Effect of Welding Parameters on GTA Weld Shape for Pure Iron Plate under Ar-O2 Mixed Shielding

Weld shape variation for different welding parameters is investigated on pure iron plate under gas tungsten arc (GTA) welding with argon-oxygen mixed shielding. Results showed that small addition of oxygen to the argon base shielding gas can effectively adjust the oxygen adsorption to the molten pool. An inward Marangoni convection occurs on the pool surface when the oxygen content in the weld pool is over the critical value, 80×10-6, for pure iron plate under Ar-0.3%O2 mixed shielding. Low oxygen content in the weld pool changes the inward Marangoni to an outward direction under the Ar-0.1%O2 shielding. The GTA weld shape depends to a large extent on the pattern and strength of the Marangoni convection on the pool surface, which is determined by the content of surface active element, oxygen, in the weld pool and the welding parameters. The strength of the Marangoni convection on the liquid pool is a product of the temperature coefficient of the surface tension (dσ/dT) and the temperature gradient (dT/dr) on the pool surface. Different welding parameters will change the temperature distribution and gradient on the pool surface, and therefore, affect the strength of Marangoni convection and the weld shape.     

Time Dependant Weld Shape in Ar-O2 Shielded Stationary GTA Welding

The stationary gas tungsten arc welding （GTA） is carried out on SUS304 stainless steel under Ar-0.1%O2 and Ar-0.3%O2 mixed shielding to observe the evolution of the molten pool and investigate the role of Marangoni convection on the weld shape. After welding, the oxygen content in the weld metal was measured by using an oxygen/nitrogen analyzer. Small addition of oxygen to the argon based shielding gas can effectively adjust the weld pool oxygen content. Oxygen plays an important role as an surface active element in determining the pattern of Marangoni convection in the stainless steel weld pool. When the weld metal oxygen content is over the critical value, 0.01 wt pct, corresponding to the Ar-0.3%O2 mixed shielding gas, the Marangoni convection changes from outward to inward direction and the weld shape dramatically changes from wide shallow shape to narrow deep shape.     

Weld Shape Variation and Electrode Oxidation Behavior under Ar-(Ar-CO_2) Double Shielded GTA Welding

Double shielded gas tungsten arc welding (GTAW, also known as tungsten inert gas (TIG) welding) of an SUS304 stainless steel with pure inert argon as the inner layer shielding and the Ar-CO₂ or CO₂ active gas as the out layer shielding was proposed in this study to investigate its effect on the tungsten electrode protection and the weld shape variation. The experimental results showed that the inner inert argon gas can successfully prevent the outer layer active gas from contacting and oxidizing the tungsten electrode during the welding process. Active gas, carbon dioxide, in the outer layer shielding is decomposed in the arc and dissolves in the liquid pool, which effectively adjusts the active element, oxygen, content in the weld metal. When the weld metal oxygen content is over 70×10^-6, the surface-tension induced Marangoni convection changes from outward into inward, and the weld shape varies from a wide shallow one to a narrow deep one. The effect of the inner layer gas flow rate on the weld bead morphology and the weld shape was investigated systematically. The results show that when the flow rate of the inner argon shielding gas is too low, the weld bead is easily oxidized and the weld shape is wide and shallow. A heavy continuous oxide layer on the liquid pool is a barrier to the liquid pool movement.     

Effects of CO2 shielding gas additions and welding speed on GTA weld shape

Gas tungsten arc (GTA) welding with deep penetration for high efficiency has long been of concern in industry. Experimental results showed that the small addition of carbon dioxide to the argon shielding gas produces an increase in the weld metal oxygen content, which is one of the compositional variables that strongly influence the Marangoni convection on the pool surface and ultimately change the weld pool shape. An inward Marangoni convection on the weld pool occurs, and hence a narrow and deep weld pool forms when the weld metal oxygen content is over the critical value of 100 ppm. When lower than this value, the weld shape becomes wide and shallow. A heavy oxide layer forms in the periphery area on the pool surface when the CO₂ concentration in the shielding gas is over 0.6%. This continuous heavy oxide layer becomes a barrier for oxygen absorption into the molten pool, and also changes the convection mode on the pool surface. A higher welding speed decreases the heat input and temperature gradient on the pool surface, which weakens the Marangoni convection on the liquid surface.     

