Synthesis and Characterization of Aluminium Alloy A356 and Silicon Carbide Metal Matrix Composite

Mohan Vanarotti1, SA Kori2, BR Sridhar3, Shrishail B.Padasalgi4 1Christ University Faculty of Engineering, Kumbalagodu (Po), Bangalore, 560 060, India

2 Visvesaraya Technological University, Belgaum, India 3 East Point College of Engineering and Technology, Bidarahally, Bangalore 560 049, India

4 Board of Research in Fusion science and Technology, Gandhinagr, Gujarat, India Abstract: Aluminum alloy and silicon carbide metal matrix composites are finding applications in aerospace, automobile and general engineering industries owing to their favourable microstructure and improved mechanical behavior. Aluminium alloy A356 and silicon carbide composites were obtained by stir casting technique. Silicon carbide content in the alloy was fixed at 5 Weight % and 10 weight % during the casting. Microstructure revealed a uniform distribution of the silicon carbide throughout the matrix. Hardness and tensile properties of the composite showed an improvement as compared to the alloy without silicon carbide additions. The present paper highlights the salient features of casting technique and characterization of aluminum alloy A356 and silicon carbide metal matrix composite.

Keywords: MMC, Matrix, Hardness, SiC, A356

1. Introduction Metal matrix composites are materials with metals as the base and distinct, typically ceramic phases

added as reinforcements to improve the properties. The reinforcements can be in the form of fibers, whiskers and particulates. Properties of the metal matrix composites can be tailored by varying the nature of constituents and their volume fraction. They offer superior combination of properties in such a manner that today no existing monolithic material can rival and hence are increasingly being used in the aerospace and automobile industries. The principal advantage MMCs enjoy over other materials lies in the improved strength and hardness on a unit weight basis.

In the present work the synthesis and characterization of MMC, Micro structural and Mechanical properties are conducted on A356 Aluminium alloy and MMC reinforced with different wt% of SiC particles. The matrix alloy of the composite used in the process is A356. The mechanical properties were evaluated by hardness

2.

Synthesis and Characterization of MMC The scope of this work is to optimize the process variables for fabrication of Aluminium SiC composite

by means of mechanical mixing. The variable speed of stirring temperature of SiC particles are optimized to obtain the suitable composite. Bottom pouring technique was used to enhance incorporation of the SiC particles by reducing the time of pouring.[5]

In the present investigation large ingots of matrix material were cut into small pieces for accommodating into the crucible. Reinforcing material (SiC) was coated with Nickel using electroless process. Composites

1 Mohan Vanarotti. Tel.: + 91 9902544599 Fax: +91 80 4012 9898.

E-mail address: mohan1.bv@gmail.com

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IPCSIT vol. 49 (2012) © (2012) IACSIT Press, Singapore

DOI: 10.7763/IPCSIT.2012.V49.3

2012 2nd International Conference on Industrial Technology and Management (ICITM 2012)

were produced by Stir casting process. Melting was carried out in a clay-graphite crucible in a resistance furnace. Cut pieces of alloy A356 were preheated at 450 °C for 3 to 4 hours before melting, and before mixing the SiC particles were preheated at 1100 °C for 1 to 3 hours to make their surfaces oxidized. Furnace temperature was first raised above the liquidus to melt the alloy scraps completely and was then cooled down just below the liquidus to keep the slurry in a semi-solid state. At this stage the preheated SiC particles were added and mixed manually [3]. Due to difficulties of mixing in semi solid state, initially manual mixing was used for the synthesis of A356 and SiC. After this, the composite slurry was re-heated to a fully liquid state and then automatic mechanical mixing was carried out for about 20 minutes at an average stirring rate of 300 rpm. In the final mixing processes, the furnace temperature was controlled to be within 730±10 °C.

At this stage the preheated SiC particles were added and mixed manually [3]. Due to difficulties of mixing in semi solid state, initially manual mixing was used for the synthesis of A356 and SiC. After this, the composite slurry was re-heated to a fully liquid state and then automatic mechanical mixing was carried out for about 20 minutes at an average stirring rate of 300 rpm. In the final mixing processes, the furnace temperature was controlled to be within 730±10 °C. The pouring temperature was controlled to be around 720 ° C. A preheated permanent graphite mould with diameters in the range of 10 mm to 25 mm was used to prepare cast bars. Finally the super heated melt was poured into the graphite mould. The preheating temperature 350 °C for Graphite moulds was maintained for slower cooling

3. Metallographic, Tensile Test and Fractography Metallographic samples were sectioned from the cast bars and were prepared using a technique specially

developed for such composites. A 0.5% HF solution was used to etch the samples wherever required. Microstructures were examined under the metallurgical microscope.

