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.     

High quality welding of stainless steel with 10 kW high power fibre laser

Abstract

The objectives of this research are to investigate penetration characteristics, to clarify welding phenomena and to develop high quality welding procedures in bead on plate welding of type 304 austenitic stainless steel plates with a 10 kW fibre laser beam. The penetration depth reached 18 mm at the maximum at 5 mm s^?1. At 50 mm s^?1 or lower welding speeds, however, porosity was generated at any fibre laser spot diameter. On the other hand, at 100 mm s^?1 or higher welding speeds, underfilling and humping weld beads were formed under the conventionally and tightly focused conditions respectively. The generation of spatters was influenced mainly by a strong shear force of a laser induced plume and was greatly reduced by controlling direction of the plume blowing out of a keyhole inlet. The humping formation was dependent upon several dynamic or static factors, such as melt volume above the surface, strong melt flow to the rear molten pool on the top surface, solidification rate and narrow molten pool width and corresponding high surface tension. Its suppression was effective by producing a wider weld bead width under the defocused laser beam conditions or reduction of melt volume out of keyhole inlet under the full penetration welding conditions. Concerning porosity, X-ray transmission in situ observation images demonstrated that pores were formed not only from the tip of the keyhole but also at the middle part because of high power density. The keyhole behaviour was stabilised using a nitrogen shielding gas, resulting in porosity prevention. Consequently, to produce high quality welds in 10 kW high power fibre laser welding, the reduction procedures of welding defects were required on the basis of understanding their formation mechanism, and 10 kW fibre laser power could produce sound deeply penetrated welds of 18 mm depth in a nitrogen shielding gas.     

Effect of welding current and speed on occurrence of humping bead in high-speed GMAW

The developed mathematical model of humping formation mechanism in high-speed gas metal arc welding (GMAW) is used to analyze the effects of welding current and welding speed on the occurrence of humping bead. It considers both the momentum and heat content of backward flowing molten jet inside weld pool. Three-dimensional geometry of weld pool, the spacing between two adjacent humps and hump height along humping weld bead are calculated under different levels of welding current and welding speed. It shows that wire feeding rate, power intensity and the moment of backward flowing molten jet are the major factors on humping bead formation.     

辅助增强匙孔气流对激光焊接不锈钢组织和显微硬度的影响

以HR-2抗氢钢为试验材料,利用板条型CO2连续激光器,采用喷嘴在前、入射角为60°的匙孔增强气流,进行了不同参数下的激光焊接试验.结果表明,在气流辅助增强匙孔激光焊中,匙孔气喷嘴在前的布局对熔池扰动小,能获得较好的焊缝成形;与传统激光焊接相比,熔深增加、熔宽明显减小,焊缝形状呈葫芦型,这是匙孔气流压制等离子体、使匙孔向底部凹陷的结果;从焊缝组织来看,增强激光焊焊缝的柱状晶较短小,焊缝中心呈现等轴晶形态,属于FA的不锈钢凝固模式.与传统激光焊不同,气流辅助增强匙孔激光焊的焊缝、热影响区显微硬度与母材相当,这可能是引入的增强匙孔气流改善了焊缝金属的冷却条件,更少铁素体析出的结果.     

脉冲激光-电弧复合焊接镁合金过程中熔池行为与气孔产生的关系研究

激光-电弧复合焊接镁合金过程中气孔是一种重要缺陷。本文针对复合热源焊接过程中熔池行为和激光匙孔引起的气孔之间的关系进行了实验研究。研究中重点考查了激光脉冲对熔池的冲击作用,匙孔的动态行为以及气孔的形成规律。研究结果表明激光脉冲的参数决定其对液态熔池的冲击作用,较强的激光脉冲作用可以有效抑制气孔的形成;液态金属的行为主要有匙孔行为主导;匙孔的尺寸和开-闭状态直接影响焊后气孔的形成;过高的激光脉冲功率会导致焊后气孔的形成;匙孔开口维持时间越长,焊后气孔数量越少。     

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.     

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.     

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.     

