Microstructural characterization and investigation of...

Indian Journal of Engineering & Materials Sciences Vol. 23, August 2016, pp. 207-222

Microstructural characterization and investigation of thermal conductivity behaviour of Al 6061-SiC-Gr hybrid metal matrix composites

S A Mohan Krishnaa *, T N Shridhar b & L Krishnamurthyb

aDepartment of Mechanical Engineering, Vidyavardhaka College of Engineering, Mysore 570 002, India

bDepartment of Mechanical Engineering, National Institute of Engineering, Mysore 570 008, India

Received 1 December 2015; accepted 6 July 2016

Metal matrix composites have been regarded to be one of the most principal classifications in composite materials. Microstructural analysis and metallography is one of the predominant terms commonly used in the research of materials. The thermal characterization of composite materials has received broad deliberation and has become governing in materials science and engineering. In this paper, the microstructural investigation of Al 6061, silicon carbide (SiC) and graphite (Gr) hybrid metal matrix composites with varying percentage reinforcements 2.5%, 5%, 7.5% and 10% have been carried out. The analysis of these hybrid compositions have been accomplished by using optical microscope, scanning electron microscope and energy dispersive spectroscopy that benefits to accomplish thermal characterization and analysis of composite material. Aluminium based composites reinforced with silicon carbide and graphite particles have been prepared by stir casting technique. It has been observed that, dispersoid concentration of the reinforcements has been homogeneous and negligible porosity has been noticed. The formations of grain boundary, interdendritic segregation, porosity and particle size have been characterized by using scanning electron microscope (SEM) and optical microscope. Elemental characterization has been achieved by using energy dispersive spectroscope (EDS).The investigation of thermal conductivity behaviour of hybrid composites with varying percentage reinforcements at lower proportions have been emphasized.

Keywords: Metal matrix composites, Microstructural analysis, Materials science, Silicon carbide, Graphite, Stir casting

Metal matrix composites are in general distinguished by the characteristics of the reinforcement namely particle reinforced, reinforcement of whisker type and metal matrix composites of layered type. Metal matrix composites (MMCs) furnish an extensive assortment of materials to simple reinforcements of castings with inexpensive refractory wool, to intricate continuous fibers lay-ups in exotic alloys. The exclusive properties of MMCs have been usually managed and maintained effectively by the prime elements, viz., matrix, the reinforcement and the interface. The distinctiveness of these composite materials can be determined by their microstructure and interfaces depicted internally, which are influenced by the production techniques and suitable thermal and mechanical properties. The microstructure describes the arrangement of the matrix and the reinforced stage. Some of the imperative attributes, viz., chemical composition, grain, consistency, behaviour and defects in the lattice are exceptionally important for the matrix. The subsequent phase can be characterized based on the percentage of volume,

variety, size, distribution and point of reference. The varying tension developed internally due to the behaviour of thermal expansion of the two stages is an additional convincing factor1-3.

Presently, research is being carried out by using two or more reinforcements to fabricate aluminium matrix composites. Such composites have been referred to as hybrid metal matrix composites. Aluminium matrix composites mixed with graphite and silicon carbide reinforcements are prominent among them. These hybrid composite materials are broadly used in structural, aerospace and automotive industries. Hybrid MMCs have greater relevance to automotive components, viz., piston rods, piston pins, braking systems, frames, valve spring caps, disk brake caliper, brake disks, disk pads and shaft. Aluminium matrix composites are more promising materials and data pertaining to thermal properties have to be established. This aspect has generated enormous interest in research pertaining to the area of composite materials. The characterization and analysis of thermal properties, viz., thermal conductivity, thermal diffusivity, thermal shock resistance, thermal expansivity and specific heat capacity are

—————— *Corresponding author (E-mail: [email protected])

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advantageous for aerospace and automotive applications, electronic packaging and thermal management equipment. Hence, emphasis needs to be given to carry out research on thermal characterization and analysis. This research work intends to enhance the comprehension of the thermal aspects of aluminium matrix composites.

Though the research work pertaining to mechanical, tribological and fatigue behaviour of composites is effectively accomplished, due emphasis needs to be given to the work related to the measurement of prominent thermal properties, viz., thermal conductivity and thermal diffusivity. The main property considered in thermal analysis of metal matrix composites is thermal conductivity. The increase in thermal conductivity of composites will depend on strength and porosity, which finds this property in aerospace and automobile applications extensively4. Thermal diffusivity is an important property for materials being used to determine the optimal work temperature in design applications referred under transient heat flow. It is the thermophysical property that determines the speed of heat propagation by conduction during changes in temperature with time. The heat propagation is faster for materials with high thermal diffusivity5-7. The assessment of thermal properties will benefit to evaluate heat capacity, variation in the intensity of heat, heat diffusion and heat release rate7. For aerospace and automotive applications, low coefficient of thermal expansion, moderate thermal conductivity, specific heat capacity and high electrical conductivity of the composites will enhance the efficiency in all perspectives. The technique that has been adopted in the experimental investigation of thermal conductivity and thermal diffusivity is laser flash apparatus. Some of the most important papers concerning the thermal properties of aluminium-silicon carbide-graphite hybrid composites have been presented.

