*4.3. Metallurgical Characterization for Assessment of Weld Microstructures*

The distinct macrostructures of different zones (base metal, stir zone, thermomechanically affected zone, heat affected zone) in the friction stir welded Al-MMCs samples are shown in Figure 11. The interfacial boundary was visible on both the advancing and the retreating side. The base material showed fine and enlarged grain particles homogeneously distributed (Figure 11a). At the extremities of the stir zone, the morphology of the microstructure significantly gets altered. (Figure 11b). In the stir zone, dynamic crystallization occurred due to the FSW process. Grain refinement occurred due to heat, mechanical deformation and stirring action of the FSW tool (Figure 11b). The boundary between the thermo-mechanical affected zone and stir zone was not visible, and the grains were distorted, aligned. A sharp transition occurred between these two regions (Figure 11c). Mechanical vibration and heat in the thermomechanical affected zone were less compared to the stir zone. *Materials* **2021**, *14*, x FOR PEER REVIEW 12 of 17

**Figure 11.** Distinct microstructures of Al-MMC: (**a**) base metal, (**b**) stir zone, (**c**) thermomechanical affected zone/stir zone. **Figure 11.** Distinct microstructures of Al-MMC: (**a**) base metal, (**b**) stir zone, (**c**) thermomechanical affected zone/stir zone.

grains at the weld zone.

The microstructural examination at the joints reveals the flow of plasticized metal

tained from the FSW are shown in Figure 12a,b. It was observed that the reinforcement particles were distributed uniformly, and no undesired higher order reactions in the dynamic recrystallized zones were seen. It is in coherence with other reports [24–26]. The formation of onion rings was observed from Figure 12b due to the composite's cylindrical FSW rotating tool material extrusion. There were no traces of common defects to FSW processes such as tunnels, wormholes and piping due to B4C dislocation in the Al matrix. The extensive stirring action of FSW resulted in the formation of homogenous and refined

The microstructural examination at the joints reveals the flow of plasticized metal from advancing to the retreating side. The dynamic recrystallized zones of samples obtained from the FSW are shown in Figure 12a,b. It was observed that the reinforcement particles were distributed uniformly, and no undesired higher order reactions in the dynamic recrystallized zones were seen. It is in coherence with other reports [24–26]. The formation of onion rings was observed from Figure 12b due to the composite's cylindrical FSW rotating tool material extrusion. There were no traces of common defects to FSW processes such as tunnels, wormholes and piping due to B4C dislocation in the Al matrix. The extensive stirring action of FSW resulted in the formation of homogenous and refined grains at the weld zone. *Materials* **2021**, *14*, x FOR PEER REVIEW 13 of 17

**Figure 12.** High-resolution SEM images of dynamic recrystallized zones of Al-MMC: (**a**) stir zone, (**b**) onion rings. **Figure 12.** High-resolution SEM images of dynamic recrystallized zones of Al-MMC: (**a**) stir zone, (**b**) onion rings.

Figure 13 reveals the microscopic image of the weld cross-section and different zones in friction stir welded samples. Considerable change was visible in the particle size of the reinforcement phase in different regions of the joint. The reinforcement particles in the weld region revealed fragmentation in the superficial zone (average particle size reduction was about 6 times, whereas in the advancing zone average particle size reduction was about 3 times). This is because the reinforcement particles at the weld are subjected to higher heat flux compared to other regions. However, in the weld nugget region, fragmentation of the particles was significantly less (average particle size reduction was about 2 times). Distinct microstructures were observed at the nugget region of the composite. The homogenous distributions of refined grains were observed in the weld region are mainly attributed to the plastic deformation due to FSW stirring tool. The reinforcement particulate precipitates were fragmented and rearranged by the stirring action. The grains splitting and size reduction lead to the evolution of microstructure that is specific in nature. There is an irregular assortment of low/ high angle boundaries along with considerable quantum of fine equiaxed grains. The microstructure revealed directional realignment indicating close to parallel to the border between the transition zone and the stir zone. This matrix structure formation may be due to the development of deformationinduced boundaries. Besides, the traces of misorientation distribution implies a close relationship between the grain-boundary formation and texture. Henceforth, the underlying microstructural process lays the platform for continuous recrystallization. The refined equiaxed grains originate from grain boundary bulging, which is reflected in concurrent development leading to discontinuous recrystallization [27,28]. Figure 13 reveals the microscopic image of the weld cross-section and different zones in friction stir welded samples. Considerable change was visible in the particle size of the reinforcement phase in different regions of the joint. The reinforcement particles in the weld region revealed fragmentation in the superficial zone (average particle size reduction was about 6 times, whereas in the advancing zone average particle size reduction was about 3 times). This is because the reinforcement particles at the weld are subjected to higher heat flux compared to other regions. However, in the weld nugget region, fragmentation of the particles was significantly less (average particle size reduction was about 2 times). Distinct microstructures were observed at the nugget region of the composite. The homogenous distributions of refined grains were observed in the weld region are mainly attributed to the plastic deformation due to FSW stirring tool. The reinforcement particulate precipitates were fragmented and rearranged by the stirring action. The grains splitting and size reduction lead to the evolution of microstructure that is specific in nature. There is an irregular assortment of low/ high angle boundaries along with considerable quantum of fine equiaxed grains. The microstructure revealed directional realignment indicating close to parallel to the border between the transition zone and the stir zone. This matrix structure formation may be due to the development of deformation-induced boundaries. Besides, the traces of misorientation distribution implies a close relationship between the grain-boundary formation and texture. Henceforth, the underlying microstructural process lays the platform for continuous recrystallization. The refined equiaxed grains originate from grain boundary bulging, which is reflected in concurrent development leading to discontinuous recrystallization [27,28].

