In this paper, by investigating the laser-induced plume morphology and the particle signal in the plume, the visual signal (average area) and particle signal (number of particles, average velocity and total signal strength) of the plume change with the input energy density of the laser was studied. The weld depth is positively correlated with the input energy density of the fiber laser. Compared to the average area of the plume, the plume particle signals demonstrate superior consistency under varying parameter conditions: during changes in defocus distance, the plume average area is negatively correlated with the laser input energy density; whereas, in other variations, a positive correlation is observed. The particle signals, under different parameter conditions, can accurately represent the positive correlation between the weld depth and the laser input energy density. Therefore, the particle signal in plume is more suitable to characterize the variation trend of weld depth with laser input energy density. In the three kinds of particle signals, it is more convenient to use the total signal amplitude which does not require a lot of calculation to characterize the change of weld depth.

It is essential to develop an accurate and fast-solving prediction model of weld bead cross-section (WBCS) morphology to select appropriate processing parameters for industrial applications of laser deep penetration welding (LDPW). In this paper, considering laser multiple reflections and absorption within the keyhole and energy balance of the keyhole central section in the direction perpendicular to welding velocity, the mathematical relationship between the fusion boundary of WBCS and the keyhole boundary was obtained. Subsequently, a predictive model of WBCS morphology during LDPW was established using the above relationship and the keyhole boundary profile calculated by the morphology-calculated model for front keyhole wall (FKW) considered the primary absorption of FKW. The results of calculations indicated that the variation of WBCS morphology with laser power, welding velocity, defocusing amount, laser wavelength, and other parameters was largely consistent with the consensus. The model feasibility was verified by comparing the results from calculations and experiments. It can be found that the keyhole depth can be obtained by calculating based on the primary absorption of FKW. However, the laser energy coupling pattern within the keyhole is characterized by multiple reflections, mainly converging towards the keyhole bottom. Therefore, the characteristics of laser energy coupling in the keyhole should be comprehensively considered while calculating the WBCS morphology during LDPW.

The laser melting processing is affected by complex physical mechanisms, exhibiting significant non-stationary characteristics across multiple dimensions, including the time and frequency domains. In view of the difficulties in quantifying the plume fluctuation during the processing and the limitations of single-dimensional analysis, this paper proposes a data processing method combining ensemble empirical mode decomposition and continuous wavelet transform (EEMD-CWT) to quantitatively analyze the time-frequency multi-dimensional of plume morphology and its correlation with processing mode transitions. The results show that during the transition from thermal conduction mode (without a keyhole) to keyhole mode, the plume area first increases and then decreases, while the fluctuation frequency and its distribution range consistently increase. Both the area and the fluctuation frequency of the plume under different processing modes present significant changes, and there is a transition stage with dual characteristics between the conduction mode and the keyhole mode. Through the analysis of EEMD-CWT, it is found that the fluctuation frequency range of the plume gradually expands from approximately 60 Hz in the heat conduction mode to 500 Hz - 2500 Hz in the keyhole mode and exhibits strong instability in the transition mode. Utilizing the multi-dimensional time-frequency analysis method proposed in this paper is expected to quickly and effectively identify the transitions among different processing states in laser melting processing, and it can provide important theoretical support and technical methods for the dynamic monitoring of the processing process, the understanding of the processing mode transformation mechanism and the process optimization.

Characterizing the dynamic behavior of the plume using particles within it is expected to avoid interference from factors such as shielding gas flow and strong light, enabling more accurate acquisition of quality information about the laser welding process. Based on the laser self-mixing interference (SMI) effect, this paper systematically investigates the key parameter system for in-situ detection of particles within the plume during fiber laser welding. The results show that when the detection sampling rate is on the order of 5 x 106 Sa/s, it can meet the acquisition requirements of interference fringe signals. When detecting particles in the plume perpendicular to the welding direction, the signal intensity is the highest. The smaller the focal length of the detection laser focusing lens and the closer the detection height is to the plate surface, the greater the obtained signal intensity. When the detection direction is perpendicular to the welding direction, the signal intensity of plume particles reaches its peak. Detection lasers focused with short focal lengths are more suitable for measuring the number of plume particles, while long focal lengths are more suitable for measuring particle size. The detection range of particles is positively correlated with the Rayleigh length of the detection laser, and the signal intensity is positively correlated with the scattered light intensity of the detection laser (i.e., the power density of the detection laser) scattered back into the laser.

