Critical power of keyhole formation in CW Nd：YAG laser deep penetration welding

The energy model was founded to calculate the critical power of keyhole formation by using the limit principle in CW （ continuous wave ） Nd： YAG laser deep penetration welding process. The model was validated by experiments. The results show that ＇.there are two errors between the calculated critical power of keyhole formation and that of experiments ： one is that the calculated results is less than those of experiments, which is caused by not considering the energy loss by heat conduction in the model of keyhole formation. The other is that there is 0. 9 mm error between the axis of the calculated curve of critical power with location of laser focus and that of experimental curve, which is induced by the excursion of laser focus in laser deep penetration welding. At last, the two errors were revised according to the analyses of the errors.     

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.     

CO_2激光焊接的同轴检测与熔透控制I.熔透状态特性及其同轴检测

对激光深熔焊的熔透状态进行了分类 ,指出了“仅熔池透”和“适度熔透”的形成条件和焊缝成形特征。建立了一套CO2 激光深熔焊熔透状态的同轴检测系统 ,研究了焊接工艺参数和熔透状态发生变化时等离子体同轴光信号的变化规律。发现在板厚不变的条件下 ,随热输入的变化 ,焊接过程由“仅熔池透”达到“适度熔透”时 ,同轴等离子体光信号强度的增量绝对值最大。     

激光焊接中的等离子体与小孔行为分析

试验采用光、声信号检测设备,对1.8 mm镀锌钢板的CO₂激光焊接过程中的蓝紫光信号、红外辐射信号、可听声信号进行了高速采集,对比激光深熔焊与热传导焊过程中三路信号的联系与区别,分析研究了激光深熔焊过程的小孔与等离子体产生过程的瞬态行为.结果表明,等离子体与小孔并不总是同时产生,等离子体要先于小孔产生,热传导焊过程中也可能存在等离子体,在试验条件下,等离子体在激光作用于工件后3~4 ms产生,稳定的小孔在焊接后约14 ms后形成,并且激光焊接中产生等离子体的激光功率密度阈值要小于产生小孔的阈值.     

CO2激光焊接的同轴检测与熔透控制I.熔透状态特性及其同轴检测

对激光深熔焊的熔透状态进行了分类,指出了“仅熔池透”和“适度熔透”的形成条件和焊缝成形特征。建立了一套CO2激光深熔焊熔透状态的同轴检测系统,研究了焊接工艺参数和熔透状态发生变化时等离子体同轴光信号的变化规律。发现在板厚不变的条件下,随热输入的变化,焊接过程由“仅熔池透”达到“适度熔透”时,同轴等离子体光信号强度的增量绝对值最大。     

双光束激光焊匙孔动态特征分析

文中研究了高功率单光束激光焊与双光束激光焊过程中匙孔动态特征的差别. 结果表明,单光束激光焊及双光束激光焊接过程中匙孔均处于从产生到湮灭的剧烈波动的过程,不同于单光束激光焊匙孔的形成、长大、维持、缩小、湮灭过程,双光束激光焊的匙孔还存在分离长大及合并缩小的过程;在相同的焊接参数及焊缝具有相同熔深的条件下,双光束激光焊匙孔的波动频率约为单光束激光焊的2/3,单光束激光焊匙孔的开口面积均值约为双光束激光焊匙孔开口面积的1/2,开口面积的波动变异系数约为单光束激光的2倍,即双光束激光焊过程中匙孔较单光束激光焊的具有较高稳定性.     

Numerical simulation of full penetration laser welding of thick steel plate with high power high brightness laser

Full penetration laser welding was carried out on a 10 mm steel plate using a 16 kW maximum power continuous wave thin disk laser. Upper surface and lower surface of molten pool were observed synchronously with two high speed CCD cameras during the welding process. The lower surface was much longer and more unstable than the upper one. A three dimensional laser deep penetration welding model in which volume of fluid (VOF) method was combined with a ray-tracing algorithm was used to simulate the dynamic coupling between keyhole and molten pool in laser full penetration welding. The calculated weld cross-section morphology and molten pool length on both upper side and lower side agree well with experimental results. Evolution of molten pool in lower side during full penetration laser welding was analyzed, periodical features of energy coupling, molten pool behavior and keyhole dynamics in laser full penetration welding were identified and discussed.     

