2.1.1. CoCrFeNiMn HEA System

Concerning the investigation of the welded joints behavior on HEAs obtained through laser-based techniques, Kashaev et al. [53] reported a study on the CoCrFeNiMn system, where the base material was fabricated via self-propagating high temperature synthesis, a process where reagents are ignited and then, due to an exothermic reaction, a given product is formed [73]. The as-produced HEA exhibited columnar FCC grains with MnS precipitates and Cr-rich carbides on its microstructure, and, as a consequence of the BM manufacturing method, its matrix composition revealed a reduced content of Mn and the existence of several impurities. These impurities did not lead to any strengthening effect, most likely due to their large grain size and low volume fraction. After welding, using a laser power of 2 kW and a welding speed of 5 m/min, several changes in texture and microstructure of the FCC matrix occurred. The precipitation of nanoscale intermetallic B2 phase compounds in the welded region promoted an increase of the microhardness in the fusion zone, as evidenced in Figure 2. The nanoscale B2 particles were seen to be mainly composed by Ni and Al, with the latter element being an impurity of the starting powders. The formation of the B2 phase was predicted by thermodynamic calculations, showing the interest of such approach to predict and explain the developed microstructures in the fusion zone of the joint. During fusion welding, there is an intense mixing of elements within the molten pool. As such, it is possible that the B2 particles are formed due to the local mixing of Ni and Al favoring the formation of this phase in the fusion zone of the joint.

**Figure 2.** Microhardness profile of a laser beam welded joint (Reproduced from [53], with permission from Elsevier, 2018).

The measured increase of hardness on the welds was suggested by the authors to be an asset for structural applications of this class of HEA. However, no assessement of the tensile properties of the welded joints was performed, thus it was not possible to state the suitabilty of these joints to be employed as structural parts. Though this was not studied in the abovementioned paper, it can be hypothesized that it may be possible to slightly change the microstructure (and resulting properties) in the fusion zone, by controlling the heat input. For example, lower heat input leads to higher cooling rates, which restricts grain growth in the fusion zone of the joint [74].

In a follow-up work, Kashaev et al. [54] addressed the impact of laser welding on the mechanical performance of the welded joints. Due to the coarse grain structure of the base material, with a grain size ranging from 250–500 μm, a refined structure in the fusion zone was observed (100–300 μm). Of special relevance in this work is the fact that the welding process did not impair the mechanical properties of the HEA joints, and the tensile and fatigue behavior of both base material and welded joints were similar (refer to Figure 3). Fracture of the welded joints occurred in the BM, which can be explained by the lower hardness of this region, which promoted strain accumulation.

**Figure 3.** Laser beam welds characterization: (**a**) Tensile testing; (**b**) fatigue testing (Adapted from [54], with permission from Elsevier, 2019).

Other researchers have also used laser welding for similar joining of CoCrFeNiMn HEAs. Jo et al. [55] also found that the hardness of the fusion zone of the joint was higher than in the base materials and also that the FCC structure was preserved. These results show the good reproducibility in terms of mechanical properties in laser welded CoCrFeMnNi HEAs obtained by different research groups. The higher hardness of the fusion zone was attributed to fine dendritic arm spacing and composition inhomogeneity. However, it must be noticed that the BM hardness was that of an as-annealed CoCrFeNiMn HEA [75]. It is known that CoCrFeNiMn alloys can exhibit higher hardness under appropriate heat treatment conditions, which would lead to a lower hardness region in the FZ if the BM was heat treated prior to welding. Obviously, the condition of the BM will impact the microstructural evolution of the welded joints, especially in what concerns strain accumulation upon mechanical testing.

