Laser directed energy deposition (LDED) is an additive manufacturing (AM) technology that uses a high-energy laser beam to melt metal powder and deposit it on a substrate. This technology can directly manufacture large-scale parts with high forming efficiency using a high-power laser. Hybrid additive-subtractive manufacturing can address the inherent problems, such as poor surface quality and low dimensional accuracy of LDED parts. Laser additive-subtractive hybrid manufacturing mainly consists of two methods. One is to perform milling surface-finishing after additive manufacturing is completed. The other is to perform milling during additive manufacturing. In this study, the influence of the interaction between adjacent layers printed using additive manufacturing along with milling subtractive manufacturing on the surface quality and mechanical properties of the final product was discussed.

A robotic hybrid additive-subtractive manufacturing system was developed in-house for fabricating 316L stainless steel samples. The laser, powder feeders, and high-speed electric spindles were integrated on two ABB robots. A 316L stainless steel powder with a particle size range of 15-53 μm was used and delivered with high-purity argon as a carrier through the nozzle. A single-factor experiment was designed to assess the optimal hatch space for robotic additive manufacturing. The hatch space was set to 2-4 mm for comparing the cross-sectional size of the single melting channel. An orthogonal design experiment with four factors and three levels was designed to determine the optimal robot parameters for the subtractive milling process. The surface quality and mechanical properties of 316L stainless steel fabricated with different processing strategies were studied using the parameters obtained above.

An excessively large or small hatch space affects the unevenness of the deposited layer, as well as the shape and mechanical properties of the fabricated 316L sample. By comparing the morphology, vertical section (Fig. 4), and mechanical properties [Fig. 5 (b)] of the samples obtained at different hatch space values, the hatch space with the best comprehensive mechanical properties was 2.5 mm. The variance analysis of the orthogonal experiment results (reported in Tables 3 and 4) demonstrates that the impact of different robot milling process parameters on the surface quality of milling additive manufacturing samples varies; specifically, we obtain the following ordering from large to small influence, the spindle speed, milling width, feed rate, and milling depth. There is no significant difference in the surface roughness and microhardness of the samples between the two processing strategies (Fig. 10), and the surface roughness and microhardness of the top surface of both are better than those of the side. In terms of mechanical properties, there is also no significant difference between the two strategies (Fig. 11). Therefore, we conclude that the additive manufacturing aacompanying with milling does not harm the surface quality and mechanical properties of the formed samples, and adding subtractive manufacturing process in the process of additive manufacturing will not have a negative impact on the samples. The 316L stainless steel valve mould parts were manufactured using additive manufacturing accompanying with milling. The length, width, and surface roughness values of the 316L part are (225±0.17) mm, (150±0.13) mm, and (0.87±0.03) μm, respectively (Fig. 15). The successful manufacture of parts verifies the feasibility of additive manufacturing accompanying with milling that is proposed in this study.

The best comprehensive mechanical properties of the samples can be obtained by additive manufacturing with 2.5 mm hatch space. With a spindle speed of 3600 r/min, feed rate of 3 mm/s, milling depth of 0.3 mm, and milling width of 3 mm, the observed surface roughness of the samples fabricated by additive manufacturing is optimal. There is no significant difference in the mechanical properties and surface quality of the samples fabricated using the process strategy of milling after additive manufacturing and additive manufacturing accompanying with milling, using the same optimized parameters, indicating that additive manufacturing accompanying with milling strategy is feasible. Valve molds are manufactured with additive manufacturing accompanying with milling strategy, which realizes high dimensional accuracy and high surface quality of valve mold parts in the nuclear power field.

With the rapid development of high-tech industry, the function and performance requirements of products are increasing on a daily basis. It has been difficult for a single metal to meet the requirements of industrial field for comprehensive performance of materials. Therefore, multi-material parts with multiple metal properties have significant development prospects. Titanium alloys are widely used in aerospace, biomedicine, automobile manufacturing, and other fields because of their high specific strength, good corrosion resistance, good biocompatibility, and high thermal strength. Aluminum alloy has the advantages of lightweight, high conductivity, and good workability. Both titanium and aluminum alloy are lightweight and high-strength materials that are focused on development in the aerospace field. If their properties are integrated, the preparation of titanium aluminum heterogeneous materials is expected to significantly improve the lightweight and comprehensive performance of components. Because of the special requirements for the manufacturing of curved surface matrix additives such as rocket body in the aerospace field, it is crucial to establish a three-dimensional model that conforms to the actual evolution process of the molten pool in the curved surface additive manufacturing process by the laser direct energy deposition and improve the surface quality of the formed components.

The laser directional energy deposition experiment with coaxial powder feeding adopts 4000 W semiconductor pumped Nd∶YAG laser. TC4 powder is used for deposition processing on 6061 aluminum alloy cylinder substrate. The substrate is sanded to remove the oxide layer before use. The integrated powder feeding laser head remains stationary above the cylinder body, which rotates around the axis with a constant angular velocity. The laser beam with constant power is used to melt the TC4 powder and the base material is sent out, thus forming a molten pool, which is protected by passing argon. The initial and steady deposition processes of Al-Ti heterogeneous materials by laser direct energy deposition are numerically simulated by using finite element simulation method. The effects of laser power and scanning speed on molten pool morphology, width-to-depth ratio, and temperature field are studied by controlling variables, and the simulation accuracy is verified by experiments.

When the laser heat source interacts with the substrate, the maximum and average temperatures of the molten pool rise rapidly within 100 ms. Subsequently, the maximum and average temperatures inside the molten pool continue to increase, but the growth rate slows down and reaches the maximum value at 300 ms (Figs. 4 and 5). Comparing the molten pool cross-section morphology of the simulated cladding layer with the actual molten pool cross-section morphology, it is observed that the morphology and temperature distribution of the molten pool obtained by the experiment are basically the same as those obtained by the finite element simulation , and the simulated spot diameter is consistent with the test spot diameter (Fig. 12). The higher the laser power is, the higher the volume energy density is. The width-to-depth ratio of the molten pool is inversely proportional to the volume energy density of the laser (Tables 4 and 5). The difference of the width-to-depth ratio between the initial deposited layer and the stably deposited layer is smaller than that between Al-Ti heterogeneous material layer and the initial deposited layer, which is due to the preheating effect of the initial Al-Ti heterogeneous material layer cladding on the stably deposited Ti layer cladding. Compared with the Al-Ti heterogeneous material layer, the cladding matrix of the stably deposited Ti layer is transformed from aluminum alloy to titanium alloy, with low thermal conductivity and high density. Additionally, when the stably deposited Ti layer is cladded, the depth of the titanium alloy matrix is higher than that of the initial deposited Ti layer. Therefore, the temperature difference between the initial deposited and the stably deposited Ti layers is lower than that between the Al-Ti heterogeneous material layer and the initial deposited layer.

The results show that the thermal behavior and morphology of the molten pool of Al-Ti heterogeneous material layer formed by laser directed energy deposition change significantly with the laser parameters. When the scanning speed is 0.32 rad/s, with the laser power increasing from 1400 W to 2300 W, the maximum temperature of the molten pool increases from 1525.5 ℃ to 3289.8 ℃, and the volume of the molten pool increases from 1.16 mm³ to 7.73 mm³. The width-to-depth ratio of the molten pool is negatively related to the laser energy density. The width-to-depth ratio of the molten pool in the Al-Ti heterogeneous material layer is the highest, 1.84, when the laser power is 2000 W and the scanning speed is 0.32 rad/s, followed by the width-to-depth ratio in the initial deposited Ti layer, 1.42, and the width-to-depth ratio in the stably deposited Ti layer is the smallest, 1.22. The width of the molten pool obtained in the experiment is 0.61 mm, and the molten pool morphology is in good agreement with that obtained by finite element simulation.

Nickel-based superalloys possess many desirable properties, including high strength, desirable oxidation resistance, and superior thermal stability, and are widely utilized as the preferred materials for crucial hot-end components in the aerospace field. As an important structural material in the aerospace industry, the GH3536 nickel-based superalloy is used to manufacture aero-engine combustion chambers and other high-temperature components of aircraft engines with high operating temperatures and complex structures. Early research on GH3536 mainly focused on deformation behavior, heat treatment, and welding. However, with increasing demand for high-performance, lightweight, and heavy-duty aerospace equipment, higher requirements are placed on traditional complex component manufacturing. Laser powder bed fusion (L-PBF), also known as selective laser melting (SLM), has been introduced to fabricate GH3536 complex-structure components. However, bulkhead, siding, and casing structures tend to be large, and conventional L-PBF technology is not capable of building large parts owing to the limited building dimensions and efficiency. Multi-L-PBF (ML-PBF) technology combines the advantages of high precision and high efficiency and is more suitable for building large-size and complex-structured GH3536 components owing to the composition of multiple single-laser beam modules. However, studies on the influence of defects, microstructures, and mechanical properties of GH3536 parts with different laser beams involved in the ML-PBF process are limited. In this study, using quadruple-laser ML-PBF equipment, the effects of different laser beams on the micro/macro properties of L-PBF-processing GH3536 parts are investigated. In addition, the differences in the defect characteristics, microstructure, residual stress, and tensile properties of the single-, dual-, and quadruple-laser-processing samples are examined. This study is expected to provide a better understanding of multi-laser interactions on the samples, and a scientific basis for the application of nickel-based materials in the aerospace fields.

Using the large-size, four-laser ML-PBF equipment and the gas-atomized GH3536 nickel-based superalloy powder particles with the particle size of 21.2-58.9 μm, GH3536 samples were prepared using single-, dual-, and quadruple-laser beams with the optimized process parameters. First, the relative densities of the samples were measured using the Archimedes method and micrograph analysis. The optical microscopy and scanning electron microscopy equipped with an electron back-scattered diffraction (EBSD) detector were employed to examine the microstructures of the cubic specimens. The residual stress of the samples was measured using an X-Ray diffraction (XRD) testing machine. In addition, a high-temperature endurance testing machine was used to test the room-temperature tensile properties of the alloys.

Re-melting during the ML-PBF process melts the unmelted powder particles on the upper surface and penetrates the powder layer better, which helps to improve the surface quality (Fig. 3). With the increase of laser beams involved in the ML-PBF process, the relative density gradually decreases from 99.82% to 92.35% and 98.97%, respectively, which is mainly due to the pores and microcracks produced during the re-melting process (Fig. 4). In the ML-PBF process, grains in the re-melting regions grow on the solidified materials, which hinders the growth of columnar crystals. With an increase in the number of laser beams, a large number of columnar crystals gradually transform into cellular crystals (Fig. 5). The texture index of the samples along the horizontal direction increases from 3.040 (single-laser) to 3.403 (dual-laser) and 3.465 (quadruple-laser), whereas the volume fraction of high-angle grain boundaries (HAGBs) gradually decreases from 65.9% to 50.1% and 46.3%, respectively (Figs. 6 and 7). This is primarily attributed to the recrystallization of grains during the ML-PBF process, which leads to the transformation of HAGBs to low angle grain boundaries (LAGBs), causing a more significant preferred growth of grains and obvious anisotropy of materials. Values of the residual stress of single-, dual-, and quadruple-laser processing samples are 192.3, 106.5, and 44.1 MPa, respectively. The tensile strengths of the samples are 858.1 (single-laser), 851.4, and 830.5 MPa, respectively, while the elongation at break is 30.3%, 25.9%, and 25.4%. The main reason for this may be that ML-PBF can induce pore and microcrack defects, which are stress concentration components that accelerate crack propagation under tensile stress, resulting in premature fracture failure, and thus reducing the elongation of the samples.

GH3536 nickel-based superalloy is prepared via ML-PBF, and the defects, microstructures, and mechanical properties in single-, dual-, and quadruple-laser-processing regions are investigated. The results indicate that the surface quality improves with an increase in laser beams introduced during the ML-PBF process, while the relative density decreases from 99.82% (single-laser) to 92.35% (dual-laser) and 98.97% (quadruple-laser). Simultaneously, after re-melting in the overlap regions during the ML-PBF inducing recrystallization, the preferred growth orientation along (001) is more apparent, the texture index increases from 3.040 to 3.403 and 3.465, and the volume fraction of LAGBs decreases from 65.9% to 50.1% and 46.3%. Under the multiple laser repeat scanning process, the residual stress in the overlap regions also reduces, where residual stress values of the single-, dual-, and quadruple-laser processing regions are 192.3, 106.5, and 44.1 MPa, respectively. All the samples display an equivalent tensile strength of more than 800 MPa, while the pores and microcracks deteriorate the ductility of the overlap regions. The elongation at break decreases from 30.3% (single-laser) to 25.9% (dual-laser) and 25.4% (quadruple-laser). This work is expected to provide an efficient reference and theoretical guidance for large-size nickel-based superalloy components fabricated via ML-PBF.

