Optimization of Al6061 Nanocomposites Production Reinforced with Multiwalled Carbon Nanotubes

Abstract

This study investigates the impact of multi-walled carbon nanotubes (MWCNTs) on the microstructure and mechanical properties of Al6061 nanocomposites. The MWCNTs were uniformly dispersed in the aluminum alloy matrix using ultrasonication following cold pressing and sintering in a vacuum. The effect of the sintered temperature on the microstructure and mechanical properties of the nanocomposites was evaluated. The addition of MWCNTs resulted in grain refinement, with the nanocomposites exhibiting smaller and more uniformly distributed grains than the pure Al6061 matrix, particularly at lower sintering temperatures of 580 and 600 °C. The nanocomposites also demonstrated an increase in hardness, with peak values observed at 580 °C, primarily due to the effective dispersion of MWCNTs, which restrict dislocation movement and reinforce grain boundaries. While higher sintering temperatures led to significant grain growth and less uniform hardness distribution, lower temperatures favored finer grain structures and more homogeneous hardness profiles. The results suggest that the optimal sintering temperature for achieving the best balance between microstructure and mechanical properties is 580 °C. However, the study also highlights the need for optimized dispersion techniques to achieve a more uniform distribution of MWCNTs.

1. Introduction

The advancement of new materials has led to the development of nanocomposites, which exhibit remarkable improvements in mechanical, thermal, and electrical properties compared to their traditional counterparts. Among these advanced materials, aluminum-based nanocomposites have emerged as a promising class due to their lightweight nature and excellent mechanical performance. Specifically, the aluminum alloy 6061, a precipitation-hardened alloy, is widely recognized for its good corrosion resistance, high strength-to-weight ratio, and favorable machinability, making it an ideal candidate for various engineering applications.

Research on the production and characterization of Al6061 aluminum alloy reinforced with nanoparticles has attracted significant attention due to its potential to enhance mechanical properties. Various studies have investigated fabrication processes, mechanical behavior, microstructural features, and the impact of different types of reinforcements on Al6061 composites [1,2,3,4,5,6,7]. A notable method for producing these composites involves the liquid metallurgy route, such as stir casting, which enables the incorporation of nanoparticles into the Al6061 matrix. For example, the tribological characteristics of Al6061 reinforced with titanium carbide (TiC) particles have been studied, illustrating the influence of different weight percentages of TiC on the composite properties [7]. The study emphasizes the importance of reinforcement content in customizing the final properties of the composite material.

Other studies have explored the addition of nanoparticles such as silicon carbide (SiC) [8], boron carbide (B₄C) [9], and graphene [10] to improve the mechanical properties of Al6061 composites. Sathyashankarasharma et al. [11] focused on Al6061 hybrid composites reinforced with B₄C and SiC particles, produced using a two-stage stir casting method. The research highlights the importance of hybrid reinforcements in achieving a synergistic effect on the mechanical properties of the composite. Additionally, the microstructural and mechanical properties of Al6061 nanocomposites reinforced with Al₂O₃ were investigated by Ezatpour et al. [12], fabricated through a stir casting process, demonstrating the potential of nanostructured reinforcements in enhancing the performance of Al6061 alloys. These studies emphasize the importance of selecting suitable nanoparticle reinforcements to achieve the desired mechanical enhancements in Al6061 nanocomposites.

