Liquid Metal Pool Research Articles

A tracer-free photographic method for the estimation of the velocity field in a liquid metal pool

Abstract Accurately measuring the velocity field (VF) of liquid metals is essential for optimizing and controlling high-temperature industrial processes, yet remains a challenging task despite the numerous existing velocimetry techniques. In this paper, a methodology for estimating the VF of a liquid metal pool is proposed. The experimental set-up used here to heat the metal sample up to the liquid state and collect data is presented. This set-up primarly intented to perform flash experimetns on liquid metals, allows contactless heating and data measurements by a high speed camera. The proposed methodology for estimating the VF consists then in implementing a Non-Rigid Registration between the different frames of the recorded temperature fields (TFs). This method is applied on TFs resulting from multiple experimental tests conducted on iron samples in the temperature range of 1 850 − 2 100 K. The consistency of the results is put to test via multiple validation tests. The influence of the main parameters is also evaluated. The results show that the proposed methodology allows a fine experimental quantification of liquid metal VFs with an estimated error of 8 % and confirm the feasibility of the proposed approach.

Electrochemical Liquid‐Liquid‐Solid Growth of Ag‐In Crystals with Liquid Indium Alloy Electrodes

AbstractSynthesis of intermetallic crystals by electrodeposition of Ag from alkaline aqueous electrolytes containing AgCN onto liquid metal electrodes via an electrochemical liquid‐liquid‐solid (ec‐LLS) process has been performed. X‐ray diffraction, scanning electron microscopy, and transmission electron microscopy were performed to identify crystalline products. Ec‐LLS experiments performed with pure liquid Hg and Ga electrodes resulted in the formation of polycrystalline Ag2Ga and Ag2Hg3. Experiments performed with In‐containing liquid metals preferentially yielded Ag9In4 and AgIn2 products with liquid Hg0.35In0.65 and Ga0.86In0.14, respectively. The product distribution with liquid Hg0.35In0.65 depended on the level of Ag supersaturation during the electrodeposition. A mechanism that accounts for the aforementioned observations is presented and discussed. This work described the formation of Ag−In intermetallic phases by the isothermal electroreduction of Ag into different liquid metal solvents via ec‐LLS. Electrodeposition of Ag into a pure Ga or pure Hg liquid metal pool yielded precisely the compounds predicted from isothermal cross‐sections of the respective binary phase diagrams. These compounds were not found when using liquid Hg or Ga containing appreciable In. The smaller enthalpy of formation for AgIn2 was consistent with its synthesis in Ga0.86In0.14. However, the observed product of Ag9In4 in Hg‐containing liquid metals could not be rationalized solely from thermodynamic factors. Instead, this observation was consistent with a kinetic pathway based on the lability of Hg‐metal bonds and nearly identical crystal structures of Ag9In4 and Ag2Hg3. Site exchange of Hg for In is consistent with our prior observations[23] of In exchange into Hg−Pd structures during Pd electrodeposition. This mechanism is not based on any direct role of electrochemistry other than aspects that dictate the operative supersaturation of the metal solute.

Review of the research on the scrubbing of fission products in liquid metal pool

In recent times, the risk assessment and design optimization of advanced fourth-generation nuclear reactors have garnered significant global scholarly attention. Specifically, researchers worldwide have been focusing on the comprehensive analysis of the scrubbing process of radioactive fission product aerosols within the reactor containment pool under severe accident scenarios. This study delves into the historical progression of investigating the pool scrubbing phenomenon, which dates back to the mid-20th century. International efforts have encompassed various aspects, including assessing the migration behavior of radioactive nuclides within molten metal environments, conducting sensitivity analyses to gauge the impact of diverse variables on pool scrubbing, formulating models for analyzing the pool scrubbing process, and conducting relevant experimental validations. Drawing upon an extensive body of literature, this article presents a synthesized overview of research into the pool scrubbing process of fission product aerosols, focusing on three principal dimensions: model elucidation, experimental investigations, and computational simulations. The insights provided herein serve as a consolidated reference point for forthcoming endeavors in this domain of research.

