Observation of keyhole in visual plasma welding robot

Stable formation of the back bead in joining of thick material is important so as to achieve high quality of welded joint. Since plasma welding employs high current density, it is suitable in the joining of thick material. The quality of plasma welding depends on keyhole which is generated from pilot gas and welding current. As a result, the relationship between welding voltage and keyhole is unclear. It is very difficult to estimate the keyhole by welding voltage and current. Authors have tried to observe the keyhole and weld pool from the top side of base metal directly. For this purpose, a CCD camera is applied with an external trigger. Moreover, the situation of keyhole is taken by high speed camera in order to determine the peak current and base current. Timing of the shutter in CCD camera is set up to take clear images of the weld pool. The formation of keyhole is investigated under two kinds of pilot gas. One is a mixture of Ar 93% and H2 7% and the other is Ar 100%. The welding condition and groove shape are found to observe the keyhole from the top side of base metal.