Zhai Linjun, Fu Yuqing, and Du Yongzhao

SignificanceBlood flow is an important parameter for measuring vital signs, and hemodynamic parameters are functional indicators of the microcirculatory system of the skin, brain, heart, liver, kidneys, and other organs. Therefore, dynamic blood flow monitoring has important application value and significance in clinical and basic life science fields, such as clinical diagnosis, intraoperative guidance, drug research, disease mechanism research, and neuroscience. Laser speckle contrast imaging (LSCI) is a full-field optical imaging technique that uses the spatial and temporal statistical properties of laser scattering intensity to monitor the blood flow of tissues in vivo. It uses simple equipment, is non-invasive, and has a fast imaging speed and high spatial resolution. Additionally, it does not require the injection of a contrast agent and can perform continuous measures for a long time. Consequently, it is widely used to measure microcirculatory blood flow parameters such as the vessel diameter, blood flow velocity, blood perfusion, and blood density in tissues and organs. It also can help doctors locate the lesion precisely with clear and accurate blood flow data,and then analyze the corresponding functional response and pathological mechanisms, which has become one of the most important tools for the clinical diagnosis of fundus diseases, skin diseases, brain diseases, and so on. In addition, it is also an important tool for basic life science research in drugs, cardiovascular and cerebrovascular diseases, and brain cognitive and behavioral sciences. Consequently, in-depth research on novel LSCI techniques with high imaging quality is valuable and significant for improving the quality of medical care and promoting the development of basic research in life science.ProgressIn the past decades, many researchers have conducted extensive researches on how to improve the quality of LSCI and expand the scope of LSCI applications, and they have had positive progress. For example, a few research groups like Luo Qingming and Li Pengcheng at Huazhong University of Science and Technology, and Tong Shanbao at Shanghai Jiao Tong University have worked on portable LSCI systems, high signal-to-noise ratio LSCI, and high resolution LSCI, which have promoted the development of LSCI in China. Researchers abroad like Boas at Boston University, Zakharov at the University of Fribourg, and Dunn at the University of Texas at Austin have worked on high-precision imaging using LSCI techniques, such as static scattered light correction and quantitative analysis of LSCI, which has also greatly promoted the development of key techniques and novel LSCI applications.In this paper, we presented a systematic, comprehensive and integrated analysis, review and summary of the current researches about key techniques and applications of novel LSCI at home and abroad emphatically from the aspect of high signal-to-noise ratio LSCI, high-resolution LSCI, high-precision LSCI, large imaging depth LSCI, and novel LSCI systems based on the investigations of current literature. In this way, we can help researchers learn more about the frontier technologies of LSCI and understand the technical challenges we faced, and we can provide ideas with a reference value to promote the development of high-quality, highly practical, and innovative LSCI systems to meet the needs of clinical diagnosis and basic biomedical research. The review consisted of the following contents: First, the technical problems of measuring deep blood flow and achieving high resolution, high signal-to-noise ratio, and high precision have been systematically summarized, and the corresponding solutions are indicated. Subsequently, we review high signal-to-noise ratio LSCI techniques based on anisotropic filtering, eigenvalue-decomposition, and transformation domain collaborative filtering methods. Meanwhile, high-resolution LSCI for motion artifact, out-of-focus blur, and non-uniform light intensity correction are also summarized. Third, we elaborate the high-precision LSCI from the perspective of static scattered light correction, quantitative analysis, and novel LSCI algorithms. After summarizing the LSCI with a large imaging depth, we introduce the latest research on the novel LSCI system and its applications in the fields of cortical blood flow imaging, surgical and therapeutic procedures, and brain and cognitive-behavioral sciences. Finally, we discuss and look forward to the development of LSCI in the future.Conclusions and ProspectsIn conclusion, LSCI has made qualitative leaps and developments in theory, imaging systems, computational methods, and clinical applications. The imaging quality of LSCI has been developed to have a high signal-to-noise ratio, high resolution, high accuracy, and large imaging depth. However, as the application scenarios of LSCI become more and more complex, which introduces greater challenges to the development of key techniques and application of LSCI. In the future, LSCI will be deeply integrated with emerging interdisciplinary fields such as biomedicine, optoelectronic information, artificial intelligence, and big data. In addition, new breakthroughs are expected in the following respects. (1) Quantitative analysis capacity. The capacity is still an important fundamental issue for LSCI in functional applications. (2) Combination of LSCI with new endoscopic technology (this will enable the noninvasive measurement of blood flow). (3) Miniaturization and integration. The development of new materials and electronic devices will certainly promote the miniaturization and integration of new LSCI systems. (4) Combination of LSCI with artificial intelligence. Artificial intelligence will further promote the development of LSCI technologies and their applications. (5) Combination with other imaging modalities (this will build a new model for LSCI-based multimodal clinical diagnostic applications). It is believed that LSCI will show a synergistic development trend in the future. We look forward to seeing the development of technologies and applications of LSCI.

