Evaluation and Characterization of Tensile Properties of Short Coated Carbon Fiber Reinforced Aluminium7075 AlloyMetal Matrix Composites via Liquid Stir Casting Method

The interface between the reinforcement and the matrix has a significant role in enhancing the property of the composite. In this work to increase the wetting ability of the reinforcement, nickel coating is done over the carbon fiber. The process of coating is carried out through three stages. It involves sensitization time, activation time and metallization time. Using the optimized time interval of the above process coating is done, on the fiber of range 0.6 to 1 micrometer. This coated fiber has the good cohesive property within each other, which increases the wettability. Stir casting process is carried out with the stirring speed of 200 rpm and the melting temperature about 780-8000C is used for the manufacturing of the composite. The results of this study revealed that, as the short coated carbon content was increased, there were significant increases in the Ultimate Tensile Strength (UTS). Furthermore, Scanning Electron Microscopy (SEM) was used in order to coordinate relationships between quality of the carbon fiber and aluminium alloy bond and thereby link with tensile properties of the metal matrix composites. The metal matrix composites are a very important role in the industries such as aerospace, automobile and sports equipment etc. The aluminium material is considered to be a light weight metal, to enhance the property of the aluminium 7075 alloy, the short coated carbon fibers added to the aluminium as a reinforcement.


INTRODUCTION
Metal Matrix Composites (MMCs) like most composite materials, provide significantly enhanced properties over conventional monolithic materials, such as higher strength, stiffness, and weight savings 1,2 .Metal matrix composites are used in a wide range of high performance applications today.The MMCs have been used in over 5.5 million kg during the year 2006 and are finding impetus in annual growth rate of over 8% 3 .The foremost applications include aviation, ground transportation, electronics and sports industries.Additionally, the applications also find themselves established in aeronautics with problems concerned to aero-structural, aero-propulsion and subsystem categories.Aluminum composites are very appealing because of their low thickness, their ability to be fortified by precipitation; their great corrosion resistance, high thermal and electrical conductivity, and their high damping limit 4 .The strong demand for weight reduction in car and aircraft fabrication urges the optimization of the design of products employing low weight materials 5 .The substitution of ordinary materials by lighter metals, for example, Aluminum alloys is, in this manner, exceedingly attractive.Notwithstanding, Aluminum alloys are not adequately solid or solid for some reasons and their reinforcement is important.Aluminum based MMCs are remarkable contender for these applications inferable from the high ductility of the matrix and the high quality of the hard reinforcing stages.The fascination for such materials is likewise because of the high modulus, quality to weight proportion, fatigue strength and wear resistance 6,7 .The presence of reinforcing particles produces potential properties non-attainable by other materials 8 .The reinforcement metal matrix offer potential for improvement in efficiency, mechanical performance and reliability over the new generation alloys 9,10 .The presentation of a ceramic material into a metal framework delivers a composite material that outcomes in an alluring mix of physical and mechanical properties which can't be acquired with solid combinations.There is an expanding requirement for information about the preparing systems and mechanical conduct of fiber reinforced MMCs in perspective of their rising generation volumes and their more extensive business applications.Composites have been developed with greater success by the use of fiber reinforcements in metallic materials 11 .Earlier study on MMCs addressed the behavior of continuous fiber reinforcement composite based on aluminum, zinc and titanium alloys matrices and the reinforcements used was Alumina fibers, carbon fiber, glass fiber etc.The required literature has been studied in the field of Metal matrix composites (MMC) for thoroughly understanding the research involved.This literature survey provides brief ideas about the developments in MMCs.The survey has been made for the different methods on the reinforcements and also the different testing procedures for the improvement of the mechanical property, and also improved strength over the weight reduction.The literature survey was also made for different techniques available for electro less copper coating of the carbon fibers which is reinforcement for Aluminium.Ramnat et al 12 .has provided a review on different techniques used for manufacturing Metal matrix composites.The Al alloys such as 6061, A356, 7075-T6 etc., and the reinforcing material such as Boron, SiC, Al 2 O 3 etc.The process was carried by squeeze casting and powder metallurgy.Similarly Raviteja et al 13 .carried out fabrication process and studied mechanical property of Al-Si 2 Cu/B 4 C composite.The composite preparation was carried out in stir casting technique using B 4 C as reinforcement added on to the matrix with the wt.% of 2, 4, 6, 8 and 10.
The result has shown B 4 C particles have distributed homogeneously over the matrix.The mechanical tests like hardness and tensile revealed that, as the percentage of reinforcement increases in the matrix, hardness and tensile improved by 6.97% and 33% respectively when it is compared with as cast.Himanshu et al 14 .have successfully prepared a particle reinforced metal matrix using aluminium as the matrix material and it is fabricated through stir casting method.Added particles are such as Alumina, B 4 C, and SiC etc., Particles in aluminium improves the hardness, yield strength, tensile strength while ductility is decreased.Addition of graphite in aluminium increases the tensile strength and elastic modulus but hardness is decreased.Also it shows a decrease in friction coefficient in case of Tribological behaviour.Zhongcai et al 15 .have studied the connection between the Cu for the electro less coating and the time and temperature of the bath solution.The work was also studied on the different quality of the carbon fibers under different bath condition.The scattering of the carbon fibers using the surfactant agents were discussed in this paper.Further Bhav et al 16 has performed an electro less copper coating on carbon fibers.The main intention behind the coating process was to avoid the interfacial reaction between the matrix and the reinforcement.Diameter of the carbon fiber which was used for the process of 6 micrometer and length 4-8mm were dipped into the different solutions for the different time and temperature and maintaining the pH between 10 and 13.Hajjari et al 17 .has studied the significant effect of tensile strength in Continuous Carbon Fibers Reinforces Aluminium Matrix Composites (CCFRAMCs).Electro less nickel coating of the Continuous CF has improved the wettability of the reinforcement.The carbon fibers coated with 0.5µm thickness of nickel layer by electro less coated with Al 2024 alloy as matrix.Investigation of the nature of fiber matrix interfaces existing in Al/C composites depending upon the presence of a nickel interlayer deposited on the carbon fibers and on the composition of the aluminium matrix was conducted by Silvain et al 18 .Results showed that the presence of this nickel layer eliminates the formation of aluminium carbide which is known to lower the mechanical properties of Al/C composites.Finally Zhang et al 19 investigated the variation of Tribological behaviour of the metal matrix composite due to the fiber length of short coated carbon fiber as a reinforcement which is dispersed in the epoxy.They found that the composite with longer short carbon fiber showed good result in wear resistance than the shorter short carbon fiber.And this study was done in the presence of graphite flake and TiO 2 nano particles.
Based on the literature cited above, the objective of this work is to study the mechanical behaviour of Aluminium 7075 alloy /short coated carbon fiber composite, like ultimate tensile strength, hardness, compression strength, ductility using universal testing machine; to carry out the micro structural study of worn out surfaces of the composite material through the optical microscopy.

