Helium Arc Research Articles

AF-FTTSnet: An end-to-end two-stream convolutional neural network for online quality monitoring of robotic welding

Online welding quality monitoring (WQM) is crucial for intelligent welding, and deep learning approaches considering spatiotemporal features for WQM tasks show great potential. However, one of the important challenges for existing approaches is to balance the spatiotemporal representation learning capability and computational efficiency, which makes it challenging to adapt welding processes with complex and drastic molten pool dynamic behavior. This paper proposes a novel approach for WQM using molten pool visual sensing and deep learning considering spatiotemporal features, the proposed deep learning network called attention fusion based frame-temporality two-stream network (AF-FTTSnet). Firstly, a passive vision sensor is used to acquire continuous dynamic molten pool images. Meanwhile, temporal difference images are computed to provide novel features and temporal representations. Then, a two-stream feature extraction module is designed to concurrently extract rich spatiotemporal features from molten pool images and temporal difference images. Finally, an attention fusion module with the ability to automatically identify and weight the most relevant features is designed to achieve optimal fusion of the two-stream features. The shop welding experimental results indicate that the proposed AF-FTTSnet model can effectively and robustly recognize five typical welding states during helium arc welding, with an accuracy of 99.26%. This model has been demonstrated to exhibit significant performance improvements compared to mainstream temporal sequence models. Available: https://github.com/Just199806/TSCNN/tree/master.

Filter-PCA-Based Process Monitoring and Defect Identification During Climbing Helium Arc Welding Process Using DE-SVM

In-process defect detection for climbing helium arc welding of aluminum alloy is challenging due to the highly complex nature of the heat and mass transfer process. This paper presents a process monitoring and defects identification method for weld quality monitoring during climbing helium arc welding based on molten pool visual sensing. High contrast molten pool images were acquired by the designed passive vision sensor. Then, an image processing algorithm based on Otsu's method and visual saliency features was proposed to extract multi-region features from the acquired images in real-time. An optimal feature subset selection approach based on filter and principal component analysis(Filter-PCA) was developed to select the components related to welding defects. Finally, a novel prediction model using Filter-PCA and the support vector machine based on differential evolution algorithm(DE-SVM) was established to identify undercut weld, snake-like weld, and sound weld. The experiment results obtained in the actual production line of 2219 aluminum alloy rocket propellent tank show that the proposed method is remarkably effective and robust. This research provides a technical approach for early identification and early warning of defects during climbing helium arc welding of aluminum alloy. It also provides a basis for stability and online control of weld formation quality under time-varying welding pose.

Modeling and Comparative Analysis of Atmospheric Pressure Anodic Carbon Arc Discharge in Argon and Helium-Producing Carbon Nanostructures.

In this work, within the framework of a unified model for the discharge gap and electrodes, a comparative numerical analysis was carried out on the effect of evaporation of graphite anode material on the characteristics of the arc discharge in helium and argon. The effect of changing the plasma-forming ion, in which the ion of evaporated atomic carbon becomes the dominant ion, is demonstrated. For an arc discharge in helium, this effect is accompanied by a jump-like change in the dependence of the current density on voltage (CVC), and smoothly for a discharge in argon. With regard to the dynamics of the ignition of an arc discharge, it is shown that during the transition from glow discharge to arc in helium, the discharge parameters are also accompanied by an abrupt change, while in argon, this transition is smooth. This is due to the fact that the ionization potentials, as well as the ionization cross sections, differ significantly for helium and carbon, and are close in value for helium and argon. For various points on the CVC, the density distributions of the charged and neutral particles of an inert gas and evaporated gases are presented.

