Al6061 alloy matrix composites reinforced with nano-reduced graphene oxide loaded Al2O3 (Al2O3/RGO) were successfully fabricated using high energy ball milling and hot extrusion techniques, with varying mass fractions. The microstructures of these Al2O3/RGO/Al6061 aluminium matrix composites (Al MMCs) were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) attached with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscope (TEM). The tensile properties and Vickers hardness of the alloy were tested. The results show that Al2O3/RGO were uniformly distributed in the Al6061 matrix and tightly bonded to the matrix. Al2O3 encapsulation on RGO surface eliminates Al4C3 brittle phase in matrix, indicating that there was no reaction between the reinforcement and the matrix Al6061. The tensile strength and vickers hardness of Al MMCs increased after Al2O3/RGO were added. Tensile tests revealed that the yield strength and tensile strength of Al MMCs with a 0.1 wt.% reinforcement were 49% and 43% higher, respectively, compared to pure Al6061 prepared under the same process. And the alloys showed the best strength and considerable plastic deformation capability for that of 0.1 wt.% Al2O3/RGO.
Article
Synergistic effect of RGO loaded Al2O3 on microstructure and mechanical properties of 6061 aluminium alloy
https://doi.org/10.21203/rs.3.rs-4097532/v1
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Aluminium (Al) alloys have become essential structural materials in the aerospace and transportation industries due to their low density, favorable mechanical properties, high thermal conductivity, and excellent corrosion resistance [1]. However, traditional aluminium alloys are gradually falling short in meeting the increasing demand for material properties, particularly in terms of strength and modulus of elasticity, in today's society. In recent years, Aluminium matrix composites (Al MMCs) have emerged as a new type of engineering material and have garnered increasing attention. Al MMCs combine the outstanding characteristics of aluminium alloys with the advantages of incorporating reinforcing materials [2–3]. They exhibit high specific strength and modulus, low coefficient of thermal expansion, excellent high-temperature properties, as well as good fatigue resistance and wear resistance. These exceptional properties have generated significant research interest for Al MMCs in a wide range of applications [4]. Al MMCs hold tremendous potential for applications in various engineering fields, including transportation and construction, where exceptional mechanical properties such as tensile strength and hardness are of utmost importance [5]. The reinforcement of Al MMCs primarily involves the use of oxide ceramic particles [6, 7], carbides [8, 9], nitrides [10, 11], and borides [12–14], employing both non-in-situ and in-situ methods. Graphene, renowned for its remarkable attributes such as high strength, excellent thermal conductivity, and an extremely low coefficient of thermal expansion [15–17], is considered an exceptional reinforcement in composite materials. Researchers have developed various methodologies to demonstrate that the incorporation of graphene enhances the mechanical, thermal, and tribological properties of metallic materials [18, 19]. However, in the production of graphene/aluminum composites, graphene is susceptible to the easy breakage of the six-membered ring structure, poor wettability at the aluminum interface, and agglomeration [20]. Additionally, the carbon element in graphene readily reacts with the aluminum element in the aluminum alloy, leading to the formation of the brittle Al4C3 phase and resulting in a decline in the performance of the Al MMCs [21, 22]. Therefore, in this study, Al2O3/RGO nanoparticles (Al2O3/RGO) with a specific layered structure will be synthesized by loading Al2O3 particles onto graphene oxide (RGO) through a hydrothermal process. Subsequently, the Al MMCs were manufactured using a powder metallurgy method by incorporating them into the matrix Al6061 as the reinforcing phase, in order to investigate the impact of the reinforcement on the mechanical properties of the matrix Al6061.
In this study, graphene oxide flakes were synthesized from flake graphite using a modified Hummers method [23]. To achieve a well-dispersed GO solution, 0.54 g of GO was initially dispersed in 30 ml of deionized water, stirred with a magnetic stirrer for 2 hours, and subsequently sonicated for 30 minutes, as illustrated in Fig. 1.
The process involved mixing 5mmol glucose with 70ml of deionized water for 10 minutes, followed by the addition of 10mmol AlCl3•6H2O3 and 10mmol NaAlO2, and stirring for 30 minutes. Subsequently, the well-dispersed GO solution was added and the mixture was stirred for an additional 1 hour. The stirred solution was then transferred to a 100ml stainless steel autoclave lined with Teflon and maintained at 150°C for 24 hours. Following the reaction, the sample was washed with deionized water and ethanol, and then dried at 60°C for 24 hours. The resulting Al2O3/RGO complex was crushed to produce Al2O3/RGO, the morphology of which is depicted in Fig. 2. (a).
