Optical techniques for real-time penetration monitoring for laser welding

Optical techniques for real-time full-penetration monitoring for Nd:YAG laser welding have been investigated. Coaxial light emission from the keyhole is imaged onto three photodiodes and a camera. We describe the spectral and statistical analyses from photodiode signals, which indicate the presence of a full penetration. Two image processing techniques based on the keyhole shape recognition and the keyhole image intensity profile along the welding path are presented. An intensity ratio parameter is used to determine the extent of opening at the rear of a fully opened keyhole. We show that this parameter clearly interprets a hole in formation or a lack of penetration when welding is performed on workpieces with variable thicknesses at constant laser power.     

Closed-loop power and focus control of laser welding for full-penetration monitoring

We describe a closed-loop control system ensuring full penetration in welding by controlling the focus position and power of a 4-kW Nd:YAG laser. A focus position monitoring system was developed based on the chromatic aberration of the focusing optics. With the laser power control system we can determine the degree of penetration by analyzing the keyhole image intensity profile. We demonstrate performance in bead-on-plate welding of Inconel 718 and titanium. The focus control system maintained a focal position on tilted and nonflat workpieces, and the penetration monitoring technique successfully controlled the laser power to maintain the full-penetration regime in the presence of linear and step changes of thickness. Finally we discuss the performances and the limits of the systems when applied to a realistic complex aerospace component.     

激光深熔焊等离子光焰图像信息提取的研究

激光深熔焊是当今制造技术发展的前沿领域。在激光深熔焊接中产生的光致等离子体光焰会随焊接过程呈周期性的变化，而焊缝的熔透性与光致等离子体光信号的累积强度有对应关系。本文通过图像处理的方法，对光致等离子光焰图像进行分析和处理，为科学分析激光深熔焊光致等离子体的机理和变化规律提供了可靠的量化数据。     

等离子弧焊接熔池和小孔形成过程数值分析

目的 研究等离子弧焊接穿孔过程中熔池内部的金属流动情况和小孔动态变化过程。方法 通过“传热-熔池流动-小孔”之间的相互耦合关系,建立了等离子弧焊接穿孔过程的数值分析模型,通过VOF方法追踪了小孔界面,采用FLOW-3D软件模拟了等离子弧焊接熔池和小孔的形成过程,定量计算了等离子弧焊接温度场、熔池流场及小孔形状;分析了等离子弧焊接熔池和小孔行为;并通过等离子弧焊接实验数据验证了模拟结果。结果 当焊接时间为0~1.0 s时,小孔深度曲线与熔深曲线几乎相同,小孔底部紧贴熔池底部;在2.8 s以后,小孔深度曲线与熔深曲线有一定距离,小孔深度曲线在一定范围内波动,等离子弧焊接电弧挖掘作用到达极限,电弧压力与其他力达到平衡状态。模拟的焊缝熔深为8.04 mm、熔宽为13.20 mm,实验测得的焊缝熔深为8.00 mm、熔宽为13.42 mm。结论 构建的随小孔动态变化的曲面热源模型和电弧压力模型可以描述等离子弧焊接过程中的电弧热-力分布;模拟出了等离子弧焊接熔池和小孔动态演变过程;模拟得到的等离子弧焊接焊缝形貌与实验测得的焊缝形貌基本吻合。     

功率密度对DP590钢激光焊缝熔深及组织的影响

目的研究相同热输入(功率/速度)下激光功率密度(功率/光斑面积)变化对焊缝熔深及组织的影响。方法在保持热输入不变的条件下,对不同功率下1.8 mm厚的DP590钢板进行光纤激光焊接试验,在光学显微镜下检测不同条件下的熔深,在扫描电镜下观察不同条件下的焊缝组织。然后,采用FLOW-3D软件对不同条件下焊接熔池/小孔行为与激光能量吸收进行了计算研究。结果随着功率密度的增加,焊缝熔深总体不断递增,但在速度为0.055 m/s和0.065 m/s时突变。结论通过模拟发现,匙孔吸收的能量影响了焊缝熔深变化。同时,焊缝冷却速率随着焊缝吸收能量的增加而降低,使得焊缝区马氏体组织大小出现差异。     

Optical Sensor for real-time Monitoring of CO(2) Laser Welding Process

An optical sensor for real-time monitoring of laser welding based on a spectroscopic study of the optical emission of plasma plumes has been developed. The welding plasma's electron temperature was contemporarily monitored for three of the chemical species that constitute the plasma plume by use of related emission lines. The evolution of electron temperature was recorded and analyzed during several welding procedures carried out under various operating conditions. A clear correlation between the mean value and the standard deviation of the plasma's electron temperature and the quality of the welded joint has been found. We used this information to find optimal welding parameters and for real-time detection of weld defects such as crater formation, lack of penetration, weld disruptions, and seam oxidation.     

