Through thickness property variations in friction stir welded AA6061 joint fatigued in very high cycle fatigue regime

•Gigacycle fatigue strength of FSW joint through the thickness was obtained.•Fatigue strength decrease from top to root of the FSW joints.•Fatigue crack initiated from the boundary between the TMAZ and HAZ.•Fatigue crack initiation process occupied over 98% of total fatigue life in VHCF. Ultrasonic fatigue tests were performed on friction stir welded AA6061 joint to investigate very high cycle fatigue (VHCF) behaviors. As a result, almost all the fatigue cracks are initiated from local plastic slip markings around the boundary between thermo-mechanically affected zone and heat affected zone. The fatigue strength decreases from the top to root of the welded joint, owing to the variation of plastic deformation history and temperature distribution through the thickness. In fractography, the fatigue crack initiation site is surrounded by a semicircular flat zone, of which the formation in VHCF regime accounts for more than 98% of the total fatigue life.