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.     

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.     

Analytical thermal model of conduction mode double sided arc welding

Abstract

An analytical thermal model of conduction mode double sided arc welding (DSAW) has been derived and used to predict the weld pool dimensions and shapes and temperatures within 2˙5 and 1˙15 mm thick AA5182 Al alloy sheets as functions of the primary DSAW parameters. Separate Gaussian distributed arc heat sources from a plasma arc welding and gas tungsten arc welding torch were assumed to act on the top and bottom surfaces of the sheets. There was excellent correlation between observed and predicted DSAW weld pool dimensions and shapes provided that suitable values for arc efficiencies and distribution coefficients for the two separate arcs were used in the model. The model is capable of predicting weld pool dimensions and shapes of both full and partial penetration conduction mode DSAW welds made in Al alloy sheet, the welding speed at which there is a transition from full to partial penetration welding and the speed above which no melting occurs.     

Pulsed gas metal arc welding of Al–Cu–Li alloy

Abstract

An experimental Al–Cu–Li–Mg–Ag–Zr type alloy in the form of 13.7 mm thick plates was studied for its fusion characteristics using gas metal arc welding (GMAW) and pulsed gas metal arc welding (P-GMAW). High copper 2319 filler of 1.6 mm diameter was used. The burn-off characteristics of 2319 filler wire in GMAW and P-GMAW were experimentally determined, including the relation between pulse current and pulse duration for the desired one-drop detachment per pulse (ODPP) condition and feasible range of pulse parameters. The effect of welding parameters on bead geometry and shape relationships was investigated through beadon-plate experiments in the welding current range above the spray transition current. Reasonably good weld beads were obtained in P-GMAW at currents as low as 194 A and welding speeds of 45 cm min^–1. P-GMAW yielded significantly higher weld penetration compared to GMAW.     

基于弧压反馈的薄板GTAW智能熔透控制

薄板GTAW的熔透控制是保障其焊接质量的关键,而焊接熔透的智能传感与识别是其中的核心难题. 传统的焊缝背面直接视觉传感可以有效提取熔透控制信息,但是往往因为可达性受限而不能使用. 因此,文中对基于GTAW弧压电信号的传感焊接熔透进行研究,基于弧压反馈提取焊接熔透信号,进而调节焊接电流波形保证焊缝均匀熔透而不发生烧穿. 试验采用4 mm厚热轧钢板、3 mm厚冷轧钢板进行表面堆焊. 结果表明,在不同电流形式下堆焊试验中验证了控制原理,反馈智能调节,变散热条件以及变间隙试验均可得到较美观的焊缝.     

Welding of magnesium alloys with activating flux

Abstract

The effects of an activating flux on AZ31B alloy welding were investigated. Alternating current tungsten inert gas (ACTIG) welding was used to weld 5·0 mm thick AZ31B alloy plates with CdCl₂, AlF₃ and TiO₂ activating flux. Applying the activating flux on the AZ31B alloy surface led to an increase in weld penetration depth. Various welding conditions, such as welding current, welding arc length, welding shielding gas flowrate, welding speed and flux thickness, influenced to different extents the ability of the activating flux to increase weld penetration. Furthermore, a high speed camera was used to monitor the arc images during welding. It was found that the brightest region of the arc was broader when CdCl₂ and AlF₃ were used, while the stability of the arc was increased when TiO₂ was used, especially in the positive electrode period. In summary, it is important that a uniform flux layer is present at the alloy plate surface and suitable welding parameters are selected.     