Solidification behaviour in plasma are welding

Melting and solidification behaviour in the deep penetrution welding process is different from that in conventional welding process in deep penetration processes there is keyhole formation and the full thickness of the plate receives the are heat input unlike conventional processes in which the heat input is received only by the surface nodes. In the present study, the thermal analysis of molten pool formation and solidification keyhole welding using plasma are welding has been done using the finite element method. The model accounts for the several phenomena associated with welding, like the distributed are heat input over the top surface and along the thickness, the temperature-dependent material properties. convection and radiation heat losses etc. The analysis is performed for different combinations of parameters. viz welding current and welding speed, which have the maximum influence on molten pool shape and solidification behaviour. The model has also been validated by conducting experimental measurement of thermal cycles experienced by the plate for different welding parameters. The weld pool dimensions. viz. the length and widlh are found to increase with inincreasing current and decereasing welding speed. Thermal cycles at locations close to the weld reach a higher value of temperature and the time for peak temperature is also less but at farther locations the peak temperature reached is lower and the time for peak temperature is higher. Details of the model, the experimental results obtained and the solidifications charateristics of the pool are discussed in this paper.     

Analytical solution for three-dimensional model predicting temperature in the welding cavity of electron beam

This paper provides an analytical solution for three-dimensional model predicting temperature in the welding cavity of electron beam. It is not easy to measure the temperature on the keyhole of electron-beam welding. Therefore it is essential to develop an analytical model that can accurately predict the temperature in the keyhole. In this study, the keyhole produced by an electron beam is assumed to be a paraboloid of revolution and the intensity of electron beam is supposed to be Gaussian profile. In order to obtain an analytical solution, the parabolic coordinate system is utilized to analyze the temperature in the keyhole and the parameter approximating convection is proposed to account for the effect of convection of molten metal. Considering the momentum balance at the bottom of the keyhole but neglecting the absorption in the plume, an analytical solution is developed for semi-infinite workpieces. As compared with other analytical solutions, the analytical solution obtained by this model provides the temperature distribution more consistent with the experimental data. The effects of various parameters on the temperature distribution in the keyhole are also discussed in this study.     

超声波塑料焊接温度测量系统

建立了基于惠普可视化工程环境(HP VEE)的实时温度测量系统来测量超声波塑料焊接的温度;并介绍了 HP VEE 的功能、该温度测量系统的组成和采用该系统进行的超声波焊接 PVC 塑料的温度测量试验. 试验结果表明超声波塑料焊接的焊接区温度具有快速升温和快速降温的特点, 在降温过程中由于焊接压力的作用会出现短暂的温度平台, 也说明该测温系统适用于超声波塑料焊接温度测量.     

Numerical simulation of fluid flow and temperature field in keyhole double‐sided arc welding process on stainless steel

Double‐sided arc welding process powered by a single supply is a type of novel high‐production process. In comparison with the conventional single‐sided arc welding, this process has remarkable advantages in enhancing penetration, minimizing distortion and improving welding production. In this paper, a three‐dimensional steady numerical model is developed for the heat transfer and fluid flow in plasma arc (PA)–gas tungsten arc (GTA) double‐sided keyhole welding process. The model considers the surface tension gradient, electromagnetic force and buoyancy force. A CCD camera is used to observe the size and shape of the keyhole and weld pool. The acquired images are analysed through image processing to obtain the surface diameters of the keyhole on the two sides. A double‐V‐shaped keyhole geometry is then proposed and its characteristic parameters are derived from the images and cross‐section of weld bead. In the numerical model, the keyhole cavum within the weld pool is treated as a whole quality, whose temperature is fixed at the boiling point of the workpiece material. The heat exchange between the keyhole and weld pool is treated as an interior boundary of the workpiece. Based on the numerical model, the distributions of the fluid flow and temperature field are calculated. A comparison of cross‐section of the weld bead with the experimental result shows that the numerical model's accuracy is reasonable. Copyright © 2005 John Wiley & Sons, Ltd.     

