Metal matrix composites have gained importance and considered as future engineering materials. These composite materials are used in various fields such as automotive, aerospace, civil and other structural fields. Composite materials possess higher specific strength, lower density, better physical and mechanical properties compared to the base materials. Aluminium Metal Matrix Composites (MMCs) have gained increasing commercial applications over the years. The property of metal matrix composite materials mainly depends on the percentage weight of reinforced material in the base material and the process of developing such composite materials. This work is focused on, Carbon Nanotube (CNT) fibres reinforcement in Aluminium 6061 matrix by a stir casting process to develop composite materials. Experimental test specimens were fabricated for the study of microstructure and mechanical property on Carbon Nanotube-Aluminium 6061 metal matrix composite. The specimens were fabricated with 0%, 2%, 4% and 6% by weight of CNT in the aluminium 6061 base metal. It is noted that on increasing percent weight of Carbon Nanotube in the aluminium metal matrix resists the compressive load up to 278 kN for 360mm2 specimens. Experimental results for wear rate show a decrease in wear loss for increasing CNT and higher hardness of 181 VHN for 6% by weight of CNT.
Keywords: Aluminium 6061, CNT, Compression strength, Hardness, Metal matrix composites.
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1. Introduction Metal matrix composites (MMC) have an important role in load-bearing engineering applications and they possess the property of high strength to weight ratio. This property helps to use such materials in various applications such as space vehicle, automotive, civil and other structural fields, where a high strength to weight ratio is predominantly expected. This leads to savings in energy and other associated costs. In the past to recent years, researchers have focused to develop such materials where their strength to weight ratio plays a major role in future engineering materials. Presently many researchers have focused on the development of carbon-nanotube (CNT) reinforced into aluminium (Al) composites to satisfy the customized requirements for the intended function. Due to the high strength of CNT reinforced in lightweight of aluminium metal matrix composites have advantages and wide applications in aviation, spaceflight, automotive and structural industries [1].
Synthesis of Carbon-nano tubes and their use in the development of composite material gained much importance, because of their uniqueness in properties. CNT has a hundred times more strength and more or less five times lower density compared to conventional materials. This makes them considerably useful material for reinforcing into composite materials [2]. Though the nano order materials on its own do not provide excellent performance in the industry, they exhibit good performance with other conventional materials when they mixed. Carbon nanotubes having young are modulus of 1TP and are in cylindrical graphite structure. CNT possess good chemical stability makes them useful as a reinforced material with aluminium matrix [3]. However, wettability, interfacial bonding and dispersion of CNT in the composite material is the challenging task and this has an impact on properties of the composite materials [4].
Aluminium 6061 is obtained from bauxite ore and further, the material is processed using the Bayer process into aluminium oxide. Aluminium oxide material is then converted into metallic aluminium by the process of smelting. Aluminium 6061 alloy has potential applications and one of the largely used alloys in aluminium 6000 series [5]. The standard structural alloy has in versatile use and they are heat-treatable alloys. Such aluminium alloys have good toughness characteristics and mainly used for medium to high strength requirements in the application field. Few applications of these alloys are in the transportation vehicle, machine, automotive components, equipment parts, recreation products and other consumer durables [6].
The research work reports that CNT reinforced into the aluminium matrix is developed by the process of spark plasma sintering and the hot-extrusion process. This process unchanged the elongation of aluminium (Al) and carbon nanotubes (CNT). The microstructure of these composites is observed using optical, scanning electron and transmission electron microscopes. It is reported that relatively low CNT incorporation and fairly thickness at the boundary layer in the composite material [7]. Carreño-Morelli et al. [8] have investigated that resonant measurement, which is improved by 9% in the CNT-magnesium composites. They have compared young’s modulus of Mg-2 wt.% CNTs with unreinforced sintered Mg and found that improvement of young’s modulus in Mg with CNT composites. Esawi et al. [9] have used the ball milling process for uniform distribution of multi-wall carbon nanotubes (MWCNTs) in the aluminium matrix
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and concluded that the dispersion of CNT is effective while restricting strain hardening of aluminium powder. Experimental results show that an increase in CNT content improves mechanical properties. The experimental results were compared with determined theoretical values based on composite theory except at 5% by weight [9]. Researchers have concluded that agglomeration of the carbon nanotubes is the major challenging task in the improvement of composite materials properties. The ball milling experiment was conducted for 48 hours by Esawi and Morsi [10] with a 10:1 ball to powder ratio. They have not observed the damaged particle in the experiment. They have established and noticed that the mechanical method of ball milling is one of the techniques to overcome the agglomeration of the nanoparticles in the matrix. Scanning Electron Microscope analysis was done and found that CNT clustering is observed in the aluminium matrix when tubular mixing was eliminated.
