**7. Summary and Conclusions**

This work developed a process termed accumulative extrusion bonding for introducing laminated structures in metallic tubes. To this end, dies are designed and evaluated using the finite element method simulations. Several multilayered tubes are produced using the process and characterized for the extent of bonding, microstructure, hardness, and strength. Significantly, bonding at the interface is achieved for the copper-copper metallic tubing to about 85% at a radial strain of 68% imparted by the AEB process. Since complete bonding is desired, it is recommended to increase the radial deformation to a value greater than 68%. The additional key findings are:


To achieve unique material properties permitted by ultrafine-laminated structures in tubes like those achieved in sheets, it is necessary to bond different material combinations and further push the layering to the thickness at and below the grain size-level. Future work will explore the possibility to extrusion bond more metal-metal combinations, push the processing to achieve finer layering, and, as necessary, improve the die design.

**Author Contributions:** Conceptualization, M.R.S. and M.K.; methodology, M.R.S. and M.K.; writing original draft preparation, M.R.S.; writing—review and editing, M.K. Both authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the U.S. National Science Foundation (NSF) under Grant No. CMMI-1727495.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data cannot be shared at this time due to technical or time limitations.

**Acknowledgments:** University of New Hampshire for laboratory support and L&S Machine Company LLC for manufacturing die components. The authors also acknowledge support and input from professors Brad L. Kinsey, Jinjin Ha, and Yannis P. Korkolis.

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