Effect Of Additional Ultrasonic Energy On Microstructure Evolution And Strain Component Of Weld Nugget Zone In Friction Stir Welding

Friction stir welding(FSW),as a solid-state joining process,has been widely used for the joining of aluminum alloys.However,despite the commercial success,the process has some concerns such as high load requirements,tool wear,narrow process parameters-window,etc.In order to deal with these problems,superimposition of ultrasonic energy on the FSW process as a variant technology is critical.The ultrasonic vibration energy can decrease the deformation resistance of materials without a significant increase in process temperature.On this basis,the ultrasonic vibration enhanced friction stir welding(UVeFSW)where.ultrasonic energy is directly imparted onto the workpieces ahead of the FSW tool using a sonotrode during welding was developed.From a series of studies,the UVeFSW seems to be beneficial towards the process in many ways,such as its lowering of welding load and improvement of the weld joint quality.However,the in-depth mechanism of the UVeFSW and the mechanism of the interaction between the ultrasonic energy and the aluminum alloy to be welded,i.e.,the way how ultrasonic energy changes the behavior of the plastically deformed material is still not studied.In this work,the mechanism of the effect of additional ultrasonic vibration on the welding process and the quality of the joint is investigated and studied through the process experiment,especially the mechanism of the effect of additional ultrasonic vibration on the microstructure and recrystallization in the weld nugget zone.For the development and application of the UVeFSW process and the improvement of the friction stir welding level,it has important theoretical significance and practical value of engineering.AA6061-T6 and AA2024-T3 aluminium alloy plates were welded by the upgraded UVeFSW process,respectively.The welding loads,morphology,mechanical properties and material flow of the joints are compared under the conditions with and without ultrasonic vibration.It is shown that the ultrasonic vibration decreases the welding loads to some extent,improves the weld formation and materials flow,enlarges the volume of the welding nugget zone or deformed zone,and improves the mechanical properties of joints.In order to reveal the influence of ultrasonic energy in UVeFSW,the electron backscatter diffraction(EBSD)technology was used to obtain information on the microstructure in the weld nugget zone.In this case,weld joints of AA6061-T6 and AA2024-T3 were prepared using friction stir welding with and without the ultrasonic vibration under optimized welding conditions.For AA6061-T6,EBSD of various locations on transverse and longitudinal of weld nugget was performed to derive the status of grain structure and recrystallization.For AA2024-T3,the continuous regions which locate on a transverse plane and various locations around the exit-hole under different plastic deformation on horizontal planes were detected by EBSD.The results show that the ultrasonic vibration improves grain refinement while it induces changes in grain orientation.Crystallographic features indicated that the complete continuous recrystallization of the nugget increased with ultrasonic vibration.The normalized ultrasonic effect based on materials deformation and grain structure evolution in the weld nugget indicate that ultrasonic energy has maximum distribution at the center of weld center axis of workpieces and the effect is reduced towards the extremities.In this study,a novel method to characterize the variation of strain component during microstructure evolution of weld nugget zone of the joint is proposed.EBSD strain component data of various locations on the transverse cross section of AA6061-T6 joint reveal that strain component distributions varied from location to location.The net strain components are higher in the presence of ultrasonic vibrations.The relative effect of ultrasonic energy on grain structure evolution was interpreted based on specific strain component(SSC).In the presence of ultrasonic vibrations,SSC was high,which caused higher strain dependent grain refinement.Consequently,based on the numerous advantages of the UVeFSW process in the butt-welding of Al-alloys,it appears rational to employ the ultrasonic vibrations in the lap-welding.To confirm the wide applicability of UVeFSW systems,several aspects such as welding loads,interface defects and failure mechanism of joints with and without ultrasonic energy were investigated.It is found that the application of ultrasonic vibrations can also improve the welding process and joint properties.