As day to day improvement of observation and measurement equipments and computer and simulation techniques have progressed, significant areas of complex arc phenomena and weld pool formation phenomena have become clarified both qualitatively and also quantitatively; these phenomena occur during tig welding and a-tig welding, which are high temperature material processes. it is an acknowledged fact that arc constriction is observed during a-tig welding and this is caused by changes within the arc plasma for the distribution of metal vapour from the weld pool, accompanied by variation in the size and the surface temperature of the weld pool. the constriction of either the arc or the arc root does not play a central role as the deep penetration mechanism during a-tig welding but cannot be ignored as an auxiliary factor for deep penetration, as seen in high s stainless steel helium tig welding. regrettably, metal vapour from the weld pool was not taken into consideration in the numerical calculation simulation introduced in this paper. an effort is necessary in the future to explain in more accurate scientific words, with a due consideration to metal vapour and the weld pool surface deformation and also careful isolation of the factors which are considered to be crucial, such as chemically reciprocal action between the arc plasma and molten metal, in order to establish arc welding techniques with even greater accuracy and reliability.
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