May. 10, 2023
Chinese Journal of Lasers
Vol. 50 Issue 9 0907106 (2023)
DOI:10.3788/CJL221200
Yu Lidong, Bian Tianxuan, Qu Yunteng, Zhang Beibei, and Bai Yang

ObjectiveSteel 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.MethodsIn 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.Results and DiscussionsFirst, 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).ConclusionsA 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802201 (2023)
DOI:10.3788/CJL220866
Ren Sixue, Li Jingbin, Shi Yameng, Wen Baoqin, Wang Xianfei, and Li Liqiao

ObjectiveThe 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.MethodsThe 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.Results and DiscussionsThe 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.ConclusionsUsing 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802207 (2023)
DOI:10.3788/CJL220806
Liang Xiaomei, Yang Yicheng, Huang Ruisheng, Tian Dexi, and Chen Xiaoyu

ObjectiveUltrahigh-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.Methods Three welding methods were adopted in the present studylaser-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.Results and DiscussionsAs 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.ConclusionsThree 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802107 (2023)
DOI:10.3788/CJL220679
Feng Qigao, Wang Linxin, Wang Lei, Ma Lijie, Tang Yongkai, and Li Bobo

ObjectiveTitanium 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.MethodsThe 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.Results and DiscussionsThe 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 (Ra) 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).ConclusionsIn 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, Ra 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802306 (2023)
DOI:10.3788/CJL220917
Li Tianyi, Shi Tuo, Li Kuan, Zhang Rongwei, Li Jianbin, Sun Yewang, and Liu Guang

ObjectiveThe 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.MethodsFirst, 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.Results and DiscussionsThe 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.ConclusionsThe 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 A90% is 99% and 96%, respectively (Fig. 11), showing an excellent prediction effect of the established model.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802102 (2023)
DOI:10.3788/CJL220746
Xie Yin, Teng Qing, Shen Muyu, Li Jikang, Ma Rui, Bai Jie, and Wei Qingsong

ObjectiveNickel-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.MethodsUsing 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.Results and DiscussionsRe-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.ConclusionsGH3536 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802303 (2023)
DOI:10.3788/CJL220975
Zhang Yi

ObjectiveLaser 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.MethodsThe 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.Results and DiscussionsThe 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).ConclusionsThis 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.

Apr. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 0802403 (2023)
DOI:10.3788/CJL220840
Wang Xiaoyue, Shi Haotian, Wang Zijian, Peng Bo, Yan Ming, and Liu Weiwei