In addition to these nanoparticles as reinforcement materials, incorporating carbon nanotubes (CNTs) has been studied to enhance the properties of Al6061 nanocomposites. Sahed [13] investigated the sintering behavior of CNT-reinforced Al6061 nanocomposites, demonstrating the successful production of carbon nanotube-reinforced Al6061 composites using ball milling and spark plasma sintering techniques. This study showcases the potential applications of CNT-reinforced Al6061 composites in industries like aerospace, automotive, and electronics, highlighting the versatility of nanomaterial reinforcements in improving the properties of Al6061 alloys. Aluminum alloys reinforced with CNTs show significantly enhanced mechanical properties compared to traditional aluminum alloys. According to Esawi and Farag [14], adding CNTs to aluminum alloys can increase tensile strength and modulus of elasticity. This increase is attributed to the high modulus of elasticity and strength of CNTs, which provide efficient reinforcement by preventing the movement of disagreements in the alloy matrix. Another study by Zhao et al. [15] highlights that using techniques such as high-energy grinding to disperse CNTs in aluminum alloys can result in a uniform improvement in mechanical strength without significantly compromising ductility.

However, the production of Al6061 nanocomposites reinforced with MWCNTs poses several challenges. Achieving a uniform dispersion of MWCNTs within the aluminum matrix is critical to enhancing the composite’s properties. Research by Li et al. [16] has shown that agglomeration of CNTs is a common problem due to their strong Van der Waals interactions. To mitigate this problem, methods such as mechanical milling, spark plasma sintering, and vapor phase chemical deposition have been explored. Zhan et al. [17] suggest that surface modification of CNTs can improve adhesion between the nanotubes and the aluminum matrix, promoting better charge transfer and consequently improving the mechanical properties of the composite. Moreover, optimizing the production parameters, such as mixing techniques, reinforcement concentrations, and sintering conditions, is essential to ensure the nanocomposites’ desired mechanical and thermal properties.

The present study aims to optimize the production process of Al6061/MWCNT nanocomposites by systematically investigating the effects of sintered temperatures on the composite’s microstructure and properties. Through a combination of experimental techniques and statistical analysis, the optimal conditions for improving the density, microstructure, and properties of the nanocomposites are determined. The findings of this research will provide valuable insights into the scalable production of high-performance Al6061/MWCNT nanocomposites, paving the way for their application in demanding engineering environments.

2. Materials and Methods

Aluminum alloy 6061 powder, the chemical composition in

Table 1. Chemical composition of aluminum alloy 6061 powder.

Al	Mg	Si	Fe	Cu
97.5	1.0	0.6	0.5	0.4

, was used as a metal matrix.

The as-received powders were characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and chemical composition by energy dispersive X-ray spectroscopy (EDS). This characterization was conducted using an optical microscope, DM 4000 M, (Leica Microsystems, Wetzlar, Germany) and with Leica Application Suite software (version 4.13.0, Leica Microsystems, Wetzlar, Germany) and Thermo Fisher Scientific QUANTA 400 FEG SEM (Thermo Fisher Scientific, Hillsboro, OR, USA) with an EBSD TSL-EDAX detector unit (EDAX Inc. (Ametek), Mahwah, NJ, USA). The EBSD data were analyzed by TSL OIM Analysis 5.2 (Ametek Inc., Devon-Berwyn, PA, USA).

SEM images in the secondary electron mode of the Al6061 powders received can be seen in Figure 1. In these images, the morphology of the powders is clear and free of defects such as pores. It can be seen in these images that the Al6061 powders are characterized by a range of particle sizes, with very small particles being observed. The particle size distribution of the powder, present in Figure 1c, shows an average size of 35 µm. Figure 1d shows the roundness distribution defined as ${\displaystyle \frac{4\pi area}{{perimeter}^2}}$, which excludes local irregularities and can be obtained as the ratio of the area of an object to the area of a circle with the same convex perimeter. Based on these results, it can be concluded that the number 1 is obtained for a perfect circle and drops to 0 when it becomes less round.

EBSD analysis revealed that the powder particles are characterized by more than one grain with different crystalline orientations. The inverse pole figure (IPF) and unique color grain maps can be seen in Figure 2. Based on these maps, the interior of the particles has a very different microstructure from the surface of the particles. Larger grains can be seen on the inside, while smaller grains are observed on the surface of the particles. The Kernel average misorientation (KAM) map shown in Figure 2c clearly indicates that the surface of the particle is the area where the greatest misorientation angle is observed. This means that the particle has undergone greater deformation. This may explain the different grain sizes observed.

