtosecond-laser-based direct writing and various composite processing techniques such as etch-assisted femtosecond-laser processing and optical-field-modulated femtosecond-laser processing are briefly introduced. Next, some hard and brittle materials commonly used in the infrared region are introduced, such as diamond, which has the best overall performance but is also the most difficult to process with high precision, and sapphire, which is widely used in a variety of infrared window devices and has extreme hardness, excellent physicochemical stability, etc. (Table 1). Next, the paper focuses on infrared micro-optical devices such as refractive and diffractive devices. These infrared micro-optical components, as the basic units of an infrared integrated optical system, are characterized by a small size, light weight, and low cost. They can realize new functions such as miniaturization, arrays, and integration, which are difficult to realize with common optical components, and have very important application value in the infrared imaging, detection, national defense, and civil fields. Finally, the paper introduces related applications based on various types of infrared micro-optical devices, which are categorized according to their different application requirements. These mainly include sensing and imaging applications based on various kinds of lenses and their array structures, various kinds of infrared detectors based on the absorption of infrared light, and various kinds of window materials based on the enhancement of infrared-light permeability. Finally, the future development trend for the femtosecond-laser processing of infrared micro-optical devices using hard and brittle materials is envisioned.

The rapid development of femtosecond-laser processing technology for the high-precision preparation of infrared micro-optical devices using hard and brittle materials leads to new ideas. Scholars at home and abroad also perform much research in this field, from the principle to the application. This leads to many achievements and greatly promotes the development of this field. Of course, at present, there are still some problems in the preparation of infrared micro-optical devices using femtosecond-laser processing. For example, because of the unique stability of the diamond material, it is difficult to process with high precision and high efficiency. In addition, there is still much room for improvement in the high-precision preparation technology used for other hard and brittle infrared materials. It is believed that through the efforts of so many researchers, the use of femtosecond-laser processing technology in the preparation of micro-optical devices using hard and brittle infrared materials will gradually improve and will bring extensive social and economic benefits.

With the development of emerging applications, such as artificial intelligence (AI), automatic drive, and high-speed computing, the information transmission capacity and processing speed required by human society are increasing exponentially. Hence, photonics is expected to be a new approach to information transfer and processing in the future for the multiple-encoding degrees of freedom, low power consumption, ultrahigh-speed information transmission, and high parallelism in information processing. Compared with traditional bulky optical systems, photonic integrated circuits (PICs) have a low loss, multifunctionality, high degree of integration, and compact size. These advantages have attracted attention from scientists worldwide. Apart from the significant progresses made in two-dimensional (2D) PICs, a rapid development trend in three-dimensional (3D) PICs consisting of 2D and 3D waveguide devices is being witnessed.

With its 3D processing capability, femtosecond-laser direct writing facilitates the transformation of PICs from 2D into 3D. This transformation provides not only a direct solution for the improvement of chip integration in the physical space but also new physical degrees-of-freedom design to achieve more complex on-chip photonic manipulation methods. Compared to 2D optical waveguide devices, 3D optical waveguide devices offer a new three-dimensional architecture for photonic chips, enabling three-dimensional integration. By spatially combining waveguide devices, multiple degrees-of-freedom of photons can be fully achieved. By writing designed optical waveguide arrays, it is possible to construct Hamiltonians, enabling the direct simulation of quantum and topological phenomena in optical systems. With these advantages, 3D PICs have important applications in various fields, including optical communications, integrated quantum optics, topological optics, astrophotonic, and optical sensing.

We reviewed the research progress of femtosecond-laser direct writing for 3D PICs. First, we introduced the mechanism of interaction between the femtosecond laser and transparent materials. On this basis, four types of femtosecond-laser direct-writing waveguides were introduced. Finally, we extensively reviewed the important applications and recent progress in 3D PICs in various fields.

