The Influence of Graphene Nanoplatelets Addition on the Electrical and Mechanical Properties of Pure Aluminum Used in High-Capacity Conductors

Abstract

The main objective of this research is to assess a graphene/Al nanocomposite with a higher strength and conductivity for use in high-capacity conductors in power transmission lines. In this study, the graphene/Al nanocomposite and pure aluminum specimens were prepared using ball milling of aluminum and graphene powders, the mechanical-electromagnetic stirrer casting process, hot extrusion and finally, annealing. The microstructural, mechanical and electrical behavior of the Al 1350 nanocomposite cast reinforced with 0.5 wt% graphene and unreinforced aluminum were studied at 20 °C and 180 °C temperatures. The results revealed that, by adding graphene to pure aluminum, the tensile strength, toughness and electrical conductivity increased, but the elongation of the Al–0.5 wt% GNP composite decreased at both temperatures.

1. Introduction

The combination of outstanding formability, high electrical conductivity, significant corrosion resistance, low density and even lower cost has led to an increased utilization of Al and its alloys in overhead conductors [1,2]. Among aluminum conductors, high-purity or electrical-grade aluminum, namely 1350-O, has been widely recognized. However, the corresponding mechanical strength and electrical conductivity indicate drastic decreases when operating at temperatures above 100 °C [3]. The temperature of the Al conductor is highly dependent on the passing current and the environmental conditions. The novel application of HTLS (high-temperature, low-sag) conductors and ACCCs (aluminum-carbon-composite conductors), for example, is indicative of its stability for continuous applications at approximately 180 °C, compared to 100 °C for traditional ACSR (aluminum conductor steel reinforced) conductors [3].

Recently, graphene has gained recognition amongst a wide array of researchers due to its exhibition of a significant set of properties. Its distinctive mechanical and electrical properties laid the groundwork for its popularity in manufacturing nanocomposite applications in order to increase aluminum’s strength and electrical conductivity [4,5,6,7]. The majority of previous investigations mainly tackled the strength enhancement [8,9,10,11,12,13,14,15,16], and a few studies [17] have been carried out at elevated temperatures. Conversely, only a small range of reports evaluated the electrical performance of Al/GNP composites, and they were conducted merely at room temperature [18,19,20]. Li et al. [18] investigated the electrical conductivity of Al/0.2 wt% GNPs that were fabricated by casting ball-milled powder followed by rolling. It was found that the interface scattering between Al and GNPs was relatively weak due to a decrease in the conductivity of the composites by approximately 0.7% IACS (International Annealed Copper Standard). Zhang et al. [19] stated that through the utilization of friction stir processing (FSP) combined with the hot extrusion method, the electrical conductivity, the tensile strength and the elongation of the graphene/Al nanocomposite were 2.1%, 17.3% and 35.4% higher than those of pure Al. Wu et al. [20] reported that the graphene/Al composite was fabricated by a novel hot AEB (accumulative extrusion bonding) method. An enhancement of electrical conductivity (about 1% IACS) was obtained with a three-cycle AEB process. Awate et al. [21] analyzed the microstructure and mechanical properties of an Al 6061 alloy via graphene nanoplate reinforcement (2, 4, 6, 8 and 10 wt%), fabricated by stir casting. They established that the as-cast tensile strength, hardness and yield strength of the graphene/Al 6061 nanocomposites were upgraded by 127%, 158% and 402%, respectively, compared to the unreinforced Al 6061 alloy. Furthermore, they reported that the nano-thickness of graphene, reinforcement quantity and the corresponding developing process are crucial elements for the improvement of the microstructure and mechanical properties of graphene/Al 6061 nanocomposites.

Among the challenges of Al–GNP composites for researchers was the issue of maintaining a uniform dispersion of graphene in the metal matrix to achieve the optimal mechanical and electrical properties. To tackle this problem, numerous studies, such as flake PM (powder metallurgy) [22], in-situ CVD (chemical vapor deposition) [23], molecular-level mixing [24] and ball milling [25] were conducted. Another setback researchers experienced was that gas-atomization Al powder typically goes along with an $A{l}_2{O}_3$ film, which is harmful for the electrical properties of Al–GNP composites [26]. Considering the importance and high application of aluminum conductors in the electrical industry, an optimum combination of conductivity and tensile strength for aluminum wires at different temperature conditions is required. Due to the softness of aluminum 1350 and its application in electrical conductors, a novel method to obtain high mechanical and electrical conductivity properties for an Al–GNP nanocomposite has been presented, for a casting process accompanied by a mechanical and an electromagnetic stirrer. The specimens were created via ball milling of Al–graphene powders, mechanical-electromagnetic stirrers, hot extrusion and annealing. In the following research, the dispersal of graphene and the microstructure of the composite were studied. Moreover, the mechanical behavior and electrical conductivity of the graphene-reinforced Al1350 matrix composites were scrutinized at 20 °C and 180 °C (operation temperature for conductors).

