Numerical analysis of keyhole establishment time in keyhole plasma arc welding

Numerical analysis of keyhole shape and keyhole establishment time is of great significance for selection and optimization of the process parameters in keyhole plasma arc welding. In this paper, a three-dimensional transient model is developed to analyze the evolutions of keyhole shape and keyhole establishment time in continuous current plasma arc welding process. Firstly, a combined volumetric heat source model is used to simulate the transient variation of temperature field. And then the surfaced formation equation is adopted to calculate dynamic features of the keyhole shape and keyhole establishment time inside weld pool, in which the force action on weld pool surface is considered. Experiment is conducted to validate the numerical simulation results. The predicted keyhole size and keyhole establishment time are in agreement with the experimental measurement. And the calculated fusion zone geometry is consistent with the measured one.     

Modeling the keyhole shape and dimension in plasma arc welding

It is of great significance to model the keyhole shape and dimensions to optimize the plasma arc welding process parameters. In this study, through employing a combined volumetric heat source mode, the weld pool in keyhole plasma arc welding is determined firstly, and then the dynamic force-balance condition on the interface between the plasma jet and the molten metal is dealt with in describing the keyhole formation inside the weld pool. The effects of welding current on the shape and size of keyhole are numerically analyzed. The sharp transformation from a partial keyhole to a full-penetration keyhole is quantitatively demonstrated.     

PAW+TIG电弧双面焊接小孔形成过程的数值模拟

综合考虑影响等离子弧小孔形成的等离子流力、重力、表面张力等力学因素，建立了等离子电弧＋钨极电弧（PAW＋TIG）双面焊接时小孔形成过程的数学模型，并与焊接传热的控制方程耦合，采用数值模拟技术，定量分析了小孔形成的动态过程，根据数值分析结果，将小孔的形成过程分为开始焊接到熔透，熔透到开始穿孔，开始穿孔到小孔形态稳定三个阶段，熔池的最小跨距可作为小孔建立的评价指标。小孔的形成是热、力共同作用的结果。     

Nd:YAG激光深熔焊接过程中同轴保护气流量对小孔的影响

通过对利用同轴视觉传感系统所采集的Nd：YAG激光深熔焊接过程中小孔图像的处理研究了同轴保护气流量对激光深熔焊接过程中小孔的影响。研究表明，同轴保护气流量的变化对小孔上部的影响较大，而对小孔内部的影响很弱。而在较高气流量下，小孔外边缘的径向尺寸随同轴保护气流量的增加而减小，而小孔内边缘的径向尺寸变化很小。小孔内、外边缘中心到激光光斑中心的距离随着同轴保护气流量的增加而减小，说明随着同轴保护气流量的增大小孔竖直轴线越来越靠近激光束轴线。     

Process control based on double-side image sensing of keyhole puddle for the VPPA welding of aluminum alloys

The double-side image sensing of the keyhole puddle in the variable polarity plasma arc welding of aluminum alloys has been investigated in this paper, to extract the characteristically geometrical size of the keyhole and to realize the feedback controlling for weld formation in the welding process. Some geometrical sizes of the visible keyhole in the front and back images such as the width, height, area, etc. can be used to monitor both the keyhole puddle and the weld formation in the welding process. Under the condition of the varied heat sink, varied gap and misalignment, the trend from normal welding to cutting can be reflected from the variations of geometrical sizes of the keyhole puddle respectively. The keyhole area, the keyhole height and the shape parameters of the keyhole puddle are the key parameters which reflect the trend from normal welding to cutting when meeting the condition of the varied heat sink, varied gap and misalignment respectively. The algorithm for the image processing of the keyhole puddle and the periphery extracting of the visible keyhole developed in the paper can be used to determine real-timely the geometrical sizes of the visible keyhole. Artificial neural network is applied to establish the model for predicting the geometrical sizes of the back keyhole puddle. The inputs of the model are the geometrical sizes of the front keyhole puddle and the weld parameters, the outputs of the model are the geometrical sizes of the back keyhole puddle. The model can be used to control the stability of keyhole and the weld formation.     

Nd:YAG激光深熔焊接过程中小孔的形态特征

通过图像处理得到了Nd:YAG激光深熔焊接过程中小孔内外边缘宽长比、内外边缘中心到激光光斑中心以及内外边缘中心之间的距离,并将其作为小孔的形态特征参数,研究了小孔形态随焊接工艺参数变化的规律.研究结果表明,激光功率的增大使小孔径向形状趋向圆形并使其在深度方向上的倾斜程度减小;在低速焊时,小孔形态基本不变,而在中高速焊接时,随着焊接速度的增大,小孔径向形状沿焊接方向拉长,小孔的倾斜程度也不断增大;在一定离焦量范围内,离焦量的变化对小孔在径向和深度方向上的形态影响较小.     

