Abstract: Narrow applicable ranges and low adjustability of process parameters of keyhole plasma arc welding result in poor stability of the keyhole and limit the wide application of this welding method. The keyhole behavior determines the stability of the welding processes and the final quality of the weld joints. Therefore, a system of controlled pulse key-holing in plasma arc welding is developed to expand the parameter ranges and improve the welding quality. Based on simple detecting device, efflux plasma voltage signal is measured in real-time during the welding process, which is employed to determine whether a keyhole is formed or not and to describe the keyhole dimension. A control strategy termed as controlled pulse key-holing is used to adjust the duration of peak and base current in time, thus the keyhole is kept in quasi-steady state. One keyhole per pulse is achieved while the duration of both peak and base currents is not constant. Key-holing is ensured, the keyhole dimension only fluctuates around the preset value, and burn-through is avoided. The usable ranges of welding process parameters are widened significantly. Plasma arc welding experiments show that the control system can make prompt adjustments when thermal loss conditions and work-piece thickness are changed so that it runs steadily and reliably and has marked control effect.

Key words: Control system, Controlled pulse key-holing, Efflux plasma voltage, Keyhole, Plasma arc welding