Arc Instability Research Articles

Metal vapor-induced arc instability in stationary TIG-welding

The stability of the arc in arc welding processes is primarily relevant to the joint quality. However, the influencing factors which determine the arc stability are still partly unknown. Combining arc welding with laser welding enhances arc stability. The laser-induced metal vapor is often named as one stabilizing factor. But this contradicts the presumptions of arc welding researchers who mentioned the metal vapor as a destabilizing factor especially when high amounts are present in the arc. This investigation analyzes the specific influence of metal vapor on arc conductivity as one aspect of arc stability by producing metal vapor using a laser process on a separately placed substrate material. Furthermore, the arc current was varied at a constant metal vapor amount. The investigations show that the metal vapor presence increases the arc voltage by at least 20% and its fluctuation amplitude by at least 51% which was presumed to mean a decreased electrical conductivity and therefore decreased arc stability. The arc instability was lower for higher arc currents at constant metal vapor amounts. Therefore, higher arc currents increase the arc stability of welding arcs in the presence of a constant metal vapor amount.

A Time-Frequency Domain Feature Extraction Approach Enhanced by Computer Vision for Wire Arc Additive Manufacturing Monitoring Using Fourier and Wavelet Transform

Wire arc additive manufacturing (WAAM) is a rapidly growing technology that offers several advantages over traditional manufacturing methods, such as high deposition rates and the ability to build large components in a cost-effective manner. However, WAAM is also prone to the generation of defects, so the timely identification of anomalies is important to reduce the waste and get components of high quality. To develop anomaly detection application, the feature extraction process represents a key ingredient which allows machine learning systems to analyze big data. Waveform GMAW welding processes are typically used in WAAM to reduce the heat input supplied to the material and avoid defects such as excessive bending of parts and residual stress. These processes are based on the controlled dip transfer principle, so the waveforms should repeat themselves during deposition. This suggests that the frequency content of the voltage and current welding signals acquired during the process can provide important information about the process state. In this research, an experimental campaign was conducted to collect data for pulsed welding and surface tension transfer (STT) processes during the deposition of mild steel ER70S6, stainless steel 316L, Aluminum 4043, and Inconel 718 alloys. Welding voltage and current signals were acquired during the building processes, and a frequency domain analysis was conducted using the Fast Fourier transform (FFT) and discrete wavelet transform (DWT) with the aim to extract features from signals aiming to better separate the feature space, which means improve anomaly detection performance in detecting defects like arc instability, porosity, geometrical defect due to arc blow and humping. Furthermore, a methodology based on time-frequency analysis enhanced by Gabor filter for texture anomaly detection of scalograms obtained by Morlet Continuous Wavelet Transform is proposed, which showed an improvement of performance in separation between normal and anomalous deposition of several materials under different welding technologies.

A Numerical Study on the Influence of External Magnetic field on Hydrogen Electric Arc Flow

We present a 3D numerical model capable of simulating hydrogen electric arc. The numerical model couples the electromagnetic field with hydrogen plasma arc dynamics (flow and heat transfer). Electric arc operates under the conditions of atmospheric pressure using direct current (DC) in the presence of external magnetic field. It is found that arc instabilities emerge spontaneously near the cathode before propagating downwards along the arc jet. Also, spontaneous splitting into several arcs is observed before merging again into one arc. The horizontal magnetic field induces arc skewing and directional flow perpendicular to the applied magnetic field. However, the axial magnetic field induces a rotational flow and arc swirling.

A New Particle Swarm Optimization Algorithm for Outlier Detection: Industrial Data Clustering in Wire Arc Additive Manufacturing