**Figure 13.** SEM images of weld zone of Al-MMC: (**a**) base material, (**b**) initial stage, (**c**) progressive stage, (**d**) advanced stage. **Figure 13.** SEM images of weld zone of Al-MMC: (**a**) base material, (**b**) initial stage, (**c**) progressive stage, (**d**) advanced stage.

and Si is observed [29,30].

The EDAX analysis was performed on the weld samples to identify the key elements in the joint and the adjoining regions (Figure 14). The crystalline phase particle distributions were examined by elemental mapping for Al, Si and B. It was observed that small size grains of Si and Al phases are distributed in narrow spaces available between B4C particle. The Al phase governs the solid solubility of various reinforcement particles. It may be noticed that Al in the SiC pellets is negligible. The uniform distribution of Al, B The EDAX analysis was performed on the weld samples to identify the key elements in the joint and the adjoining regions (Figure 14). The crystalline phase particle distributions were examined by elemental mapping for Al, Si and B. It was observed that small size grains of Si and Al phases are distributed in narrow spaces available between B4C particle. The Al phase governs the solid solubility of various reinforcement particles. It may be noticed that Al in the SiC pellets is negligible. The uniform distribution of Al, B and Si is observed [29,30].

**Figure 14.** SEM with EDAX of Al-MMC in the weld nugget region. **Figure 14.** SEM with EDAX of Al-MMC in the weld nugget region.

#### **5. Conclusions 5. Conclusions**

In this study, FSW was employed to join AA6061/SiC/B4C stir cast composites. The study aimed to identify suitable Al-MMCs with high strength, lightweight and anti-corrosion properties and explore their weldability characteristics and efficiency using FSW. The process parameters of FSW influencing the properties of the weld along with the causes attributed by the reinforcement components were investigated. The conclusions derived from this study are presented as follows: In this study, FSW was employed to join AA6061/SiC/B4C stir cast composites. The study aimed to identify suitable Al-MMCs with high strength, lightweight and anticorrosion properties and explore their weldability characteristics and efficiency using FSW. The process parameters of FSW influencing the properties of the weld along with the causes attributed by the reinforcement components were investigated. The conclusions derived from this study are presented as follows:


action. FSW is the feasible and appropriate process for welding Al-MMCs. The results promise to address the requirements of the aerospace and automobile industries. Due to the good weld efficiency, defence sectors like missile launchers and naval sectors with cruise components may adapt these materials along similar lines. The automobile and aerospace industries are the major beneficiaries; nevertheless, their applicability may be extrapo-FSW is the feasible and appropriate process for welding Al-MMCs. The results promise to address the requirements of the aerospace and automobile industries. Due to the good weld efficiency, defence sectors like missile launchers and naval sectors with cruise components may adapt these materials along similar lines. The automobile and aerospace industries are the major beneficiaries; nevertheless, their applicability may be extrapolated to marine and defence structures.

lated to marine and defence structures. **Author Contributions:** Conceptualization, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; methodology, K.S.A.A., V.M., S.A.V., M.R., A.Y., M. G. and J.W.; formal analysis, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; resources, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; data curation, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; writing—original draft preparation, K.S.A.A., V.M., S. A.V., M.R., A.Y., M.G. and J.W.; writing—review and editing, K.S.A.A., **Author Contributions:** Conceptualization, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; methodology, K.S.A.A., V.M., S.A.V., M.R., A.Y., M. G. and J.W.; formal analysis, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; resources, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; data curation, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; writing—original draft preparation, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; writing—review and editing, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; supervision, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; project administration, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W.; funding, K.S.A.A., V.M., S.A.V., M.R., A.Y., M.G. and J.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The authors confirm that the data supporting the findings of this study are available within the article.

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

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