Balamurugan et al. has worked on CNT, aluminium metal matrix composite and they have conducted the experiments from 2 to 4 percent weight of CNT contents in the matrix and concluded that tensile strength is improved with an increase in percent weight of CNT in the aluminium metal matrix composites [11]. In the literature researchers have focused on aluminium and CNT composite materials, methods of mixing them and fabrication process. However, for good mechanical properties it is necessary for uniform dispersion of the nanoparticle.
Mazahery and Shabani [12] have studied the microstructure of nanocomposites of SiC particles reinforced in A356 aluminium alloy. The samples were fabricated using the stir casting method and observed a reasonably uniform distribution of nanoparticles in the Al matrix. Nair and Joshi [13] studied the microstructure of SiC reinforced in aluminium matrix composites and found that good distribution of SiC particles. They observed that the stir casting process is the simplest and most economical method to produce good quality aluminium metal matrix composites. Therefore, in this work, a study has been focused on the stir casting process for fabrication of samples and its experimental study for compression strength, hardness and wears loss.
2. Objective The objective of the present work is to develop the CNT reinforced aluminium 6061 composite materials by stir casting process for 2%, 4% and 6% weight ratio of CNT. The experiments were conducted to study the microstructure of the composite materials using the Scanning Electron Microscope (SEM) and experimental study of mechanical properties such as compression strength, hardness and wear resistance.
3. Experimental Details Multi-walled carbon nanotubes are procured and are reinforced with 6061 aluminium alloy. The composite materials were fabricated using a stir casting method. The challenging task in this process is to achieve uniform dispersion of CNT in the aluminium 6061 base material. Due to the poor wetting property, the higher surface energy of the CNT nanoparticle and Van der walls forces, the nanoparticle gets agglomerated, which will cause non-uniform dispersion of the nanoparticle [14]. To avoid the agglomeration problem and uniform mixing a
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stirrer is used to mix CNT in the aluminium matrix. The CNT-Aluminium metal matrix composites were fabricated and examined for microstructure.
In the present study aluminium 6061(Al6061) alloy-carbon nanotube composite is prepared for weight proportions of carbon nanotubes such as 0%, 2%, 4%, and 6%. Al6061 alloy is melted in the crucible furnace and multi-wall carbon nanotubes (MWCNT) is added. For improving the dispersion of CNT, a stirrer is used to rotate at a speed of 225 RPM as shown in Fig. 1. The stirring of the molten metal is continued for 7 minutes. Nitrogen gas is used and passed to degas the molten metal. A finger metal mould is used and poured the molten metal in the mould as shown in Fig. 2. The mould is allowed to cool in the room’s environmental condition to obtain the casting. Chalk powder is used to coat the metal mould to avoid the adhering of the molten metal onto the mould surface. Then the samples were prepared and machined for testing according to ASTM E9 and ASTM G 99 standards.
Fig. 1. Furnace and stirrer used for fabrication of Al-CNT.
Fig. 2. Finger metal mould.
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4. Results and Discussions The specimens were fabricated for experimentation and observed direct view under scanning electron beam microscope for microstructure study. The microstructure observed in Fig. 3 is without CNT in the composite material. Figures 4 to 6 show the CNT distribution in the aluminium composite and found that fairly homogeneity in the dispersion of CNT in the Al6061 composite material.
Fig. 3. 0% CNT+ remaining Al6061.
Fig. 4. 2% CNT+ remaining Al6061.
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Fig. 5. 4% CNT+ remaining Al6061.
Fig. 6. 6% CNT+ remaining Al6061.
4.1. Compression test The compression test is carried out for the specimen with an 18mm diameter and 20 mm length on a universal testing machine in Fig. 7. Three Specimens for each % wt. of CNT were fabricated and tested on the Instron 4486 universal testing machine.