A study on laser welding of as-cast and as-rolled CoCrFeNiMn HEA, was a performed by Nam et al. [56,57] in order to assess their viability for cryogenic applications. On a similar laser welding case, increasing the welding speed, which varied between 6, 8, and 10 m/min, and corresponds to a decrease in the heat input, revealed that the density of shrinkage voids, primary dendrite arm spacing, and dendrite packet size decreased. It should be noted that shrinkage avoids can be avoided/minimized upon careful optimization of the process parameters [76]. The microhardness profiles showed that, on the casted samples, the FZ presented higher values than the BM, which was attributed to the differences in grain size between both regions. Nevertheless, on the as-rolled specimens, no pronounced variation in hardness was observed, due to the existence of similar grain size in both the BM and FZ.

As depicted in Figure 4, the tensile properties of the as-cast specimens were similar to those of the BMs. However, the same did not occur in the as-rolled condition, where the welded region showed lower tensile strength values when compared to the BM, which resulted from the larger grain size on the FZ than in the BM. For the as-cast samples, at a testing temperature of 298 K, fracture occurred near the HAZ/BM interface. The same did not occur on the rolled samples, where fracture occurred in the FZ, which was attributed to higher grain size of this region. Nevertheless, in both cases, the tensile properties of the samples tested at 77 K were superior to those observed as 298 K, which was attributed to the existence of deformation twinning that tends to occur at cryogenic temperatures.

**Figure 4.** Tensile testing results: (**a**) Comparison between the tensile properties of the weld at different temperatures; (**b**) fracture region (Adapted from [56], with permission from Elsevier, 2019).

In [57], an as-cast and an as-rolled CoCrFeNiMn HEA were welded together, and evident differences of the microstructure were observed between both sides of the joint, as depicted in Figure 5. This phenomenon was attributed to the distinct epitaxial dendritic growth that initiated from the different grain sizes and morphologies in the BMs.

**Figure 5.** Microstructure of the dissimilar welds: (**a**,**b**) Dendritic growth nucleated from the fusion boundary on the cast HEA (high entropy alloy) side; (**c**) dendrites from near the centerline on the cast WM side; (**d**,**e**) dendritic growth nucleated from the fusion boundary on the rolled HEA side; (**f**) Dendrites from near the centerline on the rolled WM side. BM—Base material; WM—Weld metal (Reproduced from [57], with permission from Taylor & Francis, 2019).

The microhardness increased on the as-cast BM/HAZ interface, but still, no significant change could be observed in the weld/rolled BM interface (refer to Figure 6). Overall, the tensile properties were enhanced when the material was tested in cryogenic conditions due to deformation twinning. During tensile testing, the dissimilar welds fractured on the as-cast BM, exhibiting a comparable behavior to that of the as-cast alloy. In both cases, the viability of the laser welded CoCrFeNiMn HEAs joints for applications in cryogenic environments was evidenced.

**Figure 6.** Microhardness profile of the dissimilar weld (Reproduced from [57], with permission from Taylor & Francis, 2019).

In another work on welding of the CoCrFeNiMn system, Chen et al. [58] showed that their laser welded samples had superior properties than that of the BM, since fracture of the joints occurred in that region rather than in the FZ. This could be attributed to the higher hardness of the fusion zone (≈193 HV) when compared to the base material (≈177 HV). The microstructural analysis of the weld region and BM revealed a single FCC phase with several Cr-Mn rich precipitates. In the fusion zone, the average size of the existing precipitates ranged from 0.41 to 0.49 μm, dispersed within the grains and at the grain boundaries. These, owing to their small size, have a pinning effect on dislocations [77], granting the welded region a superior mechanical performance than the BM.

Often, the resultant microstructures in fusion-based welded joints must be modified in order to improve the part mechanical properties. These microstructural modifications are often performed by post-weld heat treatments, which can induce dissolution or formation of new phases/precipitates or promote stress relieving [78–81].

