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.     

Derivation of current density distribution by arc pressure measurement in GTA welding

Abstract

Arc pressure is an important factor in understanding physical arc phenomena, especially its effects on the penetration, size and shape of GTA welding. The purpose of the present study is to derive the current density distribution on the anode base metal from the measured arc pressure distribution in GTA welding using the results from previous investigators. Using the measured arc pressure distribution on the anode base metal in GTA welding from the central part to the circumference and the equations of Lin et al. and Maecker, the current density distribution was derived. Applying the derived equation from the present work, the current density distribution was calculated from the low current region to the high current region by means of shield gas mixing ratio. It is compared and discussed with the practical welding current and the derived current by numerical integration of the current density distribution from the central part to the circumference region.     

Plasma channel formation for triggering arc welding

Abstract

A method of starting arc welding using a plasma channel formed between electrodes in a tungsten inert gas arc welding system was demonstrated. The plasma channel was generated by gas breakdown in the laser beam path. In a previous study by the present authors, the arc welding could be started using a laser produced plume. Results in the present study indicated that the laser energy required to start the process using the plasma channel was lower than that using the plume.     

New general double ellipsoid heat source model

Abstract

Based on the tridimensional Gauss distribution of power density, a general double ellipsoid welding heat source model has been developed. This model not only consists of all the characteristics of a double ellipsoid model, but also can deal with the situation where, under an external disturbance, the arc's backbone is not perpendicular to the work surface. This general double ellipsoid model is validated by measured results of the temperature field during twin wire welding. Using the non-linear finite element software Marc, the temperature field during twin wire welding was calculated using both the double ellipsoid heat source and the general double ellipsoid heat source. The thermal cycling curves and the weld pool cross-section obtained by the general double ellipsoid heat source tally well with the experimental results.     

Effects of absorptivity,shielding gas speed,and contact media on sheet metal laser welding

Abstract

A three-dimensional quasi-steady state heat conduction model is developed for laser welding of sheet metals. The heat flux at the surface of the workpiece is considered to be due to a moving Gaussian laser beam. An analytical expression is obtained for the temperature distribution by solving the conduction problem using the Fourier integral transform technique. This expression is used to locate the melting temperature isotherm, and thereby determine the weld depth and width. Experimental and theoretical results for the weld depths and widths are illustrated for different welding parameters such as the laser power, absorptivity, welding speed, and shielding gas speed. The theory and experiment are found to agree reasonably well. The effects of absorptivity, shielding gas speed, and heat loss due to different contact media at the bottom surface of the workpiece are also investigated, and are found to be significant for thin metal laser welding.     

Dimensionless correlation to estimate peak temperature during friction stir welding

Abstract

A dimensionless correlation has been developed based on Buckingham's π-theorem to estimate the peak temperature during friction stir welding (FSW). A relationship is proposed between dimensionless peak temperature and dimensionless heat input. Apart from the estimation of peak temperature, it can also be used for the selection of welding conditions to prevent melting of the workpiece during FSW. The correlation includes thermal properties of the material and the tool, the area of the tool shoulder and the rotational and translation speeds of the tool. The peak temperatures reported in the literature during FSW of various materials and welding conditions were found to be in fair agreement with the proposed correlation.     

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.     

Effects of welding parameters on melting energy of CO2 laser–GMA hybrid welding

Abstract

A series of CO₂ laser–gas metal arc (GMA) hybrid welding experiments were carried out on the mild steel workpiece to investigate the effects of the welding parameters, such as laser power, arc current and the distance between laser and arc D _LA, on the melting energy. A dimensionless parameter psi was introduced to indicate the change in the melting energy of hybrid welding. The results showed that with different welding parameters, the melting energy of hybrid welding was changed by the two heat sources (laser and arc) interaction. With an optimal combination welding parameters, psi can be increased up to 23%. Finally, the role of the two different mechanisms in the heat sources interaction was quantitatively discussed in terms of psi. It can be concluded that when D _LA<4 mm, the interaction between the laser induced plasma and the arc plasma dominates the heat sources interaction, therefore the changes of melting energy, whereas the heat sources interaction is only dominated by the preheating mechanism when D _LA≥4 mm.     

