Novel inspection of welded joint microstructure using magneto-optical imaging technology

A novel method for measuring differences of microstructure by advanced use of the Faraday magneto-optical effect is proposed. Two groups of YAG laser welds on Q235 have been investigated in order to compare MO imaging and traditional methods. Microstructure images have been compared with MO images, and MO diagrams display different colors and gray scales for the base metal, the weld zone, and the heat affected zone. Experimental results indicate that the welded joint microstructure can be inspected by MO imaging without metallographic preparation.     

Parametric optimisation and microstructural analysis on high power Yb-fibre laser welding of Ti–6Al–4V

In this work thin sheets of Ti–6Al–4V were full penetration welded using a 5 kW fibre laser in order to evaluate the effectiveness of high power fibre laser as a welding processing tool for welding Ti–6Al–4V with the requirements of the aircraft industry and to determine the effect of welding parameters including laser power, welding speed and beam focal position on the weld microstructure, bead profile and weld quality. It involved establishing an understanding of the influence of welding parameters on microstructural change, welding defects, and the characteristics of heat affected zone (HAZ) and weld metal (WM) of fibre laser welded joints. The optimum range of welding parameters which produced welds without cracking and porosity were identified. The influence of the welding parameters on the weld joint heterogeneity was characterised by conducting detailed microstructural analysis.     

焊缝结构中超声相控阵聚焦声场的数值模拟及分析*

针对焊缝内相控阵声场聚焦问题,建立焊缝结构中多高斯声束法的相控阵声场计算模型,分别用直接聚焦和底面反射聚焦两种方式对焊缝内部的相控阵声场进行数值模拟,分析不同方式下的相控阵聚焦声场特性,讨论不同区域适用的聚焦方式,分析了界面倾斜角度对聚焦效果的影响。结果表明,在相同焊缝结构条件下直接聚焦的方式更适合对厚壁焊缝的中下部进行聚焦扫描,在焊缝上表面附近,当声束偏转角大于80°时,直接聚焦方式无法有效聚焦;反射聚焦的方式更适合对焊缝上表面附近区域进行聚焦扫描,随着焦点深度增加,反射聚焦的声束偏转角增大,当声束偏转角大于55°时,反射聚焦无法形成明显焦点;两种聚焦扫描方式可形成有效的互补。当设置的焦点固定时,直接聚焦的实际焦点随界面倾斜角度增大向预设点远处偏移,而反射聚焦的效果基本不受倾斜角的影响,实际焦点始终在预设焦点附近。     

Pulsed Nd:YAG laser welding of AISI 304 to AISI 420 stainless steels

The technique to weld AISI 304 stainless steel to AISI 420 stainless steel with a pulsed Nd:YAG laser has been investigated. The main objective of this study was to determine the influence of the laser beam position, with respect to the joint, on weld characteristics. Specimens were welded with the laser beam incident on the joint and moved 0.1 and 0.2 mm on either side of the joint. The joints were examined in an optical microscope for cracks, pores and to determine the weld geometry. The microstructure of the weld and the heat affected zones were observed in a scanning electron microscope. An energy dispersive spectrometer, coupled to the scanning electron microscope, was used to determine variations in (weight %) the main chemical elements across the fillet weld. Vickers microhardness testing and tensile testing were carried out to determine the mechanical properties of the weld. The results of the various tests and examinations enabled definition of the best position for the incident laser beam with respect to the joint, for welding together the two stainless steels.     

大功率光纤激光焊焊缝跟踪偏差红外检测方法

精确控制激光束使其始终对准并跟踪焊缝是保证激光焊接质量的前提.针对大功率(激光功率10 kW)光纤激光焊接304型不锈钢紧密对接焊缝(间隙为0-0.1 mm),研究一种基于红外热像的焊缝跟踪偏差检测新方法. 采用红外传感高速摄像机摄取焊接区域熔池红外动态热像,分析激光束对准和偏离焊缝中心时的熔池温度分布和红外辐射特性,以熔池匙孔形变参数和热堆积效应参数作为激光束与焊缝中心偏差检测特征值,通过图像识别技术研究和分析特征值与焊缝偏差之间的关系. 激光焊接试验结果表明,熔池匙孔形变参数和热堆积效应参数与焊缝偏差 关键词： 大功率光纤激光焊 焊缝跟踪偏差 红外热像 检测     

