Eliminating Keyhole by Ultrasonic-Assisted Passive Filling Friction Stir Repairing Process

The keyhole in the AZ31B magnesium alloy plate was successfully repaired by ultrasonic-assisted passive filling friction stir repairing (U-PFFSR) process and the defect-free repaired joint was obtained. The keyhole was firstly filled by the filling material (FM), and then the FM and base metal (BM) was bonded by the friction stirring of pinless tool and the high-frequency vibration of ultrasonic. With the addition of ultrasonic, the joint without kissing bond defect between the FM and the BM was more easily attained and the microhardness of repaired region was increased, thereby heightening the tensile strength of repaired joint. The maximum tensile strength of U-PFFSR joint was 182.9 MPa, which was increased by 38% compared to that of repaired joint without ultrasonic. The fracture mode of repaired joint was changed into the ductile fracture from the brittle fracture when the ultrasonic was added.