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.

Mar. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 z220806 (2023)
DOI:10.3788/CJL220806
Zheng Guoliang, Liu Jianxun, He Tiefeng, Zhang Xuhui, and Wu Qingyang

On the other hand, the propagation of light beams in anisotropic media has always been of interest. In 2001, Ciattoni A discovered that when a circularly polarized (CP) beam propagates along the optical axis of a uniaxial crystal, a portion of the light beam acquires a topological charge vortex phase of ±2 due to spin reversal. In 2020, Ling X H et al. found that the conversion efficiency of spin angular momentum (SAM) to orbital angular momentum (OAM) is related to the anisotropy of the crystal and shape of the beam. To improve the “abruptly autofocusing effect” of the CAB and improve the conversion efficiency of SAM to OAM, this study investigates the propagation characteristics of a modified CAB (MCAB) propagating along the optical axis of a uniaxial crystal.Results adn Discussions In our numerical study, the incident light is a left-hand CP (LHCP) MCAB without a vortex. During the propagation, a right-hand CP (RHCP) component is generated. First, we investigate the intensity, phase, and polarization distributions of the MCAB at z=100 mm. Due to the “abruptly autofocusing effect,” the radii of the first rings for the LHCP and RHCP components become smaller [Figs. 2(c),(d)]. The phase distribution shows that the LHCP component has no vortex, whereas the RHCP component has a vortex phase with a topological charge number of 2 [Figs. 2(e),(f)]. This is the singularity of the central phase that causes the RHCP component to be a hollow beam throughout the propagation. The polarization distribution shows that the beam is no longer a uniformly CP beam (Fig. 3). Due to the anisotropy of a uniaxial crystal, the abruptly autofocusing positions of the two components differ. The “abruptly autofocusing effect” of the MCAB is approximately 3.4 times as strong as that of an ordinary CAB (Fig. 4). Furthermore, we investigate the propagation dynamics of the two components. The results show that both the LHCP and RHCP components exhibit an “abruptly autofocusing effect”. The LHCP component without a vortex forms a solid beam at the focus, whereas the RHCP with a vortex forms a hollow beam at the focus (Fig. 5). For a 10 cm long crystal, the efficiency of conversion from the LHCP component to the RHCP component with a vortex can reach 43.28%, which is approximately 10% higher than that of an ordinary CAB (Fig. 6).ObjectiveThe circular Airy beam (CAB) has received significant attention because of its peculiar “abruptly autofocusing effect”. The “abruptly autofocusing effect” has shown significant advantages in biomedical treatment, laser cutting, and other applications because the CAB can be applied solely to the target without damaging other areas. Various schemes have been designed to improve the “abruptly autofocusing effect”. For example, direct blocking of the first few rings of the CAB and modulation of the CAB’s angular spectrum can significantly enhance its “abruptly autofocusing effect”.MethodsThe method proposed by Ciattoni A is adopted to deal with the propagation of light beams along the optical axis of a uniaxial crystal. According to the results of Ciattoni A, a light field propagating along the optical axis of a uniaxial crystal can be treated as a linear superposition of ordinary and extraordinary components. Based on the angular spectrum theory, the propagation dynamics of these two components can be obtained by the Fourier transform of the MCAB’s angular spectrum. A closed-form approximation of the CAB’s angular spectrum with a suitable plane wave angular spectrum representation has been reported by Chremmos I et al. A modulation function is introduced to modulate the CAB’s angular spectrum. The “abruptly autofocusing effect” of the MCAB is superior to that of the ordinary CAB. Following the approach proposed by Ciattoni A, the propagation characteristics of the MCAB in a uniaxial crystal can be obtained.ConclusionsSimilar to other ordinary beams, when an LHCP MCAB propagates along the optical axis in a uniaxial crystal, an RHCP vortex MCAB with a topological charge number of 2 is generated. With a proper modulation function, the “abruptly autofocusing effect” of the MCAB is much stronger than that of an ordinary CAB, and the efficiency of conversion from the LHCP component to the RHCP component with a vortex is also improved.

