Showing papers on "Shielding gas published in 2007"

Predictions of weld pool profiles using plasma physics

Abstract: This paper gives a review of recent papers which have led to the capability of the prediction of weld depths for gas tungsten arc welding, for any given arc current, electrode shape or separation and welding gas. The methodology is given for deriving plasma composition as a function of temperature and pressure from basic atomic and molecular properties. Transport coefficients of density, specific heat, enthalpy, electrical conductivity, thermal conductivity, viscosity and radiation emission coefficients can then be derived as a function of temperature. The conservation equations of fluid dynamics are then used to derive weld profiles for stainless steel for welding gases such as argon, helium, carbon dioxide and a 10% mixture of hydrogen in argon. The markedly different weld depths which are obtained are related to basic material functions such as specific heat, electrical and thermal conductivity. The temperature dependence of the surface tension coefficient has a marked effect on weld depth and profiles because it can influence the direction of circulatory flow in the weld pool. Electric arcs in helium and carbon dioxide are more constricted than arcs in argon and as a consequence the magnetic pinch pressure of the arc, transmitted to the weld pool, can force strong downward flows in the weld pool and thus lead to a deep weld. It is found that because of the interactions of the arc and the weld pool through effects such as viscous drag forces of the plasma on the weld pool, it is necessary to treat the arc, the electrode and the weld pool in a unified system.

Mechanism and Microstructure of Oxide Fluxes for Gas Tungsten Arc Welding of Magnesium Alloy

Abstract: Five single oxide fluxes—MgO, CaO, TiO2, MnO2, and Cr2O3—were used to investigate the effect of active flux on the depth/width ratio in AZ31B magnesium alloy. The microstructure and mechanical property of the tungsten inert gas (TIG) welding seam were studied. The oxygen content in the weld seam and the arc images during the TIG welding process were analyzed. A series of emission spectroscopy of weld arc for TIG welding for magnesium with and without flux were developed. The results showed that for the five single oxide fluxes, all can increase the weld penetration effectively and grain size in the weld seam of alternating current tungsten inert gas (ACTIG) welding of the Mg alloy. The oxygen content of the welds made without flux is not very different from those produced with oxide fluxes not considering trapped oxide. However, welds that have the best penetration have a relatively higher oxygen content among those produced with flux. It was found that the arc images with the oxide fluxes were only the enlarged form of the arc images without flux; the arc constriction was not observed. The detection of arc spectroscopy showed that the metal elements in the oxides exist as the neutral atom or the first cation in the weld arc. This finding would influence the arc properties. When TIG simulation was carried out on a plate with flux applied only on one side, the arc image video showed an asymmetric arc, which deviated toward the flux free side. The thermal stability, the dissociation energy, and the electrical conductivity of oxide should be considered when studying the mechanism for increased TIG flux weld penetration.

Apparatus and method for welding

Abstract: The present invention relates to arc welding torch and a method of extracting fume gas from a welding site. The torch comprises a metal electrode and at least one shield gas port adapted to direct a shield gas curtain around the metal electrode and a welding site. At least one shroud gas port is spaced radially outward from the shield gas port and adapted to impart to an exiting shroud gas a radially outward component of velocity. Fume gas is preferably extracted from a position radially intermediate the shield gas curtain and the shroud gas curtain.

Effects of flux containing fluorides on TIG welding process

Abstract: The mechanisms of the inorganic powders, named activating flux and used in ATIG welding process, are not well identified. Oxides and fluorides usually compose activating fluxes. The present paper studies the fluoride activating effect. The analysis of the arc energy influence on the weld bead geometry allows determining the mechanisms that occur during ATIG welding with fluorides. It seems that fluoride activating effect could depends on the fluoride lattice energy and/or the ionic radius of the element. Fluorides act on the arc physics only. An arc energy densification is observed. Furthermore, an arc temperature increase, find out with optical emission spectrometry, characterises the fluoride activating effect. The fluorides do not affect the weld bead surface chemistry. Marangoni convection movements stay centrifugal as in TIG welding and do not favour a penetration increase.