Experimental and numerical investigation of an electromagnetic weld pool support system for high power laser beam welding of austenitic stainless steel

A three-dimensional turbulent steady state numerical model was used to investigate the influence of an alternating current (AC) magnetic field during high power laser beam keyhole welding of 20 mm thick stainless steel AISI 304 being modeled as an ideal non-ferromagnetic material. Three-dimensional heat transfer and fluid dynamics as well as the electromagnetic field equations were solved with the finite element package COMSOL Multiphysics 4.2 taking into account the most important physical effects of the process. Namely, the thermo-capillary (Marangoni) convection at the weld pool boundaries, natural convection due to gravity and density differences in the melt volume as well as latent heat of solid–liquid phase transitions at the phase boundaries were included in the model.It is shown that the gravity drop-out associated with the welding of thick plates due to the hydrostatic pressure can be prevented by the application of AC magnetic field between 80 mT and 135 mT for corresponding oscillation frequencies between 1 kHz and 10 kHz below the weld specimen. Experimentally, a value of the magnetic flux density of around 230 mT was found to be necessary to allow for single-pass laser beam welding without sagging or drop-out of melt for a 20 mm thick combination of austenitic stainless steel AISI 304 and ferritic construction steel S235JRC at an oscillation frequency of around 2.6 kHz.     

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%.     

Vision based neurofuzzy logic control of weld pool geometry

Abstract

The geometry of the weld pool contains accurate, instantaneous information about welding quality. Thus, weld pool sensing and control plays a significant role in automated arc welding. Previous studies have focused on inferring penetration through models and controlling penetration by various methods, such as adaptive control, model based fuzzy logic, etc. In the present work, a weld pool imaging system employing a LaserStrobe (tradename) high shutter speed camera is used to obtain contrasting images and eliminate arcing interference. Two image processing tools based on edge detection and connectivity analysis extract online information about the weld pool length and width. A neurofuzzy control system elicited from both human experience and experimental results has been developed to control the welding current and welding speed in real time based on changes in weld pool dimensions. Closed loop control of welding speed is used to achieve desirable weld pool geometry.     

Effect of weakly ionised plasma on penetration of stainless steel weld produced with ultra high power density fibre laser

Abstract

A weakly ionised plasma can be generated in stainless steel welding with a 10 kW fibre laser beam at the ultra high power density of ～1 MW mm^–2 in Ar shielding gas. The objectives of this study are to obtain a fundamental knowledge of optical interaction between a fibre laser beam and the weakly ionised plasma, and to evaluate effects of the plasma on weld penetration. The optical interaction was investigated by the high speed video observation or the power meter measurement of another probe fibre laser beam, which passed horizontally through the weakly ionised plasma induced during bead on plate welding of a 20 mm thick type 304 plate with a 10 kW fibre laser beam of 0˙9 MW mm^–2 in power density. The probe laser observed was refracted at 0˙6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fibre laser beam. The attenuation of the probe laser was measured to be ～4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering. Moreover, a stable laser welding process could be produced at such ultra high power density that 11˙5 mm deep penetration was obtained even if the laser peak power was modulated 1 ms periodically from 10 to 8˙5 kW. It was consequently considered that the optical interaction between the 10 kW fibre laser beam and the weakly ionised plasma was too small to exert the reduction in weld penetration.     

直流叠加脉冲型TIG焊电弧─熔池特性分析

针对乏燃料后处理溶解器腐蚀裂纹等缺陷的快速、一次性、可靠焊接修复工程技术需求,提出一种直流叠加脉冲型TIG深熔焊接方法,在10和16 mm厚304不锈钢板上进行了系列平板堆焊工艺试验.以电弧-熔池特性变化为研究对象,对比分析脉冲电流作用和直流叠加脉冲型电流作用的电弧行为、熔池流动行为及温度场的变化规律,探讨了直流叠加至...     

Welding of titanium alloys with activating flux

Abstract

In the present paper, the effects of an activating flux on Ti–6Al–4V alloy welding were investigated. Tungsten inert gas welding was used to weld 8.0 mm thickness Ti–6Al–4V alloy plates. Results show that applying the activating flux on the Ti–6Al–4V alloy surface leads to an increase in weld penetration depth, whereas the corresponding weld bead width is reduced. It was also found that various welding conditions, particularly flux thickness, influence the effectiveness of the activating flux. Furthermore, a data acquisition system was used to monitor the current and voltage signals during welding. Results from monitoring of the welding current and voltage signals reveal that there is a clear correlation between the signals and the weld penetration when the welding arc is steady. Analysis of the acquired signals can be used to identify inconsistencies in weld penetration. In summary, to take advantage of the use of activating flux in Ti alloy welding, it is important that a uniform flux layer is present at the alloy plate surface and suitable welding parameters are selected.     