In this paper, a novel outlier detection method is proposed for industrial data analysis based on the fuzzy C-means (FCM) algorithm. An adaptive switching randomly perturbed particle swarm optimization algorithm (ASRPPSO) is put forward to optimize the initial cluster centroids of the FCM algorithm. The superiority of the proposed ASRPPSO is demonstrated over five existing PSO algorithms on a series of benchmark functions. To illustrate its application potential, the proposed ASRPPSO-based FCM algorithm is exploited in the outlier detection problem for analyzing the real-world industrial data collected from a wire arc additive manufacturing pilot line in Sweden. Experimental results demonstrate that the proposed ASRPPSO-based FCM algorithm outperforms the standard FCM algorithm in detecting outliers of real-world industrial data. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —Electric arc (which is governed by the current and arc voltage) plays a significant role in monitoring the operating status of the wire arc additive manufacturing (WAAM) process. The nominal periodic current and voltage may occasionally change abruptly due to anomalies (such as arc instability, unstable metal transfer, geometrical deviations, and surface contaminations), which would affect the quality of the fabricated component. This paper focuses on detecting possible anomalies by analyzing the current and voltage during the WAAM process. A novel clustering-based outlier detection method is proposed for anomaly detection where abnormal and normal instances are categorized into two separate clusters. A new particle swarm optimization algorithm is put forward to optimize the initial cluster centroid so as to improve the detection accuracy. The proposed outlier detection method is applied to real-world data collected from a WAAM pilot line for detecting abnormal instances. Experimental results demonstrate the effectiveness of the proposed outlier detection method. The proposed outlier detection method can be applied to other industrial applications including electrical engineering, mechanical engineering and medical engineering. In the future, we aim to develop an online outlier detection system based on the proposed method for real-time for anomaly detection and defect prediction.

Unveiling arc deflection instability in narrow gap laser-arc hybrid welding of thick Ti-6Al-4V plate

Unveiling the mechanism behind arc deflection under narrow gap laser arc hybrid welding (NGLAHW) is of paramount significance in mitigating laser-arc coupling failure and welding porosity for thick Ti-6Al-4V plate structure. This paper utilizes high-speed camera technology, data statistics and model prediction to comprehensively study the influence of constrained groove space and droplet transfer mode on arc deflection. The results demonstrate that narrowing groove gap exerts a substantial compressive effect on welding arc behavior, particularly rendering it more susceptible to arc deflection when the groove gap is smaller than the arc length. The small droplet transfer mode is characterized by stable arc behavior, whereas the large droplet transfer mode tends to be susceptible to arc instability due to the droplet deviation induced by turbulent metal vapor. Particularly noteworthy is the emergence of substantial arc deflection when the droplet deviation exceeds the critical threshold of 0.51 mm, which diminishes the driving force for droplet transfer and subsequently leading to potential short-circuiting explosions and unstable welding process. Moreover, the lateral force induced by arc deflection drives a deviation in arc heat source with accelerated solidification of molten pool, consequently resulting in an elevation in welding porosity. This paper has provided a comprehensive insight into the mechanism driving arc deflection in NGLAHW, which holds paramount importance in enabling effective strategies to regulate the arc behavior.

Study of the electric arc dynamics in a cascaded-anode plasma torch

In plasma spraying process, the plasma jet stability depends on the electric arc movement inside the plasma torch. For conventional plasma torches with rod cathode and a large anodic surface, arc roots are free to move along the anode wall, leading to high plasma jet fluctuations. To overcome these instabilities, cascaded-anode plasma torches have been developed, reducing considerably the electric arc movement and thus the plasma jet fluctuations. Unfortunately, a small number of studies reports on the behaviour of theses torches including the arc instabilities and on electrode erosion process as well. In this work, several diagnostics have been set up to further understand the cascaded-anode plasma torch behaviour under different sets of operating conditions. Influence of plasma forming gas composition on the electric arc dynamic has been studied through anodic imaging. The issue of the intense plasma radiation masking is discussed since the light emitted by the arc column interferes with anodic arc roots observations. Multiple constricted arc roots have been observed and a preferential site for anodic arc root has been identified for all experimental conditions. The addition of a secondary plasma forming gas promotes a higher number of reattachments. The influence of the electric arc movement on plasma jet stability is also studied by following the plasma jet brightness fluctuations. The low voltage fluctuation frequency is also observed on brightness spectra, which means that these fluctuations could have an impact on in-flight particle treatment.