Preparation of Chopped Carbon Fibers
The PAN based continuous carbon fibers long strands which are winded in the spool as shown in the Fig. 1.The continuous carbon fibers has to be cut to smaller length of 1mm.
The continuous carbon fibers are separated from the spool and cut to length of approximately 30cm and placed on the manual printing press cutting machine.A manual feed of 1mm and less is given to machine for the cutting of carbon fibers as shown in Fig. 2.  The process of coating preparation is carried out in a sequential manner.When the electro less coating is completed the coated carbon fibers are gets attached to each other which results in decrease in wettability of nickel coated carbon fibers on the aluminium metal matrix composites.For the separation of these nickel coated fibers without affecting the coating of the surface, the fibers are immersed in the solution of 20grams/litre solution of Sodium dodecyl sulphate (NaCL 2 H 2 SO 4 ).Surfactants improves the dispersion ability of the carbon fibers in the aqueous solution and dispersion ability of the sodium dodecyl sulphate is good compare to other solutions such as polyethylene glycol or dodecyl benzene sulphonic acid.The fibers are dipped in the solution for 5 days for the proper scattering to take place.The optimized coating for prescribed sensitization, activation and metallization is selected from Table 1.

Mixture of Composites by Liquid Stir Casting Technique
The challenging factor in the preparation of composite is to maintain the proper dispersion of reinforcement to achieve a defect free microstructure other and promoted wettability.Then the melt with the crucible is removed from the heater and poured into the heated mould.After the melt is poured, it is then left out for cooling for around 3 hours and the mould specimen is separated out for any testing process.

Testing of Specimens
All tests were directed as per ASTM standards.Tensile tests were organized at room temperature utilizing a universal testing machine (UTM) as per ASTM Standard E 8-82.The specimens of width 8.0 mm and gauge length 75 mm were machined from the cast composites with the gauge length of the examples parallel to the longitudinal axis of the castings.For every composite, four test samples were tried and the general estimations of the UTS (as far as rate prolongation) were measured.Each result is a normal of four readings.

