The microstructure development in the weld key-hole along the plate thickness was investigated through scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) techniques. A fundamental understanding of the contact condition at the tool/workpiece interface in a TriflatT M designed friction stir welding (FSW) tool is presented. The development of grain structure for “stop-action” weld reveals that the plasticized material undergoes complex interacting flows due to the presence of threads and flats. The localized high material velocities induce a significant increase in the strain rate within thread space and thereby tune the Zener-Hollomon parameter. An interesting analogy is drawn between the evolution of the secondary shear zone (SSZ) in a typical metal cutting and the steady state FSW processes. The presence of different morphology of the material filling the thread space is linked to the local variation of the contact state variables along weld thickness. It was found that the contact state is governed by the intrinsic interface characteristics and the mechanical property of the material. It was also shown that the presence of larger particles (> 0.5 μm) is responsible for the formation of high angle boundaries (HAGBs) and random recrystallization texture within the stir zone, reflecting random grain orientation due to particle-stimulated nucleation (PSN). © Springer-Verlag London Ltd., part of Springer Nature 2018.