钛合金激光焊接大梯度残余应力特征分析

选用可精确描述小孔和熔池形貌的旋转高斯曲面体热源,建立了一个局部细化的钛合金激光深熔焊有限元数值计算模型,研究了激光能量密度与热输入对焊缝、小孔形貌以及焊缝及近缝区大梯度残余应力的分布特征.结果表明,钛合金激光焊接高残余应力位于小孔区域,而焊缝其余区域形成了大梯度残余应力状态;残余应力梯度在能量密度过高或过低,以及热输入过小时均较大;为避免大梯度残余应力的产生,应使焊接时激光能量密度处于中低状态.     

Full penetration welding of thick high tensile strength steel plate with high-power disk laser in low vacuum

This study was undertaken in order to investigate the effect of reduced ambient pressure from an atmospheric pressure (101 kPa) to 0.1 kPa on one-pass full penetration welding of thick high-tensile strength steel plate of 23 mm thickness. A 16 kW disk laser of 1030 nm in wavelength was employed to weld HT980 grade plates at the speed of 5–25 mm/s. In partial penetration welding, it was revealed that humping phenomena occurred easily. Full penetration welding of the high-tensile strength steel plates could not be achieved at 101 kPa. On the other hand, full penetration welding was obtained at the welding speed of less than 20 mm/s at the pressure of less than 10 kPa. Especially, at 0.1 kPa, and 17 and 20 mm/s, sound weld joints without defects were obtained. According to the observation results of a keyhole inlet and a surface molten pool during welding with a high-speed video camera, the melt in front of a keyhole was smaller and the behaviour of a keyhole and a plume was much more stable at 0.1 kPa than at 101 kPa. Moreover, in the full penetration welding, spattering was suppressed under the proper conditions. Such phenomena became more stable in fast welding. It was revealed in laser welding of thick high-tensile strength steel plates that the formation of narrow I-shaped weld beads by achieving full-penetration welding in low vacuum was essential for the production of sound welds without defect.     

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。     

New technology of the arc welding

Summary

This paper describes the effect of evaporation during laser welding, especially its effect on keyhole formation. In order to investigate the evaporation, a numerical model of evaporation at the molten pool surface of laser welding was developed. The effect of evaporation during laser welding was numerically analysed by the model and the numerical result compared with experiments. This paper may be summarised as follows:

1. It is made clear by the numerical results that the molten pool geometry in laser welding is markedly affected by the evaporation process.

2. Numerical results show that evaporation of the molten pool surface produces a recoil pressure and a keyhole is formed as a result.

3. A calculated critical condition for keyhole formation has been compared with experiments and a satisfactory result has been obtained.

     

背反射增效激光焊接熔池匙孔相变及流场分析

建立了背反射增效激光焊焊接熔池流动中气相区、液相区和固相区的统一模型，在模型中考虑了等离子体/蒸气云和小孔吸收机制，综合了表面张力、热浮力和重力的作用. 基于数值计算得到了熔池的三相匙孔相变以及流场，重点分析了表面张力对熔池流动和传热的影响. 此外，通过钛合金薄板的背反射增效激光焊接试验对模型进行了验证. 结果表明，匙孔引发等离子体/蒸气云与背面垫板诱发羽辉的耦合作用，是X形焊缝熔池形貌形成的主要原因；同时，表面张力是形成背反射增效激光焊接熔池内“椭圆回流环”的主要驱动力.     

Factors controlling the magnesium weld morphology in deep penetration welding by a CO2 laser

In laser welding with power density beyond 10⁴ W · mm⁻², the formation of plasma cavities, commonly referred to as keyholes, leads to deep penetration welds with high aspect ratios. In this paper, the morphologies of keyhole welds produced with a 6 kW CW CO₂ laser on two die-cast magnesium alloys, AZ91 and AM50, are compared. It was found that the two magnesium alloys responded differently to laser welding. Though irregular weld cross-section profiles were consistently observed on each materials, bead dimensions often varied with the welding variables in contrasting ways. For both alloys, important characteristics of the weld beads such as depth, width, crown height (hump), and surface ripples were analyzed as a function of the welding parameters, most particularly the heat input. Results show that the use of heat input, a variable grouping two welding parameters into one, was often inadequate in characterizing the bead morphology. Several explanations are given, including base metal vaporization, but the process of bremsstralung absorption explains it well and rationalizes many observed characteristics of laser weld morphology.     

