**2. Current Progress on Welding HEAs**

Among several methods for joining materials, welding comprises a broad range of techniques, and it is an expedite and often a reliable way to produce permanent and continuous joints. When proper design specifications and process optimization are used, welding technologies become unquestionably competitive for several industrial sectors, such as infrastructure construction and transportation. This category of processes is known to be capable of achieving particularly strong and resistant bonds when exposed to static and dynamic forms of loading [47,48]. As such, welding technologies can be divided according to the diverse mechanisms and processes required to achieve such joints.

Fusion-based welding processes are based on melting and solidification of metal and are currently the most established in industry. The resultant joints are typically characterized by exhibiting three

distinct regions. The fusion zone (FZ), which is the region that undergoes melting and subsequent solidification; the heat affected zone (HAZ), that is characterized by experiencing temperatures that can promote solid-state transformation and never changes its state (i.e., the material never becomes fully liquid during the process); and, lastly, the base material (BM), which is the region that remains unchanged throughout the process.

Often, fusion-based welding methods are known to impair the mechanical properties of the joints due to metallurgical incompatibility between the materials to be joined, development of high residual stresses, mismatch of thermomechanical properties of the BMs, or due to the sensitivity of the material to the weld thermal cycle. In those cases, solid-state techniques are a viable option capable of solving some of the above-mentioned problems. One well-known example is dissimilar welding of aluminium to steels [49]: While fusion welding can be used to join this dissimilar combination, the resulting mechanical properties are often poor due to the formation of brittle intermetallic compounds upon mixing of the liquid phases of the two BMs. However, solid-state methods can effectively join these materials allowing their use in structural applications.

Solid-state welding is based on intense friction, plastic deformation, and diffusion mechanisms that aid in the formation of a joint between the BMs [50]. The versatility of such approach is proven by the ability to connect materials that are difficult or impossible to join through fusion-based processes, avoiding the development of undesired phases, distortions, and high residual stresses that may occur during the liquid-solid state interchange. These processes where plastic deformation occurs, exhibit a thermomechanically affected zone (TMAZ), as well as an HAZ. The microstructural evolution of the joints will depend on the material susceptibility to the combined effect of temperature and deformation in the TMAZ, and of temperature in the HAZ.

Another possibility for materials joining is through brazing and soldering. These are mainly characterized by the introduction of a filler metal into the joint region. In such processes, the metallic filler has a melting point lower than that of the materials that are to be welded, and the welds are established by diffusion between the filler and BMs [47].

Regarding the overall developments of welding of HEAs, a survey on the contemporary literature shows that an effort is being made towards the understanding of the microstructural evolution and optimization of these joining processes [51]. Until now, the number of publications regarding welding of HEAs is increasing throughout the years, as the developments on these alloys become more evident, as it can be observed in Figure 1.

**Figure 1.** Number of publications on welding of high entropy alloys over time.

However, to the best of the authors' knowledge, most studies are focused on fusion-based welding processes. Still, it is also possible to perceive that some efforts are also being dedicated to the study of other welding techniques on HEAs, especially in what concerns solid-state welding processes. Considering this, the next topics that are presented in this paper mainly concern the overall progress

on fusion-based and solid-state welding processes on HEAs. Nevertheless, information about welding HEAs through brazing is also available, as presented by Lin et al. [52], where dissimilar joints between CoCrFeMnNi/CoCrFeNi alloy and CoCrFeMnNi/316 stainless steel were investigated.
