Showing papers on "Shielding gas published in 2000"

Electron beam welding, laser beam welding and gas tungsten arc welding of titanium sheet

Abstract: Microstructures, properties and technical parameters of welding specimen of 0.5 mm thick sheets of commercial purity titanium (C.P. Ti) have been studied via high vacuum electron beam welding (EBW-HV), CO2 laser beam welding (LBW) and gas tungsten arc welding (TIG), as well as optical microscope (OM) observation and microhardness measuring. The results indicate that the EBW is more suitable for C.P. Ti sheets welding, and the welding seam without defects can be obtained. The tensile strength and microhardness of joints are corresponding to matrix structure. The full-penetration butt welds are obtained by TIG, but they have many defects such as wide weld-seam, big deformation and coarse grains. The LBW has many advantages such as the narrowest weld-seam, the least deformation and the finest grains. The fine grains are good for properties of weld seam, but the LBW should be studied again for the reasons of unstable welding technologies and strict condition.

Investigation into arc constriction by active fluxes for tungsten inert gas welding

Abstract: Mechanisms by which active fluxes increase the penetration of conventional tungsten inert gas (TIG) welds (so called A-TIG welds) are reviewed. The most dominant mechanism for increased penetration is considered to be arc constriction rather than a change in the surface tension of the molten pool. An experimental programme of work was carried out using A-TIG flux in combination with a number of welding processes. The plasma process was investigated as it gives greater penetration than conventional TIG welding by increasing current density. The CO2 laser and electron beam processes which do not rely on a current carrying arc as the heat source for welding were also investigated. Macrosections taken from the welds made by these processes showed that the A-TIG flux was only effective when the weld pool was produced by an arc or plasma. Where there was no arc or plasma present, the flux had little effect.

Effects of activating flux on arc phenomena in gas tungsten arc welding

Abstract: Dramatic increases in the depth of weld bead penetration have been demonstrated by welding stainless steel using the gas tungsten arc (GTA) process with activating fluxes consisting of oxides and halides. However, there is no commonly agreed mechanism for the effect of flux on the process. In order to clarify the mechanism, behaviour of the arc and weld pool in the GTA process with activating flux was observed in comparison with a conventional GTA process. A constricted anode root was found in the GTA process with activating flux, while a diffuse anode root was found in the conventional process. Furthermore, it is suggested that these anode roots are strongly related to metal vapour from the weld pool, which is also related to temperature distributions on the weld pool surface.

High-speed simultaneous observation of plasma and keyhole behavior during high power CO2 laser welding: Effect of shielding gas on porosity formation

Abstract: Laser welding can produce a deeply penetrated bead at high speed. However, in high power cw CO2 laser welding, the characteristic porosity is easily formed in the weld metal, but its formation mechanism has not been well understood. Therefore, the authors have conducted systematic studies of the elucidation of porosity formation mechanism and the development of preventive remedies. They have revealed that many bubbles are formed, mainly from the bottom tip of the keyhole by intense evaporation of the metal. It has also been revealed that the keyhole fluctuates frequently and changes its size and shape, corresponding to the intermittent bubble formation. The majority of bubbles are trapped at the solidifying front in the rear part of the molten pool. However, there are few reports that deal with the simultaneous observation of keyhole and plasma dynamic behavior as well as the formation of bubbles and porosity. In this study, therefore, the interrelationship between keyhole and plasma behavior was examined...

Analysis of an arc light mechanism and its application in sensing of the GTAW process

Abstract: Sensing plays a key role in automating and controlling welding processes. In recent decades, arc light sensing has been studied for arc length control, joint tracking and droplet trans- fer detection in arc welding. However, the current technology relies on experi- mental data and lacks theoretical foun- dation. To improve measurement accu- racy, this work addresses the theoretical foundation for arc light sensing. A theo- retical model has been developed to cor- relate arc light radiation to welding pa- rameters. Distributions of different radiant sources in the arc column are studied. It is found that the distributions of the ions of the shielding gas and the vapors of the base metal and tungsten are not even, while that of the shielding gas atoms is. This suggests the spectral lines associated with the shielding gas atoms can be used to improve the accu- racy of arc light sensing. Hence, an arc light sensor has been developed to de- tect argon atom spectral lines in gas tungsten arc welding. The relationship between the argon lines and the welding parameters has been derived from the theoretical model. Joint tracking exam- ples showed the effectiveness of the de- veloped method in improving accuracy.