ObjectiveLaser-induced fluorescence spectroscopy (LIFS) is a novel spectroscopic technique with several advantages, such as multi-elemental simultaneous detection, rapid response, and real-time online monitoring; hence, it is a powerful tool for remote sensing. However, single-beam laser filamentation remains limited, including the limitation of fluorescence signal intensity and detection sensitivity owing to the influence of intensity clamping. Several methods have been proposed to address these problems. For instance, the fluorescence signal can be enhanced by the interaction between two or more filaments, which is crucial in improving the sensitivity of substance detection. Previous studies about the interactions between co-propagation filaments often adopted multiple femtosecond pulses to form plasma gratings in the medium and increase the fluorescence signal intensity. However, such methods rely on the spatial interference effect of an ultrafast light field. They have high requirements for parameters such as the excitation beam’s spatial angle and pulse delay. To circumvent the aforementioned problems, we study the interaction mechanism of collinear counter-propagating filaments and their influence on fluorescence properties. Experiment results demonstrate that although its enhancement effect on fluorescence can be comparable to that of the co-propagating filament, it is almost not affected by the laser polarization. Applying this technology to the spectroscopic detection of metal ions in compounds is expected to provide a novel approach to detecting trace contaminants by studying the linear relationship between characteristic spectral intensity and substance concentration.MethodsWe establish a collinear counter-propagating filaments (CPF) system, which comprises two collinear beams propagating in the opposite direction and focusing on the same focus through a lens, thereby forming two spatially overlapping counter-propagating filaments near the focus. We make precise adjustments using the electronic translation platform to better control the relative pulse delay of the two light beams. The atomized sample interacts with the filaments to excite the fingerprint fluorescence and the fingerprint fluorescence is transmitted to a spectrometer using an optical fiber. In addition, we configure brine solution samples of KCl, Na2SO4, and MgSO4 with different mass fractions to establish calibration curves and analyze the detection sensitivity of this system.Results and DiscussionsFirst, we compare the signal intensities induced by the CPF and single filament (SF) under the same total excitation pulse energy and successfully obtain an enhancement factor of approximately 4. Furthermore, we compare the relationship between the intensity of induced fluorescence and the pulse energy of the two filamentation systems. When the laser energy increases from 0.5 mJ to 1.5 mJ, the increase in the CPF excited signal is significantly higher than the increase in the SF excited signal; this indicates that the collinear counter-propagating filaments can obtain higher fluorescence intensity. In addition, we study the evolution of fluorescence signal intensity with time and infer that the filament interaction will prolong the fluorescence decay time of excited molecules. Simultaneously, by changing the relative pulse delay of the two lasers, fluorescence enhancement can be observed in the pulse delay range of -3-3 ps, with an excitation pulse width of 50 fs. Finally, we obtain the detection sensitivity of CPF by establishing the calibration curve of metal ions (K+, Na+, and Mg2+) in the brine solution. The experiment result demonstrates that the detection sensitivities of K+, Na+, and Mg2+ are 9.3×10-6, 4.3×10-6, and 16.7×10-6, respectively, and their corresponding determination coefficients of the calibration curves are 0.99 (KCl), 0.98 (Na2SO4), and 0.97 (MgSO4), respectively; this implies that the method adopted in this study exhibits optimal linearity in concentration measurement and can be utilized for the quantitative analysis of compound samples within a specific concentration range.ConclusionsThis study introduces a fluorescence spectroscopy detection technique induced by collinear propagation filaments. The interaction of counter-propagating filaments prolongs the fluorescence decay time of excited state molecules and enhances fluorescence signals. The fluorescence intensity excited by counter-propagating filaments is 4 times stronger than the fluorescence intensity excited by the single filament under the same pulse energy. Compared with the co-propagation filament-induced fluorescence enhancement technology, the proposed method has loose requirements for the delay control of the two excitation pulses. More importantly, this method can effectively improve the detection sensitivity of metal ions in compounds and exhibits optimal linearity; hence, it provides a novel approach to detecting trace contaminants and other applications.

Apr. 10, 2023
Chinese Journal of Lasers
Vol. 50 Issue 7 0711001 (2023)
DOI:10.3788/CJL221498
Shen Yanlong, Wan Yingchao, Wang Yousheng, Li Gaopeng, Ma Lianying, Chai Tongxin, Chen Zhengge, Zhu Feng, Huang Ke, and Feng Guobin

