Objective Ultra-narrow gap laser welding (ultra-NGLW) is a type of advanced welding technology for high-strength thick steel plates that use a laser as the heat source in an ultranarrow groove. It has the advantages of high welding accessibility and efficiency, low heat input and residual stress, and low deformation. Ultra-NGLW is suitable for welding thick high-strength steel plates in pressure vessels, ships, pipes, and hydropower equipment. Different from the common arc welded joints, in the process of multilayer filler wire welding, the complex thermal process differently changes the microstructure of each filling layer, resulting in different mechanical properties of each micro-zone. Therefore, the residual stress distribution in the ultranarrow gap laser welded high-strength steel joint is related to its special microstructure.

Methods In this paper, the B950CF bainitic high-strength steel and XK-01 wire with diameter of 1.2 mm were considered the research objects. Ultra-NGLW joints with three different thicknesses of 20, 50, and 70 mm were welded using the laser welding system comprising a TRUMPF-10002 high power laser and a Fronius automatic wire feeder.

Considering the complex microstructure and highly uneven mechanical properties of the ultra-NGLW joint, the micro-shear test was adopted to study the relationship between the mechanical properties and microstructures of the joint. The micro-shear specimen with a dimension of 1.5 mm×1.5 mm×40 mm, including weld metal (WM), heat-affected zone (HAZ), and base metal (BM), was cut from the different filling layers of the joint.

The surface residual stress of the joints with different thicknesses was measured using the μX360n X-ray residual stress tester. Considering the variance of the thermal-physical and mechanical properties of the B950CF high-strength steel with temperature, the properties of the B950CF steel at different temperatures were calculated using the JMat Pro simulation software. Based on the ABAQUS simulation software, a simulation model of the 70-mm-thick ultraNGLW joint was established and the residual stress of this joint was calculated.

Results and Discussions The microstructure of the ultra-NGLW joint is considerably more complicated than that of the arc welded joint. The specimen 2 and specimen 3 mainly comprise acicular ferrite and granular bainite. Moreover, the microstructure of the cover layer mainly comprises ferrite and upper bainite, with a small amount of columnar crystals, and low-carbon martensite. The micro-shear test results indicate that the shear stress distribution of the three specimens is “M” type. Approximately 150--200 μm lath martensite is observed at the fusion line, which is brittle and hard with high shear strength. The maximum shear strength is 658.8 MPa. It can be seen from the shear power in the micro-zone of the ultra-NGLW joint that the specimen 2 and specimen 3 exhibit the highest toughness, and the toughness of the cover layer is poor. The toughness of all specimens sharply decreases at the fusion line, which is attributed to the existence of martensite.

The residual stress of the ultra-NGLW joints is studied using X-ray nondestructive test and simulation. The results show that the residual stress distribution on the upper surface of the joints with different thicknesses is “W” shaped. The formation of martensite is the main reason for the high compressive stress(-214---476 MPa) at the fusion line. The tensile stress increases with an increase in the joint thickness. The highest residual tensile stress (measured value of 362 MPa and calculated value of 662 MPa) is located in the weld subsurface filling layer. Because the martensite decomposes in the filling layer, the remelting and high temperature tempering cause the volume shrinkage and thus induce the large residual tensile stress. It interrupts the uniformity of the residual stress distribution.

Conclusions Results show that the specimen 2 and specimen 3 mainly comprise acicular ferrite and granular bainite. The micro-shear strength (600--630 MPa) is higher than that of the BM (570 MPa), and the weld fusion line comprises lath martensite with the highest micro-shear strength of 658.8 MPa. The toughness of the middle and bottom filling layer is higher than that of the cover layer, which mainly comprises columnar crystals and low-carbon martensite.

The residual stress distribution on the upper surface of the ultra-NGLW joints with different thicknesses is “W” type. Owing to the existence of martensite, the compressive stress at the fusion line is -476 MPa (20 mm joint) and the highest tensile stress is 166 MPa (70 mm joint) in the fine-grain zone of HAZ. The tensile stress increases with an increase in the joint thickness.

The simulation results of the residual stress distribution of the 70-mm-thick ultra-NGLW joint matches the experimental results. The highest residual tensile stress (measured value of 362 MPa and calculated value of 662 MPa) is located in the weld subsurface filling layer. Because the martensite decomposes in the filling layer, the remelting and high temperature tempering cause the volume shrinkage and thus induce the large residual tensile stress.

Objective With the rapid development of China's aerospace industry, particularly considering the implementation of a series of national programs such as “Project Moonshot” and the “Large Aircraft Program”, the standards for the strength, modulus, wear resistance, and temperature resistance of materials are increasingly high. Ti and its alloys are widely used in the aerospace, biomedical, and chemical industries because of their high specific strength, excellent corrosion resistance, good heat resistance, and high biocompatibility. However, the poor friction performance and low hardness of Ti alloys limit their application in some fields. Particle-reinforced titanium matrix composites (TMCs) can maintain the excellent properties of Ti alloys. These composites have a higher specific strength and specific modulus than Ti alloys; thus, they are expected to become essential structural metal materials in the aerospace industry. In recent years, TMCs have attracted significant interest from researchers in the field of materials. TiB is a ceramic reinforcement with high hardness, and its melting point is as high as 2000 ℃; moreover, there is a very small density difference between TiB and Ti. TiB particles can be produced by an in-situ reaction between Ti and TiB₂, which leads to the formation of good interfacial bonding between the matrix and the TiB reinforcement. Recently, selective laser melting (SLM), as a newly developed additive manufacturing technology, has attracted extensive attention because it can directly process powder into materials with complex shapes and high precision. At present, the researches on the formation of TiB-reinforced TMCs via SLM mainly focus on the microstructure, distribution of the reinforcement phase, and microhardness; however, there are a few studies focusing on the mechanical properties of these materials, such as the tensile strength and plasticity. Therefore, analyzing the microstructure, phase composition, and mechanical properties of TiB-reinforced TMCs formed by SLM is necessary.

Methods TiB₂/Ti-6Al-4V mixed powders after ball milling were used as raw materials. TiB-reinforced TMCs with different B contents were prepared by SLM. The phase composition, microstructure, electron backscatter diffraction (EBSD) results, and α lamella size of the TMCs with different B contents were analyzed by X-Ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM), and the results were compared. The microhardness and tensile properties of the TMCs were analyzed by hardness and tensile tests at room temperature. The reasons for the decrease in the α lamella size and the increase in the strength of the TMCs are provided.

Results and Discussions The results show that TiB diffraction peaks are observed in the XRD patterns of the TMCs. The microstructure of the TMCs is compact, and the semi-elliptic molten pool is tightly packed to form a good metallurgical bond. The needle-like TiB reinforcement phase is observed under a scanning electron microscope. The EBSD results show that compared with that of Ti-6Al-4V, the α phase of the TMC is noticeably refined. In addition, the microhardness, tensile strength, and yield strength of the TiB/Ti-6Al-4V composites are significantly improved.

Conclusions Based on the above results, the main conclusions of this paper are as follows:

After mixed powder ball grinding, Ti-6Al-4V powder particles still showed a spherical morphology, and the TiB₂ particles were uniformly distributed on the surface of the spherical powder. XRD and SEM studies confirmed that Ti and TiB₂ could react to produce needle-like TiB particles during the SLM process. TiB has a B27 structure, where the B atoms have a zigzag and continuous arrangement in a serrated continuous pattern with strong B—B bonds in the [010] direction. Furthermore, TiB has a highly asymmetric atomic structure and a high binding strength; hence, its growth rate is higher in the [010] direction than in the [100], [101], and [001] directions. Therefore, the TiB reinforcement phase tends to have a needle-shaped/rod-shaped morphology. In the TMCs sample with a high B content, the needle-like TiB reinforcement phases are clustered together, and the phenomena of cluster growth and coupling growth are observed. Because of the presence of B and the rapid solidification step of the SLM process, the α lamella size of the TMCs significantly decreases. Compared with those of Ti-6Al-4V prepared by SLM, the microhardness, tensile strength, and yield strength of the TMCs are significantly improved. The excellent mechanical properties of the TMCs are attributed to the hardening and strengthening effects of the TiB particles and the grain refinement of the matrix. When the mass fraction of elemental B is 0.5%,the average size of the α lamellar is 0.49 μm. Compared with those of Ti-6Al-4V, the tensile strength and yield strength of the TMC increase by 25.7% (1396.4 MPa) and 30.8% (1322.2 MPa), respectively.

Objective Aircraft must be regularly overhauled during service. To repaint and obtain a new and beautiful coating and detect internal defects in the aircraft body or other key structural components, the original paint layer must be removed from the fuselage. The removal of paint from the aircraft fuselage plays an important role in maintenance. As a green and highly efficient technique, laser depainting has garnered considerable attention in recent years. The process and mechanism of laser depainting both require further investigation. The most commonly used lasers for depainting include CO₂ and nanosecond lasers, and they have been discussed and analyzed with respect to cleaning efficiency and removal mechanism. In this study, the effect of laser at different time scales on the cleaning quality of laser paint removal from Al alloy surfaces was explored. The research results lay the foundation for the laser depainting of homemade large C919 aircraft body.

Methods Millisecond and nanosecond pulsed lasers were used to clean the epoxy paint coating on the 2024 Al alloy surface. The millisecond laser has a wavelength, maximum pulsed frequency, and maximum laser power of 10.6 μm, 5 kHz, and 2 kW, respectively. The nanosecond laser has a wavelength, pulsed frequency, pulse width, and maximum single pulse energy of 1064 nm, 2--50 kHz, 30--100 ns, and 100 mJ, respectively. During the cleaning process, the laser cleaning physical phenomenon was monitored online using a high-speed camera and a secondary light source with a wavelength of 808 nm. After the laser cleaning experiment, the macroscopic and microscopic morphologies of the sample were observed and investigated using the camera and scanning electron microscopy. Lastly, the physical mechanism of two types of laser paint removal was discussed by combining the thermoelastic vibration and heat conduction models.

Results and Discussions Both lasers can effectively remove the coating and obtain a clean substrate surface with appropriate process parameters. However, the paint removal characteristics of the two lasers were quite different. Nanosecond laser paint removal showed considerably higher energy efficiency than millisecond laser paint removal. A layer of charcoal ash remained on the sample surface after millisecond CO₂ laser cleaning, which was a combustion product of paint coating. The substrate surface did not melt during the cleaning process. In the case of millisecond laser depainting, a significant amount of black smoke was generated and the paint layer was melted during the cleaning process. During the nanosecond pulse laser cleaning process, strong plasma was observed and large pieces of paint were stripped from the substrate surface. After cleaning a paint layer using millisecond laser depainting, the substrate maintained a state of unmelted surface. However, some defect micropores and microcracks appeared on the surface, which were induced by the thermal stress of the substrate. In the case of nanosecond laser depainting, the substrate surface was completely remelted when the laser power was higher than 250 W. The microstructure formation on the surface was related to a fast cooling process. Under the action of the nanosecond laser, the temperature of the substrate increased to the melting point, thus yielding a thin melting metal. Because of the large cooling velocity, the existence time of the molten pool was short. Hence, the molten pool instantaneously solidified and did not have sufficient time to spread, finally forming the microstructure. The main mechanism of millisecond laser paint removal is vaporization and combustion reaction, while that of nanosecond laser paint removal is the thermoelastic vibration effect.

Conclusions To clean a 50-μm-thick paint layer using the millisecond laser, the optimized process parameters include pulse frequency, pulse duration, and laser power of 500 Hz, 0.6 ms, and 500 W, respectively. The optimized process parameters of the nanosecond laser for cleaning a 50-μm-thick paint layer include pulse frequency, pulse duration, and laser power of 5 kHz, 60 ns, and 200 W, respectively. Compared with the nanosecond laser, the millisecond laser induces lesser remelting owing to the deeper thermal diffusion after cleaning of the paint layer. The main mechanism of millisecond laser paint removal is vaporization and combustion reaction, while that of nanosecond laser paint removal is the thermoelastic vibration effect. The energy efficiency of nanosecond and millisecond laser depainting is 300 and 5 J/mm ³, respectively. The difference in the energy efficiency is attributed to thermal diffusion depths and cleaning mechanisms of the two lasers.

Objective Compared with conventional welding repair methods, laser cladding, an advanced surface modification technology, uses nonequilibrium processing conditions, such as rapid heating and cooling, to fabricate similar alloy compositions on the surface of high-strength steel components. The coating can exhibit refined grains and high dislocation density to achieve high strength and ductility of the repair zone. Therefore, it is a potential for the laser repair of high-strength steel surfaces. Traditional welding methods are used to repair high-strength steel using multilayer and multipass repair welding. Multiple welding thermal cycles induce coarse grains in the heat-affected zone (HAZ), which can lead to significant embrittlement and poor impact toughness of the high-strength steel substrate. Similar to traditional welding, the multilayer and multipass thermal cycles in laser cladding can have multiple tempering effects on the substrate hardened zone, which can lead to grain coarsening and strength softening of the substrate in HAZ. For the multilayer and multipass laser repair of high-strength steel components, in addition to the effective control of the microstructure and performance of the cladding layer, the softening problem of HAZ in the high-strength substrate and the deterioration of mechanical properties (i.e., low strength and poor elongation) must be overcome. Therefore, in this study, the variation trends of the microstructure evolution and mechanical properties of HAZ in the 30CrMnSiNi2A substrate were shown to be beneficial in controlling the strength and ductility of the repaired high-strength steel parts.

