In this study, a pulsed current plasma arc is applied to the keyhole operating mode to control solidification of the molten pool. Further, the pulse time, peak current, and plasma gas flow rate are especially investigated.
A one-pulse/one-keyhole range of pulse times by low frequency is obtained at each of four positions (Flat, Overhead, Vertical-up, and Vertical-down). As a result, suitable pulse times for some combinations of the plasma gas flow rate (Q) and the peak current (Ip) are the peak time (tp)=0.2-0.4 sec and the background time (tb)=0.4-0.6 sec at welding speed (ν) of 120 mm/min in case of stainless steel pipe (JIS SUS304, 4 mm thickness).
Then fixed pipes are welded, and it is found that orbital welding is possible by starting from the 4:30 position and by control of the plasma gas flow rate, the peak current, and the welding speed according to the welding position at 6:00 or 7:30, 9:00, and 12:00 (o'clock) positions.
And effect of joint alignment error is studied, and it is found that permitted limits with cap pass welding are miss-matching=1.0 mm, root-gas=0.5 mm, arc-length=±1.0 mm, and arc-deviation=1.0 mm in case of mild steel pipe (JIS STPG38, 4 mm thickness).