Improving the Mechanical Properties of 2219-T6 Aluminum Alloy Joints by Ultrasonic Vibrations during Friction Stir Welding

Friction stir welding assisted by ultrasonic vibrations on the bottom surface of workpieces (UVBS- FSW) is proposed. The aim of this study is to reveal the effect of ultrasonic vibrations on the microstructure and mechanical properties of the joint. For comparative purposes, another joint without the effect of ultrasonic vibrations was obtained using the same welding parameters. The morphologies of onion skin on the top surface are changed and the distance from the wave crest to the trough is directly reduced from 90 ± 3 μm to 67 ± 3 μm as a result of the ultrasonic vibrations. The macrostructures indicate that more material around the probe is softened and takes part in the plastic deformation due to the Blaha effect of the ultrasonic vibrations. A higher intensity of the texture and typical B/B¯ texture component in the stir zone (SZ) were detected in the UVBS-FSW joint. The hardness distributions are more homogeneous and the density of geometrically necessary dislocations, stored strain energy and residual stress in the joint are all reduced due to the application of the ultrasonic vibrations. Finally, the tensile property and fracture elongation of the joint are improved by UVBS-FSW.