Electroslag Hollow Ingots for Nuclear and Petrochemical Pressure Vessels and Pipes

The paper presents ground reasoning and results of experiments and modeling of heavy hollow ingot manufacturing using advanced electroslag technology. The requirements for ingots for huge diameter reactor pressure vessels include high density, homogeneity, and minimal segregation, which are very difficult to achieve by traditional casting. In the electroslag remelting process (ESR), hollow ingots form in between two copper water-cooled molds under effective heat removal. This improves the solidification pattern due to the shortening of a solidifying volume thickness more than twice compared with a solid ingot of the same diameter. The shallow liquid metal pool and narrow mushy zone at the ESR hollow ingot solidification assure their high metallurgical quality. Due to the dense and low segregation structure, ESR hollow ingots proved to be used for as-cast pipes and heavy wall billets for further forging. The results of a mathematical simulation within the range of simulated dimensions (the outer diameter up to 2900 mm, wall thickness up to 750 mm) also predict the favorable solidification pattern for thick-wall hollow ingots of big diameters. The analysis made and the modeling results provide a framework for scaling up the sizes of hollow ingots produced by ESR and widening their application for manufacturing heavy wall large diameter shells for nuclear and petrochemical industries. The higher reachable productivity of hollow ingot formation and lower capacity of power supply source than that for solid ingots of the same diameter and weight are also preconditions of their energy saving and cost-effective manufacturing.

Experimental and numerical analysis of impingement temperature effect on the sacrificial plate's hole formation during nuclear severe accident

Liquid metal breaching onto a metal plate is an important aspect that needs to be considered in the nuclear severe accident analysis. This relates to the downward relocation of molten material to the supporting system such as sacrificial grid plate and lower head while accident happened. The molten core material impinged the sacrificial grid plate which was quenched before interacted with reactor pressure vessel. The present study aims to investigate the dynamic behaviors of holes formation due to liquid metal jet impingement onto a metal plate at initial temperatures 423 K, 473 K, and 523 K. The experiments and numerical simulation were performed with molten Wood's Metal and 5 mm thickness of Lead Bismuth Eutectic plates. The modified Moving Particle Semi-Implicit method was used to perform the numerical calculation based on the particle movement. The results showed that the plate was locally melted by the heat supplied from the liquid metal jet and induced the penetration. A larger amount of liquid metal stayed, then liquid metal pool was formed when the liquid temperature jet was lower. The temperature variation resulted unsignificant change of hole size diameter on both surfaces. The formed hole sizes in the plate's top and bottom surfaces were around 5.7–14 mm and 3.7−5 mm, respectively. The required time for the liquid metal penetration was shorter when the liquid metal temperature was higher. The increase of conduction heat transfer during the solidification of liquid metal could decrease the Nusselt number at the stagnation zone. The findings show that the experiment and numerical results can predict the sacrificial grid plate's hole formation and dynamic mechanism within critical time of molten core material impingement.

Characterization of thermal stratification using validated liquid metal pool mixing models

Weakly stratified thermal fronts in the upper plena of liquid metal cooled reactors can pose prediction complexities for safety analyses. The strong thermal diffusivity of the coolant changes gravity’s influence on the flow by reducing horizontal density gradients at time scales faster than gravity can act on them. While damped out thermal behavior results in stratification, the underlying convection currents continue contributing to enhanced mixing. Experimental turbulence spectral information, captured by distributed velocity (ultrasonic) and temperature (fiber-optic) measurements, is used to construct microscale parameters and quantify the amount of enhanced vertical thermal diffusivity in a scaled gallium surrogate of a pool type sodium fast reactor upper plenum. These constructed microscale parameters are used to validate empirical models for use in liquid metal pools. This experimental data and, separately, the validated models, are fed into a simple 1-D transport model to quantify the impacts to reactor safety and allow for testing of the efficacy of using the more easily available bulk parameters (i.e., core barrel Reynolds and Richardson numbers) to estimate the enhanced diffusivity. The 1-D model represents the experimental fronts’ progressions most closely after accounting for these characteristics of mixed convection unique to liquid metals.

