Arc Instability Research Articles

Development of novel MIG welding process with duplex current feeding

MIG welding under pure argon shielding gas atmosphere (pure argon MIGW) is suitable to obtain a high-strength and high toughness welded joint. However, pure argon MIGW is difficult to be applied practically to welding structure because of arc instability as well as low wettability and shallow penetration of weld bead. For solving the above problems, a novel MIG welding process with a duplex current feeding (DCF-MIGW) has been developed. The DCF-MIGW enables to control the wire feeding speed and the welding current independently by additionally feeding secondary current near the wire tip. In this paper, the basic characteristics of DCF-MIGW were clarified. As a result, under the same wire feeding speed, the DCF-MIGW was able to feed larger welding current than that of the conventional MIG welding. Accordingly, the quantity of heat of droplets in the DCF-MIGW was found to increase with the increase in the secondary current, leading to improved wettability and deeper penetration. The increased welding current also contributed to the arc stability. The multi-layer welding using the DCF-MIGW enabled to avoid the weld defects to prove the practical effectiveness of the DCF-MIGW.

Arc length control for efficiency enhancement of energy usage in plasma dry reforming process

Arc plasma has been applied in diverse reactions because it can provide the mechanism for highly thermal activations. However, the efficiency of conversion from electrical energy to thermal energy for the reactant is not sufficiently high, resulting in challenges in practical applications of the arc in various chemical processes. In this study, a method is introduced to enhance the energy usage efficiency for the reactant. The enhancement is based on the control of the arc length elongation. An increase in the arc length results in an enhanced heat transfer from the arc column to the reactant, followed by the reduction of the conductive heat loss to the reactor body. The existing DC arc technologies cannot easily extend the arc length because of the arc instability. However, in this study, a novel AC arc technology was adopted that can support a stable arc with a sufficiently elongated length. Correspondingly, a comparative study on different arc lengths was conducted for the dry reforming (as a model reaction). The increased arc length results in an enhanced heat transfer to the gas or a higher gas temperature. This is followed by the enhancement of the conversions of the reactants for the same supplied electric power. The results provide a firm physical basis for further optimizations of the arc process for chemical reactions.

Electric arc in a plasma spray torch under modulated current

Plasma spraying of liquid feedstock is a relevant process for the deposition of finely structured ceramic coatings. The control of arc instabilities makes obtaining the desired material properties challenging; hence efforts are devoted to developing segmented direct current torches that intend to fix the length of the arc. An alternative method involves promoting the arc oscillations and associating a pulsed liquid injection with the torch. This paper presents a study of the influence of the amplitude modulation of a direct current on the arc dynamics confined in a plasma spray torch. The dependence of the electrical features of the torch on the current amplitude modulation is presented. Plasma speed and temperature are also measured.Time-resolved imaging of the arc attachment permits us to correlate its motion, particularly the arc reattachment on the anode surface, with arc current and voltage. Measurements are compared with a simplified model providing the time-dependence of the plasma properties. It is shown that the current modulation favors the arc restrike by modulating the arc radius, but also that the electric field strength of the arc column should be modified during current variations.

Arc-to-Glow Transition Approach for Practical Use in DC Low-Power, Low-Voltage Electric Grids

This paper presents and discusses results of analysis from investigations of arc-to-glow transformation phenomenon at contact opening, under dc low-energy ( $\leq 10$ J) and low-voltage ( $\leq 250$ V) inductive loads. Dependence of the duration of arcing and glowing on current magnitude, voltage magnitude, properties of the contact material, gas quenching medium velocity and its pressure, as well as contact opening velocity and contact gap are investigated. The transition phenomenon of arc-to-glow is analyzed by means of fast photography and emission spectroscopy. Also discussed is the theoretical evaluation of conditions of arc instability. From the results, possible procedures are formulated to control the arc-to-glow transformation for practical use in dc low-voltage and low-power electrical grids.

Study on new GMA welding process with duplex current feeding

ABSTRACTGas metal arc welding (GMAW) under pure argon shielding gas atmosphere (pure argon-GMAW) is suitable to obtain a high-strength and high toughness welded joint. However, it is difficult that pure argon-GMA welding is applied practically welding structure because of arc instability. In order to perform stable pure argon-GMA welding, duplex current feeding GMAW (DCF-GMAW) has been developed. The DCF-GMAW consists of primary GMA welding current and secondary welding current by constant-current power resource. DFC-GMAW can feed larger current near wire tip. This effect makes that weld penetration depth is deeper, weld bead shape is improved using DCF-GMAW.

