Ultrasonic techniques for the detection of discontinuities in aluminum foams

Metal foams are interesting materials with many potential applications. They are characterized by a cellular structure, that is the metals or metal alloys foamed include gas voids in the material. Their particular lightweight structure and physical, chemical and mechanical properties make them suitable for a wide range of industrial applications in different sectors. For industrial applications, metal foams offer attractive combinations of low density, high stiffness to weight ratio, good energy absorption and vibration damping capacity that cannot be obtained with other materials. The control of the foaming process and the characterization of the metal foam are important issues in order to obtain a product with good properties and guarantee the quality of a mechanical component. The characterization and control of mechanical components and sandwich panels manufactured with metal foams require the assessment of the defects present in this material, like large pores or imperfections which are responsible of deteriorating the mechanical performance. Therefore, specific methods of non-destructive testing are required, both in the manufacturing process and during the life of the component. In this work, some ultrasonic transmission techniques developed for detection of defects associated with the manufacturing process of aluminum foams are proposed. These techniques were used on plates and structures of different thicknesses and geometries formed with this material. Ultrasonic transmission techniques were carried out both, with low frequency air coupled transducers, and higher frequency transducers, focused and unfocused, by contact and immersion. To validate the results, the ultrasonic images obtained were compared with radiographic images of the foam.