Grating-based X-ray dark-field computed tomography for the characterization of friction stir welds: A feasibility study

Friction Stir Welding (FSW) is a relatively new joining technology. It is mostly applied to weld aluminium alloys, which are difficult to fusion weld. Characterization of these welds has thus far been done almost exclusively by optical or electron microscopy, which only provide surface information about the material. The third dimension is only accessible with other methods, the most common being X-ray computed tomography. In order to image microstructural changes introduced by the welding process, conventional X-ray analysis needs to be performed with very high spatial resolution, which limits the investigations to small volumes. In this paper we present how the recently developed method of grating-based X-ray dark-field imaging is able to retrieve information of the micro-structure in macroscopic friction stir welds. We compare results obtained with optical microscopy and dark-field imaging and show how equivalent geometrical measures can be retrieved from both methods. As dark-field imaging is based on X-rays we extend the analysis to the third dimension. These first results demonstrate the use of grating-based X-ray dark-field imaging as a supplement to the established characterization methods for FSW welds.