ObjectiveThe laying environment of petroleum drill pipe is always poor, and the laid soil contains oxidizing bacteria, moisture, air, etc. During service, rust often occurs at the internal and external threads of the drill pipe joint, resulting in unstable connection of internal and external threads. In order to improve the utilization rate of steel, derusting methods are often used to repair the surface of metal parts in industrial field. The traditional rust removal methods are mainly mechanical, manual and chemical methods, among which the mechanical methods are sand blasting and high-pressure water flow. Compared with traditional cleaning methods, laser cleaning has the characteristics of green environmental protection, high efficiency and non-contact. It has gradually become a common cleaning method in industry. As an advanced cleaning technology, laser cleaning focuses high-energy laser on the corrosion layer to be cleaned. After the surface corrosion layer absorbs laser energy, the thermal coupling phenomenon and plasma shock wave effect appear, and the effect of surface rust removal is realized. Therefore, the pulse laser is used to clean the corrosion on the surface of NC50 petroleum pipe thread, and the significance of the influence of different factors on the cleaning effect is systematically analyzed to provide theoretical guidance for the laser cleaning of industrial petroleum pipe thread.MethodsThe laser cleaning experiment is carried out on the petroleum pipe thread by the orthogonal experiment method, and the orthogonal data table is analyzed with the surface roughness as the evaluation index, and the significance order of the factors influencing the laser cleaning effect is obtained. The surface roughnesses and oxygen contents under different process parameters are obtained by the single factor experiment. The surface damage of cleaning samples is measured with oxygen contents, and the process parameters are optimized by taking surface roughness as the optimization objective. Based on the response surface methodology, the mathematical model is established to describe the relationship between the optimization objective and laser process parameters. By combining the mathematical model with the optimized particle swarm optimization algorithm, the optimized process parameters are obtained.Results and DiscussionsAccording to the orthogonal experimental data, the influence of laser power on the experimental results is significant. On this basis, single factor experiments are carried out to analyze the variation of surface roughness and oxygen content of cleaned workpiece with laser power, scanning speed, and defocusing amount. Among them, when the scanning speed is 1500 mm/s, the defocusing amount is + 1 mm and the laser power is variable, the workpiece surface roughness value (Fig. 6) and oxygen content (Fig. 7) first decrease and then increase, and both reach the lowest value when the laser power is about 500 W. With the increase of the laser power (below 500 W), the corrosion layer vaporizes obviously, the corrosion layer on the workpiece surface is removed, the surface roughness value decreases, and the oxygen content decreases. Increasing the laser power continuously causes the melt to adhere around the spot pit and increase the surface roughness. In order to expand the particle search space and improve the population diversity in the iterative process, a particle swarm optimization algorithm is proposed by using the combination of power function and learning factor as an improved operator fused into the inertia weight. The convergence speed of the curve obtained by the improved algorithm is faster than that obtained by the traditional particle swarm optimization algorithm (Fig. 16). The optimized process parameter combination is obtained by combining the improved particle swarm optimization algorithm with the mathematical model of response surface method. The particle swarm optimization algorithm predicts that the surface roughness after cleaning is about 4.73 μm. According to the optimization algorithm, the combination of process parameters is as follows: the laser power of 488 W, the defocusing amount of + 3 mm and the scanning speed of 3000 mm/s. Using this parameter combination for laser cleaning and micro-morphology detection experiments, the surface roughness of the sample is about 4.64 μm. The accuracy of the prediction of the improved particle swarm optimization algorithm is improved obviously.ConclusionsIn this study, the significances ranking of the factors (the laser power, defocusing amount, and scanning speed) influencing the cleaning effect of petroleum pipe threads is obtained by the orthogonal experiment. The relationship mathematical model between the workpiece surface roughness and laser process parameters is established based on the response surface method, which is combined with the optimized particle swarm optimization algorithm to obtain the optimized process parameter combination. The factors influencing the cleaning effect of petroleum pipe threads are the laser power, defocusing amount, and scanning speed according to their significances ranking from the highest to the lowest. The combination of power function and learning factor is used as the improved operator of the particle swarm optimization algorithm to dynamically adjust the inertia weight. The improved algorithm is obviously better than the traditional particle swarm optimization algorithm in convergence speed. According to the optimization algorithm, the combination of process parameters is as follows: the laser power is 488 W, the defocusing amount is + 3 mm, and the scanning speed is 3000 mm/s. The laser cleaning experiment is carried out by using the optimized process parameters. The results show that the micro-morphology of the workpiece cleaned under the combination of process parameters is relatively smooth in the molten pool, and the shape of the pit is close to the shape of the Gaussian light source. The oxygen content is removed, and the optimization effect is obvious. The accuracy of the method proposed in this paper is proved, and it can provide theoretical guidance for the optimization of process parameters of laser cleaning.

ObjectiveFor transportation, aviation, aerospace, and defense military equipment components, the need for lightweighting is particularly urgent. Magnesium alloys have the benefits of high specific strength and stiffness, damping and vibration reduction, electromagnetic shielding, remarkable machining performance, and easy recycling. With the continuous research and innovation of new materials and technologies of magnesium alloys, their application potential will be infinite. Researchers suggested an approach of applying a tunable ring spot in laser processing that has attracted a lot of attention in the field of laser processing. Although domestic and foreign scholars have performed a lot of research investigations on magnesium alloy laser welding and have attained good findings. There is still a big gap between the existing research investigations and practical engineering applications, and it is crucial to develop novel laser welding technology.MethodsIn this study, AZ31B magnesium alloy butt joints with a thickness of 5 mm are welded by fiber laser with an adjustable ring spot. Based on ensuring the total power of 3 kW, pure center laser, center laser/ring laser, and pure ring laser are employed to weld magnesium alloy joints. The surface shapes, microstructures, and mechanical properties of the joints are discovered and examined.Results and DiscussionsIn terms of weld formation, first, compared with that when using pure center laser welding, the melting width in the upper part of the weld is considerably increased when using ring laser welding, and the combination of center laser and ring laser has a greater impact on the weld cross-sectional shape. Second, when the ring laser power is greater than the center laser power, the formations of the upper and lower surfaces of the welds become unstable (Figs. 3 and 4). In terms of metallographic structure, during pure center laser welding, the columnar crystal region near the fusion line of the weld is very narrow, and the grains in the center of the weld are finer, but due to the fast cooling rate, there are small pores and cracks around the weld [Figs. 5(a1)-(a4)]. When the center is welded with a ring laser, the convection heat transfer time is extended in the molten pool’s upper part, increasing the grain size in the center of the weld. Even in pure annular laser welding, a wide columnar grain region is formed near the weld fusion line [Figs. 5(c1)-(c3)]. The weld zone hardness is higher than that of the base metal, and as the ring laser power increases, the weld center hardness decreases. When the central laser power is 2000 W and the ring laser power is 1000 W (sample 2#), the tensile strength and elongation of the welded joint are the largest, reaching 215 MPa and 14.0%, respectively, which are 79.6% and 96.6% of those of the base metal [Fig. 7(a)]. The joint fracture is a brittle-ductile mixed fracture.ConclusionsThe existence of a ring laser has a considerable influence on the weld formation. Based on the center laser, the ring laser application can substantially increase the fusion width in the upper part of the weld. The combination of center laser and ring laser has a great influence on the cross-sectional shape of the weld, the best weld formation is generated when the center laser power is 2000 W and the ring laser power is 1000 W. The existence of ring laser has a certain effect on the microstructure of the weld. Only in the central laser action area, there is no clear heat-affected zone or columnar grain zone near the weld fusion line, and the equiaxed grains are relatively fine. In the ring laser action area, there are heat-affected and columnar crystal zones near the weld fusion line, and the equiaxed crystal grains are coarse. Further, with the ring laser power increasing, the hardness value of the central area of the weld decreases. The ring laser beam can enhance the elongation of laser welded joints of the magnesium alloys, and the welded joints have better tensile strength produced under the optimized process parameters.

ObjectiveWith the electronic industry’s rapid development, the preparation of the functional surface of electronic devices has become a critical link in the high-precision electronic components’ application. The demand for 316L stainless steel with functional gold plating is increasing. Given the local functional requirements of electronic devices and the high-cost limitations of precious metal coatings, enterprises typically use the local electroplating approach for production. However, with the continuous enhancement of the performance of electronic devices, the product structure tends to be complex and puts forward higher requirements for the dimensional accuracy of the deposition area, as well as the design and manufacture of profiling fixtures are becoming more challenging, and the production efficiency and yield by this approach are declining, which poses a challenge to meet the production demand of high-end products. Thus, the development of new high-efficiency micro-electrochemical local gold deposition technology offers new ideas for developing domestic electronic devices, chip packaging, and other high-precision technologies, and is of certain importance for China’s industrial upgrading and meeting international demand for high-end products.MethodsTo solve the problems of difficult direct electrodeposition, complex pretreatment, and poor flexibility of existing local electrodeposition technology on 316L stainless steel substrate, the laser is presented into the electrodeposition system to eliminate the oxide film on the surface of stainless steel and induce maskless localized electrodeposition on the substrate surface. The machining mechanism and coating properties are investigated theoretically and experimentally using scanning electron microscopy (SEM), X-ray dispersion spectroscopy, cyclic voltammetry (CV), and current-time curve. The influence of laser single pulse energy, scanning speed, and pulse frequency on the surface morphology of the coating is explored, and the heat accumulation influence on the deposition accuracy is examined.Results and DiscussionsLaser scanning 316L stainless steel surface can efficiently eliminate the surface oxide film, activate the substrate, and realize the simultaneous elimination of oxide film and electrodeposition, without disrupting the plated area (Fig. 3). The limited increase of laser single pulse energy aids to enhance the coating surface morphology and reduces the coating roughness. However, when the single pulse energy is increased further, the bare substrate, pores, and other defects on the coating surface increase, and the surface quality decreases (Fig. 6). By gradually increasing the laser scanning speed, the flatness of the coating is effectively enhanced, and the best state is realized when the scanning speed is 10 mm/s. When the scanning speed is improved further, the coverage and density of the coating are considerably increased, and the particles are coarsened (Fig. 10). The increase of laser pulse frequency improves the laser thermal effect’s accumulation, effectively improves the electron transfer rate, speeds up the nucleation rate in the electrodeposition process, and refines the grains (Fig. 11). The prolonged accumulation of the laser thermal effect will lead to stray deposition in the laser’s unscanned area, reduce the dimensional accuracy of the coating, as well as cause low adhesion and density of the stray deposition layer, and uneven element distribution (Fig. 12). The optimal laser parameters are generated through the optimization test. The gold coating prepared under the optimal laser parameters has good service properties like adhesion and corrosion resistance, and the typical local gold coating pattern has high precision and beauty (Fig. 18).ConclusionsWe propose a laser induced localized electrodeposition process on a stainless steel surface. This process integrates the elimination of oxide film on the stainless steel surface and the electrodeposition phase to realize high-efficiency localized electrodeposition on the 316L stainless steel surface. First, we examine the test mechanism and then obtain the influence of different laser parameters on the gold coating through a series of single-factor tests. Finally, we get the optimal parameters of laser induced electrodeposition of gold coating: the laser scanning speed, laser pulse frequency, laser single pulse energy, number of laser scannings are 10 mm/s, 3000 kHz, 5 μJ, and 2, respectively. We assess the service performance of the coating prepared under the optimized parameters by bending test, thermal shock test, and corrosion test. Through observation, it is discovered that the service performance of the coating is good and can meet the application requirements. This process is predicted to be used in the high-precision electronic component industry and offers new ideas for the surface treatment and packaging of electronic components.

