I. INTRODUCTION II. MATERIALS AND METHODS

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1 International Journal of Advances in Engineering, 2015, 1(3), ISSN: (printed version); ISSN: (online version); url: RESEARCH ARTICLE Microstructural Analysis of Aluminium Hybrid Metal Matrix Composites Developed Using Stir Casting Process M.D.Antony Arul Prakash and M. Arockia Jaswin Department of Mechanical Engineering, Aksheyaa College of Engineering, India. Received 19 February 2015 / Accepted 10 March 2015 Abstract: Manufacturing of aluminum alloy based casting composite by stir casting is one of the most economical methods of processing MMC. In this project silicon carbide and boron carbide particulate reinforced aluminium alloy matrix composites are produced by stir casting process by varying the particulate addition by weight fraction of Al (90%), SiC (5%, 6.5%, 8%) and B 4 C (5%, 3.5%, 2%). Micro structural changes of the cast composites are studied by using metallurgical microscope and scanning electron microscope (SEM). Dispersion of the materials is equal in all the three composites by employing stir casting process and is studied from the characterizations study. Keywords: Aluminium; SiC; B 4 C; Composite; SEM. I. INTRODUCTION The process of metal matrix composite materials is to combine the desirable attributes of metals and ceramics. The addition of high strength, high modulus refractory particles to a ductile metal matrix produces a material whose mechanical properties are intermediate between the matrix alloy and the ceramic reinforcement [1-3]. Metals have a useful combination of properties such as high strength, ductility and high temperature resistance, but sometimes have low stiffness, whereas ceramics are stiff and strong, though brittle. Among the variety of manufacturing processes available for discontinuous metal matrix composites, stir casting is generally accepted as a particularly promising route, currently practiced commercially. Aluminium silicon carbide alloy composite materials are widely used for a many number of applications like engineering structures, aerospace and marine application, automotive bumpers, sporting goods and so on [4-5]. As Aluminium alloy materials are constructed by casting in specified sequence of orientation. Hence, the failure of a single aluminium alloy specimen does not give the total failure of casting. However, it leads to progressive failure of the casting. Several performance characteristics are expected from these materials. They are the materials to be used for sophisticated applications like aircraft and space applications should have higher performance, efficiency and reliability. Materials have to be of light-weight for many applications so that the resulting products can be efficient and cost effective [6]. Composites with proper composition and manufacturing can withstand corrosive and high temperature environments. With all these advantages it is obvious to think why the composites have not replaced the metals. One major drawback linked with the composites is its high cost which is often due to the use of expensive raw materials and not due to the manufacturing processes [7,8]. The experiments with the addition of SiC and fly ash into Al2024 alloy. Finally they concluded that the increase in area fraction of reinforcement in matrix result in improved tensile strength, yield strength and hardness and also the percentage rate of elongation of the hybrid MMCs is decreased significantly [9]. The stir casting experiment by the combination of Aluminum Al6061 with Silicon Carbide (grid size 60) has been studied by varying the mass fractions of 5%,10%,15% and 20% [10]. The stir casting experiments by the combination of Aluminum and varying the weight fraction of SiC (SiCp 320 grit size) (5%, 10%, 15%, 20%, 25%, and 30%). Final samples were tested by hardness using Brinell hardness machine, impact strength test using Charpy-V-Notched machine and Microstructure examination conducted on Trinocular type Metallurgical Microscope [11]. II. MATERIALS AND METHODS Al6061 is a precipitation hardening aluminium alloy, containing magnesium and silicon as its major alloying elements. It has good mechanical properties and exhibits good weld ability. It is one of the most common alloys of aluminium for general purpose use. Silicon carbide and Boron carbide are used as additives. Table 1 shows the % of reinforcing materials in composites. Base material Aluminium 6061 were bought in rods and are weighed as per the composition and are cut into small piece for the convenience of placing them inside the crucible no 6 and are heated to 750 o C in an induction furnace for three to four hours. Reinforcing materials Silicon Carbide and Boron Carbide are also weighed separately as per the weight ratio and are preheated to 800 o C at a separate furnace for two to three to improve the wettability of the material.