Davis et al.8 in their research paper have explained the thermal conductivity of metal matrix composites, which are potential electronic packaging materials and have been calculated by using effective medium theory and finite element techniques. The thermal boundary resistance, which occurs at the interface between the metal and the included phase (typically ceramic particles), has a large effect for small particle sizes. It has been found that, silicon carbide particles in aluminium must have radii in excess of 10 μm to

obtain the full benefit of the ceramic phase on the thermal conductivity.

Cem Okumus et al.9 in their paper have studied on thermal expansion and thermal conductivity behaviour of Al/Si/SiC/graphite hybrid composites. Aluminium-silicon based hybrid composites reinforced with silicon carbide and graphite particles has been prepared by liquid phase particle mixing and squeeze casting. The thermal expansion and thermal conductivity behaviour of hybrid composites with various graphite contents and different silicon carbide particle sizes (45 µm and 53 µm) have been investigated. Results have indicated that, increasing the graphite content improved the dimensional stability, and there has been obvious variation between the thermal expansion behaviour of the 45 µm and the 53 µm silicon carbide reinforced composites.

Molina et al.10 have investigated the behaviour of thermal conductivity of aluminium-silicon carbide (SiC) composites based on high volume fraction of silicon carbide particles. It has been investigated by comparing data for composites fabricated by infiltrating liquid aluminium into preforms made either from a single particle size, or by mixing and packing SiC particles of two largely different average sizes 170 μm and 16 μm. For composites based on powders with a monomodal size distribution, the thermal conductivity increases steadily from 151 W/mK for particles of average diameter 8 µm to 216 W/mK for 170 µm particles. For the bimodal particle mixtures the thermal conductivity increases with increasing volume fraction of coarse particles and reaches a roughly constant value of 220 W/mK for mixtures with 40 or more vol% of coarse particles. It has been shown that all present data can be accounted for by the differential effective medium (DEM) scheme taking into account a finite interfacial thermal resistance.

Parker et al.11 have explained the flash method of determining thermal diffusivity, heat capacity and thermal conductivity. A high‐intensity short‐duration light pulse is absorbed in the front surface of a thermally insulated specimen a few millimeters thick coated with camphor black, and the resulting temperature history of the rear surface has been measured by a thermocouple and recorded with an oscilloscope and camera. The thermal diffusivity has been determined by the shape of the temperature versus time curve at the rear surface, the heat capacity

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by the maximum temperature indicated by the thermocouple, and the thermal conductivity by the product of the heat capacity, thermal diffusivity and the density.

Chen et al.12 have reviewed on metal matrix composites with high thermal conductivity for thermal management applications. The latest advances in manufacturing process, thermal properties and brazing technology of SiC/metal, carbon/metal and diamond/metal composites have been presented. Key factors controlling the thermophysical properties have been discussed.

Hohenauer et al.13 have experimented on flash methods to examine thermal diffusivity and thermal conductivity of metal foams. The results of thermal conductivity have been obtained by using a laser flash device. In particular, a magnesium alloy has been investigated. To meet the requirements of flash technique, coplanar samples have been prepared. A finite element model has been generated to study the influence of the preparation method and measurement techniques.

It has been evident from the literature review that, thermal characterization and analysis of aluminium matrix composites (AMCs) have to be given greater emphasis. However, investigations concerning the thermal characterization and analysis of composite materials of AMCs have been meagre. Many experimental investigations have been carried out pertaining to thermal characterization of aluminium silicon carbide composites, but limited work has been accomplished concerning aluminium-silicon carbide-graphite hybrid MMCs. The literature review has indicated the need for further investigations on thermal characterization and analysis of aluminium matrix composites. If these materials are to be used for many engineering applications, the thermal aspects of AMCs need to be given more emphasis. Hence, it becomes important that the evaluation of thermal aspects and characteristics of hybrid composites cannot be ignored in order to transform the material from design stage to manufacturing stage.

In the present scenario, research has been accomplished on mechanical and tribological properties of hybrid composites substantially, but limited research has been carried out on aluminium-silicon carbide-graphite hybrid composites relating thermal characterization and analysis. It has been reported in the literature that, the experimental investigation on thermal characterization and analysis

of aluminium and silicon carbide has been carried out concerning low and high weight proportions9. But, research on thermal characterization and analysis of Al 6061 with silicon carbide (SiC) and graphite (Gr) hybrid metal matrix composites pertaining to low and high weight fraction have been deficient. The pertinent thermal properties of aluminium based hybrid MMC reinforced with silicon carbide and graphite have to be investigated in terms of varying percentage reinforcements and smaller particle sizes. Casting techniques, viz., stir casting, centrifugal casting and squeeze casting have been extensively used.

One of the major advantages of aluminium-silicon carbide-graphite hybrid metal matrix composites is that, these composites are self-lubricating materials comprising graphite, yet their strength can be improved by the presence of silicon carbide ceramic phase. These AMCs possess better property of friction and excellent wear resistance due to the combined effect of the strength of silicon carbide influenced on the matrix and lubrication property of graphite9,12. Silicon carbide exhibits mechanical and thermal properties and has been used extensively as a prominent reinforcement. Graphite represents an optimal combination of strength, ductility and thermal properties for pertinent engineering applications. Graphite has superior characteristics, viz., tensile strength, improved castability, damping capacity and machinability. From literature review, graphite has been used scarcely. Hence in this research work, graphite has been used as reinforcement concurrently with silicon carbide and aluminium matrix alloy considering low weight fraction of hybrid composites. Fabrication of composites

