REVIEW ON EFFECT OF VARIOUS REINFORCEMENTS ON THE THERMAL PROPERTIES OF LM-25 AL ALLOY AND SOME COMMON LIGHT METAL ALLOYS

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International Journal of Mechanical Engineering and Technology (IJMET)
Volume 10, Issue 05, May 2019, pp. 143-159. Article ID: IJMET_10_05_015
Available online at http://www.iaeme.com/ijmet/issues.asp?JType=IJMET&VType=10&IType=5
ISSN Print: 0976-6340 and ISSN Online: 0976-6359
© IAEME Publication
REVIEW ON EFFECT OF VARIOUS
REINFORCEMENTS ON THE THERMAL
PROPERTIES OF LM-25 AL ALLOY AND SOME
COMMON LIGHT METAL ALLOYS
Ishfaq Ahmad Ganaie, Adarsha Hiriyanniah, Kaushik Vijaya Prasad, V. Ravinarayan
School of Engineering and Technology-Jain (Deemed – to be – University)
Kanakapura, Ramanagara, Karnataka, India
Vishnu K. R
Department of Mechanical Engineering, PESU-EC, Bangalore
ABSTRACT
For thermal management in applications like automobiles, aviation, marine etc. we
need materials with high thermal conductivity, low coefficient of thermal expansion and
at the same time the materials should have high strength, high corrosion resistance and
low density. Aluminum alloys are excellent choices as the alloys possess all these
properties. In order to improve the properties of the alloys further to make them more
suitable for the applications, alloys are reinforced with different materials. This review
focuses on influence on thermal properties of LM25 and some common light metal
alloys reinforced with different materials, effect of heat and solution treatments on
LM25/SiC MMC, effect of aging treatment on thermal fatigue of LM-25 alloy and the
most common synthesis technique used.
Keywords: LM25 (A356), thermal conductivity, coefficient of thermal expansion,
thermal fatigue, reinforcement.
Cite this Article: Ishfaq Ahmad Ganaie, Adarsha Hiriyanniah, Kaushik Vijaya Prasad,
V. Ravinarayan and Vishnu K. R, Review on Effect of Various Reinforcements on the
Thermal Properties of Lm-25 Al Alloy and Some Common Light Metal Alloys,
International Journal of Mechanical Engineering and Technology, 10(5), 2019, pp.
143-159.
http://www.iaeme.com/IJMET/issues.asp?JType=IJMET&VType=10&IType=5
1. INTRODUCTION
Among the hypoeutectic aluminium silicon alloys, LM-25 also called A 356 or Al-Si7-Mg is
one of them. This is picking up far reaching fame and discovers applications in vehicle, aviation,
marine, defense, sports, bio-medical, electrical, food, chemical and other industries. Because of
its magnificent mix of properties, for example, good casting characteristics, machinability, high

Review on Effect of Various Reinforcements on the Thermal Properties of Lm-25 Al Alloy and
Some Common Light Metal Alloys
strength to weight ratio, low density, low coefficient of thermal expansion and corrosion
resistant in sea-water and marine atmosphere [1,2]. A356 aluminum alloy is utilized to create
numerous parts in car industry in light of the more noteworthy interest for light weight and high
quality materials which decrease the fuel consumption [3]. Adequate thermal conductivity is an
important physical property for alloys used in motor components. In case of pistons it is
necessary that the heat generated in the course of the compression process is removed as quickly
as possible to avoid thermal stresses and hot spots on the piston surface. High thermal
conductivity will therefore play a major role in determining the life time of certain motor
components [4].
Key necessities of the alloys created for the uses of thermal management in electronics,
optoelectronics, automobiles, aerospace, etc. are a high thermal conductivity (TC) and an
adequately low coefficient thermal expansion (CTE) [5,6]. Therefore alloys used for thermal
management should have as high thermal conductivity as possible for the high rate of heat
transfer and as low coefficient of thermal expansion as possible to avoid the volume expansion
of the component due to high temperatures.