Heat Transfer Mechanisms in Arcs of Various Gases at Atmospheric Pressure

This work expands findings about the dominant heat transfer mechanisms in argon and helium arcs at atmospheric pressure for monoatomic (Ar, He, 50% Ar–50% He), diatomic (air, {hbox{N}}_{2}, {hbox{O}}_{2}, {hbox{F}}_{2}, {hbox{Cl}}_{2}), and triatomic ({hbox{CO}}_{2}) gases. The objective is to understand the dominant mechanisms in atmospheric plasmas through validated numerical modeling for GTAW welding process. Arcs of all gases have lengths of 10 mm and 200 A current. Five heat transfer mechanisms are considered: Joule heating, convection, radiation, conduction, and Thomson effect. Results indicate that the general structure of the arcs and dominant mechanisms are qualitatively similar for all gases; sizes change depending on the gas. The dominant energy input near the cathode is Joule heating, while that near the anode is convection. The dominant energy output always follows the same sequence: Thomson effect next to the cathode followed by convection, radiation in the arc column, and conduction near the anode. Joule heating is the most significant in Ar, while in He, it has the lowest significance. This is due to differences in electric conductivity of He (higher up to 21,000 K and lower beyond 21,000 K than other gases) and high viscosity of He, which creates a small Joule heating versus a large convection region. He transfers the most heat towards the anode by convection while {hbox{N}}_{2} has the lowest; due to the high enthalpy and viscosity of He compared to {hbox{N}}_{2}. Finally, Ar has the most significant radiative emission and He the smallest due to their net emission coefficient.

Evaluation of self-absorption effect of He I resonance line by measurement of forbidden emission in helium arc jet plasma

We have proposed a new method to estimate the optical escape factor (OEF) in high-density helium (He) plasma. Plasma with an electron temperature of ca. 3 eV and density of 5 × 1013 cm−3 was generated by a cascaded arc discharge and rapidly cooled by the introduction of additional He gas, which resulted in a transition from ionizing to recombining plasma. With an increase in the gas pressure, the plasma became optically thick, and the He I forbidden line (spin-exchange intercombination line, 1 1S-2 3P: 59.1 nm) with resonance lines were simultaneously observed using a vacuum ultraviolet spectrometer. Comparison of the intensity ratio of the He I 58.4 nm resonance line to the forbidden emission with those by determined from the collisional-radiative model considering the self-absorption enabled the successful estimation of the OEF. The OEF was decreased with the ambient He gas pressure and was 6.6 × 10−4 and 3.5 × 10−5 for He gas pressures of 1.59 and 20.22 Pa, respectively.

Comparison of Langmuir probe and optical emission spectroscopy on a volume-cusp filament ion source using helium.

D-Pace has a Penning ion source that runs with helium for studies of α-particle production. We want to study its plasma properties as a function of varying operational parameters, which results in varied output ion beam characteristics. In order to diagnose the ion source plasma, a collisional-radiative code for helium to be used with optical emission spectroscopy measurements is developed. This has the advantage of being non-invasive, which allows for measurements using the emitted light from the plasma. This collisional-radiative code is shown to compare well with the Yacora on the Web model developed at IPP-Garching, and improves upon it with the addition of radiation trapping. Furthermore, the sensitivity of this model to the inclusion of additional excited state populations and non-Maxwellian electron energy distribution functions is investigated. It is shown that non-Maxwellian distributions can significantly affect spectroscopy measurements. This diagnostic is benchmarked to Langmuir probe measurements on the TRIUMF-licensed volume-cusp ion source to determine whether it can replicate the measured electron density and electron temperature as a function of varied operational parameters. The operational parameters are helium gas flow (10-40 SCCM), arc voltage (100-200V), and arc current (1-5A). The measured plasma properties, while different in absolute value, have similar trends for each operational parameter except when varying arc voltage. It is shown that this mismatch as a function of arc voltage is likely due to high energy non-Maxwellian electrons from the cathode sheath, which are not included within the collisional-radiative model.

