Abstract

The tremendous thermal and mechanical capabilities of carbon-based nanomaterials have drawn from researchers across the world. Composites reinforced with graphene nanoplatelets (GNPs), multiwall carbon nanotubes (MWCNT), and fullerenes (C20) were utilized in this study to increase their strength. A hot extrusion approach and a solution-based semipowder metallurgical technology were employed. Microscopically and mechanically, the samples were tested. Mechanical properties were assessed through the use of roughness and tensile tests. Even a small amount of nanocarbon (0.25 wt %) significantly improved the toughness and hardness qualities of AA7075. Composite reinforced with C20 was found to have higher hardness and yield strength than any other samples.

1. Introduction

Metal matrix composites (MMCs) have fetched much attention in the last half of the 20th century because of their superior properties versus unreinforced metals and alloys [1, 2]. These materials have excellent heat conductivity, high tensile and elasticity modulus, and excellent creep and wear resistance [3]. Aluminium-based composites are becoming increasingly popular among MMCs due to their small weight, high strength-to-weight proportion, and inexpensive cost [4, 5]. They have the potential to revolutionize the automotive, aerospace, and aviation industries. However, aluminium alloys such as AA7075 are brittle and have poor mechanical properties like toughness and ultimate tensile strength. AA7075’s mechanical performance has been improved by researchers utilizing reinforcement materials in recent years [6, 7].

It is essential to consider the features of the filler materials that make up Al-based MMCs, for instance, their size, shape, morphology, surface area, and contact among the matrix and strengthening [8]. A variety of micron-sized materials has been used to enhance the mechanical properties of aluminium (Al), conferring to the literature. High weight or volume fractions of these particles increase strength, but this effect dramatically reduces the flexibility [9, 10]. Density can be improved in aluminium-based alloys by using substantial weight fractions of micron-sized particles. There is some evidence that the flexibility of metals can be enhanced by using nano-sized reinforcements [11].

It has been proven that nanoparticles can improve the performance of the aluminium matrix [12]. In addition to the existing nanoparticle materials, carbon-based nanoreinforcement (MWCNT, GNPs, and C20, among others) is being examined as an option [13]. High strength, outstanding thermal and electrical properties, and exclusive mechanical and physical features of CNT make them an attractive option for use as reinforcements in Al-based composites [14]. They have a toughness of 1 TPa, a strength of 50 GPa, and an aspect proportion up to 1000. CNT’s lower density (1.2-2.1 g/cm), as well as solid mechanical qualities, also makes them a good candidate for use in the creation of lightweight composite materials.

It was [15] that first reported on the use of an aluminium matrix carbon nanotube-reinforced composite. After this, CNT continued to be used as a reinforcement for aluminium [16]. The study of CNT-reinforced aluminium matrix composites is popular among scientists. Poor dispersal of carbon in the matrix is the primary impediment to production because of the strong C=C bonding [17]. Prior research has concentrated on high-energy ball milling, spherical plasma sintering, and frictional stir to achieve an equal distribution of material. MWCNT-reinforced aluminium alloy matrix composites were developed by [18] using a multipass friction stir process. The MWCNT reinforcement in the matrix was found to be considerably homogenously distributed, and the composite was found to be two times as hard as the primary alloy [19]. The sp2-hybrid structure of two-dimensional graphene is a single-atom-thick layer. This material has an extremely high modulus of elasticity (1000 TPa) and a higher strength (140 GPa) with higher thermal conduction (5250 W/(mK)).

Graphene’s inimitable electrical, thermal, and mechanical qualities have made it a hot issue in composite technology for decades. Literature records numerous attempts to create graphene-reinforced aluminium matrix compounds. According to [20, 21], the semipowder method can be used to make graphene-reinforced aluminium matrix compounds. The inclusion of graphene increased the yield and tensile strength, and this was reported in [22]. As with carbon nanotubes and graphene, fullerenes can be used in metal matrix composites to increase their mechanical properties [23]. It is a carbon allotrope with a zero-dimensional structure. Fullerene’s carbon atoms are organized in pentagons and hexagons and feature sp2 and sp3 bonds. Carbon nanotube and graphene have been extensively studied in comparison to fullerene-reinforced aluminium matrix composites [24, 25].

