Arc Weld Pool Research Articles

Influence of reduced pressure on arc and weld pool properties in TIG welding

In this article, transient transport phenomena in the arc and weld pool for different ambient pressures during tungsten inert gas welding were reported. Arc plasma and weld pool behaviour vary with ambient pressure because of changes in arc thermophysical properties. The maximum arc temperature decreases with decreasing ambient pressure because of reduced plasma electrical conductivity. Decreased arc mass density results in a lower maximum weld pool surface temperature and a decrease in maximum metal vapour content as ambient pressure decreases. Arc pressure decreases with decrease in ambient pressure, resulting in shallower penetration. Calculated arc temperature distribution and weld cross-section are compared with experiments to validate the modeling method for simulating heat and mass transfer phenomena in low ambient pressure welding.

Comparative numerical analysis of electromagnetic forces in DC and HFPC TIG welding

The paper presents a detailed theoretical analysis of the electromagnetic forces in the arc plasma and the weld pool metal in Tungsten Inert Gas (TIG) welding at direct current (DC) and high-frequency pulse current (HFPC) modulation with a frequency of 10 kHz. The electromagnetic (Lorentz) force, acting on arc plasma and metal is presented via potential (magnetic pressure) and rotational (equivalent electromagnetic force) force components. This approach allows us to substantially simplify the analysis of the Lorentz force effect on the arc plasma and weld pool metal under different arc burning modes. It was shown that in HFPC TIG welding with a given frequency, the effect of modulated current on the arc plasma is inherently non-stationary, whereas the effect on the weld pool metal is determined by the time-averaged magnitude of electromagnetic force throughout the period of current modulation. The proposed approach serves as a basis for numerical analysis of the electromagnetic forces in arc plasma and weld pool metal during DC and HFPC modes of the welding process. We established that the application of HFPC modulation promotes a greater intensity of both the gas-dynamic processes in the welding arc, thereby significantly increasing the pressure on the weld pool surface, and convective heat transfer from the most heated central region near the weld pool surface towards the weld pool bottom. Both these factors lead to an increase of the arc penetrability in the case of the HFPC mode compared with the DC mode, all other conditions being equal.

The mechanism study of TIG-MIG hybrid welding process based on simulation

Tungsten inert gas-metal inert gas (TIG-MIG) hybrid welding combines the advantages of tungsten inert gas (TIG) welding and metal inert gas (MIG) welding. Due to the benefits of the two welding techniques, it is an effective strategy to enhance welding quality and productivity. To investigate the mechanism of arcs interaction in TIG-MIG hybrid welding, a three-dimensional transient model for arc-droplet-weld pool behavior was established. The temperature field, metal vapor content, flow fluid, current density and electromagnetic force in the arc space and weld pool were simulated and analyzed. The calculation results showed that the introduction of the TIG arc decreased the content of metal vapor and increased the arc temperature. Compared with single MIG welding, the amplitudes of maximum arc temperature and plasma velocity in TIG-MIG hybrid welding were lower. Experimental results demonstrated that the droplet deflection angle, droplet transfer frequency, weld bead cross-section, and arc temperature distribution all agreed well with calculated values.

Research on Arc Morphology and Keyhole Behavior of Molten Pool in Magnetically Controlled Plasma-GMAW Welding

In the magnetically controlled Plasma-GMAW welding process, the composite arc forms a keyhole in the workpiece to be welded. In order to explore the effect of process parameters on arc coupling, weld pool and keyhole, and the behavior characteristics of keyhole, the arc behavior and side weld pool information were collected using a welding arc acquisition system and a high-speed camera during bead-on-plate welding. The arc image is processed by pseudo-color enhancement technology, and the collected molten pool information is analyzed by boundary extraction algorithm and coordinate conversion algorithm, and the molten pool boundary and keyhole entrance width are obtained. It is found that the coupling degree of the two arcs increases with the increase in plasma current, GMAW current and magnetic field intensity. With the increase in plasma current, the size of keyhole inlet increases; with the increase of GMAW current, the size of keyhole inlet decreases, and the wave crest increases. With the increase of magnetic field intensity, the intensity of metal oscillation between the two arcs increases, and so does the wave crest.