The CNTs used in this study have consistent diameters, typical of multi-walled carbon nanotubes (MWCNTs). These MWCNTs have been well characterized in previous works [18]. In the transmission electron microscopy (TEM) images in those works, it is possible to observe the structure of the CNTs in more detail, showing their concentric layers. The CNTs have a uniform outer diameter, indicating their quality and consistency. With the conditions used, this dispersion method reduces the diameter of the CNTs, concentrating them into smaller outer diameter bands due to the elimination of the outer walls, as observed in previous work [18].

The nanocomposites were produced by powder metallurgy route using the Al6061 matrix powder and MWCNTs. Mixing and dispersion were performed using ultrasonication in isopropanol for 15 min to ensure uniform distribution of MWCNTs within the Al6061 matrix. This mixing and dispersion method has proven effective in producing nanocomposites with different matrices [18,19]. The mixing was then cold-pressed at 550 MPa to form green compacts. The green compacts were sintered using different temperatures (540, 560, 580, 600, and 620 °C) to enhance the bonding between the Al6061 matrix and the MWCNTs. The density of the sintered composites was measured using the Archimedes principle. The theoretical density was calculated based on the rule of mixtures, and the experimental density was compared to determine the relative density of the composites. Optical microscopy (OM), digital microscope (DM), SEM, and EBSD were used to characterize the microstructure of nanocomposites.

Vickers hardness tests were conducted on polished cross-sections of the sintered samples using a load of 196 mN and a dwell time of 10 s, considering 30 hardness values in each sample. The equipment used was a FALCON 400 micro/macro-Vickers hardness tester (INNOVATEST Europe BV, Maastricht, The Netherlands). For the study of the hardness in the areas, matrices with four columns and eight rows were used to evaluate the evolution of hardness with the addition of reinforcement.

The tensile test was conducted at a speed of 1 mm/s using Shimadzu EZ Test equipment (Shimadzu Corporation, Kyoto, Japan). Three samples from each nanocomposite and matrix were tested.

3. Results and Discussion

The sintering temperature plays a crucial role in the densification of the Al6061 matrix and the Al6061/MWCNT nanocomposites, proving to be an essential parameter in production. Figure 3 shows the evolution of density for the matrix and the nanocomposites at different sintering temperatures. Both samples show an increase in density with rising sintering temperatures, highlighting the role of thermal energy in reducing porosity and enhancing particle bonding. At 580 °C, the highest value is observed, which may indicate an optimal sintering temperature. However, a slight decrease in matrix density is observed for higher temperatures. This decrease may be related to some reaction occurring in the matrix at this temperature. For the nanocomposites, achieving a significantly higher density may indicate an improvement in the bonding between the reinforcement and the matrix and a reduction in porosity compared to lower sintering temperatures. As expected, the presence of CNTs impacts the densification process.

Microstructural characterization was performed to observe and understand the effect of sintering temperature. In Figure 4, SEM images of an enlarged area of the samples can be observed, characterizing the presence of pores and other microstructural features and the effect of sintering temperature and reinforcement presence. These images show the samples sintered at 620, 600, and 580 °C for the matrix and the composites. Observing these images, the composites are characterized by more dark phases, likely corresponding to pores and CNT agglomerates. The samples sintered at the highest temperature exhibit a lighter phase, which decreases with a reduction in sintering temperature. The samples sintered at 580 °C display the most homogeneous microstructure and appear more densified, confirming the results in the graph in Figure 3.

For the samples sintered at 620 °C, the matrix presents a more homogeneous, coarse microstructure, and some dark areas are visible, which may correspond to pore regions and areas of secondary phases. The microstructure of the Al6061 matrix reinforced with CNTs is visibly different from the pure matrix. There is a more complex distribution of phases and structures. The addition of CNTs appears to have resulted in a more refined grain structure. The grains are smaller and more uniformly distributed compared to the pure matrix. The image shows a higher density of dark areas, which may indicate the presence of CNTs and the possible formation of intermetallic compounds or other secondary phases due to the interaction between the CNTs and the Al6061 matrix.