Marangoni convection and weld shape variations in He–CO<Subscript>2</Subscript> shielded gas tungsten arc welding on SUS304 stainless steel

Bead-on-plate GTA welding (gas tungsten arc welding) on a SUS304 substrate is carried out to investigate the effect of carbon dioxide gas in the helium base shielding on the oxygen content in the weld pool and the weld shape variations. Experimental results show that small addition of carbon dioxide to the shielding gas can precisely adjust the weld metal oxygen content and change the weld shape from wide shallow type to narrow deep one when the weld pool oxygen content is over the critical value, which is from 68 to 82 ppm, due to the Marangoni convection reversal from the outward to inward mode on the pool surface. The weld depth/width ratio increases two times suddenly when the carbon dioxide content in the torch gas is over 0.4 or 0.2% for 1 mm or 3 mm arc length, respectively. The GTA weld shape depends to a large extent on the pattern and magnitude of the Marangoni convection on the pool surface, which is influenced by the active element oxygen content in the SUS304 pool, temperature coefficient of the surface tension (dσ/dT), and the temperature gradient on the pool surface (dT/dr, r is the radius of the weld pool surface). Changing the welding parameters will alter the temperature distribution and gradient on the pool surface, and thus, affect the magnitude of the Marangoni convection and the final weld shape.     

Marangoni convection and weld shape variations in Ar–O2 and Ar–CO2 shielded GTA welding

Increasing the oxygen or the carbon dioxide concentration in the argon-based shielding gas leads to an increase in the weld metal oxygen content when the oxygen or carbon dioxide concentration is to be lower than 0.6 vol.% in the shielding gas. However, when the O₂ or CO₂ concentration is higher than 0.6 vol.% in the Ar-based shielding gas, the weld metal oxygen is maintained around 200 ppm–250 ppm. An inward Marangoni convection mode in the weld pool occurs when the weld metal oxygen content is more than 100 ppm. When it is lower than 100 ppm, the Marangoni convection would change to the outward direction and the weld shape varies from a deep narrow to a shallow wide shape. The effective ranges of O₂ and CO₂ concentrations for deep penetration are same. A heavy layer of oxides is formed when the O₂ or CO₂ concentration in the shielding gas is more than 0.6 vol.%. Based on the thermodynamic calculation of the equilibrium reactions of Fe, Si, Cr and Mn with oxygen in liquid iron for the oxide products, FeO, SiO₂, Cr₂O₃ and MnO and the experimental oxygen content in the weld metal, Cr₂O₃ and SiO₂ oxides are possibly formed at the periphery area of the liquid pool surface under the arc column during the welding process. One model is proposed to illustrate the role of the oxide layer on the Marangoni convection on the pool surface at elevated temperature. The heavy oxide layer inhibited the fluid flow induced by the Marangoni convection and also became a barrier for the oxygen absorption into the molten weld pool.     

Development of simplified active flux tungsten inert gas welding for deep penetration

A new advanced active flux tungsten inert gas (AA-TIG) welding technique, named cap active flux tungsten inert gas (CA-TIG) welding using atmospheric oxygen, was proposed to increase the penetration depth of a weld. Only a simple nozzle cap with an air inlet was used for the welding. Flowing inert gas used as a shielding gas through a nozzle center led to the aspiration of oxygen from the atmosphere to the molten pool. The penetration depth was increased by the reversal of the Marangoni convection due to the entrained oxygen, and it reached three times deeper than that of the conventional TIG welding. Additionally, no degradation of the tungsten electrode was observed because it was protected by the inert gas. The penetration depth was changed by the oxygen content in the molten pool and it could be easily controlled by the nozzle cap design and the welding parameters.     

Activated-TIG Welding of Different Steels: Influence of Various Flux and Shielding Gas

In tungsten inert gas (TIG) welding, a low depth of penetration (DOP) is achieved during single pass. To achieve the required DOP, the speed of welding should be reduced; thus productivity reduces significantly. In this work, influence of 14 different oxide-, chloride-, and fluoride-based fluxes are evaluated on DOP and width-to-penetration ratio during flux-activated TIG (ATIG) welding of low alloy steel (AISI 4340), austenitic (AISI 304 and AISI 316) and duplex (Duplex 2205) stainless steels. The effect of welding current and three different shielding gas compositions is also studied during ATIG for these workpieces. Arc and weld metal pool behaviors are captured in order to study the physical behavior of the process. Results revealed that oxide-based fluxes like SiO₂, MoO₃, MoS₂, CrO₃, and TiO₂ increases DOP significantly and in many cases through penetration (penetration reaches beyond plate thickness) is achieved. There is a noteworthy enhancement in penetration because of the addition of H₂ in shielding gas. Addition of helium also helps to increase DOP. Arc behavior reveals the constriction of arc column during activated TIG welding, and positive surface tension-induced flow in centripetal (inward) direction is observed.     