Mar. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 5 z220737 (2023)
DOI:10.3788/CJL220737
Liu Wenjin, Xu Bing, and Sun Wenhan

Results and Discussions A simulation is used to validate the performance of the project-constraint decoupling method for the aberration correction, coupling error elimination, stability, and computation complexity. It can compensate the aberration better than the traditional method to decoupling method for the wave front sensor-free system (Fig. 2). Additionally, it can effectively eliminate the decoupling error between the Woofer and the Tweeter (Fig. 3), as the aberration is broken down into low order Zernike modes and high order modes before being corrected by the Woofer and Tweeter. During the decoupling operation, it is more stable than the conventional approach, and the advancements have improved its performance in the control process. Finally, the decoupling method suggests in the research has a lower computational complexity than the conventional method (Fig. 4). An experimental system was built to evaluate the effectiveness of the method. The experiment demonstrates that the decoupling algorithm can effectively compensate for phase distortions (Fig. 6 and Fig. 7) and significantly suppress the coupling error between the dual deformable mirrors and decompose the aberration accurately (Fig. 8).ObjectiveDual deformable mirrors are often used to create wave front-sensor-free adaptive optics systems that can be used to correct aberrations with broad strokes and high spatial frequencies. The Woofer, which has big amplitude and is used to correct low order aberrations, and the Tweeter, which has a high spatial resolution and is used to correct high order aberrations, are two examples of dual deformable mirrors. However, without the decoupling process, it is difficult to avoid the coupling error, which would cause the deformable mirrors to generate an opposite surface shape and waste the ability of aberration correction in the dual deformable mirror adaptive optics system. To solve this problem and make the Woofer and Tweeter could work efficiently together; a decoupling method must be developed. Even the decoupling algorithms are the subject of considerable study, most of them focus on dual deformable mirror adaptive optics systems with wave front sensors. These techniques frequently employ the data from the wave front sensor to aid in decoupling. A few decoupling methods are used for the wave front sensor-free adaptive optics system, and their performances are usually not satisfactory for the engineering project. To improve the performance in the aberration correction, coupling error reduction, stability, and computation complexity for the wave front sensor-free adaptive optics system, a new decoupling technique must be developed. This might lead to further applications for the adaptive optics technology in things like large-scale telescopes, vision equipment, and laser beam cleanup.MethodTo make the dual deformable mirrors in the system work together to correct the aberration, a straightforward but effective decoupling method based on the mode project-constraint is proposed. The Woofer is controlled by a low order Zernike mode coefficient to avoid correcting the high order modes, and the Tweeter is constrained by the project-constraint method to eliminate the low order modes in its corrections. Obtaining the related matrix of the mode coefficients to the Woofer control signal is essential to the decoupling control process. It can be obtained through the Woofer’s influence functions and the low order Zernike modes which will be corrected by the Woofer. The project-constraint, which requires the following processes, can also limit the use of low order modes in the Tweeter. To start, a relationship matrix between the Zernike mode coefficients and the Tweeter’s control signals needs to be produced. Then, the component of the signals in the Tweeter-induced coupling error can be solved by the relation matrix. Finally, by subtracting the component-induced coupling error from the initial Tweeter control signals, the signals free of coupling error can be obtained. These techniques result in the realization of the Woofer and Tweeter’s decoupling.ConclusionsIn this paper, a simple and effective method was proposed based on project-constraint to restrict the coupling error and eliminate the aberration in a wave front sensor less adaptive optics system with a dual deformable mirror. This method can outperform the conventional method in terms of aberration correction, coupling error elimination, stability, and processing complexity. It can be used to make the Woofer and Tweeter cooperate efficiently to correct the aberration by Zernike mode decomposition. Then the low order Zernike modes of the aberration can be compensated by the Woofer, and the other Zernike modes of the aberration can be corrected by the Tweeter.

