CASTING OF A356/TiB 2p COMPOSITE BASED ON THE TiB 2p /CMC/PPS MORTAR M.HIZOMBOR 1 ; S.M.H. MIRBAGHERI 1 ; AND R. ABDIDEH 2

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1 CASTING OF A356/TiB 2p COMPOSITE BASED ON THE TiB 2p /CMC/PPS MORTAR M.HIZOMBOR 1 ; S.M.H. MIRBAGHERI 1 ; AND R. ABDIDEH 2 1) Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran, hizombor@yahoo.com 2) Department of Metallurgical Engineering, Islamic Azad University(Young Researchers Club), Ahwaz, Iran. Abstract Composite materials are among those new materials used for many industrial applications. Many Methods have been proposed to overcome the problem of poor wettability between ceramic reinforcement particles and molten aluminum for metal matrix composite (MMC) production in casting processes. In this investigation, an innovation procedure has been proposed for casting of metal matrix composites by adding a mortar consist of expandable polystyrene beads, carboxy methyl cellulose paste, water and TiB 2p particles as a mould pattern. This process was examined for A356/TiB 2p composite. The use of pretreated TiB 2p particles, 1wt% magnesium as a wetting agent and mechanical mold vibration while MMC s slurry is solidifying were found to promote wettability of TiB 2p with molten matrix alloys. Produced composites were characterized using optical and scaning electron microscopy. Then mechanical properties of the composites, such as hardness, wear, tensile testing and porosity levels of produced Al/TiB 2p composites were measured and results has been discussed. Results show the mechanical properties strongly dependent on the distribution of the TiB 2p particles. Keywords: Casting, Metal Matrix Composite, Wettability, TiB 2p /CMC/PPS mortar, Aluminum A356, porosity. 1. INTRODUCTION The requirement for strong, light and stiff materials has extended an interest in metal matrix composites (MMC s ). During the past two decades, MMC s have received substantial attention because of their improved strength, high elastic modulus and increased wear resistance over conventional base alloys. The wide Scale introduction of MMC s has been increasing simultaneously with the technological development [1, 2]. Aluminum matrix composites posses many advantages such as low specific density, high strength and good wear resistance, with the development of some non- continuous reinforcement materials, whisker, fibres or particles. In particular, the particulate reinforced aluminum matrix composites not only have good mechanical and wear properties, but are also economically viable [3,4]. Therefore, TiB 2p - particulate reinforced aluminum composites have found many applications in aerospace and automotive industry [5]. There are many methods for fabrication of particulate reinforced metal matrix composites (MMC s ) Such as powder metallurgy, Squeeze casting, compocasting, and so on [5]. Among the variety of manufacturing processes available for discontinuous metal matrix composites, casting is generally accepted as a particularly promising rout, currently practiced commercially. Its advantages lie in its simplicity, flexibility and applicability to large quantity production. According to Skibo at al. [6], the cast of preparation composites materials using a casting method is about on- third to half of competitive methods, and for high volume production, it is projected that, the cast will fall to on- tenth[7]. In general, the solidification synthesis of metal matrix composites involves producing a melt of the selected matrix material followed by the introduction of a reinforcement ceramic into the melt, obtaining a suitable dispersion. The next step is the solidification of the melt containing suspended dispersions under selected condition to obtain the desired distribution of the dispersed phase in the cast matrix.

