*2.1. Fabrication of Composites (Phase-I)*

A graphite crucible in electrical furnace (SwamEquip, Chennai, India) was used to prepare the Al-MMCs reinforced B4C and SiC. Hexachloroethane tablets (M/s Madrad Fluorine Private Ltd., Chennai, India) were used for degassing (eradicate dross) from the composites. At 650 ◦C, B4C and SiC were pre-oxidized for 2 h and transferred into the liquid matrix (AA6061) with constant stirring. To improve the metal bonding, heat treatment was given to B4C to form a layer on SiC. After adding B4C and SiC at an optimal speed (500 rpm) and time (10 min), the melt was then transferred into an iron die mould. There were no traces of casting defects. To improve the wettability of the reinforced particles, magnesium was added during the stirring and melting of the alloy. Zirconia coated AISI310 stainless steel material was employed with speed variations ranging between 300 and 500 rpm. The furnace operated at 230 V, 6 kW, and the molten metal working temperature was 990 ± 0.1 °C with a maximum sustainable temperature of 1200 °C. The stir casting furnace specifications are given in Table 1.

The Al-MMCs were prepared using the stir casting unit (Figure 1). The scientico electrical furnace of dimension 600 mm × 600 mm × 600 mm of the outer shell and furnace height 0.75 m was used. The crucible (dimension 200 mm × 300 mm × 50 mm) made of

liquid matrix (AA6061) with constant stirring. To improve the metal bonding, heat treatment was given to B4C to form a layer on SiC. After adding B4C and SiC at an optimal speed (500 rpm) and time (10 min), the melt was then transferred into an iron die mould. There were no traces of casting defects. To improve the wettability of the reinforced par-

ticles, magnesium was added during the stirring and melting of the alloy.

*Materials* **2021**, *14*, x FOR PEER REVIEW 3 of 17

#### *2.2. Experimental Setup* **Table 1.** Process parameters for stir casting.

*2.2. Experimental Setup* 

The Al-MMCs were prepared using the stir casting unit (Figure 1). The scientico electrical furnace of dimension 600 mm × 600 mm × 600 mm of the outer shell and furnace height 0.75 m was used. The crucible (dimension 200 mm × 300 mm × 50 mm) made of Zirconia coated AISI310 stainless steel material was employed with speed variations ranging between 300 and 500 rpm. The furnace operated at 230 V, 6 kW, and the molten metal working temperature was 990 ± 0.1 ◦C with a maximum sustainable temperature of 1200 ◦C. The stir casting furnace specifications are given in Table 1. **S. No. Parameters Value**  1 Spindle speed 300–500 rpm 2 Stirring time 10 min 3 Temperature of melt 990 °C 4 Preheated temperature of SiC and B4C particles 650 °C 5 Powder feed rate 0.75–1.0 g/s

**Figure 1. Figure 1.**  Schematic of stir casting setup. Schematic of stir casting setup.

**Table 1.** Process parameters for stir casting.


#### *2.3. Experimental Procedure for FSW of Al-MMC's (Phase-II)* **Sample No. Al % SiC % B4C % Mg %**

**Table 2.** Composition of various samples in wt. %.

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*2.3. Experimental Procedure for FSW of Al-MMC's (Phase-II)* 

To strengthen Al-MMCs, SiC (325 nm mesh size) and B4C (30 nm) particles were reinforced in the metal matrix (Figure 2). Hexachloroethane tablets was used to improve SiC and B4C powder's wettability and enhance their behaviour in Al melts [23]. The composition percentages of different samples fabricated in this study are listed in Table 2. 1 85 10 3 2 2 85 8 5 2 3 85 5 8 2 4 85 3 10 2

To strengthen Al-MMCs, SiC (325 nm mesh size) and B4C (30 nm) particles were reinforced in the metal matrix (Figure 2). Hexachloroethane tablets was used to improve SiC and B4C powder's wettability and enhance their behaviour in Al melts [23]. The composition percentages of different samples fabricated in this study are listed in Table 2.

**Figure 2.** Scanning electron microscope micrograph of reinforcement particles: (**a**) SiC, (**b**) B4C. **Figure 2.** Scanning electron microscope micrograph of reinforcement particles: (**a**) SiC, (**b**) B4C.