The Electrical Behaviour of Railway Pantograph Arcs

Electric arcing is an unavoidable consequence of the current collection process by sliding contact in railways and metros, and in general in many electrified transportation systems (ETSs). The most relevant consequences in an electrical perspective are: the occurrence of transients triggering resonant behaviour and transient responses, reduction of the energy efficiency of the system, conducted and radiated disturbance, in particular for the new radio systems widely employed for signalling and communication. The involved parameters are many (type of materials, current intensity, DC and AC supply, relative speed, temperature), as well as the studied characteristics (arc instability and lifetime, dynamic behaviour, electrical system response, radiation efficiency and coupling to external radio systems). This work reports the state of the art in arc modelling, arcing experimental characterisation, interaction with the supply system, radiated emissions and disturbance to radio systems, providing a complete description of phenomena and of reference data, critically discussing similarity and differences between sources. Proposed arc models are many with different assumptions and simplifications for various applications, so that a critical review and discussion are a necessity, considering the many different approaches and not-so-obvious applicability. The comparison with experimental results highlights unavoidable discrepancies, also because of intrinsic arc variability and for the many involved parameters and operating conditions. The impact of the arc as embedded in the railway system is then considered, speaking of conducted and radiated phenomena, including interference to radio communication systems and arc detection. The most prominent effect for conducted emissions is the excitation of system resonances, including the LC filters onboard rolling stock and substations in DC railways, with consequences for disturbance and energy efficiency, and this is discussed in detail. Conversely, for high frequency emissions, the attenuation along the line circuit is significant and the effective distance of propagation is limited; nevertheless radiated electromagnetic field emissions are a relevant source of disturbance for radio systems within the ETS premises and outside (e.g., at airports). The published approaches to quantify performance reduction are discussed with emphasis on experimental methods.

Visualization of cathode spot control using laser irradiation and oxide addition in wire arc additive manufacturing of titanium alloys

Arc instability is one of the most critical problems in gas-metal-arc (GMA) based wire arc additive manufacturing of titanium (Ti) alloys. It can result in a poor bead surface, surface oxidation, and spattering. In particular, the relocation of the cathode spot area is the main cause of big spatters because of the high thermal energy of the molten droplet at the molten pool surface. In this study, two cathode spot control techniques were applied using auxiliary laser heating and prelaid oxides, and the behaviors of the cathode spots and arc were visualized using high-speed photography. When the laser beam was irradiated in front of the GMA, a cathode spot was formed at the laser irradiation position, and the cathode jet did not interfere with the arc plasma and droplet transfer from the GMA. However, when the distance between the GMA and the laser irradiation position increased by more than 8 mm, multiple cathode spots were established, and spattering increased. The prelaid Ti oxide particles increased the metal deposition efficiency by establishing multiple and dispersed cathode spots rather than a concentrated cathode spot by droplet impingement. It was found that the volumetric transfer efficiencies (excluding spattering) for the laser-assisted control and Ti oxide powder were up to 99.87% and 91.2%, respectively.

Three-dimensional unsteady model of arc heater plasma flow

Arc jets are unique facilities used to evaluate the performance of Thermal Protection Systems (TPS) for hypersonic vehicles. They produce high pressure and high enthalpy plasma flow to simulate the extreme heat encountered during atmospheric entry. The constricted arc heater part of an arc jet increases the test gas temperature by Joule heating. This study details the development of the three-dimensional unsteady plasma flow analysis tool, ARCHeS (ARC Heater Simulator). Coupled Navier-Stokes, radiative transfer and Maxwell equations yield current density, magnetism, radiation, and flow field solutions. The present work constitutes the first demonstration of an unsteady three-dimensional plasma flow simulation of high pressure and high enthalpy arc heater that captures kink instabilities of the electric arc. It is found that the arc attachment is mainly driven by upstream arc instabilities.

Relationships between arc plasma jet properties and plasma/liquid interaction mechanisms for the deposition of nanostructured ceramic coatings

Ceramic nanostructured coatings with intermediate thicknesses between 10 and 100 µm give improved thermal and mechanical properties when used as thermal barrier coatings or wear-resistant coatings. Such coatings comply with the technical requirements of aeronautical and automotive applications. This implies the development of deposition processes with high throughput and deposition rates to promote the formation of nanostructured coatings. The use of a liquid phase as a carrier medium for nanoparticles or solution precursors is of major interest for injection within a thermal plasma jet. The as-sprayed materials can form ceramic nanostructured coatings provided the liquid injection, encompassing the physicochemical properties of the liquid and its injection method, can withstand the plasma properties. The repeatability of the interaction phenomena between the liquid phase and the arc jet has a key role in the efficiency of deposition, and some research efforts are devoted to stabilizing the arc while the liquid jet is continuously injected within the plasma. Alternatively a pulsed arc plasma jet can be generated and is associated with a time-phased injection of droplets. This paper presents the different issues related to the arc plasma properties produced by direct plasma torches, including arc instabilities and their influence on plasma/liquid interaction mechanisms leading to the formation of nanomaterials. There is a focus on pulsed plasma spraying associated with synchronized injection of microsized droplets by means of an inkjet printing method.