Mar. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 6 z220548 (2023)
DOI:10.3788/CJL220548
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.

Mar. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 z220840 (2023)
DOI:10.3788/CJL220840
Yu Lidong, Bian Tianxuan, Qu Yunteng, Zhang Beibei, and Bai Yang

Results and Discussions 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).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.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.

Mar. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 8 z220866 (2023)
DOI:10.3788/CJL220866
Xiong Zhengdong, Jiang Lingling, Cheng Tingqing, and Jiang Haihe

Many materials have been proved to be suitable for passively Q-switched lasers in the 3 μm waveband, and only a few relatively stable materials such as Fe2 +∶ZnSe crystals can achieve a large energy output. However, as the Fe2+∶ZnSe crystal has a low damage threshold (1.5-2.0J/cm2@100 ns) in the 3 μm waveband, it can easily be damaged when operating at high peak power and high repetition frequency. Hence, to reduce the risk of damage during normal operation, it is necessary to analyze the pulse characteristics of Fe2+∶ZnSe crystals in passive Q-switched lasers to reduce the possibility of damage to the saturable absorber and realize laser operation at a high peak power and high repetition rate.Based on the measurement results, a high-repetition-rate Fe2+∶ZnSe crystal passively Q-switched laser system with a concave-convex resonator structure is developed to compensate for the thermal focal length. The Er,Cr∶YSGG crystal has dimensions of Ф3 mm×100 mm. Concave mirror M1 is used as the all-reflection mirror (R1=200 mm), M2 is used as the output mirror (R2=-216 mm), and the reflectivity of the convex surface is 70% at 2.79 μm.By optimizing the internal component layout of the 60 Hz Er,Cr∶YSGG passively Q-switched laser, the high repetition frequency and high peak power of the 2.794 μm passively Q-switched laser can be achieved. Figure 5 shows the experimental waveforms of the 60 Hz passively Q-switched laser with two Fe2+∶ZnSe crystals. The single pulse energy of the lasers is 4.7 mJ and 7.0 mJ, with pulse widths of 97.0 ns [Fig.5 (a)] and 72.6 ns [Fig.5 (c)], respectively.ObjectedLasers with high repetition rate and nanosecond pulse width around 3 μm waveband are required to improve the conversion rate of optical parameters and reduce the thermal effect when they are used in mid-infrared parametric pumping and hard tooth tissue ablation. The Q-switched technology is widely used to generate lasers with high peak power and narrow pulse width. Currently, the high-peak-power laser output at 3 μm waveband has been obtained using electro-optic Q-switched laser technology. However, because of the thermal depolarization effect of polarized laser under high-power pump, the repetition frequency of electro-optic Q-switched technology cannot be increased. The high repetition frequency can be achieved using acousto-optic Q-switched technology, but the large laser pulse energy cannot be realized owing to the limitation of the diffraction efficiency of the acousto-optic device. Mechanical Q-switched technology cannot produce stable laser pulses because it is difficult to accurately control the motor during high-speed operations. Theoretically, a passively Q-switched laser can achieve nanosecond laser pulses with high repetition frequency and high peak power as long as the damage threshold of the optical components is sufficiently large. Moreover, as a passively Q-switched laser has a compact cavity structure, its use is advantageous in laser applications.MethodsUsing output mirrors with different reflectivities, the values of the output pulse width of a Fe2+∶ZnSe saturable absorber are theoretically calculated (Fig.1 and Table 1). The values provide theoretical guidance for the design of passively Q-switched lasers. The pulse widths under two initial transmittances of the Fe2+∶ZnSe crystal (91.9% and 93.6%) and two reflectivities of the output mirror (30% and 40%) are measured using Er,Cr∶YSGG laser crystal rods with two sizes (Ф3 mm×100 mm and Ф4 mm×100 mm) pumped by a xenon lamp (Fig.2). The theoretical calculation results are verified through relevant experiments.Results and DiscussionsThe results in Table 1 and Fig.3 show that the pulse width of the output lasers with different initial transmittances narrows with an increase in the reflectivity of the output mirror. A passively Q-switched laser output with large energy and narrow pulse width can be obtained more easily when the initial transmittances of the saturable absorber are lower. The experimental results verify the accuracy of the calculation. Moreover, the pulse width of the laser output has little relation with the size of the laser crystal rod, and the pulse widths obtained with the crystal rods with two different sizes are similar. From the beam diameter in the cavity, it is observed that a larger crystal rod diameter changes the mode volume in the cavity and increases the output laser energy; however, the laser energy density in the cavity does not increase because the bleaching process of the saturable absorber is not affected by the increase in the beam diameter.ConclusionsThe results show that a saturable absorber with low initial transmittance can achieve a low pulse width, whereas a saturable absorber with high initial transmittance can compress the pulse width by enhancing the reflectivity of the output mirror. Based on these results, the xenon lamp pumping Er,Cr∶YSGG laser is optimized, and Fe2+∶ZnSe passively Q-switched laser pulses with high repetition rate (60 Hz) and high peak power (7.0 mJ) are realized.