Analysis of a fibre laser welding case study, utilising a matrix flow chart

For fibre laser welding of an eccentric corner joint, the quality of the resulting weld cross section was studied with respect to the dependence on process parameters like lateral laser beam alignment, beam inclination, focal plane position or welding speed. The complex load situation of the support beamer was simplified to bending of one corner. Due to fatigue load, the weld properties causing the peak stress are essential, in particular the top and root shape of the weld cross section. For the parameters varied, the resulting shapes were categorized into different top and root classes, determined by certain key dimensions, considering also welding defects like undercuts. The shapes are boundary conditions for Finite Element Analysis of the joint under load for quantitative comparative analysis of the maximum stress. As two high strength steel grades were joined, the hardness transition across the weld was of interest, too. High speed imaging of the weld pool surface shape provided additional information on the relation between the parameter input and quality output. The different trends identified were discussed and guidelines were derived. As the systematic documentation of results is unsatisfactory in welding, a new method was developed and applied for the first time, called the Matrix Flow Chart. It enables an illustrative view on the resulting welding trends in a combined manner and is extendable by other researchers.     

Laser welding with filler wire

New applications such as welding of material combinations and the ability to fill opening gaps between the workpieces offer new prospects for laser beam welding processes with filler wire. To guarantee good quality, vertical distance variations between wire tip and weld pool are, above all, not permissible as this causes globular metal transfer and would accordingly result in strongly rippled, unclean welds. A process-internal signal, recorded by a sensor, helps to solve this problem. The automatic tracking of the vertical wire position is possible on-line via a controller. In this way, the running process can be optimized and a consistently good weld quality can be achieved.     

奥氏体不锈钢焊缝中的声学仿真模型与全聚焦成像检测

声波在奥氏体不锈钢焊缝中传播时,由于声束弯曲,为超声成像带来了困难。我们基于Ogilvy焊缝模型,建立了奥氏体不锈钢焊缝非均匀各向异性声场仿真模型,采用Dijkstra路径搜索算法对各向异性条件下的声传播路径和声传播时间进行了数值模拟和分析。在此基础上,采用Verasonics超声相控阵成像系统,进行了奥氏体不锈钢焊缝的全聚焦成像实验。结合理论模型计算的声传播路径和相应的传播时间,对采集到的全矩阵回波数据进行了成像检测。结果表明,与均匀介质模型的全聚焦成像结果相比,基于本文模型的焊缝全聚焦成像检测结果具有更高的缺陷定位精度和分辨率,验证了本方法的可行性,为奥氏体不锈钢焊缝成像检测提供了新的思路。     

Magnetooptical images of inhomogeneous magnetic fields in metallic films with in-plane anisotropy

The magnetooptical (MO) images of the inhomogeneous field created by permanent magnets in magnetic metallic films with in-plane anisotropy are experimentally studied. The MO images recorded using the longitudinal Kerr effect are the superposition of two pictures, namely, a polar-sensitivity MO image and a longitudinal-sensitivity MO image. An analysis of these images after separation shows that the polar-sensitivity MO image reflects the distribution of the inhomogeneous field component that is normal to the surface of an indicator film in an analog manner. The longitudinal-sensitivity MO image reflects the angular distribution of the in-plane component of a stray field in an analog manner. The coincidence of the experimental and corresponding simulated MO images makes it possible to interpret the experimental images. In particular, it is shown that the specific features detected in the topological characteristics of the inhomogeneous field correspond to experimental singular points. Hidden magnetic images (magnetic bar codes) are shown to be visualized with metallic CoFe films. As an example, the stray field of a magnetic system made of cylindrical magnets is mapped.     

Effect of magnetic field on plasma control during CO2 laser welding

During high power CO₂ laser beam welding, the plasma above the keyhole has a shielding effect that it not only absorbs part of the laser energy but also defocuses the laser beam. As a result, the welding efficiency and the aspect ratio of the welds are influenced. In order to reduce the effect of plasma, helium as a plasma control gas has been used successfully and effectively. However, the cost of helium in Southeast Asia is extremely high and therefore the production cost is significantly increased when helium is used as a continuous bleeding plasma control gas. To search for an alternative plasma control technique, feasibility in using magnetic effect as a control tool is explored in this paper. The influences of the magnetic field strength, laser power, welding speed, field direction and shielding gas (e.g. helium and argon) on the penetration depth and the width of bead were also investigated. Experimental results indicated that the magnetic field can influence the shielding effect of the plasma without using plasma control gas. It was found that at a suitable magnetic field strength the penetration depth was increased by about 7%, but no significant difference on the width of bead was found. Moreover, it was shown that the plasma control effect can be achieved at low magnetic field strength and the penetration depth can be increased significantly under argon atmosphere.     

