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.     

Time–frequency diagram applied to stability analysis in gas metal arc welding based on acoustic emission

Abstract

Time–frequency diagram is applied to estimating the stability of gas metal arc welding process. A methodology has been developed to obtain indexes based on image processing of the spectrogram in order to evaluate the relationship between the spectrogram and the stability of process. The acoustic emission (AE) signals have been processed and characterised to obtain the spectrogram of the acoustic signals generated by arc. Statistical analysis of image generated by spectrogram and temporal parameters of AE signals has been used as a tool to obtain a method for stability evaluation. As a main result of the research, it demonstrates the effectiveness of the application of image processing of the time–frequency diagram for evaluating the stability in the welding processes. The results demonstrate the validity of this method to characterise the stability using the image characterisation.     

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.     

The humping phenomenon during high speed gas metal arc welding

Abstract

A commonly observed welding defect that characteristically occurs at high welding speeds is the periodic undulation of the weld bead profile, also known as humping. The occurrence of humping limits the range of usable welding speeds in most fusion welding processes and prevents further increases in productivity in a welding operation. At the present time, the physical mechanisms responsible for humping are not well understood. Thus, it is difficult to know how to suppress humping in order to achieve higher welding speeds. The objectives of this study were to identify and experimentally validate the physical mechanisms responsible for the humping phenomenon during high speed gas metal arc (GMA) welding of plain carbon steel. A LaserStrobe video imaging system was used to obtain video images of typical sequences of events during the formation of a hump. Based on these recorded video images, the strong momentum of the backward flow of molten metal in the weld pool that typically occurred during high speed welding was identified as the major factor responsible for the initiation of humping. Experiments with different process variables affecting the backward flow of molten weld metal were used to validate this hypothesis. These process variables included welding speed, welding position and shielding gas composition. The use of downhill welding positions and reactive shielding gases was found to suppress humping and to allow higher welding speeds by reducing the momentum of the backward flow of molten metal in the weld pool. This would suggest that any process variables or welding techniques that can dissipate or reduce the momentum of the backward flow of molten metal in the weld pool will facilitate higher welding speeds and productivity.     

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.     

Microstructures of gas metal arc weld metal of 950 MPa class steel

Abstract

The effects of shielding gas composition on the properties and microstructure of single pass weld metals produced by GMA (gas metal arc) groove welding of 950 MPa class steel plates have been investigated. The shielding gas employed was a mixture of argon (Ar) and carbon dioxide (CO₂) (0–25%), and the weld heat input was ～3 kJ mm. With increasing CO₂ content, the hardness of the weld metal decreased from 380 HV to 280 HV, and the absorbed energy of the Charpy impact test decreased from 130 J to 90 J. The microstructures of the weld metal, consisting primarily of low carbon martensite and carbide free bainite, became more bainitic as the CO₂ content of the shielding gas was increased. It was also found that the MA constituent, embrittling microstructure, was formed in the granular bainitic area, the volume fraction of which increased with the CO₂ content of the shielding gas.     

Modelling weld bead geometry using neural networks for GTAW of austenitic stainless steel

Abstract

The authors analyse the importance of different weld control parameters on the weld pool geometry of gas tungsten arc welding using an online feature selection technique that suggests weld voltage and vertex–angle pair as more important than the weld voltage and torch speed pair. Using the selected features multi layer perception and radial basis function networks are developed for prediction of bead width, penetration depth, and bead area. With cross-validation the authors have extensively studied the performance of composite models (one model for all outputs) and individual models (one model for each output). The individual models are found to work better than composite models. Usually, radial basis function networks are found to work better than the multi layer perception networks. To assess the influence of weld control parameters the authors have studied the performance of both networks using different combination of inputs. Overall, the performance of the proposed models is found to be quite satisfactory.     

