Liquid Weld Pool Research Articles

Применение нано- и ультрадисперсных материалов для управления структурой и свойствами металлов в соединительных технологиях: мировая практика (обзор)

The paper presents an overview of the use of nano- and ultrafine powders as functional additives in various methods of arc welding (fusion surfacing). Methods for introducing nano- and ultrafine powders into a liquid weld pool are considered. The results of experimental studies of soldering processes are presented. The objectives of further research in the field of practical application of nano- and ultradisperse materials for connecting technologies are formulated.

The Current State of Application of Ultra- and Nanostructured Powders in Technologies in Welding Technologies (Review)

This paper presents an overview of applying nano-and ultra-disperse powders as modifiers in various procedures of arc welding, as well as in electro-slag welding and plasma powder spraying. Consideration of some methods of embedding nano-and ultra-disperse powders into a liquid weld pool is provided.

Modeling and real-time prediction for complex welding process based on weld pool

A major assumption in developing intelligent robot in industrial fields is that the intelligence has to be from senior human workers. However, in many industrial applications, this assumption may not hold. For example, in welding process, a senior welder can continually choose proper weld parameters and tune weld performance based on their observations of the liquid weld pool. But a robot with better pool shape measurement system and quicker respond speed may outperform human welders. In such cases, intelligent welding agents, if developed successfully, would greatly reduce welding cost by avoiding much expensive human training efforts and improve the welding performance. In recent years, machine learning techniques have emerged as a new learning framework to address such problem. The theory of machine learning provides a normative account of how agents may predict their future action based on a model structure, in which the training and future data have to be in the same feature space and have the same distribution. In this paper, Gaussian Process Regression (GPR) is proposed to model real-time welding process. Gas tungsten arc welding experiments were performed and the experimental data are utilized to validate the proposed method. We train the model using 1284 pairs of data in one experiment and test this model on a challenging domain of 4 other experiments by 11,642 pairs of data, using the same algorithm, GPR architectures, and hyperparameters. Bayesian Optimization Algorithm (BOA) surrogated with GPR is proposed to provide robust prediction results. This paper opens a door for enabling robot with industrial skills, resulting in the first artificial welder that is capable to respond real-time observations and provide robust predictions.

Effects of filling status of cold wire on the welding process stability in twin-arc integrated cold wire hybrid welding

Twin-wire metal inert gas/metal active gas (MIG/MAG) arc welding is one typical type of the high-efficient welding technologies. In order to further improve the welding efficiency without increasing the welding heat input, a novel welding technology entitled twin-arc integrated cold wire hybrid welding was developed. The addition of cold wire not only increased the welding deposition rate but also improved the welding stability. In this paper, the influential mechanism of cold wire on welding stability was studied by the high-speed photography and electrical signal acquisition. It was found that the cathode spot could be stabilized on the surface of the weld pool by increasing the cold wire feed speed, which significantly improved the welding stability. The main reasons are as follows: the addition of cold wire significantly decreased the temperature of the liquid weld pool metal around the cold wire. This led to increasing the electron emission difficulty and reducing the gradient distribution of surface tension. Therefore, the cathode spot would be more likely to stabilize in the high temperature zone instead of drifting with the rear flow of the liquid weld pool metal. Besides, the cathode spot could also be stabilized because of the less dramatic fluid flow of the liquid metal. Finally, the welding stability was evaluated by a mathematical statistic method, which further verified that the welding stability could be improved with the increase of cold wire feed speed to a proper range.

Reaction Kinetics of Mg<sub>2</sub>Si in the Laser Welding Pool of Aluminum Alloys

The metallurgy reactions in the weld pool were analyzed with the application of reaction kinetics. Furthermore a formula to quantificationally calculate the amount of Mg2Si was generated. By means of calculating the thermodynamic condition of the reaction and combined with the verification testing, this article indicates that the metallurgical product Mg2Si was created in the liquid weld pool but not separated out from the arborescent crystal during its solid state. This thesis summarizes the precipitation mechanism of Mg2Si and explains the configuration, amount and location where Mg2Si was separated out by analyzing the phase diagrams and constitution diagrams. Relevant data suggests that: the content of Mg2Si is slightly less than Mg.