The sample sintered at 620 °C exhibits more significant porosity than that at 580 °C. The lower porosity in the 580 °C sample indicates denser and potentially more resistant composites. Sintering at 620 °C results in more prominent grains and a coarser microstructure, while sintering at 580 °C results in smaller grains and a more homogeneous microstructure. The presence of secondary phases is more evident in the sample sintered at 620 °C, suggesting that the high temperature may favor the formation of these phases and intermetallic compounds.

High-magnification SEM images of the samples sintered at 620 and 580 °C can be observed in Figure 5.

From these images, it can be confirmed that CNTs significantly impact the microstructure of the samples sintered at 620 °C. The introduction of CNTs into the Al6061 matrix promotes the formation of a finer microstructure with more refined second phases. At 620 °C sintering, bright and gray particles can be observed in the matrix and the nanocomposites. These particles are also observed with the reduction in sintering temperature, although the bright particles appear less frequently.

EDS analyses were performed to characterize the chemical composition of the samples. Figure 6 and Figure 7 show the elemental composition maps of the samples sintered at 620 °C. The analysis of the EDS maps reveals that the Al6061 matrix exhibits a relatively homogeneous elemental distribution in aluminum. The presence of iron, magnesium, and silicon in specific areas suggests the formation of secondary phases or precipitates, which can affect the mechanical properties and the overall microstructure of the material. These observations are crucial for understanding the influence of alloying elements and impurities on the material’s performance. In the EDS elemental maps for the nanocomposites (Figure 7), zones with iron, magnesium, and silicon are also visible. However, the regions richer in Fe and Si are smaller than those observed in the matrix sintered at the same temperature. These maps confirm that the bright particles are rich in Al, Fe, and Si, while the dark gray particles contain Mg and Si. A point EDS analysis of the areas marked in Figure 6a is presented in

Table 2. EDS chemical composition (in at. %) of the regions identified in Figure 6a.

	Al	Si	Cu	Mg	Fe	Possible Phases
Z1	78.7	7.6			13.6	Al8Fe2Si
Z2	96.4	1.8		1.8		(Mg, Si)ss
Z3	99.3	0.6	0.1			α-Al

. Based on this analysis, it can be verified that the bright particles are possibly Al₈Fe₂Si, while the dark gray particles could be solid solutions of Mg and Si in α-Al. For the nanocomposites, it is possible to observe that the CNTs are most likely concentrated at the triple points and in the pores, as confirmed by the elemental distribution map of carbon (Figure 7d).

The high-magnification SEM image of the fracture surface present in Figure 8 reveals essential details about the interaction between the Al6061 matrix and the carbon nanotubes. The blue arrows indicate locations where the CNTs are pulled out from the Al6061 matrix. These CNTs appear well anchored in the matrix but protrude from the aluminum grains. The fracture surface shows a typical ductile fracture morphology, with rough regions and cavities, indicating that the material underwent considerable deformation before fracturing. CNTs at the fracture points suggest that they contribute to the composite’s mechanical strength, possibly delaying crack propagation. The good integration of the CNTs in the matrix can be observed, which is crucial for efficient load transfer between the matrix and the reinforcements. The CNTs emerging from the grains may be located at the grain boundaries, acting as effective reinforcements in these critical regions.

