Arc Craters Research Articles

Suppression of humping bead in high-speed triple-wire gas indirect arc welding

Triple-wire gas indirect arc welding (TW-GIA) is a new technology with the advantages of low heat input and high deposition rate. However, the humping bead restricts the improvement of welding efficiency. In this article, the high-speed cameras and infrared thermal imaging system were used to characterize the formation mechanism of the TW-GIA humping bead. The physical model was established to discuss the force mechanism of the weld poor. And the influence of process parameters on humping bead was studied. The results show that the increase of the welding angle reduced the arc pressure and droplet impact force in the opposite direction of welding, which was beneficial to eliminate the hump. When the welding height was lower than 3 mm, the excessive arc pressure led to the humping bead with arc crater. When the welding height was greater than 20 mm, the distance between the droplets of side wires was far, forming the humping bead with “double bead” defect. In addition, the flow distance of the liquid metal shortened as the welding current decreased, which could suppress the humping bead. When the ratio of the total wire feeding speed to the welding speed was 10, the critical current was 360 A, and the critical current increased as the ratio increased. The maximum TW-GIA welding speed of bead-on-plate was 4.2 m/min, which was 137% higher than that of high-speed GMAW.

Numerical simulation of the interaction between double cathode spot craters in vacuum arc

In this paper, a numerical simulation on the interaction between the double cathode spot craters in the vacuum arc is carried out. By establishing a double-cathode spot ablation model, the crater development process when two spots coexist is simulated, and the formation mechanism of cathode spot groups is analyzed. The simulation results show that the two cathode spots appearing on the flat electrode would squeeze each other to form a liquid metal ridge, which changed the ablation morphology of the cathode spots. When the double-spot arc craters appear simultaneously, the metal ridge will be squeezed into a straight line, otherwise, when the spots appear one after another, the liquid metal ridge will shift toward the side of the spot that appears first. The relative experimental results are adopted to compare with the simulation results. By comparison, it is found that the morphology of the multi-spot crater in the experiment is in agreement with the simulation results.

Rapid solidification and metastable phase formation during surface modifications of composite Al-Cr cathodes exposed to cathodic arc plasma

A combination of both conventional and advanced high-resolution characterization techniques was applied to study the modified layers on the surface of three composite Al-Cr arc cathodes with identical nominal composition of Al-50 at.% Cr but varying powder grain sizes. The results revealed that the modified layers consist mainly of metastable phases such as Cr solid solution, high temperature cubic Al8Cr5, supersaturated Al solid solution, and icosahedral quasicrystal. The metastable phase formation indicates that high cooling rates were involved during the solidification of molten material produced in the arc craters during cathode spot events. The average cooling rate was estimated to be 106 K/s based on secondary dendrite arm spacing measurements and supporting phase-field based simulations. The formation mechanisms of the modified layers are discussed based on the obtained results and the current literature.

Simulation of arc crater formation and evolution on plasma facing materials

• A 2D axially symmetric arc crater model was developed by COMSOL software. • Formation and evolution of arc craters on PFMs were simulated and analyzed. • Refractory metals W and Mo are more difficult to be damaged by arcs than Cu and Al. Arc erosion on plasma facing materials in tokamak devices is a potential source of impurities and dust in plasma. A two dimensional axially symmetric COMSOL model with heat transfer, fluid dynamics including phase transition and surface tension effect has been used to describe the formation and evolution of arc craters on plasma facing materials. The formation and evolution of arc crater on W cathode is described in detail. The energy flux loading causes melting of central area within a few nanoseconds. Due to the gradients of incident pressure, the melted layer is extruded out, and thus forms the melt jets. The differences of the arc craters on several related materials in tokamaks under the same pressure and energy flux density are also discussed. The crater temperature of W and Mo is much higher than that of Cu and Al. And, the melting volume of refractory metals W and Mo is significantly lower than that of Cu and Al. Refractory metals are more difficult to be damaged by the arcs, and more suitable for plasma facing materials.

