Controlled bridge transfer (CBT) gas metal arc process for steel sheets joining

In non-pulsed gas metal arc welding (GMAW), spatter can be reduced by lowering the short-circuit current to a low level just before the re-arcing. The reduction in spatter requires an improvement in the accuracy of predicting the re-arcing by stabilizing the metal transfer and improving the robustness of the accuracy against disturbances. The controlled bridge transfer (CBT) process optimizes the accuracy of predicting the re-arcing in real time in response to the metal transfer, realizes spatter reduction and stable arc in non-pulsed GMAW. Traditionally, GMAW is carried out using electrode positive polarity. However, this polarity is not sufficient for welding extra-thin steel sheets, specifically those thinner than 1.0 mm. With electrode negative (EN) CBT process, although slight arc voltage fluctuation occurs caused by the behaviour of cathode spots on the tip of the wire during EN polarity GMAW, instantaneous voltage uses command computation to improve the transient response against the disturbance. Consequently, a stable arc can be obtained without increasing the number of short circuits in a unit time to obtain spatter-free welds.     

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.     

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.     

Wear mechanism in contact tube

Abstract

Stable sliding contact is the most important factor in the stable arc discharge of solid wire. If the contact tube wears seriously, arc discharge will become unstable and the wire position will be shifted. Ordinary non-Cu-coated solid wire causes much wear of the contact tube, about 10 times that caused by Cu-coated solid wire. The mechanism of contact tube wear is not only friction at high temperature, but also a molten bridge formed between the contact tube and wire surface. A complex of superfine particles decreases the amount of contact tube wear. For example, the mixture of graphite and magnetite reduces the amount of contact tube wear by 80%. These particles form an interface layer on the inner surface of the contact tube, and the layer prevents the transfer of molten Cu to the wire surface. The interface layer mainly consists of FeO and complex oxides.     

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.     

Neural network model of heat and fluid flow in gas metal arc fillet welding based on genetic algorithm and conjugate gradient optimisation

Abstract

Although numerical calculations of heat transfer and fluid flow can provide detailed insights into welding processes and welded materials, these calculations are complex and unsuitable in situations where rapid calculations are needed. A recourse is to train and validate a neural network, using results from a well tested heat and fluid flow model to significantly expedite calculations and ensure that the computed results conform to the basic laws of conservation of mass, momentum and energy. Seven feedforward neural networks were developed for gas metal arc (GMA) fillet welding, one each for predicting penetration, leg length, throat, weld pool length, cooling time between 800°C and 500°C, maximum velocity and peak temperature in the weld pool. Each model considered 22 inputs that included all the welding variables, such as current, voltage, welding speed, wire radius, wire feed rate, arc efficiency, arc radius, power distribution, and material properties such as thermal conductivity, specific heat and temperature coefficient of surface tension. The weights in the neural network models were calculated using the conjugate gradient (CG) method and by a hybrid optimisation scheme involving the CG method and a genetic algorithm (GA). The neural network produced by the hybrid optimisation model produced better results than the networks based on the CG method with various sets of randomised initial weights. The CG method alone was unable to find the best optimal weights for achieving low errors. The hybrid optimisation scheme helped in finding optimal weights through a global search, as evidenced by good agreement between all the outputs from the neural networks and the corresponding results from the heat and fluid flow model.     

Novel rotation arc system for narrow gap MAG welding

Abstract

The present paper develops a hollow axis motor driven high speed rotation arc system for narrow gap welding (NGW), and introduces the features of this system. Some welding experiments were then carried out to investigate the characteristics of welding wire melting and weld formation for this new process. Experimental results show that the melting rate of wire increases and the residual melting ball diameter of wire tip decreases respectively with an increase in rotation speed, and this melting rate is higher in pulsed welding and NGW respectively than in dc and flat plate welding. Furthermore, the arc rotation can obviously improve the penetration into NGW groove sidewalls and bead shape, and thus the system has been used to weld practical NGW joints successfully.     

AC–GTAW of aluminium – new perspective for evaluation of role of positive polarity time

Abstract

Experiments were carried out to determine the effect of the positive polarity time on cathodic cleaning, arc voltage, arc power and weld bead geometry on AC–GTAW of aluminium. Welding current was kept constant in both positive (EP) and negative polarity. During the experiments current and voltage monitoring was performed to further analysis of arc voltage and arc power. There was an increase in the weld depth and fusion areas when longer EP times were used. This behaviour differs from the traditional literature relating to welding processes with non-consumable electrodes. However, this increase occurs from a certain point, when the EP time is >4·0 ms over a total period of 20·0 ms. The use of a minimum EP time was found to be sufficient to perform the oxide removal, yielding weld beads with satisfactory fusion characteristics and minimum wear of the tungsten electrode.     