Prediction of weld quality in pulsed current GMAW process using artificial neural network

Abstract

Weld joint dimensions and weld metal mechanical properties are important quality characteristics of any welded joint. The success of building these characteristics in any welding situation depends on proper selection-cum-optimisation of welding process parameters. Such optimisation is critical in the pulsed current gas metal arc welding process (GMAW-P), as the heat input here is closely dictated by a host of additional pulse parameters in comparison to the conventional gas metal arc welding process. Neural network based models are excellent alternatives in such situations where a large number of input conditions govern certain outputs in a manner that is often difficult to adjudge a priori. Six individual prediction models developed using neural network methodology are presented here to estimate ultimate tensile strength, elongation, impact toughness, weld bead width, weld reinforcement height and penetration of the final weld joint as a function of four pulse parameters, e.g. peak current, base current, pulse on time and pulse frequency. The experimental data employed here are for GMAW-P welding of extruded sections of high strength Al–Zn–Mg alloy (7005). In each case, a committee of different possible network architectures is used, including the final optimum network, to assess the uncertainty in estimation. The neural network models developed here could estimate all the outputs except penetration fairly accurately.     

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.     

Hardness changes on pass by pass basis in mild steel gas metal arc welds

Abstract

A series of welds were fabricated in mild (ASTM A36) steel. The first pass was the entire length of the plate and each subsequent pass was indented ～25 mm. This allowed us to determine the change in hardness on a pass by pass basis by mapping the hardness over each weld pass.     

Through arc sensing in gas metal arc welding with signature images

Abstract

This work presents a new method for through the arc estimation of welding parameters in gas metal arc welding based on signature images. The method is generally applicable and makes use of a novel data fitting procedure. The work covers unassisted through the arc estimation, without auxiliary techniques such as welding head oscillation. The technique is demonstrated for both pulse and short circuiting welding, with estimation of position relative to an overlap joint and a narrow gap weld, and estimation of the width of a 'V' groove. The accuracy of the technique is affected by how closely the welding signatures match the fitting data and the signature image quality match provides a valuable check which shows when through the arc estimates cannot be relied on. Overall, the results demonstrate that through the arc measurements can have application in specialised situations, although accuracy is ultimately limited by phenomena that affect the arc and are not correlated with the measurement.     

Coating composition,weld parameter and consumable conditioning effects on weld metal composition in shielded metal arc welding

Abstract

Pipeline construction has been moving towards the use of higher strength low carbon steels as these become available. In order to achieve the required high strength and toughness to match those of the parent pipe, girth weld metals have to rely mostly on their composition. The aim of the present study was to characterise the influence of the consumable chemistry and moisture content, and the welding parameters on the composition of weld metals produced by E8010 and E9010 shielded metal arc (SMA) cellulosic consumables. The alloying compounds in the consumable coating were isolated using a caustic soda solution, analysed with electron backscattered diffraction and identified as a variety of oxides. Making use of a mechanised SMA welding machine, the effect on weld metal alloying element contents of the welding conditions was evaluated. Smaller welding arc voltages and lower consumable coating moisture content produced weld metals with richer chemistry. The element transfer mechanisms from the consumable coating to the weld metal are also discussed.     

On-line prediction of quality and shear strength of spacer pad welds of nuclear fuel pins by applying neural network analysis of acoustic emission signals

Abstract

In the present study, the development of an acoustic emission technique (AET) based methodology is reported for online prediction of quality and shear strength of spacer pad welds of nuclear fuel pins of pressurised heavy water reactors (PHWRs). The quality evaluation of spacer pad welds was made by classification of different weld categories using cluster analysis and artificial neural network (ANN) study of acoustic emission signals generated during welding. The ANN approach was also effective in arriving at the quantitative estimation of percentage correct classification between any two classes. For assessment of shear strength of individual coins of spacer pad welds by ANN, the properties of basic sigmoidal function were exploited and this could predict the strength of each coin with an accuracy of 97%. The results assume significance because instrumentation methodology is suitable for online application and complement the currently followed statistical quality control approaches for spacer pad weld assessment.     

Control of nitrogen content and porosity in gas tungsten arc welding of high nitrogen steel

Abstract

In welding of high nitrogen steel (HNS), it is essential to control the nitrogen content and porosity in the weld metal. In this paper, the influence of shielding gas composition and heat input on the nitrogen content and porosity in the weld metal of HNS was investigated by gas tungsten arc welding. The experimental results indicate that the weld nitrogen content increases as N₂ in the shielding gas is increased in the same heat input of welding. The weld nitrogen content decreases with increasing the heat input for pure argon used as a shielding gas, whereas it increases with increasing the heat input for the shielding gas including some nitrogen. The nitrogen pore can be avoided when the nitrogen content in the shielding gas is <4% in the heat input range of 528–2340 J mm^–1.     

