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.     

Wire melting phenomena in gas metal arc welding

Abstract

The wire melting rate in gas metal arc welding using constant voltage power supplies is well described by a parabolic model that considers the effects of electrical resistance and arc heating. However, tests performed with constant current power supplies indicate that the melting rate can deviate from this model when welding current is close to the globular–spray transition value. Although the causes of this anomaly have not yet been well established, most authors consider them to be related to variations in liquid metal temperature close to the metal transfer mode transition current. The present work evaluates the effect of welding parameters on the wire melting rate when a constant current power supply is used. A numerical model that considers the contribution of both Joule and arc heating was developed to calculate the temperature distribution in the wire and its melting rate. The model was used to assess the changes in either arc heating or metal vaporisation that might explain the melting irregularities. High speed cinematography was used to analyse metal transfer in the current range associated with the melting irregularities.     

Analysis of dynamic plasma behaviours in gas metal arc welding by imaging spectroscopy

For gas metal arc welding, the effect of CO₂ mixture in a shielding gas on a metal transfer process was investigated through the observation of the plasma characteristics and dynamic behaviour at the droplet’s growth-separation-transfer by the temperature measurement methods which were suitable, respectively, to the argon plasma region and the metal plasma region. At the present experimental conditions, the metal transfer process was a spray transfer type with 100%Ar shielding gas. On the other hand, with 85%Ar + 15%CO₂ shielding gas, the metal transfer process was a globular transfer type in which the arc length was shorter, the width was narrower and the time interval of the droplet separation was longer. For both shielding gases, the metal plasma region near the arc central axis exhibited 6500–7500 K, which was lower than the argon plasma region. With 85%Ar + 15%CO₂ shielding gas, when the metal droplet grew below the electrode wire, the region below the droplet has a high plasma temperature and a high concentration of iron vapour which surrounded the droplet. The region also exhibited a remarkably high electron number density. At the spray transfer process, the argon plasma region had an electron number density twice as high as the metal plasma region. Meanwhile, at the globular transfer process, the metal plasma region had a higher electron number density than the argon plasma region, which corresponded to a higher electrical conductivity near the arc axis. This means that the electric current goes through the arc axis easier than the spray transfer process. This condition increases the temperature below the droplet. The thermal expansion increases the force preventing the droplet from falling down. In consequence, the metal transfer takes the globular transfer type.     

Analysis of arc voltage,current, and light signals in pulsed gas metal arc welding of aluminium

Abstract

The use of readily acquired weld arc data such as voltage, current, or light emissions to identify droplet transfer events or transfer mode, or for quality purposes such as seam tracking is of interest for thin section applications in arc welding of aluminium. In the present study, voltage, current, and light emission signals are acquired at high rates, and synchronised with high speed shadowgraphing. The usefulness of the various signals for identifying droplet detachment events and transfer modes during pulsed gas metal arc welding of aluminium is compared. It is found that individual droplet detachments are consistently identifiable during globular, spray, and streaming transfer, and during peak and background current detachment, using the voltage signal. Although the arc light signal also contains droplet detachment data, it is difficult to identify droplet detachments occurring during the background phase at low background current levels. Preliminary measurements show that these signals can also be used for seam location in thin section lap fillet welding.     

Study on metal transfer modes and welding spatter characteristics of self-shielded flux cored wire

Abstract

The metal transfer behaviour of self-shielded flux cored wire and the resultant welding spatter were investigated by using a high speed camera. Three modes of metal transfer were found, i.e. bridging transfer without arc interruption in explosive form or by surface tension, globular repelled transfer and droplet transfer, while the former two modes played a key role in the weld metal transfer. Correspondingly, the bridge explosion, discontinuous globular repelled process and the misalignment of droplets transfer were the main factors causing the welding spatter.     

Metal transfer and arc behaviour of novel consumable and non-consumable electrode indirect arc droplet welding

Abstract

The consumable and non-consumable electrode indirect arc droplet welding is a novel method that applies an indirect arc generated between a consumable electrode and a non-consumable electrode to welding. It was successfully used to overlap join the thin zinc coated steel plate. In the point to point (consumable electrode and non-consumable electrode) configuration, the stable metal transfer was obtained. The globular, projected spray and streaming spray transfer modes were promoted. With the increase in welding power, the metal transfer became faster and more stable. At the same time, the indirect arc became brighter, smaller and more stable. The arc self-regulation guaranteed the stable burning of the indirect arc. The desire stability was observed from the fluctuation of the welding current and the arc voltage waveforms. The side concentrated melting on the root of the droplet produced high pinch effect, which facilitated the detachment of the droplet.     

