Conventional Gas Tungsten Arc Research Articles

Studies on microstructure, mechanical and pitting corrosion behaviour of similar and dissimilar stainless steel gas tungsten arc welds

In the present study, an attempt has been made to weld dissimilar alloys of 5mm thick plates i.e., austenitic stainless steel (316L) and duplex stainless steel (2205) and compared with that of similar welds. Welds are made with conventional gas tungsten arc welding (GTAW) process with two different filler wires namely i.e., 309L and 2209. Welds were characterized using optical microscopy to observe the microstructural changes and correlate with mechanical properties using hardness, tensile and impact testing. Potentio-dynamic polarization studies were carried out to observe the pitting corrosion behaviour in different regions of the welds. Results of the present study established that change in filler wire composition resulted in microstructural variation in all the welds with different morphology of ferrite and austenite. Welds made with 2209 filler showed plate like widmanstatten austenite (WA) nucleated at grain boundaries. Compared to similar stainless steel welds inferior mechanical properties was observed in dissimilar stainless steel welds. Pitting corrosion resistance is observed to be low for dissimilar stainless steel welds when compared to similar stainless steel welds. Overall study showed that similar duplex stainless steel welds having favorable microstructure and resulted in better mechanical properties and corrosion resistance. Relatively dissimilar stainless steel welds made with 309L filler obtained optimum combination of mechanical properties and pitting corrosion resistance when compared to 2209 filler and is recommended for industrial practice.

Influence of Welding Process and Post Weld Heat Treatment on Microstructure and Pitting Corrosion Behavior of Dissimilar Aluminium Alloy Welds

Welding of dissimilar Aluminum alloy welds is becoming important in aerospace, shipbuilding and defence applications. In the present work, an attempt has been made to weld dissimilar aluminium alloys using conventional gas tungsten arc welding (GTAW) and friction stir welding (FSW) processes. An attempt was also made to study the effect of post weld heat treatment (T4 condition) on microstructure and pitting corrosion behaviour of these welds. Results of the present investigation established the differences in microstructures of the base metals in T4 condition and in annealed conditions. It is evident that the thickness of the PMZ is relatively more on AA2014 side than that of AA6061 side. In FS welds, lamellar like shear bands are well noticed on the top of the stir zone. The concentration profile of dissimilar friction stir weld in T4 condition revealed that no diffusion has taken place at the interface. Poor Hardness is observed in all regions of FS welds compared to that of GTA welds. Pitting corrosion resistance of the dissimilar FS welds in all regions was improved by post weld heat treatment.

Effect of Welding Process on Microstructure, Mechanical and Pitting Corrosion Behaviour of 2205 Duplex Stainless Steel Welds

An attempt has been made to weld 2205 Duplex stainless steel of 6mm thick plate using conventional gas tungsten arc welding (GTAW) and activated gas tungsten arc welding (A- GTAW) process using silica powder as activated flux. Present work is aimed at studying the effect of welding process on depth of penetration, width of weld zone of 2205 duplex stainless steel. It also aims to observe the microstructural changes and its effect on mechanical properties and pitting corrosion resistance of 2205 duplex stainless steel welds. Metallography is done to observe the microstructural changes of the welds using image analyzer attached to the optical microscopy. Hardness studies, tensile and ductility bend tests were evaluated for mechanical properties. Potentio-dynamic polarization studies were carried out using a basic GillAC electro-chemical system in 3.5% NaCl solution to observe the pitting corrosion behaviour. Results of the present investigation established that increased depth of penetration and reduction of weld width in a single pass by activated GTAW with the application of SiO2 flux was observed when compared with conventional GTAW process. It may be attributed to the arc constriction effect. Microstructure of the weld zones for both the welds is observed to be having combination of austenite and delta ferrite. Grain boundary austenite (GBA) with Widmanstatten-type austenite (WA) of plate-like feature was nucleated from the grain boundaries in the weld zone of A-GTAW process. Mechanical properties are relatively low in activated GTAW process and are attributed to changes in microstructural morphology of austenite. Improved pitting corrosion resistance was observed for the welds made with A-GTAW process.