Friction welding phenomena of aluminium

Active flux TIG (ATIG) welding is a simple variant of the conventional TIG process that allows increased penetration of the weld and enables welding in one pass, with total penetration and without chamfer opening, for joints with thicknesses of 5 mm or more. Different mechanisms have been proposed to explain this effect, with emphasis on contraction of the arc due to the presence of negative ions and alteration in the movement of liquid metal in the weld pool, associated with variations of surface tension as a function of temperature. This study evaluates the effect of the amount of one flux of one component (Cr₂O₃) placed on the surface of the work piece, and the additions of KClO₄ and Al₂O₃, on the shape of the weld bead. Three sets of bead-on-plate weld tests were performed on 5-mm-thick ABNT 304 stainless steel plates. In the first set, the amount of flux used varied; in the second set, the effect of the additions of KClO₄ was studied; and in the third, Al₂O₃. Electric current and voltage were measured during the welding, and width, penetration and area were measured on cross sections of the weld bead. Results indicated a small variation in the welding voltage (increase less than 1 V) during the transition from TIG to ATIG welding. Surface flux concentration affected the weld penetration, with a rapid increase of penetration and weld bead area occurring, to concentrations between 7.5 and 15 g/m², followed by a milder variation for greater concentrations (up to 120 g/m²). On the other hand, whereas the addition of KCLO₄ clearly reduced the increase of penetration caused by the flux, the addition of Al₂O₃ had a less significant effect on the process.     

大厚度船用高强钢激光-电弧复合焊技术研究

为实现大厚度高强钢全熔透单道对接焊,针对厚度20mm的AH32船用高强钢,采用15kW大功率CO_2激光进行激光-电弧复合焊接.分析了工件坡口、焊接速度、送丝速度、离焦量、装配间隙等规范参数对焊缝成型影响;通过金相观察以及显微硬度测定分析了接头组织性能.结果表明:通过激光功率等焊接规范匹配,激光-电弧复合焊接能实现20mm厚板的全熔透单道对接;钝边为8mm的Y形坡口有助于提高厚板激光-电弧复合焊缝熔透能力;降低焊接速度有利于提高熔深能力;工件厚度较大时,装配间隙对焊缝熔深能力的影响较为显著;接头硬度表明厚板激光复合焊焊缝纵向热循环模式存在较大差异.     

Fibre laser welding of aluminium alloy

The objectives of this research are to investigate the effects of various welding conditions on penetration and defect formation, to clarify their welding phenomena and to develop the procedure of reduction of the defect. Fibre laser bead-on-plate welding was performed on several aluminium alloys, in particular A5083, at the power of 6 or 10 kW and several power densities from 0.4 kW/mm². It was found that the weld beads were narrower and deeper with an increase in the laser power density. For example, fully penetrated weld beads in 10 mm thick plates were produced at the laser power density of 640 kW/mm² and the welding speed of 10 m/min. However, convex–concave bead surfaces were formed. Moreover, in the case of the high power density, no porosity and many pores were present at high and low welding speeds, respectively. On the other hand, in the case of the ultra-high power density, few pores were generated in high speed welding. These reasons were interpreted by observing keyhole behaviour, bubble formation and the molten pool geometry during high power fibre laser welding with a high-speed video camera and microfocused X-ray transmission in situ observation method. Moreover, the porosity in the weld bead was reduced and prevented by the utilization of nitrogen gas instead of Ar gas, or the forward inclination angle of 40° (50° from the right angle) in Ar shielding gas.     