It is important to note that thermal conductivity is reduced by the addition of alloying
components, where components in solid mixture result in a higher thermal resistance than a
similar measure of components used as reinforcements in the composite materials. The last
mentioned regularly decreases thermal conductivity relative to the increment of volume fraction
[7]. In order to enhance the thermal conductivity and reduce the coefficient of thermal
expansion composite materials are being developed. There are different types of composites
like metal matrix composites (MMCs), ceramic matrix composites and polymer matrix
composites. Thermal properties of matrix composite materials of metals are influenced by many
parameters, for example, size and shape of particles, reinforcement distribution in the matrix,
volume fraction and matrix/reinforcement particle interface interaction [8, 9]. Not all thermal
properties but TC of MMC is mostly influenced by the microstructural parameters like void,
porosity and matrix/reinforcement interface interaction [10, 11].
In this review LM 25 alloy is discussed regarding the effect of various reinforcements on
its thermal properties (TC & CTE), commonly used synthesis technique for LM 25 alloy
composites, aging effect on thermal fatigue of the alloy and the relationship between the thermal
properties of composites.
2. CHEMICAL COMPOSITION, THERMAL AND MECHANICAL
PROPERTIES OF LM 25 ALLOY
Table 1 Chemical composition of LM-25 alloy [12]
Element Cu Mg Si Fe Mn Ni Zn Pb Sn Ti Al
% wt. 0.2 max 0.2-0.6 6.5-7.5 0.5 max 0.3 max 0.1 Max 0.1 Max 0.1 max 0.05 max 0.2 max Rest
Table 2 Thermal and mechanical properties of LM-25 alloy [12]
Properties Values
Density 2.67 g/cc
Tensile strength 234 Mpa
Hardness 79.2 BHN
Melting point 557-613 ˚C
Thermal conductivity @ 25°C 151 ˚C
CTE @ 20°C-100°C 2.15 Χ 10 ̄⁵ / ˚C
Specific heat capacity 963 J/kg ˚C

Ishfaq Ahmad Ganaie, Adarsha Hiriyanniah, Kaushik Vijaya Prasad, V. Ravinarayan and Vishnu
K. R
For a homogeneous isotropic material, the relationship between thermal properties is given
by the well-known equation;
α = k / ρc
Where, α = thermal diffusivity
k= thermal conductivity
ρ= density of the material and
c= specific heat capacity of the material.
G. J. Pearson et al. [13] showed that this equation does not hold for heterogeneous
anisotropic materials like matrix composite materials; α ≠ k / ρc.
3. SYNTHESIS TECHNIQUE
Essential processes for synthesis of AMC's at commercial or large scale can be arranged into
the following three primary categories [14];
1. Liquid state techniques
a. Stir casting
b. Infiltration process
c. Reactive preparing technique and
d. Spray statement
2. Solid state techniques:
a. PM preparing
b. Diffusion holding
3. Vapor deposition technique:
a. Physical vapor deposition
Table 3 Comparison of various MMC synthesis techniques [15]
Synthesis
Technique
Allowed size
and shape
range
Metal
yielding
Volume
fraction
range
Reinforcement
Damage
Cost
Stir casting
Wide range of
Shapes up to
500kg
Very high,
more Than
90%
Up to 0.3 No damage
Least
Expensive
Squeeze casting Limited shapes Low Up to 0.45 Severe damage
Moderately
Expensive
Powder metallurgy
Wide range,
restricted size
High
Reinforcement
Damage
Expensive
Spray casting
Limited shape,
large size
Medium 0.3 – 0.7 Expensive
Lanxride technique
Limited by pre-
form shape,
restricted size
Expensive
3.1. Stir Casting Technique-a liquid state fabrication technique
S. Ray, in 1968, took ceramic particles (Al₂O₃) and mixed into the molten aluminium by stirring
process which started the stir casting technique [16] .Among the various synthesis techniques
available for the production of matrix composites, stir casting technique is typically

acknowledged due to its simplicity, capability of commercial production and being economic.