Numerical Simulation of Helium Arc at High Pressure and Low Current

A computational fluid dynamics (CFD) model has been developed to investigate the time evolution of a helium plasma discharge at high pressures (from 2 to 8 MPa) and low electric current (0.35 A), including the interaction between the plasma and the electromagnetic fields, under local thermodynamic equilibrium (LTE) assumption. To account for pressure dependence, novel thermodynamic and transport properties have been calculated in a wide pressure and temperature range. The model has been further improved by considering the effect of plasma–electrode interactions and the formation of the plasma sheath. High-performance computing (HPC) was used to solve the CFD simulation, focusing on reference cases at 8 MPa and 0.35 A. Numerical results have shown that the sheath model and updated transport and thermodynamic properties have a significant impact on the electric potential, resulting in very good agreement between the simulation and experimental values.

Internal dynamic simulation analysis of new DC load switch arc chamber

The DC load switch which relies on the rapid increase of arc voltage to realize natural commutation is actively promoted in the flexible DC distribution network. When it is disconnected, there will be an arc in the gas arc chamber. The arc voltage directly determines the speed and success or failure of commutation. It is proposed to apply the gas arc chamber with metal grid to the main flow branch of natural commutation DC load switch. Based on magnetohydrodynamics (MHD), the arc model of gas arc chamber with metal grid is established by finite element method, and the segmentation of arc by metal grid is realized by assuming the plasma sheath, Then the calculation of the rapid rise process of arc voltage is realized. Through the temperature field distribution and arc voltage variation curve in the arc chamber, the process of helium arc moving along the arc channel and being cut by metal grid under the action of magnetic field is analysed, and then the feasibility of rapidly increasing the arc voltage by using this scheme is revealed. The research results provide some theoretical guidance for the design of natural commutation direct current load switch.

Modelling and experimental investigations of composition-dependent heat and mass transfer during Cu–Ni alloy nanoparticle synthesis in a transferred arc helium plasma

In the synthesis of alloy nanoparticles (NPs) via arc evaporation and fast quenching, the composition of the synthesized NPs differs significantly from that of the precursor. The properties of the NPs formed following evaporation of a copper–nickel anode by a helium arc are investigated using a nucleation model coupled to the thermal and flow fields derived from a magneto-hydrodynamic simulation of the arc, and experimentally. Results are obtained for three different Cu:Ni precursor compositions, 10:90, 50:50 and 90:10 at%. The synthesized particles are spherical with different size distributions and have a higher concentration of Cu than the precursor in all three cases, in contrast to previous observations, but in accordance with the predictions of the model. Emission spectroscopic measurements of copper and nickel lines indicate that the concentration of atomic copper in the plasma region near the anode is much higher than that of the nickel, in accordance with the predictions of the model. The higher vapour pressure of copper compared to nickel, and its higher activity in the alloy precursor (anode), immersed in a self-consistent thermal and flow field maintained by the transferred arc are found to play a critical role. The thermodynamic, radiative and transport properties of the helium plasma contaminated with evaporated precursor are important in determining the thermal and flow fields.

Fiber-form nanostructured tungsten formation by helium arc discharge plasma irradiation under a gas pressure of 5 kPa

Fiber-form nanostructured tungsten (W) formation was successfully demonstrated using a helium (He) arc discharge plasma generated at a gas pressure of 5 kPa. The W substrate was exposed to the He arc discharge plasma at a surface temperature of about 1000 °C. A bias voltage, Vb, was applied between the W substrate and the vacuum chamber to control the incident He ion energy, Ei. Scanning electron microscope (SEM) observations showed that the W surface was slightly roughened, but no fiber-form nanostructure was formed at a Vb of −22 V. On the other hand, fiber-form nanostructure growth was observed at a Vb of −112 V. Furthermore, cross-sectional SEM observations revealed the presence of He bubbles inside the W fibers and the sub-surface layer of the W substrate. As collisions between ions and neutrals in the sheath cannot be ignored at 5 kPa, a theoretical evaluation of Ei distribution was performed using the collisional sheath model. Furthermore, an experimental evaluation of Ei using the sputtering yield of carbon substrates by He arc discharge plasma irradiation was carried out, and then Ei was obtained to be 40–50 eV for Vb = −112 V. Therefore, the distribution of Ei at Vb = −112 V is considered to contain more than 20 eV, which is necessary for the growth of fiber-form nanostructured W found in conventional low-gas pressure plasma irradiation experiments.