Microstructure characteristics of resistance spot welds of AZ31 Mg alloy

Abstract

The experimental investigation was carried out to study the weld microstructure of resistance spot welding of AZ31 Mg alloy 1 mm thick. A fine and homogeneous non-equilibrium microstructure of globular α grains, surrounded by eutectic mixtures of α and β (Mg₁₇Al₁₂), was achieved. The thermal–electrical–mechanical analysis model was employed to simulate the thermal history and the temperature gradient. It was found that a combination of the welding conditions and the particular thermophysical properties of the AZ31Mg alloy established a uniform temperature distribution throughout the weld pool and this thermal condition is ideal for nucleation throughout the melt metal and equiaxed grain structure forming.     

大厚板双面双TIG电弧打底焊熔池成形特性

采用视觉图像监测法对新开发的双面双TIG电弧错位同步打底焊熔池成形特性进行了研究.结果表明,根部预留间隙是保证打底焊根部熔合的有效措施,可以实现大厚板不清根焊接.脉冲TIG焊是大厚板打底焊的优选方法,峰值期间电弧熔化能力强,坡口根部两侧边缘熔化良好.基值期间熔池迅速凝固,防止了下淌.双TIG电弧错位同步打底焊时,后电弧的加入导致两弧之间工件高温区增加,前电弧熔池拉长.焊缝宏观金相显示母材充分熔化,没有侧壁未熔合,焊缝正表面成形圆滑过渡,没有气孔、裂纹、夹渣、咬边等成形缺陷.     

双面激光打底焊根部熔合模拟及组织演变分析

提出了一种双面摆动激光错位同步焊接新工艺,用于厚板大钝边的自熔打底焊接,开展了不同激光间距下打底焊接试验,建立了双面激光打底焊接有限元模型,获取熔池形貌,并与试验焊缝形貌进行对比验证,在此基础上分析了接头粗晶区的焊接热过程及组织演变.结果表明,双面摆动激光错位同步焊接可以实现大钝边打底焊缝的良好熔透成形;与单面激光打底焊接相比,双面激光焊接粗晶区800 ~ 500 ℃的冷却时间(t_8/5)和800 ~ 300 ℃的冷却时间(t_8/3)延长;当激光间距不低于50 mm时,粗晶区发生重新奥氏体化. 单面摆动激光焊接粗晶区的显微组织为板条马氏体,双面摆动激光焊接粗晶区的显微组织为板条马氏体和板条贝氏体,而且随着激光间距的缩短,板条贝氏体含量逐渐增加;双面摆动激光错位同步焊接利于改善接头性能,降低冷裂倾向.     

Detection and analysis of weld penetration in stationary TIG arc welding — a preliminary study

Abstract

A constant voltage microcomputer controlled welding source is used for on-line detection of weld penetration. By adding an instantaneous pulse at regular intervals to the normal welding current the molten pool is excited to oscillate in stationary TIG arc welding. By analysing the signal, the relationship between the frequency of molten pool oscillation and weld penetration is studied and a mathematical model, the weld pool oscillation equation, is established. Using a finite difference equation to solve the eigenvalue problem of the molten pool oscillation, a numerical solution can be obtained. Experimental results obtained from welding 1Cr18Ni9Ti, 20 # and 16Mn steel show good agreement with the theoretical predictions.     

Weld geometry and tensile strength in laser welded thin sheet metals

Abstract

Weld geometries and their effects on the ultimate tensile strength (UTS) of welded specimens were studied experimentally for type 316 stainless steel sheets. Bead on plate welding with 0.1 and 0.2 mm thick workpieces and lap welding with 0.1 mm thick workpieces were carried out. The results showed that the strength of lap welds is influenced by the weld geometry to a large extent. A three-dimensional quasi-steady state heat conduction model was developed to understand the heat flow mechanism during laser welding of sheet metals of finite thickness. The temperature distribution and weld geometry were calculated using the mathematical model and the theoretical results were found to compare well with experimental data. More energy is used to produce melt pools in thinner workpieces than in thicker ones. The model can be used to select suitable process parameters to achieve a predetermined UTS by controlling the weld geometry.