2.1.2. Other HEA Systems

Most of the works on welding of HEAs currently focus on the CoCrFeNiMn system, as exemplified above. However, some researchers started to pay attention to the weldability of other HEA compositions.

The use of GTAW for welding of an Al0.5CoCrFeNi HEA was attempted by Sokkalingam et al. [66]. The BM was composed by a near-equiaxed microstructure and, after welding, the HAZ exhibited a microstructure with a size, approximately, double of that of the BM. This is explained by the weld

thermal cycle, that is known to induce grain growth in the HAZ, especially closer to the fusion boundary. Additionally, the FZ was characterized by dendritic growth, and near the weld centerline, a fine equiaxed microstructure could be observed. Both the BM and the FZ exhibit a mixed FCC and BCC structure. However, the volumetric fraction of the BCC phase was reduced due to the thermal history experienced by the material during the welding procedure. Mechanical testing showed that the weld region exhibited an inferior microhardness, and the tensile properties experienced a reduction of 6.4% in strength and 16.5% in ductility, when compared to the BM.

The same authors also performed dissimilar welding of Al0.1CoCrFeNi HEA to AISI304 stainless steel by GTAW [65]. As depicted in Figure 8, microstructural characterization revealed that the HEA side of the weld was characterized by epitaxial grain growth, while the AISI304 stainless steel side exhibited non-epitaxial grain growth. Nevertheless, towards the weld centerline, the joint tends to exhibit an equiaxed dendritic grain structure. During tensile testing, fracture occurred in the fusion zone, which was attributed to the heterogeneous distribution of the microstructure and lower hardness of the weld. Overall, the applicability of these dissimilar joints for structural applications was confirmed by means of mechanical testing, where the dissimilar joint exhibited superior values for the yield strength and ultimate tensile strength (≈265 and ≈590 MPa, respectively) than that of the HEA side of the BM (≈148 and ≈327 MPa, respectively).

**Figure 8.** Microstructural characterization by means of EBSD (Electron Backscatter Diffraction) inverse pole figure analysis of the dissimilar welds. BM-1: Al0.1CoCrFeNi HEA; BM-2: AISI304 stainless steel (Reproduced from [65] with permission from Cambridge University Press, 2019).

The corrosion behavior of laser welded Al0.5CoCrFeNi HEA was studied by Sokkalingam et al. [64], by evaluating the corrosion potential and corrosion current density obtained by potentiodynamic polarization tests. The BM was composed by an equiaxed microstructure composed by Cr-Fe and Al-Ni rich phases and Al-rich particles. During welding, the dissolution of the Al-Ni rich and Al-rich compounds into the CoCrFeNi matrix occurred, resulting in a microstructure exhibiting Cr-Fe rich columnar dendrites with an Al-Ni rich interdendritic region. Overall, the results from the corrosion resistance tests evidenced that the welded joints exhibited a higher corrosion resistance than the BM alone, when exposed to aqueous corrosion environments. This was attributed to the solubility of the Al in the alloy matrix during welding that causes an increase of its corrosion potential, resulting on the reduction of the galvanic circuit in the joint.

More recently, weldability studies on AlxCoCrCu*y*FeNi HEAs have been performed by Martin et al. [67,68]. Cu segregation was seen to promote solidification cracking in the fusion zone of the GTAW joints. By changing the alloy composition, it was possible to mitigate the cracking susceptibility of this HEA class. These works, which were supported by thermodynamic calculations to predict the existing phases on the FZ as function of the alloy composition, show the importance of optimizing the BM starting composition when cracking phenomena, such as hot cracking and liquation cracking, are prone to occur in the materials to be welded. Though this was only observed in the AlCoCrCuFeNi HEA system, it is likely that other HEA compositions may exhibit the problems. If that

is the case, the addition of filler materials [48] can also be a potential solution to adjust and improve the chemical composition of the fusion zone.

Panina et al. [69] reported the effects of pre-heating temperature (400, 600, and 800 ◦C) on the laser weldability of a Ti1.89NbCrV0.56 refractory high entropy alloy. Initially, the BM microstructure was mainly composed by BCC grains presenting also small C15 Laves phase particles. Hot cracking occurred when welding was performed with the BM at room temperature and at 400 ◦C, which was attributed to the low ductility of the alloy. Since during welding thermal stresses are generated, materials with poor ductility can suffer cracking if those stresses are not relieved. Using pre-heating temperatures of 600 ◦C and 800 ◦C, however, resulted in defect-free joints. The microstructure of the welds was characterized by columnar grains, where the grain size tended to increase with the increase of pre-heating temperatures. This can be explained based on the effect of changing the pre-heating temperature before welding: Higher pre-heating temperature leads to a slower cooling rate, which promotes more significant grain growth. Due to these slower cooling rates, the grain size was larger in the different FZ. As such, the microhardness of the FZ tended to decrease with the increase of the selected pre-heating temperatures. The mechanical performance of the welds was also assessed through tensile testing at 750 ◦C, and enhanced tensile properties were observed when pre-heating at 800 ◦C (ductility of ≈10%, yield strength of 265 MPa and ultimate tensile strength of 285 MPa vs. the 250 MPa maximum stress obtained when fracture occurred in the elastic region on the as-cast specimens). Overall, the results obtained in this study highlight the need for optimizing the welding parameters, such as the pre-heating temperatures, in order to obtain high performing joints.

Currently, it is clear that welding of HEAs is a growing research topic. However, most of the work is focused on the CoCrFeNiMn alloy system using high power beams (laser and electron beam). Though some recent studies have addressed the weldability of other HEA systems, the existence of a significant research gap regarding the weldability of these materials is highly noticeable.