MyMUSCLE, a New Multiphysics, Multiscale Simulation Coupling Environment

Increasing the computational power enables the nuclear community to combine existing knowledge on the variety of different physical phenomena that take place in reactors and to develop tools that can simulate these combined, interacting phenomena simultaneously. This includes phenomena related to structural mechanics, fluid dynamics, and reactor physics among others. Coupling different codes developed specifically for the analysis of separate phenomenon is currently a topic high on the research and development agenda of the international community. Based on the experience of successfully computing the dissymmetric benchmark in the Phénix reactor by coupling the system thermal-hydraulic (STH) code SPECTRA to the computational fluid dynamics (CFD) code CFX in the H2020 SESAME project, the Nuclear Research and Consultancy Group (NRG) is currently developing the code-coupling tool myMUSCLE: MultiphYsics MUltiscale Simulation CoupLing Environment. MyMUSCLE is an independent, external, Fortran-based code that arranges the efficient and robust coupling of different codes. It aims at being flexible with respect to the codes being coupled, i.e., commercial and open-source codes, while having a single coupling tool that enhances quality assurance. It is currently set up to couple SPECTRA as a STH code to CFX, Fluent, STAR-CCM+, or OpenFOAM as a CFD code. This paper presents the proof of principle and first verification of the myMUSCLE tool under development by applying it to multiscale thermal-hydraulic applications. First, a flow through a pipe is modeled as proof of principle for explicit coupling at a single coupling interface. Second, in preparation for modeling liquid-metal-cooled fast reactors, a piping system with a pool with natural convection is modeled. The results of the multiscale calculations show good agreement among the different coupled CFD codes. Finally, the preparations for simulating the TALL-3D experiment, used for generating data for validation of simulation tools for liquid-metal pools, are presented.

Evolution and Formation of Non-Metallic Inclusions during Electroslag Remelting of Ce-Bearing 15Cr-22Ni-1Nb Austenitic Heat-Resistant Steel

The evolution of inclusions in austenitic heat-resistant steel with different Ce content during protective argon gas atmosphere electroslag remelting (ESR) was studied. All oxide inclusions in the Ce-free consumable electrode are MgO·Al2O3. A part of these MgO·Al2O3 inclusions was removed before metal droplets entered the liquid metal pool during the ESR. The soluble oxygen (arising from the reoxidation) reacted with soluble aluminum, calcium, and magnesium in liquid steel to form MgO·Al2O3 and CaO–Al2O3 inclusions in liquid steel. All oxide inclusions in the electrode with 0.016 mass% Ce are Ce2O2S. A portion of these Ce2O2S inclusions was dissociated into soluble oxygen, cerium, and sulfur in liquid steel during the ESR process, whereas the others were removed by absorbing them into molten slag. The oxide inclusions in the liquid metal pool and remelted ingot were Ce2O3, CeAlO3, and Ce2O2S. The CeAlO3 and Ce2O3 inclusions were reoxidation products formed by the chemical reaction between the soluble oxygen, soluble aluminum, and cerium. The oxide inclusions in the electrode with 0.300 mass% Ce are CeS and Ce2O2S. These CeS inclusions were removed by molten slag adsorption during the ESR. A part of these Ce2O2S inclusions was removed by slag adsorption, and the remaining entered into the liquid metal pool. The oxide inclusions in the liquid metal pool and the ingot were Ce2O3 and Ce2O2S. The Ce2O3 inclusions were formed through the chemical reaction between the soluble oxygen and cerium in the liquid metal pool. The Ce2O2S inclusions in the liquid pool originate from reoxidation products during the ESR process and the relics from the electrode.