The workpieces of dimensions 150 mm × 150 mm × 6 mm were prepared in butt welding configuration (Figure 3). The FSW tool was made of tool steel (H-13) with a shoulder diameter of 25 mm. The cylindrical tool dimensions of 6 mm diameter and 5.7 mm height were considered (Figure 4). After a few trial-and-error experiments using FSW 3T 300NC, the optimal process parameters ranges were identified. The subsequent trials were based on the design of experiments (DOE) with different levels of process parameters accounting for 81 trials. However, the detailed information of the DOE based trials is not discussed here as the scope of the paper focuses on properties and behaviours of the weld. The FSW tool rotating speed, travelling speed and plunge force used for the trials were 1000 rpm, 75 mm/min and 10 KN, respectively. The welded specimens are cut along the cross-sections for tensile test specimens per ASTM (E8M) standards. The workpieces of dimensions 150 mm × 150 mm × 6 mm were prepared in butt welding configuration (Figure 3). The FSW tool was made of tool steel (H-13) with a shoulder diameter of 25 mm. The cylindrical tool dimensions of 6 mm diameter and 5.7 mm height were considered (Figure 4). After a few trial-and-error experiments using FSW 3T 300NC, the optimal process parameters ranges were identified. The subsequent trials were based on the design of experiments (DOE) with different levels of process parameters accounting for 81 trials. However, the detailed information of the DOE based trials is not discussed here as the scope of the paper focuses on properties and behaviours of the weld. The FSW tool rotating speed, travelling speed and plunge force used for the trials were 1000 rpm, 75 mm/min and 10 KN, respectively. The welded specimens are cut along the cross-sections for tensile test specimens per ASTM (E8M) standards. *Materials* **2021**, *14*, x FOR PEER REVIEW 5 of 17

**Figure 3.** Friction stir welded specimen. **Figure 3.** Friction stir welded specimen.

**Figure 4.** Schematic representation of FSW process and tool.

of heat energy and expressed by Equation (1).

rotational (*Qr*) heat energies as shown in Equation (2).

**3. Estimation of Energy Generated in FSW for Heat Input Examinations** 

where *η*—power transformation coefficient, and *P*—power transferred.

(*M*) and angular frequency (*ω*) and is expressed by Equations (3) and (4) .

*dP* = ω*dM* =

The amount of heat generated in FSW is the power transformed to the total amount

The total amount of heat energy transferred is the sum of the translational (*Qt*) and

The total amount of mechanical power derived is dependent on the amount of torque

ω*rdF* = ω*r*τ

*P* = ω

*P* (1)

*M* (3)

*dA* (4)

*Q* = *Qt* +*Qr* (2)

*Q* =η

**Figure 4.** Schematic representation of FSW process and tool. **Figure 4.** Schematic representation of FSW process and tool.


**Figure 3.** Friction stir welded specimen.


#### rotational (*Qr*) heat energies as shown in Equation (2). **3. Estimation of Energy Generated in FSW for Heat Input Examinations**

*Q* = *Qt* +*Qr* (2) The amount of heat generated in FSW is the power transformed to the total amount of heat energy and expressed by Equation (1).

The total amount of heat energy transferred is the sum of the translational (*Qt*) and

$$\mathcal{Q} = \eta \mathcal{P} \tag{1}$$

*P* = ω*M* (3) where *η*—power transformation coefficient, and*P*—power transferred.

*dP* = ω*dM* = ω*rdF* = ω*r*τ*dA* (4) The total amount of heat energy transferred is the sum of the translational (*Qt*) and rotational (*Qr*) heat energies as shown in Equation (2).

$$Q = Q\_t + Q\_r \tag{2}$$

The total amount of mechanical power derived is dependent on the amount of torque (*M*) and angular frequency (*ω*) and is expressed by Equations (3) and (4).

*dP* = *ωdM* = *ωrdF* = *ωrτdA*

$$P = \omega M \tag{3}$$

$$dP = \omega dM = \omega r d\mathbf{F} = \omega r \tau dA \tag{4}$$

where *r*—radial distance, *F*—force exerted by the FSW tool, *τ*—sheer contact stress in the material, and *A*—area of the weld.