Robust pattern recognition for measurement of three dimensional weld pool surface in GTAW

Ability to observe and measure weld pool surfaces in real-time is the foundation for next generation intelligent welding that partially mimics welders' sensory capability, i.e., acquiring weld status information from the observation of weld pool. To image and measure the mirror-like weld pool surface under the strong arc radiation, a structured light laser pattern, dot matrix, has been projected onto the weld pool surface and its specular reflection is intercepted and imaged by an imaging plane placed with a distance from the arc. The reflection pattern, deformed by the specular liquid weld pool, contains three-dimensional (3D) geometry information of the weld pool. In this paper, a robust recognition procedure has been proposed to identify the reflection pattern in real-time. In particular, an adaptive thresholding algorithm is proposed to distinguish the laser dots in the reflection pattern. Then a reflection pattern recognition algorithm is proposed to determine the row and column number for each reflected laser dot such that reflection pattern can be matched with the projection pattern. The identified reflection pattern can be used to reconstruct the 3D weld pool surface. Experiments with different welding conditions have been conducted to verify the real-time performance of the proposed procedure, including the effectiveness, robustness and time complexity. It has been found that the procedure is capable of identifying the reflection pattern from a captured image in less than 19 ms. The real-time performance meets the most widely used arc welding process for precise welding-gas tungsten arc welding (GTAW) that typically requires to be controlled a few times per second.

Investigação sobre a eficiência da fibra de vidro e da fibra cerâmica como suporte de solda na soldagem unilateral

O objetivo deste trabalho foi avaliar uma nova categoria de suporte de solda para a soldagem unilateral do aço carbono, usando tecidos de fibra de vidro e fibra cerâmica. Trata-se de uma proposta de inovação tecnológica, uma evolução do suporte cerâmico convencional. A soldagem unilateral com este tipo de suporte de solda foi executada em chapas de aço carbono com arame tubular (FCAW) e MIG-MAG (GMAW). A eficiência da fibra de vidro e da fibra cerâmica em suportar o metal de solda líquido foi verificada durante a soldagem do passe de raiz. Várias espessuras e combinações de tecidos de fibra de vidro e fibra cerâmica foram testadas, bem como foram identificados pontos de trabalho. As seguintes variáveis do processo foram estudadas: a abertura de raiz, a corrente e a velocidade de soldagem. Os resultados mostram que os materiais à base de fibra de vidro e fibra cerâmica são eficientes para suportar a poça de solda líquida. O modo de fixação do suporte de solda na junta soldada também mostrou ser efetivo. Foi possível estimar uma faixa de operação do processo, com abertura de raiz de 2,0 a 4,0 mm, a energia de soldagem menor que 1,8 kJ/mm, a técnica de soldagem ligeiramente puxando e com pequena oscilação da tocha, espessura do suporte de solda de tecido de fibra de vidro maior que 1,6 mm. Macrogafias da juntas soldadas mostraram que houve penetração nas laterais do chanfro, reforço de solda adequado na raiz e não foram notados defeitos.

High-power laser welding of contaminated steel sheets

A deep penetration laser welding process differs from a heat conduction laser welding process in the generation of a steam capillary through the material. The so called keyhole is formed if high energy densities of the laser beam vaporize the base material, leading to a vapor-filled channel which is surrounded by a liquid weld pool. The shape and the oscillations of the keyhole are affected by various physical parameters of the welding process. Under constant process conditions a balance of pressure keeps the capillary open. If the material is contaminated by a layer with physical properties differing from the bulk material (e.g. oil), the equilibrium is disturbed in a negative way. The coating evaporates during the melting of the base material, resulting in vapor locks at the capillary and the weld pool, leading to pores and failures along the weld seam. In order to understand and avoid this effect, it is the aim to analyze and describe the failure generating mechanisms. Additionally, a control method of the laser power is investigated which influences the keyhole’s oscillations and could suppress an unwanted collapse.

