Weld Pool Research Articles

Study of dynamic welding pool for AZ31B magnesium alloy with adjustable ring mode laser welding

Study of dynamic welding pool for AZ31B magnesium alloy with adjustable ring mode laser welding

Effect of pool temperature on microstructure and corrosion resistance of PTAW Ni layer

In plasma transferred arc welding (PTAW), the weld pool temperature often fluctuates. The spray weld layer is prone to overheating, leading to adverse effect on its performance. This study investigated the influence of spray welding current on the weld pool temperature and in turn on the microstructure, microhardness and corrosion resistance of the PTAW-Ni layer on a steel substrate. Weld pool temperature increased with increasing spray welding current and varied along the welding gun path before reaching the steady state. With increasing pool temperature, more Fe diffused from the substrate into the spray weld layer, leading to a widened fusion zone, an increased dilution rate and a decreased microhardness. The spray weld layer has a dendritic microstructure. The dendrites have a higher Cr content, while the interdendritic regions have a higher Ni content. Increasing weld pool temperature increased diffusion, resulting in reduced enrichment of the Cr and Ni elements. The increase of Fe and decrease of Cr in the spray weld layer caused the decreasing corrosion resistance.

Modelling the weld cladding process to predict weld clad position and shape error

Wire arc additive manufacturing (WAAM) is one of the most productive metal additive manufacturing methods. One of its most promising applications holds in the manufacturing of difficult-to-cut materials where production costs can be reduced with minimizing the time of machining and total tool costs. To develop a correct WAAM, technological processes for manufacturing complex-shaped components welding torch path corrections and welding power corrections have to be made especially in critical sections such as corners and sharp edges. A predictive mathematical model of the material cladding during the WAAM process has been developed for the purposes of generating an optimal toolpath of the WAAM clads. This predictive mathematical model is simplified to reflect the important physical phenomena in the weld pool but also to optimize computing time. In this paper, the principle of the mathematical model is described, and its functionality is verified by the welding experiments with five different welding power settings. For the initial calibration of the model parameters single straight-line weld clads with 5 different welding power settings (wire feeds) ranging from 5.0 to 8.6 m/min were investigated. 3D scans of these welded samples are used for the verification. With the calibrated simulation model, it was possible to predict the precise shape with a maximum deviation circa 0.20 mm. The start portions of the weld clads seem more complex having the deviation circa 0.30 mm. These are valuable results as the WAAM technology is generally considered to be reasonably rough.

The Formation Mechanism and Suppression Strategies of Spatter in Pulsed Gas Metal Arc Welding for Titanium Alloy

The GMAW process for titanium alloy is not commonly applied within the industry due to the occurrence of severe spatter. This research endeavors to elucidate the mechanism underlying spatter formation and explore efficacious strategies to suppress spatter. The experimental results demonstrated the existence of two distinct spatter types: large and small spatter particles. The high- speed images and synchronous electrical signals were utilized for determining the spatter formation mechanism, with force analysis serving to mutually validate the inferences. The large spatter particles originated from the whole transitional molten droplet as it descended within the arc space, while the small spatter particles were formed by the partial transitional molten droplet as it contacted the weld pool. The cathode jet force accounted for the formation of large spatter particles, whereas the electromagnetic force was responsible for the small spatter particles. To suppress spatter, increasing detachment current and decreasing pulsing frequency were employed. Consequently, the spatter rate witnessed a remarkable decrease from 14.00% to 3.33% with a progressive increment in detachment current from 100 A to 300 A, and a corresponding decline from 12.67% to 1.33% upon decrementing the pulsing frequency from 90 Hz to 50 Hz. This research suggests that a judicious increase in the detachment current can effectively decrease the spatter rate while concurrently preserving welding efficiency.

Rotating Sonotrode Design for Ultrasonic-Assisted Arc Welding of Metal Materials.