Nd∶YAG激光深熔焊接过程中小孔的形态特征

通过图像处理得到了Nd∶YAG激光深熔焊接过程中小孔内外边缘宽长比、内外边缘中心到激光光斑中心以及内外边缘中心之间的距离,并将其作为小孔的形态特征参数,研究了小孔形态随焊接工艺参数变化的规律。研究结果表明,激光功率的增大使小孔径向形状趋向圆形并使其在深度方向上的倾斜程度减小;在低速焊时,小孔形态基本不变,而在中高速焊接时,随着焊接速度的增大,小孔径向形状沿焊接方向拉长,小孔的倾斜程度也不断增大;在一定离焦量范围内,离焦量的变化对小孔在径向和深度方向上的形态影响较小。     

Determining the keyhole geometry based on double-elliptic distribution of plasma arc pressure

The plasma arc pressure plays an important role in determining the keyhole formation and size. So it is of great significance to choose adaptive distribution mode of the plasma arc pressure for determining keyhole shape and size. In this study, through employing a double-elliptic distribution mode of plasma arc pressure, three-dimensional keyhole was numerically simulated. The unsymmetric feature of the keyhole inside the weld pool was described. The development of keyhole was demonstrated under different levels of welding current. The critical current required to form an open keyhole was obtained for the study cases.     

等离子弧焊接小孔形状和穿孔过程的数值分析

根据小孔壁面上的力学平衡条件,建立了穿孔等离子弧焊(PAW)的小孔模型.利用该模型,对6 mm厚不锈钢板PAW准稳态小孔的形成过程进行了数值计算,得到了不同工艺参数下准稳态小孔的形状和尺寸.根据达到准稳态时小孔壁面的受力状态,对PAW过程中小孔从盲孔到穿孔的转变机制进行了探讨,分析计算了小孔壁面上各区域的作用力大小.随...     

耐热铝合金的研究及应用现状与展望

针对2219铝合金激光镜像焊接过程,建立激光镜像焊接过程热-流耦合模型,对匙孔动态特征进行定量求解与分析. 结果表明在激光镜像焊接过程中,两侧匙孔迅速耦合并保持小幅度波动. 匙孔耦合前,两侧匙孔横截面面积差异较小,最多相差仅0.35 mm²;匙孔耦合后,匙孔横截面面积持续增加,并于120 ms后在一定范围内波动. 热输入的增加会导致匙孔的耦合程度增大,并提高匙孔稳定性,其中激光功率对匙孔耦合区域影响较大,焊接速度对匙孔开口面积影响较大,基于上述规律总结出针对6 mm厚2219铝合金镜像焊接试验的优化焊接工艺窗口.     

基于小孔形状的TCS不锈钢激光+GMAW-P复合焊热场模型

为合理描述激光能量在小孔内的分布特征,采用光线追踪法处理光线在小孔内的多次反射和孔壁的Fresnel吸收,对线热源小孔模型做出了改进.根据小孔形状尺寸的计算结果,确定激光焊体积热源的分布参数.将标定后的激光焊体积热源分布参数应用于激光+熔化极脉冲电弧(激光+GMAW-P)复合焊的组合式体积热源模型,对TCS不锈钢复合焊准稳态温度场进行了数值分析.开展了TCS不锈钢复合焊工艺实验,将复合焊焊缝形状尺寸的模拟结果与实测结果进行了对比,验证了所建立的复合焊热场模型.基于小孔形状的复合焊热场模型能较好地模拟TCS不锈钢复合焊温度分布与焊缝成形.利用该模型计算了不同工艺条件下TCS不锈钢焊接HAZ形状尺寸以及HAZ内不同位置处的热循环曲线,分析了TCS不锈钢复合焊的热循环特征,为接头组织与性能的预测分析奠定了基础.     

电子束深熔焊匙孔的研究现状

综述了电子束深熔焊匙孔特性及行为研究的现状.简述了国内外研究者在电子束匙孔形成机理及匙孔行为数值模拟研究领域已取得的部分研究成果,并对电子束深熔焊匙孔的进一步研究工作做了展望.指出在电子束深熔焊匙孔行为数值模拟研究中考虑到金属蒸汽流的作用,对于进一步揭示电子束深熔焊机理和匙孔的形成过程及其特性,控制熔透质量具有重要的理论意义.     

基于Level-Set方法的小孔及熔池动态形成数值模拟

构建了激光深熔焊接三维混合态连续模型,模拟了固液界面过渡层的固、液态共存和熔池高于焊接表面的特征,并采用Level-Set方法追踪了气液界面的移动,计算得到了小孔和熔池的动态形成过程.结果表明,熔池在小孔前沿薄后沿厚,温度梯度在小孔前沿大后沿小,小孔及熔池前沿和后沿存在明显的非对称性;孔壁上蒸发的金属蒸气由孔壁流向小孔中轴线,且向孔外喷射;孔底吸收的激光功率密度最大,最高温度3700 K位于孔底,高于汽化温度567 K;小孔形成的初期阶段孔深的变化较快,但随着小孔深度的增加,孔深变化速率逐渐下降.     

