Modelling of electromagnetic processes in system 'welding arc – evaporating anode' Part 1 – Model of anode region

Abstract

A mathmatical model of electromagnetic processes occurring in the 'arc column – anode region – evaporating anode' system is presented. The anode region of electric arc with an evaporating metallic anode is described by a model, under which the non-equilibrium near anode plasma containing atoms and ions of the evaporated metal, along with atoms and ions of the ambient (inert) gas, is subdivided into a space charge layer immediately adjoining the anode surface and ionisation region adjacent to the arc column. This model allows determining the potential drop between welding arc column plasma and anode surface depending on the current density and plasma temperature near the anode, as well as upon the temperature of its surface.     

Modelling of electromagnetic processes in system 'welding arc–evaporating anode' Part 2 – model of arc column and anode metal

Abstract

A mathematical model of electromagnetic processes occurring in the 'arc column–anode region–evaporating anode' system is suggested. Electric charge transfer processes in the multicomponent plasma of the electric arc and in the bulk of the metallic anode are described using the Ohm's law and charge conservation law generalised for a case of discontinuity of the electric field potential at the boundary between the arc plasma and the anode surface. A procedure was developed for finding the numerical solution of the stated problem by the shock capturing method, allowing modelling of the electromagnetic processes in the 'welding arc–evaporating anode' system in the presence of a reverse potential jump in the anode region (negative anode drop). Results of modelling of the said processes for gas metal arc and plasma transferred arc welding of steel in an inert gas (Ar) atmosphere are given.     

Soviet welding inventions

Summary

The configuration of anode spots on the molten pool in GTA welding of thin plate is discussed on the basis of observations of the behaviour of slag particles under varying welding conditions and with regard to the base metal content of minor elements.

The positions of slag particles on the molten pool are largely influenced by surface flow on the pool caused by changes in the plasma stream direction due to small arc inclinations and by the steel's content of minor elements, such as sulphur and oxygen. Especially when the surface flow is slow in a stationary state, the anode spots are moved over a wide range by minor disturbances, and irregular weld beads due to anode spot movement are liable to occur.

The effect of the anode region size on the molten pool on melting of the reverse side during welding of thin plate is also discussed. The anode region size is largely influenced by the arc length and arc force (tungsten electrode tip shape), which spread the plasma stream in the higher‐temperature region, and by the content of minor elements which change the metal flow direction on the molten pool.

The anode region radius to molten pool radius ratios in a stationary arc with an arc length of 3 mm are 0.73 for a 45° conical tip at a low content of minor elements, 0.64 for a truncated conical tip at the same content, and 0.61 for a 45° conical tip at a high content of minor elements.     

低压环境对等离子弧穿透能力的影响

设计了一套水冷铜阳极电弧力测试装置,测量了不同环境压力下等离子弧在穿透一定厚度工件后电弧压力的径向分布情况,研究了焊接电流、环境压力对于穿透电弧压力的影响规律. 结果表明,等离子穿透电弧压力在电弧中心区域最大,沿着径向从中心至边缘迅速减小. 在同一环境压力下,随着焊接电流的增加,等离子穿透电弧压力增大;当焊接电流相同时,在一定的压力范围内,随着环境压力的降低,等离子穿透电弧压力显著增大,且环境压力变化对于等离子穿透电弧压力的影响比焊接电流变化对其的影响更显著.     

Effects of activating flux on arc phenomena in gas tungsten arc welding

Abstract

Dramatic increases in the depth of weld bead penetration have been demonstrated by welding stainless steel using the gas tungsten arc (GTA) process with activating fluxes consisting of oxides and halides. However, there is no commonly agreed mechanism for the effect of flux on the process. In order to clarify the mechanism, behaviour of the arc and weld pool in the GTA process with activating flux was observed in comparison with a conventional GTA process. A constricted anode root was found in the GTA process with activating flux, while a diffuse anode root was found in the conventional process. Furthermore, it is suggested that these anode roots are strongly related to metal vapour from the weld pool, which is also related to temperature distributions on the weld pool surface.     