Mar. 10, 2023
Chinese Journal of Lasers
Vol. 50 Issue 6 0601001 (2023)
DOI:10.3788/CJL220800
Xu Fan, Li Xing, Liu Zheng, Bo Youfeng, and Chen Kunyu

ObjectiveLiquid ramjet is the optimum power device for high-dynamic near-space vehicles because of its high thrust-weight ratio, simple structure, and lightweight. Ramjet technology is being extensively developed by the military around the world. However, during a rigorous test, the welding spots between the flame tube and reinforcement ring of a ramjet, which was identified as the weak link, were damaged. The flame tube and strengthening ring are both made of GH3230 superalloy. First, resistance spot welding is used to weld the two parts. Moreover, laser welding was chosen as the welding method because of its high energy density, fast welding speed, and excellent welding quality. However, there is still the possibility of weld tearing between the two parts. Currently, the shape of the laser welding spot is a popular study area, and optimizing it can significantly increase welding strength and quality. Thus far, several researchers have proposed various welding shapes, such as ring- and C-welding spots. It was found that the shear performance of welding spots has a positive connection with the area of fusion surfaces. Furthermore, the ring-welding spot welding path is considered a closed curve, which is unfavorable to stress release. The C-welding spot welding path is open and can release stress; thus, it is superior to the ring-welding spot. However, the existing weld shapes still have insufficient welding strength, which reduces the reliability and assembly precision of the products. Therefore, four weld shapes, such as ring-, C-, oval-, and S-welding spots, were designed based on the real structure of the product. The differences in performance between the welding spots were compared to determine the best welding scheme. This study can be used as a reference for welding ramjet products, and it has both innovative and practical values.MethodsLaser welding experimental research was conducted for lap welding of GH3230 superalloy sheets. The differences in the weld formation, mechanical properties, fracture behavior, and microstructure of the welding spots were compared. The thickness of plate Ⅰ, the thickness of plate Ⅱ, and the width of the overlap region were 1, 0.6, and 12 mm, respectively. The welding equipment is a UPRB4600 laser welding machine, which is equipped with a 2 kW fiber laser system. During welding, a single pass was performed; the defocusing distance was set to 0 mm and three specimens were welded for every process parameter. After welding, the microstructure of the weld joint was observed using a digital optical microscope; the welding strength was tested using a universal testing machine, and the fracture morphologies was analyzed using a scanning electron microscope.Results and DiscussionsFour weld shapes were designed when the overlap zone was certain: ring-, C-, oval-, and S-welding spots (Fig. 1). The differences in the weld formation, mechanical properties, fracture behavior, and microstructure of the welding spots were compared, and the following results were obtained: 1) four weld shapes showed significant adaptability in the weld formation and microstructure (Fig. 3 and Fig. 12 ). The weld width increased as the heat input increased, and the increase in the back weld width was greater than that of the top weld width (Fig. 4). 2) The shear performance of the welding spots was related to the area of the fusion surface, which increased as the fusion area increased. The S- and oval- welding spots had higher welding strength followed by the C- and ring-welding spots (Fig. 6). In the ring- and C-welding spots fracture modes, the weld departed from the interface, and the weld broke along the cross-section of the oval- and S-welding spots (Fig. 11). 3) The S-welding spot showed the best performance among four welding spots when the overlap zone was certain because of its geometric shape. The product of the flame tube was welded using the S-welding spot, resulting in a good welding quality. The welding strength of the S-welding spot was 1.8 times stronger than that of the ring-welding spot. Other thin-wall products that require high welding strength can benefit from the S-welding spot.ConclusionsHerein, the effects of four welding spots shapes on welding properties were studied and the optimal welding scheme was determined. The results show that the S-welding spot has better tensile-shear performance and weld formation when the overlap zone is certain because it has a large welding area, and the welding path is open, thus releasing stress. The tensile-shear strength of the improved weld is 1.8 times stronger than that of the original welding spot (C-welding spot). The S-welding spot is suitable for welding thin-wall products, such as flame tubes.