Time resolved in-situ multi-contrast X-ray imaging of melting in metals

In this work, the application of a time resolved multi-contrast beam tracking technique to the investigation of the melting and solidification process in metals is presented. The use of such a technique allows retrieval of three contrast channels, transmission, refraction and dark-field, with millisecond time resolution. We investigated different melting conditions to characterize, at a proof-of-concept level, the features visible in each of the contrast channels. We found that the phase contrast channel provides a superior visibility of the density variations, allowing the liquid metal pool to be clearly distinguished. Refraction and dark-field were found to highlight surface roughness formed during solidification. This work demonstrates that the availability of the additional contrast channels provided by multi-contrast X-ray imaging delivers additional information, also when imaging high atomic number specimens with a significant absorption.

Multi‐Stage Enhanced Removal of Inclusions During Electroslag Remelting Process by a Static Magnetic Field

An axial static magnetic field (ASMF) is superimposed during the electroslag remelting (ESR) process to regulate the inclusion removal (GCr15 bearing steel) in the current study. After applying a 50 mT ASMF, the metal pool of the ESR ingot becomes shallower and the solidified dendritic structure tends to grow axially, together with a higher inclusion removal efficiency. The number of inclusions in ESR ingot decreases, and the inclusion with larger size (>10 μm) is almost eliminated. By tailoring the morphology of the liquid melt film (LMF), droplet, and metal pool, the ASMF applied in this study strengthens the kinetic conditions (inclusion migrated to slag/metal interface) for the inclusion removal, thus realizing the multi‐stage enhanced removal of inclusion during ESR process.

BWR lower head penetration failure test focusing on eutectic melting

Decommissioning work occasioned by the Fukushima Daiichi Nuclear Power Station (1F) accident of March 2011 is in progress. Severe accident (SA) analysis, testing, and internal investigation are being used to grasp the 1F internal state. A PWR system that refers to the TMI-2 accident is typical for SA codes and testing, on the other hand, a BWR system like 1F is uncommon, understanding the 1F internal state is challenging. The present study conducted the ELSA-1 test, a test that focused on damage from eutectic melting of the liquid metal pool and control rod drive (CRD), to elucidate the lower head (LH) failure mechanism in the 1F accident. The results demonstrated that depending on the condition of the melt pool formed in the lower plenum, a factor of LH boundary failure was due to eutectic melting. In addition, the state related to the CRD structure of 1F unit 2 were estimated.

Three-dimensional integro-differential model of unstationary hydrodynamic process in a liquid-metal pool of underwater arc welding

Wet underwater arc welding is now widely used. At the same time, obtaining high-quality welds with this welding method is an urgent scientific and technical problem due to their saturation with hydrogen and oxygen and the formation of pores. One of the promising directions for solving this issue is the use of an external electromagnetic effect on the liquid metal in the weld pool in order to control the movements of the molten metal flows to improve the degassing processes of welded joints. It is possible to estimate the parameters and efficiency of external electromagnetic influence by means of mathematical modeling of related electromagnetic, hydrodynamic and thermal processes occurring in the welding installation. The article proposes a three-dimensional integro-differential model of a non-stationary hydrodynamic process occurring in the liquid metal of a weld pool in an underwater arc welding system with an external electromagnetic effect. For the equations of hydrodynamics boundary value problems are formed, which, using potential theory, are reduced to a system of integro-differential equations for the vorticity function in the volume of a liquid conductor and a simple vector layer on its surface. For a numerical solution, the resulting system of integro-differential equations is approximated by an algebraic system according to the Krylov-Bogolyubov method. This system of equations makes it possible to determine the velocity field in the liquid metal of the weld pool for any welding modes.

