Microstructure and Mechanical Properties of as Cast

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S E Vannan S P Vizhian, 5 5 million kg in the year 2006 and is increasing at an annual growth rate of over 8 5 Most of their current. applications are in aviation ground transportation electronics and sports industries The applications of metal. matrix composites in aeronautics have been established in the aerostructural aeropropulsion and subsystem cat. egories The Aluminium alloys are quite attractive due to their low density their capability to be strengthened by. precipitation their good corrosion resistance high thermal and electrical conductivity and their high damping. capacity 6 The strong demand for weight reduction in car and aircraft fabrication urges the optimization of the. design of products employing low weight materials 7 The replacement of conventional materials by lighter. metals such as Aluminium alloys is therefore highly desirable However Aluminium alloys are not sufficiently. stiff or strong for many purposes and their reinforcement is necessary Aluminium based MMCs are outstanding. candidates for these applications owing to the high ductility of the matrix and the high strength of the hard rein. forcing phases The attraction for such materials is also due to the very high specific modulus strength to weight. ratio fatigue strength and wear resistance 8 10 The presence of reinforcing particles produces potential. properties non attainable by other materials 11 The reinforcement metal matrix offer potential for improve. ment in efficiency mechanical performance and reliability over the new generation alloys 12 14 The intro. duction of a ceramic material into a metal matrix produces a composite material that results in an attractive. combination of physical and mechanical properties which cannot be obtained with monolithic alloys There is an. increasing need for knowledge about the processing techniques and mechanical behaviour of fiber reinforced. MMCs in view of their rising production volumes and their wider commercial applications Composites have. been developed with greater success by the use of fiber reinforcements in metallic materials 15 Earlier study. on MMCs addressed the behavior of continuous fiber reinforcement composite based on aluminum zinc and ti. tanium alloys matrices and the reinforcements used was Alumina fibers carbon fiber glass fiber etc. In recent years considerable work has been done on fiber reinforced metal matrix composites which exhibit. low friction low wear rate and excellent antiseizing properties composite fiber reinforced metal matrices pos. sess great potential to be the next generation of advanced composites offering many advantages compared to fi. ber reinforced polymers Specific advantages include high temperature capability superior environmental stabil. ity better transverse shear and fatigue properties 16 17 Fiber reinforced composites are often characterized. by their high specific strength and specific modulus parameters i e strength to weight ratios and are widely. used for applications in low weight components 18 Metals reinforced by short fiber have the advantage of. being machinable and workable using conventional processing techniques especially short fiber reinforcements. have more favorable influence on the stiffness and elastic plastic tensile properties If good fiber alignment is. obtained the tensile properties are much improved 19 Fiber alignment is often obtained during processing by. using either contracting flow or expanding flow in the extrusion The mechanism of strengthening and the me. chanical properties of metal matrix composites have attracted a considerable number of investigations 20 23. Very little literature is available on mechanical properties of fiber reinforced metal matrix composites Most of. the published data pertain to the mechanical properties of particulate reinforced MMCs deal with tensile proper. ties while only a relatively small amount of data has been obtained dealing with compression properties al. though it is generally known that the compressive strength of an MMC is invariably higher than its UTS Hence. in the present investigation importance is also given to the compressive properties of the MMCs together with. the tensile properties such as the UTS ductility hardness and Young s modulus The mechanical properties of. MMCs are also affected by the residual stresses which form as a result of the differences in the thermal expan. sion coefficients between the matrix and reinforcement There are various models which have been developed. to estimate the residual stresses in MMCs For instance the models developed by Eshelby 24 Mura and Taya. 25 and Tanaka and Mori 26 can be utilized to predict the yield stress both in tension as well as compression. Analysis of the theoretical model proposed by Eshelby 24 in combination with X ray diffractrometry studies. conducted by Arsenault and Taya 27 reveals some interesting trends First the theoretical model predicts yield. stress which is higher in compression than in tension this is in agreement with experimental results Second the. predicted values of the yield stress of the MMCs were found to be less than those experimentally determined. both in tension and compression This discrepancy was attributed to the high dislocation density present in the. annealed MMCs and which is not considered in the development of the models Third although the average re. sidual stress in the MMCs is relatively small there can be relatively substantial compressive stress at the ma. trix reinforcement interface In addition the state of stress in the matrix region adjacent to the reinforcement. S E Vannan S P Vizhian, may be either tensile or compressive depending on the size distribution and loading of the strengthening phases. Fourth in the matrix region between the reinforcements the residual stress will be tensile and plastic deforma. tion is likely to initiate in this since it contains fewer dislocations when compared to the reinforcement matrix. In the present investigation aluminium alloy Al 7075 was used as the matrix material Al 7075 alloy has the. highest strength and ductility of the aluminium alloys with excellent machinability and good bearing and wear. properties 28 Most of the fiber reinforced metal matrix composites are produced by liquid metallurgy some. times known as the vortex method 29 although many different processes for fabricating these as cast com. posites are also available which have been reported by various researchers In the present work the vortex me. thod of producing AMC s in which basalt short fiber have been used as the candidate reinforcements of fiber. sizes ranging from 1 to 1 5 mm and added to the vortex formed in the Al 7075 melt above its liquidus tempera. ture Since the ductility ultimate tensile strength UTS compressive strength Young s modulus and hardness. of the composite material are all vital properties of a structural material the present investigation aims at study. ing these properties in the Al 7075 alloy basalt fiber composites. 