As Al6061 is a precipitation-solidified aluminum alloy with magnesium and silicon as its primary alloy components, it possesses excellent mechanical properties. Therefore, in this study, Al6061 alloy powder with an average diameter of 10 mm was utilized as the base material (Fig. 2b), while Al2O3/RGO with an average diameter of ranging from 100 nm to 400 nm was employed as the reinforcement (Fig. 2a). The content of other metallic elements in Al6061 is detailed in Table 1.
|
Al |
Si |
Mg |
Fe |
Cu |
Mn |
Gr |
Zn |
Ti |
O |
|---|---|---|---|---|---|---|---|---|---|
|
97.47 |
0.63 |
1.08 |
0.17 |
0.32 |
0.052 |
0.18 |
0.032 |
0.02 |
0.16 |
The Al6061 powder was introduced to a high-speed mixer and blended with varying ratios (0.1, 0.3, and 0.5 wt.%) of Al2O3/RGO for one hour to achieve a preliminary mixture. Following the blending process, the powder was transferred to a ball mill tank and subjected to planetary ball milling at 200 rpm for 10 hours, alternating between 20 minutes of forward rotation and 20 minutes of reverse rotation. The milling was conducted with a ball to material ratio of 5:1, consisting of 20% of 10 mm balls, 60% of 8 mm balls, and 20% of 5 mm balls.
To create the preforms, the resulting mixture from ball milling was loaded into a 40 mm diameter extrusion barrel die and compressed at 300 MPa for 5 minutes. Once the extrusion die with a 16:1 extrusion ratio was mounted onto the extrusion barrel and securely fastened, the entire assembly, including the preforms, was transferred to a heating furnace. The furnace was preheated to 500°C at a rate of 200°C/h. After holding at 500°C for 50 minutes, the process of hot extrusion commenced, yielding a composite aluminum alloy bar with a diameter of 10 mm.
For comparison, pure Al6061 samples without the incorporation of Al2O3/RGO were prepared using the same methodology. Figure 3 provides a schematic representation of the preparation process.
Density measurements were carried out on Al6061 and Al MMCs specimens using Archimedes' principle, and the density of the composite was calculated using the equation (Eq. [1]).
where m is the mass of the composite sample in air, m1 is the mass of the same composite sample in distilled water and ρw is the density of distilled water. The density of distilled water at 20℃ is 998 kg/m3.
The microstructure of the obtained Al MMCs was observed using scanning electron microscopy (SEM) with the GeminiSEM 500. The phase composition was determined through X-ray diffraction (XRD) analysis using an energy dispersive X-ray spectrometer (EDS) and a Mini Flex 300/600. The specimens were exposed to Cu Kα radiation (0.15418 nm) with a scanning speed of 2°/min, and the 2θ scans were conducted in the range of 20° to 90°. Sheet samples of the alloy cross sections were manually crushed into thin foils less than 100um thick and punched into 3mm diameter plates with a puncher. The plates were electrolytically flattened with a twin-jet electropolishing machine before being inspected using a JEM-2100F transmission electron microscope (TEM) at 200 KV. The electrolyte was a solution of 10% perchloric acid in ethanol.
The hardness of extruded bars was evaluated using a Wilson|VH1102-01-0087 hardness tester in accordance with ISO 6507. The mechanical properties of the Al MMCs were assessed following ISO 6892-1 standards. Tensile specimens with a diameter of Φ10 were machined from the extruded bars and tested at room temperature using an E45.305 electronic universal testing machine (300 kN) at a constant rate of 0.1 mm/min. To ensure the statistical significance of the results, at least three samples were tested for each condition. After the tensile tests, the fracture surfaces and regions near the fractures of the specimens were examined using SEM.
Figure 4 illustrates the changes in density and porosity of Al6061 and Al MMCs. It is evident that with an increase in the Al2O3/RGO ratio, the density of Al MMCs decreases while the porosity increases, aligning with the theoretical calculations. The inclusion of low-density Al2O3/RGO in the Al MMCs contributes to the reduction in density, which can be attributed to factors such as pressure magnitude and entrapment of gases during the cold compression molding process.
