Showing papers on "Gas metal arc welding published in 1999"

Modeling macro- and microstructures of gas-metal-arc welded HSLA-100 steel

Abstract: Fluid flow and heat transfer during gas-metal-arc welding (GMAW) of HSLA-100 steel were studied using a transient, three-dimensional, turbulent heat transfer and fluid flow model. The temperature and velocity fields, cooling rates, and shape and size of the fusion and heat-affected zones (HAZs) were calculated. A continuous-cooling-transformation (CCT) diagram was computed to aid in the understanding of the observed weld metal microstructure. The computed results demonstrate that the dissipation of heat and momentum in the weld pool is significantly aided by turbulence, thus suggesting that previous modeling results based on laminar flow need to be re-examined. A comparison of the calculated fusion and HAZ geometries with their corresponding measured values showed good agreement. Furthermore, “finger” penetration, a unique geometric characteristic of gas-metal-arc weld pools, could be satisfactorily predicted from the model. The ability to predict these geometric variables and the agreement between the calculated and the measured cooling rates indicate the appropriateness of using a turbulence model for accurate calculations. The microstructure of the weld metal consisted mainly of acicular ferrite with small amounts of bainite. At high heat inputs, small amounts of allotriomorphic and Widmanstatten ferrite were also observed. The observed microstructures are consistent with those expected from the computed CCT diagram and the cooling rates. The results presented here demonstrate significant promise for understanding both macro-and microstructures of steel welds from the combination of the fundamental principles from both transport phenomena and phase transformation theory.

Hybrid welding with arc and laser beam

Abstract: This paper deals with a combination of two different welding processes, i.e. the synergic action of two different heat sources for fusion welding. The major part of the paper is focused on the combined action of a welding arc and a laser beam. The main advantage of the use of both heat sources is more efficient use of the energy supplied. With certain parameters, the quantity of molten material increases by 100% compared with the sum of the individual quantities of molten material in the individual processes. The paper further presents several practical applications of the combined welding process. Eventually, development of arc augmented laser welding may progress in two directions; the first being the synergic effect of a laser beam and plasma welding arc and the second the combination of tandem laser welding and consumable electrode welding. This paper presents two such cases schematically.

Process behavior and stability in short circuit gas metal arc welding

Abstract: In this paper, the results of an investigation dealing with short circuit gas metal arc welding with the emphasis on process stability are presented. Welding runs were made under different conditions and, during each run, the different process parameters were continuously monitored. It was found that maximum process stability is reached under specific welding conditions. Outside this maximum, either irregular material transfer takes place with a tendency for open arc droplet transfer or stubbing of the welding wire in the weld pool starts to occur, accompanied by highly irregular melt-off behavior. The results show that process stability is directly related to weld pool oscillation. More specifically, it appears that process stability is maximum when the short circuit frequency equals the oscillation frequency of the weld pool. Under these conditions, the weld pool touches the droplet at the end of the electrode at every oscillation, which results in regular droplet transfer and high stability of the overall welding process.

On-line quality monitoring in short-circuit gas metal arc welding

Abstract: This paper addresses the problems involved in the automatic monitoring of the weld quality produced by robotized short-arc welding. A simple statistical change detection algorithm for the weld quality, the repeated Sequential Probability Ratio Test (SPRT), was used. The algorithm may similarly be viewed as a cumulative sum (CUSUM) type test, and is well-suited to detecting sudden minor changes in the monitored test statistic. The test statistic is based on the variance of the weld voltage, wherein it will be shown that the variance decreases when the welding process is not operating under optimal conditions. The performance of the algorithm is assessed through the use of experimental data. The results obtained from the algorithm show that it is possible to detect changes in weld quality automatically and on-line.

Droplet formation, detachment, and impingement on the molten pool in gas metal arc welding

Abstract: A mathematical model is developed to describe the globular transfer in gas metal arc welding (GMAW). This work is both theoretical and experimental. Using the volume-of-fluid (VOF) method, the fluid-flow and heat-transfer phenomena are dynamically studied during the following processes: droplet formation and detachment, impingement of a droplet on a solid substrate, impingement of multiple droplets on the molten pool, and solidification after the arc extinguishes. 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 in close agreement, suggesting that the theoretical treatment of the model is good.