Insights into surface modification and erosion of multi-element arc cathodes using a novel multilayer cathode design

Nowadays, multi-element cathodes are frequently employed to grow multi-element thin films and coatings using cathodic arc deposition processes. During cathode erosion, the cathode spot sequentially ignites on the cathode surface and imposes melting–solidification cycles that lead to material intermixing and the formation of a modified layer on the cathode surface. To allow us to study these surface modifications, a 10 μm thick Mo/Al multilayer coating was sputter-deposited onto a standard Ti arc cathode. This cathode was eroded by a dc steered arc discharge for a short duration enabling the observation of single craters formed by type 1 and 2 cathode spots. Furthermore, separated clusters of overlapping craters and a fully eroded surface caused by different stages of erosion were differentiated when scanning the erosion track in the lateral direction. Cross sections of single craters were prepared by focused ion beam techniques while metallographic methods were applied to obtain cross sections of overlapping craters and the modified layer. The layers of the multilayer coating acted as trace markers providing new insights into the material intermixing within craters, the material displacements during crater formation, the plasma pressure acting on the craters, and the temperature gradient (heat-affected zone) below the craters. The observations are discussed within the framework of established arc crater formation models.

ITER-grade tungsten limiters damage under high turbulent heat flux in the T-10 tokamak

The tungsten limiters were tested in the T-10 tokamak during the 2015–2017 experimental campaign. The limiters were made from the ITER-grade WMP “POLEMA” tungsten. Inspection of the tungsten limiters after experimental campaign has revealed their strong destruction due to the combination of the high heat and particle fluxes from the edge plasma toward the limiters surface. The influence of the edge tokamak plasma on tungsten limiters leads to significant cracking of the tungsten. Heat load up to 2 MW m−2 leads to micro-cracks at the grain boundaries. Heat loads exceeding 5 MW m−2 lead to macro-cracks formation. In the areas of highest heat loads, tungsten plates are melted and close to full destruction. In these zones, intensive sparking and arcing were observed. Arc craters have been scattered across the surface, and along the cracks. It is supposed that the nonambipolar flow due to the arcs and sparks can explain the observed overheating and melting of the tungsten surface. Disruptions and runaway electrons beams have driven extreme heat loads of more than 1 GW/m2 causing strong melting of the tungsten on the outer board of the ring limiter.

On the current density at the cathode of a vacuum arc

In view of the revival of activity on vacuum arcs, it is important to analyze the basic experimental data used in theory. Current density is one of the most important parameters. It is shown that the current density obtained by measuring postmortem arc craters could substantially underestimate the real current density.

Hot Cracking and Microstructure of Welding Joint of Magnesium Alloy AZ91D

The microstructure and cracking characteristics of MIG welded magnesium alloy (AZ91D) joint, and the effect of welding speed on cracking susceptibility have been investigated. The result indicates the welded joint consists of primary α-Mg and divorced phases (eutectic α-Mg + eutectic β-Mg17Al12), the latter mainly distributing along the α-Mg grain boundaries. The weld of the magnesium alloy displays a high cracking susceptibility. The cracks are mainly formed in the arc crater at the end of the weld. These cracks propagate along the α-Mg grain boundary, and they belong to the solidification cracking. These solidification cracks are resulted by the joint function of the low melting point liquid film in the weld and the tensile stress suffered by the weld metal during the solidification process. The low melting point liquid film is the internal cause to form the solidification cracks, while the tensile stress is a necessary condition. Limiting the amount of the low melting point eutectic and decreasing the tensile stress of the welding joint are two effective methods to improve the solidification cracking susceptibility of the magnesium alloy weld.

Entire Symmetric Structure Inchworm-Type Piezoelectric Linear Actuator

The feature of the hot cracks of the welding joint of the MIG welded magnesium alloy AZ91D was studied systematically. The result indicates that the weld of the magnesium alloy displays a high cracking susceptibility. The cracks are mainly formed on the centerline of the weld and in the arc crater at the end of the weld. These cracks propagate along the α-Mg grain boundary, and they belong to the solidification cracking. These solidification cracks are resulted by the joint function of the low melting point liquid film in the weld and the tensile stress suffered by the weld metal during the solidification process. The low melting point liquid film is the internal cause to form the solidification cracks, while the tensile stress is a necessary condition. Limiting the amount of the low melting point eutectic and decreasing the tensile stress of the welding joint are two effective methods to improve the solidification cracking susceptibility of the Mg alloy weld.