Feb. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 4 0402006 (2023)
DOI:10.3788/CJL211526
Liao Dasong, Wang Qian, Wang Feisen, Chen Hui, Ji Fei, Wen Ting, and Zhou Lei

Results and Discussions As the pulsed laser scanning speed is increased, the surface oxide film and oil stain are gradually removed, and the original dark-gray surface of the TC4 titanium alloy becomes golden-yellow, then light-yellow, and finally silvery-white (Fig. 2). The microscopic morphology of the TC4 surface changes with increasing scanning speed. When the scanning speed is low, ridge structures and cracks form on the surface, along with severe thermal oxidation. The surface becomes smoother and the thermal oxidation decreases with increasing scanning speed, but the effect of removing the oxide film is reduced at a large scanning speed (Fig. 3). The scanning speed affects the number of pulses in the unit spot area, the cumulative input energy, and the overlap rate of the spot; therefore, pulsed laser at different scanning speeds produces different surface morphologies of the titanium alloy substrate (Fig. 4). With an increase in scanning speed, the O content on the TC4 surface first increases then decreases, and then increases, while the Ti content shows the opposite trend. The main component of the oxide film on the TC4 surface is TiO2. When the scanning speed is 500 mm/s, TiO forms on the surface by thermal oxidation. When the scanning speed is increased to 9000 mm/s, the surface oxygen content is the lowest, the substrate material is exposed, and the cleaning quality is the best (Figs. 5, 6, and 7). The roughness of the TC4 surface first increases and then decreases with increasing scanning speed, which is closely related to the changes in surface morphology (Fig. 8). Due to the formation of a remelted layer with grain refinement and acicular martensite α′ and laser shock strengthening effect on the surface, the hardness of the TC4 surface increases after laser cleaning, and then gradually decreases with increasing scanning speed (Figs. 9 and 10). Compared to the original, the laser-cleaned TC4 surface has increased corrosion resistance due to the remelted layer, reduced roughness, and the formation of a dense oxide film by slight oxidation (Figs.11 and 12).ObjectiveTC4 titanium alloys have excellent overall performance and are widely used in the manufacture of key components for aerospace and military hardware. However, the oxide film on the surface can adversely affect its formation and performance, which results in porosity in the weld, reduced mechanical properties, reduced electrical conductivity, and weakened bonding of the plating or coating with the substrate. Therefore, the oxide film must be removed from the titanium alloy. Traditional methods for cleaning the oxide film on the surface of titanium alloys have many limitations. However, laser cleaning technology compensates for the shortcomings of traditional cleaning approaches with features such as being green, being efficient, and causing low damage. Several studies have discussed the influence of laser energy density on the cleaning effect. In this study, the effects of scanning speed on the cleaning effect of the oxide film on TC4 titanium alloy surfaces are studied. The results can help optimize the laser cleaning process and improve the cleaning quality and efficiency of titanium alloys.MethodsIn this study, a nanosecond pulsed fiber laser is used to remove oxide film and oil stain from a TC4 titanium alloy surface at different scanning speeds. The surface morphology of the cleaned TC4 titanium alloy is observed by scanning electron microscopy (SEM). The effect of different scanning speeds on the surface chemical composition and chemical bonds of the sample surface is analyzed by energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The surface roughness is measured using a laser confocal microscope and the hardness of the surface is measured using a Vickers hardness tester with a load of 200 g. Finally, the corrosion resistances of TC4 titanium alloy before and after laser cleaning are measured on an electrochemical workstation.ConclusionsThe scanning speed has a significant effect on the cleaning of oxide film and oil stain on the surface of TC4 titanium alloy. When the scanning speed is 500 mm/s, the surface becomes golden-yellow due to thermal oxidation, and significant remelting and cracking occur. The O content (mass fraction) reaches 24.86%, and the oxide TiO forms. With an increase in scanning speed from 3000 mm/s to 9000 mm/s, the original dark-gray surface of TC4 becomes silvery-white, and the amount of thermal oxidation is gradually reduced. When the scanning speed is 9000 mm/s, the O content(mass fraction) reaches its lowest value of 4.54%, and the Ti content (mass fraction) reaches its highest value of 84.2%. The oxide film and oil stain are removed, and the substrate material is exposed. The surface roughness of TC4 titanium alloy gradually decreases with increasing scanning speed. Laser cleaning technology can improve the hardness and corrosion resistance of a TC4 surface, and the enhancement effect is affected by the scanning speed.