The cost of synthesis of composites material utilizing the technique is around 33% to a half
portion of that of other techniques, and for high volume synthesis, it is anticipated that the cost
will tumble to one-tenth [17]. It permits a conventional composites synthesis course to be
utilized, hence limits the last cost of the item. MMCs are typically synthesized by Liquid
Metallurgy method or stir casting technique [18].
The synthesis of MMCs includes preparation of chosen molten matrix in a crucible kept
inside a furnace for heating. Then pouring of reinforcement particles or short fibers into the
molten matrix is done which is kept inside the crucible and getting a proper matrix-
reinforcement mixture by stirring with the help of mechanical stirrer powered by an electric
motor. The following stage is the solidification of the mixture containing suspended dispersed
particles under chosen conditions to get the proper mixture particles in the matrix.
In order to synthesize the MMCs by the stir casting technique, there are several factors
which need to be taken into account some of them are; reinforcement particles distribution in
the matrix material; reinforcement-matrix physical contact (wettability), porosity in the MMCs
and chemical reaction between the two components of MMC [19]. To get improved properties
of the MMCs, there should be proper distribution of reinforcement particles in the matrix
material because distribution tends to disturb during molten stage and during casting process.
This occurs due to density difference between the two materials.
For proper MMC synthesis, molten matrix should wet the solid reinforcement particles. The
essential means used to enhance wetting are (a) enhancing energy of the surface of
reinforcement particles (b) diminishing the surface tension of the molten matrix material, and
(c) decreasing the energy at the reinforcement-matrix interface [20, 21]. A few methodologies
have been taken to enhance the wetting of the particles with a molten matrix, including the
reinforcement coating, introducing of alloying components to the liquid matrix compound, the
particle treatment, etc. [22]. Porosity is a measure of void spaces in a material which is
undesirable and should be minimized. The ratio of the volume of this void space to the total
volume of the MMC, size of this void space and its distribution in the MMCs greatly decides
the mechanical properties of the MMCs. Porosity and other defects emerge from improper
casting process [23, 24]. It was found that the measure of gas porosity in casting depends more
on the ratio of volume of particles to the total volume of MMC than on the measure of hydrogen
gas present [25]. Chemical interactions must not happen between the reinforcement particles
and the matrix material for proper synthesis of the MMCs. For synthesis of composite material
by the technique, learning of its various working parameter are exceptionally fundamental. If
these process parameters are correctly handled, we can get the cast MMCs with improved
properties.
Figure 1 Schematic diagram of liquid phase fabrication technique (stir casting method) [56].

K. R
The critical procedure parameters are [19].
1. Stirrer Design.
2. Stirrer Speed.
3. Mixing Temperature.
4. Mixing Time.
5. Preheat temperature of support.
6. Preheat temperature of form.
7. Support sustain rate
8. Stirrer Blades.
3.1.1. Stirrer design
For vortex formation, this parameter is vital in the stir casting technique. The molten matrix
flow behavior depends on number of blades and blade angle. The stirrer is submerged till two
third profundity of liquid matrix. For uniform distribution of particles, good interface bonding
and to dodge aggregation of particles above parameters are essential. Stirrer blade angle should
be 45˚ or 60˚ and number of blades 3 to 4. There are different stirrer designs available. The one
which is quite popular is turbine stirrer. Mild steel impeller at 500-700rpm and at a depth of 2/3
of height of molten metal from the bottom can be used for stirring molten metal [55]. Proper
design of stirrer forms homogeneous suspension state with least grooves and high strength.
3.1.2. Stirrer speed
This parameter is vital to enhance wettability of reinforcement particles by the liquid matrix.
Mixing speed is responsible for the development of vortex which allows the proper mixing of
particulates in fluid metal. A stirring speed of 300-600 rpm is found to be optimum and
increases the wettability and ensures uniform distribution of particles in matrix [19].
3.1.3. Stirring temperature
We know viscosity of liquids decrease with increasing temperature so increasing temperature
above the melting point decreases its viscosity which improves wettability between the matrix
and the particulates. Good wettability is obtained by keeping temperature at 800°C.