ObjectiveThe 304L stainless steel membrane sheets in the MARK III LNG containment system are extensively applied, which should be connected by welding in the form of fillet joint. In practical production, plasma arc welding (PAW) is more employed in the joint production. However, the production efficiency is low. So it is considered to apply laser welding in the production to increase the welding speed and enhance the production efficiency. Thus, several investigations are necessary for the application.In this research, the flow of molten pool and weld formation in 304L stainless steel fillet welding by the circular scanning laser are investigated. Thus, the reason why undercut occurs and how various welding parameters affect its appearance are highlighted, which contributes to preventing undercut from occurring.MethodsIn this study, 304L stainless steel sheets whose depth is 1.2 mm are used. A single factor experiment is carried out to study the influence on the formation of the weld by different welding parameters. Among all of the welding parameters, scanning amplitude and scanning frequency are considered to have a substantial effect during the welding process, so the high-speed photography is employed to study the impact of these parameters on the molten pool’s fluid flow, which is employed to explain the formation of weld with the molten pool solidification and the energy distribution in circular scanning.Results and DiscussionsThe molten pool’s dynamic process from the establishment to stabilization during circular scanning laser welding is observed using high-speed photography. The experimental findings reveal an undercut on the bottom plate welded by circular scanning mode, particularly with larger scanning amplitude, and larger undercut. The scanning laser moves along the preset trajectory, and the high-speed photographic findings reveal a molten pool forming along the moved path of the laser spot during the first scanning period, while unmelted solid remains inside the circular area. In a single scanning cycle, there is a melted loop and an unmelted round area (Fig. 5). The moving of the laser spot superimposes the moving along the welding direction except for its scanning movement, and then the unmelted area gradually decreases because of repeated stacking of the molten pool and eventually reaches a steady-state (Fig. 6). In aggregate, the laser energy is concentrated on both sides of the weld, while the energy in the central region is low (Fig. 10). Under small scanning amplitudes, the unmelted area disappears while it is always there under large scanning amplitudes. Under the scanning laser’s agitation, molten metal of the upper plate spatters at the molten pool’s front due to the impetus from the laser (Fig. 7), which also leads to the lower plate’s molten metal flowing to the upper at the molten pool’s trailing end (Fig. 8). Meanwhile, as the scanning amplitude increases, the absorbed laser energy per unit length decreases, leading to the lower molten pool temperature. Then the molten pool’s front solidifies fast, and thus the unmelted area can not be further reduced. Therefore, the lower plate’s molten metal cannot not be supplemented, which leads to the undercut’s occurrence.ConclusionsThe following conclusions can be drawn from the above experiments.First, the penetration depth and the degree of undercut of the circular scanning laser overlap joint are negatively correlated with scanning frequency. When other welding parameters are fixed, the higher the scanning frequency is, the smaller the penetration depth is. While welding, the proposed scanning frequency is below 200 Hz, and the scanning amplitude is below 2 mm.Second, there is base metal loss including spatters or evaporation during welding, which requires melted metal from the upper sheet to supplement. When the scanning amplitude is large or the frequency is high, the molten pool’s solidification speed is fast and the solidification time is short, and then the downward flow channel of molten metal at the molten pool’s front becomes narrower, which leads the downward flow of molten metal to be reduced. Thus, the molten metal at the lower side is not enough and the undercut occurs.Third, the unmelted area in the molten pool is caused by the energy distribution in the circular scanning. The laser energy is concentrated on both sides of the weld, while the energy in the central region is low. When the scanning amplitude is large (more than 1.5 mm) or the frequency is high (200 Hz), the molten pool’s temperature is low, so the molten pool solidifies fast, creating a large unmelted zone, and then the undercut intensifies.

ObjectiveDue to the high friction and wear caused by the development of high-speed and heavy-duty railway transportation, parts and components are prone to failure in the forms of wear, peeling, fatigue cracks, and so on. Higher requirements for the uniformity of the work piece's hardened layer after laser quenching are put forward to improve the wear resistance and service life of the surface of parts and components. The surface hardening effect of laser quenching is closely related to the light intensity distribution of the beam. When the commonly used flat-top rectangular light spot acts on the material, the topography of the hardened layer presents a "crescent" shape, deep in the middle and shallow at both ends, as the consequence of the obvious lateral heat loss at the edge of the light spot. To increase the depth of the hardened layer at the edge, a specific light field distribution is required to increase the energy injection on both sides of the spot, so that the temperature field under the action of the laser is as uniform as possible. As a result, developing a broadband laser optical system with adjustable light intensity distribution to realize the real-time output of a broadband beam with a specific light intensity distribution is an effective way to improve quenching quality. We present a laser quenching optical system based on galvanometer variable-speed scanning in this paper. The light field and temperature field distributions in the quenching process are regulated by changing the variable-speed scanning mode of the galvanometer, so that more heat is injected into the quenching regions at both ends to overcome the energy loss caused by lateral transfer, improving the uniformity of the phase transformation hardened layer. This study provides a reference for the high-quality surface strengthening of high-end equipment parts under heavy load conditions.MethodsThe ANSYS software is used to establish the thermophysical model of the laser quenching process, and the initial conditions and boundary conditions are set according to the working conditions. A static rectangular light field with an equivalent thermal effect is established in the simulation model as a result of the complex calculation of the moving heat source model scanned by the galvanometer. The heating curve at the characteristic point and the maximum surface temperature distribution confirm the equivalence of temperature field evolutions in the two laser quenching methods. The 45 steel is selected as the base material, and the model considers the thermophysical parameters of the material at different temperatures. Because the temperature gradient in the laser quenching process is large enough to meet the cooling rate requirements of self-cooling quenching, the hardened layer distribution can be obtained by making isotherms based on the material’s phase transition temperature. The effects of the variable speed coefficient and variable speed range of galvanometer scanning on the uniformity of the phase transformation hardened layer are investigated, and a quenching experiment is performed using the equal power, equal quenching area, and variable scanning mode scheme.Results and DiscussionsWhen the scanning frequency of the galvanometer is above 333 Hz, the surface temperature field distribution and cyclic heating process after repeated scanning by a small spot are essentially the same as those after static heating by an equivalent rectangular large spot (Figs. 6 and 7). Two types of laser quenching modes have equivalent temperature field characteristics. The light field distribution and hardened layer distribution after galvanometer scanning with different variable speed coefficients are simulated (Figs. 8 and 9), and the light field distribution and hardened layer distribution after galvanometer scanning with different variable speed ranges are simulated (Figs. 10 and 11). The variable-speed coefficient or variable-speed area in the galvanometer variable speed scanning is increased, allowing more heat flow into the two-stage quenching area, compensating for the loss of heat transfer in the lateral direction, and improving the hardened layer uniformity. A laser quenching experiment with the same quenching parameters as the simulation ones is carried out using the developed system device. Taking the position where the depth of the hardened layer is reduced to 90% of the maximum value as the boundary of the homogenization area, the quenching area width is 10 mm, and the homogenization area width after galvanometer scanning at constant speed is 3.6 mm. When the variable-speed area at both ends is 2.5 mm wide and the variable-speed coefficient is 0.8, the homogenization area width in the hardened layer is 5.1 mm, which is approximately 42% higher than the homogenization area width (Fig. 13).ConclusionsIn the present study, a laser quenching optical system based on variable speed scanning of a galvanometer is proposed. The system consists of a QBH (quartz block head), a collimating and focusing integrated mirror, a single-axis galvanometer, and a galvanometer variable-speed scanning control system. The "saddle" shape light field with low energy in the middle and high energy at both ends is realized by setting the widths of the galvanometer variable-speed scanning areas at both ends and the width of the galvanometer constant speed scanning area in the middle, as well as the variable-speed coefficient. The laser quenching process based on galvanometer variable-speed scanning is studied, and the equivalent thermal light field model based on repeated variable-speed scanning is established. The simulation is used to examine the effects of the variable-speed coefficient and galvanometer variable-speed scanning area on the morphology of the hardened layer, and the laser quenching test based on repeated variable speed scanning is performed through the system. The results show that increasing the variable-speed coefficient and galvanometer variable-speed scanning area can improve hardened layer uniformity, which can be used to guide high-quality quenching of equipment parts under heavy load conditions.

ObjectiveWind energy, as a renewable and clean new energy source, has great potential to meet the world's energy demand. Wind turbines typically are installed in harsh environments, such as the sea and the Gobi Desert. Therefore, higher requirements are proposed for the wear resistance and corrosion resistance of wind turbine equipment, especially the surfaces of wind turbine bearings and their raceways. According to relevant regulations, the hardness of the bearing raceway surface should be up to 55-62 HR, and the depth of the hardened layer should exceed 3 mm. 42CrMo, as a common material for wind turbine bearings, has high toughness and excellent fatigue resistance. However, the hardness of 42CrMo is relatively low (350-450 HV), so the surface of the 42CrMo raceway should be strengthened to meet the application requirements of wind turbine equipment. Induction quenching is a common surface treatment technology for wind turbine bearings. However, the limited quenching depth, the existence of quenching soft bands, and quenching cracks limit the development of large-scale wind turbine bearings. In this study, the microstructure, solidification process, and residual stress of boron-doped 4Cr13 stainless steel martensite coating are studied, and the differences in properties such as hardness, frictional behaviors, and salt spray corrosion are analyzed and compared with those of induction-hardened 42CrMo. A high hardness crack-free martensitic stainless steel coating with a thickness of more than 3 mm is successfully prepared on the surface of a wind turbine bearing raceway simulator with a diameter of 1 m under preheated conditions. We hope that our research will help advance laser cladding technology for surface strengthening of wind turbine bearings.MethodsThe 42CrMo low alloy high strength steel is selected as the substrate, and the 4Cr13 martensitic stainless steel powder doped with 1%-1.5%(mass fraction) boron element is used as the cladding powder. Before laser cladding, the 42CrMo substrate is polished and cleaned with acetone, and the powder is dried at 120 ℃ for 3 h. Then, the laser cladding is performed using optimized test parameters. Afterward, the microstructure of the coating is observed by the X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the solidification process of the coating is analyzed by the differential scanning calorimetry and coefficient of thermal expansion measurements, and the residual stress of the coating is measured using the contour method. A microhardness tester is used to test the hardness of the coating, and a vertical universal friction and wear tester is used to test the friction and wear of the sample at room temperature. An electrochemical workstation is used to test the potentiodynamic polarization of the coating. A neutral salt spray test is conducted.Results and DiscussionsThe thickness of the coating exceeds 3 mm, and no defect, such as cracks, pores, and inclusions, is found in the coating. In addition, the interface between the coating and the substrate is smooth, and the dilution rate is less than 5% [Fig. 4(a)]. The microstructure of the coating consists of martensite, residual austenite, and reinforcement phase (Fig. 3). Contrary to the residual tensile stress distributed in the common coating, the residual stress in the boron-doped 4Cr13 stainless steel martensite coating is residual compressive stress (Fig. 6). This phenomenon is primarily related to the transformation stress caused by martensitic transformation during the solidification of the coating. The hardness of the coating is more than 800 HV (Fig. 8), which is 1.2 times that of induction-quenched 42CrMo (650 HV). The high hardness of the coating is caused by the high content of reinforcement phases in the coating and the solid solution strengthening of the martensite matrix by the Cr element. The wear results show that the wear loss of the coating is only 50% of that of induction-quenched 42CrMo under the same wear conditions (Fig. 9). Because the coating's high hardness reinforcement phases effectively protect the matrix from direct grinding by abrasive particles during the wear process, the coating's wear resistance is improved. According to the neutral salt spray test and electrochemical test results, the coating exhibits better corrosion resistance than induction-quenched 42CrMo.The corrosion rate of the coating is 68.8% lower than that of induction-quenched 42CrMo (Fig. 11). Furthermore, the self-corrosion potential of the coating is 0.128 V higher than that of induction-quenched 42CrMo.ConclusionsIn this study, the high-hardness crack-free martensitic stainless steel coating with a thickness of more than 3 mm is successfully prepared on the surface of a wind power turbine bearing raceway simulator by laser cladding technology. The microstructure of the coating consists of martensite, residual austenite, and reinforcement phases (M2B and M23C6). Due to the synergistic effects of thermal stress and phase transformation stress, the residual compressive stress in the cladding layer reduces the risk of cracking. The hardness of the coating is more than 800 HV, which is 2.4 times that of the 42CrMo matrix (335 HV) and 23% higher than that of induction-quenched 42CrMo (650 HV). Under the same wear conditions, the wear loss of the coating is 0.15 g, which is only 50% of that of induction-quenched 42CrMo (0.30 g). According to the neutral salt test results, the average corrosion rate of the coating is 0.352 mg·m-2·h-1, which is significantly lower than that of induction-quenched 42CrMo (1.131 mg·m-2·h-1). According to the polarization curve, the self-corrosion potential of the coating is -0.173 V, which is 0.128 V higher than that of induction-quenched 42CrMo (-0.301 V).

