1. INTRODUCTION:

Carbon nanotubes (CNTs) have experience several discriminating applications and evolved tremendous amazement by virtue of its extraordinary mechanical and thermal properties [1–3]. Carbon nanotubes exhibit almost five times elastic modulus (~1 TPa) and closely 100 times tensile strength (~150 GPa) than those of high strength steels [4,5] because of their length and diameter. Because of the unimaginable high strength of CNTs makes them most important reinforcement for the composite materials. Besides, the carbon nanotubes also exhibit superior dispersion strengthening to the composite structures. Numerous researches on CNT based composites has been focused on polymer–matrix composites (PMCs) in order to improve the electrical conductivity beside with the enhancement in mechanical strength fabricated by different techniques [6-8]. However, the research on CNT reinforced ceramic matrix composites and metal matrix and has been rather limited.

The fabrication of CNTs reinforced ceramic matrix composites (CMCs) have been synthesized by hot pressing and high temperature extrusion and in situ production, sonication and dry pressing to achieve homogenized dispersion of carbon nanotubes [9,10]. Previously, in the field of CNT reinforced metal matrix composites (MMCs) a very limited research has been done because of lack of a suitable synthesis technique, and the complexity associated with the interfacial reaction between CNTs and metal matrices. But now a day, Aluminum matrix composites (AMCs) have become more research focus field and gained more attention due to their superior properties as compared to pure aluminum, such as greater strength, wear resistance and better formability. Now a day, in the aviation and automotive industries, AMC are used due to increase in demand for lightweight and high strength materials. Research across the globe is being carried out to enhance the properties of AMCs using novel reinforcement and fabrication methods.

This paper is organized as follows. The processing technique is discussed in Section 2. In section 3, literature review of the metal matrix composites is carried out and fabrication technique used for fabrication discussed in Section 4. Results and discussion are discussed in Section 5 and conclusion is done in section 6.

2. PROCESSING TECHNIQUE:

Carbon Nanotubes reinforced metal matrix composites can be prepared by variety of processing techniques such as stir casting technique, powder metallurgy, friction stir processing etc. But mainly they can divide into two category named as liquid state processing and solid state processing.

2.1 Liquid state processing technique (Stir casting):

AMCs can be fabricated by means of two techniques i.e. the liquid-phase processing and the solid-phase processing (Powder metallurgy). The liquid state processing offers advantages of improved interface bonding, economical and resultant net shape. In this technique, the reinforcement is incorporated into the molten matrix and the composites are fabricated by means of a casting process. Stir casting technique is the most conventional form of fabrication of MMCs [11]. It is a liquid state method of MMCs fabrications in which dispersed phase of reinforcement is mixed with a molten metal matrix by means of continuous stirring either takes place under vacuum or in an inert gas environment. In sir casting process the main disadvantage is that in this reinforcement are often tend to make agglomerates, which can be avoided by only intense mechanical stirring. One should be careful while dispersing the reinforcement into the molten metal matrix to coordinate the reactivity of the component used with the temperature of molten metal and duration of mechanical stirring. Stir casting process is easy to operate and equipment requirement is not so high and also this method has high production efficiency. In stir casting method speed of stirring and particle size play an important role.

2.2 Solid state processing technique (Powder metallurgy):

Powder metallurgy fabrication route have the advantage that, unlike liquid metal processing, almost any material composition can be processed [12]. In this method the material wastage is less because of minimal need for machining. In powder metallurgy two phases can be easily mixed and dispersed in the solid state. This technique involves placing powder in a rotating chamber along with balls. Due to the impact of balls powder gets deformed and fractures. This technique is widely used for fabrications of MMCs. In powder metallurgy due to longer pressure and ball mill processes CNTs may be damage, in composites there may be more pores, cracks etc. In this process manufacturing cost is high and preparation of complex shapes is not easy task.

3. LIERATURE REVIEW:

From past few decades the AMCs have been widely explored for their excellent combination of properties. They exhibit many superior qualities, such as low weight, low cost etc., due to which they are preferred over many other materials already being used. Moreover, the characteristics profile for these materials can be easily modified by changing the processing techniques, changing refinements and controllable parameters. The first step during the manufacturing of composites is to obtain uniform distribution of refinement in the composites. And it is also essential to prevent the agglomeration in the composites. In the evaluation of distribution of reinforcement in the composite microstructure analysis is quite important. The results of various microstructure analyses of CNTs are presented below.

