Metal vapors in gas tungsten arcs: part ii. theoretical calculations of transport properties
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I.
INTRODUCTION
THEcalculation of plasma transport properties can provide valuable information concerning the behavior of the gas tungsten welding arc. The electrical and thermal conductivities of the plasma will determine the arc configuration and, subsequently, the transport of energy to the weldment. ~,2 The thermal and electrical conductivities of argon and helium plasmas containing small amounts of iron, calcium, and aluminum vapors were calculated from established plasma physics formulae. Qualitative analysis of the results can lead to a fundamental understanding of the effects of metal vapors on the welding arc. A more rigorous study will involve the solution of the Elenbaas-Heller equation in cylindrical coordinates by the finite element method, with the calculated conductivities as input data. Such an investigation is beyond the scope of this work but will be presented elsewherefl Theoretical calculations of plasma transport properties are very approximate due to the complex nature of plasma behavior and inexact knowledge of collision cross-section values. The calculations of various investigators are often in marked disagreement with experimental data and with each other. 4 However, such calculations can be very helpful in determining the relative effects of compositional variations on the plasma properties, and subsequently on the transport of energy to the weldment.
II.
PREVIOUS WORK
A large number of investigators have reported theoretical and experimental data for the transport properties of pure argon or pure helium plasmas. 4 Investigations regarding mixtures of gases are less numerous. Glickstein 1 calculated the thermal and electrical conductivities of argon and helium plasmas with small additions of aluminum vapor. He found that even concentrations as small as 0.01 pct AI will significantly increase the electrical conductivities of the inert gases at low temperatures, particuG. J. DUNN is Materials Scientist, IBT, Beverly, MA. T. W. EAGAR is Associate Professor, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted August 26, 1985.
METALLURGICALTRANSACTIONS A
larly in the case of helium. This was attributed to the low ionization potential of aluminum, which results in an increase in the electron density at lower temperatures. The changes in thermal conductivity were less significant, with small increases at low temperatures for that of argon, and little change in that of helium, except at high concentrations and temperatures. Glickstein substituted these calculated values into a onedimensional model of the welding arc in order to explain the effects of aluminum on stainless steel welds reported in the literature. 5 However, Glickstein's model is inadequate in that it considers only the addition of aluminum to a pure inert gas. This is not the true nature of the welding arc. In practice, an arc on stainless steel will contain not only argon or helium, but also a mixture of iron, manganese, chromium, and other metal vapors .6 These elements have ionization potentia
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