基于实测小孔的激光深熔焊接三维传热模型

激光深熔焊接的传热过程包括热传导和焊接熔池内熔融材料的对流流动.考虑了熔池的对流和材料热物性参数的温度依存性,建立了激光深熔焊接的三维传热模型,以试验所得小孔为原型;考虑了熔池中流质的轴向流动及轴向的温度变化;采用了"固液同一法",让温度低于液相线的材料粘性系数趋于无限大,而高于液相线的材料则采用实际值.采用有限单元法对模型数值求解的结果表明,激光焊接温度中的等温线为一组类似椭圆的曲线;小孔前沿的温度梯度、速度梯度大;后沿的温度梯度、速度梯度小;焊接熔池中的对流流动是在表面张力驱动下产生的.     

焊丝熔化方式对激光焊接过程的影响

研究了两种焊丝熔化方法(电弧预熔丝激光焊、激光填丝焊)激光焊接过程对匙孔稳定性以及焊缝成形的影响,进一步研究了焊丝熔化方法对焊接接头质量的影响,并对比分析了两种焊丝熔化方式对焊接速度的适应性. 结果表明,电弧预熔丝激光焊过程中,熔池表面匙孔开口尺寸变化不大,匙孔较为稳定;激光填丝焊方法由于熔化的液态金属距离匙孔边缘很近,焊接过程中熔池表面匙孔开口尺寸变化较大,而且容易出现熔池表面匙孔的闭合. 与激光填丝焊相比,电弧预熔丝激光焊熔化的焊丝端部可以沿熔池边缘流入,与匙孔边缘的距离较远,匙孔稳定性较好,焊缝气孔数量较少. 当焊接速度为8 m/min时,电弧预熔丝激光焊的焊缝成形良好;而激光填丝焊焊缝背面成形不连续,并且出现了未焊透的缺陷.     

6061铝合金激光深熔焊等离子体光谱特征与气孔的相关性

采用光纤激光器对6061铝合金进行焊接,获得了表面成形良好的焊缝. 利用光谱仪和高速摄像机获取等离子体的光谱和图像,分析了激光深熔焊时等离子体的光谱特征,讨论了光谱强度及其波动程度与焊缝气孔的位置及气孔率之间的相关性. 结果表明,6061铝合金激光深熔焊时等离子体的电离度低,光谱中只有金属原子谱线,AlI 396.152 nm谱线的强度能够反映焊接过程中等离子体的光谱特征;等离子体光谱强度及其波动程度与焊缝中氢气孔的形成位置和气孔率均不存在必然联系;等离子体光谱强度与小孔型气孔的位置不存在相关性,但其波动程度能够反映小孔型气孔的气孔率.     

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.     

铝合金激光-MIG复合填丝焊稳定性分析

通过与铝合金激光-MIG复合焊方法相对比,主要研究了铝合金激光-MIG复合填丝焊的焊缝成形、熔深稳定性、余高稳定性、焊缝气孔率、激光匙孔特征、等离子体特征. 试验结果表明,在合适的工艺参数条件下,铝合金激光-MIG复合填丝焊焊接过程稳定,焊缝成形良好,额外填入的焊丝可以连续稳定地过渡到熔池中,激光匙孔具有明显的形成、长大、湮灭周期性变化特征;相同工艺参数条件下,铝合金激光-MIG复合填丝焊的焊缝熔深稳定性、余高稳定性与激光-MIG复合焊相当,激光匙孔开口面积约增加15.34%,等离子体+电弧总面积约增加1.95%.     

光纤激光焊接熔池和小孔的高速摄像与分析

采用主动光源和光学窄带滤光片等辅助器件,利用高速摄像技术对光纤激光焊接过程中的熔池和小孔进行了拍摄,获得了较为清晰的熔池和小孔图像,以及不同激光功率下光纤激光焊接熔池和小孔的实际尺寸,可以为光纤激光焊接熔池和小孔的模拟提供可靠的参考依据。对高速摄像图片和焊缝熔深波动以及焊缝形貌进行了分析。结果表明:小孔前沿附近是光纤激光焊接过程中飞溅产生的主要区域;利用高速摄像可以监测焊缝的熔深变化;熔池温度最低的区域为熔池后部的中轴线两侧,而非熔池边界处。     

Prevention of porosity by oxygen addition in fibre laser and fibre laser–GMA hybrid welding

