CO2激光焊接不锈钢光致等离子体动态特性分析

光致等离子体是CO2激光焊接中一个重要的物理现象，它与焊接的稳定性、焊接质量及能量利用率等都有密切的关系。利用高速摄像及光信号监测两种手段对等离子体的动态变化过程及其对焊接稳定性的影响进行了深入的研究。提出了等离子体的变化每个周期内分为四个阶段，而影响焊接稳定性的根本原因是等离子体在穿透与未穿透之间的波动。     

激光焊接质量实时检测技术的研究现状

从检测信号和内容两个方面介绍了国内外激光焊接质量实时检测技术的研究现状。用于检测的信号主要有可听声波、超声波、紫外辐射、可见光、红外辐射、等离子体电信号以及机器视觉等，分析了每种信号的优缺点；目前实时检测的内容主要有焊接过程的稳定性、装配质量、离焦量、焊缝熔深和表面形貌、熔池与小孔的形态等。     

Weld pool geometry during keyhole laser welding of thin steel sheets

Abstract

In industrial applications of laser welding it is often essential to obtain full penetration welds at high processing rates using minimal heat input. Keyhole welding meets these requirements when process parameters are kept close to the boundary where complete penetration switches to partial penetration welding. In the present work weld pool behaviour at the edge of the full penetration regime has been studied. Four types of keyhole penetration mode were observed. The first type is a completely developed keyhole through the material thickness and open in the root region, whereas the second type is closed at the root. The third mode is unstable and results in intermittent penetration involving periods of open and closed keyhole conditions interspersed with periods of lack of fusion. The fourth mode is a partial penetration mode. A possible explanation of the weld pool transient behaviour is presented based on three-dimensional reconstructions of the weld pools.     

Study on thick film resistor and electrode fabricated by laser micro-cladding electronic pastes

Nowadays, electronic devices are required to be smaller, to have a high-density integration and to be of lower cost. In order to meet the developing demands, some new techniques have appeared in recent years. In this paper, a technique named laser micro-cladding electronic pastes is used to fabricate the thick film resistor and electrode units on insulator substrates without using a mask. The results demonstrate that the high-quality resistors could be fabricated by laser micro-cladding. However, the processing steps are very important since they have significant effects on the surface finish, the extent of diffusion between electrode film and resistor film, and on the reliability of the resistors. Laser micro-cladding electronic pastes may become a novel technique to fabricate high quality conductive lines (electrodes) and resistors with good properties that will come into industrial application in the future.     

Deformation of specimen during laser surface remelting

During laser surface remelting, thermal expansion in heating, and contraction of material in cooling, and due to microstructure changes in the heated surface layer, the specimen deforms. This induces volume changes during the process of remelting the thin surface layer and causes internal stresses. By measuring the deformation of the specimen during the remelting process as well as after the laser beam interaction, i.e., to the moment when the specimen cools down to the ambient temperature, it is possible to follow the progress of deformation in the specimen material. Detailed information on the deformation events in the specimen material during the laser remelting process of a thin surface layer enables the engineer to optimize the process variables in terms of amount of deformation or magnitude of residual stresses on completion of cooling.     

Microstructures and properties of laser/arc hybrid welds and autogenous laser welds in pipeline steels

Abstract

Lasers are capable of producing welds with deep penetration, low distortion and faster travel speeds, compared to arc welding. More recently, laser/arc hybrid welding processes have also been generating interest for industrial fabrication. In this paper, six carbon–manganese, mainly pipeline, steels were welded using both autogenous Nd:YAG laser welding, and Nd:YAG laser/MAG hybrid welding. The improvements in weld microstructures and weld metal toughness that are possible when using the hybrid process are described and illustrated. Laser/arc hybrid welding is shown to be a process that can generate good quality welds in commercially available pipeline steels. It also has the potential to complete girth welds in these steels with significantly fewer welding passes than are currently required for arc welded pipelines, reducing the joint completion time.     