The influence of post-weld heat treatments on laser welded CoCrFeNiMn HEAs was also reported in the literature. Nam et al. [59] studied the effect of post-weld heat treatments on a temperature range of 800 and 1000◦C for one hour, on laser beam welds of a cold-rolled CoCrFeNiMn HEA. Before the heat-treatment, the welded region exhibited a larger grain size and inferior tensile strength and hardness than the BM. After being heat-treated, the welds showed superior hardness than the BM, with the FZ preserving the original BM FCC crystal structure and a decrease in the size and fraction of Cr-Mn oxide inclusions. With the increase of temperature of the heat treatment to 1000 ◦C, the variation in grain size between the weld metal and heat-affected zone decreased, which resulted in approximately the same tensile properties between the welded joint and the BM. This initial work shows the fundamental role of critically selecting the base material initial condition and subsequent post-weld heat treatments. These are fundamental to improve the joint microstructure and consequently its mechanical properties.

Concerning other fusion-based welding techniques, Wu et al. [60,61] investigated Electron Beam Welding and Gas Tungsten Arc Welding (GTAW) on a CoCrFeMnNi HEA. For this purpose, ingots were produced via arc-melting and then thermomechanically processed to achieve a homogeneous equiaxed microstructure. After welding, microstructure characterization evidenced that no major defects existed in the joints, and that the microstructure was mainly composed of dendrites and large columnar grains. Overall, the yield strength of both welded joints was higher than that of the BM. Differences between both welding methods resided on the dendrite arm spacing and on the amount of elemental segregation, which were less evident on the electron beam welds. This can be attributed to the fast cooling rate of the process, which can decrease grain growth and elemental segregation when compared to arc-based techniques [47,48]. The tensile strength of the GTAW samples exhibited, approximately, 80% of the tensile strength and 50% of the ductility of the BM, while the electron beam-welded samples presented a similar behavior to that of the BM. Though sound joints were achieved in both cases, the ability of electron beam welding to localize the heat in a restrict region can

be considered an advantage and a potential justification on the superior mechanical properties when compared to arc-based welding of the same alloy.

More recently, Oliveira et al. [62] performed a comprehensive study on GTAW of as-rolled CoCrFeNiMn HEA. Using synchrotron X-ray diffraction analysis, the authors observed that the extension of the HAZ was larger than that determined based on electron microscopy and hardness measurements techniques. In fact, due to the large deformation imposed by cold rolling of the starting BM, recovery phenomenon was seen to occur far away from the weld centerline. This phenomenon translated into a decrease of the residual stresses of the material in that region, though no variations in grain size and hardness were observed.

The use of filler materials during fusion-based welding is often used to control and adjust the chemical composition and resulting microstructures [82,83]. Moreover, in hard-to-join dissimilar pairs, careful selection of the filler material can aid in the inhibition of solidification cracking or other defects than can occur upon solidification [84,85].

Nam et al. [63] evaluated the use of two different filler materials during similar GTAW of an CoCrFeMnNi HEA. The selected filler materials were a 308 L stainless steel, while the other had the same composition as the BM. The results evidenced that both types of welds exhibited a single FCC phase, nevertheless the elemental percentage of Fe increased with the proximity to the weld centerline when the stainless steel filler was used. Regarding the mechanical properties, both welds exhibited superior values than the original BM. The joint obtained using the CoCrFeMnNi filler presented the higher values for the microhardness (≈165 ± 1 HV), as depicted in Figure 7a. The tensile properties were also assessed at room and cryogenic temperatures, exhibiting a comparable behavior to that of the cast BM, as presented in Figure 7b. Overall, the tensile testing provided an insight for the possibility to use stainless steel 308 L as a filler metal to guarantee the applicability of these HEAs in cryogenic environments.

**Figure 7.** Mechanical behavior of the welded joints: (**a**) Tensile testing; (**b**) microhardness distribution (Adapted from [63], with permission from Elsevier, 2020).

Due to the possibility to control the composition, and therefore the resulting microstructures using filler materials, we hypothesize that in the future, dedicated filler materials can be developed to control and tune the HEA joints properties.