2. Materials and Methods

In this work, the graphene/Al nanocomposite and pure aluminum specimens were developed using ball milling, a mechanical-electromagnetic-stirrer casting process, hot extrusion and annealing. For the fabrication of the nanocomposite specimens, Al 1350 (

Table 1. Chemical composition of Al 1350 in weight percentages.

Al	Fe	Si	B	Zn	Cu	Ga	Ti	V	Mn	Cr	Others
99.8	$0.126$	$0.026$	$<0.0001$	$0.011$	$<0.0001$	$0.008$	$0.004$	$0.0136$	$0.003$	$0.001$	$<0.1$

) and a pure Al powder (95%) with an average size of 20 μm (Figure 1a) were utilized as the matrix. Additionally, GNPs with an average thickness of 15 nm, approximately 5–10 layers and a lateral dimension of 5 μm were used as the reinforcement (Figure 1b).

To fabricate the composite, the GNP powder was first dispersed for 20 min using ultrasonic velocity in ethanol to sever the particle agglomerations. Subsequently, the aluminum and graphene powders (25% graphene and 75% aluminum) were ball-milled for 2 h in an argon atmosphere (>99.99%) to avoid oxidation. The ball-to-powder weight ratio and rotational speed were equal to 10:1 and 250 rpm, respectively. Ball milling was carried out in ambient conditions and the amount of stearic acid employed as the PCA (process controlling agent) was 2 wt%. The FESEM image illustrates that the morphology of the aluminum powder altered from disordered to flaky, which increases the specific surface area of the matrix powder (Figure 2a). This method boosts the wettability of the graphene nanoplates and their uniform distribution in the matrix phase. The composite powder was poured into the holes of an aluminum ingot with a diameter of 6 mm and a depth of 50 mm (Figure 2b). The perforated aluminum ingots were filled with the composite powder, which melted in a furnace under an argon atmosphere at 780 °C. Following the complete melting of the aluminum composite, the mechanical stirring process was applied using a stainless steel mechanical impeller to rotate the molten at 2000 rpm for 10 min. Next, the electromagnetic stirring process was implemented with a frequency of 30 Hz and 5.8 kW from 780 °C to 650 °C. Figure 3 identifies the developed casting machine. The cooling rate was 8 °C/min, and the speed of the resultant electromagnetic force that caused the rotation of the molten metal in the desired direction was 400 rpm. After the stirring process was complete, the chamber cooled in the ambient air. For the sake of comparison with the nanocomposite specimens, the Al 1350 ingots without GNPs were used in the present study (similar to the production process of composite specimens). Following the casting process, in order to decrease porosity and improve the mechanical and microstructural properties, a hot extrusion process was conducted at 300 °C using a 20:1 extrusion ratio. Annealing treatment was also carried out at 343 °C for 1 h. Barker’s reagent was used for the electro-etching of the samples, and the microstructures were investigated using a field emission scanning electron microscope (FESEM, Mira 3-XMU) and Raman spectroscopy (wavelength: 532 nm). The hardness test was performed by the Vickers (M5C-1000) method, which was exercised under a load of 50 g and a dwell time of 15 s. Tensile tests were performed at temperatures of 20 °C and 180 °C according to the ASTM B557M-10 standard. Samples with a diameter of 6 mm and a gauge length of 30 mm were utilized to perform the tensile test with an applied constant strain rate of $1\times {10}^{-3} {\mathrm{S}}^{-1}.$ Fracture surface images were examined, as well.

