Abstract: To make the keyhole status transformation of “open-closure-open” more stable and smooth during keyhole plasma arc welding (PAW), further improvement is made on the previously developed system of pulsed keyhole PAW with square waveform of welding current. The current waveform with two dropping slopes at the pulse trailing-edge is specially designed and employed. Through adjusting the magnitudes of these two dropping slopes, both heat input and plasma force can be controlled more flexibly. Based on the optimization of pulse parameters, the keyhole status transformation from open to closure becomes more smooth. The improved system takes the plasma efflux voltage depicting the keyhole size as the controlled variable, and the pulse current level and its two dropping slopes at the trailing-edge as the controlling variables. The control algorithm PI (proportional and integral) and the strategy of controlled pulse keyholing are employed to ensure the formation of a complete keyhole and maintain the keyhole size close to the preset value. The experimental results show that the system accurately operates in the mode of one keyhole per pulse. Compared to the square waveform, the specially designed current waveform can make the keyhole shift between open and closed states more stable and smooth. Thus, the weld formation quality is fine, and the improved system has significant control effects.

Key words: Control system, Controlled pulse keyholing, Keyhole, Plasma arc welding, Plasma efflux voltage