Titanium alloys have ultra-high specific strength and excellent high-temperature corrosion resistance and are widely used in manufacturing key components in the aerospace industry. Many issues arise during the mechanical processing of titanium alloy materials, including serious tool wear, residual stress, burns, and adhesion. Consequently, the reliability and yield of the current processes are relatively low, and the cost is high. Synchronous nanosecond laser-assisted electrochemical machining has been proposed for environment-friendly, efficient, and precise machining of the Ti-6Al-4V titanium alloy. This method takes advantage of the high efficiency and local temperature rise of laser processing and the good surface quality of electrochemical machining (ECM). Experimental studies were conducted to determine the effects of key process parameters, such as electrolysis voltage, laser power, and feeding rate. The results revealed that a electrolysis voltage of 20 V, laser power of 3-5 W, and feeding rate of 1.8 mm/min were preferred for high-efficiency and high-precision machining of Ti-6Al-4V titanium alloy. The method of “high→low laser power” was proposed to obtain high-precision and efficient machining using laser-assisted ECM.

We use self-designed and developed special equipment for experimental research (Fig.2). The processing object is Ti-6Al-4V titanium alloy sheet with a specimen size of 50 mm×50 mm×5 mm and a polished surface. The tool tube electrodes with an inner diameter of 0.53 mm and an outer diameter of 1.20 mm was used in experiment. In order to ensure that the electrolyte flow state was laminar flow and achieve high laser coupling efficiency, the electrolyte flow was set to 100 mL/min. Before the processing test, Ti-6Al-4V specimens were cleaned sequentially for 15 min in acetone and absolute ethanol by ultrasonic cleaning, and then dried with a hair dryer. Sodium nitrate (NaNO₃) solution with mass fraction of 12.5% at room temperature (24 ℃) was adopted as electrolyte. By controlling the relative movement of the workpiece and the tool electrode, the short-circuit current real-time detection method is used to set the initial machining gap between the tool tube electrode and the workpiece. In the actual machining, there is a short circuit phenomenon when the machining gap is 0.1 mm, and there is no obvious machining when the gap is too large, so the initial gap is selected as 0.2 mm. Each group of tests is done three times, after the test, the sample is cleaned with absolute ethanol and deionized water in turn, dried with a hair dryer. Laser confocal microscope (VK-X200K, KEYENCE) and scanning electron microscope (Scanning Electron Microscopy, SEM, S5000, Hitachi) were used to measure the surface morphology and contour of processing, mainly including the groove width, groove depth and bottom surface roughness. Finally, the data is processed to obtain various charts to analyze.

To investigate the effects of electrolysis voltage, laser power, and feeding rate on the machining performance of Ti-6Al-4V titanium alloy, single-layer synchronous nanosecond laser-assisted ECM experiments were performed. At electrolysis voltages lower than 18 V, the surface color of the workpiece was yellow-brown, and the main surface component was a yellow passivation film structure of TiO₃ formed without laser assistance (Fig. 3). The electrochemical machining process stability of the titanium alloy was relatively poor without laser assistance. In contrast, the surface processing quality of the microgrooves was better, and the machining contour accuracy was higher with laser assistance. A small amount of white substance formed inside the microgrooves as a result of the electrolytically activated TiO₂. Thus, the stability of the ECM can be increased by laser assistance. With a laser power of 3 W, the end-face gap material was removed via laser-assisted electrochemical machining, and the processing area at the bottom of the microgroove was flat (Fig. 5). With a laser power of 6 W, the laser center processing area was larger than that of electrochemical machining, and a deep groove was formed in the center of the workpiece processing area. When the feeding rate was less than 1 mm/min (Fig. 7), a microgroove was formed at the center of the machining area. The stray current corrosion and taper of the groove side were significant. When the feeding rate was greater than 1.5 mm/min, the bottom surface of the groove was relatively flat. Furthermore, increasing the feeding rate decreases the depth of the microgroove. A novel processing mode (Fig. 9) with a high laser power followed by a low laser power was also proposed. Using the control strategy, the average groove width was decreased, and the bottom of the groove was smoothened. It was demonstrated that using a high laser power first and then low laser power processing mode could not only improve the localization of processing but also effectively reduce the roughness of the bottom surface of the microgroove.

The laser-assisted ECM was used to achieve precise and efficient machining of the microgrooves on Ti-6Al-4V titanium alloy material using a self-designed experimental apparatus. Laser-assisted ECM can achieve three-dimensional machining of Ti-6Al-4V titanium alloy using a passive salt solution and low electrolysis voltage at room temperature. A laser power of 3 W, low electrolysis voltage, and feeding rate of 1.8 mm/min were preferred for the efficient and precise machining of titanium alloy materials. A gradient laser power (5 W→3 W) was applied to enhance both machining efficiency and accuracy. A rectangular table and a deep and narrow groove with a width of 1583.12 μm and a depth of 3724.63 μm were processed using the optimized machining parameters.

Ti65 is a new type of high-temperature titanium alloy that is designed and applied in the field of high temperature of 650 ℃. Due to its poor plasticity, Ti65 is difficult to create and process using traditional methods. Laser deposition manufacturing technology is a new processing method that has several technical advantages when it comes to the preparation of new alloys. The Ti65 titanium alloy samples used in this study were created using this technology. Due to the principle of laser deposition manufacturing, the formation of column crystals is generally unavoidable. These crystals lead to anisotropy in the microstructure and the mechanical properties of the deposited sample. Therefore, it is important to analyze the microstructure and mechanical properties of the sample in different sections and directions. We hope that our findings will be useful in the design and application of laser-deposited Ti65 alloys.

The experiment was carried out on the laser deposition manufacturing equipment at the National Defense Key Laboratory of Shenyang Aerospace University. Pure argon was used as powder feed and protective gas to prevent Ti65 from being contaminated by H and O during deposition. The forged TA15 was used as the substrate with a thickness of 30 mm. During the deposition process, the laser power was 5 kW, the spot diameter was 5 mm, the scanning speed was 10 mm/s, the powder-feeding rate was 10-20 g/min, the single layer height was 0.8 mm, and the overlap rate was 50%. In this experiment, short-edge and one-way reciprocating layer-by-layer scanning was used. The size of the deposited sample was 250 mm×15 mm×80 mm. An abrasive paper of 400-2000 mesh was used for grinding, and a polishing cloth was used on the sample for 50 min. The sample was equally divided in two by wire cutting, the first piece was annealed at 900 ℃/4 h/air cooling, while the second was kept in the as-deposited. The test blocks were cut along the XOY, YOZ, and XOZ planes by wire cutting, and the metallographic specimen was embedded and tested for microhardness. The test blocks were located in the middle of the sample. Three tensile specimens with a diameter of 5 mm and standard distance of 25 mm were cut along the XOY and XOZ planes, respectively. The tensile tests were carried out on an electronic universal testing machine, and the tensile results were averaged. The dimensions of the different phases were measured using Image J software. The microstructure and fracture morphology were observed using a GX51 optical microscope and ΣIGMA scanning electron microscope. The hardness of the specimen was tested using an HVS-1000A microhardness tester. Finally, ten values were measured on each test surface, and the average value was obtained (loading load of 1.96 N and duration of 10 s).

The macroscopic morphology of the deposited sample on distinct sections differed (Fig.2). Coarse columnar crystals and parallel-distributed layered structures were formed along the deposition direction in the laser-deposited Ti65 samples and α-lath coarsening was obvious in the layered structure. A section in the perpendicular (to the deposition) direction exhibited an equiaxed crystal structure. The microstructures on different sections were similar as they had lamellar structures (Fig.3). After annealing, the grain boundaries in different sections were intermittent, the α-lath was coarsened, the microstructure was a basketweave structure, and the anisotropy of the microstructure was not notable (Fig.4). The strengths of the as-deposited samples stretched along the deposition direction and perpendicular (to the deposition) direction were 1015 and 1055 MPa and their corresponding elongations were 11.4% and 8.4%, respectively (Table 2). After annealing, the strengths along the deposition direction and perpendicular (to the deposition) direction increased to 1025 and 1079 MPa, and the corresponding elongations increased to 12.7% and 10.5%, respectively. The fracture models of the as-deposited and annealed samples in different directions were quite disparate. The difference in macroscopic morphology led to the difference in microhardness in distinct sections. Also, the large number of equiaxed crystals contributed to the highest microhardness in the XOY section. The microhardness of the as-deposited sample along the deposition direction was slightly lower compared to the microhardness in the perpendicular (to the deposition) direction by 20 HV0.2_{(Fig.6). After annealing, the microhardness values in different directions were similar and close to 400 HV0.2.In this study, Ti65 samples were successfully created using laser deposition manufacturing method. The microstructure and mechanical properties of the as-deposited samples exhibited anisotropy. After annealing at 900 ℃/4 h/air cooling, the tensile strength, plasticity, and microhardness increased but the anisotropy was not notable.}

Annealing can change the microstructure, improve the mechanical properties and weaken the anisotropy of the comprehensive mechanical properties of laser-deposited Ti65.

Titanium alloys are lightweight alloys with excellent properties including high strength, high stiffness, and good corrosion resistance. Hence, they are widely used in aerospace, automobile manufacturing, and other fields and are one of the most widely studied engineering materials in the field of additive manufacturing. Metal wire feed deposition forming is an important metal additive manufacturing process that has the advantages of low cost and 100% material utilization rate. However, the process characteristics easily lead to problems of poor surface roughness and low dimensional accuracy of parts, which limits the wide application of this process. Accordingly, to improve the surface roughness and dimensional accuracy of such parts, fine metal wires can be deposited by controlling the energy input. However, a method for obtaining high-quality titanium alloy parts has not been reported in the literature. In this study, the composite heat source of the laser and joule heat is used to fuse and deposit the fine titanium alloy wire with the diameter of 0.3 mm. The influence of process parameters on the geometrical characteristics of the deposited single bead is systematically investigated, and a stable combination of the forming process parameters is obtained. Then, based on a stable single bead, aiming at shape control, high-quality thin-walled parts are obtained using the gradient transition deposition method.

The process uses the synergy of a laser and joule current to deposit metal wires on a traveling substrate. Metal wires are continuously fed into the molten pool for continuous deposition as the substrate moves and rapidly solidify to form continuous smooth single beads. In this experiment, the effects of process parameters on the geometric properties of single bead are systematically studied, metallographic sample of single bead is prepared, and pictures and geometric characteristic data are collected. A high-quality titanium alloy thin wall is deposited by a stable single-layer deposition process parameter combination, the length and wall thickness of thin-walled parts are measured, and the line roughness and surface roughness of the thin-walled titanium alloy are determined. Finally, the thin-walled parts are cut into non-standard tensile specimens to test the mechanical properties in the deposition and travel directions.

The width and height of the deposited single bead are significantly affected by laser power. Under univariate conditions, with an increase in the laser power, the width of the single bead increases, the height decreases, and the wetting angle decreases (Fig. 3); with an increase in the wire feeding speed, the width remains stable, the height increases, and the wetting angle decreases (Fig. 4); with an increase in the travel speed, the width of the single bead decreases, the height tends to remain stable after reaching a certain speed, and the wetting angle does not change significantly (Fig. 5); the current does not affect the single bead deposition geometric features, but excessive current could worsen the formed morphology (Fig. 6). Thin-walled titanium alloys are deposited based on optimized process parameters, and it is found that the main factors affecting the deposition quality are the heat input and interlayer increment. By controlling the gradient input of the laser power and optimizing the interlayer increment and wire drawing method, the deposition quality is improved, and defects in the deposited parts are avoided. Finally, a titanium alloy thin-walled part without defects and with uniform width and height is obtained (Fig. 14). The average wall thickness of the titanium alloy thin-walled parts without post-treatment is 0.648 mm, with the thickness deviation of 0.004 mm (Fig. 15) and surface roughness (R_a) of 1.776 μm (Fig. 17). Results regarding the mechanical properties show that the tensile strength of the titanium alloy sample is 905.05-957.64 MPa (Fig. 18), and the mechanical properties are comparable to those of forging and casting (Table 3).

In this study, the effects of process parameters on the geometrical characteristics of deposited single bead are investigated using filament melting deposition process with the composite heat source of laser and joule heat. Using the composite energy generated by the laser and joule heat as the heat source and by controlling the heat input, titanium alloy is continuously deposited during the process of heating the wire, and high-quality thin-walled parts can be obtained at low laser power. By optimizing the deposition process, the defects and deficiencies in the deposition process are solved, and a thin-walled titanium alloy part with high surface quality and high dimensional accuracy is obtained. Accordingly, R_a is determined as 1.776 μm. The forming quality is much higher than that of mainstream wire feed additive manufacturing, and the maximum ultimate tensile strength is 957.64 MPa. The mechanical properties are comparable to those of forgings and casting.