Feb. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 4 0402020 (2023)
DOI:10.3788/CJL220819
Zhao Kai, Chen Kai, Jiang Gaoqiang, Wang Xingyu, Sun Panjie, and Cui Chengyun

ObjectiveFacing the long-term problem of poor corrosion resistance of magnesium alloys, it is considered that preparing coatings on the surface of magnesium alloy substrates is an effective means. Dense coatings can effectively isolate Mg alloys from external corrosive environments. Compared with coating preparation techniques, such as thermal spraying, which requires molten material, cold spraying avoids the melting and recrystallization of materials, high-temperature oxidation, stress cracking, and other problems. However, the surface of the coating generally has high roughness, which has an adverse effect on the anti-corrosion performance. Laser shock peening can effectively improve the surface morphology of the cold-sprayed coating and reduce the roughness of the coating surface. Simultaneously, problems, such as oxidation and material cracking, caused by thermal effect are avoided, maintaining the advantages of cold spraying. In addition, laser shock peening can improve the residual stress state of the coating surface and increase its microhardness.MethodsIn this paper, the mechanism of the effect of multiple laser shock peenings (LSPs) on the surface morphology of cold-sprayed pure aluminum coatings is discussed. A pure aluminum coating is prepared on the surface of a magnesium alloy by cold spraying. A nanosecond-laser transmitter is used to shock the surface of the cold-sprayed coating. The laser pulse energy is 4 J, spot diameter is 3 mm, and overlap rate is 50%. The effect of LSPs with different impact numbers on the surface morphology, roughness value, phase, residual stress state, and microhardness of the coatings are studied. The surface morphologies of the coatings before and after laser shock are measured using a confocal laser microscopy. The 3D and 2D profiles are used to describe the change in the coating surface morphology before and after laser shock, and the evolution process and mechanism of the coating surface morphology during laser shock are analyzed. The phase composition of the coating before and after laser shock is detected using an X-ray diffractometer. The effect of laser shock peening on the phase retention ability of a cold-sprayed pure aluminum coating is studied. The states of the residual stress on the coating surface before and after laser shock are measured using an X-ray stress tester. The effect of laser shock on the transformation of residual stress on the surface of the cold-spray coating is investigated. The hardnesses of the coating surface before and after laser shock are measured using a microhardness tester. The effects of different impact numbers on the microhardness of the coating surfaces are compared.Results and DiscussionsThe original cold-sprayed coating without laser shock has a rough surface morphology. There are obvious micro-peak and micro-valley morphologies on the surface. After laser shock, the surface maintains good integrity, although the coating is not metallurgically bonded (Fig. 6). The