Effect of Graphene Content on Tensile Strength and Microstructure of Aluminum Matrix Composites

In this study, graphene nanoplatelets (GNPs) reinforced aluminum (Al) composites with various GNPs content (0.1, 0.3, 0.5wt.%) were fabricated by powder metallurgy (PM) method. In this method, ultrasonication, mixing, filtering, drying, compacting and sintering processes were performed. The crystal structure and microstructure of powders and fabricated composites were analyzed with X-Ray diffractometer (XRD) and scanning electron microscopy (SEM). With this study, the effects of GNPs content were investigated on the density, Vickers hardness, ultimate tensile strength, and microstructure of Al-GNPs composites. The Vickers hardness (57±2.5 HV) and ultimate tensile strength (120MPa) of Al-0.1%GNPs improved up to +90% and +23.3% when compared with pure aluminum (30±2 HV, 92MPa). From the microstructural analysis, homogeneously distributed GNPs was located at aluminum grain boundaries. The mechanical properties of Al-GNPs composite decreased due to the agglomeration of GNPs above 0.1wt.% GNPs content.


Introduction
Aluminum-based composites are finding increased use in military, automotive, and aerospace industries due to their advanced properties [1]. In general, these composites can be fabricated through various methods such as powder metallurgy (PM), melting and squeeze casting [2]. PM is a remarkable method due to its ability of more homogeneous dispersion. The samples have been fabricated by this method owing to having its minimal finishing requirement, and economic/technical advantages [3][4][5].
Graphene has been used as reinforcement element in metal matrices such as aluminum [27][28][29], magnesium [30], and titanium [31] to enhance the properties of metal matrix nanocomposites. However, there are a limited number of studies related to mechanical or tribological properties of Al-GNPs composites. Bastwros et al. [32] examined the bending strength of Al-GNPs composites fabricated by PM method. The strength of Al-1%wt.GNPs increased ~47% when compared with Al6061 alloy. Rashad et al. [33] researched the effect of GNPs content on the compressive, tensile strength, Vickers hardness of aluminum composites. Comparing with pure aluminum, the tensile strength, hardness, and compressive strength of Al-0.3wt.%GNPs composite increased nearly +11.1%, +11.8%, -7.8%, respectively. Li et al. [34] investigated the tensile properties of bulk nanostructured aluminum/graphene composites. The results showed that the bulk nanostructured aluminum/graphene composite exhibited increased strength over pure aluminum with an addition of only 0.5wt.% graphene nanoflakes. Above 1.0wt.% of graphene nanoflakes, however, this strengthening effect sharply dropped due to the clustering of graphene nanoflakes.
Most of the given studies concentrated on just one GNPs content and mechanical properties. Also, the sizes of their raw materials are greater than the size of raw materials in this paper. In this study, Al-GNPs composites with various GNPs content (0.1, 0.3, 0.5wt.%) were fabricated by PM method. The effects of GNPs content were deeply investigated on the apparent density, Vickers hardness, ultimate tensile strength, and microstructure of Al-GNPs composites.

Materials
In this study, the pure aluminum powder was used as matrix material which was purchased from Alfa Aesar (United Kingdom). Also, GNPs powders were used as reinforcement element which was supplied from Grafen Chemical Industries (Turkey). The general properties of purchased Al and GNPs powders were given in Table 1. SEM images of pure aluminum and GNPs powders were presented in Figure 1. As seen from the figure, aluminum powders contain nearly spherical, rod-like morphology, and the particle size of ~10µm. On the other hand, GNPs have few-layered morphology and the layer thickness of ~10nm.

Method
The experimental procedure and sintering conditions were studied in our previous study [28]. In summaries, Al-GNPs composites were fabricated by PM method as given in Figure 2. In this method, aluminum powders were blended in an ethanol medium using a mixer. Dispersion of GNPs (0.1, 0.3, 0.5wt.%) was carried out for 1 hour using ultrasonic homogenizer. GNPs slurry added drop by drop into the aluminum-ethanol mixture. Mixing performed until homogeneous slurry obtained. After the mixture filtered and dried overnight at 50°C under vacuum. The composite powder was pressed in a mold under 600MPa to form green cylindrical samples. After compacting, the samples were sintered under vacuum in the tube furnace at the constant sintering time and temperature (t S = 180 min and T S =630°C).
The experimental density of Al-GNPs composites was measured according to the Archimedes principle, by using a balance with the accuracy of 0.1 mg. A micro-Vickers hardness tester (HV1000B micro Vickers hardness tester) under a load of 200g was used to measure the hardness of the samples. The average values of at least six measurements at polished cross-sections for various areas of each sample were considered. Tensile tests were performed by the universal test machine (Mares Test-10 tons) with the tensile rate of 5 mm/min.
The morphology and microstructure of powders and sintered composites were analyzed by scanning electron microscopy (SEM, Jeol JSM-7001F). X-ray diffraction analysis (XRD, Rigaku Smartlab) were used to investigate the phase analysis of powders and composites.