外加纵向磁场GTAW焊接机理：Ⅱ。电弧模型

针对外加纵向磁场GTAW（gas tungsten-arc welding)焊接过程,采用小孔气体微压传感器法和钨极探针法分别测定了外加纵向磁场GTAW焊接电弧在水冷Cu阳极上的等离子流力和电流密度的分布,并对其规律进行了研究,建立了外加纵向磁场GTAW焊接电板等离子流力和电流密度的径向分布数学物理模型。     

直流磁场作用下铜蒸气对电弧特性的影响

为研究直流纵向磁场作用下金属蒸气对熔化极气体保护焊(gas metal arc welding,GMAW)电弧特性的影响,将钨铜复合材料制成特殊钨极代替熔化极产生铜蒸气,利用高速摄像法、光谱测温法以及小孔探测法对其进行了测试研究.结果表明,铜蒸气进入电弧等离子体后,电弧出现分层,随铜蒸气含量的增加,弧芯外围区域半径随之增加,弧芯区的尺寸减小.当铜含量为0%时,外加直流磁场后,电弧在阴极区收缩阳极区扩张,其轴向最高温度明显上升;电弧压力峰值偏离轴线,在外加磁场强度为0.015 T时呈现双峰分布,电流密度与电弧压力分布趋势相似;随着铜蒸气的介入,弧芯区电弧表现为阴极区收缩,阳极区扩张,弧芯周围的铜蒸气则明显收缩,电弧轴向最高温度上升的幅度明显降低.随着铜含量的增加,电弧的导电面积增加,环向电磁力作用减弱,电弧中心压力下降幅度显著降低,阳极电流密度的分布趋势逐渐趋于扁平化.     

Metal transfer instability in gas metal arc welding

Abstract

This work presents a simplified model of metal transfer in gas metal arc welding. The model incorporates key features of metal transfer including the change in droplet diameters as welding moves from the globular into the spray metal transfer region, and the increase in welding voltage that is observed to occur as the pendant droplet grows. The model predicts that an instability arises in the globular metal transfer region, which leads to deterministic chaos and complex limit cycles with many droplet sizes. The instability also causes deterministic chaos with a characteristic gap in droplet diameters at the transition to spray mode metal transfer. The model explains observed features of metal transfer in some detail, including the existence and location of preferred bands of droplet sizes. Whether the instability is present or not defines the boundary between chaotic globular metal transfer and the stable drop spray transfer mode. The identification of deterministic chaos in gas metal arc welding metal transfer opens the way for new approaches to welding control.     

基于图像分析的焊接电弧空间电流密度分布测量

焊接电弧空间电流密度分布是反映电弧热-力耦合作用、影响焊接冶金与成形质量的重要因素之一.文中提出了一种针对非熔化极气体保护焊（gas tungsten arc welding, GTAW）电弧的电流密度空间分布的测量方法:首先通过拍摄GTAW电弧在一定特征谱线波长上的图像,利用标准温度法计算得到电弧空间温度场的分布;然后通过建立电势场偏微分方程,根据温度与电导率的关系、边界条件、截面电流守恒条件,求解得到电流密度分布.经对比,该方法计算所得电流密度与文献中同等条件试验测得数据具有较好的一致性.该方法采用非侵入式方法实现电弧电流密度分布的测量,为实现焊接过程的实时监测和质量控制提供了技术基础.     

Gas tungsten arc welding studies on similar and dissimilar combinations of Al-Zn-Mg alloy RDE 40 and Al-Li alloy 1441

Abstract

The present study is concerned with gas tungsten arc welding of two high strength aluminium alloys, namely, an Al-Zn-Mg alloy (RDE 40) and an Al-Li based alloy of Russian grade 1441. One of the critical requirements of these two alloys is that they should be weldable. In the present work, weldability aspects of these alloys were studied in terms of solidification cracking tendency, microstructure, tensile properties, and microhardness across the welds. These studies were extended to dissimilar welds between RDE 40 and 1441 produced via conventional gas tungsten arc (GTA) welding as well as pulsed current GTA welding. It was found that RDE 40 welds were less sensitive to solidification cracking and weld metal porosity compared with 1441 alloy. The superior weldability of RDE 40 was related to the equiaxed nature of the fusion zone and a lower sensitivity to moisture pickup. It was possible to produce RDE 40-1441 welds without defects. Pulsed current welding of RDE 40 to 1441 showed improved mechanical properties compared with conventional GTA welding, and these were related to the refinement of the fusion zone microstructure.     

水冷铜阳极法测量阳极等离子电弧力径向分布

设计了一套水冷铜阳极装置,用压力传感器测量等离子弧焊阳极电弧力的径向分布情况,研究了焊接电流、离子气流量和弧长对阳极电弧力的影响规律.结果表明,阳极电弧力在电弧中心最大,随径向距离的增大而迅速减小,电弧力作用半径可以认为等于约束喷嘴出口半径;随焊接电流和离子气流量的增大阳极电弧力显著增大,电弧力作用半径变化不大;而弧长对阳极电弧力的影响不大;测量结果表明,等离子弧焊电弧力传统经验公式忽略电流对电弧力的影响是有一定缺陷的.     

