A mechanical and microstructural investigation of friction stir processed AZ31B Mg alloy-SiC composites

3.1.1 Effect of tool rotational speed 545 RPM

At a rotational speed of 545 RPM, the test results exhibited variations in the UTS across the specimens. Specimen S10 registered the highest UTS at 198.73 MPa, followed by S12 at 183.50 MPa, while S11 had the lowest value at 159.18 MPa. This suggests that the lower rotational speed facilitated a moderate heat input, which promoted proper grain refinement and maintained the overall tensile strength. It is observed that the UTS of the S10 sample is around 25% better than S11, while 8% better than that of sample S12. The percentage of elongation (POE) measurements showed the highest elongation of 10.09% in specimen S12, indicating better ductility due to the sufficient heat generation and plastic deformation.

The S10 sample exhibits 12.8 and 15.26% greater hardness values than the S11 and S12 samples, respectively. The moderate hardness observed at this rotational speed implies that the grain size and structural integrity were appropriately maintained.

In summary, the UTS varied between 159.18 and 198.73 MPa, with an average hardness ranging from 58.60 to 67.55. The highest elongation of 10.09% was observed in S12, suggesting improved ductility. These findings indicate that at the lower rotational speeds, the reduced heat generation allowed for better material flow, ultimately preserving the tensile strength.

3.1.2 Effect of tool rotational speed 765 RPM

The UTS of the S15 sample is better by 17 and 20.5% than the S13 and S14 samples. Similarly, POE is an indication of improved ductility, and with S15, it is 37 and 91% higher than S13 and S14 samples, respectively. But the hardness value was critically reduced by 14 and 12%, respectively, than the S13 and S14 samples.

The hardness values slightly decreased, with S15 showing a value of 56.09, likely due to a better balance of heat input and cooling rates. The increased UTS up to 205.26 MPa, combined with the relatively lower hardness values, suggests a trade-off between strength and ductility, and the moderate rotational speed of 765 RPM appears to enhance the stirring effect, leading to a more uniform grain distribution and improved overall mechanical properties.

3.1.3 Effect of tool rotational speed 1,070 RPM

At a rotational speed of 1,070 RPM, the mechanical properties exhibited a mixed response. In the UTS, the 39% improvement was noted in S17 compared with the S16 specimen, whereas in the S18 specimen that increment is 80% as that of the S17 specimen, suggesting that this speed facilitated some degree of grain refinement and material mixing.

However, specimen S18 recorded a notably lower UTS of 112.73 MPa, indicating that the excessive heat generation at this high rotational speed led to material softening. Similarly, the percent of elongation data shows that S17 had a moderate elongation of 6.57%, while specimens S16 and S18 exhibited much lower values of 3.45 and 3.84%, respectively, pointing to embrittlement of the material due to high-speed processing.

The findings suggest that increasing the rotational speed initially enhances the UTS and percent of elongation up to an optimal level, but further increases lead to material degradation due to excessive heat input. The maximum UTS were recorded at 765 RPM, while the highest hardness values were observed at 1,070 RPM, likely due to rapid cooling effects. Increasing the rotational speed improves heat generation and material flow, but beyond a certain point, excessive heat results in grain coarsening, which reduces the overall strength. The excessive heat input at 1,070 RPM appears to have caused material softening, leading to a decrease in UTS. The intermediate rotational speed of 765 RPM appears to provide an optimal balance between heat generation and material mixing, resulting in improved mechanical properties. In summary, higher rotational speeds enhance UTS and POE up to an optimal point, after which excessive heat generation at 1,070 RPM reduces UTS and ductility while increasing hardness due to grain coarsening.

3.2.1 Effect of feed rate 20 mm/min

At a feed rate of 20 mm/min, the test results showed the highest UTS of specimen S10. The UTS of sample S10 is 13 and 36% greater than that of S13 and S16 samples, respectively, indicating a well-consolidated microstructure

The highest hardness value of 86.83 was measured in S16, suggesting work hardening effects from the slow feed rate. Notably, S10 had the highest UTS of 198.73 MPa with a moderate hardness of 67.55, whereas S16 at the same feed rate showed the highest hardness of 86.83, indicating a localized strengthening effect. The lower feed rate of 20 mm/min allowed for sufficient heat input and material consolidation, improving the overall mechanical strength of the Mg alloy.

