It is well known that the welding arc becomes unstable and the erosion of the tungsten electrode increases under hyperbaric TIG arc welding, especially in helium atmosphere, because the arc tends to constrict and the cathode spot concentrates at the electrode tip. As the result, the arc length increases, the arc column moves irregularly and the weld bead tends to fluctuate. Accordingly, sometimes it becomes" difficult to maintain the arc and sound weld cannot be obtained in hyperbaric welding. Therefore, arc length control to maintain an optimum arc length was tried by controlling arc voltage. Especially, a- fuzzy control process, in which the stable control can be performed easily under any atmospheric pressure, was applied. As the result, it is made clear that, if optimum parameters are selected, sound arc length control was executed by the fuzzy control method regardless ambient pressure.
A number of studies on hyperbaric welding for constructing or repairing underwater constructions have been made widely1). The author has also studied on the applicability of TIG arc welding to hyperbaric welding up to 5.1MPa in argon and helium atmosphere2-S). As the results, it is made clear that the TIG arc has a tendency to constrict, the diameter of arc column decreases considerably and the cathode spot concentrates at the electrode tip under high pressure helium atmosphere. Accordingly the erosion of the electrode tip and the arc length tends to increase with an increasing pressure. In case the arc length becomes too long, the arc becomes unstable and it is difficult to obtain a sound weld bead. In order to stabilize the arc under high pressure, some procedures have been proposed6, 7). In this paper, the effect of arc length control on the arc stability was studied.
