Comparison between contact and immersion ultrasonic techniques to evaluate longitudinal residual stress in friction stir welding of aluminum plates

Ultrasonic stress measurement is a nondestructive technique that has increasingly been employed to evaluate the welding residual stress of different materials. This study compares contact and immersion ultrasonic waves employed to measure residual stresses that are produced by friction stir welding (FSW) of 5086 aluminum plates. The ultrasonic stress measurement technique employs critically refracted longitudinal (LCR) waves produced by 2 MHz contact and immersion ultrasonic probes. The LCR waves are longitudinal ultrasonic waves propagated parallel to the surface within an effective depth inside the material. A finite element simulation of the welding process, verified by a hole-drilling technique, was also used to validate the results of ultrasonic stress measurement. The combination of using finite element analysis along with the LCR technique is known as the FELCR technique. The ultrasonic technique is able to measure the average of stresses within the depth of material; hence, subsurface stress analysis by the finite element technique was used for comparison with the ultrasonic measurement results. The distribution of longitudinal residual stress was determined by employing the FELCR technique and utilizing the contact and immersion probes separately. The results were then compared, showing no considerable difference between using the contact and immersion probes in ultrasonic stress measurement of FSW on the aluminum plates.