Arc Spot Research Articles

Evaporation Erosion of Contacts Under Static Arc by Gas Dynamics and Molten Pool Simulation

Two mass erosion mechanisms or mass loss modes for electrodes during the repetitive arcing processes should be responsible for the failure of electromagnetic relays, and there are evaporation erosion and sputter erosion. Of these two, the evaporation erosion is believed to be the main failure mechanism for the electrodes used in low-current relays. Unfortunately, there is still no straight-forward model to predict the characteristics of the evaporation process on the electrodes. In this paper, heat transfer and fluid dynamics in the molten pool in silver contacts are coupled with the gas dynamics in the vicinity of arc spot to determine the evaporation rate due to static arc. The influences of input parameters, such as arc current and material properties, on the evaporation rate are analyzed. Of all those parameters, boiling point and thermal conductivity of the electrode are found to be the most significant influences on the evaporation rate. The transfer direction of mass due to electrode evaporation is discussed on the assumption of zero net material loss, and it is found to be directly related to the current and heat flux from the arc to the electrodes. The method used in this paper to evaluate the evaporation erosion characteristics can serve for strategies to alleviate evaporation erosion for relay electrodes.

Effect of cathode composition and nitrogen pressure on macroparticle generation and type of arc discharge in a DC arc source with Ti–Al compound cathodes

Thin films deposited with unfiltered DC arc plasma from Ti, Ti0.75Al0.25, Ti0.50Al0.50, Ti0.30Al0.70, and Al cathodes were characterized with a scanning electron microscope for quantification of extent of macroparticle incorporation. Depositions were performed in N2 atmosphere in the pressure range from 10−6Torr up to 3·10−2Torr, and the formation of cathode surface nitride contamination was identified from X-ray diffraction analysis. Visual observation and photographic fixation of the arc spot behavior was simultaneously performed. A reduction in macroparticle generation with decreasing Al content and increasing N2 pressure was demonstrated. A correlated transformation of the arc from type 2 to the type 1 was visually detected and found to be a function of N2 pressure and at% of Al in the cathode. For the Ti cathode, no arc transformation was detected. These observations can be explained by a comparatively high electrical resistivity and high melting point of Al rich surface nitrides, promoting an arc transformation and a reduction in macroparticle generation.

The Study of the Thermal Processes in an Electrode Influenced by an Electric Arc

The effect of a moving arc spot on a material of a plasmatron electrode is considered in the present paper. Temperature-dependence of material thermophysical properties is taken into account. Dynamics of temperature and thermal stresses fields are calculated. The characteristic feature of temperatures distribution along radius of an electrode is large gradient of temperatures in a narrow zone near to a surface which can cause large thermostresses and microcracks.

Investigation on Behavior of Macro-Particles in TiN Film by Arc Ion Plating.

Macroparticle contamination deteriorates the qualities and performances of protective coatings by arc ion plating, resulting in a limitation in their applications. In this work, the effects of transverse magnetic field (TMF), pulsed bias, gas pressure, and substrate position on behavior of the macro-particles (MPs) in TiN films are quantitatively investigated. It is demonstrated that the key factor of the deposition process on the MPs behavior is magnetic field, which controls the movement of arc spot significantly. At relatively low magnetic field intensity, the MPs behavior is greatly influenced by the other three process parameters. The sensibilities of the three parameters on MPs behavior are decreased with the increasing magnetic field intensity. At high magnetic field intensity, the MPs distribution keeps almost the same even when the other parameters are varied.

Electrical and plasma flow characteristics of a segmented plasmatron operating with mixture of gases

The electric characteristics of a segmented plasmatron and the results of optical emission spectroscopy of Ar-air, N2, and N2–CO2 are presented. The main working gas forming the plasma stream was fed near the cathode into the arc region and another additional gas was injected into the plasma stream beyond the arc. It is shown that the gas injected into the plasma stream is drawn to the arc area due to arc spot movement and cyclic arc shrinking and expanding due to the power supply pulsation. It was found that when the anode spot moves upstream, the additional gas is retracted into the arc region, changing the operating conditions of the plasmatron. The retraction mechanism depends on the gas type and is different in argon and molecular plasmas. The results of the plasma emission spectroscopy show differences in the electron excitation and rotational temperatures for the plasmas studied and are used to explain the mechanism of functioning of a segmented plasmatron.

