Showing papers on "Gas metal arc welding published in 1998"

A theoretical model for gas metal arc welding and gas tungsten arc welding. I.

Abstract: A recently developed theory for predicting arc and electrode properties in gas metal arc welding (GMAW) has been generalized to include arc–electrode interfaces, variation of surface tension pressure with temperature, Marangoni forces and handling of weld pool development in stationary gas tungsten arc welding (GTAW). The new theory is a unified treatment of the arc, the anode, and the cathode, and includes a detailed account of sheath effects near the electrodes. The electrodes are included as dynamic entities and the volume of fluid method is used to handle the movement of the free surface of the molten metal at one electrode. Predictions can be made of the formation and shape of the welding droplets as a function of time in GMAW and also of weld pool development in GTAW, accounting for effects of surface tension, inertia, gravity, arc pressure, viscous drag force of the plasma, Marangoni effect and magnetic forces, and also for wire feed rate in GMAW. Calculations are made of current densities, electric potential, temperatures, pressures and velocities in two dimensions, both in the arc and also within the molten metal and solid electrodes. Calculations are presented for GMAW and GTAW for an arc in argon and the results are compared with experimental temperature measurements for the plasma and the electrodes.

A dynamic model of drops detaching from a gas metal arc welding electrode

Abstract: A dynamic model of drop detachment in gas metal arc welding is presented for low and moderate welding currents in an argon-rich plasma. Simulations performed with this model are compared with extensive experimental measurements of constant-current welding images and with limited experimental measurements of pulsed-current welding images. The comparisons indicate that the experimental axial magnetic forces are much less potent than the calculated axial magnetic forces when welding-current transients are not present. To explain this finding the hypothesis that internal flows are able to develop under the relatively quiescent conditions that exist during drop development in constant-current welding is advanced.

The dynamic analysis of metal transfer in pulsed current gas metal arc welding

Abstract: The dynamic characteristics of metal transfer in the pulsed current gas metal arc welding are analysed using the volume of fluid method incorporating the electromagnetic force. The surface profile and the velocity distribution of fluid in the drop are computed numerically. The viscous flow and inertial force of a molten metal generated by the peak current are found to have significant effects on the metal transfer and drop detachment. The ranges of the pulsing frequency for which one drop is detached per current pulse are predicted and calculated results are in good agreement with the experimental data with some discrepancy for low load duty cycles.

Chromium, nickel and manganese in shipyard welding fumes

Abstract: Tests were conducted to determine the effects of base metal and welding electrode composition on welding fume. Materials included HY-100 and HSLA-100 high-strength, low-alloy steels. Shielded metal arc welding (SMAW) was performed with E1 1018-M electrodes and gas metal arc welding (GMAW) with MIL-100S-1 electrode wire. These tests included measurement of fume composition, fume generation rates and worker breathing zone fume. Sampling of welding fume also was conducted in a shipyard. This study concludes that some shipyard welding and cutting operations, materials and processes will be impacted by the recent and anticipated reductions in exposure limits. Additional controls will be required to comply with these reductions. Results indicate: . Exposure to hexavalent chromium can be expected when welding or cutting materials that contain chromium or chromates. These materials include stainless steels, high-chromium nickel alloys and some low-alloy steels. . The highest nickel levels occurred during SMAW and GMAW of stainless steels and nickel alloys. However, only the samples in enclosed spaces exceeded the proposed limit for nickel. . SMAW, GMAW and flux cored arc welding (FCAW) of stainless steels, carbon steels and low-alloy steels produced the highest manganese levels. . Eight-hour TWA levels of hexavalent chromium of up to 1-2 μg/m 3 were found during shipyard and laboratory sampling of SMAW of HY-100 using E11018-M and E12018-M electrodes. Similar levels also may be possible when welding with these electrodes on other materials.

