Process And Mechanism Of Ultrasonic Vibration Enhanced Friction Stir Welding

In conventional friction stir welding (FSW), large axial force and tool torque are necessary to guarantee sufficient heat generation, which may cause the tool wear and limit the welding speed, especially in welding high strength and high hardness materials. To solve these problems, some researchers used secondary heat sources (laser beam, induction, electric resistance, and electric arc) to assist softening of the material, thus, the required axial force and the tool torque are lowered, and the welding speed is increased. However, the extra heating may lead to some inherent disadvantages such as mechanical properties degradation caused by the increase of heat affected zone and the growth or dissolution of precipitated phase. It is well known that ultrasonic vibration can remarkably soften the metallic materials without significant heating. Therefore, ultrasonic vibration is of application potential for lowering the welding load and improving the welding speed on the precondition of good weld quality in FSW.In this study, a new modified ultrasonic assisted FSW process was developed, which is called the ultrasonic vibration enhanced friction stir welding (UVeFSW). In this process, the ultrasonic vibration is transmitted into the localized area of the workpiece near and ahead of the rotating tool by an ultrasonic vibration tool head directly.AA6061-T4 plates and AA2024-T4 plates were welded by this new FSW process, respectively. The morphology, microstructure and mechanical prosperities of the joints were examined. The experiment results show that the ultrasonic vibration improves the weld formation, reduce or even eliminate the void defect caused by inadequate material flow, enlarges the volume of the stir zone, refines and homogenizes the grain structure in weld nugget zone and thermo-mechanically affected zone, and improves the mechanical prosperities.Next, the welding loads, thermal cycle and material flow during UVeFSW were investigated. The welding loads were measured by capturing the electrical parameters of the FSW motors simultaneously and transforming them to the torque and force. The thermal cycles were measured by thermocouples at different locations. The results reveal that the main spindle torque and the axial force decrease significantly but the welding transverse force has no obvious change. Although the peak temperature almost remains unchanged, the ultrasonic vibration has a little preheating effect.The marker insert technique and metallographic analysis were combined to characterize the material flow in FSW and UVeFSW, respectively. A thin pure aluminium foil with different configurations was used as marker material. The tool "stop-action" technique and "section" technique were used to illustrate the transient and steady state of the material flow around the tool. The effect of ultrasonic vibration on material flow is demonstrated by comparing and analyzing the material flow velocity and strain rate under the condition with/without ultrasonic vibration. The experimental results reveal that the ultrasonic energy can soften the material and enhance the material flow during FSW. Three different analysis methods were developed to estimate the effect of ultrasonic energy quantitatively based on thorough experimental investigations. The data reveals that under the ultrasonic vibration the softened material volume around the tool, the material flow velocity and the strain/strain rate all increase obviously.