3D femtosecond-laser direct-writing PICs provides a crucial solution for manipulating optical information. However, significant scientific and technical problems must be addressed before practical applications can be achieved. For example, the spherical aberration and Kerr nonlinearity that hinder the high-quality fabrication of 3D waveguides need to be addressed. In addition, the dynamic modulation efficiency of 3D PICs is limited by the large spacing between the surface and internal components. If these problems are solved in the near future, the femtosecond-laser direct-writing technique can be fully utilized in fabricating 3D PICs, with significant economic benefits to human society.

An ultrashort pulse exhibits an instantaneous high irradiance, which typically induces nonlinear interactions within various materials. In comparison to a short pulse, an ultrashort pulse has an exceedingly brief interaction duration, resulting in a relatively limited energy redistribution induced by thermal transfer. Therefore, ultrashort lasers possess the inherent potential to effectively mitigate the processing constraints encountered in the production of components with superior quality, exceptional precision, elevated hardness, and arduous machinability, surpassing the capabilities of conventional processing methodologies. In the domain of ultrashort laser micro/nano manufacturing, the paramount significance of theoretical inquiries cannot be overemphasized because they establish the fundamental basis for achieving precise control and manipulation. In addition, the intricate nature of the interaction between ultrashort lasers and materials is a subject of profound interest in the field of optical physics.

In stark contrast to the conventional paradigm of laser thermal processing, the interaction between ultrashort lasers and materials manifests a myriad of complex phenomena unfolding across various temporal and spatial scales. When an ultrashort laser interacts with a material, the photons are primarily absorbed by charge carriers. Simultaneously, the excitation and motion of the electrons induce a modification in the potential of the atoms, facilitating the transfer of electron energy from the optical phonon wave to the acoustic phonon wave within a time frame measured in picoseconds (10^-12-10^-10 s). The duration of the plasma motion, material ablation, and sputtering can range from nanoseconds to microseconds. Therefore, it is imperative to develop an all-encompassing framework that incorporates the intricate dynamics of laser beam propagation, electron ionization and energy transfer, plasma motion, thermal and non-thermal phase transitions, and laser ablation. This holistic model will be indispensable in unraveling the underlying principles governing the intricate interplay between ultrashort laser pulses and materials. Nevertheless, the advancement of such a theoretical framework poses a significant impediment in the ultrashort laser field.

Considering the inherent disparities in the properties of metals, semiconductors, and dielectric materials, this scholarly article commences by elucidating the intricacies of the electron dynamics and intricate interplay between photons, electrons, and ions during ultrashort laser irradiation at the atomic level. This paper first introduces the computation of the electron dynamics of materials under ultrashort pulses. This manuscript initially presents the utilization of time-dependent density function theory (TDDFT) to scrutinize the impact of laser parameters on the rate of electron excitation. Furthermore, it introduces the concept of employing TDDFT to compute the optical properties. Subsequently, taking into account the adherence of a metallic system to the Fermi-Dirac distribution, a streamlined approach that employs density functional theory (DFT) is introduced to derive the electron excitation parameters. Then, the utilization of real-time TDDFT in conjunction with molecular dynamic simulation is introduced to explore the intricate mechanisms underlying the coupling between photons, electrons, and ions. Additionally, a streamlined approach known as ab initio molecular dynamics is presented as a means to investigate the non-thermal phase transition phenomena exhibited by crystalline materials. Finally, the paper highlights that the utilization of an atomic scale model is inherently constrained when investigating phenomena occurring within a few picoseconds or even femtoseconds.

Then, this paper provides a comprehensive overview of the current cross-scale multi-physics coupling models employed in the simulation of ultrashort laser machining. The two-temperature equation is introduced as a means to explore the intricate dynamics of the energy exchange between electrons and ions within the context of metallic systems. Furthermore, we introduce a methodology that combines the two-temperature equation and electron excitation rate equation to analyze the energy transfer dynamics between electrons and ions within semiconductor and dielectric materials. This paper elucidates the two methods capable of manifesting the electron-ion energy in the realm of macroscopic scales.