Correlation between weld formation and processing parameters in CO2 laser welding of steels

Abstract

Laser welding, which has undergone rapid development in the past few decades, is one of the most important applications in laser materials processing. Although some general data are available, precise welding parameters are equipment specific. In the present study, a series of autogenous laser welds on mild and stainless steels has been investigated, using a Trumpf 3·0 kW CO² laser system, to establish welding parameter windows. The correlation between laser power, welding speed, and weld bead profile for bead on plate welding has been obtained. For a constant laser power, penetration depth reaches a stable value as welding speed exceeds 11 000–13 000 mm min^-1. This value is defined as the penetration threshold. Lower welding speed produces deeper penetration. However, under such conditions, the unstable keyhole and weld pool could result in undercut and porosity. The maximum penetration achievable for sound welds on both mild steel and stainless steel was investigated. The correlation between penetration threshold and power level was also established. The parameter windows established for autogenous welds can be adopted effectively on butt jointsif welding speed is reduced by 25%.     

Porosity formation mechanism and its prevention in laser lap welding for T-joints

A high-speed camera and X-ray transmission observation system were used to observe the keyhole and molten pool dynamic behavior in laser lap welding T-joints. The oscillation frequency of the molten pool and the keyhole increases with increasing gap. The lower keyhole becomes slant with the large gap and large quantities of bubbles are formed at the bottom tip of the keyhole. The molten pool is divided into three different zones by the large gap and a small eddy is formed at the lower molten pool. The bubbles are difficult to escape from the lower molten pool and the gap when the gap is large, resulting in the formation of porosity at the gap and root of weld seam. The distribution characteristics of porosity in different gap have an excellent agreement with the keyhole and the molten pool dynamic behavior. Porosity can be suppressed by maintaining a small gap or adopting high welding speed. The paper provides fundamental insights into the mechanism of porosity formation during laser lap welding T-joints and guidance to aid in its elimination.     

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.     

EXPERIMENTAL INVESTIGATION ON FORMING PROCESS OF HUMPING BEAD IN HIGH SPEEDMAG ARC WELDING

开展了高速活性气体保护( MAG)电弧焊接工艺实验, 确定出了不同焊接电流条件下形成驼峰焊道时的临界焊接速度、相邻驼峰之间的距离以及同一驼峰焊道“波峰”和“谷底”的断面形貌. 基于高速MAG电弧焊熔池的视觉检测图像, 分析了驼峰焊道的产生机理, 并利用上坡焊和下坡焊实验进行了验证. 同时, 也分析了保护气体成分对高速MAG电弧焊焊缝成形的影响.     

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.     

Weldability of 1.6 mm thick aluminium alloy 5182 sheet by single and dual beam Nd: YAG laser welding

Abstract

The weldability of 1.6 mm thick 5182 Al–Mg alloy sheet by the single- and dual-beam Nd:YAG laser welding processes has been examined. Bead-on-plate welds were made using total laser powers from 2.5 to 6 kW, dual-beam lead/lag laser beam power ratios ranging from 3:2 to 2:3 and travel speeds from 4 to 15 m min^-1. The effects of focal position and shielding gas conditions on weld quality were also investigated. Whereas full penetration laser welds could be made using the 3 kW single-beam laser welder at speeds up to 15 m min^-1, the underbead surface was always very rough with undercutting and numerous projections or spikes of solidified ejected metal. This 'spikey' underbead surface geometry was attributed to the effects of the high vapour pressure Mg in the alloy on the keyhole dynamics. The undesirable 'spikey' underbead geometry was unaffected by changes in focal position, shielding gas parameters or other single-beam welding process parameters. Most full penetration dual-beam laser welds exhibited either blow-through porosity at low welding speeds (4–6 m min^-1) or unacceptable 'spikey' underbead surface quality at increased welding speeds up to 13.5 m min^-1. Radiography revealed significant occluded porosity within borderline or partial penetration welds. This was thought to be caused by significant keyhole instability that exists under these welding conditions. A limited range of dual-beam laser process conditions was found that produced sound, pore-free laser welds with good top and underbead surface quality. Acceptable welds were produced at welding speeds of 6 to 7.5 m min^-1 using total laser powers of 4.5–5 kW, but only when the lead laser beam power was greater than or equal to the lagging beam power. The improved underbead quality was attributed to the effect of the second lagging laser beam on keyhole stability, venting of the high vapour pressure Mg from the keyhole and solidification of the underbead weld metal during full penetration dual-beam laser welding.