Pendant droplet oscillation during GMAW

Abstract

This paper deals with pendant droplet oscillation in pulsed current gas metal arc welding (GMAW). Using a constant current power source, bead on plate welds were made on mild steel plates. The frequency of the pendant droplet oscillation was monitored visually (using a high speed video camera) and electronically (by analysis of the voltage signals). The results obtained are compared with the results predicted by a model based on a mass spring system. It was found that the oscillation frequency of small droplets revealed by both monitoring techniques matches that predicted by the model. For larger droplets, the oscillation frequency measured by voltage signal analysis tends to be higher than that revealed by video analysis and that predicted by the model. This implies that during droplet oscillation in (GMAW) the arc behaviour may change and as a result the arc voltage is not any more proportionally related to the arc length.     

Arc power and efficiency in gas tungsten arc welding of aluminium

Abstract

A calorimetric study of gas tungsten arc welding of aluminium is described. The present study comprised experiments in which autogenous welding runs were each made on a block of electrical conductor grade aluminium. The blocks were all approximately cubic in shape which, when combined with the high thermal conductivity of aluminium, ensured that their temperature equalised soon after the completion of a run. Each sample was immersed in insulating material before welding so that heat losses to the surroundings were minimised. Thermocouples were attached to the block in each experiment and the bulk temperature rise was related to the energy input associated with the welding run. The effects of arc polarity, alternating current balance, shielding gas composition, arc length and welding current on the arc power and arc efficiency were investigated. The results obtained with alternating current are compared to those for direct current, and the differences are explained.     

Sensing and seam tracking of welding line in backingless V groove welding

Abstract

The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors tried to apply the switch back welding method to butt welding to join discs with flat metal plates without backing plates. During the welding, the gap was detected by processing the weld pool image taken with a CCD camera. Moreover, the tip of the electrode wire is changing owing to a curve of its wire extension in accordance with a rotation of the torch axis. Even though teaching is achieved before the welding, the weaving centre is shifted from the gap centre. The controller of the seam tracking was designed to prevent any steady state error.     

Parametric approach to positional fault detection in short arc welding

Abstract

This work investigates a parametric technique for short arc welding fault detection. The independent component analysis algorithm is applied to split welding voltage and current signals into arcing and short circuiting regions. A series of parameters including means, medians and normalised inner products are calculated by processing the different sections of welding electrical data. Out of position welds in overlap joints are studied with investigation of the connection between parameters and short arc welding features. The effect of changing shielding gas is also illustrated. Figures of merit to quantify the change in various parameters between faulty and good welds are compared and discussed. It is shown that the data separation into two regions with independent component analysis is useful in providing various parameters that can assist in short arc welding fault detection under different conditions.     

Sub-nanosecond monitoring of micro-hole electrical discharge machining pulses and modeling of discharge ringing

Monitoring the gap voltage and current in micro-hole electrical discharge machining (EDM) using high-speed data acquisition with 0.5 ns sampling period is conducted. The spark and arc pulses at three stages, namely electrode dressing, drilling, and penetration, of the micro-hole EDM are recorded. The EDM process parameters are setup to use negative polarity to blunt the electrode tip and positive polarity for micro-hole drilling and penetration. A new phenomenon of pre-discharging current is discovered. In the first 20–30 ns of spark and arc pulses, the current starts to rise while the voltage remains the same. Effects of EDM process parameters, including the open voltage, electrode diameter, and polarity, on the rate of spark and arc pulses and electrode feed rate are investigated. A model based on the RLC circuit is developed to study the ringing effect at the end of a discharge. The intrinsic parasitic capacitance and resistance of a RLC circuit are calculated from the decaying voltage signal and compared under two sets of experiments with varying wire electrode diameter and gap voltage to validate the ringing model. The calculation and experimental results validate the proposed RLC model for ringing phenomenon. The model shows the electrode diameter has negligible effect on ringing and high open voltage increases the parasitic resistance and damping in ringing. The monitoring technique and ringing model developed in this research can assist in the selection and optimization of micro-hole EDM process parameters.     

Effect of power source characteristic on CO2 short circuiting arc welding

Abstract

Some problems are reported concerning the observation of the weld pool and effects of the power source characteristic on arc stability in CO₂ short circuiting arc welding. First, the effect of a power source with a constant voltage characteristic on CO₂ short circuiting arc welding is investigated by analysing the behaviour of the voltage and the current. From the results of this analysis, the cause of spatter generation is identified. A new power source characteristic is then proposed to improve the stability and the self-regulation of the arc. By adjusting the power source characteristic, the metal transfer can be stabilised in the CO₂ short circuiting arc welding process, i.e. the present authors have developed a power source having a non-linear characteristic. Its performance is verified by carrying out experiments.     