An analysis of the formation of metal droplets in arc welding

Abstract: A dynamic two-dimensional arc model has been used to investigate the effects of the various forces acting on the droplet in gas metal arc welding (GMAW). The model is based on the equations of conservation of mass, energy, momentum and current, Ohm's law and a Maxwell equation. The model treats the welding wire, the plasma and the workpiece. For molten metal droplets at the tip of the welding wire, we account for effects of inertia, gravity, surface tension, magnetic force, viscous drag force and arc pressure. Calculations are presented for a 1.6 mm diameter wire of mild steel for arcs in argon to determine the separate effects of these forces on droplet formation. It is found that, for arcs in pure argon at currents around the transition from the globular transfer mode to the spray transfer mode, viscous drag and arc pressure effects are approximately self-cancelling. It is also found that forces have a much larger effect than do forces on the transition from globular to spray modes of metal transfer.

Method and system for gas metal arc welding

Abstract: An improved method of gas metal arc welding (GMAW) is disclosed. The method includes utilizing a pulsed current having a variable waveform to ensure the detachment of one-droplet-per-pulse of current. During the welding process, the current is sufficient to produce a droplet at the end of a consumable electrode wire. After the droplet reaches a desired size, the current is lowered to induce an oscillation in the droplet. The current is then increased which, in combination with the momentum created by the oscillation, effects droplet detachment. The oscillation may be monitored by observing the arc voltage to determine a preferred detachment instant. A computer implemented method allows for the adaptive control of the current waveform to accommodate for anticipatable variations in the welding conditions, while maintaining ODPP transfer and a constant pulse period. An accompanying system is disclosed for implementing the method of adaptive welding.

Magnetic forces acting on molten drops in gas metal arc welding

Abstract: In gas metal arc welding, magnetic forces arising from the interaction of the welding current with its own magnetic field play an important role in the detachment of drops from the molten welding electrode These forces drive the dynamic evolution of the drop and also depend on the instantaneous shape of the drop In this paper, experimentally observed manifestations of magnetic forces are shown and a technique for approximating the temporal evolution of the axial magnetic force from experimentally measured drop shapes is reported The technique provides quantitative data illustrating the large increase in the magnetic forces when a drop detaches from the electrode

Dynamic simulation of metal transfer in GMAW, Part 1: Globular and spray transfer modes

Abstract: The dynamic characteristics of the globular and spray metal transfer modes are simulated by adopting the Volume of Fluid (VOF) method. The electromagnetic force due to the welding current is included in the formulation of the VOF algorithm. The effect of the welding arc is considered under the assumptions of a uniform and linear current density on the pendant drop surface. The free surface profiles, pressure and velocity distributions within the drop are calculated numerically. It is found that axial flow and radial pinch force promote drop detachment, and the taper of the elongated molten drop in the spray mode is maintained by the electromagnetic force. The current density distribution on the drop surface has significant effects on the molten drop profile, detaching drop volume and taper formation at the molten tip. Predicted results are in reasonably good agreement with the experimental data.

Predictions of metal droplet formation in gas metal arc welding. II.

Abstract: Predictions of metal droplet formation in gas metal arc welding (GMAW) are made for mild steel wires with argon as a shielding gas, using a generalized theory for predictions of arc and electrode properties in arc welding The theory is a unified treatment of the arc and the electrodes and includes a detailed analysis of the anode sheath region, together with a free surface treatment for the molten metal at the tip of the welding wire Results of calculations made for a mild steel wire of 012 and 016 cm diameters are in good agreement with experimental measurements of droplet diameter and droplet detachment frequency at currents between 150 and 330 A The results indicate that the anode sheath region and the effects due to variation of surface tension with temperature have an important influence on the dynamics of droplet growth at the tip of the wire in GMAW Predictions are made for the transition from the globular to the spray mode of metal transfer, in agreement with experimental observations in arc welding

Mechanisms of electrode degradation when spot welding coated steels

Abstract: The present work compares the behaviour of precipitation hardened and dispersion strengthened electrodes during the manufacture of resistance spot welds in coated steel sheet. The primary mechanism causing the failure of spot welding electrodes was growth of the electrode tip. Under normal welding conditions electrode tip growth was primarily dependent on local alloying morphology so that deterioration was mainly a function of the type of coating present on the steel. However, welding with high currents or use of current stepping programmes could extend electrode performance such that softening became a more dominant electrode degradation mechanism. Under these conditions, use of dispersion strengthened material could extend electrode campaign life.