An image analysis was conducted on the area presented in Figure 4 based on the phases identified for the samples sintered at different temperatures. The results are shown in the graphs in Figure 9. Figure 9a–c show the phase fraction results for the matrix sintered at 600, 620, and 580 °C. The values in blue correspond to the α-Al phase, red to the solid solution of Mg and Si in the α-Al phase, green to the Al₈Fe₂Si, and black to the pores. Figure 9d–f show the same results for phase fraction results for the nanocomposites sintered at 600, 620, and 580 °C. Based on the analysis of these graphs presented in Figure 9, for the Al6061 matrix, with the reduction in sintering temperature, there is a decrease in secondary phases. The percentage of Al₈Fe₂Si and (Mg, Si)ss decreases significantly with the reduction in temperature, suggesting less formation of intermetallic compounds. The amount of porosity is relatively constant, with a slight decrease at the temperature of 580 °C. For the nanocomposites, the presence of CNTs seems to be reflected in the pores + CNTs amount. With the reduction in sintered temperature, the number of pores + CNTs decreases, suggesting better integration of the CNTs in the matrix. The secondary phases Al₈Fe₂Si and (Mg, Si)ss show variations but do not follow a clear trend, as seen in the Al matrix. Regarding the quantity of Al₈Fe₂Si and (Mg, Si)ss phases, CNTs do not appear to have a significant influence, only affecting their size.

The image in Figure 10 presents the unique color grain maps of the Al6061 matrix sintered at 620 and 580 °C, the unique color grain map of Al6061/MWCNTs sintered at 580 °C, and a grain size distribution graph for the Al6061 matrix and MWCNT-reinforced Al6061 nanocomposites sintered at different temperatures. For the Al6061 matrix, the grains are large and well-defined, while for the nanocomposites the grains are smaller than the pure matrix, suggesting that the MWCNTs inhibit grain growth during sintering. The graph compares different materials and processing temperatures: the Al6061 matrix processed at 620, 600, and 580 °C (in various shades of blue) and the Al6061/MWCNT (multiwalled carbon nanotube composite) processed at 600 °C and 580 °C (in shades of red). The main results observed in the grain size distribution are as follows: the Al6061 matrix sintered at 620 °C presents the largest area fraction in large grains (above 80 µm), indicating significant grain growth at high sintering temperatures. Sintering of the matrix at 600 °C and 580 °C shows a smaller grain distribution, with most grains in the 10–20 µm range. For Al6061/MWCNT sintered at 600 °C and 580 °C, even smaller grains—with a significant area fraction in the range of 10–15 µm—are observed, indicating that the addition of MWCNTs results in more pronounced grain refinement. The Al6061 matrix sintered at 620 °C shows significant grain growth, while the grains are smaller at lower temperatures of 600 °C and 580 °C. The grain size distribution in the nanocomposites is more uniform and concentrated in smaller sizes, which can improve the mechanical properties due to the increased grain boundary density; adding MWCNTs results in grain refinement, inhibiting grain growth during sintering. This is evidenced by the smaller area fraction of large grains in the nanocomposites.

Figure 11 shows the evolution in hardness of the matrix and nanocomposite evolution for different sintering temperatures. The results indicate that adding MWCNTs to Al6061 improves the hardness of the material at lower sintering temperatures, with a notable peak at 580 °C (with a 28% increase in hardness). However, very high sintering temperatures (620 °C) seem to impair the hardness of the CNT-reinforced composite, possibly due to factors such as thermal degradation of the CNTs or the formation of porosities and defects in the matrix. Therefore, the selection of sintering temperature is crucial for optimizing the mechanical properties of MWCNT-reinforced Al6061. For the matrix, a maximum value is observed at 620 °C due to a higher density of precipitates. However, this temperature cannot be considered optimal due to poor densification. For the nanocomposites, incorporating MWCNTs tends to increase the hardness of Al6061, especially at temperatures of 560 and 580 °C. Sintering at 580 °C appears to be the most effective for increasing hardness when adding MWCNTs, showing a significant increase compared to the sample without reinforcement.