Effects of shielding gas composition on arc characteristics and droplet transfer in tandem narrow gap GMA welding

The effects of shielding gas composition in tandem narrow gap gas metal arc welding were studied. The shielding gas included argon, carbon dioxide and helium. The arc characteristics and droplet transfer process were analysed. The results show that in the same welding parameters, the trail wire welding current is higher than the lead wire welding current. With the increase of carbon dioxide content, the welding currents of two wires decrease, and the trail wire droplet transfer mode transforms from spray transfer to projected transfer. With the increase of helium content, the welding currents increase and the lead wire droplet transfer mode transforms from projected transfer to spray transfer. The weld width is the largest when the shielding gas mixture is 80%Ar10%CO₂10%He.     

Stability of droplets in gas metal arc welding

Abstract

A method is developed to compute the stability of droplets detaching from the consumable electrode in gas metal arc welding. An aluminium electrode is used as the model system. Theoretical predictions and supporting experiments are used to ascertain the rate of transfer of droplets for a range of current encompassing globular and spray transfer, and to examine the transition between these distinct modes of metal transfer. The method uses dimensional analysis to correlate the mass, rate of transfer, and force acting to detach droplets. The correlation is found to undergo a notable change in slope at the transition between globular and spray transfer. The accuracy of the correlation is assessed by comparing the theoretical and measured rate of transfer of droplets in gas metal arc welding using aluminium electrodes of variable diameter.     

Mapping transfer modes for stainless steel gas metal arc welding

Abstract

A methodology for the construction of transfer mode maps for stainless steel gas metal arc welding, with argon and argon–oxygen shielding gases, is presented. A back lighting laser and high speed video camera were used for visualisation and measurement of droplets and electrode extension. The reasons for the use of a groove, instead of the traditional bead on plate method, and of same volume beads are discussed and the results assessed. Unlike in other mapping procedures, mapping was conducted as a function of welding current and arc length. In addition, transfer rate v. welding current or wire feedrate curves were plotted. The results show the importance of the use of both maps and curves for identification and quantification of the shielding gas effects on the transfer mode. The results also suggest that an increase in oxygen content in the shielding gas reduces the values of transition current and transition wire feedrate (as expected), but also that it reduces the transfer rate and droplet size at the globular–spray transition.     

Hybrid laser-arc welding of advanced high-strength steel

This study investigated the synergetic effect between laser beam and electrical arc during hybrid welding by using a spectral diagnostic technique. The synergetic effect increased the energy density in the keyhole and deepened the weld penetration, resulting in a lower plasma electron temperature. The metal transfer mode was a globular one at a small offset distance while a spray mode was achieved with an increase in the offset distance. The decrease in the arc voltage and arc current due to the synergetic effect caused this transition from spray to globular modes. Globular transfer mode destabilized the molten pool and keyhole with the large droplet impingement, leading to the formation of porosity in the corresponding weld bead. The presence of porosity was on-line detected by identifying serious fluctuations in the Fe I electron temperature signals based on the fact that the instability of the molten pool and keyhole is strongly related to the signals coming from the plasma.     

Droplet transition for plasma-MIG welding on aluminium alloys

The synchronous acquisition system of droplet image inspection and arc electric signals were established and the droplet transition characteristics of aluminum alloys were researched in the plasma-MIG welding process.Typical droplet transition modes include globular transfer mode,short circuiting transfer mode,metastable spray transfer mode and projected transfer mode.The result indicates that MIG droplet transfer frequency and droplet transfer modes are changed by introducing the plasma arc in the plasma-MIG welding process compared with the MIG welding on the aluminum alloys,which broadens the range of welding parameters when the stable welding process proceeds.The metastable spray transfer and projected transfer mode are proved to be the most optimal modes by comparing the stability of electronic signal,droplet transition,weld appearance and weld penetration.     

A novel molten wire tungsten inert gas welding process

ABSTRACT

A novel molten wire tungsten inert gas welding process was presented. In this process, a welding arc is used to melt the workpiece, and a melting arc is used to melt the feeding wire. Metal transfer is separated from the melting of the wire and the workpiece, and the arcs are stable no matter the droplet transfer mode is spraying or globular. The spatter rate in the process is close to zero. Mechanical properties of low carbon steel joint bonded with this process are superior to those bonded with the metal inert gas welding process.     

Visualization of gas metal arc welding on globular to spray transition current

The transfer modes in gas metal arc welding have important effects on welding quality. However, present study of metal transfer modes is not yet fully understood. In this study, welding arcs was visualised using the optical emission spectroscopy technique. The carbon steel wire electrode was used for welding with 80% Ar?+?20% CO₂ shielding gas. The results showed that the globular to spray transition current was 330–350?A. During globular to spray transition, argon,CO₂ and Fe plasma tended to gradually change from localising near the arc axis to a two-layer structure having 11,000?K in high-temperature region away from the arc axis and around 7000?K in low-temperature region near the arc axis.     