Based on the principles of energy transfer, this paper presents a comprehensive overview of the contemporary cross-scale multi-physics coupling models utilized in the simulation of ultrashort laser ablation. This manuscript presents a novel approach that synergistically merges the principles of two-temperature equation and molecular dynamics simulations. This combined methodology enables a comprehensive description of material ablation phenomena, encompassing non-equilibrium phase transition thresholds and intricate chemical reactions. It is important to note, however, that the applicability of this method is primarily confined to the realm of nanoscale laser ablation. This paper also presents a novel approach that integrates the lattice temperature with fluid mechanics and heat and mass transfer models, employing the framework of the two-temperature equation (or the two-temperature model-coupled electron excitation rate equation). This methodology facilitates the visualization of laser-induced ablation phenomena at the microscale level.

In addition, it suggests potential avenues of research that could be pursued in the future within this field.

In recent years, researchers have undertaken extensive investigations into the intricate interaction between ultrashort laser pulses and materials. By utilizing DFT and ab initio molecular dynamics, these scholars have simulated the intricate processes of electron excitation, energy transfer, and atomic motion within materials. Furthermore, they have successfully constructed a comprehensive theoretical framework that encompasses the cross-scale coupling of electron excitation, the two-temperature equation, molecular dynamics, and fluid mechanics. The interconnection of these models facilitates the comprehensive characterization of the intricate phenomena occurring in the realm of ultrashort laser processing. This successful integration allows for the accurate simulation of material ablation, nano-ripple generation, and microstructure evolution.

Nevertheless, the intricate examination and formulation of models for ultrashort laser processing at various scales present a series of challenges that will undoubtedly shape the trajectory of future advancements in theoretical modeling. First and foremost, it is imperative to propose a precise and straightforward methodology for the calculation of the photo-induced alteration of material properties. This methodology must take into account the intricate crystal modifications that occur within materials when subjected to multiple ultrashort pulses. Furthermore, it is imperative to develop a novel theoretical framework that can seamlessly integrate the precision of molecular dynamics with the efficiency of fluid dynamics. Finally, novel approaches for coupling and spatial-temporal resolution optimization are being actively pursued in order to maintain the computational precision while enhancing the computational efficacy, enabling the simulation of ultrashort laser processing involving tens of millions of pulses.

The ionic liquid electrospray thruster, characterized by its high specific impulse, compact size, controllable thrust, and self-neutralizing beam, is effectively utilized in attitude control and orbit modification of micro and nano satellites. The emitter, which is comprised of a microcone array, serves as a critical component of the thrusters. A passive supply of ionic liquid is realized by employing sintered porous glass as the emitter material. This type of glass offers superior resistance to electrical corrosion than porous metal, thereby enhancing the thruster’s lifespan. However, processing porous glass presents a challenge due to its inherent brittleness. The ultrafast laser, notable for its low thermal impact and excellent adaptability to various materials, offers a viable solution to this issue by enabling minimal-damage processing of the porous glass. However, the fabrication of porous glass emitters using ultrafast lasers is not extensively reported, necessitating further investigation into the machining process and resultant performance. In this study, the ultrafast laser machining process for producing high-precision, uniform, and densely arranged porous glass microcone arrays is examined. The outcomes of this research will pave the way for the feasible production of porous glass emitters for ionic liquid electrospray thrusters.

In this study, G5 grade porous borosilicate glass is employed, and it is processed via ultrafast laser with a pulse width of 8 ps. First, the material removal behavior of porous glass is examined. The morphology differences between porous glass and K9 glass after laser pulse are compared via scanning electron microscopy, and their average material volumes removed by single laser pulse are measured via laser confocal microscopy. Subsequently, an in-situ high-speed camera is used to observe the material removal behavior of porous glass and K9 glass. This enabled an in-depth analysis of the material removal mechanism of porous glass. Furthermore, the ultrafast laser machining process of porous glass microcone array is examined. The effects of scanning strategy and machining allowance on the geometric features of microcone array are analyzed, and a microcone array with good uniformity and high sharpness is fabricated. Finally, porous glass emitters are manufactured using an ultrafast laser. These emitters served as the foundation for the assembly and testing of an electrospray thruster.