Aluminium-silicon carbide-graphite hybrid metal matrix composites specimens have been stir cast by using aluminium alloy Al 6061 as the matrix material and reinforcements silicon carbide and graphite particulates containing different percentage compositions (2.5%, 5%, 7.5% and 10%). Aluminium alloy (Al 6061) has been used as the matrix material to which the particulates of silicon carbide of average particle size of around 15 to 25 microns and particulates of graphite of average particle size of around 60 to 70 microns have been used as reinforcements. The purpose of considering smaller particle size of silicon carbide is that, it exhibits better thermal characteristics with acceptable interfacial thermal resistance. The purpose of considering larger


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particle size for graphite may not affect the thermal characteristics, but it will improve the tribological and machinability characteristics of aluminium matrix composites. Hybrid metal matrix composites specimens have been cast by mixing equal proportions of silicon carbide and graphite reinforcements maintaining the total percentage of reinforcements same (2.5%, 5%, 7.5% and 10%). A specimen of matrix alloy Al 6061 has been cast without the inclusion of any reinforcements. The powders of silicon carbide and graphite have been added by direct mixing and top pouring stir casting system has been adopted. In this research paper, the evaluation of thermal properties, viz., thermal conductivity, thermal diffusivity and specific heat capacity have been accomplished. Different specimens have been considered as per American Society for Testing and Materials (ASTM) standards. The specimen size for the determination of thermal conductivity is diameter 12.7 mm and thickness 3 mm and for the determination of CTE, length is 10 mm and diameter is 5 mm. The specimen size for determination of specific heat capacity is powder form or pellets, approximately 20 mg14. The hybrid composite specimens have been fabricated commercially. Also, from room temperature to 800°C, the atmosphere for fabrication process has been maintained.

The composite specimens having aluminium matrix reinforced with silicon carbide and graphite reinforcements have been stir cast. A known amount of Al 6061 alloy pieces in the sintering furnace has been heated and allowed the same to melt at 780°C. Complete melting of aluminium has been ensured while preparing the specimen. The alloy pieces have been kept in the crucible and preheated the mould at the required temperature range 750°C-800°C. The reinforcements silicon carbide and graphite have been preheated at the above mentioned temperature range. Magnesium (about 3 to 5 g) has been added to the molten alloy of aluminium to improve wettability. Slag has been removed by using scum powder to avoid poor quality casting. In order to remove moisture content in the casting the melt has been maintained at the above mentioned temperature for about 20 min. Approximately 5% mass of solid dry hexachloro-ethane tablets or degassing tablets have been used to degas the molten metal at 780°C. Stirring of the molten metal maintained at around 750°C, has been accomplished using a mechanical stirrer, to a create vortex. The molten metal has been

stirred at a 400 rpm for about 10 min. The stirring of the mixture has been carried out to ensure uniform dispersoid concentration of reinforcements in the matrix material. After the process of solidification, mould is cooled to avoid shrinkage of casting metal for about 3 h to complete the process1-3. Then the casting has been separated from the mould which is subsequently cleaned. The required test specimens were 22 mm diameter and 220 mm length. They have been machined thoroughly. The dimensions chosen agree well with the available literature. The samples are fabricated to the required sizes. Five specimens have been separately considered for the determination of thermal conductivity with different specimen sizes. Metallographic Specimen Preparation

Metallography is an investigative study of the microstructure of the materials. Metallographic analysis is considered to be a very precious tool. The analysis of the microstructure of materials benefits in determining the exact procedure about material processing and being regarded as a critical step for the evaluation of product reliability. Generally, the fundamental steps involved in the metallographic specimen preparation are documentation, sectioning and cutting, mounting, grinding, rough and final polishing, buffing and etching. To investigate the prominent microstructural features in hybrid metal matrix composites, it is extremely essential to accomplish polishing and etching4,15. One of the vital challenges in functional and engineering materials is the study of morphology controlled process based on the growth of crystal. Microstructural analysis of composite materials have been advantageous for the examination of cohesive interfacial bonding, porosity, particle size, grain size and boundaries, interdendritic segregation and dispersoid concentration of the reinforcements. During microstructural examination, the structure of the material is studied under high magnification. The properties of a material determine the level of performance for a specific application and the properties are independent on the structure of the material.

The samples with varying percentage reinforcements 2.5%, 5%, 7.5% and 10% have been prepared for the study of microstructure by using standard metallographic procedure. The samples have been grinded and polished by using an emery paper. The process of polishing has been accomplished by using powder of alumina to achieve surface finish. Keller’s reagent has been applied to the samples and


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has been observed under an optical microscope to characterize the formation of grain boundary and interdendritic segregation. The microstructural analysis has been carried out for Al 6061 with varying percentage reinforcements of silicon carbide and graphite. Microstructural analysis of composites is advantageous for mechanical and thermal characterization of composite materials. The examination of dispersoid concentration of the reinforcements, cohesive interfacial bonding, formation of grain boundaries and interdendritic segregation in hybrid composites will influence the determination of mechanical and thermal properties of composites, viz., tensile strength, modulus of elasticity, thermal conductivity, specific heat capacity and thermal expansivity. Hence, microstructural characterization has been carried out to accomplish thermal characterization and analysis with varying percentage reinforcements of hybrid composites.