By estimating the coefficients, the total heat input generated by the FSW tool is given by Equations (5) and (6):

$$Q = \int\_0^{2\pi} \int\_0^R \omega r^2 \tau d\theta dr \tag{5}$$

$$Q = \frac{2\pi\eta}{\text{3S}} \times \mu \times F\_{\text{N}} \times \omega \times \text{R} \tag{6}$$

Heat input in Equation (6) is the amount of energy per unit length of the weld.

#### **4. Results and Discussion**

#### *4.1. Mechanical Testing for Strength Assessment of Welds*

4.1.1. Tensile Tests

The tensile tests were carried out at room temperature using a 40 tonnes capacity universal testing machine (UTM) INSTRON 8801 (High Wycombe, UK). The specimens were prepared according to the ASTM (E8M) standard. It was observed that sample #1 had the lowest B4C content had the highest tensile strength (172 MPa) compared to the other samples (Figure 5). This is attributed to the good plasticity and density of Al [13]. With the increase in B4C and a decrease in SiC content in the friction stir welded Al-MMCs samples, the tensile strength decreases. This can be attributed to the even distribution of fine Al-SiC particles. Higher percentages of SiC contributed to particle agglomeration of Al-SiC during the solidification that caused the decrease in strength of composites. The maximum value of yield stress was 142.58 MPa for sample #1, within the acceptable range of most applications. The strengthening of the composites joints after the FSW process may be attributed to dislocation density of SiC and B4C particles, plastic deformation, and interactions between dislocations. The high yield strength of the Al-MMCs was due to the high amount of dislocations in its matrix. During the synthesis of Al-composite, additional dislocations were introduced by SiC and B4C particles due to the induced internal stresses. It is attributed to the different cooling rates of the reinforcement particles from the weld process temperature due to the thermal expansion coefficients mismatch. The percentage elongation drastically reduced with a decrease in SiC and an increase in B4C content. SiC exhibits high strength to weight ratio and high strength retention at elevated temperatures. With this, composite may also improve chemical stability and eliminate catastrophic failures when added to a metal matrix. However, it is important to account that processing of SiC at high temperature is preferable only when the reinforcement is to be processed at high temperature. There is also maximization of monolithic ceramics causing fragility in the composites. During the FSW process, the thermal expansion coefficient between reinforcement and the matrix contributes to the thermal cooling stress from processing temperature. *Materials* **2021**, *14*, x FOR PEER REVIEW 7 of 17

**Figure 5.** Tensile test properties measured for different sample compositions. **Figure 5.** Tensile test properties measured for different sample compositions.

4.1.2. Hardness Test

pacity leading to prominent improvement in the mechanical properties.

higher hardness followed by samples #3, #2 and #1. This observation emphasizes that the hardness is directly proportional to the B4C and inversely proportional to the SiC content. Unreinforced Al alloy had an average hardness value of 23 HV, which improved considerably by adding reinforcement and subsequent thermal action during the FSW process, indicating an optimal increase as it eliminates brittleness. SiC and B4C particles are relatively harder and tightly bonded, constricting dislocation movements in the Al matrix, thus enhancing the hardness. SiC has lower thermal expansion and can withstand high temperature due to its high creep and oxidation resistance reflected in the mechanical properties when added as reinforcement in Al-MMCs. Hence, a higher SiC content results in a decline in hardness. The hardness observed at the weld zone was higher than other zones for all the samples. It indicates the strengthening of Al-MMCs after it was subjected to the FSW process. This is due to the friction action in FSW, which aids in the uniform distribution of the reinforcement particles in the Al matrix. B4C exhibits low density and high hardness, which improved the structural integrity and hardness of the Al-MMCs. It also possesses excellent stability with high thermal loading, wettability and abrasive ca-