Finite element analysis of resistance spot welding in aluminium

Abstract

In the present work, an integrated simulation model has been developed for analysis of the resistance spot welding process in aluminium alloys via the finite element method. A coupled electrothermal analysis for an axisymmetric sheet-electrode geometry has been carried out that can predict the expected nugget diameter, penetration, and electrode face heating at any instant throughout the welding time. Several calculations have been carried out for different welding currents, welding times, and electrode forces and for different surface conditions of the aluminium sheets. Non-linear, temperature dependent, thermophysical material properties have been considered. An observation of interest is that in most instances the maximum nugget diameter is reached well within 0·02-0·04 s and further flow of welding current simply increases the electrode face heating. Also, the initial surface condition influences the growth of the fusion zone to a great extent. Various other conclusions have also been drawn.     

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.     

Experimental study on temperature distributions within the workpiece during friction stir welding of aluminum alloys

This study aims to experimentally explore the thermal histories and temperature distributions in a workpiece during a friction stir welding (FSW) process involving the butt joining of aluminum 6061-T6. Different types of thermocouple layout are devised to measure the temperature histories during FSW at different locations on the workpiece in the welding direction. Successful welding processes are achieved by appropriately controlling the maximum temperatures during the welding process. Regression analyses by the least squares method are used to predict the temperatures at the joint line. A second-order polynomial curve is found to best fit the experimental temperature values in the width direction of the workpiece. The Vickers hardness test is conducted on the welds to evaluate the hardness distribution in the thermal-mechanical affected zone, the heat affected zone, and the base metal zone. Tensile tests are also carried out, and the tensile strength of the welded product is compared with that of the base metal.     

Effect of shielding gas composition on low temperature toughness of Al5083–O gas metal arc welds

Abstract

There is an ever increasing range of shielding gases, which vary from the pure gases to complex mixtures based on argon, helium, oxygen, and carbon dioxide. The commercially available gas mixtures should be considered in terms of their suitability for ensuring arc and metal transfer stability, performance, and weld quality. The objective of the present paper is to study the toughness of Al5083–O aluminium alloy, to evaluate the variation of welding zone toughness as a function of the shielding gas composition and the testing temperature. To achieve these objectives, gas metal arc welding was performed with four different shielding gas compositions (100%Ar?0%He, 67%Ar+33%He, 50%Ar?50%He, and 33%Ar+67%He), and tests were carried out at four different temperatures, namely,+25°C (+77°F), ?30°C (?22°F), ?85°C (?121°F), and ?196°C (?321°F). The welding zone was divided into four subzones for analysis, namely, weld metal, fusion line, heat affected zone, and base metal according to the notch position. Tensile and yield strengths did not show a great effect of testing temperature at +25°C to ?85°C, but increased greatly at ?196°C. Also, strain tended to increase as test temperature decreased. Shielding gas composition does not have a great influence on mechanical properties. The size and number of defects were least in the 33%Ar?67%He mixture. This shows that the higher the helium gas content, the lower the number of defects detected via radiographic inspection. In the impact test, the maximum load was lowest in the weld metal and highest in the base metal at room temperature, and the maximum load and displacement were higher and lower respectively at ?196°C than those at other test temperatures, showing that the lower the test temperature, the higher the maximum load, without any special features related to the phase composition being observed in the load–deflection response. The absorbed energy of the weld metal notched specimens did not depend significantly on test temperature and shielding gas mixture. Conversely, the other specimens showed that as temperature was decreased, absorption energy increased slightly up to a maximum at ?85°C, but then decreased markedly at ?196°C.     

Study of grain size and microhardness of submerged arc welded joint of AISI 1060 steel

Abstract

Change in microstructure, grain growth, hardness and residual stress in a weldment are very much dependent on the temperature distribution, peak temperature and cooling rate. In the present work, three-dimensional transient finite element analysis has been used to predict the cooling rate and peak temperature at different points of the submerged arc welded joint. Grain size and microhardness of the submerged arc welded joint of AISI 1060 steel were experimentally measured and explained on the basis of estimated peak temperatures and cooling rates.     

