Effect of ultrasonic vibration on fatigue performance of AA 2024-T3 friction stir weld joints

Abstract

Fatigue test of conventional friction stir welding (FSW) joints, ultrasonic vibration enhanced friction stir welding (UVeFSW) joints and base metal in 2024-T3 aluminum alloys are performed to investigate the influence of ultrasonic vibration on the fatigue strength and life of joints. It is found that for 50% survival probability the fatigue strength of UVeFSW joint is greatly improved, up to 96.13% of the base metal. On the advancing side, there exists a weak bonding region at upper part of transverse cross section of welds due to material flow. The stress concentration reaches maximum in the weak bonding region, which usually becomes the fatigue crack initiation site of FSW weld. The application of ultrasonic vibration narrows the weak bonding region and reduces the stress concentration there. In the crack propagation zone, the fatigue striations of UVeFSW joints fracture are parallel with each other and spaced apart, while the regular fatigue striations are not obvious but with larger spacing and poorer parallelism for FSW joint fracture. In the final fracture region, the application of ultrasonic vibration makes the secondary-phase particles smaller in size, more in quantity, and more uniform in distribution. All these contribute the improvement of fatigue performance in UVeFSW.