In-situ time-resolved neutron diffraction measurements of microstructure variations during friction stir welding in a 6061-T6 aluminum alloy

The microstructure change is one of the most important research areas in the field of friction stir welding (FSW). However, in-situ observation of microstructure changes has been extremely challenging because many measurement techniques are not applicable. Recently developed in-situ time-resolved neutron diffraction method, which drastically improves the temporal resolution of neutron diffraction, enables observervation of the transient microstructure changes during FSW. A portable FSW system was installed in the Spectrometer for MAterials Research at Temperature and Stress (SMARTS) at Los Alamos Neutron Science Center and the FSW was made on 6.35mm-thickness 6061-T6 Al alloy plate. At the same time, the neutron beam was centered on the mid-plane of the Al plate at 8 mm from the tool center (underneath the tool shoulder). The diffraction peak was continuously measured during welding. The peak broadening analysis has been performed using the Williamson-Hall Method. The result shows that the dislocation density of about 3.2 × 10¹⁵ m^-2 in that position duing FSW, which is a significant increse compared to before FSW (4.5 × 10¹⁴ m^-2) and after FSW (4.0 × 10 ¹⁴ m^-2). This quantitatively analysis of the grain structure can provide an insight to understand the transient variation of the micro structure during FSW.