Showing papers on "Gas metal arc welding published in 2005"

3D welding and milling: Part I-a direct approach for freeform fabrication of metallic prototypes

Abstract: Solid Freeform Fabrication (SFF) gives engineers a new freedom to build parts that have thus far proved difficult to manufacture using conventional machining. However, the surface finish and accuracy of SFF parts are lower than those of conventionally machined parts. A process combination of additive and subtractive techniques is currently being developed in order to overcome this problem. A novel hybrid approach of our group called ‘3D welding and milling’ uses gas metal arc welding as an additive and milling as a subtractive technique, thereby exploiting the advantages of both processes. Compared to other deposition processes, gas metal arc welding is the most economic way of depositing metals. In this paper, the initial results of the process development and the characterization of the parts fabricated by this process are reported.

Advancements in pulse gas metal arc welding

Abstract: Environmental concerns have driven manufactures to look for alternative materials like aluminium that are lighter in weight and possess good thermal and electrical conductivity. However, fabrication of alternative materials presents a considerable challenge for fabrication in volume production. Welding is one of the most common fabrication processes. Good thermal and electrical conductivity act as a drawback for welding and generally results in excessive heating of base material. Pulse gas metal arc welding (GMAW-P) overcomes this drawback by producing spray transfer at lower mean currents. Modern welding has become complex due to need for setting up of combination of large number of welding parameters to achieve best quality of weld. Trial and error methods are impractical. In addition, there are many facets of disturbances and each has its own source and mitigation techniques. This need has resulted in several advancements in GMAW-P technology. This paper reviews progress in performance of GMAW-P technology.

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.

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 ...

3D welding and milling: part II—optimization of the 3D welding process using an experimental design approach

Abstract: A process of combined additive and subtractive techniques for the direct freeform fabrication of metallic prototypes and tools is being developed by the authors. This hybrid process, called ‘3D welding and milling’, uses gas metal arc welding (GMAW) as an additive and conventional milling as a subtractive technique, thereby exploiting the advantages of both processes. In this paper, the results of the optimization of the deposition process using a statistical approach as well as the result of plastic injection molding with the inserts fabricated by this hybrid process are described. The result proves the applicability of the 3D welding and milling process for direct fabrication of metallic prototypes and tools.

Particle size distribution of gas metal and flux cored arc welding fumes

Abstract: A cascade impactor was employed to separate the fume particles in order to determine the size distribution of welding fume. Clear images of coarse welding fume particles (microspatter) from scanning electron microscopy are presented. The particle size distribution of the welding fume reveals that gas metal arc welding (GMAW) fume consists predominately of particle agglomerates smaller than approximately one micrometer. Less than 10% of the fume by weight is microspatter, which is larger than a micrometer. This fraction of microspatter does not change greatly as the GMAW parameters are changed. Flux cored arc welding (FCAW) fume contains more microspatter, approximately 30% by weight.

Pore formation during hybrid laser-tungsten inert gas arc welding of magnesium alloy AZ31B—mechanism and remedy

Abstract: One of the major concerns during high speed welding of magnesium alloys is the presence of porosity in the weld metal that can deteriorate mechanical properties. This study seeks to analyze the presence method and quantity of pore during hybrid laser-tungsten inert gas arc (TIG) welding of magnesium alloy AZ31B by radiography, optical microscopy and electron probe microanalysis (EMPA). At the same time, it identifies both the mechanism of pore formation and a remedy for this problem. The experimental results indicate that lacking of shielding gas for laser beam is the dominant cause of macroporosity formation during the hybrid of laser-TIG welding of magnesium Alloys AZ31B plate, and hydrogen is not main cause to form large pores. A favorable weld without porosity can be obtained by appending lateral shielding gas for laser beam.

Laser induced stabilisation of the welding arc

Abstract: This paper deals with the influence of laser radiation on the stability of the welding arc. Experiments were conducted using a low power (500 W) Nd:YAG laser in combination with a gas tungsten welding arc. The laser induced arc stabilising effect was measured under various experimental conditions. It was found that the stabilising effect can be explained in terms of two phenomena: the absorption of laser energy by the arc plasma and the change of the arc plasma composition caused by strong evaporation of workpiece material. Both phenomena lead to a reduction of the effective ionisation potential of the plasma and thus provide a more conductive, stable plasma channel for arc root and column that overcomes disturbance by external forces. The proposed stabilisation mechanism was validated by measuring the absorption of laser energy by the arc plasma using a laser energy meter and the changes of arc plasma composition caused by the laser radiation by means of emission spectroscopy.