Figure 5 displays the transverse SEM microstructure of Al MMCs with varying mass percentages of Al2O3/RGO. The microstructure of the four specimens, produced using the same process but with different mass fractions of Al2O3/RGO, exhibited general similarities, as depicted in Fig. 5 (a)-(d), with no discernible metallurgical defects such as shrinkage, bubbles, or cracks observed. Following the addition of Al2O3/RGO reinforcement, fine black strips emerged within the Al6061 matrix. In Fig. 5b (Al2O3/RGO: 0.1 wt%), the flaky Al2O3/RGO reinforcement is visible on the surface of the Al6061 composite (indicated within a red box), with its size ranging from and the elements in its region (as shown in Fig. 5e) confirmed through EDS analysis.
The XRD diffraction patterns of Al MMCs with varying Al2O3/RGO content after hot extrusion are depicted in Fig. 6. It is evident that the primary peaks correspond to the Al alloy, and the crystal structure remains unchanged following the addition of Al2O3/RGO reinforcement and the hot extrusion process. As per JCPDS card number 85-1327, the peaks at 38.3°, 44.6°, 65.1°, 78.2°, and 82.3° correspond to the crystallographic indices of (111), (200), (220), (311), and (222), respectively. Notably, peaks related to the RGO, Al2O3 and Al4C3 phase are absent in the XRD patterns due to their minimal presence. There was no brittle phase of Al4C3 at the end of the hot extrusion preparation, as has been found in other studies[20, 21]. This is attributed to the RGO, which is coated in Al2O3 particles, exhibiting limited interaction with the Al6061 matrix, thereby hindering the formation of the brittle phase Al4C3. The weave coefficient of the alloy was calculated using the equation (Eq. [2]), where hkl denotes the (111), (200), or (222) orientation [24].
The weave coefficients of the composites at various mass percentages are presented in Fig. 6(b), revealing a reduction in the weave coefficient for the (200) orientation following the addition of the reinforcement. These texture modifications suggest that the grain orientation underwent transformation due to the presence of the Al2O3/RGO phase.
Typical bright-field TEM and EDS images of 0.1wt.%Al2O3/RGO/Al6061 composites are shown in Fig. 7. As can be seen in the high magnification image in Fig. 7(a), the large black stripes of RGO and small particles of Al2O3 are embedded in the grey Al6061 matrix. The Al2O3/RGO fragments and matrix are tightly bound together. The brittle Al4C3 phase was not found in the TEM image, in agreement with the XRD results. Figure 7(b) shows the low magnification TEM images of graphene sheets in the composites. And Fig. 7(c) is the EDS mapping images of 0.1wt.%Al2O3/RGO/Al6061 composites in the red-framed area of Fig. 7(b). The RGO was identified by C element distribution (Fig. 4c) and O element enrichment (Fig. 4d). It can be seen that there is an enrichment of Mg elements near the RGO, while a Si rich phase is found in the Al matrix.
Figure 8 depicts the average microhardness of Al MMCs, along with a corresponding graph showcasing their microhardness. Notably, as the Al2O3/RGO content in the Al 6061 matrix rises, the microhardness values of the Al MMCs experience a substantial boost. Specifically, their hardness escalates from 69.02 Hv to 97.78 Hv, marking a remarkable 41.6 percent increase. This unequivocally demonstrates the advantageous impact of Al2O3/RGO on Al6061.
Tensile tests were conducted at room temperature to examine the mechanical properties of Al MMCs with varying Al2O3/RGO contents. Figure 9(a) showcases the true stress-strain curves of the Al MMCs. As depicted in Figs. 8(b) and 8(c), the addition of Al2O3/RGO led to a slight decrease in plasticity but an increase in strength for the Al MMCs. Notably, the incorporation of different amounts of reinforcement significantly enhanced the yield strength and tensile strength of the composites compared to pure Al6061. The maximum improvement was observed at a content of 0.1%. Specifically, the yield strength increased by 49%, rising from 181 MPa to 270 MPa, while the tensile strength improved by 43%, increasing from 200 MPa to 286 MPa. Moreover, the reinforcement had a considerable impact on the elastic modulus of the composites, initially increasing and then declining. These results unequivocally demonstrate the substantial enhancement of the mechanical characteristics of the composites through the inclusion of the Al2O3/RGO phase,. Similar findings were also reported by SARAVANAN [9].