Methods Multilayer laser cladding 30CrMnSiA powders were processed on thick 30CrMnSiNi2A steel plates with geometric sizes of 120 mm×60 mm×10 mm using the 8-kW semiconductor laser (Laserline LDF-8000-60). The laser cladding parameters were as follows: 2100-W laser power, 7.3-mm beam diameter, 9-mm/s laser scanning speed, 10-g/min powder feed rate, 10-L/min powder feed gas flow, 20-L/min coaxial shielding gas flow, and 0.3-mm single clad layer. The substrates were separately cladded using 1--8 layers, and the samples were retained in the air-cooling state. The microstructure was characterized using the Zeiss-AxioCam MRc5 optical microscope and TESCAN-LYRA3 scanning electron microscope. The microhardness was characterized using the Zwick/Roell ZHμ Vickers microhardness tester with 0.5-kgf load and 15-s holding time. To investigate the mechanical properties of HAZ subjected to different laser thermal cycles, the cladding coating was cut off, and the hardened layer and tempered zone of the substrate were retained. Tensile and impact samples were prepared using the Zwick/Roell Z100 tensile testing machine and 300 J impact testing machine.

Results Owing to rapid laser heating, the first cladding layer did not significantly decompose the residual austenite of the substrate for tempering; however, the following layers had an obvious effect on the residual austenite decomposition, which slightly decreased the sample ductility and impact toughness. With an increase in the number of cladding layers, the ductility of the specimen samples increased, and initial crack and crack growth occurred in the high-temperature tempering zone (HTTZ); however, when the cladding layers did not quench the substrate, initial crack and crack growth occurred in the incomplete quenching zone (IQZ) and HTTZ. Moreover, the uniform plastic deformation decreased, resulting in a significant decrease in the elongation.

Conclusions and Discussions Each cladding layer can repetitively quench the substrate during multiple cladding. However, the quenching depth gradually decreased with an increase in the number of cladding layers because the former deposited layers absorbed the heat. IQZ occurred when the thermal cycle could not heat the substrate above the austenitizing temperature and the cladding layer that did not quench the substrate began to produce a tempering effect with an increase in the number of cladding layers. Each cladding layer had the tempering effect on the 30CrMnSiNi2A substrate during the multilayer process. With the increasing number of cladding layers, the residual austenite among the substrate martensite lath bundles first decomposed, the carbides gradually precipitated, and the martensite laths coarsened, becoming wider and blocky until the lath martensite completely transformed into a sorbite microstructure. In terms of the mechanical properties of the substrate in HAZ, the tensile strength gradually decreased and the impact toughness gradually increased with the number of cladding layers. Owing to rapid laser heating, the first cladding layer will not decompose the tempered retained austenite (RA) in the substrate; however, the double cladding layers will significantly decompose RA. Because of the decrease in the ductile RA phases, the tensile elongation and impact toughness of the double cladding layers were slightly reduced.

Objective Copper-steel laminated plates exhibit the advantages of both copper and steel. Such plates are extensively used in the aerospace, instrumental, and military fields. However, some problems, such as high reflectivity, serious oxidation of the oxygen-assisted cutting surface, and large seam widths after cutting, can be observed when cutting 1-mm-thick copper-steel laminated plates with a fiber laser. Although some solution methods have been proposed in the existing researches, the protection of the original properties of the material surface, the processing precision, and the product processing technologies are still limited. The effects of lasers and materials can be improved using the pretreatment method proposed in this study, in which the element compositions, morphologies, and physicochemical properties of processing areas are adjusted to ensure processing quality, expand the processing range of a 1064 nm fiber laser, and design the experiment for processing slit gratings. The feasibility of this method is verified.

Methods Based on the analysis of the material properties and the laser processing characteristics, a shallow melting surface could be obtained by controlling the energy input. The plate surface was scanned using a low-power fiber laser by considering oxygen as the auxiliary gas to obtain morphology changes and material properties conducive to laser processing. The processing quality of this method is compared with those of other methods. A three-dimensional (3D) microscope was used to observe and measure the surface morphology, heat-affected zone (HAZ), and slit width. In addition, the Plus software was used to estimate the oxide distribution in the shallow melting area formed by pretreatment. Thus, the proportion of oxide area in the shallow melting area could be calculated. The surface roughness was measured using a Keynes laser confocal microscope. Further, the surface morphological changes in the shallow melting area were analyzed to investigate the influence of pretreatment on laser absorption and subsequent processing. The advantages of this method with respect to quality and technology were deduced via experiments in which different pretreatment methods were used. After reasonable processing parameters were obtained, the slit grating was processed by planning the processing path, and the deformation and accuracy were controlled within the required range.

Results and Discussions In this study, the plate surface is guided to obtain shallow melting and black oxide via pretreatment. The width of shallow melting on the material surface is smaller than the spot diameter, and the oxide and molten substance overlap with each other. Therefore, the main energy absorption area with respect to the spot diameter is segmented and reduced, and black oxide can effectively improve the laser absorption rate. Thus, the problem of low laser absorptivity on the surface of the copper-steel laminated plates can be solved and the appropriate energy utilization can be achieved. Hence, most of them are used to remove the materials within the irradiation range and reduce heat accumulation at the edge of the slit. The slit width is smaller than the spot diameter, and the HAZ of the slit is reduced. Thus, the cut accuracy and quality of the laser-cut copper-steel laminated plates are improved when the proposed pretreatment method is applied. Through pretreatment, a pretreated surface containing 36.54%--77.84% of oxidation area can be obtained. The larger the proportion of the oxidation area, the lesser is the amount of oxide copper spots in the shallow melting zone, the greater is the density of the molten pool, and the more uniform is the edge line. Compared with that of the oxygen-assisted cutting method, the seam width of the pretreatment method decreases by approximately 45% and that of the carbon black coating method decreases by 36%.

Conclusions The material surface melts in the form of a shallow layer when the energy density is controlled and the oxidation reaction is guided. The width of shallow melting on the material surface is smaller than the spot diameter. Compared with other methods, the material around the slit is not affected by oxidation or pollution before and after processing. Compared with other methods, the laser pretreatment method achieves a better cutting quality and requires shorter time. By conducting the complete experiment with respect to two factors and three levels, a pretreated surface in agreement with the shallow melting state can be obtained. In this treatment range, the processing parameters are selected based on the proportion of oxidation area to perform the cutting experiment, and the grating sample is produced. The results show that a slit grating satisfying all the quality requirements can be obtained using the proposed method. Further, the HAZ is considerably smaller after cutting, and there is no slag on the back. The properties of copper cladding before and after cutting are protected to ensure that the quality of the copper-steel laminated plates cut by the fiber laser meets the application requirements.

Objective Inconel 625 is one of the main materials for jet engines and various industrial gas turbines with a stable structure and an excellent performance. It is used in extreme environments with high temperature and wear. The service environmental conditions of components have become more severe with the rapid development of the marine and aerospace industries. Moreover, the demand for higher-performance Inconel 625 components has increased. Particle-reinforced Inconel 625 metal matrix composites can improve the strength and wear resistance of materials, which has become a research hotspot in the recent years. However, only a few studies have been published on the effects of different amounts of reinforcing phase on the matrix, especially on the microstructure and friction and wear properties. The advantages of WC are high hardness and low coefficient of thermal expansion. It can also easily be wetted by molten metal and is an ideal material for improving the wear resistance of components. As a reinforcing phase, WC can meet the requirements of components under high-wear conditions, such as in gas turbines. WC-12Co with a particle size of 45-100 μm was selected for use in this study. Cobalt has excellent thermodynamic properties, which prevents laser from directly acting on WC, thus relieving WC melting, maintaining the particle integrity, and improving the composite properties. WC-12Co particle-reinforced Inconel 625 metal matrix composite coatings were prepared herein by laser cladding. The effects of the WC-12Co particles with mass fractions of 5%, 10%, and 15% on the microstructure and wear resistance of the Inconel 625 matrix were studied, which have a high reference value for component repair technology and material selection in practical industrial applications.

Methods The Inconel 625/WC-12Co composite coatings were prepared by laser cladding in argon atmosphere. The microstructure, phase composition, microhardness, and friction and wear properties of the composite coatings were obtained by metallographic and scanning electron microscopy (SEM), X-ray diffraction (XRD), and microhardness and ring-block wear testers, respectively. The width and depth of the wear marks were measured by a white light interference 3D surface profiler, and a three-dimensional model was established. The effects of different WC-12Co additions (with mass fractions of 5%, 10%, and 15%) on the microstructure and wear resistance of the Inconel 625 matrix were obtained according to the abovementioned characterization and analysis.

Results and Discussions The WC-12Co particle-reinforced Inconel 625-based composite coatings were successfully prepared by laser cladding. The composite coatings with different WC-12Co contents were analyzed by XRD, SEM, microhardness, and friction and wear properties. The results are as follows:

1) After the WC-12Co addition, the microstructure was refined and accompanied by the precipitation of NbC, M₂₃C₆, and other carbides. The content and kinds of carbides also increased with the increase of the WC-12Co content.

2) Most WC-12Co kept their complete form and were uniformly distributed in the coatings. Some were decomposed or melted at the edge. The unmelted WC-12Co in the coatings inhibited the growth of coarse columnar crystals. The equiaxed and short columnar crystals were dominant around the WC-12Co. More Mo and Nb elements were found in the matrix intercrystalline. The higher the addition amount, the higher the content of the two elements, indicating that the two elements segregated at the grain boundary.

3) The composite coating microhardness gradually increased with the increase of the WC-12Co content. Compared with the pure Inconel 625 (264 HV_0.2), the average microhardness of the composite coatings was 274 HV_0.2, 289 HV_0.2, and 308 HV_0.2 respectively. The increase of the microhardness was caused by the eutectic precipitation, fine grain strengthening, and secondary phase strengthening.

4) The mass loss, wear rate, wear width, and depth of the composite coatings significantly decreased, and the wear rate decreased from 10.5004×10^-4 to 0.5768×10^-4 mg·m^-1, indicating that the WC-12Co addition can significantly enhance the wear resistance of Inconel 625. Adhesive and abrasive wears occurred alternately in the friction and wear process. Due to the extremely high hardness of the WC-12Co particles, the duration of the abrasive wear increased when the amount of WC-12Co was high, and the strengthening effect of the carbides precipitated from the grain boundaries reduced the wear of the Inconel 625 matrix, significantly improving the wear resistance.

Conclusions The WC-12Co particle-reinforced Inconel 625 metal matrix composite coatings were prepared by laser cladding. The WC-12Co addition significantly affected the microstructure and the wear resistance of the Inconel 625 matrix. The composite coating microstructure was refined; carbides, such as NbC and M₂₃C₆, were precipitated at the grain boundaries; and equiaxed and short columnar crystals were dominant around the WC-12Co. The augmentation of the WC-12Co content significantly improved the microhardness and the wear resistance of the composite coatings. In summary, adding WC-12Co particles can refine the microstructure of the Inconel 625 matrix and improve its hardness and wear resistance.

Objective Recently, with the development of laser technology, increasingly complex components of high entropy alloy (HEA) can be prepared using laser three-dimensional (3D) printing technology. However, HEA prepared using this method exhibit low strength and plasticity. Therefore, AlCoCrFeNi_2.5 HEA with high plasticity is introduced in laser melting deposition (LMD) technology, which is a laser 3D printing technology. Herein, the microstructure and mechanical properties of AlCoCrFeNi_2.5 HEA prepared using LMD are studied. We aim to fabricate HEA with excellent mechanical properties using the laser 3D forming method.

