Physical basis for the transition from globular to spray modes in gas metal arc welding

Abstract

In gas metal arc welding with argon gas, there is a fairly sudden transition current above which diameters of the molten metal drops detached from the welding wire change from being greater than the wire diameter in the ‘globular’ mode to less than the wire diameter in the ‘spray’ mode. It is concluded that the primary cause of this transition is that at higher currents the magnetic pinch pressure from current within the molten metal becomes larger than the pressure induced by the surface tension of the molten metal. A formula expressing this condition is I = 2π(γD/μ0)1/2, where I is the transition current, D is the diameter of the wire, γ is the surface tension coefficient of the molten metal and μ0 = 1.26 × 10−6 N A−2 is the permeability of free space. This formula predicts transition currents in fair agreement with previously published experimental results from various authors for both steel and aluminium, for wire diameters varying from 0.4 to 3.0 mm. The formula is not valid for carbon dioxide, helium or hydrogen where, unlike argon, there is arc constriction at the base of the welding wire. Nevertheless, the formula represents a useful approximation for the change in metal transfer modes using various welding wire materials if, as is usual, argon is the principal component of the welding gas.