The Oscillation Behaviour of Liquid Metal in Arc Welding

This paper gives an historic overview and new developments of research activities in the field of the oscillatory behaviour of liquid metal in arc welding. Early work focused on the oscillation behaviour of the weld pool in Gas Tungsten Arc Welding (GTAW). Agitated weld pools exhibit specific modes of oscillation, the frequency of which can be measured from the arc voltage data and is conditioned by the geometry of the weld pool and the properties of the liquid metal. Of technological interest is the alteration of the oscillation behaviour for partially and fully penetrated situations, which can be used for penetration control during welding. A logical extension of the research activities was related to the influence of filler wire addition on the oscillation behaviour. An intermediate step towards the description of Gas Metal Arc Welding (GMAW), is the situation of GTAW with cold filler wire supply. It was found that both the liquid weld pool and the pendant liquid droplet at the tip of the filler wire experience an oscillation, which obscures the influence of the individual contributions of both liquid masses on the voltage data. It was shown that online penetration control is still possible, provided that the metal is transferred in an uninterrupted way, i.e. the filler wire flows smoothly into the weld pool. For GMAW, in which detached droplets collide with the weld pool surface, the difficulties are even more prominent. Recent work is related to this issue. Monitoring of the phenomena occurring at the weld pool and the pendant droplet become problematic by means of the voltage data. Observations by means of high-speed video imaging will be discussed. Apart from the experimental studies, efforts are undertaken in numerical simulations of the processes. A good correlation is obtained between experimental data and the results of the numerical models.

Heat Transfer and Fluid Flow during Gas-Metal-Arc Fillet Welding for Various Joint Configurations and Welding Positions

Gas-metal-arc (GMA) fillet welding is one of the most commonly used welding processes in the industry. This welding process is characterized by the complex joint geometry, a deformable weld pool surface, and the addition of hot metal droplets. In this work, a three-dimensional numerical heat-transfer and fluid-flow model is developed to capture the effects of the tilt angle of the fillet joint and the welding positions, i.e., V, L, and other configurations on the temperature profiles, velocity fields, weld pool shape, weld pool free surface profile, thermal cycles, and cooling rates during GMA welding in spray mode. The governing equations of conservation of mass, momentum, and energy are solved using a boundary fitted curvilinear coordinate system. The weld pool free surface deformation is calculated by minimizing the total surface energy. A dimensional analysis is performed to understand the importance of heat transfer by conduction and convection and the role of various driving forces on convection in the liquid weld pool. The computed shape and size of the fusion zone, finger penetration characteristic of the GMA welds, and the solidified free surface profile are in fair agreement with the corresponding experimental results. The calculated cooling rates are also in good agreement with independent experimental data. The results reported here indicate a significant promise for understanding the effect of joint orientations and welding positions on weld pool shape, size, and the cooling rates based on fundamental principles of transport phenomena.

Heat and fluid flow in complex joints during gas metal arc welding—Part II: Application to fillet welding of mild steel

A numerical model described in part I [W. Zhang, C.-H. Kim, and T. DebRoy, J. Appl. Phys. 95, 5210 (2004)] was used to investigate the heat transfer and free surface flow during gas metal arc fillet welding of mild steel. Dimensional analysis was used to understand the importance of heat transfer by conduction and convection and the role of various driving forces on convection in the liquid weld pool. The calculated shape and size, finger penetration characteristic and solidified surface profile of the fillet welds were in fair agreement with the experimental results for various welding conditions. The calculated cooling rates were also in good agreement with independent experimental data. The effect of welding parameters on important weld bead characteristics was quantitatively studied using the numerical model. The results reported here indicate a significant promise for understanding and control of gas metal arc fillet welding processes based on fundamental principles of transport phenomena.

In situ observations of phase transitions in Ti–6Al–4V alloy welds using spatially resolved x-ray diffraction

In situ spatially resolved x-ray diffraction (SRXRD) experiments were used to directly observe the heat-affected zone phases present during gas tungsten arc welding of a Ti–6Al–4V alloy. The experiments were performed at the Stanford Synchrotron Radiation Laboratory using a 250 μm diam x-ray beam to gather real-time experimental information about the α−Ti→β−Ti phase transition during weld heating. Six different welding conditions were investigated using SRXRD to experimentally determine the extent of the single phase β-Ti region surrounding the liquid weld pool. These data were compared to predicted locations of the β-Ti phase boundary determined by calculated weld thermal profiles and equilibrium thermodynamic relationships. The comparison shows differences between the experimentally measured and the calculated locations of the β-Ti boundary. The differences are attributed to kinetics of the α−Ti→β−Ti phase transition, which requires superheating above the β-Ti transus temperature to take place during nonisothermal weld heating. Analysis of the results reveal that the transition to β-Ti requires an additional 3.96 s (±0.29 s) of time and 169 °C (±25.7 °C) of superheat above the β-Ti transus temperature to go to completion under an average weld heating rate of 42.7 °C/s.

Spatially resolved X-ray diffraction mapping of phase transformations in the heat-affected zone of carbon-manganese steel arc welds

Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite; however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld.

