Corrosion behavior of Aluminium-Boron carbide-graphite composites C. Muthazhagan 1, a, A. Gnanavelbabu 2, b*, K. Rajkumar 3, c and G.B.

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1 Applied Mechanics and Materials Submitted: ISSN: , Vol. 591, pp Accepted: doi: / Online: Trans Tech Publications, Switzerland Corrosion behavior of Aluminium-Boron carbide-graphite composites C. Muthazhagan 1, a, A. Gnanavelbabu 2, b*, K. Rajkumar 3, c and G.B. Bhaskar 4, d 1 Assistant Professor, Department of Mechanical Engineering, S.K.P Engineering College Tiruvannamalai , Tamil Nadu, India * 2 Professor & Head, Department of Mechanical Engineering, Agni College of Technology OMR, Thalambur, Chennai , Tamil Nadu, India 3 Associate Professor, Department of Mechanical Engineering, SSN College of Engineering Chennai , Tamil Nadu, India 4 Professor & Head, Department of Mechanical Engineering, Tagore Engineering College Rathinamangalam, Chennai , Tamil Nadu, India a jeevamudhan@yahoo.co.in, * b dr.agbabu@gmail.com, c rajkumark@ssn.edu.com, d bhaskarang01@yahoo.com Key words: Corrosion, Erosion-Corrosion, Graphite, Boron Carbide. Abstract. The corrosion behaviour of Al (6061)-B4C-Graphite was investigated. The Aluminium Metal Matrix Composites (AMMC) was fabricated through two step stir casting method. The composites were fabricated with various volume percentages of Boron Carbide (5, 10 &15%) and Graphite (5, 10& 15%). Corrosion studies of AMMC was investigated with 4%, 8%, 12% wt. % NaCl solution at room temperature. Also erosion-corrosion test were performed on the specimens in the NaCl solution with silica sand. Erosion-corrosion tests indicated that the rate of material loss mechanism is mechanical abrasion with enhanced corrosion. The material loss mechanism was significantly higher in the case of erosion-corrosion tests. Introduction Metal matrix composites are combination of metallic properties such as high ductility with ceramic properties such as high strength [1]. AMMC are being considered as good candidates for replacing conventional alloys in many industries such as aerospace, automotive, and sport due to their potential engineered properties. Particulate reinforced metal matrix composites are promising materials for applications due to their favourable mechanical properties such as improved strength, stiffness and increased wear resistance compared to unreinforced alloy [2]. Aluminium metal matrix reinforced with Boron Carbide (B 4 C) is a novel composite, which is used in automotive industries especially in brake pads and brake rotor, due to high wear resistance [3], high strength to weight ratio, elevated temperature toughness and high stiffness. B 4 C is also used in the nuclear industry [4] as radioactivity containment vessels and control rods fixture, since B 4 C is a neutron absorber [5]. Self-lubricant reinforcement like graphite improves antifriction properties due to its lamellar structure [6]. Cast aluminium-graphite composites displayed excellent corrosion resistance in SAE-40 engine oil. Good corrosion resistance displayed while studying AA6061 Composites containing 7 wt. % graphite particles in SAE 30 lubricating oil [7]. This work concentrated on corrosion and erosion-corrosion behaviour of AMMC for the understanding of corrosion mechanism. Experimental Details Preparation of composites: Boron Carbide particles of 25 µm size were used in this study. Two step stir casting method was used to fabricate the Al-B 4 C-Graphite. Aluminium alloy was melted up to C. Varying volume % (5, 10 & 15%) of reinforcement (B 4 C and graphite) was added with this molten Al-6061 alloy. It was allowed to cool to solidus temperature and stirred at 120 rpm using steel impeller. In this way 100% of the B 4 C particles were transferred to the metal. Simultaneously, graphite was added with Al-B 4 C, and then stirred composite material is transferred to metal mould. Composites were cast into required length and cut to size 15x15x5mm. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,08:19:17)

2 52 Advanced Manufacturing Research and Intelligent Applications Heat Treatment (T6) was carried out for the all the specimens. The heat treated composites were polished according to standard metallographic procedures, etched with Keller s reagent and observed in Optical Microscope. Immersion test: The corrosion tests were carried out in 4, and 12 wt.%nacl solutions which were prepared following standard procedures. The specimens for the test were cut to size 15x15x5mm, after which the sample surfaces were mechanically polished with emery papers. The samples were de-greased with acetone and then rinsed in distilled water before immersion in the prepared still solutions of 4,&12 wt.% NaCl, which were all exposed to atmospheric air. The results of the corrosion tests were evaluated by weight loss and corrosion rate measurements on 24 hours. Weight loss for each sample was evaluated by dividing the weight loss by its total surface area which is in accordance with ASTM standard recommended practice ASTM G31. Corrosion rate for each sample were thus evaluated from the weight loss measurements. Erosion-corrosion test: E-C test is conducted using the rotating chamber with in a saline abrasion media. The rotation of chamber is maintained at 150 rpm. The tests were conducted for MMC composite in 12 wt. % NaCl solution media with 30 wt. % of silica sand particles. Erosioncorrosion test results were reported as weight loss method. Results & Discussion Microstructure: The microstructure of Al-B4C-Graphite is shown in Fig.1and Fig.2. A typical micrograph of Al6061-B4C-graphite composites shows a reasonably even distribution of graphite particles and boron carbide. It is to be noted that the B4C & graphite particles were simply entrapped by the primary aluminium 6061 during the solidification of the composite melt. It is also observed that porous sites were minimal. Fig. 1: Al-B4C (5%)-Graphite (5%) [8] Fig. 2: Al-B4C (5%)-Graphite (15%) [8] Fig.3 shows corrosion rate of Al MMC (5% graphite) with 4%NaCl. Corrosion rate shows a steady increase over the days and increase in boron carbide percentage increase the corrosion rate due to poor wettability between the reinforcement and matrix. Corrosion behaviour of Al MMC is influenced by incorporation of graphite particles. Corrosion at interfaces has been attributed due to the presence of aluminium carbides formed during fabrication. Presence of graphite particles might be reacted with Al matrix to form the aluminium carbides. These aluminium carbides are formed at grain boundary which increases the intergranular corrosion. The accelerated corrosion at these sites has been attributed to imperfect bonding and fissures in the composite and emphasizes the need for eliminating fabrication flaws to reduce corrosion of MMCs in chloride environments. Further corrosion also occurs due to the oxidation of free carbon in the B4C. Presence of boron carbide increases the corrosion rate of A6061 alloy in chloride environments.

