Effect Of Ultrasonic Vibration On The Molten Pool Flow And Keyholing Behavior Of Plasma Arc Welding

Plasma arc welding(PAW)has a wide application prospect in the welding of medium-thick steel plates structures because of its deep penetration in one step.However,in the conventional PAW process,the dynamic stability of weld pool and keyhole is poor,and the welding process parameter window is narrow.These problems restrict its widespread application.In order to overcome this limitation of traditional PAW,our research group developed the ultrasonic vibration assisted keyholing plasma arc welding process(U-PAW).U-PAW connects the ultrasonic unit with the tungsten electrode,and the ultrasonic vibration is directly applied to the plasma arc through the tungsten electrode,which changes the plasma arc heat-pressure characteristics and improves the keyholing/penetration ability.In the early stage,the experimental research on the new U-PAW technology was mainly carried out.but the influence mechanism of ultrasonic vibration on the molten pool flow and keyholing ability of plasma arc welding has not been revealed.In this thesis,numerical simulation is used to quantitatively analyze and study the temperature-flow field in welding pool and dynamic keyhole evolution process in U-PAW,which is of great theoretical significance and engineering application value for optimizing new U-PAW technology and promoting the application of U-PAW.Ultrasonic wave acting on plasma arc medium will produce acoustic radiation force.Based on the work done by acoustic radiation force and electric field force,the analytical formulas of plasma arc pressure and gas shear stress are derived,and the influences of welding current,plasma gas flow rate,sound speed,ultrasonic frequency and amplitude are comprehensively considered.Ultrasonic vibration increases plasma arc pressure and gas shear force.Both plasma arc pressure and gas shear force promote the keyholing ability of molten pool,but plasma arc pressure is the main factor.In order to study the influence of ultrasonic vibration on the heat-pressure distribution of plasma arc on anode surface,a bidirectional coupling mathematical model of ultrasonic field and plasma arc was established,and the numerical analysis of electromagnetic-temperature-flow field of plasma arc was carried out.The calculation results show that ultrasonic vibration improves the thermal conductivity of plasma arc and decreases the electrical conductivity.The further contraction of the plasma arc increases the current density,heat flux density,arc pressure and gas shear force on the anode surface.A mathematical model of U-PAW molten pool-keyhole behavior is established by taking the numerical results of heat-pressure distribution on the anode surface as boundary conditions.The numerical simulation results show that there are two circulations in PAW and U-PAW molten pools,and clockwise circulations exist near the upper surface of the molten pool under the action of gas shear stress and Marangoni force.Under the action of plasma arc pressure and electromagnetic force,a counter-clockwise circulation is formed in the lower part of the molten pool.In U-PAW,ultrasonic makes the plasma arc contract,as well the plasma arc pressure increase.This makes the liquid metal in the molten pool flow to the bottom,and the size of the keyhole inlet becomes smaller.Under the condition of controlled pulse welding current,the electromagnetictemperature-flow of the plasma arc itself changes instantaneously,and the sound pressure in the plasma arc caused by ultrasonic vibration also changes periodically with the welding current,which leads to the periodic changes of the heat flux density,arc pressure and gas shear stress of the plasma arc on the anode surface.Using the established model,the periodic change process of these parameters is numerically analyzed.Based on the heat-pressure characteristics of plasma arc on the anode surface corresponding to the controlled pulse current waveform,a mathematical model of U-PAW molten pool-keyhole behavior that changes dynamically with welding current is established.The dynamic process of forming through keyhole in U-PAW weld pool at the peak welding current stage,closing keyhole at the basic welding current stage,and periodically changing shapes of weld pool and keyhole is numerically simulated.Ultrasonic vibration improves the keyholing ability of controlled pulsed plasma arc.The ultrasonic-assisted plasma arc welding experiment process was carried out under the condition of continuous welding current and controlled pulse welding current.The plasma arc pressure and current density were measured.The back keyhole image was captured.The keyhole center deviation distance and the size were extracted.The shape and size of the weld cross section were measured.These experimental measurement results are in good agreement with numerical simulation results.