Arc instabilities during split anode calorimetry with the TIG welding process

Split anode calorimetry investigations of tungsten inert gas (TIG) arcs were conducted using a large-diameter shielding gas nozzle. Some conditions displayed arc symmetry whilst others proved distinctly asymmetric. The variation of welding current and electrode tip to workpiece distance (ETWD) was studied. Decreasing the ETWD was found to increase the current density towards the arc axis but similar to a previous study of the authors, Gaussian distributions were not observed. The gas nozzle was designed to produce laminar gas flow and sound shielding behaviour; however, anode surface oxidation was found after welding, presumably caused by shielding gas contaminated with oxygen through the welding sequence. Therefore, axial arc symmetry was influenced by random effects. The conditions and reasons for the observed phenomena are explored within this paper.

Experimental Investigation on Transition Characteristics of Arcing Stages Under Transverse Magnetic Field

Transverse magnetic field (TMF) contacts can prevent local overheat by creating a TMF and driving the vacuum arc to rotate around the contact periphery, thus improving the breaking performance of vacuum circuit breaker. The arcing behavior and the transition between arcing stages have great influence on the heat distribution on the contact surfaces, and they will finally affect the interruption at current zero. In this paper, a demountable vacuum chamber was used to investigate the arc behavior between two kinds of TMF contacts. A digital high-speed camera was used to record the arcing process. By combining the arcing appearance with arc voltage waveform, the arcing process was divided into four stages: initial expansion stage, arc instability stage, arc motion stage, and diffuse stage. The voltage characteristic and arc appearance of each stage were summarized and analyzed, and the transition condition between different stages was acquired. Moreover, the influence of current level and contact parameters on the duration and transition moment of each stage was also studied. The experimental results indicate that the initial stage is influenced by the arc ignition mode, whereas the arc motion and diffuse stage are significantly affected by the current level, contact type, and diameters. With the increase of peak current level, the duration of arc motion stage will increase, while the transition moment from arc motion to diffuse stage will move toward current zero.

Instant detection of porosity in gas metal arc welding by using probability density distribution and control chart

A novel porosity detection technique from the voltage and current transients is introduced in this paper. An online weld monitoring that detects the porosity at an earlier stage is much demanding in the industry due to their adverse effects on structural integrity. In this research work, control chart and probability density distribution have been employed as tools to detect arc instability and weld porosity. The results showed that the pattern of probability density distribution changes for the defect and defect-free welds significantly. The mean and standard deviation control charts plotted with voltage clearly distinguished the quality of the weld based on sample points spread within or outside the control limits. For minute internal porosities, the sample points at the corresponding region in the standard deviation control chart were outside the limits whereas it is well within the control limits in the mean control chart. Inspector can predict the presence and near location of porosity using these tools by simple mathematical calculations easily and instantly. The results proved that the developed approach is successful and promising for the weld inspection.

Influence of Axial Magnetic Field on Cathode Plasma Jets in High-Current Vaccum Arc

The cathode plasma jets in vacuum arc and the effect of magnetic field on them have been studied for several years and these studies are helpful to understand the arc characteristics and improve the controlling effect of magnetic field on arc plasma. In this paper, the experiments of the cup-shaped axial magnetic field (AMF) contacts were conducted in which a Helmholtz coil was introduced to generate different external imposed AMFs. It was found that the appearance of arc plasma was changed by variation of the external imposed AMF. If the composite AMF in arc column increased gradually, the obvious individual cathode plasma jets were detected, the arc constriction decreased, and the arc stability increased. Meanwhile, the inclination of cathode plasma jets was observed and the inclination direction was found to be consistent with the magnetic field vector direction which is closely related to the composite AMF. By variation of the external imposed AMF and its direction, the inclination direction of cathode plasma jets also changed. The criterion to detect obvious cathode jets in arc column is obtained that at the contact peripheral where R> 80.5% R0 (R0 is the contact radius), the AMF strength is no less than 4.5 mT/kA. It was also found that if the composite AMF in interelectrode gap points to different directions at a certain moment, it may lead to the transfer of arc column from the center to the edge of the interelectrode region and the arc instability until when the composite AMF in the interelectrode gap points to the same direction. It can be concluded that the appearance of individual cathode plasma jets can be manipulated by the variation of AMF, which is helpful to study the interaction between arc plasma and electrodes, especially the anode, and improve the interruption performance at higher current.