#### 4.1.2. Hardness Test

The interface bonding strength between the Al matrix and reinforcement particles was recorded using the microhardness tests (Figure 6). It was observed that sample #4 had higher hardness followed by samples #3, #2 and #1. This observation emphasizes that the hardness is directly proportional to the B4C and inversely proportional to the SiC content. Unreinforced Al alloy had an average hardness value of 23 HV, which improved considerably by adding reinforcement and subsequent thermal action during the FSW process, indicating an optimal increase as it eliminates brittleness. SiC and B4C particles are relatively harder and tightly bonded, constricting dislocation movements in the Al matrix, thus enhancing the hardness. SiC has lower thermal expansion and can withstand high temperature due to its high creep and oxidation resistance reflected in the mechanical properties when added as reinforcement in Al-MMCs. Hence, a higher SiC content results in a decline in hardness. The hardness observed at the weld zone was higher than other zones for all the samples. It indicates the strengthening of Al-MMCs after it was subjected to the FSW process. This is due to the friction action in FSW, which aids in the uniform distribution of the reinforcement particles in the Al matrix. B4C exhibits low density and high hardness, which improved the structural integrity and hardness of the Al-MMCs. It also possesses excellent stability with high thermal loading, wettability and abrasive capacity leading to prominent improvement in the mechanical properties. *Materials* **2021**, *14*, x FOR PEER REVIEW 8 of 17

**Figure 6.** Comparison of hardness for friction stir welded Al-MMCs samples. **Figure 6.** Comparison of hardness for friction stir welded Al-MMCs samples.

Wear tests were performed on SiC and B4C reinforced Al-MMCs to study the wear behaviour of Al-MMCs. B4C particles showed an insignificant impact on the wear properties, and consequently, the study was restricted to only the SiC content. SiC exhibits

Al MMC's tribological properties. For varying SiC content, the friction coefficient was recorded. The coefficient of friction was higher for samples whose SiC content ranged between 2 to 4% (Figure 7). It was also observed that the friction coefficient increased with an increase in load from 10 N to 20 N (while maintaining a constant velocity of 1 m/s). However, the friction coefficient decreased when the load was further increased to 30 N. This may be attributed to the increased brittleness of the samples [5]. The wear rate prominently increased with higher loading, and this observation was for all loading (Figure 8). The wear resistance gets transformed from abrasion to adhesion, reducing the average friction coefficient [5]. The wear rate of composites increased with the increase in load. This is due to the direct metal to metal contact, which leads to wear. The reinforcement particles reduced the plastic deformation by constricting the movement of dislocations. With the increased normal load, the reinforcement particles eroded from the surface leads

4.1.3. Wear Analysis

to base Al resulted in a higher wear rate.

#### 4.1.3. Wear Analysis

Wear tests were performed on SiC and B4C reinforced Al-MMCs to study the wear behaviour of Al-MMCs. B4C particles showed an insignificant impact on the wear properties, and consequently, the study was restricted to only the SiC content. SiC exhibits excellent wear and corrosion resistance for a wide range of temperature reflected in the Al MMC's tribological properties. For varying SiC content, the friction coefficient was recorded. The coefficient of friction was higher for samples whose SiC content ranged between 2 to 4% (Figure 7). It was also observed that the friction coefficient increased with an increase in load from 10 N to 20 N (while maintaining a constant velocity of 1 m/s). However, the friction coefficient decreased when the load was further increased to 30 N. This may be attributed to the increased brittleness of the samples [5]. The wear rate prominently increased with higher loading, and this observation was for all loading (Figure 8). The wear resistance gets transformed from abrasion to adhesion, reducing the average friction coefficient [5]. The wear rate of composites increased with the increase in load. This is due to the direct metal to metal contact, which leads to wear. The reinforcement particles reduced the plastic deformation by constricting the movement of dislocations. With the increased normal load, the reinforcement particles eroded from the surface leads to base Al resulted in a higher wear rate. *Materials* **2021**, *14*, x FOR PEER REVIEW 9 of 17

**Figure 7.** Variation of coefficient of friction of Al-MMCs with SiC content at different load and constant velocity of 1 m/s.

constant velocity of 1 m/s.

ity of 1 m/s.

**Figure 7.** Variation of coefficient of friction of Al-MMCs with SiC content at different load and

**Figure 8.** Variation of wear rate of Al-MMCs with SiC content at different load and constant veloc-

constant velocity of 1 m/s.

