**4. Conclusions**

Copper-clad aluminium rods with different copper sheath thicknesses were produced by SPD at room temperature, followed by annealing rather than by conventional extrusion at elevated temperatures. Detrimental formation of intermetallic phases with low conductivity was not observed. An interface between constituents formed by their severe intermixing due to large shear, high hydrostatic pressure and possibly diffusion during annealing.

Severe plastic deformation was performed using ECAP for one or two passes along Routes A and BC. Extreme grain refinement down to the nano-range size as a result of SPD processing was shown. The UFG microstructure remained stable during annealing. It is demonstrated that to achieve an interface without defects, ECAP must be performed on copper-clad aluminium rods with thickness of copper cladding at least 0.7 mm or higher.

The effective conductivity of aluminium copper-clad conductors dropped after deformation in proportion to the number of ECAP passes (more when using Route BC than A). Annealing, especially short annealing at 200 ◦C; however, resulted not only in recovery of conductivity but in conductivity values exceeding the theoretically predicted ideal ones.

In addition, annealing resulted in an increase of hardness at the interface and in the copper sheath, which confirms the possibility of hardening UFG metals by annealing at temperatures in the range of 0.25 to 0.3 Tm. Thus, SPD and short annealing at the temperature within this range could be used to produce lightweight conductors with high conductivity and strength.

**Author Contributions:** R.L.—conceptualization, funding acquisition, supervision, writing—original draft, analysis of results; M.D.—properties measurements; A.K.—microstructure characterization, analysis of results, writing—review & editing; G.R.—samples preparation and processing, analysis of results.

**Funding:** This research was funded by the EU Framework Program for Research and Innovation 'HORIZON 2020 (Grant-742098).

**Acknowledgments:** R. Lapovok acknowledges the Marie Curie Fellowship within the EU Framework Program for Research and Innovation 'HORIZON 2020 .

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