Analysis of Beam Material Interaction in Welding of Titanium with Fiber Lasers

Abstract: Ti-6Al-4V is one of the most widely used titanium alloy in industrial applications because of its lightweight and corrosion resistance. The new generation of high power fiber lasers presents several benefits, namely, high power, low beam divergence, and compact size. These lasers can be used in a diversity of materials as the low wavelength that characterizes them allows absorption by almost all metals and alloys. This article presents a research about the weldability of the Ti-6Al-4V alloy using a fiber laser. Weld beads produced with different processing parameters were morphologically characterized under optical microscopy and the microstructures obtained were investigated.

Pulsed arc welding method

Abstract: In a pulsed arc welding method of the invention, a shield gas made mainly of carbon dioxide gas is used, and a low frequency pulse of 30 to 100 Hz is continuously generated, on which a high frequency pulse with a pulse frequency of 500 to 2000 Hz is superimposed. In this case, the welding current is determined such that an average peak current IP avg is at 300 to 700 A, an average base time T b is at 5 to 30 ms, a current amplitude IP- a at a peak time of a high frequency pulse is at 50 to 600 A, and a current amplitude IP b at a base time of a high frequency pulse is at 20 to 200 A. This method ensures a reduced degree of biasing of a drop and an arc, for which the size, release time and release direction of a drop are kept substantially completely constant, and can achieve drop transfer wherein regularity of one pulse group-one drop transfer is very high. Hence, spatter and fume generation rates can be remarkably reduced.

Effect of oxygen content in He–O2 shielding gas on weld shape in ultra deep penetration TIG

Abstract: The effect of the shielding gas concentration on the weld shape was studied for the moving bead on plate TIG welding of SUS304 stainless steel under He–O2 mixed shielding. The small addition of oxygen to the helium base shielding gas can precisely control the oxygen content in a liquid pool and the weld shape. Oxygen is a surface active element for stainless steel. When the oxygen content in the liquid pool is above the critical value of ∼ 70 ppm, the weld shape suddenly changes from a wide shallow type to a deep narrow one due to the change in the Marangoni convection from the outward to inward direction on the liquid pool surface. Weld shape variations influenced by the welding parameters including welding speed, welding current and electrode tip work distance under pure He and He–0.4%O2 mixed gas shielding were systematically investigated. The investigation results showed that the final shape of the TIG weld depends to a large extent on the pattern and magnitude of the Marangoni convection on t...

Hot cracking in tungsten inert gas welding of magnesium alloy AZ91D

Abstract: Plates of 3–5 mm in thickness were extracted from an AZ91D ingot and then butt joints of the plates were produced using tungsten inert gas (TIG) welding method. The TIG arc was also used to deposit welding beads on some of the thin plates. No cracking was found in the butt joints. However, hot cracking was always observed to propagate from the heat affected zone (HAZ) under the welding bead into the weld metal right after a welding bead was deposited on the thin plate. Metallographic and fractographic evidence was obtained to show that the hot cracking is 'liquation cracking' in the partially melted HAZ under the high thermal stresses. In the butt joints, the weld metal has the finest grains, highest strength and best ductility, and the HAZ was found to be the 'weakest link'.

Plasma properties of helium gas tungsten arc with metal vapour

Abstract: The energy source characteristics of gas tungsten arc (GTA) strongly depend on the physical property of arc plasma. In welding processes, it has been experimentally confirmed that metal vapour evaporated from a high temperature weld pool drastically changes the property of arc plasma and decreases its temperature. However, the effect of metal vapour on the characteristics of heat flux into a base metal is still not clear owing to the difficulty in experimental studies of arc plasma. In the present paper, the energy source property of helium GTA mixed with metal vapour was numerically analysed. It was found that the intense radiation generated from dense metal vapour decreases heat flux into a base metal and contracts the current density distribution especially near the arc axis.