Macro-micro numerical simulation and experimental study of Mg-Gd-Y-Zn-Zr alloy fabricated by cold metal transfer wire arc additive manufacturing

The Mg-Gd-Y-Zn-Zr alloy components were fabricated by cold metal transfer (CMT) based wire arc additive manufacturing (WAAM). The heat transfer and equiaxed grain growth in the weld pool bonding zone of the alloy were investigated through macro-micro numerical simulation. The results shows, the morphology of the molten pool in the bonding area is ellipsoid with a major semi-axis of 13 mm and a semi-minor axis of 6 mm. The size of the molten pool increases with increasing temperature. With the increase in the number of layers of material added, the high temperature zone is enlarged continuously. This is due to the weakening of the heat dissipation of the substrate and the preheating effect of the first material added on the subsequent material added. The phase field model of equiaxed grain growth during the solidification process of the CMT arc welding pool, was established. The growth of equiaxed crystals in the bonding region temperature gradients of 4.9 °C/m and 3.6 °C/m, the propulsion velocity of 1.5 m/min and 2.3 m/min were simulated, respectively. The further away the molten pool from the keyhole center, the smaller the temperature gradient, the faster the dendrite growth speed, and the more vigorous the dendrite morphology. The unique layered processing method and Marangoni effect of the melting pool in the bonding zone of CMT-WAAM arc additive promote the β-Mg24(Gd,Y)5 and RE-rich phases of body-centered cubic structure. These precipitated phases are evenly distributed at the grain boundaries, it hinders the migration of grain boundarie st, which in turn inhibits grain growth.

Analysis of Arc-weld Pool Behavior of Double-Frequency Modulated Pulse TIG Root Pass Welding

Analysis of Arc-weld Pool Behavior of Double-Frequency Modulated Pulse TIG Root Pass Welding

Probing Element Transfer Behavior during the Submerged Arc Welding Process for CaF2-SiO2-Na2O-Cr2O3 Agglomerated Fluxes: A Thermodynamic Approach

Submerged arc welding joins metal by the heating of the electrode, base metal, and flux in the arc plasma, while the weld pool is protected under the granular flux and molten slag. Due to complex chemical reactions occurring between the arc plasma, weld pool, and molten slag (flux), flux essentially affects the weld metal composition, which, in turn, dictates the mechanical properties of the weldment. Therefore, fine-tuning the weld metal composition is essential to ensure a sound weld, and efforts worldwide have been focused on the control mechanism of flux on the weld metal composition. Recently, agglomerated fluxes have been widely applied due to low energy consumption during manufacture. The Cr2O3-bearing agglomerated flux is one of the most commonly used flux types in fields of heavy industrial applications. However, few works concern the element transfer behavior when Cr2O3-bearing agglomerated fluxes are used. Within this framework, typical agglomerated CaF2-SiO2-Na2O-Cr2O3 fluxes with varying Cr2O3 content from 10 to 50 wt.% are designed and applied to Q345A steel. The influence of Cr2O3 content upon the transfer behaviors of essential elements, including O, Cr, and Mn, is quantified and interpreted from the point of thermodynamics. By incorporating a gas-slag-metal equilibrium consideration, the assumptions made in previous studies are justified. Additionally, evidence regarding the loss of Cr and Mn to the arc plasma is provided, and a possible thermodynamic approach to predict element transfer levels is proposed. It is revealed that the gas-slag-metal equilibrium consideration is able to qualitatively analyze the transfer behaviors involved in the submerged arc welding system, even under high temperatures. Based on the quantitative data, the practical implications as well as limitations of the gas-slag-metal equilibrium model are proposed.

An easy-to-use multi-physical model to predict weld pool geometry in keyhole plasma arc welding

Plasma arc welding (PAW) can produce deep-penetration weld when a keyhole mode appears with high welding currents. The process involves complex thermal-hydrodynamical effect and electromagnetic effect, which requires fundamental research in multi-disciplinary fields to give full understanding. Therefore, an easy-to-use model, which includes an arc zone and a workpiece zone, was developed to calculate the keyhole PAW process with full consideration of multi-physical mechanisms. It calculates thermal plasma process and its heat transfer to workpiece. Meanwhile it predicts an accurate arc pressure and then treats it as a pressure source term in workpiece zone. In this way, the model can display a keyhole-like welding process with great convenience. Experimental results of arc pressure and weld pool geometry verify the model. Further research shows that the choice of different parameters in solid-liquid phase change model may cause a difference to calculated results. It's found that solid viscosity 20 N s/m2 and Mushy zone constant 108 are good choice. Welding characteristic concerning three different welding parameters is predicted to guide practical engineering application. It's found heat flux and arc pressure have a non-linear positive correlation with welding current and a significantly negative correlation with orifice diameter and distance from torch to workpiece. It also shows the noteworthy arc heating area is much larger than the noteworthy pressure striking area in PAW. Fusion depth is more sensitive than weld width to the variation of welding parameters.