The electrical conductivity of the Al/0.5 wt% GNPs samples was analyzed using a SG5068 DC resistance tester, according to the ASTM B193-16 standard, and the four-point probe method. For each temperature, three nanocomposite cylindrical samples with a diameter of 10 mm and a length of 200 mm were employed. The following equations were used to convert the electrical resistance (R) to electrical conductivity ($\mathsf{\sigma }$):

$$\mathrm{R}=\mathsf{\rho } (\frac{\mathrm{L}}{\mathrm{A}})=\frac{1}{\mathsf{\sigma }}\times \frac{\mathrm{L}}{\mathrm{A}}=\frac{\mathrm{V}}{\mathrm{I}}$$

(1)

$$\mathsf{\sigma }=\frac{\mathrm{I}\times \mathrm{L}}{\mathrm{V}\times \mathrm{A}}$$

(2)

where $\mathsf{\rho }$ is the resistivity, I the current, V the voltage, L the length of samples and A the area of the cross-section of samples.

For the purpose of comparison, the electrical and mechanical properties evaluation of pure Al was carried out using the same parameters.

3. Results and Discussion

3.1. Characterizations of the Composites

Figure 4 demonstrates the Raman spectra of GNPs and the ball-milled Al–GNP powder, which can further explain the influence of ball milling on the structure of graphene. It clearly exhibits the peaks at 1351 and 1577${ \mathrm{cm}}^{-1}$, corresponding to the D-peak and G-peak of graphene, respectively. Moreover, the Raman spectrum of ball-milled Al–GNP powder locates its peaks at 1351 and 1583${\mathrm{cm}}^{-1}$ for the D-peak and G-peak, respectively. This analysis could be utilized to distinguish the number of layers, disorder, strain and doping in graphene [27]. Raman spectroscopy also shows the structure and defects of graphene [20], with higher $I_D/I_G$ values revealing more defects [28]. The $I_D/I_G$ of raw GNPs was 0.61, whereas the value of milled powders was 1.04. Comparing these results, it could be deduced that ball milling produces defects in graphene.

Based on the optical microscope results (Figure 5) and the ASTM E-112 standard, the grain size of pure aluminum was approximately 22.5 and that of the composite was 15.9. This proves that the addition of graphene significantly reduces the grain size. The distribution of graphene nanoplates in aluminum, indicated in Figure 6a (low magnification) and Figure 6b (high magnification), confirmed that the distribution of graphene in the matrix was homogenous and that graphene agglomerates could not be observed. The stir-casting process was executed via mechanical and electromagnetic stirring. The mechanical stirring created shear stress in the melt and distributed the reinforcing particles. The electromagnetic stirring exerted a body force on the molten metal and assisted with the distribution of the reinforcing particles as well as the decrease in the dendritic structures and the refinement of the grains during solidification [28,29,30,31]. EDX was applied to reveal the existence of graphene (Figure 6c) and confirm the existence of a carbon composition in the matrix. To further confirm the presence of graphene, Raman analysis was deployed in different locations of the composite cylindrical sample (Figure 6d). The FESEM results and the distribution of graphene are in agreement with the preceding studies that used the same fabrication method as the present work [21]. The $I_D/I_G$ ratios of the composite in this study are much lower than that of the composites fabricated by other production methods [20,32]. As seen in Figure 4, the $I_D/I_G$ ratio for raw graphene powder is 0.61 and the $I_D/I_G$ ratios of the composite are proximate to this value. A decrease in the value of the $I_D/I_G$ ratio indicates that the composite production has improved the structure of graphene compared to the outcome of ball milling the aluminum–graphene powder. The characteristic peaks of graphene oxide and graphitized carbon were not observed, indicating that the GNPs were not oxidized, graphitized or agglomerated. Considering the amount of GNPs used and the clear Raman analysis for different locations of the composite, $A{l}_4{C}_3$ was not observed. The GNP content has a primary effect on the content of the aluminum carbide phase [33]. Similarly, the ball milling duration [32], the casting temperature and the cooling rate [34,35] are the dominant factors governing the formation of this phase, all of which were considered in this work. In general, the reduction in grain size, the satisfactory distribution of graphene according to FESEM images, the absence of defects in the graphene after producing the composite, and the absence of secondary phases, such as aluminum carbide based on the Raman analysis, confirmed the feasibility of the fabrication process.

3.2. Density and Hardness

Theoretical and empirical densities of pure aluminum and composite were measured using Archimedes’ principle, and are listed in

Table 2. Density variation of pure Al and Al–0.5 wt% GNP composite.