Dependence of the Chopping Current Level of a Vacuum Interrupter on Parallel Capacitance

The distribution of chopping currents of a vacuum circuit breaker using CuCr25 contact material was measured in a laboratory circuit consisting of a three-phase 20-kV cable system and a dry-type distribution transformer rated 900 kVA. The cable length between the upstream feeding transformer and the circuit breaker was several hundreds of meters. An inductive load connected to the distribution transformer provided a 50-Hz current of 10 A rms. Under these conditions, the mean chopping current level was between 3.2 and 3.5 A and showed no significant dependence on the arcing time, which was varied between 2 and 8 ms. After addition of a surge capacitor of 130 nF between the transformer terminals and ground, which very much reduces the overvoltage during switching, the chopping current level increased to 5.6 to 7.7 A in dependence of the arcing time. In order to explain this, the interaction of the arc with the electric circuit in particular with the cables on both sides of the breaker has to be evaluated. If the cables are long, travelling waves need to be considered as well as all return current paths in a three-phase circuit. The coincidence of excited network oscillations with random arc instabilities may result in either momentary amplification or damping of such oscillations. An additionally installed capacitor serves as a source sustaining a momentary current reduction after an arc instability and allows the excitation of a subsequent arc instability leading to an increase of the chopping current with longer arcing times. With low capacitance on the load side, there is no interaction and therefore no influence from the arcing time.

Double Arc Instability in the Solar Corona

Abstract The stability of the magnetic field in the solar corona is important for understanding the causes of solar eruptions. Although various scenarios have been suggested to date, the tether-cutting reconnection scenario proposed by Moore et al. is one of the widely accepted models to explain the onset process of solar eruptions. Although the tether-cutting reconnection scenario proposes that the sigmoidal field formed by internal reconnection is the magnetic field in the pre-eruptive state, the stability of the sigmoidal field has not yet been investigated quantitatively. In this paper, in order to elucidate the stability problem of the pre-eruptive state, we developed a simple numerical analysis in which the sigmoidal field is modeled by a double arc electric current loop and its stability is analyzed. As a result, we found that the double arc loop is more easily destabilized than the axisymmetric torus, and it becomes unstable even if the external field does not decay with altitude, which is in contrast to the axisymmetric torus instability. This suggests that tether-cutting reconnection may well work as the onset mechanism of solar eruptions, and if so, the critical condition for eruption under a certain geometry may be determined by a new type of instability rather than by the torus instability. Based on them, we propose a new type of instability called double arc instability (DAI). We discuss the critical conditions for DAI and derive a new parameter κ, defined as the product of the magnetic twist and the normalized flux of the tether-cutting reconnection.

Pulsed arc plasma jet synchronized with drop-on-demand dispenser

This work concerns with the liquid injection in arc plasma spraying for the development of finely structured ceramics coatings. Nanostructured coatings can be now achieved with nanopowders dispersed in a liquid (SPS: Suspension Plasma Spraying) or with a salt dissolved into a liquid (SPPS: Solution Precursor Plasma Spraying) injected into the plasma jet. Controlling electric arc instabilities confined in non-transferred arc plasma torch is therefore a key issue to get reproducible coating properties. Adjustment of parameters with a mono-cathode arc plasma allows a new resonance mode called “Mosquito”. A pulsed arc plasma producing a periodic regular voltage signal with modulation of enthalpy is obtained. The basic idea is to synchronize the injection system with the arc to introduce the liquid material in each plasma oscillation in the same conditions, in order to control the plasma treatment of the material in-fly. A custom-developed pulsed arc plasma torch is used with a drop-on-demand dispenser triggered by the arc voltage. A delay is added to adjust the droplets emission time and their penetration into the plasma gusts. Indeed, the treatment of droplets is also shown to be dependent on this injection delay. A TiO2 suspension and an aqueous solution of aluminium nitrate were optimized to get ejectable inks forming individual droplets. The feasibility of the process was demonstrated for SPS and SPPS techniques. Coatings from the suspension and the solution were achieved. First synchronized sprayings show a good penetration of the droplets into the plasma. Coatings show a fine structure of cauliflowers shapes. The synchronization of the ejection allows a control of morphology and a better deposition efficiency. Further investigations will find the optimal operating parameters to show the full potential of this original liquid injection technique.