Real-time recognition of arc weld pool using image segmentation network

This work aims at automatically tracking the weld pool boundary to monitor the process and collect operation and process data for analysis and modeling of the operator. The strong arc radiation and less predictable operation add challenges. It is relatively difficult to design a static feature pattern to consistently detect the weld pool boundary/position under different welding conditions. A deep learning network, such as a conventional neural network (CNN), could provide a solution. However, its training requires large amount of data. In a classification problem where each image corresponds to one label, an automated method may be available to calculate the needed labels. Unfortunately, for the problem of image segmentation being addressed in this study, i.e., detecting the weld pool, the labels cannot be generated automatically or the problem would have been solvable by conventional image processing. An effective deep learning approach that does not require large datasets is needed. As such, we propose to use an architecture of the U-Net, which extracts contextual information by combining low-level feature maps with high-levels. It not only can be trained end-to-end from few images to perform well but is also known for its capability to be real-time. To train the network, various welding experiments have been conducted by randomly changing the welding current and welding speed. Various weld pool boundaries/shapes are generated under various welding conditions. As such, the data is representative to better ensure the reliability and robustness of the trained network. Experimental results verified that the trained network can detect the weld pool boundary under various conditions accurately in different welding current, welding speed, and weld pool shape.

Joint Penetration Monitoring in Low-Frequency Pulsed GTA Root Pass Welding of Medium-Thick Steel Plates

Welder-dependent manufacturing is no longer suitable for the modern production of a high-performance nuclear pressure container. The high-quality root pass welding of medium-thick steel plates is the main challenge to obtain a sturdy reactor vessel, especially to generate one-sided welding with back-formation bead without a backing. Herein, low frequency and large duty-cycle pulsed gas tungsten arc welding (GTAW) was employed to weld the medium-thick steel plates with a 5-mm root face and 2-mm root opening. The arc characteristic and weld pool dynamic behavior in the proposed GTA root pass welding was investigated by a high-speed camera, and a deflection phenomenon of arc tail flame was first found. The correlations of the characteristic parameters of the arc tail flame, including the deflected angle and length, with the weld joint penetration and welding speed were also analyzed in detail. The results showed a negative correlation to the welding speed and a positive correlation with the weld joint penetration. A sound weld bead was formed at a range from 15 deg and 20 mm to 19 deg and 27mm. Based on the above relationship, a new method using these two characteristic parameters was proposed to identify the weld joint penetration in the root pass welding, and its fundamentals were completely demonstrated by the dynamic change of the keyhole. Its feasibility was also demonstrated by the experiment combined with the weld pool dynamic-dependent theoretical analysis.

Mapping flow evolution in gas tungsten arc weld pools

The complex flow within the molten metallic pool, during advanced manufacturing and joining processes such as welding and additive manufacturing, directly affects the performance of the final components; through the generation of various microstructures and strain gradients. Understanding of the flow within such melt pools can enhance the predictability of the final microstructures and therefore component performance. In situ synchrotron X-ray imaging is employed to observe and map the evolving flow patterns within gas tungsten arc weld pools using tracking particles. The experimentally observed flow patterns are compared with numerical simulations to gain additional insights on accurate predictability in relevance to the physical driving forces within the flow. The spatio-temporal distribution of the flow using the mapped data is analyzed by considering the evolution of driving forces acting throughout the weld pool. The results demonstrate quantitative benchmark flow mapping with confirmation of the overall mechanisms of weld pool flow.

Numerical Analysis of Keyhole and Weld Pool Behaviors in Ultrasonic-Assisted Plasma Arc Welding Process.

The acoustic radiation force driving the plasma jet and the ultrasound reflection at the plasma arc-weld pool interface are considered to modify the formulas of gas shear stress and plasma arc pressure on the anode surface in ultrasonic-assisted plasma arc welding (U-PAW). A transient model taking into account the dynamic changes of heat flux, gas shear stress, and arc pressure on the keyhole wall is developed. The keyhole and weld pool behaviors are numerically simulated to predict the heat transfer and fluid flow in the weld pool and dynamic keyhole evolution process. The model is experimentally validated. The simulation results show that the acoustic radiation force increases the plasma arc velocity, and then increases both the plasma arc pressure and the gas shear stress on the keyhole wall, so that the keyholing capability is enhanced in U-PAW.

