Research On Arc Bubble Behavior And Welding Characteristics In Ultrasonic Wave-assisted Underwater Wet Welding

Underwater wet welding?UWW?has been widely used in the construction and maintenance of underwater marine structures due to its low cost,simple operation and strong applicability.In conventional UWW process,a local gas-phase region around the arc is called an arc bubble.Due to the interaction of arc bubble and the surrounding water medium,the dynamic behavior of the bubble affects the arc stability,which will directly determine the quality of the welding process.Hence,it will be a promising method to stabilize welding process by improving the bubble dynamics.In view of wide application of power ultrasound in the field of welding,the use of its unique acoustic field characteristic to influence the welding process has a good potential for development.Therefore,from the perspective of arc bubble behavior,ultrasonic wave-assisted UWW?U-UWW?was proposed for the first time.The new method controlled the dynamic change of arc bubble under the action of acoustic radiation force to ensure the stability of the welding process.In this dissertation,the bubble evolution process,arc stability and the quality of welded joint in U-UWW were analyzed step by step.Together with the results of spatial acoustic field simulation,the influence of ultrasonic introduction on arc bubble was discussed in detail,and the influence mechanism of bubble change on arc stability and droplet transfer behavior was revealed.Based on thermal process changes,the performance improvement of U-UWW joint was clarified.The research of this subject not only provides a theoretical basis for the acoustic regulation of arc bubble,but also has important significance for improving the weld quality in U-UWW.The bubble behaviors in conventional UWW and U-UWW were analysed.Using high-speed camera and electrical signal analysis system,four different bubble evolution modes were obtained by observing bubble evolution process under different welding parameters.After analysing the size changes of arc bubble,the conditions of forming stable bubble in conventional UWW were determined,and the feasibility of the bubble stability evaluation was verified by mechanical constraint method.At the same time,the effects of different bubble evolution modes under the acoustic field were investigated and it was found that the shape and motion feature of the bubble changed significantly,among which the common necking phenomenon was generally eliminated and bubble dynamic stability was improved,but the degree of improvement varied with welding parameters and acoustic field parameters.The variation in arc stability in the U-UWW process was investigated.It could be observed that under different welding parameters and acoustic field parameters,the arc stability of U-UWW was better than that of conventional UWW.By means of electrical signal characteristics and high-speed images,the variation in droplet transfer behavior under acoustic field was investigated,and the ultrasonic effect was evaluated from the aspects of droplet morphology,size,frequency and transfer stability.With using ultrasonic wave,the droplet repelling phenomenon was weakened,the droplet diameter was significantly reduced from 4.5mm to 3.83mm,and the transfer frequency was increased from 7.9Hz to 10.5Hz.Thus,the droplet transfer period was shortened and the transfer process was relatively stable.Combined with the change of weld formation,the relationship between process stability and weld formation in U-UWW was discussed.The variations in arc stability and droplet transfer process had a significant effect on weld formation.Under the action of ultrasonic wave,large and stable bubble was always attached to the surface of weld pool,which reduced the water cooling and thus more heat was transferred to the weld pool.Hence,weld penetration and width were both increased.Furthermore,the bubble-acoustic interaction was also related to the acoustic field parameters and welding parameters resulting in the changes in weld geometries,and the fusion ratio of U-UWW increased with different welding parameters in comparison with conventional UWW.According to experimental data,a two-dimensional acoustic field axisymmetric model was established.The acoustic pressure,time-average potential and acoustic radiation force were extracted and the distribution of spatial acoustic field in water medium was discussed.It could be seen that the presence of the bubble caused a significant change in acoustic pressure,and the downward axial acoustic radiation force at the bubble surface was the main factor that constrained the bubble floating.The action mechanism of acoustic radiation force and arc bubble was then analyzed.The acoustic radiation force changed the bubble force balance condition in conventional UWW,which allowed the bubble to have different degrees of lateral shift during motion.Based on the motion characteristics of gas bubble in water,the influence essence of acoustic radiation force was further clarified,and the reasons for the difference in four bubble evolution modes under ultrasonic effect were revealed.The change of bubble motion feature maked the effect of gas flow drag force and bubble buoyancy on molten droplet be weakened,which was the main reason for the decrease in droplet size and repulsive angle and the increase in transfer frequency.The differences in microstructure and mechanical properties between conventional UWW and U-UWW joints were analyzed.The influence mechanism of ultrasonic on the microstructure and mechanical properties of welded joints was investigated.The products and their contents of phase transformation in U-UWW joint changed.Acicular ferrite was observed in weld metal?WM?,and in fully transformed heat affected zone?FTHAZ?,the martensite content decreased while upper bainite content increased.Compared with conventional UWW,the impact toughness of weld metal and fusion line in U-UWW were improved to varying degrees,indicating superior mechanical properties.For joint hardness,WM and HAZ of U-UWW both had lower hardness values than conventional UWW,in which the fluctuation of hardness value was reduced.Comparing the effects of ultrasonic exerted method on the solidification process of weld pool and combined with thermal cycle characteristics,the improvement mechanism of joint performance was clarified.Unlike the acoustically regulated bubble,when the ultrasonic vibration was transmitted to weld pool via base metal,without and with using ultrasonic vibration,the phase transformatio products of weld metal were similar,but grain size and morphology changed,and there was no influence on the HAZ microstructure.The change in bubble evolution characteristics under acoustic regulation effect significantly influenced the thermal process,which decreased the cooling rate in HAZ and thus increased the cooling time of t_8/5 near the fusion line.Finally,the microstructure of the welded joints changed,and the change trend was basically consistent with the characteristics of the welding thermal process.Hence,it could be considered that the change of the welding thermal process in U-UWW was the main reason for the improvement of joint microstructure and mechanical properties.