Coordinated design of the cartridge-type blanket and the ceramic pebble divertor for the helical reactor FFHR-b3

The cartridge-type blanket for the helical reactor has been updated and named the CARDISTRY-B3. Together with this, the ceramic pebble ergodic limiter/divertor system named the REVOLVER-D3 has been adopted in the design. There was a difficulty in fabricating the blanket cartridges due to the first wall shape with three-dimensional surface. This difficulty has been removed by introducing the slit first wall, where the first wall surface is formed by alternately stacking solid metal plates and porous plates made of metal or ceramic. The liquid metal flowing inside the blanket cartridge oozes out through the porous plates and forms the surface flow covering the first wall. Solid target plates for the helical divertor has been omitted, because the divertor region is already covered and protected by the liquid metal surface flow. The liquid metal freely falls inside the blanket cartridges by the gravity force and then exhausted to the pool set on the lower port. The ceramic pebbles, which are dropped to the inboard-side ergodic layer to form the flowing limiter divertor, also flow into the liquid metal pool. The pebbles and liquid metal are individually elevated again by screw conveyers. Since the free surface of the liquid metal is exposed to the plasma, the vapor pressure of the working liquid metal should be low enough. Ternary or quadruple alloys, which include Li, Sn, Pb (or Bi), and Er, have been selected as the candidates of the functional liquid metal for the CARDISTRY-B3 that satisfies the requirements of low vapor pressure, low density, low melting point, and high tritium breeding ratio. In this study, the coordinated design of CARDISTRY-B3 and REVOLVER-D3 for the FFHR-b3 is presented. The FFHR-b3 is one of the design options that aims at demonstration of 100 MW net electricity with two times larger devise size than LHD.

Numerical Simulation on Motion Behavior of Inclusions in the Lab-Scale Electroslag Remelting Process with a Vibrating Electrode

In order to meet the requirement of high-quality ingots, the vibrating electrode technique in the electroslag remelting (ESR) process has been proposed. Non-metallic inclusions in ingots may cause serious defects and deteriorate mechanical properties of final products. Moreover, the dimension, number and distribution of non-metallic inclusions should be strictly controlled during the ESR process in order to produce high-quality ingots. A transient 2-D coupled model is established to analyze the motion behavior of inclusions in the lab-scale ESR process with a vibrating electrode, especially under the influence of the vibration frequency, current, slag layer thickness, and filling ratio, as well as type and diameter of inclusions. Simulation model of inclusions motion behavior is established based on the Euler-Lagrange approach. The continuous phase including metal and slag, is calculated based on the volume of fluid (VOF) method, and the trajectory of inclusions is tracked with the discrete phase model (DPM). The vibrating electrode is simulated by the user-defined function and dynamic mesh. The results show that when the electrode vibration frequency is 0.25 Hz or 1 Hz, the inclusions will gather on one side of the slag layer. When it increases from 0.25 Hz to 1 Hz, the removal ratio of 10 μm and 50 μm inclusions increases by 5% and 4.1%, respectively. When the current increases from 1200 A to 1800 A, the flow following property of inclusions in the slag layer becomes worse. The removal ratio of inclusions reaches the maximum value of 92% with the current of 1500 A. The thickness of slag layer mainly affects the position of inclusions entering the liquid-metal pool. As the slag layer thickens, the inclusions removal ratio increases gradually from 82.73% to 85.91%. As the filling ratio increases, the flow following property of inclusions in the slag layer is enhanced. The removal ratio of 10 μm inclusions increases from 94.82% to 97%. However, for inclusions with a diameter of 50 μm, the maximum removal ratio is 96.04% with a filling ratio of 0.46. The distribution of 50 μm inclusions is significantly different, while the distribution of 10 μm inclusions is almost similar. Because of the influence of a vibrating electrode, 10 μm Al2O3 and MnO have a similar removal ratios of 81.33% and 82.81%, respectively.