Microstructural characterization with varying weight fraction of hybrid metal matrix composites has been accomplished by using scanning electron microscope, optical microscope and energy dispersive X-ray spectroscope. Optical microscope has been used to investigate the formation of grain boundaries and interdendritic segregation. The microstructures with magnification 200X and 500X have been used to characterize the behavior of hybrid composites.

Scanning electron microscope (SEM) has been used to examine the porosity, particle size and dispersoid concentration of the reinforcements. The microstructures with magnification 500X have been used to investigate the behavior of hybrid composites. Scanning electron microscope attached with energy dispersive X-ray spectroscope (EDAX) has been used for the chemical characterization of hybrid composites. EDAX characterizes the elemental composition of hybrid composites comprehensively. Analysis using Optical Microscope

Nikon Microscope LV 150 with Clemex Image Analyzer is used. It is basically an upright Optical Metallurgical Microscope and high performance stereo zoom microscope. This is ideally suited to the basic needs of manufacturing and research and development in the field of metallurgy for the analysis of microstructure, grain size, inclusion rating, nodular count, particle size, coating layer thickness, depth of carburization etc. This high-resolution microscope has magnification ranging 50X to 1000X with computer interface and is equipped with sophisticated and high resolution Clemax Charged Coupled Device (CCD) camera with user friendly Clemax software4.

Figures 1-5 depict the microstructures of hybrid metal matrix composites reinforced with the particles of silicon carbide and graphite. The samples with

Fig. 1—Microstructures of Sample 1



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varying percentage reinforcements have been prepared for the study of microstructure by using standard metallographic procedure.

Figure 1 indicates the microstructure of Al 6061 without any reinforcements. It has been revealed that the fine precipitates of alloying elements have been dispersed in the matrix of aluminium. The microstructures of hybrid composites for a consistent reinforcement of particles of graphite with silicon

carbide have been presented by using optical microscope. The uniform distribution of the reinforcements is an indispensable condition for a composite material to accomplish its performance at extreme superiority5,6,15,16. Figures 2-5 depict the microstructures of aluminium matrix alloy with varying weight fraction of the reinforcements signifying that the fine precipitates of alloying elements have been dispersed along the grain





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boundary in the matrix of aluminium. The distribution of the particles of silicon carbide and graphite has been homogeneous with no parallel striations and detrimental pores. The segregation in the internally formed dendrites has been observed because the particles of silicon carbide and graphite have been driven depending of the process of solidification. The presence of these particles in the matrix substantially improves the microstructure, encumbering the coarsening of the initial phase of dendrites during the process of solidification.

The particles of silicon carbide have been shifted towards the primary phase of the dendrite boundaries of aluminium matrix alloy, although some particles have been observed within the grains of the matrix alloy. It has been observed that, the grain boundary appears between the matrix alloy and the grains of silicon carbide disperse uniformly near the region of the boundary. It has been reviewed in the literature that, comparatively homogeneous microstructure of aluminium matrix composites leads to high hardness 7. The matrix alloy Al 6061 has been agglomerated with the grains of silicon carbide segregating near the grain boundary, thus forming intra-granular grains. The experimental investigations have demonstrated that by the addition of the particles of graphite divulged analogous effect with the addition of silicon carbide. It has been noticed that, by enhancing the proportion of the particles of graphite led to the refinement of grains. It has been reported in the literature that, the particles of silicon have been nucleated consistently depending on the distribution of the particles of silicon carbide8,16,17,18. It has been reported in the literature that, larger particles of silicon carbide have been necessary in high volume fraction of composite materials. The particles of silicon carbide have been distributed homogeneously and silicon carbide particles occupy the interstitial positions around coarse particles. It has been reviewed that, a dense and uniform microstructure has been advantageous for electronic packaging equipment due to better mechanical strength and high thermal conductivity9,19. Microstructural Analysis using SEM

The microstructural examination of hybrid composites to determine the cohesive interfacial bonding between the matrix and the reinforcements, dispersoid concentration of the reinforcements, particle size and porosity have been carried out by using Hitachi S-3400N Variable Pressure Scanning Electron Microscope (SEM). This device has been

developed by improving the design of S-3000N Variable Pressure Scanning Electron Microscope. It allows the study of wet, oily, and non-corrosive samples without metal coating or other complicated specimen preparation techniques. Better electron optics, larger specimen chamber, faster, cleaner, evacuation system and fully automated software are some of the major features of S-3400N10.

The evolution of microstructure, specifically in terms of size, shape and distribution of the reinforcements has been assessed by using a systematic approach. The microstructural analysis of hybrid composites has been advantageous to study the morphology and presence of porosity. Microstructural investigation of metal matrix composites can be discussed in terms of distribution of reinforcements and reinforcement matrix interfacial bonding11-13,19,20.

Figures 6-10 designate the microstructures of the different hybrid MMCs percentage reinforcements. Figure 6 depicts Al 6061 with no reinforcements. Figures 7-10 represent the microstructures of Al 6061 with the addition of reinforcements silicon carbide and graphite with varying weight fraction 1.25%, 2.5%, 3.75% and 5% (equal proportions of SiC and Gr). It has been examined that, with the addition of

Fig. 6—Al 6061

Fig. 7—Al 6061 with 1.25% SiC and 1.25% Gr


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silicon carbide and graphite of varying percentage reinforcements, the distribution of the reinforcements has been homogeneous with the absence of cracks and detrimental pores. It has been reported in the literature that, when the volume fraction of the reinforcements is higher the uniform dispersoid distribution has been obtained by melt stirring for a slightly longer time14-16,21,22.