基于激光视觉的钢结构焊缝图像处理系统

针对钢结构件品种多、批量小,焊缝形状位置一致性差,机器人重复定位过程复杂等缺点,设计了一种基于激光视觉的钢结构焊缝图像处理系统. 运用CCD工业相机和激光器,采集带有激光条带的焊缝图像,分别利用中值滤波柔化噪音,Otsu算法自适应阈值分割,开操作和形态学处理相结合去除图像中除目标像素外的小连通区域,提取激光条带的中心线,最终利用Hough变换对中心线直线拟合,得到特征点位置,并通过骨支架试验验证该技术的可行性. 结果表明,该方法可快速准确地检测到焊缝特征点,满足实际要求.     

Deep penetration dispersing of aluminum with TiB2 using a single mode fiber laser

Today laser alloying/dispersing of Aluminum is a well-known technique to increase the wear resistance of parts. The alloyed/dispersed layer is typically limited in depth to about 1–1.5 mm. For Aluminum the absorptivity of near infrared wavelengths is comparably low. This makes conventional laser alloying/dispersing inefficient. In contrast, the overall absorption of the laser power using the deep penetration effect is comparatively high. The comparison of the principles of the conventional laser alloying process and the deep penetration alloying process is shown in this study. In deep penetration alloying, the track geometry and the distribution of the filler material can both be influenced by the process parameters. It is shown that a deeper alloying is possible by using the deep penetration effect caused by a highly intensive laser beam. The laser beam is scanned in a circular or other motion in order to control the shape of the track. By this manner of alloying it is possible to realize nearly rectangular and comparable deep track geometries. The dependencies of the penetration depth on the process parameters will show a way how to compromise between the optimal distribution of the filler material and the geometry of the track in order to optimize the whole process.     

Spectroscopic monitoring of penetration depth in CO2 Nd:YAG and fiber laser welding processes

A method for on-line monitoring of the joint penetration depth is reported, based on the spectroscopic analysis of the optical emission collected from laser-metal interaction zone. This technique has been applied to quite diverse welding procedures (c.w. CO₂ laser, cw fiber laser and pulsed Nd:YAG laser) of stainless steel plates. Although the acquired spectra reveal different characteristics according to the employed laser source, a discrete contribution of several iron lines can be highlighted in both types of investigated welding processes. Starting from the measurement of the intensities of those lines it is possible to calculate the plasma electron temperature, which was found to decrease as far as the laser power is enhanced together with the penetration depth, in static as well as dynamic conditions. Such a behavior does not correspond to a real decrease of the plasma plume temperature but has to be ascribed to the position of the optical collimator collecting light only from the top of the keyhole. Indeed, for deeper penetrations the hottest core of the plume moves down into the vaporized capillary so that the plasma temperature measured on top of the keyhole appears lower at higher incident powers. The electron temperature signal could be useful for the development of a feedback loop system of the laser power to control the weld penetration depth.     

Experimental study on welding characteristics of CO2 laser TIG hybrid welding process

Abstract

The experiments of CO₂ laser TIG paraxial hybrid welding with 4 mm thick AISI 321 stainless steel sheet have been performed. The arc images and welding characteristics have been investigated with different energy ratios between laser and arc. The experimental results indicate that the hybrid welding is similar to laser welding and has also two welding mechanisms: deep penetration welding and heat conduction welding. Because of the effect of keyhole induced by laser, the arc root can be stabilised and compressed, and the current density and the penetration depth are all increased significantly, which show the characteristics of deep penetration welding. However, when the current is increased to a critical value, the laser induced keyhole disappears and the arc expands obviously, which decreases the penetration depth, so that the welding mechanism has been changed from deep penetration welding to heat conduction welding. Furthermore, the effects of distance between laser beam and electrode, pulsed laser and hybrid manners on hybrid welding characteristics have also been studied.     

Hybrid laser arc welding of X80 and X120 steel grade

The aim of the present work was to investigate the possibilities of hybrid laser arc welding regarding reliable production of longitudinal welds of high strength pipe steels X80 and X120 and to evaluate achievable mechanical properties of laser hybrid welds. The study focused on weld toughness examination in low temperature range up to ?60°C. Suitable filler materials were identified in the context of this task. It could be shown that metal cored electrodes guaranteed sufficient Charpy impact toughness at low temperature for both investigated materials. Modern arc welding technologies such as modified pulsed spray arc were used to promote deeper penetration of the filler material into the narrow laser welding gap. Edge preparation with a 14 mm deep root face was considered as optimum, because no penetration of the filler material could be detected beyond this depth limit, and therefore, any metallurgical influences on the weld metal properties through the welding wire could be excluded.     

