Thermal modelling of laser welding and related processes: a literature review

The main emphasis of this review is on thermal modelling and prediction of laser welding in metals. However as similar techniques are employed to model conventional welding processes such as arc, resistance and friction, as well as related processes such as alloying, cladding and surface hardening, part of this review is given over to the modelling of these processes where appropriate. The time frame of the review is up to the year 2002.     

Effect of welding speed on butt joint quality of Ti–6Al–4V alloy welded using a high-power Nd:YAG laser

Annealed Ti–6Al–4V alloy sheets with 1 and 2 mm thickness are welded using a 4 kW Nd:YAG laser system. The effects of welding speed on surface morphology and shape, welding defects, microstructure, hardness and tensile properties are investigated. Weld joints without or with minor cracks, porosity and shape defects were obtained indicating that high-power Nd:YAG laser welding is a suitable method for Ti–6Al–4V alloy. The fusion zone consists mainly of acicular α′ martensite leading to an increase of approximately 20% in hardness compared with that in the base metal. The heat-affected zone consists of a mixture of α′ martensite and primary α phases. Significant gradients of microstructures and hardness are obtained over the narrow heat-affected zone. The laser welded joints have similar or slightly higher joint strength but there is a significant decrease in ductility. The loss of ductility is related to the presence of micropores and aluminum oxide inclusions.     

Surface preparation of Ti–3Al–2.5V alloy tubes for welding using a fiber laser

Ti–3Al–2.5V tubes are widely used in aircraft hydraulic systems. Meticulous surface preparation before welding is necessary to obtain a sound weld involving these alloy tubes. Conventionally this is done by cleaning with environmentally malign toxic chemicals, such as, hydrofluoric acid and nitric acid. This paper describes the laser-cleaning process of the surface of these tubes with a fiber laser as a preparation for pulsed gas tungsten arc welding and results obtained. A simple one-dimensional heat equation has been solved to evaluate the temperature profile of the irradiated surface. It is shown that surface preparation by laser cleaning can be an environmentally friendly alternative process by producing acceptable welds with laser-processed tubes.     

基于标准温度法的电弧高温区自动判别研究

在电弧等离子体的光谱诊断中,标准温度法测温原理与目前先进的图像传感技术相结合,通过特征谱图像完成电弧全场温度信息采集,因其良好的时、空分辨率而被广泛应用于电弧温度测量。但是谱线的发射系数与等离子体温度不是单调变化关系,传统标准温度法选取一条ArⅠ谱线完成对电弧等离子体的测量,在电弧内部的高温电离区域产生谱线辐射强度降低的现象,需要人为判定电弧不同位置所处的温度区间才能完成温度的计算,整个过程无法通过软件自主完成。针对此问题,根据电弧等离子体的局部热力学平衡条件,探索一种基于双特征谱线的标准温度法测温原理,通过融合电弧在外层低温区域聚集的Ar原子发出的ArⅠ谱线发射系数场,和在高温区域的Ar一次电离离子所发出的ArⅡ特征谱线发射系数场,将达到ArⅠ谱线标准温度的位置处的ArⅡ谱线发射系数作为电弧不同温度区域的判定依据,完成电弧等离子体高温区域的自动判别,继而应用ArⅠ谱线发射系数与温度对应关系在电弧高、低温区域分别计算电弧温度,消除单一的ArⅠ谱线发射系数场暗区给计算带来的不利影响;设计并搭建了一种镜前分幅采集系统,其中分光镜将弧光等能量分成两束,利用两组反射镜和窄带滤光片建立起两路光学通道,使CMOS在一次曝光中完成两组电弧特征谱图像的采集,并且两幅图像的采集时刻、焦距、光圈等拍摄参数完全一致,达到良好的时间、空间一致性,从而减小谱线融合时误差的输出,满足了原位获取两组电弧特征谱图像的需求;为验证测量系统可行性以及后期的电弧图像提取,以黑白棋盘为标靶,用Harris算子对系统采集的图像进行扫描,根据角点坐标证明系统所采集的两幅图像具有良好的一致性,并且据此将两幅图像做归一化处理,以便后期的电弧特征谱图像的提取;通过假设所测电弧等离子具有轴对称属性,以CMOS所采集的特征谱图像亮度信息作为电弧发射系数场在不同角度下的投影依据,经过中值滤波降噪后,利用ML-EM迭代重建算法求解电弧的三维发射系数分布。实验中,选择受自吸收效应影响较小的ArⅠ696.5 nm谱线和ArⅡ480.6 nm谱线为测量目标,并且在696.5 nm谱线的光通路中加入OD0.4的中性减光片,使两幅特征谱图像的最高亮度值保持一致。选取150A焊接等离子弧为测量对象,经ML-EM法三维还原后,将两条谱线发射系数场等像素融合,在ArⅠ谱线发射系数达到最大值的像素点位置处,ArⅡ谱线发射系数达到ε_rp,判定ArⅡ谱线发射系数大于ε_rp的像素点位置为电弧高温区域,其余位置为低温区域,最终在不同温度区域自动完成焊接等离子弧的温度计算。实验结果表明696.5 nm谱线和480.6 nm谱线发射系数场融合后可以自动识别电弧高温区域,继而完成电弧等离子体的自动测量,为电弧温度实时监测的实现提供更多可能。     