Heat transfer enhancement by Marangoni convection in the NH3–H2O absorption process

The objectives of this paper are to quantify the effect of Marangini convection on the absorption performance for the ammonia–water absorption process, and to visualize Marangoni convection that is induced by adding a heat transfer additive, n-octanol. A real-time single-wavelength holographic interferometer is used for the visualization using a He–Ne gas laser. The interface temperature is always the highest due to the absorption heat release near the interface. It was found that the thermal boundary layer (TBL) increased faster than the diffusion boundary layer (DBL), and the DBL thickness increased by adding the heat transfer additive. At 5 s after absorption started, the DBL thickness for 5 mass% NH₃ without and with the heat transfer additive was 3.0 and 4.5 mm, respectively. Marangoni convection was observed near the interface only in the cases with heat transfer additive. The Marangoni convection was very strong just after the absorption started and it weakened as time elapsed. It was concluded that the absorption performance could be improved by increasing the absorption driving potential (x_vb−x_vi) and by increasing the heat transfer additive concentration. The absorption heat transfer was enhanced as high as 3.0–4.6 times by adding the heat transfer additive that generated Marangoni convection.     

Activated flux TIG welding of Inconel 718 super alloy in presence of tri-component flux

In this study, we have explored the influence of newly developed tri-component oxide flux (Cr₂O₃, FeO, and MoO₃) on weldability, bead geometry, weld pool temperature variation, and mechanical strength of Inconel 718 welded joints. Moreover, the influence of used flux on weld pool, the surface morphology of electrode and penetration capability of tungsten inert gas (TIG) welding on Inconel 718 plates have been well elucidated. Results indicate that the flux mixture significantly increases the penetration depth as well as aspect ratio almost 200% as compared to conventional TIG welding. The arc constriction caused by newly developed oxide flux upsurges the heat density and the weld pool temperature of joints. The alloying effect caused by entrapped oxide particles greatly improves the hardness as well as the tensile strength of joints. The reported reinforcement in the welding performance may increase potential utility of the developed methods for real-world applications.     

High strain rate deformation and cracking of AA 2219 aluminium alloy welded propellant tank

Aluminium alloy AA 2219 (Al–6.6Cu–1Mn) is the candidate material for the fabrication of propellant storage tank of launch vehicle. Cold rolled sheets of 6.5 mm thickness are used to make the cylindrical shell, while sheets of 4.5 mm thickness are used for the construction of dome through petal forming technique. Petals, formed through cone rolling, treated to T87 temper condition are welded together by TIG welding to configure the dome. Such domes are joined to the cylindrical shell through a ring by TIG welding.The upper stage consists of two tanks, one oxidizer tank (liq. O₂) and other fuel tank (liq. H₂). After completing various developmental qualification tests, propellant flow rate test of one of the system was carried out. Almost all the liquid oxygen of the tank was removed and only a little quantity remained at the bottom. During one of the subsequent tests; when dry nitrogen gas was purged to evaporate the remaining liquid oxygen, the oxidizer tank dome catastrophically fractured with an audible sound. Fracture of oxidizer tank dome, placed at lower part of the system caused excessive deformation and subsequently it also caused fracture of fuel tank dome placed just over it.Detailed metallurgical investigations were carried out on the failed components and it was found that the tank failed under very high strain rate deformation. This paper brings out the details of the investigation carried out.     

AC TIG welding with single-component oxide activating flux for AZ31B magnesium alloys

Magnesium-based alloys are finding extensive applications foreground in aerospace and automotive applications. Weldability of magnesium alloys has recently been investigated with a variety of processes. In this article, the activating flux TIG (ATIG) welding of magnesium alloys with three single-component fluxes (TiO₂, Cr₂O₃ and SiO₂) under alternating current (AC) mode was studied. The effects of welding speed, weld current and electrode gap on the weld shape and the weld arc voltage in AC TIG welding with oxide fluxes were investigated on an AZ31B magnesium alloy substrate. The mechanisms of oxide fluxes on the arc shape and the arc voltage on the weld shape are discussed. The result showed that the TiO₂ and Cr₂O₃ increase the weld penetration of AC TIG welding of magnesium with good bead cosmetics. The SiO₂ increased the weld penetration with very poor formation of the weld surface. However, the arc voltage decreased with the used of TiO₂ flux, and increased with the used of Cr₂O₃ flux. The mechanism of TiO₂ and Cr₂O₃ fluxes increasing penetration should not accord with the “arc constriction”. It would comply with some potential effects of the flux interacting with the liquid metal of fusion zone.     

