Any conductor through which current is flowing is surrounded by a magnetic field. The interaction between the current and the field produces a mechanical force, the direction of which is radially inwards into the conductor. If the conductor is not cylindrical in shape but, for some reason, is increasing or decreasing in diameter, there will also be a longitudinal force component. As the filler wire melts, it turns into a droplet of metal which, as a result of surface tension, remains hanging at the end of the wire. However, as the welding current is flowing through the droplet, it also is subject to electromagnetic forces. Depending on conditions, these magnetic forces may tend either to detach the droplet from the tip of the wire or to retain it there.
If the arc is narrow, the current is concentrated through a small point on its way out through the droplet. As a result, upward-acting forces (see Figure) will act on the droplet. This is the case when the welding current is low, or when CO2 is used as the shielding gas. Under these conditions, the droplets can grow and become quite large. The opposite applies when welding with a thin filler wire, an argon-rich shielding gas, and high current and high voltage. Under these conditions, the arc is wider than the filler wire, and the current paths expand downwards. The forces acting on the droplet are then directed downwards, producing a downward stream of small droplets of molten material. This produces a stable, short-circuit-free spray arc.
Reference: ESAB pulsed MIG welding literature
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