Methods LMD has been developed to synthesize AlCoCrFeNi_2.5 HEA. The laser process parameters are as follows: laser power, scanning speed, powder feeding speed, shielding gas flow rate, spot diameter, defocusing amount, and lifting amount are 700 W, 400 mm/min, 8 g/min, 5 L/min, 2 mm, 11 mm, and 0.25 mm, respectively. The material used for LMD is AlCoCrFeNi_2.5 high entropy prealloyed powder (sphericity ≥90%), and the range of the alloy particle size measured using the laser particle size analyzer is 45--105 μm. The alloy powder is placed in a vacuum drying oven, heated to 120 ℃, and retained for 2 h. Then, it is cooled to room temperature in a vacuum environment, poured into a powder feeder, and placed in a feeding barrel for standby. Further, a 316L stainless steel plate with dimensions of 100 mm×60 mm×10 mm is selected as the base plate, and the oxide layer on the surface is removed using a grinder. Additionally, an electric spark cutting machine is used to cut the AlCoCrFeNi_2.5 HEA samples into different sizes based on the test requirements. A heat setting machine is used to inlay the samples that required grinding and polishing. The samples are polished with 240 ^#, 400 ^#, 800 ^#, 1200 ^#, 2500 ^#, 4000 ^# metallographic sandpaper and SiC polishing solution with particle size of 0.05 μm and 0.02 μm, respectively The appropriate amount of aqua regia is prepared to corrode the polished samples. The X-ray diffractometer (XRD) is used to perform phase analysis of the sample, and the metallographic microscope (OM) and scanning electron microscope (SEM) are used to observe the structure and fracture morphology of the sample. Moreover, an energy spectrometer (EDS) is used to perform surface analysis of the alloy samples scan to obtain the element distribution, and the electron backscatter diffraction device (EBSD) is employed to conduct crystallographic analysis of the alloy sample. The mechanical properties of the plate-shaped tensile sample are investigated using a tensile testing machine.

Results and Discussions The surface of the AlCoCrFeNi_2.5 HEA sample prepared using the LMD technology shows metallic luster without macro or microcracks. Compositional analysis revealed that AlCoCrFeNi_2.5 HEA prepared using LMD exhibit epitaxy columnar dendrite textures, which are primarily composed of the face-centered cubic structure (FCC) at the primary and secondary dendrites and body-centered cubic structure (BCC) at the dendrite gap, respectively. The columnar dendrites grow along the maximum temperature gradient direction in the molten pool, which is parallel to the direction of the laser deposition (DD). The FCC phase located at the trunk of the dendrite grows preferentially along the <100> crystallographic direction. Stretching results show that the tensile strength and elongation of the alloy are 1428 MPa and 25.8%, respectively, along DD. In the laser scanning direction (SD), the yield strength, tensile strength, and elongation at break of the alloy are 586 MPa, 1288 MPa, and 16.1%, respectively. Because columnar dendrites grow epitaxially along DD, DD shows fewer dendrite walls and phase boundaries than SD. Further, fewer “obstacles” are encountered by the dislocation slip during the stretching process, and it can store more dislocations to provide more plasticity and work-hardening ability; thus, the alloy shows more excellent mechanical properties in DD than in SD. The fracture morphology analysis revealed abundant dislocation slippages in the FCC phase region. The BCC phase located in the dendrite clearance effectively hinders the propagation of slippage during the deformation process, thereby further increasing the dislocation density in the FCC phase. Thus, the tensile sample undergoes continuous work hardening in the middle and late stages of deformation. Therefore, the high strength and ductility of AlCoCrFeNi_2.5 HEA are primarily ascribed to the coupling synergy between the FCC and BCC phases.

Conclusions Plate-like AlCoCrFeNi_2.5 HEA samples with excellent comprehensive mechanical properties are prepared using the LMD technology. The alloy prepared using this method exhibits a uniformly distributed structure, no component segregation, and excellent comprehensive mechanical properties. The addition of the Ni element to the AlCoCrFeNi_2.1 eutectic HEA (EHEA) leads to the uniform precipitation of the BCC hard phase only in the dendrite gap, thus ensuring high strength and good plasticity of the alloy. The tensile strength and elongation of the alloy reach 1428 MPa and 25.8%, respectively. The solidification structure of the plate-like AlCoCrFeNi_2.5 HEA sample prepared using LMD shows columnar dendrite with epitaxial growth. The columnar dendrites grow along the maximum temperature gradient direction in the molten pool, which is parallel to DD. The FCC phase at the dendrite stem grows preferentially along the <100> crystallographic direction. This study provides a new strategy for controlling the microstructure of dual phase HEAs and preparing HEA with high strength and plasticity.

Objective Femtosecond laser micromachining technology has excellent three-dimensional (3D) processing capabilities and provides significant advantages in the production of experimental materials with complex 3D structural features. However, the continuous improvement of ablation efficiency and accuracy is still an eternal topic. The femtosecond laser ablation of monocrystalline silicon is accompanied by the luminescence phenomenon derived from the plasma. During the movement of the three-degree-of-freedom motion control platform in 3D space, the plasma spot produces different forms, particularly during the reciprocating ablation process in the X direction, and two distinct spot forms appear. The trailing direction of the light spot is the upper and lower left when moving forward and backward, respectively. The optimized cascaded support vector machine (SVM) classifier is used to accurately classify and analyze the light spot and can explore the ablation efficiency and accuracy in different ablation directions.

Methods First, the SVM classifier uses the feature of the spot centroid to classify the light spots at the first level. Then, we introduce the means of upper and lower peer lines and obtain two types of light spots. One type is correctly classified into the corresponding ablation direction, called the R light spot, which includes the first-level UP light spot (the trailing direction is the upper left) and first-level DN spot (the trailing direction is the lower left). The other type is incorrectly classified into the opposite ablation direction, called the E spot. Next, the first-level DN spot is superimposed, and the average value is calculated to obtain the average spot. To further obtain the standard model to maximize the similarity of each first-level DN spot, the mean spot is placed into a generative adversarial network (GAN) for training and generation. Compared with random noise, the use of average light spots can reduce the number of training and produce a final generated image more similar to the standard model. Finally, SSIM is used to calculate the similarity between the E spot and standard model, and the E spot is classified using the second-level SVM to generate the second-level UP and DN. Combining the E spot with the first-level UP and DN spots, the final classification result is achieved.

Results and Discussions Using this method, the classification accuracy is 100% under the processing power of 10 mW. In the entire ablation cycle, 34 spots are produced corresponding to the two trailing directions in the two ablation directions. Under 20 mW, the classification accuracy is also 100%. Each half of the ablation cycle produces 33 light spots in the same trailing direction. The deviation is the classification result under 50 mW, and its accuracy is 98.5%. There should be 66 light spots in the same motion state every half cycle; in the second half cycle, one light spot is not correctly classified in the classification result. In the entire time series, only two spots are misclassified, which is close to 100%, and the classification effect is significantly improved. To verify the accuracy of the cascaded SVM classifier in the classification of different states of light spots generated under different ablation directions, three classification methods of histogram of oriented gradient (HOG)-SVM, local binary mode (LBP)-SVM, and Gaussian pyramid (GP)-SVM are compared. Among them, HOG is constructed by calculating and counting the histogram of the gradient direction of the local area of the image operating on the image local grid unit and maintaining good invariance of the image deformation. LBP is an operator that can effectively measure and extract local texture information of an image. It has significant advantages such as nonrotational deformation and gray invariance. GP downsamples the image to obtain partial information of the image. Compared with the traditional HOG-SVM, LBP-SVM, and GP-SVM classification methods, the classification accuracy of the cascaded SVM classifier is increased by 5 to 9 percentage points, 12 to 16 percentage points and 9.0 to 15.5 percentage points 10, 20, and 50 mW, respectively. The cascaded SVM classifier delivers nearly 100% classification accuracy for the spot when using each level of processing power, which has obvious advantages.

Conclusions To classify the different forms of light spots in the femtosecond laser ablation process of single crystal silicon, an optimized cascaded SVM classifier is used. First, the first-level classification is performed based on the spot centroid feature. Then, the standard model is established by generating confrontation GAN. Next, the second-level SVM classification is performed using the structural similarity SSIM of the misclassified spot and the standard model. The classification results are remarkable. A better understanding of the movement state of the light spots can aid further exploration of the law of ablation. It has an indelible effect on the improvement of ablation efficiency and accuracy.

Objective For hot stamping high strength boron steel B1500HS and Q235 steel, dissimilar materials laser welding tailored blanks combine the excellent properties of the two materials and can meet the special performance requirements of the structure. For example, B-pillar of automobile body structure requires that the upper and lower sections of B-pillar have low strength, while the middle section has high strength. However, due to the difference in mechanical properties of the materials connected at the end of the welded joint interface, the stress singularity and other mechanical mismatching effects are caused in the interface among weld, heat-affected zone (HAZ), and base metal (BM), forming a plastic gradient. So far, there are few researches on controlling the plastic gradient in each region of the welded joint during laser welding, so it is very necessary to find a method to control the plastic gradient of the welded joint. In this paper, welding with synchronous thermal field (WSTF) method is proposed in order to regulate the cooling rate of the joint, intervene the structural transformation of each region of the welded joint, reduce the plastic gradient and coordinate the plastic of each region. A comparative investigation of 2 mm boron steel/Q235 steel laser welded joint prepared by conventional laser welding and the WSTF conditions is carried out to systematically evaluate the differences about plasticity between them, and further provide a beneficial reference for the selection of controlling the plastic gradient of dissimilar material laser welded joint in practical engineering applications.

Methods Boron steel/Q235 steel laser welding tailor blanks are performed using YAG laser device. The welding conditions are conventional laser welding with 300, 450, 600 ℃ thermal fields, respectively. After that, high temperature tensile test, fracture morphology, and microstructure are observed by using electronic high temperature tensile testing machine, scanning electron microscope, and metallographic microscope, respectively. The high temperature tensile tests are conducted under 700, 750, 800, 850, 900 ℃, respectively. By comparing the above results, the plastic gradient difference of boron steel/Q235 steel welded joint under conventional laser welding and WSTF conditions is obtained.

Results and Discussions By comparing the high temperature tensile test and microstructure of boron steel/Q235 steel laser welded joint under conventional laser welding with 300, 450, 600 ℃ synchronous thermal field, respectively, it can be found that: 1) elongation: compared with the conventional condition, the elongation of 300, 450, 600 ℃ increases by 19.70%、 20.69%、 21.21% respectively (Table 5). 2) fracture angle: under conventional laser welding conditions, the fracture Angle of the joint is only 115°. Under the synchronous thermal field conditions of 300, 450, 600 ℃, the fracture angle increases by 13.04%, 21.74% and 41.74%, respectively(Fig. 8). 3) plastic strain: when the thermal field temperature is 600 ℃, compared with the conventional conditions, the plastic strain in the weld increases by 31.47%, the HAZ of Q235 steel increases by 28.23% and the HAZ of boron steel increases by 28.61%(Fig. 10). The plasticity of each region is more gradual and harmonious. 4) microstructure: under conventional conditions, the majority of the weld and the HAZ of boron steel are martensite, while the content of martensite in the HAZ of Q235 steel is relatively small, and ferrite and pearlite account for the majority. Under the condition of 600 ℃ thermal field, the microstructure of the weld is mostly ferrite and pearlite, while the microstructure of the HAZ of boron steel is mostly ferrite and the difference with the ferrite microstructure of the weld is smaller. There is almost no martensite in the HAZ of Q235 steel, but all ferrite and pearlite, and the fusion line interface transition is more uniform (Fig. 13).

Conclusions The results show that through synchronously applying the preset thermal field during the laser welding process and laser heat source for boron steel/Q235 steel welded tailor blanks, under the synchronous effect of thermal field and laser heat source, the WSTF method improves the microstructure transformation of weld and HAZ of boron steel and Q235 steel. The method can effectively improve the plasticity of integrated boron steel/Q235 steel welded joint, significantly reduce the plastic gradient in each region of the joint and make it to be gradual and harmonious. For integrated boron steel/Q235 steel laser welded joint, the higher the temperature of the thermal field, the higher the elongation of the integrated welded joint, and the more flat the fracture surface. For each region of boron steel/Q235 steel laser welded joint, the plasticity of the weld is obviously improved, the difference of stress-strain relationship among the weld, the HAZ and the BM is reduced, and the deformation of each region of the welded joint is more coordinated. Due to the reduction of cooling rate of weld and HAZ after welding, the temperature is higher and the retention time is longer, the formation of the brittle organization is avoided. The difference between the weld and its both sides is smaller, the transition at the interface of each region is more uniform, and the plastic gradient is greatly reduced. Therefore, the plasticity of boron steel /Q235 steel laser welded joint is higher than that of conventional laser welding under WSTF, and the plasticity is consistent and coordinated.

Objective Renovating surface defects of Ni-based single crystal superalloy blades is the key to prolong their manufacturing life, and repair mechanism is also a hot research topic in material physics and chemistry. In this study, laser melting method combined with theoretical model calculation is used, and the self-developed DD series Ni-based superalloys in China is taken as the research object. On the basis of previous studies, the substrate orientation has rotated 45°around the direction of [010] during laser melting of single crystal alloys, then rotate different angles ξ along the direction of [001], the distribution and variation of different crystal orientation regions with the increase of ξ are studied. It is found that with the increase of ξ, the distribution of single crystal domain increases on the side of molten pool. Substrate orientation 112/[201ˉ] with single crystal region on one side of molten pool is obtained. Repairing operation on its substrate, because of the disappearance of [010] domain in the molten pool of epitaxial growth repair zone, the grain boundary [001]/[010] also disappears, the ability of stray grain formation is greatly reduced. The substrate orientation suitable for laser cladding repair is obtained, and the mechanism of the effect of changing substrate orientation on the formation ability of stray grain in the epitaxial growth structure is demonstrated.