ObjectiveGH4169 alloy is extensively employed in nuclear reactors, aerospace gas turbine production, and pressure containment because of its good fatigue resistance and outstanding corrosion resistance. For GH4169 alloy, the strengthening impact of γ″ on the matrix is better than that of the γ′ phase. When the temperature exceeds the limit service temperature (650 ℃), the γ″ phase, a metastable phase, is transformed into the more stable δ phase with an orthorhombic structure, decreasing the mechanical properties of GH4169 alloy. Thus, it is an urgent problem for GH4169 alloy to enhance the service temperature and microstructure stability. Presently, the common solutions are to increase the content of secondary strengthening phase γ′ and reduce the content of primary strengthening phase γ″. As a crucial component of γ′, the Al element's content will directly influence its precipitation amount. In the traditional process of casting and forging, many researchers modify the γ′ precipitated phase's amount and morphology by changing the volume fraction of Al, to achieve more stable microstructure and stronger mechanical properties. The element segregation and melting loss of adding elements are prone to occur in as-cast alloys, making it impossible to meet the desired design. In the laser deposition manufacturing (LDM) process, scholars primarily focus on the influence of Nb element on the LDMed GH4169 alloy, but there are few studies on the effect of Al element content on the evolution of microstructure of LDMed GH4169 alloy. In this research, we hope to enhance the microstructure and stability of GH4169 alloy by adding Al element using laser deposition manufacturing technology.MethodsConventional GH4169 powder and a certain percentage of Al powder are combined using the mechanical blending approach. Laser deposition manufacturing is used to fabricate bulk specimens. The microstructures of as-deposited and heat-treated GH4169 alloys with various Al contents are examined, and the high-temperature microstructure stabilities of alloys with various Al contents are also examined deeply. First, the added Al element is distributed uniformly in the blended powder and LDM sample, and Al element loss is identified using the inductively coupled plasma (ICP)detection. Then, the microstructures of the as-deposited and heat-treated GH4169 alloys with various Al contents are observed, and the effect of Al content on the samples is examined. Second, the heat-treated samples are subjected to a long-term high-temperature aging treatment, and each sample's high-temperature microstructure stability is assessed. Finally, the effects of the Al content on the solidification process of GH4169 alloy are observed.Results and DiscussionsThis study reveals that the uniformly mixed GH4169 powder and Al powder can be attained by mechanical mixing (Fig. 2). The added Al element only indicates a small amount of loss (Table 3), and the Al element is evenly distributed in the sample without visible segregation (Fig. 3). The as-deposited GH4169 alloy's microstructure with various Al contents consists of non-uniform columnar dendrites and equiaxed grains, which changes as the Al content increases. The secondary dendrite arm gradually develops (Fig. 4), accompanied by the main columnar structure, and the 2θ angle corresponding to the (200) crystal plane also gradually increases and shifts to the right (Fig. 5). Here θ is the diffraction angle. After the solid-solution+ aging heat treatment, the lamellar structure of each sample does not disappear, the increasing Al content results in the δ phase's fragmentation (Fig. 8). After further long-term aging, the number of δ phases decreases first and then increases, and the GH4169+ 0.50Al alloy reveals better high-temperature microstructure stability (Fig. 10). The increase in Al content cannot change the alloy's solidification sequence but decreases the alloy's solidification interval (Fig. 11).ConclusionsBased on the above analysis, the morphology analysis findings reveal that the as-deposited GH4169 alloys with different Al contents show the morphology of epitaxially columnar dendrites. The addition of Al inhibits the element segregation in the Laves phases and the interdendritic region. The alloy secondary dendrite morphology is gradually developed. The findings of differential scanning calorimetry (DSC) demonstrate that the melting points of NbC and the γ matrix decrease as the Al contents increase, but the alloy solidification sequence does not change. After the solid-solution+ aging heat treatment, the needle-like δ phase is precipitated, but the Laves phase is dissolved in large quantities. Furthermore, the increase in Al content results in the δ phase's fragmentation. After the long-time high-temperature aging heat treatment, the numbers of δ phases decrease first and then increase with the increase of Al content. The GH4169+ 0.50Al alloy shows better thermal stability.

ObjectiveDroplet fragmentation is a common natural phenomenon. The study on droplet deformation and rupture caused by pulsed laser impact has not only theoretical importance in fluid mechanics but also practical application value in industrial fields. In the field of extreme ultraviolet nanolithography, it is required to understand the dynamic response of droplets to laser pulse impact to optimize the laser-produced plasma light sources. A laser-induced phase change is a method for moving liquids by optical radiation, allowing for large deformations and flow speeds. In this process, the speed distribution generated by the pulse within the droplet is a crucial factor influencing the droplet deformation and fragmentation. However, less attention has been paid to this speed distribution. Here, the effect of speed field formed by laser impacting droplets on droplet deformation and propulsion is mainly studied.MethodsBased on a physical model of the interaction between the laser and the droplet, numerical simulation is employed to investigate the influence of speed fields formed under various laser energies on the droplet deformation and propulsion. Employing the volume of fluid approach, combined with the laminar flow model, the primary characteristics of deformation and the precise details of flow are determined. The pressure implicit with the splitting of operators is employed to solve the problem. The pressure staggering option discretization scheme for pressure is employed. The pressure staggering option discretization gives more accurate findings since interpolation errors and pressure gradient assumptions on boundaries are avoided. For cases with strong surface tension and a high density ratio, this scheme works better. The momentum is discretized by employing the second-order upwind scheme and the volume fraction equation is discretized by the geometric reconstruction scheme. Meanwhile, a Rayleigh-Taylor instability analysis is performed to estimate the dependences of droplet deformation and breakup time on the Weber number. The kinetic process of droplet fragmentation is qualitatively examined.Results and DiscussionsIn the validation simulations, the obtained droplet propulsion speed are generally consistent with experimental results (Fig. 4). Then, a Gaussian-type initial speed distribution is set up inside the droplet. The time evolution of droplet deformation when We=4921.6 is demonstrated in (Fig. 5). Because of the Rayleigh-Taylor instability, the axial thickness of droplet gradually compresses with time, while the radial radius gradually stretches with time. The laser-induced drop deformation mainly depends on the Weber number. The time evolutions of droplet deformation parameters (Fig. 6) show the relationship between the rupture time and the Weber number. The thin layer radius and rim rupture time for droplets are in general agreement with theoretical models. The findings reveal that when the Weber number increases, the droplet deformation becomes more severe. A thinner sheet is formed in shorter time with a larger radial expansion radius. Furthermore, the greater the forward motion, the smaller the corresponding rupture time. This speed distribution can be employed to simulate the deformation dynamics of droplet impacted by laser in a moderately focused state. The early-time laser-matter interaction influences late-time fragmentation. The droplet advances without overall fragmentation at a Weber number below 110.7. The higher the Weber number, the greater the corresponding speed in the flow field, the more drastic the droplet deformation, and the smaller the debris formed by droplet rupture (Fig. 7). Additionally, after droplet fragmentation, due to the vortex volume, there are interesting phenomena including spin, fusion, and rupture of fine droplets.ConclusionsIn this research, numerical simulation is employed to investigate the influence of speed fields formed under various laser energies on droplet deformation and propulsion. The droplet’s response to laser shock is modulated by laser energy and spatial distribution (the Weber number and the speed distribution inside the droplet). Meanwhile, a Rayleigh-Taylor instability analysis is performed to estimate the dependences of droplet deformation and breakup time on the Weber number. The kinetic process of droplet fragmentation is qualitatively examined. Understanding these dynamics is required in the field of droplet target shaping to predict the maximum target surface size before disintegration at given laser parameters.

ObjectiveRecently, two-photon three-dimensional laser direct writing technology has realized fast development and gradually developed into a mature micro/nanofabrication technology. However, this technology is currently largely employed in laboratories. A limitation is the low direct writing speed. A simple and efficient approach to enhance the direct writing speed is to employ multibeam parallel writing instead of single beam writing. Some research has already employed a spatial light modulator or a diffractive optical element to produce multibeam parallel writing. However, till now, multibeam direct writing cannot be separately modulated for each channel, or the modulation speed is too slow, which has been unable to achieve a high-speed direct writing scheme with industrial application value. In addition, employing various motion scanning devices is also an effective means to improve the direct writing speed. In the previous work, piezoelectric stage is usually used for motion control. However, it can only attain a scanning speed of 0.1-10 mm/s. A faster, conventional solution is to employ a galvanometer, which can increase the speed by 1-2 orders of magnitude because of its low inertia. Recently, a study group employed the resonant galvanometer to achieve a higher scanning speed, up to 8 m/s, but the precision was low and the feature size was 1-4 μm. In this study, we design and verify a six-channel high-speed two-photon laser direct writing system, with a maximum direct writing speed of 7.770 m/s per channel, a feature size of 150 nm, and a comprehensive direct writing speed of 46.62 m/s in parallel writing.MethodsIn this research, we employ a 780 nm femtosecond laser as the light source and precompensate the group velocity dispersion using a grating pair module. The laser beam was modulated using a spacial light modulator to produce multiple beams. A weighted Gerchberg-Saxton algorithm is employed to create a six-beam hologram and produce a multibeam with high uniformity through continuous iteration. The multibeam is focused on a multichannel acoustic optical modulator (AOMC), to achieve high-speed independent modulation of the multibeam. Then the spot is imaged to a polygon laser scanner’s plane, which is employed to conduct the X-axis scan, and the 3D scanning is achieved using a linear stage with air bearings as Y-axis and a piezoelectric stage as Z-axis. Furthermore, a reflective image rotator is designed in this study (Fig. 3), which is positioned before the polygon laser scanner to adjust channel spacing continuously.Results and DiscussionsIn this study, IP-dip two-photon photoresist made by Nanoscribe in Germany is employed as the standard photoresist sample. The first test was channel spacing uniformity. The distance between each adjacent channel is 8.7 μm [Fig. 4(a)], which is extremely close to the computed value of 8.75 μm. Compared with independently designing optical paths for each beam in the multibeam writing, employing SLM and AOMC to achieve the same function cannot only simplify the design but also has the benefit of high uniformity in channel spacing. The feature size measurement is the second test [Fig. 4(b)]. At the highest direct writing speed (~8 m/s), the line width decreases from 260.5 nm to 74.31 nm as the laser power decreases from 35 mW to 18 mW; however, after 150 nm, the lines start to form a lattice. The feature size of this system is approximately 150 nm, although a smaller feature size can be achieved by switching to a laser with a higher repetition frequency or employing low-speed direct writing. The third test was scanning speed [Fig. 4(d)]. In the single-channel mode, our maximum direct writing speed is 7.770 m/s and more than 46 m/s in six channels. The fourth test is to calibrate the image rotator [Fig. 4(c)]. The rotating stage’s precision employed in this system is 0.02°, according to equation (1), the adjustment accuracy of channel spacing is up to 0.3 nm. Additionally, this research performed the dot [Fig. 5(a)] and a programming test [Fig. 5(b)]. Table 1 compares the most recent literature studies on two-photon laser direct writing. In terms of scanning speed, our system obtains a speed of about 8 m/s at the single-channel mode, and in terms of feature size, our system reaches 150 nm, which is better than most studies.ConclusionsIn this study, SLM is employed to create six beams, and AOMC is employed to separately control the switching and intensity of each beam. High-speed writing is achieved based on a polygon laser scanner and two-dimensional linear stage with air bearings. The direct writing speed is 7.770 m/s in single-channel mode, and over 46 m/s at parallel writing. The feature size is approximately 150 nm. The system’s potential in high-speed writing was preliminarily confirmed. Furthermore, this system’s AOMC devices and laser energy serve as a limitation on the number of beams it can produce. It is easy to increase the number of beams by replacing more channel AOMC and higher power laser sources, therefore enhancing the writing’s effectiveness. However, there is still room for enhancement in the system’s writing strategy, which will be further investigated in the future. This study is of significant benefit in enhancing the writing speed of two-photon laser direct writing technology, which is helpful for the technology to advance toward greater industrial use.