Generalized representation of arc shape, arc column characteristics and arc-weld pool interactions for DC electric arcs burning in monoatomic gases

The shape of a DC electric arc in arc welding with monoatomic shielding gases (Ar, He and Ar/He blends) is generalized in dimensionless form with algebraic correlations. The novel expressions capture the main magnetohydrodynamic arc characteristics (temperature, velocity, magnetic field) and also yield the main arc–weld-pool interactions (heat flux, current density and pressure). The proposed correlations are validated against numerical results presented in this work and numerical and experimental results found in the literature. Simulated electric arcs conditions considered include arc currents and arc lengths in the range of 200–300 A and 5–10 mm respectively, burning in Ar and Ar-He mixtures and arc lengths ranging from 7–10 mm for He.

Numerical Investigation of Arc-Pool-Metal Vapor Behavior in GTAW with an External Magnetic Field

Gas tungsten arc welding with an external magnetic field is proven to suppress weld defects while improving welding speed. A three-dimensional numerical model that considers interactions among the arc plasma, weld pool, metal vapor, and external magnetic field is developed. The influences of the external magnetic field and metal vapor on arc and weld pool behaviors are investigated. The external magnetic field has an important influence on the arc shape and the weld pool flow field. The metal vapor increases the arc radiation loss but decreases the conductivity and local current density.

In situ X-ray observations of transient states in arc weld pools

Metallic alloys coalesce via extremely rapid melting and subsequent solidification to form fusion welded joints. The melt pool evolution in melting and solidification sequences during the welding process determines the formation of the final weld joint shape, microstructure and defects. The scientific insight on weld pool evolution and related phenomena can be a key contribution to enhance structural integrity and resilience of the welded structures or components. However, inherent complexity with multi-physics phenomena, associated high temperatures and the rapidness of the processes make direct experimental investigation of welding is extremely demanding. Thus, internal flow behaviour during welding or other melt-pool-based metal processing such as additive manufacturing remains unclear and hinders progression to process optimisation. In this contribution we report the observation of melt pool dynamics that take place during electric arc welding, obtained through in situ synchrotron imaging at millisecond scale. The analysis flow patterns along with the quantified weld pool surface dynamics revealed us to how different contributing forces dictate the flow conditions over the distinct durations of the relatively short existence of the liquid phase. Our preliminary results suggest the existence of arc, surface tension and gravity dominant regimes during the evaluation of the weld pool. Further, we present our observations on how different welding parameters influence these regimes and develop into different transient conditions.

Effect of Magnetic Arc Oscillation on the geometry of single-pass multi-layer walls and the process stability in wire and arc additive manufacturing

Abstract Magnetic Arc Oscillation was applied during the construction of single-pass multi-layer walls of low carbon steel and Ti6Al4V by the Gas Tungsten Arc Welding-based Wire and Arc Additive Manufacturing process, and the influence on the geometry and the process stability was evaluated. The geometric features were assessed using transverse section macrographs and the effects of different patterns and frequencies of oscillation on the arc characteristics, metal transfer and weld pool behavior during the layer deposition were investigated using high speed and welding cameras. The results show that this technique is efficient for making thinner walls by minimizing the overflow of the weld pool and, in some conditions, increasing deposition efficiency. Furthermore, the distribution of material along the wall length becomes more homogeneous. An explanation of the effects of Magnetic Arc Oscillation on the wall geometry based on forces that act on the molten metal during layer deposition was made. Because of the swinging movement of the welding arc, the heat is distributed over a larger area, and the power density decreases. Thus, fewer previous layers are remelted, and the volume and the weight of the weld pool reduce. The weld pool temperature drops, and the surface tension force and the viscous friction increase. The distribution of arc pressure also becomes less concentrated, and the arc force on the molten metal decreases. Additionally, a magnetic force appears on the molten metal, which contributes to a change in the direction of the resultant force on the weld pool.

