Preliminary investigation by synchrotron radiation of small 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 the 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, 100 microns or more, makes them hardly detectable by means of conventional control methods. Further investigation of these small defects will provide a new unique insight on the defects that can be found in FSW samples. During the third experiment, that has not been finished yet, both a long and a small crack could be detected on the two sides of a notched sample. The images show that the crack 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.