Arc Plasma Column Research Articles

Development of a novel GTAW process for joining ultra-thin metal sheets

The butt weld of ultra-thin sheet raw material coils in production lines is a crucial challenge with traditional welding processes because of the troubles related to the control of heat input, arc stability, application reliability, and investment costs. To overcome such challenges, a novel GTAW process has been developed for joining ultra-thin metal sheets for the first time. The novel welding torch with a novel orifice is the main discovered point of this process. In this paper, the novelties of the developed technology are clarified by comparing it with the conventional GTAW process. The results show that the novel GTAW process features much distinctiveness as compared to the conventional GTAW process: (i) the arc plasma column is more concentrated; (ii) the heat input is considerably reduced, but the temperature at the arc center is greatly increased; (iii) the metal evaporation and the metal vapor amount attached to the tungsten electrode surface are enormously reduced, and (iv) the corrosion of the tungsten electrode tip is vastly reduced. As a result, the novel GTAW process can successfully perform the butt weld of thin/ultra-thin metal sheets (up to 0.03 mm) with high quality and reliability.

Dominant Heat Transfer Mechanisms in the GTAW Plasma Arc Column

Dominant Heat Transfer Mechanisms in the GTAW Plasma Arc Column

Characterization of Reversed Arc Hydrocarbon Plasma in Material Processing

A comprehensive study of the reversed arc plasma enhanced CVD (RACVD) reactor utilizing an Ar + H2 + CH4 plasma-creating mixture in the pressure range 1–100 Torr with a plasma flow direction opposite to the direction of the arc current was carried out. The reversed arc discharge has rising current–voltage characteristics showing voltage increasing with pressure and hydrogen concentration. The spectrum of the Ar-H2-CH4 plasma column includes CH, C2, and H2 molecular bands, in addition to Hα, Hβ, Hγ, and Hδ lines. The dissociation degree of H2 was estimated from the intensity ratio IHα/IArI of the Hα and ArI 750 nm lines using the optical actinometry method, yielding an average dissociation degree of hydrogen in the arc plasma of 15–20%. The average vibrational and rotational temperatures of CH radicals are Tv = Tr = 3000 K ± 300 K. The dissociation degree of hydrogen in the reversed arc discharge was calculated by the advection–diffusion-reaction model and showed reasonably good agreement both with experimental findings and with LTE calculations. The high concentration of nascent hydrogen and hydrocarbon radicals in the reversed arc plasma and its uniform distribution across the arc column makes it suitable for diamond coatings. The results obtained on the interaction of reversed arc plasma with substrates suspended within the current-carrier arc plasma column were applied to the description of a dusty reversed arc plasma in fluidized bed reactors. It was found that the energy effectiveness of the treatment of nanoparticles in the RACVD fluidized bed reactor exceeds 90%.

Effect of a bidirectional coupling of an LTE arc column to a refractory cathode in atmospheric pressure argon

An appropriate coupling of an arc plasma column in the state of local thermodynamic equilibrium to a refractory cathode necessarily involves the non-equilibrium boundary layer between them. A model has been developed that combines a model of an equilibrium direct current arc plasma in atmospheric pressure argon with the assembly of a cathode made of tungsten and the boundary layer. A bidirectional coupling has been realized that allows us to consider a variable voltage drop across the boundary layer for different positions on the cathode. The results are obtained for arc currents between 10 and 150 A in the cases of both a unidirectional and a bidirectional coupling. The results show differences in the distributions of the temperature and the normal current density on the cathode surface and the radial and axial distributions of the plasma temperature. Comparison with the results of a fully non-equilibrium model of the arc plasma and experimental findings from optical emission spectroscopy shows a fair agreement for currents, where the deviations from equilibrium in the arc column can be ignored. For arc currents beyond 100 A, the arc attachment on the cathode appears in two forms, which differ from each other in the distributions of the temperature and the normal current density on the cathode surface, whereas the values of the total arc voltage are close to each other.

Thermal Plasma of Electric Arc Discharge Between Cu-Cr Composite Electrodes

This work deals with investigations of model plasma source realised as electric arc discharge in gas atmosphere between Cu-Cr composite electrodes. Radial distributions of temperature and electron density in arc plasma column were studied by optical emission spectroscopy. The content of electrode metals' vapours in discharge was calculated on the base of experimentally obtained plasma parameters as initial data. So, in this way the erosion properties of such composition can be determined.

