2004 Volume 22 Issue 2 Pages 240-247
A general way of producing pulsed alternating current waveforms for metal inert gas welding is to reverse the polarity in part of the base current period from electrode positive (EP) to electrode negative (EN). In the case of reversing the polarity from EP to EN at the right beginning of a base current period, new cathode spots tend to be produced on the surface of elongated residual molten metal that remains on the tip of a welding wire right after the detachment of a droplet. A strong cathode jet breaks the elongated residual molten metal and the fragments deposit on both sides of weld bead as tiny spatters. Delaying the polarity reversing by a time of 1.5 ms at the beginning of a base current period can effectively reduce tiny spatters because in the delay time period the molten metal on the tip of a welding wire becomes steady with a hemispheric shape.
At the end of EN current period the polarity is again switched from EN to EP and a new cathode spot is occasionally produced at a location away from the molten pool on the surface of base metal. Arc voltage becomes abnormally high so that the arc length control system extends automatically the base current period. If the cathode spot moves in a direction away from the molten pool arc interruption occurs in the extended base current period. Elimination of the abnormal arc voltage before it is used for arc length control can avoid the abnormal extension of base current period and probable arc interruption in the extended period as well. If the cathode spot moves more quickly in a direction away from the molten pool arc interruption occurs in the original base current period. In this case the rate of abnormal arc voltage increasing is very high. When the rate is higher than a preset value the present base current period is terminated immediately and arc current is switched from base current to pulse peak current. Consequently, the strong arc stiffness of pulse peak current brings the cathode spot back to the molten pool or its vicinities and arc interruption is avoided.