Preliminary investigation by synchrotron radiation of cracks and defects in AA FSW samples

A very good definition of defects position and dimension in welded joints is desirable for application of fracture mechanics techniques and improvement of industrial acceptability criteria. The aim of this paper is to illustrate the preliminary results achieved in the investigation of defects in aluminum welded joints, using the phase contrast X-ray imaging techniques available at SYRMEP beamline of the Elettra synchrotron radiation facility. Both CCD and high-resolution X-ray films were employed as detection systems. Three sets of experiments have been performed so far. The aim of the first one was to investigate feasibility and to optimize the experimental set-up. A good definition of artifact defects on joints, obtained by both conventional welding techniques and Friction Stir Welding, was achieved. Successively, the investigation was restricted to joints obtained by Friction Stir Welding, which is currently the most interesting technique also from an industrial point of view. In the FSW samples prepared for the second experiment some original very small flaws were visible. The flaws size makes the defects hardly detectable by means of conventional control methods. Further investigation will provide a new unique insight on the defects that can be found in FSW samples. During the third experiment, long and a small cracks could be detected on the sides of notched AA samples. A long crack is shown in Fig.1. The images and the tomographic reconstructions (as the one shown in Fig.2) show that the cracks propagates along several planes, a condition that has not been deeply investigated in a quantitative way up to now and will be the object of further investigation. Figure 1: PHC radiography of a long crack (extension approx 3.5 mm). Figure 2: PHC, 3D tomographic reconstruction of a crack (extension approx 1.24 mm).