Sample	$\mathbf{Theoretical} \mathbf{Density} \mathbf{(}\mathbf{g}\mathbf{/}\mathbf{c}{\mathbf{m}}^{\mathbf{3}}\mathbf{)}$	$\mathbf{Experimental} \mathbf{Density} \mathbf{(}\mathbf{g}\mathbf{/}\mathbf{c}{\mathbf{m}}^{\mathbf{3}}\mathbf{)}$
Pure Al	2.72$\pm 0.02$	2.7$1\pm 0.02$
Al–0.5 wt% GNPs	2.70 ± 0.03	2.71$\pm 0.04$

. The results corresponding with the experimental density calculation indicate that the density of pure aluminum increases with the addition of GNPs, which could stem from the homogeneous dispersion of GNPs in the aluminum matrix [36].

Table 3. Hardness test results of pure Al and Al–0.5 wt% GNP composite.

Sample	Hardness (HV)
Pure Al	34.3$\pm 3.6$
Al–0.5 wt% GNPs	41.0$\pm 4.2$

discloses the average hardness values of pure Al and GNP-reinforced composite. Results are reported for five measurements from various regions of the samples. Pure Al has a value of approximately 34 HV, and the Al–GNP sample has a value of 41 HV.

The increase (+19.7%) in hardness could be attributed to the incorporation of harder particles in the soft aluminum matrix. This nano-reinforcement can have a prominent role in constraining and localizing deformation during the indentation phase [36,37].

3.3. Tensile Strength, Fracture Surfaces and Electrical Conductivity

The mechanical strength and electrical conductivity of pure Al and Al/GNP composite at 20 °C and 180 °C are presented in

Table 4. Electrical conductivity and mechanical properties of pure Al and Al–0.5% wt GNP composite.

T (°C)	Sample	Tensile Strength (MPa)	Elongation at Break (%)	Electrical Conductivity (MS/m)
20	Pure Al	61.02$\pm 3.28$	55.6$\pm 3.8$	37.58$\pm 0.06$
	Al–0.5 wt% Graphene	147.63$\pm 8.32$	51.8 $\pm 2.9$	38.35$\pm 0.06$
180	Pure Al	35.5$\pm 2.25$	45.1$\pm 3.5$	15.57$\pm 0.04$
	Al–0.5 wt% Graphene	56.97$\pm 5.3$	41.4$\pm 3.28$	16.37$\pm 0.04$

and Figure 7. The existence of 0.5 wt% graphene in pure aluminum enhances its mechanical and electrical properties. At 20 °C, the tensile strength and electrical conductivity increased 142% and 2%, respectively, while the elongation decreased 7%. At 180 °C, the tensile strength and the electrical conductivity increased 60.1% and 5%, respectively, though the elongation declined by 8%. As can be seen in Figure 7, Young’s modulus increased substantially after the addition of graphene. In addition, the toughness increased in the aluminum–graphene composite.

Graphene has a large cross-section and is easily clustered due to the strong absorption of Van der Waals force among the plates in the preparation process [9]. These agglomerates act as a source of cracking and cause the composite to be torn at low strength [38,39,40]. Nevertheless, with the uniform distribution of GNPs inside the matrix, it can be deduced that these particles boost the strength of the nanocomposite by locking the cracks, switching their direction inside the field with the extension of the crack and reducing the grain size [41,42]. Hence, both the reduction in grain size and the uniform distribution from the ball milling and the mechanical-electromagnetic stirrer until the composite cooled enhanced the tensile strength at both temperatures.

Despite the superb intrinsic electrical conductivity of graphene [43] and the doping effect in Al matrices [44], which proved to be advantageous factors in improving their electrical conductivities, the extra interfaces between graphene and Al represent an unfavorable characteristic for the enhancement of the electrical conductivity of the nanocomposite. Formation of ${Al}_4{C}_3$ [35] and $A{l}_2{O}_3$ [26] as electrical insulators can reduce the electrical conductivity of the composite. Different interfaces had large variations in electrical resistivity, and the adverse effect of the interfaces on the electrical conductivity of the system was notably weakened when forming clean and intimate contact interfaces [45,46]. It is asserted that the structure in our nanocomposite, including the graphene distribution and the interfacial bonding, account for the superiority in the electrical conductivity and the mechanical strength compared to Al 1350.

An examination of the fracture surface of the samples (Figure 8) showed that the number of dimples in the composite decreases when the surface of the composite is flat compared to pure aluminum.