The simultaneous observation of both plume morphology and particle in plume is crucial for characterizing the plume dynamic behavior with the particle signals during fiber laser keyhole welding. In this study, synchronous triggering of the photodetector and the particle in-situ measurement system facilitated the simultaneous observation of both plume morphology and particle within the fiber laser beam, as well as the relationship between the plume’s visual image and the optoelectronic signal. In the early stages of fiber laser welding, optoelectronic signals and particle signals are generated nearly simultaneously. During the welding process, the plume can be categorized into a bottom oscillating section and a narrow section resembling the focusing pattern of the fiber laser beam. Notably, when the bottom plume oscillates in the direction of the fiber laser beam, the maximal amplitude of the particle signals is observed. This characteristic is independent of various parameters, including laser power, welding speed, and the defocus distance. When the bottom plume swings toward the fiber laser beam, the particles carried by the plume into the focused beam contribute to the maximal signal amplitude. This feature can be utilized to establish a correspondence between particle signals and the oscillating behavior of the plume.

The penetration depth is an important indicator for evaluating the laser penetration ability in the laser welding process. Plume is the main information carrier of welding process signals. In this paper, by synchronously collecting the plume vision signal and the plume particle signal (based on the principle of self-mixing interference, SMI), and combining the algorithms of ensemble empirical mode decomposition and fast Fourier transform (EEMD-FFT) to extract the time-domain and frequency-domain features of the plume signals, a method designed for penetration depth monitoring through the fusion of multiple plume signals is introduced. The results show that both the plume area and the total intensity of the SMI signal are positively correlated with the weld penetration depth, and the frequency-domain features of the plume signal have a higher correlation than the time-domain features. Compared with the peak frequency, the centroid frequency of plume signal has higher sensitivity and adaptability to working conditions when reflecting the changes in the penetration depth. The SMI signal has obvious advantages in signal processing efficiency. Its storage space is only 0.47 % of that of visual signal, and the overall processing time can be shortened by 93.4 %. The two types of signals have good complementarity in terms of information. The signal strategy can be flexibly selected according to actual needs to achieve a balance between the efficiency and accuracy of welding monitoring. The research can provide a novel technical scheme for the in-situ monitoring during laser welding.

The keyhole, as the primary feature of laser deep penetration welding, represents the principal site for achieving light-thermal energy conversion in the welding process. To study the three-dimensional morphology of keyhole, it was assumed in this paper that the temperature of molten liquid at the edge of keyhole aperture is boiling point temperature, and the boiling temperature isotherm on melt pool surface was calculated based on two different treatment methods of heat source to estimate keyhole aperture profile. Then it was subsequently verified by using in-situ optical observation and the method about melt pool quick freezing to retain keyhole. The results show that keyhole aperture in laser deep penetration welding can be divided into the zone formed by laser direct heating and the zone formed by laser-induced vapor eruption from front keyhole wall. The former is closely related to the laser energy distribution on material surface rather than the laser energy distribution in the direction of keyhole depth, while the latter is more susceptible to the influence of erupted laser-induced vapor related to the energy distribution on front keyhole wall. When the welding speed is low, the tilt angle of front keyhole wall is larger, the zone formed by laser-induced vapor eruption from the front keyhole wall is not obvious, and the zone formed by laser direct heating is approximately circular in shape. The profile of boiling temperature on the laser-induced melting pool surface calculated by the direct calculation method of laser heat source acted on material surface is consistent with the zone formed by laser direct heating. When the inclination angle of front keyhole wall is reduced by changing welding speed or material, the impact of laser-induced vapor emitted from front wall on rear wall is increased, resulting in an elliptical or “gourd” morphology of the keyhole aperture.

The establishment of an accurate prediction model of keyhole depth during laser welding is of great significance for predicting the weld depth or pre-selecting suitable welding process parameters. In this paper, based on the primary absorption of the incident laser by the front keyhole wall, the tilt angle of the keyhole wall was calculated point by point according to the equilibrium relationship between the laser energy absorbed by the material and the energy required for evaporation at any position of the front keyhole wall. A prediction model for the keyhole depth in laser deep penetration welding has been established by taking into account the laser optical parameters, the focusing system parameters, the welding process parameters, and the material properties. The transformation law of the front keyhole wall profile, the Fresnel absorption coefficient, and the keyhole depth with laser power, welding speed, focusing focal length, and other parameters was calculated in the model, which is basically in line with the consensus. At the same time, the keyhole depth results of calculations and measurements under some parameter conditions were compared. It can be found that the prediction model of keyhole depth during laser deep penetration welding can be used to approximate the calculation of weld depth, and the feasibility of the model was preliminarily verified. Moreover, combined with previous experimental results, the conclusion that the primary absorption of front keyhole wall is the key factor determining the keyhole depth can be re-verified by using mathematical calculations. It also shows that the complex energy coupling law in keyhole has a noticeable difference in the influence of different physical phenomena during laser deep penetration welding.

