Improving tensile properties of Al/Mg joint by smashing intermetallic compounds via ultrasonic-assisted stationary shoulder friction stir welding

Continuous intermetallic compounds (IMCs) at the nugget zone (NZ) of dissimilar friction stir welded Al/Mg joint easily become crack initiation and propagation path, deteriorating mechanical properties. To reduce or eliminate the disadvantages induced by the continuous IMCs, ultrasonic-assisted friction stir welding (UaFSW) based on stationary shoulder system was employed to join 6061-T6 aluminum alloy and AZ31B magnesium alloy. Defect-free joint without the shoulder marks was obtained under the synergistic effect of the stationary shoulder and the ultrasonic. Vibration and acoustic streaming induced by the ultrasonic broke the continuous IMCs layer near the thermo-mechanically affected zone (TMAZ) of advancing side (AS) into pieces or particles. Facture path from the short Al/Mg interface near the TMAZ at the AS of the conventional joint was changed to the long Al/Mg interface at the retreating side (RS) of the UaFSW joint, improving tensile properties. Maximum values of tensile strength and elongation of the UaFSW joint were 152.4 MPa and 1.9%, which were 17 MPa and 0.8% higher than those of the conventional joint, respectively.