Methods In our previous research, the mechanism of substrate orientation for the formation of equiaxed crystals has been elaborated in detail. It is defined that when the laser scans along the [100] direction on the crystal plane, that is, the substrate orientation relationship is the initial substrate orientation. When the initial matrix is rotated 45° around the crystal direction (i.e., the y-axis), the growth of heterocrysts can be effectively suppressed. On this basis, this study continues to take the crystal direction as the rotation axis. Through the second rotation of different ξ and laser melting technology, the mechanism of inhibiting the growth of heterocrystals is revealed, and the optimal substrate orientation is obtained to effectively control the single crystal nature of the repaired area.

Results and Discussions According to the experimental results, for the initial substrate crystal orientation (001)/[100], the value of the single-layer repair height is limited by the position of the [001]/[100] crystal zone boundary in the laser cladding epitaxial growth structure, and the position of the [001]/[100] crystal zone boundary in the weld pool is related to the initial substrate crystal orientation. For the matrix (101)/[101]ˉ crystal orientation rotated 45° around the y-axis, as shown in Fig. 2 (a) and Fig. 2 (b). In this case, because there is no crystal zone boundary in the epitaxial growth structure by laser cladding, the height can be effectively raised in the single-layer repair. However, the monocrystal property of the repaired structure is affected by the existence of the sum zone on the left and right sides. However, when the substrate with ξ=45° is used (as shown in Fig. 10), due to the disappearance of the [010] crystal zone in the weld pool of the epitaxial growth repair structure, the intersection line of the [001]/[010] crystal zone on one side of the corresponding molten pool is generated, and all the crystal regions are [001] crystal regions on one side of the molten pool, so the substrate orientation with only a single crystal zone can be obtained. Based on the above matrix orientation combined with multi-channel and multi-layer laser cladding experiment, the single crystal structure of laser cladding repair can be obtained completely, which plays a great role in improving the single crystal property of laser cladding epitaxial growth structure.

Conclusions In this paper, we propose a laser cladding repair method for the secondary rotating matrix, and obtain the substrate orientation suitable for the growth of single dendrite direction. The specific conclusions are as follows:

1. In the experiment of laser cladding epitaxial growth and repair, the choice of the initial substrate orientation can affect the ability of heterocrystal formation in cladding epitaxial growth structure under the condition of other repair parameters such as laser parameters, scanning speed and so on.

2. When the initial crystal direction rotates 45° around the y-axis, the crystal direction is rotated by the rotation axis (secondary rotation). When ξ=45°, the laser cladding repair process is carried out along the crystal direction on the surface. The single-layer repair height increases and the single-layer dendrite growth direction is ensured on one side of the weld pool, so as to ensure the monocrystalline of the laser cladding epitaxial growth structure.

3. The internal mechanism of this situation is that after changing the initial crystal substrate orientation, the crystal area in the laser melting pool expands and occupies the area where the original crystal zone is located, and the boundary of the crystal zone disappears in the molten pool. Therefore, a large single crystal area can be adopted, which ensures the repair efficiency and single crystal property of the laser cladding epitaxial growth structure.

Objective Titanium and its alloys are indispensable and satisfactory owing to its superior physical and chemical properties: high specific strength and modulus, excellent thermal strength, and corrosion resistance. Against the background of the industrial rapid development, the aerospace manufacturing industry has put forward requirements for light-weight and large-scale aircraft, which intensely increased the use of titanium alloys. There are considerable studies based on laser welding of titanium alloy sheets, and significant effort has been made. However, a few studies have been conducted on the joining of thick titanium alloy plates, leaving many technical problems unsolved. For the welding of a thick titanium alloy plate, the arc welding process is a low-cost method, with poor weld beam formation, high residual stress, and wide heat-affected zone. Electron beam welding (EBW) is one of the high energy density welding processes. The challenges of EBW is obvious, and its application is severely limited by the vacuum condition. Several studies stated that narrow-gap laser filling welding is an optimal choice for the joining of thick plates, compared with other welding processes. In this paper, we investigate and analyze the effects of laser beam swing parameters on the formation and porosity of TC4(Ti-6Al-4V) titanium alloy narrow-gap laser filling welding. We obtain that it could eliminate the lack of fusion and reduce porosity with matched laser beam oscillation mode, amplifier, and frequency. It could provide basic data and theoretical supports for thick titanium alloy welding.

Methods Titanium alloy plate (TC4(Ti-6Al-4V)) and wire (TC3(Ti-5Al-4V)) are used in this study. First, 2 mm-wide-gap TC4(Ti-6Al-4V) is filled with TC3 wire by laser swing welding, which takes different oscillation modes: amplitude and frequency. A well-formed surface and cross-section with low porosity are selected as suitable oscillation parameters. Next, 20 mm-thick TC4(Ti-6Al-4V) is welded by selected parameters. Apart from surface formation and cross-section, the weld joint has been analyzed by X-ray pore detection and tested tensile strength. Then, the tensile fracture is analyzed by a scanning electron microscope.

Results and Discussions The weld surface formation cloud improves a lot despite the laser beam oscillation mode considered. The fusion depth gets smaller, and the fusion width gets bigger in the cross-section (Table 2), which could reduce the lack of fusion. When the oscillation amplitude is between 1.5 mm and 2 mm, and the frequency is between 100 Hz and 200 Hz with circular oscillation, the surface of the weld becomes continuously smooth, and the cross-section turns wider and shallower (Table 3 and Table 4). A bigger form factor of weld applies to narrow-gap welding to reduce lack of fusion matched oscillation parameters could be selected with form factor and porosity (Fig. 4). When the oscillation frequency is between 100 Hz and 200 Hz with 2 mm amplitude and circular swing, no pore was detected on the X-ray films (Fig. 5). 20-mm-thick TC4(Ti-6Al-4V) has been welded by narrow-gap laser beam swing with circular 2 mm and 100 Hz oscillation. The results showed that the weld joint gets a well-formed surface and has no pore in the X-ray film. In the tensile strength tests, the weld joint maximal tensile strength reaches 930 MPa, which is the same as base metal and the tensile samples are ductile fracture (Fig. 10).

Conclusions In this study, narrow-gap TC4(Ti-6Al-4V) plates are welded by swing laser wire filling welding. Compared with non-oscillating laser welding, the surface forming can be improved after the beam swings, and the continuous smooth weld can be obtained. Under the circular oscillation, when the oscillation amplitude is between 1.5 mm and 2 mm, and the frequency is between 20 Hz and 100 Hz, the well-formed weld can be obtained. When the oscillation mode is linear and circular, the porosity is relatively small, while when the oscillation mode is non-oscillating and infinite oscillation, the porosity is relatively large. When the oscillation amplitude is 2 mm, the porosity is small, and no obvious pore is found from the X-ray films when the oscillation frequency is between 100 Hz and 200 Hz. Owing to comprehensive consideration of weld formation and porosity, with the circular oscillation 2 mm amplitude and 100 Hz frequency of the oscillation parameters, we have completed the 20 mm-thick TC4(Ti-6Al-4V) narrow-gap laser wire filling welding with well forming joint surface, and no obvious pore was detected from X-ray detection. The maximum tensile strength of the 20 mm-thick weld joint reaches 930 MPa, which is the same as base metal, and the fracture mode is a ductile fracture with a dimple fracture surface under the scanning electron microscope. We obtain matched oscillation parameters. The data has been validated with well formation and property from the test of the 20 mm-thick weld joint, which plays a significant role in thick titanium alloy welding.

Objective As a new advanced manufacturing technology, laser cladding rapid prototyping has been widely used to laser forming without any need for a mold or die. However, the traditional laser cladding usually adopts the low-power forming below 2000 W, which is inherent with problems such as low deposition efficiency, long-forming time, and insufficient material use. In contrast, the high-power hollow ring laser cladding can effectively improve the forming efficiency by optimizing the experimental process parameters using model analysis. In recent years, neural networks have been gradually applied to optimize multi-parameter objective in laser cladding, laser welding, and laser communication. The neural network prediction model is capable of fitting and modeling nonlinear data through iterative learning without the need to specify the function form in advance, demonstrating excellent ability to deal with multivariate nonlinear problems. However, the single neural network model may suffer from problems like slow and easy to fall into local extremum training speed. In this study, not only a neural network model optimized by a genetic algorithm is proposed to predict and optimize the laser cladding deposition efficiency, but also the parallel random search is employed to effectively solve the aforementioned two problems of the training process in the model. We expect that our research can contribute to improving deposition efficiency and shortening forming time in high-power laser cladding forming.

Methods The experimental equipment adopts the hollow ring internal light powder feeding cladding system, which mainly consists of a 6 kW Raycus laser, a 6-axis KUKA robot, and a hollow ring internal light powder feeding nozzle. The cladding material is Fe314 powder and the experimental substrate is 304 stainless steel. The effects of the laser power, scanning velocity, powder feeding velocity, and defocusing on the deposition rate of the cladding layer were studied systematically by an orthogonal experiment. The tissue differences of the samples under high and low deposition rates were consistently observed by scanning electron microscope. The Box-Behnken(BBD) experiment was then designed by the response surface method to study the influence of several interaction factors on the deposition rate. Meanwhile, the multiple regression model was also established to predict and optimize the deposition rate. Additionally, a series of randomized trials were conducted as a supplement. Both the results of the BBD experiment and the samples of the randomized trial were taken as the training data of the genetic neural network, and eventually, the genetic neural network model was trained to predict and optimize the deposition rate. By comparing the modeling ability, generalization ability, and optimization ability of the two models, the most suitable model was selected as the estimator for the following experiments to accomplish closed-loop control of the deposition rate.

Results and Discussions The range analysis method was adopted to analyze the orthogonal experimental results. It is indicated that powder feeding rate has the greatest influence on the cladding deposition rate followed by laser power and scanning speed, and the defocusing amount on the cladding deposition rate was of the least influence (Table 3). When it comes to the reciprocal influence of various factors on the deposition rate, the response surface methodology (RSM) model shows that the interaction effect between laser power and powder feeding rate is the most significant. This is probably because the laser energy density improves as the laser power enhances, resulting in the enlargement of the high-temperature range in the molten pool. Within a certain power range, the deposition rate increases significantly by increasing the molten powder in the molten pool [Fig. 5(a)]. The second interaction effect is between scanning speed and powder feeding rate [Fig. 5(b)], and the interaction effect between defocusing amount and laser power is the least significant [Fig. 5(c)]. Simultaneously, the comparison of the predicted experimental values of genetic algorithms-back propagation (GA-BP) and RSM models after training shows that both models have good fitting accuracy, but the value of R² in GA-BP is closer to 1 (Fig. 8). By comparing the generalization ability of GA-BP and RSM models, the absolute average deviation(AAD) values of RSM and GA-BP models were 8.762% and 4.938%, respectively (Fig. 9). The maximum deposition rate obtained by the optimized GA-BP model was 61.74 g/min, which was higher than the value of 53.55 g/min obtained by the response surface method (Table 7). The above studies show that the prediction, generalization, and optimization capabilities of the genetic neural network model are superior to those of the response surface model, and the neural network model optimized by the genetic algorithm can provide a more effective prediction method for the achievement of laser cladding forming with high deposition rate.

Conclusions In this study, considering the orthogonal experiment in the hollow ring internal light powder feeding cladding system, the BBD experiment designed by response surface method was adopted and an RSM model using deposition rate as the response target was established. Subsequently, the influence of laser power, powder feeding speed, scanning rate, and defocusing amount on the deposition rate of high-power laser cladding was systematically investigated, individually or interactively. Based on the results of the BBD experiment and the samples of the randomized trial, a GA-BP model was set up. Comparing the performance of the two models by analyzing the modeling and generalization abilities, the technological parameters of laser cladding with high deposition rate were optimized. In conclusion, the performance of the GA-BP model is slightly better than that of the RSM model as the root mean square error and the average absolute deviation of the GA-BP model are smaller than those of the RSM model, and the decision coefficient values obtained in the RSM model and GA-BP model are 0.9479 and 0.9726, respectively. The prediction abilities of GA-BP and RSM models are fairly close, but under the condition of high deposition rate, the optimal deposition rate in the GA-BP model is 61.74 g/min, which is higher than the value of 53.55 g/min optimized by the RSM model. Briefly, the GA-BP model can be considered an effective method to optimize the high deposition rate of cladding. Because of the high reliability, the model can find the optimal deposition rate within a specific process range, which can be used as an estimator for operating closed-loop control over the following forming system to obtain high-efficiency and high deposition rate in the later cladding.