A fuzzy-logic based optical sensor for online weld defect-detection

Abstract: This paper describes an intelligent optical sensor for real time defect detection in gas metal arc welding processes. The sensor measures the radiations emitted by the plasma surrounding the welding arc, and analyzes the information in real time to determine an index of local quality of the weld. The data processing algorithm encompasses a Kalman filter to reduce the heavy amount of noise affecting the measured signals, and an intelligent fuzzy system to assess the degree of acceptability of the weld. The fuzzy system is also able to detect the risk of specific problems (e.g., anomalies in the current, voltage or speed of the arc, contamination with other materials, holes) and the position of defects along the welding line. In an extensive experimental comparison, the fuzzy system outperforms a former version of the detection algorithm based on a statistical approach.

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 mar...

Influence of welding processes on microstructure and mechanical properties of dissimilar austenitic-ferritic stainless steel welds

Abstract: Dissimilar metal welding of austenitic (AISI 304)-ferritic (AISI 430) stainless steel has been taken up to understand the influence of the welding process on microstructure and mechanical properties. Fusion welding processes, namely, gas tungsten arc welding (GTAW), electron beam welding (EBW), and friction welding, have been employed. The GTAW and EBW processes were selected to understand the heat input effects, while friction welding was included to compare fusion and solid-state welding processes. The material used for fusion welding studies is 20-mm-thick, hot-rolled, and annealed plate. Rods of 18 mm diameter machined from the same plate material were used for friction welding studies. In GTAW, ER 430 filler material was employed for dissimilar metal combination, while other welds are autogenous. Gas tungsten arc welds consisted of coarse columnar grains. In electron beam welds, the microstructure consisted of predominantely equiaxed grains on the austenitic stainless steel side, while colum...

Effect of current waveforms on metal transfer in pulsed gas metal arc welding

Abstract: With new types of welding power supply based on higher performance power electronic devices and digital control techniques, advanced pulse waveforms can be produced to offer more characteristics that can be fine tuned to optimize the welding process. In this paper, pulsed current waveforms with four basic parameters and six secondary parameters are used to study the effect of waveform parameters on the mode of metal transfer in pulsed gas metal arc welding (GMAW-P). An experimental system is developed to sense, observe and analyse the images of droplet/wire, and the transient data of welding current and arc voltage. Experiments are conducted to study the influence of the ratio of the pulsing-current time to the droplet-detachment time as well as the droplet-detachment current level on the mode of metal transfer. Appropriate sets of welding conditions including pulse waveform and parameters are obtained to achieve the ideal transfer mode of one-droplet-per-pulse in GMAW-P.

Deformation modelling in layered manufacturing of metallic parts using gas metal arc welding: effect of process parameters

Abstract: Residual stress induced deformations are a major cause of loss in tolerances in solid freeform fabrication processes employing direct metal deposition. In this paper a 2D finite element thermo-mechanical model is being presented to predict the residual stress induced deformations with application to processes where material is added using a distributed, moving heat source. A sequentially coupled thermo-mechanical analysis is performed using a kinematic thermal model and a plane strain structural model. Temperature dependent material properties are used with the material modelled as elastic perfectly plastic. The material used is mild steel. The numerical results are checked against experimental data by manufacturing plate-shaped single layered specimens using an indigenously developed semi-automatic deposition system. The simulation results are compared with experimental data for successive sections along deposition and it is found that, with the exception of plate edges, the two are in very good agreement. The error at plate edges can be as high as 45% and the reason is that a 2D model cannot capture the effect of plate bolting accurately. The computational model is extended further to study the effects of various process parameters, like heat sink characteristics, rate of deposition and deposition sequence, on the buildup of residual stress and deformations. It has been observed that these parameters affect not only the magnitude of deformations but also its distribution. The residual stress distribution depends upon the sequence of deposition and the highest stresses are found at the last deposited row. In order to minimize distortions a proper combination of process parameters is essential.

Residual welding stresses in laser beam and tungsten inert gas weldments of titanium alloy

Abstract: The residual welding stresses in laser beam (LB) and tungsten inert gas (TIG) weldments of a titanium alloy in thin plate form were investigated experimentally in the present work. A hole drilling technique was used to measure the residual stresses in the weldments. The effects of the welding method and post-weld heat treatment (PWHT) on the residual stresses were analysed. The results show that (i) the residual stress distribution in the LB welded joints is similar to that obtained for traditional fusion welding processes, although the distribution zone is much narrower in LB welding, (ii) the residual stress in the heat affected zone for LB welding is about 100 MPa lower than that for TIG welding, and (iii) PWHT in vacuum greatly relieves the welding residual stress.

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.

Cold metal transfer has a future joining steel to aluminum

Abstract: Tests showed good results for tensile strength, corrosion resistance, and limiting fatigue strength when welding steel to aluminum with a modified GMAW process.

