Consideration of Arc Welding Conditions from the Standpoint of Heat Conduction Theory

Abstract

Arc welding conditions are discussed for a mother plate which is n times as large as and geometrically similar to the original plate. From the theory of heat conduction, it easily follows that the temperature of the geometrically similar point is equal if we select the values of current, effective arc voltage for heat input, welding velocity and time as n, 1, 1/n and n² times those of the original plate, respectively.
This is based on the assumption that the current distribution in the arc obeys the law of similitude which holds in metal conductor parts. It is well known that the melting rate of welding rod is proportional to the current and the heat input to the base metal is also proportional to the current and is independent of arc voltage when we neglect the heat from the arc column. However the relation of similitude fails when we consider the phenomena in welding arc, i.e., the electromagnetic pinch effect, plasma stream, surface tension and convection in the molten pool. Some discussions on each theme are described.
From the standpoint of heat conduction, we presume that the welding speed as well as the welding current should be much larger in aluminum compared to steel. However the practical recommended conditions contradict the presumption above mentioned. It is well known in practise that the tendency of finger type penetration is very strong in aluminum. This is based on the reason that the liquid pressure which counteracts the digging action due to arc is much smaller in aluminum owing to its smaller specific density compared to steel.