In the process of the ultrasonic-assisted arc welding of metal materials, traditional ultrasonic application methods, such as the low-frequency impact of ultrasonic horns on a base material, can easily cause the non-fusion defect. In order to solve this problem, a rotating sonotrode with a groove and double thin ends was designed in this study. The ultrasonic vibration is transmitted into the weld pool by the rolling of the sonotrode on both sides of the weld. The resonant frequency was set at 50 kHz. Firstly, based on the Mindlin theory, a rotating sonotrode without a groove was designed by solving the frequency equation and by conducting a finite element simulation. Secondly, the effects of the groove, perforation, and transition mode on the resonant frequency, stress distribution, and amplification factor were investigated by finite element simulation. Finally, the optimum rotating sonotrode with a groove was obtained. The results show that the size of a rotating sonotrode that has a small frequency error can be obtained by using the discrete interval solver method combined with finite element simulation. The groove can significantly reduce the resonant frequency. The stress concentration can be effectively reduced by using the elliptical transition mode. The resonant frequency and amplification factor of a rotating sonotrode with a groove could be effectively adjusted by a method of double-position joint perforation. The final resonant frequency was 49.721 kHz and the amplification factor was 3.02. This study provides an effective design method for a sonotrode with double thin ends and a groove structure.

Features of a weld pool crystallization during laser welding of copper and steel

Features of a weld pool crystallization during laser welding of copper and steel

Robotic welding system for adaptive process control in gas metal arc welding

Changing process conditions such as distortion, varying seam preparation or gap width during welding is a major challenge in automated gas metal arc welding (GMAW). While human welders can adjust the process during welding (e.g. welding speed, torch orientation), an automated welding system needs sensors to detect and actuators to adjust the process. Adjusting the process in response to changing process conditions is usually referred to as adaptive welding. The aim of this work is to build a robotic welding system capable of automatically adapting the welding process using some of the approaches of a human welder. To enable adaptive process control, a robotic welding system is built. It consists of four main components: a six-axis industrial robot for mechanical guidance of the welding torch, a welding power source, a monochrome visual camera as an image sensor and a process controller that combines the three components. The camera captures images of the weld pool during welding and processes the images to provide geometrical information such as the width of the weld pool and the position of the weld pool front. Changes in the weld pool geometry are quantified, and an adjustment strategy is generated in the process control unit in real time. Process adjustments can be mechanical (e.g. welding speed, torch orientation) and electrical by adjusting synergic process settings (wire feed speed, arc length, process dynamics). Validation tests demonstrate the functionality of the welding system. Two use cases were investigated. Firstly, a deposited weld bead was examined, and variations in the width of the weld pool were induced by varying the welding speed. The second application was a seam tracking application. The path is pre-programmed, and the specimen is positioned with an offset to the path. Compensation for the offset is implemented.

Numerical Analysis of Thermal Flow Dynamics of Arc Plasma and Molten Pool in Hollow Cathode Arc Welding with Oxygen Content

The mechanism of the pulsed hollow cathode arc welding (HCAW) process was revealed using a fully coupled model with a hollow cathode. We solved the governing equations with the Marangoni effect to study the dynamic behaviors of a molten pool with a square pulsing current (200~400 A, 900 Hz) and varying O2 content; the dynamics of the arc plasma and the weld pool in the HCAW process were investigated quantitatively. The results show that the intensity of the arc plasma was more significantly weakened by the design of the hollow cathode in HCAW than that in GTAW with a solid hollow cathode. We could obtain a stable molten pool even with a large pulsing current section (200 A–400 A) at higher frequencies. The flow dynamics of the molten pool were mainly dominated by the Marangoni effect with varying oxygen content, and we could promote penetration by increasing O2 content in HCAW.