Low heat input gas metal arc welding for dissimilar metal weld overlays part II: the transition zone

Dissimilar metal weld overlays (DMWOL) of nickel base alloys on low alloy steel components are commonly used in the oil and gas, petrochemical, and power generation industries to provide corrosion and oxidation resistance in a wide range of service environments and temperatures. Traditionally, dissimilar weld overlays are produced using cold or hot wire gas tungsten arc welding. This study aims to identify and evaluate potential advantages of low heat input gas metal arc welding processes over the conventional gas tungsten arc welding in the production of dissimilar weld overlays. In order to evaluate the quality of these overlays regarding resistance against hydrogen-assisted cracking, their transition zone region is investigated in this part of the publication series. Metallurgical characterization, including energy-dispersive x-ray spectroscopy, is performed on Alloy 625/grade 22 steel overlays. The transition zone is characterized by a narrow planar growth zone and steep compositional gradients from the fusion boundary towards the weld metal. Evidence of low carbon contents in the planar growth zone, as well as for carbide precipitation in the cellular growth zone was found. The microstructure in the transition zone region of the fusion zone shows characteristics known to be suitable for good resistance against hydrogen embrittlement.

An experimental and numerical study of forces and residual stresses in AISI 316L stainless steel joints due to conventional and pulse gas tungsten arc welding

This paper aims to present the results of a numerical and experimental study of the temperature field, internal forces and the residual stresses in 2 mm thick autogenous welds of AISI 316L stainless steel produced by continuous and pulse current gas tungsten arc welding. A special experimental device was used to measure the temperature and the internal forces due to the welding. The welds were qualified for internal and external weld imperfections according to ISO 15614-1. FEM software ANSYS® Multyphysics™ was applied in order to solve the thermal and mechanical problems. Normal residual stresses were measured by the hole-drilling strain gauge method in the continuous current weld. The peak value of the longitudinal stress was 80 % of the base metal yield stress. The magnitude of the numerically obtained residual stress values was found to be 16 % to 19 % above the measured one in the longitudinal and transverse direction, respectively. The experimental device used in this study allowed for a real time measurement of forces far from the weld seam. On the basis of the correspondence between the calculated and measured forces the numerical results were verified. Therefore, this device might open up new possibilities for determining thermo-mechanical material data.

Development of welding technique to avoid the sensitization in the alloy 600 by conventional Gas Tungsten Arc Welding method

Even though Nickel based superalloy (alloy 600) exhibits high corrosion resistance at elevated temperatures, the main limitation of this alloy is the formation of Cr-depleted zones in the grain boundary regions. In this regard, there is a constant demand for the reduction of Cr-depleted (sensitization) zones towards various applications. In order to achieve this, the present work is focussed on the fabrication of weld joints by Gas Tungsten Arc Welding (GTAW) and Pulsed Current Gas Tungsten Arc Welding (PCGTAW) using the filler wires such as ERNiCr-3, ERNiCrMo-3, and ERNiCrMo-14 to avoid sensitization. The fabricated materials were analyzed by optical and scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). The macro examination of these welded materials clearly reveals the presence of defect-free weld joints. As per the results from the microstructural analysis, the rapid cooling was achieved in the pulsed current technique, and subsequently, the equiaxed dendrites were observed in the fusion zone. EDS analysis showed the presence of Nb-rich (MC) carbide phases near the joints fabricated by filler wires i.e. GTA ERNiCr-3 and ERNiCrMo-3 indicating the preferential formation of Nb-carbide over Cr-carbide. In a similar way, Mo-rich M23C6 phases were observed near the joint fabricated by the filler wire, GTA ERNiCrMo-14. There was no microsegregation observed during the fabrication through the PCGTA welding technique.

Microstructure and Fatigue Property of Ti–6Al–4V by Ultrahigh Frequency Pulse Welding

Butt welding tests of 1.5 mm thickness Ti–6Al–4V were treated by conventional gas tungsten arc welding (C-GTAW) and ultrahigh frequency pulse GTAW (UHFP-GTAW). The low cycle fatigue (LCF) experiments were conducted on the welded joints. The results of fatigue experiment showed that the number of fatigue cycles was increased with UHFP-GTAW. Changes in the microstructure resulting from reduced heat input were expected to enhance the fatigue propagation resistance. The morphology of the martensites in fusion zone was smaller compared to C-GTAW process, and a larger distribution density of basketweave structure was also obtained by UHFP-GTAW. Furthermore, the decreased fatigue crack rate was accompanied as the increased grain boundaries produced by the reduced grain size in fusion zone. Observation of fatigue fractographs revealed that the UHFP-GTAW has obvious slip traces at fatigue initiation sites and more deep secondary cracks in the crack propagation regions associated with the smaller dimples of final fracture zones. The proportion of propagation regions was much larger than C-GTAW. As a result, it can be considered as the representation of the improvement in ductility.