Effect of three-dimensional melt pool convection on process characteristics during laser cladding

A three-dimensional heat transfer model is developed to simulate the cladding process that include the different physical phenomena such as heat transfer, phase changes, addition of powder particles and fluid flow due to Marangoni–Rayleigh–Benard convection. It is found that the Rayleigh–Benard convection is insignificant and Marangoni–Benard convection is dominant for the studied cases. By varying the scanning speed and Marangoni number the melt pool size and strength of convection are changed and its influence on clad built-up geometry, dilution level, maximum and average melt pool temperatures and the form and scale of the microstructure of the solidified clad track has been studied.     

粉末熔池耦合活性TIG焊接方法

提出了一种新型活性焊接方法——粉末熔池耦合活性TIG焊(Powder pool coupled activating TIG welding,PPCATIG)。该方法采用双层气体进行焊接,内层利用惰性气体保护钨极,外层通过自动送粉装置将活性剂粉末随保护气体送入电弧-熔池区域,增加熔深,提高焊接效率,实现机械化自动化焊接。针对SUS304不锈钢进行了直流正接PPCA-TIG表面熔深,通过与传统TIG焊对比,研究了SiO_2活性剂对电弧形态、焊缝成形、组织和力学性能的影响。结果表明:SiO_2能使电弧等离子体收缩、熔池金属流态改变,并且焊缝熔深能达到传统TIG焊的3倍以上,焊接效率明显提高。焊缝组织主要为奥氏体和铁素体,铁素体形态以骨架状为主。焊缝抗拉强度略低于母材,但相比传统TIG焊,焊缝屈服强度略有提高,其焊缝低温冲击韧性达到了传统TIG焊的96.8%,表现出了良好的力学性能。同时,采用该方法可有效避免活性剂粉末对钨极的污染。     

Study of the characteristics of duplex stainless steel activated tungsten inert gas welds

The purpose of this study is to investigate the effects of the specific fluxes used in the tungsten inert gas (TIG) process on surface appearance, weld morphology, angular distortion, mechanical properties, and microstructures when welding 6 mm thick duplex stainless steel. This study applies a novel variant of the autogenous TIG welding, using oxide powders (TiO₂, MnO₂, SiO₂, MoO₃, and Cr₂O₃), to grade 2205 stainless steel through a thin layer of the flux to produce a bead-on-plate joint. Experimental results indicate that using SiO₂, MoO₃, and Cr₂O₃ fluxes leads to a significant increase in the penetration capability of TIG welds. The activated TIG process can increase the joint penetration and the weld depth-to-width ratio, and tends to reduce the angular distortion of grade 2205 stainless steel weldment. The welded joint also exhibited greater mechanical strength. These results suggest that the plasma column and the anode root are a mechanism for determining the morphology of activated TIG welds.     

CO2-shielded arc as a high-intensity heat source

The characteristics of a CO₂-shielded arc are studied to evaluate its potential as a novel heat source for material processing, with lower costs and higher productivity than that of the tungsten–inert gas (TIG) arc. A double-gas-shielded system, using both CO₂ and an inert gas, is employed for the arc torch; this minimizes consumption of the tungsten electrode and gives arc stability equivalent to an argon TIG arc for 1800 s operation. The arc voltage of the CO₂-shielded arc is about 19 V for an arc current of 150 A and an arc gap of 3 mm, which is much higher than the 12 V obtained for an argon TIG arc. The CO₂ constricts the arc, resulting in an increase in the maximum heat flux density at the anode surface by a factor of about 10 relative to the TIG arc. The penetration depth of stainless steel melted by the CO₂-shielded arc is much larger than that for the argon TIG arc. It is concluded that the greater heating power of the CO₂-shielded arc, which is due to the greater arc constriction, in turn a consequence of the greater specific heat of CO₂, should lead to a large increase in material processing productivity.     

Laserstrahlschweißen von Titanwerkstoffen unter Berücksichtigung des Einflusses des Sauerstoffes