2 334 Int. J. Adv. Eng., 2015, 1(3), Table.1 % of reinforcing materials Composite Aluminium (Al-6061) Silicon Carbide (SiC) Boron Carbide (B4C) A 90% 5% 5% B C 90% 90% 6.5% 8% 3.5% 2% Development of Hybrid composites: First of all stirring system has been developed by coupling motor with gearbox and a mild steel stirrer. All the melting was carried out in a graphite crucible in an oil-fired furnace shown in figure 1. Aluminium rods were preheated at 850 oc for 3 to 4 hours before melting and mixing the SiC and B4C particles were preheated at 840 oc for 1 to 3 hours to make their surfaces oxidized. The furnace temperature was first raised above the liquids to melt the aluminium alloy completely and was then cooled down just below the liquid us to keep the slurry in a semi-solid state. At this stage the preheated SiC and B4C particles were added and mixed manually. Manual mixing was used because it was very difficult to mix using automatic device when the alloy was in a semi-solid state. After sufficient manual mixing was done, the composite slurry was reheated to a fully liquid state and then automatic mechanical mixing was carried out for about 10 minutes at a normal stirring rate of 600 rpm. In the final mixing process, the furnace temperature was controlled within 760 ± 10 oc. Figure.1 Experimental setup used for stir casting Characterization using Scanning Electron Microscope: The scanning electron microscope (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing signals that contain information about the sample's surface topography, composition and other properties such as electrical conductivity. The types of signals produced by an SEM include secondary electrons, back-scattered electrons (BSE), characteristic X-rays, light, specimen current and transmitted electrons. Secondary electron detectors are common in all SEMs, but it is rare that a single machine would have detectors for all possible signals. The signals result from interactions of the electron beam with atoms at or near the surface of the sample. In the most common or standard detection mode, secondary electron imaging or SEI, the SEM can produce very high-resolution images of a sample surface, revealing details about less than 1 to 5 nm in size. Due to the very narrow electron beam, SEM micrographs have a large depth of field yielding a characteristic three-dimensional appearance useful for understanding the surface structure of a sample. This is exemplified by the micrograph of pollen shown to the right. A wide range of magnifications is possible, from about 10 times (about equivalent to that of a powerful hand-lens) to more than 500,000 times, about 250 times the magnification limit of the best light microscopes. Optical microscopes analysis made to identify the dispersion reinforcing particles, and to check the formation of agglomeration. III. RESULTS AND DISCUSSIONS SEM Analysis: For the micro structural studies, specimens were cut from cast bars and mounted in Bakelite, ground with grit paper, using copious amounts of water as lubricant. The mounted samples were then mechanically polished

3 335 Int. J. Adv. Eng., 2015, 1(3), using a 1μm alumina-powder suspended in distilled water. Fine polishing to near mirror like finish was achieved using 0.5μm diamond paste and etched with Keller s reagent. Reinforcement morphology and its distribution in the metal matrix along with other intrinsic micro structural features were identified by examining the samples in a JEOL JSM 3.5 CF Japan make Scanning Electron Microscope (SEM). Fig.2 to Fig.10 shows SEM image of fractured surface of the composites and it reveals that the additives are ingredient in composites. Figure.2 SEM micrograph of composition A at 100x Figure.3 SEMmicrograph of composition A at 500x Figure.4 SEM micrograph of composition A at 1500x

4 336 Int. J. Adv. Eng., 2015, 1(3), It can be seen from the figures that there is an increase the particles cluster corresponding to an increase in the processing temperatures. Which were stirred for a con-stant speed with longer period at higher temperature, the particles were agglomerated in the melt. Although there is an increase in the particle clustering with increase processing temperature, it was observed that the ten-dency for formation of particle cluster was greater in the higher holding time than in the low holding time. During the higher holding time with temperature, the geometry of the capturing of the particles does not restrict their movement inside the liquid metal as well as solidification. Also the presence of a low viscosity of liquid metal tents to physically not restricts growth of porosity. Thus, the tendency for particle cluster or porosity is high in the higher temperature with prolonged contact between ma-trix and reinforcement. Figure.5 SEMmicrograph of composition B at 100x Figure.6 SEM micrograph of composition B at 500x Figure.7 SEMmicrograph of composition B at 1500x

