Heat and metal transfer in gas metal arc welding using argon and helium
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INTRODUCTION
THE shielding gas composition is a critical process variable that influences the operation of gas metal arc welding (GMAW). ElI In general, the shielding gas protects the electrode and the workpiece from contaminants in the atmosphere, acts as a medium in which a current can flow to sustain an arc, and affects the shape of the weld bead and the resulting mechanical properties of the weldment. This investigation focuses on the influence of argon and helium upon the arc parameters, tapering of the electrode, and metal transfer in GMAW of mild steel. Though both argon and helium are inert gases, most of their other properties are markedly dissimilar. One of the important characteristics of a shielding gas is its ionization potential, which reflects the tendency of a gas to ionize. The first ionization potentials, representing the loss of one electron, for argon and helium are 15.755 and 24.580 V, respectively. L21The lower ionization potential of argon indicates that it is ionized at a lower voltage than helium and therefore can strike an arc more easily. The lower ionization potential of argon also means a lower power (arc voltage • current) in the arc, which results in a more shallow weld penetration, undercutting and a poor weld bead contour, t3] Helium's higher ionization potential requires a higher voltage to ionize the gas and to provide a current flow large enough to sustain the arc. The higher arc voltage of helium, resulting in high arc power density, produces a more contracted arc and a smaller cathode spot. The intense and contracted helium arc column also results in greater penetration than for the argon shielding gas. The cost of helium is higher than for argon, which is a commercial disadvantage. Helium, however, is still used in the industry for high conductivity materials due to its ability to produce welds at higher speeds, t41 Originally, the intent of this investigation was to provide a comparison of the arc parameters of a pure helium and a pure argon shielded arc. However, the arc calculations using the properties for pure helium did not P.G. JONSSON, formerly Graduate Student, Massachusetts Institute of Technology, is Head of Secondary Metallurgy Group with MEFOS, Luleh, Sweden. T.W. EAGAR and J. SZEKELY, Professors, are with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted August 11, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
converge. The electrical conductivity of pure helium was not high enough to sustain an arc at the lower temperatures that exist close to the anode and the cathode. Instead, a 90 pct helium-10 pct iron vapor gas mixture was used in the calculations. A more in-depth explanation is given in Section IV. In Section II, the general characteristics of the GMAW process are discussed. Section III presents details of the theoretical calculation of the arc parameters. The thermophysical properties for argon and helium are then compared in Section IV. Thereafter, some results a
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