Tensile test samples were prepared as per ASTM E8 specification. Fractured surface after tensile test was subjected to SEM examination.

4. Results and Discussions

4.1.

Microstructure

Fig.1: A356/5%SiC (600X) Fig.2: A356/5%SiC (600X)

Fig.3: A356/10%SiC (100X) Fig.4: A356/10% SiC (600X)

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Figures 1, 2 3 and 4 reveal the microstructure of the composite at 5% and 10% silicon carbide content. Microstructure reveals a reasonably homogeneous distribution of SiC particles in the cast composite. It was found that the particles showed a strong tendency to accumulate in the colonies which froze in the last stage of solidification and usually contained eutectic phases. The SiC particles were also observed to be accommodated on the grain boundaries.

4.2. Mechanical Properties The mechanical properties such as hardness, UTS, % Elongation obtained as averages of several tests are

presented in Table 1.

Table1: Mechanical properties of Al 356 + SiC MMC Sl.No Composition Tensile Strength (MPa) % Elongation Hardness (BHN)

01 A356 187 3.1 61

02 A356+5%SiC 187 2.8 70

03 A356+10%SiC 188 2.4 78

Hardness is found to increase with increasing silicon carbide content in the material. As compared to pure Al 356 alloy, 5% silicon carbide addition shows an increase of 9 BHN (15%). In contrast, composite with 10 % silicon carbide shows an increase of 17 BHN (28%). This increase in hardness is expected since SiC particles being very hard dispersoids contribute positively to the hardness of the composite [1]. The increased hardness is also attributable to the hard SiC particles acting as barriers to the movement of dislocations within the matrix.

Table 1 also reveals that UTS shows a marginal increase with increase of silicon carbide content in the matrix. The increase in strength is not commensurate with corresponding increase in hardness. This perhaps can be attributed to insufficient homogeneity obtained on account of improper stirring during the casting of the composite.

The % elongation of the composite decreases as the percentage of the reinforcement content increases in the composite and this appears to be quite obvious from the enhanced hardness associated with higher SiC content.

4.3.

Fractographic Observations Figures 5-7 reveal the fractured surfaces of the pure alloy and the composite under an SEM.

(a)

(b) Fig. 5: Fractographic features of Al Alloy 356

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(a) (b) Fig.6: Fractographic features of Al 356 + 5% SiC

(a) (b)

Fig.7: Fractographic features of Al 356 + 10% SiC

Figures 5 and 6 reveal dimples indicating overload failure under tension. The fractured surface displays a dendritic structure typical of castings. The fractured surface, the unreinforced material contained a rather uneven distribution of large dimples connected by sheets of smaller dimples.

A higher SiC (10%) composite shows brittle fracture failure (Fig.7). Interdendritic cavities seen in Figures 5-7 could have been generated during casting. Featureless regions observed in Figs. 6 and 7 is characteristic of brittle mode of fracture in localized regions possibly due to high hardness of the material owing to SiC particulate composite

5. Conclusions Al 356 reveals reasonable increase in hardness and decrease of ductility with increasing silicon carbide

content. This can be attributed with increase in volume fraction of silicon carbide in the alloy with increasing silicon carbide.

Marginal increase in UTS in the alloy can be attributed in inadequate homogenity of SiC particles in the matrix. This perhaps could be due to inefficiency of stirring during casting of the composite.

Both ductile and brittle mode of fracture is observed in the composite. With increasing silicon carbide content the material tends to fail in brittle mode.

6. Acknowledgement The authors gratefully acknowledge the financial support from the Board of research in Fusion Science

and Technology, Gandhinagr, Gujarat. Under grant No. NFP-MAT-F11-01.

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[3] Zhengang Liuy, Guoyin Zu, Hongjie Luo, Yihan Liu and Guangchun Yao “Influence of Mg Addition on Graphite Particle Distribution in the Al Alloy Matrix Composites” -J. Mater. Sci. Technol., 2010, 26(3), 244-250.

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[4] Metals Handbook” 10th edition. Vol 2: Properties and Selection, Nonferrous alloys and Special Purpose Materials, ASM International, Davis, J.R

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[6] B Agarwal and D. Dixit “Fabrication of aluminium based composites by foundry techniques”, Transition of Japn Institute of Metals Vol 22 No8 1981

[7] M. Mares “Some issues on tailoring possibilities for mechanical Properties of particulate reinforced metal matrix Composites” Journal of Optoelectronics and Advanced Materials, Vol. 3, No. 1, March 2001

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[9] Vikram Singh and R.C. Prasad. “Tensile and fracture behavior of 6061 al-SiCp metal matrix Composites International Symposium of Research Students on Materials Science and Engineering December 20-22, 2004

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