2 In preparing metal matrix composites by casting method, there are several factors that need considerable attention, including: The difficulty of achieving a uniform distribution, wettability between two main substance, porosity and chemical reaction between the reinforcement material and the matrix alloy. In order to achieve the optimum properties of the metal matrix composites, the distribution of the reinforcement material in the matrix alloy must be uniform, and the wettability or bonding between these substances should be optimized. The porosity levels need to be minimized, and chemical reactions between the reinforcement materials and the matrix alloy must be avoided [7]. 2. MATERIALS AND EXPERIMENTAL PROCEDURE 2.1. Materials A commercial casting aluminum A356 alloy was utilized as the matrix materials, the chemical composition of A356 matrix alloy used in this investigation are given in table1. This alloy has been selected because of its good fluidity as well as the presence of silicon and magnesium. The Silicon content of A356 alloy is sufficiently high, therefore it can be casted in high temperature with out giving rise to the extensive formation of Al 4 C 3 [8]. In the present study, composites containing 5, 7.5 and 10 %vol. TiB 2p were manufactured, the average size of TiB 2 particles were 6µm. The TiB 2p particles were oxidized in air at 1100ºC for 4 hours to improve their wettability with molten A356 alloy. Fig.1 Show the SEM images of TiB 2 particles. EPS/ TiB2p performs were fabricated using pre- expandable polystyrene and carboxy methyle cellulose in infiltration methods has been mentioned in other refrences [9] Fabrication of EPS/ TiB 2p performs In order to EPS/TiB 2p fabrication, TiB 2 particles must be coated on the pre-expandable polystyrene. Solution of water and carboxy methyle cellulose was utilized as a binder. Suitable volume percentage of carboxy methyle cellulose soluble in water, was obtained experimentally (2-3) % wt., in the less amounts, there was no suitable adherence between TiB 2 particles and pre-expandable polystyrenes, and in above of 3% wt. carboxy methyle cellulose, because of high concentration solution, the coat of TiB 2 particles on the preexpandable polystyrenes is not uniform. Pre-expandable polystyrenes was poured into a container, mixture of binder and particles was added gradually and stirred continuously for about 5min, using an impeller at a speed of 400rpm. Coated preexpandable polystyrenes were formed as a preforms with dimension of cm. To annihilate humidity, the EPS/TiB 2p performs was dried at 60ºc for approximately 2 hours Production of Al/TiB 2p composites The composites were manufactured by molten metal of A356 alloy using an electric induction furnace, which is 30 KW power. The molten alloy was suitably degassed with hexachloroethane (C 2 Cl 6 ) and fluxed with 45%wt. KCl-45%wt. NaCl-10%wt. NaF. In this work, pouring temperature of A356 molten alloy was selected 780ºc and pure magnesium was added to molten metal approximately 1, and 5 wt. % in order to wettability enhancement. EPS/TiB 2p performs were put in the permanent mould (Fig.2), subsequently molten metal was poured into it and allowed to cool down to room temperature. In some experiments, mechanical mold vibration with 180 Cycle/sec and 2-3 mm amplitude was used to improve the wettability of molten aluminum and silicon carbide particles.

3 2.4. Chemical composition analysis Content magnesium in A356 aluminum alloy is lost in high temperature by oxidation. Pure magnesium was added to molten A356 to restitute the magnesium and improvement the wettability of the alloy. The chemical composition analysis was carried out using D47533KELEVE coantimeter Microstructural characterization The microstructures of the manufactured A356/TiB 2p composites were investigated using Scanning electron microscope (SEM) to determine the distribution of the TiB 2 particles and presence of porosity. Sample for metallographic observation were sectioned and prepared by standard methods. Fabricated A356/TiB 2p composites microstructures were invest perused by image analysis (IA) techniques to obtained the volume percentage and distribution of TiB 2p in them Density measurements The density of the A356/TiB 2p samples was measured according to ASTM D797 standard and using Archimedes principle [10]. The samples were precision weighed in an electron balance to an accuracy of 0.1 mg. The theorical density of composites and alloy matrix specimens was then calculated according to the rule of mixtures [10]. The volume fraction porosity of the composites was also determined by use of equation 1. % vol. prosity ρ ρ cal exp = (1) ρ cal 2.7. Hardness and wear tests The hardness of the Al/TiB 2p composites and matrix alloy were preformed using Vicker method (30 sec. At a load of 31 kgf) according to ASTM E92 standard. In order to eliminate possible segregation effect, the mean of at least three tests was taken for each specimen. Dry sliding wear tests were carried out according to the ASTM G99 standard using a pin-on-disc type apparatus [10]. The test material in the form of rings of diameter 5mm and length 15mm were slide against a steel disk equivalent to 3343 grade with hardness of 60HRC. Wear tests were preformed under normal pressure of 1Mp and with sliding distance of 100, 300, 600 and 1000 m. In this study, the sliding distance of specimens was selected 0.5 m, and in all tests, each experiment was s repeated at least 3 times. After the tests, the specimens were cleaned with acetone, dried and weighted again to determine the weight loss due to wear. The weight loss was calculated from the difference in weight of the specimens measured before and after the test to the nearest 0.1 mg, using an electronic balance. Finally weight loss values were converted into volume loss and wear rate known density data. 3. RESULTS AND DISCUSSION 3.1. Investigation of effective parameters Oxidation of TiB 2 particles, magnesium percentage in molten metal and mold vibration before and during solidification are important process parameters for fabrication of low volume fraction composites using EPS/TiB 2p performs. The addition of magnesium to molten A356 alloy to promote the wetting of silicon carbide particles is particularly successful [11]. Indeed, the addition of magnesium to molten aluminum can modify the matrix metal alloy by generating a transient layer between the particles and molten metal. This transient layer has a low wetting angle, decrease the surface tension of liquid, and surround the particles with a structure that is similar to both the particle and matrix alloy [7]. The addition of magnesium to