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.     

New stability index for short circuit transfer mode in GMAW process using acoustic emission signals

Abstract

A new index for the analysis of stability of GMAW processes taking as a base the acoustic emission generated by the arc during the short circuit metal transfer is presented in the present work. The incidence of parameters of process is analysed in the indices of the acoustic emission signal, as well as its influence on the stability, evaluated through a new proposed index. The results obtained allowed having a relationship between acoustic signals and arc voltage signals, and demonstrated the validity of proposed index of acoustic emission for analysis of stability of GMAW process. The obtained results were then compared with other classic stability analysis methods based on statistical analysis of temporal signals of arc voltage. Finally a statistical analysis for the validation of the obtained experimental results was carried out. As a result of the investigation, the effectiveness of the method proposed as a new way for the analysis of stability is demonstrated. The research can contribute towards a new standard to evaluate the stability in welding processes.     

Analysis of electrical characteristics for hybrid pulsed micro-tungsten inert gas welding arc

Abstract

The present paper experimentally investigates and analyses in detail the electrical characteristics of the arc for the hybrid pulsed micro-tungsten inert gas process. In particular, an average voltage–current (V–I) graph plotting approach is proposed to describe the dynamic characteristics of the hybrid arcs using the filtered voltage and current of the arc. It is shown that the average voltage and electrical resistance of the heavily hybrid pulsating (HHP) arc are lower than those of the slightly hybrid pulsating (SHP) arc, particularly during the background current stage. Furthermore, the analyses reveal that the average V–I characteristic graph for the arc forms a hysteresis loop, primarily because of the thermal inertia of the arc, during a pulse period. This closed loop is larger for the SHP arc than for the HHP arc, and shrinks longitudinally with increasing frequency and width of the current pulse. These results thus demonstrate that the HHP arc has a greater thermal inertia, particularly at higher current and pulse frequency. Experimental results show also that the mean voltage of the arc and the features of the hysteresis loop vary markedly with weld penetration in addition to pulsating frequency and pattern, and thus suggest the possibility of monitoring the hybrid pulsed weld penetration via arc sensing.     

Arc heating hot wire assisted arc welding technique for low resistance welding wire

Abstract

An assistant arc was used to preheat the wire for hot wire tungsten inert gas (TIG) welding which was independent on the resistance of the welding wires and avoided the drawbacks of the traditional hot wire preheating method. The new method is suitable for low resistance wire such as copper and aluminium. The experimental results showed that the wire temperature varied linearly with preheating current and hyperbolically with wire feeding speed. The temperature of wires achieved 60% of their melting points when LF6, H90, HS201 and H08Mn2Si welding wires were used at a current less than 50 A. With arc assisted hot wire, the welding deposition efficiency of the HS201 wire was increased by 96% compared with the traditional TIG welding method, while its microstructure was similar to that of the cold wire welding.     

Dependence of the melting rate of the electrode in CO2 welding on electrode extension

The data obtained for the variation of the melting rate and voltage in CO₂ welding with the change of the electrode extension are analysed. The dependences of the mean resistance of the unit length of the extension, the increase of the melting factor and the arc voltage on the electrode wire diameter are determined. The appropriate approximating dependences are derived. The average value of the melting factor in the reverse polarity arc at zero extension is determined.     

稀土钨电极在电推进系统中的耐烧蚀性能研究

阴极作为电弧加热发动机的“心脏”,担负着加热工质气体产生推进力的作用,由于卫星服役周期长,点火次数频繁,对阴极材料的寿命及耐烧损性能提出了很高的要求。为寻求耐烧损性能优异的阴极材料,以W-La-Ce-Y与W-La-Y-Zr电极为基础,采用氢还原-中频烧结的方法制备W-La-Ce-Y-Re与W-La-Y-Zr-Re电极材料。模拟发动机实际工况在250A电流下进行燃弧试验,测量燃弧后电极的质量损失并对电极尖端的组织形貌进行观察表征。结果表明,加铼电极在燃弧2h后的烧损质量更少,尖端形貌变化程度更小,耐烧损性能更好。在扫描电镜下观察到铼的加入抑制了尖端表面枝晶的生长,弱化了钨基体晶粒的再结晶程度,并探讨了稀土氧化物的迁移途径;在金相显微镜下观察到加铼电极的再结晶晶粒更细小,铼起到了细化再结晶晶粒的作用。在不同工作电流下测得加铼电极的阴极压降更低,铼的加入提升了电极材料的电子发射性能。