According to the morphology after several laser pulses, the diameter of the material removal range for porous glass is approximately twice the spot size, and the average volume of porous glass removed by single laser pulse is approximately 16.2 times that of dense K9 glass (Fig.4). An in-situ observation reveals a large quantity of debris eject from the porous glass during laser processing. The diameter of this debris is similar to that of a porous glass particle (approximately 50 μm), a phenomenon not observed when processing K9 glass (Fig.5). Therefore, the material removal mechanism for porous glass involves laser-induced particle spalling, which differs from that of K9 glass, where ablation is dominant (Fig.6). Furthermore, factors influencing the geometric features of the microcone array are investigated. It is determined that the apex size and height uniformity of the microcone array decrease with the reduction in machining allowance (Figs.8 and 9). Local spalling at the apex of cones can be suppressed by adopting a scanning strategy with feeding from the edge to the center of the microcone, resulting in better uniformity (Fig.9). In the final stage of the study, porous glass emitters with various geometric features are manufactured using an ultrafast laser. Each emitter comprises 1836 microcones, with a density of up to 2174/cm² (Fig.11).

In this study, the ultrafast laser machining process of porous glass emitter microcone array is examined. By comparing the morphology after ablation, average material volume removed by single laser pulse, and in-situ observation, it is verified that sintered porous glass exhibits the material removal behavior of ultrafast laser induced particle spalling. Next, the ultrafast laser machining process of porous glass microcone array with high sharpness and good uniformity is examined. Porous glass microcone array, with an average apex size of 20 μm, can be fabricated using the scanning strategy with feeding from the edge to the center of the microcone and machining allowance of 60 μm. Based on this processing technology, porous glass emitters are fabricated. During testing, each emitter demonstrates a measured thrust of 90 μN under a 3.0-kV condition.

Laser cleaning is an important laser application and is known as the “most promising green cleaning technology of the 21st century.” It has unique advantages that make it effective in the efficient and precision cleaning of large and complex components. This technology can be used to clean parts that cannot be cleaned using traditional technology, significantly improving the cleaning efficiency and reliability of the product. Although developed countries abroad have performed much work on the basic theory, process exploration, and engineering application of laser cleaning, there are still common problems such as a low component cleaning efficiency, an unclear coupling mechanism, incomplete evaluation standards, and insufficient online monitoring technology. Therefore, researching the basic theory and equipment for efficient laser cleaning is a specific implementation goal of the “Made in China 2025” initiative, which aligns with China’s sustainable development strategy. This will assist in improving the automation level of equipment maintenance in areas such as the aerospace, rail transit, and ocean shipping sectors, and has important significance in promoting the upgrading and optimization of China’s industrial structure.

