Research progress in ultrasonic testing for friction stir welding of aluminum alloy

The defects in friction stir welding (FSW) of aluminum alloy, e.g., tunnel defect, lack of penetration (LOP) and kissing bond, are with complex shape and narrow gap by inappropriate welding parameters. Firstly, the characteristics of FSW welding and typical defects were briefly described, and the difficult points in ultrasonic testing were summarized from the low time resolution, incomplete characterization of irregular defects, close acoustic impedance between kissing bond and Al alloy and the reduction of detection sensitivity. Subsequently, the research work on the ultrasonic testing for FSW of aluminum alloy were reviewed from the following four aspects, including conventional ultrasonic testing, time-of-flight diffraction (TOFD), phased array ultrasonic testing and other ultrasonic testing techniques. Finally, the research on the ultrasonic signal processing methods and machine learning methods was prospected. The signal characteristics were analyzed and extracted to further improve the resolution and signal-to-noise ratio of ultrasonic testing and realize the accurate identification and quantification for complex shape defects and subtle defects