ObjectiveWith the development of the aerospace industry, increasing attention has been paid to large-scale integral structural parts that contribute to aircraft's lightweights. Aviation aluminum alloy parts face the urgent need for larger sizes and more complex structures. A single additive manufacturing method cannot meet the needs of aviation parts. The structure and size requirements can be realized by combining the high-precision advantages of selective laser melting (SLM) technology with the size advantages of wire arc additive manufacturing (WAAM) through connection technology to realize an additive manufacturing scheme of large aviation aluminum alloy parts. A new type of connection technology is required for large and complex structural components to meet the high-quality connection requirements of large-scale integral components. The laser additive jointing (LAJ) technology can adjust the process parameters during the layer-by-layer joining to realize reasonable control of the heat input, thereby reducing the workpiece deformation. LAJ is used in this study to join SLMed AlSi10Mg aluminum and WAAMed 2024 aluminum alloys. The microstructure and mechanical properties of the jointing areas are compared and analyzed. We hope that our basic strategies and research results can help advance efficient, high-quality intelligent manufacturing of large aluminum alloy components for aerospace.MethodsIn this study, SLMed AlSi10Mg aluminum alloy and WAAMed 2024 aluminum alloy are joined by adding the AlSi10Mg alloy powder and 2024 alloy powder, respectively. The joining process is realized using LAJ. During the LAJ, specimens using different filling powders are annealed at 240 ℃ for 2 h to prevent deformation due to stress concentration, and the microstructure is systematically investigated using an optical microscope (OM) and energy dispersive spectrometer (EDS). The microhardness of SLMed AlSi10Mg substrate-jointing area-WAAMed 2024 substrate is measured. The mechanism of hardness change in the jointing area is revealed. In addition, tensile tests at room temperature are performed to evaluate the mechanical properties of the specimens using two different powders.Results and DiscussionsThe results show that the microstructure of the jointing area for the AlSi10Mg joining specimen is mainly composed of columnar crystals, and its growth direction is roughly parallel to the deposition direction (Fig. 4), whereas the grain size of the jointing area of 2024 jointing specimen is substantially refined (Fig. 5). The microhardness of the substrate on both sides of the AlSi10Mg jointing specimen is higher than that of the jointing area (Fig. 7), and the average microhardness of the 2024 jointing area is higher than the average microhardness of two types of heterogeneous aluminum alloy substrates. The room temperature tensile properties of the specimens using different powders and the two heterogeneous aluminum alloy substrates are tested (Fig. 8). The results show that the fracture positions of the two specimens are in the jointing area (Fig. 9). The tensile strength in the jointing areas of the two specimens is lower than that of the substrate (Fig. 10). In addition, the fracture mechanisms of both jointing specimens are quasi-cleavage fracture (Fig. 11).ConclusionsAccording to the above analysis, the results of the morphology analysis show that the microstructure of the AlSi10Mg jointing area is columnar crystals with obvious growth direction characteristics. The microstructure of the grain in the jointing area is finer when the 2024 aluminum alloy powder is used as filling powder, and bright white strengthening phase is precipitated at the grain boundary. The microhardness and tensile strength of the jointing specimen using 2024 aluminum alloy powder are better than those using AlSi10Mg aluminum alloy powder under the same annealing heat treatment system. The generation of the second strengthening phase and fine grain strengthening can improve the mechanical properties. The fracture positions of the jointing specimens using two powders are all in the jointing area, indicating that the tensile strength of the jointing specimen does not reach the tensile strength of the substrate. The tearing edges forming around the pores in the jointing area are observed in the tensile fracture. All fracture mechanisms are quasi-cleavage fractures.

ObjectiveThe coal mine machinery and equipment are susceptible to wear failure and their service life is reduced because of the harsh working conditions, including high speed, heavy load, vibration, impact, and so on. Preparing wear-resistant coating on worn parts' surface is one of the most economical and efficient approaches to address the challenge of wear failure of coal mine machinery and equipment. The laser cladding technology has the benefits of fast heating speed, small heat-affected zone of the substrate, small thermal deformation of the workpiece, fine grain size, compact microstructure and low dilution rate of the coating, and metallurgical bonding between the coating and the substrate, which can efficiently enhance the workpiece's surface hardness and wear resistance. Thus, the laser cladding technology has a broad use prospect in the field of coal mine machinery production, maintenance, and remanufacturing. The Fe-based alloy powder has exceptional qualities of low cost and excellent anti-wear properties, and it is always employed in the repair and remanufacturing applications of coal mine machinery and equipment. Thus, the microstructure and tribological properties of Fe-based coating fabricated using laser cladding technology on the 27SiMn steel surface are studied. This offers a theoretical basis for Fe-based alloy powder's application in the coal mine machinery and equipment.MethodsFe-based alloy powder is produced on the surface of 27SiMn steel using the laser cladding equipment. The spot size, laser power, scanning speed, lap rate, and powder feeding rate are 15 mm×2 mm, 3000 W, 6 mm/s, 45%, and 30 g/min, respectively. The phase compositions and morphologies of the Fe-based alloy powder and coating are examined using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The changes in microhardness are measured using the hardness tester, and the substrate and coatings' tribological properties at room temperature are analyzed by the friction and wear tester .Results and DiscussionsFrom the coating surface [Fig. 2a)], the dendrite is presented in the microstructure. The growth changes of the microstructure in the molten pool zone during laser cladding are presented in Fig. 2(b), and the cellular structures are generated in the bonding area between the coating and the substrate. The Cr and Mo elements are enriched around the grain boundary and the Fe element is presented in the grain (Fig. 3). From Fig.4, the Fe and Cr elements are changed visibly in the bonding zone between the coating and the substrate. It implies that the Fe and Cr elements' mutual diffusion has happened. This element diffusion phenomenon demonstrates that the Fe-based alloy powder and substrate are melted during the process of laser cladding because of the laser beam's action. The coating's XRD analysis result is demonstrated in Fig.5, and the coating comprises of the body-centered cubic α-Fe phase and (Fe-Cr) solid solution phase. The coating's microhardness is demonstrated in Fig.6. The coating area's average microhardness is (652.62±49.00)HV, and the average microhardnesses of the heat-affected zone and the substrate are (515.29±82.00)HV and (292.68±19.00)HV, respectively. The coating's average microhardness is about 2.1 times that of the substrate. In Fig. 7(a), the average friction coefficients of the coating and the substrate are 0.3265 and 0.3344, respectively. The wear volumes of the coating and the substrate are 2.77×10-4 mm3 and 1.2×10-2 mm3, respectively (Table 4 and Fig. 8). The wear rates of the coating and the substrate are demonstrated in Fig. 7(b). Compared with the substrate's wear rate, the coating's wear rate decreases by 97%. Figure 9 demonstrates the wear morphologies of the coating and the substrate. The substrate's wear is more serious, and the coating's wear track is relatively smooth.ConclusionsIn this research, the microstructure and tribological properties of Fe-based coating fabricated using laser cladding technology on the 27SiMn steel surface are studied. The primary phases of laser cladding Fe-based coating contain the α-Fe phase and (Fe-Cr) solid solution phase. The coating surface microstructure is composed of dendrite structure, which is dense and uniform, without visible cracks or other defects. The cross-sectional microhardness shows a gradient change. The coating's average hardness is about 2.1 times that of the substrate. The substrate's wear rate is 70.8×10-7 mm3·N-1·m-1, and the coating's wear rate is 1.634×10-7 mm3·N-1·m-1. The wear mechanisms of the substrate are primarily adhesive wear and abrasive wear, while the coating's wear mechanisms are primarily abrasive wear, and oxidation wear exists in both the substrate and coating.

ObjectiveDynamic compaction (DC) technology generates a shock wave in the powder, allowing the final compact to achieve theoretical density in a very short time . The compacts formed by DC technogy have smaller porosity, more even density, and better mechanical properties than those formed by conventional powder metallurgy (PM) process. However, the study on dynamic compaction of micro-scale parts is relatively absent, and the current dynamic compaction technologies are unsuitable for the dynamic compaction on a micro-scale. The laser shock wave has the loading characteristics of high strain rate (106-107 s-1), controlled energy, and a small affected area. In this study, a novel approach for compacting powder on a micro-scale by laser shock wave is presented, which integrates the unique advantages of pulse laser with a high strain rate and is appropriate for micro-forming.MethodsIn the experiment, Nd∶YAG laser with the Gaussian distribution beam was employed to accomplish the dynamic compaction research on the self-designed experimental platform. The density, surface morphology, microstructure, and mechanical properties of the finished compact were investigated using an optical microscope, scanning electron microscope, and Vickers hardness tester. The effect of laser energy, powder morphology, and impact frequency on the copper compact’s density was examined.Results and DiscussionsCopper powder is compacted under different laser energies and the copper powder compact samples are ?2.5 mm×(0.2-0.15) mm in size.Figure 3 demonstrates that all blanks are complete and regular in shape with no evident crack defects and there is no flaking at the blank’s edge. With the laser energy increase, this phenomenon gradually reduces, and a compact with the best forming quality is generated at 1800 mJ laser energy. Figure 4 indicates that the pore in the compact also increases with the increase of laser energy, and it can be discovered that the pore size and distribution uniformity in the compact’s center and edge areas are accordant. Figure 5 reveals that with the increase of laser energy from 360 mJ to 1800 mJ, the compact’s relative density increases from 76.1% to 91.3%, and the trend of relative density increase slows down. The effect of three kinds of copper powder morphologies on the density of copper compacts is explained. Irregular copper compacts have good surface quality under 1800 mJ laser energy impact, whereas spherical powder compacts show an evident flaking phenomenon. According to the microstructure investigation of the cross-section of three various morphologies of copper compacts in Figure 7, there is an obvious plastic deformation of the copper particles and solid bridging between the particles in the spherical powder compacts. In addition to the pressure welding mechanism, there is an additional mechanical interlocking force between irregular copper particles. The maximum relative density of the irregular copper powder (15-20 μm) compact, spherical copper powder (0.5-2.0 μm)compact, and spherical copper powder (10-20 μm) compact is 91.35%, 94.35%, and 93.12%, respectively. Comparing the relative density and forming quality of copper compact, irregular copper powder is suitable for fabrication on a micro-scale part. The compact’s mechanical properties were examined. The finding shows that the average microhardness of the final copper compact increases to 67.0 HV, 82.8 HV, and 91.7 HV, respectively, with the increase of laser energy. This is because the powder material undergoes plastic deformation during shock wave propagation, which fills the inter-particle pore and causes strain hardening in the particles’ deformation area. The shock wave pressure increases with the increase of laser energy, increasing the compact’s strain hardening. The impact frequency also influences the final copper compacts’ relative density. Figure 10 reveals that the relative density of compacts increases with the impact frequency increases. The contribution of the first and the second impact to the relative density increase under the five laser energies are 3.03%, 4.77%, 3.23%, 1.75%, and 1.29% while the total contribution of the third and fourth impact is only 1.00%, 0.93%, 0.56%, 0.55%, and 0.75%, respectively. The best performance of the final compact can be generated under the double impact. Double impact of 360 mJ+ 1800 mJ laser energy was adopted to reduce the negative influence of stain hardening effect on the performance improvement of compact, and the relative density of the compact reaches 96.5%.ConclusionsThis study systematically examines the influence of process parameters, including laser energy, powder morphology, and impact frequency on relative density and mechanical properties of final compacts dynamically compacted using laser shock waves. With the increase of laser energy, the forming ability of copper compact gradually enhanced. The increase speed of relative density slows down due to the deformation resistance’s production. The compact’s microstructure investigation shows that the connection mechanism between irregular particles is solid pressure welding and mechanical internal locking, while the connection mechanism between spherical particles is only solid-state pressure welding. Thus, the irregular copper powder compact has relatively high connection strength and is not easy to generate defects, including crack and peeling, which is appropriate for the production of micro-scale parts. Multiple impacts (two or more impact times) can efficiently improve the densification of compacts, and the increase in densification is primarily contributed by the first two impacts. Double impact with first low laser energy and then high laser energy can reduce the influence of strain hardening on the improvement of compact properties.