Experimental study and kinetic modeling of methane decomposition in a rotating arc plasma reactor with different cross-sectional areas

Methane decomposition into hydrogen and carbon is analyzed in a plasma reactor, with a rotating arc and different cross-sectional areas for the passing gas. This novel setup helps the arc discharge to sweep a larger fraction of the reactant which could cause a better interaction of methane molecules with plasma phase causing higher conversions. The effects of angular velocity of arc discharge, feed flow rate, and cross-sectional area for the passing gas were investigated on the reactor performance. Methane conversion increased significantly by changing the arc mode from stationary to rotating. Increasing the cross-sectional area for the passing gas causes conversion drop for stationary arc whereas a slight increase in conversion is observed for rotating arc mode. Hydrogen production rate of 100 ml/min with an energy yield of 26.8 g/kWh achieved at a methane flow rate of 150 ml/min. The residence time is estimated to be 0.2–3.9 s in the range of the present study, which is a much longer period compared to the plasma process time. Therefore, it is suggested that the mass transfer rate between the gas and plasma phase is the controlling factor for methane conversion. In this respect, an apparent reaction rate constant is derived by considering methane conversion as that fraction of gas, which is exposed to the active area of the plasma arc column.

Experimental investigation of gliding arc plasma fuel injector for ignition and extinction performance improvement

Flame extinction at high-altitude is one of the serious safety problems for aero-engines. The low pressure and temperature at high-altitude make it difficult to relight after extinction. Plasma-assisted combustion has been widely investigated in aerospace propulsion systems, while the mechanism is still not clear. In this study, a novel gliding arc plasma fuel injector using alternating current discharge is proposed to obtain reliable altitude relight performance and wider lean blow-out limits. The ignition and extinction experiments of gliding arc plasma fuel injector using 5 °C and −30 °C kerosene (RP-3) were conducted. A thin and string-like gliding arc plasma column was observed with repeating phases of breakdown between the fuel atomizer and venturi nozzle, extension with rotating airflow and column quenching. The optical emission spectroscopy of gliding arc plasma in gliding arc plasma fuel injector indicated the significant thermal effect caused by the arc column. Using gliding arc plasma fuel injector, the ignition limit was greatly extended, almost to the lean blow-out limits for the low temperature fuel. Low temperature and low injection pressure lead to much larger droplets and irregular injection cone. Influenced by heating and hydrodynamic effect of gliding arc plasma, the drop size at −30 °C kerosene spray decreases from around 150 μm to 30 μm, which is a main reason for ignition range extension. The equivalence ratios of lean blow-out were extended by up to 48% and 47% using 5 °C and −30 °C kerosene, respectively. Several light hydrocarbons, such as CH4 (1.26 SLM), C2H4 (1.31 SLM) and C2H2 (2.30 SLM) with larger laminar flame propagation velocity, were detected as reforming products. Both heating and reforming effects contribute to the lean blow-out limit extension. The residence time of kerosene sprays in gliding arc plasma fuel injector is the determining factor for conversion capability.

The Effect of the Magnetic Grids and Arc Runner on the Arc Plasma Column in the Contact System of a Molded Case Circuit Breaker

This paper examined the extent of the dependence of the interrupting capability on the shape of the contact system in a molded case circuit breaker (MCCB). Thus, a simple analysis method was suggested for estimating the interrupting capability and used to investigate the magnetic flux density distribution in a contact system caused by a short-circuit current. However, the MCCB produced by a joint-research company turned out to have the poor interrupting capability and a low magnetic flux density distribution for driving arc plasma between two contacts in the contact system. To build up additional interrupting capability without making many changes, a new arc runner was devised to prevent arc plasma from returning to its original position in the contact system. The arc voltage characteristics were tested with a simplified synthetic testing facility using an LC resonant circuit. The result showed that the new MCCB had a much higher arc voltage than the existing MCCB. This implies that investigating the arc voltage is fairly comparable to predicting the arc plasma extinction and the interrupting capability.

Experimental study on electrical property of arc column in plasma arc welding

Electrical property is a key factor to determine performance and characteristic of welding arc. In this study, the electrical property distribution of welding plasma arc was investigated by means of a novel dual active probes method. Experiments were conducted with different positions and polarities of two active probes in plasma arc column, to address the distribution of electrical conductivity in arc column and the effect of detection current direction in both radius and axis. The trend of radial electrical conductivity in different cross section of arc column could be got via calculating the V-I ratio. When the probe tips are symmetric about the center of the arc column, the gradient of the arc electrical conductivity in radial is greater than that in axial. When the probe tips are asymmetric about the center of the arc column in the same arc cross section, the V-I ratio get from the situation that the negative probe far from arc center is larger than that as negative probe closer to arc center. When the different polarity probes moved in axis direction, the negative pole effect and the arc voltage fall both act on the V-I ratio of axis direction, to make it appear contradictory trends. Such a study is significant in helping to develop and optimize novel plasma arc heat source such as cross arc and coupling arc.

