Supervised descent method for weld pool boundary extraction during fiber laser welding process

In order to obtain a high-quality weld during the laser welding process, extracting the characteristic parameters of weld pool is an important issue for automated welding. In this paper, the type 304 austenitic stainless steel is welded by a 5 kW high-power fiber laser and a high-speed camera is employed to capture the topside images of weld pools. Then we propose a robust visual-detection approach for the molten pool based on the supervised descent method. It provides an elegant framework for representing the outline of a weld pool and is especially efficient for weld pool detection in the presence of strong uncertainties and disturbances. Finally, welding experimental results verified that the proposed approach can extract the weld pool boundary accurately, which will lay a solid foundation for controlling the weld quality of fiber laser welding process.

Remote Laser Welding with High Power Fiber Lasers

The introduction of the high power fiber laser with brilliant beam quality has enabled the rapid development of remote laser welding (RLW). This paper presents a theoretical review of remote laser welding. As a promising technology, RLW offers increased flexibility, high operational speed, and reduced cycle time to process a wide range of workpieces. This study presents the typical characteristics of RLW with high power fiber lasers. It also investigates the influence of process parameters such as laser power, welding speed, shielding gas supply, beam inclination and focal position on the weld seam quality.

A comparative evaluation of microstructural and mechanical behavior of fiber laser beam and tungsten inert gas dissimilar ultra high strength steel welds

The influence of different welding processes on the mechanical properties and the corresponding variation in the microstructural features have been investigated for the dissimilar weldments of 18% Ni maraging steel 250 and AISI 4130 steel. The weld joints are realized through two different fusion welding processes, tungsten inert arc welding(TIG) and laser beam welding(LBW), in this study. The dissimilar steel welds were characterized through optical microstructures, microhardness survey across the weldment and evaluation of tensile properties. The fiber laser beam welds have demonstrated superior mechanical properties and reduced heat affected zone as compared to the TIG weldments.

HAZ Characterization and Mechanical Properties of QP980-DP980 Laser Welded Joints

The QP980-DP980 dissimilar steel joints were fabricated by fiber laser welding.The weld zone(WZ)was fully martensitic structure,and heat-affected zone(HAZ)contained newly-formed martensite and partially tempered martensite(TM)in both steels.The supercritical HAZ of the QP980 side had higher microhardness(~549.5 Hv)than that of the WZ due to the finer martensite.A softened zone was present in HAZ of QP980 and DP980,the dropped microhardness of softened zone of the QP980 and DP980 wasΔ21.8 Hv andΔ40.9 Hv,respectively.Dislocation walls and slip bands were likely formed at the grain boundaries with the increase of strain,leading to the formation of low angle grain boundaries(LAGBs).Dislocation accumulation more easily occurred in the LAGBs than that of the HAGBs,which led to significant dislocation interaction and formation of cracks.The electron back-scattered diffraction(EBSD)results showed the fraction of LAGBs in sub-critical HAZ of DP980 side was the highest under different deformation conditions during tensile testing,resulting in the failure of joints located at the sub-critical HAZ of DP980 side.The QP980-DP980 dissimilar steel joints presented higher elongation(~11.21%)and ultimate tensile strength(~1011.53 MPa)than that of DP980-DP980 similar steel joints,because during the tensile process of the QP980-DP980 dissimilar steel joint(~8.2%and 991.38 MPa),the strain concentration firstly occurred on the excellent QP980 BM.Moreover,Erichsen cupping tests showed that the dissimilar welded joints had the lowest Erichsen value(~5.92 mm)and the peak punch force(~28.4 kN)due to the presence of large amount of brittle martensite in WZ and inhomogeneous deformation.

High-power femtosecond laser generation from an all-fiber linearly polarized chirped pulse amplifier

An all-fiber high-power linearly polarized chirped pulse amplification(CPA)system is experimentally demonstrated.Through stretching the pulse duration to a full width of approximately 2 ns with two cascaded chirped fiber Bragg gratings(CFBGs),a maximum average output power of 612 W is achieved from a high-gain Yb-doped fiber that has a core diameter of 20μm with a slope efficiency of approximately 68%at the repetition rate of 80 MHz.At the maximum output power,the polarization degree is 92.5%and the M^(2)factor of the output beam quality is approximately 1.29;the slight performance degradations are attributed to the thermal effects in the main amplifier.By optimizing the B-integral of the amplifier and finely adjusting the higher-order dispersion of one of the CFBGs,the pulse width is compressed to 863 fs at the highest power with a compression efficiency of 72%,corresponding to a maximum compressed average power of 440.6 W,single pulse energy of 5.5μJ and peak power of about 4.67 MW.To the best of our knowledge,this is the highest average power of a femtosecond laser directly generated from an all-fiber linearly polarized CPA system.

