Abstract
Since arc narrows in the plasma welding, heat concentration ratio is higher than other welding. This heat source generates the keyhole in the high current region, which penetrates base metal. The base metals are joined by closing the keyhole. The quality of welding depends on state of the keyhole. However, the keyhole may become unstable due to behavior of molten metal. In this research, observations of the keyhole and molten metal under various welding conditions were performed by using high speed video camera to investigate the behavior of the keyhole in the plasma welding. Length from torch cap to bottom edge of the keyhole in high speed video camera picture was proportional to plasma gas flow rate. On the other hand, the length was mostly kept constant regardless of change of welding current. When plasma gas flow rate was changed during the welding in real time, appearance of the keyhole and shape of bead were changed. Control of stabilization of the keyhole is tried based on relationship between the length found by image processing and plasma gas flow rate. The authors tried to design the digital controller controlling the length by plasma gas flow rate to obtain the stable keyhole.