非穿透激光深熔焊熔深与等离子体光信号的关系

对于非穿透激光深熔焊而言 ,焊接熔深是最重要的质量参数之一 ,对其进行实时监测具有重要意义。通过采集和分析激光焊接过程中等离子体光信号以探索焊接熔深的实时监测技术。研究结果表明 ,当激光功率不变时降低焊接速度 ,或焊接速度不变时提高激光功率 ,在焊缝熔深加大的同时 ,采集到的等离子体光信号强度亦呈明显上升趋势 ;在工艺参数不变的同一焊缝内 ,对应焊缝光信号的频谱图中存在明显主频 ,且随着焊接线能量的提高 ,主频率下降 ;同一焊缝内由于某些不稳定因素导致局部焊缝熔深变化超过 8%时 ,等离子光强明显下降 ,而对应该段信号的幅值谱不存在明显主频率。     

Sub-nanosecond monitoring of micro-hole electrical discharge machining pulses and modeling of discharge ringing

Monitoring the gap voltage and current in micro-hole electrical discharge machining (EDM) using high-speed data acquisition with 0.5 ns sampling period is conducted. The spark and arc pulses at three stages, namely electrode dressing, drilling, and penetration, of the micro-hole EDM are recorded. The EDM process parameters are setup to use negative polarity to blunt the electrode tip and positive polarity for micro-hole drilling and penetration. A new phenomenon of pre-discharging current is discovered. In the first 20–30 ns of spark and arc pulses, the current starts to rise while the voltage remains the same. Effects of EDM process parameters, including the open voltage, electrode diameter, and polarity, on the rate of spark and arc pulses and electrode feed rate are investigated. A model based on the RLC circuit is developed to study the ringing effect at the end of a discharge. The intrinsic parasitic capacitance and resistance of a RLC circuit are calculated from the decaying voltage signal and compared under two sets of experiments with varying wire electrode diameter and gap voltage to validate the ringing model. The calculation and experimental results validate the proposed RLC model for ringing phenomenon. The model shows the electrode diameter has negligible effect on ringing and high open voltage increases the parasitic resistance and damping in ringing. The monitoring technique and ringing model developed in this research can assist in the selection and optimization of micro-hole EDM process parameters.     

CO2激光焊接的同轴检测与熔透控制Ⅲ.板厚变化时熔透状态同轴光信号分析

利用CO2激光深熔焊等离子体光信号的同轴检测系统,对工件板厚变化时激光焊接熔透状态的同轴光信号进行了时域和频域分析.结果表明,"适度熔透"状态时对应的等离子体同轴光信号强度最大,"未熔透"与"过熔透"状态对应的信号强度均小于"适度熔透";频域分析表明,"适度熔透"状态对应的2～6kHz段频谱重心最小.     

短电弧数控铣削加工的优化

基于不同电极材料的加工特性,采用空心柱状石墨电极对特硬超强难切削材料——镍基高温合金GH4169进行高速电弧放电加工。实验表明:在气液混合介质下(气体0.3 MPa,液体0.1 MPa),电压为25 V,占空比为50%,频率为500 Hz时,空心柱状石墨电极能够达到相对最优的材料去除率和表面质量,且电极损耗相对较小。同时为了避免加工中出现短路现象,基于高速数据采集系统,以PCI-1716数据采集卡为硬件平台,借助于Advantech 32位DLL驱动程序接口,采用Borland Delphi 7.0 Pascal语言进行硬件驱动和控制,实现短电弧加工中电流信号的实时监控。     

激光深熔焊声源发声机理

声信号是激光焊接过程中重要的质量检测参量，目前对于激光声信号的发声机理认识还不够深刻.文中对激光深熔焊过程中声源发声机理和熔池小孔形貌进行了深入研究.首先参考声学中亥姆霍兹效应，通过声电类比，得到小孔形貌与声音信号强度和共振频率的关系.采用CCD图像采集系统获得小孔实际形貌，验证理论研究所得结果的合理性.通过声信号采集系统获取给定焊接工艺参数下的激光焊接声场数据，进行快速傅里叶变换处理，获得声场幅值谱图.结果表明，试验数值与理论值相吻合，证明所提出的小孔发声机理具有较好的适用性.     