Objective Currently, the development of lightweight materials has become the primary objective of manufacturing, and the application of high-strength steel has emerged. Compared with traditional lightweight materials such as aluminum and magnesium alloys, 1000 MPa ultrahigh-strength steel exhibits good strength and toughness, high safety performance, and low application costs. It also exhibits good economic benefits. However, the application of high-strength steel requires reliable welding joints. Recently, the welding joints obtained via traditional welding methods are softened, the heat-affected zone is embrittled, the impact toughness of welding joints is low, the joint strength is lower than that of the base metal, the welding deformation is large, and the welding efficiency is low. As a new type of heat source welding method, laser-arc hybrid welding can effectively restrain the softening of the heat-affected zone, reduce damage to the base material, and exhibits a high welding efficiency and small deformation, etc. Furthermore, the performance of welding joints of 1000-MPa-grade tempered ultrahigh-strength steel increasingly deteriorates with the increase of strength, limiting the popularization and application of this type of high-strength steel. Therefore, it is considerably important to select appropriate filling materials and adopt laser-arc hybrid welding for studying the performance of welding joints of this type of high-strength steel.

Methods Equal-strength matching welding wire MG90-G and low-strength matching welding wire ER80YM were selected to perform laser-arc hybrid single-pass welding on the 1000-MPa-grade quenched and tempered ultrahigh-strength steel BS960E to well solve the deterioration of the welded joint performance of this type of high-strength steel. The welding equipment is manufactured using a 10 kW fiber laser (TRUMPF LASER TruDisk 10002) and a welding machine. The laser-guided hybrid welding method is adopted. In addition, the groove form is type I, and the butt gap is 1 mm. The welding process is optimized through the single factor variable method, and the welding quality is evaluated via non-destructive flaw detection. The heat input changes are collected based on the thermal cycle. The tensile strength, hardness, and low-temperature impact toughness of the welding joints for two types of welding materials were tested, and the fracture was analyzed via microscope and scanning.

Results and Discussions The mechanical properties of the BS960E laser-arc hybrid welded joints were analyzed when using different welding speeds and welding materials, and the following results were obtained. 1) Welding quality: The weld formation was good when the welding speeds were 1.32 m/min and 0.72 m/min. With the increase of welding speed, the porosity of welding seam decreased significantly. Thus, a stable molten pool at high welding speed can inhibit the generation of porosity. At a high speed, the fusion ratio of welding joints increases, the beneficial elements of welding wire transition to welding seam are reduced, and the impact property of welding seam deteriorates. 2) Hardness and microstructure: Under the condition of high-speed welding, the hardness of welds increases by 9.7% compared with that observed under low welding speeds. Laser-arc hybrid welding can effectively improve and restrain the softening of welding joints, and the hardness of the softening zone is reduced by 8.8% compared with that of the base material. The microstructure of the weld with a welding speed of 1.32 m/min includes bainite and M-A component, and the microstructure in the heat-affected zone is lath martensite. 3) Tensile properties: The tensile strengths of 90 wire and 80 wire welded joints are 1129, 1117, 1145, and 1084 MPa, respectively. The tensile strength is equivalent to that of the base metal, and the elongations observed in the aforementioned cases are 93%, 98%, 104%, and 102% of the base metal, respectively. Thus, low-strength matching has better ductility. 4) Impact performance and fracture: The impact performances of the two welds at low welding speeds increased by 28.5% and 15.7%, respectively, compared with those observed at high welding speeds. The impact fracture of the welding seam of the two types of welding wires exhibits the characteristics of ductile fracture, and the impact fracture of the heat-affected zone is mainly manifested as a brittle fracture. With the decrease of welding speed, the brittleness of the MG90-G welding joint in the heat-affected zone increases, but the impact performance deteriorates less obviously because of the small difference in the welding input energy. However, the impact toughness of the ER80YM wire welded joint in the heat-affected zone increases with the decrease of welding speed.

Conclusions Research results show that laser-arc hybrid welding can effectively improve the softening of joints and reduce the width of heat-affected zone of joints; therefore, the tensile fracture can be observed in the base metal. Under the action of the laser heat source, laser-arc hybrid welding can effectively improve defects such as undercuts caused by traditional welding. Further, under the conditions of fast cooling and heating, the weld porosity can be effectively reduced by increasing the welding speed. From the matching analysis of welding materials, the laser-arc hybrid welded joint with low-strength matching wires exhibits better ductility than that obtained in case of equal-strength matching wires. Further, the tensile strength is equivalent to that of the base metal. Under the same welding process parameters, the impact performance of different welding wire welded joints is small, which can be attributed to the unevenness of the weld metal composition and the influence of fusion ratio. Currently, the impact performance of the welded joints of this type of high-strength steel is a weak link, which must be further improved. Therefore, under the condition of laser-arc hybrid welding, the selection of appropriate heat input and low-strength matching welding wires can effectively improve the low-temperature impact performance of welding joints.

Objective Sucker rod coupling failure is a major problem in sucker rod pumping systems, which are frequently used in oil fields worldwide. Surface modification of sucker rod couplings is an economical method to address this problem. High speed laser cladding (HSLC), which is proposed on the basis of laser cladding technology, is a novel additive manufacturing technology for surface modification. A HSLC modifies the relative positions of laser beam, spray powder, and molten pools, thus powder particles can be heated to their melting point before being guided into the molten pool. Due to limited transmitted laser energy, a micro-molten pool can form on the substrate such that a coating with low dilution ratio and metallurgical bonding is produced. The HSLC in this study has a higher cladding efficiency (500 cm ²/min) and a suitable coating thickness (25 μm to 500 μm), which overcomes the efficiency obstacle of conventional laser cladding technology. It provides a highly efficient and low-cost production method for the fabrication of thin coatings on sucker rod couplings to improve their surface properties (such as hardness, wear resistance and corrosion resistance). In this paper, martensitic stainless-steel coating is prepared on the surface of a 35CrMo sucker rod coupling by HSLC, followed by laser remelting (LRM), to explore a new way to improve wear resistance and corrosion resistance of the sucker rod couplings.

Methods Martensitic stainless-steel powder, with particle diameters ranging from 15--53 μm, is selected as the cladding material. Coatings are prepared by a ZKZM-4000 HSLC system. The laser cladding parameters are: 3500 W laser power, 7 m/min scanning speed, and 60% overlap ratio. The remelting parameters are: 3500 W laser power, 15 m/min remelting speed, and 50% overlap ratio. After the laser cladding and remelting processes, specimens are cut from the substrate, then mounted, ground, and polished for microstructure observation and property testing. The microstructure of the HSLC and LRM coatings are characterized by optical microscopy. The phases of the coatings are determined by X-ray diffraction (XRD). The hardness distribution of the coatings is studied using a hardness meter. The tribological properties are tested using a MFT-EC4000 friction and wear tester. Potentiodynamic polarization and electrochemical impedance spectroscopy of the coatings and substrate, in a 3.5% NaCl solution, are also measured using a Perkin-Elmer M398 electrochemical workstation.

Results and Discussions The HSLC coating is fully dense, smooth, and without any noticeable stomata, inclusions, or cracks. The thickness of the coating is approximately 512 μm, while the heat affected zone is only about 85 μm. The surface roughness of HSLC is 15.7 μm. LRM can reduce the surface roughness to 5.4 μm because of the remelting of surface powders. The HSLC coating is composed of single martensitic structure. The phase composition does not change after LRM processing. However, the width of the diffraction peak varied due to the changes in grain size. The HSLC coating shows a special multi-layer lapped character. The coatings form a metallurgical bond with the substrate due to the maximum temperature gradient and slow growth rate; planar crystals appear at the combined zone. The middle cladding layer contains dendrites with a typical epitaxial growth tendency along the temperature gradient. Near the surface of the coatings, the dendrites become fine equiaxed grains without an obvious preferential growth direction. LRM can improve the multi-layer lapped character and refine the dendrites. Some lump-structure dendrites are formed in the surface because of the high cooling rate.

The average hardness of the HSLC and LRM coatings is 470 HV and 494 HV, respectively, which is about 2.2 times that of the substrate. The average friction coefficient of the substrate, HSLC coating, and LRM coating are 0.22, 0.24, and 0.33, respectively. However, the wear loss of the LRM coating, HSLC coating, and substrate increases in that order. The wear loss of the samples does not show an obvious relationship with their friction coefficients, which suggests contributions from various wear mechanisms. The worn surface of substrate shows various ploughed groves parallel to the sliding direction, which is a typical feature corresponding to the abrasive wear mechanism. However, wear pits can be found on the worn surface of the HSLC and LRM coatings. The worn scar of the HSLC coating or LRM coating is shallower and wider than those of the substrate, which contributes to the adhesion wear mechanism.

Both HSLC and LRM coatings show excellent corrosion resistance with passive regions. The self-corrosion potential (E_corr) for the LRM coating is highest, at approximately -0.370 V, and the self-corrosion current density (I_corr) is lowest, at 2.599 μA/cm². The HSLC coating and substrate have E_corr and I_corr values of -0.5261 V and 6.195 μA/cm² and -0.7469 V and 9.259 μA/cm², respectively. Nyquist plots for the substrate, HSLC coating, and LRM coating are all unfinished capacitance arcs with different radii and impedances. The analog circuit could be expressed by R(QR). The corrosion resistance of the coatings is also evaluated by the model value of impedance (|Z|) and the phase value; a higher value of impedance and phase angle suggest a more stable passive film. The maximal value of capacitive arc radius, impedance, and the phase angle are all higher for the LRM coating. HSLC improved the wear resistance and corrosion resistance of the substrate and LRM further improved the performance of the HSLC coating.

Conclusions A novel HSLC and LRM technique successfully developed a pore and crack-free martensitic coating on a sucker rod coupling surface. The HSLC coating shows a gradient structure, consisting of planar crystal, columnar dendrites, and equiaxed crystals from bonding zone to surface. The coating exhibits good wear resistance and corrosion resistance. LRM leads to refined dendrites and a uniform distribution of composition, which is helpful for enhancement of surface properties. HSLC and LRM are potential methods for improving the wear resistance and corrosion resistance of sucker rod couplings for industrial applications.

Objective Bronze-based diamond grinding wheels have been widely used, and their applications have increased sharply. However, they are difficult to dress after being blunt. Traditional dressing methods, such as mechanical and electrical dressing, have the disadvantages of large loss of dressing tools, low dressing efficiency, and serious environmental pollution. The laser dressing method has significant advantages such as high efficiency, environmental protection, controllability, and wide applicability. However, if using traditional long-wavelength continuous, millisecond, or nanosecond infrared lasers, their melting/vaporization ablation mechanism can easily cause carbonization damage of the diamond abrasive grains on the surface of the grinding wheel. The short-wavelength picosecond laser has the technical advantage of “electronic state” cold processing, which can simultaneously ensure the sharpening effect and inhibit the carbonization damage of diamond abrasive grains due to high temperatures. It has significant technical advantages when dressing the grinding wheel. In this paper, the picosecond green laser was used to radially sharpen the bronze-based diamond grinding wheel. The protocols of using a picosecond green laser to sharpen the bronze-based diamond grinding wheel were explored. Moreover, selectively and quantitatively removing of the bronze matrix at the grinding wheel was achieved.

Methods Firstly, a 10 ps green laser was focused on the bronze/diamond surface, and the damage thresholds were calibrated by the S-on-1 damage measurement method. This method allowed to determine the suitable working conditions for picosecond green laser to sharpen the bronze/diamond grinding wheels. Secondly, a picosecond green laser was used to sharpen the surface of the bronze/diamond grinding wheel. Thirdly, the surface morphology and roughness were characterized using the laser confocal microscope. Finally, the effects of laser peak power density, repetition frequency, and scanning times on the sharpening effect were studied.

Results and Discussions 1) The damage thresholds of the bronze matrix and the diamond abrasive grains differed in two orders of magnitude and amounted to 1.23×10 ⁹ W/cm ² and 3.71×10 ¹¹ W/cm ², respectively (Fig. 4). The difference in damage threshold was conducive to the selective micro-removal of the bronze matrix and the selection of the sharpening process parameters. 2) The picosecond laser damage characteristics of bronze diamond grinding wheels were studied. Next, these characteristics are compared with the traditional approach using a continuous or short-pulse laser. Picosecond laser has greatly reduced the carbonization of diamond abrasive grains. If the appropriate peak power density was selected, the diamond abrasive grains were not easy to be carbonized even at high repetition frequency, there was no obvious heat trace. 3) The laser power density played a major role in the sharpening effect (Fig. 7). When the laser power was constant, adjusting the number of scans quantitatively removed the bronze matrix at the surface of the grinding wheel (Fig. 9). When the power density was constant, a proportional increase in both the laser power and repetition frequency achieved a good sharpening effect. However, the gradual accumulation of heat has increased the chances of carbonization (Fig. 8).

