Dramatic increases in the depth of weld bead penetration have been demonstrated by welding stainless steel using the gas tungsten arc (GTA) process with activating fluxes consisting of oxides and halides. However, there is no commonly agreed mechanism for the effect of flux on the process. In order to clarify the mechanism, behaviour of the arc and weld pool in the GTA process with activating flux was observed in comparison with a conventional GTA process. A constricted anode root was found in the GTA process with activating flux, while a diffuse anode root was found in the conventional process. Furthermore, it is suggested that these anode roots are strongly related to metal vapour from the weld pool, which is also related to temperature distributions on the weld pool surface.

The energy source characteristics of gas tungsten arc (GTA) strongly depend on the physical property of arc plasma. In welding processes, it has been experimentally confirmed that metal vapour evaporated from a high temperature weld pool drastically changes the property of arc plasma and decreases its temperature. However, the effect of metal vapour on the characteristics of heat flux into a base metal is still not clear owing to the difficulty in experimental studies of arc plasma. In the present paper, the energy source property of helium GTA mixed with metal vapour was numerically analysed. It was found that the intense radiation generated from dense metal vapour decreases heat flux into a base metal and contracts the current density distribution especially near the arc axis.

In this research, two colour pyrometry was conducted to obtain the surface temperature of weld pools, in which the weld pool was photographed by a high speed camera during arc welding. Two wavelengths (950 and 980 nm) of light in the infrared range were selected from the thermal radiation light emitted from the weld pool at the instant when the arc was extinguished, using an imaging spectroscope. Consequently, in gas tungsten arc welding, it was shown that the surface temperature distribution of a weld pool is affected by the sulphur content in the base metal. It is thought that this temperature distribution is determined by the balance between the driving forces of viscous drag from the cathode jet of plasma and Marangoni surface tension. In gas metal arc welding, it was seen that the surface temperature distribution becomes uniform and the temperature is 1715-1845 K, which is obviously lower than that of the metal droplet.

Nakata (2010) The measurement of metal droplet temperature in GMA welding by infrared two-colour pyrometry, Welding International, 24:2, 81-87,The temperature of metal droplets is essential for clarifying the phenomenon of metal droplet transfer and the melting behaviour of wire; also, it governs the emission of fumes. On the other hand, in situ measurement of the temperature of a metal droplet formed at the tip of a wire during welding was difficult. Hence, this temperature was obtained in many experiences of measurements by such a way that several numbers of metal droplets were collected in a calorimeter to measure the amount of heat content of metal droplet and the heat was converted to temperature. With this way, however, the reliability of the measurement is not necessarily high because the heat loss of the metal droplet during the time when detaching from the wire tip and entering into the calorimeter has to be estimated properly. In this research, two-colour pyrometry has been conducted to obtain the temperature of metal droplets, in which metal droplets have been photographed by a high-speed camera during arc welding, two wavelengths (950 and 980 nm) of light in the infrared range have been selected from the thermal radiation light emitted from the metal droplet at the instant of arc extinguishment by using an imaging spectroscope, and the temperature has been obtained from the intensity ratio of the two waves of light. Consequently, in CO 2 arc welding, it has been revealed that the constricted arc causes high-heat input density locally at the arc root portion of a metal droplet and thereby the arc root portion exhibits a higher temperature. By contrast, in MAG (80% Ar-20% CO 2 ) arc welding, it has been disclosed that because the arc covers metal droplets, the temperature distribution in a metal droplet is relatively uniform and the average temperature is lower than in CO 2 arc welding.

Heile-1) CO 2 CO 2 2 3) 4 7) 8) MAG Ar-20%CO 2 689 nm 590.2 nm 950 nm 980 nm [ 26 p. 214-219 (2008)]