4 aluminum reduces its surface tension. The reduction is very sharp for initial 1%wt magnesium addition [10]. According to sukumaran et al. [12], increasing the magnesium content over 1%wt., increases the viscosity of matrix alloy and decrease the wettability, moreover, addition of magnesium over this value lead to the formation of a Mg 5 Al 8 phase, which devastates the mechanical properties of composite because of its low melting point. Heat treatment of TiB 2 particles before introduce into the melt annihilates absorbed gases from the particle surface, and alter the surface composition by forming an oxide layer on the surface [13]. In composites manufacturing, a mechanical force can usually be used to dominate surface tension and improve wettability between TiB 2p and molten metal. In the experimental work of Zhou et al. [13] proposed that is necessary to break the gas layer surrounding the ceramic particles in order to achieve good wettability. Using mechanical and ultrasonic vibration of molten aluminum in order to improve the wettability of ceramic particles has been reported in other refrences [14] Distribution of TiB 2p in EPS/ TiB 2p preforms Fig.3 show image of coated pre-expandable polystyrenes with uniform TiB 2p distribution. This uniformity aids uniform TiB 2p distribution in final Al/TiB 2p composites Microstructure of Al/TiB 2p composites The properties of the metal matrix composites depend not only on the matrix, particles and the volume fraction, but also on distribution of reinforcing particles and interface bonding between the particle and matrix. The SEM micrograph of the aluminum composite reinforced with approximately 5, 7.5 and 10 vol. % of TiB 2p is shown in fig.4 (a), (b) and (c) Density and porosity The results of theorical and experimental densities and porosities of the composites according to the volume percentage of TiB 2p particles are shown in table2. Table2 shows that, the theorical and experimental density of the composites increase linearly (as expected from the rule of mixture) [3]. The experimental value are lower than that of the theorical density, therefore, the density measurement showed that, these composites contained some porosity. The amount of porosity and density in the composites increased with increasing volume percentage of the TiB 2 particles. The increased porosity of the cast Al/TiB 2p composites with increased TiB 2p content has also been reported by other researchers [15]. The porosity of the composites was found between 3.3 and 4.6 vol. %. These values indicate that, the porosities of the composites are at acceptable levels for low volume percentage of composites. Kok et al. [15] demonstrated that porosity level of metal matrix composites was found approximately 4%. SEM micrograph of porosity in TiB 2p reinforced composite are shown in fig Hardness, Wear test The hardness variation of the manufactured particle reinforced composite are shown in fig.6. Results indicate that, the hardness of the manufactured composites increase more and less linearly with volume percentage of TiB 2 particle phase. The variation in weight loss with sliding distance for unreinforced matrix alloy and Al/TiB 2p composites containing different volume percentage of TiB 2 particles are summarized in table3 and fig.7. It is clear that, the weight loss of the composites is lower than that of the unreinforced alloy. The weight loss also decreases