A semi-empirical model of the fume formation from gas metal arc welding

Abstract: The control of exposure to welding fume is necessary to meet health and safety obligations. The work reported here examines the fundamentals of welding-fume formation. A physical chemistry model of the metal vapour mechanism for fume formation has been developed for non short-circuiting transfer gas metal arc welding (GMAW). The model includes the important contribution made by direct condensation of metal vapour onto the weldpool and workpiece, in removing a substantial fraction of the fume. The model shows that droplet size and wire feed speed control the fine fume formation rate. The understanding developed so far, indicates that the smaller the detached droplet size, the lower the total fume formation rate. The physics behind this is explained. The model gives an insight into how process modification might be used to control fume at source. Control at source is believed to be the most cost-effective and energy-efficient technique for dealing with welding fume. It is anticipated that the understanding gained from this project will be applied to determine the practical limits for the control of welding fume at its source.

Numerical Modeling of Enhanced Nitrogen Dissolution during Gas Tungsten Arc Welding

Abstract: Weld-metal nitrogen concentrations far in excess of Sieverts-law calculations during gas tungsten arc (GTA) welding of iron are investigated both experimentally and theoretically. A transient, threedimensional mathematical model has been developed to calculate the residual nitrogen concentrations during GTA welding. This model combines calculations for the plasma phase with those for nitrogen absorption and for the transport of nitrogen by convection and diffusion in the weld metal and diffusion throughout the weldment. In addition, the model takes into account the roles of turbulence and the nitrogen desorption reaction in affecting the residual nitrogen concentration in the weldment. Autogeneous GTA welding experiments in pure iron have been performed and the resulting nitrogen concentrations compared with the modeling results. Both experimental and modeled nitrogen concentrations fall in a range between 2.7 and 4.7 times higher than Sieverts-law calculations at a temperature of 2000 K. Modeled nitrogen concentrations correlate well with the experimental results, both in magnitude and in the general trends, with changes in the travel speed and nitrogen addition to the shielding gas.

A hot-cracking mitigation technique for welding high-strength aluminum alloy

Abstract: A hot-cracking mitigation technique for gas tungsten arc welding (GTAW) of high-strength aluminum alloy 2024 is presented. The proposed welding technique incorporates a trailing heat sink (an intense cooling source) with respect to the welding torch. The development of the mitigation technique was based on both detailed welding process simulation using advanced finite element techniques and systematic laboratory experiments. The finite element methods were used to investigate the detailed thermomechanical behavior of the weld metal that undergoes the brittle temperature range (BTR) during welding. As expected, a tensile deformation zone within the material BTR region was identified behind the weld pool under conventional GTA welding process conventional GTA welding process conditions for the aluminum alloy studied. To mitigate hot cracking, the tensile zone behind the weld pool must be eliminated or reduce to a satisfactory level if the weld metal hot ductility cannot be further improved. With detailed computational modeling, it was found that by the introduction of a trailing heat sink at some distance behind the welding arc, the tensile strain rate with respect to temperature in the zone encompassing the BTR region can be significantly reduced. A series of parametric studies were also conducted to derive optimal processmore » parameters for the trailing heat sink. The experimental results confirmed the effectiveness of the trailing heat sink technique. With a proper implementation of the trailing heat sink method, hot cracking can be completely eliminated in welding aluminum alloy 2024 (AA 2024).« less

Formation mechanism of porosity in high power YAG laser welding

Abstract: With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N2 gases except Type 304 in N2 gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also included in the porosity. This mechanism is similar to that in high power CO2 laser welding. Fast liquid flows occurred circularly from the bottom keyhole to the rear upper part of the molten pool, from the rear to the front near the pool surface, and from the top to the bottom behind the keyhole in weld molten pools of both A5083 alloy and Type 304 steel in He, Ar or N2 shielding gas. Slightly different flows were noticed in the molten pool of Type 304 steel between YAG and CO2 lasers.With the objectives of clarifying the formation mechanism of porosity and producing a sound weld bead, welding conditions of porosity formation were investigated in A5083 alloy and Type 304 steel welded with a high power YAG laser, and the behavior of a keyhole, bubbles and porosity as well as liquid flows were observed during laser welding through X-ray transmission imaging system using markers. It was confirmed that a lot of bubbles and pores were formed in 3.5 kW YAG laser weld beads produced in Ar, He and N2 gases except Type 304 in N2 gas. Porosity was reduced at high welding speed in Type 304 steel even in He and Ar gases. A lot of bubbles were formed by the evaporation of metals from the bottom tip of the keyhole and flowed upwards in front of the solid-liquid interface. Some bubbles disappeared out of the molten surface especially in A5083 alloy welded at low welding speed, but the majority of bubbles were trapped at the solidifying front of the weld beads in most cases. The shielding gas was also...