3.2.2 Effect of feed rate 31.5 mm/min

At a feed rate of 31.5 mm/min, the data exhibited better mechanical characteristics. The highest UTS was observed in specimen S17, indicating that this feed rate facilitated optimal heat input and material flow for improved consolidation. The UTS of sample S17 is higher by 28 and 19% as compared to S11 and S14, respectively. The same case is observed with POE also. The POE showed a moderate value of 6.57%, suggesting a balanced trade-off between strength and ductility. The POE of sample S17 is higher by 3.5 and 13% as compared to S11 and S14, respectively.

The hardness value for S17 was 70.18, confirming a moderate and well-balanced hardness level. Across the specimens tested at 31.5 mm/min, the UTS remained relatively stable, ranging from 159.18 to 203.54 MPa, further corroborating the balanced heat generation and material flow characteristics at this feed rate, which resulted in enhanced overall mechanical properties.

3.2.3 Effect of feed rate 50 mm/min

At a feed rate of 50 mm/min, the results exhibited a mixed response. The highest UTS of 205.26 MPa was observed in specimen S15, indicating that this feed rate facilitated optimal material consolidation and flow. The UTS of sample S15 is higher by 12 and 82% as compared to S12 and S18 samples, respectively.

However, specimen S18 recorded notably lower UTS 82 and 63% as compared to S15 and S12 samples, suggesting that the excessive feed rate led to inconsistencies in the mechanical properties. This observation suggests that high feed rates can result in uneven material properties.

The findings suggest that a feed rate of 31.5 mm/min provided the optimal balance among the mechanical properties, exhibiting the best combination of UTS, percent of elongation, and hardness. Lower feed rates resulted in higher hardness values, indicating significant work hardening, but reduced elongation. Conversely, higher feed rates of 50 mm/min led to the highest UTS values, but also introduced inconsistencies in the mechanical properties. This is likely due to the feed rate’s influence on material plasticization and consolidation – lower rates allow for better consolidation, while higher rates can cause defects like voids and incomplete fusion, ultimately reducing the overall strength of the material.

3.3.1 Effect of hole diameter 0.8 mm

At a hole diameter of 0.8 mm, the UTS values were consistently high, peaking at 205.26 MPa, indicating minimal structural degradation. The UTS of sample S15 is higher by 3.2 and 0.8% as compared to S10 and S17 samples, respectively. The percent of elongation exhibited a moderate value of 6.57%, and for the S15 sample, it is greater by 29 and 69% than S10 and S17 samples, respectively, ensuring a balanced set of mechanical properties.

Additionally, the hardness values were relatively low, with a minimum of 56.09, suggesting proper material consolidation. These findings suggest that a smaller hole diameter of 0.8 mm helps reduce the extent of material damage and minimize stress concentration during the hole processing operation.

3.3.2 Effect of hole diameter 1.2 mm

As the hole diameter was increased to 1.2 mm, the mechanical properties exhibited a noticeable decline. The UTS values were more moderate, with a significant decrease observed in specimen S18, which recorded a UTS of 112.73 MPa. This suggests that increasing the hole diameter begins to negatively impact the structural integrity of the material, likely due to the increased effect of material discontinuities. Similarly, the percent of elongation further decreased, with S18 exhibiting a value of 3.84%, indicating a loss in ductility.

The hardness values, however, remained moderate, with S18 recording a value of 64.62, potentially attributable to the increased stress concentration caused by the larger hole diameter.

3.3.3 Effect of hole diameter 1.6 mm

When the hole diameter was further increased to 1.6 mm, the mechanical properties exhibited a more pronounced deterioration. The UTS values were substantially reduced, with specimen S16 recording a UTS of 145.62 MPa, indicating a weaker material integrity. Additionally, the percent of elongation reached the lowest level observed at 3.45%, suggesting an embrittlement of the material.

Furthermore, the hardness values were the highest across all specimens, with S16 exhibiting a hardness of 86.83 HV, which can be attributed to strain hardening effects. The increased hole diameter of 1.6 mm appears to have introduced more discontinuities in the material, leading to a significant decline in ductility and an increase in localized strain hardening.