Figure 10 presents the fracture surfaces of Al6061 with the Al2O3/RGO phase. Dimples and tearing ridges were observed in the alloys, regardless of the presence of the Al2O3/RGO phase. Initially, the size of the dimples decreased with increasing content of the enhancement phase, but then increased, aligning with the variation in tensile elongation of the Al MMCs investigated. In Figs. 10(b)-(d), it can be observed that the Al2O3/RGO phase was pulled out from the matrix, forming strip-like voids. This observation clearly demonstrates the effective strengthening effect of the enhancement phase.
Figure11 presents an analysis of the fracture mechanism of Al MMCs, revealing their behavior under tensile forces. Initially, the matrix undergoes plastic deformation, resulting in the formation of numerous microcracks. As the tensile force increases, these microcracks progressively expand inward alongside the matrix, accompanied by an increase in dislocation movement. However, when these cracks encounter the obstacles presented by Al2O3/RGO particles, their extension is impeded. It becomes evident that strengthening occurs when the movement of dislocations is hindered. Al2O3 particles were deposited onto RGO to increase its surface roughness and facilitate better interaction with the Al matrix. This could lead to the formation of an interlocking effect between the Al2O3/RGO reinforcement and the Al6061 matrix, which further strengthen the material[25]. Furthermore, due to the disparate thermal expansion coefficients of Al6061 and Al2O3/RGO nanoparticles, strain fields are created around the Al2O3/RGO during the cooling process following hot extrusion. These strain fields act as barriers to dislocation movement during stretching. Consequently, a higher load is required to transfer these dislocations around the strain field. The fracture morphology of the stretched specimen in Fig. 10 further supports and elucidates this phenomenon.
In this study, Al6061 alloy matrix composites reinforced with 0.1, 0.3, and 0.5 weight percent nano-reduced graphene oxide loaded Al2O3 were successfully manufactured by high-energy ball milling and hot extrusion processes. As the Al2O3/RGO content increased, the density of the Al-MMCs decreased while the porosity increased slightly. There were no detectable metallurgical defects such as shrinkage, bubbles or cracks. The Al2O3/RGO is uniformly embedded in the Al6061 matrix and strongly bonded to the Al6061 matrix. Because Al2O3 covered the RGO surface and inhibited the reaction between the RGO and the Al alloy matrix, there was no formation of the brittle Al4C3 phase. The alloy's yield strength and tensile strength exhibited a sharp increase followed by a slow decrease with increasing Al2O3/RGO content. The highest values were achieved at 0.1 wt.% reinforcement, which were 49% and 43% higher than those of pure Al6061, respectively. The Al6061 aluminium matrix composites with Al2O3/RGO exhibited comprehensive and excellent mechanical properties due to the synergistic effect of RGO and Al2O3.
Data availability
All data, models generated or used during the study are available from the corresponding author by request.
Author Contributions Statement
Hongding Wang, Haitao Zheng and Zhonglei Ma wrote the main manuscript text, Haitao Zheng prepared figures 1-6 and 8-11, Mingshuai Hu prepared figure 7. All authors reviewed the manuscript.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (52361023).
- Heinz A, Haszler A, Keidel C, et al. Recent development in aluminium alloys for aerospace applications[J]. Materials Science & Engineering A, 2000, 280(1):102–107.
- Suresh S. The Golden Jubilee Issue - Selected topics in Materials Science and Engineering-Past, Present and Future - Preface[J]. Acta Materialia, 2003, 51(19):5647–5647.
- D.V. Lohar, Literature Review of Graphene Composites, International Journal of Innovative Research in Science, Engineering and Technology 6 (1) (2017) 475–478.
- Kamat S V, Hirth J P, Mehrabian R. Mechanical properties of particulate-reinforced aluminum-matrix composites[J]. Acta Metallurgica, 1989, 37(9):2395–2402.
- Amol Mali1, Sourabh Kherde, Indranil Sutar, Sagar Wani, Shubham Dhodare, A Review Paper on Study of Aluminum Matrix Composite, International Research Journal of Engineering and Technology (IRJET) 5 (5) (2018) 2321–2324.