This study focuses on the significant demand for laser cleaning in the aerospace, high-speed rail, and ocean shipping fields in China. It considers large and complex components such as the TA15 titanium alloy intake ports for the new generation of aerospace solid-liquid ramjet engines, high-speed rail body features or bogie components, and hatch covers in oceangoing ship manufacturing as research objects. It introduces the research progress of the Harbin Institute of Technology in laser cleaning mechanisms and processes, the online monitoring of multiple parameters during cleaning processes, and intelligent equipment technology in recent years in order to provide valuable references for the sustainable development of intelligent laser manufacturing in China in the future.To overcome the shortcomings of online monitoring technology for laser cleaning at home and abroad, the team of Guo Bin and Xu Jie from the Harbin Institute of Technology established a coupled multivariate rapid identification method for laser cleaning and its key technologies for short-term online regulation. They established a multi-parameter online detection and regulation system based on spectroscopy (Fig.17), providing technical support for the subsequent development of intelligent, flexible, and selective precision laser cleaning equipment and efficient laser cleaning equipment for large components. This system achieves real-time control of the laser cleaning quality, and the accuracy errors of laser spot size and average power output are better than 1%. The Harbin Institute of Technology has completed the development of a complete set of equipment for large-scale component cleaning in fields that include ocean shipping, high-speed rail, and nuclear power, integrating systems and devices such as lasers, computer numerical control (CNC) systems, industrial robots, cleaning end effectors, water cooling equipment, dust removal equipment, and safety protection devices for the first time in China (Figs.26 and 27). Based on the complete set of laser cleaning equipment for large components, research has been carried out on laser cleaning processes such as rust removal for ship hatch cover features, paint removal for high-speed rail bogie wheelset features, and the removal of marine microorganisms on nuclear power floating bucket features, as well as functional verification of gantry-type CNC laser cleaning equipment. A complete set of process solutions for the laser cleaning of large components in the ocean shipping and high-speed rail fields has been provided, with a cleaning efficiency exceeding 50 m²/h.

Different methods are involved in the binding of objects such as coatings, dirt, marine microorganisms, and small particles to a substrate, and it is necessary to distinguish and research different physical removal mechanisms based on the physical characteristics of various objects. When cleaning the oxide film on the surface of a titanium alloy inlet, nanosecond pulse laser cleaning can not only completely remove the oxide film on the titanium alloy surface but can also prevent secondary oxidation of the substrate as a result of the low thermal effect characteristics of nanosecond laser, making it an optimal laser cleaning method (Fig.3). When cleaning a painted high-speed railway aluminum alloy car body, different colors and thicknesses of paint require different laser cleaning methods (Fig.4). When the paint is thin (≤40 μm), a laser light source with a lower absorption rate for the paint is selected, and the paint is removed through thermal vibration, which achieves better results. When the paint is thick, it is necessary to choose a laser light source with a higher absorption rate for the paint, and the paint is removed using an ablation mechanism, which is a good choice. For the laser cleaning of high-strength steel hull rust, the main removal mechanism during dry cleaning involves energy absorption by the oxide film and gasification (Fig.5). When the oxide on the surface undergoes gasification and evaporation, the downward reaction force is generated on the sample surface, making the removal of the deeper oxide film easier. Laser cleaning using a narrow pulse width and high peak power is effective at removing marine microorganisms (Fig.6). The laser removal mechanisms for the extracellular polymeric substances (EPS) layer and barnacle substrate are ablation gasification and shock wave peeling, respectively. Establishing a cleaning thermal vibration model will assist in better elucidating the change laws of the laser cleaning behavior, temperature field, and stress field with the laser spatiotemporal energy characteristic parameters and predicting the relationship between the different cleaning parameters and cleaning quality in the laser cleaning process. The team of Guo Bin and Xu Jie from the Harbin Institute of Technology established a thermal vibration model using the finite element method to simulate the temperature and stress fields during laser cleaning (Figs.7‒10). The results were compared with experimental results. The final calculation accuracy of the temperature and thermal stress fields exceeded 85%.

Laser cleaning technology can significantly improve equipment manufacturing, protect the environment, and reduce labor requirements. It will bring users direct economic benefits of tens of millions or even billions of yuan. Moreover, innovative core technologies can be promoted by appropriate enterprises, universities, and research institutes, with significant economic and social benefits. The different application fields for laser cleaning lead to a significant demand for laser cleaning equipment. Domestically, much attention is being given to the remanufacturing industry, including a massive demand for engineering machinery, automotive parts, and machine tool remanufacturing. Thus, laser cleaning equipment has broad market prospects. Establishing optimization process specifications for the laser cleaning of typical component surface contaminants, developing a laser multi-parameter online detection system for the laser cleaning process, selecting intelligent and flexible robots for precision laser cleaning equipment, and developing efficient laser cleaning equipment for large components will assist in maintaining China’s leading position in the field of innovative laser manufacturing.

