Dynamic microstructural evolution during friction stir processing of an Al4Si binary model alloy: an in situ study

We use in situ synchrotron X-ray diffraction during friction stir processing (FSP) of a binary Al – 4 at.% Si model alloy to understand its dynamic microstructural evolution as a function of a comprehensive matrix of experiments. We position the synchrotron beam within the stir zone and study the microstructural evolution via two approaches: static (fixed region of interest) and trailing (constant 0.16 mm distance behind the FSP tool). In static configuration, we study the stir zone microstructure during cooling as the FSP tool moves away from the region of interest. In trailing configuration, we study the microstructural changes developing along the processing length. Complementary postmortem microstructural characterization via energy dispersive spectroscopy, electron backscatter diffraction, transmission electron microscopy, and atom probe tomography are provided, relating in situ observations to the final microstructures. Our results provide valuable insights on the relationship between lattice expansion/contraction of Al and Si under different combinations of tool rotation speeds and processing speeds. We calculate the evolution of the processing temperatures along the processing length based on the linear expansion coefficients of Al and Si. The non-deformable brittle nature of the Si phase, alongside its limited solubility in Al, allows Si to serve as a crystallographic thermocouple. The occurrence of dynamic recrystallization is studied through the evolution of the full width at half maximum (FWHM) of the Al peaks. Our results demonstrate the practicality of in situ synchrotron X-ray diffraction as a tool to expand the current understanding of dynamic recovery and recrystallization during FSP.