The findings indicate that a smaller hole diameter of 0.8 mm maintained higher UTS, while larger hole diameters led to increased hardness but reduced UTS. This suggests that increasing the hole diameter weakens the material structure by altering the stress distribution. Larger hole diameters create stress concentration zones, which can result in premature failure. Therefore, optimizing the hole diameter is crucial for preserving the structural integrity of the material. The sharp rise in hardness observed at a hole diameter of 1.6 mm implies that larger holes increase localized stress concentrations, making the material more susceptible to failure. Conversely, smaller hole diameters retained higher UTS and percent of elongation, whereas larger holes increased hardness but decreased ductility. These findings emphasize the importance of carefully selecting the optimal hole diameter to avoid material weakening.

3.4.1 Effect of tool pin profile TT

Tapered threaded pin profile (TT) demonstrated a moderate range of UTS values, spanning from 198.73 to 112.73 MPa, which is there is a decrement by 43 and 14% observed in S14 and S18 as compared to the S10 sample, indicating a moderate degree of variation. The percent of elongation also varied significantly, though the elongation values remained within a moderate range. Notably, the hardness values were balanced, with a value of 64.62 HV, suggesting uniformity in the mechanical properties.

Overall, the TT profile facilitated a moderate UTS range and balanced hardness, which can be attributed to its ability to enable uniform heat generation and material mixing during the hole processing operation.

3.4.2 Effect of tool pin profile square headed (SQ)

The SQ tool pin profile exhibited a range of UTS values, with the highest recorded at 205.26 MPa but also a lower value of 145.62 MPa. This suggests that the SQ profile was capable of producing high UTS results in certain cases, but also demonstrated inconsistencies, indicating variability in the deformation process.

Additionally, the percent of elongation was higher, implying improved ductility. However, the hardness values were inconsistent across specimens, suggesting that the SQ profile led to uneven material behavior. Overall, the SQ tool pin profile provided the highest UTS observed, but lacked the consistency observed with other pin profiles.

3.4.3 Effect of tool pin profile TH

The TH pin profile demonstrated the most consistent mechanical performance. It achieved the highest UTS value of 203.54 MPa, indicating excellent structural integrity. Additionally, the moderate elongation of 6.57% ensured good ductility, while the balanced hardness of 70.18 HV confirmed optimized mechanical properties.

These findings suggest that the TH pin profile facilitated uniform heat generation and material flow, leading to a desirable combination of strength, ductility, and hardness. Thus, this pin profile can be considered a suitable choice for applications requiring balanced and consistent mechanical attributes.

The TH tool pin profile exhibited the most consistent and balanced mechanical characteristics. In contrast, the SQ tool displayed considerable variability, suggesting inconsistent heat generation and material flow during processing. The TT tool maintained moderate values, making it a suitable option for applications requiring uniform material properties. The tool pin profile influences material flow, heat generation, and grain refinement. A well-designed pin profile can enhance mechanical interlocking and weld quality. The TH pin facilitates more uniform and predictable heat input, leading to a desirable balance of mechanical properties.

3.5.1 Optical microscopy to analyze grain refinement and reinforcement particle distribution

The optical microscopy images provide insights into the grain refinement and particle distribution across different specimens processed using FSP. Figure 15(a)–(i) represents the optical microscopy images for the L9 specimen with various process parameter variations. Specimens processed at lower rotational speeds (545 RPM) with a TT tool, for sample S10, exhibit fine and uniform grain structures due to moderate heat input and efficient plastic deformation. The SiC reinforcement is well-dispersed for sample S10, contributing to enhanced mechanical properties. In contrast, specimens like S12, processed at higher feed rates (50 mm/min) and with a threaded (TH) tool, display coarser grain structures. The limited stirring effect results in uneven SiC distribution, leading to localized clustering and reduced strengthening effects.

