Weld Pool Surface Research Articles

Effect of oxygen in shielding gas on weldability in plasma-GMA hybrid welding process of high-tensile strength steel

This study aims to clarify the effect of oxygen in shielding gas on weldability in the plasma-GMA (Gas Metal Arc) hybrid welding process of high-tensile strength steel plates. The difference in keyhole profile and bead formation, when the GMA shielding gas was pure Ar, Ar + 2% O2, or Ar + 20% CO2, was investigated for plate thicknesses of 6 and 9 mm for the first time. It was found that the weld beads were in good condition for 6 mm thickness plates for all shielding gases, which implied that the window of welding conditions for this thickness is wide. In contrast, for 9 mm thickness plates, a fully penetrated weld bead was achieved only in Ar + 20% CO2, and weld bead penetration in Ar + 20% CO2 is higher than in pure Ar and Ar + 2% O2 in the same welding condition. Due to decreased surface tension caused by sufficiently increased oxygen absorbed into the weld pool, the keyhole diameter increased to penetrate the bottom side of the plate, and the depressing weld pool surface under GMA allowed the heat input from the GMA to be directly applied to a deeper position. Consequently, the plasma-GMA hybrid welding process with Ar + 20% CO2 achieved a complete penetration for a plate of 9 mm thickness, owing to the effects of both phenomena. It proved a potential to increase penetrability in welding thicker plates by controlling oxygen content in shielding gas of GMA adequately.

Effect of current waveform in MIG arc on weld bead formation in plasma-MIG hybrid welding

Plasma-metal inert gas (MIG) hybrid welding enables to join thick steel plates in single pass. However, arc coupling occurring between the plasma and MIG arcs disturbs its heat source characteristics, lowering the welding quality. This arc coupling phenomenon is not yet understood due to the complexity. This study aims to clarify the effect of current waveform of arc on weld bead formation according to the arc coupling in plasma-MIG hybrid welding. The metal transfer characteristics and bottom side weld pool were observed for direct current (DC) and pulse-MIG current waveforms. In addition, Ni element was used for visualizing the transport process of high-temperature molten metal provided by MIG welding within the weld pool. From these results, the effects of differences in MIG arc current waveforms on heat and mass transport processes within the weld pool and also on weld bead formation on the bottom side through changes in the occurrence of arc coupling were discussed. As a result, it was clarified that the droplets after detachment from the wire were transferred to the weld pool surface under the wire tip for DC MIG current, while those were transferred along the wire axis to the weld pool surface behind the keyhole for pulse-MIG current. When the droplet was transferred to the weld pool region with the forward flow such as the pulse-MIG current case, the heat was transported to the bottom side together with the counter-clockwise eddy behind the keyhole, strongly contributing to increasing the penetration depth. In the case of pulse-MIG current, the plasma arc is oscillated due to the arc coupling. According to this oscillation, the accumulation of molten metal behind the keyhole is prevented to suppress the humping bead formation. Consequently, pulse-MIG current was found to be suitable for increasing the penetration depth and suppressing humping bead formation on the bottom side comparing with DC MIG current.

Interaction Mechanism of Arc, Keyhole, and Weld Pool in Keyhole Plasma Arc Welding: A Review.

The Keyhole Plasma Arc Welding (KPAW) process utilizes arc plasma highly constricted by a water-cooled cupper nozzle to produce great arc pressure for opening a keyhole in the weld pool, achieving full penetration to the thick plate. However, advanced control of welding is known to still be difficult due to the complexity of the process mechanism, in which thermal and dynamic interactions among the arc, keyhole, and weld pool are critically important. In KPAW, two large eddies are generally formed in the weld pool behind the keyhole by plasma shear force as the dominant driving force. These govern the heat transport process in the weld pool and have a strong influence on the weld pool formation process. The weld pool flow velocity is much faster than those of other welding processes such as Tungsten Inert Gas (TIG) welding and Gas Metal Arc (GMA) welding, enhancing the heat transport to lower the weld pool surface temperature. Since the strength and direction of this shear force strongly depend on the keyhole shape, it is possible to control the weld pool formation process by changing the keyhole shape by adjusting the torch design and operating parameters. If the lower eddy is relatively stronger, the heat transport to the bottom side increases and the penetration increases. However, burn-through is more likely to occur, and heat transport to the top side decreases, causing undercut. In order to realize further sophistication of KPAW, a deep theoretical understanding of the process mechanism is essential. In this article, the recent progress in studies regarding the interaction mechanism of arc, keyhole, and weld pool in KPAW is reviewed.

