3.4.1. Cross-Section under the Weld Shoulder

The wavy pattern visible in Figure 9a represents the large deformation in the weld crown. The microstructure exhibits a morphological variation from a coarse grain structure at the top (Region A, Figure 10b), altered to an elongated pattern (Region B, Figure 10c) which eventually leads to a large number of globular equiaxed grains observable at the inside region (region C, Figure 10d). This microstructural subdivision with a gradual trend from the edge of the sub-shoulder region

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towards the inner parts of the SZ can be attributed to the grain shape adjustment during the continuous recrystallization induced by the large plastic deformation.

**Figure 10.** Grain and flow details of the sub-shoulder region of the weld crown (Reagent B). (**a**) macrograph of the sub-shoulder region; different microscopic features are enlarged as Regions A, B, C in Figure 10b–d, respectively; (**b**) Regions A; coarse grain structure, (**c**) Regions B; elongated grains, (**d**) Regions C; ultrafine grained microstructure.

The curving area near the top surface exhibits a random distribution of the coarse grains (Region A, Figure 10b). This can be caused by the thermal dissipation gradient near the surface, similar to the chill zone in casting.

At region B (Figure 10c) a lamellar morphology is initiated at the crown region where the slope increases at the edge and a large shear strain is imposed through the sub-shoulder area which simultaneously experiences the maximum compression rate. In this situation the dynamic recrystallization takes place very fast which pins the grain boundaries and rearrange the grains to an elongated morphology.

Stirring is inherently classified as a large strain warm deformation. During the stirring action at the inner layers of mass located far away from the top surface, the pin-driven plastic deformation prevails over the shoulders. This can also be considered a severe grain deformation due to the mechanical stirring action. Consequently, the elongated grains undergo a fragmentation and the subsequent formation of an ultrafine grained microstructure (Region C, Figure 10d). On the other hand, because this location is far from the surface, rapid cooling is unlikely. Therefore, the grains distribution is homogeneously equiaxed. This is also the region where other abnormal microscopic features have been observed, such as metallic-glass amorphous structure and localized shrinkage [33].

3.4.2. Plan View under the Shoulder (Longitudinal Axis)

To explore the relationship between the pin and sub-shoulder flow, we investigated the flow features of the weld in the plan view under the shoulder in the longitudinal axis of the weld-line. The results of the sub-shoulder flow observation are illustrated in Figures 11 and 12.

**Figure 11.** Surface flow patterns at the centre of the weld-line. (**a**) Plan view of the weld surface, (**b**) Macro-etched sub-shoulder (0.5 mm deep) flow patterns at the AS, and (**c**) Macro-etched sub-shoulder (0.5 mm deep) flow patterns at the RS.

**Figure 12.** The top view of the sub-shoulder region with a proposed model for the flow-lines 0.5 mm underneath the shoulder during the stirring action. (**a**) the flow model of the sub-shoulder area, (**b**) the macro-etched sub-shoulder region including the actual flow patterns, at the position of the exit zone.

Figure 11 shows the plan view of the sub-shoulder flow for the weld-seam at the centre of the weld-line, where the tool was half way through the weld trial. Figure 11a shows a plan view of the actual weld-line, including periodic weld pitches at the weld crown, representative of the deposited stirred mass layers at the trailing edge of the tool. Figure 11a,b show the macro-etched flow patterns of the weld-seam for the same position of the weld in Figure 11a. The samples were polished to approximately 0.5 mm in depth, and were macro-etched to show the surface flow pattern both in the AS (Figure 11b) and RS (Figure 11c).

As shown in Figure 11b,c, the flow patches at the plan view of the weld-line exist at both sides; AS and RS. This is consistent with our interpretation that the sub-shoulder flow lines arise from the deposition of the stirred layers of mass at the trailing edge of the tool. The periodic deposition of the mass flow creates a pitch pattern for the surface flow which extends to some depth, driven by the action of the shoulder during the stirring. These features are not created by the pin, but rather the shoulder.

The features in Figure 11a,b are smeared material that extends to some depth below the surface, reveal that the flow lines at the edge of the weld-line, and different features are evident at the AS and RS.