Optimization of laser welding of DP/TRIP steel sheets using statistical approach

Generally, the quality of a weld joint is directly influenced by the welding input parameter settings. Selection of proper process parameters is important to obtain the desired weld bead profile and quality. In this research work, numerical and graphical optimization techniques of the CO₂ laser beam welding of dual phase (DP600)/transformation induced plasticity (TRIP700) steel sheets were carried out using response surface methodology (RSM) based on Box–Behnken design. The procedure was established to improve the weld quality, increase the productivity and minimize the total operation cost by considering the welding parameters range of laser power (2–2.2 kW), welding speed (40–50 mm/s) and focus position (?1 to 0 mm). It was found that, RSM can be considered as a powerful tool in experimental welding optimization, even when the experimenter does not have a model for the process. Strong, efficient and low cost weld joints could be achieved using the optimum welding conditions.     

Development of computer-assisted minimal-dose system with beam blanker for TEM

A computer-assisted minimal-dose system has been developed for the high-resolution observation of radiation-sensitive samples using a transmission electron microscope (TEM). This system consists of a CCD camera, a beam blanker and a control computer (PC) that also controls the TEM. A sample is illuminated by an electron beam only when the CCD camera takes images; otherwise, the beam blanker cuts off the electron beam. Emulated images, which are calculated from the images taken and the variable parameters of the TEM, such as magnification and sample stage position, are displayed on the control PC display. After a few times of repetition of exposures and emulations, the sample is positioned to final observation area. Subsequently to select the observation area, the system automatically adjusts the focus position from two different illuminating angle images and the TEM conditions are appropriate for taking a final image. The total electron dose before the final image is taken can be markedly decreased because the sample is irradiated by an electron beam only when images are taken. For the final image, a three-dimensional Fourier filtering method (3DFFM), which corrects the spherical aberration for the through-focus image series and enables the selection of the optimum focus condition later, is also included in our system.     

W波段同轴静磁波荡器自由电子激光优化设计

基于同轴静磁波荡器的自由电子激光主要利用环形电子束与同轴TE模式的相互作用产生电磁辐射,是一种重要的毫米波辐射源。分析了同轴结构作用区内外半径、工作模式、电子束电压、波荡器周期等参数对辐射频率的影响规律,研究了电子束平均半径的选取原则和束波互作用腔的设计方法,设计了辐射频率在W波段的基于同轴静磁波荡器的自由电子激光,并进行了粒子模拟,在电子束电压为720 kV,电子束流为1 kA的情况下,获得了频率107 GHz、辐射功率35 MW的TE01模,束波转换效率为4.9%,束波作用腔总长度小于200 mm,同轴静磁波荡器的磁场幅度0.34 T。     

Detecting magnetic field direction by a micro beam operating in different vibration modes

A new method to detect the magnetic field direction by using a silicon structure is presented in this paper. The structure includes a micro beam and an in-plane coil electrode. When the electrode under a magnetic field is applied with an alternating current, the micro beam is actuated under the effect of the Lorentz forces. Magnetic fields of different directions cause different vibration profiles. The direction of the magnetic field is obtained by measuring the vibration amplitudes of the micro beam, which is driven to work at first- and second-order resonant modes. A micro structure has been fabricated using the bulk micromachined silicon process. A laser Doppler vibrometer system is implemented to measure the vibration amplitudes. The experimental results show that the amplitude of the structure, which depends on the different modes, is a sine or cosine function of the angle of the magnetic field. It agrees well with the simulation result. Currently a resolution of 10° for the magnetic field direction measurement can be obtained using the detecting principle.     

Characterization of Kovar-to-Kovar laser welded joints and its mechanical strength

For the packaging of a pump laser in butterfly package, the most crucial assembly step is the fiber-to-laser diode coupling and attachment. The use of laser welding as the joining method offers several advantages if compared with the adhesive joints: strong joining strength, short process time and less contamination. This paper reports on laser welding process characteristics; weld strength and its fracture mode. The penetration depth and melt area of laser spot welds were found to be complicated functions of laser pulse energy, intensity, and beam diameter. Effects of pulse width, input power and size of the focal spot on the rate of energy input to the workpieces and consequently, the weld strength were reported. The weld strength was found to be dependent on the overlapping area between the two joining materials. Surface roughness, R_a, has influence on the fraction of energy absorbed, A, and therefore, affecting the penetration depth. Thermal analysis was carried out on the laser-welded joints and its heat-affected zone (HAZ) induced by various power densities was examined. These data are important in order to optimize and utilize the laser welding process as an effective manufacturing tool for fabrication of reliable pump laser.     