Metal part wear and corrosion are predominant forms of damage in engineering applications, prevalent in various fields of national life. Wear and corrosion initially occur on the surface of the part and further extend into the part. Fatigue, cracks, and other forms of component failure also stem from the component surface. Therefore, strengthening treatment for the surface of parts is gaining increasing attention. In material processing, most laser energy can be converted into thermal energy over a short time period. Laser processing involves a high-energy density laser beam which interacts with a metal surface, causing it to melt, gasify, change phases, and solidify. Laser surface treatment is a non-contact process in which the laser beam interacts with the material surface during processing, making it straightforward to control. Laser has the characteristics of high power and high precision, so that laser processing materials have been widely used in engineering, military, industry, communications and other almost all fields.

Surface heat treatment technology has a long development history. For example, surface heat treatment methods such as induction and flame quenching, and carburizing and nitriding processes effectively improve surface hardness, corrosion and wear resistance, and surface fatigue performance. Laser surface heat treatment achieves some advantages over traditional heat treatments. First, accuracy is the most prominent feature. In laser surface heat treatment for parts with complex 3D shapes and structures, controlling the laser beam size significantly reduces the heat affected zone, accurately controls treated and untreated areas, and minimizes heat treatment deformation and residual stress. Using traditional heat treatment methods, parts are heated to the required temperature and subsequently quenched in oil or water for high-speed cooling to achieve the required surface hardness. For heat treatment of thin-walled and high-precision parts, uncontrollable heat input such as overheating or melting leads to a large heat affected zone, surface deformation and uneven hardness distribution on the part surface. In most industrial applications, failure behaviors such as wear and corrosion only occur in specific part areas. Therefore, strengthening areas prone to failure to enhance the performance of parts and extend their service life. Hence, the advantages of using laser surface processing are prominent owing to its highly oriented nature and ability to deliver controllable energy to the desired area.

Second, laser beams have high energy density. For example, in the laser beam irradiation area, a significant amount of heat accumulates over an extremely short time period to reach the phase transition temperature in the area to be processed. Owing to the fact that laser processing only involves local areas, when the laser is inactive, heat quickly diffuses towards the substrate, causing the heating zone to cool rapidly. Therefore, compared to traditional heat treatment methods, an additional cooling medium is not required.

In terms of sustainable development, laser beam heat sources are clean, green, and pollution-free. Compared to nitriding and carburizing heat treatment processes, laser surface heat treatment does not involve chemical reactions during metal melting and pool cooling, and completely avoids chemical pollution. In the mass processing production line, the laser beam improves both laser utilization and higher heat treatment efficiency through the photosystem and the workbench, thus, reducing its cost in contrast to traditional heat treatments.

Traditional metal surface strengthening treatment methods include flame quenching, induction heating surface heat treatment, and nitriding and carburizing chemical heat treatment. Since the 1970s and 1980s, owing to laser development, laser surface heat treatment has been applied to the research of steel metal surface strengthening treatment. Therefore, this study first discusses the advantages and disadvantages of laser-energy-field surface heat treatment technologies (Table 1) and the multiphysics simulation equations of temperature, fluid and phase fields in the simulation process (Fig. 1). After which, the research and development status of five typical laser-energy-field heat treatment technologies, namely, laser surface hardening, remelting, surface alloying, cladding, and shock peening, are summarized (Figs. 2‒8). Subsequently, based on engineering application requirements, surface wear and corrosion resistance improvements, alongside the residual stress of parts are summarized (Figs. 9‒11). Finally, potential research directions for future laser-energy-field heat treatment are outlined (Fig. 12).

Laser-energy-field surface heat treatment technology is capable of improving the surface properties of metal parts. The wear and corrosion resistance alongside residual stress on the surface of a part determine its service life. In future, the combination of laser additive manufacturing, and in-depth heat treatment and multiphysics simulation assistance are expected to become key development fields for laser-energy-field surface heat treatment technologies.