3.5.1. Proposed Composition of Flow Layers

The metallurgical analysis in Figures 8 and 9 shows that the flow line patterns evident in optical microscopy are in fact oxidation effects in the AS, and the accumulation of the shearing and formation of high-density sub-grain boundaries during the DRX procedure in the RS. Thus, we propose that what are conventionally called flow arms are better understood as 'flow layers'.

We believe the more accurate interpretation, for the AS, is that they represent the oxidised fronts between packets of material that have been transported through the weld by tool motion and rotation. Where they occur, they are evident in all cross sections along the weld. An oxidization explanation is feasible considering the high temperature of the stirring process, though an explanation is needed for how the air enters the weld - more on this below. We interpret the layers as being a packing (or stacking) mechanism in the AS, i.e., the flow motion is orthogonal to the flow layer in this region. It is interesting to note that the metallographic macro/micrographs showed that the defect-free samples (see Figure 2) do not reveal flow-arms to the same extent.

For the RS the flow layers represent shearing between different layers, i.e., the flow motion may be in/out of the page. This is inferred from by the microstructure in Figure 9, which shows elongated grains with internal accumulation of sub-grain boundaries that have been subject to self-rearrangement during dynamic recrystallization. More specifically, the proposed DRX mechanism is that the stirred flow packets experience severe shearing, and hence stored strain is induced. By using the heat generated during re-cooling, the elongated grains cause a self-rearrangement to relieve the stored strain, leading to a high-density accumulation of sub-grain boundaries inside the grain. As the elongated grains have similar crystallographic orientation, so too the newly formed sub-grain boundaries have similar crystallographic direction. Consequently, both the grain boundaries and sub-grain boundaries etch similarly, and more so than the matrix. The dark colour of these layers under optical microscopy arises

from the etching of these sub-grain boundaries with the characteristic darker contrast compared to the matrix.

3.5.2. Proposed Principles of Motion of Flow Layers

In this regard, and based on the metallurgical improvements, the suggested model in Figure 13 provides an explanation for the entering of the air into the weld region and beginning of the oxidation in flow layers. We introduce the concept of a flow patch. This is a chip of material that is excised from the base material, transported around the weld as a lump, and deposited in the wake of the weld as a flattened layer.

**Figure 13.** Schematic of the entering of the tool into the workpiece, with a proposed model for the entering of the air into the weld region and formation of the oxidation layer between the deposited mass layers at the trailing edge of the tool (Steps **1**–**5** explain the details of the discontinuous flow mechanism).

As shown in step 1 in Figure 13, the entering of the tool into workpiece initiates the excision of the flow patches on the tool, as chip formation on the flat area of the tool. These flow patches are softened by the heat generated from the friction, and in contact with the air form an oxide surface layer. By the further embedding of the tool into the workpiece, the flow patch is also pushed into the stirring zone between the AS and the RS (step 2). Simultaneously, a volume of the air also enters into the material which is mostly because of the angled geometry of the flat-thread configuration (steps 2 and 3). Air is readily available in the weld due to the shaking and vibration of the tool-substrate. This trapped air forms as an air-pocket at the corners of the flat-thread geometry and intensifies the oxidation layers of the plasticized mass during the stirring (steps 3 and 4). Eventually, the stirred mass is transported to the trailing edge of the tool, where they plough into the RS and are deposited as the flow layers of the weld (step 5). The oxidized surfaces of these flow patches are the flow layers that are observed in the cross section. They are deposited as periodic layers, corresponding to multiple flow patches.