2 Experimental,2 1 Materials, In the present study Al 7075 alloy having the chemical composition as per the ASTM ingot specification given. in Table 1 was used as the base matrix alloy Basalt short fibers were used as reinforcement The weight per. centage of basalt short fiber was varied from 2 5 10 steps of 2 5 wt The compocasting technique was. used to prepare the composite specimens which is similar to the one used by Sharma et al 30. 2 2 Preparation of Composite, In this process the cu coated basalt short fiber was first pre heated to temperature of 500 C and maintained at. that temperature till it was introduced into the Al alloying elements melt The preheating of the reinforcement is. necessary in order to reduce the temperature gradient and to improve wetting between the molten metal and the. basalt short fiber A known quantities of these metals ingots were pickled in 10 NaOH solution at room tem. perature for ten minutes Pickling was done to remove the surface impurities The smut formed was removed by. immersing the ingots for one minute in a mixture of 1 part nitric acid and 1 part water followed by washing in. methanol These cleaned ingots after drying in air were loaded into different alumina crucibles These crucibles. kept in different furnace which were setting metals respected melting temperature The melts were super heated. and maintained at that temperature The temperatures were recorded using a chromel alumel thermocouple The. molten metals were then degassed using purified nitrogen gas Purification process with commercially pure ni. trogen was carried out by passing the gas through an assembly of chemicals arranged in a row concentrated. sulphuric acid and anhydrous calcium chloride etc at the rate of 1000 cc minute for about 8 minutes A stain. less steel impeller or stirrer coated with basalt short fiber was used to stir the molten metal and create a vortex. The impeller used for stirring was of centrifugal type with three blades welded at 45 inclination and 120 apart. The stirrer was rotated at a speed of 500 rpm and a vortex was created in the melt The depth of immersion of. the impeller was approximately one third the height of the molten metal above the bottom of the crucible The. reinforcing basalt short fiber which were preheated in the muffle furnace were introduced into the vortex at the. rate of 120 gm min Stirring was continued until interface interactions between the basalt short fiber and the ma. trix promoted wetting Then the melt was degassed using pure nitrogen for about 3 4 minutes and after reheat. Table 1 Chemical composition of Al 7075 alloy and basalt fiber. Element Si Fe Cu Mn Mg Cr Zn Ti Al,0 4 0 5 1 6 0 3 2 5 0 15 5 5 0 2 Bal.
Element SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 MnO. 69 51 14 18 3 92 2 41 5 62 2 74 1 01 0 55 0 04,S E Vannan S P Vizhian. ing to super heat temperature 540 C it was poured into the pre heated lower half die of the hydraulic press. The top die was brought down to solidify the composite by applying a pressure of 100 kg sq cm Both the lower. die and the upper dies were preheated to 280 C before the melt was poured into it The pressure applied enables. uniform distribution of the basalt short fiber in the developed composite. 2 3 Testing of Specimens, All tests were conducted in accordance with ASTM standards Tensile tests were conducted at room temperature. using a universal testing machine UTM in accordance with ASTM Standard E 8 82 The tensile specimens of. diameter 8 0 mm and gauge length 75 mm were machined from the cast composites with the gauge length of the. specimens parallel to the longitudinal axis of the castings For each composite four tensile test specimens were. tested and the average values of the UTS Young s modulus and ductility in terms of percentage elongation. were measured The hardness tests were conducted in accordance with ASTM Standard E 10 using a Brinell hard. ness tester with a ball indenter of 2 5 mm diameter and a load of 31 25 kg The load was applied for 30 secs Eight. hardness readings were taken for each specimen at different locations to circumvent the possible effects of par. ticle segregation Compression tests were conducted on a UTM in accordance with ASTM Standard E 9 at room. temperature In this test the compression loads were gradually increased and the corresponding strain was meas. ured until the specimen failed Each result is an average of four readings The results are tabulated in Table 2. 3 Results and Discussion,3 1 Microstructure, The micrograph illustrating the microstructure of the metal matrix composites was used in this investigation. Samples for the microscopic examination were prepared by standard. Metallographic procedures etched with killer s agent and examined under optical microscope The optical. microstructure of as cast Al 7075 alloy and Al 7075 short basalt fiber composite are shown in Figures 1 a c. Micrograph indicates the nearly uniform distribution of the short basalt fiber in the Al 7 5 basalt short fiber. Al 10 basalt short composite, The feasibility by compo casting to produce Al based composite containing basalt short fiber was confirmed. the incorporation of basalt short fiber in the Al matrix was successful in all the castings Figures 1 a c show. the microstructure of a as cast Al 7075 alloy b Al 7 5 percent basalt short fiber reinforced MMCs and c Al 10. percent basalt short fiber reinforced composites Visual examination shows that the feasibility by liquid metal. lurgy to produce Al based composite containing basalt short fiber was confirmed and the incorporation of basalt. short fiber in the Al matrix was successful in all the castings Microstructural analyses of the as cast Al 7075 al. loy and basalt short fiber reinforced composite showed large grain size of the Al 7075 alloy matrix and a quite. non homogeneous distribution of the reinforcing fibers Spheroidal grains observed are due to casting limiting. dendrite formation and growth This is due to overlapping solute diffusion fields which homogenizes solute. gradients and prevents constitutional super cooling This leads to spheroidal grains The grain size of matrix is. reduced by great extent in all the Al basalt short fiber composite prepared in this study under different experi. mental conditions 125 m to 45 m The physical properties depend on the microstructure and particle size. shape and distribution in the matrix alloy The grain size of matrix alloy is somewhat larger than that of the. Table 2 Mechanical properties of as cast Al 7075 basalt . How to cite this paper Vannan SE and Vizhian S P 2014 Microstructure and Mechanical Properties of as Cast Alumi nium Alloy 7075 Basalt Dispersed Metal Matrix Composites Journal of Minerals and Materials Characterization and Engi neering 2 182

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