Monitoring Welding Torch Position and Posture Using Reversed Electrode Images – Part I: Establishment of the REI-TPA Model

Sensing, modeling, and control are the key technologies of intelligent welding manufacturing. The position and attitude of the welding torch relative to the weld seam affects the quality of the produced weld directly, which is the basis of control and offline programming of the welding robot. Based on the existing research of reversed electrode image (REI) on monitoring of weld pool surface and penetration state, this work focused on the application of REI on monitoring of welding torch position and attitude (TPA). In this paper, a REI-TPA model was established to quantitively relate REI to TPA by considering the surface of the weld pool as a spherical mirror. With the REI-TPA model, the offset distance and deflection angle of the welding torch relative to the correct position and attitude can be calculated. The calculation is based on the measurement of the relative distance between the tip of REI and the electrode in a passive visual image of the weld pool, arc length, and weld pool geometry. This model can be applied to the real-time control of TPA, which is a supplement to the application of welding passive visual image and an extension to multi-information acquisition and processing methods in the robotic welding process.

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.

Observation of Arc and Metal Transfer Behavior according to Shielding Gas in the WAAM of Ti–6Al–4V Alloy Using the Pulsed Gas Metal Arc Process

In arc welding and wire arc additive manufacturing (WAAM) of Ti alloys, pulsed gas metal arc (GMA) processes have a higher deposition than shortcircuiting GMA mode processes, such as cold metal transfer, surface tension transfer, and controlled short-circuit processes. In this study, pulsed GMA WAAM of Ti–6Al–4V alloy was conducted under Ar, Ar50%/He50% mixed, and He shielding gases. Owing to the thermionic emission of electrons from the Ti substrate, cathode jets were emitted from the high-temperature region of the weld pool, which interfered with droplet transfer into the weld pool. The arc shape surrounding the droplet varied according to the shielding gas, and the arc was established at the bottom of the hanging droplet under the He shielding gas, which disturbed droplet detachment. Two spatter generation modes of droplet ejection from the weld pool surface and inflight droplet repelling were observed, and droplet ejection was the most frequent spatter generation mechanism. The mixed shielding gas showed the best performance in terms of arc stability, wire melting, droplet transfer, and spatter suppression. The arc, cathode, and metal transfer characteristics were elucidated in this study, and a suitable gas composition for pulsed GMA WAAM of Ti alloys was proposed.

Influence of weld pool surface depression on burning conditions of an arc with a refractory cathode

Influence of weld pool surface depression on burning conditions of an arc with a refractory cathode

Numerical analysis of arc and molten pool behaviors in high speed tandem TIG welding of titanium tubes

A high speed tandem TIG welding process was adopted to manufacture titanium welded tubes. A coupled electrode, arc and weld pool numerical model was also developed to investigate the temperature distribution, arc plasma and molten metal flows, and energy propagation of this welding process. Under the driving of the electromagnetic force, arc plasmas under the leading and trailing electrodes flowed towards each other. The Marangoni stress was much higher than the arc shear stress, and was the main driving force for the backward molten metal flow on the top weld pool surface. The calculated arc efficiency of this welding process was 79.8%. The high speed tandem TIG welding process can improve the manufacturing efficiency and quality of titanium welded tubes.

Penetration state recognition based on stereo vision in GMAW process by deep learning

100 % penetration state with consistent and reliable weld is typically required in Gas metal arc welding (GMAW). Three-dimensional (3D) information of the weld pool surface is more sensitive to the welding penetration state. In order to take advantage of this critical information, a biprism passive stereo visual system based on a single camera is designed and established in this paper. A novel high-performance deep learning strategy is invested and discussed to recognize penetration state. Based on 3D information of the weld pool surface, the proposed strategy directly correlating the depth of the weld pool to penetration states. Deep learning stereo matching network based on residual structure and multiscale spatial aggregation is built and the depth of the weld pool is estimated based on the input left and right image pairs. A reliable dataset for optimizing stereo matching network to estimate the depth of the weld pool surface is established by using the globally optimized variational stereo matching algorithm. Output weld pool depth information is input into a high-performance classification network, which has been weighted with efficiency and precision. In this study, deep learning intelligibility approach is adopted to visualize the class activation regions (CAM), and it is found that CAM shows high activity around the concave regions on the weld pool surface. The optimized model comes into effect on the test set, recognition accuracy reached 99.6 %, which demonstrates sufficient performance.