Low temperature vaporisation of Cr from fluoride flux reacted at 1350 °C with Al–Cr–Fe powder: Thermochemical analysis of gas phase reactions and nano-strand formation

The submerged arc welding (SAW) process is complex because multi-phase reactions occur simultaneously across a large temperature interval from 2500 °C in the arc cavity, down to the weld pool liquidus temperare at the slag-weld pool interface. This complexity hinders research on specific process metallurgy aspects, such as gas formation from the oxy-fluoride slag. The main objective of this work is to illustrate a low temperature experimental technique as an accurate reaction simulation experiment of SAW flux oxy-fluoride slag behaviour in terms of gas formation and metal powder assimilation reaction mechanisms as observed in the SAW process. The oxy-fluoride slag behaviour was confirmed by identification and analyses of nano-strand formation in the reacted slag formed from the reaction of welding flux with Al-Fe-Cr metal powders at 1350 °C. The observed nano-strand formation agrees with similar observations in SAW post-weld slags used to confirm of oxy-fluoride vaporisation and re-condensation. Element distribution from energy dispersive X-ray spectroscopy (EDS) maps and thermochemical calculations were used to gain insights into the reactions in nano-strand formation. The nano-strands contained chromium patches and spots of less than 1 μm in scale, confirming that added Cr metal powder assimilated via the gas phase. Cr-fluoride formed part of the oxy-fluoride slag and likely formed Cr-fluoride gas. Cr was recovered via re-condensation of Cr vapour formed from the reaction of Cr-fluoride with Al gas. This work presents an accurate low temperature simulation method of gas formation and metal powder assimilation reactions in oxy-fluoride slags as in the SAW process.

In situ chemical analysis of duplex stainless steel weld by laser induced breakdown spectroscopy

The high corrosion resistance and good mechanical properties of duplex stainless steel (DSS) are due to its special chemical composition, which is a balanced phase ratio of ferrite (α) and austenite (γ). Many industrial applications require the integration of DSS components. For this, Gas tungsten arc welding (GTAW) is an excellent choice, as it allows an automated operation with high reproducibility. However, when the weld pool solidifies, critical ratios of α- and γ- phases can occur, which lead to solidification cracking, increased susceptibility to corrosion, and a decrease in ductility and critical strength. Previous studies have shown that these defects can be caused by the accumulation of manganese and chromium in the heat affected zone (HAZ), requiring ongoing monitoring of this accumulation. A suitable method for such monitoring is laser-induced breakdown spectroscopy (LIBS), which can be used in two operating modes: calibration using standard reference samples and calibration-free. Unlike conventional quantitative LIBS measurements, which require reference samples to generate a calibration curve, calibration-free LIBS (CF-LIBS) allows chemical compositions to be determined solely from the emission spectrum of the plasma. Numerous publications show that CF-LIBS is a fast and efficient analytical method for the quantitative analysis of metal samples. In this work, CF-LIBS is applied to spectra obtained during GTAW DSS welding and the result is compared with those obtained by PLS analysis. A good correlation was found between both types of analysis, demonstrating the suitability of the CF-LIBS method for this application. The CF-LIBS method has a significant advantage over conventional LIBS due to the rapid in situ measurement of concentrations of major alloying elements without calibration procedure. This, combined with fast feedback and appropriate adjustment of welding parameters, helps prevent welding defects.

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.

Numeric simulation and experimental study in magnetic field-assisted laser scanning welding of T2 red copper

To address the challenges of low energy coupling efficiency and poor process stability in the laser welding of T2 red copper, this study establishes a thermal-fluid-coupled three-dimensional numerical model; and we conduct experimental tests to investigate the effects of different magnetic field strengths on laser scanning welding of T2 red copper. The results indicate a high degree of conformity between the simulated calculations and experimental tests in terms of the cross-sectional fusion morphology. As the magnetic field strength increases from 0 to 160 mT, the lowest and highest temperatures, melt width, and melt depth of the weld pool initially exhibit an initially increasing trend followed by a decreasing trend. At a magnetic field strength of 120 mT, these parameters peak at 1383 K, 4810 K, 2.388, and 1.753 mm, representing improvements of 2.44 %, 58.75 %, and 1.68 %, respectively, compared with those without an applied magnetic field. However, the maximum flow velocity within the weld pool exhibits a trend of initially decreasing and subsequently increasing, reaching a minimum value of 0.4102 m/s at a magnetic field strength of 120 mT, representing a 12.76 % reduction compared with that without an applied magnetic field. As the magnetic field strength increases, the surface morphology of the weld seam initially improves and then deteriorates. In addition, the grain structure within the weld zone first becomes finer and coarser. When the magnetic field strength reaches 120 mT, the surface-forming quality of the weld seam improves; the internal grain structure of the weld zone becomes finer; and the hardness, tensile strength, and electrical conductivity of the weld joint reach their respective maximum values of 47.13 HV, 270 MPa, and 78.24 mS/m. This represents significant improvements of 18.45 %, 56.07 %, and 25.2 %, respectively, compared to those without an applied magnetic field. Under a steady magnetic field. the effective thermal input of the laser energy to the base material is enhanced, thereby improving the stability of the weld pool during the welding process. This enhancement leads to improved welding quality, yielding new insights and theoretical foundations for the laser welding of T2 red copper.