Artificial neural networks and acoustic emission applied to stability analysis in gas metal arc welding

Abstract

The present paper describes the application of neural networks to obtain a model for estimating the stability of gas metal arc welding (GMAW) process. A neural network has been developed to obtain and model the relationships between the acoustic emission (AE) signal parameters and the stability of GMAW process. Statistical and temporal parameters of AE signals have been used as input of the neural networks; a multilayer feedforward neural network has been used, trained with back propagation method, and using Levenberg Marquardt's algorithm for different network architectures. Different welding conditions have been studied to analyse the incidence of the parameters of the process in acoustic signals. The AE signals have been processed by using the wavelet transform, and have been characterised statistically. Experimental results are provided to illustrate the proposed approach. Finally a statistical analysis for the validation of the experimental results obtained is presented. As a main result of the study, the effectiveness of the application of the artificial neural networks for modelling stability analysis in welding processes has been demonstrated. The regression analysis demonstrates the validity of neural networks to predict the stability of welding process using the statistical characterisation of the signal parameters of AE that have been calculated.     

Comparative study of small scale and ‘large scale’ resistance spot welding

Abstract

An incrementally coupled electrical–thermal–mechanical model is developed to simulate small scale resistance spot welding (SSRSW) using the finite element method. This numerical model is then employed to study the differences between SSRSW and ‘large scale’ resistance spot welding (LSRSW). The variations in contact area, current distribution, and temperature profile at the workpiece/workpiece interfaces are compared. The computation shows that the difference in electrode force could be the essential reason for other differences between SSRSW and LSRSW. Compared with LSRSW, a much lower electrode force (pressure) applied in SSRSW results in a relatively small contact area and hence a much higher current density, which in turn leads to a greater heating rate and higher temperature at the workpiece/workpiece interface. This small contact area also results in a relatively small nugget diameter in SSRSW, which is only about 30% of the electrode tip diameter. In contrast, the nugget diameter in LSRSW is comparable to the electrode tip diameter. The predicted nugget diameters in both SSRSW and LSRSW of mild steel sheets compare well with experimental results.     

Thermoelectric method for temperature measurement in friction stir welding

Abstract

Previous research within friction stir welding (FSW) has demonstrated that online control of welding parameters can improve the mechanical properties and is necessary for certain applications to guarantee a consistent weld quality. One approach to control the process is by adapting the heat input to maintain a stable welding temperature, within the specified operating boundaries. This requires accurate in-process temperature measurements. This paper presents a novel method to measure the temperature at the interface of the FSW tool and workpiece. The method is based on the thermoelectric effect between dissimilar materials. The measurements are compared to thermocouple measurements and to a physical model and show good correspondence to each other. Experiments demonstrate that the method can quickly detect temperature variations, due to geometrical variations of the workpiece or due to parameter changes. This allows use of the method for online control of robotic FSW.     

Signature images for arc welding fault detection

Abstract

The present work elucidates the signature image approach to welding fault detection, covering the calculation of signature image data objects from blocks of welding electrical data (voltage and current), the definition of appropriate vector operations, and the manipulation of the signatures to permit detection of welding faults. Detection of out of position welds in overlap joints is illustrated, and the relevant changes in signature features are related to the physics of the short arc welding process. Finally, it is shown how fault detection performance can be improved by optimising the way the signatures are calculated.     

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.     

Mathematical modelling of rotational arc sensor in GMAW and its applications to seam tracking and endpoint detection

Abstract

High speed rotational arc sensing is an important method to detect the torch deviation during automatic seam tracking of arc welding. In the present paper, a mathematic model of high speed rotational arc sensing is analysed. Simulations have been implemented in two conditions, namely considering or without considering weld bead profile. The simulation results are consistent with the experimental results. The current waveforms at the beginning of the welding are different from those at middle of the welding because of the formation of the weld bead profile. The signal patterns for seam tracking and endpoint detection are proposed. A phase shift between the rotation and the current variation is also discovered in the experiments. The mathematical model can be helpful for the interpretation and improvement of arc sensing systems.     

Limitations of pulsed MIG‐MAG welding for obtaining welded joints with constant penetration and bead shape

Summary

This article presents a study of the MIG‐MAG welding process with pulsed‐arc transfer applied to joints in motor vehicle exhaust pipes and silencers for which the mean thickness of the base material is approximately 2 mm, and the ratio of the latter to the workpiece diameter is about 0.04. It is shown that, for this type of welded joint, constraints on the maximum and minimum values for penetration into the base metal are incompatible with those relating to the shape and size of the weld bead due to progressive heating of the workpieces during the process.