Technical Study of Exhaust Valve Overlaying Based on Micro-Plasma Arc Welding

The micro-plasma arc welding technology（MPAW） was used to remanufacture the exhaust valve of W615 engine. The ST6 Co alloy powder was selected as overlaying material, the processing parameters of overlaying was optimized considering the geometry demand of deposited layer, and the current downslope method was adopted to weaken the arc crater. The microstructure characteristics and the micro-hardness of the weld were investigated, no defects were found in the deposited layer or at the interface, and the heat-affected zone among joint was not evident. The maximum hardness situated at the top layer being 657.8HV, with a slight decrease towards the bottom layer and fusion zone.

Vacuum breakdown on metal surfaces

The unipolar arc model is described. Experimental proof that unipolar arcing represents a discharge form which easily leads to explosive plasma formation is provided. Using a laser-produced plasma, it has been demonstrated that unipolar arcs ignite and burn on a nanosecond time scale without any external electric field being applied. Similar unipolar arc craters have been observed on the cathode surface of a pulsed vacuum diode with an externally applied field of 0.5 MV/cm. The experimental results show that cathode spots are formed by unipolar arching. The localized buildup of plasma above an electron-emitting spot naturally leads to a pressure gradient and electric field distribution which drives the unipolar arc. The high current density of a unipolar arc provides explosive plasma formation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Surface analysis study of electrical-arc-induced wear

A fundamental study was conducted to understand the changes in electrical- arc-induced wear. 303 stainless steel and a bearing made of 52100 steel were selected for the study. An arc was drawn between the two surfaces and experiments were conducted with external voltages of 10–90 V. The arc crater and mountain volumes, the average disturbance diameters, and the maximum crater depth and mountain heights were measured and correlated with the arcing power. A relatively large amount of energy was received by a small area over a short period of time. It was found to cause considerable melting, corrosion, hardness changes and other phase changes in the heat-affected zone and possibly direct ablation of the material. Lips were formed, which were sheared or fractured during subsequent relative movement of the mating member, leading to abrasion, corrosion, surface fatigue and fretting.

Studies of unipolar arcing in the mini-torus

Unipolar arcs have been investigated in a small tokamak, the Mini-Torus, on probes made of graphite, aluminium, stainless steel, titanium, and molybdenum, exposed to the plasma by time-resolved in situ observations with an endoscope system and a high speed framing camera in conjunction with photoelectric spectral recordings from an optical monochromator. Various types of arcing tracks are observed. Visual and SEM examination of the details of arc craters reveal several interesting features of the structure and motion of unipolar arcings. Special attention has been paid to the “stationary arcs” with fixed arc spots (giant craters), some of which show a rotational structure.

The observation of unipolar arc damage on stainless steel and TiC coatings on stainless steel

Unipolar arcing represents a plasma-surface interaction process leading to surface damage and metal impurity influx in tokamaks. It develops if the sheath potential formed in the plasma-surface contact is high enough to ignite and sustain a micro-arc. A laser-produced plasma was used for a comparison of the arc damage produced on stainless steel surfaces with that on surfaces protected with a coating of TiC a few microns thick. Smooth coatings of superior-quality TiC were produced by the activated reactive evaporation process, which is a plasma-assisted physical vapor deposition process. For a sufficiently high electron temperature in the laser-produced plasma a large number (about 300 000 cm −2) of unipolar arc craters were observed on the stainless steel surface which had been exposed to the expanding laser-produced plasma cloud for a few hundred nanoseconds. In comparison, no similar arc craters were detected on the surfaces protected by the TiC coating which showed minimum damage limited to the laser beam impact area.

Formation time and heating mechanism of arc cathode craters in vacuum

With clean cathodes in UHV it is shown experimentally that (i) the arc craters are formed successively and displaced without spatial interruption, (ii) there are only a few active craters at one instant for currents below the spot-splitting limit (generally 1-2), and (iii) the crater formation is much faster than is compatible with the heat conduction time scale, the formation time amounting to only a few nanoseconds. From these observations and with the measured values of crater radii and formation times the heating mechanism of the craters is estimated. The calculations show that Joule heating is insufficient to explain the short time scale, therefore ion impact heating is concluded to be the dominant process. Also this energy source is effective only by heating thin layers of a well defined thickness (about 0.1 mu m) which are removed immediately after melting. Therefore, the melting front proceeds faster into the interior than by heat conduction. This model explains the high heat conduction losses at the cathode as measured by Daalder (1977), and also the reason for the arc spot movement.