Figure 12 shows a hardness contour map of the Al6061 matrix and Al6061/MWCNTs sintered at 580 °C. The regions are color-coded according to the hardness scale on the side HV0.02. Based on the results, the hardness varies from 23 to 35 HV0.02, with some areas reaching up to 38 HV0.02. The distribution is relatively uniform but there are slight, localized variations. For the nanocomposites, the hardness varies from 25 to 56 HV0.02, with some regions reaching up to 58 HV0.02. The overall hardness of the nanocomposite is significantly higher than that of the pure matrix. The distribution is less uniform than the pure matrix, with more pronounced regional variations. The nanocomposite shows significantly higher hardness compared to the pure Al6061 matrix. The presence of MWCNTs contributes to the increase in hardness due to the restriction of dislocation movement and the reinforcement of grain boundaries. The pure Al6061 matrix shows a more uniform hardness distribution, while the nanocomposite presents a more heterogeneous distribution. The heterogeneity in hardness distribution in the nanocomposite may be due to the uneven distribution of MWCNTs or the formation of secondary phases at the interfaces. In the nanocomposite, regions with very high hardness (above 50 HV0.02) indicate that adding MWCNTs effectively improves mechanical properties locally. These high-hardness regions may be critical points for wear resistance and deformation resistance.

Table 3. Mechanical properties of matrix and nanocomposites obtained by tensile tests for samples sintered at 580 °C.

	Tensile Strength (MPa)	Elongation (%)
Al6061	63 ± 8	8.2 ± 0.7
Al6061/MWCNTs	87 ± 7	4.4 ± 1.2

shows the mechanical properties of the Al6061 matrix and the nanocomposite sintered at 580 °C obtained from tensile tests—the two properties analyzed were tensile strength (in MPa) and elongation (in %). The Al6061 aluminum matrix has a tensile strength of 63 MPa. When reinforced with MWCNTs, the tensile strength increases significantly to 87 MPa. The increase in tensile strength (from 63 MPa to 87 MPa) when adding MWCNTs indicates that the presence of carbon nanotubes strengthens the material, probably due to the reinforcing effect of the nanotubes, which improves the tensile strength of the aluminum matrix. The decrease in elongation (from 8.2% to 4.4%) suggests that although the Al6061/MWCNTs nanocomposite is stronger, it becomes more brittle, typically when stiffer materials or reinforcements are added. Adding multi-walled carbon nanotubes (MWCNTs) to 6061 aluminum results in a material with improved strength properties but a reduction in ductility. This reflects a typical trade-off between strength and flexibility. For applications where tensile strength is more critical than ductility, such as in structures that need to withstand large loads without deforming significantly, the Al6061/MWCNTs nanocomposite may be preferable. However, the pure Al6061 matrix could be more suitable for applications requiring more ductile materials.

4. Conclusions

This study explored the effects of multi-walled carbon nanotubes (MWCNTs) and sintering temperatures on the microstructure and mechanical properties of Al6061 nanocomposites. Higher sintering temperatures, especially 620 °C, led to grain growth and less uniform hardness. In contrast, lower temperatures (580 °C and 600 °C) promoted finer grains and a more uniform hardness distribution.

Adding MWCNTs to the Al6061 matrix resulted in significant grain refinement and a substantial increase in hardness and tensile strength, particularly at a sintering temperature of 580 °C. This improvement is attributed to the clamping effect of the MWCNTs, which restricts grain growth and improves mechanical properties by preventing dislocation movement and promoting the load transfer mechanism. However, the hardness distribution of the nanocomposites was more heterogeneous than that of the pure matrix, probably due to the uneven dispersion of the MWCNTs or the formation of secondary phases.

The best results were observed for the samples sintered at 580 °C. The increase in hardness is mainly due to the effective dispersion of the MWCNTs and the formation of secondary phases in smaller quantities than those observed at higher temperatures. However, attention must be paid to the uniform distribution of the MWCNTs to make the most of their strengthening potential. This knowledge provides a solid basis for further development and optimization of MWCNT-reinforced Al6061 nanocomposites for advanced engineering applications.