Deng et al. [13] developed composite of 2024Al – CNT by isostatic pressing. He analyzes using SEM micrograph and found that the density of the composites is very homogeneous, and the morphology of 2024Al powders remains unchanged. However, the peak of Al₄C3 phase does not found signifying that no reaction of carbon nanotubes and Al take place. A. Esawi and K. Morsi [14] used mechanical alloying (MA) to generate a homogenous distribution of CNT within Al powders and using The FESEM the morphology and size of the mechanically alloyed composite powders was used to characterize during different stages of milling. It can be seen that after 48h milling the CNTs appear embedded in the aluminum matrix which do not appear damaged when milling intensity to be 200 rpm and ball to powder ratio 10:1. Xu et al. [15] by using optical microscopy revealed that CNTs are located mainly at aluminium grain boundries. Dense microstructure and low porocity observed after hot pressing process. Esawi et al.[16] using both XRD and TEM analysis showed the nanostructure of the matrix was retained in the final products which contributed to the enhanced strength displayed by all samples compared to un-milled aluminum. CNTs have been found to act as nucleation sites for void formation during tensile testing. In addition, both CNT pullout and CNT inner tube slippage were observed in fractured surfaces H.J. Choi and D.H. Bae [17] reported the change in dimension of SWNTs during fabrication process. When specimen tested for 10% compression the length of SWNTs reduced to be ~2nm and diameter enlarged to be ~10nm. Choi et al. [18] study the morphology of 3 Vol% MWCNTs and reveals that the interface in the composite is very clean, no voids or carbides are seen. YufengWuand Gap-Yong Kim [19] investigated the effect of processing temperature and revealed that composite fabricated at 620⁰C and 640⁰C achieved relative densities higher than 99%. Relative density of the order of 97.4% is can be obtained when temperature is 600⁰C because small amount of liquid is available during solidification. Hosseini et al. [20] using friction sir processing technique and observed microstructrural modification, better distribution of reinforcement and grain size reduction of intermettalic particles in thenuggest zone. The average grain size of the particles reduces to 6.31μm from 21μm after treatment and further reduces to 3.98 μm in nuggest zone when CNT was incorporated. Xiao-ning HAO et al. [21] observed that particle morphology size is influenced by the ball milling time, rotation speed etc.

Fig1:- EDS analysis of the Al2024/MWCNT

FABRICATION TECHNIQUE:

The effectiveness of multi-walled tubes for using it as reinforcement in composite is also a critical issue. The temperature of aluminium when reinforcement has to added to metal matrix plays an important role because the nanotubes react with aluminium when temperature is 6530C. In this attempt, a novel processing approach has been undertaken to synthesize a MWCNT–Al2024 nanostructured composite by stir casting technique. Hereof, it must be mentioned that the exercise of stir casting to synthesize MWCNT reinforced Al2024-based composite has never been reported in the literature. For fabrication of Al2024 and CNT (multiwalled) the Carbon nanotubes of 95 % purity was used as reinforcement for synthesizing the composite structure. The bulk density of the CNTs was 0.04 – 0.06 g/cm3 with a dimension of 10 – 30 nm diameter, and 10 micron length. Microstructural evaluation of the composite has also been carried out in order to characterize the composite and to validate the presence of carbon nanotubes in stir casting technique.

4. RESULTS AND DISCUSSION:

After successful fabrication of the Al2024 multiwalled carbon nanotubes composite, microstructural evaluation of the composite has also been carried out in order to characterize the composite and to validate the presence of carbon nanotubes in Al2024-MWCNT composite fabricated using stir casting technique.

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Fig2: SEM micrograph of stir cast Al2024/MWCNT composite

Energy Dispersive Spectroscopy (EDS) analysis was performed on Al2024 – MWCNT composite to carry out the elemental analysis of the composite. The SEM image exhibits grayish matrix of Al2024 - MWCNT. EDS performed on one of the specimen, report presence of aluminium, carbon and oxygen as 53.98, 22.03 and 5.2 wt.%, respectively.

To visualize the presence of MWCNTs in the Al2024/MWCNT composite SEM images were taken using SU8000 instrument. Different SEM images were taken by keeping visualization level to 500nm, 2μm and 30μm. From the images it is clear that MWCNT is present in the composite.

5. CONCLUSION:

Multiwalled carbon nanotubes is a favorable reinforce material because of its special physical and chemical properties and mechanics properties. Though the study of MWCNTs reinforced metal materials has received some progress, it is still far from the practical application. The main objective of this study was to fabricate the Al2024 - MWCNT composite with stir casting process, which we achieved. While the fabrication of Al2024 – MWCNT composite, the temperature of the base metal play an important role. When the temperature of base metal is above 650⁰C, the reaction of the MWCNT with Al2024 takes place.