工艺参数对耦合AA-TIG焊电弧阳极电流密度的影响

针对耦合电弧AA-TIG焊,采用基于不锈钢阳极的钨探针法研究了主要工艺参数对电弧阳极电流密度分布的影响规律.与常规TIG焊电弧相比,在相同条件下耦合AA-TIG焊电弧的阳极电流密度明显降低,并随着电弧电流和辅助电弧中氧气流量的减小以及钨极间距和弧长的增大而减小.当主钨极和辅助钨极的间距较小时,耦合AA-TIG焊电弧的阳极电流密度符合高斯分布;当钨极间距较大时,阳极电流密度向双峰分布过渡,电弧边缘部位符合二次高斯分布,而中心部位偏离二次高斯分布.     

Characterization of welding arc and weld pool formation in vacuum gas hollow tungsten arc welding

An experimental method is suggested to obtain the effective arc radii for various welding conditions in vacuum gas hollow tungsten arc welding. The irradiance distribution of welding arc next above the anode workpiece is obtained by applying Abel inversion algorithm to the CCD arc image, and then used to determine the distribution of arc heat flux, arc pressure and current density from the physical relations of arc irradiance, temperature and current density in gas tungsten arc welding. The resultant arc models are then adopted to simulate the gas hollow tungsten arc welding process for various gas flow rates.     

外加纵向磁场GTAW焊接机理：Ⅰ电弧特性

针对外加纵向磁场GTAW（gas tungsten-arc welding)焊接过程,采用红外热像伪着色法测定了外加纵向磁场GTAW焊接电弧的温度场,并建立了外加纵向磁场GTAW焊接电弧热流密度径向分布模型,对焊接电反外形的变化,焊接电弧电流,电压与外加纵向磁场强度变化的关系进行了研究。     

Arc characteristics of laser-TIG double-side welding

Abstract

Based on the experiments of laser-TIG double-side welding (LTDSW) for aluminium alloys, the influence of laser radiation on the arc behaviours of the opposite side was investigated. Generally, with the variation of laser power, there are three typical arc shapes: arc column convergence, arc root constriction and arc expansion. An important point to notice is that the laser keyhole preheating will induce the arc column convergence in the LTDSW. The arc voltage in the LTDSW is lower than that in TIG welding over the entire range of the experimental currents. Moreover, with increasing welding current, the difference in arc voltage between TIG welding and LTDSW is diminished because of the self-stabilisation of the arc burning at high currents. The complex transformation of arc behaviours has a great effect on the arc current density and its stability. The laser generated hot spot or laser induced plasma will have a higher temperature and greater electron density than neighbouring regions, and will offer the line of least resistance or the lowest potential drop. Hence, it is very reasonable that the arc voltage should descend under the influence of laser radiation, and the arc electrons should compress and root to the hot spot or plasma zone.     

Laser stabilisation of arc cathode spots in titanium welding

Abstract

Cathode spot formation is very pronounced during arc welding of titanium and titanium alloys. The dynamic behaviour of these spots was observed to interfere with metal transfer during welding, this interference being a fundamental cause of poor weld quality in these alloys. In the present work, stabilisation of the arc cathode spot with a focused Nd–YAG laser beam during pulsed gas metal arc welding of titanium was investigated. The laser beam was focused near the leading edge of the weld pool and the laser power and focus spot size were varied to determine the values required to confine the cathode spot to the laser focus position. The results showed that, for fixed welding conditions, the laser power required to prevent cathode spot motion varied as a function of focus spot size. The required laser power was minimised at 200 W for a spot size of 0.6 mm. The laser stabilised arcs had lower voltage but approximately the same current density as stabilised arcs. Increased welding speeds required marginally higher laser powers to stabilise the spot, but the minimum power was still attained with a 0.6 mm focus spot diameter. The laser power density required for stabilisation decreased as spot size was increased, varying from almost 10⁶ W cm^?2 at the smallest spot size to approximately 10⁴ W cm^?2 at the largest. Cathode spot stabilisation improved weld quality by reducing spatter generation and weld bead irregularity.     

Voltage change in the GMAW process due to the influence of a droplet travelling in the arc

Abstract

Experimental measurements have been made to investigate the meaning of the fluctuation or noise of electrical signals for the gas metal arc welding process through globular to spray transfer mode, with particular attention being paid to the so-called 'drop spray' transition mode. The results reveal that the welding arc voltage is significantly affected by the molten droplet travelling in the arc. A sharp drop in arc voltage appears just after the detachment of the droplet, reaching its minimum quickly and then recovering, according to the location of the droplet in the arc. Although the full explanation of the feature requires further study, we believe that an important influence is the geometrical effect. The existence of the droplet in the arc path significantly affects the geometrical shape of the arc and arc attachment at the anode, and hence the overall voltage of the arc.     

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.