**4. Conclusions**

A high velocity plastic deformation joining technique was performed on dissimilar Al/Cu tubular assemblies through the application of magnetic pulse forces. This study evaluated the influence of the surface roughness of the copper target component on the quality of the joint considering the microstructure of the interface and the fracture load obtained from the uniaxial tensile test.

The surface condition of the target component influences the welding quality. However, it is not possible to correlate the surface roughness of the target with the waviness interface and also with the tensile load. Nonetheless, the formation of a waviness pattern at the joint interface requires a moderate roughness since neither the smoothest surface (polished copper) nor the least smooth surface (sandblasted surface) established a waviness pattern, and the best pattern was formed in the joint produced by the use of chemically treated surface. The surface roughness should have influenced the jet formation and consequently the pattern at the joint interface. Though the MPW is called a solid-state process, the occurrence of localized melting was verified. The formation of the intermetallic compounds (mainly Al2Cu phase) at the Al/Cu joint interfaces which found wavy patterns is confirmed. Moreover, some microvoids and a few microcracks were also formed in these reacted zones.

The strongest joint was obtained from the use of a threaded target, and it was achieved without the formation of any intermetallic or waviness at the interface. This joint failed at the Al tube base during the tensile test, indicating a sound joint though it was not a metallurgical type. Thus, in the MPW process, it seems very challenging to achieve an e ffective metallurgical bond.

The joint assembly prepared by lubrication on the target surface failed during the MPW process, meaning that the preparation of a clean surface appears a prerequisite for this process.

For this study, the parameters such as the stando ff distance, the voltage level, and LWZ distance were kept constant, and this arrangemen<sup>t</sup> was implemented to study the influence of the surface conditions for the same working conditions. However, it seems that the samples' length may influence joint formation. Thus, for any design, a new welding window should be optimized.

**Author Contributions:** Contributed to the conceptualization of this study, A.R., I.V.d.O., and G.A.T.; contributed to the experiments, A.M.R.; contributed to the conceptualization and writing the manuscript, O.E.; provided the revision, O.E. and A.R. All authors have read and agreed to the published version of the manuscript.

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

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