*2.2. Solid-State Welding of HEAs*

As previously mentioned, welding materials in the solid state can be a reliable and advantageous way to achieve sound joints. The current information regarding welding HEAs using solid-state techniques shows that most studies are focused on friction stir welding (FSW) [55,86–92]. Nevertheless, other possibilities for joining these materials are rotary friction welding [91] and diffusion bonding [92].

Concerning FSW, the literature shows that an effort for the development and comprehension of the microstructural evolution of FSWed HEAs joints is underway. For instance, FSW of a CoCrFeNiMn HEA manufactured by vacuum induction melting, followed by thermomechanical processing was conducted by Jo et al. [55]. After the welding process, the tensile strength and the ductility of the samples exhibited a similar behavior to that of the BM. Ductile fracture occurred in the BM, indicating that the microstructure evolution in the processed region promoted a higher joint strength. The welding process was characterized by inducing dynamic recrystallization, which resulted in grain refinement aided by the temperature increase, coupled with the massive deformation imposed during the process. The microhardness distribution on the welds exhibited higher values than the BM. An EBSD (Electron Backscatter Diffraction) inverse pole analysis on the cross section of the weld exhibited significant grain refinement and a lower proportion of twins at the center of the weld, indicating also that the fraction of low angle grain boundaries tends to decrease with the increase of distance from the weld center. These boundaries have an important role on the mechanical performance of the material acting as barriers to plastic deformation and inhibiting the grain growth mechanisms induced by the increase of temperature.

Typically, it is often preferred that failure of a welded joint occurs in the BM. This is an evidence of the higher resistance of the welded region, which implies that, for structural parts, the main limiting aspect will be the BM mechanical properties. Provided that those properties are ensured to be constant over time, and since FSW is known to be a very reliable process, unlike some arc-based welding processes, the welded joints can be safely used as structural parts in key engineering applications.

A step further into the investigation of the FSW process applied to the CoCrFeNiMn HEA was taken by Xu et al. [86]. In this work, forced cooling was applied to the processed material, aiming at improving the joint properties. The mechanical results showed that it is possible to enhance the mechanical properties of the welds without a ductility loss through the fast cooling of the joint. This enhancement of the material mechanical properties was explained by the inhibition of the static recovery and selected grain growth that can occur during post-annealing.

Zhu et al. [87] performed FSW on cast CoCrFeNiAl0.3. Defect-free welds were obtained using two different speeds, 30 and 50 mm/min. In both cases, apart from the original FCC matrix, the results from X-ray diffraction analysis showed that no phase changes occurred. The morphology of the welds exhibited four typical thermomechanically affected areas after FSW: (i) SZ, the stir zone, where refined equiaxed grains resultant from recrystallization were observed; (ii) TMAZ, the thermomechanically affected zone, which exhibited both coarse and fine grains; (iii) HAZ, where columnar grains were observed with an average size of 132 μm; (iv) BM, characterized by columnar grains resultant from the casting process, preferentially oriented in the solidification direction. Because of these microstructural differences, the microhardness was higher at the center of the weld, decreasing with the distance from the center. Additionally, reducing the speed of the tool, which increases the heat input, resulted in slightly a larger grain size on the SZ, which is in good agreement with the effect of FSW process parameters on other materials [93].

In another study, Zhu et al. [88] studied a quaternary HEA composition, Co16Cr28Fe28Ni28, in order to study the effects of the reduced Co content on the material mechanical performance. Their work showed that after recrystallization, through thermomechanical processing, the tensile properties were superior to that of common HEAs, as depicted in Figure 9a. Such evidenced the possibility for the enhancement of the alloy mechanical properties through precipitation hardening, given the reduced proportion of Co. By performing FSW on this HEA, varying only the welding speed (ranging between 30 and 50 mm/min), a refined microstructure composed of equiaxed grains was obtained in the SZ, which remained with its original FCC crystal structure. However, at the higher level of welding speed, the formation of a kissing bond [94], which is characterized by the partial penetration of the weld, was inevitable, due to low heat input. In both cases, the formation of a white band was evidenced. Further analysis of this feature revealed the presence of W-rich and Cr-rich particles. The presence of W-rich particles can be explained by the welding tool wear, which is a common occurrence during FSW [95]. The presence of the Cr-rich particles was not explained, requiring further experimental work to justify its presence. Regarding the hardness of the joints, the SZ exhibited a relatively higher hardness than the BM, which was attributed to the distorted crystalline network, high fraction of deformation twins, and refined grain structure (see Figure 9b regarding the hardness profile obtained across the joint).

**Figure 9.** Sample characterization: (**a**) Tensile properties; (**b**) microhardness distribution (Adapted from [88], with permission from Elsevier, 2018).

During FSW, tool wear can occur, and debris can be incorporated in the processed material. The influence of tungsten and chromium carbide particles, caused by the tool deterioration during FSW of a CoCrFeNiMn HEA was accessed by Park et al. [89]. The process parameters comprised a welding speed of 30 mm/min, while the tool rotation varied between 400, 600, 800, and 1000 rpm. The results showed that both the welds and the BM exhibited a single FCC crystal structure. No cracks or voids were found on the welds, although the increase of tool rotation resulted in thinning near the center line, which corresponds to an inferior thickness of the weld when compared to the BM. The formation of a tornado-shaped region on the SZ was also evidenced when performing FSW with rotations speeds higher than 600 rpm (refer to Figure 10a). This tornado-shaped region was characterized by the formation of a secondary phase correspondent to W- and Cr-rich carbides, aided by the tool wear. Overall, superior characteristics regarding the hardness, tensile strength, and joint efficiency were obtained with a rotation speed of 800 rpm, where the grain size was at its lowest value. This was attributed to the different heat inputs that govern the solid-state transformation during the process. As depicted in Figure 10b, a comparison on the carbide size and concentration can be observed between a lower heat input (800 rpm) and a higher heat input sample (1000 rpm). These results show that higher rotation speeds lead to more severe wear of the FSW tool.