Variable AC polarity GTAW fusion behavior in 5083 aluminum

Abstract: Aluminum alloys are typically welded on AC with the gas tungsten arc welding (GTAW) process. Many power sources have "max penetration" indicated when more than 50% of the AC cycle is spent on electrode negative polarity and "max cleaning" when more than 50% of the cycle is on electrode positive polarity. In the work reported here, weld bead dimensions, notably penetration and bead width, increase with the percentage of electrode positive polarity during the unbalanced square wave AC welding of aluminum alloys with the GTAW process. This is in direct contradiction of conventional assumptions about the role of electrode positive and electrode negative contributions to surface cleaning and fusion behavior during AC welding. The primary source of the extra base metal melting during electrode positive operation is in the nature of cold cathode field emission of electrons from the base metal. The dielectric breakdown of surface oxide as electrons are emitted also contributes to the increased fusion, but this is not a contributory factor once the weld metal surface is completely clean. Both phenomena require extra energy to be supplied to the cathode, which results in the increased fusion. Earlier works on the gas metal arc welding (GMAW) process confirm this behavior of enhanced melting at the cathode. Positive ion bombardment, thermal convection from the plasma jet, and radiation from the plasma complete the thermal input to the cathode for metal fusion.

Welding Stability System and Method

Abstract: A weld stability system for an arc welding apparatus and method of operation is disclosed. The weld stability system may comprise a shielding gas supply and a control assembly. The shielding gas supply may include a first source of gas, a second source of gas, a mixing chamber, a first valve selectively connecting the first source of gas to the mixing chamber, a second valve selectively connecting the second source of gas to the mixing chamber, and a shielding gas supply line configured to direct gas from the mixing chamber to a weld gun. The control assembly may include a controller operatively engaging the first and second valves, and at least one sensor configured to monitor a parameter of an arc welding process and communicate with the controller.

Laser arc hybrid welding method for surface coated metal parts, the surface coating containing aluminium

Abstract: The process of welding a steel part (1) by a laser beam, comprises combining the laser beam with an electric arc to operate a fusion of the metal and the welding of parts. The steel part has a coating surface (2) having aluminum (90%) and silica (10%) and a thickness of 30 mu m. The part has a thickness of 0.8-2.5 mm, and comprises a deposit of aluminum or silica on the surface of a side edge (1a) before welding the part. An arc is delivered by a welding electrode with tungsten, or formed at an end of the fusible wire. A gaseous protection of the part is maintained during welding. The process of welding a steel part (1) by a laser beam, comprises combining the laser beam with an electric arc to operate a fusion of the metal and the welding of parts. The steel part presents a coating surface (2) having aluminum (90%) and silica (10%) and a thickness of 30 mu m. The part has a thickness of 0.8-2.5 mm, and presents a deposit of aluminum or silica on the surface of a side edge (1a) before welding the part. An arc is delivered by a welding electrode with tungsten, or formed at an end of the fusible wire. A gaseous protection of the part is maintained during welding. The laser beam is generated by a laser generator of carbon dioxide and neodymium-doped yttrium aluminum garnet with diodes or an ytterbium fiber.

Effects of Different Shielding Gases and Power Waveforms on Penetration Characteristics and Porosity Formation in Laser Welding of Inconel 690 Alloy

