Compaction sintering and mechanical properties

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –
INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING
6340(Print), ISSN 0976 – 6359(Online) Volume 3, Issue 3, Sep- Dec (2012) © IAEME
AND TECHNOLOGY (IJMET)

ISSN 0976 – 6340 (Print)
ISSN 0976 – 6359 (Online) IJMET
Volume 3, Issue 3, September - December (2012), pp.565-573
© IAEME: www.iaeme.com/ijmet.asp
Journal Impact Factor (2012): 3.8071 (Calculated by GISI)
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COMPACTION, SINTERING AND MECHANICAL PROPERTIES OF
Al-SiCp COMPOSITES

Jeevan.V1, C.S.P Rao2 and N.Selvaraj3
1,2,3
Department of Mechanical Engineering, National Institute of Technology Warangal,
Warangal, Andhrapradesh, India.
Email: vemula.jeevan@gmail.com

ABSTRACT
A trend has been observed in the field of aluminum based composite materials to
employ silicon carbide as reinforcement material in developing composites of unique
properties. In the present study, an attempt has been made to fabricate the unreinforced Al
and its composites were synthesized using the Powder Metallurgy (P/M) manufacturing route
with blending, pressing and sintering allows the near net shape fabrication of precision parts.
The composites are further solution heat treated at 5290C for two hours and artificially aged
at 1750C for 18 hours. Optical Microscopy, Scanning Electron Microscopy has been carried
out to analyze powder morphology and composite structure. An increasing trend towards
micro-hardness and compressive strength with increase in weight percentage of silicon
carbide has been observed.
KEYWORDS: Al-SiCp, Mechanical Properties, Microstructure, Powder Metallurgy.
1. INTRODUCTION
Particulate Reinforced Aluminum Matrix Composites (PR AMCs) have evoked a
vehement interest in recent times for potential applications in aerospace, defence and
automotive industries. PR AMCs exhibit improved physical, mechanical and wear resistant
properties such as higher stiffness, superior strength-to-weight ratio, improved wear
resistance, increased creep resistance , low coefficient of thermal expansion, improved high-
temperature properties, and high workability of the composites over those of the monolithic
metals oralloys [1-5].
Earlier studies on Metal Matrix Composites (MMCs) addressed the behaviour of
continuous fiber reinforcement composite based on aluminum, zinc and titanium alloys
matrices. The wide usage of these composites is restricted because of high production cost of
composite and composite fiber. MMCs that include both particulate and whiskers have
attracted considerable attention than fiber reinforced MMCs, because of their low cost and
considerable ease of manufacturing. A wide range of PR AMCs manufacturing processes has

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been developed. These are generally manufactured either by solid state (Powder Metallurgy
processing) or by liquid state (stir casting) processes effectively [6].

To fabricate PR AMCs, among the various manufacturing technologies powder metallurgy is
one the most advantageous techniques to fabricate isotropic distribution of particles in matrix, good
dimensional accuracy, complex, net shape lightweight components can be produced cost effectively.
Powder metallurgy is especially suitable for producing PR AMCs as it prevents some wettability
problems of silicon SiCp and deleterious reactions that may appear during casting routes. Blended
fine powder mixtures in the solid state with particulates, whiskers or platelets along with binders
produce materials of uniform microstructure. The conventional powder metallurgy process can easily
formulate different composition by mixing elemental or premixed powders along with reinforcement,
and pressing the powder mixture to form green compact by applying hydraulic pressure and sintering
the green compact in inert gas atmosphere. Few microstructural parameters control and contribute to
the advancement in the properties of PR AMCs. These involve the matrix alloy, the morphology, size,
and weight fraction of the reinforcement particulate; the material processing technique; and the heat
treatment adapted [1-7].

PR AMCs powder is highly compressible. Mostly, green densities of more than 90 % of
theoretical can achieve utilizing low compacting pressures around 200MPa, allowing the use of
presses with smaller capacity. Sintering of PR AMCs parts is more economical than for most other
PM materials due to the relatively low sintering temperatures. Due to the low density of PR AMCs,
more than twice number of parts can be produced from unit weight of powder as compared to ferrous,
copper and tungsten based powders.

During last decade, several researchers have reported the fabrication of Al-SiCp composites
and testing of their properties such as tensile strength, hardness, wear resistance and microstructural
characterization. Most of the researchers have observed that an increase in tensile strength, hardness
and wear resistance while decrease in ductility with increase in reinforcement content and aluminum
alloy powders are difficult to sinter because of the stable aluminum oxide film covering the powder
particles and thus reducing sinter-ability. In addition, the presence of hard ceramic particles in
aluminum ductile matrix increases the processing difficulty. Related work carried out on aluminum
alloy by reinforcing ceramic particles such as SiC, Al2O3, ZrO2, TiO2 etc., with varying
reinforcement sizes, volume/weight fractions, lubricants, compaction pressures, sintering
temperatures, sintering time, and sintering atmospheres. By varying these parameters will result
optimal set of parameters lead to resultant microstructure and properties [7-22].

