| Applying ultrasonic energy to assist friction stir welding(FSW)provides various advantages over the conventional FSW process such as improve welding speed,reduce welding loads,and enhance material flow.Several modes of exertions of ultrasonic vibration were adopted.These are,horizontal vibration exertion onto the FSW tool via bearings,vertical vibration exertion on the FSW tool,and lateral vibration exertion to the workpiece,ultrasonic energy exertion into the localized area of the workpiece near and ahead of the FSW tool and ultrasonic energy exertion on the back of the workpiece.However,some problems were associated with these modes of exertions.Development of new equipment and a new process for highly efficient coupling of ultrasonic vibration and FSW is of great significance to improve the welding quality,improve the welding efficiency,and enhance the welding manufacturing level of lightweight structures.The XHL7132 CNC milling machine was converted into an FSW machine.Ultrasonic vibration was transmitted to the FSW tool from the transverse direction through rollers,the ultrasonic vibration assisted friction stir welding(UVaFSW)system was successfully developed.The amplitude measurement showed that,in UVaFSW,the effective coupling of ultrasonic vibration to the rotating tool was achieved by rollers,avoiding the energy loss of bearing-type coupling.The ultrasonic vibration was transmitted indirectly to the weld zone via the FSW tool,the workpieces realized simultaneous coupling of FSW thermo-force effect and ultrasonic softening effect.An investigation was carried out into the application of ultrasonic vibrations energy exerted to the workpieces’ weld zone via the FSW tool in friction stir welding of 6061-T6 aluminium alloy plates.The results showed that UVaFSW could broaden the process window,refine the grains in the weld nugget zone(WNZ)and improve the mechanical properties of the joint.At a rotation speed of 600 rpm and a welding speed of 200 mm/min,the application of ultrasonic completely eliminated the hole-shaped defects on the advancing side and increased the welding speed of obtaining a good joint to 280 mm/min.The microhardness of the WNZ,tensile strength and elongation of the weld were improved,and the impact toughness of the centre and both sides of the weld increased.Force sensors were used to examine the welding loads.It was found that UVaFSW could reduce the torque and axial force of the rotating tool.At a rotation speed of 600 rpm and a welding speed of 160 mm/min,the axial force and torque at the steady welding stage were reduced by 11.35%and 7.1%,respectively.The thermal cycle in welding was measured by thermocouple.The results showed that the UVaFSW produced a less significant influence on the thermal effect on the workpiece.The peak temperature of the thermal cycle was comparable to that of FSW except for a slight increase in heating rate and a slight decrease in cooling rate.The 6061-T6 and 2024-T3 dissimilar aluminium alloys plates were used to study the material flow in UVaFSW.The results showed that ultrasonic vibration was effectively transmitted to the bottom of the weld,which promoted the migration of plastic material on both sides of the weld.The depth and width of the shoulder-affected zone expanded;at different depths of this zone,the horizontal migration distance of the material increased.The boundary of the pin-affected zone expanded to both sides;at different depths of this zone,the horizontal migration distance of the material decreased,while the width of the plastic deformation zone showed an increasing trend,which indicated that more material moved from the bottom up.On the advancing side,there was a clear intersection of the horizontal migration and vertical migration.In the horizontal section,at the top and bottom of the weld,when the material transitioned from the keyhole,generated by tool"stop-action" technique,to the steady-state zone,the curvature of the transition interface increased,indicating that the rotation of the tool could drive more rear material flow. |
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