Conclusions In this study, the damage rules and mechanisms of bronze-based diamond grinding wheels sharpening with picosecond laser were studied. Moreover, the damage threshold of the picosecond laser ablation of the bronze matrix/diamond was quantified, and the laws of different process parameters acting at the surface of the grinding wheel were analyzed. The removal mechanism of the picosecond green laser on the bronze matrix is mainly vaporization. It allowed avoiding the carbonization of diamond abrasive grains. Even at high repetition frequencies, there was no obvious heat accumulation. The study shows that the damage threshold of the bronze matrix and diamond abrasive grains are different in two orders of magnitude. The bronze matrix can be selectively removed by adjusting the peak power density and quantitatively removed by adjusting the number of scans. Moreover, the picosecond green laser is capable of ensuring the integrity of the diamond abrasive grains by selectively and quantitatively removing the bronze matrix.

Objective Selective laser melting (SLM), because of its capacity to fabricate complex precision parts with high forming accuracy, has been hailed as one of the most promising manufacturing technologies for rapid prototyping. However, the mechanical properties of metal materials formed by SLM have the characteristics of anisotropy, high strength, and low plasticity. Therefore, heat treatment is always needed to control the microstructure to meet application requirements. Annealing treatment is typically adopted to improve the mechanical properties of selective-laser-melted titanium alloys. Therefore, study of the effect of annealing temperature and holding time on the microstructure, mechanical properties, and fracture mechanism of TC11 titanium alloys formed by SLM is of great significance.

Methods The resulting microstructure, mechanical properties, and fracture morphology of selective-laser-melted TC11 titanium alloys under different heating temperature and holding time were studied, and the fracture mechanism under different conditions was explored. Firstly, compact TC11 titanium alloys were obtained by SLM. Secondly, different annealing heat treatments were performed on the samples. Thirdly, the phase composition of the different samples was analyzed by X-ray diffraction, and the microstructure morphology was observed by optical microscopy (OM) and scanning electron microscopy (SEM). Finally, the change in the micro-hardness of different samples was tested using a micro-hardness tester, the tensile properties at room temperature were tested, and the fracture morphology was observed by SEM.

Results and Discussions The as-deposited TC11 titanium alloys are composed of hexagonal close-packed Ti (HCP/Ti), with lattice parameters a and c of 0.2934 nm and 0.46757 nm, respectively. The annealed samples consisted of HCP/Ti and body center cubic Ti (BCC/Ti), where a and c for HCP/Ti are 0.29172 nm and 0.46817 nm, respectively (Fig.3). Based on the observation of microstructure morphology by OM and SEM, it is deduced that the selective-laser-melted TC11 titanium alloys consisted of columnar grains, within which acicular α' martensite was present (Fig.4). After annealing at 850 ℃ for 4 h, fine α+β mixed structures were formed in the alloys, this was the result of the nucleation and growth of α' martensite. Due to the sufficient atomic diffusion leading to coarsening α lamellae, basket-weave structures were formed in the samples annealed at 950 ℃ for 4 h. Moreover, α clusters with the same orientation and continuous grain boundary α phase (GB α) were also observed in the grains and at the grain boundaries, respectively (Fig.5). When annealing temperature remained at 950 ℃ with holding time of 1 h or 2 h, basket-weave structures were also formed in the samples, but the width of α lamellae was about one-half and one-quarter of that in the samples annealed at 950 ℃ for 4 h, respectively. In addition, GB α phase began to transform into a discontinuous distribution (Fig.6). According to the results of the hardness test, the average micro-hardness of the as-deposited samples is about 402 HV_0.5, whereas the hardness of the samples annealed at 950 ℃ for 4 h is only about 85% that of the as-deposited samples. Moreover, the micro-hardness of the samples annealed at the same temperature for 2 h and 1 h is about 381 HV_0.5 and 393 HV_0.5, respectively. The increase in micro-hardness with the decrease in holding time could be due to the effect of fine-grained strengthening (Fig.7). The tensile strength and percentage elongation after fracture is 1557 MPa and 2.5%, respectively, because acicular martensite is characterized by high strength and poor plasticity. However, after the samples were annealed at 850 ℃ and 950 ℃ for 4 h, the tensile strength is 72% and 64% of that of the as-deposited samples, respectively. In this case, the percentage elongation after fracture is 4.5 times and 5.7 times that of the as-deposited samples, respectively. When the samples were annealed at 950 ℃, their tensile strength increased from 996 to 1051 MPa, and the percentage elongation after fracture increased from 14.3% to 19.8% (Fig.8, Table 3). This is because the fine basket-weave structures have the effect of fine grain strengthening, and GB α has an effect on the percentage elongation after fracture. Based on fracture analysis, no obvious plastic deformation was observed in the macroscopic fracture of the as-deposited samples, and the fracture exhibited granular grain surfaces with different orientations. The microscopic fracture showed the characteristics of intergranular dimples. The above analysis indicates that the as-deposited sample underwent intergranular fracture when it was stretched at room temperature (Fig.9). However, macroscopic fractures of the annealed state had obvious plastic deformation, and the microscopic fractures showed the characteristics of dimples with larger size, indicating that ductile fracture occurred in the annealed state (Fig.10).

Conclusions The effect of annealing temperature and holding time on the microstructures, mechanical properties, and fracture mechanism of selective-laser-melted TC11 titanium alloys is investigated, and the following conclusions can be drawn: The microstructures of the alloys are composed of acicular martensite within columnar grains parallel to the building direction. After annealing at 850 ℃ and 950 ℃ for 4 h, the microstructures of the alloys are fine α+β mixed and α+β basket-weave structures, respectively. As a result of decomposition of the acicular martensite, the alloys are softened, and the softening effect is more obvious with the increase in annealing time or temperature. Furthermore, the fracture mechanism changes from being intergranular to ductile, which is consistent with the regular variation of the percentage elongation after fracture. When the alloys are annealed at 950 ℃, with shorter holding time, finer lamellar α can be obtained. Continuous GB α transforms into a discontinuous distribution, finally resulting in the simultaneous increase in tensile strength and elongation after fracture. Selective-laser-melted TC11 titanium alloys with better strength and plasticity can be obtained through annealing at 950 ℃ for 1 h, with a tensile strength and percentage elongation of 1051 MPa and 19.8%, respectively, after fracture.

Objective Laser shock imprinting (LSI) is a manufacturing technique for material strengthening and forming using high-pressure plasma shock waves induced by laser pulses. It has been widely used in many fields. Warm laser shock peening (WLSP) combines the advantages of laser shock peening, dynamic strain aging, and dynamic precipitation and can produce microstructures with high stability. The LSI technology can produce regular large-area periodic microstructures with different shapes from hundreds of microns to nanometers on a metal foil surface. Corresponding to WLSP, the temperature-assisted LSI technology changes the forming process, forming quality, and forming mechanism of an aluminum foil. Therefore, it is important to conduct a detailed investigation on warm laser shock imprinting (WLSI) and reveal the mechanism of high strain rate plastic deformation hardening and dynamic recovery softening during multiple WLSI.

Methods WLSI of an aluminum foil at different imprinting temperatures and imprinting times was conducted using the WLSI experimental devices. The imprinting temperature was controlled using an electric heating plate. We tested the forming height, surface quality, surface hardness, and microstructure of the aluminum foil using an optical profilometer, scanning electron microscope, nano-indenter, and transmission electron microscope, respectively. The ABAQUS/Explicit module was used to analyze the transient mechanical effect of the aluminum foil during the WLSI process, in which the residual stress and deformation speed of the forming parts were also analyzed.

Results and Discussions For WLSI at different temperatures, when the imprinting temperature was 25 ℃, the forming height was about 8.2 μm. When the imprinting temperatures were 150 ℃ and 225 ℃, the forming height was increased to 9.3 μm, and the microstructure on the aluminum foil surface had a good forming quality. When the imprinting temperature was 300 ℃, the forming height was dropped to about 8.35 μm, and the formed part surface had a poor oxidation phenomenon (Fig. 3). The simulation results by the ABAQUS/Explicit module showed that the maximum deformation speed of the aluminum foil at 300 ℃ was about 41.8 m/s, 10.6% higher than that at 25 ℃ (37.8 m/s). Furthermore, the WLSI introduced high residual stress at the top of the microstructure. With the increase of imprinting temperature, the area of the high residual stress was increased, the maximum residual stress was decreased, and the difference between the maximum and the minimum internal stresses was decreased gradually (Fig. 5). In the WLSI at different imprinting times, compared with the forming height after single imprinting (3.8 μm), those after two (5.8 μm) and three (7.4 μm) successive imprintings were increased by 52.6% and 94.7%, respectively (Fig. 6). It should be noted that slip and twinning are the main deformation mechanisms of materials. Aluminum is a high-fault-energy metal with a small expansion dislocation width but does not easily form twin. In this study, the laser-induced shock pressure was 3.8 GPa, and the strain rate was greater than 10 ⁴s ^-1 during the WLSI shock hardening and softening process. The WLSI process triggered dislocation slip in different slip planes and formed dense dislocation. The grains’ dislocation entanglement and chaotic dislocation entanglement separated the high- and low-density dislocations to form cellular substructure and sub-grains. The high-density dislocation, small cellular substructure, and sub-grains made the strength and hardness of the aluminum foil increased, which led to the second and third WLSI deformation increments. We divided the softening process at multiple WLSI into two processes, namely, dynamic recovery and dynamic recrystallization, based on the deformation conditions. Furthermore, we compared the refinement methods for high- and low-fault-energy materials with that for the medium-fault-energy materials. Our results show that the refinement methods for high- and low-fault-energy materials are more simple than that for the medium-fault-energy materials. Owing to the high-fault-energy material of the aluminum foil, dislocation slip and dynamic recrystallization result in grain refinement. However, owing to the low temperature, short deformation time, small deformation degree, and high-fault-energy, only dynamic recovery occurred in our experiment. Moreover, no dynamic recrystallization and grain refinement occurred, and a large number of cellular substructures and high-density dislocation were retained in the grains (Fig. 9). Therefore, compared with dynamic recovery softening, shock hardening plays a dominant role in multiple WLSI.

Conclusions This study demonstrates that an increase in imprinting temperature reduces the flow stress of aluminum foil and makes its formation easy. WLSI leads to a high forming height and good surface quality at 150 ℃ when the imprinting temperature is 300 ℃. Furthermore, the springback and shrinkage of the aluminum foil lead to a small forming height, whereas the oxidation leads to bad surface quality. With the increase of imprinting times, the forming height of the aluminum foil gradually increases, whereas that of each imprinting decreases. After three imprinting times at 200 ℃, the forming part surfaces maintain good oxidation state and surface quality. Multiple WLSI can enhance the deformation resistance of the formed parts and strengthen the mechanical properties of the aluminum foil. Thus, the foil is subjected to the dual effects of shock hardening and recovery softening. The shock hardening plays an important role in the experiment, which ultimately leads to the successive increment of the hardness and decrement of the forming height of the aluminum foil.

Objective It is essential to initially establish a precise recognition model to achieve accurate control for a penetration state in laser welding. Although the recognition method of the penetration state using visual signals is widely proposed, there are still interferences, such as plasma, vapor, and spattering, in the laser welding process. Besides, there is no significant change in adjacent penetration state. These are the problems in vision-based recognition of the laser welding penetration state. Feature engineering and deep learning seem to be the only methods to solve these problems. Although the feature engineering-based method is interpretable, it requires many subtasks to decrease overall recognition efficiency. Also, the value of data cannot be fully developed. On the other hand, the deep learning-based method realizes an end-to-end recognition from the original image to the penetration state. It improves the overall recognition efficiency and data value. The deep learning-based method becomes a major research focus because of integrating intelligent technologies into manufacturing systems. However, deep learning-based methods require a large amount of data, because fewer data result in overfitting. The boundary between adjacent penetration states is unclear, making it difficult for supervised learning methods to be applied. Inspired by the fact that skilled welders consider asymptotic information in deciding on the welding process, we propose a laser welding penetration state identification method that incorporates timing information. The timing information is expected to improve the determining factor of a deep learning method for the weld penetration state and increase the amount of data.

Methods The frame of the proposed method consists of a spatial feature extraction module (SFEM) using a convolutional neural network (CNN), a time domain feature extraction module (TDFEM) using a bi-directional long short-term memory (BiLSTM) neural network, and a classification module (CM) using a SOFTMAX function. In the SFEM, we used two convolutional layers and two max-pooling layers to extract the input image sequence's spatial features. Afterward, we applied TDFEM to extract the features in the time domain from the input sequence. In the TDFEM, the feature sequence was simultaneously input into the forward LSTM and reversed the LSTM to obtain the forward and reverse outputs. Then, we summed the forward and reverse outputs of the same input as the final output of the current input. In the CM, we first input the spatiotemporal features into the fully connected network for dimensionality reduction. Subsequently, we mapped the low-dimension features to categorical probabilities using the SOFTMAX function. For data acquisition, the optimum penetration condition was obtained through a welding test. We incremented the laser power corresponding to the optimum penetration state, and then decremented it to obtain excessive penetration and incomplete penetration conditions.

