**5. Conclusions**

The internal flow features of the AA6082-T6 BFSW were evaluated using a set of developed reagents and optical microscopy. The key findings are that the dark curved patterns (conventionally called "flow-arms"), are actually oxidation layers at the advancing side, and at the retreating side are elongated grains with a high-density of accumulation of sub-grain boundaries due to DRX. A model of discontinuous flow within the weld is proposed, based on the microscopic observations. It is inferred that the internal flow is characterized by packets of material ("flow patches") being transported around the pin. At the RS they experience high localized shearing at their mutual boundaries, as evidenced in high density of sub-grain boundaries. Flow patches at the AS are stacked on each other and exposed to oxidization.

**Author Contributions:** Conceptualization, A.T., D.J.P.; methodology and formal analysis, A.T.; supervision, D.J.P., D.C., K.S.; validation, A.T., D.J.P.; writing–original draft, A.T.; writing–review & editing, A.T., D.J.P., D.C., K.S.

**Funding:** This research received no external funding.

**Acknowledgments:** Thanks are extended to Mike Flaws and Kevin Stobbs for assistance with electron and optical microscopy, respectively.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