3.1.4. Stirring Time
Stirring time should neither be too long to cause aggregation of particulates nor too short to
disturb the uniformity of distribution of the reinforcement. Stirring time used by S. S. Shinde
[19] is 5 min.
3.1.5. Reinforcement pre-heat temperature
The reinforcement is first heated to 500˚C for 40 min. to remove any moisture or gases present.
3.1.6. Mould pre-heat temperature
In order to eliminate any gases, present in the slurry, which is undesirable for porosity, the
mould is heated to about 500˚C for about an hour.
3.1.7. Reinforcement feed rate
The reinforcement feed rate should be uniform in order to avoid the agglomeration of
particulates which causes porosity and inclusion defects in the cast MMCs. The flow rate of
reinforcements measured is 0.5 gram per second [53].

3.1.8. Stirrer blades
For uniform distribution of reinforcements by stirring process, stirrer blade angle should be 45˚
or 60˚ and number of blades 3 to 4. Stirrer with two blades results in the presence of grooves of
varying sizes. Stirrer with 4 blades increases the hardness of a matrix composite although, at
the same temperature, there is increase in particle clustering which is due to homogeniety in
mixing. In case of stirrer with 5 blades there is alao increase in clustering at the same
temperature but not as much as 4 blades [54].
4. EFFECT OF REINFORCEMENTS ON THE THERMAL
PROPERTIES OF LIGHT METAL ALLOYS
4.1. Effect of alumina (Al₂O₃) particulate reinforcement on LM25 alloy
Youssef El-Kady et al. [26] worked on the effect of alumina particles on the thermal
conductivity of LM25 (A 356) alloy. Alumina particles of sizes 60nm and 200nm with volume
fraction of the reinforcement particles of 1%, 3% and 5% were dealt. The Al2O3 nanoparticles,
in general, reduce the thermal conductivity of the MMCs when matched with the LM-25 alloy.
The metal matrix composite in which 3% volume fraction of Al2O3 particles with 60nm size
were used indicated preferable thermal conductivities over those containing 200 nm
nanoparticles. The metal matrix composites containing 3% of alumina having size 60 nm and
5% with the size of 200 nm of particles indicated almost similar thermal conductivity as A356.
The decrease in the thermal conductivity of the MMCs because of the Al2O3 particles may
be due to the low thermal conductivity of the alumina particles itself and the porosity which
enhances with the volume fraction of the particles. Among different MMCs used the one with
5% volume fraction and 60nm sized nanoparticles showed most percentage reduction in thermal
conductivity of 47%. The variation of thermal conductivity with time for all the MMCs worked
with by is shown in the figure-2. Thermal conductivity varies with time initially for about 160
minutes then attains steady state condition.
The variation of thermal conductivity with different volume fraction of the reinforcement
particles is shown in the figure-3. The outcomes acquired from the work of Kady et al. [26]
demonstrated that the agglomeration percentage of the nanoparticles affects the thermal
conductivity of the MMCs. More the volume fraction of the particles of reinforcement more is
the agglomeration percentage of the particles which can prompt irritating aftereffects of the
normal thermal conductivity of the MMCs. Locations where Particles are not present in clusters
have higher thermal conductivity than those locations where particles are present in bunches.
(a) (b) (c)
Figure 2 Variation of average TC with time for a) 1%, 3% and 5% of nanoparticles of size 60nm b)
1%, 3% and 5% of nanoparticles of size 120nm (c) A356 alloy[26].

K. R
Type of synthesis technique employed and its processing conditions also affects the thermal
conductivity of the synthesized MMC. If process parameters are selected properly, this can
reduce the porosity and agglomeration of the particles.
Figure 3 Variation of thermal conductivity and percent thermal conductivity change with volume
fraction of all MMC samples [26].
