NDE of friction stir welds of Al alloys using high-frequency acoustic microscopy

This paper presents results on high-frequency ultrasonic imaging of bonds between different materials with application to friction stir welding (FSW). The unique feature of this paper is the use of high numerical aperture lenses that allow the excitation of leaky surface acoustic waves. These interact with weak bonds to produce wave interference of incident and reflected waves at weak bond interfaces. The interference provides a vivid contrast marking the weak interface bond as an easily recognisable feature. Presented are a description of the high-frequency lenses, simulation of the contrast phenomenon and the application to friction stir welds. The welds were between cast A356 and wrought 6061 Al alloys obtained under four different processing conditions. The selected sections of these were imaged at high frequencies in the GHz range by bringing the acoustic lenses towards the specimen's surface far enough so that the peripheral lens rays were incident at large enough angles to exceed the first and second critical angles and to excite leaky surface acoustic waves. The interference of these waves allowed the imaging of the bond details with a resolution of about 1.5-3m. The features and flaws observed in the images were correlated with those reported on in the literature as well as those obtained from scanning electron microscopy and tensile property evaluation. The quality of weld bonds inferred from the images was correlated to the tensile fracture experiments, which showed that improperly bonded specimens showed a relatively larger number of flaws. The results suggest that high-frequency acoustic microscopy is indeed a useful method for the diagnostic evaluation of FSW, especially since acoustic microscopes are now available as portable instruments commercially.