The 6xxx series aluminum alloys have a widespread application, especially in the building,
aircraft and automotive industry due to their properties. Increasing demand for these materials have
resulted in increasing research and development for high strength and high-formability
aluminum alloys. Among 6xxx series aluminum alloys AA6082 one of the most common engineered
aluminum alloy. It offers a combination of better corrosion resistance and weldability due to its lower
strength values in the welding zone. In numerous applications, AA6061 can be replaced with AA6082
due to its higher strength [11-12].

The objective of the present investigation is to fabricate the unreinforced Al and its
composites. Hence, the present studies are aimed at fabrication of Al and Al-5 wt% of SiCp
composite that is fabricated by powder metallurgy route followed by solution heat treatment and
artificially aged. Microstructure, micro-hardness and compressive strength of the developed
unreinforced Al and its composites are studied. alloys [1-5].

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2. EXPERIMENTAL PROCEDURE

2.1 Materials

It is necessary to select pure metal powder and optimal processing parameters for the
preparation of specimens. Commercial pure aluminum is obtained from M/S Metal Powder
Company Ltd, Tamil Nadu, India. Silicon, Magnesium, and Manganese are supplied by
premier industrial corporation limited Maharashtra, India. Silicon carbide is obtained from
outside vendor at Tamil Nadu, India. The morphology of raw powders (Al, SiCp) was made
with Scanning Electron Microscopy (SEM), JSM-6390 Model (JOEL) shown in figure 1(A),
1(B), 1(C), 1(D) and 1(E). The EDAX analysis has shown in figure 2(A) and 2(B).Particle
size and purity details for raw materials are given in table 1.

Fig. 1(A) SEM of Aluminum Powder Fig. 1(B) SEM of Silicon Powder

Fig. 1(C)SEM of Magnesium Powder Fig. 1(D)SEM of Manganese Powder

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Fig. 1(E) SEM of Silicon Carbide Powder

Fig. 2(A)EDAX of Aluminum Powder Fig. 2(B)EDAX of Silicon Carbide Powder
Table 1: Details of Raw Material
Sl.No Raw Material Particle Size Purity
1 Aluminum -200/+325 mesh 99.50%
2 Silicon -325 mesh 99.57%
3 Magnesium -150 mesh 99.67%
4 Manganese -325 mesh 99.78%
5 Silicon Carbide -1200 mesh 98.0%

2.2Mixing
The chemical composition of the AA6082 prepared by elemental mixing is as follows: Al–
1.0Si–0.9Mg–0.7Mn/5.0 SiCp (all concentrations by weight). Contech Precision Balance (Type: CA
223) is used for weighing elemental powder. Metal and ceramic powders were blended in a Turbula
mixer with Jar container. Blending is one of the crucial processes in powder metallurgy where the
metallic powders have mixed with the ceramic reinforced particles.Good blending produces no
agglomeration of both the metallic and ceramic powders. 1.5% of acrawax by weight was added to the
base Aluminum powder and mixed separately for 15 minutes. In general lubricant was added and
homogeny blended to reduce friction between the powder mass and the surface of the die and obtain a
good compaction. Addition of 1.0 Si, 0.9 Mg, 0.7Mn as elemental were made to the lubricated base
powder and mixed for 15 min each, after which a composition similar to that of wrought 6082 Al
alloy was gained. Finally by addition of 5% of SiC particulates by weight to the 6082 Al alloy powder
and mixed for 20minutes. The obtained powder mixtures with ceramics were homogeny at
macroscopic level.
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2.3 The Specimens Compacting

For pressing, a hydraulic press (Model: plus one machine fabric) was used to obtain
green compacts. Die wall is brushed with zinc stearate powder for easy ejection of pallet and
to reduce the friction between them. Blended Powders were compacted at 200 ± 5 Mpa in a
hardened steel die. In order to avoid damage of the samples during ejection, the compaction
pressure was decreased to 5Mpa after maximum pressure was obtained. The dimensions of
green compacts are 13.3 x 13.3 x 13.3 mm3. The theoretical density assuming zero porosity
was calculated by Rule of Mixture (ROM). The green density of the compacts was
determined from weight and volume measurements. The AA6082 and AA6082-SiCp powder
mixtures exhibit uniform die filling and provides good reproduction of part configuration.
Theoretical density and green density are shown in Table 2.