The elongation of the pure aluminum compared to the graphene aluminum nanocomposite specimens seemed to cause this phenomenon. Nevertheless, the addition of graphene increased the strength and toughness of the Al–0.5 wt% GNP composite at both temperatures [47,48].

The addition of 0.5 wt% graphene to pure aluminum improves the electrical conductivity by 2% at 20 °C. This percentage of improvement is minute, yet the results could be considered acceptable owing to the significant increase in the mechanical strength and no perceivable decrease in the electrical conductivity compared to numerous previous studies [49,50,51,52,53,54,55,56,57]. On the contrary, a 5% increase in the electrical conductivity and a 60% increase in the tensile strength at 180 °C, the operating temperature of high-capacity conductors, is noteworthy.

To clarify the contribution of the present study, the results for the mechanical and electrical properties of pure Al, Al 1350–0.5 wt% GNP composite and Al 1350-O used in conductors [58,59] are set out in

Table 5. Electrical conductivity and mechanical properties of Al 1350-O used in conductors, pure Al and Al 1350-0.5 wt% GNP composite of the present study at 20 °C.

Property	Al 1350-O Used in ACCCs	Pure Al of the Present Study	Al 1350-0.5 wt% GNP Composite of the Present Study
Theoretical density (g/${\mathrm{cm}}^3$)	2.703	2.72	2.71
Hardness (HV)	20	34.28	41
Tensile strength (MPa)	58–99	61.02	147.63
Elongation at break (%)	$\ge 23$	55.6	51.8
Volume resistivity (Ohm.mm2/m)	0.027899	0.02660	0.02607
Electrical conductivity (MS/m)	35.84	37.58	38.35

. At 20 °C, the hardness, elongation and electrical conductivity for pure Al in the present study were 71.4%, $\le 141.7\%$ and 4.63%, respectively. These are higher than the Al 1350-O used in conductors, though the volume resistivity is 4.85% lower. Additionally, the hardness, elongation and electrical conductivity for Al 1350–0.5 wt% GNP composite were 105%, $124.3\%$ and 7%, respectively. These are also higher than Al 1350-O, while the volume resistivity is 7% lower. The values of the theoretical density and tensile strength for pure Al and Al 1350–0.5 wt% GNP composite fall in the range of desired properties.

Conversely, Knych et al. [60], who studied aluminum–graphene and copper–graphene composites, investigated the electrical, mechanical and structural properties of rods obtained after the extrusion process and wires after the drawing process. The results of their research revealed an increase in the strength and the electrical resistance. They stated that one of the most prominent setbacks of the synthesis of Al–GNP composites was the poor wettability of aluminum with graphene. They attributed the decrease in the electrical conductivity of the composite to poor wettability. In the present study, the issue of poor wettability of aluminum with graphene was overcome through ball milling.

Achieving such outstanding properties for aluminum wires accounts for the remarkability of the composite and manufacturing methods in this work. It could be claimed that utilizing mechanical-magnetic stirring casting as the composite cools down helps to remove the impurities in the melt in the form of slag. These impurities are targeted by machinery resulting in a higher purity material that provides more favorable electrical and mechanical properties for aluminum. On the other hand, as mentioned, through the utilization of the stirring technique followed by reducing the grain size, modifying the microstructure, reducing the dendritic structures, using ball milling and increasing the wettability of graphene, as well as the mechanical and electrical properties of graphene, better properties were obtained for the pure aluminum and the composite in the present study.

4. Conclusions

In this work, ball-milled aluminum–graphene powder, as well as the processes of casting with a mechanical-electromagnetic stirrer, hot extrusion and annealing for the fabrication of Al–0.5 wt% GNPs, were employed. It was proven that the addition of graphene reduces the grain size. The uniform distribution of graphene in aluminum was confirmed by FESEM and Raman spectroscopy. The results of the research exhibited that the utilization of graphene in pure aluminum improves both the mechanical and electrical properties. The results revealed that the tensile strength, elongation and electrical conductivity were altered by +142%, $-7\%$ and $+2\%,$ respectively, at 20 °C, while the tensile strength and electrical conductivity rose by 67.3% and 5%, respectively. However, elongation declined by $-8\%$ at 180 °C. When adding 0.5 wt% graphene to pure aluminum, the toughness increased at both temperatures. Economizing the production method can facilitate the use of this composite in high-capacity conductors.