Ultrasonic evaluation of friction stir welds and dissimilar intermixing using synthetic aperture focusing technique

Friction stir welding (FSW) is a recently developed solid-state joining process that uses a specially shaped rotating tool to produce lap or butt joints. At the National Research Council, an inter-institute collaboration was started in 2007 with the goal of exploiting the NDE expertise and applying it for the characterization of friction stir welds for various industrial applications. In particular, very good performance was obtained using ultrasonic immersion or laser-ultrasonics combined with the synthetic aperture focusing technique (SAFT) for detecting lack of penetration in butt joints, discontinuities such as wormholes and hooking in lap joints. Dissimilar metal welds of aluminum and magnesium by FSW are also considered for automotive and aerospace applications. Complex vortex flows are produced during the FSW process that may create intercalated lamellar structures with the possible formation of intermetallic compounds, causing variable hardness and degradation in mechanical properties. A modified version of SAFT that takes into account the difference of ultrasonic velocity in the joint between that of Al and Mg has been developed to study the dissimilar intermixing. Welded samples in the butt configuration with different welding speeds and seam offsets are tested using the immersion technique with the modified SAFT. Results will be presented for both defect detection and weld characterization, and the capabilities and limitations will be discussed.