The porosity has been investigated both experimentally and theoretically. Using water displacement method based on Archimedes principle, experimental values of the density of the hybrid

composites have been evaluated and the theoretical values of the density have been computed using rule of mixtures. Water displacement method allows the determination of the density in air compared to its displacement in water or other liquid of known density. Depending upon the nature of the specimen, the resultant value may deviate from the true mass. A clean specimen is weighed accurately in air using a laboratory balance. The same specimen is weighed while suspended in water or other liquid of such density that the specimen will sink. Deducting the mass of the suspension wire from the weight in liquid, the volume of the specimen is calculated from the effect of displacement by a liquid of known density (Archimedean principle). This allows the determination of density of specimens with irregular shapes, uneven surfaces, or porosity. Caution must be exercised to assure that no air is trapped within the specimen. Placing the specimen in a vacuum while submerged in the displacement liquid will usually avoid error. Rule of mixtures has been applied to evaluate the magnitude of density of hybrid MMCs depending on the theoretical values of density and volume fraction of matrix and reinforcements. Porosity in composite materials can be evaluated by noticing the difference between the theoretical and experimental values of the density.

Figures 7-10 characterize that, cohesive interfacial bonding between the matrix alloy and reinforcements has been accomplished. Also, due to the variation in weight fraction of the reinforcements by performing constant stirring, the dispersoid concentration has been uniform with negligible porosity and no massive clustering has been observed. It has been reported that, the clustering of particles may occur due to the stirring speed, duration of stirring and presence of porosity21-24. The particle sizes of the hybrid composites with varying weight fraction have been shown in the micrographs. It has been described in the literature that, by increasing the content of graphite in the composite matrix has been led to the refinement of grain for aluminium and eutectic silicon and reduction in porosity. It has been accounted that, the evaluation of the distribution of reinforcements and porosity has been favourable to carry out thermal characterization and analysis of composites. Also, it has been investigated that, porosity will deteriorate the thermal and mechanical properties of MMCs16. It has been mentioned that, due to the increase in volume fraction of the reinforcements, the distribution is more reliable with negligible porosity. It has been illustrated in the



Fig. 10—Al 6061 with 5% SiC and 5% Gr


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literature that, the microstructures of aluminium-silicon carbide-graphite hybrid metal matrix composites have been beneficial for mechanical characterization16 -19. It has been highlighted that, by the addition of silicon carbide with aluminium can improve the flexural strength, modulus of elasticity, thermal conductivity and distribution of stress20,21. Elemental Characterization using EDS

Microstructural analysis of hybrid composites has been carried out by using scanning electron microscope attached with energy dispersive X-Ray spectroscope (EDS). Energy dispersive X-ray spectroscope is an investigative procedure used for the elemental examination or chemical categorization of a sample. It relies on contact of some source of X-ray excitation and a sample. Its potential of categorization has been due in large part to the elementary principle that each element has an exclusive atomic structure allowing set of peaks on its emission spectrum of X-ray. The four primary components of the EDS setup are the excitation source, X-ray detector, pulse processor and analyzer. An X-ray detector has been used to convert X-ray energy into voltage signals, and later sent to a pulse processor, which measures clearly the signals and passes them onto an analyzer for data display and analysis. The most common detector that has been used commonly is Si-Li detector.

Figures 11-15 illustrate the elemental compositions of hybrid metal matrix composites with varying weight fraction. It has been shown that, the

percentage of aluminium is maximum and the percentage composition of other elemental compositions, viz., silicon and magnesium are very trivial. The matrix alloy Al 6061 possesses maximum percentage of aluminium with 0.06% of magnesium to enhance the property of wettability. The other samples with varying percentage reinforcements possess aluminium and silicon as the major elements. This elemental characterization reveals perfection in the process of fabrication by the method of stir casting. Due to the phenomenon of diffraction based on the focus of the electron beam, Graphite has been absorbed completely. Table 1 indicates the comparison of elemental characterization of the hybrid metal matrix composites. Determination of Density and Porosity of Hybrid Composites

The density of hybrid MMCs have been determined by using the relationship between volume and mass. Experimentally, it has been determined by water displacement method (Archimedes principle) and theoretically by using rule of mixtures1. Rule of mixtures (ROM) has been regarded as the most fundamental concept used in the theoretical principles of composite materials. In materials science, rule of mixtures is a weighted mean used to predict different properties made of a composite material made up of continuous and unidirectional fibers. It provides a theoretical upper bound and lower bound on properties, viz., modulus of elasticity, mass density, ultimate tensile strength, thermal conductivity and electrical conductivity. Also, rule of mixtures has

Fig. 11—Element compositions of Al 6061


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Fig. 12—Element compositions of Al 6061 with 1.25% SiC and 1.25% Gr




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Fig. 15—Element compositions of Al 6061 with 5% SiC and 5% Gr

been beneficial for the theoretical evaluation of different mechanical and thermal parameters1-3. The Eq. (1) has been used to calculate density of the hybrid composites using rule of mixtures. ρC = ρm Vm + ρp Vp … (1)