- Kok M. Production and mechanical properties of Al2O3 particle-reinforced 2024 aluminium alloy composites[J]. Journal of Materials Processing Technology, 2005, 161(3):381–387.
- Al-Salihi H A, Mahmood A A, Alalkawi H J. Mechanical and wear behavior of AA7075 aluminum matrix composites reinforced by Al2O3 nanoparticles[J]. Nanocomposites, 2019, 5(3):1–7.
- Arivukkarasan S, Dhanalakshmi V, Stalin B, et al. Mechanical and tribological behaviour of tungsten carbide reinforced aluminum LM4 matrix composites[J]. Particulate Science and Technology, 2018, 36(8): 967–973.
- Basavarajappa S, Chandramohan G, Mukund K, et al. Dry sliding wear behavior of Al 2219/SiCp-Gr hybrid metal matrix composites[J]. Journal of Materials Engineering & Performance, 2006, 15(6):668.
- Mohanavel V. Synthesis and evaluation on mechanical properties of LM4/AlN alloy based composites[J]. Energy Sources Part A Recovery Utilization and Environmental Effects, 2019:1–10.
- Sahin Y, Acılar M. Production and properties of SiCp-reinforced aluminium alloy composites[J]. Composites Part A: Applied Science and Manufacturing, 2003, 34(8): 709–718.
- Kumar, Narendra, Gautam, et al. A Study on Mechanical Properties and Strengthening Mechanisms of AA5052/ZrB2 In Situ Composites[J]. Journal of engineering materials and technology, 2017, 139(1).
- Cheneke S, Karunakar D B. The effect of solution treatment on aging behavior and mechanical properties of AA2024-TiB2 composite synthesized by semi-solid casting[J]. SN Applied Sciences, 2019, 1(11):1–17.
- Guan C, Chen G, Kai X, et al. Strengthening-toughening of graphene nanoplates and in situ ZrB2 nanoparticles reinforced AA6111 matrix composites with discontinuous layered structures[J]. Materials Science and Engineering: A, 2022, 853: 143750.
- Park S, Ruoff RS. Chemical methods for the production of graphenes[J]. Nature Nanotechnology, 2009, 4(4):217.
- Wehling T O, Novoselov KS, Morozov S V, et al. Molecular Doping of Graphene[J]. Nano Letters, 2007, 8(1):173–177.
- Sattar T. Current Review on Synthesis, Composites and Multifunctional Properties of Graphene[J]. Topics in Current Chemistry, 2019, 377(2).
- M Güler, Bac N. A short review on mechanical properties of graphene reinforced metal matrix composites[J]. Journal of Materials Research and Technology, 2020, 9(3):6808–6833.
- Chu K, Wang X H, Li Y B, et al. Thermal properties of graphene/metal composites with aligned graphene[J]. Materials & Design, 2017, 140(FEB.):85–94.
- Wen-ming Tian, Song-mei Li, Bo Wang, Xin Chen, Jian-hua Liu, Mei Yu. Graphene-reinforced aluminum matrix composites prepared by spark plasma sintering[J].International Journal of Minerals Metallurgy and Materials, 2016, 23(06):723–729.
- Zhang H, Xu C, Xiao W, Ameyama K, Ma C. Enhanced mechanical properties of Al5083 alloy with graphene nanoplates prepared by ball milling and hot the extrusion. Mater Sci Eng A 2016;658:8–15.
- Khodabakhshi F, Arab SM, Švec P, Gerlich AP. Fabrication of a new Al-Mg/graphene nanocomposite by multi-pass friction-stir processing: dispersion, microstructure, stability, and strengthening. Mater Charact 2017;132:92–107.
- Besenhard J O. Preparation of Graphite Oxides[M]. John Wiley & Sons, Ltd, 2007.
- Wang H, La P, Shi T, et al. Predeformation and Subsequent Annealing—A Way for Controlling Morphology of Carbides in Large Dimensional Bulk Nanocrystalline Fe-Al-Cr Alloy[J]. Metallurgical and Materials Transactions A, 2014, 45: 522–528.
- Shang C, Zhang F, Xiong Y, et al. Spark plasma forging and network size effect on strength-ductility trade-off in graphene reinforced Ti6Al4V matrix nanocomposites[J]. Materials Science and Engineering: A, 2023, 862: 144480.
No competing interests reported.