On the Gas Heating Mechanism for the Fast Anode Arc Reattachment in a Non-transferred Arc Plasma Torch Operating with Nitrogen Gas in the Restrike Mode

The present work provides a detailed kinetic analysis of the time-resolved dynamics of the gas heating during the arc reattachment in nitrogen gas in order to understand the main processes leading to such a fast reattachment. The model includes gas heating due to the relaxation of the energy stored in the vibrational as well as the electronic modes of the molecules. The results show that the anode arc reattachment is essentiality a threshold process, corresponding to a reduced electric field value of E/N ~ 40 Td for the plasma discharge conditions considered in this work. The arc reattachment is triggered by a vibrational instability whose development requires a time of the order of 100 µs. For E/N < 80–100 Td, most of the electron energy is transferred to gas heating through the mechanism of vibrational–translational relaxation. For larger values of E/N the electronic–translational energy relaxation mechanism produces a further intensification of the gas heating. The sharp increase of the gas heating rate during the last few µs of the vibrational instability give rises to a sudden transition from a diffuse (glow-like) discharge to a constricted arc with a high current density (~107 A/m2). This sudden increase in the current density gives rise to a new anode attachment closer to the cathode (where the voltage drop between the original arc and the anode is the largest) thus causing the decay of the old arc spot.

Initiation of arcing on tungsten surface exposed to steady state He plasmas

Arcing was initiated in steady state helium plasmas by negatively biasing a tungsten electrode to around −500 V. On the tungsten electrode, nanostructures were grown by the plasma irradiation. In this study, we characterized the property of the initiated arcing by measuring the temporal evolutions of the electrode potential and the arc current. The ignition frequency and duration of arcing were presented from the potential measurements; the arc duration was in the range of <10 ms and the distribution altered when changing the biasing voltage. The behavior of arc spots was observed with a fast framing camera. It was shown that the spots split frequently, and sometimes, they run on the surface independently. From the fluctuation of the arc current, the fractal feature of arcing was revealed.

Anode Current Density Distribution Measurements for Different Vacuum Arc Modes Subjected to Axial Magnetic Field

We report our measurements of anode current density distributions for different modes in vacuum arc discharge using a split-anode and a cup-shaped axial magnetic field cathode configuration system. The modes investigated included the diffuse arc and anode spot. The anode surface was divided into four regions: one central region and three symmetrical-disposed peripheral annular-arc regions. Three different-sized central regions were selected with diameters of 12, 18, and 20 mm. The contact material was CuCr25 (25% Cr). The arc current in the tests ranged from 6 to 14 kA (rms) at 50 Hz. The opening velocities were 1.8 and 2.4 m/s, respectively. The currents of the four areas on the anode contact were measured by four Rogowski coils situated outside the vacuum chamber. The experimental results show the current density distribution in different arc modes. For the diffuse arc mode with arc currents of 6 and 8 kA (rms), the average current density in the central region at current peak ranged from 8.5 to 15.9 A/mm $^{2}$ . The anode spot current density ranged from 31.9 to 37.7 A/mm $^{2}$ , estimated from the current density in the eroded region caused by the anode spots covering the entire central region of diameters 12 and 18 mm, at currents of 12 and 14 kA, respectively, regarded as the lower limit current density of the anode spot. Moreover, the dependence of the current density distribution on opening velocity in diffuse arcs was slight. For the anode spots, the current density distribution with higher opening velocity was more concentrated.

Interaction Between Arc Spot Plasma and Steel Surface in Descaling

Cathode spots of low-pressure arc discharge have been used for cleaning oxides on the steel surface. A phenomenon of two-step plasma scanning on the oxidized steel surfaces is observed, where arc spots first move on the entire surface of the sample with strong evaporation and explosion of oxides. Afterward, there is a brief scanning, and a remaining thin oxide layer is removed while a clean surface of the bulk metal appears. It is believed that the two-step plasma scanning is due to the layered structure of oxides, which results in the self-adjusting of cathode spots on the oxidized surface. The corresponding spot plasmas are investigated by a high-speed camera and an oscilloscope. The voltage waveform and plasma spot images exhibit various characteristics during the two-step scanning process. Compared with the case of Ar arc discharge cleaning, the surface hardness of the sample cleaned with N2 arc spot plasma is obviously enhanced, which should be attributed to the formation of Fe4N during descaling.

