| The application of aluminum/steel composite structure can not only meet the demand for lightweight,but also take into account the advantages of performance and cost,and plays an irreplaceable role in the development of automobile lightweight.Due to the great differences in thermophysical properties,chemical properties,and mechanical properties between aluminum alloy and steel,the high-quality connection of aluminum/steel dissimilar metals is facing severe challenges.Friction stir welding(FSW)technology has been successfully applied to the joining of dissimilar alloys,but there are still some problems,such as poor interface bonding ability,low welding efficiency,and short service life of the FSW tool.In order to solve the above problems,a new FSWmodified process,which applies ultrasonic vibration directly to the surface of the workpiece to be welded,was used for welding dissimilar metals of aluminum/steel.The influence mechanism of ultrasonic vibration on joint quality and welding process was clarified by process test.Firstly,the conventional friction stir lap welding(FSLW)and ultrasonic vibration enhanced friction stir lap welding(UVe FSLW)of 6061-T6 aluminum alloy /Q235 steel were carried out,and the effects of ultrasonic energy on weld formation,joint macromorphology,and mechanical properties were compared and analyzed.The results show that ultrasonic vibration can effectively soften the workpiece to be welded,improve the forming quality of the weld surface,eliminate tunnel defects caused by poor material fluidity,expand the volume of the weld nugget zone,and broaden the welding process window.Ultrasonics improves the failure load of the lap joint,changes the failure form and fracture mode of the joint,and improves the microhardness of the aluminum alloy side and steel side.Subsequently,the microstructure of different areas of the joint was characterized,and the optimization mechanism of ultrasonic on the quality of the joint was studied.It was found that additional ultrasound can change the distribution characteristics of steel structure near the interface,refine the grain structure of the weld nugget zone and thermomechanic affected zone,make the precipitated phase in the weld nugget zone dispersed,reduce the generation of intermetallic compounds on the retreating side of the interface,and promote the mixing of materials on the advancing side of the interface.The increase in failure load of the UVe FSLW joint was the result of joint optimization with the increase of effective bonding width,the improvement of macro/micro mechanical interlocking ability,and the thinning of the intermetallic compound layer.On this basis,the corrosion resistance of the joint was studied,and it was found that ultrasonic vibration improved the corrosion resistance of the joint.Besides,the influence of ultrasonic energy on the FSLW welding process(welding load,thermal cycle,and material flow)of aluminum/steel dissimilar metals was further explored.It was found that ultrasonic can significantly decrease the welding load,and the reduction of welding axial force was the largest.The addition of ultrasonic vibration reflects its preheating effect and reduces the peak temperature during welding.The instantaneous state of the weld was obtained by "emergency stop+rapid cooling" technology,and the material flow near the keyhole was characterized by industrial CT.It was found that ultrasound increased the flow range of steel particles near the pin of the FSW tool and changed the flow path of the steel structure at the same time. |
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