光纤激光深熔焊接羽辉可分为底部摆动羽辉和类似于激光束聚焦形态的狭长形羽辉.采用 6 kW的光纤激光焊接低碳钢,对比研究了旁轴高速保护气流对这两部分羽辉的影响.结果表明,高速保护气流对狭长形羽辉具有明显的抑制效果,但其流向对焊接过程稳定性的影响则明显不同:气流逆焊接方向时飞溅多、熔深浅、焊缝成形差;气流沿焊接方向时飞溅少、熔深加深、焊缝成形良好,保护气流逆焊接方向时,小孔口前部凸起液柱发生偏折,小孔口缩小、熔池波动更剧烈.进一步分析表明,保护气流逆焊接方向时,小孔口前部凸起液柱向小孔口偏折并堵塞小孔、影响底部摆动羽辉的喷发是致使焊接过程稳定性恶化的主要原因.在光纤激光焊接中,布置保护气时应该考虑其流向对底部摆动羽辉喷发状态的影响.

采用准分子激光对PMMA板进行加工,通过旋转台完成打孔,并探究多项参数对于微孔深度与锥度的影响.研究发现:在一定范围内,当脉动频率和单脉冲能量增加时,微孔的深度线性增加、锥度线性减小;旋转速度对于微孔的深度和锥度无太大影响;采用较大的三角形掩模加工微孔的锥度最佳.通过选择合适的掩模与参数,加工出的微孔比冲孔法效果更佳.

International thermonuclear experimental reactor correction coil cases are made of heavy, thick, high strength, and high toughness austenitic stainless steel 316LN. The BTCC (bottom and top correction coils) case has the dimension of 2.5 × 7 m2 and cross section of 239.8 × 146.7 mm2, side correction coil case has the dimension of 7.2 × 7.6 m2 and cross section of 147.8 × 168 mm2, and they will be closure welded after winding pack insertion. The 20 mm welding depth, dozens of meter of welding length, strict welding requirements, large size, and complex configuration bring a big challenge to this closure welding work. 20 kW high power laser welding is selected as the main welding method because of the advantage of potential welding deformation control and its penetrating ability and cracking resistance. The welding parameter is developed that can cover the assembly gap from 0 to 0.5 mm with a good welding quality. A special test coupon is designed for the welding procedure qualification, and related tests are carried out to qualify the joint properties of bend, tensile, and impact. Finally, a full-scale BTCC case is welded. After welding, ultrasonic testing confirms that almost all welds satisfy the weld seam quality requirement. The recorded temperatures less than 250° indicate that the temperature induced by welding will not harm the internal winding pack. The dimensional deviation of the inner face is less than 4 mm and also satisfies the tolerance requirement of ±2 mm for the BTCC case.

To resist the alternating electromagnetic loads, the International Thermonuclear Experimental Reactor (ITER) correction coils are protected by a 20-mm-thick 316LN austenitic stainless steel case. According to the strict tolerance requirement of the case manufacture, laser welding was applied to enclosure welding of the cases with a 20-kW high-power laser for root pass and Tungsten Inert Gas welding was applied for filler and cover passes. In this study, the microstructure and mechanical properties of enclosure welding joint were investigated to evaluate the quality of the joints. No porosity, crack or lack of fusion was found on the cross-section of the joints. The microstructure in the weld zone is single austenite phase and the morphology of the joint consists of cellular crystal, columnar crystal, and small amount of the equiaxed grain. The average tensile strength of the joint is 590 MPa, which is 98.3 & x0025; of the strength of the base material of 600 MPa. The fracture position is located at the base material with an apparent necking morphology, which confirmed the good plasticity. The side bend test specimens were subjected to significant plastic deformation, which bent through 180 & x00B0; and no surface crack can be found. The impact toughness of the weld joint at 4.2 K is larger than 180 J & x002F;cm(2), and the fracture surface is composed of fine equiaxed dimples and tearing ridges, which indicates that the specimens failed in a ductile manner. All of the test results satisfy the ITER requirements.

The present work envisages the development of a novel and low-cost self-mixing interferometry (SMI) technology-based single particle sensing system in a microchannel chip for real time single micro-scale particle sizing. We proposed a novel theoretical framework to describe the impulse SMI signal expression in the time domain induced by a flowing particle. Using Hilbert transform, the interferometric fringe number of the impulse SMI signal was retrieved precisely for particle size discrimination. For the ease of particle sensing, a hydrodynamic focusing microfluidic channel was employed by varying the flow rate ratio between the sample stream and the sheath liquid, and the particle stream of a controllable width was formed very easily. The experimental results presented good agreement with the theoretical values, providing a 300 nm resolution for the particle sizing measurement.

