**4. Conclusions**

The present paper studied different welding techniques applied to hybrid aluminum alloys (EN AW-6181 T6 and EN AC-42100 T6) joined each other in all possible combinations. First, a real case study was analyzed, which consisted of a sports-car frame where the cast EN AC-42100 T6 and EN AW-6082 T6 aluminum alloys were welded with the traditional MIG technique. The MIG, CMT, and fiber laser-MIG hybrid joints of EN AW-6181 T6 and EN AC-42100 T6 bars were studied to extend the study to other techniques that are difficult to find in real applications and compare the results in a more reliable way in simpler and more reproducible geometries. The microstructure and hardness properties were evaluated and the main conclusions can be summarized as follows:


and its width was greater in the C-C case. In general, the innovative techniques (CMT and fiber laser hybrid) allow a better microstructure to be obtained than the traditional technique (MIG).

	- Re-solubilization area (PMZ, T > 450 ◦C) with a complete dissolution of hardening precipitates and subsequent re-precipitation related to cooling parameters;
	- Intermediate area (about 10 mm from FZ, T > 380 ◦C) where the β transformation takes place;
	- Over-aging area: (380 < T (◦C) < 240) with the β" to β' transformation; and
	- Slightly altered zone (T < 240 ◦C) where any marked over-aging phenomena of β" are avoided.

It can be concluded that the present study demonstrated the higher quality of innovative CMT and fiber laser-MIG hybrid welding over the traditional MIG, either in terms of metallurgical or mechanical properties. The properties achieved during these experiments could be useful information during the design of real automotive applications due to the increasing demand of highly loaded and lightened structures. Thus, there will be an increasing need for thinner and hybrid combinations of Al alloys that can be addressed by looking at the present database of CMT or laser-MIG hybrid welding results. In addition, the comparison between casting and extrusion techniques provided some input for future development in this field, which, for example, would require better thermal management of the welding input in order to increase the casting welding quality.

**Author Contributions:** Conceptualization, G.C., S.C.; Investigation, G.C., S.C.; Methodology, G.C., S.C.; Supervision G.C., S.C.; Writing—original draft, G.C.; Writing—review & editing S.C. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** The authors are grateful to Ing. Enrico Lena, Ing Stefano Manesta, and Ing. Andrea Panvini for their collaboration in carrying out the experiments.

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