Influence of ultrasonic vibration on keyholing/penetrating capability in plasma arc welding with controlled pulse waveform

Ultrasonic vibration-assisted plasma arc welding with controlled pulse current waveform was developed to weld the 8 mm thick stainless steel plates. Both bead-on-plate and butt welding experiments were carried out with open-loop and closed-loop control strategies under different welding conditions. The signals of welding current and efflux plasma voltage and the images of keyhole exit were sensed to monitor the keyhole status during the welding process. It was found that the ultrasonic vibration-assisted plasma arc can completely penetrate the stainless steel plates with the thickness of 8 mm in a single pass with higher welding speed or lower peak welding current in controlled pulse keyhole mode. By continuous real-time adjustment of welding current waveform in closed-loop strategy, the “one-pulse-one-open keyhole” mode was achieved successfully with lower welding heat input. Due to the reduction of heat input and further constriction of plasma arc under the action of ultrasonic vibration, the weld was narrowed and the depth-width ratio was raised.