Laser Hybrid Welding Method and Laser Hybrid Welding Torch Using a Zinc and/or Carbon and/or Aluminum-Containing Rod

Abstract: The invention relates to a method for welding coated sheet metal (3) by way of a laser hybrid welding method, whereby at least one laser method and one shielded arc-welding method are carried out, and a weld metal (12) is supplied to the weld by way of the shielded arc-welding method. The invention also relates to a laser hybrid welding torch for welding coated sheet metal (3), which comprises at least one laser (13) and at least one shielded arc unit (5) and a rod feeding device (11) for a welding rod (12). The aim of the invention is to provide a method or a device of the aforementioned kind which allows for reducing or completely avoiding inclusions such as are e.g. caused by the evaporation of the coating of the metal sheet (3). For this purpose, a clamping device (6) is used for positioning the coated metal sheet (3) without substantial gaps between the individual sheets. A material having a zinc and/or carbon and/or aluminum content is used as the weld metal or welding rod (12).

Developing psycho-acoustic experiments in gas metal arc welding

Abstract: To better understand the acoustic characteristics used by professional welders, psycho-acoustic experiments are designed. Using custom built experimental apparatus and a digital signal processor, time delay and band-reject filters are implemented while welding to assess welders' reliance on sound.

Friction stir welding of 2219 aluminum: Behavior of θ (Al2Cu) particles

Abstract: An experimental study was conducted to determine if the maximum temperature in the workpiece can reach the lower bound of the melting temperature range and trigger liquation during friction stir welding (FSW) of aluminum alloys as some computer simulation has suggested. Alloy 2219, which is essentially a binary Al-Cu alloy, was selected as the material for study because of its clear lower bound of the melting temperature range, that is, the eutectic temperature 548°C. In addition to FSW, gas metal arc welding (GMAW) of Alloy 2219 was also conducted to provide a benchmark for checking liquation in FSW of Alloy 2219. The microstructure of the resultant welds was examined by both optical and scanning electron microscopy. It was found that in GMAW of Alloy 2219, θ (Al 2 Cu) particles acted as in-situ microsensors, clearly indicating the onset of liquation by reacting with the surrounding aluminum matrix and forming distinct composite-like eutectic particles upon reaching the eutectic temperature. In FSW, on the other hand, no evidence of θ-induced liquation was found as the welds contained θ particles alone and no eutectic particles, suggesting that the eutectic temperature was not reached during FSW. However, in most friction stir welds large θ particles were observed, some exceeding 100 μm and even 1 mm in length as compared to the normal θ particles of only about 10-15 μm in length in both the base metal and the weld, that is, the stir zone or nugget. The large θ particles appeared to have formed during FSW from agglomeration of fractured θ particles and the smaller ones of the θ particles in the workpiece. No apparent correlation between the extent of agglomeration and the welding condition was found.

Pipe seam tack welding methods and apparatus using modified series arc welding

Abstract: Welders and methods are presented for short-circuit arc welding a workpiece using a modified series arc welding configuration with two electrodes via a sequence of welding cycles, in which each cycle includes an arc condition a short-circuit condition, wherein one or both electrode currents are selectively reversed during a reverse boost portion of the welding cycle to transfer molten metal from the second electrode to the first electrode prior to a short-circuit condition of a subsequent welding cycle.

Influence of Welding Parameters and Shielding Gas Composition on GTA Weld Shape

Abstract: The interaction between the variable welding parameters and shielding gas composition in determining the weld shape in Ar-CO2 shielded gas tungsten arc (GTA) welding with SUS304 stainless steel is discussed. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is controlled by the content of surface active element, oxygen, in the weld pool and the welding parameters. Results showed that oxygen absorption into the liquid pool during the welding process is sensitive to the CO2 concentration in the shielding gas. An inward Marangoni convection occurs on the pool surface when the oxygen content is over 100 ppm in the welding pool under Ar-0.3%CO2 shielding. A low oxygen content in weld pool changes the inward Marangoni convection to an outward direction under the Ar-0.1%CO2 shielding. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension (dσ/dT) and the temperature gradient (dT/dr) on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.

Numerical simulation of welding-induced distortion in thin-walled structures

Abstract: A sequentially coupled thermal stress analysis approach is presented for modelling temperature and distortion profiles resulting from welding thin-walled structures. The material is modelled as thermo-elastic–plastic with isotropic strain hardening. The heat source is modelled as a three-dimensional (3-D) double ellipsoid, and 3-D finite element (FE) models are employed for predicting ensuing distortions. Comparisons between the simulation results and experiments performed for eight weld configurations are presented. The weld configurations include bead-on-plate, butt weld and tee joint welds with varying plate thicknesses. Temperature measurements using thermocouples and an infrared (IR) imaging radiometer are directly compared to the thermal simulations. Likewise, distortions measured directly on the experimental set-ups are compared to the FE distortion predictions. Very good correlation is obtained for temperature as well as distortion predictions between experimental and proposed numerical a...