Valerian Nemchinsky

Dross formation and heat transfer during plasma arc cutting

Plasma arc cutting can be characterized in terms of two distinct speeds. At cutting speeds above , the plasma jet does not cut through metal plate. At speeds below , the molten metal from the kerf sticks to the bottom of the plate, forming the so-called dross. In the first part of this work, both speeds, and , have been measured in a wide range of cutting parameters (currents, metal thicknesses and nozzle orifice diameters) for oxygen plasma arc cutting of steel. In the second part of the work, models by which to calculate and are presented. Comparison of calculated and measured values of allowed us to obtain the efficiency with which the arc power is consumed by the cutting process. It is shown that the efficiency rises as the cutting speed increases. It is suggested that the speed separating dross-producing and dross-free modes of cutting corresponds to a specific value of the Weber number. Calculations performed according to this hypothesis agree well with our measurements of .

Plasma flow in a nozzle during plasma arc cutting

A simple model describing the plasma temperature, pressure and velocity distributions inside the nozzle during plasma arc cutting is developed. Temperature dependences of plasma properties are considered. Predicted and measured values of the plasma pressure inside the arc chamber are compared to validate the model. Calculations demonstrated that a substantial portion of the power dissipated inside the nozzle is radiated; the rest heats the plasma jet. The proportion of the power lost due to radiation increases with the arc current, length of the nozzle and gas-flow rate. The double-arcing phenomenon is hypothesized to result from the electrical breakdown of the gas at the nozzles exit. Calculations of the electrical field at the nozzles exit support this hypothesis.

The effect of the type of plasma gas on current constriction at the molten tip of an arc electrode

During welding, the Lorentz (electromagnetic) force is the major force that detaches the molten metal droplet from the electrode - anode. The magnitude of this force is determined by the current distribution within the droplet. Experiments show that the Lorentz force very much depends on the type of plasma gas used. No explanation of this very important fact exists at the present time. An explanation based on the influence of the negative voltage - current characteristic of the anode layer on the current distribution near the anode is proposed. The reason for the falling shape of the V - I curve of the anode layer is discussed in detail. It is shown qualitatively and by calculations that the negative voltage - current characteristic leads to the current constriction at the anode tip. The steeper the slope of the V - I curve the more the current constriction is pronounced. According to calculations, the Lorentz force is several times higher when argon (slowly falling V - I curve) is used as a plasma gas compared to the case when the plasma gas is helium (steeply falling V - I curve). An explanation of the small Lorentz force for an arc in a molecular gas is proposed.

Heat transfer in a liquid droplet hanging at the tip of an electrode during arc welding

The motion of melted metal in a droplet hanging at the tip of an arc electrode during arc welding is considered. The motion is induced by a surface tension gradient due to the non-uniformly heated surface of the droplet. It is shown that the melt flow is confined within a narrow boundary layer. The thickness of this layer and the melt velocity within it are estimated. The influence of the metal motion on heat transfer in the droplet is considered. A simple formula for effective thermal conductivity, which takes into account thermocapillary convection, is obtained. Estimates show that, for conditions typical for arc welding, the effective coefficient of thermal conduction exceeds the normal value by approximately tenfold. Calculated heat fluxes agree with those obtained from the observed electrode melting rates.

Erosion of Thermionic Cathodes in Welding and Plasma Arc Cutting Systems

Thermionic arc cathodes are the cathodes where thermionic emission is the main electron emission mechanism. They are used in welding (Tungsten pure or doped with rare earth oxides) and plasma arc cutting (Tungsten for cutting with inert gas or Hafnium for cutting with Oxygen plasma). There are two different sources of erosion: constant current (CC) erosion and erosion during arc initiation and termination (cycling erosion). Available experimental data for both types of cathode and both types of erosion (CC and cycling) are presented and discussed. For quite some time, it has been clear that CC erosion is due to cathode evaporation. It has been shown that as almost all the evaporated atoms return back to the cathode, the net erosion rate is much lower than the evaporation rate. The existing model allows one to calculate the ratio of these two (escape factor). It is in the range 10-2-10-3. The important role of cathode geometry and plasma flow pattern in the cathode proximity is discussed. The nature of cathode erosion during arc start and arc termination is much less understood in spite the fact that the corresponding erosion could be very important and even dominate for multiple cycles. Different processes that lead to this type of erosion are considered.