4.2. Effect of silicon carbide (SiC) on LM25 alloy (by powder metallurgy)
4.2.1. Thermal conductivity (TC)
P. S. Bains et al. [27] worked on the SiC particulates of size 37μm and studied its effect on A
356 alloy synthesized through powder metallurgy technique. Three important parameters that
is the effect of volume fraction, sintering temperature and sintering time on the TC of the MMC
were studied. It was shown that the TC varies in the range of 170 to 235W/mK. Also by
increasing volume fraction percentage of SiC while keeping sintering temperature and sintering
time constant, the TC of the MMC enhances. By varying sintering temperature no significant
variation was found on the values of TC. In addition to this there is no significant change in TC
of the MMC when we vary sintering time with sintering temperature. But by varying sintering
time while keeping sintering temperature constant, the TC values change.
4.2.2. Coefficient of thermal expansion (CTE)
P. S. Bains et al. [27] also showed that CTE of the MMC varies in the range of 3.31ppm/K to
9.99ppm/K. Particles are uniformly distributed in the matrix at medium sintering temperature
and time. Particles of two different sizes i.e. fine and course SiC of 37μm and 74μm were
closely packed and used to fill the voids in the MMC. The sintering temperature and uniform
distribution of particles affect the CTE of the MMC. When the percentage of volume fraction
used was least i.e. 10-25%, the CTE of the MMC was least. When the sintering time is lowest
and combination of reinforcements i.e. more course SiC and less fine SiC, the CTE is also
minimum.
4.3. Effect of silicon carbide (SiC) on LM25 alloy (liquid pressing method)
4.3.1. Thermal conductivity (TC)
Lee et al. [28] worked on the silicon carbide particulates as a reinforcement material for the
A356 alloy and used 45% volume fraction of SiC particulates. Cast MMC was fabricated by
liquid pressing method.
The value of TC of the MMC was found to be 155W/mK with amount of pore 0.4% which
is higher than the alloy. It was also shown that porosity greatly affects the TC of the MMC. The

variation of TC with porosity is shown in the figure-4 (a) [28] along with the predicted values
by Hasselson-Johnson modal [29].
(a) (b)
Figure 4 Variation of thermal conductivity (k) and coefficient of thermal expansion (CTE) with
porosity in the MMC[28].
4.3.2. Coefficient of Thermal Expansion (CTE)
Lee et al. [28] measured the value of CTE which was in the range of 8-10ppm/K which is less
than the alloy. The data with the ROM (rule of mixture) and Turner`s modal was compared and
found that measured values lie between the values as calculated by the two modals shown in
the figure-4 (b). The CTE does not vary significantly with the porosity in the MMC [30].
4.4. Effect of nickel (Ni) reinforcement on LM25 alloy
ALU (2012) report [31] shows that the TC of the MMC reduces nearly linearly with increasing
Ni content in the matrix. The measured values of the TC of the MMC were plotted against
weight % of Ni in the matrix. The variation of TC of the MMC is shown in figure-5 (a) [31].
TC of the same MMC was also calculated using the models of Maxwell [32] and Hashin-
Shriktman (HS) [33]. Both the measured values and the calculated values are also plotted
against each other and are shown in the figure-5 (b) [31]. The calculated values of TC were also
plotted against wt. % of Ni and the variation is shown in the figure-5 (c) [31] which shows the
comparison among the measured TC values and the calculated TC values by the two models.
(a) (b) (c)
Figure 5 Variation of (a) measured thermal conductivity values with Ni content (b) measured TC
values with calculated values and (c) comparison between measured TC values and the two
models[31].

K. R
4.5. Effect of silicon carbide (SiC) on LM25 alloy ( vaccum hot pressing)
Jayakumar et al. [34] worked on SiC particulate reinforcement with volume fraction of
5%,10%,15% and 20%. The CTE of the alloy is around 19.3 Χ 10 ̄⁶ /˚C and the corresponding
values for CTE were calculated and was found that CTE decreases with increase in volume
fraction of SiC particulates. The CTE values of the experiment are tabulated and shown in the
table-4.The CTE depends on matrix percentage, reinforcement percentage and porosity in the
MMC. For constant porosity CTE decreases with increase in SiC content. It was also observed
that MMC with 5% vol of SiC particulates has most porosity and hence from the table it is clear
that porosity also tend to decrease CTE.