Natural convection phenomena in a liquid metal pool due to relocated and heap of heat-generating core debris: Numerical study

In the present numerical study, authors have investigated the natural convection heat transfer and fluid flow characteristics in a pool due to the heat generating core debris, i.e. decay heat in a typical fast breeder reactor. Two significant aspects of decay heat removal from debris under core disruptive accidents have been analyzed. These key aspects are (i) relocation of heat-generating core debris at the different locations inside the main vessel and (ii) effect of debris heap on heat transfer from source to ambient. These cases are analyzed extensively on full-scale three-dimensional reactor model with all major components. Multiple tray-type core catcher has incorporated with passive pipes over one collection tray. The present numerical study provides a robust and practical feasible core debris retention arrangement after a severe accident in fast breeder reactors. Also, the study is focused on a feasible array of core collection trays that maintained heat-generating debris and heat shield trays under safe thermal designed limit (∼1200 K for debris and ∼ 923 K for trays). This eliminates the further advancement of accidents and leakage of radioactive material into environments. During our analysis, it has been found that single tray with passive pipes maintained the heat-generating debris and tray under safe thermal limit for a fraction of 50 % destroyed core relocated to lower plenum while remaining heat-generating debris is present in its original position. Multiple tray arrangements accommodate the whole core debris within safe thermal design limits. With the possibility of debris heap after its relocation to the lower plenum, it has been found that heap angle less than 3° does not affect the integrity of core catcher.

Control of the Properties of Welded Joints in Wet Underwater Welding

The aquatic environment will negatively affect the processes that occur during the formation of a welded joint and leads to a deterioration in its properties. The welding conditions do not allow the use of traditional methods to improve the quality of the weld metal - preheating or subsequent heat treatment. One of the possible effective ways to solve this problem when welding with flux-cored wire is the use of external electromagnetic exposure. Generation of axial alternating control magnetic fields in the welding zone allows controlling the hydrodynamics of the molten weld pool. Because of the interaction of an external electromagnetic field with an electric field of the arc, ponderomotive forces appear in the weld pool, which leads to mixing of the liquid metal. Changing the direction of electromagnetic induction with a given frequency and interval, you can periodically change the direction of flow in the weld pool. Intensive reverse mixing of the melt occurs, during which the hydrogen accumulated in front of the crystallization front as a result of concentration compaction is removed from it, the melt temperature in the central and peripheral areas of the pool is averaged, which slightly reduces the crystallization rate of the weld pool, thereby increasing its degassing time. As a result, it was possible to significantly reduce the porosity of the weld metal (more than 8 times) and reduce the hydrogen content (more than 2.5 times). Metallographic studies showed a significant refinement of the structure both weld metal and the heat-affected zone (2 and 1.3 times, respectively), its homogenization and a decrease in the degree of defectiveness. Thus, the investigations showed that the external electromagnetic effect on the liquid metal pool in the process of wet welding underwater is a useful tool that allows you to control the processes that occur during the crystallization stage of the weld pool and significantly increase the ductility of the weld metal (more than 1.5 times).

Microstructure and Properties of Ti-6Al-4V Articles 3D-Printed with Co-axial Electron Beam and Wire Technology

Advanced additive manufacturing technology of direct energy deposition type called x Beam 3D Metal Printing was employed for manufacturing of articles of various size and shape using titanium Ti-6Al-4V alloy wire as a feedstock material. The key distinctive feature of this process is applying of the hollow conical electron beam generated by low-voltage (< 20 kV) gas-discharge electron beam gun for heating and melting of the substrate and wire, which ensures precisely controllable liquid metal transfer from the wire end to the substrate, specific temperature gradients at the fusion area and heat flow from liquid metal pool. Such conditions of heating, melting and cooling during deposition of molten additive material provide controllable microstructure formation, including grain size and material texture. Influence of processing parameters and cooling conditions on fusion, crystallization, grain growth and intragrain structure of manufactured “chunky” Ti-6Al-4V blocks was discussed. The post-processing heat treatment was used to relieve residual stresses formed during article manufacturing. Tensile and fatigue testing has been performed for samples cut in various directions of produced articles to investigate the material uniformity. Optimization of processing parameters allowed production of 3D printed articles from Ti-6Al-4V with isotropic microstructure and mechanical characteristics which met standard requirements for Ti-6Al-4V material.