Fullerene-reinforced composites were made via high-pressure torsion, according to [26]. A 2 vol % reinforcement of fullerene was found to increase microstructure and hardness performance significantly. Aluminium and fullerene’s microstructure and interfacial characteristics were studied in [27, 28]. Using liquid metal infusion, they created a new composite material. Using XRD and Raman spectroscopy, Al4C3 intermetallic phases were discovered in [29]. Noncomposite materials were not examined so far for their mechanical properties. Using powder metallurgy, researchers [30] created fullerene-reinforced aluminium matrix compounds annealed at 500°C. In the above study, extensive microstructure analyses yielded a stable nanoscale network topology.

The mechanical dampening capabilities of fullerene-reinforced aluminium matrix compounds were studied in [31] and found to be particularly successful in producing damping behavior with a controlled lattice. Fullerene (0D), MWCNT (1D), and GNP (2D) materials were employed in this analysis to enhance the mechanical characteristics of soft [32] AA7075. An aluminium matrix with fullerene reinforcement has been hot extruded using a semipowder method. In addition, the mechanical performance of AA7075 was compared to that of nanocarbon materials in this study.

2. Experimental Studies

2.1. Production of Composites

Composite materials are made using various techniques, including ultrasonication, powder mixing, drying, pressing, and a hot extrusion method. Figure 1 shows a diagram of the production process for samples. Before ultrasonication, carbon materials (each 0.25 wt % in the matrix) were first treated with 100 ml of ethanol. To prevent aggregation of nanocarbon, a vibrational separation procedure was utilized in the process. A vacuum distillation apparatus and a mixed method (matrix and reinforcement) have been established. The mixture was heated to 1500°C and stirred up to 600 rpm by using a magnetic stirrer. To ensure that all of the ethanol have been eliminated, the powders were dried out in an environment-controlled furnace. Table 1 displays the pressing and sintering parameters. Cylindrical samples of were made at the end of the press-and-sintering operation. The extrusion proportion was 15 : 1, and the speed was 0.4 mm/s at 400°F during hot extrusion.

3. Characterization of Produced Composites

The phase determination of samples was done using the Rigaku X-ray diffraction test instrument. Metallographic methods (grinding with 400 to 2000 SiC grit sheet, improving with diamond solutions of 6 μm) were applied to all specimens in order to examine their mechanical properties. SEMs equipped with EDX machines were utilized to examine the microstructure (Figure 2) of carbon-reinforced composites. The dispersion of carbon in the matrix of AA7075 was examined using JEOL UHR-TEM scanning transmission electron microscopy.

The sample for transmission electron microscopy (TEM) was generated using the focused ion beam (FIB) method for this study. Vickers hardness testing was performed using a QNESS Q10A hardness tester. The weight was 500 g, and the dwell time was 15 seconds. Trial specimens for tensile testing were created using the extrusion method. A Zwick Roell tensile tester was employed to conduct the test at 0.02 mm/s.

4. Results and Discussion

4.1. Results of Density

For AA7075 and carbon-reinforced composites, theoretical densities were determined using the mixing rule. Archimedes’ principles were used to calculate densities in the laboratory experiments. Table 2 lists the theoretical and experimental densities. The theoretical density of AA7075 drops when reinforcements are added. The reduced density of nanocarbon can be attributed to AA7075. Using mass differences, it is evident that actual densities are lower than expected values. The sintering procedure may be to blame for these discrepancies. Because the atoms are more easily diffused while sintering at a high temperature. As a result, the density of the nanocarbon is lower than theoretical values because it absorbs gas constituents such as O, N, and CO.

4.2. X-Ray Diffraction

Investigation of carbon-reinforced compounds by X-ray diffraction in 2θ to the 10-90° range was carried out. Figure 3 clearly shows AA7075-related peaks. GNPs and C20 when added result in the creation of additional peaks with low intensities. New peak formation does not change when MWCNT is incorporated into the AA7075. Consequently, a fixed time scanning method was used in addition to completing continuous scanning. To establish the existence of carbon-based compounds, scientists focused their attention solely on the 2-3° range.

The highest intensities of the purchased MWCNT, GNPs, and C20 at 2° are, respectively, 23.5°, 26.4°, and 20.8°. The 0.05° step width and the 120 s count duration were both found to be acceptable. This shows that microcarbon materials can be used in composite materials. According to some literature, the development of brittle aluminium carbide (Al4C3) can be caused by a chemical interaction between the aluminium matrix and carbon-based reinforcements. Carbide production may decrease interfacial connection between the matrix and reinforcements of nanocarbon materials.