ObjectiveGH3536 is a nickel-base superalloy with outstanding oxidation resistance. It has good metallurgical properties and forming ability. For the 3D printing process superalloy, it has become the preferred material of choice. Rolled solid solution GH3128 (R-GH3128) has the benefits of strong corrosion resistance, oxidation resistance, and high fatigue strength. The two materials are extensively employed in the manufacture of high-temperature parts in the aerospace field. 3D printing can rapidly and accurately prepare complex geometric structures, which are challenging to achieve by traditional forging, casting, and other approaches. However, because of the travel limitations of printing equipment, the size of parts manufactured using the 3D printing process is restricted, which is challenging to meet the demands of large-size precision components manufacturing in the aerospace field. Laser welding technology has the benefits of small heat input, and adjustable beam transformation and being implemented in the atmospheric environment. It has developed into a crucial superalloy connection technology. In this study, the butt welding process test is conducted, the impact of fiber laser welding parameters on the weld morphology is investigated, and the structure and properties of the 3D-GH3536/R-GH3128 joint are examined.MethodsThe welding equipment uses a fiber laser, with a wavelength of 1060-1070 nm and a transmission fiber core diameter of 200 μm. The collimator’s focal length is 200 mm, the focusing lens’ focal length is 300 mm, and the focal spot diameter is 0.3 mm. The test materials are 3D printed GH3536 and rolled solid solution GH3128 flat plates, and the specification of the butt sample is 60 mm×40 mm×4 mm. In the welding process, the laser beam is incident perpendicular to the plate surface, and the focus is on the plate’s upper surface. The argon lateral protection is used. The circular nozzle’s inner diameter is 10 mm, the gas flow is 10 L/min, the included angle between the protective gas nozzle’s axis and the plate surface is 50°, the phosgene spacing is 2 mm, and the protective gas’ output length is 6 mm. The welding test is conducted using drag welding. In the welding process, a special fixture is employed to keep the sample plate’s butt joint in good condition, and the welding direction is perpendicular to the 3D printing forming direction. Maintaining a 2 m/min welding speed, while changing the laser’s defocus and power for welding. After welding, the sample’s cross-section is cut to prepare the metallographic sample. The joint’s microstructure is observed using a metallographic microscope, and the tensile fracture sample’s cross-section morphology is observed using a scanning electron microscope. The joint’s microhardness is tested using a hardness tester. The hardness tester’s indenter load is 100 g and the loading time is 15 s. With the tensile testing machine, tensile samples are prepared and the welds’ tensile properties are evaluated.Results and DiscussionsUsing the 3D-GH3536/R-GH3128 butt welding test, it is discovered that when the speed is constant, the weld depth is positively correlated with the welding power; however, when the power is 3800 W and the defocus is -5-+ 5 mm, concave defects develop on the back of the weld (Fig. 5). Employing the defocus of 0 mm, welding speed of 2 m/min, and laser power of 3800 W, the well-generated weld is created, and the butt welding test of a 4 mm thick 3D-GH3536/R-GH3128 plate is conducted under these parameters. The butt joint presents a typical nail head weld shape. The joint structure is primarily made of columnar and equiaxed crystals. The joint structure’s columnar crystals are essentially symmetrically distributed along the weld center and converge at the weld center [Fig. 8(b)]. On the weld side near the fusion line of R-GH3128 [ Fig. 8(c)], there is no visible equiaxed fine-grained region, demonstrating a mixed region of columnar and equiaxed grains with shorter lengths. On the weld side near the fusion line of 3D-GH3536, the weld structure is followed by a fine-grained region and a mixed-grain region made of equiaxed and columnar crystals [Fig. 8(d)]. The average grain size at the weld center is 58.11 μm [Fig. 10(b)], the number proportion of grains with a weld size less than 50 μm is 62.2%, and the number proportion of grains with a weld size of 50-100 μm is 18.8%. The number proportion of grains with a grain boundary dislocation angle of less than 15° is 61.8% [Fig. 10(c)]. The tensile test findings demonstrate that the average tensile strength of the upper part of the welded joint is 722.3 MPa, and the average tensile strength of the lower part is 723.1 MPa (Fig. 13). The tensile strength of the upper and lower parts of the welded joint is essentially the same, which is 93% of that of the 3D-GH3536 base metal. The fracture mode is the ductile fracture.ConclusionsThe 4-mm thick 3D-GH3536/R-GH3128 dissimilar alloy butt joint is successfully obtained by laser welding. The welded joint exhibits nail head morphology. The welded structure is primarily columnar crystal and equiaxed crystal. The number proportion of grains with a weld size less than 50 μm is 62.2%. The tensile test findings demonstrate that the sample’s fracture occurs at the weld position, and the upper tensile strength and lower tensile strength are essentially the same, about 93% of that of the 3D-GH3536 base metal. The fracture mode is the ductile fracture.

ObjectiveMany structural components, such as twisted variable cross-section structures, are used extensively in various industries, including aerospace, biomedical devices, ocean machinery, or other fields, such as marine propellers and combustor chambers of jet engines. Most parts have geometric characteristics, such as large-angle twists, large-angle bends, and spatial gradient-change cross-section structures. Under some working conditions and application requirements, the abovementioned twisted variable cross-section structure parts are difficult to process using conventional subtractive manufacturing techniques, such as the casting and milling processes. Using powder-feeding metal additive manufacturing, such as laser melting deposition (LMD) technology, can effectively avoid this problem and achieve excellent precision and near-net shaping of complex structure parts without a physical model. There have recently been a few reports on the route planning for this type of geometric characteristics structure in the field of laser melting deposition technology, both at home and abroad. Thus, a continuous gradual slicing and discrete method is proposed to obtain the route information and complete the forming.MethodsThe typical horizontal slicing can no longer meet the forming needs of the abovementioned structure, given that the structure sections examined in this research exhibit spatial geometric characteristics of large twist angles, large bending angles, and spatial gradient-change cross-section structures. A continuous gradual slicing and discrete method was proposed to layer, slice, and separate the forming parts to realize laser metal deposition forming of this structure. The original three-dimensional model is first constructed and segmented using the aforementioned procedure. The six-axis KUKA robotic arms can then be equipped with position and posture data. Finally, multiple experiments were performed to obtain the best forming process parameters and excellent forming parts. After the forming experiment, the geometric parameters of the formed part are measured based on model analysis, and three points along the deposition angle are selected for cutting and sampling.Results and DiscussionsThe continuous gradual slicing method is proposed to complete the layering of twisted variable cross-section structure parts, and discrete deposition units with different geometric characteristics are obtained [Fig. 7(a)]. The homogeneous transformation matrix is obtained by rotating each deposition discrete unit as a point in the base coordinate and tool base coordinate systems. Thus, its position and posture information is determined (Fig. 9). According to the information above, the deposition units with different spatial orientation information (Fig. 10) are spliced horizontally to gain the actual deposition track. In the forming stage, a powder-feeding head and six-axis robot were used to achieve the twisted variable cross-section structure parts.ConclusionsA continuous gradual layering method is proposed that is based on the inside-laser powder-feeding technology and the principle of laser metal deposition; the twisted variable cross-section structure is sliced according to its geometric characteristics, and the sliced layer and deposition units are successfully obtained. The layering problem of the twisted variable cross-section structure is solved, and the laser metal deposition accumulation of the twisted variable cross-section structure is also realized. The results of the formed parts are as follows: the formed part has a high surface smoothness and forming accuracy; the bending angle and torsion angle of the formed parts are 46.18° and 44.79°, and the errors with the original design angle are 2.62% and -0.47%, respectively; the diameter of the initial circular section and the side length of the end square section of the formed part are 59.63 mm and 60.72 mm respectively, and the errors with the original design size are -0.62% and 1.20%. There are no obvious defects, such as pores and cracks, on the formed part's surface, and each formed part's structure is dense and uniform. Finally, the continuous gradual layering forming technique effectively improves the forming ability of LMD technology for the twisted variable cross-section structure parts. This provides support for its wider application in the additive manufacturing field.

ObjectiveQuench-partitioning(QP) steels are extensively employed to reduce automobile weight for its ultra-high-strength and ductility. As an example, QP1180 steels have the total elongation of 15% and the strength of more than 1200 MPa. They are often welded employing laser beam to achieve a low heat input, narrow heat-affected zone, and high production efficiency. Generally, the QP steel laser-welded joints are tensile fractured at the region of the base metal, but the testers fail at the softening zone. The extreme narrow softening zone contains tempered martensite, which is restricted and strengthened by the neighboring hard heat-affected zone and base metals. Thus, the deformation is evenly distributed along the gage length. However, the fracture position varies according to compositions, tensile strain rates, and various welding parameters. For other advanced high-strength steels, the softening zone are not always fracture positions. QP980, DP1000, and MS1500 sheets of steel have softened zone, but CP800 and TRIP980 steels have no softening. Most of failures occur at the weakest point. Recently, the oscillation scanning technique is coupled with the laser welding process for the advantages of high efficiency, low gap requirement, low crack and porosity, and fine grains. We hope to examine the effects of this scanning technique on the microstructure and mechanical properties of QP1180 steel welded joints.MethodsQP1180 steel sheets with the 1.2 mm thickness are welded at the laser power of 3 kW, travel speed of 3 m/min, and zero defocusing distance with various scanning strategies. There are three linear scanning rotation angles of 90°, 45°, and 0°. The weld surfaces are compared and the weld cross-sections after etching are observed and measured employing an optical microscope and scanning electron microscope. Using electron back scattered detection, the microstructure and grain information are characterized. The microhardness and tensile properties are also measured under the three linear scanning rotation angles. Finally, the fracture surfaces are characterized to explain the deformation and fracture mechanism.Results and DiscussionsThe weld formations of QP1180 high-strength steel welded joints under different linear scanning rotation angles demonstrate various weld penetrations, fusion zone widths, the distances between two intercritical heat impacted zone, and widths of coarse grain heat affected zone (CGHAZ) and fine grain heat affected zone (FGHAZ). Only the samples under the linear scanning rotation angles of 45° and 0° obtain full penetration at the same total heat input. The samples under the linear scanning rotation angle of 45° achieve the widest heat impacted zone and the similar fusion zone width with that of the sample under the linear scanning rotation angle of 90°. The sample microstructure under the linear scanning rotation angle of 45° reveals the phase compositions that are identical to those of the normal laser welds: the dendritic grain lath martensite in the fusion zone, coarse grain lath martensite in CGHAZ, fine grain lath martensite in FGHAZ, mixed phases of lath martensite and original base metal in intercritical heat affected zone (ICHAZ), and the tempered martensite in subcritical heat affected zone (SCHAZ). The grain orientation map and band contrast map reveal the difference between lath martensite and ferrite in morphology and gray scale. The hardness result reveals that there are slightly higher hardness in FGHAZ and lower hardness in SCHAZ than that of the base metal. The hardness distribution is unaffected by various rotation angles, but the average hardness in fusion zones is respectively (482±6)HV, (471±5)HV, and (472±7)HV under linear scanning rotation angles of 90°, 45°, and 0°. The full penetration welds under linear scanning rotation angles of 45° and 0° demonstrate ultimate tensile strength of 1271 MPa and 1245 MPa, respectively. The sample’s tensile failure position is the same as the previous investigation result: the fracture occurrs in the softening zone. It may be explained by the widening of softening zone. The variation of rotation angle during linear scanning laser welding can change the maximum and minimum absolute speeds of the moving heat source and the energy transfer direction, and it finally leads to the difference in energy distribution. The overlap ratio in remelting plays a role in enlarging the fusion zone and heat-affected zone. A large rotation angle can increase the transportation heat in the horizontal direction. The linear scanning with various rotation angles then results in various weld formation and mechanical properties.ConclusionsIn this research, three linear scanning strategies with various rotation angles are employed in laser welding of QP1180 steel. Through the rapid and space-limited scanning, the weld formation significantly changes and is adaptive to a large gap. A large rotation angle results in a small weld depth and a large fusion zone/heat affected zone. Here, the rotation angle of 45° is appropriate for large weld width and weld penetration. The microstructure and tensile properties of QP1180 high-strength steel welded joints using oscillation scanning are not significantly altered compared with those of normal QP1180 high-strength steel welded joints, while the failure position moves to the softening zone in QP1180 steel, which normally occurs with a high heat input. The deformation and fracture mechanisms of QP1180 steel welded joints are altered by the scanning strategy.