**Figure 8.** Variation of wear rate of Al-MMCs with SiC content at different load and constant veloc-**Figure 8.** Variation of wear rate of Al-MMCs with SiC content at different load and constant velocity of 1 m/s.

ity of 1 m/s. SEM analysis was used to investigate the material loss from the surface. The SEM micrographs of worn surface are shown in Figure 9a,b in 10 N and 20 N loads, respectively. The presence of wear debris and ploughing marks are visible, which are insignificant to cause any behavioural changes of the material. The reinforced particles agglomerated in the weld region due to the frictional heat produced during welding. The SEM micrographs of 30 N load are shown in Figure 9c,d. The corresponding EDAX analysis is also presented. The peaks of Al and reinforcement particles were observed. Small peaks of Fe were also observed, suggesting the abrasion of the steel surface by the reinforcement particles. Particles were homogeneously distributed, and no damage of worn surface was observed for higher load. In few cases, the presence of delamination with intense plastic deformation suggests adhesion wear [5]. The peaks of O in the EDAX analyses indicate the presence of oxidative driven wear. It is well known that Al readily reacts with atmospheric oxygen and forms aluminium oxide at the surface when undergone the counter steel abrasion.

**Figure 7.** Variation of coefficient of friction of Al-MMCs with SiC content at different load and

## *4.2. Radiography Testing of Welds for Quality Assessment*

Al-MMCs were placed between the radiation source (Ir-192, Co-60 and Cs-137) and radioactive film. The variation in the image intensities was observed to analyse pores, cracks or discontinuities. Figure 10 shows that the friction stir welded samples (representative) were free from any defect and discontinuity. Pores or pore clusters in the weld samples may weaken the strength in the region but do not alter the weld properties. No pores or pore clusters were observed in the samples. Hence, the samples were free from defects and discontinuities.

steel abrasion.

SEM analysis was used to investigate the material loss from the surface. The SEM micrographs of worn surface are shown in Figure 9a,b in 10 N and 20 N loads, respectively. The presence of wear debris and ploughing marks are visible, which are insignificant to cause any behavioural changes of the material. The reinforced particles agglomerated in the weld region due to the frictional heat produced during welding. The SEM micrographs of 30 N load are shown in Figure 9c,d. The corresponding EDAX analysis is also presented. The peaks of Al and reinforcement particles were observed. Small peaks of Fe were also observed, suggesting the abrasion of the steel surface by the reinforcement particles. Particles were homogeneously distributed, and no damage of worn surface was observed for higher load. In few cases, the presence of delamination with intense plastic deformation suggests adhesion wear [5]. The peaks of O in the EDAX analyses indicate the presence of oxidative driven wear. It is well known that Al readily reacts with atmospheric oxygen and forms aluminium oxide at the surface when undergone the counter

**Figure 9.** SEM/EDAX for AA 6061 composites with different reinforcement: (**a**) 10%SiC/3%B4C, (**b**) 8%SiC/5%B4C, (**c**) 5% SiC/8% B4C, (**d**) 3% SiC/10% B4C. **Figure 9.** SEM/EDAX for AA 6061 composites with different reinforcement: (**a**) 10%SiC/3%B4C, (**b**) 8%SiC/5%B4C, (**c**) 5% SiC/8% B4C, (**d**) 3% SiC/10% B4C. samples may weaken the strength in the region but do not alter the weld properties. No pores or pore clusters were observed in the samples. Hence, the samples were free from defects and discontinuities.

sentative) were free from any defect and discontinuity. Pores or pore clusters in the weld

**Figure 10.** (**a**) Radiographic images of AA6061 welded composite plates reinforced with: (**a**) 5% SiC/8% B4C, (**b**) 3% SiC/10% B4C. **Figure 10.** (**a**) Radiographic images of AA6061 welded composite plates reinforced with: (**a**) 5% SiC/8% B4C, (**b**) 3% SiC/10% B4C.

*4.3. Metallurgical Characterization for Assessment of Weld Microstructures* 

The distinct macrostructures of different zones (base metal, stir zone, thermomechan-

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.
