Using ultrasonic waves and finite element method to evaluate through-thickness residual stresses distribution in the friction stir welding of aluminum plates

The main goal of this study is ultrasonic measuring of through-thickness residual stresses in friction stir welding of aluminum plates. A 3D thermo-mechanical finite element analysis is used to evaluate residual stresses caused by friction stir welding of 5086 aluminum plates. The finite element (FE) model has been validated by the hole drilling method. Residual stresses obtained from the FE analysis is then compared with those obtained from ultrasonic stress measurement. The ultrasonic measurement technique is based on acoustoelasticity law which describes the relation between the acoustic waves and the stress of material. The ultrasonic stress measurement is accomplished by using longitudinal critically refracted (L_CR) waves which are propagated parallel to the surface within an effective depth. Through-thickness distribution of longitudinal residual stresses is evaluated by employing the L_CR waves produced by four different frequencies (1. MHz, 2. MHz, 4. MHz and 5. MHz) of ultrasonic transducers. Utilizing the FE analysis along with the L_CR method (known as FEL_CR method), the through-thickness distribution of longitudinal residual stress could be achieved. The comparison between ultrasonic and FE results show an acceptable agreement, hence the FEL_CR method could be successfully applied on the FSW plates. It has been concluded that the longitudinal residual stresses of aluminum plates joined by friction stir welding can be evaluated by using the FEL_CR method. The good potential of FEL_CR as a nondestructive method is also confirmed in through-thickness stress measurement of aluminum plates.