兰姆波在搭接焊缝上的反射和透射

采用数值模拟方法和实验研究了兰姆波在搭接焊缝上的反射和透射。数值模拟了频率500 kHz的兰姆波S0与.A0模态,分别在2 mm厚不锈钢板上的不同宽度搭接焊缝模型上的反射和透射,并计算了相应的反射系数和透射系数。发现:当搭接焊缝宽度小于4 mm时,S0模态的反射系数和焊缝宽度具有近似的线性关系,而焊缝余高和两板之间的间隙对S0模态反射系数的影响很小。基于兰姆波S0模态反射系数与焊缝宽度的线性关系。提出了通过测量S0模态反射系数快速测量搭接焊缝宽度的方法。实验测得的2 mm厚不锈钢板上不同宽度搭接焊缝对500 kHz的入射兰姆波S0模态的反射系数,验证了上述线性关系,说明了可用兰姆波反射回波测量搭接焊缝宽度。      

Optimal design for laser beam butt welding process parameter using artificial neural networks and genetic algorithm for super austenitic stainless steel

Laser welding input parameters play a very significant role in determining the quality of a weld joint. The joint quality can be defined in terms of properties such as weld bead geometry, mechanical properties and distortion. Therefore, mechanical properties should be controlled to obtain good welded joints. In this study, the weld bead geometry such as depth of penetration (DP), bead width (BW) and tensile strength (TS) of the laser welded butt joints made of AISI 904L super austenitic stainless steel were investigated. Full factorial design was used to carry out the experimental design. Artificial Neural networks (ANN) program was developed in MatLab software to establish the relationships between the laser welding input parameters like beam power, travel speed and focal position and the three responses DP, BW and TS in three different shielding gases (Argon, Helium and Nitrogen). The established models were used for optimizing the process parameters using Genetic Algorithm (GA). Optimum solutions for the three different gases and their respective responses were obtained. Confirmation experiment has also been conducted to validate the optimized parameters obtained from GA.     

非稳腔输出环状激光束的传输特性

本文从菲涅耳衍射积分公式出发研究了激光器非稳腔输出环状光束在空间的传输规律.由轴上光强的极值点位置给出了划分非稳腔传输空间的近场、中场和远场区域公式.理论计算结果表明,非稳腔输出端环状光束在中场区域激光能量逐渐向轴心方向转移,而在远场变为爱里型圆盘状光束的过程.在非稳腔脉冲TEA CO_2激光器上测得的实验结果与理论计算结果很好地符合.     

Aberrations theory of a time dependent magnetic field lens

We analyze new forms of aberrations affecting a time dependent magnetic field lens recently proposed. The lens consist of an axially symmetric ellipsoidal solenoid producing a spatially uniform but time pulsating homogeneous field. The dynamics of electrons emitted by some object and injected into this coil shows that this system is capable of focusing a parallel beam or produce images free of geometrical aberrations. The cardinal elements of this lens change periodically in positions and therefore to obtain a stationary image it is necessary to employ a chopped electron beam. Two forms of aberrations originate from the finite duration of the electron pulse and the energy dispersion of the incoming beam (chromatic aberration). Their image degradation effects are quite similar and result from recording different axially displaced images at some fixed stationary plane. We show how, by adjusting the parameters characterizing the magnetic pulse and introducing small apertures that increase the depth of focus, their effects can be reduced or in some cases eliminated. Other forms of instrumental aberrations characteristic of this time dependent magnetic field lens are briefly discussed.     

Aberrations theory of a time dependent magnetic field lens

We analyze new forms of aberrations affecting a time dependent magnetic field lens recently proposed. The lens consist of an axially symmetric ellipsoidal solenoid producing a spatially uniform but time pulsating homogeneous field. The dynamics of electrons emitted by some object and injected into this coil shows that this system is capable of focusing a parallel beam or produce images free of geometrical aberrations. The cardinal elements of this lens change periodically in positions and therefore to obtain a stationary image it is necessary to employ a chopped electron beam. Two forms of aberrations originate from the finite duration of the electron pulse and the energy dispersion of the incoming beam (chromatic aberration). Their image degradation effects are quite similar and result from recording different axially displaced images at some fixed stationary plane. We show how, by adjusting the parameters characterizing the magnetic pulse and introducing small apertures that increase the depth of focus, their effects can be reduced or in some cases eliminated. Other forms of instrumental aberrations characteristic of this time dependent magnetic field lens are briefly discussed.