Abstract: A high power Nd-YAG laser was used to perform bead-on-plate (BOP) welding on high viscosity Inconel 690 alloy plates of 3 mm in thickness with three different shielding gases (Ar, He, and N 2 ). Adopting a rectangular laser power waveform, four different peak-base power differentials (ΔP) were applied with a constant average power of 1.5 kW. A comprehensive investigation was performed into the influences of the shielding gas, the flow rate, and the value of AP on the characteristics of the resulting welds, including the weld morphology, the penetration depth, the plume volume, and the porosity formation. The results showed that the weld penetration depth, the depth-to-width ratio, the weld surface roughness and the degree of weld spattering all increased with increasing AP. The choice of shielding gas had a significant effect on the porosity ratio (P r ) of the weld. The weld formed under Ar shielding had the highest P r , while that formed under N 2 shielding had the lowest. Under He shielding, the gas flow rate had a significant effect on the porosity ratio. However, under the higher density gases of Ar and N 2 , the porosity appeared to be insensitive to the flow rate. Finally, an increased ΔP yielded a significant reduction in P r for the welds with higher porosity.

Prediction of element transfer in submerged arc welding

Abstract: The transfer of elements across the molten weld pool has been predicted by developing quadratic models in terms of flux ingredients with the application of statistical experiments for mixture design. Bead-on-plate weld deposits were made at fixed welding parameters using submerged are welding fluxes prepared as per extreme vertices algorithm of mixture experiments in a CaO-MgO-Caf 2 -Al 2 O; flux system. The results show that some of the individual flux ingredients and their binary mixtures have a predominant effect on weld metal transfer of oxygen, manganese, silicon, and carbon contents. The analysis of experimental data also indicates that transfer of oxygen is affected by several properties of flux ingredients such as oxygen potential, thermodynamic stahility, and viscosity. In the element transfer of silicon, both thermochemical and electrochemical reaction mechanisms operate simultaneously. Transfer of manganese is principally related to the weld metal oxygen contents as well as electrochemical reaction in the molten weld pool. The transfer of carbon was generally governed by the oxidation reaction. Iso-response contour plots were also developed to quantify the transfer of elements against different flux compositions.

Predicting onset of high speed gas metal arc weld bead defects using dimensional analysis techniques

Abstract: The onset of geometric defects such as humping or discontinuous weld beads during gas metal arc welding (GMAW) frequently limits the use of higher welding speeds and increased productivity. In the present study, a dimensional analysis of the GMAW process was performed in order to identify a number of dimensionless groups formulated based on various GMAW process parameters and material properties that could be used to predict when humping or discontinuous weld beads would occur. Experimental data from bead on plate GMA welds in plain carbon steel plate made using argon and two different reactive shielding gases, welding powers between 5 and 12 kW and a range of welding speeds were then used to create dimensionless process maps. These maps showed the limiting welding speed above which the high speed weld defects occurred as a function of all influential process parameters. It was shown that all experimental data for limiting welding speeds could be collapsed onto two collinear dimensionless curves. ...

Porosity and Nitrogen Content of Weld Metal in Laser Welding of High Nitrogen Austenitic Stainless Steel

Abstract: Some problems such as nitrogen desorption and pores always occur in the weld metal during welding of high nitrogen steel. In order to study the nitrogen content and porosity of the weld metal of high nitrogen steel 1Cr22Mn16N, the steel was welded by CO2 laser welding, and the influence of the shielding gas composition and heat input on the nitrogen content and porosity of the weld metal was investigated. The experimental results indicate that the weld nitrogen content increases slightly with the increase of the nitrogen content in shielding gas under the same laser welding conditions. The nitrogen content of the weld metal decreases with the increase of the heat input when pure argon is used as the shielding gas, whereas that of the weld metal is improved with the increase of the heat input when some nitrogen is added to the shielding gas. The higher the heat input, the less the porosity in the weld metal, and the more nitrogen in the shielding gas, the less the porosity becomes.