Results and Discussions Figure 5 shows that the CNN-BiLSTM method's accuracy converges to 1 after 5000 iterations, whereas the CNN method's accuracy only converges to approximately 0.93. Using the CNN-BiLSTM method, the difference in accuracy on training and validation sets is insignificant, suggesting no overfitting using the proposed method (Fig. 6). The identification accuracy and overall evaluation index of the CNN-BiLSTM method reach 99.26% on the test set, much higher than the those of the conventional CNN method. Although various CNN-LSTM method indicators are about 97%, the CNN-LSTM method only considers the previous information of current input in the time domain without considering subsequent information of current input in the time domain. The proposed method takes only 9.43 ms to identify a single image in a PC (Table 2). The CNN and CNN-LSTM methods misclassify the penetration state as the true label. Moreover, Fig. 7 shows that the proposed method can suppress misclassification. In this paper, the training process's convergence tends to be consistent when the learning rate (LR) is close to 10 ^-3. The model does not converge when the LR is 10 ^-5, suggesting no overfitting. When the optimizer (OM) is set to Adagrad or Adam, the training process's convergence is similar to that of the stochastic gradient descent method applicable to this paper. The proposed method could not converge within three epochs when the Abadelta OM is used because the Adadelta easily falls into the local optimum in the middle and later training stages (Fig. 8). Table 3 shows that all accuracy metrics of CNN-BiLSTM are above 97.66% when the LR is around 10 ^-3 or OM is replaced, which suggests that the proposed method is robust.

Conclusions In this paper, timing information was not considered in deep learning-based methods for penetration state recognition. Our proposed method, CNN-BiLSTM, can adaptively extract spatiotemporal context information, as the method demonstrated good convergences and stability. The introduction of temporal information can indirectly play the role in data augmentation, and the proposed method does not overfit. The overall recognition accuracy of the proposed method on the test set is 99.26%. As such, the proposed method meets the standard requirements of vision-based laser welding in terms of penetration condition monitoring. Furthermore, the method is robust to changes in LR and optimizer. Although the proposed method has many advantages, the following aspects still need to be focused in future research. Better classification of penetration states will be a future interest choice in terms of research objects. Also, making the network structure fit the parallel computing system will be a good future research direction. In terms of model optimization, making the timing model lightweight without reducing the accuracy will be another good option.

Objective Q960E steel is a low alloy high-strength steel. It is widely used in construction machinery, pressure vessels, and subway vehicles due to its good weldability. The Q960E steel plate used in construction machinery often comes into direct contact with sand and gravel, causing wear and tear which can lead to failures of mechanical parts and increase usage costs. The key to laser cladding of Q960E steel is to produce no obvious cracks. At present, the most widely used laser cladding wear-resistant powder is Ni60-WC, due to the high hardness of the Ni60 cladding layer, it is difficult to avoid cracking. Therefore, in this study, we designed a Ni-based WC composite powder with both high toughness and high wear resistance. The powder is deposited on the Q960E substrate by laser cladding; through preheating and heat preservation treatment, a wear-resistant coating without obvious cracks can be prepared, and the wear resistance is improved.

Methods The test substrate material is Q960E steel, which was wire-cut into a test block of 200mm×50mm×20mm. The low crack sensitivity nickel-based tungsten carbide alloy powder was designed, and the WC reinforced phase and NiCuBSi bonded phase was prepared by aerosolization. The two were mixed at a mass ratio of 3:7 (Table 1). The nickel-based tungsten carbide powder was dried in an electric furnace at 100 °C for 1 h. Before the cladding, the Q960E substrate was put into the electric furnace at 150 °C for 1 h. The resistive cast aluminum heating plate was placed on the cladding platform, and the holding temperature was set at 150 °C. After the preheating in the furnace was complete, the sample was taken out and placed on the heating plate to keep the preheating temperature of the matrix under control at 150 °C. After the completion of cladding, the sample was put into the furnace at 150 ℃ for 4 h to reduce the residual stress. The microstructure, element distribution, phase, wear morphology, and cross-sectional hardness of the cladding layer were analyzed by scanning electron microscope(SEM), EDS, X-ray diffraction(XRD), optical microscope, and Vickers hardness tester. The wear resistance of the cladding layer and that of the 960E steel were tested and compared.

Results and Discussions A good metallurgical bonding was formed between the cladding layer and the substrate. No obvious pores or cracks appeared in the coating, and it was well-formed with spherical WC particles diffusely distributed in the coating (Fig. 1). The tested shear strengths of the cladding and substrate are 411.25MPa, 366.46MPa, 382.56MPa, and 427.18MPa. The location of the fracture is at the interface between the cladding and the base material, indicating that the presence of WC spherical particles will reduce the bonding strength between the cladding and the substrate, resulting in fracture at the interface. The spherical phase is WC particles with a slightly soluble structure at the edge. The surrounding irregular shape flocs for the laser cladding process of spherical WC decomposition of W₂C particles, increasing the bonding strength of the reinforced phase WC and the bonding phase Ni, Cu (Fig. 2). The main phases of the coating are composed of Ni, Cu, WC, W₂C, and Ni₃B (Fig. 3). In the process of cladding, under the action of the laser, part of the spherical WC particles decomposed and W₂C floc-like small particles were generated. Part of Ni reacted with B in-situ to generate Ni₃B, which enhanced the strength of the coating. In the cladding layer, the hardness of the coating varied greatly due to the existence of WC spherical particles. When the hardness head was all pressed into WC particles, the hardness reaches more than 1500HV; where no WC particles existed in the coating, the average hardness is 404HV. Because the Q960E had been tempered, the effect of laser heat input on the hardness of the heat-affected area and the base material was not obvious; the average hardness of heat-affected area and base material hardness area is about 374HV (Fig. 4). The wear morphology of the Q960E steel showed obvious plowing grooves and cutting scratches; the wear morphology of the cladding layer showed that the large particles of WC balls effectively prevented the abrasive particles from pressing in and plowing, resulting in an overall shallow wear groove depth (Fig. 5). Comparison of the wear mass and volume showed that the mass losses of the Q960E steel and the Ni-WC melt-coated samples are 7.568 g and 1.243 g, respectively. The wear volume was obtained by dividing the mass loss before and after wear by the density, to obtain 0.967cm ³ and 0.138cm ³, respectively.

Conclusions A Ni-based tungsten carbide alloy powder with low crack susceptibility was designed, and WC and NiCuBSi spherical powders prepared by aerosolization were mixed according to the mass ratio of 3:7. The Ni-based WC wear-resistant coating without obvious cracks was prepared on the surface of Q960E steel by a preheating and holding treatment. The main phases in the cladding layer are WC, W₂C, Ni, Cu, and Ni₃B. WC and W₂C are the main reinforcing phases, and the hardness of spherical WC is more than 1500HV, which improves the wear resistance of the cladding layer. Ni and Cu are the main bonding phases, which enhance the toughness of the coating. The wear resistance of the Ni-WC coating after cladding is more than 6 times that of the Q960E substrate, mainly due to the large particles of WC spheres effectively hindering the abrasive particles from pressing in and plowing. The overall depth of the wear groove was not deep; therefore, the cladding coating can effectively hinder the wear of hard gravel on the substrate.

Objective A paint layer can be applied to metals to enhance their surface characteristics. However, in many cases, paint often needs to be removed from the metal surface because of its potential damage to the environment. Paint removal using laser provides several advantages over the conventional techniques such as mechanical or chemical cleaning. Specifically, an accurate removal area, minimal detrimental effects to the substrate, reduction in contaminated waste, and fast cleaning rate are the key favorable factors in paint removal using laser. Several studies have been published in the literature that dealt with the effect of different process parameters for paint removal including the change of the temperature. Other processes that affect the relationship between the laser beam and paint have not been determined. In the present study, we report a novel type of research methods to understand the detailed micro process of paint removal, such as the plasma effect near the paint surface and the microscopic destruction process in the paint. We expect that our basic strategy and findings can help in understanding the characteristics and mechanisms of paint removal.

Methods In this work, 2024 aluminum alloy and polyacrylate resin-based paint were employed. A laser paint-cleaning test was carried out using pulsed laser with a wavelength of 1064nm and a pulse width of 1μs. In the experiment, the focal spot diameter of the Gaussian beam was approximately 78μm. The whole apparatus was completely automatic, that is, a computer controlled the laser power, repetition rate, and scanning speed. The cleaning residues were deposited on a silicon wafer, which was located 17 mm from the surface of the sample, as shown in Fig.1. The effects of scanning speed, pulse frequency, and laser power on the laser-cleaning quality were investigated. According to the morphology and element-valence changes in the cleaned surface and by combining the morphology of the cross section of the paint and particles generated during the cleaning process, the underlying process and mechanisms of the paint removal using pulsed laser were thoroughly investigated. Simultaneously, the temperature and stress-field distributions of the finite-element simulation using COMSOL Multiphysics software were also used for the auxiliary analysis.

Results and Discussions The paint in the experiments could be removed using pulsed laser. The laser-cleaning quality first increased and then decreased (Fig.3, Fig.4) and the surface roughness first decreased and then increased (Table 2, Table 3) with the increase in the scanning speed and pulse frequency. Furthermore, the laser-cleaning quality increased (Fig.5) and the surface roughness first decreased and then increased (Table.4) when the laser power increased. The morphologies and elements of the cleaned-surface study illustrate that the laser plasma and thermal combustion were affected by the absorption of laser energy by the paint during the laser-cleaning process (Fig.6). In addition, the X-ray photoelectron spectroscopy analysis indicates that C—H, C—C, O—H, C=O, C—O, and other covalent bonds in the polymer molecular chain of the paint were broken under the action of the pulsed laser (Fig.7). During the cleaning process, a layered structure was formed in the paint. Obvious cracks appeared that were parallel to the surface of the paint at the fracture section, which extended inside the paint. This result indicates the presence of a mechanical effect perpendicular to the surface of the paint. The cohesion of the lacquer was destroyed, which damaged the paint between the layers, and the paint layer was ejected (Fig.8). The study of the collected particles illustrates that the presence of mechanical mechanisms in the paint-damage process, such as vibration and impact, and the vaporized paint nucleated and grew in the high-energy limited area formed by the pulsed laser, which resulted in the formation of nanoparticles (Fig.9).

Conclusions In the present study, three different process parameters, namely, scanning speed, pulse frequency, and laser power, influence the laser paint-cleaning quality at different levels. The laser-cleaning quality first increases and then decreases with the increase in the scanning speed and pulse frequency and increases as the laser power increases. The laser-cleaning quality is good when the process parameters are as follows: laser power=16.5W, scanning speed=600mm/s, and pulse frequency=30kHz. Under different process parameters, the main mechanism of the laser paint removal is different. With regard to the analysis characterization, we conclude that the effect of the cohesive-failure and crack-propagation-fracture mechanisms is more efficient than the chemical bond-fracture combustion.

Objective Silicone rubber has been widely used in aerospace and power transportation because it has stable and reliable physical properties. However, the hydrophobicity of its surface should be improved to enhance its stability in practical applications. This property can be improved more quickly and effectively by laser etching than by surface coating, plasma processing, imprinting, and other methods. Various surface microstructures can also be obtained through laser etching. The main factor that causes the change in hydrophobicity is the rough microstructure on silicone rubber surfaces after laser irradiation. However, the specific influence of its surface morphology on hydrophobicity has not yet been confirmed. Fractal dimension is a measure to characterize the irregularity of complex shapes, which can indicate the effectiveness of the space occupied by complex shapes, and has been widely used in studies on the physical properties of rough surfaces. Therefore, in this study, fractal theory and fractal dimension are introduced to explore the rough structure and geometric characteristics of silicone rubber surfaces after laser etching, establish their association with surface hydrophobicity, and provide a method for explaining the change in the wettability of rough surfaces.

Methods A silicone rubber surface was etched with an SPI nanosecond fiber laser at a maximum power of 70 W and a wavelength of 1064 nm. Silicone rubber surfaces in different wetting states were obtained by modifying the laser fluence. The wettability of the surfaces was characterized by measuring their contact and rolling angles. Fourier transform attenuated total reflection infrared spectroscopy(ART-FTIR) and energy dispersive spectromete(EDS) were then conducted to detect the chemical elements and groups on the sample surfaces, and the influence of chemical factors on surface wettability was excluded. After the rough surface microstructure was determined as the main cause of the change in wettability, the contour curves of the sample surfaces collected with a white light interferometer (BRUKE, ContourGT-K0) were drawn to calculate the fractal dimension. Combined with the scanning electron microscope(SEM) micrograph of the sample surfaces, fractal theory was introduced to analyze the micro-nanocomposite structures produced on the laser-etched silicone rubber surface.

