Observation of the keyhole during plasma arc welding

Keyhole plasma arc welding is a unique arc welding process for deep penetration. To ensure the quality of the welds, the presence of the keyhole is critical. Understanding of the keyhole will certainly benefit the improvement of the process and weld quality. Currently, the size of the keyhole is assumed to be correlative with the robustness of the keyhole process in maintaining the keyhole. To verify this assumption, the keyhole and the weld pool were simultaneously monitored from the back side of the workpiece. It was found that once the keyhole is established, the width of the keyhole does not change with an increasing welding current and a decreasing welding speed. This implies the width of the keyhole gives no adequate information about the stable state of the keyhole and should not be used as an indication of the robustness of the keyhole process in plasma arc welding.