Influence Mechanism Of Ultrasonic Vibration On The Transport Coefficients And The Heat-pressure Characteristics Of Plasma Arc

Plasma arc has the characteristics of high stiffness,concentrated energy density and capability of form keyhole in the molten pool.It has great application potential in joining of medium thickness plates.However,conventional plasma arc still has some problems,such as insufficient keyholing ability and poor stability of the keyhole.To solve the above problems,a new process variant ultrasonic vibration assisted keyholing plasma arc welding(U-PAW)was developed,ultrasonic vibration(UV)was applied to plasma arc and molten pool through tungsten electrode of plasma arc welding torch.The previous process experiments have proved that the stiffness of plasma arc is further strengthened,and the keyholing ability of plasma arc is improved.However,the interaction mechanism of ultrasound and plasma arc has not been studied systematically.To analyze the propagation process of ultrasound in plasma arc and the action mechanism of ultrasonic vibration on micro-behaviors such as particle motion state,ionization and recombination in plasma arc,and to understand the influence of ultrasonic vibration on macroscopic heat-pressure characteristics of plasma arc,it has important theoretical significance and practical engineering value which can promote the application of the U-PAW process in joining of medium thickness steel platesThe mathematical model of ultrasonic propagation in plasma arc is established,and the numerical analysis of ultrasonic field is carried out with the plasma arc flow field and density field as the background fields.The results of characteristic frequency analysis,frequency analysis,and transient analysis show that the ultrasonic working frequency of 25 kHz is well matched with the structure and size of plasma arc torch,and the ultrasound effect is the best under this frequency.The ultrasonic propagation path in plasma arc is influenced by the structure of plasma nozzle,and the sound field obtained from the nozzle exit of plasma arc torch is the superposition of reflected sound,direct sound and mixed sound.The amplitude of ultrasonic field is closely related to the density of plasma,and the flow velocity of plasma will affect the ultrasonic field,and when the flow velocity increases,more acoustic energy will be transmitted to the workpiece surface.The formulas for calculating the transport coefficients of ultrasonic vibration assisted plasma arc welding were derived.Ultrasonic action on plasma arc will affect the transport coefficients of the plasma arc.Ultrasound will affect the velocity distribution function of particles in plasma arc,and it is necessary to expand the velocity distribution function from fi(0)+fi(1)to fi(0)+fi(1)+fi(2).According to Boltzmann equation and Maxwell velocity distribution function,the relationship between ultrasonic vibration and the second-order approximation of velocity distribution function is established,and the formulas for calculating thermal conductivity and electrical conductivity of plasma in U-PAW are derived by using the new velocity distribution function.The formulas of transport coefficients considering the influence of ultrasound are applied to the numerical model of plasma arc.The results show that ultrasound can increase the thermal conductivity and decrease the electrical conductivity of plasma arc.The influence of ultrasonic vibration on the transport coefficient of plasma arc is dependent on the ultrasonic intensity and temperature of plasma arc at the corresponding positions.The numerical model of ultrasonic vibration assisted plasma arc is established.The effect of ultrasound on plasma arc is systematically considered.Macroscopically,the terms of acoustic radiation force and acoustic energy are added to the governing equations.Microscopically,the influence of ultrasound on particle motion(change of transmission coefficient)is taken into consideration,and the influence of ultrasound on thermal-pressure characteristics of plasma arc is calculated and analyzed.The results show that,macroscopically,ultrasonic energy has little effect on plasma arc,while acoustic radiation force is benefical to increase the flow velocity of plasma.Ultrasound increases the thermal conductivity of the plasma arc and decreases electric conductivity.At the same welding current,U-PAW arc column is radially compressed.In general,the application of ultrasound can increase plasma flow velocity,arc column stiffness,arc pressure,and current density on anode surface.Therefore,the heat-pressure characteristics of plasma arc are enhanced,and the keyhole ability is improved.The arc pressure and current density on the anode surface were measured by static hole method and split anode method respectively,and the numerically calculated and measured results were consistent with each other.CCD cameras were used to capture the images of the keyhole exit and the plasma arc.The keyholing ability of ultrasonic vibration assisted plasma arc is verified experimentally by both the keyhole exit images and the macrographs of welds.