ObjectiveIn recent years, the laser-directed energy deposition (L-DED) of titanium alloys has attracted extensive attention. However, most relevant research is focused on traditional titanium alloys. These alloys are developed by traditional casting or forging technologies without taking into account the metallurgical properties of L-DED. The intrinsic high cooling rate and high thermal gradient of the L-DED process often lead to a tendency toward almost exclusively columnar or dendritic grains, making anisotropic mechanical properties, and being, therefore, undesirable. Attempts to optimize the L-DED processing parameters reveals that it is difficult to change the conditions to promote equiaxed growth of titanium grains, limiting L-DED potential advantage in the fabrication of high-performance titanium alloys.L-DED titanium alloys have good compatibility between melt and solid structure to effectively control the structure, as it directly dominates the microstructure formation and property control throughout the forming process. The Ti-Zr congruent alloy is a good candidate in this regard because it has a single melting temperature, which gives it high structural stability. But a considerable disadvantage of this alloy is its insufficient strength (747 MPa). Alloying is a promising potential in resolving the problem associated with the congruent alloy. The basic structure in this study is Ti-Zr congruent alloy, the alloying component is Nb with effective strengthening, and the cluster formula of Ti-Zr-Nb alloys is constructed using the cluster model. The as-fabricated alloys’ microstructure and properties are thoroughly investigated.MethodsThe alloys are designed using a cluster model and then fabricated by the laser additive manufacturing on a pure titanium plate. An X-ray diffractometer is used to identify constituent phases. The microstructure and composition are studied using the scanning electron microscopy in conjunction with an energy dispersive spectrometer microprobe system. A digital microhardness tester is used to assess the microhardness. A room-temperature compressive test is performed on a universal tester. The dry sliding friction and wear properties are evaluated by a wear tester. An electrochemical workstation is used to measure the electrochemical properties in the HCl solution with concentration of 1 mol/L. Finally, using a laser confocal microscope, the surface roughness is determined.Results and DiscussionsAll as-deposited alloys are made up of a single β-Ti solid solution with near-equiaxed crystal morphology. The difference is that the lattice constant of β-Ti increases and its grain refines as Nb content increases (Fig. 3), which results from increased Nb in β-Ti and an expanded growth-limiting factor, respectively. Owing to the combined effects of solid solution strengthening and grain refinement, the hardness (Fig. 6), strength, and plasticity (Table 4) of the as-deposited alloys monotonously increase with the increase in Nb content. Increasing Nb content improves the antifriction and wear resistance of as-deposited alloys (Fig. 9) because increased hardness improves the antiabrasion ability of as-deposited alloys. Because Nb2O5 improves passive film passivation by reducing anion vacancy concentrations, and because Nb ions can replace Ti cations to increase passive film stability, the corrosion resistance of the as-deposited alloys gradually improves with increasing Nb content (Fig. 11). As the Nb content increases, the surface roughness of the as-deposited alloys decreases due to the fluidity and spreadability of the melt (Fig. 13).ConclusionsThe microstructure of all as-deposited alloys consists of single β-Ti near-equiaxed crystals. As the Nb content increases, the phase lattice constant increases, and its grains refine, resulting in a monotonous increase in hardness, strength, wear, and corrosion resistance that are superior to those of as-deposited Ti-Zr congruent alloy. However, the formability changes in the opposite direction. To balance the relationships between formability and other properties, the Nb content(atomic fraction) should be limited to 3.75%-5.00%.

ObjectiveTC4 titanium alloy is prone to β-phase and ordering transformation near the phase transition temperature due to its dual-phase composition characteristics. When the laser cladding is used to manufacture TC4 titanium alloy moldings, the high-temperature gradient and rapid solidification during the cladding process occur. The accumulation of thermal stress, an easy occurrence, will cause cracks to develop inside the molded part. Additionally, the structure of TC4 titanium alloy is unstable under high-temperature environments, and the rapid cooling and heating during the laser cladding process will make the base metal grains at the bottom of the molten pool epitaxial growth leading to the arrangement of the original β columnar grains along the deposition direction to form a typical solidified texture. The resulting columnar grain structure and nonequilibrium phase will further reduce the mechanical strength of the material. Furthermore, in the preparation of multilayer and multichannel samples, the change in the interlayer temperature will cause a change in the cooling rate, which will affect the transformation of the metastable β phase, and the different transformation products will affect the properties of the material. Therefore, given the uneven mechanical properties of the TC4 titanium alloy laser cladding structure during the reheating thermal cycle, the thermal simulator is used to reproduce the effect of the reheating thermal cycle during the multipass laser cladding process. The laser cladding thermal cycle and the effects of its characteristics on the microstructure and properties of TC4 titanium alloy are also investigated.MethodsIn this paper, the thermal simulation testing machine is used to conduct thermal cycle tests on TC4 titanium alloys to explore the effects of thermal cycle peak temperature and cooling rate on its microstructure transformation and mechanical properties. The coaxial powder feeding laser additive manufacturing (LAM) technology is used to prepare the TC4 titanium alloy cladding material on the surface of the titanium alloy substrate. Subsequently, the thermal simulator is used to prepare the samples based on the TC4 titanium alloy cladding material for thermal cycle simulation experiments at varying peak temperatures and cooling rates. The metallographic structure analysis, scanning electron microscope observation, X-ray diffraction composition analysis, and microhardness and microshear tests are conducted to determine the effect of thermal cycle on the microstructure and properties of the formed parts.Results and DiscussionsAfter thermal cycle simulations at different temperatures, as shown in Fig. 6, the β phase inside the sample decomposes, forming a microscopic region with alternating rich and poor solute atoms, and the microstructure exhibits bright and dark partitions. As the peak temperature increases, the proportion of dark parts gradually decreases, indicating that the β-segregation phenomenon has improved, especially when the peak temperature reaches above the phase transition temperature of TC4 titanium alloy (995 ℃), the matrix transformation effect dominates. The orientation of α-clusters is diverse, the number of primary phases αp generated by β-phase transformation increases, the size becomes thicker, the aspect ratio decreases, and a small amount of flaky α is separated by the remaining β phases, forming the lamellar structure with alternating α/β distribution. As the cooling rate increases, the type of material phase transition changes. When the cooling rate is 21 ℃/s, the unstable β phase undergoes a diffusive phase transition and an orderly transition, as shown in Figs. 7(c), (g), and (k); in addition to the primary phase αp, a small number of small particles dispersed in the interlamellar space also appear. When the cooling rate is 30 ℃/s, the α′ phase is formed. The results in Fig. 9 show that as the peak temperature increases, the original α-phase and αp are more fully transformed into β-phase due to the effect of diffusion, so the content of α-Ti in the material decreases. The results in Fig. 10 show that the distribution of β phase is not uniform and the hardness change is not obvious. When the peak temperature of the thermal cycle is lower than 960 ℃, with the increase in cooling rate, the hardness of the postweld material is generally higher than that of the cladding layer, showing a trapezoidal change, rising sharply at first, then smoothly, and then slightly decreasing. There is no phase change in the sample, and the grain growth time is shortened with the acceleration of the cooling rate, thereby increasing the hardness. When the peak temperature of thermal cycle is higher than 960 ℃, the dissolution of the α phase and the transformation of the β phase occur during the heating and cooling of the material. With the increase in cooling rate, the hardness of the material first decreases and then stabilizes. According to the microshear test results (Fig. 11) and fracture morphology analysis (Figs. 12 and 13), the fracture mechanism of the material is micropore aggregation and fracture at the peak temperature below 960 ℃ has good toughness. The fracture mechanism changes from ductile fracture to brittle fracture with increased peak temperatures.ConclusionsThe results show that microstructure coarsening occurs inside the material when the peak temperature is lower than 960 ℃. The aspect ratio of primary phase αp grows with the increase of temperature. The allotropic transformation and ordering transformation of the metastable β phase at the grain boundary occur as the peak temperature reaches 1100 ℃, resulting in the formation of Ti3Al ordered phase, the reduction of the aspect ratio of the primary phase αp, the refinement of the structure, and the significant increase of the hardness and the crack sensitivity. When the cooling rate increases, the phase transformation type changes from metastable β-phase allotropic transformation to martensitic transformation, the primary phase αp morphology changes from lamellar to needle-like, and the material hardness gradually tends to be stable, but when the cooling rate is higher than 12 ℃/s, the hardness of the material increases slightly due to the formation of Ti3Al.

ObjectiveLaser welding, which uses a high-energy laser beam as a welding heat source, has been widely used in material processing because of the advantages of energy concentration, high flexibility, and high production efficiency. As an important physical phenomenon in deep penetration laser welding, plasma generation contains considerable information about laser welding processes. The study of plasma behavior is critical for the quality monitoring of laser welding processes. In recent years, plasma electrical signal detection technology has been used because of its advantages of simultaneously acquiring plasma temperature and oscillation characteristics. With the further study of plasma electrical signals, the characteristics of electrical signals under different welds are becoming crucial parameters for monitoring laser welding processes. Some scholars studied the relationship between the oscillation of plasma electrical signals and weld penetration depth. However, the difference in plasma electrical signal characteristics between partial and full penetration welds have received little attention. Therefore, it is essential to analyze the plasma electrical signals under different penetration conditions and improve plasma detection methods for monitoring the laser welding process.MethodsIn this study, titanium alloy TC4 is used to investigate the characteristics of the plasma electrical signals under partial and full penetration welds. The titanium alloy plate with a 2-mm thickness is welded using an Nd∶YAG laser. Partial and full penetration welds are realized by adjusting the laser power and changing the welding speed. During the welding process, a synchronous acquisition system for plasma photoelectric signals is used. The short-time autocorrelation analysis method is used to analyze the difference in the oscillation frequency of plasma electrical signals between partial and full penetration welds.Results and DiscussionsComparing the collected plasma photoelectric signals, the oscillation characteristics of electrical signals at different periods are different (Fig. 5). The electrical signals are segmented and every section is analyzed using short-time autocorrelation method to investigate the oscillation frequency variation of the plasma electrical signals in the entire welding process. The characteristic oscillation frequency of the plasma electrical signals in the initial stage is 241-669 Hz, which is evidently lower than that in the relatively stable stage, and the characteristic oscillation frequency decreases as the welding heat input increases (Fig. 6). The fluctuation interval length of the plasma characteristic oscillation frequency under the partial penetration weld in the relatively stable stage is only 200 Hz, whereas that under the full penetration weld can reach 500 Hz (Fig. 7), which is an important feature that can be used to distinguish the partial penetration weld from the full penetration weld.ConclusionsIn this study, a photoelectric observation system is constructed to obtain plasma photoelectric signals during laser welding of titanium alloy. The difference in the characteristic oscillation frequencies of plasma electrical signals is analyzed under different conditions of partial and full penetration welds. The main findings of this study are as follows.In the initial stage, the characteristic oscillation frequency of the plasma electrical signals under the partial penetration weld is much higher than that under the full penetration weld. Simultaneously, the characteristic oscillation frequency of the plasma generated under the partial penetration changes faster than that under the full penetration . The differences in oscillation frequency and variable speed are important features that distinguish the partial penetration weld from the full penetration weld.In the relatively stable stage, the characteristic oscillation frequency of the plasma electrical signals under the partial penetration weld fluctuats and the deviation is 7%-9% of the average. The characteristic oscillation frequency under the full penetration varies dramatically and the maximum deviation is over 40% of the average. The stability of the characteristic oscillation frequency is another important feature that distinguishes the partial penetration weld from the full penetration weld.