Optimization of the Weld Pool Boundary Calculated by the LSTM-Based Measurement Method in GTAW

The shape of the weld pool surface contains a lot of important features to reflect the quality of the weld. However, it is difficult to obtain the weld pool boundary precisely. In this paper, a boundary extension method is designed first to optimize the boundary profile of the interpolated weld pool surface calculated by the long-short-term memory (LSTM)-based measurement method. Experimental results show that after boundary extension, the errors of the left and right part of the boundary of the weld pool are slightly improved. The weld width error in the X direction is reduced to 2.43%, and the weld width error in the Y direction is slightly increased to 8.68%. Then the robustness of the LSTM-based model is analyzed by studying the phenomena of missing points and more points. To solve the problem caused by the missing/adding the first few imaging points, a forward-reverse united reconstruction optimization method is then designed. After optimization by forward-reverse united reconstruction optimization method, the boundary error of the reconstructed weld pool surface is obviously reduced. The weld width error is reduced to 1.62% in the X direction and 3.94% in the Y direction.

A measurement method for the forced weld pool surface in GTAW based on simulation model and LSTM network

A measurement method for the forced weld pool surface in GTAW based on simulation model and LSTM network

Influence Mechanism of Metal Vapor in Plasma Arc Lap Welding

A three-dimensional coupled tungsten electrode–plasma arc–metal vapor– weld pool model was developed to investigate plasma arc and metal vapor characteristics, weld pool convection, and energy transfer in conduction plasma arc lap welding. The arc energy efficiency was also calculated. The numerical results showed that in conduction plasma arc lap welding, the constraint effects of the plasma arc by a small constricting nozzle, plasma gas, and electromagnetic force were strong, and no keyhole was formed inside the weld pool, so the heat flux on the weld pool surface was high as well as the weld pool temperature and mole fraction of Fe vapor above the weld pool surface. The high concentration of Fe vapor in the arc decreased the conduction energy from the plasma arc to the weld pool along with the arc energy efficiency. The calculated arc energy efficiency was only 50.2%. Without considering Fe vapor, the calculated weld pool had complete joint penetration. When considering Fe vapor, the calculated weld geometry agreed well with the experimental result.

Numerical investigation for dominant factors in slag transfer and deposition process during metal active gas welding using incompressible smoothed particle hydrodynamics method

Numerical simulation based on an incompressible smoothed particle hydrodynamics method was performed to clarify dominant factors of slag transfer and deposition processes in a metal active gas welding. The computational model considered the effects of a shielding gas flow on a slag floating on a weld pool surface. The computational result showed that the slags were generated on the weld pool surface near the centre of a heat source and were transferred to the edge of the pool. Those tendencies were similar to those observed in an experiment. The slags stayed at the trailing region of the pool and then deposited on a weld bead. These behaviours supported the validity of the present computational result. To identify the dominant factors of the slag behaviour on the weld pool, numerical experiments were conducted with individually acting forces due to the Marangoni effect, the shearing force, the Lorentz force, and the drag by the shielding gas flow. The computational results suggested that the shearing force and the drag due to the shielding gas were dominant near the centre of the heat source, and the slag was transferred from the vicinity of the heat source to the end of the weld pool. On the other hand, the force due to the Marangoni effect was dominant in the trailing region of the weld pool, and consequently, the slag was transferred from there to the front region in the welding direction.

Study on the molten pool behavior and porosity formation mechanism in dual-beam laser welding of aluminum alloy

Based on the mechanism of dynamic coupling between molten pool and keyhole, a three-dimensional (3D) transient model for the dual-beam laser welding of aluminum alloy is established by considering the surface tension, Marangoni force, and recoil pressure. The morphology of the molten pool, porosity formation process, and the heat transfer mechanism during the process of laser welding under different parameters are analyzed. A double rotating 3D Gaussian heat source is used to represent the laser beam, and the volume of fluid method is used to track the gas-liquid free surface, and the gas-liquid interface force is transformed by the continuous surface force model. The results show that in the process of dual-beam welding, the interaction between the two keyholes enhances the fluid flow perpendicular to the spot line, and the shape of weld pool surface changes from elliptical to circular. Furthermore, welding speed and spot spacing have a significant effect on the shape of the molten pool. Furthermore, it is observed that dual-beam welding can improve the stability of the keyhole and reduce the maximum oscillation amplitude and the number of keyhole breakups. At a specific spot spacing, a unique process of separation and fusion is discovered in addition to the common stages of growth, maintenance, breakup, and shrinkage of the keyhole. The simulation results are in good agreement with the existing experimental results. Overall, this paper provides useful insights into the dynamics of the molten pool in dual-beam welding and reveals the molten pool behavior and porosity formation mechanism.