Laser welding process and strength enhancement of carbon fiber reinforced thermoplastic composites and metals dissimilar joint:A review

Carbon fiber reinforced thermoplastic composites(CFRTP)and metals hybrid structures have been widely used in aircraft lightweight manufacturing.However,due to the significant difference in physical and chemical properties between CFRTP and metals,there are lots of challenges to connect them with high quality.Laser welding has a good application prospect in CFRTP and metals connection,and a significant research progress has been made in the exploration of CFRTP-metal laser joining mechanism,joining process optimization,joining strength improvement and joining defects controlling.However,there are still some problems need to be solved for this technology application.In this paper,the research progress of CFRTP-metal laser joining was summarized in three major aspects:theoretical modeling and simulation analysis,process exploration and parameter optimization,joint performance improvement and process innovation.And,problems and challenges of this technology were discussed,and the outlook of this research was provided.

微织构特征对铝合金-CFRTP激光焊接头力学性能的影响

该文研究了激光毛化微织构特征对铝合金-CFRTP激光焊接接头性能的影响,分析了激光毛化微织构的作用机理.结果表明,利用激光毛化技术可以在铝合金表面构建微织构,获得成形良好且性能优异的铝合金-CFRTP激光焊接搭接接头,接头处形成了有效的异种材料紧密嵌合.随着激光毛化微织构深宽比的增大,熔融CFRTP在铝合金表面微织构中由全部填充转变为部分填充,接头拉伸剪切性能呈现先增大后减小的趋势,接头失效机制呈现“混合断裂(粘附断裂+内聚断裂)-CFRTP母材断裂-混合断裂”的变化.当深宽比达到1.2~1.6时接头拉伸剪切性能值最大,此时断裂位置位于CFRTP母材.激光毛化微织构对铝合金-CFRTP接头强化机理为增强CFRTP在铝合金表面的机械嵌合作用,以及激光毛化微织构产生飞溅的“钉扎”作用.

Pump scheme optimization of an incoherently pumped high-power random fiber laser

Optical signal-to-noise ratio(OSNR) is one of the most significant parameters for the performance characterization of random fiber lasers(RFLs) and their application potentiality in sensing and telecommunication. An effective way to improve the OSNR of RFLs is pump scheme optimization, for example, employing a temporally stable source as the pump. In this paper, the output performance of an incoherently pumped RFL dependence on the pump bandwidth has been investigated both in experiment and theory. It is found that a high-OSNR RFL can be achieved with broadband amplified spontaneous emission(ASE) source pumping, and a relatively broad pump bandwidth can also help suppress the spectral broadening while maintaining an ultra-high spectral purity.By optimizing the pump bandwidth to ~10 nm, maximum OSNR of ~39 dB(corresponding to a spectral purity of ~99.96%) with more than 99 W output power can be obtained. Moreover, for the pump bandwidth of 0.6–40 nm, the spectral purity can reach as high as >99% with the pump power ranging from ~85 to ~117 W.In addition, with the aid of theoretical simulation based on a modified power balance model, we find that the increment of pump bandwidth can decrease the effective Raman gain coefficient, further influencing the gain characteristics, nonlinear effects, and eventually the output performance. This work provides new insight into the influence of the pump characteristics on the output performance of incoherently pumped RFLs.