Electric Arc Fluctuations in DC Plasma Spray Torch

Direct current plasma torches for plasma spraying applications generate electric arc instabilities. The resulting fluctuations of input electrical power hamper a proper control of heat and momentum transfers to materials for coating deposition. This paper gives an overview of major issues about arc instabilities in conventional DC plasma torches. Evidences of arc fluctuations and their consequences on plasma properties and on material treatments are illustrated. Driving forces applied to the arc creating its motion are described and emphasis is put on the restrike mode that depends on the arc reattachment and the boundary layer properties around the arc column. Besides the arc root shown as a key region of instability, the Helmholtz oscillation is also described and accounts for the whole plasma torch domain that can generate pressure fluctuations coupled with voltage ones.

A Study of the Influence of the Surrounding Gas on the Plasma Jet and Coating Quality During Plasma Spraying

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of Hα emission was found to become gradually stronger while Hβ and Hγ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.

Microsecond Crystallization of Amorphous Silicon Films on Glass Substrates by Joule Heating

Large area crystallization of amorphous silicon (a-Si) thin-films on glass substrates is one of the key technologies for manufacturing flat displays on commercial scales. Low-temperature polycrystalline silicon (LTPS) is a crucial active-layer material for polycrystalline silicon thin-film-transistors (poly-Si TFTs) mounted on thermally susceptible glass substrates. LTPS-TFT backplanes are required for fabricating flat panel displays such as active-matrix liquid-crystal-displays and active-matrix organic-light-emitting diodes (AMOLEDs). The LTPS-TFT backplanes are strongly preferred over a-Si TFTs since several devices, AMOLEDs in particular, operate in a current-driven mode. The crystallization methods used for producing LTPS include solid phase crystallization, metal induced crystallization, and excimer laser crystallization (ELC). The ELC method is currently used for crystallizing the a-Si films in mass production lines. Recently, we developed a novel and highly promising crystallization method named Joule-heating induced crystallization (JIC) by which an entire a-Si film could be crystallized on a glass backplane within tens of microseconds. However, arc instability was observed during the crystallization, which is attributed to the dielectric breakdown in the conductor/insulator/a-Si sandwich-structures at high temperatures during electrical pulsing for crystallization. We devised a method for crystallizing the a-Si films while preventing arc generation; in this method an a-Si active layer was pre-patterned into islands followed by the deposition of a gate oxide and gate electrode. An electric pulse was then applied to the gate electrode formed using a Mo layer. The Mo-layer was used as a Joule-heat source for the crystallization of the pre-patterned active islands of the a-Si films located below the Mo-layer. The JIC-processed poly-Si TFTs were fabricated successfully and the proposed method was found to be compatible with the standard processing of coplanar top-gate poly-Si TFTs. In this report, we will discuss some critical issues concerning the arc generation, glass backplane deformation, and phase-transformation phenomena observed during the JIC process on the basis of theoretical investigations as well as in-situ and ex-situ experiments. Finally, we will discuss the procedure for fabricating stable JIC poly-Si TFTs and optimize the process flow and processing conditions such as the power density and pulsing time for fabricating the JIC poly-Si TFTS. Figure 1

Analytical interpretation of arc instabilities in a DC plasma spray torch: the role of pressure

Arc instabilities in a plasma spray torch are investigated experimentally and theoretically thanks to a linear simplified analytical model. The different parameters that determine the useful properties of the plasma jet at the torch exit, such as specific enthalpy and speed, but also pressure inside the torch and time variations of the flow rate are studied. The work is particularly focused on the link between the recorded arc voltage and the pressure in the cathode cavity. A frequency analysis of the recorded voltage and pressure allows the separation of different contributions following their spectral characteristics and highlights a resonance effect due to Helmholtz oscillations; these oscillations are responsible for the large amplitude fluctuations of all the parameters investigated. The influence of heat transfer, friction forces and residence time of the plasma in the nozzle are taken into account, thanks to different characteristics’ times. The volume of the cathode cavity in which the cold gas is stored before entering the arc region appears to be of prime importance for the dynamics of instabilities, particularly for the non-intuitive effect that induces flow-rate fluctuations in spite of the fact that the torch is fed at a constant flow rate.