Table 4 CTE of Al alloy and MMC
MMCs CTE(μm/m˚C)
Al alloy
Alalloy/5%SiC
Al alloy/10%SiC
Al alloy/15%SiC
Al alloy/20%SiC
19.3
17.4
16.0
16.4
16.1
4.6. Effect of bonding thermal cycle, solution and aging treatment on SiC MMC
Xu Zhiwu et al. [35] worked on an MMC in which matrix material was A 356 alloy and the
reinforcement particulates were of SiC. CTE of A 356/ SiC composites and their joints was
dealt after subjecting to thermal bonding cycle (BTC) and heat treatment processes. The MMC
was synthesized using stir casting technique where 20% vol and 12.6μm average particle size
of SiC was used. The MMC was then subjected to VLP diffusion bonding process for 15
seconds in air at different temperatures. Solution treatment was given to the samples before and
after the bonding process and also aging treatment was given after the bonding process. In 20-
100˚C temperature range, CTE values were measured at 5 ˚C/min. Various processes through
which the samples were subjected are shown in the figure-6.
(a)
(b)
Figure 6 Flow charts (a) Heat treatment for base metal (b) Bonding and heat treatment of the VL
bonded joints as discussed in ref.[35]

Table 5 CTE of the MMC after (a) bonding thermal cycle (b) BTC, solution & aging treatment
(a)Peak temperatute of bonding thermal cycle (˚C) CTE (μm/m ˚C)
20
460
500
530
16.015
16.026
15.854
15.951
(b)Peak temperature of Bonding thermal cycle (˚C) CTE (μm/m ˚C)
460
500
530
15.551
15.556
15.561
There is a slight change in CTE of the MMC after subjecting to solution treatment and heat
treatment as shown in the table-5 (a). And little effect on CTE values due to bonding thermal
cycle as shown in the table-5 (b). CTE of the VLP bonded joints of the MMC decrease with
increasing bonding temperature and is lower than that of the MMC material itself.
4.7. Effect of quarry dust reinforcement on LM25 alloy
Ramesh et al. [36] worked on the effect of quarry dust on the TC of the A356 alloy. Vol % of
5%, 7.5% and 10% was studied. TC of the MMC specimen was measured by guarded hot plate
method. With increase in temperature of the MMC, TC also increases which is shown in the
figure-7 [36]. The TC of the MMC decreases with increaes in vol. % of quarry dust this is
caused by the scattering of energy carriers (electrons and phonons). Using different particle
sizes also affect the TC of an MMC [37] which is the another cause. The same work showed
that for the specimen with 7.5% vol. % of quarry dust reduction in TC is 13.4% than it is for
A356 alloy which is because of having 5.2% porosity in the MMC sample.
Figure 7 Variation of TC with Temperature for the alloy and the composite[36].
4.8. Effect of borassus flyash reinforcement
Karthick selvam et al. [38] worked on the borassus flyash reinforcement in the matrix of A356
alloy and studied the effect on thermal conductivity of the MMC. Vol. % of 4% was used and
the sample was subjected to different test loading conditions as shown in the table-6. The TCs
calculated are 186, 205.69 and 242.91 W/mK for different temperature differences. From the
table it is clear that TC of the MMC material with 4% flyash increases with increase in
temperature across the samples.

K. R
Table 6 Loading conditions of TC test for 4% borassus flyash
S.No. T₁ (K) T₂ (K) V (volt) I (Amp) Q (W) K (W/Mk)
1. 43.5 41.3 161 0.51 41.05 186
2. 45.2 43.1 161 0.51 41.055 205.69
3. 46.3 44.5 165 o.53 43.725 242.91
TC values are calculated from the Fourier heat conduction equation; k = Q L / A ΔT
Where, k= thermal conductivity
Q= heat transfer rate
L= thickness of the material
A= surface area of the material and
ΔT= temperature difference across the ends.