Helium isotope, C/3He, and Ba‐Nb‐Ti signatures in the northern Lau Basin: Distinguishing arc, back‐arc, and hotspot affinities

AbstractThe northern Lau Basin hosts a complicated pattern of volcanism, including Tofua Arc volcanoes, several back‐arc spreading centers, and individual “rear‐arc” volcanoes not associated with these structures. Elevated 3He/4He ratios in lavas of the NW Lau Spreading Center suggest the influence of a mantle plume, possibly from Samoa. We show that lavas from mid‐ocean ridges, volcanic arcs, and hotspots occupy distinct, nonoverlapping fields in a 3He/4He versus C/3He plot. Applied to the northern Lau Basin, this approach shows that most of Lau back‐arc spreading systems have mid‐ocean ridge 3He/4He‐C/3He characteristics, except the NW Lau spreading center, which has 3He/4He‐C/3He similar to “high 3He” hotspots such as Loihi, Kilauea, and Yellowstone, but with slightly lower C/3He. Niua seamount, on the northern extension of the Tofua Arc, falls squarely in the arc field. All the NE Lau rear‐arc volcanoes, including the recently erupting West Mata, also have arc‐like 3He/4He‐C/3He characteristics. Ba‐Nb‐Ti contents of the lavas, which are more traditional trace element indicators of mantle source enrichment, depletion, and subduction input, likewise indicate arc and hot spot influences in the lavas of the northern Lau Basin, but in a more ambiguous fashion because of a complex prior history. This verifies that 3He/4He‐C/3He systematics are useful for differentiating between mid‐ocean ridge, arc, and hotspot affinities in submarine volcanic systems, that all three of these affinities are expressed in the northern Lau Basin, and provides additional support for the Samoan plume influence in the region.

Experimentally established correlation between ion charge state distributions and kinetic ion energy distributions in a direct current vacuum arc discharge

DC arc plasmas from Al, Ti, Cu, Mo, and W cathodes have been characterized with respect to plasma chemistry and charge-state-resolved ion energy. The evaluated average ionization energies in the plasmas were found to be linearly correlated with the kinetic ion energies. This was further supported by evaluation of previously published data for 42 elements. A comparison of the total ion kinetic energy distribution and the corresponding ion charge state distribution, as defined by the ionization energies of the constituent ions, showed close to equivalent shapes and widths, for all cathodes analyzed. This suggests that the energy provided for ionization and acceleration varies simultaneously during plasma generation in the arc spot. The presented results provide a link between the ionization and acceleration processes, and may provide further insight into the fundamentals of cathode spot evolution and plasma generation.

Arc spot grouping: An entanglement of arc spot cells

In recent experiments, clear transitions in velocity and trail width of an arc spot initiated on nanostructured tungsten were observed on the boundary of the thick and thin nanostructured layer regions. The velocity of arc spot was significantly decreased on the thick nanostructured region. It was suggested that the grouping decreased the velocity of arc spot. In this study, we try to explain the phenomena using a simple random walk model that has properties of directionality and self-avoidance. And grouping feature was added by installing an attractive force between spot cells with dealing with multi-spots. It was revealed that an entanglement of arc spot cells decreased the spot velocity, and spot cells tend to stamp at the same location many times.

Unusual Cathode Erosion Patterns Observed for Steered Arc Sources

A cathodic arc source with a magnetron-type magnetic field was investigated for stability, erosion, and compatibility with a linear macroparticle filter. Here we report about unusual arc spot erosion patterns, which were narrow (~ 2 mm) with periodic pits when operating in argon, and broad (~ 10 mm) with periodic groves and ridges when operating in an argon and oxygen mixtures. These observations can be correlated with the ignition probability for type 2 and type 1 arc spots, respectively.

Modelling and Optimizing of Joint’s Fracture Toughness between A7075-T651 and AZ31B Dissimilar Alloys Welded by GMA Spot Welding Method

Innovative technologies in dissimilar welding for lightweight metals like aluminium and magnesium alloys would achieve weight reduction, high specific strength, fuel efficiency and less environmental pollution in automobiles, aviation and marine industries. The objective of this research was to investigate, model and optimize the fracture toughness (FT) of lap weld joint between A7075-T651 aluminium and AZ31B magnesium alloys welded by gas metal arc (GMA) spot welding method using ER308L stainless steel filler. The FT of the welding joint was calculated from yield strength and absorbed charpy impact energy of the joints using Rolfe-Novak-Barsom correlation and was compared with the FT of AZ31B parent metal. The results showed that, maximum FT up to 93.08% of the FT of AZ31B parent metal can be achieved. The mathematical model had high accuracy to predict the FT and the optimization for FT has been matched with the validation test.

High-current cathode and anode spots in gas discharges at moderate and elevated pressures

Physical properties of the high-current density spots (which are not the hot arc spots) formed on the electrodes due to development of electro-dynamical and ionization instabilities in the near-electrode layers are the subjects of this paper. The influence of these spots on homogeneity of the plasma column between the electrodes due to initiation of the discharge constriction is discussed and illustrated by the proper experimental data. The geometrical and gas-dynamic effects stabilizing the formation of the anode and cathode current spots at higher gas pressures are described.