3 Applied Mechanics and Materials Vol Fig. 3: Corrosion rate of Al in 5, 10 & 15% of B 4 C with constant 5% of Graphite in 4%NaCl Fig. 4: Corrosion rate of Al in 5, 10 & 15% of B 4 C with constant 10% of Graphite in 4%NaCl The corrosion rate of Al MMC (10% graphite) with 4%NaCl is shown in Fig.4. It is observed that the corrosion rate is higher order when compared to 5% graphite. The reason believed to be more amount formation of the aluminum carbide. This leads to increased corrosion rate. The observed corrosion trend is similar to the 5% graphite composite. Fig.5 shows corrosion rate of Al MMC (5% graphite) with 12%NaCl. Corrosion rate shows a steady increase over the days with increasing NaCl wt%. Under normal conditions of temperature and pressure, graphite is relatively stable in water. Graphite is also an electrical conductor and efficient cathode for oxygen reduction in aerated solutions, promoting galvanic corrosion in graphite reinforced MMCs. Galvanic corrosion has been identified as a primary corrosion mechanism for graphite reinforced aluminium matrices in aerated solutions. Intergranular corrosion occurs, which is a localized attack along the grain boundaries or on immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity. Therefore further increase in corrosion rates are attributed due to these reasons. Fig. 5: Corrosion rate of Al in 5, 10 & 15% of B 4 C with constant 5% of Graphite in 12% NaCl Fig. 6: Corrosion rate of Al in 5, 10 & 15% of B 4 C with constant 10% of Graphite in 12%NaCl The corrosion rate of Al MMC (10%graphite) with 12%NaCl is shown in Fig.6. The observed corrosion trend is similar to the 5% graphite composite. However order of corrosion rate is increased with increasing graphite volume fraction. The reason is clearly elucidated in previous section.

4 54 Advanced Manufacturing Research and Intelligent Applications Fig. 7: Erosion-corrosion rate of Al in 5, 10 &15% of B 4 C with constant 5% of Graphite The erosion-corrosion rate of Al MMC (5%graphite) is shown in figure 7. It is observed that weight loss increased with increasing of testing hours from 0.01 gm at 2 hrs to 0.015gm at 12hrs. Erosion-corrosion behavior of composite at higher volume fraction of B4C (15%) is decreased to some extent due to increased hardness may elevate the erosion attack. The result shows that the weight-loss in erosion-corrosion is greater than that for the corrosion type (NaCl solution). This increase in weight-loss may be attributed to the increase in severity of the erosive-corrosive attack over the specimens. It is believed that material loss mechanism might be driven by mechanical erosion of the material of the formed oxide layer on the surfaces with increased corrosion of the material. Conclusion Hybrid metal matrix composites were successfully fabricated using stir casting method. The corrosion rate of the composite is increased with increasing of boron carbide and graphite particles in Aluminium matrix. The mechanism of corrosion is that Intergranular corrosion which is a localized attack along the grain boundaries or on immediately adjacent to grain boundaries. The weight-loss in erosion-corrosion is greater than that for the corrosion (NaCl solution). Erosioncorrosion mechanism for the AMMC is that mechanical erosion of formed oxide layer on the surfaces with increased corrosion of the material. The addition of graphite leads to more mechanical erosion attack due to lower hardness of composite which is result of soft nature of graphite. References [1] I.A.Ibrahim, F.A.Mohammed, E.J.Lavernia, Particulate reinforce metal matrix composites: review, Journal of material science, 26 (1991), pp [2] J. Hashim, L Looney, M.S.J.Hashmi, Metal matrix composites: production by the stir casting method, Journal of material processing science, 92 (1999), pp.1-7. [3] A. Canakci& F. Arslan, Abrasive wear behavior of B4 C particle reinforced Al2024 MMCs, International Journal of Advance Manufacturing Technologies, 63 (2012), pp [4] K. Reinmuth, A. Lipp, H. Knoch, K.A. Schwetz, Boron carbide as neutron absorbent, Journal of Nuclear Materials, 124 (1984), pp [5] H. Werheit, Boron rich solids-a chance for high-efficiency high temperature thermoelectric energy conversion, Materials Science and Engineering B, 29(1995), pp [6] M. Kestursatya, JK Kim, PK Rohatgi., Wear performance of copper graphite composite and a leaded copper alloy, Materials Science and Engineering A, 339 (2003), pp [7] M. Saxena., O.P Modi,., B.K Prasad,., and A.K Jha, Wear, 169 (1993), pp [8] C. Muthazhagan, A. Gnanavelbabu, G.B. Bhaskar and K. Rajkumar, Influence of Graphite Reinforcement on Mechanical Properties of Aluminum-Boron Carbide Composites, Advanced Materials Research, 845(2014), pp

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