It has been reported in the literature that, the density of Al 6061 is 2.7 g/cc, silicon carbide is 3.21 g/cc and graphite is 2 g/cc. Table 2 refers to the density of hybrid composites for the varying percentage compositions (2.5%, 5%, 7.5% and 10%) reinforced with precipitation hardening matrix alloy Al 6061. Equation (1) has been beneficial to evaluate the density of composite materials and they have been validated with experimental results. The difference between the theoretical and experimental density of hybrid composites is very marginal and has been proved to have negligible porosity. Experimental Investigation on Thermal Conductivity of Composites using Laser Flash Apparatus

Laser flash technique is exceedingly resourceful for the evaluation of thermal conductivity and thermal diffusivity. The thermal diffusivity has been measured

by using a NETZSCH model LFA 447 Nano Flash diffusivity apparatus. The unit used in this work has been equipped with a furnace, capable of operation from 25°C to 300°C. The system has been equipped with a software-controlled automatic sample changer allowing measurement of up to 4 samples at the same time. The temperature rise on the back face of the sample is measured using an infrared detector. Data acquisition and evaluation have been accomplished using a comprehensive 32-Bit MS-Windows software package.

In the past experimental investigations and review, it has indicated clearly the potential prospects of further investigations on thermal characterization and analysis of aluminium matrix composites. From the literature review, it is clear that, the investigation pertaining to aluminium matrix composites have been given greater prominence. If these materials are to be used for many prominent engineering applications, the thermal aspects of AMCs need to be given more importance. Hence, it becomes important that the evaluation of thermal characteristics of hybrid composites cannot be ignored in order to transform the material from design stage to manufacturing stage. The research has been accomplished on hybrid composites based on mechanical and tribological properties substantially, but a limited research has been carried out on aluminium-silicon carbide-graphite hybrid composites pertaining to thermal analysis and characterization. It has been reported in the literature that, the experimental study on aluminium and silicon carbide has been carried out based on low and high weight fraction16. But, very limited work has been carried out on thermal analysis and characterization of Al 6061 with silicon carbide

Table 1—Comparison of elemental characterization of percentage compositions of hybrid MMCs

Composition of the elements Percentage (wt%) reinforcements

Aluminium Silicon Magnesium (Mg) and chromium (Cr)

0% 99.69 0.25 Mg- 0.06 2.5% 98.67 1.33 -------- 5% 97.62 2.38 ------- 7.5% 96.12 3.7 Cr- 0.18 10% 95.99 4.01 --------


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(SiC) and graphite (Gr) based on low and high weight fraction of hybrid metal matrix composites. Hence, graphite (Gr) has been reinforced concurrently with silicon carbide considering low weight fraction of hybrid composites.

For the determination of thermal conductivity and thermal diffusivity, the sample should be disc shaped and size is as per ASTM standards. Five samples have been considered with different percentage compositions. Al 6061 is the base alloy and reinforcements silicon carbide and graphite with different percentage compositions or weight fraction 1.25%, 2.5%, 3.75% and 5% (equal proportions of SiC and Gr) have been selected. All the specimens have been tested from room temperature to 300°C. This temperature range has been selected so as to include the entire usable range of the composites, without the formation of liquid phase in the matrix. The sample has been measured using a standard sample holder (diameter of 12.7 mm and thickness 3 mm). The sample has been coated with graphite on the front and back surfaces in order to increase absorption of the flash light on the sample’s front surface and to increase the emissivity on the sample’s back surface.

Table 3 depicts the determination of specific heat capacity of hybrid composites. Thermal conductivity has been determined by taking the product of thermal diffusivity, density and specific heat capacity of hybrid composites.

It is mandatory to estimate the specific heat capacity of hybrid composites for the determination of thermal conductivity. The specific heat capacity of hybrid composites has been determined by using differential scanning calorimeter (NETZSCH DSC 200 Maia F3). Figure 16 depicts the variation of thermal conductivity and temperature for the different compositions of hybrid metal matrix composites. Figure 17 indicates the variation of thermal diffusivity and temperature for the different compositions of hybrid metal matrix composites. From Fig. 16, it has been observed that, Al 6061 exhibits maximum

thermal conductivity with 168 W/m K. Generally, the thermal conductivity increases as the temperature increases significantly. It has been noticed that, with the addition of reinforcements silicon carbide and graphite to Al 6061, there is reduction in the thermal conductivity at maximum temperature for the different percentage compositions of hybrid metal matrix composites. Addition of graphite with aluminium matrix alloy and silicon carbide with varying weight fraction resulted in the reduction in thermal conductivity. It has been reported in the literature that, the thermal conductivity of hybrid MMCs considerably increases by reinforcing silicon carbide over the different range of temperatures16,22,30. It has been inferred that, the reinforcement of graphite with aluminium and silicon carbide does not enhance thermal conductivity and thermal diffusivity significantly. It has been examined that, addition of silicon carbide and graphite reinforcements with high volume fraction results in higher values of thermal conductivity. This has proved that, the addition of reinforcements silicon carbide and graphite has insignificant influence in the increase of thermal conductivity. It has been reviewed that, graphite improves the dimensional stability, but there will be no variation in thermal behaviour of hybrid composites. Based on these investigations, it can be concluded that, the thermal conductivity of hybrid composites reduces due to the variation of graphite. It can be concluded that, the hybrid composites with 5%

