**3. Summary and Conclusions**

The present article offers an overview on the current status and progress on welding of high entropy alloys. The literature reveals that the main developments on this topic are mainly focused on fusion-based process by means of laser welding, whereas solid-state ones focus on being put on friction stir welding. Nevertheless, it is highly noticeable that the improvement of the welding processes regarding the feasibility of structural and functional applications of HEAs is at its beginning stage.

Regarding fusion-based processes, the current developments show that a step towards the optimization of laser-based methods is being taken. Nevertheless, a need for research on the joining of HEAs through other fusion-based techniques, that are economically more viable than laser-based approaches, is of great interest. Additionally, beyond the welding process, the initial condition of the alloy affects the weld microstructure and its behavior. Such a matter is important to study the four core effects characteristic of HEAs and their impact on the microstructural transformations on the molten pool and mechanical performance of the welds.

Concerning solid-state processes, the literature shows that success on joining CoCrFeNiMn HEAs has been achieved, although there is still a need for the optimization of the process parameters to accomplish high performing joints. Regarding their mechanical behavior, the joints show good mechanical properties, although there is still a need to enhance such solid-state processes into what that makes HEAs competitive, e.g., their outstanding behavior in extreme conditions.

Currently, special emphasis is being put on the most studied CoCrFeNiMn system. However, to further expand the potential applications of HEAs, other alloy systems need to be explored. No major work on dissimilar joints exists yet. This is another key area of interest where the need to couple the properties of different materials is of great interest.

Currently, it is well-known that HEAs can incorporate impurities during their casting. These impurities can lead to the formation on unexpected phases, especially under the non-equilibrium solidification of fusion-based processes. These unexpected phases were already observed for the CoCrFeMnNi system, and were seen to improve the joint mechanical properties. However, it is not necessarily true that these impurities will always be beneficial for the joint microstructure and performance, and as such, efforts should be made to improve the chemical homogeneity of the starting BMs, but also address the potential formation of these phases and propose methods to mitigate them.

All in all, the future of welding of HEA is at its early stages with the potential to further expand the potential applications of these advanced engineering materials.

**Author Contributions:** Both authors contributed equally to the research of the literature and writing of the review paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fundação para a Ciência e a Tecnologia (FCT - MCTES) via the project UIDB/00667/2020 (UNIDEMI).

**Acknowledgments:** JGL and JPO acknowledge Fundação para a Ciência e a Tecnologia (FCT - MCTES) for its financial support via the project UIDB/00667/2020 (UNIDEMI).

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