sharply protruding micro-peak area on the surface suffers severe plastic deformation and sinks after being crushed by the laser shock wave. The micro-valley area is reduced under the plastic deformation and extrusion of the material. The original uneven surface morphology of the coating is flattened owing to the sinking of micro-peaks and micro-valleys . With an increase in the number of laser shocks, the degree of surface flatness gradually improves (Figs. 7 and 8). The line and surface roughness values of the coating surface continuously decrease (Fig. 9). The X-ray diffraction (XRD) patterns reflect the phases of the coating surface before and after laser shock. The results demonstrate that no high-temperature oxidation occurs on the coating before or after laser shock (Fig. 10). Owing to the high thermal expansion coefficient of pure aluminum, the volume of the particles is easily affected by heat, and there is low residual tensile stress on the surface after spraying. After laser shock, the residual stress on the coating surface gradually transforms from tensile to compressive (Fig. 11). Owing to the severe plastic deformation of the coating surface layer caused by laser shock, a deformed hardened layer forms on the surface layer, the surface microhardness gradually improves, and the ability of the material to resist plastic deformation improves. However, the ability of the subsequent impact to produce plastic deformation of the coating is attenuated; thus, the increase in microhardness gradually decreases with an increase in the number of impacts (Fig. 12).ConclusionsIn this study, the effect of laser shock peening on the surface morphology of cold-sprayed pure aluminum coatings is investigated. The specific results are as follows: 1) After laser shock peening, the coating surface maintains good integrity. The originally uneven coating surface is flattened by the rolling deformation of the laser shock wave, and the surface morphology is reshaped. The surface roughness value gradually decreases with an increase in the number of laser shocks. After three impacts, the surface and line roughness values decrease from 31.85 μm and 21.39 μm to 12.88 μm and 8.87 μm, respectively. 2) After laser shock peening, the coating does not exhibit serious oxidation phenomenon, and the material maintains the original powder characteristics. With an increase in the number of impacts, the residual stress on the coating surface transforms from tensile to compressive, the surface microhardness value gradually increases, and the maximum surface microhardness reaches 51.67 HV. 3) With an increase in the number of laser shocks, the surface roughness value decreases, together with the magnitude of each drop; the increase in the microhardness value demonstrates the same trend. The effect of the first laser shock is the most obvious; the effects of subsequent shocks continuously weaken, and the overall effect increases with the increase of the number of shocks.

Feb. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 4 0402004 (2023)
DOI:10.3788/CJL220638
Xiong Junzhe, Kong Ming, Hong Bo, Shi Feiyang, Jian Juan, Zhan Honghui, and Shan Liang