ObjectiveMicrofabrication of semiconductors is crucial because semiconductor materials are extensively employed in solar cells, microelectronic machinery, optical components, etc. Due to the characteristics of semiconductor materials, including a fast increase in electrical conductivity with temperature, and high hardness and brittleness, traditional mechanical processing has been unable to meet the demands of microfabrication. Simple electrochemical processing has low processing efficiency, severe stray corrosion, environmental pollution from electrolyte solutions, and other challenges. Because the laser has the benefits of high precision and strong domain fixation to produce thermal impact on materials and the integration of electrochemical processing has the benefit to eliminate microcracks and heat-affected zones, the laser and electrochemical machining can be combined to produce good surface processing quality.Thus, the single-crystal germanium is employed as the experimental material, and a neutral and non-polluting NaNO3 electrolyte is employed to perform backward laser-controlled electrolytic processing to confirm the processing mechanism’s feasibility, and on this basis, a auto-coupled laser-electrochemical co-machining approach is employed to perform experimental research on micro grooving of single-crystal germanium materials.MethodsIn this research, an experimental investigation of single-crystal germanium through auto-coupled laser-electrochemical co-machining is performed. First, a scanning electron microscope is employed to observe the processing morphological characteristic, an energy spectrometer is employed to detect the elements and their occupancy, a confocal microscope is employed to obtain 3D morphology, and a Raman spectrometer is employed to examine the residue composition on the dimple and record the current changes during processing. To examine the dimple’s depth, entrance diameter, removal volume, and sidewall taper produced by processing in terms of both heat and mass transfer. Based on this, the experimental research of microgrooves is conducted using laser-electrochemical co-machining to investigate the morphological characteristics of microgrooves under the combined laser and laser-electrochemical processing and to examine the trends in microgroove width and depth under the auto-coupled hybrid laser electrochemical machining.Results and DiscussionsAuto-coupled laser-electrochemical machining is employed to achieve non-ablation on the upper surface and electrolytic micro-dimples on the lower surface to confirm the processing mechanism. The benefit of this approach is that the irradiation position of the incident laser corresponds to the electrolytic dimple’s position, and no special cathode design is needed for the automatic coupling process. The obtained electrolytic dimples are free of microcracks and heat-affected zones, which are typical characteristics of electrolytic processing (Fig. 5). Isolated and non-dense oxide GeO2 attached to the machined surface can hinder the electrolytic processing and influence the surface quality of single-crystal germanium (Figs. 6 and 7). The current variation’s trend demonstrates that the large change in current between the laser beam’s withdrawal and the addition of the processing beam is attributed to the localized increase of the conductivity of single crystal germanium by laser irradiation. The processing results demonstrate that the maximum entrance diameter, depth, removal volume, and sidewall taper are obtained at the offset distance of 7-9 mm. This finding may be related to the impact of offset distance on both aspects of heat and mass transfer. For the small offset distance, the cooling impact is not conducive to material reduction, and is conducive to the discharge of processing products and suppression of concentration polarization.For the large offset distance, the cooling impact is weak, but the product discharge ability is also reduced, both of which contradict each other and jointly determine the processing process and findings (Fig. 8). Based on this, the characteristics of microgroove morphologies processed by various processing approaches are investigated, and it is concluded that auto-coupled laser-electrochemical co-processing can eliminate the defects including recast layer and scatters caused on the surface, and obtain better microgroove morphology (Figs. 11 and 12). Meanwhile, this AHLECM is employed to investigate the impacts of various processing voltages on the processing findings, and it is found that the microgroove’s width and depth gradually widen and deepen with the increase of used voltage (Table 1).ConclusionsIt is proposed that laser irradiation can cause a fixed-domain conductive channel within the material, therefore obtaining the processing of single-crystal germanium through auto-coupled laser-electrochemical co-machining. When a neutral sodium nitrate solution is employed as the electrolyte and a horizontal copper sheet is employed as the cathode, the laser irradiation on the upper surface can generate fixed-domain electrolysis on the backside of single-crystal germanium to produce micro-dimple. The offset distance’s effect on the dimple morphology is studied. The maximum processing current, dimple diameter and depth, sidewall taper, and removal volume all increase first and then decrease with the increase of offset distance. Their turning points occur at the offset distance of 7-9 mm, and the possible effects generated by various offset distances are examined in terms of heat and mass transfer. By linking laser etching and electrochemical dissolution at the same processing position, the auto-coupled laser-electrochemical co-processing can be achieved, and the preliminary experimental research of microgrooves is conducted. The microgrooves’ structural and morphological characteristics are examined, and the processing voltage’s influence rules on the microgroove dimensions are investigated. It is found that the microgroove’s depth and width increase as the processing voltage increases, which demonstrates that electrochemical dissolution is the crucial phenomenon in the process of auto-coupled laser-electrochemical co-machining.

ObjectiveWith the rapid development of modern industry, aluminum alloy has been widely used in industrial production due to its excellent physical and chemical properties. Meanwhile, the preparation of high-quality microholes in aluminum alloys is in high demand. Laser drilling technology, when compared with other traditional microhole machining techniques, has the advantages of high processing accuracy and efficiency, good flexibility, and no tool loss. However, the high reflectivity and thermal conductivity of aluminum alloys make it challenging to prepare high-quality microholes. Hence, it is crucial to study the influence of laser parameters on microhole morphologies and tapers and develop laser drilling technology for microholes in aluminum alloys.MethodsIn this study, we use a femtosecond laser to drill microholes with a diameter of 200 μm in a 1 mm thick 1060 aluminum alloy. First, the influence of the laser repetition rate, scanning time, scanning speed, and feed time on microhole morphologies and tapers is studied using the control variable method. Then, based on the single-step trepanning drilling method [Fig. 2 (a)], a three-step trepanning drilling method [Fig. 2 (b)] is proposed to solve the problems of big taper and poor entrance and exit morphologies. The three circles are set as one ring in the three-step trepanning drilling method. The innermost circle (C1) is first scanned clockwise, and the laser moves toward the first ring’s outer circle (C3). After the first layer scanning, the laser feeds down a distance and the above process of C1-C3 is repeated until the center perforated hole forms. Subsequently, the laser is focused on the C4-C9 circles to complete the second and third rings similarly and finally the microhole processing is finished. In the three-step trepanning drilling, the drilling process is divided into three steps: the first ring is used to form the center microhole, the second ring is used to widen it, and the third ring is used to improve the hole morphology [Fig. 3 (b)]. The entrance and exit morphologies and the surface slag of microholes are analyzed using a laser confocal microscope. The corresponding tapers are calculated.Results and DiscussionsThe influence of laser repetition rate, scanning time, scanning speed, and feed time on microhole morphologies and tapers using the single-step trepanning drilling method is analyzed. According to the results, the low repetition rate makes the heat accumulation unsustainable, which causes low material removal efficiency and a large taper. The heat accumulation at the lower part of the microhole reaches saturation when the repetition rate is increased to 50 kHz, and the microhole taper is the smallest (Fig. 4). Scanning time has a similar effect as feeding time on the taper and entrance and exit morphologies of the microhole. Multiple scans can improve the roundness of the outlet (Fig. 5). With the increase in feed times, the laser energy density in the lower part of the microhole exceeds that in the upper part of the microhole, so the microhole exit diameter increases faster than the entrance diameter. Too many feed times will increase taper (Fig. 6). The laser scanning speed and repetition rate determine the spot overlap ratio; the higher the spot overlap ratio, the more material is ablated and the smaller the taper (Fig. 7). Based on the above research results, the appropriate laser parameters are selected for the comparative experiment of single-step and three-step trepanning drilling methods. The morphologies and tapers of microholes processed by three-step trepanning drilling method are greatly improved (Fig. 8). This is because, in the single-step drilling process, the slag and plasma generated by laser ablation will cause interference and plasma shielding effect on the subsequent laser pulse, making processing the lower part of the microhole difficult and affecting the exit morphology and taper. However, most slag and plasma produced in the three-step drilling process will be discharged through the bottom of the center hole, which reduces scattering interference to the laser beam. Therefore, the morphologies of the microholes will be improved, and the tapers will be reduced. Observations of slag on the surface of the microhole processed by two methods support this analysis (Fig. 9). Although the three-step trepanning drilling method requires multiple feeds, it has little influence on processing time and efficiency (Table 1).ConclusionsIn this study, the influence of laser repetition rate, scanning time, scanning speed, and feed time on microhole morphologies and tapers is studied using the control variable method. A three-step trepanning drilling method is proposed to solve the problems of a big taper and poor morphology caused by single-step trepanning drilling. The morphologies and tapers of the microholes prepared by the three-step trepanning drilling method are improved. The minimum taper is 0.78°, which is 31% lower than those of the microholes prepared by single-step trepanning drilling method with the same laser parameters. The main reason is that the three-step trepanning drilling method can effectively promote the discharge of slag and plasma in the drilling process, which reduces disturbance to laser energy and hence improves the stability and uniformity of laser energy.

ObjectiveHigh-nitrogen steel has high strength, good corrosion resistance, and low cost. However, its difficulty in processing hinders its application. In this study, selective laser melting (SLM) is used to manufacture high-nitrogen stainless steel, which is suitable for manufacturing complex components and ensuring good mechanical properties. This study focuses on the effects of the laser volume energy density on the nitrogen content, relative density, phase composition, microstructure, and mechanical properties of manufactured samples.MethodsIn this study, the self-made high-nitrogen steel powder is used, and a vacuum heating furnace is used to dehydrate the powder before the experiment. Three laser parameters are designed for additive experiments. The nitrogen content and relative density of the specimens are measured after grinding and polishing. Phase analysis is performed using X-ray diffraction (XRD). The tensile strength is measured using a universal testing machine and the fracture morphologies of the tensile specimens are observed using scanning electron microscopy. The relationship between the microstructure and mechanical properties is studied. In addition, the phase compositions of the sample before and after a tensile test are measured using high-energy synchrotron XRD to analyze the strengthening mechanism.Results and DiscussionsThe study shows that, when the laser volume energy density increases from 190.5 J·mm-3 to 285.7 J·mm-3, the tensile properties first increase and then decrease (Fig. 9). When the laser power is 200 W, the laser scanning speed is 400 mm/s, and the laser volume energy density is 238.1 J·mm-3, the specimen exhibits the best mechanical properties with the tensile strength of 1275.0 MPa and elongation of 14.7%. The nitrogen content(mass fraction) of the specimen reaches 0.76% (Table 3), and the relative density of the specimen is 99.8% (Fig. 6). An appropriate laser volume energy density can improve the relative density and increase the solubility of nitrogen in the material. Supersaturated nitrogen plays an important role in solid solution strengthening in the austenite lattice, which then improves the tensile strength and elongation of the additive-manufactured components. Plastic transformation triggers the deformation-induced ferrite transformation (DIFT) effect. The transformation of austenite to ferrite enhances the strength and toughness of the material.ConclusionsIn this study, high-nitrogen stainless steel is manufactured via SLM. The microstructure primarily comprises austenite with a small amount of ferrite. After process optimization, with a laser power of 200 W and laser scanning speed of 400 mm/s, the manufactured parts demonstrate the highest relative density of 99.8%. They show excellent mechanical properties, with tensile strength of 1275 MPa and elongation of 14.7%. Microstructural studies show that the laser volume energy density affects the nitrogen content of the specimen. An increase in the austenite content leads to an improvement in the mechanical properties. Simultaneously, Cr2N precipitates remain in the sample at low volume energy densities. When the volume energy density is high, numerous circular pores appear in the sample, which cause premature failure during the tensile test. Our study shows that high-nitrogen stainless steel components with high relative density and excellent mechanical properties can be obtained by optimizing the laser volume energy density, which leads to the development of SLM for high-nitrogen steel.

ObjectiveNowadays, rare-earth ion-doped fiber lasers and amplifiers are widely used in optical communication, industrial processing, military, and medical applications. Among them, the Er-Yb co-doped fiber amplifiers (EYDFA) with high power, low noise, and small size have a great potential for long-range space communication applications. However, rare-earth-doped fibers produce many color centers when exposed to various types of cosmic radiation such as X-rays, γ-rays, etc. Some of the color centers generated during irradiation originate from co-dopant elements like aluminum and phosphorus in rare-earth-doped fibers, whereas others originate from precursors formed during fiber fabrication. The absorption bands of these color centers are mainly located in the UV and NIR bands, which can cause radiation-induced absorption (RIA) in the pump and signal bands of the Er-Yb-doped fiber (EYDF), resulting in a severe degradation of the performance of the fiber amplifier. Therefore, it is crucial to improve the radiation resistance of the EYDF.MethodsA radiation-resistant Er-Yb co-doped fiber (RREYDF) was prepared via modified chemical vapor deposition (MCVD), and the concentration and ratio of doping components such as erbium, ytterbium, phosphorus, and cerium were adjusted to enhance the radiation tolerance of the fibers. The Er, Yb, and P doping ratio was 1∶22∶536, and the core and cladding dimensions were 10.5 μm and 130 μm, respectively. The irradiation doses for current space missions range from 300 Gy to 1000 Gy, so those values are chosen to test the RIA and radiation-induced gain variation (RIGV) of RREYDF. The RIA and RIGV were tested using Photon Kinetics 2500 and a typical EYDFA, respectively.Results and DiscussionsRadiation-induced absorption (RIA) was 0.10 dB/m and 0.19 dB/m (300 Gy) at 940 nm, and 0.46 dB/m and 0.37 dB/m (1000 Gy) at 1550 nm. For gain testing, an Er-Yb co-doped fiber amplifier (EYDFA) was built, and the radiation-induced gain variation (RIGV) was 0.2 dB (300 Gy) at 1550 nm and 0.7 dB (1000 Gy) at a pump power of 7.3 W. The mechanisms of the relevant irradiation resistance studies were analyzed. Cerium co-doping was used in Er-Yb fibers to enhance the radiation resistance, taking advantage of the coexistence of Ce3+ and Ce4+ in silicates, where Ce3+ can trap the holes created during radiation. Therefore, the latter competes with the precursor P2O3 and reduces the formation of P1 color centers, thus improving the radiation resistance of the fibers.ConclusionsIn summary, the RIA of the radiation-resistant Er-Yb co-doped fibers prepared by MCVD were 0.10 dB/m and 0.19 dB/m (300 Gy), and 0.46 dB/m and 0.37 dB/m (1000 Gy) at 940 nm and 1550 nm, respectively, after irradiation at 300 Gy and 1000 Gy and 0.2 Gy/s average dose rate. The RIGV was tested by a typical EYDFA, and the results showed that the RIGV was 0.2 dB (300 Gy) at 1550 nm and 0.7 dB (1000 Gy) at a pump power of 7.3 W. In addition, the radiation-resistance mechanism of the fibers was analyzed. This Er-Yb co-doped optical fiber has excellent radiation performance and extensive applications in the fields such as long-range space communication, remote sensing, and space navigation.