Laser powder bed fusion (LPBF), as a typical additive manufacturing process, has the unique capability to consolidate powder in a layer-by-layer fashion according to user-defined configurations, using a laser as the energy source. Some typical metals, such as aluminum alloys, Ti alloys, Ni alloys, and stainless steel, have been successfully manufactured via LPBF and widely used in the aerospace, automobile, and marine industries. Inconel 718, an austenitic Ni-Cr-based superalloy, has an improved balance of creep, tensile properties, and corrosion resistance at 700 °C. To further improve the high-temperature performance of Inconel 718, incorporating hard and temperature-resistant ceramic particles within the Inconel matrix to produce metal matrix composites is regarded as a promising method. TiC particles are the most common ceramic reinforcements owing to their high melting point, high hardness, low density, and excellent friction and wear properties. In this study, LPBF was applied to prepare TiC-particle-reinforced Inconel 718 matrix composites with different TiC contents, and the effect of TiC content on the forming quality, microstructure, and mechanical properties of Inconel 718 composites was investigated.

The powders used were gas-atomized spherical Inconel 718 powder with a particle size distribution of 15-53 μm and irregularly shaped TiC powder with an average size of 1 μm. Powders with 0.5% (mass fraction) TiC and 1% TiC were prepared using a roller mixer. The TiC/Inconel 718 composite specimens were then fabricated with a BLT-S200 selective laser melting system using the optimized process parameters in an argon atmosphere. In addition, homogenization and solution aging heat treatment were performed on some of the tensile specimens. Subsequently, a roughness tester was used to evaluate the surface quality of the specimens. The porosity and defect distribution of the specimens were examined using optical microscopy (OM) and industrial computed tomography. The microstructures of the composites were observed using OM and scanning electron microscopy (SEM). Archimedes' method was used to measure the relative densities of the specimens. Vickers hardness was measured, and the tensile properties were examined using an electronic universal testing machine at room temperature. Moreover, the fracture morphology of the composite was characterized by SEM.

Compared with the as-deposited Inconel 718 superalloy, the addition of 0.5% and 1% TiC particles has little effect on the surface roughness of the sample. By adding 0.5% and 1% TiC particles, the increase in roughness of different surfaces of the composite block can be controlled within 5% (Fig. 3). The surface-forming quality of the Inconel 718 superalloy and its composites is good. The polished section of the Inconel 718 sample did not exhibit any apparent pores. However, several near-circular pores are observed in the polished section of the TiC/Inconel 718 samples (Fig. 4). The diameters of the pores in the Inconel 718 superalloy and its composites are concentrated between 20 and 70 μm, and most of the pores in the samples are <50 μm. The number of pores in the composites significantly increases with TiC content (Fig. 5). The relative densities of 0.5% TiC/Inconel 718 and 1% TiC/Inconel 718 are 99.55% and 99.32%, respectively (Table 2). The OM images of the section along the deposition direction of all samples exhibit the interlaced “fish scale” structure as normally observed in LPBF-fabricated metal parts (Fig. 6). The high-resolution SEM image reveals typical epitaxial crystallization in the microstructure of the Inconel 718 superalloy and its composites (Fig. 7). In addition, the reinforcing particles are homogeneously dispersed within the matrix, and their irregular edges and corners almost disappear (Fig. 8). The hardness of the composites increases with the TiC content. When the TiC content is increased to 1%, the hardness of the composite increases from 273 HV (Inconel 718) to 302 HV (Table 3). Compared with the as-deposited Inconel 718 superalloy, the tensile strength and yield strength of the 0.5% TiC/Inconel 718 composite increase by 36 and 34 MPa, respectively, but the elongation decreases slightly. When the TiC content is increased to 1%, the average tensile strength, yield strength, and elongation of the 1% TiC/Inconel 718 composite reach 1111 MPa, 781 MPa, and 20.6%, respectively. Compared with the heat-treated 718 superalloy and its composites, the addition of TiC particles also increases the tensile strength and yield strength and reduces the elongation (Fig. 9). Numerous pores and a few cracks are observed in the tensile fractures of the TiC/Inconel 718 composites [Figs. 10(c) and 10(d)]. The high-resolution SEM image and scanning results of the Ti element distribution show that TiC particles are distributed in these pores, which is the reason for the decreased elongation of the composites.

TiC/Inconel 718 composites were manufactured using the LPBF process. The surface-forming quality of the Inconel 718 superalloy and its composites is good, and adding 0.5% and 1% TiC particles can enable control of the increase in the roughness of different surfaces of the composite block to within 5%. Compared with the pore distribution of the Inconel 718 superalloy, the number of pores in the composite with 0.5% TiC particles increases by 37.5%; when the TiC content increases to 1%, the number of pores in the composite increases significantly, and the relative density of the composite decreases from 99.70% (Inconel 718) to 99.32%. The microstructure of the TiC/Inconel 718 composite is similar to that of the Inconel 718 superalloy, and typical epitaxial crystallization is observed, in which TiC particles are uniformly distributed in the matrix of TiC/Inconel 718 composites. With an increase in TiC content, the average Vikers hardness of the composites gradually increases from 273 to 302 HV. Compared with the Inconel 718 superalloy prepared via LPBF, the tensile strength and yield strength of the 1% TiC/Inconel 718 composite increase by 66 and 45 MPa, respectively. The addition of TiC particles improves the tensile strength and yield strength but decreases the elongation.

Carbon fiber reinforced plastics (CFRP) and titanium alloys are used in modern equipment owing to their high specific strength, corrosion resistance, and fatigue resistance. Heterogeneous joints consisting of CFRP and metals are widely used in the aerospace industry. Laser joining of CFRP and metals has recently attracted significant interest owing to its high joining efficiency and superior joining quality compared to mechanical joining, adhesive bonding, ultrasonic welding, etc. Because the process is both time-consuming and labor-intensive, the accurate prediction of appropriate process parameters is highly desired. In this study, a realistic three-dimensional finite element model is developed for the numerical simulation of the temperature field during the laser joining of carbon-fiber reinforced polyetheretherketone matrix composites (CFPEEK) with a poly ether ether ketone (PEEK) resin matrix and TC4 titanium alloy, and the laser joining process is investigated.

A fiber laser with a laser wavelength of 1070 nm and collimator focal length of 200 mm is used in the laser joining experiment (Fig. 1). A rectangular spot is obtained by beam shaping using an integrating mirror, with a focal length of 200 mm and a spot size of 0.6 mm×5.8 mm. The laser power is 3500 W, and the welding speed ranges from 5 mm/s to 40 mm/s. The dimensions of the TC4 titanium alloy and CFPEEK are 60 mm×30 mm×2 mm and 60 mm×25 mm×2 mm, respectively. The CFPPEK is composed of a PEEK matrix and carbon fibers. The carbon fiber layers are alternately layered with PEEK layers and have a 0°/90° cross-weave (Fig. 2). The PEEK layer is 0.1 mm thick on average, and the carbon fiber layer is 0.2 mm thick on average.

A three-dimensional model of laser joining of TC4/CFPEEK is established based on the fact that laser-induced heat input is transferred to the bonding interface via heat conduction and actual experimental conditions. The grid size is graded to improve the simulation accuracy while maintaining computational efficiency (Fig. 3). Importantly, in the CFPEEK module, the PEEK layer thickness (0.1 mm) is uniform for an isotropic homogeneous material, and the fiber layer thickness is uniform (0.2 mm) for an orthotropic material with a large difference in the thermophysical characteristics between the radial and axial directions. The thermal and physical parameters of the TC4 and CFPEEK used in the simulation are listed in Table 1. It is worth noting that the thermal conductivity of TC4 is relatively low, and it is therefore difficult to obtain the maximum melting depth in both TC4 and CFPEEK simultaneously (Fig. 4); thus, the experimental and simulation results of TC4 and CFPEEK are compared separately. The results reveal that the temperature field distribution obtained by the numerical simulation agrees well with the experimental results (Figs. 5-7). Our developed model is then used to estimate the temperature field distribution as a function of laser power (Figs. 8-10), welding speed (Figs. 11-14), and beam size (Figs. 15 and 16). Finally, a simulation-predicting laser joining process window is provided to guide the parameter selection for laser joining of TC4 and CFPEEK (Fig. 17).

Considering the carbon fiber and resin distributions in CFPEEK, a more realistic finite element model for the laser joining of CFPEEK and TC4 is developed. In this model, CFPEEK is composed of carbon fiber layers and resin layers that vary in thickness. The resin layer is isotropic, whereas the carbon fiber layer is orthogonal. This model is used to investigate the effects of laser parameters on the temperature field of the joint. The accuracy of the model is confirmed by the experimental results, and the process window for laser-joining CFPEEK and TC4 is predicted.

The fabrication of an overhang or large inclination structure with laser cladding must be completed on an inclined substrate. Regarding the parameters and morphology of laser cladding layer, previous studies have mainly been conducted on a horizontal base plane. Few studies have focused on the influence of different inclination angles of the base plane on forming morphology. The molten pool is often stretched or even displaced by gravity when conducting multilayer deposition with a large inclination, which affects the height and width of the single pass after solidification. A slight change in the height and width can affect the final forming accuracy; whereas, large changes in the height and width, particularly when the actual layer height is inconsistent with the preset layer height, will directly affect the forming quality and continuity. Therefore, this study explores the influence of different base plane inclinations on the height and width of a single track, and uses the base plane inclination as one of the inputs to establish a neural network prediction model.

First, a single-factor experiment method was used to scan a single layer to determine the working range of each process parameter and the change step of the parameters. The laser power was varied from 800 to 1200 W in steps of 100 W. The scanning speed was varied from 4 to 8 mm/s in steps of 1 mm/s. The angle was varied from 0° to 135° at the step rate of 15°. Thin-wall deposition experiments were then carried out at 10 selected angles, and five groups of deposition with different power parameters at each angle were considered. Each group of thin wall was deposited with 30 layers, and the process included five groups of selected scanning speeds, which was changed every six layers. A CCD layer height measurement system was used to collect layer height data in real time during the deposition process of the thin wall.

The thin wall was cut from the middle, and the width of each layer of the cut section was measured. The mean values of the last three layers of every six layers in the measured layer height data are valid (Fig. 6). Finally, 250 sets of height and width data were obtained. Based on this data (Figs. 7 and 8), a BP neural network prediction model was established. The model considers the inclination of the cladding base plane, scanning speed, and power as the input, and the height and width of the cladding layer as the output. The data containing various angles, power, and speed information were regarded as the training set to enhance the comprehensiveness of the test set. The model was built using only the training set, and the remaining data were used as the test set. The test set was only used to test the predictive ability of the model and evaluate its generalization ability.

The influence of variable angle cladding of 0°-135° on the single-pass morphology was studied. The experimental results show that the layer height first decreases and then increases with the change in the inclination angle, and that at 90° yields the lowest layer height, which can be attributed to the constant change in the angle between the gravity direction and the growth direction. The layer width first increases and then decreases with the angle change, reaching the highest value at 90°. The root mean square error of the two established neural network prediction models is controlled below 0.1, and the 90% confidence prediction accuracy A_90% is 99% and 96%, respectively (Fig. 11), showing an excellent prediction effect of the established model.

Cutting flexible copper clad laminates (FCCLs) using UV lasers is an excellent process; however, it has certain drawbacks. Multiple laser cuts are often required for double- and multi-layer FCCLs, resulting in lower productivity. In addition, laser cutting leaves debris on the product surface, which requires secondary cleaning, thereby increasing costs. Most importantly, the high temperature and high laser energy density can easily exceed the thermal damage threshold of the product, resulting in carbonization. Carbide can cause short circuits, which are the most severe defects in electronics. Moreover, process defects such as excessive heat-affected zones of FCCL caused by improper parameter selection, over-melting of the material, or cutting energy not reaching the cutting threshold result in product failure and increased power loss, which in turn increases process costs. Therefore, we designed a laser cutting process that can effectively reduce the photo-thermal effect on the surface and cutting edge of the FCCL, and cut the FCCL using minimum energy to reduce the generation of carbide. It is expected that this method can be applied to various thin-film materials that require laser processing.

The 49 μm polyimide double-sided adhesive-free flexible copper clad laminate was cut using a 355 nm UV nanosecond laser cutter as follows. First, the minimum energy for cutting the FCCL, the critical cutting energy , was determined by varying the laser scanning speed, laser scanning power, and the number of repetitive scans through pre-cutting experiments. Second, three laser cutting parameters were traversed to verify and correct the critical cutting energy value. Finally, the critical cutting energy value was used to examine the effect of the aforementioned laser cutting parameters on the surface profile, cross-sectional profile, degree of carbonization, width of the etched slit, and changes in the surface and cross-sectional composition of the FCCL, while ensuring the cutting through the FCCL. This allows the process parameters to be further adjusted to achieve high quality and low power consumption cutting.