Study of arc characteristics using varying shielding gas and optimization of activated-tig welding technique for thick AISI 316L(N) plates

• In-house developed activated flux for welding of 316L(N) stainless steel. • Identification of optimum shielding gas mixture based on arc stability studies. • Full-depth of penetration in 11 mm thick plate in single pass welding with optimised parameters at reduced heat input. • Joining of 20 mm thick plate by double side A-TIG welding technique. In the present study, type 316L(N) bead-on-plates were welded using Activated-Tungsten Inert Gas (A-TIG) welding process with argon and helium gas at various ratios to evaluate the arc characteristics. The welding process parameters including shielding gas composition were optimized using the design of experiments to join 11 mm thick plate in a single pass. In addition, the A-TIG welding technique using optimized process parameters was developed to weld plates up to 20 mm thickness with significantly reduced heat input. High power density of helium arc shielding leads to constricted arc column that enhances arc efficiency and produces deeper penetration.

Vision sensing and feedback control of weld penetration in helium arc welding process

Consistent joint penetration is essential to mechanical properties of weldments, yet difficult to realize in gas tungsten arc welding due to heat accumulation, varying welding conditions, and uncertain disturbance, especially for manufacturing large-scale structural components. The solution is to monitor welding penetration in real-time and implement closed-loop control. In this paper a directional light-assisted vision sensing scheme is proposed to measure the transient profile of the backside weld pool/bead for monitoring and further controlling the joint penetration state. The developing zone and the developed zone of the backside weld bead are recognized, helping determining its final width. And the relationship between the transient backside bead width and the corresponding final width is established. Furthermore, the backside bead profile is employed as a feedback signal and the closed-loop control of weld formation is implemented to achieve full penetration and a constant backside bead width. A multiple-input single-output model predictive controller is utilized in the control system, creatively combined with an anti-windup integrator eliminating the steady-state error caused by nonlinearity in welding. Experimental results have proved that the developed control system is effective in keeping a consistent backside bead width with a rapid response, a small overshoot and robustness against varying heat transfer conditions.

Investigation of transient metal vapour transport processes in helium arc welding by imaging spectroscopy

In order to understand the transient transport mechanisms of metal vapour in an arc plasma, imaging spectroscopy was performed for helium arc plasmas burning on pure metals. The measured images were converted into the spectral intensity distributions at the central cross-section of the arc plasma by image processing and Abel inversion. The transient transport process of metal vapours inside the arc plasma are discussed in terms of the diffusion coefficients of metal vapour. The metal vapour generated from the weld pool surface was transported into the arc plasma and to the region near the tungsten electrode within a few seconds. As time passed, the metal vapour spectral intensities gradually increased near the weld pool surface, near the electrode and in the intermediate region. In contrast, the He I spectral intensity was weak and the He I distribution shrunk toward the centre of the arc. This is because the helium arc plasma was cooled by the strong radiation from metal vapour. For an arc burning on pure chromium, chromium ions reached to centre of the arc after about 40 s. In contrast, on pure iron, iron ions did not reach to the centre of the arc. This is because the helium arc plasma on pure chromium was cooled to a temperature at which the electric field and ordinary diffusion coefficients of chromium are largest, whereas the temperature of the plasma on pure iron remained high. Two factors contributed to this; the higher boiling point of iron, which reduces the iron vapour concentration, and the lower radiative emission coefficient of iron.