Spatially resolved X-ray diffraction phase mapping and α → β → α transformation kinetics in the heat-affected zone of commercially pure titanium arc welds

Spatially resolved X-ray diffraction (SRXRD) is used to map the α → β → α phase transformation in the heat-affected zone (HAZ) of commercially pure titanium gas tungsten arc welds. In situ SRXRD experiments were conducted using a 180-µm-diameter X-ray beam at the Stanford Synchrotron Radiation Laboratory (SSRL) (Stanford, CA) to probe the phases present in the HAZ of a 1.9 kW weld moving at 1.1 mm/s. Results of sequential linear X-ray diffraction scans made perpendicular to the weld direction were combined to construct a phase transformation map around the liquid weld pool. This map identifies six HAZ microstructural regions between the liquid weld pool and the base metal: (1) α-Ti that is undergoing annealing and recrystallization; (2) completely recrystallized α-Ti; (3) partially transformed α-Ti, where α-Ti and β-Ti coexist; (4) single-phase β-Ti; (5) back-transformed α-Ti; and (6) recrystallized α-Ti plus back-transformed α-Ti. Although the microstructure consisted predominantly of α-Ti, both prior to and after the weld, the crystallographically textured starting material was altered during welding to produce different α-Ti textures within the resulting HAZ. Based on the travel speed of the weld, the α → β transformation was measured to take 1.83 seconds during heating, while the β → α transformation was measured to take 0.91 seconds during cooling. The α → β transformation was characterized to be dominated by long-range diffusional growth on the leading (heating) side of the weld, while the β → α transformation was characterized to be predominantly massive on the trailing (cooling) side of the weld, with a massive growth rate on the order of 100 µm/s.

The influence of thermofluids phenomena in gas tungsten arc welds in high and low thermal conductivity metals

A comprehensive thermofluids numerical model of stationary gas tungsten arc (GTA) welding has been developed and used to examine the effects of thermofluids phenomena on the temperatures and flow velocities in the weld pool and their impact on the resultant weld dimensions for welds produced in high vs low thermal conductivity metals. A dynamic grid re-mapping technique was used to map a block of finite elements into the liquid weld pool to permit use of a k– ε turbulence model in the liquid. Simulations for low conductivity AISI 304 stainless steel show that good correlation with experimental data was only possible if the effects of fluid flow and turbulent mixing in the weld pool were modeled accurately. Conversely, simulations in higher conductivity metals, 6061 and 1100 aluminum, showed that while the average flow velocities and level of turbulence were higher, their effect on the final temperatures and weld pool dimensions were less significant.

Analysis of heat-affected zone phase transformations usingin situ spatially resolved x-ray diffraction with synchrotron radiation

Spatially resolved X-ray diffraction (SRXRD) consists of producing a submillimeter size X-ray beam from an intense synchrotron radiation source to perform real-time diffraction measurements on solid materials. This technique was used in this study to investigate the crystal phases surrounding a liquid weld pool in commercial purity titanium and to determine the location of the phase boundary separating the high-temperature body-centered-cubic (bcc) β phase from the low-temperature hexagonalclose-packed (hcp) α phase. The experiments were carried out at the Stanford Synchrotron Radiation Laboratory (SSRL) using a 0.25 × 0.50 mm X-ray probe that could be positioned with 10-µm precision on the surface of a quasistationary gas tungsten arc weld (GTAW). The SRXRD patterns were collected using a position-sensitive photodiode array in a φ-2φ geometry. For this probe size, integration times of 10 s/scan at each location on the specimen were found adequate to produce high signal-to-noise (S/N) ratios and quality diffraction patterns for phase identification, thus allowing real-time diffraction measurements to be made during welding. The SRXRD results showed characteristic hcp, bcc, and liquid diffraction patterns at various points along the sample, starting from the base metal through the heat-affected zone (HAZ) and into the weld pool, respectively. Analyses of the SRXRD data show the coexistence of bcc and hcp phases in the partially transformed (outer) region of the HAZ and single-phase bcc in the fully transformed (inner) region of the HAZ. Postweld metallographic examinations of the HAZ, combined with a conduction-based thermal model of the weld, were correlated with the SRXRD results. Finally, analysis of the diffraction intensities of the hcp and bcc phases was performed on the SRXRD data to provide additional information about the microstructural conditions that may exist in the HAZ at temperature during welding. This work represents the first directin situ mapping of phase boundaries in fusion welds.