Through the study of the intrinsic relations among the laser cutting parameters used in this experiment, the critical cutting energy value of a 49 μm FCCL was initially derived as 0.4000 J/mm using the relevant equations and experimental data. Then, the critical cutting energy value was corrected to 0.3867 J/mm by traversing each laser parameter (Fig. 3). The results also indicate that an increase in laser scanning speed is beneficial in reducing carbide generation, inhibiting the phenomenon of melt reflux due to thermal effect, avoiding slag accumulation, and obtaining a flatter and cleaner cross section, which are beneficial to improve the dimensional accuracy of the product and reduce the short circuit phenomenon of the product more effectively (Figs. 4 and 10). An increase in laser scanning power can reduce the copper content at the cross-section and increase the spacing between slits. However, the high power is likely to cause defects such as rough cross-sections and over-melting of the copper layer, whereas a low power can fail to cut through the FCCL (Fig. 6). Increasing the number of repetitive cuts can alleviate the over-melting phenomenon during cutting and improve the surface quality of the etched seam; however, it tends to increase the spacing of the etched seam. Although it does not have a significant effect on the degree of charring and carbonization, repetitive cutting results in loss of energy (Fig. 8). The optimal parameters for cutting the FCCL under the conditions of this study are a laser scanning speed of 2000 mm/s, laser scanning power of 12 W, and 70 repetitive cuts (selected at the expense of partial processing efficiency).

In this study, the influence of different process parameters on the laser-cutting quality of FCCL is examined in detail, and a method to optimize the laser cutting process is proposed. The principle behind the selection of the process parameters is to select cutting parameter values according to the evaluation parameter table used for pre-cutting experiments after calculating and verifying the required critical cutting energy value. Therefore, priority is given to moderate cutting power, followed by a high cutting speed, and finally, a minimum number of repetitive cuts. The results of this study provide a guide for achieving high-quality, low-energy laser cutting of FCCLs. In addition, the experimental method and the optimization of laser cutting parameters provided in this study are applicable to different thicknesses of FCCL, flexible circuit boards, polyimide coverlay, and reinforcing plates shaped by laser cutting.

Laser welding processes with high energy density, precision, and efficiency are widely employed in the automotive, aerospace, and medical industries. They have significant advantages in joining materials, such as aluminum alloys, stainless steel, magnesium alloys, and dissimilar metals.

The welding penetration state is one of the most critical indicators for the quantitative evaluation of laser welding quality. The precise identification of the weld penetration state in real-time is a key bottleneck in monitoring and controlling dynamic laser welding processes. The complex physical-chemical metallurgical interaction between the laser beam and the metal workpiece releases intense optical, thermal, and acoustic radiation. The acoustic information is derived from the thermal vibration under a high heat input, and the pressure shock wave generated when the keyhole is internally stressed. Its acoustic characteristics (sound pressure amplitude and frequency characteristics) are closely related to the state of the keyhole. In this paper, we present a new method for quantitatively assessing weld penetration state based on acoustic time-frequency characteristics and deep learning for pulsed laser welding of thin-walled aluminum alloys. This study will contribute to research on the high correlation between acoustic information and weld penetration state in laser welding.

First, as shown in Fig. 1, a visual-acoustic-emission multi-information real-time synchronous sensing system was developed to acquire visual images and acoustic signals reflecting the dynamic behavior of the keyhole, and the acoustic signals were preprocessed using frame splitting and wavelet-packet threshold denoising methods. Second, a smoothed pseudo-Wigner-Ville distribution (SPWVD) was used to extract time-frequency domain images from each frame of acoustic signal, and a gray-level co-occurrence matrix (GLCM) was introduced to extract the time-frequency domain texture features and feed them into the back-propagation neural network (BPNN) for prediction. Finally, a convolutional neural network (CNN)-based weld penetration state classification model was established using the SPWVD acoustic time-frequency map as the original input.

During the preprocessing of the acoustic signal, wavelet-packet threshold denoising effectively filters out some burrs in the signal, and the denoised signal is framed in one pulse period, as shown in Fig 2. Second, the time-frequency maps of the acoustic signals extracted using the SPWVD method exhibit significant differences in the texture features with different penetration states, as shown in Fig 6. Here, the four texture features of the SPWVD time-frequency maps extracted from the GLCM show a clear trend as they change from non penetration to partial penetration and then to full penetration states. Finally, we constructed GLCM-BPNN and SPWVD-CNN classification models and compared the advantages and disadvantages of both classification models. Despite the high correlation between the acoustic time-frequency map and the dynamic behavior of the keyhole and welding penetration, the CNN classification model based on the SPWVD time-frequency map shows a higher accuracy (98.8%) than the traditional BPNN classification model (85%). This indicates that the deep learning model based on the SPWVD time-frequency map as a direct input to the CNN model yields improved recognition results.

(1) The preprocessing method of acoustic signals using frame splitting and wavelet packet thresholding can effectively intercept useful segments and obtain a signal with good denoising results.

(2) The SPWVD method extracts a time-frequency map of the pulsed laser welding acoustic signal. The texture features of SPWVD time-frequency map is highly correlated with the dynamic behavior of the laser-welded keyhole and the weld penetration state.

(3) The SPWVD-CNN deep learning weld penetration state recognition model has a classification accuracy exceeding 98%. The proposed model provides a new approach and technical path for reliable monitoring of the thin-walled aluminum alloys laser welding process.

Seam tracking technology based on laser structured light vision sensing, which transforms weld positioning into the positioning of structured light stripe feature points, has strong universality and robustness. It is regarded as the most promising seam tracking solution for engineering implementation. However, arc light, splashes, and fumes in real-time seam tracking can severely contaminate the structured light image, which affects the accuracy and robustness of weld positioning. In addition, the welding site typically provides limited computing power, and the real-time performance of weld positioning directly affects welding efficiency and quality. Accurate and efficient filtering of noise in structured light images can effectively improve the accuracy and efficiency of weld feature positioning, which is valuable in improving welding quality. This study proposes a structured light image segmentation method based on a lightweight DeepLab v3+ semantic segmentation network. It implements noise filtering of arc light, spatters, and fumes, to achieve accurate and efficient noise filtering of weld structured light images by segmenting laser structured light stripes.

A method for weld structured light image segmentation based on a lightweight DeepLab v3+ semantic segmentation network was proposed in this study to filter the noise in the weld structured light image. First, the image characteristics of the structured light of the weld in the dataset were analyzed. Most positions of structured light stripes in the structured light images can be easily distinguished from the image background, except for the region with significant aliasing between structured light stripes and noise. Using a shallow network can also result in significant expressiveness for this problem. Therefore, the Resnet-18 network was adopted to replace the original backbone network. This improved the inference speed of the DeepLab v3+ semantic segmentation network. Second, the ratio of pixels occupied by structured light stripes to the image background in the weld structured light image dataset of this study was significantly imbalanced. The original loss function would have made the model predict more pixels of structured light stripes as the image background, a result unconducive to weld feature point positioning. In this study, a weighted cross-entropy loss function was designed with the complement of pixel occupancy as the weight to improve the segmentation accuracy of structured light stripes.

The segmentation results of the structured light images show that the proposed method can precisely segment the structured light stripe under noise interference from sources such as arc light and spatter (Fig. 7). The backbone network test and comparison results show that the segmentation performance and efficiency of the DeepLab v3+ semantic segmentation network improve when the Resnet-18 network is used as the backbone network (Table 2). The loss function test and comparison results show that the weighted cross-entropy loss function significantly improves the pixel accuracy A_L of structured light stripe segmentation of the model. The model has the highest average score when the ratio of structured light stripe loss-gain coefficient α₁ to image background loss-gain coefficient α₀ is 1/15. This result indicates that the model achieves a balance between false and missed detection of structured light stripes and exhibits optimal overall performance (Table 3). Finally, the proposed method has an average single-image inference time of 15.9 ms, a pixel accuracy of 96.47% for structured light stripes, and an average intersection-over-union of 89.04% for structured light stripes, which is superior to the comparison methods in terms of segmentation performance and efficiency (Table 4). In the region with severe aliasing of structured light stripes and noise, the proposed method outperforms the comparison methods in segmenting the boundary of structured light stripes, demonstrating the effectiveness and superiority of the proposed lightweight DeepLab v3+ semantic segmentation network in the segmentation of weld structured light images (Fig. 8).

This study proposes a weld structured light image segmentation method based on a lightweight DeepLab v3+ semantic segmentation network to filter out noise, from sources such as arc light, splash, and soot, in the weld structured light image and improve the accuracy and robustness of weld tracking. The proposed network achieves noise filtering by segmenting the foreground image of the weld structured light image. First, a shallow Resnet-18 network is used to replace the original backbone network to improve the efficiency of the DeepLab v3+ semantic segmentation network. Next, a weighted cross-entropy loss function is designed with the complement of pixel occupancy as the weight to improve the pixel accuracy of the DeepLab v3+ semantic segmentation network for structured light stripe segmentation and reduce the missed detection rate of structured light stripes. The experimental results show that: (1) by using the shallow Resnet-18 network instead of the original backbone network, the DeepLab v3+ semantic segmentation network can improve the inference speed in structured light image segmentation without degrading the segmentation performance; (2) the improved weighted cross-entropy loss function can effectively improve the pixel accuracy and intersection-over-union ratio of the structured light streak segmentation model; (3) the proposed lightweight DeepLab v3+ semantic segmentation network exhibits better segmentation performance and higher efficiency in weld structured light image segmentation than the classical semantic segmentation model, indicating the effectiveness and superiority of the proposed method in weld structured light image segmentation.

Nickel-based superalloys exhibit high potential as raw materials for heated-end-core components because of their superior structural stability, oxidation resistance, corrosion resistance, fatigue resistance, and creep resistance. When compared to electrical discharge machining and normal pulsed laser machining, femtosecond laser machining offers higher precision and a lower heat effect. When a femtosecond laser pulse irradiates a metallic surface, electrons absorb photons and reach high temperature while the lattice remains unchanged, resulting in a low thermal effect. So, the femtosecond laser is expected to have the potential for machining GH3230 superalloys. At present, some reports have focused on femtosecond laser drilling technology and the increase of hole depth of nickel-based alloys, whereas femtosecond laser ablation technology and its characteristics on nickel-based alloys are yet to be discussed. In this study, a green femtosecond laser with a wavelength of 515 nm was used to investigate the ablation threshold, ablation rate, and ablation depth of a GH3230 nickel-based superalloy. Furthermore, an increase in the scanning width is proposed to expand the ablation depth. The effect of plasma induced by a femtosecond laser on the ablation depth is analyzed.

An as-forged GH3230 superalloy with a thickness of 1.2 mm was used as the base metal in laser ablation. First, a green femtosecond laser with a wavelength of 515 nm was used to ablate the GH3230 surface. The ablation width was measured using a white-light interferometer. The ablation thresholds at different scanning speeds were calculated by fitting the relationship curve between ablation width and laser fluence. The multi-pulse threshold incubation coefficient of GH3230 and ablation threshold of GH3230 under a single laser pulse were obtained by fitting the relationship curve between the ablation threshold and equivalent pulse number. The absorption spectrum of GH3230 was obtained using a spectrophotometer. After various scanning times ranging from 800 to 20000, the ablation depth and width were measured using an optical microscope (OM) with laser fluences of 1.27, 2.54 and 3.81 J/cm². The saturation of the ablation depth and the limitation of the ablation rate were analyzed to reveal the advantages of green femtosecond laser machining. Finally, the ablation characteristics of GH3230 were adjusted using the overlapping scanning method. The ablation depth and width were measured by OM to obtain the ultimate ablation depth, ablation rate, and depth-to-width ratio. The effect of the scanning spacing is further discussed.

In this study, the ablation threshold of GH3230 using a green femtosecond laser was lower than the previously reported ablation threshold of nickel-based alloys using an infrared femtosecond laser. The main reason for this is that the absorption of the green light wave band is much higher than that of the red band. The green laser also reduces the ablation threshold owing to its higher photon energy, which possibly removes materials more efficiently. In the overlapping scanning method, both the ablation width and depth increase. The ablation efficiency of the green femtosecond laser is higher than that of the infrared femtosecond laser. As the scanning width increases, the ablation width increases, which causes the plasma to diffuse laterally and weakens the shielding effect of the plasma on the laser. The energy at the bottom of the groove increases the ablation depth and efficiency.

In this study, the single-pulse laser ablation threshold of as-forged GH3230 superalloy using a green femtosecond laser with a wavelength of 515 nm was 0.27 J/cm². With an increase in the number of scanning, the ablation depth increases and the beam energy entering the groove bottom decreases. Finally, the ablation depth tends to saturate. In the overlapping scanning method, with an increase in ablation width, the plasma diffuses laterally and reduces the plasma density. The reduction in plasma density increases the ablation depth limit but decreases the depth-to-width ratio of the groove. Compared with an infrared femtosecond laser, the green femtosecond laser can significantly reduce the ablation threshold and improve ablation efficiency.