Development of High-Performance Cascade Arc Discharge Device for Applications to Plasma Window

We fabricated a cascade arc discharge source with a channel diameter of 8 mm for application to plasma window as an alternative differential pumping. Helium arc plasmas up to a discharge current of 100 A were generated, and the performance as the plasma window was investigated. The pressure ratio of high-pressure discharge section to low-pressure vacuum side was about 200 at a gas flow rate of 0.05 L/min. In order to characterize the plasma parameters around the anode electrode, vacuum UV and visible emissions of He I were measured. The electron density was determined to be 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for a discharge current 100 A and a gas flow rate of 0.05 L/min, while the electron temperature was 0.11 eV. Moreover, the forbidden transition of He I 2s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> S- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> P was observed. The effect of radiation trapping of He I Ly-α was studied by the intensity ratio of Lyα to forbidden line and the collisional radiative (CR) model. As a result, it was found that the plasma generated was optically thick with an optical escape factor of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> .

Constraints on Metastable Helium in the Atmospheres of WASP-69b and WASP-52b with Ultranarrowband Photometry

Abstract Infrared observations of metastable 23S helium absorption with ground- and space-based spectroscopy are rapidly maturing, as this species is a unique probe of exoplanet atmospheres. Specifically, the transit depth in the triplet feature (with vacuum wavelengths near 1083.3 nm) can be used to constrain the temperature and mass-loss rate of an exoplanet’s upper atmosphere. Here, we present a new photometric technique to measure metastable 23S helium absorption using an ultranarrowband filter (FWHM 0.635 nm) coupled to a beam-shaping diffuser installed in the Wide-field Infrared Camera on the 200 inch Hale Telescope at Palomar Observatory. We use telluric OH lines and a helium arc lamp to characterize refractive effects through the filter and to confirm our understanding of the filter transmission profile. We benchmark our new technique by observing a transit of WASP-69b and detect an excess absorption of 0.498% ± 0.045% (11.1σ), consistent with previous measurements after considering our bandpass. We then use this method to study the inflated gas giant WASP-52b and place a 95th percentile upper limit on excess absorption in our helium bandpass of 0.47%. Using an atmospheric escape model, we constrain the mass-loss rate for WASP-69b to be ( ) at 7000 K (12,000 K). Additionally, we set an upper limit on the mass-loss rate of WASP-52b at these temperatures of ( ). These results show that ultranarrowband photometry can reliably quantify absorption in the metastable helium feature.

Numerical study of the metal vapour transport in tungsten inert-gas welding in argon for stainless steel

Metal vapour emanating from the weld pool during tungsten-inert-gas (TIG) welding affects the arc welding process. To understand the transport mechanisms of metal vapour in a TIG arc, an axisymmetric computational model is developed that includes the tungsten cathode, stainless-steel anode workpiece and the arc plasma region self-consistently. The combined diffusion coefficient method, which calculates diffusion coefficients due to mole fraction gradients (ordinary diffusion), temperature gradients, pressure gradients and the electric field is used to treat iron–chromium–argon and iron–chromium–helium plasmas. It was found that in both cases, metal vapours can reach the cathode region. The effect of different diffusion coefficients on metal vapour transport was investigated. It was found that ordinary diffusion is the main driving force for upward metal vapour diffusion away from the anode workpiece in an argon arc. The diffusive transport carries the metal vapour into the recirculating convective flow, which then transports the metal vapour to the cathode region. Here the upward diffusion driven by the temperature gradient and electric field leads to the build of high concentrations of the metal vapours adjacent to the cathode. In the helium arc, in contrast, metal vapour is transported upwards from the workpiece by electric field diffusion, which is much stronger in this case. Spectroscopic measurements of atomic chromium emission show that metal vapour can reach the cathode region in an argon TIG arc, providing support for the predictions of the model. Only by taking into account all diffusion driving forces is it possible to predict the distribution of metal vapour in a TIG welding arc.

