Arc interference and a unique push–pull arc solution in alternating current plasma arc welding of aluminium alloys

This paper analyses the arc interference in an a.c. combined (both the pilot arc and the plasma arc exist in the welding process) plasma arc welding of aluminium alloys. This phenomenon results in an unstable welding process and weld formation. To maintain the pilot arc during the period when the electrode is positive, a cathode region must be formed in front of the tungsten electrode tip. However, to guarantee plasma arc initiation during the same period, this region must be the anode region. Consequently, neutralization of the positive and negative charge in the region in front of the tungsten electrode tip will occur. This will result in the instantaneous disappearance of the conducting region in front of the tungsten electrode tip, which will cause the plasma arc or pilot arc or both to be instantaneously extinguished. Arc interference appears because the potential of the nozzle is unable to adapt to that of the conducting plasma arc column in the d.c. positive electrode mode. A unique push–pull arc solution for solving the trade-off between the arc interference and the reliable initiation of the a.c. plasma arc is proposed and implemented. In addition, high-quality weld formation in the keyhole mode, which is energy efficient and avoids a double arc, is achieved.