The energy distribution of electrode in hollow cathode centered negative pressure arc

As a new type of welding heat source, hollow cathode centered negative pressure arc (HCCNPA) has evenly distributed energy density in the center of the arc and a higher energy gradient at the boundary in comparison with traditional gas tungsten arc (GTA). With these changes, the brightness of the cathode increased significantly, which is the effect of inner arc plasma on the hollow cathode. To better understand of this, in present study the measurements of energy distribution between anode and cathode in HCCNPA were performed respectively by calorimetric calculation. Experiments were conducted to evaluate the influence of welding parameters on heat distribution of HCCNPA, and the results were compared with conventional GTAW process. It was found that the application of negative pressure in the hollow cathode greatly modified the energy distribution between anode and cathode. Though the energy demand of maintaining a stable arc increased with the application of negative pressure which reflected as the increase of arc voltage, the total energy used to electrode became lower. In particular, the energy in cathode increased and that in anode decreased as the negative pressure applied, and these differences increase as the current and arc length increase. Otherwise, the application of negative pressure in the hollow made the result arc constrained, so the HCCNPA has a more evenly distributed energy density in the center and a higher energy gradient at the boundary in comparison with the conventional GTA, which would enable the development of high quality applications.

Phenomena in GTA and GHTA welding in Mars-like atmosphere

NASA is advancing the project of manned Mars exploration. In the future, Martian outposts and structures will be constructed. To realize this, welding technology is expected to be applied. The main atmospheric component of Mars is carbon dioxide, and the atmospheric pressure is approximately 700 Pa. In this study, welding experiments were carried out in a simulated Mars atmosphere of 99.5% carbon dioxide and a pressure of 700 Pa. Conventional gas tungsten arc (GTA) welding and gas hollow tungsten arc (GHTA) welding, in which arc operating gas is allowed only to flow out from the electrode tip of a hollow tungsten electrode, were investigated. The arc discharge behaviour and the melting characteristics in the simulated Mars atmosphere were studied. As a result, it was shown that GTA welding and GHTA welding might be applicable even in the Mars atmosphere.

The effect of pulsed frequency on the plasma jet force with ultra high-frequency pulsed arc welding

Plasma jet force is considered to be the key component of the arc force during arc welding. This is known to be the important factor for surface penetration into the molten pool. With pulsed welding technology, the arc plasma constricts significantly which affects the plasma jet force. The experimental results in this work indicate that the arc force increases with increasing pulse frequency. The plasma jet force is found to be the most important contributor to a larger arc force during ultra high-frequency pulsed gas tungsten arc welding (UHFP-GTAW). In this work, the reason for the increasing force is studied with support from a mathematical model. The plasma jet force was recognized to change with the distribution coefficient a at various pulse frequencies. With increasing frequency, the value of the distribution coefficient decreases. As known, the distribution coefficient represents attenuation of the curve. The results suggest that with a high pulse frequency, a high level of plasma jet force occurs in a constricted arc plasma and significant attenuation of the plasma jet force can be found. In contrast, the plasma jet force was low with little attenuation during conventional gas tungsten arc welding (C-GTAW).

Development and evaluation of SUS 304H — IN 617 welds for advanced ultra supercritical boiler applications

At moderately high temperature sections of Advanced Ultra Super Critical (AUSC) boilers, welding of superalloys to austenitic steels is inevitable owing to economic aspects of boiler. Welding of SUS 304H and Inconel 617 (IN 617) was attempted using IN 617 filler material employing conventional Gas Tungsten Arc Welding (GTAW) process and the procedure was successfully established along with optimized welding parameters. Microstructural characterization was carried out to identify various zones on either side of the fusion boundaries. Unmixed Zone and Heat Affected Zone (HAZ) were observed towards SUS 304H fusion boundary while no distinct HAZ was observed towards IN 617 fusion boundary. Micro-hardness profiling indicated decrease in hardness at the HAZ towards SUS 304H fusion boundary. Mechanical properties evaluation at both ambient and elevated temperatures was carried out and data obtained was compared with those of base metals. The tensile strength of the cross weld specimens at high temperatures were observed to be marginally lower than that of IN 617 but significantly more than that of SUS 304H, hence, tolerable. Stress-rupture properties of the cross-weld specimens as tested in this study were found to be intermediate to the base metals’ data, thus, suitable for AUSC power plants' boiler applications. Hence, this work gives an insight into welding procedure establishment, microstructural development, variation of mechanical properties at elevated temperatures and stress-rupture properties of the dissimilar metal welds at elevated temperatures.