6. REFERENCES:

1. A.G. Mamalis, L.O.G. Vogtländer, A. Markopoulos, Precision Eng. 28 (2004) 16–30.

2. M. Baxendale, J. Mater. Sci.: Mater. Electron. 14 (2003) 657– 659.

3. E.T. Thostenson, Z. Ren, T.W. Chou, Compos. Sci. Technol. 61 (2001) 1899–1912.

4. X.F. Zhang, X.B. Zhang, G.V. Tendeloo, S. Amelinckx, M. Beeck, J.V. Landuyt, J. Cryst. Growth 130 (1993) 368–382.

5. R.B. Pipes, P. Hubert, Compos. Sci. Technol. 62 (2002) 419–428.

6. A.P. Davey, J. Coleman, A. Dalton, S. Maier, A. Drury, D. Gray, M. Brennan, K. Ryder, M. Lamy De La Chapelle, C. Journet, Synth. Met. 103 (1999) 2559–2562.

7. L. Valentini, J. Biagiotti, J.M. Kenny, S. Santucci, Compos. Sci. Technol. 63 (2003) 1149–1153.

8. W.K. Maser, A.M. Benito, M.A. Callejas, T. Seeger, M.T. Mart´ınez, J. Schreiber, J. Muszynski, O. Chauvet, Z. Osváth, A.A. Koós, L.P. Biró, Mater. Sci. Eng.: C 23 (2003) 87–91.

9. Z. Balázsi, F. Kónya, L. Wéber, L. Biró, P. Arató, Mater. Sci. Eng.: C 23 (2003) 1133–1137.

10. S. Rul, F. Lefèvre-schlick, E. Capria, Ch. Laurent, A. Peigney, Acta Mater. 52 (2004) 1061–1067.

11. J. Hashim, L. Looney, M.S.J. Hashmi.Metal matrix composites: production by the stir casting method. Journal of Materials Processing Technology 92-93 (1999) 1-7.

12. Jinzhi Liao and Ming-Jen Tan. Mixing of carbon nanotubes (CNTs) and aluminum powder for powdermetallurgy use.PowderTechnology 208(2011) 42-48.

13. C.F. Deng, D.Z. Wang, X.X. Zhang, A.B. Li. Processing and properties of carbon nanotubes reinforcedaluminum composites. Material science and engineering A 444 (2007) 138-145

14. A. Esawi and K. Morsi.Dispersion of carbon nanotubes (CNTs) in aluminum powder. Composites: Part A 38 (2007) 646-650.

15. C.L. Xu, B.Q. Wei, R.Z. Ma, J. Liang, X.K. Ma, D.H. Wu.Fabrication of aluminum–carbon nanotube composites and their electrical properties. Carbon 37 (1999) 855-858.

16. A.M.K. Esawia, K. Morsib, A. Sayeda, A. Abdel Gawada, P. Borahb. Fabrication and properties of dispersed carbon nanotube–aluminum composites. Materials Science and Engineering A 508 (2009) 167-173.

17. H.J. Choiand D.H. Bae. Strengthening and toughening of aluminum by single-walled carbon nanotubes.Materials Science and Engineering A 528 (2011) 2412-2417.

18. H.J. Choi, B.H. Min, J.H. Shin, D.H. Bae. Strengthening in nanostructured 2024 aluminum alloy and its composites containing carbon nanotubes. Composites: Part A 42 (2011) 1438-1444.

19. Yufeng Wu and Gap-Yong Kim.Carbon nanotube reinforced aluminum composite fabricated by semi-solid powder processing. Journal of Materials Processing and Technology 211 (2011) 1341-1347.

20. S.A. Hosseini, Khalil Ranjbar, R. Dehmolaei, A.R. Amirani. Fabrication of Al5083 surface composites reinforced by CNTs and cerium oxide nano particles via friction stir processing. Journal of alloys and compounds 622 (2015) 725-733.

21. Xiao-ning Hao, Hai-ping Zhang, Rui-xiao Zheng, Yi-tan Zhang, Kei Ameyama, Chao-li MA. Effect of mechanical alloying time and rotation speed on evolution of CNTs/Al-2024 composite powders. Trans. Nonferrous Met. Soc. China 24(2014) 2380−2386.

Received on 12.04.2017 Accepted on 16.05.2017

Research J. Engineering and Tech. 2017; 8(3): 191-194.

DOI: 10.5958/2321-581X.2017.00031.9