Table 2: Theoretical Density and Green Density
Material Theoretical Green Density
Density (%)
(g/cm3)
AA6082 2.62 94.32
AA6082-SiCp 2.64 91.36

2.4Sintering and Heat Treatment

The mild steel boat with dimensions 30x15x5 cm3 and 0.4cm thickness is filled with
fine sand and the green compactswith achieved dimensions are placedin the boat. The boat is
moved slowly inside pre heating zone with hydraulic arm. The temperature within the furnace
rises slowly in the preheat zone till it reaches the actual sintering temperature. The green
compacts are de-lubricated in the preheat zone at 3500C for 30 minutes. After de-lubrication
of pallets the boat enters into hot zone or sinter zone where the temperature raised slowly to
6200C it remains essentially constant for 45 minutes in a protective atmosphere cracked
ammonia. The sintering temperature is kept below the melting point of the base metal. The
boat is pushed into the cooling zone where the drop in part temperature is controlled precisely
and cooled to room temperature. As the parts travel through the furnace, the temperature
cycle results change in composition, microstructure and properties. In the preheat zone, the
lubricant volatilizes, leaves the part as a vapor, and is carried out by the dynamic atmosphere
flow. In the hot zone, metallurgical bonds develop between particles and solid state alloying
takes place. The part then moves through the cooling zone. The microstructure developed
during sintering determines the properties of the part. Dimensional changes encountered after
sintering. The premixed elemental AA6082 specimens are subjected to volumetric expansion.
Sintered densities of specimens were measured by the Archimedes principle (water
displacement technique). The porosity is increased during the sintering process compared to
the green one. The large porosities reduced the sintering densities due to wide polymer burn
off range leaving residual porosity. Proper bonding between metallic matrix and ceramic
particles at interface and the morphology and distribution of pores and carbides in the matrix
are achieved. The composites are further solution heat treated at 5290C for two hours and
artificially aged at 1750C for 18 hours in a muffle furnace.

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3. RESULT AND DISCUSSION
3.1 Microstructure
The purpose of microstructure examination was to investigate grain size and shape
morphology and distribution of the silicon carbide particles. The microstructures of the
unreinforced Al and Al-SiCp composites were studied using optical microscope. For this
purpose small samples were cut from the cube fabricated by powder metallurgy process.The
flat samples were polished using silicon carbide paper (320, 400, 800, 1000, and 1500 grit)
and finally using a short-nap cloth with fine alumina powder as slurry. The samples were then
etched using the Keller’s reagent. Figure 4(A) and 4(B) shows the optical microscope
photographs for the the unreinforced Al and Al-SiCp. Micrograph indicates the nearly
uniform distribution of the SiCp particles in the aluminum matrix and some clustering of
silicon carbide arise reinforcement in the matrix.

Fig. 4(A) Microstructure of AA6082 Fig. 4(B) Microstructure of AA6082-5SiCp
3.2 Micro-hardness Test
Vickers Microhardness measurements were performed on polished flat specimens
according to ASTM E384-08 with indenting load of 200gf and dwell time 15 seconds. The
average microhardness data given in this paper resulted from five measurements. The
position of indentation on the sample was chosen randomly. The microhardness test gives a
good indication on the strength of the material. As the SiCp increases from 0 to 5 percentage
hardness also increased. The results were shown in Figure 5.

50
48
Micro-Hardness

46
44
42
40
38
36
AA6082 AA6082-5SiCp

Fig. 5 Microhardness of P/M AA6082 and AA6082-5%SiCp

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3.3 Compression Test

The compression test was chosen as it requires small size specimen. The samples
which are a problem in case of powder metallurgy produced aluminum silicon carbide
composites. For each combination, four compression Specimens were tested. Figure 6,
illustrates the effect of Silicon carbide particulate reinforcement content on the compression
strength of the composite. It is observed that the compressive strength of the composite
increases as the reinforcement content increases from 0 to 5 weight percent. This increase in
the compression strength is attributed to the presence of hard particles, which imparts high
strength to the composite. This may be due to very small amounts of particulates at different
orientations, which can make significant difference in stress-strain behavior. The rigidity and
crushing strength of particles is much higher than that of matrix material hence the strength
increases.

600
580
Compression Test

560
540
520
500
480
460
AA6082 AA6082-5SiCp

Fig. 6Ultimate Compressive Strength of P/M AA6082 and AA6082-5%SiCp

4. CONCLUSION

During compaction of powders, the shape and quality of final component depends
upon the quality of initial manual compact. Therefore, the manual compact should be
prepared carefully and proper allowances should be in dimensions to get the desired final
component. Compaction at 200 MPa followed by sintering at 6200C has been successfully
used to produce Al alloy and Al-SiCp composites. During thepecipitation hardeningthe alloy
is transformed to a homogeneous, one phase solution. Micro-hardness, compressive strength
of powder metal Al alloy and Al-SiCp composites increases with increase in reinforcement
content from 0 to 5% weight of SiCp.

5. Acknowledgements

The authors wish to thank Mr.VinayChoudary (C.E.O) and Mr. Saibaba (GM),
Innomet Powders, Hyderabad, for their support and encouragement during the research
studies.

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