Heat treatment for weld beads

Abstract: Two parts are welded together at a weld spot in a groove. A spot adjacent the weld spot is heated simultaneously and independently of the welding so that the heating and welding form a heat-treated weld bead along the groove. Preheat treatment, postheat treatment, or both, may be used simultaneously with welding to maximize performance of the resulting weld joint. Preheat and postheat treatment may be performed using lasers while the welding may be performed by TIG welding or laser welding. The welding torch and preheat and postheat lasers are coplanarly aligned.

Arc sensing for defects in constant-voltage gas metal arc welding

Abstract: A method for detecting flaws in automatic, constant-voltage gas metal arc welding using the process current and voltage signals was developed. Seven algorithms process the current and voltage signals to give quality parameters. A defect-detection algorithm processes the quality parameters and flags welds that are different from a baseline. The baseline is constructed from the recorded quality parameters of previously made, defect-free welds. Two series of tests were conducted to test the sensitivity of the algorithms for defects and to test the entire sensor system in production. In the first series, on a 3-mm lap joint, the algorithms detected a lack of shielding gas, oily parts and melt-through from inadequate part cross section, but could not detect off-joint welding. The algorithms showed a mixed sensitivity to melt-through from large root openings. In production tests, 520 welds were monitored. The defect detection algorithm was tuned on data from the first 82 welds. The sensot flagged five of the six defects monitored and did not flag any defect-free welds The defect not detected was a small pore, < 3 mm diameter.

Numerical Study of a Free-Burning Argon Arc with Copper Contamination from the Anode

Abstract: The present modeling of a free-burning argon arc accounts for copper vapor contamination from the anode. Simulations are made for an atmospheric arc that has a length of 10 mm and an electric current of 200 amps. Predicted results for two different anode evaporation rates are compared to those from a pure argon arc with no copper vapor contamination. Copper vapor concentration, temperature, electric potential, and current density profiles are presented. Included in this analysis are radiation losses from both the argon and copper by using recently calculated net emission coefficients. It was found that evaporation of copper from the anode results in a cooling of the arc in a region close to the anode, but has an insignificant influence on the arc close to the cathode. Due to the arc flow characteristics most of the copper vapor tends to be confined to the anode region.

Twin-wire GMAW : Process characteristics and applications

Abstract: Although more complicated than regular GMAW, twin-wire gas metal arc welding offers benefits that include high deposition rates and increased travel speeds.

An analysis of heat transfer and fume production in gas metal arc welding. III

Abstract: A two–dimensional dynamic theory for predictions of arc and electrode properties in arc welding has been used to investigate heat transfer phenomena in the welding wire in gas metal arc welding (GMAW). The theory is a unified treatment of the welding wire, the plasma and the workpiece and includes a free surface treatment for the welding drops, accounting for the effects of inertia, gravity, surface tension, arc pressure, magnetic forces, and viscous drag by the gas flow around the drop. Also, the theory accounts for the variation of the surface tension coefficient with temperature and includes thermal and dynamic phenomena within the solid and liquid phases of the wire, together with a detailed treatment for the electrode sheath regions. Calculations are made for arcs in argon with wires of mild steel at currents between 150 and 325 A. Results of calculations for heat fluxes within the wire suggest that evaporation from the surface of the droplet during droplet growth has an important influence on the he...

Influence of nitrogen in the shielding gas on corrosion resistance of duplex stainless steel welds

Abstract: The influence of nitrogen in shielding gas on the corrosion resistance of welds of a duplex stainless steel (grade U-50), obtained by gas tungsten arc (GTA) with filler wire, autogenous GTA (bead-on-plate), electron beam welding (EBW), and microplasma techniques, has been evaluated in chloride solutions at 30 °C. Pitting attack has been observed in GTA, electron beam welding, and microplasma welds when welding has been carried out using pure argon as the shielding gas. Gas tungsten arc welding with 5 to 10% nitrogen and 90 to 95% argon, as the shielding gas, has been found to result in an improved pitting corrosion resistance of the weldments of this steel. However, the resistance to pitting of autogenous welds (bead-on-plate) obtained in pure argon as the shielding gas has been observed to remain unaffected. Microscopic examination, electron probe microanalysis (EPMA), and x-ray diffraction studies have revealed that the presence of nitrogen in the shielding gas in the GTA welds not only modifies the microstructure and the austenite to ferrite ratio but also results in a nearly uniform distribution of the various alloying elements, for example, chromium, nickel, and molybdenum among the constitutent phases, which are responsible for improved resistance to pitting corrosion.