Problems encountered in underwater welding and remedies- a review

Wet underwater welding has proved to be the most suitable solution for underwater repairs and fabrication works. However, number of difficulties are faced that are specifically depicted in the testing and analysis of welds obtained while welded underwater. Major issues that are faced include immediate contact of workpiece with water leading to severe quenching effect, hydrogen diffusion due to dissociation of water which causes hydrogen induced cracking, arc instability, porosity in welds due to infused gases, loss of alloying elements leading to degradation of mechanical properties and microstructure of weld metal, increased hardness of HAZ and associated cracking. Since its inception during world war I, number of researchers have worked on finding the way out to control the harmful effects due to problems encountered during underwater welding. This paper discusses the experimental works conducted for detailing the problems encountered and the solutions suggested by researchers to overcome them. Controlling the input parameters during welding, addition of alloying elements into the electrode coating and multipass welding are the suggested solutions for obtaining desired characteristics in the welds.

MHD instability at the cathode spot as the origin of the vortex formation in high-intensity plasma arcs

Magnetohydrodynamic instability in a high-intensity arc, similar to typical arcs in DC electric arc furnaces, is simulated using an induction based model under 2D axisymmetric conditions. Time-averaged results show a good agreement with steady-state calculated results expected for a stable arc. The transient results declare that z-pinch close to the cathode, occurring due to the high electrical current density, is responsible for arc instability in this region. The unstable behavior of the arc can be evaluated in a periodic procedure. Moreover, correlations between the fluctuations in total voltage drop curve and the arc shape are investigated: when the arc is in form of column (or bell) the total voltage drop is on a minimum peak; if there is an irregular expansion of the arc in form of arms, the total voltage drop shows a maximum peak.

The Onset of a Substorm and the Mating Instability

AbstractThe paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 RE. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high‐beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high‐resolution videos of two substorm onsets. We conclude that they may be low‐entropy content bubbles with a balanced field‐aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non‐MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high‐beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos

Effect of electrode wire surface integrity and surface oxides on the stability of gas metal arc welding

ABSTRACT ER70S-6 gas metal arc welding (GMAW) wire electrodes are widely used in various market segments. Stability of GMAW process is vital for obtaining desired welding performance. The random variations in voltage transients can represent welding consumable quality. In the present study, wire electrodes from three sources were analysed in standard synergic and pulse synergic GMAW using digital storage oscilloscope (DSO) and high-speed camera setup. The recorded voltage data were analysed using time domain and probability density distribution (PDD) analysis. High speed camera images were used to understand abnormal voltage variations observed in voltage oscillograms and PDD patterns. Results confirmed the presence of signatures of wire quality in the variations of analysed voltage transients along with high speed camera images. Microscopic examination confirmed damaged wire surface and presence of remnant oxide layer post manufacturing on the wire surface/sub surface as the main reasons for poor product performance and arc instability. The method adopted in this work can help end users in selecting appropriate welding wire to enhance productivity and can also help wire manufacturers to improve their product quality.

Improving arc stability during wire arc additive manufacturing of thin-walled titanium components

Fabrication of thin-walled titanium structures by a gas metal arc based wire arc additive manufacturing process is considered challenging due to arc instability. This instability is primarily caused by the cathode spot oscillations. In this work, attempts were made to improve the cathode spot stability by using a surface oxidised titanium feedstock. The deposition behaviour was analysed by a high-speed camera analysis under straight and reverse polarities. Current-voltage waveforms were suitably modified to achieve a stable short-circuiting transfer in bare and surface oxidised conditions. The surface oxidised wire was found to provide a stable arcing and reduction in arc wandering. Based on the correlation between current-voltage waveforms and metal transfer characteristics, optimum deposition procedures were identified to manufacture thin-walled titanium components.

Improvement of Weld Joint Strength by Applying Random Vibrations Along With External Magnetic Field

Welding is a metal joining process induces high residual stresses. These are strongly influencing the mechanical properties of weldment. In earlier days heat treatment and shot peening techniques were used to relieve these stresses. Due to the time consuming of these processes, in this research work we have applied random vibrations to relive the residual stresses to improve mechanical properties. Along with that, welding arc may get affected by magnetic field during welding. This leads to arc instability which is responsible for welding defects like lack of fusion, porosity. These reduce the quality and strength of weld. To overcome this, arc spattering with external magnetic field need to be reduced. In this research work, a setup has been designed for vibration assisted welding along with external magnetic field set up to improve the mechanical properties of Mild Steel weld joints by means of hardness and ultimate tensile strength. Welding had been performed with and without these setups. After performing welding work pieces have been tested both welding conditions and results have been compared.