SignificanceMiniaturization of electronic and information devices has become the development trend in the age of science and technology. Owing to its benefits, including simple integration without contact, flexible and controllable fabrication, and low material loss, femtosecond laser micro/nano manufacturing has gradually become a crucial research direction in micro/nano manufacturing technology. The femtosecond laser can achieve ultra-precision, high-efficiency, and high-quality micro/nano fabrication of almost all materials. The interaction between femtosecond laser and materials is distinct from that of the traditional laser-material interaction process, owing to its ultra-short pulse characteristics, which is a nonlinear and unbalanced multi-temporal-scale ultrafast process. It involves various physical processes, like photon absorption and electron excitation, phase transition, plasma/shockwave radiation and eruption, material removal, and other ultrafast dynamics processes. These physical processes fundamentally influence the final structure of laser processed materials and can directly regulate the structure’s morphology and properties. Thus, to attain the constrained breakthrough and extensive application in femtosecond laser micro/nano fabrication, it is important to understand and regulate the ultrafast dynamic evolution of femtosecond laser interaction with materials. An in-depth study and understanding of the ultra-fast dynamic evolution mechanism in femtosecond laser processing will offer a theoretical fundamental and guidance for the realization of high-efficiency, high-precision, and high-quality femtosecond laser micro/nano fabrication. Therefore, facilitating the quick development of femtosecond laser micro/nano fabrication technology and its application.ProcessFemtosecond laser pump-probe technology is employed to investigate the ultrafast dynamics evolution in femtosecond laser processing. With the development of the time delay translation stage and optical imaging technology, temporal and spatial resolution has been improved. The imaging types include transmission type and reflective type, interferometric type, and holographic imaging. Numerous dimensions of ultrafast dynamics imaging in femtosecond laser fabrication have been achieved. Moreover, to probe the material response in femtosecond laser processing more completely, a multi-scale pump-probe system has been constructed to probe the whole process of femtosecond laser-material interaction.There are several investigations about the probe of photon-electron interaction, electron-lattice interaction, and plasma radiation and eruption in the process of femtosecond laser-material interaction, which indicates the physical mechanism of each stage. Moreover, the mechanism research for shaped femtosecond laser processing and femtosecond laser-excited chemical reaction synthesis of materials are also performed.The crucial physical parameters of the femtosecond laser-material interaction theoretical model can be evaluated by studying the photon-electron interaction process based on pump-probe technology that can guide the theoretical model’s enhancement and development. There is a lot of research to determine the crucial factors like electron decay time, electron-hole combination mechanism, and electron relaxation time, therefore regulating the electron density evolution, electron/lattice temperature, phase transition mechanism, and ablation findings. Furthermore, the probe of the photon-electron interaction process can show the nonlinear ionization mechanism in laser-material interaction, which guides the optimization of material processing conditions and parameters, and attains the effective and controllable manufacturing of structures.The evolution of transient optical properties during femtosecond laser materials interaction can be employed to investigate the electron-lattice interaction process, and further, show the phase transition and removal mechanism of materials in the picosecond-nanosecond time scale. Presently, femtosecond laser processing mechanisms on traditional materials like fused silica, silicon, and germanium under the picosecond-nanosecond time scale have been investigated. Recently, research on new materials, including two-dimensional materials has emerged, which shows the phase transition mechanism and ablation mechanism of emerging materials.The process of plasma eruption and shockwave propagation in the picosecond to nanosecond time scales after femtosecond laser processing plays a crucial role in the evolution of the final morphology and the investigation of material properties based on plasma. Current studies primarily focus on the influences of femtosecond laser parameters (including laser fluence, wavelength, and pulse width), material properties, and processing environment on the generation and propagation of plasma/shockwave. The findings show the impacts of air/material plasma excitation and shockwave expansion on the final morphology of laser-induced micro/nanostructures.New approaches based on electron dynamics control have been suggested in recent years. Therefore, in addition to investigating the traditional Gaussian pulse’s mechanism, some studies have also been reported about the interaction between spatial-temporal shaping femtosecond laser and materials. Currently, the studies focus on double-pulse femtosecond laser processing, Bessel laser processing, and simultaneous spatial and temporal focusing of femtosecond laser processing, which improves the development of spatial-temporal shaping of femtosecond laser processing. There are also investigations on the probe of the ultrafast dynamics of chemical reaction excited by femtosecond laser, showing the physicochemical mechanism in the process of material chemical reaction excited by femtosecond laser, expanding the application and development prospect of femtosecond laser micro/nano fabrication.Conclusions and ProspectsFemtosecond laser manufacturing is forecasted to become the primary means of high-end manufacturing in the future, which will offer crucial manufacturing support to attain leapfrog development in new energy, aerospace, national defense, and other fields. It is the ideal technology to break through the manufacturing technology challenges of numerous core components, but there are still a lot of issues. To enhance the manufacturing accuracy, efficiency, quality, and controllability of femtosecond laser, it is crucial to have a deep understanding of the complex mechanism in femtosecond laser processing. In observing the temporal and spatial evolution of local transient electron dynamics in ultrafast laser manufacturing, the accuracy of temporal and spatial resolution, three-dimensional panoramic observation from various angles, and multi-scale ultrafast probe are the three major problems. In this study, the ultrafast dynamic in femtosecond laser micro-nano manufacturing is summarized, and the interaction between femtosecond laser and matter is summarized. The development history of femtosecond laser pump-probe technology is introduced, and the multi-scale ultrafast dynamic in various stages of femtosecond laser micro-nano manufacturing is summarized. Distinct material systems, processing environments, and mechanisms based on electrons dynamics control are summarized and compared, which offers a crucial observation fundamental and guidance for femtosecond laser micro-nano manufacturing.The current study about the ultrafast dynamics of femtosecond laser micro-nano manufacturing has broken through the drawbacks of traditional image sensors to attain higher frame rates and shutter speeds. To enhance the image acquisition’s speed, these studies more or less sacrifice one or more specific parameters. Additionally, traditional methods still have challenges, including challenges in light field reconstruction, single observation means, and difficulty in attaining precise coordination and coupling of numerous observation means. Combining the pump-probe technology, ultrafast continuous imaging technology with the four-dimensional scanning ultrafast electron microscopy technology, can develop a multi-scale quasi three-dimensional pump-probe system with substantial spatial-temporal resolution and dynamical continuous observation capability, which can be employed for observation of electron ionization and phase change during the evolution of the structures and properties in the femtosecond laser extreme manufacturing. High spatial-temporal resolution observation of multi-scale processes will revolutionize the research on ultrafast dynamics in femtosecond laser manufacturing.

SignificanceBecause of the outstanding photochemical and optical properties, the titanium dioxide (TiO2) has been used for applications such as energy materials and optical information. Compared to other semiconductor photocatalysts, TiO2 has attracted more attentions since the report of its ability for water splitting. In the recent half century, TiO2 is considered one of the ideal semiconductor photocatalysts because of its abundance, high stability, nontoxicity, and low cost. Because of its characteristics of high transmittance and high refractive index, TiO2 also has application prospects in the field of optical information. With the development of micro/nano technology, advanced micro/nano processing technologies have been used to prepare TiO2 micro/nano structures and functional devices (Fig. 1). In the field of energy materials, TiO2 micro/nano structures can be used for photoelectrochemistry water-splitting, dye-sensitized solar cells, photocatalysis, photovoltaic cells, photodetectors, and lithium-ion batteries. In the field of optical information, TiO2 micro/nano structures can be used for metasurface structure color, optical encryption, holography, and optical metadevices. The micro/nano processing of TiO2 is essential for the applications of TiO2 micro/nano functional devices.The ultrafast laser processing minimizes heat affected zone due to its ultra-short interaction time with the material. The ultrafast laser can be used to process almost any material due to the ultra-high power density. This provides an ideal choice for the processing of TiO2 micro/nano structures and functional devices. The ultrafast laser micro/nano processing of TiO2 has attracted attention in recent years. In this review, we introduce the research progress of ultrafast laser processing of TiO2 micro/nano structures and functional devices. Processing approaches and applications of TiO2 micro/nano structures are discussed. In addition, the prospects of ultrafast laser processing of TiO2 micro/nano structures and functional devices are discussed.ProgressFour ultrafast laser processing approaches are commonly used for TiO2 micro/nano structures (Fig. 4), including ultrafast laser ablation of material surface, laser-induced periodic surface structure, ultrafast laser induced phase transformation of TiO2 and ultrafast laser writing TiO2 micro/nano structures on titanium. Numerical calculation models for ultrafast laser ablation of TiO2 have been established. The ablation thresholds are calculated, and the results are consistent with the experimental results (Fig. 5). A method is developed to achieve processing of amorphous TiO2 nanotubes and their transformation to anatase using the ultrafast laser (Fig. 6). Compared with thermal annealed pure TiO2 nanotubes, TiO2 nanotubes with exposed reactive anatase {010} facets are prepared after the ultrafast laser induced phase transformation. Based on the method of laser induced in situ growth of TiO2, a "THU" pattern with height smaller than the laser wavelength is realized (Fig. 7).TiO2 micro/nano functional devices, such as devices for photoelectrochemistry water-splitting, structural color, and optical encryption, are also discussed. TiO2 nanotubes with exposed reactive anatase {010} facets can be prepared by the ultrafast laser. Due to the exposure of reactive facets, the TiO2 nanotubes show an improved photocurrent density about five times of those of the thermal annealed pure anatase TiO2 nanotubes (Fig. 9). Based on the method of laser induced in situ growth of TiO2, grating-type TiO2 nanostructures with various periods can be processed. The grating-type TiO2 nanostructures is used for grating-based structural colors in the visible range. The observed colors are determined by the grating period with a fixed incident angle (Fig. 10). Due to the anisotropic optical behavior of grating-type structures, an optical encryption strategy is developed. The horizontal and vertical nanostructures demonstrate a remarkable difference in scattered intensity when visualized under a dark field optical microscope. The scattered intensity is the strongest when the nanostructures are illuminated along the direction perpendicular to the nanostructures. While it is the weakest when the nanostructures are illuminated along the direction parallel to them. Two different images can be encrypted on the same position (Fig. 11).Conclusions and ProspectsWe introduce the research progress of ultrafast laser processing of TiO2 micro/nano structures and functional devices. The photochemical and optical properties of TiO2 are introduced. The photochemical and optical properties of TiO2 can be adjusted by preparing TiO2 with different micro/nano structures. A model of transient local electron density is established. The theoretical predictions of ablation threshold and diameters are realized. Different approaches, including ultrafast laser ablation of material surface, laser-induced periodic surface structure, ultrafast laser induced phase transformation of TiO2 and ultrafast laser writing TiO2 micro/nano structures on the surface of titanium plate, have been studied for the processing of TiO2 micro/nano structures and functional devices. Ultrafast laser provides a choice for the processing of TiO2 micro/nano structures and functional devices. The method shows promising applications in fields such as photoelectrochemistry water-splitting, structural color, and optical encryption devices. The processing of TiO2 micro/nano structures and functional devices using the ultrafast laser also has some challenges, for example, the efficiency needs to be improved. With the further research on the mechanism and processing technology, it is expected to further expand the application ranges of TiO2 micro/nano structures and functional devices.