Validation of evaporation-determined model of arc-cathode coupling in the peak current phase in pulsed GMA welding

A first experimental validation of the evaporation-determined arc-cathode coupling (EDACC) model is performed by comparing the experimental and simulated current in the peak current phase of a pulsed gas metal arc welding process. For this, the EDACC model was extended to limit the cathode surface temperature to a realistic value of <2400 K. The information on the plasma for the EDACC model was gathered from literature and extrapolated and extended according to qualitative reasoning. The information on the cathode surface of the EDACC model was derived from a steady-state simulation of the weld pool, using an averaging approach over time for the energy and current. The weld pool surface temperature was compared to pyrometric measurements, that were performed for this work, and the agreement was found to be fair. The observed agreement between the modeled and experimentally determined current was within 10%. As strong assumptions were made for the comparison, the validation cannot be considered as final, but the assumptions are thoroughly analyzed and discussed. However the critical link between surface temperature, plasma temperature and total current transmitted could be reconstructed.

Numerical analysis of molten metal behavior and undercut formation in high-speed GMAW

• A self-adaptive integrated arc heat-force distribution model is developed for GMAW. • Backward flow, gouging region and thin metal layer are processes initiating undercut. • Two dimensionless numbers are developed to predict the growth rate of undercut. A computational fluid dynamics model was established to investigate the undercut formation in high speed gas metal arc welding (GMAW). A double-ellipse integrated "arc current density-arc pressure-electromagnetic force-arc heat" distribution model, which could be self-adaptive to weld pool surface evolution, was developed. The multi-coupling transport phenomena in weld pool were simulated, and the effect of driving forces on the behaviors of molten metal and the formation of undercut was analyzed quantitatively. The results show that the high-velocity backward molten metal flow, large-size gouging region and prematurely solidified thin metal layer at weld toe were typical processes that initiated undercut. The evolution of undercut morphology was determined by the characteristics of inertia force, hydrostatic pressure and arc pressure in the different stages. Based on Buckingham- π theorem, the dimensionless growth rate of undercut area was a parabolic function of a dimensionless group including characteristics of stress state, molten metal flow and morphology of weld pool. This study clarified the physical origin of undercut defect in GMAW and might also provide some basic guidelines for its suppression.

The reconsitution of the weld pool surface in stationary TIG welding process with filler wire

The reconsitution of the weld pool surface in stationary TIG welding process with filler wire

Relationship among Welding Defects with Convection and Material Flow Dynamic Considering Principal Forces in Plasma Arc Welding

The material flow dynamic and velocity distribution on the melted domain surface play a crucial role on the joint quality and formation of welding defects. In this study, authors investigated the effects of the low and high currents of plasma arc welding on the material flow and thermodynamics of molten pool and its relationship to the welding defects. The high-speed video camera (HSVC) was used to observe the convection of the melted domain and welded-joint appearance. Furthermore, to consider the Marangoni force activation, the temperature on the melted domain was measured by a thermal HSVC. The results revealed that the velocity distribution on the weld pool surface was higher than that inside the molten weld pool. Moreover, in the case of 80 A welding current, the convection speed of molten was faster than that in other cases (120 A and 160 A). The serious undercut and humping could be seen on the top surface (upper side) and unstable weld bead was visualized on the back side (bottom surface). In the case of 160 A welding current, the convection on the weld pool surface was much more complex in comparison with 80 A and 120 A cases. The excessive convex defect at the bottom side and the concave defect at the top surface were observed. In the case of 120 A welding current, two convection patterns with the main flow in the backward direction were seen. Almost no welding defect could be found. The interaction between the shear force and Marangoni force played a solid state on the convection and heat transportation processes in the plasma arc welding process.

Physical mechanisms of fluid flow and joint inhomogeneity in variable-polarity plasma arc welding of thick aluminum alloy plates

A variable-polarity plasma arc (VPPA) is an effective energy source for the welding of thick aluminum alloy plates. However, the mechanisms influencing the fluid flow and the inhomogeneous distribution associated with thick-plate VPPA welding remain unclear, restricting the application of this technology in welding of thick aluminum alloys. Here, the relationship between the microstructure of the weld bead, energy transfer, and fluid flow is clarified by combining in situ three-dimensional x-ray imaging and multi-physics modeling. We find that heat conduction at the keyhole wall is the main factor influencing the morphology of the weld pool. The plasma arc pressure hinders the upward flow of liquid metal, while shear forces promote this flow. This causes the metal close to the weld pool surface to flow slowly, while that inside the weld pool has much higher velocity. It is also concluded that the large crystal size observed in the lower layer of the weld is partly caused by heat treatment from the upper layer of the thick plate. An eddy with a high flow velocity to the rear of the weld pool destroys the crystal-growth process, and this is considered to be one of the reasons for fine crystals appearing in the upper part of the weld. The mechanisms revealed here will help us to guide the use of VPPA technology in the production of stable, high-quality welding of thick aluminum alloys.