Time evolved force domination in arc weld pools

The flow within the melt pool during welding is a major factor that dictates the formation of the final fusion zone shape, solidification microstructure and defects. In this paper, we report the evolution sequence of the arc weld pool flow that observed via fast in-situ synchrotron X-ray imaging. Varying flow regimes attribute to the dominance of characteristic forces within the weld pool during rapid solid-liquid-solid phase transformation. Our analysis indicates the general sequence in the arc, surface tension and gravity driven force domination. Welding process parameters appear to influence significantly in determining the domination interval and the intensity of individual force. In some instances, arc and surface tension driven forces can prevent pores, which causes porosity in final welded structures, escaping from the melt pool. Preliminary relations between power input levels, diffract force domination regimes, flow patterns and pool surface changes in fusion welds are suggested by considering the behavior of multiple weld pools.

Investigation of heat transfer and fluid flow in high current GTA welding by a unified model

This research is aimed to understand the heat transfer and fluid flow of the arc plasma and weld pool with a significant deformation of the weld pool surface during high current gas tungsten arc welding (GTAW). A unified model including the tungsten electrode, arc plasma and the weld pool of low carbon steel is developed by presetting a free surface deformation from the experimental observations. Buoyance, Lorentz force, plasma drag force and Marangoni force are taken into account to investigate the weld pool dynamics with a great depressed free surface. Heat flux, current density, Marangoni force and plasma drag force at the weld pool surface are presented to understand the heat and momentum transfer from the arc plasma to the weld pool. It is found that the distributions of the heat flux and current density as well as temperature at the depressed weld pool surface are double-peak profiles; the molten metal flow of the weld pool with severe surface depression are determined by the plasma drag force rather than the Marangoni force. The effect of the Lorentz force is more important than that of the buoyance, but weaker than the Marangoni force. The results are consistent with the existing results of both the experiment and the theory.

A unified model for coupling mesoscopic dynamics of keyhole, metal vapor, arc plasma, and weld pool in laser-arc hybrid welding

Although building a model of laser-arc hybrid welding has attracted substantial attention in recent decades, many problems remain. In particular, existing models failed to reproduce the highly transient (down to several tens of nanoseconds) physical interactions between solid, liquid, vapor plume and plasma occurring in the welding process. These interactions control the key physical processes of laser-arc hybrid welding and play dominant roles in process defect formation and final joint quality. Development of a unified model for laser-arc hybrid welding capable of modeling the multiphase physical interactions is faced with three major challenges. The first is how to decouple the highly correlated and mesoscopic interactions between the solid, liquid, vapor plume and plasma phases. The second and third are respectively the numerical treatments of the nonlinear discontinuous sheath layer between the arc plasma and the weld pool and the Knudsen layer between the vapor plume and the weld pool. In this study, we have developed a three-dimensional unified mathematical model that couples the mesoscopic dynamics of the arc plasma, keyhole, metal vapor, and weld pool in the laser-TIG hybrid welding process. To decouple the highly correlated gas, liquid, solid and plasma interactions, we proposed a novel approach in which the electromagnetic field in the workpiece (including solid and liquid phases) and outside the workpiece (gaseous phase and plasma) were continuously modeled globally, but in which the heat transfer and fluid flow are modeled separately within and outside of the workpiece and then coupled through boundary conditions. To treat the nonlinear interface between the arc plasma and weld pool outside the keyhole as well as between the vapor plume/plasma and the weld pool inside the keyhole, we proposed a novel ghost-fluid-based multiple timescale stepping algorithm. The results indicate that our model can be used to predict the time-dependent mesoscopic dynamics of vapor plume, plasma, keyhole and weld pool in a coupled manner. Good agreement was obtained between the numerical predictions and experimental results and literature data.

Turbulent molten pool analysis of tandem GMA automotive steel sheet welding

In this research, a three-dimensional turbulent weld pool simulation technique of tandem gas tungsten arc welding (GMAW) in a lap joint fillet is described and compared to a conventional laminar flow simulation. Basically, four governing equations are adopted for continuity, namely, continuity, Navier-Stokes, energy, and the popular volume of fluid equations. According to the basic theory of an arc weld pool, all known characteristics such as the arc heat input, arc pressure, electromagnetic force, and Marangoni flow are applied to the analysis model as a body force term or boundary conditions. A conventional arc weld pool analysis usually adopts a laminar flow; however, its results have shown a weld bead shape that is quite different from an experiment of a tandem GMA lap joint fillet welding of an automotive steel sheet. Therefore, the authors suggest the use of a k-εturbulent analysis model and show that its results coincide with the experiment results.