Fluid and arc behavior with ultra high frequency pulsed GTAW

During ultra high frequency pulsed gas tungsten arc welding (UHFP-GTAW), the root radius of arc decreased with arc constriction that caused the narrower weld bead. The larger arc force created the more depression of surface, which had been important to increasing of weld penetration. Simultaneously, molten liquid metal is the key factor during welding process, and the mobility of molten pool was enhanced by weld behavior compared with conventional gas tungsten arc welding (C-GTAW). In the paper, the characteristics of arc behavior and fluid of molten pool was discussed with UHFP-GTAW. The results indicated the significant effect of the arc plasma on heat and force of molten pool with UHFP-GTAW. Further, the temperature diffusion of molten pool would reduce, which can limit the impact of arc heat on base metal effectively.

Microstructure, tensile properties of Ti-6Al-4V by ultra high pulse frequency GTAW with low duty cycle

The paper compares the characteristics of Ti-6Al-4V titanium alloy for conventional gas tungsten arc welding (C-GTAW) process with ultra high frequency pulse gas tungsten arc welding (UHFP-GTAW) on the conditions of 50% and 20% duty cycles. The effects of heat input on average intercept of grain size in the fusion zone (FZ), microstructure, tensile properties and fracture behavior were determined. These findings suggest that a lower heat input by UHFP-GTAW process was responsible for better grain size, microstructure and tensile properties except for the effect of ultra high frequency pulse current. In contrast with C-GTAW process, the average intercept of grain size in FZ decreased by 30% at most with 20% duty cycle. The morphology of basketweave structure predominated in the microstructure of FZ by UHFP-GTAW, moreover, a large density and uniform distribution of resultant basketweave were obtained with 20% duty cycle. The samples obtained by UHFP-GTAW generally displayed better ductility, in particular, when the duty cycle was reduced to 20%, the elongation and percentage reduction of area were respectively increased by 140% and 275%. The fractures with 20% duty cycle were mainly located in the base metal, such fractograph showed development of intense shear and deep dimple aggregation, which might be caused by improvement of the ductility.

Numerical Analysis of Gas Tungsten Arc Welding with a Constricted Nozzle for Butt Joint of Thin-Sheet Metals

GTA (Gas Tungsten Arc) welding process has been studied as a higher level welding process. The “GTA welding process with constricted nozzle” is one of new welding processes [1]. In this new process, arc stiffness is greatly improved by attaching an additional gas nozzle which is called “constricted nozzle” inside the shielding gas nozzle. The design of a constricted nozzle is shown in Figure 1. With this new process, a butt joint of thin-sheet metals can be achieved although that is difficult with a conventional GTA welding process. In this study, numerical simulation is conducted to understand phenomena of arc plasma in a GTA welding process with a constricted nozzle scientifically.

擬似火星大気での GTA 溶接および GHTA 溶接実験

NASA is advancing the project of manned Mars exploration. In the future, Martian outposts and structures will be constructed. To realize this, welding technology is expected to be applied. The main atmospheric component of Mars is carbon dioxide, and the atmospheric pressure is approximately 700 Pa. In this study, welding experiments were carried out in a simulated Mars atmosphere of 99.5% carbon dioxide and a pressure of 700 Pa. Conventional Gas Tungsten Arc (GTA) welding and Gas Hollow Tungsten Arc (GHTA) welding, in which arc operating gas is allowed only to flow out from the electrode tip of a hollow tungsten electrode, were investigated. The arc discharge behavior and the melting characteristics in the simulated Mars atmosphere were studied. As a result, it was shown that GTA welding and GHTA welding might be applicable even in the Mars atmosphere.

