S W Simpson

Metal transfer measurements in gas metal arc welding

The present work reports results obtained using combined laser shadowing and cross correlation techniques to investigate the metal transfer process. Experimental results of the droplet size and velocity of 0.9 mm and 1.2 mm wires from the globular region to the spray transfer region are presented. The results indicate that the droplet size is not continuous in the narrow region of mode transition, and also preferred bands of droplet size exist. Droplet velocity measurements show good agreement with a simple model based only on the electromagnetic pinch force.

Formation of molten droplets at a consumable anode in an electric welding arc

A theoretical dynamic model has been developed to predict the formation of molten droplets on a moving wire electrode in a gas metal welding arc. Calculations have been made of the droplet shape and size as a function of the welding current by accounting for the electromagnetic pinch effect, surface tension, gravitation and momentum transfer due to motion of the solid wire electrode. Our calculations start with an artificial cylindrical liquid column which, for low currents, develops into a droplet which is close to spherical. However, for currents above about 250 A, the magnetic pinch constricts the column such that a smaller elongated droplet is formed.

A semi-empirical model of the fume formation from gas metal arc welding

The control of exposure to welding fume is necessary to meet health and safety obligations. The work reported here examines the fundamentals of welding-fume formation. A physical chemistry model of the metal vapour mechanism for fume formation has been developed for non short-circuiting transfer gas metal arc welding (GMAW). The model includes the important contribution made by direct condensation of metal vapour onto the weldpool and workpiece, in removing a substantial fraction of the fume. The model shows that droplet size and wire feed speed control the fine fume formation rate. The understanding developed so far, indicates that the smaller the detached droplet size, the lower the total fume formation rate. The physics behind this is explained. The model gives an insight into how process modification might be used to control fume at source. Control at source is believed to be the most cost-effective and energy-efficient technique for dealing with welding fume. It is anticipated that the understanding gained from this project will be applied to determine the practical limits for the control of welding fume at its source.

Langmuir probe measurements in a vacuum arc plasma centrifuge

This work presents experimental data on several important parameters related to isotope enrichment and throughput in a vacuum arc plasma centrifuge. Using Langmuir probe measurements in the rotating magnetized plasma column, the axial drift velocity, electron temperature, and ion density were investigated for eight different metallic plasmas: C, Mg, Al, Ni, Cu, Zn, Cd, and Pb. For magnesium, the angular velocity of the plasma column was estimated from the axial velocity measurements using a modified slit technique, and the result was compared to values obtained from probe signals. Cathode erosion measurements were also made and checked against the plasma parameters for consistency. It was found that although the centrifuge operation at high magnetic field does not offer any obvious advantages, higher currents should increase throughput without degrading performance.

Ionisation and recombination rates in argon plasmas

An approximate model for treating multistep ionisation and recombination in inert gas plasmas is presented. The model makes specific use of the typical electronic level structure of inert gases with a large energy gap between the ground state and first excited state, followed by levels which are relatively closely spaced. Formulae for ionisation and recombination rates in argon are derived which can be rapidly evaluated. The method is useful for inert gas plasmas in which radiation effects are secondary in importance or negligible.

Speed of rotation in a vacuum arc centrifuge

Results of measurements made on a vacuum arc centrifuge are reported. The rotational velocity of the plasma column has been deduced by cross correlating the floating potential detected by two Langmuir probes inserted into the plasma. The main result is that the rotational velocity is slightly lower than the Alfven critical velocity, for the four different elements used as cathode material to form the plasma (Mg, Zn, Cd, Pb). A tentative explanation for the results is proposed based on an extension of the global energy balance argument which describes a conventional plasma centrifuge.

Experiments with background gas in a vacuum arc centrifuge

A vacuum arc centrifuge has been operated with an initial filling gas of either argon or hydrogen for pressures ranging from 10/sup -3/ to 10/sup -1/ Pa. The angular velocity /spl omega/ of the plasma has been determined by cross-correlating the signals from potential probes, and the electron temperature T has been deduced from Langmuir probe data. At high gas pressures and early times during the 14 ms plasma lifetime, high-frequency nonuniformities frequently observed in the vacuum discharge disappear, suggesting that the associated instability is suppressed. Under the same conditions, nonuniformities rotating with much lower angular velocities are observed in the plasma. Temperatures are reduced in the presence of the background gas, and the theoretical figure of merit for separation proportional to /spl omega//sup 2//T is increased compared to its value in the vacuum discharge for both argon and hydrogen gas fillings.

Performance of a vacuum arc centrifuge with a nonuniform magnetic field

The effect of a variable magnetic field on behavior in the rotation region of a vacuum arc centrifuge is analyzed using a first order perturbation of the fluid equations describing the plasma. It is found that the plasma contracts and rotates more rapidly as it moves into an increasing magnetic field. Under conditions typical of current devices, the separative performance is predicted to improve significantly.

The effect of collisions on near-steady rotation in a vacuum arc centrifuge

A two-dimensional fluid simulation of the plasma column present in a vacuum arc centrifuge after the anode grid has been developed. The simulation accounts for the effect of collisions on the plasma column, which is assumed to consist of multiply charged ions and electrons. It is found that the radial outwards plasma drift associated with collisions produces a spreading of the column density profile. With the plasma assumed to be in electrical contact with the anode grid, this in turn leads to a flow of current directed radially inwards, which increases the angular velocity of the plasma.

Analytical description of a collisional plasma column in a vacuum arc centrifuge

In this work the effects of electron-ion collisions in the plasma column of a vacuum arc centrifuge are modelled using a perturbation technique. It is found that the model agrees reasonably with an earlier fluid simulation, in which ion viscosity effects were also included. Using the perturbed solutions, the axial evolution of the steady-state separation profile is resolved, showing that the effect of electron-ion collisions is to improve separative performance with increasing axial position. The conditions under which separative performance are optimized are suggested. The non-uniformity in the axial magnetic field of a vacuum arc centrifuge caused by plasma rotation is also investigated. It is found that the non-uniformity is weak for standard operating conditions.