Research On Ultrasonic-arc Plasma Interaction Mechanism And Welding Characteristics

As a newly developed arc welding method,ultrasonic wave assisted arc welding successfully introduced power ultrasound into arc and weld pool during which the ultrasonic acts on the top of the arc in the coaxial alignment direction.The advanced process for molten metals can be realized by utilizing additional ultrasonic field.Joint microstructure and mechanical properties can be improved.During the welding process,there is an acoustic radiation and scattering field in the region between the radiator and the workpiece to be welded,the acoustic energy acts on the arc,droplet and the welding pool.It has been found that compared with the conventional arc welding,the arc is bound significantly and becomes brighter,the frequency of droplet transfer increases and the weld spatter phenomenon reduces for ultrasonic wave assisted arc welding.Ultrasonic wave assisted metal inert gas?U-MIG?welding has been proved to be an excellent approach to promote the droplet transfer.For welded joints,under the action of ultrasonic,the grain structure in the weld zone is refined,and porosity is significantly reduced.These findings from previous studies have shown that ultrasonic wave assisted arc welding has good application prospects.In this thesis,the research mainly focuses on improving the ability of the ultrasonic field,determining the interaction mechanism between the ultrasonic and the arc plasma and the characteristics of ultrasonic acting on droplet.These issues belong to the key issues constraining the welding process for further development and need to be analyzed in depth from the perspective of acoustic.The study will not only help reveal ultrasonic,plasma and two overlapping physics phenomena,but also has important practical significance to the better application for the welding method.A two-dimensional acoustic field model for the acoustic emission device of ultrasonic wave assisted arc was established and velocity potential ? was introduced.By dimensionless processing of variables,the dimensionless boundary integral equation was derived to describe the entire acoustic field.This direct relationship between the velocity potential and the geometric parameters of the radiator was obtained.The model could describe the relevant laws between the performance of acoustic field and the radiator geometry parameters.In the evaluation process of the acoustic field,the dimensionless acoustic radiation power ???,the dimensionless acoustic pressure ???,the dimensionless time average potential ???,the dimensionless acoustic radiation force ??? and the restoring force constant ??? were calculated,respectively and the calculation results were validated by RT styrene ball suspension test.Based on the relationship between the acoustic field property and the geometric parameters of radiator,the effect of the radiator radius R₁,the central pore radius R₂,and the curvature radius R or depth D of concave radiator surface on the acoustic radiation force and the restoring force constant were analyzed in detail.In addition,for different concave depths,three concave radiator surfaces including spherical cap surface,paraboloid of revolution,and rotating single curved surface were investigated systematically by observing the variation of the maximum acoustic radiation force in the axial and radial direction.The calculation suggested that,to obtain the best acoustic binding ability,the ultrasonic wave assisted arc setup should be operated under the first resonant mode using a radiator with a spherical cap surface,a small central pore,a large section radius and an appropriate curvature radius.These results were verified by experiment combined with arc acoustic binding theory,the results agreed well with the calculation results.For the ultrasonic wave assisted arc,the feature interaction between ultrasonic and arc was studied.In ultrasonic wave assisted tungsten inert gas?U-TIG?welding at low current,the pulsed arc was first observed.Compared with the conventional TIG arc,the high-temperature region in the arc column greatly expanded,and there were internal particle agglomerations shaking up and down at a constant frequency.The arc shape varied with ultrasonic excitation current and the height of ultrasonic radiator.In the vicinity of the resonance point,the ultrasonic excitation current was 150 mA and the height of radiator was 14 mm,the straight-degree of the arc was the strongest and the ripple frequency was also the largest?522 Hz?.Analyzing the plasma particles showed that it had a certain relationship between the oscillation frequency of plasma particles and density,and the acoustic pressure could be calculated based on the dependence of the electron temperature and density on time and space.Arc electronic temperature and density variations before and after the application of ultrasonic to arc were systematically studied by the arc spectrum experiment.Under the action of ultrasonic,when the collection point closed to the edge of the arc,the intensity of Al spectral lines increased more pronounced.The Helmholtz equation for acoustic propagation in arc was derived,acoustic wave generation and propagation processes were fully described.Acoustic propagation in different arc voltages was calculated by the finite element method.The results showed that the modal frequency increased with the increase of arc voltage,the calculation of acoustic pressure in arc provided a theoretical basis for droplet analysis for the next step.The characteristic of ultrasonic on the droplet was analyzed.The date of droplet transfer process was collected by high-speed camera,the object of research included droplet displacement,acceleration,volume,and combined force acting on the droplet.According to the result of the force in different welding currents,it could be determined the additional force from ultrasonic field acting on the droplet during transfer process.For the analysis of variation accelerated motion of a single droplet,the results showed that the additional force met the characteristics of ultrasonic radiation force.In addition to flat position,U-MIG welding was also applied to other welding positions.Compared with conventional MIG welding,the arc stability was significantly enhanced for U-MIG welding,wherein the U-MIG vertical-up welding process,this droplet shape was more uniform and the trajectory for droplet transfer was more stable.Based on the research of the effect of ultrasonic on arc and droplet,pulsed ultrasonic was first introduced in the arc welding in order to overcome the adverse effect of temperature in large-scale welding.For pulsed ultrasonic wave assisted arc welding was also a new welding method.Arcs,droplet transfers,joint microstructures and properties for continuous and pulsed ultrasonic wave assisted MIG?PU-MIG?welding were preliminary studied by the welding experiment of 7A52 aluminum alloy.From the experimental results,it could be seen in the PU-MIG welding,the arc compression was more significant,the smallest droplet size and the largest metal transfer frequency were obtained,the weld area increased significantly.The grains refined by pulsed ultrasonic was much finer than those refined by continuous ultrasonic.Due to enhanced ultrasonic agitation,there was a significant inhibitory effect to the Zn element gasification loss in PU-MIG welding.Comparing with the ultimate tensile strength and elongation of the U-MIG joint,those of the PU-MIG joint increased 9.1%and 18.2%,respectively.While the joint hardness test showed that the hardness of the PU-MIG weld reached 80%of the hardness of the base metal,weld zone softening improved significantly.The study could lay the theoretical and experimental basis for the research of the acoustic regulation during welding method in the future.