4.1.2. Visualization of Flow Layers

In this work, based on the microscopic observations, we elucidated the internal flow features of the plastic deformation in different regions of the stirring zone during the bobbin friction stir processing. Key concepts that emerge are:


#### 4.1.3. Proposing a Model of the Internal Flow Processes

Many other studies have been based on the assumption of continuous flow within the stirring zone. However, such approaches have not had very much success in predicting the actual flow. It has been particularly difficult to model the tunnel void as a flow-based defect using continuous flow assumptions. The present work puts this into context, by finding that the flow is highly discontinuous, based on observation of the microstructure. In turn this has been facilitated by discovery of a suitable reagent and metallographic measurement. Consequently, the present results imply that methods based on continuous flow computational fluid dynamics (CFD) are unlikely to be successful in explaining flow defects. In CFSW the flow lines ("onion rings") are attributed to the tool-material interaction [18]. However, these flow features are not defects per se. Likewise for BFSW the tool-material interaction has been identified as contributing to discontinuous flow as a bulk effect [34]. The present work is consistent with these findings, but extends them to the defect situation. In particular here it is proposed that the thread-flat features of the pin further to a finer scale discontinuity of flow, in terms of packets of material. These are deposited at the trailing edge of the tool, and squashed in the process.

#### *4.2. Implications for Practitioners*

Industry users of AA6082-T6 should note that tunnel defects are associated with internal oxidization of the weld cross section. This has the potential to cause reduced mechanical properties, corrosion resistance and the fatigue strength as the tunnel void as a macro-size discontinuity can deteriorate the integrity-related properties of the weld.

#### *4.3. Limitations of this Work and Implications for Future Research*

Our analyses of the flow layers were limited in the number of such layers investigated. We cannot exclude the possibility that flow layers may have features of both oxidization and DRX sub-grain boundary formation. A potential future research project could examine multiple flow layers, at different positions along their length, and evaluate both microstructure (e.g., using SEM or TEM) and elemental composition.

3D visualization of the flow layers was not possible during optical and electron microscopy. A possible research question could be to progressively re-polish the surface in a controlled manner to build up a 3D representation of the flow layers.

Another possible future line of research could be measurement of the mechanical properties of the weld in correlation with the flow features. Progressive loading of a sample of the cross section, with microscopy inspection in between, could identify the evolution of the crack propagation and failure mechanisms. Regarding the oxide layers, it may be useful to apply fractography or creep-fatigue tests to examine crack propagation.

It is to be expected that the formation of the macro-size tunnel void and the micro-cracks during stirring action would extensively affect the strength of the final weldment. Macroscopic defects are likely to be unacceptable to industry users, and hence preventing these is the first priority. Even if there is no tunnel defect, the internal micro-cracks are likely to cause adverse outcomes in the three-point bending and other tests used. Hence there could be value in further research into welding process settings, including tool features, that minimise these defects.