Results and Discussions Laser treatment could significantly improve the hydrophobicity of the silicone rubber surface. The surface of the untreated silicone rubber exhibited a weak hydrophobicity with contact and rolling angles of ~110° and >90°, respectively. As the laser energy input increased, the contact angle of the silicone rubber surface increased rapidly. When the laser fluence increased to 10 J/cm ², the contact angle increased to ~160°, whereas the rolling angle decreased to ~3°. ART-FTIR and EDS revealed that the input laser energy did not induce the changes in the chemical elements and groups on the silicone rubber surfaces. The surface wettability of the laser-treated silicone rubber was mainly determined by its three-dimensional microstructure. The silicone rubber surface was pyrolyzed locally when the laser fluence was low. Consequently, a coarse structure with a high self-similarity and a composite state of large and small particles formed, thereby improving the fractal dimension of the surface and slightly increasing the surface hydrophobicity. As the laser fluence increased, the large particles on the silicone rubber were pyrolyzed to the micro-nanoparticles, which reduced the fractal dimension of the silicone rubber surface. Droplets were only in contact with the convex surface of the small particles on the surface, creating a superhydrophobic surface. As the laser fluence further increased, a plate-like structure with trenches was produced because of thermal effects, and the roughness of the processed surface increased. When the balance between the inputted laser energy and the surface pyrolysis of silicone rubber was reached, the rough structure of the surface no longer changed significantly. As a result, a stable superhydrophobic surface with a high self-similarity was created.

Conclusions When silicon rubber is etched with a nanosecond laser, the chemical element composition and groups on the surface do not vary significantly, and wettability changes mainly because of the surface microstructure. Therefore, the fractal characteristics of the rough structure of the laser-treated silicone rubber surface are analyzed to establish the relationship between surface microstructure characteristics and hydrophobicity. As the laser fluence increases, the highest fractal dimension of 1.65 is obtained when the silicone rubber surface is irradiated with a laser fluence of 7.5 J/cm ². A micro-nanocomposite structure with a high self-similarity simultaneously appears on the surface of the silicone rubber, thereby improving its hydrophobic properties. When the laser fluence further increases to 10 J/cm ², the large particles on the silicone rubber surface become refined into small particles and disperse on the surface. Consequently, the surface roughness of the silicone rubber and the fractal dimension decrease to 4--5 μm and 1.40, respectively. As a result, the contact state between the silicone rubber surface and the water droplets transforms from a Wenzel model to a Cassie model. In other words, the processed surface changes from a hydrophobic state to a superhydrophobic state. When the laser energy fluence further increases, the fractal dimension increases again and stabilizes at about 1.55. When the silicone rubber is irradiated with larger laser energy, small micro-nano particles continue to be generated on the surface. These small micro-nano particles are continuously stacked on the basis of the original particles, thereby forming a composite structure with a high self-similarity again. However, when the balance between the rate of the thermal cracking of the large particles and the formation of the small micro-nano particles is obtained, the self-similarity of the surface micro-nano structure no longer changes, and the surface hydrophobicity remains stable. Therefore, the analysis of the fractal characteristics of the micro-nano structure on the silicone rubber surface after laser etching helps establish the relationship between surface structure and hydrophobicity. It also provides a basis for rapidly preparing superhydrophobic silicone rubber surfaces and regulating their surface microstructure.

Significance Laser-assisted near-field nanomanufacturing uses a near-field focused laser beam to break down the diffraction limit and heat materials to induce phase change or phase explosion to fabricate nanoscale materials and complex structures. It has an outstanding feature in both academic and industrial fields due to its highly coherent features, including continuously adjustable incident laser power, highly controllable processing position, and accessible to nanodomains. Among various near-field technologies, the tip-based near-field technique utilizes the tiny but sharp geometry of scanning probe microscope (SPM) tips to focus the incident optical field into an extremely small area in proximity to the tip apex. The highly enhanced electromagnetic field generates huge but localized heat in the surface to be manufactured and modifies its morphology through photon absorption and consequent phase change at the nanoscale.

Progress In the processing domain, in-situ information about the optical field, temperature rise, stress, and material structure evolution is critical for understanding and refining nanomanufacturing. It is helpful for in-depth understanding of the physical mechanism of multiphysics interaction and further optimization and process control. The enhanced electromagnetic field in and around SPM tips has been fully studied in recent decades; however, a knowledge gap remains relative to temperature rise and thermal stress evolution in the same region. A high-fidelity simulation on atomic force microscopy (AFM) tungsten tip under laser irradiation demonstrated that the electric field was primarily concentrated at the apex outside the tip rather than inside ( Fig. 1). The geometry and irradiating position on the tip were found to be important factors affecting the temperature rise of the tip. A systematic experiment measuring the temperature of the tip revealed that the temperature rise was a trade-off between the absorption area size and heat conduction to the base of the AFM tip ( Fig. 2). In addition to temperature rise in the tip, the temperature in the substrate is also important. Yue et al. performed the first experimental study on a silicon substrate under an AFM tip. Based on the Raman thermometry theory, the Raman shift of scatterings from the silicon substrate was collected by a self-developed temperature measurement system. A temperature rise of ~240 ℃ in a sub-10-nm area in the silicon substrate was achieved when the laser was focused on the tip end ( Fig. 3). A simulation based on the experimental conditions further uncovered the nonlinear photon absorption in this limited area ( Fig. 4).

Micro/nanoparticles represent an alternative geometry employed to introduce near-field focusing and simultaneous heating at the sub-wavelength scale, especially in the application of laser-assisted nanopatterning and nanolithography on a large-area substrate. Knowledge of the temperature and thermal stress beneath particles is critical; however, acquiring this knowledge remains a significant challenge for immediate sensing. Tang et al. has conducted pioneering experimental work on direct measurement of temperature and thermal stress in a silicon substrate beneath various transparent microstructures, including a single silica particle, a glass fiber, and a layer of particles (Fig. 5). The corresponding simulation of the same particle-substrate structure was performed to correlate the simulated optical field with the experimental result (Fig. 8).

Although various experimental methods have been developed for nanoscale probing, the theoretical simulation, e.g., molecular dynamic simulation, is a predominant method to predict atomic-level phenomena in an ultrafast period in laser-assisted near-field nonmanufacturing processes. In addition, the previous work primarily focused on enhancing the optical field under SPM tips, the light confinement effect, and the temperature evolution of the tip and substrate only under the condition of no phase change. However, the incident laser is extremely focused and enhanced in a nanodomain in the surface to be processed under the tip; thus, the material in this domain undergoes intense heating, phase change, phase explosion, stress generation and propagation, and rapid structural evolution in a very short time. Computer simulation research has focused more on small-scale and short-time heating processes, which would lose important information on material/structure evolution. Wang established a large-scale system with hundreds of millions of atoms and studied the long-term behavior and structural evolution of heating, melting, phase transition, solidification, and defect formation in laser-assisted tip-based near-field nanomanufacturing (Figs. 9--14). In addition, the shock wave during the nanomanufacturing process has been studied extensively (Figs. 16--18). Although simulation results were based on a few assumptions, the models used in the simulations were designed according to real dimensions, and this provided cutting-edge and detailed knowledge and understanding of physical images.

Conclusions and Prospects This review primarily focuses on experimental and theoretical investigations of the optical, temperature, and stress fields, and simulation study of the structural evolution in SPM tip-based laser-assisted near-field nanomanufacturing. However, unsolved problems remain in laser-assisted near-field nanomanufacturing. In the ultrafast and dynamic process of nanostructure manufacturing, the physical phenomena in such a small domain (e.g., nano to subnano domains) are difficult to observe and detect. Although newly developed laser-assisted probe methods are nanoscale accessible, they take much more time to satisfy signal collecting than the entire time range of ultrafast dynamic process of nanomanufacturing. In addition, SPM tips are fragile and can be easily damaged and contaminated, and their geometric shape change significantly affects the near-field electromagnetic field distribution. The unexpected change in tips shape will then widen the near-field light field distribution around the needle apex and make the manufactured structure deviate from expectation. The high temperature rise in laser-assisted near-field nanomanufacturing will introduce large thermal expansion in both the tip and the processed surface, and yield unstable measurements because the distance between the tip and substrate is only several nanometers. In terms of rapid response measurement, large-scale manufacturing, and improving and expanding industrial applications, this technology still has room for significant development and improvement.

Significance Because of their special chemical properties, nanoparticles have a wide range of application prospects in optoelectronics, catalysis, medicine, military, and other fields. Researchers have developed many methods for preparing nanoparticles, such as solid phase method, liquid phase method, and gas phase method. These methods all have some shortcomings. For example, the liquid phase method is easy to introduce impurities difficult to be removed, the gas phase method has high costs and harsh conditions, and the solid phase method yields particles with uneven distribution and easy agglomeration. Different from these traditional preparation methods, pulsed laser ablation in liquid (PLAL) can create an ultra-high temperature and ultra-high pressure environment in the liquid, which provides a possibility to prepare nanoparticles that are difficult to be prepared under conventional conditions. It does not need to build complex experimental setups or add various catalysts. The morphology and size of the nanoparticles can be controlled by changing the parameters such as the wavelength, pulse width and frequency of laser and the type of solvents and target materials.

Progress At present, there are four major types of nanoparticles prepared by PLAL: metal nanoparticles, metal oxide nanoparticles, alloy nanoparticles, and non-metal nanoparticles. Metal nanoparticles mainly include noble metal nanoparticles and active metal nanoparticles. The chemical properties of noble metals are stable and the corresponding nanoparticles are easily prepared. Therefore, researchers are more inclined to prepare nanoparticles with a small size and a uniform distribution by controlling different parameters. However, active metal nanoparticles easily react with oxygen atoms in the solution to form metal oxide nanoparticles. To address this problem, researchers try to inhibit the oxidation of active metal nanoparticles by adding surfactants (sodium dodecyl sulfate, hexadecyl trimethyl ammonium bromide) or polymers to the solution. Some progress has been achieved, but the generation of metal oxide nanoparticles is inevitable. How to prepare highly active metal nanoparticles in the solution is still a difficulty for researchers. When metal oxide nanoparticles are prepared, the methods can be generally divided into two types: (1) with metal oxides as the targets, the metal oxide nanoparticles are produced by pulsed laser in the solution; (2) with pure metal as the target, the nanoparticles react with the solution to obtain the corresponding metal oxide nanoparticles. In addition, there are many types of oxides for the same metal. During the laser ablation process, metal oxide nanoparticles with different crystal forms or valence states may be produced in the solution. How to prepare a single type of metal oxide nanoparticle in the solution remains a question. Currently, most alloy nanoparticles prepared by PLAL are composed of two noble metals, or one noble metal and one active metal. Although alloy nanoparticles containing two active metals have been prepared, the products still contain some metal oxides or hydroxide nanoparticles. It is also found that alloy nanoparticles with a certain molar ratio can be prepared by adjusting the ratio of two metal elements in the alloy target. The non-metal nanoparticles prepared by PLAL mainly focus on carbon and silicon that generally do not react with the solution, so non-metal nanoparticles with special morphology can be prepared by adjusting energy, wavelength, and other parameters. Taking carbon as an example, graphene sheets or diamond nanoparticles can be prepared by PLAL. In addition to the above two materials, attempts have been made to obtain some other non-metal nanoparticles by PLAL.

Conclusions and Prospects Compared with the traditional nanoparticle preparation methods, PLAL has simple operation and strong applicability. In some cases, the size and structure of nanoparticles can be controlled by adjusting the laser parameters and other factors. With the increasing demand for nanomaterials, PLAL will be more widely used. However, PLAL also has some room for improvement, such as the low yield of nanoparticles. Researchers have taken a variety of measures to increase the yield of nanoparticles, such as changing the target shape, combining PLAL with microfluidic technology or ultrasonic treatment technology. Although the yield of nanoparticles has been increased, it can not meet the requirements of industrial production. Moreover, the preparation of pure metal nanoparticles by PLAL is still a challenge. Although many kinds of additives or solvents have been added, the desired results have not yet been achieved, which still needs much work. Furthermore, it is also an important research direction to combine PLAL with other nanoparticle preparation methods to prepare composite nanoparticles with excellent properties. In addition, PLAL has been applied to the preparation of nanoparticles for decades. Due to the complex reaction process, numerous factors affecting the morphology of nanoparticles, and a lack of effective characterization methods, the research on the growth mechanism of nanoparticles progresses slowly. In recent years, many researchers have put forward their own theoretical analysis in combination with some simulation methods, but the detailed reaction mechanism has not been conclusive until today. Therefore, researchers should focus on the basic principles of PLAL to provide a theoretical basis for the preparation of pure metal nanoparticles.