Simulation of the hot core mode of arc attachment at a thoriated tungsten cathode by an emitter spot model

Recently, a constricted attachment of an atmospheric pressure low-current argon arc in the centre of the flat end face of a thoriated tungsten cathode was observed and spectroscopically analysed. Its diameter of 0.6 mm and its length of the free standing part of 10 mm are the typical dimensions of electrodes for high-intensity discharge lamps. This paper gives a physical interpretation of the axially symmetric arc spot by a simulation of its properties with a cathodic sheath model which takes into account a reduction in the work function above a critical temperature of the cathode surface by a thorium ion current.At first the optical observation and spectroscopic investigations are recapitulated. Then, an overview is given on the essential elements which are needed to simulate the cathodic arc attachment on a hot electrode. A simulation of a central cathode spot with these elements gives results which are far away from the experimental findings if a constant work function φ is used. Therefore, a temperature-dependent work function φ(T) is introduced. This φ(T) transitions from 4.55 to 3 eV above temperatures of the order of 3000 K. With this emitter spot model a constricted arc attachment is obtained by simulation in the centre of the flat end face of the cathode in accordance with experiment. For currents below iarc,max ≈ 15.5 A, two spot solutions with different cathode falls are found. They form a current–voltage–characteristic consisting of two branches which extend from a turning point at iarc,max to lower currents. For iarc > iarc,max, only a diffuse mode of cathodic arc attachment is obtained. It is shown by a comparison with measured data for iarc = 7.5, 10, 12.5 and 15 A that the solution with the lower cathode fall is observed experimentally.

GMA Spot Welding of A7075-T651/AZ31B Dissimilar Alloys Using Stainless Steel Filler

The aim of this research was to investigate the welding joint's fracture toughness and fracture surface morphologies between A7075-T651 aluminum and AZ31B magnesium dissimilar alloys welded using a new technique of gas metal arc spot lap welding with unconventional ER308L-Si stainless steel filler. The fracture toughness of the welding joints was estimated from yield strength and absorbed charpy impact energy of the joints using Rolfe–Novak–Barsom correlation, and was compared with the fracture toughness of AZ31B parent metal. Fracture surface morphologies were investigated by scanning electron microscopy. The results showed that the joint can achieve a maximum fracture toughness of 93.08% of the AZ31B parent metal. Most of the joints failed at AZ31B alloy, while some failed at A7075-T651 alloy and ER308L-Si nugget. Brittle fracture mechanism was observed for all the joints. It was found that the new gas metal arc spot lap welding technique with ER308L-Si filler could offer very good mechanical properties.

Effect of arc behaviour on the temperature fluctuation of carbon electrode in DC arc discharge

Diffuse and multiple arc-anode attachment modes were observed in a DC arc discharge with a carbon electrode. During the arc discharge, the surface temperature of the electrode was successfully measured by two-colour pyrometry combined with a high-speed camera which employs appropriate band-pass filters. The relationship between the arc-anode attachment mode and the temperature fluctuation of electrode surface was investigated. The diffuse arc-anode attachment mode leads to relatively large temperature fluctuation on anode surface due to the rotation of the arc spot. In the case of diffuse mode, the purity of synthesized multi-wall carbon nanotube was deteriorated with temperature fluctuation.

Optical diagnostics of low-pressure quasi-stationary diffuse discharge

This paper is devoted to the investigation of low-pressure high-current impulse diffuse discharge (HCIDD), which is a specific quasi-stationary form of a pulsed magnetron discharge. The high-speed gated camera imaging and optical emission spectroscopy results are reported. Four distinct discharge forms were observed: high-current impulse magnetron discharge, HCIDD, columnar impulse diffuse discharge and cathodic spot arc. The magnetic probes arranged along axial and radial directions were used to study the behaviour of magnetic field fluctuations. The characteristic oscillations of the magnetic flux density occur in HCIDD and columnar diffuse discharge. Their frequencies range from 0.4 to 2 MHz depending on the magnetic field configuration.

Interface microstructure and mechanical properties of arc spot welding Mg–steel dissimilar joint with Cu interlayer

A novel technology was developed for the arc spot welding of AZ31Mg alloy to Q235 steel with Cu as interlayer. The mechanisms of bonding dissimilar materials were investigated using mechanical and metallurgical examinations. Results show that the joining of Mg alloy to steel with Cu involved two bonding mechanisms: weld-brazing by the Cu transition layer at the interface edge and bonding by a micron-scale composite transition layer of Al3Cu4Fe3 and Fe4Cu3 intermetallic phases at the interface center. The additional reaction of Cu increased the reaction temperature and composition ranges at the interface. It also elicited a bridge effect that improved the weldability of Mg alloy and steel by new formed phases.