Comparison of the crystallographic textures of AA7075 with carbon-reinforced composites was done with the same tools (carbon-based XRD). Vertical to the extrusion direction, as indicated in Table 3 and Figure 4, the estimated extrusion-direction pole figures are as follows: {111}, {200}, and {220}. Carbon reinforcements resulted in minor increase in the maximum pole intensity () values in the middle and perimeter, according to an examination of {111} pole figure intensities. The tensile behavior may be affected by the contrast between the unreinforced and carbon-reinforced samples. The above results regarding {111} pole figures are similar to the findings of [33]. However, the behavior of the {200} and {220} pole figures does not alter significantly when carbon elements were added to AA7075.

4.3. Microstructure Analysis

Carbon-reinforced aluminium matrix composites are depicted in the Scanning Electron Microscope (SEM) images (Figure 5). The figures make it easy to identify grain boundaries. Although a few micropores can be seen in the surface, there are no significant flaws. When looking at the porosities, they were found to be identical. According to the report [34], there is a correlation between fabrication pressure and micropore development.

Through X-ray mapping, distribution of carbon reinforcements in aluminium was mapped out. In the matrix, a significant improvement in the carbon distribution was found out. However, the MWCNT-reinforced composite showed a modest amount of partial agglomeration. MWCNT has a strong affinity for clustering owing to the large van der Waals attraction energy among carbon atoms. The metal matrix can have an issue with uniformity, as making carbon nanotubes to embed in these materials is more difficult. MWCNT-reinforced samples had very few agglomerations, but adding GNPs and C20 improved the homogeneous distribution.

As a result of the scanning transmission electron microscopy (STEM) mapping, it was possible to determine the exact location of carbon atoms having a low molecular weight (C20) in the matrix. Figure 6 illustrates that carbon atoms were integrated into the matrix without any obvious agglomerations. Good surface and interface behavior qualities can be achieved by dispersing C20 effectively.

In general, aluminium matrix composites enhanced with fullerenes are developed using casting and powder metallurgy techniques. Because of this, aluminium matrix could not get a homogeneous distribution of hardness with the addition of C20. This study used a semipowdered technique to provide a consistent distribution of C20 in the matrix. Using the semipowder approach, we were able to attain uniform distribution of the ultrasonicated reinforcement in the matrix. An ethanol solution injected a uniform weak van der Waals bonding in the C20 reinforcements.

4.4. DSC Results

To investigate chemical interactions between matrix and carbon reinforcements, samples underwent differential scanning calorimetry (DSC) analysis. Figure 7 indicates the DSC curvatures for all specimens. An STA 7300 Hitachi equipment was used to raise the temperature of the samples from 500 to 700°C at the rate of 100°C/min. The melting points of AA7075, MWCNT, GNPs, and C20-reinforced composites were found as 655°C, 656°C, 662°C, and 655.8°C, respectively. A total of 288, 258, 252, and 240 mJ/mg were determined for these samples, respectively, as their melting enthalpies. At these temperatures, weight loss is less due to carbon reinforcements. As a result, the DSC curves do not show any notable phase transitions or the generation of carbides like Al4C3. Aluminium matrix and carbon reinforcements do not appear to react with one another. No intermetallic phases were found among aluminium alloy and nicotinamide (NCT) samples in this DSC investigation, and this is in contrast to the findings of [35].

4.5. Mechanical Tests of Composites

For unreinforced aluminium and composites, the average of hardness measurements is shown in Figure 8. The average is calculated by taking three trials for each specimen. Carbon reinforcements indicate a boost in the hardness of AA7075. Rigid reinforcements (MWCNT, GNPs, and C20) added to the soft matrix resulted in this improvement. The reinforcements might have increased the strength of the material and therefore limited the amount of indentation. The addition of MWCNT, GNPs, and C20 increased hardness by 17%, 22%, and 26%, respectively. Table 2 shows that the C20 sample has the highest porosity value among the composites. Because of this, it can be deduced that the hardness of composite materials is not related to their interior porosity.

Figure 9 depicts the stress and strain values for AA7075 and produced composites. Recorded strain values include tensile, yield, and failure strains. The addition of carbon reinforcement significantly improves the tensile and yield strengths of AA7075. MWCNT increases AA7075’s yield and tensile strengths by 18% and 27%, respectively. When GNPs were added to AA7075, a 26% improvement in yield strength and a 33% rise in tensile strength are seen. With adding C20, the tensile and yield strengths of AA7075 improve by 36% and 48%, respectively. The above results show that the mechanical characteristics of soft AA7075 can be enhanced by using carbon reinforcements. An increase in mechanical properties in the produced composites may be due to hot press and longer sintering times.