NUMERICAL SIMULATION OF DYNAMIC BEHAVIOR OF KEYHOLE AND MOLTEN POOL AT K-PAW QUASI STEADY PROCESS

The keyhole plasma arc welding (K-PAW) is widely applied in engineering project now as a high energy beam welding with its advantages of low-cost and easy operation. However, the arc instability may arise and welding defects will be produced in K-PAW due to the high current and strong plasma penetrating force when medium thickness plates are welded, finally weakening the efficiency of K-PAW. Furthermore, it is found that the flow field of liquid metal in the molten pool and the stability of keyhole have a critical influence on welding quality. Therefore, modeling and simulating molten pool, keyhole and flow field in the K-PAW quasi steady process except for arc starting and ending phases are helpful to understand the welding process theory completely and promote its application further. But to date, there is little study on the coupled analysis of molten pool and keyhole in the quasi steady welding process due to the difficulty to make keyhole stable. In this work, based on the principles of fluid dynamics with considering arc pressure, surface tension, electromagnetic force, buoyancy and gravity, a *国家自然科学基金资助项目51205176 收到初稿日期: 2015-10-07,收到修改稿日期: 2015-12-23 作者简介:徐斌,男, 1988年生,博士生 DOI: 10.11900/0412.1961.2015.00518 第804-810页 pp.804-810

二段給電式ミグ溶接プロセスの数値シミュレーション

GMA welding under pure argon shielding gas atmosphere (pure argon-GMA welding) is suitable to obtain a high-strength and high toughness welded joint. However, it is difficult to apply pure argon-GMA welding practically to welding structure because of arc instability. In order to perform stable pure argon-GMA welding, duplex current feeding GMA welding has been developed. The duplex current feeding GMA welding consists of primary GMA welding current by constant-voltage power source and secondary current by constant-current power resource. In previous experimental study, it was found that the temperature of a droplet by duplex current feeding GMA welding was higher than the conventional GMA welding. In this study, mechanism and basic characteristics of duplex current feeding GMA welding are investigated by numerical simulation with a simplified duplex current feeding GMA welding model. As a result, it was found that total welding current was largely increased by conducting the secondary current in addition to the primary GMA welding current under the same arc voltage and wire feed speed. Furthermore, the droplet temperature was also increased due to the increase in total welding current.

Experimental Investigation of High-Current Vacuum Arc Instability Modes Under Transverse Magnetic Field

The behavior of vacuum arc influences the heat distribution on the contacts and the contact melt and erosion, and thus, it greatly affects the breaking capacity after current zero. In this paper, high-current arc instability modes were investigated in a demountable vacuum chamber with a high-speed charge-coupled device video camera. Three kinds of spiral contacts were used with CuCr50 as the contact material. We observed and summarized three types of arc instability modes: 1) multicolumn arc mode; 2) single stationary jet mode; and 3) single moving jet mode. As the peak current increases, the instability mode will transition from multicolumn arc mode to single stationary jet mode, and then to single moving jet mode. We also found the periodical transition between the plasma jet and the relatively diffuse columnar arc during single moving jet mode. It was shown that the occurrence of arc instability modes, especially the single moving jet mode, is usually accompanied by the increase in mean value and high fluctuations on the arc voltage. Besides, during the instability mode, a large number of droplets and macroparticles will be produced in the gap. These will have a great influence on the interruption capacity of the vacuum interrupter.

Heat input analysis of variable polarity arc welding of aluminum

Influence of direct current electrode positive (DCEP) duty ratio in variable polarity (VP) arc welding of aluminum has been investigated in the current work. VP arc adopts alternate current (AC) of DCEP and direct current electrode negative (DCEN) as power source, and the duty ratio indicates the polarity’s durational portion in the current pulse. Experimental results showed proportional relationship between heat input and DCEP duty ratio although there was a fluctuation due to the arc instability increase caused by wearing of the tungsten electrode tip. This observation violates the conventional arc theory of anode heating, and therefore, a new theoretical approach combining the tunneling effect and random walk of cathode spot has been proposed. During DCEP duration, arc is concentrated on thinner oxide cathode spot, which further melts and breaks the oxide layer of the spot. This process is repeated at another cathode spot at a high frequency. Therefore, the higher the DECP duty ratio, the more cathode spots are generated and the longer was the arc concentration time. This in turn resulted in a higher heat input for longer DCEP polarity duration. The proposed mechanism combining the tunneling effect and random walk of cathode spot can not only explain the anomalous results of the current research but also could address the ambiguous result of deeper penetration in thicker oxide in the past research.