Ultrahigh-power laser welding is an important development direction for plates with medium-thickness welding. The laser-arc hybrid welding method has obvious advantages in improving the appearance, quality, and efficiency of the weld. Therefore, the 10 kW level high power laser-arc hybrid welding technology has developed rapidly. However, when the laser power reaches more than 10 kW, the vaporization behavior of the materials, the interaction between the laser beam and plasma, the stable state of the molten pool flow, the mechanism of heat transmission, and the metallurgical behavior of the weld all change to different degrees, which will affect the stability of the welding process, leading to a poor appearance of the weld and generation of weld defects, and seriously limiting the popularization and application of 10 kW laser welding. The variation in the plasma morphology during the welding process indirectly reflects the stability of the welding process. In this study, the characteristic parameters are collected, which reflect the plasma morphology and appearance of welds of three different hybrid welding methods with different laser powers: laser-MAG single-wire hybrid welding, laser-MAG single-wire hybrid welding with filler wire, and laser-MAG double-wire hybrid welding, to seek the characteristic parameters for predicting the quality of welds and providing reference values for ultra-high-power laser-arc hybrid welding with different heat sources.

laser-MAG single-wire hybrid welding, laser-MAG single-wire hybrid welding with filler wire, and laser-MAG double-wire hybrid welding. The weld width and penetration were extracted when the laser power increased from 5 kW to 30 kW. Then, the plum and spatter, which were produced in the welding process and investigated by a high-speed camera, the plasma diffusion height, area, and plasma splash area with different laser powers were extracted for the three welding methods. The goal is to explore the relationship between the size of the weld and the morphological characteristics of the welding plasma for different welding methods and laser energy, which lays the foundation for 10 kW high power laser-arc hybrid welding.

As shown in Figure 4, the weld face of the three welding methods becomes worse with the increase in laser power, especially when the laser power is 20 kW. The appearance of the weld changes differently, and the differences among the three welding methods are gradually highlighted. The increase in the feature size of the weld is proportional to the increase in the laser power, but the relationship is not linear. Before and after the laser power reaches 20 kW, the increase in the weld feature size decreases slightly, and concave-convex points appear in the size curve; when the power is the same, the penetration of the laser-MAG single-wire hybrid welding is small, while that of the laser-MAG single-wire hybrid with filler wire is large. The former increases slightly with an increase in laser power, whereas the latter increases significantly. The variation law of the weld width with laser power is similar to that of penetration, and the weld size curve of the laser-MAG double-wire hybrid welding method is always in the middle position, as shown in Figure 6. For the three welding methods, the plasma area and the fluctuation increase with an increase in the laser power, and the variation trend of plasma fluctuation is the same as the fluctuation of penetration and the fluctuation of plasma spatter, but the fluctuation of weld width is smaller, as shown in Figures 9 and 11.

Three different welding methods were used to explore the regular appearance of the weld and plasma morphology with different laser powers. The results showed that when the power was increased, the plasma area and fluctuation of the three welding methods increased, and the weld width, penetration, and fluctuation values increased. When the power was increased to 20 kW, the increment in the plasma area and fluctuation decreased, the increment in the weld size decreased, the maximum increment of weld penetration for laser-MAG single-wire hybrid welding decreased by 71.64% compared with the other two welding methods, and the appearance of the weld worsened. In addition, when the power was constant, compared with laser-MAG single-wire hybrid welding, the plasma area and standard deviation increased, the penetration depth decreased, and the appearance of the weld deteriorated. When laser-MAG double-wire hybrid welding was adopted, the changes in the plasma morphology and appearance were not obvious. When the power was increased to 20 kW, the increment in the amplitude of the variation decreased. In addition, there is a correlation between the appearance of the weld and plasma morphology. The plasma morphology is related to the laser power and wire feeding mode: when the laser power increases or the filler wire is added, the plasma concentration in the incident direction of the laser increases, the stability worsens, and the attenuation and interference of the laser enhance, which leads to a decrease in penetration and an increase in the spatter. Therefore, the change in plasma shape can be used as a reference to predict the appearance quality of the weld.

The fabrication strategy for nonlinear photonic crystals has drawn substantial research interest because of their highly efficient nonlinear optical interactions. Femtosecond laser engineering has distinct advantages over conventional methods for the fabrication of nonlinear structures. These advantages include its high precision, resolution, and flexibility. This paper summarizes the research progress of femtosecond laser processing technology for constructing nonlinear photonic crystals and provides a brief introduction to the quasi-phase matching theory involved. The processing mechanism of femtosecond-laser-induced ferroelectric domain inversion and laser erasure of second order nonlinear polarization coefficients (χ2) are discussed, and the experimental results and applications of nonlinear photonic crystals in different dimensions realized by these two approaches are demonstrated. Finally, the challenges of the femtosecond laser technique in the processing of nonlinear photonic crystals are analyzed, and the prospects for future development are presented.

This paper summarizes the research progress of femtosecond laser processing technology for constructing nonlinear photonic crystals and also provides a brief introduction to the quasi-phase matching theory involved. The processing mechanism of femtosecond-laser-induced ferroelectric domain inversion and laser erasure of χ2 are discussed, and the experimental results and applications of nonlinear photonic crystals in different dimensions realized by these two approaches are demonstrated. Finally, the challenges faced by the femtosecond laser technique for processing nonlinear photonic crystals are analyzed, and the prospects for future development are discussed.

Tightly focused femtosecond laser pulses can induce a thermoelectric field in the ferroelectric crystal that inverts the direction of spontaneous polarization. On the basis of this mechanism, an arbitrary arrangement of 2D inverted domains can be constructed to enhance the second-harmonic emission from the crystal, and quasi-phase-matching structures can be integrated in the LiNbO₃ waveguide to achieve efficient frequency conversion (Fig. 4). This technique can also be used to fabricate 3D nonlinear photonic crystals in multi-domain/single-domain Ba_0.77Ca_0.23TiO₃(BCT)/ Ca_0.28Ba_0.72Nb₂O₆(CBN) crystals, which demonstrate second harmonic diffraction with 3D quasi-phase matching (Fig. 5). Another technique that relies on the laser-induced amorphization of the crystal to partially erase χ2 shows versatility for processing non-ferroelectric crystals. Multiple quasi-phase-matching structures can be inscribed into the waveguide core to realize a parallel multiwavelength output (Fig. 6). A 3D nonlinear photonic crystal has also been obtained using this technique in LiNbO₃ to provide abundant 3D reciprocal vectors for second-harmonic generation in various directions (Fig. 7).When processing inside the crystal, the aberration resulting from the mismatch of the refractive index causes an axial shift of the focal spot, which seriously limits the axial resolution as well as the fabrication quality of the structures. Reasonable diffractive optical components for aberration compensation must be implemented during fabrication. One method is to introduce a spatial light modulator into the femtosecond laser processing system, thereby eliminating the effect of aberration by loading a specific phase hologram.To date, few attempts have been made to combine nonlinear photonic crystals with other optical devices to extend their functionalities. Various functional optical devices, such as electro-optic modulators, resonators, waveguides, and nonlinear frequency converters, can be integrated within a single ferroelectric crystal by combining the flexibility of the femtosecond laser and other processing techniques. The integrated photonic chips exhibit more powerful functions in modern optical signal processing and quantum computing.Currently, χ2 can only be reduced by 20% using the femtosecond laser erasure technique, which restricts the modulation efficiency of the structures. Therefore, a deep understanding of the femtosecond laser interaction mechanism with the lattice is required to determine the optimal fabrication parameters for large-amplitude χ2 erasure, thereby improving the frequency conversion efficiency of the as-prepared structures.In addition to the aforementioned development trends, certain topics, such as the development of a fabrication strategy with high efficiency to lay the foundation for mass production, must be investigated further. With improved femtosecond laser processing technology, nonlinear photonic crystals show promising prospects.

The spatial distribution ofχ2can modulate wavefronts in a new wavelength range; thus, it can be applied in optical communication, optical storage, and quantum information processing. Nonlinear patterns constructed flexibly by a femtosecond laser are capable of the nonlinear generation of vortices and Hermite–Gaussian beams. The as-prepared 3D nonlinear photonic crystal can realize the simultaneous conversion of the fundamental beam into multiple structured beams (Fig. 9) or efficient beam shaping based on full-dimensional phase matching and nonlinear volume holography (Fig. 10). In the past few years, researchers have also introduced detour phase encoding by femtosecond laser fabrication into nonlinear holography to realize the reconstruction of arbitrary target images (Fig. 11). Moreover, the strategy of erasing nonlinear coefficients using femtosecond lasers can be applied to quartz crystals to obtain efficient frequency doubling in the challenging deep-ultraviolet region (Fig. 13).

While substantial progress has been made in the femtosecond laser processing of nonlinear photonic crystals, some challenges remain.

When processing inside the crystal, the aberration resulting from the mismatch of the refractive index causes an axial shift of the focal spot, which seriously limits the axial resolution as well as the fabrication quality of the structures. Reasonable diffractive optical components for aberration compensation must be implemented during fabrication. One method is to introduce a spatial light modulator into the femtosecond laser processing system, thereby eliminating the effect of aberration by loading a specific phase hologram.

To date, few attempts have been made to combine nonlinear photonic crystals with other optical devices to extend their functionalities. Various functional optical devices, such as electro-optic modulators, resonators, waveguides, and nonlinear frequency converters, can be integrated within a single ferroelectric crystal by combining the flexibility of the femtosecond laser and other processing techniques. The integrated photonic chip will exhibit more powerful functions in modern optical signal processing and quantum computing.

Currently, χ2 can only be reduced by 20% using the femtosecond laser erasure technique, which restricts the modulation efficiency of the structures. Therefore, a deep understanding of the femtosecond laser interaction mechanism with the lattice is required to determine the optimal fabrication parameters for large-amplitude χ2 erasure, thereby improving the frequency conversion efficiency of the as-prepared structures.

In addition to the aforementioned development trends, certain topics, such as the development of a fabrication strategy with high efficiency to lay the foundation for mass production, must be investigated further. With improved femtosecond laser processing technology, nonlinear photonic crystals show promising prospects.

The supply of natural diamond can no longer meet the normal production and living needs of humans due to the increasing number of applications for diamond. High-quality and large-scale production of synthetic diamond has been a topic of research since Tennant discovered diamond was an allotrope of carbon in 1796. Diamond substrates used for optical devices can be classified as single-crystal diamond (SCD) or polycrystalline diamond (PCD). Diamond materials can be divided into four types according to their composition and nitrogen impurity content: I a, I b, Ⅱ a, and Ⅱ b, as listed in Table 1 with their specific parameters. Diamond manufacturing methods can be divided, depending on the characteristics of the technology used, into static, dynamic, and low pressure. Diamond formation can be divided into direct, melting, and epitaxial methods based on the characteristics of the diamond produced. Gem-grade diamonds are usually prepared by high-temperature and high-pressure (HPHT) and chemical vapor deposition (CVD).

A window material with high transmittance is an important guarantee of the high precision and stability of a photoelectric system. Diamond has high transmittance in the wavelength range from ultraviolet (0.225 μm) to microwave (~8000 μm) and has become one of the best choices for manufacturing wide-band window materials for photoelectric systems with the invent of increasingly sophisticated diamond preparation technology. Diamond is a third-generation ultra-wide bandgap semiconductor material and has a typical face-centered cubic structure (lattice constant is 0.35668 nm, bond length is 0.155 nm, bond angle is 109°28') with several characteristics such as high hardness (Mohs hardness 10), high thermal conductivity (2000 W/mK), and low thermal expansion coefficient (1.2×10^-6 K^-1).

The high refractive index of the visible and infrared bands of synthetic diamond results in a diamond transmittance of only 71%, which limits its in-depth application in high-precision photoelectric systems. At present, the most commonly used method is to construct a surface anti-reflection film to reduce the reflection loss of the diamond surface. The preparation principles and processes involved in this method are relatively simple. The anti-reflection effect is achieved using a film by selecting different materials so that the reflected light on the upper and lower surfaces of the film interfere and counteract the reflection. However, there are a limited selection of viable anti-reflection coating materials for some wavelengths, resulting in a narrow bandwidth and insufficient angular spectrum range. The damage threshold of anti-reflection coatings is lower than that of diamond, which significantly reduces the service life, stability, and output power of high-power lasers. Recent studies have suggested this problem can be solved by exploiting micro-nano structures designed by theoretical calculation to produce diamond anti-reflection micro-nano structures with sizes ranging from 1 nm to 100 μm by laser etching and ion etching.

Although research in this area has been widely reported, there is no systematic summary in the literature of the design and preparation of anti-reflection micro-nano structures on diamond surfaces. Therefore, this paper classifies and summarizes the design theory and preparation technology of diamond surface anti-reflection micro-nano structures, which provides a technical reference for preparing diamond micro-nano structures and expanding the anti-reflection applications of diamond micro-nano structures in the future.