ObjectiveThere are increasing demands for mid-infrared lasers at 3-5 μm, which overlaps with the transparency window of atmosphere, for their potential applications in various fields, including laser surgery, spectroscopy, infrared countermeasures, environmental monitoring, and laser communication. There are lots of approaches to achieve lasers at 3-5 μm band, which can be roughly divided into two major categories. The first category is based on population inversion, namely the linear method, which includes HF/DF gas lasers, semiconductor cascade lasers, fiber lasers, and solid-state lasers (typically Fe∶ZnSe or Fe∶ZnS lasers). The second category is based on the nonlinear effect, including optical parametric oscillators (OPO, typically using PPLN and ZGP crystals as nonlinear media), difference frequency generation (DFG), and frequency doubling. Compared to these lasers, the Fe∶ZnSe or Fe∶ZnS lasers, emitting at the mid-infrared range of 4-5 μm, enjoy several advantages, including high efficiency, wide wavelength-tuning range, and compactness of optical cavity. As a consequence, lots of efforts have been made in the development of Fe∶ZnSe/Fe∶ZnS lasers in the past decade. For some practical applications, continuous-wave (CW) Fe∶ZnSe/Fe∶ZnS lasers are required. Several CW Fe∶ZnSe lasers, all of which operates at the wavelengths of over 4 μm, have been demonstrated by using various CW pumping sources, including the Cr:ZnSe laser, Er:YAG laser, Er:ZBLAN fiber laser, and Er:Y2O3 laser. Limited by matured pump sources at ~3 μm, CW Fe∶ZnSe lasers were seldom domestically reported. In this paper, a CW Fe∶ZnSe laser at ~4 μm is demonstrated by using a self-developed continuous-wave Er-doped fiber laser at 2.8 μm.MethodsThe pump source used in our study is a continuous-wave Er-doped fiber laser at ~2.8 μm developed in our lab. It has a maximum output power of about 4 W. The gain medium, i.e., the Fe∶ZnSe crystal is 3.5 mm in length and has a cross section of 10 mm×10 mm, with the Fe2+ ion concentration of 1.0×1019 cm-3. For obtaining effective CW lasing, the crystal is wrapped in a piece of indium foil and clamped to a copper mount cooled to ~77 K by liquid nitrogen in a cryostat, owing to the lifetime of the upper laser level in a Fe∶ZnSe crystal being as short as 370 ns at room temperature while around 57 μs at 77 K. Longer lifetime of upper laser level makes continuous-wave emission much easier. The faces of the gain crystal are anti-reflection coated at 2.7-4.8 μm. Windows of the cryostat are 3 mm CaF2 plates with AR coated at 2.7-4.8 μm. In the laser cavity arrangement, the feedback is a special coated CaF2 plano-concave mirror, while the output mirror is another CaF2 plano-concave mirror with a coupling ratio of ~35%. The radius of each cavity mirror is identical at 50 mm. A dichroic mirror is placed with an incidence angle of ~45° to separate the pump beam and output laser beam. An uncoated CaF2 lens with a focal length of 50 mm is used to couple the pump beam into the crystal. Unlike the previous demonstrations of CW Fe∶ZnSe lasers, the pump direction in our experiment is counter-pumping, i.e., the pump beam and laser beam propagate in opposite directions.Results and DiscussionsAn optical spectrum analyzer is used to monitor the lasing of the 4 μm signal. When the pump power is increased to about 0.4 W, there is a little peak in captured spectrum. Subsequently, fixing the pump power slightly higher than the threshold, we adjust the cavity mirrors and the coupling lens to maximize the output power. The output power as a function of pump power is recorded and shown in Fig. 3. The maximum power is 0.97 W, which is limited by the pump capability. The slope efficiency is fitted to be ~38.6%, which nearly approaches the limited efficiency of the current laser. The measured laser spectra are shown in Fig. 4. The central wavelength shifts from 3747.1 nm at low output power to 3773.3 nm at high output power with a signal to noise ratio (SNR) of as high as 40 dB, which is also named with “red-shift” and is common in free-running solid-state lasers. The wavelength in this laser is approximately 3.8 μm. The laser spot indicates that the beam profile has a desirable fundamental Gaussian distribution.ConclusionA watt-level high efficiency 3.8 μm mid-infrared all-solid-state continuous wave laser is demonstrated in this study. The output power of 0.97 W with a central wavelength at 3.8 μm with a slope efficiency of 38.6% from Fe∶ZnSe crystal is obtained by employing a self-developed continuous-wave Er-doped fiber laser at 2.8 μm as the pump source. The pump source is employed under liquid nitrogen cooling, and the beam profile has a desirable fundamental Gaussian distribution.

Apr. 10, 2023
Chinese Journal of Lasers
Vol. 50 Issue 7 0701008 (2023)
DOI:10.3788/CJL221087
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