Propagation of high-power fiber laser with high-order-mode content

Propagation properties of high-power fiber laser with high-order-mode(HOM) content are studied numerically for the first time to the best of our knowledge. The effect of HOM on the propagation property is evaluated by the power in the bucket(PIB) metric. It is shown that PIB is mainly dependent on HOM content rather than the relative phase between the fundamental mode and HOM. The PIB in vacuum is more than 80% when the power fraction of the HOM is controlled to be less than 50% at 5 km. The relative phase has an impact on the peak intensity position and concentration of the far-field intensity distribution. If an adaptive optics system is used to correct the peak intensity deviation, the results indicate that there exists a maximal value of PIB as relative phase increases. Such effect is weakened when propagating in turbulence. Compared to the laser beams without HOM, laser beams with HOM content are less influenced by the turbulence and can reduce average intensity fluctuation. The results may be useful in the design of a high-power fiber laser system.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot.At present,research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser.However,the suppression of spectral broadening,caused by self-phase modulation or four-wave mixing,is still a challenging attribute to the close distance between the broadened laser and signal laser.If using a traditional fiber grating with only one stopband to suppress the spectral broadening,the signal power will be stripped simultaneously.Confronting this challenge,we propose a novel method based on phase-shifted long-period fiber grating(PS-LPFG)to suppress spectral broadening in a high-power fiber master oscillator power amplifier(MOPA)laser system in this paper.A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique.The resonant wavelength of the fabricated PS-LPFG is 1080 nm,the full width at half maximum of the passband is 5.48 nm,and stopband extinction exceeds 90%.To evaluate the performance of the PS-LPFG,the grating is inserted into the seed of a kilowattlevel continuous-wave MOPA system.Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%,providing an effective and flexible way for optimizing the output linewidth of highpower fiber MOPA laser systems.

2080 nm long-wavelength, high-power dissipative soliton resonance in a dumbbell-shaped thulium-doped fiber laser

We demonstrate a 2080 nm long-wavelength mode-locked thulium(Tm)-doped fiber laser operating in the dissipative soliton resonance(DSR) regime. The compact all-fiber dumbbell-shaped laser is simply constructed by a 50/50 fiber loop mirror(FLM), a 10/90 FLM, and a piece of large-gain Tm-doped double-clad fiber pumped by a 793 nm laser diode. The 10/90 FLM is not only used as an output mirror, but also acts as a periodical saturable absorber for initiating DSR mode locking. The stable DSR pulses are generated at the center wavelength as long as 2080.4 nm, and the pulse duration can be tunable from 780 to 3240 ps as the pump power is increased. The maximum average output power is 1.27 W, corresponding to a pulse energy of 290 nJ and a nearly constant peak power of 93 W. This is, to the best of our knowledge, the longest wavelength for DSR operation in a mode-locked fiber laser.

400 μm stripe lasers for high-power fiber coupled pump modules

We present a 940 nm quasi-continuous wave semiconductor laser designed as a building block for high-power fiber coupled pump modules.The laser comprises a 400μm narrow-stripe array mounted on an aluminum nitride substrate using hard solder.The chip has been optimized for high optical power and low lateral far-field angles.Two vertical and six lateral structure variations have been investigated to determine the best achievable performance.Operating at 1 ms pulse width and a repetition rate of 10 Hz,the laser device reaches a maximum pulse power of 86 W from a 400μm aperture and more than 62%maximum conversion efficiency.Low lateral far-field angles(95%power enclosed)of11.5 and 13.5,depending on the epitaxial design,enable efficient multimode fiber coupling.The potential for highly reliable applications has been demonstrated.

激光功率对2195铝锂合金光纤-半导体激光复合焊接形貌与气孔的影响

光纤-半导体激光复合焊接技术充分结合了光纤与半导体激光热源的优势,在激光加工领域拥有巨大的潜力.针对2195铝锂合金开展光纤-半导体激光复合焊接试验,并定量研究激光功率对焊接形貌与气孔的影响.结果表明,光纤激光功率显著影响焊缝熔深,半导体激光功率显著影响焊缝上熔宽.基于回归分析方法建立焊缝横截面积预测模型.此外,光纤与半导体激光均对焊缝气孔缺陷的控制起着重要的作用,较高的光纤激光功率有利于降低气孔缺陷.对于4 mm厚2195铝锂合金,采用光纤激光功率为3.0 k W、半导体激光功率为2.5~3.0 k W时,熔池温度高且光纤-半导体激光复合作用范围大,焊接接头气孔缺陷少.

Dendritic Boundary Corrosion of AA2198 Weld Using Fiber Laser Welding with Al–Cu Filler Wire

The microstructures and corrosion behaviors of AA2198–T851 alloy and weld were analyzed under corrosive conditions.Weld was formed using an innovative fiber laser welding process with AA2319 Al–Cu filler wire. The metallurgic morphology and distribution of the chemical compositions were determined using imaging techniques such as optical micrograph, scanning electron micrograph, high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffraction. Corrosion was evaluated using an immersion test and electrochemical impedance spectroscopy in 3.5% NaCl solution at room temperature. Results indicate that the parent alloy suffered from pitting corrosion during the initial 4-h immersion which was caused by the inhomogeneous distribution of its chemical components and the different intermetallics formed during the rolling process. The weld experienced dendritic boundary corrosion under the same conditions due to the addition of the Al–Cu filler and rapid solidification during laser welding, which led to the precipitates Cu enrichment along the grain boundary. When a welding joint was immersed in the solution for 5 days, a big crack was observed across the center of the weld. In comparison, there was good corrosion resistance in the heataffected zone with a compact protective film.