Influence of Electrode Shape and Size on Electric Arc Channel and Crater

Electrical arc machining (EAM) is a developing thermal process which benefits from high energy density arc column for machining process. In EDM though, many researches have been conducted to make a better understanding of material removal mechanism, plasma temperature distribution, plasma density etc. using simplified tool and workpiece as pin and plate electrode in single discharge. However, in EAM due to high energy density and long discharge duration, plasma arc column behavior is different from plasma discharge in EDM. It can be further deducted that besides the electrical parameters, it is highly possible that the electrode shape and some other conditions have significant influence on the discharging process and formation of the crater. Therefore, in this paper effects of different electrode shapes and size on plasma channel growth and crater formation was investigated based on time resolved imaging of single arc discharge and crater foot prints. Using an ultra-fast four-channel ICCD camera, changes in plasma channel in presence of different electrode tips and diameters was studied. Different currents were also applied to investigate light emission of plasma column during the discharge. Afterwards, shape of craters in different conditions were compared. Observation of images in definite time steps from the beginning of breakdown to the end of discharge showed that plasma channel diameter changes over the time and along the gap distance. Comparing crater geometry, tool erosion and plasma arc column in different conditions give an insight to efficient design of discharge equipment for precise observation of arc discharge in EAM.

Influence of Gap Air Flushing on Plasma Channel and Crater Geometry in Single Blasting Erosion Arc Discharge

Blasting erosion arc machining (BEAM) is a promising method for machining difficult-to-cut materials. In BEAM, high pressure inner flushing is implemented to increase machining efficiency in terms of material removal. However, effects of flushing on plasma arc evolution and material removal mechanism in BEAM are still unknown. In this research, air was blown to a single arc discharge as an alternative to water based dielectric in BEAM process. Using an ultra-fast four channel ICCD camera, influence of cross directional air jet on plasma channel development and crater formation was studied. Observation showed that plasma arc channel is obviously disturbed in presence of high speed air flushing. In other word, arc plasma column moves along the air flushing direction. Consequently, crater geometry and electrode wear will highly be influenced by air flushing in the gap. High speed air flushing increases molten metal flushed away from molten pool compare to the stationary air medium condition. This indicates that there is a decrease in recast layer on machined surface and as a result, volume of removed material will increase. Moreover, using high speed air flushing in single arc discharge decreases tool electrode wear volume significantly. Finally, based on the experimental results and theoretical analysis, high speed air flushing proves aerodynamic arc disturbing phenomenon during single arc discharge which is a potential evident to hydrodynamic arc breaking mechanism in BEAM.

A Survey of Chemical Nonequilibrium in Argon Arc Plasma

The widespread application of electric arcs is closely related to the continuous research interest over the course of many years. The present survey is concerned with chemical and excitation nonequilibrium in atmospheric pressure argon plasma generated between a sharpened tungsten cathode and a flat copper anode at current levels of 35–200 A. Advanced fully nonequilibrium modelling is applied to simulate the combination of the wall-confined arc plasma column and the open region in front of the anode in a self-consistent manner in order to pay tribute to the tremendous research work that E. Pfender has done. The new modelling results are presented along with experimental and modelling results of the studies of E. Pfender and his group and other works of relevance.

Coupled gas flow-plasma model for a gliding arc: investigations of the back-breakdown phenomenon and its effect on the gliding arc characteristics

We present a 3D and 2D Cartesian quasi-neutral plasma model for a low current argon gliding arc discharge, including strong interactions between the gas flow and arc plasma column. The 3D model is applied only for a short time of 0.2 ms due to its huge computational cost. It mainly serves to verify the reliability of the 2D model. As the results in 2D compare well with those in 3D, they can be used for a better understanding of the gliding arc basic characteristics. More specifically, we investigate the back-breakdown phenomenon induced by an artificially controlled plasma channel, and we discuss its effect on the gliding arc characteristics. The back-breakdown phenomenon, or backward-jump motion of the arc, as observed in the experiments, results in a drop of the gas temperature, as well as in a delay of the arc velocity with respect to the gas flow velocity, allowing more gas to pass through the arc, and thus increasing the efficiency of the gliding arc for gas treatment applications.

Modeling of weld pool phenomena in tungsten inert gas, CO2-laser and hybrid (TIG+CO2-laser) welding

The efficiency of the welding process in terms of weld penetration and weld width is greatly determined by the heat, mass, and charge transfer phenomena in the weld pool. These phenomena, in turn, depend on the thermal and electromagnetic interaction of the heat source used with the metal being welded. The most adequate models of the welding processes should consider the interaction of the phenomena in the heat source, on the base metal surface and inside its volume by a self-consistent way. This paper is devoted to the development of a self-consistent model of weld pool dynamics in tungsten inert gas (TIG), laser and hybrid (laser + TIG) spot welding without a keyhole formation. The proposed model allows simulation of processes taking place in the weld pool and on its surface. The model takes into account free surface deformation, influence of plasma shear stress, thermocapillary Marangoni effect, and Lorentz forces on the weld pool, as well as of the processes in the arc plasma including laser-arc interaction. For this purpose, the model of the weld pool is combined with a model of arc plasma column where the interaction processes between Gaussian beam radiation emitted by a continuous-wave CO2 laser and the argon arc plasma are described. The equations of the model proposed are solved numerically by means of finite element method. The simulation results are compared with real welding experiments performed with steel S-235JR. A good accordance between simulation and experimental results is observed.

