Vibration Exerting Modes And Experimental Study Of Ultrasonic Assisted Friction Stir Welding

Conventional friction stir welding (FSW) technology has been widely employed in the manufacturing of light alloy structures. However, FSW has disadvantages such as low welding speed and high welding load. To encounter these, ultrasonic assisted FSW has been developed. Ultrasonic vibration is advantageous in reducing the deformation resistance of material and improving the welding efficiency. In this modified FSW process, ultrasonic exertion on the material is possible through various modes. However, all the modes of ultrasonic exertion are not equally effective. Development of a dedicated ultrasonic vibration assisted FSW technology necessitates a broad investigation on the effects of different vibration exerting modes which would aid to explore a more suitable way to transmit ultrasonic energy.In this study, two experimental platforms for ultrasonic assisted FSW were set up. The first set up, the so called ultrasonic assisted FSW (UaFSW), consisted of arrangements where ultrasonic vibration was exerted on the workpiece plane by the roller of an ultrasonic seam welder. Second set up was ultrasonic vibration enhanced FSW (UVeFSW) where ultrasonic vibration was transmitted into the region of the workpiece near and ahead of the rotating tool in the direction of depth. Welds of 6 mm thick AA2024-T3 plates were prepared using FSW, UaFSW and UVeFSW processes. The morphology, microstructure and mechanical properties of the joints were examined. Welding of dissimilar materials and marker insert techniques were employed to investigate the material flow characteristics around the tool and the underlying physical mechanisms of the processes. Tool traverse force, torque and axial force were measured during the welding process. Thermal cycle of each process was measured by placing thermocouples at different positions during the welding.Compared with FSW, the width of the weld nugget zone was wider, and the material flow around the tool was enhanced in UaFSW, which would be beneficial for the diminution or elimination of the weld inner defects when welding at higher speeds. The grain refinement in the nugget zone was not obvious with ultrasonic vibration, while, it promoted expansion of the nugget zone to the thermo-mechanically affected zone. The tensile properties of the joints changed slightly in this mode of ultrasonic exertion. The ultrasonic energy was divergent during UaFSW, still it could improve the microhardness in the upper weld nugget zone. Ultrasonic vibration had no significant effect on the axial force during welding, however, the traverse force and torque of the tool increased in UaFSW compared with conventional FSW.In UVeFSW process, results of dissimilar welding showed that material mixing was higher on both sides of the joint line during the welding with ultrasonic vibration while the results of marker insert studies indicated that ultrasonic vibration could enhance the backward flow of the material. Besides, the tool stirring action became stronger and the nugget width was wider with ultrasonic vibration which would help eliminate weld defects. Microstructure of the weld cross-section showed no significant change in grain size with ultrasonic exertion. However, the boundary of the nugget zone and the thermo-mechanically affected zone in the UVeFSW weld became smoother compared with that in FSW. Results demonstrated the percentage elongation of the joints was improved in UVeFSW at low tool rotating speeds which also caused variation in the mode of cracking in UVeFSW. The crack in the conventional FSW joint propagated along the thermo-mechanically affected zone and the weld nugget zone, while in UVeFSW, it was located in the heat affected zone. In UVeFSW, ultrasonic energy was effectively transmitted to the bottom of the weld nugget which improved the microhardness there. Contrary to UaFSW, the welding load was lower in UVeFSW than in conventional FSW, which could help decrease the tool wear and prolong its service life. Ultrasonic exertion showed preheating effect in the region to be welded ahead of the tool in UVeFSW, but only insignificant. During the heating period of the weld thermal cycle, comparing with conventional FSW, the maximal temperature increment in the measured locations with ultrasonic vibration was about 40 ℃. The peak temperature, about 330 ℃, was almost unaffected by the ultrasonic vibration.