V. Jeevan, C.S.P Rao & N. Selvaraj
Abstract – The increase in development of new metal matrix composites is ever increasing and thereby spurt increase and thrust in development of aluminum based composites research. Al and its MMCs have wider applications in automobile and aircraft industries due to their high strength to weight ratio. Sintered aluminum matrix composites are quite attractive material for the automobile industry due to their superior mechanical, tribological properties and the fabrication advantages associated with a powder metallurgy process. The objective of this work was to conduct a detailed assessment of the microstructure and mechanical properties of AA6082-SiC composites. The sintering temperatures are varied with respect to soaking time in nitrogen atmosphere. An increasing trend was observed in hardness with increase in volume percentage of silicon carbide.
Keywords—Aluminum Matrix Composites, Powder
Metallurgy, Mechanical Properties, Microstructure Sprut
.
Department of Mechanical Engineering, National Institute of Technology Warangal, India
E-mail : vemula.jeevan@gmail.com
I. INTRODUCTION applied [2]. The reinforcements are being used in the form of particles, short fiber or whisker, continuous fiber [3]. The technical difficulties such as fibre damage, improper fibre orientation, fibre to fibre contact and high cost are involved in manufacturing of the continuously reinforced aluminum matrix composites.
Also, the problems associated with fabrication of whisker reinforced aluminum matrix composites are faulted internal structure of whiskers and irregular surface which may contain particulate contamination with metallic matrix and relatively high cost over irregular particles. The simple processing and low material cost has made the particulate reinforced aluminum matrix composites (PR AMCs) preferable to the other form of composite material for many commercial applications and gaining their importance due to their low density in combination with high stiffness, improved wear resistance, low coefficient of thermal expansion. However, the processing costs and machining difficulties associated with PR AMCs component parts has restricted their range of application to that of specialised components [4,5].
Aluminum and its alloys have many outstanding attributes that lead to a wide range of applications which require weight reduction and energy savings due to their low density [1]. Their applications have often been limited because of their low mechanical and tribological properties at elevated temperatures. However,
Aluminum and its alloys exhibit poor tribological properties. Therefore, Aluminum and its alloys can meet various demands of several applications by the addition of reinforcement materials which have enhanced mechanical properties and better thermal stability. These
Aluminum matrix composites (AMCs) combine the characteristics of metallic matrix along with the characteristics of reinforcement which improve in superior mechanical, tribological and thermal expansion characteristics, but this is usually achieved at the expense of other properties such as ductility which depends on the matrix alloy, morphology, size, volume/weight fraction of the reinforcements, the material fabrication method and the heat treatment
There are several manufacturing methods to produce these composite materials, which could be grouped into three major processing routes depending on the state of matrix during the fabrication process, either liquid or semisolid or solid state processing route
[6]. Among them powder metallurgy (P/M) is a proven technology for producing light weight parts in an economical manner. Composites fabricated by P/M route has merits such as less residual voids and dissolved gases in products, it has better control on the microstructure and uniform distribution of the reinforcement especially at higher volume fractions, a good interface between matrix material and inclusions because of lower manufacturing temperatures that lead to less destructive interfacial reactions, near-net-shape forming of compacts [7]. Among all the powders aluminum or aluminum alloy based composite powders are light weight and extremely compressible by
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International Journal on Mechanical Engineering and Robotics (IJMER) employing low compacting pressures. Sintering of aluminum matrix composite parts are highly energy efficient when compared to ferrous or copper based powders due to the relatively low sintering temperatures
[8]. It has proved its position in automobile, aerospace, and many other engineering applications due its wear resistance properties and substantial hardness [9]. A major drawback in producing PR AMCs is high cost.
The reasons are twofold. Initially the material itself is costly due to presence of specially synthesized prealloyed powders. These powders are cold pressed, then hot pressed or sintered and extruded. The second reason that these alloys are expensive is due to secondary processing. They are not produced to near net shape and therefore require extensive forging or machining, which can be particularly problematical because of the ceramic component. Conventional P/M processing can overcome these problems. Only two processing steps are required
(press and sinter) and the part is formed into its final shape. Thus there is significant potential for press-andsinter processed aluminum P/M composites in order to provide high stiffness at low cost.