Rapid Solidification and Surfaces Alloying Using Continuous CO&lt;sub&gt;2&lt;/sub&gt; Laser to Generate Ni-Aluminide on Steel

Rapidly solidified thin micro-alloyed surface layers are generated by laser melting of plasma thermal sprayed steel surfaces. Samples of carbon steel are plasma sprayed with fine nickel and aluminum powders. Laser surface melting generated a thin localized molten pool of metal with steep horizontal thermal gradient. The latter triggered intense vortex formation in the molten pool which thoroughly mixed the nickel and aluminum powders within the molten pool in a fraction of a second. As the sample is moved away with a predefined velocity, the cold substrate quenched the melt pool, generating rapidly solidified micro-alloyed surfaces. A 2.5 kilowatts continuous carbon dioxide laser was used for surface melting; laser power was maintained at 800 watts while the samples were moved with respect to the laser beam at linear velocities in the range of 100-200 mm/min. The technique generated metallurgical bonded novel surfaces. Depth of the laser modified layer was achieved in the range of 0.2-0.4 mm. Refined microstructures of pre-austenite grain size in the range of 4±2 µm were generated. Micro-hardness measurements of the modified layer indicate an almost three times enhancement in the hardness values; the latter are, in general indicative of mechanical strength of the material. The shape of the solid/liquid interface of the advancing molten pool determines the orientation of the growing dendrites; at higher velocities of sample translation with respect to the laser beam, these are almost parallel to the sample surface. The orientation of the dendrites, the uniformity in surface alloying (within fraction of a second) and the resulting hardness values are explained with the help of the modeled shape of the liquid metal pool. The laser processed material proved to be a flexible technique to synthesize novel surfaces for surface sensitive applications.

A more precise unified model to describe comprehensive multiphysics and multiphase phenomena in plasma arc welding

Plasma technology involves complex and coupled electric, magnetic, thermal and hydrodynamical interaction. It’s important to understand all the multiphysics mechanism for a wider industrial application. A 3D unified model was established, which includes a combination of hydrodynamical equations, heat transfer equation and electromagnetic equations, to comprehensively describe the electro-thermal conversion, electromagnetic effect, heat transfer, gas-liquid two-phase flow and solid-liquid phase change in plasma arc welding (PAW). Volume of Fluid method was used to track the gas-liquid interface of plasma arc and liquid metal pool, and an enthalpy-porosity technique was used to deal with the solid-liquid phase change during the welding process. The transient evolutions of temperature field, flow field and keyhole in the entire PAW process were predicted, and the distribution of electric potential and current density was obtained. It is found that there are high potential drop and current density near the electrode tip, which generates very high arc temperature and arc velocity. Plasma arc gradually melts the metal, then pushes the liquid metal to flow upwards along the keyhole surface, and finally penetrates through the workpiece, forming a “reversed bugle-like” configuration. Experiment was conducted with a plasma arc welding system, and it is found that both weld pool and keyhole geometry predicted by the model generally agree with experimental results. The calculated electric potential and full-penetration time also coincide with the experiment. It’s for the first time to give a relatively accurate prediction of electric potential drop and full-penetration time in unified PAW models. The research provides a deep and fundamental understanding of the comprehensive electric-magnetic-thermal-hydrodynamical multiphysics and gas-liquid-solid multiphase interaction in PAW process.

Polarimetric imaging method for surface quality evaluation of a liquid metal pool obtained during welding

Conventional methods used to inspect the molten pool surface induced by arc welding processes are based on post-welding evaluation using destructive or semi-destructive testing methods. Many attempts were performed to develop alternative and nondestructive tests. In this context, a polarimetric vision methodology is evaluated. The polarimetric parameters are measured from the thermal radiations emitted by the hot liquid metal at a wavelength within an arc plasma blind spectral window. The Stokes parameters and their segmentation within a Gaussian mixture model allow to discriminate artifacts at the surface of the liquid metal. After weld pool solidification, the use of scanning electron microscopy and energy dispersive spectroscopy allow to identify the artifacts, i.e., floating alumina particles.