Morphology of Electro Less Nickel Coating
The SEM image which is shown in the Fig. 4(a) shows the uncoated carbon fibers which has the mean diameter of 7.16µm.The nickel coating that is deposited on the carbon fibers under the coating procedure.The Fig. 4(b) shows the coating thickness of 0.64µm over the carbon fiber as seen under SEM model JEOL JSM 6360 with a magnification capacity of X500, X1000, X2000 and accelerating voltage of 15 kV with a working distance of 10 mm.The uniform coating is maintained over the carbon fibers by employing uniform wettability over the matrix in order to give good adhesion and protection.The microscopic image reveals that the thickness and the morphology of the coating is maintained uniform under the time variant of sensitization, metallization and activation at 5, 15 and 3 minutes respectively.The X-ray Diffraction graph describes the mixture of solution that undergone for the preparation of a nickel coating on the carbon fiber.This can be determined with the graph plotted intensity vs. the angle 2â and the software PCPDFWIN to determine the mixture solution which is allotted for the peaks.The X-ray diffraction has an emitter and collector to study the diffraction occurred when the light beam comes in contact with the object.For the uncoated and nickel coated carbon fibers as shown in the Figs.5(a) and 5(b), the angle 2q shows the angle of refraction of light to the reference beam.Accordingly for the certain angle of refraction and the certain input in the software the presence of solution at the particular angle can be determined which is the solution that accommodate for achieving the nickel coating on carbon fiber.The uncoated carbon fiber shows a Carbon content of 92% present in the processed carbon fiber.

Tensile Properties
The plot for ultimate tensile strength (UTS) with a variety of short carbon covered fibers in metal matrix composite is appeared in the Fig. 6(a).It is observed that there is an increase in the ultimate tensile strength (UTS) over the variety of scattered fibers.This phenomenon is elaborated by studying the percentage variation in the UTS value from as cast Aluminum 7075 combination for various weight percentages of reinforced carbon fibers as appeared in Fig. 6(b).It has been noted that for variation in the fibers from 0 to 1% there was increment in the UTS estimation of 4.35%.Further increment up to 3% has demonstrated the UTS esteem addition to 26.11%.The presence of carbon filaments has ascribed the expansion in the UTS esteem which credits to the increment in the quality of general metal lattice

Fracture Surface
T h e g e n e r a l c o m p o n e n t s o f t h e microstructure, e.g.fiber orientation, and volume fraction were concentrated on utilizing a microscope and fractured surfaces were studied utilizing an examining electron magnifying lens.Average fractured surfaces of short carbon fiber composites, acquired from tensile tests, are demonstrated Fig. 7(a).As per Sood et al. [21], the experiments conducted likewise propose that short fibers once in a while crack.Just not very many fibers can be seen as cracked and there is likewise a confirmation of ductile failure in the matrix.The disappointment is couple of strands split longitudinally and transversally as appeared in the Fig. 7(b).The failure of fibers in the composite might be credited to the expansion in weight on the stress of the specimen Sanders et al 22 .

CONCLUSIONS
The tensile properties of the Al7075 alloy short coated carbon fiber composites are altered by the addition of the different weight percentage of carbon fiber.

Fig. 7 :
Fig. 7: SEM of the tensile specimen shows (a) fractured short Basalt fibers and (b) fiber puIl out from the specimen

Table 1 : Mean thickness of nickel coating on carbon fiber Trial S time A time M Time Mean no (min) (min) (min) thickness (µm)
NaOH pellets onto the solution.This stage is carried out with constant agitation at 80 o C.

Table 2 Samples
The electro less coating of the carbon fibers • were uniform with the coating thickness of 0.6 -1µm with Activation, sensitization and metallization stages at different time intervals.The stir casting technique with the addition of • Magnesium metal powder improved the mixture ability of the AA7075 metal matrix with the coated carbon fiber.The ultimate tensile strength is found to be • increasing by 13.80, 29.45, and 41.25% with increase of weight percentage of carbon fiber value which attributes to the increase in the strength of overall metal matrix composite The increase in UTS is due to the hard carbon short fiber, which confers quality to the lattice combination giving improved tensile strength There has been a considerably good agreement • between the theoretical model values and the experimental results obtained for Ultimate tensile strength value of short carbon coated as cast Al 7075 alloy composite.The fracture was ductile showing fiber debonding, • and fiber cracking.