Fabrication of low thermal expansion Fe–Ni alloys by in-situ alloying using twin-wire arc additive manufacturing

The binary Fe–Ni system offers alloys with notably low linear coefficients of thermal expansion (CTE), contingent upon their Ni content. In this respect twin-wire arc additive manufacturing (T-WAAM) presents the opportunity of in-situ alloying through the simultaneous feeding of two metal wires into a weld pool to obtain desired alloy compositions. This study aims to deposit a graded wall with Ni contents targeted at 42, 46, and 52 wt% in the building direction of the wall, along with a block comprising of 46 wt% Ni, employing the T-WAAM approach. The results show effective incorporation of additional Ni into the weld pool and geometrically stable weld beads in the continuous metal transfer mode during the T-WAAM process. This mode led to the defect-free and chemically stable deposition of the graded wall and the block with average Ni contents of 42.3 ± 1.1, 45.8 ± 1.4, 52.6 ± 0.8 wt%, and 46.4 ± 0.9 wt%, respectively. The measured Curie temperatures of the as-deposited alloys and the mean CTE values of alloy 46 were found to be comparable to the commercial alloys. In summary, this study validates the feasibility of in-situ deposition of low thermal expansion alloy compositions, thereby enabling the possibility of on-demand thermal expansion properties.

A novel optimizing Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) method for improving weld quality

To enhance the reliability of aluminum alloy welding quality, a novel Pulsed Plasma Gas Variable Polarity Plasma Arc Welding (PPG-VPPAW) system is proposed. This system enables independent control and rapid adjustment of the plasma gas, allowing for flexible modulation of arc pressure output under constant thermal conditions, thereby reducing porosity and refining grain structure. The research analyzed the impact of parameters of different plasma gas on arc characteristics, welding porosity, and welding microstructure. The results reveal that the periodic changes in pulsed plasma gas induce corresponding variations in arc temperature, arc shape, arc pressure, arc voltage, and the morphology of the weld pool keyhole. The magnitude of the difference between the base value and peak value of pulsed plasma gas determines the extent of arc pressure fluctuations and the behavior of the backside keyhole. The maximum oscillation amplitude of the backside keyhole can reach up to 4.5 mm². Additionally, the oscillation of pulsed plasma gas within the molten pool has been demonstrated to effectively reduce weld porosity and refine grain size. Under this process, the grain size of the weld metal can be reduced by up to approximately 25%. Current work provides an important direction for fabricating highly reliable aluminum alloy weldments.

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.

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.