4.9. Effect of carbon nanotube reinforcement on Al-12 wt.% Si
CNT have high thermal conductivity which makes them good for electronic packaging (40).
Individual single-walled CNT have 6600W/mK (41) and individual multi-walled CNT have
3000W/Mk (42,43).
Srinivasa R. Bakshi et. al. (40) worked with the carbon nanotubes. Al-12 wt.% Si containing
10 wt.% multiwall carbon nanotubes (CNTs). Object oriented finite element method was used.
The thermal conductivity values were measured experimentally at 50°C. It was found that the
overall thermal coductivity of the composite decreases largely. This very large reduction is due
to presence of CNT clusters because CNT clusters have thermal conductivity three times less
than individual CNTs.
4.10. Effect of diamond particles reinforcement on 5056 type Al-Mg alloy
Kiyoshi Mizuuchi et. al. (44) worked on effect of diamond particle dispersed aluminium matrix
composite fabricated in solid-liquid coexistent state by spark plasma sintering process (SPS).
5056 type Al-Mg alloy was dealt and the thermal conductivity of the matrix composite was
examined. It was found that the maximum thermal conductivity reached to 403W/mK at 45.5%
volume fraction of diamond particles. Relative packing density of the diamond matrix was 97%
or higher between volume fraction of 25.5% to 45.5%. Beyond that it the thermal conductivity
again decreases which is due to reduction in relative packing density of the diamond matrix
composite. They compared the values with the experimental values of Johnson and Sonuparlak
(45) which is 259W/mK at 50% volume fraction and with Chen et. al. (46) which is 240W/mK
at 60% vol.
The difference in the values can be due to; (1) the above metal matrix composite can be
created at a temperature less than the melting point of pure Al metal by the above process which
reduces the chances of diamond particles damage. (2) evaporation and condensation are the
bonding mechanisms in the above SPS process. The evaporation is instant which occurs due to
the flow of high current because of skin effects (47, 48) of spark discharge followed by
condensation process.
Different values of thermal conductivity are shown in the following figure [44] againt the
volume fractions.

Figure 8 Experimental data and theoritical curves from values obtained using Maxwell–Eucken
equation[44].
4.11. Effect of SiC fine particles reinforcement on AA6061 aluminium alloy
Yibin Xua et. al. (49) worked on the effect of SiC fine particles on the AA6061 Al alloy. 0% ,
10%, 20%, 30% and 40% volume fraction of the reinforcement named as AMC600, AMC610,
AMC620, AMC630 and AMC640 respectively was used. The high energy mixing powder
metallurgy technique was used to fabricate the specimens.
Thermal conductivity of a light metal alloy should expectedly enhance due to the addition
of higher thermal conductivity dispersed particles. But mostly opposite happens and it reduces.
There are various reasons behind this. One of the main reason is interfacial thermal resistance
which increases especially with increase in volume fraction.Also strength of the MMC also
decreases with increase in reinforcement size (50,51). Interfacial thermal resistance was also
dealt and it was found that it is less at lower volume fractions. Adhersion of particles is good
at lower percentages and adhersion becomes bad at higher values.They even noticed a slight
enhancement in thermal conductivity at lower volume fractions. The experimental values found
were compared with the theoritical modal of Maxwell and it was found that the difference is
initially very small but then increases as the graph goes opposite to the modal as shown in the
following graph [49].The main reason is increasing interfacial thermal resistance at higher
volume percentage values.
Figure 9 Variation of thermal conductivity with volume fraction of SiC fine particles using Maxwell`s
expression and experimental values[49].

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4.12. Effect of reinforcement of graphite flakes on Al 2024 alloy
Valerio Oddone et. al. (52) worked on the effect of graphite flakes as on thermal properties of
light metal alloys. The MMC was prepared by spark plasma sintering method. The in-plane
thermal conductivity of the Al alloy increases with the volume percentage of the graphite and
becomes equal to that of sintered copper at the 50%. The variation is shown in the graph (a)
[52] below.