Experimental approach to selection of pulsing parameters in pulsed GMAW

Abstract: An efficient method of iden- tifying power supply pulsing parameters for pulsed gas metal arc welding based on statistical experimental design is pre- sented. Fractional factorial screening ex- periments are combined with D-optimal experimental designs to allow the user to develop an accurate wire feed rate model for varying pulsing conditions and to characterize the desirable one droplet per pulse (ODPP) operating region for a given wire type and diameter. Equations defining the wire feed rates and time at a given peak current required for ODPP transfer are presented. Compared to con- ventional techniques, a very small num- ber of experiments is required. A power- supply-dependent approach and a more generic method employing measured rather than nominal current values are presented. Joints produced using this ap- proach are evaluated and found to meet applicable bead geometry standards.

Application of fuzzy logic to spatial thermal control in fusion welding

Abstract: This paper considers the problem of sensing and controlling torch position in the pulsed gas metal arc welding (P-GMAW) process. The attitude and positional control described is essential to the production of quality welds with a specified geometry. For constant current arc welding processes, as normally employed with P-GMAW, the arc voltage signal variations that occur as a function of changes in the contact-tube-to-work distance can be used to automatically control the welding system with respect to bead placement and proper sidewall fusion. However, the arc voltage signals are uncertain and noisy because of many inherent disturbances associated with the electrode tip, droplet formation, droplet detachment, and droplet transfer through the arc. To deal with the nonlinear time-varying process with its inherent stochastic disturbances associated with the metal transfer, the theory of fuzzy sets was used as a general framework to interpret the uncertain arc signals and provide logic for control. The fuzzy logic controller weld joint tracking system was implemented and tested with pulsed gas metal arc welds under a variety of conditions. The goal was to obtain quick and accurate response to tracking errors in the presence of disturbances. A series of experiments was conducted to evaluate the performance of the fuzzy logic controller. The experimental results show that the fuzzy logic controller was found to be suitable for these purposes and better than methods based on signal averaging and bipolar decision levels under these criteria.

The classification of metal weld deposits in terms of the amount of oxygen

Abstract: In analogy to the classification of metal weld deposits and arc welding processes of low-carbon and low-alloy steel in terms of the amount of hydrogen in metal weld deposits, a similar classification was given in terms of the amount of oxygen. Different methods of arc welding processes were chosen, such as welding with coated electrodes (basic, rutile, acid, oxide electrodes), shielded arc welding process MIG/MAG, submerged arc welding process (various wires, fluxes, shielded gases). The amount of oxygen and the percentage of acicular ferrite of metal weld deposits were mainly analysed and the impact toughness of it. Metallographical structures and fractography tests of metal weld deposit with varied amount of acicular ferrite were presented by putting attention to non-metallic inclusions and their morphology presented in metal weld deposit. Additional inclusions observation and measurements were done using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer. The studies were also made on the classification of metal weld deposits and arc welding processes of low carbon and low alloy steel in terms of the amount of oxygen in metal weld deposits on the following processes: low-oxygen processes, medium-oxygen processes, high-oxygen processes.

Laser welding superalloy articles

Abstract: A process is provided for laser welding a nickel or cobalt based superalloy article to minimize cracking by preheating the entire weld area to a ductile temperature, maintaining such temperature during welding and solidification of the weld, with the welding utilizing a powder alloy feed and the speed of the laser controlled to less than 10 inches per minute.