ObjectiveFluid motion is a common phenomenon in observed nature and utilized in industries. Mastering the fluid flow is an important prerequisite for an in-depth study of fluid mechanics. The particle image velocimetry (PIV) is a non-contact global flow-field measurement and display technology that provides accurate data for flow-field measurements without affecting the flow field. The particle image velocimetry is mainly divided into two categories: cross-correlation and optical flow algorithms. The optical flow algorithm is primarily used in small-displacement scenarios. When the particle displacement is significantly larger than the particle size, the optical flow method cannot yield accurate results. The cross-correlation algorithm is mainly used in large displacement scenarios, and the combination of the two algorithms can satisfy more application scenarios. Although the hybrid algorithm has higher accuracy than the traditional algorithm in large- and small-displacement scenarios, the angle information is not well retained in the case of complex fluid. Because the image of the particle conforms to the Airy spot model and the light intensity satisfies the two-dimensional Gaussian distribution, if the Gaussian radial basis function interpolation is used, the velocity field refinement will be transformed into a surface reconstruction problem, and the reconstructed velocity field will have a higher accuracy. Therefore, we propose a cross-correlation optical flow mixing algorithm based on the Gaussian radial basis function interpolation to reduce the angular error.MethodsBased on the traditional hybrid algorithm, in this study, the Gaussian radial basis function interpolation is used to replace bicubic interpolation and design a cross-correlation optical flow hybrid algorithm. First, a pair of particle images is inputted, and a cross-correlation method is used to extract the relatively large particle motion in each query window. A Gaussian radial basis function is used for data interpolation to fill the speed vector in each pixel. For each pixel, the image displacement is processed to remove the speed vector detected in the image. Subsequently, the initial velocity vector is determined using the HS optical flow method, and the residual velocity field is refined using the variable spectral flow method based on the dynamic illumination equation. The Gaussian radial basis function interpolation method is used to interpolate the velocity field at each layer, and the more refined velocity field vectors are obtained. Finally, the velocity field vectors obtained by the cross-correlation and optical flow algorithms are superimposed to obtain an accurate velocity field. The algorithm is quantitatively evaluated through a Rankine vortex simulation experiment. The influence of displacement and particle size on the accuracy of the algorithm is studied. Subsequently, a two-dimensional PIV experimental system is built, and rotation and water injection experiments are performed to simulate the vortex current field and jet field, respectively. The practicability of the proposed algorithm is verified.Results and DiscussionsIn the Rankine vortex simulation experiment, the manifold reconstructed by the proposed method is more in line with the characteristics of the Rankine vortex and closer to the ground truth (Fig. 3). The root mean square error (RMSE) and average angular error of the cross-correlation optical flow hybrid algorithm based on Gaussian radial basis function interpolation are 27.36% and 38.32% lower than those of the Hybrid method 2020, respectively (Table 1). With an increase in the maximum displacement, the root mean square error gradually increases. In most cases, the hybrid algorithm based on Gaussian radial basis function interpolation is superior to the Hybrid method 2020. In the case of a small displacement, the RMSE can be decreased by approximately 45%, whereas in the case of a large displacement, the RMSE can be decreased by approximately 15% (Fig. 5). With an increase in particle size, the angle error first decreases and then the best reconstruction result is obtained when the particle size is 3 pixel. The proposed method can obtain good reconstruction results in the cases of both small and large particle sizes. In the case of particle size of 2-4 pixel, the average angle error of the proposed method is approximately 15% lower than that of the Hybrid method 2020 (Fig. 6). The results of water injection and rotation experiments verify the performance of the proposed algorithm in practical applications.ConclusionsIn this study, based on the traditional hybrid algorithm, the Gaussian radial basis function interpolation is used to replace bicubic interpolation, and a cross-correlation optical flow hybrid algorithm based on Gaussian radial basis function interpolation is proposed. This approach preserves the angle information in the complex flow field, which is not possible using the traditional hybrid algorithm. It changes considerably with velocity, and the method can accurately reconstruct flow fields. The proposed algorithm and the Hybrid method 2020 algorithm are used to reconstruct the velocity field in an experiment. The results show that the two algorithms can maintain high consistency in the entire manifold, and the proposed algorithm can retain more angle information. This verifies that the proposed algorithm can accurately reconstruct the actual complex flow field and has potential for practical applications.

Feb. 25, 2023
Chinese Journal of Lasers
Vol. 50 Issue 6 0604001 (2023)
DOI:10.3788/CJL220822
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