Peculiarities Formation of Welded Joints under the External Electromagnetic Influence

Controlling the movement of liquid metal by selecting the parameters of an external electromagnetic effect makes it possible to change the conditions of dynamic equilibrium of the weld pool and, as a result, the formation of a weld. Magnetic process control has advantages over mechanical control methods, since it is carried out without contact with the welding zone. The study of processes leading to a decrease in the concentration of defects in metals, recombination of dislocations, polygonization, recrystallization, defect healing, etc., is an urgent task for technologists. The purpose of the work is to study the laws of formation of phase composition, microhardness, grain, lath, subgrain, dislocation structures of low-alloy steel welds in underwater welding and the relationship of structural parameters with the properties of strength and crack resistance of these joints. Microstructure studies were carried out by light, scanning, and transmission electron microscopy. Mathematical modelling was carried out to optimize the research efficiency. The developed computer application implements the idea of sequential calculation of quantities, where the value of the welding current and the current in the inductor is selected by the researcher. The influence of structural factors at the dislocation level on local internal stresses, which determine the deformation localization zones in the structures of the upper and lower bainites in the deposited metal, is analyzed. The conditions for obtaining high-quality welded joints in the welding of low-alloy steels, which ensure their strength and crack resistance, are established.

Limits on Ti Element Transfer in Submerged Arc Welding: Thermochemical Analysis

TiO2 inclusion formation in the weld pool is required to produce potent nucleation sites for acicular ferrite microstructure formation. Weld metal oxygen content must be limited to ensure acceptable materials properties, although sufficient oxygen is required for inclusion formation. Weld pool oxygen is sourced from the decomposition of flux oxides. Weld metal oxygen is controlled with flux chemistry formulation by CaF2 dilution of oxides. In conventional submerged arc welding (SAW), the flux is the source of weld metal Ti. Transfer of Ti from the slag appears to be limited to 400 parts per million (ppm). SAW modification by metal powder additions changes the element transfer reactions. Thermochemical analysis is applied to explain the limitations in Ti transfer from the slag, compared to improved reaction conditions for Ti element transfer in the aluminum‐assisted Ti alloying of weld metal. Low TiO2 activity due to low flux TiO2 content from CaF2 dilution and Ti loss from Ti‐fluoride gas formation limits Ti transfer from slag. Aluminum‐assisted alloying of the weld metal shifts the gas composition from Ti‐fluoride formation to Al‐fluoride formation and lowers the system partial oxygen pressure to increase the weld metal Ti content, with acceptable ppm O remaining in the weld metal.

On the interpretation of Varestraint and Transvarestraint hot cracking test results

The Varestraint test and its variant Transvarestraint are one of the most widely used techniques for evaluating a material solidification cracking sensitivity during welding. The result of such tests is a crack length which is proportional to the material’s cracking susceptibility. Nevertheless, the welding and load parameters can unintentionally influence the crack length, which in some cases can distort the material evaluation. An approach is described as to how these effects can be assessed with the aid of a digital crack analysis. The crack lengths are compared position-dependently with their possible propagation due to the weld pool movement during continuous loading. The index derived from this can be used by the operator to evaluate his test parameters. In addition, a comparison of the results of different Varestraint setups is made possible. Alongside experimental results, a numerical sensitivity analysis is presented on how individual welding and loading parameters can affect the crack lengths.

Keyhole Welding with Hybrid Plasma-Free Arc Source

Improving the keyhole welding process window is challenging work because the force and thermal state in the weld pool are hard to control. A hybrid plasma-free arc source was developed based on a plasma arc torch, sided tungsten was added close to the constraint orifice to form a free arc, and hybrid arcs formed after the free arc was fully absorbed into the constraint arc. In such a one-anode, twocathode hybrid arc system, the added free arc acts in an assistant role to adjust arc heat output without influencing the arc pressure peak, and the constraint arc acts as the guiding arc to control the arc pressure and slightly incline to the free arc side. Bead-on welding experiments were done to test keyhole welding process behavior with the hybrid arc source, including arc column, keyhole state, weld surface performance, and melting state in the weld pool. Results showed that (1) the hybrid arc can enlarge the stable keyhole welding process window in aspects of welding current and welding speed, and a more-obvious improving effect is obtained with the leading-sided tungsten; (2) the hybrid arc with the rear-sided tungsten has a smooth front weld surface; (3) the increased heat will enlarge the weld cross-section area with no obvious influence to the backside weld width; and (4) the leading keyhole wall is related to the outer tungsten position. The research gives a new method for improving keyhole welding stability.