In this paper, the research progress in diamond anti-reflection micro-nano structures in recent years is reviewed. First, the anti-reflection mechanism of the micro-nano structure (vector diffraction theory, EMT) is introduced and the viability of the two mechanisms are discussed before the anti-reflection mechanism of EMT is theoretically deduced (Fig. 1 and Fig. 2). Second, the basic principles and technological conditions of laser processing and ion etching are summarized. The different pulse widths determine the micro-nano structures on the diamond surface prepared by laser processing and that allows them to be divided into two categories: micro-nano structure etched by short-pulse lasers (ns) and periodic structure induced by ultrashort pulse (fs) lasers. Ion etching uses high-energy ions to bombard the diamond surface to obtain a smaller feature size on the diamond surface. Through different process conditions, a vertical or angled-side wall morphology can be obtained. The influence of the micro-nano structure on the transmittance of diamond prepared by the two methods is summarized (Table 2), and the advantages and disadvantages of various preparation technologies in the application of diamond anti-reflection are compared (Table 2). The application prospects of diamond are briefly described, with the aim of providing a technical reference for related fields.

This paper gives a systematic summary of the basic principle and processing method for laser machining and ion etching anti-reflection micro-nano structures on diamond surfaces. The influence of micro-nano structures with different periods and contours on transmittance properties is explored, and the advantages and disadvantages of various preparation technologies are clarified. This provides a systematic reference for researchers in related fields from which they can design and prepare anti-reflection micro-nano structures on diamond. This research shows that it is necessary to use simulation software to optimize the design of micro-nano structure parameters, such as size, period, duty cycle, and height, before the diamond surface anti-reflection micro-nano structure is prepared (using the technology presented in this paper), and to design the method for micro-nano processing based on theoretical calculation results. However, current preparation processes cause errors in the micro-nano structure size, period, and other parameters that may lead to deviations in the peak transmittance. Therefore, understanding how to match the preparation technology with the simulation results and achieve the ideal anti-reflection effect on a diamond surface is still an urgent problem requiring a solution.

Laser is a tool widely used in industrial manufacturing that has the advantage of non-contact technology. Lasers can be used to produce complex structures without photomasks in air, vacuum, or water. In addition, lasers can be easily focused down to the micrometer scale; therefore, they can be used in microdevice fabrication. In particular, they are widely used in marking, drilling, annealing, surface modification, and other processes in the microelectronics industry. However, because of the diffraction limit, the minimum achievable resolution of a laser is limited by its wavelength. The microsphere provides a mechanism to manipulate light in a way that cannot be achieved using traditional optical components. The focusing and scattering of light can be manipulated at the microscopic scale using microspheres. The limitation caused by the diffraction limit is overcome based on near-field optics. Therefore, optical dielectric microspheres are used to modulate the laser and realize micro-nano processing with a resolution above the diffraction limit. On this basis, researchers have also overcome the difficulties of traditional micro-nano processing techniques, such as slow processing and inability to achieve large-area one-time processing, through self-assembled microsphere array technology. At the same time, researchers have also realized the processing of arbitrary micro-nano patterns using off-axis laser irradiation technology. In this study, micro-nano processing was realized by modulating the laser with a densely packed single-layer dielectric microsphere array. Pattern processing, which breaks through the diffraction limit resolution, was realized on a gold film on the surface of the microsphere.

The near-field optical enhancement effect of the microspheres was simulated and analyzed, and the mechanism of the effect of laser direct writing technology on the gold micro-nano structure using the microsphere array was obtained. The experimental method (Fig. 3) includes the following steps: preparing the polydimethylsiloxane (PDMS) thin film, closely laying the dielectric microsphere array on the PDMS film (Fig. 4), ion sputtering the gold plating film, laser vertical irradiation for single-hole processing, laser changing angle irradiation for line processing (Fig. 5), and multi-point processing to realize patterning.

The optical field intensity of the microspheres was simulated (Fig. 1). The effects of the microsphere size and laser wavelength on the optical field enhancement and full width at half maximum (FWHM) of the laser peak were determined (Fig. 2). The micro-nano-processing technique of microspheres using a Mie scattering laser was studied. Process parameters such as laser wavelength (Fig. 6), size of microspheres (Fig. 7), thickness of ion sputtering coating (Fig. 8), laser off-axis irradiation offset angle (Fig. 10), and laser irradiation energy density (Fig. 11) were optimized. The morphological characteristics of the gold micro-nano structure were characterized by scanning electron microscopy, and the influence laws of each process on the processing results were summarized to optimize the process parameters. The experimental results show that 100 nm diameter holes can be machined under the following process parameters: laser wavelength of 532 nm, gold film thickness of 25 nm, microsphere size of 1.49 µm, and laser energy density of 25 mJ/cm2 (Fig. 9). Simple pattern processing was performed, and the line width of the processed pattern was close to 280 nm at half wavelength under the following process parameters: laser wavelength of 532 nm, gold-film thickness of 25 nm, microsphere size of 2.53 µm, laser energy density of 30 mJ/cm2, and processing line width of 1/3 for each step (Fig. 12).

This paper introduces a method for processing gold films on the surface of microspheres by modulating laser with a single-layer optical dielectric microsphere array. Using this method, the gold film on a large-area microsphere array can be processed at a high rate and resolution in the micron order. The optical near-field of the dielectric microsphere array was analyzed to realize the convergence of light beyond the diffraction limit. Along with the software simulation of the regulation of the light field by microspheres, the influences of the size of the microsphere and laser wavelength on the machining accuracy were discussed. Then, through experiments using different fabrication processes, the influences of the laser wavelength, size of the dielectric microspheres, thickness of the ion sputtering coating, and energy density of the laser irradiation on the processed gold micro-nano structures were studied and discussed. Finally, the optimal processing parameters were obtained, and a gold single-hole structure of approximately 100 nm was obtained. The step and line widths suitable for patterning were studied by changing the incident angle of the laser. Simple pattern processing was performed, and the linewidth of the processed pattern was close to 280 nm.

Steel corrodes easily in an air environment. To improve its corrosion resistance, black mixed-crystal phases of Fe₃O₄, FeO, and Fe₂O₃ can be generated on its surfaces using blackening technology. Chemical oxidation, electrochemical oxidation, heat treatment, and other traditional blackening technologies cannot satisfy the requirements of green development owing to the use of toxic blackening agents, high energy consumption, environmental pollution, and low density of blackening film. Laser melting technology has been actively studied for improving the corrosion resistance of metal surfaces because of its high quality, high efficiency, and environment-friendliness. Research on blackening and rust prevention of steel surfaces using laser melting technology mainly focuses on the corrosion resistance of laser melting layers prepared with different laser powers or galvanometer scanning rates. However, the laser spot energy has a Gaussian distribution, and the single-pulse energy density and spot overlap rate cause rapid changes in the instantaneous heat accumulation and temperature field of the material surface during laser melting. In addition, repeated laser scanning leads to continuous heat accumulation and temperature field variations on the material surface during laser melting. These changes significantly influence laser melting, resulting in significant differences in the corrosion resistance of the prepared laser melting layers.

In this study, a laser melting layer was prepared on the surface of a Q235B steel plate sample using a 1064 nm pulsed laser. Based on the electrochemical analysis method, the effects of the single pulse energy density, spot overlap rate, and the number of laser scanning on the corrosion resistance of the laser melting layer of the Q235B steel plate were investigated. The optimal parameters of the laser melting were determined, and the laser melting layer with the best corrosion resistance was prepared. The corrosion resistance of the laser melting layer prepared based on the optimal laser parameters and that of the traditional alkaline blackening layer were compared and analyzed to verify the influence of the optimal laser parameters in improving the corrosion resistance of the laser melting layer.

First, laser melting experiments were performed on steel plate surfaces, each with a single laser scanning at a energy density interval of about 1.27 J/cm² ranging from 1.27 to 6.36 J/cm² (Fig. 2). At a single pulse energy density of 3.82 J/cm², the laser melting layer on steel plate surfaces with 70%, 80%, and 90% laser spot overlap rates had the maximum self-corrosion potential and minimum self-corrosion current density (Fig. 3). Therefore, the best single pulse energy density of the laser was determined to be 3.82 J/cm². Second, for a single laser scanning with a single pulse energy density of 3.82 J/cm², the laser melting layer with an 80% laser spot overlap rate had the largest self-corrosion potential and the lowest self-corrosion current density; in addition, the number of microcracks per unit area of the surface was the lowest, and the crack width was the narrowest (Figs. 3 and 4). Therefore, the optimal laser spot overlap rate was determined to be 80%. Third, laser melting experiments with different laser scanning times were conducted with the laser single-pulse energy density of 3.82 J/cm² and laser spot overlap rate of 80%. When the number of laser scanning was four, the laser melting layer showed the highest self-corrosion potential and lowest self-corrosion current density; furthermore, the number of microcracks per unit surface area was the lowest, and the crack width was the smallest (Figs. 5 and 6). Finally, energy spectrum and X-ray diffraction pattern tests revealed that the optimal laser melting layer prepared based on the optimal laser parameters mainly comprised Fe₃O₄ and FeO, thus complying with the national aviation industry standard (HB/Z 5079—1996) for steel blackening, with Fe₃O₄ as the main component of the corrosion-resistant layer (Fig. 7). The impedance arc radius and charge transfer resistance of the Q235B steel plate increased by approximately three times, and the impedance modulus was high (Figs. 8 and 9). A comparison of the surface roughness and scanning electron microscopy (SEM) data of the two corrosion-resistant layers further revealed that the optimal laser melting layer had a reduced surface roughness and good uniform density. This is more conducive to isolating the steel substrate from the corrosive environment and thus achieving improved corrosion resistance (Fig. 10).

A laser melting layer with high corrosion resistance was prepared on a Q235B steel plate surface using laser melting technology. The effects of the laser single-pulse energy density, spot overlap rate, and the number of laser scanning on the microstructure and electrochemical corrosion resistance of the laser melting layer were investigated. The following conclusions were drawn. First, the laser single-pulse energy density, spot overlap rate, and the number of laser scanning significantly influence the microcrack distribution, self-corrosion potential, and self-corrosion current density in the unit area of the laser melting layer. The optimal laser parameter can help achieve the strongest corrosion resistance of the laser melting layer. Second, based on the laser single-factor experiments of the single‐pulse energy density, spot overlap rate, and the number of laser scanning, the optimal laser parameters can be determined, and the laser melting layer with the strongest corrosion resistance can be prepared. Finally, the microstructure of the optimal laser melting layer prepared by the optimal laser parameter combination from the inside to the outside can be regarded as the transition from the gradual Fe oxidation layer to the stable Fe oxidation layer mainly composed of Fe₃O₄-FeO mixed crystals. The stable Fe oxidation layer exhibits decreased surface roughness and microcrack density, fewer oxidation leakage points, and prevention of excessive oxidation, thereby improving the corrosion resistance of the laser melting layer.

Wheels and rails of high-speed trains are prone to severe damage, fatigue, and fracture damage on the wheel surface owing to wear, corrosion, strength reduction, fatigue cracking, and other reasons, thus affecting the stability and safety of train operation. The commonly used repair process to eliminate wheel surface defects causes material wastage and economic losses. To improve the service life of a wheel, laser cladding technology is used to prepare a cladding layer on the surface of a wheel and rail to enhance their damage resistance. Therefore, in this study, Fe-, Ni-, and Co-based alloy coatings, widely used in the field of laser cladding, are prepared on the surface of the ER9 wheel material using laser cladding technology. The mechanical properties, damage mechanism, and corrosion behavior of the substrates and coatings are investigated.

The base material of the laser cladding experiment was taken from the ER9 wheel steel tread, and three types of self-fluxing alloy powders—Fe-, Ni-, and Co-based—were used as cladding materials. Laser cladding technology was used to prepare the powder coating with thickness of 15 mm on sample surface by coaxial powder feeding. All samples was cut using the wire-cutting method. First, after the prepared metallographic samples were corroded, a SU8010 scanning electron microscope (SEM) and X-ray diffractometer (XRD) were used to study the microstructure and phase of the cladding layer. The microhardness of the samples was measured with a Vickers hardness tester (Qness-Q60). The prepared tensile and impact specimens were then tested for mechanical properties using an MTS universal testing machine and a Charpy pendulum impact testing machine, respectively. Furthermore, the fracture morphologies of the tensile and impact specimens were observed by SEM. Next, the prepared friction and wear samples were characterized by an MFT-EC4000 tester, and the wear surface, wear debris morphology, and element content of the samples were characterized and analyzed using SEM and its accompanying EDS. An electronic balance scale with an accuracy of 0.1 mg was used to measure the average wear. Finally, potentiodynamic polarization curves (Tafel) and electrochemical impedance spectroscopy (EIS) of the samples were obtained using an electrochemical workstation in a 3.5% NaCl solution at room temperature.