Advances in Plasma Arc Cutting Technology: The Experimental Part of an Integrated Approach

The experimental part of an integrated approach to design and optimization of plasma arc cutting devices will be presented; in particular results obtained through diagnostics based on high speed imaging and Schlieren photography and some evidences obtained through experimental procedures. High speed imaging enabled to investigate start-up transition phenomena in both pilot arc and transferred arc mode, anode attachment behaviour during piercing and cutting phases, cathode attachment behaviour during start-up transient in PAC torches with both retract and high frequency pulse pilot arc ignition. Schlieren photography has been used to better understand the interaction between the plasma discharge and the kerf front. The behaviour of hafnium cathodes at high current levels at the beginning of their service life was experimentally investigated, with the final aim of characterizing phenomena that take place during those initial piercing and cutting phases and optimizing the initial shape of the surface of the emissive insert.

Dissociation reactive thermal conductivity in a two-temperature plasma

Dissociation reactive thermal conductivity (DRTC) is the transfer of dissociation energy in plasma with temperature and, therefore, composition gradient. In the existing theories, calculation of the DRTC coefficient consists of the calculation of diffusion coefficients and plasma composition. The heat flux is then calculated by assigning to every molecule the dissociation energy and by the multiplication of the molecule flux density by this energy.This approach, correct for the LTE plasma, is not adequate for the non-equilibrium two-temperature plasma: it does not allow one to separate the total DRTC coefficient into two components responsible for the heat transfer by electrons and heavy particles (atoms, molecules, ions). Only at LTE, during atom reassociation, do the electrons recuperate the energy they spent during the molecule dissociation. Therefore, in order to separate these two components of DRTC, the kinetics of the dissociation?reassociation processes should be considered. This is done in this paper for nitrogen plasma at atmospheric pressure. Fe, the electron fraction of the total DRTC coefficient, was calculated for Te (electron temperature) in the range 0.4?1.0?eV and Th (heavy particles temperature) from 0.2?eV to Te. It is shown that Fe depends mostly on the electron temperature and increases with increasing electron temperature.

Heat transfer in an electrode during arc welding with a consumable electrode

The heat balance of the solid part of the electrode during arc welding with a consumable electrode is considered. Solution of the heat conduction equation makes it possible to obtain a relation between the droplet temperature, the wire feed rate and the electrode extension. Comparison of the droplet temperature obtained in this way with the measured droplet temperature showed good agreement. Analysis of experimental data has shown that the droplet heat content primarily depends on the wire feed rate.

Electrode melting during arc welding with pulsed current

The electrode-melting rate and molten-droplet parameters during arc welding with pulsed current are considered. The discreteness of the droplet detachment is taken into account. Calculations agree well with the experimental fact that there is a range of operational parameters within which one droplet is transferred per current pulse. Variations of the droplet parameters and the melting rate within this range are calculated. It is shown that the high-frequency limit of this one-droplet-per-pulse range is characterized by the least thermal load on the weld, the least amount of fumes and the highest melting rate.

The rate of melting of the electrode during arc welding. The influence of discrete removal of the melt

Molten droplets are removed periodically from the electrodes tip during arc welding. During the process the thickness of the molten layer that separates the heat source (plasma) from the solid wire to be melted also changes periodically. Correspondingly, the instantaneous rate of melting experiences periodic variations. It is shown that the periodicity of removal of the melt affects the average rate of melting. The effect is small for small P?clet numbers (, where v is the rate of melting, L is the size of the detaching droplets and is the thermal diffusivity), but it is very pronounced for high P?clet numbers. Comparison of the calculated rate of melting with the rate measured during arc welding shows that good agreement has been obtained taking convection of the melt into account.