In this study AA6082-SiC composites have been fabricated using a “press-and-sinter” P/M process comprised of blending, compaction, and sintering.
Mixture of five different compositions viz. 0, 5, 10, 15, and 20vol.% of SiC particulates in aluminum alloy matrix were prepared using terbula blender. Then, the sinterability of these composites was investigated at different levels of temperatures and soaking times. The effects of the sintering temperature of each composite on the density and hardness of the sintered parts are also examined.
II.
E
XPERIMENTAL
P
ROCEDURE
Initially, series of raw powders were weighed to the desired alloy chemical composition, placed into a
Nalgene bottle and blended in a turbula blender for 30 minutes, after which a chemical composition similar to that of wrought AA 6082 alloy was obtained as shown in Table 1 and Particle size and Purity details were shown in Table 2. Finally by addition of 5, 10, 15 and
20% of SiC particulates by volume to the base AA 6082 powder and mixed for 20minutes. The obtained powder mixtures with ceramics were homogeny at macroscopic level.
The blends were compacted at into cylindrical pellets (10 mm in diameter and 10 mm in height) using a uniaxial semi-automatic hydraulic press (svs, India).
Some of the green compacts shown in Fig. 1. The theoretical density assuming zero porosity was calculated by Rule of Mixture. The green density of the compacts was determined from weight and volume measurements. The AA6082 and AA6082-SiC powder
35 mixtures exhibit uniform die filling and provides good reproduction of part configuration.
Table 1. Chemical composition of AA6082 by weight percentage
Chemical composition
Si
Mg
Mn
Fe
Cr
Zn
Cu
Ti
Al
AA 6082
1.0
0.9
0.7
0.5
0.25
0.20
0.10
0.10
Bal.
Table 2. Particle size and Purity details
Raw Material
Aluminum
Silicon
Magnesium
Manganese
Copper
Iron
Zinc
Titanium
Silicon Carbide
Particle Size (Mesh)
-200/+325
-325
-140
-325
-325
-100
-400
-140
-1200
Purity
99.50%
99.57%
99.67%
99.78%
99.81%
99.39%
99.65%
99.10%
99.00%
Fig. 1 Green compacts of different compositions
Green parts were then sintered in a horizontal tube furnace (metrex scientific instruments) in a nitrogen atmosphere. Typically, the sintering temperatures are
570 o C and 600 o C and all the samples is furnace cooled to room temperature. The heating profiles at two intervals are shown in Fig. 2 and Fig. 3. The sintered density was obtained through dimensional measurements as well as by Archimedes displacement method [10]. The microstructural analyses were carried out using a scanning electron microscope (SEM) and hardness tests were carried out using rockwell hardness
B scale as shown in Fig. 4.
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International Journal on Mechanical Engineering and Robotics (IJMER)
The hardness measurements were conducted on the sintered compacts at 100 kgf test load and dwell time 15 seconds. The indentations at macroscopic level are shown in Fig. 5. The final readings are taken on average of five indentations.
Fig. 2 Heating profiles for AA6082-SiC upto 570 o C
Fig. 5 Rockwell hardness indentations on the sintered cylindrical pallets
I II .
R ESULTS A ND D ISCUSSION
Samples for metallography were polished using standard techniques and etched with kellers reagent. The sintered composites were analysed for its microstructure by Scanning Electron Microscopy. The SEM images of the AA6082 alloy powder with different volume percentages of SiC are shown in Fig. 6-10.