Table 2—Density of hybrid composites with varying percentage reinforcements

Serial number Hybrid composites Density (g/cc) (Experimental) Density (g/cc) (Theoretical) Difference in density 1. Al 6061 2.7 2.7 0 2. Al 6061 + 1.25% SiC + 1.25% Gr 2.694 2.696 0.002 3. Al 6061 + 2.5% SiC + 2.5% Gr 2.685 2.692 0.007 4. Al 6061 + 3.75% SiC + 3.75% Gr 2.676 2.679 0.003 5. Al 6061 + 5% SiC + 5% Gr 2.661 2.668 0.007

Table 3—Specific heat capacity of hybrid composites with varying weight fraction at 300°C

Serial number

Hybrid composites Specific heat capacity (J/kg K)

1. Al 6061 980 2. Al 6061 + 1.25% SiC +

1.25% Gr 968

3. Al 6061 + 2.5% SiC + 2.5% Gr

947

4. Al 6061 + 3.75% SiC + 3.75% Gr

924

5. Al 6061 + 5% SiC + 5% Gr

918


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silicon carbide and 5% graphite reinforced with Al 6061 exhibited the lowest thermal conductivity compared with those of other hybrid composites for a consistent level of porosity. From literature, aluminium-silicon carbide composites are attractive with many outstanding features, including high thermal conductivity, low thermal expansivity and low density. With any aluminium matrix alloy, the addition of only silicon carbide enhances thermal conductivity and flexural strength16,22-25,27.

The thermal conductivity of composites has intrigued enormous interest for about a century. Effective thermal conductivity of composites is generally determined based on the thermal conductivity of constituents, volume fraction, shape, microscale arrangement and interfacial thermal resistance between the constituent phases. The interfacial thermal resistance plays a vital role in understanding the effect of thermal conductivity behavior of composites. The interfacial thermal resistance in a composite refers to the conglomerated effect of the thermal contact resistance and the thermal boundary resistance. The particle sizes of the reinforcements are very important to examine the significant effect of interfacial thermal resistance for the investigation of thermal conductivity behavior of

composites. Many experiments have been shown that, the interfacial thermal resistance has a remarkable effect on the effective thermal conductivity of composite materials. In this research work, the thermal conductivity of hybrid composites is gradually varying as the particle or grain size of silicon carbide is relatively smaller and as a result, interfacial thermal resistance has a strong influence in comprehending the effect of thermal conductivity behavior. It has been reported that, due to an increasing volume fraction of silicon carbide with any aluminium matrix alloy, the variation in thermal conductivity of composite materials depicts an increasing trend for smaller particle sizes and decreasing trend for larger particle sizes. As a matter of fact, the interfacial thermal resistance radically decreases and matches with the experimental results. It has been noticed that, due to the increase in volume fraction of silicon carbide the thermal conductivity of silicon carbide gradually increases29,30.

It has been reported in the literature that, the dependence of the overall thermal conductivity on the particle diameter for spherical particles of equal size has been investigated with several predictions. The reason for the decrease of the thermal conductivity values with decreasing grain size of the different

Fig. 16—Variation of thermal conductivity and temperature for compositions of MMCs

Fig. 17—Variation of thermal diffusivity and temperature for compositions of MMCs


220

compositions of silicon carbide can be attributed to the interfacial properties between the aluminium matrix and SiC. It is evident that, decreasing the grain size results in a larger surface area between aluminium matrix and SiC. The interfacial reaction between aluminium matrix and SiC can reduce the thermal conductivity of composites. The decrease in thermal diffusivity dominates the temperature dependence of thermal conductivity in the high temperature region. The specific heat capacity decreases strongly at temperatures below room temperature and dominates the temperature dependence of thermal conductivity16.

Experiments have been conducted on Al 6061-silicon carbide-graphite hybrid metal matrix composites for varying percentage reinforcements to determine the tensile strength and moduli of elasticity. Tensile test has been carried out at room temperature by using a Universal Testing Machine (UTM) in accordance with Bureau of Indian Standards guidelines. The modulus of elasticity and Poisson’s ratio are essential for the determination of mechanical and thermal properties of composite materials. In this study, tensile strength, moduli of elasticity and Poisson’s ratio have been determined for hybrid metal matrix composites for varying percentage reinforcements 2.5%, 5%, 7.5% and 10%. Five specimens have been tested and the magnitude of tensile strength, moduli of elasticity and Poisson’s ratio have been presented in Table 4. From the literature, moduli of elasticity and Poisson ratio of Al 6061 are 70 GPa and 0.3, silicon carbide is 150 GPa and 0.2 and graphite is 10 GPa and 0.27. Moduli of elasticity and Poisson’s ratio

have been calculated for hybrid metal matrix composites by using Eqs (2) and (3). E= EP VP + (1-Vp) Em ... (2)

νc = νm Vm + νp Vp … (3) where, Em is the modulus of elasticity of the matrix alloy, Ep is the moduli of elasticity of the reinforcements, Vm is the volume of the matrix, and Vp is the volume of the particles or reinforcements. Also, νm is the Poisson’s ratio of the matrix alloy and νp is the Poisson’s ratio of the reinforcements.

Table 4 confirms the fact that, the tensile strength depicts an increasing trend in hybrid metal matrix composites as the percentage reinforcement of silicon carbide and graphite particulates are increased. The increase in tensile strength is because of the existence of silicon carbide reinforcement particles.

Figures 18-20 depict the variation of tensile strength, moduli of elasticity and Poisson’s ratio with the percentage reinforcements of hybrid composite specimens.