Matrix and reinforcing interfacial bonding can be utilized to determine the transfer of load from soft material to the tougher particle. It is possible to achieve this effect by distributing carbon evenly across metals. There was some accumulation with MWCNT, but the GNPs and C20 were inserted more uniformly. Because of the zero-dimensional structure of C20 reinforcements and the weak van der Waals connection among carbon atoms in GNPs, a more uniform dispersion is possible than that of MWCNT. The homogenous distribution of reinforcement significantly impacts the load transmission characteristics of composite materials. In order to determine a composite’s yield strength, one can use equation (1) and the load transfer system. represents the reinforcement fraction, while (about 80 MPa) represents the matrix yield strength. The material’s mechanical properties are affected by the small quantity of reinforcement.

Thermal expansion coefficient (CTE) which mismatches between aluminium and nanocarbon materials can significantly affect mechanical characteristics. The CTE of MWCNT, GNPs, and C20 is about 106 K1 while AA7075 is about . When displacement density increases around the grains, the strength of the material rises as a result. The area of carbon reinforcement is crucial in determining the density of dislocations in addition to varying CTE values. As a result, the material’s strength increases due to a greater surface area, and this is seen obviously in the case of C20. As the dislocation density grows, the dislocations’ motion is constrained, thereby increasing strength. For GNP-reinforced composites, researchers [36] revealed that the following formula (2) can be used to define the improvement in strength to yield vs. CTE. A similar procedure is offered for CNT-reinforced composites.

Because of the MWCNT’s minimal average diameter (8–18 nm), this theoretical model is functional to multiwall carbon nanotube-strengthened composites, resulting in yield strength of 127 MPa. Because of this, we can conclude that the MWCNT-reinforced sample has a significantly larger CTE than GNP samples. However, this equation has never been used to describe C20-reinforced composites in any published research. However, 115 MPa can be reached when this technique is used on C20 reinforcements.

Orowan looping mechanisms play an essential role in defining materials’ mechanical properties. When carbon materials are introduced into an unstrengthened matrix, displacement loops and posterior stress may occur around strengthening [37]. Consequently, the strength of the pure matrix increases in both tensile and yielding modes. GNPs and multiwall carbon nanotubes can be employed in equation (2) to calculate the increase in yield strength.

Calculations based on equation (3) yield 100 MPa for GNPs and 118.6 MPa for MWCNT-reinforced compounds. Their size strongly influences the mechanical properties and dispersion capabilities of reinforcement particles. C20-reinforced composites can be evaluated using the following dispersal hardening mechanism equation.

The yield strength of 120 MPa is found by using equation (3) for C20-reinforced composites. According to the literature, MWCNT- and GNP-reinforced composite yield strengths are calculated using the same theoretical formulas. For GNP-reinforced composites, the predicted and measured values are nearly identical. An increase of 19.1 MPa has been calculated theoretically with all collected parameters. An increase in yield strength of 21 MPa for GNP-reinforced composites was found through experimental data.

For MWCNT-based composites, the predicted yield strength far exceeds the experimental strength. When comparing MWCNTs to GNPs, MWCNTs have a greater strength than carbon fibres based on theoretical equations. GNP-reinforced composites, on the other hand, have produced better mechanical test results, as demonstrated by this research. The mechanical properties of the composites can be derived from various sources, including Orowan looping, load transfer, CTE mismatches, and interactions between the matrix and reinforcement, carbon atom bonding structures, and porosity level. As a result, there may be discrepancies between theoretical and experimental findings.

It is possible to account for load transmission and dispersion hardening process formulas in calculating mechanical properties of C20-reinforced composites, which is distinct from other reinforced composite materials. The hot-extruded C20 composite has a fracture surface. Hollows and carbon gatherings clearly mark the crack surface. An energy-dispersive X-ray spectroscopy (EDX) investigation shows that the sample contains carbon (Figure 10). There are also a few small voids and dimples visible.

5. Conclusions

Semipowder metallurgy successfully incorporated MWCNT, GNPs, and C20 reinforcements into AA7075. The distribution of carbon elements, particularly C20, was uniform. Intermetallic phases such as Al4C3 were not detected by XRD and DSC analyses. The nanocarbon compounds were found in fixed time scanning approach. According to DSC data, the weight loss of AA7075 at melting temperatures was reduced when carbon reinforcement is added. Texture intensity differences between AA7075 and its composites were indistinguishable. With the inclusion of carbon reinforcement, aluminium yield and tensile strength values increased significantly. There is no correlation between density and hardness in the investigated AA7075 metal matric composites reinforced with carbon compounds.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.