5 337 Int. J. Adv. Eng., 2015, 1(3), Figure.8 SEMmicrograph of composition C at 100x Figure.9 SEMmicrograph of composition C at 500x Figure.10 SEMmicrograph of composition C at 1500x Optical Microscopic Analysis: Metallographic samples were sectioned from the cast bars and were prepared using a technique specially developed for such composites. Figure 11 to16 shows the optical microstructure at three different magnifications. A 0.5% HF solution was used to etch the samples wherever required. Microstructures were examined under the metallurgical microscope. From micro structural analysis, it is observed that Aluminium Silicon Carbide composite having cluster particles and some places are identified without SiC inclusions. This was due to varying the contact time between the SiC particles and molten aluminium during processing and high surface tension and poor wetting behaviour between Aluminium and SiC particles. To overcome the surface tension problem and improve wetting properties, a mechanical force can be applied uniformly during distribution of reinforcement in the metal matrix composites

6 338 Int. J. Adv. Eng., 2015, 1(3), Figure.11 Optical micrograph of composition A at 50x Figure.12 Optical micrograph of composition A at 100x Figure.13 Optical micrograph of composition B at 50x Figure.14 Optical micrograph of composition B at 100x

7 339 Int. J. Adv. Eng., 2015, 1(3), Figure.15 Optical micrograph of composition C at 50x Figure.16 Optical micrograph of composition C at 100x CONCLUSION Characterization study was made by SEM and optical microscopic analysis the result predict that the dispersion of the SiC and B 4 C particles are equal in all over the specimens and also shown in the images. It can be seen from the figures that there is an increase the particles cluster corresponding to an increase in the processing temperatures. REFERENCES [1] C.Neelima Devi, N.Selvaraj, V.Mahesh Micro structural aspects of Aluminium Silicon Carbide Metal Matrix Composite, Int. Journal of Applied Sciences and Engineering Research, Vol. 1, No. 2, [2] Eliasson and R. Sandstorm, Applications of Aluminium Matrix Composites, Part 1, G. M. Nawaz, H. Neber-Aeschbacherand F. H. Wohlbier eds., Trans. Tech.publications, Switzerland, 1995, pp [3] Gopal Krishna U B, Sreenivas Rao K V & Vasudeva B Effect of Boron Carbide Reinforcement on Aluminium Matrix Composites, International Journal of Metallurgical & Materials Science and Engineering (IJMMSE) ISSN Vol. 3, Issue 1, Mar 2013, pages: [4] Gopal Krishna U B, Sreenivas Rao K V and Vasudeva B Effect of percentage reinforcement of B4C on the tensile property of Aluminium Matrix Composites,ISSN Vol. 1, No. 3, October [5] Gowri Shankar M.C, Jayashree P.K, Ravi raj Shetty, AchuthaKini and Sharma S.S Individual and Combined Effect of Reinforcements on Stir Cast Aluminium Metal Matrix Composites, International Journal of Current Engineering and Technology ISSN [6] Khalid Mahmood Ghauri, Liaqat Ali, Akhlaq Ahmad, Rafiq Ahmad, KashifMeraj Din, Ijaz Ahmad Chaudhary, Ramzan Abdul Karim Synthesis and Characterization of Al/SiC Composite Made by Stir Casting Method, Pak. J. Engg. & Appl. Sci. Vol. 12, Jan., 2013 (p ). [7] M. Marimuthu, L. John Berchmans Preparation and Characterization of B4C Particulate Reinforced Al-Mg Alloy Matrix Composites, International Journal of Modern Engineering Research (IJMER) Vol. 3, Issue. 6, Nov - Dec pp [8] MahendraBoopathi, M, K.P. Arulshri and N. Iyandurai Evaluation of Mechanical Properties of Aluminium Alloy 2024 Reinforced with Silicon Carbide and Fly ash Hybrid Metal Matrix, American Journal of Applied Sciences, 10 (3): , [9] Naher, Brabazon, Looney Simulation of the stir casting process, Journal of Materials Processing Technology (2003) M. M. Dave, K. D. Kothari Composite Material-Aluminium Silicon Alloy, Indian Journal of Research, Volume : 2, Issue : 3,March [10] ShubhamMathur, AlokBarnawal Effect of Process Parameter of Stir Casting on Metal Matrix Composites, International Journal of Science and Research (IJSR) ISSN (Online): , Volume 2 Issue 12, December [11] VibuNanthan.M, Vidhusan.C, and Vignesh.S Machinability Studies of Turning Al/SiC/B4C Hybrid Metal Matrix Composites using ANOVA Analysis, International Conference on Thermal, Material and Mechanical Engineering (ICTMME'2012) July 15-16, 2012 Singapore.