钛合金激光焊接过程中等离子体光信号的检测与分析

通过试验的方法探讨了钛合金激光焊接过程中的等离子体光信号与焊接质量的关系，结果分析表明，钛合金的激光焊接过程比低碳钢产生更强烈的等离子体云，等离子体光信号可以较好地反映焊接过程的稳定性和焊接质量的变化。焊接质量良好时，等离子体光信号幅值在某一固定值上下波动，当改变焊接参数使焊接熔深或熔宽改变时，信号的平均幅值亦有明显改变；当较大间隙或错边存在时，等离子体光信号的平均值明显降低，但两种信号的特征不尽相同，前者造成等离子体光信号剧烈动荡而均值很低，而后者一定范围内，随着错边量增加，信号幅值逐渐减小。上述试验结果表明，可以用等离子体光信号的幅值高低和波动大小作为焊接质量实时监测和控制的特征信号。     

A laser straightness measurement system using optical fiber and modulation techniques

In this paper a laser straightness measurement system is proposed. Using the characteristics of travelling straight and parallel, a new diode laser system utilizing optical fiber and modulation techniques was developed. This optically aligned beam is used to establish a reference line for the straightness measurement of any mechanical system. A four-quadrant photo detector was used to receive the laser beam. A phase sensitive detecting technique was adopted to process the opto-electrical signals from the detector. A single board microcomputer employing an 8098 processor was developed to conduct all data acquisition and signal processing. The influence from disturbing lights, circuit drift and beam drift can be removed from the system. The system was calibrated using an HP5528 laser interferometer. Results showed good accuracy of 0.3 μm within the range of ±100 μm and pointing stability of 0.3 μm at a 1 m distance was obtained. Two practical testing examples are carried out on a CMM and a grinding machine, respectively. Repeatability of both tests was found within 0.5 μm. This low-cost system has been successfully applied to the straightness measurement of a precision CMM.     

薄板钛合金激光焊熔透稳定性临界条件的计算

钛合金薄板熔透激光焊研究发现,在一定焊接参数条件下,由于金属蒸气和光致等离子体的作用,即使焊接过程的工艺参数稳定不变,也存在全熔透不稳定过程,其特征是焊缝表面成形均匀不变,而焊缝背面出现熔透与未熔透之间交替跳跃的成形特征,这种不稳定焊接过程属于激光深熔焊过程的本征特性,主要取决于焊接过程穿孔形成的稳定性。基于小孔形成机理和孔壁能量平衡的分析,提出了小孔穿孔速度与焊接速度相匹配的熔透稳定性物理模型,将穿孔速度与激光功率密度、焊接速度、材料物理性能、板厚联系起来,并建立了熔透焊所需最小激光功率密度计算关系式,理论计算结果与试验结果基本一致。所建立的关系式可用于判断焊接工艺参数深熔焊熔透稳定性。     

基于高通量测试方法的高功率激光焊接工艺特性分析

为了更加高效、精准的获得工艺参数对高功率激光焊接工艺特性的影响规律，提出了激光焊接特性分析用高通量测试方法，并借助高速摄影和图像处理技术对测试过程中的光致羽辉和飞溅进行了统计分析. 结果表明，随着激光功率增加，焊接过程中飞溅数量与波动程度增加，对应焊后表面成形质量变差，内部裂纹增加；对于万瓦级激光焊接，相较于正离焦焊接，负离焦量焊接时的焊缝成形更为优异，焊接过程飞溅数量得到有效抑制，熔深波动程度也得到明显改善；实际焊接结果与激光焊接特性高通量测试结果高度一致，这说明基于高通量测试方法的激光焊接特性分析方法可以真实、快速的反映出工艺参数变化时对焊接特性的影响规律，是一种切实可行的激光焊接特性分析新方法.