Effect of vacuum on penetration and defects in laser welding

Abstract: The effect of vacuum on weld penetration and porosity formation was investigated in high-power CW CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and was improved in austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation and liquid flow in the molten pool during high power YAG laser welding under various conditions through the microfocused X-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas that a lot of bubbles were generated near the bottom part of molten pool from the tip of a fluctuated keyhole and resulted in large pores. On the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, although the middle and bottom parts of the keyhole swelled in the molten pool probably because the evaporation of metals was so intense. Moreover, quite different liquid flows were observed between the normal and vacuum welding. Namely, there was a strong molten flow from the bottom of molten pool near the keyhole tip along the solidification interface to the upper rear part in the normal welding, while the liquid flowed upwards along the rear keyhole wall probably due to the strong stream of metallic vapors in vacuum. It is considered in vacuum welding that the liquid flow into the bottom part of the molten pool from the keyhole does not occur because of the direction of evaporated metals toward the upper keyhole outlet. This may exert a beneficial effect on the reduction or prevention of pores or porosity.The effect of vacuum on weld penetration and porosity formation was investigated in high-power CW CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and was improved in austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation and liquid flow in the molten pool during high power YAG laser welding under various conditions through the microfocused X-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas that a lot of bubbles were generated near the bottom part of molten pool from the tip of a fluctuated keyhole and resulted in large pores. On the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, althoug...

Gas control system for a plasma arc torch

Abstract: An apparatus and process is provided for controlling the selection and supply pressure of gases used in a plasma arc torch. A plurality of pressurized feed gases are selectively routed by solenoid gas valves to one or more motorized pressure regulators. A separate regulator controls the pressurized output of a selected pre-flow gas, plasma gas, shield gas, and post flow gas. A microprocessor establishes recommended pressures for each type of gas and prevents operating pressures from being used which may damage a plasma arc torch.

Torch for gas shielderd arc welding using consumable electrode

Abstract: A welding torch comprises a torch body having a wire guide cylinder for guiding a welding wire; an internal cylinder fitted to the torch body in such a manner as to surround the wire guide cylinder; and an external cylinder fitted to the internal cylinder in such a manner as to surround the internal cylinder. The primary interception of outside air is effected with a second shielding gas layer blown off from a second gas passageway on the outer side, and the complete interception of outside air is effected with a first shielding gas layer blown off from a first gas passageway on the inner side. Consequently, it is made possible thereby to sufficiently prevent outside air from reaching a molten pool and also sufficiently suppress the generation of an unfavorable surface oxide, thus enabling welding work to be satisfactorily practiced by using the above welding torch.

A quality and cost approach for welding process selection

Abstract: The aim of this work was to propose, apply and evaluate a methodical approach to select welding processes in a productive environment based on market requirements of Quality and Costs. A case study was used. The welds were carried out in laboratory, simulating the joint conditions of a manufacturer and using several welding processes: SMAW, GTAW, pulsed GTAW, GMAW with CO2 and Ar based shielding gases and pulsed GMAW. For Quality analysis geometrical aspects of the beads were considered and for Cost analysis, welding parameters and consumable prices. Quantitative indices were proposed and evaluated. After that, evaluation of both Quality and Costs was done, showing to be possible to select the most suitable welding process to a specific application, taking into account the market conditions of a company.