An investigation of gas metal arc welding sound signature for on-line quality control

Abstract: On-line quality control in automated welding operations is an important factor contributing to higher productivity, lower costs and greater reliability of the welded components. However, the development of on-line inspection techniques and feedback control is still in its infancy. Sound signature produced by GMAW contains information about the behavior of the arc column, the molten metal and the metal transfer mode. Manual welders rely on sound signature as important feedback for production of good welds. High-speed data acquisition and computer-aided analysis of the sound signature may indicate conditions that generate weld defects. This paper presents an experimental investigation that attempts to establish correlations between sound signature, welding parameters and weld transfer mode with a view toward detection of weld defects. The results indicate that the sound signature is a usable, practical information source for process-linked quality control of welds.

Elevated-temperature, plasma-transferred arc welding of nickel-base superalloy articles

Abstract: A nickel-base superalloy article which is susceptible to strain-age cracking and has a directionally oriented, single crystal, or equiaxed grain structure is repaired with minimal welding heat input into the article. The article is first heated to a welding temperature of from about 1650° F. to about 2000° F. in an inert atmosphere. A damaged area of the article is weld repaired using a plasma-transferred arc welder which vaporizes a filler metal in a plasma arc and deposited the vaporized metal onto the article to form a weld overlay. Minimal additional heat is added to the article during welding, as the weldment metal is vaporized remotely from the article.

Method of arc welding using dual serial opposed torches

Abstract: An improved method of arc welding uses a single power supply connected to opposed welding torches placed on both sides of a workpiece Each torch is connected to a different polarity lead of the power supply and forms a separate arc with the workpiece When a current is supplied to the first torch, it is guided from the first electrode, through the first arc, the workpiece, the second arc, and to the electrode of the second torch This guiding function improves the penetration, concentration, as well as the directional stability of the arc This permits the effective and efficient welding of relatively thick workpieces using existing equipment at low current levels

Dynamic analysis of globular metal transfer in gas metal arc welding—a comparison of numerical and experimental results

Abstract: A mathematical model to describe the globular transfer in gas metal arc welding is developed. This work is both theoretical and experimental. Using the volume-of-fluid (VOF) method, the fluid-flow and heat-transfer phenomena during the impingement of a droplet on a solid substrate, arc striking, the impingement of multiple droplets on the molten pool and finally the solidification after the arc extinguishes are dynamically studied. A He-Ne laser in conjunction with the shadow-graphing technique is used to observe the metal-transfer processes. Theoretical predictions and experimental results are shown to be in good agreement, suggesting that the theoretical treatment of the model is sound.

Effects of welding parameters on hard zone formation at dissimilar metal welds

Abstract: An experimental study was conducted to determine effects of welding parameters and to optimize those parameters that have the most influence on eliminating or reducing the extent of hard zone formation at dissimilar metal welds (DMWs). Preheat, base metal thickness and welding electrode composition were found to have the most influence. Maintaining an optimum preheat for a given base metal thickness and controlling the maximum interpass temperature throughout welding resulted in drastic reduction and often complete elimination of hard zones at DMWs fabricated with ENiCrFe-3 electrodes, but not those welds fabricated with E309 stainless steel electrodes. This finding indicates that depending on the cooling rate and composition of the welding electrode, hard zones in DMWs can be eliminated. The cooling rate must be slow enough to avert formation of hard allotropic structures (i.e., martensite) and fast enough to avoid precipitation of hard intermetallic phases. The optimum welding electrode composition is one that will retard formation and precipitation of intermetallic phases during welding while the preheat needed to prevent the formation of allotropics is being maintained. Unfortunately, this unique characteristic is not available in most, if not all, austenitic stainless steel electrodes; nickel-based welding electrodes have been demonstrated to be more receptive.