As shown in Fig. 2, the coating surface is uniform and dense, without noticeable cracks, pores, and other defects. Furthermore, the microstructure is mainly composed of dendrites and eutectic structures. XRD spectrum analysis (Fig. 3) shows that the Fe-based coating is mainly composed of α-Fe, (Fe, Ni), Cr₇C₃, and other solid solutions. The Ni-based coating is mainly composed of solid solution γ-Ni, intermetallic compound FeNi₃ and hard Cr₂₃C₆ phase. The crystal phases of the Co-based coating are mainly the FeNi₃, γ-Co, and Cr₂₃C₆ phases. The investigation of mechanical properties indicates that the surface hardness after laser cladding treatment improves significantly (Fig. 4), and the Fe- and Ni-based alloy coatings have the highest microhardness (approximately 716.5 HV). The average hardness of the Ni-based alloy coating and Co-based alloy coating is approximately 384.2 HV and 456.1 HV, which are an increment 45.6% and 72.8%, respectively. The hardness of the coating structure is enhanced to achieve a strengthening effect. Figures 5 and 6 show that the elongation of the Fe-based tensile specimen is the lowest (1.34%), and the tensile fracture has cleavage steps. The tensile strength of the Co-based alloy coating is the highest (approximately 976.41 MPa), and the tensile fracture exhibits a river pattern feature. The tensile strength of the Ni-based alloy coating tensile specimen (approximately 813.95 MPa) decreases compared with the substrate, but the elongation reaches 34.5%, and the tensile fracture exhibits a dimple-like morphology. Figure 7 shows that the impact fractures of Fe- and Co-based coatings are brittle, while the Ni-based coating exhibits good ductility and an impact toughness considerably higher than that of the former two. In terms of friction and wear research (Figs. 9 and 11), the wear amount and wear rate of the coatings are significantly reduced, while those of the Co-based alloy coating are the lowest [4 mg and 0.4×10^-4 g/(N·m), respectively], which is 78.9% lower than that of the base material. Only furrows appeared on the wear surface. The wear mechanism is mainly abrasive wear. The wear rate of the Fe-based alloy coating was reduced by approximately 52.6% compared with the substrate, and the wear surface is slightly damaged. The wear mechanism is characterized by abrasive and adhesive wear. The Ni-based alloy coating has a rough grinding surface and a large amount of wear debris accumulation because of the coupling effect of abrasive and adhesive wear. In the electrochemical corrosion study, the Nyquist curves of the substrate and cladding layer in a 3.5% NaCl solution showed capacitive arc characteristics (Fig. 12). The maximum impedance of the cladding layer is two orders of magnitude higher than that of the substrate. According to the test parameters of the polarization curve (Table 4), the self-corrosion potentials of the Fe-, Ni-, and Co-based coatings are -0.475, -0.415, and -0.408 V, respectively, and the self-corrosion densities are 2.980, 0.249, and 0.172 μA/cm², respectively.

The microstructure of the laser cladding coating on the surface of the wheel material is mainly composed of dendritic and eutectic structures. The hardness of the coating is significantly improved. The Ni-based alloy coating has good tensile strength and impact toughness, and the fracture is characterized by toughness, whereas the Co- and Fe-based alloy coatings have a brittle fracture; however, the difference is marginal. Compared with the matrix, the cladding coatings have a lower friction factor, wear rate, and better corrosion resistance, and the Co-based alloy coating has higher hardness (the microhardness was increased by 72.8%). The wear resistance of the Co-based alloy coating is the best (the friction factor is 0.31, the wear amount is approximately 4 mg, and the wear scar depth is 10.70 μm). The corrosion resistance of the Co-based alloy coating is the best (the impedance value is two orders of magnitude higher than that of the substrate). A comparative analysis of the three coatings shows that the Ni-based coating has a rough surface, high wear rate, poor wear reduction effect, and weak hardness and strength. The wear and corrosion resistance of the Co-based coating is higher than that of the Fe-based coating, but the latter has lower engineering costs and also provides overall wheel protection.

Laser milling has the advantageous characteristics of wide material adaptability, adjustable laser energy density, and no mechanical force, which make it suitable for the material removal processing of difficult-to-machine materials such as Ni-based superalloys. In order to expand the process system of laser processing and explore the material removal mechanism of novel laser milling methods to process nickel-based superalloys, the multi-beam coupled nanosecond laser milling of the DZ411 nickel-based superalloy was investigated in this work. The influence mechanisms of laser processing parameters on the surface quality and processing efficiency were analyzed, the results of which can provide technical support for the optimization of laser milling parameters.

The material used in this paper was the DZ411 nickel base superalloy. First, the laser milling of the Ni-based superalloy was carried out with the nanosecond pulse laser under different laser process parameters. Then, the surface morphologies of the machined nickel-based superalloy groove were observed using laser scanning confocal microscope (LSCM) and scanning electron microscopy (SEM), and the element distributions on the sample surface before and after laser milling were analyzed by EDS. Finally, the influence mechanisms of process parameters such as pulse frequency, scanning speed, scanning pitch, laser power, and scanning times on the milled surface morphology, surface roughness, milling efficiency, and element distribution were analyzed.

The surface roughness of the material after laser milling increased with scanning times, and the trend became slower; the laser milling efficiency tended to firstly increase, then decrease, and then slightly increase again with scanning times. The highest milling efficiency was achieved at the scanning times of 10 (Figure 3). As the number of scanning times increased, the size of the bumps and pits at the bottom of the face grooves increased and the shape gradually became irregular (Figures 4 and 5). The surface roughness decreased with the increase of scanning speed, and the milling efficiency tended to increase and then decrease with increasing scanning speed (Figure 7). As the scanning speed increased, the laser overlap along the scanning feed direction decreased, the machining depth decreased, and the size of the craters and bumps at the bottom of the face grooves decreased (Figures 8 and 9). With an increase in the scanning pitch, the surface roughness tended to firstly decrease and then increase, and the milling efficiency showed an overall trend with the increase of the scanning pitch (Figure 10). When the scanning pitch was less than 30 μm, a large amount of raised melt appeared at the bottom of the face groove, and the smaller the scanning pitch was, the larger was the size of the raised feature size. When the scanning pitch was 30 μm, the surface roughness was the smallest; when the scanning pitch was larger than 30 μm, more obvious ridge-like bumps were formed between two adjacent scan paths, which made the surface roughness higher (Figures 11 and 12). The surface roughness at the bottom of the face groove increased with increasing laser power, and the milling efficiency increased with increasing laser power and decreasing pulse frequency (Figure 13). At a pulse frequency of f=30 kHz and a power of P=5 W, the convex surface microstructure formed by laser radiation scanning on the material surface was larger than the original surface. When the power increased, the depth of the surface groove increased, and the bottom materials were further melted and vaporized to form the larger protrusion (Figures 14 and 15). The laser interacted with the substrate material to form metal oxides and intermetallic compounds such as Al₂O₃ (Figure 16).

In this work, a new type of multi-beam coupled laser was used in the machining process, and the material removal mechanism was analyzed. The effects of repeated scanning times, scanning speed, scanning pitch, laser power, and pulse frequency on surface morphology, bottom surface roughness, milling efficiency, and surface material element distribution were studied. The main mechanism behind material removal using a laser involves the laser irradiating on the surface of the material so that it melts and even vaporizes instantaneously. Partial melts leave the bottom of the groove through a jet force, and the partial melts condense at the bottom of the groove to form bulges. When the pulse frequency is high, the scanning speed is low, the laser power is high, and the scanning speed is low, the laser energy absorbed in the unit area at the bottom of the groove is large, and the ablation effect at the bottom of the groove is intense, resulting in a relatively high surface roughness at the bottom of the groove. When the scanning pitch is 30 μm, the surface roughness is the minimum. When the scanning times is 10 and the scanning speed is 100 mm/s, the milling efficiency is the highest, and it generally increases with the increase in the scanning pitch, the increase in the laser power, and the decrease in the pulse frequency. During the laser milling process, the material undergoes complex physical and chemical changes, and some metal oxides and intermetallic compounds are formed on the machined surface.

The composition distribution of the deposited layer in the laser cladding overlapping process is investigated numerically and experimentally. To achieve large-scale remanufacturing, an overlapping method is often implemented. The current results indicate that the characteristics of the second-track molten pool, including its size and flow state, will be changed, or even deformed, under the influence of the first track and gravity during overlapping cladding, and the final distribution of components will be affected by the change in molten pool flow, which directly affects the molding quality. Additionally, the laser overlapping cladding process exhibits highly complex heat transfer and thermo-elastic-plastic-flow multi-physics field coupling changes, which are accompanied by physical phenomena, such as melting, solidification, and phase transitions in the metal powder that cannot be directly observed through experiments. The fluid flow and heat transfer in the molten pool during the overlapping cladding process are presented in a 3D view in this study; this is done to deeply analyze the composition distribution characteristics in the two tracks and the interaction mechanism of the first and second tracks.

First, a 3D Eulerian-Eulerian multiphase flow model based on the volume averaging approach is developed in this study to investigate the laser cladding overlapping process with 316L steel powder on a 45 steel substrate, which is coupled with multi-physical phenomena of molten flow, heat transfer, and mass transfer. Appropriate process parameters (molten height, molten width, and molten depth) are obtained through the previous orthogonal test to obtain the appropriate powder utilization and laser absorption rates by adjusting the model. Finally, the model is solved, and the distribution states and evolution laws for the temperature, velocity, and solute fields in the laser cladding overlapping process are obtained. The composition distribution mechanism in the overlapping process is analyzed by comparing the temperature, flow, solute fields, and experimental results of the two track cladding layers.

By comparing the simulation results of two tracks during the overlapping process, the geometric morphology of the cladding layers is observed to have a better consistency (Figs. 6 and 7). The temperature field of the second track is slightly higher than that of the first track (Figs. 4 and 5) for the same process parameters. The cross-sectional temperature field of the second track is asymmetric. Similar to the flow field evolution of the first track, the second track exhibits clockwise and counter-clockwise vortices in the longitudinal section of the molten pool [Figs. 8(b) and 9(b)]. Unsimilar to the first track, the cross-section of the molten pool of the second track is asymmetric, owing to the inconsistency of the temperature gradient [Fig. 9(c)]. In addition, the element distribution in the deposition layer obtained by the two-track cladding layer simulation is compared with the experimental data, and the results show that the chromium content in both tracks is nearly the same from above to below (Figs. 12 and 13). The overlapping area of the first track is partially remelted by the second track. Some elements of the first track will enter the molten pool of the second track, and the content of the powder metal will be increased, which results in the Cr content of the overlapping region being slightly higher than those of the two tracks.

In this study, the effects of the first track on the morphology, temperature field, flow field, and solute field of the second track, during the laser cladding overlap process, are investigated. Comparing the simulation results for the two tracks reveals that the temperature field evolution of the two tracks is exceedingly similar. However, the evolutions of the flow fields and distributions of the solute fields of the two tracks are vastly different owing to the influence of the first track cladding layer in the overlapping cladding process. Our study shows that the evolution of the molten pool and element distribution in the deposition layer of the second track are greatly influenced by the first track. To achieve better repair of high-end parts with the lower defect tendency, a better overlapping rate should be considered in the laser cladding overlapping process.

High-speed laser cladding has the characteristics of fast speed, high efficiency, good finish, a random cladding layer thickness, low heat input, low dilution rate, energy saving, and environmental friendliness. The binding strength, microstructure, and coating performance are strongly influenced by the scanning path, which is an important factor affecting the thermal field distribution during high-speed laser melting. Compared with unidirectional scanning, reciprocating scanning, and other scanning methods, the heat accumulation of the circular scanning method is greater, and the temperature of the adjacent passages is higher during cladding. This study analyzed the temperature change of the high-speed laser cladding process in a circular scanning path. The iron-based TY-1 coating was sintered on the surface of 27SiMn steel with a circular scanning path using high-speed laser cladding, and the influence of temperature changes from inside to outside on the grain growth, hardness and corrosion resistance of coatings in different regions under the same parameters was analyzed.

Monitoring the molten pool temperature in real-time is difficult during high-speed laser cladding. The influence of temperature changes on the material microstructure and performance was studied in the high-speed laser cladding process. In this study, the aforementioned relationship was clarified through simulations and experiments.

A simulation clarified the temperature change of high-speed laser cladding. The microstructure, hardness, and electrochemical corrosion were studied by the experimental analysis of the cladding process, and the finite element analysis software ANSYS Workbench was adopted to simulate the high-speed laser melting process to obtain the thermal field distribution law of the coatings at different positions in the circular path. Then, some samples were prepared using the high-speed laser melting technique under conditions identical to the simulated conditions. Microscopic structures perpendicular and parallel to the laser scan direction were observed using an optical microscope. Subsequently, hardness and electrochemical corrosion experiments were conducted. The influence of temperature on the changes in the microscopic structures and performance of the cladding coating was analyzed.

