Plasma arc welding is a joining technique in which an arc is struck between a tungsten electrode and the workpiece. The flow of plasma gas (argon) is constricted by an orifice surrounding the electrode which increases the velocity of the gas and raises its temperature producing a higher energy density weld than conventional arc welding methods.1,2 In variable polarity plasma arc (VPPA) welding, the electrode is switched positive and negative several times per second so that efficient workpiece heating (electrode negative) is combined with surface cleaning by the argon ions (electrode positive).3
Hartman, et al.,4 have reported that excess eutectic at the surface of the partially melted zone (PMZ) exists in VPPA-welded 2219 aluminum. The enrichment has been proposed to occur by exudation caused by transient stresses during welding, preferential vaporization of aluminum, and sputtering of aluminum by argon ions.
The distribution of anode and cathode sites...