**Figure 10.** Characterization of friction stir welded (FSWed) CoCrFeNiMn joints: (**a**) Morphology of the welds at different rotation speeds (from top to bottom: 400, 600, 800, 1000 rpm); (**b**) carbide content at 800 rpm and 1000 rpm (Adapted from [89], with permission from Elsevier, 2019).

On another perspective, Shaysultanov et al. [90] performed FSW on a carbon-doped CoCrFeNiMn HEA, with the intent of studying the influence this controlled C addition on the welded joints mechanical performance. After being produced via thermite-type self-propagating high-temperature synthesis, the samples were cold-rolled and annealed at 900 ◦C for 1 h, to obtain an equiaxed microstructure. The welds resultant from the FSW process were defect-free, while microstructural differences were observed on the grain size on the BM and the SZ, with a change from 9.2 to 4.6 μm, respectively. With this joining process the proportion of M23C6 carbides increased, which was attributed to the rise in temperature triggered by intense plastic deformation, aiding in the precipitation of this phase. Overall, the mechanical performance of the welds exhibited higher values than the BM in both the microhardness (an increase of ≈40 HV) and tensile properties (an increase of ≈80 MPa on the ultimate tensile strength and of ≈200 MPa on the yield strength), which can be attributed to the carbides' precipitation.

As described above, most of the research work on solid-state welding of HEA focuses on FSW. However, other solid-state techniques have also started to be used to join this class of advanced materials.

Rotary friction welding was conducted in a eutectic AlCoCrFeNi2.1 HEA by Li et al. [91]. As depicted in Figure 11a–c, similarly shaped joints were obtained with friction pressures of 80 and 120 MPa, while welding at 200 MPa resulted in an increase of the burn-off length and on the size of the resulting flash. Microstructurally, at the center of the weld, on the dynamic recrystallization zone (DRZ), the grains exhibited a refined and equiaxed structure. Additionally, on the TMAZ, the microstructure is composed of bent and elongated grains, while the HAZ exhibited fewer eutectic cells when compared

to the BM. An EBSD analysis of the samples produced under the friction pressure of 120 MPa, revealed both an FCC phase, composed mainly by Fe, Co, and Cr, and a B2 phase, composed of AlNi intermetallic compounds and BCC structured-type CrFe precipitates. The tensile properties were superior when the friction pressure was of 200 MPa, where fracture of the joints occurred in the BM. As depicted in Figure 11d–f, the rough fractured surface is the result of the diferent ductility of the hard B2 phase and the soft FCC phase. Nevertheless, the specimens welded with 80 and 120 MPa of friction pressure yielded inferior tensile performance, fracturing at the joint interface, which is due to the the existence of a weld interface and discontinuous distribuition of the B2 phase on the DRZ region.

**Figure 11.** Morphology of the welds and fractured surfaces after tensile testing. The joints were obtained under the friction pressures of: (**a**) and (**d**) 80 MPa; (**b**) and (**e**) 120 MPa; (**c**) and (**f**) 200 MPa (Adapted from [91], with permission from Elsevier, 2020).

Another possibility for welding materials in the solid state is through diffusion bonding. Unlike other solid-state welding processes, this technique proves its purpose when joining materials with a high susceptibility to cracking, as in the case of refractory metals [96–98]. Lei et al. [92] studied vacuum diffusion bonding between the single-phase FCC Al0.85CoCrFeNi HEA and a TiAl alloy. For this purpose, an axial pressure of 30 MPa, a temperature range of 750 to 1050 ◦C, and a holding time of 30 to 120 min were used. Given the sluggish diffusion effect, characteristic of HEA systems, the atomic diffusion from the TiAl substrate into the HEA substrate was drastically inferior, when compared to the diffusion resultant from the HEA side. As depicted in Figure 12, the obtained bonds were characterized by having three distinct regions, which could be divided according with their microstructural composition: Region I, composed by α2-Ti3Al + solid strengthened γ-TiAl; region II, which can be expressed as Al(Co, Ni)2Ti; and region III, characterized by an Cr(Fe, Co) solid solution phase. The formation of voids was noticeable in the interlayer between regions II and III, which was caused by the atomic flux imbalance provided by the process parameters in use. Overall, the optimal results yielded a maximum microhardness of 923 HV in region I, and a maximum shear strength of 71 MPa (at 850 ◦C and after 90 min of holding time).

**Figure 12.** Characterization of the diffusion bonded TiAl/Al0.85CoCrFeNi joints using 950 ◦C/1 h/30 MPa: (**a**) microstructure under SEM; (**b**) Compositional element distribution taken from the scanning line region (Adapted from [92], with permission from Elsevier, 2020).

As it can be inferred from this work, multiple welding works on HEA currently exist, and increasing attention to these alloys' weldability is emerging. Table 2 compiles the existing studies on welding of different HEA, while Figure 13 details the relative importance of each welding technique already applied to HEAs. As it can be noted, most studies concern the CoCrFeNiMn HEA, showing the need to further extend these research works to other alloys systems.


**Table 2.** Summary of the welding techniques currently used on HEAs.

**Figure 13.** Percentage distribution of the number of papers considered in this study: (**a**) by welding technique; (**b**) by alloy system.