Figure 15

(a)–(i) Optical microscopy images for L9 specimen with various process parameter variations: (a) S10: 545 RPM, 20 mm/min, 0.8 mm pin, TT Tool. (b) S11: 545 RPM, 31.5 mm/min, 1.2 mm pin, SQ. (c) S12: 545 RPM, 50 mm/min, 1.6 mm pin, TH. (d) S13: 765 RPM, 20 mm/min, 1.2 mm pin, TH. (e) S14: 765 RPM, 31.5 mm/min, 1.6 mm pin, TT. (f) S15: 765 RPM, 50 mm/min, 0.8 mm pin, SQ. (g) S16: 1,070 RPM, 20 mm/min, 1.6 mm pin, SQ. (h) S17: 1,070 RPM, 31.5 mm/min, 0.8 mm pin, TH. (i) S18: 1,070 RPM, 50 mm/min, 1.2 mm pin, TT.

At intermediate rotational speeds (765 RPM), specimens like S13 and S14 show well-refined grain structures, with dynamic recrystallization enhancing grain boundary formation. The TT tool in S14 facilitates better mixing, resulting in improved particle dispersion. However, S15, processed at 50 mm/min with a SQ tool, exhibits a combination of fine and coarse grains, indicating that high traverse speed limits heat input, affecting the uniformity of grain refinement.

At the highest rotational speed (1,070 RPM), the grain structures become more refined in specimens like S16, which employs a square tool and a low feed rate (20 mm/min). The higher tool rotation enhances plastic flow, leading to fine grain refinement and a homogeneous microstructure. However, S18, processed at 50 mm/min with a TT tool, shows coarse grains and uneven SiC dispersion. The excessive traverse speed reduces stirring efficiency, preventing uniform grain refinement. The optical microscopy results confirm that optimal process parameters, particularly moderate rotational speeds (765 RPM) and feed rates (31.5 mm/min), lead to refined grains and uniform reinforcement dispersion, which directly impact mechanical properties.

3.5.2 SEM to analyze grain refinement and reinforcement particle distribution

The SEM images validate or confirms the findings obtained from optical microscopy by providing a detailed view of grain boundaries, reinforcement distribution, and defect formation. Specimens with optimized processing conditions, such as S10 and S16, exhibit a well-dispersed SiC phase with minimal clustering. The TT tool in S10 promotes uniform particle distribution, which enhances interfacial bonding between the reinforcement and the Mg matrix. This uniform dispersion contributes to the improved hardness and UTS observed in mechanical tests. Similarly, S16, processed at 1,070 RPM and 20 mm/min, exhibits strong interfacial bonding and homogeneous particle distribution, leading to the highest recorded hardness among all specimens. In contrast, SEM images of specimens processed at high feed rates, such as S12 and S18, reveal significant particle clustering and agglomeration. The inadequate stirring effect at higher traverse speeds limits the uniform distribution of SiC particles, resulting in stress concentration points that reduce mechanical performance. Specimens like S17 also show non-uniform particle dispersion with some reinforcement segregation at grain boundaries, which affects localized stress distribution and may contribute to reduced elongation properties. At intermediate speeds, specimens such as S14 and S15 show excellent dispersion with minimal porosity, confirming that a balanced combination of rotational and traverse speeds improves reinforcement distribution. The SEM analysis highlights the importance of processing parameters in achieving optimal microstructural characteristics. Moderate feed rates and moderate rotational speeds ensure effective material flow, preventing defects such as porosity and particle agglomeration, ultimately improving the composite’s mechanical performance (Figure 16).

Figure 16

(a)–(i) SEM images for L9 specimen with various process parameter variations. (a) S10: 545 RPM, 20 mm/min, 0.8 mm pin, TT Tool. (b) S11:545 RPM, 31.5 mm/min, 1.2 mm pin, SQ. (c) S12: 545 RPM, 50 mm/min, 1.6 mm pin, TH. (d) S13: 765 RPM, 20 mm/min, 1.2 mm pin, TH. (e) S14: 765 RPM, 31.5 mm/min, 1.6 mm pin, TT Tool. (f) S15: 765 RPM, 50 mm/min, 0.8 mm pin, SQ. (g) S16: 1,070 RPM, 20 mm/min, 1.6 mm pin, SQ. (h) S17: 1,070 RPM, 31.5 mm/min, 0.8 mm pin, TH. (i) S18: 1,070 RPM, 50 mm/min, 1.2 mm pin, TT.