Fig. 3 Heating profiles for AA6082-SiC upto 600 o C
Fig. 6 Micrograph of AA6082
Fig. 4 Rockwell hardness tester
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Fig. 7 Micrograph of AA6082-5.vol% SiC
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International Journal on Mechanical Engineering and Robotics (IJMER)
The micrographs clearly indicate the evidence of minimal porosity in the entire 6082 aluminum alloy and its SiC composites. The distribution of particles in the matrix is fairly uniform. However the tendency of clustering could not be avoided for composites containing fine SiC. In Fig. 11, the matrix and the reinforcement can be seen clearly that SiC is uniformly distributed in the aluminum matrix
Fig. 8 Micrograph of AA6082-10.vol% SiC
Fig. 9 Micrograph of AA6082-15.vol% SiC
Fig. 11 Micrograph of AA6082-15.vol% SiC at high magnification
Hardness measurements were performed on the sintered specimens according to ASTM E18-08 with indenting load of 100kgf and dwell time 15 seconds.
The average hardness data shown in Fig. 12 resulted from five measurements. The position of indentation on the sample was chosen randomly. The hardness test gives a good indication on the strength of the material.
As the SiCp increases from 0 to 20 volume percentage hardness also increased.
Fig. 10 Micrograph of AA6082-20.vol% SiC
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FiG. 12 The variation of hardness with SiC
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International Journal on Mechanical Engineering and Robotics (IJMER)
IV. CONCLUSION
During compaction of aluminum alloy based composite powders, the shape and quality of final component depends upon the quality of initial manual compact. Therefore, the manual compact should be fabricated carefully and proper allowances should be given terms of dimensions to achieve the desired final component. In the present study, AA6082 composite powders accompanied with 0–20 vol.% of SiC particles were prepared using the conventional press and sinter method. The microstructure revealed the uniform dispersion of fine SiC in the aluminum matrix. High hardness values are obtained at 600 o C sintering temperature under nitrogen atmosphere.
V. REFERENCES
[1] C. Padmavathi, A. Upadhyayaa and D. Agrawal,
“Effect of microwave and conventional heating on sintering behavior and properties of Al–Mg–
Si–Cu alloy,” Materials Chemistry and Physics, vol. 130, pp. 449-457, 2011.
[2] J.U. Ejiofor and R.G. Reddy, “Developments in the Processing and Properties of Particulate Al-Si
Composites,” JOM, pp. 31-37, 1997.
[3] Karl U. Kainer, “Metal Matrix Composites,
Custom-made Materials for Automotive and
Aerospace Engineering,” WILEY-VCH, 2006.
[4] X.Zhang and M.l.Tan Selection of particulate reinforcement in P/M metal matrix composites,”
Journal of Materials Processing Technology, vol.
63, pp. 913-917, 1997.
[5] Adnan Ahmed, Andrew J. Neely, And Krishna
Shankar, “Experimental comparison of the effects of nanometric and micrometric particulates on the tensile properties and fracture behavior of al composites at room and elevated temperatures,”
The Minerals, Metals & Materials Society and
ASM International, metallurgical and materials transactions A, vol. 42A, pp. 795-815, 2011.
[6] J.W. Kaczmar, K. Pietrzakc and W. Woosin Aski,
“The production and application of metal matrix composite materials,” Journal of Materials
Processing Technology, Vol. 106, pp. 58-67,
2000.
[7] [7] A. Ibrahim, F. A. Mohamed and E.
J. Lavernia, “Particulate reinforced metal matrix composites - a review,” Journal of Materials
Science, Vol. 26, pp. 1137-1156, 1991.
[8] K.S. Dunnett, R.M. Mueller and D.P. Bishop,
“Development of Al–Ni–Mg–(Cu) aluminum
P/M alloys,” journal of materials processing technology, Vol. 198, pp. 31-40, 2008.
[9] C.D. Boland, R.L. Hexemer Jr, I.W. Donaldson and D.P. Bishop, “ Industrial processing of a novel Al–Cu–Mg powder metallurgy alloy,”
Materials Science & Engineering A, vol. 559, pp.
902-908, 2013.
[10] Jeevan.V, C.S.P Rao and N.Selvaraj,
“Compaction, sintering and mechanical properties of Al-SiCp composites,” International
Journal of Mechanical Engineering and
Technology, Vol. 3, pp.565-573, 2012.
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