Relationships governing the movement of the cathode and anode spots of an electric arc

The main relationships governing the movement of the cathode (anode) spot of an electric arc, in particular, the effect of rotation of the arc spots, which can be visually investigated at low arc currents, are investigated. The reasons for this phenomenon are explained on the basis of the vortex electrodynamic model of the electric arc in which the current-conducting channel in the plasma arc column has the structure of the dynamic cylindrical coaxial condenser. This phenomenon must be taken in the analysis of the processes taking place in the weld pool because the rotation of the channel of the plasma arc column is the reason for the effective rotational displacement of the material of the weld pool.

Effect of electric arc on laser keyhole behavior based on direct observation during low power pulsed laser–arc hybrid welding process

This paper presents an investigation on effect of electric arc on laser keyhole behavior based on direct observation during pulsed laser–arc hybrid welding process. Direct and synchronous observation of laser keyhole outlet and arc plasma is achieved by means of an assistant illuminating laser and corresponding optical modulation. Experimental results show that after laser pulse action the keyhole outlet does not close immediately and the time depends on the welding parameters. Larger arc current will decrease the existing time of keyhole outlet. Extension of arc plasma column into the laser keyhole gives the pressure to the inner keyhole wall, and consequently hinders the keyhole from collapse, resulting in the longer existence of keyhole outlet. The surface tension of the liquid metal around the keyhole outlet increases due to the extension of arc column into the keyhole, which also prevents the keyhole outlet from collapse.

Wire electrical discharge machining applied to high-speed rotating workpieces

In order to evaluate the influence of high-speed rotating workpieces on wire electrical discharge machining (WEDM), single discharge experiments were carried out inside a grinding machine, in a self-designed wire electrical discharge dressing device (WEDD-device). The shape and size of eroded craters, measured on the workpiece/anode, were found to be highly influenced by the applied relative speed. Based on the crater's shape, its radial expansion speed can be calculated and the slip of the plasma arc column can be measured. Additionally, it was found that the volume of eroded craters increases as relative speed is increased, indicating that higher melting efficiencies are achieved for higher relative speeds. Finally, an electro-thermal model is described and simulation results are discussed, which help to better understand the influence of relative speed on erosion.

Characteristic Analysis of Plasma Arc Adjusted by a Magnetic Field

The image collecting devices, which consist of optical lens, neutral density filter, narrow-band interference filter, CCD and the other equipments are set up to collect the images of plasma arc column with and without external transverse alternating magnetic field (ETAMF). On the basis of de-noising and enhancing morphological characteristics to the collected images, the characteristics of plasma arc with and without ETAMF are analyzed comparatively. The results show that the application of ETAMF is capable to get oscillation of plasma arc, and change its location, shape and heat flux density distribution, which can get efficiently control in the width of heated area and flatten heat flux density of workpiece surface.

The Production of Plasma Pulses With Convoluted Arc Columns in Atmospheric Pressure Air

Experimental results for the operation of a device which generates radially propagating pulses of plasma are reported. These are produced from a convoluted arc plasma column formed using a suitably configured magnetic field (B-field). Both quasi-steady-current and alternating-current arc plasmas in atmospheric pressure air have been studied. High-speed photographic records of the convoluted plasma column during the main arcing phase are presented along with video images of propagating plasma clouds formed when the B-field and arc current collapse. Measured time variations of current and arc voltage during the main current phase are correlated with features on the photographic records. Results are presented, showing the effect of different operating conditions upon the propagation rates of the plasma pulses.

Computation of emission characteristics of Ar–Fe arc plasma column during the synthesis of nano particles of Fe-oxides

The article reports the results of the computationally determined radiative emission characteristics of a transferred arc argon plasma source used in plasma-assisted nano particles synthesis of Fe-oxides. A thermodynamic code uses a spectroscopic database to compute internal partition functions and densities of various charged states in argon–iron vapor plasma system during the synthesis of Fe-oxides. The net amount of radiation emitted by the plasma at different axial positions along the column has been monitored by optical emission spectroscopy. It was also used to determine the amount of metal vapors mixing inside the plasma column during the synthesis process. The net emission coefficient estimated along a certain line of sight shows an effective increase even for a small inclusion of Fe vapors (0.11%) inside the column. It has been noticed that the radiations emitted by the plasma inside the reactor chamber might also be one of the factors, controlling the crystalline phase of Fe-oxide particles.