Functional Fibers and Functional Fiber‑Based Components for High‑Power Lasers

The success of high-power fiber lasers is fueled by maturation of active and passive fibers,combined with the availability of high-power fiber-based components.In this contribution,we first overview the enormous potential of rare-earth doped fibers in spectral coverage and recent developments of key fiber-based components employed in high-power laser systems.Subsequently,the emerging functional active and passive fibers in recent years,which exhibit tremendous advantages in balancing or mitigating parasitic nonlinearities hindering high-power transmission,are outlined from the perspectives of geo-metric and material engineering.Finally,novel functional applications of conventional fiber-based components for nonlinear suppression or spatial mode selection,and correspondingly,the high-power progress of function fiber-based components in power handling are introduced,which suggest more flexible controllability on high-power laser operations.

光纤激光工艺参数对Q235钢穿透点焊缝形状和尺寸的影响

为了研究光纤激光工艺参数对穿透点焊缝形状和尺寸的影响,采用单因素试验方法,进行了不同激光功率、摆动幅度和焊接时间下的穿透点焊试验,分析了激光的功率密度分布、点焊缝的形状和尺寸的变化规律。结果表明,光纤激光光斑内的功率密度分布影响点焊缝的形状和尺寸。随着激光功率的增大,未摆动高斯光束形成点焊缝的形状由抛物线状逐渐转变为钟铃状,点焊缝的熔宽、熔深、深宽比和熔宽比都随之增大。光束摆动增大了激光与材料的作用面积,改变了功率密度分布和点焊缝的形状和尺寸。随着摆动幅度的增大,点焊缝的形状由钟铃状逐渐转变为碗盆状,点焊缝的熔深和深宽比都逐渐减小而熔宽比逐渐增大。当激光功率密度不变时,焊接时间对点焊缝的形状影响不大。随着焊接时间的增大,点焊缝的熔深和熔宽都逐渐增大,但深宽比基本保持不变。

高速保护气流向对光纤激光深熔焊接羽辉的影响

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.

匙孔内光纤激光致喷发蒸汽对焊接过程的影响

光纤激光深熔焊接羽辉由匙孔内激光致蒸汽喷发所致,对焊接过程存在严重的负面影响.文中通过改变匙孔内激光致蒸汽的喷发特征,研究羽辉对光纤激光深熔焊接过程的影响规律.结果表明,随着焊接速度的提高,沿焊接方向的匙孔口长度逐渐增大,匙孔前壁的倾斜角则逐渐减小.该现象导致孔内激光致喷发蒸汽的特征发生变化:底部摆动羽辉的喷发方向逐渐沿焊接反方向偏离激光束,狭长形羽辉的高度则逐渐降低直至消失;羽辉对焊接熔深的负面影响也逐渐减小直至消失,但飞溅数量逐渐增多,焊缝表面成形则逐渐恶化.进一步分析表明,匙孔前壁激光致蒸汽的喷发方向变化是底部摆动羽辉的喷发方向和狭长形羽辉高度均发生改变的主要原因;提高焊接速度可降低羽辉对焊接过程的负面影响,但匙孔前壁激光致蒸汽对匙孔后壁的冲击作用将导致孔口沿焊接方向的长度变大、飞溅增多、焊缝表面成形质量变差.创新点:(1)原位观测了不同焊接速度下羽辉和匙孔的形态,获得了底部摆动羽辉、狭长形羽辉随焊接速度的变化规律及其影响焊接过程的规律.(2)基于匙孔内激光致蒸汽喷发方向和匙孔前壁倾斜角度的变化,揭示了底部摆动羽辉和狭长形羽辉形态发生变化的原因.(3)分别揭示了底部摆动羽辉和狭长形羽辉对焊接过程的影响,为光纤激光焊接羽辉的控制及光纤激光焊接技术的优化奠定了基础.