Characterizations of Composites
XRD patterns of pure Al, GNPs powders and Al-GNPs composites were shown in Fig. 3. As shown in the figure, GNPs peaks (2θ=26.5°) are not detected in any composite structure because of the low detection limit of XRD and the low content of GNPs. Also, in-situ formed secondary phase (Al 4 C 3 ) is not detected in all Al-GNPs composite.

Density and Hardness Test Results
The theoretical and experimental density variations of pure aluminum and graphene reinforced aluminum composites were given in Table 2. The maximum density value was obtained at Al-0.1%GNPs composite due to the homogeneous dispersion of GNPs in aluminum matrix. After this graphene content (0.1wt%), both theoretical and experimental density were decreased due to the agglomeration tendency of GNPs. In addition, the low theoretical density of graphene (ρ=2.25 g/cm 3 ) affected the decrease in the density of Al-GNPs composites after 0.1%GNPs content. The best sintering conditions were determined as given the previous paper (T S =630°C, t S =180 min) [28]. By using these sintering conditions, the variation of Vickers hardness for pure aluminum and Al-GNPs composites was presented in Figure 5. The highest hardness value (57±2.5 HV) was detected at Al-0.1%GNPs composite. After 0.1wt.%GNPs content, the poor interface between aluminum and graphene formed due to the large agglomerated graphene clusters which can lead to the higher porosity and the lower Vickers hardness. Also, GNPs act as solid-lubricant which promotes the easy sliding between the particles during plastic deformation.

Tensile Test Results
The variation of ultimate tensile strength for pure Al and Al-GNPs composites was shown in Fig. 6. Similarly, the ultimate tensile strength (+23.3%) for Al-0.1%GNPs composite increased when compared with pure aluminum. In Al-GNPs composites, GNPs act as a barrier at aluminum grain boundaries in order to preserve the grain growth. The dislocations cannot move easily due to the restricted dislocation movement which increases in strength of aluminum composites. After 0.1wt.% graphene content, the ultimate tensile strength of aluminum composites reduced due to the agglomeration tendency of GNPs. This agglomeration was confirmed by the SEM images of Al-0.3%GNPs and Al-0.5%GNPs composite.

Conclusions
In this study, pure aluminum and Al-GNPs composites with various graphene content (0.1, 0.3, 0.5wt.%) were fabricated by PM method. The effects of GNPs content on the density, mechanical properties (Vickers hardness, tensile strength), and microstructure of Al-GNPs composites were investigated. The results obtained from the study performed were summarized as follows:  The highest apparent and experimental densities were determined as 2.50±0.02 g/cm 3 and 2.52±0.02 g/cm 3 for Al-0.1%GNPs, respectively. Similarly, the maximum Vickers hardness value was obtained as 57±2.5 HV at the same composition. Density and hardness of Al-GNPs composites began to decrease above 0.1%wt.GNPs content due to agglomeration tendency and easy-sliding of GNPs.  The highest ultimate tensile strength was obtained as 120MPa in Al-0.1%GNPs. This increment is due to the superior mechanical properties of GNPs.  By considering all mechanical test results, The Vickers hardness (57±2.5 HV) and ultimate tensile strength (120MPa) of Al-0.1%GNPs improved up to +90% and +23.3% when compared with pure aluminum (30±2 HV, 92MPa).  XRD analysis showed that graphene peaks were not seen in these composites owing to the low content of GNPs and the low detection limit of XRD analyzer.  From the SEM analyses, bonding between the particles and well neck formation were observed at pure Al and Al-GNPs composites. GNPs were detected along the aluminum grain boundaries in Al-GNPs composites.  As a result, graphene addition to aluminum matrix enhanced the mechanical properties of Al-GNPs composites up to 0.1wt.%GNPs. After this content, the mechanical properties of Al-GNPs composites decreased due to the agglomeration of GNPs.