**5. Summary**

The present work describes the microstructure of the Ta/Cu and Cu/SS interfacial layers of explosively welded Ta/Cu/SS clad. It is shown that the hardening and softening processes are strongly related to the microstructure and clad strength evolution. The processes that increase the yield strength of the material are severe plastic deformation, the formation of thin layers of ultra-fine grains, and solidified melt zones, while dynamic recovery, recrystallization, and crack formation inside the solidified melt cause softening.

SEM/EBSD analyses revealed a complex microstructure of parent sheets, which consists of characteristic features such as interlock microstructures, elongated grains, or twins. Strain hardening processes predominate softening ones in the interfacial regions far from large solidified melt zone. It has been established that a high-rate shearing of interfacial layers leads to a strain-hardening due to

intense slip and deformation twinning in copper sheets at both interfaces. In layers surrounding the melted zones, the as-deformed grains are replaced by new recrystallized (equiaxed or columnar) ones due to the heat transfer from the melt to the severely deformed metals. The microhardness of welded sheets increases significantly as the joining interface is approaching excluding the volumes directly adhering to large melted zones, where a noticeable drop of microhardness due to recrystallization is observed.

However, precise analysis of dynamic recrystallization and recovery is exceptionally complicated as both phenomena strongly depend on both temperature and strain distributions near the interface as well as the melting point of the joined metals. Another observation is a quite di fferent morphology of the solidified melt zones at the Ta/Cu and Cu/SS interfaces. The melted zones close to the Ta/Cu interface consist mainly of a mixture of pure Cu and Ta particles of di fferent sizes, whereas the melted zones near the Cu/SS interface are composed of nano-grained compounds based on elements of both neighboring joined sheets. The microhardness of solidified melt regions formed near both interfaces is significantly lower than that measured inside the strain hardened layers of stainless steel. Therefore, the integrity of all clad components is fully conserved on lateral bending.

**Author Contributions:** Conceptualization, H.P.; methodology, H.P. and R.C.; formal analysis, H.P.; investigation, R.C. and I.M.; writing—original draft preparation, H.P.; writing—review and editing, R.C.; visualization, H.P.; project administration, I.M; funding acquisition, H.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Centre for Research and Development of Poland under gran<sup>t</sup> TECHMATSTRATEG2/412341/8/NCBR/2019 (EMuLiReMat).

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