Method and apparatus for hybrid welding under shielding gas

Abstract: A hybrid welding from a combination of laser beam welding with gas-shielded welding by electric arc uses at least two electrodes are used The electrodes can be flooded by a common shielding gas curtain or separately or in groups The hybrid welding increases the possibility of influencing the welding process and especially provides improved possibilities for automation

Nitrogen Absorption by Iron and Stainless Steels during YAG Laser Welding

Abstract: The nitrogen absorption by iron, Fe-20Cr-10Ni and SUS329J1 stainless steel YAG laser welding in the atmosphere of Ar-N2 mixture gas was investigated in comparison with those during arc welding using the same materials as in this experiment and equilibrium data. Although the nitrogen contents of YAG laser weld metal increase with the nitrogen partial pressure were as well as those of arc weld metal of arc welding, the nitrogen content during YAG laser welding were quite less than those during arc welding. Blowholes can not be observed in Fe-20Cr-10Ni and SUS329J1 stainless steel and can only be found in iron at lower nitrogen partial pressure during YAG laser welding. A discussion on the difference in nitrogen absorption between YAG Laser and arc welding has suggested that small amount of nitrogen absorption results from less opportunity of nitrogen to touch the surface of molten metal due to the active evaporation of metal which covers the major surface of molten metal during laser welding metal. Additionally, the short-time thermal cycle compared with arc welding may bring insufficient nitrogen absorption in the weld metal during cooling. It can be considered that the nitrogen absorption during YAG laser welding is basically different from that during arc welding.

Experimental investigation of gas tungsten arc welding and comparison with theoretical predictions

Abstract: An experimental investigation in gas tungsten arc welding with the hydrogen addition to argon was made. The hydrogen addition to argon makes the arc constrict and the energy concentrate in the arc, which produces increase in arc power. Experimental welding was carried out on three base metals, i.e., low-alloy steel, high-alloy stainless steel, and an aluminum alloy, using no filler material and in argon with the addition of 0.5-20% hydrogen. In welding of stainless steel with the addition of 20% hydrogen to argon, the quantity of the base metal melted increases by three to five times, with the other parameters remaining the same, in welding of low-alloy steel by three times, and in welding of aluminum by six to nine times. The experimentally obtained results were compared with the theoretical ones. With both steels, the experimental and theoretical results agreed well, but with aluminum, they differed very much.

Mapping transfer modes for stainless steel gas metal arc welding

Abstract: A methodology for the construction of transfer mode maps for stainless steel gas metal arc welding, with argon and argon–oxygen shielding gases, is presented. A back lighting laser and high speed video camera were used for visualisation and measurement of droplets and electrode extension. The reasons for the use of a groove, instead of the traditional bead on plate method, and of same volume beads are discussed and the results assessed. Unlike in other mapping procedures, mapping was conducted as a function of welding current and arc length. In addition, transfer rate v. welding current or wire feedrate curves were plotted. The results show the importance of the use of both maps and curves for identification and quantification of the shielding gas effects on the transfer mode. The results also suggest that an increase in oxygen content in the shielding gas reduces the values of transition current and transition wire feedrate (as expected), but also that it reduces the transfer rate and droplet siz...

Flux-containing wire for gas shield arc welding

Abstract: PROBLEM TO BE SOLVED: To provide a flux-containing wire for gas shield arc welding capable of obtained weld metal having good welding operability and excellent low temperature toughness even in the case gaseous CO2 is particularly used as shield gas in a CaF2 series flux-containing wire. SOLUTION: In this flux-containing wire for gas shield arc welding, the inside of the outer surface made of steel is filled with flux containing CaF2 of 1.0 to 5.0%, one or two kinds of the multiple oxide of alkali metal and/or alkaline-earth metal and Ti and the multiple oxide of alkali metal and/or alkaline-earth metal, Ti and Si (hereinafter referred generically as alkali TiO2 multiple oxide) of 0.3 to 3.5% and TiO2 of 0.5 to 3.0% (inclusive of the value expressed in terms of TiO2 of Ti in the alkali TiO2 multiple oxide), to the total weight of the wire, and, in the flux-containing wire, CaF2/ alkali TiO2 multiple oxide=1.0 to 5.0, and CaF2/TiO2=1.0 to 5.0 are satisfied, moreover, the total content of hydrogen in the wire is 90 ppm or less, and furthermore, carbonate is contained in 2% or less.