EAST装置第一壁热沉冷却结构关键工艺的研究

对由埋管引起的面对等离子体第一壁热沉冷却结构冷却效果的下降进行了实验测量与数值模拟。在相同加热源条件下,比较了热沉内光孔、埋管打压胀管及高温真空埋管钎焊三种工艺实验件的换热效率,并与数值模拟的结果进行比较,确定了合理的计算模型和参数。对高场测的热沉原型件,比较了埋管打压胀管和高温真空钎焊连接换热效率,为EAST热沉冷却结构优化提供了依据。     

Laser welding of thin polymer films to container substrates for aseptic packaging

Keyhole laser welding of polymers is a subject well covered and researched, but relatively little information exists regarding the welding of thin polymer films, particularly to a heavier substrate. This paper presents the design of a suitable test apparatus for laser welding thin film to a heavier substrate, and shows the results of an investigation into the feasibility of laser welding multi-layer polymer film lids to tubs for the manufacture of aseptic food containers. A consistent weld, free from defects, is the key to process success. Typical welding defects have been synthesised in order to investigate, and consequently remove, their cause. The result is a reliable welding method based on even film clamping. With careful attention to machine design, a seal of high mechanical strength and chemical integrity is possible.     

SPV490Q钢焊接冷裂纹声-磁定位方法研究

声发射源定位方法可以精准判断信号发生时间及位置,但由于其检测数据量大,难以进行实时评价,而磁记忆定位方法能实时观察数据结果,所以用声-磁结合的方法对冷裂纹进行定位分析。焊后即进行声发射监测,待焊缝冷却后,按一定时间间隔用磁记忆技术进行检测,这样既能精确得到冷裂纹起裂时间及其位置,又能实时观察冷裂纹扩展走向。同时,通过SPV490Q钢斜Y型坡口焊接冷裂纹实验验证了该方法的可行性。     

RAFMs 中厚板TIG 焊下的温度场和应力场研究

应用ANSYS 软件对低活化马氏体/铁素体钢(RAFMs)中厚板多道焊接过程进行三维有限元模拟,将得到的温度场和应力场分别与实验对比,对焊缝区和热影响区宽度进行预测,并通过焊接实验进行验证,同时对焊后的试件进行了金相分析和硬度测试。结果表明：温度场和应力场模拟结果和实测结果基本一致,焊缝区以及热影响区宽度的模拟结果和实测结果基本吻合;采用316L 焊丝填充的焊缝,焊缝与母材熔合较好,焊缝没有明显缺陷;硬度检测呈现：焊缝的硬度大大高于热影响区和母材。     

Sampling-Based RBDO of Ship Hull Structures Considering Thermo-Elasto-Plastic Residual Deformation

We present a shape optimization method using a sampling-based RBDO method linked with a commercial finite element analysis (FEA) code ANSYS, which is applicable to residual deformation problems of the ship hull structure in welding process. The programming language ANSYS Parametric Design Language (APDL) and shell elements are used for the thermo-elasto-plastic analysis. The shape of the ship hull structure is modeled using the bicubic Ferguson patch and coordinate components of vertices, tangential vectors of boundary curves are selected as design variables. The sensitivity of probabilistic constraint is calculated from the probabilistic sensitivity analysis using the score function and Monte Carlo Simulation (MCS) on the surrogate model constructed by using the Dynamic Kriging (DKG) method. The sequential quadratic programming (SQP) algorithm is used for the optimization. In two numerical examples, the suggested optimization method is applied to practical residual deformation problems in welding ship hull structures, which proves the sampling-based RBDO can be successfully utilized for obtaining a reliable optimum design in highly nonlinear multi-physics problem of thermo-elasto-plasticity.     

A detailed analysis of the MIG spectrum for the development of laser-based seam tracking sensors

This paper presents a detailed series of measurements of the spectrum of the light emitted from a MIG welding arc. This work was done in the framework of a larger project concerning the development of a seam tracking sensor. Detailed measurements and analysis of the spectrum produced from welding arcs have been performed. The measurements extend from the ultraviolet region of the spectrum (150 nm) to the near infrared region (970 nm) and have revealed the presence of many strong emission lines in the spectrum. The results are of great importance for the design of any optical or vision system working close to a welding arc, because the wavelength of the optical system can be selected to be close to a value where the spectrum of the arc causes the minimum interference to the laser light.