并行双光束激光焊接匙孔瞬态振荡和熔池流动行为数值模拟——I.模型的建立和匙孔瞬态行为

建立了并行双光束激光焊接匙孔瞬态振荡和熔池流动行为的三维数学模型,模拟了并行双光束激光焊接过程中匙孔瞬态行为.结果表明,并行双光束激光焊接过程中,匙孔形貌演化过程复杂,其深度随时间的变化具有阶段性和一定的周期性;深度的振荡频率与单光束激光焊接在数量级一致,可达到数千赫兹;焊接速度增大,匙孔深度振荡的振幅将减小,当焊接速度增大到一定程度时,匙孔甚至可能处于稳定状态;光斑间距过大或者过小,匙孔深度振荡的振幅将变大;光斑间距太大时,熔深存在着较大的起伏.     

气流辅助增强匙孔激光焊熔深增加机理

采用直径为0.8 mm的气流喷嘴直吹匙孔,开展了不同吹气方向、气流入射点位置及流量下的激光焊试验,为获得增强的匙孔效应和增加的熔深.通过等值线图分析获得了优化的气流参数,最大熔深较传统激光焊增加了约38%.合适入射点位置和流量的增强匙孔气流,不仅压制了等离子体,还将匙孔口部的液态金属向下压,使得匙孔口部明显扩大、熔深增加、焊缝成形良好,匙孔内等离子体的流向发生了改变,因而熔池内液态金属的流向也发生了变化;入射点位置偏后时,其作用区域为匙孔后方熔池,将液态金属向熔池后方推,会导致驼峰焊道的产生.     

激光深熔焊接抛物面小孔模型

激光深熔焊接由于其无与伦比的优良特性成为高功率激光工业应用的主要方面.在假设小孔为旋转抛物面结构的基础上,通过分析小孔内的压力平衡、蒸发压力、表面张力、能量平衡、小孔壁的温度以及小孔底部的能量平衡问题,采用数值迭代方法求解出小孔的深度和宽度.将焊接中的理论分析由假设小孔形状为圆柱面只能适合低速焊接推广到中高速焊接.计算了在不同焊接速度下小孔的深度、宽度以及与焊接试验的焊缝深度,与焊接试验中得到的结果进行了比较,理论计算结果也比较吻合实际焊接情况.     

Distinct morphology of keyhole wall during high power fibre laser deep penetration welding

Abstract

The keyhole wall is the interaction interface between the laser and the material during the laser deep penetration welding. Measuring the morphology of the keyhole wall is thus of significance for understanding the high power fibre laser deep penetration welding process. In this paper, the clear keyhole wall was reserved by suddenly closing laser during high power fibre laser welding of copper alloy. The results indicate that the keyhole can be divided into laser action region and metallic vapour pressure maintenance region. The laser action region is on the front wall of keyhole, and a series of concentric elliptical rings are observed in this region. In another region, its diameter is significantly larger than the spot diameter and the keyhole wall is basically smooth. The results are different from those generally accepted viewpoints, which clarify the flowing law of molten fluid in the keyhole and are thus of great guiding significance for optimising the welding technology.     

利用等离子体反翘控制穿孔等离子弧焊中小孔的稳定性

利用高速摄影照片试验研究了等离子弧焊接时小孔形成过程中等离子体反翘的变化情况,以及小孔尺寸与等离子体反翘之间的关系.结果表明,等离子体反翘随着小孔的形成而不断变强,喷射角不断变大;随小孔尺寸的增大,等离子体反翘逐渐变弱,喷射角逐渐减小.通过分析等离子体反翘与小孔尺寸的关系,根据等离子体反翘导电特性设计了一种简单适用的探针法检测小孔状态的电路,可以检测出小孔的不同状态.提出了一种新的穿孔等离子弧焊中小孔稳定性的控制思想.     

基于“三明治”方法的激光焊接能量耦合模型(英文)

建立激光与能量耦合模型以研究激光在小孔内的传输和孔壁能量的分布。该模型的主要特点包括:1)小孔和孔内等离子体的逆韧致吸收系数均为实验测量所得;2) 入射激光为高斯分布的聚焦光束而非平行光束;3) 同时考虑了激光光束在孔内多次反射的菲涅尔吸收和逆韧致吸收。计算结果表明:孔壁所吸收的激光能量并不一致;尽管激光未能直接照射孔底,但是小孔孔底所吸收的激光能量最多。基于聚焦光束的特征分析,焦平面的位置对小孔前沿所吸收的激光能量较后沿更重要。     

Keyhole formation and its characteristics in laser welding mode transition

Keyhole is the most important characteristic for laser deep penetration welding, and its formation indicates the beginning of laser deep penetration welding mode. The keyhole developing process was analyzed and the keyhole formation time was calculated according to welding speed and the length of weld bead formed in the keyhole formation process. The results showed that the keyhole forms in 40-70 ms at different rate of change of laser power. In laser deep penetration welding process, the variation of light intensity radiated by laser induced plasma can identify the keyhole formation, but it can not be used to estimate the keyhole formation time because of delay effect.