CO 2 laser welding of magnesium alloys

Abstract: Metallic alloys with a low mass density can be considered to be basic materials in aeronautic and automotive industry. Magnesium alloys have better properties than aluminum alloys in respect of their low density and high resistance to traction. The main problems of magnesium alloy welding are the inflammability, the crack formation and the appearance of porosity during the solidification. The laser tool is efficient to overcome the difficulties of manufacturing by conventional processing. Besides, the laser processing mainly using shielding gases allows an effective protection of the metal against the action of oxygen and a small heat affected zone. In this paper, we present experimental results about 5 kW CO2 laser welding of 4 mm-thick magnesium alloy plates provided by Eurocopter France. The focused laser beam has about 0.15 mm of diameter. We have investigated the following sample: WE43, alloy recommended in aeronautic and space applications, is constituted with Mg, Y, Zr, rare earth. More ductile, it can be used at high temperatures until 250 degrees Celsius for times longer than 5000 hours without effects on its mechanical properties. A sample of RZ5 (French Norm: GZ4TR, United States Norm ZE41) is composed of Mg, Zn, Zr, La, rare earth. This alloy has excellent properties of foundry and it allows to the realization of components with complex form. Also, it has a good resistance and important properties of tightness. The parameters of the process were optimized in the following fields: laser power: 2 to 5 kW, welding speed: 1 to 4.5 m/min, focal position: -3 mm to +3 mm below or on the top of the metal surface, shielding gas: helium with a flow of 10 to 60 l/min at 4 bars. Metallurgical analyses and mechanical control are made (macroscopic structure, microscopic structure, interpretations of the structures and localization of possible defects, analyse phases, chemical composition, hardness, tensile test etc.) to understand the parameters influence of welding on the obtained beads. For a given laser power, we considered that the welding speed as well as the focal position strongly influence the macroscopic and microscopic welding aspect, whereas the dependence with the flow of the protection gas is weak. For WE43, the bead appears correct in the macroscopic scale for a laser power of 2 kW, a speed of 2 m/min, a focal position on the metal surface or 1 mm under; and an output helium gas of 50 l/min. For RZ5, a correct weld is obtained with a 3 kW laser power, a welding speed of 2 m/min, a focal position of 1.5 mm under the surface and a 50 l/min output helium gas. The microscopic examination showed that the size of the grains has clearly reduced (reduction factor can be up to 35) without formation of porosities, neither cracks nor inclusions; indeed the measured Vickers microhardness of the weld bead is slightly higher than the basic metal. Experiments show that we obtained adequate parameters for high quality welding without using filler material. In future, we plan to weld at higher speed by optimizing the various parameters of the laser welding (power, focal position welding speed and gas flow, ...). Furthermore, we will try to weld samples with a thickness superior than 4 mm.

Narrow groove welding gas diffuser assembly and welding torch

Abstract: A diffuser assembly is provided for narrow groove welding using an automatic gas tungsten arc welding torch. The diffuser assembly includes a manifold adapted for adjustable mounting on the welding torch which is received in a central opening in the manifold. Laterally extending manifold sections communicate with a shield gas inlet such that shield gas supplied to the inlet passes to gas passages of the manifold sections. First and second tapered diffusers are respectively connected to the manifold sections in fluid communication with the gas passages thereof. The diffusers extend downwardly along the torch electrode on opposite sides thereof so as to release shield gas along the length of the electrode and at the distal tip of the electrode. The diffusers are of a transverse width which is on the order of the thickness of the electrode so that the diffusers can, in use, be inserted into a narrow welding groove before and after the electrode in the direction of the weld operation.

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.

Shielding gas mixture for gas-metal arc welding

Abstract: A shielding gas mixture for gas-metal arc welding of austenitic stainless steel is provided in which the gas mixture comprises from about 2 to about 5% carbon dioxide, from about 1 to about 4% nitrogen, and the balance being argon. Also, a process for welding austenitic stainless steel is provided by forming an electric arc between a nonconsumable electrode and the workpiece and in which the gas mixture is used.

Composite welding process

Abstract: PROBLEM TO BE SOLVED: To provide a composite welding process which forms efficiently a weld zone of high quality in a welding member. SOLUTION: When in this composite welding process, a weld zone is formed on an welding member 17, an welding rod 16 is added to a laser beam 12, an arc 20 and a shielding gas 19 so as to promote the fluidity of a molten-metal part 18.

Gas shielded-system arc welding method

Abstract: PROBLEM TO BE SOLVED: To improve properties such as wear resistance, corrosion resistance, etc., by making small the dilution of overlaying metal by a base metal 3. SOLUTION: When performing overlaying by gas shielded-system arc welding using an electrode wire 1, by emitting a laser beam 7 to the rearward of molten metal 5 and by introducing an arc 4 backward, the zone overlaying is made by penetration by the only heat of the molten metal 5, the dilution of the overlaying metal by the base metal 3 is made small, and the properties such as the wear resistance, the corrosion resistance, etc., are improved.