Effects of laser induced metal vapour on arc plasma duringlaser arc double sided welding of 5A06 aluminium alloy

In order to study the effects of laser induced metal vapour on arc plasmaduring laser arc double sided welding (LADSW), emission spectroscopytechniques were utilised. The arc plasma in LADSW of 5A06 aluminium alloywas compared with conventional gas tungsten arc welding (GTAW) inthis work. The electron temperature and density were estimated by the Saha–Boltzmannequation and the Stark broadening effect. The results indicated that the amountof metal vapour in the arc plasma during LADSW was larger than that of GTAW.Furthermore, the electron temperature of arc plasma in LADSW was lower thanthat in GTAW, and the electron density of arc plasma in LADSW was higher thanthat in GTAW near the arc anode. It was shown that the differences in arcproperties between LADSW and the conventional GTAW processes were the resultof laser induced metal vapour changing the composition of the arc plasma.

Numerical analysis of energy source properties of hollow cathode arc

Since a plasma torch can stabilize high temperature arc plasma by employing a shielding gas, it is widely utilized as a heat source. Gas Tungsten Arc (GTA) is the most widely employed type of the plasma torch. GTA enables to produce the arc plasma with high energy density. Therefore it is suitable especially for processes to require concentrating the heat input at a point but less effective for processes to require heating the target material uniformly. On the other hand, Hollow Cathode Arc (HCA) to be a kind of GTA produces the arc plasma by introducing the shielding gas from the tip of the cathode. It has been studied as a heat source especially in low pressure, for example, for space welding or plasma CVD. As an application at the atmospheric pressure, it can be utilized for processes such as thermal spraying. In this paper, the basic heat source properties of argon HCA at the atmospheric pressure are numerically analyzed. Furthermore, the properties are compared with those for the conventional GTA.

Grain Refinement in Ferritic Stainless Steel Welds Through Magnetic Arc Oscillation and Its Effect on Tensile Property

Grain size reduction in weld fusion zone confers the advantage of increased resistance to solidification cracking and improved mechanical properties. In an effort to refine the fusion zone grain structure in AISI 430 ferritic stainless steel, Gas tungsten arc welding (GTAW) was performed during which transverse oscillations of the arc were induced through the use of an alternating external magnetic field. It was observed that the structure in the as - soIidified weld was predominantly columnar in the case of the conventional (i.e. continuous current) gas tungsten arc welding process. Arc oscillation results in columnar to equiaxed transition in the grain structure; under optimum condition of arc oscillation frequency and amplitude, the grain size of the weld bead was finer than in conventional welds. This could be attributed to the factors that include enhanced fluid flow, reduced temperature gradients and continually changing weld pool size and shape due to the action of the imposed magnetic field. An oscillation variable, which resulted in maximum grain refinement in the fusion zone, was employed to evaluate longitudinal all-weld tensile properties, and residual stresses across weldments. The tensile properties of arc oscillation welds were found to be superior to those of conventional welds. Arc oscillated welds exhibited low tensile residual stresses in the fusion zone compared to conventional gas tungsten arc welds.

Characteristics of an arc column in a hollow cathode arc

The hollow cathode arc (HCA) was developed as a plasma source for low-pressure applications in the 1960s, since when it has been variously researched to clarify its underlying mechanism.1, 2 When arranged to incorporate a relatively high arc current, HCA can also be used as a welding heat source.3–5 Through being a plasma source in low-pressure environments, HCA has further attracted attention as a welding heat source for use in space.6, 7 Through providing a degree of base metal fusion incommensurably greater than that obtainable with a conventional GTA (gas tungsten arc) heat source at atmospheric pressure, HCA is also regarded as being effective for industrial applications in low-pressure terrestrial environments.8

Grain refinement and high performance of titanium alloy joint using arc-ultrasonic gas tungsten arc welding

Arc-ultrasonic welding technology is one approach for refining the grain structure in titanium alloy welds. This paper reports work in which welds were produced in an α-β titanium alloy under a variety of conditions, including conventional gas tungsten arc welding (GTAW) and arc-ultrasonic GTAW. The results show that arc-ultrasonic energy refined β grain structure in the weld metal, and improved the fatigue life of weld joints. Ultrasonic energy enhances fluid flow, reduces temperature gradients, and causes a continual high frequency oscillation in the welding pool. These effects are believed to be responsible for refining the solidification structure and improving the performance of titanium weld joints.