Laser welding of modified 316LN steel with a thickness of 20mm was conducted using a YLS-20000 fiber laser. The microstructure of the weld joint was characterized and tensile and fracture toughness tests were carried out. The microstructure evolution of the fracture specimens was studied systematically to elucidate the fracture mechanism. The weld was composed of the single austenite phase and was characterized by cellular and columnar grains, and the grain size became coarser with a weak orientation. The R-m values of the laser welded joints were almost equal to that of the base metal both at RT and 4.2 K, and the fracture surfaces were featured by ductile fracture with quantities of dimples and microvoids. The fracture toughness of the weldments decreased to (similar to)84% of that of the parent metal at 4.2 K. Enhancement of the fracture toughness was attributed predominantly to the presence of twins and to the fine grain size, and the reduction was caused by the partially stress-induced phase transformation of the austenite to martensite. The synergistic effects of these factors result in a favorable improvement in the fracture toughness of the weldment.

Binder-free porous Si/Cu architecture with the unique Cu walls wrapped Si/Cu porous network as lithium-ion battery anode was fabricated by laser remelting, diffusion bonding and dealloying processes. The porous network had enough space to allow for the large expansion of Si during lithation. The Cu walls and Cu skeletons could prevent loss of electrical contact between pulverized Si and current collector. The laser remelting influenced the Cu diffusion behavior during diffusion bonding, and was the most critical process for the fabrication of porous Si/Cu architecture.

The IC10 single crystal superalloys are welded by the laser welding process, and the effects of welding speed on joint weld forming, cross-section morphology and microstructure are analyzed. The results show that the surface and back widths of welds increase with the decrease of welding speed. The cross-sections of welds under different welding speeds all show a typical goblet shape. The welds mainly consist of fine grains, cellular crystals and columnar crystals. The welding cracks extending along the grain boundaries always occur in welds under different welding speeds. The growing direction of the grains in joints tends to be the same, which increases the cracking susceptibility of joints.

The 100 mm thickness SUS304 stainless steels arc welded by the ultra-narrow gap laser welding, and the microstructures and mechanical properties of the joints arc analyzed. The results show that the hundred-millimeter-grade thickness plates can be effectively welded by the ultra-narrow gap laser welding and the welds present good surface appearances. The microstructures of the joints arc composed of austenite and small amount of ferrite, and the grains arc with the cellular cquiaxcd shapes. The tensile strength of the joints is 658 MPa and the tensile fracture is located at the cellular grain zone near the fusion line. The fracture morphology shows fine cquiaxcd dimples and tearing ridges, which indicates that the fracture is ductile. The microhardness at the cquiaxcd grain zone is obviously higher than that at the cellular grain zone.

A defect-free welded joint of 100 mm-thick SUS 304 steel plates is produced by ultra-narrow gap laser welding with filler wire in the laser conduction mode in 42 layers. The laser beam concurrent heating of the groove side walls and bottom ensures the adequate side wall fusion, with the angle distortion belowed 1°, and the average fusion ratio of the whole weld of about 11%. The microstructure is composed of cellular grains adjacent to the fusion line and fine equiaxed grains. The average tensile strength of the weld joint is 658 MPa, which corresponds to ∼95% of that of the base metal. The fracture occurs near the fusion line in the cellular grain zone of the weld joint. The microstructure morphology difference in the weld joint implies the uneven distribution of microhardness which values in the equiaxed grain zone significantly exceed those the cellular zone.

为了改善2060铝锂合金激光填丝焊接的焊缝组织并进一步提高接头力学性能,使用了辅助激光填丝焊接的新方法.文中采用光纤激光器焊接2 mm厚2060-T8铝锂合金薄板,通过填充焊丝引入直流电流,研究电流对焊缝成形、焊缝结晶组织和接头力学性能的影响.结果表明,施加电流后,焊缝表面鱼鳞纹变得细密均匀,表明焊接过程更加稳定.同时,焊缝上、下熔宽趋于一致,焊缝结晶组织细化,熔合线附近等轴细晶区宽度减小.与无辅助电流相比,接头抗拉强度提高约5.8%.

The laser induced plasma during CO2 laser deep penetration welding have been studied through the real-time observation and analysis with high-speed camera and multichannel spectrometer.Under the condition of local thermodynamic equilibrium(LTE),the temperature of the plasma have been calculated using the method of multispectral diagonal lines,and then the plasma time characteristic have been quantitative studied in the stable deep penetration laser welding process.Furthermore,the relationship between plasma oscillation characteristics and welding stability and its influence on the weld formation have been researched.The results show that plasma oscillates with a certain frequency in a stable welding process.Meanwhile,the up and down frequency of plasma is consistent with its size fluctuation approximately,and the frequency of plasma is also basically in agreement with the surface stripe change of weld.In the unsteady laser deep penetration welding process,the frequency of formation of stripe and the oscillation frequency of the plasma on the surface of the corresponding weld are very different.