Experimental Study On Weld Pool Flow And Keyholing Behavior In Ultrasonic Vibration Assisted PAW

Ultrasonic vibration assisted keyhole plasma arc welding(U-PAW)can enhances keyholing capability.However,in the previous study,a thin workpiece was used for the overlay welding test.And there is no in-depth study on the effect of ultrasonic vibration on the liquid flow of the weld pool and the dynamic behavior of keyhole.This paper studies the dynamic behavior of the weld pool and keyhole by visual detection.And carried out surfacing and butt welding tests on 8 mm thick stainless steel workpiece.This is of great significance for optimizing U-PAW process parameters and expanding its practical application range.The visual sensing system for weld pool is developed.The image of weld pool and keyhole entrance was detected by a dual CCD camera visual sensing system,and the effect of applying ultrasonic vibration on the size of the weld pool and keyhole was studied.The tracer particle technique was used to track the flow state of the liquid metal on the surface of the weld pool under different welding process conditions,and the backward flow velocity of the liquid was calculated.Ultrasonic vibration causes more plasma shear force to act downward along the back wall of the keyhole,which promotes the downward flow of the weld pool liquid,so the longitudinal flow velocity of the weld pool liquid is slowed down to varying degrees.In addition,the volume of the plasma arc shrinks further after applying ultrasonic vibration.The above two reasons have reduced the size of the weld pool and keyhole.Open-loop controlled pulsed keyhole PAW was carried out on the 8 mm thickness stainless steel workpiece by surfacing and butt welding.The workpiece cannot be completely penetration under the condition of conventional PAW process parameters.The images of the keyhole on the back of the workpiece can be captured and stable efflux plasma voltage signals were detected after applying ultrasonic vibration,which proved that the weld pool was completely perforated and complete penetrated weld was obtained.Because ultrasonic vibration improved the stiffness of the plasma arc,then the area of the keyhole exit was increased and the backward offset of the keyhole was reduced.The stable keyhole welding mode can be maintained by increasing the welding speed and applying ultrasonic vibration.Experiments on ultrasonic vibration assisted controlled pulsed keyhole PAW were carried out on 8 mm thickness stainless steel workpiece.After applying ultrasonic vibration,the peak welding current is decreased by 5%-14%when the welding speed increased by 10%-20%.This shows that the application of ultrasonic vibration can use a lower peak current and a faster welding speed to achieve a stable one-pulse-one-keyhole mode and the weld width was narrower and the depth-to-width ratio was larger.The application of ultrasonic vibration can reduce welding heat input and improve welding efficiency.It is beneficial to expand the application range of keyhole plasma arc welding.