Prevention of porosity in partial penetration fibre laser and fibre laser–gas metal arc hybrid welding is investigated. It is found that modulation of laser power prevents porosity formation in fibre laser welding, but that this method is not effective in hybrid welding. However, the addition of a small amount of oxygen to the molten pool can prevent porosity formation in both fibre laser and hybrid welding. This is attributed to stabilisation of the keyhole. During welding, oxygen reacts with dissolved carbon to form carbon monoxide (CO) in the keyhole. The CO partial pressure in the keyhole prevents the intense interaction between laser beam and molten metal, thus stabilising the keyhole. The role of CO formation is confirmed by the enhancement of porosity suppression with increased carbon content of the base metal.     

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.     

激光深熔焊过程熔池小孔发声数值计算与试验分析

激光深熔焊过程中产生的声信号是激光焊接过程质量检测的重要参量.文中对3.5 mm厚的低碳钢板进行焊接速度分别为3,4和5 cm/s的工艺试验,得到了三种典型的熔池小孔,即平底形熔池小孔、锥形熔池小孔和匙形熔池小孔.通过对声信号的频谱分析,得到这三种不同形状的熔池小孔的共振频率分别为:1 503.9 Hz、1 894.53 Hz和2 792.96 Hz.同时,文中对试验得到的三种形状的熔池小孔发声过程进行了数值分析,得到了对应的共振频率,分别为:1 453.125 Hz、1 890.625 Hz和2 750 Hz.数值计算得到的结果与试验结果误差在5%范围内.结果表明,文中所建立的熔池小孔模型能很好的反映出实际的焊接过程声信号的声场分布,为声信号在质量检测中的应用提供理论基础.     

Hump formation mechanism in gas-jet-assisted keyhole laser welding of HR-2 stainless steel

Gas-jet-assisted keyhole laser welding offers the possibility of a breakthrough in the limitations of penetration depth in laser welding,which currently suffers from equipment restrictions.A gas jet of sufficient intensity to assist the keyhole should be used to obtain suppressed plasma,a deepened keyhole,and increased penetration depth.However,an excessively strong gas jet gives rise to humps.The incident angle of the keyhole-assisted gas jet is 60°,with a nozzle ahead of the laser beam.A series of experiments were carried out with different welding velocities and gas parameters by using HR-2 hydrogen-resistant stainless steel and a slab CO_2 continuous-wave laser welding machine.The weld profiles can be categorized into four types,welds of traditional laser welding,welds of enhanced laser welding,undercut welds,and humping welds with increased gas pressure.A high-speed camera was employed in the experiments to monitor the formation of humps under an excessively strong gas jet.The results of analysis show that hump formation can be divided into six stages.Its main driving force is the intense turbulence of gas jet.There are two main reasons for hump formation:premature solidification of the molten pool caused by the large temperature gradient between the front and rear parts of the molten pool,and the emergence of a thin layer liquid bridge with one-directional flow under the enhanced cooling effect of excessively strong gas.     

铝合金双光束激光焊接过程稳定性分析

从成形、气孔、熔深的波动、激光匙孔、光致等离子体、飞溅等方面研究了铝合金高功率单光束激光和双光束激光焊接过程的稳定性。结果表明,在相同工艺参数下,双光束激光焊缝表面成形明显优于单光束,熔深的波动较单光束小;在相同工艺参数及相同熔深条件下,双光束焊缝气孔率较单光束小,小孔及等离子体的波动周期较单光束长,面积波动变异系数较单光束小,飞溅较单光束更加细小均匀。     

激光深熔焊中匙孔形成过程的动态模拟

为了准确的模拟出激光深熔焊接中匙孔变化的动态过程,根据匙孔内激光的能量吸收机制,采用射线跟踪法产生热源,建立了激光深熔焊接过程中气、液、固三相统一的数学模型,并采用了VOF方法追踪自由液面,模型中考虑了表面张力、热浮力和反冲压力的作用,通过数值计算得到了匙孔变化的动态过程与相应的温度分布.结果表明,匙孔形成的主要驱动力是反冲压力,在加热初期,激光主要用来加热金属基体,随着激光辐照时间的增加,熔池表面温度急剧升高,金属进而汽化,产生匙孔.针对304不锈钢的工艺试验表明计算结果与实际相符合.     

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.