Porous tungsten nanostructure formation using a helium arc discharge plasma under sub-atmospheric pressure

Porous tungsten (W) nanostructure formation was performed for the first time using a helium (He) arc discharge plasma under sub-atmospheric pressure of 80 kPa. Scanning electron microscope observations showed that micron-sized bubble and hole structures were formed in the W substrate at a surface temperature of 1700 °C, indicating He penetration into the W substrate. At the surface temperature of 800 °C, surface blackening of the W substrate due to the He plasma irradiation for two hours was found. The unique surface morphology has a W nanofiber network with a thickness of ~10 µm which consists of assembly of W nanoparticles with a diameter of 20–30 nm. It was suggested that an interaction between W vapor evaporated from the W electrodes and the surrounding He plasma plays a key role for physical processes of the nanostructured W formation identified in this study. It could be formed due to a process of cluster–cluster aggregation of W nanoparticles produced by nucleation and condensation from the W vapor phase. The W nanoparticle network deposition is considered to be a new process for the porous W nanostructure formation as an alternative to standard fuzz formation associated by bubble formation on the surface of base W substrate.

Highly Ionized Helium Arc Plasma. Determination of the Temperature in Nonequilibrium Conditions and the Effect of Plasma Microfields

Methods are proposed for the spectral measurement of electron temperature Те in a highly ionized nonequilibrium He plasma at atmospheric pressure based on a comparison of the intensities of atomic and ionic lines with a whole complex of experimental data (primarily, ne). The large energy gap between the emitting HeI and HeII excited levels (of more than 50 eV) and allowance for the nonequilibrium manifested in the overpopulation of the atomic levels with respect to the ionic levels make it possible to make reliable and simple Те estimates, especially if the ionization energies of the emitting levels of the atom and the ion are close. For practical use, three pairs of HeI and HeII lines observed in the spectral range of 200–1100 nm are recommended. The radial and temporal (upon the quasistationary pulsed heating of the electric arc plasma) dependences of Те are obtained in the range of axial values of 3.2–4.7 eV.

A computational model of gas tungsten arc welding of stainless steel: the importance of considering the different metal vapours simultaneously

A 2D computational model of the mixing of multiple metal vapours into a helium arc in gas tungsten arc welding of stainless steel is presented. The combined diffusion coefficient method, extended to three-gas mixtures, is used to treat helium–chromium–iron and helium–manganese–iron plasmas. It is found that all metal vapours penetrate to the arc centre and reach the cathode, with iron vapour confined near the cathode tip, while chromium and manganese vapours accumulate about 1.5 mm above the tip. The predicted distributions of chromium, manganese and iron show reasonable agreement with published photographic images and radial distributions of atomic line emission intensities. The results are also consistent with published measurements of the deposition of the metals on the cathode surface. A detailed examination of the influence of the different diffusion coefficients, net emission coefficients and vapour pressures of the metals on the metal vapour transport in the arc plasma is presented. It is shown that cataphoresis (diffusion due to applied electric fields) leads to the penetration of the metal vapours into the arc. The different distribution of iron vapour from those of chromium and manganese vapours near the cathode is strongly influenced by the lower ordinary diffusion coefficients of iron at low temperatures. Radiative emission is found to be important since it leads to cooling of the arc, which decreases the influence of cataphoresis. The vapour pressure only influences the concentration of the metal vapour close to the workpiece. Results for the two-gas helium–chromium and helium–iron systems are compared to those for the three-gas helium–chromium–iron system. It is shown that it is important to consider the different metal vapours simultaneously to obtain an accurate calculation of the metal vapour and arc temperature distributions.

Generation of a Large Diameter He Cascade Arc Plasma for a Plasma Window Application

We fabricated a cascade arc discharge source with a channel diameter of 8 mm for various applications to virtual vacuum interface (plasma window). Stationary helium arc plasma up to a discharge current of 100 A was generated, and the performance as plasma window was investigated. The pressure ratio of high-pressure discharge section to low-pressure vacuum side was above 70 at a 50-A He discharge with a gas flow rate of 0.2 L/min. To characterize the plasma at the anode electrode, emissions of vacuum UV and visible wavelengths were observed. Spectral analysis of He I continuum spectra and H- $\beta $ Stark broadening showed that the plasma temperature was around 0.9 eV and density was $3.4\times 10^{13}$ cm−3 for a 100-A He discharge with a gas flow rate of 0.05 L/min.