Temperature dependence of the erosion of A1 and TiC by vacuum arcs in a magnetic field

The erosion yields by vacuum arcs of 16.6 A in a magnetic field of 0.05 T have been measured by weight loss as a function of temperature for A1, A1 + 3 % Mg (proposed for INTOR) [1] and TiC which was deposited by chemical vapour deposition (CVD) in a 5 μm thick layer on Mo and stainless steel. For Al the erosion increases steeply at a temperature above 0.8 T m, arc velocity decreases with temperature and the arc craters become larger.

Surface damage by sheath effects and unipolar arcs

Unipolar arcing develops if the sheath potential is high enough. A laser-produced plasma of short duration was used to study the onset and development of arcing on a stainless steel surface. The laser-produced plasma with kT e ∼ - 100 eV expands rapidly from the focal spot on the target surface in the normal and in radial directions. After one laser shot the damage on the polished surface was observed with an optical and a scanning electron microscope. Although no external voltage was applied, about 20 000 unipolar arc craters are observable on the stainless steel surface which was exposed to the radially expanding plasma for the short time of a few hundred nanoseconds. The size of the arc craters decreases with increasing distance from the focal spot. The initial cathode spot is 1 μm in diameter and 3–6 μm deep, acting like a hollow cathode. The arcs obviously contribute to the erosion of wall material. The experimental results also show that some of the eroded material is redeposited on stainless steel surfaces in loosely bound form.

Arcing studies in ISX-B

Results of experimental measurements of arcing in ISX-B are described. Samples of stainless steel were exposed at the plasma edge and arcing currents to the sample were measured. Currents from several amperes up to 100 A were observed. It was possible to trigger arcs during well-behaved discharges by applying a high voltage to the sample. Elemental analysis of surface features of samples has shown that sulfur segregates to surfaces of arc craters in type 304 stainless steel. Detection of sulfur on surface samples may be related to arcing during cleanup processes in tokamaks.

On the Nature and the Motion of Arc Cathode Spots in UHV

AbstractCathode spot types and spot motion of arcs in ultra high vacuum have been investigated with large area cathodes that consisted of two adjacent pieces of Mo and Cu. Arc currents were 20–60 A dc and 8–20 kA pulse (duration about 1 ms). Two spot types occured with different velocities and surface erosion: Type 1 spots are typical for surfaces covered by oxides or thick adsorption layers, whereas clean surfaces show only type 2 spots.During arc‐conditioning both types exist simultaneously in a complex mutual dependence. Type 1 spots react weakly on the cathode material, while type 2 spots burn preferentially on Cu and at the boundary line between Mo and Cu. The motion of type 1 spots is determined by the expanding spot plasma, whereas type 2 spots show a step‐by step motion, determined by explosions in the arc craters. Generally a spontaneous formation of type 2 spots beneath the arc plasma takes place only with contaminated surfaces (probably by a transition from type 1 to type 2 spots). Thus a breakdown between plasma and cathode surface requires the presence of contaminations. The observed effects occur in low current dc‐arcs as well as in high current pulse arcs. They are discussed for different spot models.

Anode crater formation by pulsed arcs

Anode damage due to short−duration (3 ns−1 μs) arcs between a tungsten cathode and copper, gold, palladium, and platinum anodes has been studied. The electrode arrangement was mounted inside the specimen chamber of a scanning electron microscope so that the surfaces to be discharged, the interelectrode spacing, and the resultant damage were readily observable. The effects of arc current and arc duration on crater dimensions were studied. Anode crater diameters were found to vary as the square root of the arc current for fixed arc duration. A weak dependence of anode crater diameter on arc duration was found at constant current for arcs lasting less than ∼100 ns. The depth of cratering was also measured and relationships between the crater diameter and depth were found for platinum and gold anodes. Large differences in arc crater diameters were not found for each of the anode metals, despite the differences in their thermal properties. The transition from anode to cathode arcs with increasing gap could also be observed and was found to occur between 1.3 and 2.1 μ for a plantinum anode.