Switching Behavior of a Gas-Filled Model DC-Contactor Under Different Conditions

The switching behavior of gas-filled contactors was investigated using a model switch. High-speed imaging and spectroscopy provided information regarding the characteristics of the arc plasma. The impact of external magnetic fields, fill gas, fill pressure, and current amplitude was investigated. The most important factor, influencing the switching behavior for currents below 100 A, is provided by external magnetic fields, which increase arc voltage and arc instability considerably. The observed changes of the side-on profiles of the characteristic emission lines indicate changes in particle densities and arc temperature.

Conception, implementation and evaluation of induction wire heating system applied to hot wire GTAW (IHW-GTAW)

A notable advantage of the GTAW process with the use of a hot wire is the higher deposition rate, which can be twice as high as that achieved with a cold wire for the same welding current. In the conventional procedure, the wire is heated through the passage of current to create heat by the Joule Effect. However, due to the interaction of the magnetic fields generated by the currents which circulate through the wire and the arc, the deflection of the arc (by magnetic blow) occurs. This paper describes the conception, construction and evaluation of a wire heating system based on electromagnetic induction, aimed at minimizing or even eliminating the problems inherent to the conventional heating technique of passing a current. The induction heating method was applied in the hot wire GTAW process (resulting in the IHW-GTAW technique), using different values for the welding current (100, 150 and 200 A) and coil currents between 20 and 120 A. For the same welding current, a gain of up to 220 % could be achieved in the deposition rate by preheating the wire with the IHW-GTAW technique, when compared with the rate achieved with the cold wire (CW-GTAW) method, showing a high heating efficiency. No magnetic blow or arc instability was observed with the induction heating method. In addition, because it can operate without the need for permanent contact of the wire with the molten pool (as the secondary arcing observed with the conventional heating method does not occur), other applications are envisaged, such as GTAW with tangential wire feeding, GTAW with dynamic feeding, hot wire techniques using laser and GMAW processes, as well as brazing.

Onset Mechanism of M6.5 Solar Flare Observed in Active Region 12371

Abstract We studied a flare onset process in terms of stability of a three-dimensional (3D) magnetic field in active region 12371 producing an eruptive M6.5 flare in 2015 June 22. In order to reveal the 3D magnetic structure, we first extrapolated the 3D coronal magnetic fields based on time series of the photospheric vector magnetic fields under a nonlinear force-free field (NLFFF) approximation. The NLFFFs nicely reproduced the observed sigmoidal structure which is widely considered to be preeruptive magnetic configuration. In particular, we found that the sigmoid is composed of two branches of sheared arcade loops. On the basis of the NLFFFs, we investigated the sheared arcade loops to explore the onset process of the eruptive flare using three representative magnetohydrodynamic instabilities: the kink, torus, and double arc instabilities (DAI). The DAI, recently proposed by Ishiguro &amp; Kusano, is a double arc loop that can be more easily destabilized than a torus loop. Consequently, the NLFFFs are found to be quite stable against the kink and torus instabilities. However, the sheared arcade loops formed prior to the flare possibly become unstable against the DAI. As a possible scenario for the onset process of the M6.5 flare, we suggest a three-step process: (1) double arc loops are formed by the sheared arcade loops through the tether-cutting reconnection during an early phase of the flare, (2) the DAI contributes to the expansion of destabilized double arc loops, and (3) finally, the torus instability makes the full eruption.

Influence of Current Feeding Position of Duplex Current Feeding MIG Welding on Droplet Heat Quantity.

Pure argon metal inert gas (MIG) welding is expected to offer the possibility to obtain high toughness weld joints. However, due to its arc instability and low wettability, it is difficult to apply pure argon MIG to a practical welding structure. In order to solve these problems, an improved MIG welding process with a duplex current feeding (DCF-MIG) mechanism was developed. In the DCF-MIG process, the welding current and the wire feeding speed are independently controlled by an additionally feeding secondary current from a secondary power source. Thereby, DCF-MIG can supply a large current compared to conventional MIG under the same deposition rate. In this study, to consider the influence of the secondary current feeding position of DCF-MIG on droplet heat quantity, droplet heat quantity was measured by calorimetry. As a result, the droplet heat quantity was found to be increased significantly with the increase of the distance between the primary current feeding point and secondary current feeding point. The increase of the droplet heat quantity in the DCF-MIG process had a strong effect on improving bead shape and penetration. The droplet heat quantity with the effective current value of DCF-MIG was derived from the simplified calculation and the results roughly agreed with the experimental data.