5 with increasing the TiB 2p volume percentage, which is in agreement with the previously reported results and can be attributed to the increased hardness of composites. 4. CONCLUSION The following major conclusion can be drown from the present investigation. 1. Processing parameters such as magnesium content in the molten metal alloy, TiB 2 particle oxidation and mechanical mold vibration during and before solidification are among the important factures to be considered in the production of Al/TiB 2p using EPS/TiB 2p preforms. 2. Microstructural investigation of EPS/TiB 2p preforms showed that, there is a uniform distribution of TiB 2 particles on pre-expandable polystyrene, this uniformity aids uniform TiB 2p distribution in fabricated Al/TiB 2p composites. 3. Al/TiB 2p composites consisting of 5, 7.5 and 10 vol. % TiB 2 particles could be produced successfully by EPS/TiB 2p preforms. 4. SEM observation of the Al/TiB 2p microstructures showed that, there is a uniform distribution of TiB 2 particles in manufactured composites. 5. The density and the porosity of manufactured composites increased with increasing volume percentage of TiB 2 particles. 6. The hardness of Al/TiB 2p composites increased with increasing volume percentage of particulates. 7. in the produced Al/TiB 2p composites, the weight loss and wear rates are lower, compared those to the unreinforced matrix alloy REFERENCES [1] M. Muratoglu, M. Aksoy,' The effects of temperature on wear behaviors of Al-Cu alloy and Al-Cu/SiC Composite', Mater.Sci.Eng., A282, 2000, pp [2] D. B. Miracle, ' Metal Matrix Composites from Science to technological significance ' composite science and technology, 65, 2005, [3] Y. Sahin,' Preparation and some properties of SiC particle reinforced aluminum alloy composites', Material&Design, 24, 2003, pp [4] Y. sahin, ' The effect of sliding speeed and microstructure on the dry wear properties of metal- matrix composites', Wear, 214, 1998, pp [5] J. R. Davis, Aluminum and Aluminum Alloys, ASM Specialty Handbook, 1998,160. [6] D. M. Skibo, D. M. Schuster, L. Jolla, 'Process for preparation of composite materials containing nonmetallic particles in a metallic matrix and composite materials', U. S. Patent No , [7] J. Hashim, L. Looney, M. S. J. Hashmi,' Metal matrix composites: production by the stir casting method', Mat. Proc. Tech., 92, 1999, pp [8] S. Vaucher, O. Beffort, 'Bonding and interface formation in Metal Matrix Composites', EPMA report Nr 250, Thun, Switzerland, [9] A. Mortensen, J. Thomas,' Method for producing a microcellular foam', US. Patent, No [10] Y. Sahin, M. Acilar, ' Production and Properties Of SiC- reinforced Aluminum Alloy Composite ', Composite:Part A,34,2003,pp [11] J. Hashim, L. Looney, M. S. J. Hashmi,' The enhancement of wettability of SiC particles in cast aluminium matrix composites', Mat. Proc. Tech., 119, 2001, pp [12] K. Sukumaran, S. G. K. Pillai, V. S. Kelukutty, B. C. Pai, K. G. Satyanarayana, K. K. Pavikumar, J. Mater. Sci. 30, 1995, pp

6 [13] W. Zhou, Z. M. Xu,' Casting of SiC reinforced metal matrix composites', J. Mat. Proc. Tech., 63, 1997, pp [14] Y. Tsunekawa, H. Nahaneshi, M. Okumia, N. Mohri, Key Eng. Mater., , 1995, pp [15] M. Kok, ' Production and mwchanical properties of Al 2 O 3 particle- reinforced 2024 aluminium alloy composites', Mat. Proc. Tech., 161, 2005, pp Table1. Chemical Analyses of used A356 Alloy and standard element range in this alloy Si Mg Fe Cu Mn Zn Ti Al Used alloy Bal Standard range < Bal (a) (b) Fig. 1. SEM images of TiB 2p a) Low magnification b) High Magnification Mortar dried (EPS/TiB 2p ) Mould Fig. 2. View of fabricated EPS/TiB 2p after moulding and drying

7 3 mm Fig.3. Images of coated pre-expandable polystyrenes Uniform TiB 2p distribution Fig.4. SEM micrograph (a) of the aluminum composite reinforced (b) with approximately a) 5 b) 7.5 (c) and c) 10 vol. % of TiB 2p Table2. Theorical and experimental densities and porosities of the composites and unreinforced A356 alloy Samples Theorical density (gr/cm 3 ) Experimental density (gr/cm 3 ) Porosity (%) A A356-5% TiB 2p A %TiB 2p A356-10%TiB 2p Fig.5. SEM micrograph of porosity in TiB 2p reinforced composite

8 Hardness (HV30) TiB 2p volume percentage(%) Fig.6. Hardness variation of the manufactured TiB 2p reinforced composite Table3. Variation in weight loss with sliding distance for unreinforced matrix alloy and Al/TiB 2p composites Samples Weight loss( mg) 100m 300m 600m 1000m A A356-5% TiB 2p A %TiB 2p A356-10%TiB 2p Weight loss (mg) Sliding Distance (m) Fig.7. Weight loss variation according to the sliding distance and TiB 2p volume percentage