The effect of N2 addition upon the MIG welding process of duplex steels

Abstract: This paper presents the results of a study on the weldability of a duplex stainless steel, Avesta 2205, carried out by GMAW (MIG) pulsed arc welding process. An AISI 2209 electrode (AWS A/SFA 5.9, ER2209) was used as filler metal. The study was focused on the N2 content in the shield gas, from 0% to 6.4%. Firstly, a microstructural characterization of the welds using scanning electron microscopy (MEB-EDX) was carried out. Also, in order to study the microstructural changes originated by the welding thermal cycles and the % content of the N2, the ferrite content in the weld pool and heat affected zone (HAZ) were determined. Vickers hardness, tensile and bending tests were performed to determine the mechanical properties of joints and hence the influence of N2 addition without decrease in the mechanical properties. Finally, the joints were examined for susceptibility to intergranular corrosion using the Standard ASTM 262 93, practice A. The optimal content of N2 in the shield gas is included between 3% and 5%, which attain to obtain a 94% base material UTS.

Arc heating hot wire assisted arc welding technique for low resistance welding wire

Abstract: An assistant arc was used to preheat the wire for hot wire tungsten inert gas (TIG) welding which was independent on the resistance of the welding wires and avoided the drawbacks of the traditional hot wire preheating method. The new method is suitable for low resistance wire such as copper and aluminium. The experimental results showed that the wire temperature varied linearly with preheating current and hyperbolically with wire feeding speed. The temperature of wires achieved 60% of their melting points when LF6, H90, HS201 and H08Mn2Si welding wires were used at a current less than 50 A. With arc assisted hot wire, the welding deposition efficiency of the HS201 wire was increased by 96% compared with the traditional TIG welding method, while its microstructure was similar to that of the cold wire welding.

Effect of active flux addition on laser welding of austenitic stainless steel

Abstract: The use of active flux in tungsten inert gas (TIG) welding is known to increase its weld depth. The present paper involves study of active flux laser beam welding (ALBW) of austenitic stainless steel sheets with respect to its effect on plasma plume, microstructure and mechanical properties of the resultant weldments. ALBW performed with SiO2 as the flux significantly modified shape of the fusion zone (FZ) to produce narrower and deeper welds. Plasma plume associated with the process was considerably smaller and of lower intensity than that produced during bead on plate laser beam welding (LBW). Flux addition during LBW produced thin and rough weld bead associated with humping. The development of such a weld bead is cause by reversal in the direction of Marangoni flow by oxygen induced inversion of surface tension gradient, widely fluctuating plasma plume and presence of oxides on the weld pool surface preventing free flow of the melt. Active flux laser weldments exhibited lower ductility than tha...

Metal cored electrode for open root pass welding

Abstract: A method and apparatus of forming a root bead in a gap between spaced ends of at least one workpiece. The gap includes an open root. A metal cored electrode having a metal sheath and core materials is used to for the root bead in the open root. The core materials include little or no slag forming agent. The metal cored electrode is advanced at a given wire feed rate toward the open root to weld the ends together by at least partially filling the open root in a first weld pass. A welding current having a controlled waveform is used. The waveform includes a succession of welding cycles each having a short circuit portion and a metal the metal cored electrode and to cause the metal cored electrode to melt and transfer to the ends in the open root to form a root bead. The formed root bead has little or no slag on the upper surface of the root bead. A shielding gas is used to at least partially protect the molten metal in the open root from the atmosphere.

Pulsed Current GMAW for Superior Weld Quality of Austenitic Stainless Steel Sheet

Abstract: The performance of pulsed current gas metal arc welding (P-GMAW) and conventional gas metal arc welding (GMAW) processes in welding of 2.5 mm thin stainless steel sheet at different heat input has been studied. The use of P-GMAW at low heat input has been found superior to the use of GMAW process with respect to significant reduction in intergranular corrosion (IGC) susceptibility along with considerable improvement in some other characteristics of weld joint by maintaining comparable mechanical properties. It was further observed that the IGC, weld geometry and microstructure of P-GMA weld joint are largely governed by pulse parameters, and varies as a function of factor φ, defined as a summarized influence of pulse parameters such as peak current, base current, pulse-off time and pulse frequency. The increase of φ has been found favourable to reduce IGC, weld geometry along with refinement of the cast (dendritic) structure of weld deposit.