The finite element simulation shows that the maximum temperature ranges from 1800 to 2065 ℃ (Fig. 2). Influenced by the temperature of the cladding layers, the coating temperature gradually increases from the inside to the outside. The insulation time of region A1 without preheating is longer than those of regions A2 and A3 with preheating. The preheating time of regions A2 and A3 gradually increases. In contrast, the insulation time gradually decreases (Fig. 4). Owing to the thermal influence from adjacent coatings, the microstructure at the bottom of the coating is mostly columnar crystals (Figs. 10 and 11), and the primary dendrite spacing of the columnar crystals ranges from 5 to 25 μm (Fig. 10). According to the fine crystal reinforcement theory, grain refinement increases the number of grain boundaries; therefore, the maximum average hardness of the coating section at region a3 (the region corresponds to region A3 in the simulation) is 579 HV (Fig. 12). The potential difference between the interface and core of the dendritic crystal is reduced because the coating is meticulous and uniform. Therefore, the maximum self-corrosion voltage of the coating at region a3 is -0.466 V, and the minimum self-corrosion current is 0.7943×10^-6 A·cm^-2 (Table 5). The best coating performance is demonstrated at region a3.

The highest temperatures of the melting pool at regions A1, A2, and A3 were 1890, 1955, and 1998 ℃, respectively, obtained by circular scanning path simulations using high-speed laser melting technology. The temperatures of the coatings at different positions were related. Influenced by the temperatures of the cladding layers, the coating temperature gradually increased from the inside to the outside; the insulation time of region A1 without preheating was longer than those of regions A2 and A3 with preheating. The preheating time of regions A2 and A3 gradually increased while the insulation time gradually decreased. The preheating and heat preservation of the coatings at regions A1, A2, and A3 reduced the temperature gradient and cooling rate. The temperature gradient and cooling rates reduced with increasing preheating and insulation time of the coatings at regions A1, A2, and A3. The slender columnar crystals at the bottom grew laterally and evolved to equiaxed and thick columnar crystals. The average hardness of the coating section at regions a1, a2, and a3 were 512, 466, and 579 HV, respectively, and the hardness gradually increased along with the increase in cooling rate. When the cooling rate was high, the dimensions of the grains did not grow significantly. Thus, the microstructure remained small and compact and the plastic deformation resistance and coating hardness were high. The corrosion resistance voltages of the coatings at regions a1, a2, and a3 were -0.525, -0.514, and -0.466 V, respectively. The corrosion resistance increased with a decrease in the insulation time. Because the Cr element in the powder was partially consumed during heat preservation process, the corrosion resistance of the coating was reduced; thus, the coating at region A3 had relatively good corrosion resistance owing to the short insulation time.

Previous studies on laser paint removal from aircraft skin mainly focused on the optimization of laser paint removal parameters and the improvement of efficiency. However, to improve the reliability and safety of laser paint removal technology and promote its application in the field of aviation engineering, the potential impact of laser energy absorption on the performance of the substrate during the laser paint removal process must be clarified. According to Fourier’s law of heat conduction and the law of conservation of energy, laser paint removal generally affects the properties of the surface material of the matrix, whereas surface integrity plays an important role in material performance and service life. Therefore, this study aims to investigate the effect of laser paint removal on the surface integrity of aircraft aluminum alloy skin substrates, including the substrate surface morphology and roughness, microhardness, and microstructure.

In this work, paint removal experiments on 2024 aluminum alloy aircraft skins at energy densities of 12.89-25.48 J/cm² were performed using pulsed fiber laser. Then, the qualities of paint removal at different energy densities were analyzed via trinocular continuous zoom stereo microscopy (SM), scanning electron microscopy (SEM), laser confocal microscopy (LSCM), microhardness tester, and X-ray diffractometer (XRD). Subsequently, changes in the surface morphology and roughness of the substrate after removing the coating, as well as the microhardness and microstructure versus energy density, were investigated. Finally, the temperature field distributions at different energy densities were studied using finite element analysis. The effects of the temperature field on paint removal and substrate surface integrity were further discussed. Consequently, the internal relationship between the evolution of the substrate microstructure and hardness change during laser paint removal was revealed.

When the energy density is relatively high (≥22.90 J/cm²), the paint layer is completely removed (Fig. 5). Moreover, the surface roughness (S_a) and peak valley height difference (PVHD) of the substrate gradually increase as the energy density increases (Figs. 6-7). Meanwhile, under high energy density conditions, the refinement of sub-grains on the material surface, an increase in dislocation density (Fig. 9), and precipitation of strengthening phase σ (Al₅Cu₆Mg₂) are observed (Fig. 10). As a result, a small increase in the surface hardness occurs (Fig. 8). At the energy density of 22.90 J/cm², the PVHD is 8.28 μm. Compared with that of the original sample, the microhardness increases by 2.8%, which meets the requirements of the aircraft skin recoating process and application standards. Meanwhile, the calculation results show that the temperature at the junction of the paint layer and the substrate is 415.46 °C. The paint layer is then completely ablated and gasified. Because the temperature of the substrate surface is lower than its melting threshold (500 °C) (Fig. 13), thermoplastic deformation does not occur. The best cleanliness and surface integrity are obtained at 22.90 J/cm².

After laser paint removal, the temperature of the substrate surface increases rapidly with the increase in energy density owing to the thermal effect of the laser. When the energy density increases to 22.90 J/cm², although the roughness of the aluminum alloy substrate surface increases slightly, it still can meet the roughness requirements of the surface coating process. In addition, the surface layer of the substrate hardens owing to the plastic deformation of the material, precipitation of the strengthening phase, and refinement of subgrains. At the energy density of 22.90 J/cm², the hardness of the base material increases by 2.8%, meeting the requirement that the property change of the material after paint removal should not exceed 5% in the aircraft skin material standard.

The flat die, a key component of flat die granulators, is subject to severe wear. Laser cladding technology is used widely, and the wear resistance of the flat die can be improved using laser cladding technology. Nickel-based self-fluxing alloy powder has excellent wear resistance and corrosion resistance at a lower cost. TiC ceramic particles were added to the nickel-based self-fluxing alloy powder to enhance the wear resistance of the coating. The previous study showed that the coating had the best all-round performance when the volume fraction of additive TiC was 25%. However, few studies have examined the optimal process parameters for the laser cladding of Ni60A-TiC composite coatings with 20CrMnTi steel as the substrate. Therefore, the Ni60A-25%TiC composite coating was prepared on the surface of 20CrMnTi steel by laser cladding. This study examined the effects of the laser power, scanning speed, and powder feeding speed on the microstructure and wear resistance of the Ni60A-25%TiC coating.

The Ni60A-25%TiC powder was mixed evenly using a QM-QX4 ball mill. A three-factor, three-level orthogonal experiment was designed with the test factors of laser power, scanning speed, and powder feeding speed. Cladding coatings were prepared with different technological parameters. A CFT-I surface comprehensive tester was used for the friction and wear tests. The mass before and after wear was measured using a BSM-220.4 electronic balance. X-ray diffraction (XRD), three-dimensional surface topography, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and microhardness tester were used to characterize the phase composition, 3D morphologies, microstructure, element distribution, and element valence and microhardness of the coatings, respectively.

The coating after laser cladding was dense and showed good metallurgical bonding with the substrate (Fig. 3). The dilution rate and microhardness of the cladding layer were used as evaluation indices. The factors affecting the quality of the cladding layer in descending order were the powder feeding speed, scanning speed, laser power which was obtained by the extreme difference (Table 6) and variance (Table 7) analysis. XRD revealed the main phase composition in the coating to be SiO2, Cr2O3, and TiC. The coating phase varied slightly with the different process parameters (Fig. 4). The friction and wear test showed that the frictional state differed according to the process parameters. The friction coefficient of the coating samples was small, and the wear process was stable. Among them, S3 sample had the lowest wear rate of 1.5×10-5 mm3/(N∙m). The microscopic morphology at the abrasion area of the sample was analyzed (Fig. 7). Abrasive wear occurred on the surfaces of the S3 and S4 samples; the wear surfaces were relatively smooth, and the coatings were covered with oxide films, such as SiO2 and Cr2O3, in the friction process. The surface of the S1, S5, and S7 samples mainly showed adhesive wear. The surface of S2, S6, S8, and S9 samples mainly showed abrasive and adhesive wear. The wear resistance of the S10 substrate was poor, and the surface showed abrasive wear, adhesive wear, and plastic deformation, and severe furrows and pits appeared. The above analysis showed that S3 showed better wear resistance. The hardness and wear resistance of the coating was enhanced by the synergistic effect of dispersion strengthening and solid solution strengthening. XPS showed (Fig. 10) that the solid lubricant film of the S3 coating was comprised mainly of oxides, such as SiO2, Cr2O3, TiO2, and NiO.

Using the dilution rate and microhardness as evaluation indices, the factors affecting the quality of the cladding layer from the largest to smallest were the powder feeding speed, scanning speed, and laser power. The composite coating showed a significantly lower wear rate compared to the substrate. The Ni60A-25%TiC composite coating with the best all-around performance was produced at a laser power of 1.4 kW, scanning speed of 7 mm/s, and powder feeding speed of 21 g/min. Severe furrows and fatigue wear were observed on the substrate surface, and the wear of the cladding layer was mainly abrasive. Oxide particles, such as SiO₂, Cr₂O₃, TiO₂, and NiO, generated by friction can be used as solid lubricants to form oxide films on the friction layer surface that can prevent further wear of the friction layer and improve the wear resistance of the coating.

The effect of lasers on mechanical properties, control of coloring, and exploration of new coloring processes has been explored in recent years. However, limited research has been conducted on the effect of fluctuations in the process parameters on the stability of titanium alloy surface coloring. In this study, a 355 nm wavelength ultraviolet nanosecond laser is used to investigate the effect of laser process parameter fluctuation on the stability of the titanium surface coloring, to determine the reasons for the formation of different colors from the perspective of thin-film interference, and to explore the effect of different laser process parameters on the surface morphology and elemental composition of the colored samples.

The matrix method was adopted to design different laser process parameter ranges, determine the parameter range of stable color formation, and prepare stable colors such as orange, golden, blue, green, purple, and other colors on the sample. The CIE1976 standard chromaticity system of the International Commission on Illumination was used to calibrate the color samples based on the stable colors formed by laser process parameters, and the variation intervals of the scanning speed, laser power, and hatching distance were set according to gradients of equal value or equal ratio, respectively. The color stability of each sample was tested under different process parameter changes using a spectrophotometer. Laser confocal microscopy was used to observe the microscopic topography of samples of different colors, and the coloring stability of each color sample under different process parameters was examined. The samples were analyzed using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), and the change characteristics of the samples were further explained based on the microstructure and elemental composition of the samples. X-ray diffraction analysis was performed on each colored sample and the titanium alloy substrate to analyze whether the laser action formed a diffraction peak on the surface of the sample that was different from that of the titanium alloy substrate.

Various color samples, such as orange, golden, blue, green, and purple, are prepared on the titanium alloy using 355 nm wavelength ultraviolet nanosecond laser processing equipment. The stable colors of the titanium alloy are achieved under a relatively low accumulated fluence, as shown in Table 2. This paper quantitatively analyzes the three laser-induced process parameters through a color difference experiment, examining the effect of fluctuations in the power, scanning speed, and hatching distance within a small range on the color of the titanium alloy surface. The change in hatching distance has the greatest impact on coloring stability, as shown in Fig 6. Microstructural observation and elemental composition analysis of the prepared samples of different colors are carried out. From the perspective of the microstructure, a comparison of the structure and thickness of the oxide film and the proportion of the element content of each color sample are analyzed to illustrate the changing characteristics of the color.

By laser induction of the titanium alloy using 355 nm wavelength ultraviolet nanosecond processing equipment, two different microscopic morphologies are observed under a laser confocal microscope after the laser acts on the surface of the titanium alloy. When the cross-section is analyzed and tested by SEM, a two-layer structure with noticeable structural differences is found. In the XRD analysis, the color samples form diffraction peaks different from those of the substrate, confirming that the superstructure observed under SEM analysis is a titanium oxide layer. It is further confirmed that the newly formed diffraction peak has the structure of TiO/TiO₂ by searching for relevant information. In the EDS elemental composition analysis test, the samples formed at low speed, low power, and high defocus amounts, such as purple and green, are compared with samples showing orange, blue, and golden colors formed at high speed, high power, and low defocus amount. The former samples form an oxide layer that is thicker, higher content in oxygen, and lower content in carbon. Under the effect of a nanosecond laser, the different samples formed on the surface of the titanium alloy are directly related to the thickness of the oxide film. The thickness of the oxide film results from the combined effect of material heating and surface oxidation under the thermal effects of different laser process parameters.