Application of schlieren interferometry to temperature measurements during laser welding of high-density polyethylene films

Schlieren interferometry is found to be an alternative tool for temperature measurement during thermoplastic laser welding with regard to methods based on thermocouples or optical pyrometers. In fact, these techniques are not easily applied when materials to be processed have reduced thickness, negligible heat conduction, and low emissivity, as is the case of welding high-density polyethylene films with 10.6-microm CO2 laser radiation, even if the method reaches its applicability limit after approximately 1 s of the interaction process. The schlieren method provides the means and the results to probe the thermal variations of the laser-thermoplastic interaction on both the surface and the interface between the sample material and the air.     

Keyhole depth instability in case of CW CO<Subscript>2</Subscript> laser beam welding of mild steel

The study of keyhole (KH) instability in deep penetration laser beam welding (LBW) is essential to understand welding process and appearance of weld seam defects. The main cause of keyhole collapse is the instability in KH dynamics during the LBW process. This is mainly due to the surface tension forces associated with the KH collapse and the stabilizing action of vapour pressure. A deep penetration high power CW CO₂ laser was used to generate KH in mild steel (MS) in two different welding conditions i.e. ambient atmospheric welding (AAW) and under water welding (UWW). KH, formed in case of under water welding, was deeper and narrower than keyhole formed in ambient and atmospheric condition. The number and dimensions of irregular humps increased in case of ambient and under water condition due to larger and rapid keyhole collapse also studied. The thermocapillary convection is considered to explain KH instability, which in turn gives rise to irregular humps.     

Online Monitoring of Welding Status Based on a DBN Model During Laser Welding

In this research, an auxiliary illumination visual sensor system, an ultraviolet/visible (UVV) band visual sensor system (with a wavelength less than 780 nm), a spectrometer, and a photodiode are employed to capture insights into the high-power disc laser welding process. The features of the visible optical light signal and the reflected laser light signal are extracted by decomposing the original signal captured by the photodiode via the wavelet packet decomposition (WPD) method. The captured signals of the spectrometer mainly have a wavelength of 400–900 nm, and are divided into 25 sub-bands to extract the spectrum features by statistical methods. The features of the plume and spatters are acquired by images captured by the UVV visual sensor system, and the features of the keyhole are extracted from images captured by the auxiliary illumination visual sensor system. Based on these real-time quantized features of the welding process, a deep belief network (DBN) is established to monitor the welding status. A genetic algorithm is applied to optimize the parameters of the proposed DBN model. The established DBN model shows higher accuracy and robustness in monitoring welding status in comparison with a traditional back-propagation neural network (BPNN) model. The effectiveness and generalization ability of the proposed DBN are validated by three additional experiments with different welding parameters.     

The mechanism of penetration increase in A-TIG welding

The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel. The effect of flux on the flow and temperature fields of weld pool is simulated by the PHOENICS software. It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tension grads from a negative value to a positive value, can cause significant changes on the weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld. This change is the main cause of penetration increase. Moreover, arc construction can cause the weld width to become narrower and the penetration to become deeper, but this is not the main cause of penetration increase. The effects of flux on fluid flow of the weld pool surface and arc profiles were observed in conventional TIG welding and in A-TIG welding by using high-speed video camera. The fluid flow behavior was visualized in real-time scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow patterns leading to weld beads with narrower width and deeper penetration could be apparently identified in the case of A-TIG welding. The flux could change the direction of fluid flow in welding pool. It has a good agreement with the simulation results.     

Energy Transfer Modes in Pulsed Laser Seam Welding

A systematic parametric study has been made in pulsed Nd:YAG laser welding to understand the energy transfer modes. Four different energy transfer zones, namely conduction, transition, penetration, and keyhole, have been identified. The traditional classification of energy transfer modes based on the power density value of 10⁶ W/cm² is not strictly applicable as the transfer mode varies with pulse duration. The threshold power density to form keyhole is not constant, but the threshold energy density has been found to be invariant around 2.4 kJ/cm². The pulse duration has been optimized to be of about 8 ms to achieve welds of higher aspect ratio.     

