*3.2. Microstructure Evolution*

The transverse section of a BFSW weld-seam should reveal three typical zones; HAZ, TMAZ and SZ, distinguished from the BM towards the centre of the weld. However, macro-sections of the BFSW welds (Figure 2) only indicated a primary macrostructural region for the stir zone (SZ) at the centre of the weld. The other transitional zones could not be identified using macro etching. To make the TMAZ and HAZ regions visible we re-polished, used etchant B and higher magnification for the micrographs and then montaged them together.

Figure 3 shows a photomontage of the micrographs taken from the transverse cross-section of the BFSW weld (selected from the sample of Figure 2b) close to AS and the RS hourglass boundaries, etched by Reagent B. This reveals the deformation-induced grain refinement at the transition region (HAZ/TMAZ) on either side of the SZ.

The latter region exhibits a grain orientation transformation. The transformation is represented in grain size and morphology. The transition region has rotated-elongated grains morphology detectable with a distortion in grains compared with the parent material. Compared to the large columnar grain structure of the BM, the microstructure of the SZ reveals a homogenous distribution of the fine equiaxed grains. This abrupt change in structure separates the TMAZ/HAZ region from the plastically deformed finer-grained SZ, and the large directional grains observed in the BM structure. This is attributed to dynamic recrystallization (DRX), which in turn arises from the thermomechanical (heating & strain) mechanisms during stirring.

**Figure 3.** Photomontage of the microscopic features for the BFSW weld structure, near the AS and RS borders (etched by Reagent B). (**a**) transition region in AS border (regions 3 in Figure 2b), (**b**) transition region in RS border (regions 4 in Figure 2b).
