Process And Mechanism Of Ultrasonic Vibration Assisted Keyholing Plasma Arc Welding

The plasma arc was used as the heat source and pressure source in keyhole plasma arc welding.During the welding process,the weld pool and open(fully penetrated)keyhole were formed by the plasma arc.However,conventional PAW still has some limitations,especially insufficient keyholing capability for penetrating plate thickness as well as the poor dynamic stability of keyhole and weld pool,which restrict the application of keyhole PAW.Ultrasonic vibration,as a kind of mechanical energy,has advantages of easy application and high energy usage efficiency.Some researchers' found that ultrasound assisted TIG and MIG welding produced satisfactory process effectiveness.Is it feasible to apply ultrasonic vibration in PAW process to improve the keyholing capability of the plasma arc?In this study,a specially designed ultrasonic transducer is connected to the tungsten electrode directly,ultrasound is transmitted into the plasma arc via the electrode itself,and ultrasonic vibration assisted keyholing PAW system is developed.This system consists of ultrasonic vibration system,PAW system,control system and vision sensing system.A laser Doppler vibrometer(LDV)was employed to aim at the end of the tungsten electrode in a coaxial way to determine the vibration parameters of the ultrasonic system.Experiments were performed to measure the pressure distribution and column geometry of the plasma arc.The CCD camera used to observe the plasma arc column generated between the torch and the water-cooled copper block.The captured images of plasma arc show that the ultrasonic vibration can contract the column diameter of the plasma arc.To detect the pressure of the plasma arc,the stagnation pressure measurement apparatus was used.The results reveal that with the increases of welding current,the effect of ultrasonic vibration on the arc pressure was decreased.The welding tests on stainless steel plates are carried out to examine if the additional ultrasonic vibration is able to play a positive role in improving the heat-pressure features and the keyholing capability of the plasma arc.The CCD camera was employed to observe the keyhole exit from the backside of the workpiece.The experimental results show that the ultrasonic vibration can enhance the keyholing capability of the plasma arc.Under the same conditions,the additional ultrasonic vibration can produce open keyhole with a lower welding current and a higher welding speed.Compared to the conventional PAW,the establishment of the open keyhole is accelerated and the quasi-steady state is reached in a shorter time in ultrasonic vibration assisted PAW.Meanwhile,ultrasonic vibration could increase the keyhole size and reduces the keyhole centre deviation distance.Also,macrographs of the weld at cross section showed that ultrasonic vibration can reduce the weld width at the upper part and increase the weld width at the lower part.At the same time,the ultrasonic vibration can improve the quality of the weld surface morphology.With ultrasonic vibration,U-PAW produces the microstructure of weld joint with more equiaxed grains.There are more equiaxed grains at the upper region than that at the lower region at macrograph of weld at cross section.It shows that ultrasonic vibration changes the physical properties of the plasma arc,and then affect the weld microstructure.Concurrently,some of the ultrasonic vibration passed through the plasma arc into the weld pool and further impact on the weld solidification process.