Fig. 18—Variation of tensile strength with percentage reinforcements

Fig. 19—Variation of moduli of elasticity with percentage reinforcements

Table 4—Moduli of Elasticity and Poisson’s Ratio of Hybrid Composites

Serial number

Percentage reinforcements

Tensile strength (MPa)

Moduli of elasticity

(GPa)

Poisson’s ratio

1 0 125 70 0.3 2 2.5% 132 71.5 0.2984 3 5% 138 73 0.2945 4 7.5% 144 74.5 0.2935 5 10% 146 76 0.2912


221

Graphite represents an optimal combination of strength, ductility and thermal properties for pertinent engineering applications. It has received considerable attention during recent years. Graphite has superior characteristics, viz., tensile strength, improved castability, damping capacity and machinability. One of the main advantages of aluminium-silicon carbide-graphite hybrid composites is that, they are self lubricating materials containing graphite, and strength can be enhanced by the presence of silicon carbide ceramic phase. Aluminium matrix hybrid composites are expected to exhibit excellent wear resistance due to the strengthening effect of silicon carbide in the matrix and lubrication property of graphite31.

It has been examined that, the presence of graphite particulates in hybrid metal matrix composites has a tendency to vary tensile strength, moduli of elasticity and Poisson’s ratio. Thus, it has been essential to understand to what extent the combined effect of silicon carbide and graphite particles influences the tensile strength, moduli of elasticity and Poisson’s ratio of hybrid metal matrix composites. In has been reported in the literature that, for aluminium graphite composite there has been a decrease in tensile strength, as the amount of graphite addition can be attributed to weaker graphite particulates. Also, the main drawback by using graphite as reinforcement is resulting loss in strength of the entire composite. This implies that, the addition of graphite directly with any aluminium matrix alloy will result in the gradual decrease in tensile strength of the composite material and do not exhibit any substantial variation in the properties. Conclusions

Based on the characterization and analysis of hybrid composites, the following conclusions have been drawn: (i) The microstructure of Al 6061 without any

reinforcements has been clearly revealed that the

fine precipitates of alloying elements have been dispersed in the matrix of aluminium. The microstructures of hybrid composites for a consistent reinforcement of particles of graphite with silicon carbide has been presented using optical microscope.

(ii) It has been observed that, the particles of silicon carbide and graphite have been dispersed in the matrix alloy Al 6061. The distribution of the particles of silicon carbide and graphite has been homogeneous with no parallel striations and detrimental pores. The interdendritic segregartion have been observed because the particles of silicon carbide and graphite have been driven depending of the process of solidification. The presence of these particles in the matrix substantially improves the microstructure, encumbering the coarsening of the initial phase of dendrites during the process of solidification.

(iii) The particles of silicon carbide have been shifted towards the primary phase of the dendrite boundaries of aluminium matrix alloy, although some particles have been observed within the grains of the matrix alloy. It has been observed that, the grain boundary appears between the matrix alloy and the grains of silicon carbide disperse uniformly near the region of the boundary.

(iv) The matrix alloy Al 6061 has been agglomerated with the grains of silicon carbide segregating near the grain boundary, thus forming intra-granular grains. It has been noticed that, by enhancing the proportion of the particles of graphite led to the refinement of grains.

(v) The images of SEM depict that, cohesive interfacial bonding between the matrix alloy and reinforcements has been accomplished due to the variation in the weight fraction of the reinforcements. Also, due to the variation in weight fraction of the reinforcements by performing constant stirring, the dispersoid concentration has

Fig. 20—Variation of Poisson’s ratio with percentage reinforcements


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been uniform with negligible porosity and no massive clustering has been observed.

(vi) Al 6061 exhibits maximum value of thermal conductivity, whereas there is a decline in thermal conductivity at maximum temperature for the different percentage compositions of hybrid metal matrix composites with the addition of reinforcements silicon carbide and graphite to Al 6061. The thermal conductivity of hybrid composites reduces due to the enhancement of graphite.

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vol 21, May 2002. 2 Habbu N R, Development and Application for transport

sector, IISC Bangalore, 2000. 3 Karl Ulrich Kainer, Basics of Metal Matrix Composites. 4 Hull D & Clyne T W, An Introduction to Composite

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(1999) 4461. 8 Davis L C, J App Phys, 77 (2009) 4954-4960. 9 Cem Okumus S, Mater Sci, 18 (2012). 10 Molina J M, Narciso J & Louis E, Mater Sci Eng, 480 (2008)

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J Mech Mater Eng, 6 (2011) 41-45. 18 Srivastava Ashish, Garg Prayag, Kumar Avdhesh, Krishna

Yamini & Varshney Kanu Kumar, Int J Adv Res Innov, 1 (2014) 242-246.

19 Zhu Xiao-min, Yu Jia-kang & Wang Xin-yu, Trans Nonfer Met Soc China, (2012) 1686-1692.

20 Yigezu Belete Sirahbizu, Mahapatra Manas Mohan & Jha Pradeep Kumar, J Min Mater Charact Eng, (2013) 124-130.

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22 Antonyraj Arockiaswamy, German R M, Wang, Suri & Park, J Therm Anal Calorim, (2010) 361-366.

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IOSR J Mech Civ Eng, (2010) 12-16. 26 Hai Duong, Papavassiliou D V, Mullen K J & Muruyama S,

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