Hybrid simulation of laser deep penetration welding

In laser deep penetration welding, the knowledge on the temperature history of the material is of great interest for the assessment of the quality properties of the weld. For this purpose a hybrid process model that enables the fast calculation of temperature distributions as a function of process parameters is applied. The interaction between laser and material is taken into account by a reduced keyhole model, which exploits a hierarchy in the spatial dimensions occurring at high feed rates. The resulting shape of a stationary keyhole is introduced as a Dirichlet boundary into a thermal finite element simulation in which it is moved through the workpiece according to the process control of the laser beam. The boundary is mathematically described by a level set function and immersed in a fixed computational mesh. The Dirichlet boundary condition is imposed using an embedded boundary method. The calculated temperature distributions are evaluated by means of bead on plate welds conducted in 0.9 mm thick sheets of 1.4301 (AISI 304) stainless steel.     

Melt pool vorticity in deep penetration laser material welding

In the present study, the vorticity of melt motion in the keyhole and weld pool has been evaluated in case of high power CO₂ laser beam welding. The circulation of vorticity is obtained as a function of Reynolds number for a given keyhole volume which is linked to Mach number variation. The shear stress and thermal fluxes present in the turbulent pool are linked to diffusivity and Prandtl number variation. It was shown that below a critical value of Rayleigh number, the conduction mode of melt transfer signifying beam absorption becomes dominant. Above this value, convective heat transfer indicates melting and evaporation occurring in the weld pool during laser welding. The evaporative recoil pressure expels the liquid while surface tension and hydrostatic pressure help to retain the melt in the keyhole cavity in this high power laser beam welding. The understanding of several hydrodynamic phenomena occuring in the weld pool is valuable not only for understanding basic mechanistic aspects but also for process optimization involved in laser beam welding.     

The influence of adhesive on the Al alloy in laser weld bonding Mg–Al process

Laser weld bonding is a new welding technology, being used to join Mg–Al alloys. The penetration depth of LWB Mg–Al joint was larger than that in simply laser welding joint in same welding parameters. The temperature at the edge of the Al fusion zone in LWB Mg–Al joint was higher than that in laser welding joint, which was measured through the thermal couples. The laser-introduced plasma in LWB Mg–Al process is observed by the high-speed camera, which is different from that in laser welding process. The surface temperature and state of the Al alloy were changed because of the addition of the adhesive, thus the laser absorptive of Al alloy was increased in LWB process, comparing with that in laser welding process; and the decomposition of the adhesive would make a depression in the Al fusion zone, which would be beneficial to the formation of keyhole welding in LWB Mg–Al joint.     

锁孔TIG焊接技术的研究进展

锁孔TIG焊接技术具有不开坡口、单面焊双面成形的特点,尤其适用于中厚板焊接。首先介绍了锁孔TIG焊的基本原理,然后分析了目前锁孔TIG焊接波形控制技术的研究现状,主要包括恒流锁孔TIG焊、单脉冲锁孔TIG焊和高低频双脉冲锁孔TIG焊。针对现有锁孔TIG焊接技术存在热输入量过大、接头组织晶粒粗大等缺点,重点介绍了一种新型的快频脉冲TIG焊接技术,它通过快频脉冲电流产生的电磁场对电弧进行压缩,控制焊接过程的热输入量,达到提高焊接质量的目的。与传统脉冲电弧相比,快频脉冲电弧的收缩效果更为明显,可细化焊接接头组织,将快频脉冲焊接技术应用于锁孔TIG焊是今后研究的热点和主要发展方向。     

Effect of process parameters on bead properties of A359/SiC MMCs welded by laser

Metal matrix composites are well known materials that present several positive features, mainly in terms of mechanical strength to weight ratio. On the other side, MMC’s are extremely difficult to machine (in particular by chip removal processes) and this difficulty has limited a wider diffusion. Welding, together with other joining technologies, could be a solution to limit the chip removal processes in the production of a MMC part, but this is not a problem-free process. This paper presents an experimental study on the laser welding of MMCs using both a CO₂ and a diode laser source. The CO₂ welding is performed in keyhole conditions (deep penetration), whereas the diode laser performs a conduction welding. Results show that the CO₂ welding is greatly affected by the formation of Al₄C₃, that compromises the toughness of the bead. The content of Al₄C₃ can be reduced by acting on the process parameters, but cannot be completely eliminated. Instead, diode laser welding generates a sound bead, with negligible Al₄C₃ formation and of superior quality. Considering also the other positive features of this kind of sources (high efficiency and radiation wavelength, that is, highly absorbed by MMCs), diode laser welding seems to be highly recommendable for MMCs laser welding.