Dynamic simulation of metal transfer in GMAW, Part 2: Short-circuit transfer modes

Abstract: Short-circuit transfer is simulated dynamically by adopting the Volume of Fluid (VOF) method to analyze the effects of welding parameters on metal transfer. Appropriate initial and boundary conditions are imposed to simulate short-circuit transfer. The free surface profiles, pressure and velocity distributions are computed numerically during short-circuit transfer. The effects of the welding current, drop volume, contact area and wire feed rate on metal transfer are analyzed through variations of the pinch radius and break-up time. In the early stage of transfer, the molten metal in the bridge is transferred to the weld pool mainly due to the capillary pressure. The electromagnetic force becomes a dominant factor in the later stages of transfer. The effects of a current waveform on the characteristics of the metal transfer also are simulated.

Double-sided arc welding increases weld joint penetration : Aluminium welding

Abstract: Connecting two torches to the two terminals of a single power supply achieves more concentrated arcs that increase weld joint penetration.

Friction welding of aluminum alloy hollow members

Abstract: The root shape of a flash formed in the friction-welded joint is controlled by introducing an upset delay time interposed between termination of the friction step and commencement of the upset step and by controlling the upset displacement speed. This provides a friction-welded joint having a tensile strength and a fatigue strength which are comparable with those of TIG- or MIG- welded joints and enables aluminum alloy hollow members to be produced by friction welding instead of TIG or MIG welding. Typically, the upset delay time is 0.4 to 0.8 sec and the upset displacement speed is 5.0 to 70.0 mn/sec to form a flash having a flash root radius of 0.5 mm or more and a flash root angle of 75° or less.

Cathodic cleaning and heat input in variable polarity plasma arc welding of aluminum

Abstract: For variable polarity plasma arc welding (VPPAW) of 1,100 Al, it was found that the net heat input to the aluminum workpiece did not decrease as independent changes in polarity balance enabled the tungsten electrode to become the predominant anode in the alternating current arc. For the thin sheet edge welds made in this study, the independent parameters used to vary the arc current polarity balance were very effective in delivering a wide range of actual arc power polarity balance. The ratio of electrode positive polarity arc energy to the total arc energy ranged from as little as 0.03 to as high as 0.99. Despite this pronounced difference in arc polarity, no significant variation in the average arc efficiency (net heat input/arc energy) of 0.51 was found. Substantial heating of the workpiece during electrode positive polarity was attributed to field type emission of electrons from the low boiling point aluminum cathode. Unlike thermionic emission at the tungsten, field emission electrons do not cool the cathode. While the actual arc efficiency were relatively constant, there were significant differences in the measured heat input, the weld size, and the effectiveness of the cathodic cleaning.

An electronic welder control circuit

Abstract: This paper presents a 6 kW electronic welder control circuit. The DC welding current is controlled with a simple and robust control circuit. Fast dynamic performance and low current ripple are achieved. Paralleled IGBTs are used with a switching frequency of 50 kHz. The welder machine with this control stage can be applied to shielded metal arc welding and gas tungsten arc welding processes. The welding DC voltage is also controlled with an outer controller acting in the current reference value. Fast dynamic is also obtained for the DC voltage control without steady state error. With this control level, gas metal arc welding and flux-cored arc welding can be applied. The current control and the DC voltage control can also be applied in different cycles depending on a external signal. In the peak and background current cycles. the current control and DC voltage control are applied, respectively, enabling the use of the electronic welder for pulsed gas metal arc welding. In the paper, the system modelling is also realised. Some modelling and experimental results are presented. The annoying problems caused by this type of equipment in low voltage networks are also shown.