Observations of the phenomenon of abnormal arc voltage occurring in pulsed metal inert gas welding of aluminium alloy

Abstract

At the time of arc reignition after short circuiting during electrode positive polarity, cathode spots are newly formed in the centre of the weld pool surface, where oxides scarcely exist. The work function of the cathode surface increases and the cathode spots concentrate because of the lack of sufficient oxides, leading to an increase in the potential gradient across the cathode fall space and the adjoining contraction space. Consequently, the arc voltage becomes abnormally high in spite of the short arc length. Moreover, when electrode polarity is switched from positive to negative immediately after a droplet has detached from the wire tip, cathode spots are newly formed on the surface of the molten metal remaining at the wire tip, where little oxide exists, leading to an abnormal increase in arc voltage as well. Therefore, the change in arc voltage does not necessarily indicate a fluctuation in the arc length.     

3D numerical study of effects of temperature field on sensitisation of Alloy 690 butt welds fabricated by gas tungsten arc welding and laser beam welding

Abstract

This study examines the effects of the temperature field on the sensitisation of Alloy 690 butt welds fabricated using the gas tungsten arc welding (GTAW) method and the laser beam welding (LBW) method respectively. The welding thermal cycles of the two welding methods are simulated using ANSYS software based upon a moving heat source model. The validity of the numerical model is confirmed by comparing the simulation results with the corresponding experimental findings. Agreement is found between the numerical results for the temperature field and the experimental temperature measurements. In addition, it is shown that the LBW weldment experiences a more rapid heating and cooling effect than the GTAW weldment, and therefore has both a smaller heat affected zone and a narrower sensitisation region. Thus, the validity and general applicability of the thermal welding model are confirmed.     

Effects of torch configuration and welding current on weld bead formation in high speed tandem pulsed gas metal arc welding of steel sheets

Abstract

Undercut and humping bead are the common defects that limit the maximum welding speed of tandem pulsed gas metal arc (GMA) welding. In order to increase the maximum welding speed, effects of the inclination angle, interwire distance and welding current ratio between the leading wire and trailing wire on bead formation in high speed welding are investigated. The undercut and humping bead is attributed to the irregular flow of molten metal towards the rear part of the weld pool. This irregular flow can be prevented by the trailing wire with a push angle from 5° to 13° , which provides an appropriate component of arc force in the welding direction. The irregular flow is also related to the distance between the leading wire and the trailing wire, and the flow becomes regular when the distance is in the range 9–12 mm. Moreover, the stabilisation of the bulge of the weld pool between the two wires, the presence of enough molten metal below the trailing arc, and the reduced velocity of molten metal flow towards the rear part of the weld pool, are essential to increase the maximum welding speed. These conditions can be obtained by adjusting the ratio of the leading arc current to the trailing arc current. A maximum welding speed as high as 4–4·5 m min^?1 is achieved by setting the current ratio to a value ranging from 0·31 to 0·5.     

气体保护焊短路过渡熔滴成形的建模分析

提出了气体保护焊短路过渡过程中的熔滴成形的概念,并建立了模型.通过微距高速摄像技术和数字图像处理技术对熔滴成形过程中小滴状熔液所受的重力、电磁力以及表面张力提供的支持力进行了分析和定量计算,认为表面张力所提供的支持力远大于电磁力和重力共同导致的促使小滴状熔液下落的力.同时考虑了焊丝的熔化,认为以上因素最终造成小滴状熔液能以滴状的形态不断在焊丝端面进行扩展,形成熔滴.试验证明了熔滴成形这一模型的正确性.     

Effect of magnetic field applied during gas metal arc welding on the resistance to localised corrosion of the heat affected zone in AISI 304 stainless steel

Resistance to localised corrosion at the heat affected zone (HAZ) of AISI 304 stainless steel gas metal arc welded (GMAW) in presence of magnetic fields of different intensity was studied. Samples of HAZ, 8 mm away from the weld centre, were subjected to electrochemical potentiokinetic reactivation to assess the degree of sensitisation (DOS). The application of magnetic field during welding induced lower DOS and better resistance to pitting corrosion in 3.5% NaCl solution than samples welded without it. Experimental findings suggest that magnetic fields enable Cr redistribution in the austenitic base metal during the welding thermal cycle reducing Cr depletion.