Mig gun with axially aligned offset motor

Abstract: A mig welding gun handle is in two sections that are offset from each other by an angle of approximately 20 degrees. The offset handle makes the gun exceptionally comfortable for an operator to maneuver when welding various workpieces. A non-right angle gear set in which the pinion axial centerline intersects the gear axis of rotation at an angle of approximately 70 degrees makes the offset handle possible. The gun further comprises a gas seal that blocks atmospheric air in the weld wire liner from reaching the welding arc. The gas seal comprises a passage that bleeds inert gas to the interior of the liner. The relatively high pressure of the inert gas in the liner interior blocks atmospheric air from flowing through the hole in the contact tip to the arc.

Depth of penetration in gas metal arc welding

Abstract: A model is presented of the depth of penetration in gas metal arc welding. This model is based on the assumption that the heat and mass transfer to the weld pool and the depth of penetration may be correlated by a dimensionless relation. This correlation leads to an analytical expression for depth of penetration, which involves empirical constants that are related to the efficiency of heat and mass transfer to the pool. The accuracy of the model is examined by comparing the theoretical depth of penetration and the measured depth of the weld pool for a range of processing variables encompassing short arc and free flight mass transfer. Measurements are obtained from bead on plate welds of stainless steel using a stainless steel electrode and a shielding gas that is rich in argon. The results confirm that the depth of penetration is affected by variations in the rate of mass transfer.

Consumable electrode type pulse arc welding method

Abstract: In a consumable electrode type arc welding method, when short circuit shift welding is conducted at the start of an arc in the process of pulse arc welding and also when an inclinations of a rise or fall is decreased in the case of starting an arc in the process of pulse welding and also when an inclination of a short circuit current is increased, short circuit, which irregularly occurs at the start of an arc, can be released in a short period of time and the arc can be stabilized early. When the generation of a magnetic arc blow is suppressed, it becomes possible to form proper welding beads. Due to the foregoing, the efficiency of welding work can be enhanced.

A Comparison of Gas Metal Arc Welding with Flux-Cored Wires and Solid Wires Using Shielding Gas

Abstract: In the present work, gas metal arc welding (GMAW) with flux-cored wires and solid wires using shielding gas has been adopted for welding stainless steel. Five different compositions of shielding gas are used with flux-cored wire and three with solid wire. Spatter rates, chemical compositions, tensile strength and elongation tests have been performed and are reported. The spotter rates of the sample made using flux-cored wires are less than that for the sample made using solid wire. The ultimate tensile strength and elongation are not influenced by the composition of the shielding gas.

A detailed analysis of the MIG spectrum for the development of laser-based seam tracking sensors

Abstract: This paper presents a detailed series of measurements of the spectrum of the light emitted from a MIG welding arc. This work was done in the framework of a larger project concerning the development of a seam tracking sensor. Detailed measurements and analysis of the spectrum produced from welding arcs have been performed. The measurements extend from the ultraviolet region of the spectrum (150 nm) to the near infrared region (970 nm) and have revealed the presence of many strong emission lines in the spectrum. The results are of great importance for the design of any optical or vision system working close to a welding arc, because the wavelength of the optical system can be selected to be close to a value where the spectrum of the arc causes the minimum interference to the laser light.

Fume formation rates in gas metal arc welding

Abstract: An improved fume chamber was constructed, and fume rates were measured with unprecedented precision for both steady- and pulsed-current welding of mild steel using 92% argon/8% CO2 shielding gas. Compre- hensive fume maps were constructed de- picting fume rates over a wide range of currents and voltages. Fume generation was generally lower under pulsed-cur- rent conditions. Theoretical arguments explaining this difference are presented.

Welding aluminum alloys 6061 with the opposing dual-torch GTAW process

Abstract: Cracking is a major concern in welding aluminum alloys. Although weld solidification cracks can be eliminated through the addition of filler metal, the additives modify the alloy or base metal constituents and may not always be desirable. High-energy beam processes, such as electron beam welding, that result in minimal heat input reduce crack sensitivity, but their high cost limits their applications. In this study, the conventional gas tungsten arc welding process is modified by disconnecting the workpiece form the power supply and placing a second torch on the opposite side of the workpiece. Such a modification changes the direction of the current flow, improves the weld penetration and reduces the heat input. Using this modified process, 6061-T651 alloy was welded without filler metals. Analysis suggested the reduced heat input, the changed direction of the current flow and the symmetric heating were responsible for the observed reduction of the cracking sensitivity.