Hard coatings on soft metallic substrates

Abstract: The application of PVD CrN coatings onto soft, electropolished nickel moulds and PVD TiN, CrN and double-coating (TiN + CrN) onto copper welding contact tips is reported. Both studies and tests were made in practical conditions, i.e. in industrial manufacturing. The first case study was on protection of the surface of a precision mould with a complicated shape and used for artificial tooth production. The substrate was 4 mm thick Ni, electrodeposited and electropolished. Such a sensitive surface is used together with polymethylmethacrylate paste in a tooth production pressure process at 150°C. We found that in this case, the only feasible deposition technique was DC reactive sputtering of Cr and CrN at 140°C in the SPUTRON plasma beam sputtering equipment of Balzers. A 3 μm thick CrN coating with high microhardness (1400 HV, 0.25 N on the Ni substrate), good adhesion (Lc AE= 12-16 N) and excellent corrosion and oxidation resistance, successfully protected the nickel mould from mechanical damage occuring during production cycles, even after its use for 18 months. The second case was improvement of copper welding contact tips used in the MIG welding technique. TiN and CrN coatings were used both as single and double coatings, directly deposited on the copper substrate and also using 5 and 16 urn thick Ni galvanic coatings as an intermediate layer. Good adhesion (Lc AE=15–22 N), low scratching coefficient, high microhardness (1400–2500 HV, 0.10 N) and temperature resistance up to 600°C (TiN) or 800°C (CrN and TiN + CrN) provided an improvement in welding contact tip life from the standard value of 10–16 h of interrupted work to 85–210 h. The best results were obtained with the “hard” tip substrate made of CuZnZr alloy. For all substrate-coating combinations — various substrate quality and applied coatings — the abrasion wear of the electrode wire was predominant, while any welding material stuck onto the coated contact tip was easy to clean off.

Hybrid welding apparatus

Abstract: A hybrid welding apparatus for subjecting a base metal to gas-shielded arc welding and laser welding comprises a laser generator for emitting a laser beam and a condenser optical system for condensing the laser beam to laser weld the base metal. The condenser optical system includes a plurality of lenses, each having a hole portion in the center thereof. A tubular supply holder for supplying a welding wire is passed through these respective hole portions of the lenses and located substantially coaxially with the axis of the condenser optical system. The laser beam and welding wire to be applied to the base metal are arranged coaxially, so that laser-welding and arc-shielded welding can be simultaneously performed with a wide angle between the welding head and a bevel of the base metal, thereby deepening penetration of the weld at an increased welding speed.

Droplet detachment and plate fusion characteristics in pulsed current gas metal arc welding

Abstract: Molten metal droplet detachment and plate fusion characteristics, which have an important bearing on the weld quality, were influenced by the parameters of pulsed current, viz peak current (I p ), peak duration (T p ), background current (I B ) and background duration (T B ) The complex interdependence of the above parameters of pulsed current makes it difficult to select the most suitable combination of parameters for welding (Refs 1-4) To resolve this problem, a first estimate of the pulse parametric zone based on burnoff, droplet detachment and arc stability criteria was obtained It has been further restricted taking into account the short circuit and burnback characteristics Within this narrowed zone, the suitable combination of the pulsed current parameters has been recommended on the basis of a combined criteria, viz uniformity in arc length, uniformity in droplet detachment and weld bead surface quality This is done to obtain good quality welds These studies have been carried out for a fixed pulse cycle time and one droplet detachment per pulse conditions Further studies have been carried out for different types of droplet detachments, namely background detachment, one droplet detachment per peak, two droplets detachment per peak and three droplets detachment per peak by varying pulse cycle time The suitable type of droplet detachment has been selected on the basis of uniformity in arc length, uniformity in droplet detachment and uniformity in weld bead surface undulation The effect of types of droplet detachment on weld bead geometry has been studied The peak energy, arc noise level, reinforcement height and heat input were found to be low for background detachments Also with background detachments, uniformity in arc length and weld penetration were found to be high