低碳钢熔化极等离子弧焊接工艺

对Q235低碳钢进行熔化极等离子弧焊接工艺试验,并研究其工艺参数对焊缝成形和熔滴过渡的影响.结果表明,与常规的熔化极惰性气体保护焊相比熔化极等离子弧焊接在提高焊接效率、减少飞溅和气孔等方面具有优越性.当其它条件不变时,焊缝熔宽随等离子电流增大而变宽,而对熔深和熔滴过渡类型影响不大;熔化极等离子弧焊接电流增大时,U - I相图的线簇随着电流增大而集中,熔滴过渡依次为短路过渡、大滴过渡和射流过渡.     

药芯焊丝CO2焊熔滴过渡现象的观察与分析

药芯焊丝CO2气体保护焊熔滴行为对药芯焊丝工艺性有直接的影响.采用焊接过程的高速摄影技术和汉诺威焊接质量分析系统对药芯焊丝的熔滴过渡形态进行观察分析.证实随着焊接参数的变化,药芯焊丝可能形成排斥过渡、表面张力过渡和细熔滴过渡.在排斥过渡时,由于大熔滴在焊丝端部较长时间的停留,出现明显的熔滴自身的爆炸飞溅,气体的强烈逸出飞溅等现象,表面张力过渡是介于大熔滴的排斥过渡与细熔滴过渡二者之间的一种过渡形式.细熔滴过渡时,焊接过程稳定、飞溅小、焊缝成形良好、生产效率较高,是药芯焊丝理想的过渡形式.在细熔滴的过渡的条件下形成的渣柱对熔滴的过渡起着导向作用.     

Microstructural analysis of the anode in gas metal arc welding (GMAW)

The nature of the applied shielding gas has a strong influence on arc stability and transfer metal mode of the welding process. In particular, increase of the percentage of carbon dioxide in argon induces the increase of the transition current value from the globular to spray metal transfer mode. This work shows that these effects are linked to the chemical and microstructural modifications of the anode tip during the gas metal arc welding process. The microstructure of the anode is investigated for various experimental conditions. Transition between the two transfer modes is linked to the existence and disappearance of a rather insulating oxide “gangue” at the wire extremity whose nature depends of the shielding gas. Chemical reactions at high temperature such as oxidation–reduction reactions between shielding gas and melted metal govern the transition of the spray-arc to globular transfer mode.     

双丝间接电弧氩弧焊的熔滴过渡

采用氙灯背光高速摄像系统及示波器对双丝间接电弧氩气保护焊的熔滴过渡及其对应的电压、电流波形进行了研究.结果表明,双丝间接电弧氩弧焊焊接电流与电弧电压的不同匹配选择,熔滴具有短路过渡、大滴过渡、混合过渡、射滴过渡、射流过渡等不同过渡形式.随着焊接电流的增大熔滴尺寸减小,熔滴细化,随电弧电压的增大,熔滴尺寸减小.熔滴过渡形式与电压、电流的波形之间有很好的对应关系.     

面向工程应用的BA-GMAW熔滴过渡形态

综述了涉及工程应用的潜弧熔化极气体保护焊(Buried arc gas metal arc welding,BA-GMAW)熔滴过渡形态特征。结果表明,在大电流和相应的焊接参数下,BA-GMAW工艺的熔滴过渡形态有3种,即呈滴状过渡、摆动过渡和旋转过渡。BA-GMAW电弧空腔中电弧形态属于连续、敞开、非活动型;电弧空腔内氛围发生了质的变化,电弧爬升到熔滴上方,满足了喷射过渡形成3要素。焊接工艺参数对BA-GMAW熔滴过渡形态的影响,主要是焊接电流和电弧电压的影响;前者主要改变熔滴上作用力方向和大小,后者涉及潜弧的稳定性。潜弧焊熔滴过渡形态与工艺质量的利好关系,是受稳弧技术控制的熔滴过渡特性改善所决定的。创新点：通过典型工程应用试验案例中的焊接工艺参数,分析、判断相应的电弧和熔滴过渡形态,完成从实践到理论的转换,为后续论点论述打下坚实基础。     

Metal transfer characteristics of GMAW with strip electrode

Abstract

The metal transfer process of gas metal arc welding with strip electrode is observed by a high speed digital camera system. Because the rectangular strip electrode has a large width/thickness ratio, the pendant droplet is elliptical in shape, and multicurrent channels are generated. The Lorentz force induced between the multicurrent channels drives the droplet and welding arc to move along the strip electrode end, but the droplet always lags behind the welding arc. The movement results in uncertain droplet’s detaching location and transition trajectory. The projected transfer mode is promoted, and the streaming and rotating transfer modes are restrained. With the increase in welding power, the droplet motion is faster but more stable. The stability can be reflected from the fluctuation of the welding current and arc voltage waveforms.