Globula and Spray Transfer in Mig Welding

Abstract

“Globular” and “spray” are classified by IIW as two distinct modes of metal transfer in “Metal Inert Gas” (MIG) welding. In argon there is a fairly sudden transition current above which the diameter of the molten droplets from the welding wire changes from having a diameter larger than the welding wire in the “globular” mode to being smaller than the welding wire in the “spray” mode. An approximate physical model is considered where, for currents much below the transition current, the forces on the molten drop attached to the welding wire are dominated by surface tension, tending to hold the drop to the wire, and gravity, tending to pull the drop from the wire. Above the transition current, however, the magnetic pinch pressure from the self-magnetic field of the current becomes greater than the increase in pressure inside the drop produced by surface tension so there is then spray transfer. Equating these pressures, a formula for the transition current I, is I = 2π(γD/μ0)1/2 where D is the diameter of the wire, γ is the surface tension coefficient of the molten metal and μ0 is the permeability of free space. Predicted transition currents are in fair agreement with experimental results for both steel and aluminium, for wire diameters varying from 0.4 to 3.0 mm.