The test is carried out according to ASTM E9 standards the test specimens were prepared and axial compression strength was recorded. The ultimate yield point is a loading condition at which, the specimens started developing the crack, which is recorded. Strain values were measured by an extensometer. The results of three specimens for each % wt. of CNT are plotted in Fig. 8 for ultimate stress and in Fig. 9 for strain rate. Observed Fig. 8, that the maximum error of three samples is less than 7.2% for ultimate stress and 1.6% for strain. The average values of three specimens are taken as ultimate strength and are tabulated in Table 1. The resultant graph of the average values for the stress-strain curve was obtained, which is as shown in Fig. 10.
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Fig. 7. Universal testing machine.
Fig. 8. Ultimate compression strength of three specimens
for the various percentage weight of CNT.
Fig. 9. Strain of three specimens for the various percentage weight of CNT.
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Table 1. Average of three specimen ultimate compression strength kN/mm2 with strain.
Fig. 10. Stress-strain graph for compression test.
4.2. Wear test Wear test was conducted on Ducom pin on disc apparatus with the 8 mm a pin diameter and pin length of 25 mm. The disc of diameter 180 mm with a thickness of 12 mm is used. The wear loss in the micron of three test specimens for load 5, 10 and 15 N is plotted with various % wt. of CNT and shown in Figs. 11 to 13. The average values of the wear loss of three specimens are tabulated in Table 2 and plotted the graph as shown in Fig. 14.
Fig. 11. Wear loss of three specimens under
5 N load for the various percentage weight of CNT.
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Fig. 12. Wear loss of three specimens under
10 N load for the various percentage weight of CNT.
Fig. 13. Wear loss of three specimens under
15 N load for the various percentage weight of CNT.
Fig. 14. Wear loss with various percentage weight of CNT.
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Table 2. Three specimen average wear loss in microns. Percentage
of CNT Load in Newton
5 N 10 N 15 N 0% CNT 140.56 180.33 192.23 2% CNT 138.72 168.95 162.44 4% CNT 135.25 161.22 155.66 6% CNT 125.63 151.46 138.49
4.3. Vickers hardness test Vickers hardness tests were conducted for the specimens. 6% by weight of CNT in the Al6061 is observed that VHN was 181 for 10 kg load. The hardness of the composite material increases with an increased weight percentage of the CNT. Figure 15 shows the VHN for 0%, 2%, 4% and 6% weight of CNT in MMC.
Fig. 15. Hardness for the various percentage weight of CNT.
5. Conclusion The specimens of multi-walled carbon nanotubes reinforced in Al6061 metal matrix composites were prepared using a stir casting method. Microstructure of the CNT reinforced in Al6061composite were studied. These SEM images show a fair amount of dispersion of carbon nanotubes in the composite material. The experimentation for compression strength is conducted and found that ultimate compression strength is increased to 0.63 kN/mm2 (14.28%), 0.7 kN/mm2 (22.85%) and 0.77 kN/mm2 (29.87%) with CNT contents of 2% wt., 4% wt. and 6% wt. respectively as compared to 0 % wt. of CNT in the composite. The results show that ultimate compression strength increases to 60% for 4% wt. of CNT with respect to 2% wt. of CNT. It is observed, 30.68% increase in ultimate compression strength for 6% CNT with respect to 4% wt. of CNT. Compression test results indicate that the ultimate compression strength for the specimen with 6% by weight of carbon nanotubes has a maximum value of 0.77 kN/mm2. Wear loss decreases with an increase in CNT content for the same load condition, but wear rate
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increases at higher loads for the same percentage weight of CNT reinforcement in Al6061 composite material. The non-uniform dispersion of nanoparticles causes more wear rate in the case of 15N load as compared to 10 N load for CNT, which contents of 2%, 4% and 6% weight of CNT. The experiment exhibits that the wear behaviour of the composites Al6061 with a 6% weight of multi-walled CNT has more resistance to wear and the lowest wear for the same load conditions. The wear loss tends to decrease with increase CNT content in the matrix, which shows that the addition of multi-walled CNT helps decrease the wear loss. The hardness test is conducted on the Vickers hardness apparatus and noted that the Hardness of the CNT-Al 6061 composite material increases with an increase in the percent weight of CNT in the matrix. It is observed, the microstructure of the fabricated specimen, a better process further would improve the dispersion of CNT in the composite materials. This would further improve the rate of wear loss and compression strength particularly at a higher percentage of CNT.
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