Experimental Study On Controlled Pulse Keyholing Plasma Arc Welding Assisted By Ultrasonic Vibration

Conventional keyholing plasma arc welding(PAW)is sensitive to the change of process parameters,and the stability of keyhole is poor,which restricts its wide application in industry.In order to solve these problems,our group developed a new process and equipment of ultrasonic vibration assisted plasma arc welding(U-PAW).In the first stage,only constant current PAW with or without ultrasonic vibration was studied,and it was fo und that ultrasonic vibration could increase plasma arc pressure and enhance keyholing ability.However,the change of plasma arc current density caused by ultrasonic vibration has not been measured,and the effect of ultrasonic vibration has not been investigated for controlled pulse keyholing plasma arc welding.In this study,further testing and research work has been carried out in the above two aspects.The current density distribution of plasma arc on the anode surface was measured under different welding currents and plasma gas flow rates in both PAW and U-PAW.The results show that ultrasonic vibration can enhance the current density of plasma arc on the anode surface,and the increase is most significant at the center of plasma arc,which is more than 20%.With the increase of welding current from 80 A to 140 A and plasma gas flow rate from 2.5 L/min to 3.5 L/min,the increase of plasma arc current density caused by ultrasonic vibration is more obvious.Open-loop experiments of plasma arc welding with controlled pulse current waveform was carried out.For a group of process parameter,the workpiece cannot produce an open keyhole in conventional PAW;but U-PAW produces an open keyhole.and a stable efflux plasma voltage signal with a certain amplitude is formed in U-PAW.With the increase of welding speed,a stable one-pulse-one-keyhole mode can still be formed after the application of ultrasonic vibration.And ultrasonic vibration can increase the weld penetration.These results prove that ultrasonic vibration enhances the penetration and keyholing ability of plasma arc and enlarges the process parameter window of stable keyhole.Experiments on controlled pulse keyholing plasma arc welding assisted by ultrasonic vibration were carried out.The peak welding current of controlled pulse decreased by 7%-20%when the welding speed increased by 17%-27%with different base current durations,different current descent times and different descending currents.That is to say,ultrasonic vibration can make controlled pulse plasma arc welding achieve stable one-pulse-one-keyhole mode at a lower peak welding current and a faster welding speed.Under the same process parameter,the area of keyhole exit can be increased by 38%and the deviation distance of keyhole exit can be reduced by 34%.The above results prove that ultrasonic vibration reduces the average welding current and heat input,improves the welding efficiency and enhances the plasma arc keyholing ability.