Influence of the oxygen content in the shielding gas on microstructure and mechanical properties of laser welds of titanium and titanium alloys In the present work, a new tool concept for laser welding of titanium in high volume production has been presented and evaluated. Through the innovative application of a six‐layer metal web it is possible to calm the argon gas flow and avoid pernicious turbulences during welding. The integration of the mentioned metal web at the base of an open welding chamber allows the automated welding of highly reactive materials, such as titanium, under atmospheric pressure and inert shielding conditions. The higher density of argon relative to air offers the unique possibility to leave the chamber open on the top, so that a higher degree of flexibility than gas shielding devices for TIG welding, especially for industrial robots, is attained and can be successfully used for industrial mass production. Furthermore this device is important for welding three‐dimensional contours or to shield the regions of overlap (in overlapped joints) where shielding gas trailers are unsuccessful. By means of the presented gas shielding procedure and a modern laser welding process such as Nd:YAG laser welding, systematic investigations on the effect of oxygen on the microstructure as well as on the mechanical properties of reference bead‐on‐plate weldments could be performed for the first time. As a result of these welding trials it can be concluded that in order to avoid discolorations and hardness increase, lower restrictions to the purity of the shielding gas, in comparison to TIG welding condition, can be allowed. The maximum tolerable value of oxygen in the welding atmosphere was found to be approximately 1000 ppm for laser welding. On the contrary the maximum value for TIG welding is about 30 ppm. Further investigations on the microstructural and mechanical properties of the joints confirm that the optical quality assurance criteria for TIG welding due to the standards of aircraft construction transferable to Nd:YAG welding are.     

The mechanism of penetration increase in A-TIG welding

The mechanism of the increasing of A-TIG welding penetration is studied by using the activating flux we developed for stainless steel. The effect of flux on the flow and temperature fields of weld pool is simulated by the PHOENICS software. It shows that without flux, the fluid flow will be outward along the surface of the weld pool and then down, resulting in a flatter weld pool shape. With the flux, the oxygen, which changes the temperature dependence of surface tension grads from a negative value to a positive value, can cause significant changes on the weld penetration. Fluid flow will be inward along the surface of the weld pool toward the center and then down. This fluid flow pattern efficiently transfers heat to the weld root and produces a relatively deep and narrow weld. This change is the main cause of penetration increase. Moreover, arc construction can cause the weld width to become narrower and the penetration to become deeper, but this is not the main cause of penetration increase. The effects of flux on fluid flow of the weld pool surface and arc profiles were observed in conventional TIG welding and in A-TIG welding by using high-speed video camera. The fluid flow behavior was visualized in real-time scale by micro focused X-ray transmission video observation system. The result indicated that stronger inward fluid flow patterns leading to weld beads with narrower width and deeper penetration could be apparently identified in the case of A-TIG welding. The flux could change the direction of fluid flow in welding pool. It has a good agreement with the simulation results.     

Influence of Current and Shielding Gas in TiO2 Flux Activated TIG Welding on Different Graded Steels

In tungsten inert gas (TIG) welding, limited depth of penetration can be achieved during single pass welding. To achieve the desired depth of penetration, the speed of welding needs to be significantly reduced and hence, the productivity decreases. In the present work, the effect of TiO₂ activated flux on penetration is evaluated for different workpieces namely AISI 1020, AISI 304, AISI 316, and Duplex 2205 steels at different currents and shielding gas compositions. The results show a significant increase in the depth of penetration and reduction in the width-to-penetration ratio using the activated flux for all the workpiece materials considered here. Current increases the depth of penetration, however, the influence of flux becomes more significant with higher welding current. Maximum of 37.8%, 44.3%, 47%, and 124% increase in depths of penetration is measured for AISI 1020, AISI 304, AISI 316, and Duplex 2205 steels, respectively, when activated flux is used. Also, maximum of 70% increase in the depth of penetration is further achieved when Ar along with 5% H₂ is used as the shielding gas compared to that when pure Ar is used. The constriction of arc column increases the energy density, which increases the depth of penetration. Measurement of microhardness and metallurgical observations are carried out for samples after TIG welding and activated tungsten inert gas (ATIG) welding and compared to observe the solidification phenomenon during the process.     

MGH956合金TIG焊原位合金化对其组织性能的影响

采用TIG焊对氧化物弥散强化(ODS)高温合金MGH956进行原位合金化焊接.在相同的焊接条件下,填加两种不同的填充材料:与母材化学成分相似的基体填充材料,以及在基体填充材料基础上加入了合金元素Al和Fe2O3的Al-Fe2O3填充材料.通过对比分析两组试样在焊接过程中发生的原位合金化反应机理,及其对焊缝微观组织和力学性能的影响,研究原位合金化反应对ODS合金TIG焊接头组织与性能的影响.结果表明:在填充材料中加入Al和Fe2O3合金元素时,焊缝处的气孔数量明显减少,气孔尺寸也较为减小;焊缝中原位生成了新的增强相颗粒Al2O3、TiC以及YAlO3,同时,基体中的纳米级增强相Al-Y复合氧化物团聚倾向降低.力学性能试验结果表明,填加Al-Fe2O3填充材料时焊缝显微硬度值明显提高,接头抗拉强度达到了578 MPa,为母材强度的80.3%.