Coefficient of thermal expansion decreases with the vol.% of graphite. Through-plane CTE
decreases more than in-plane CTE. The variations are shown in the following graph (b) [52].
(a) (b)
Figure 10 Variation of (a) k and (b) CTE of Al2024/graphite MMC with vol. % of graphite[52].
5. THERMAL FATIGUE (LOW CYCLE) OF LM-25 ALLOY
Tsuyosh et al. [39] worked on the low cycle thermal fatigue of LM-25 alloy. The sample was
first heat treated using solution annealing process at 530°C for four and a half hour then
quenching process was done in water followed by aging treatment for one hour at 200°C. The
alloy sample was then subjected to extra aging treatment at 250°C for different durations of
one, ten and hundred hours. Various test conditions are shown in the table where one cycle is
of ten minutes.
Table 7 Conditions and results of the thermal fatigue test.
Test No.
Strain
Range Δϵ
(%)
Temperature
Range ΔT
(°C )
Peroidic
Time (min)
Aging
Time (hr)
Aging
Temperature
(°C)
No.of
cycls to
fatige
failue (Nf)
Time To
Fatige
Failure (Tf)
1 ±0.7 250-100 10 0 250 97 58200
2 ±0.5 250-100 10 0 250 530 318000
3 ±0.5 250-100 10 1 250 702 421200
4 ±0.6 250-100 10 1 250 480 288000
5 ±0.8 250-100 10 10 250 205 123000
6 ±0.5 250-100 10 10 250 870 522000
7 ±0.65 250-100 10 100 250 472 283200
8 ±0.5 250-100 10 100 250 1700 1020000
Decreasing strain range and increasing aging time elongates fatigue life as shown in the
figure-8 (a) [39] which is due to the fact that tempering affects more due to longer aging time.It
can be infered from the table that if the aging is more fatigue life is longer irrespective of large
strain amplitude. Also from figure-8 (b) [39] it can be seen that aging time increment from 10

to 100 hr has less effect on increasing fatigue life than from 0 to 10 hr. So, due to T6 heating
followed by aging, fatigue life is increased but strength is decreased.
(a) (b)
Figure 8 (a) Total strain vs no. of cycles to fatigue failure (b) no. of cycles to failure vs aging time
plots[39].
6. CONCLUSION
Among the various methods of synthesis of MMCs of LM-25 alloy, the stir casting method is
simple, economical, allows wide range of shapes of particulates and causes no damage to
reinforcement. Alumina particulates reinforcement in general decrease TC of the LM-25 alloy.
In P/M synthesis method of LM-25/SiC system, vol. % and sintering time affects the TC of the
system. At constant sintering temperature and sintering time vol. % increases the TC of the
alloy. In liquid pressing synthesis method of LM-25/SiC system with vol. % of 45% enhances
the TC and reduces the CTE of the MMC. The reinforcement of Ni reduces the TC of the alloy
as its wt. % is increased and the experimentally measured values are more close to HS model
than Maxwell`s model. CTE of LM-25/SiC system, synthesized through vaccum hot pressing
method, decreases with increase in SiC content at constant porosity in the MMC. Heat and
solution treatments slightly change CTE of the LM-25/SiC system and increasing bonding
temperature reduces the CTE of the joints of MMC even lower than the MMC itself. TC of the
alloy reduces with increase in quarry dust content. This is due to scattering of heat carriers,
porosity and different sizes of the particles. TC of the alloy reinforced with 4% volume fraction
of the borassus flyash enhances with increase in temperatures across the MMC. The TC of Al-
12 wt.% Si composite decreases largely due to the presence of CNT clusters. The maximum
thermal conductivity of 5056 type Al-Mg alloy reaches to 403W/mK at 45.5% volume fraction
of diamond particles and then again decreases. Interfacial thermal resistance increases with
increase in volume percentage of reinforc ement which happens because adhersion of particles
at higher percentage becomes weak. TC of 2024 Al alloy increases with increase of graphite
flake percentage and its CTE decreases. Due to T6 heating followed by aging treatment
lengthens the thermal fatigue life but reduces the strength. Increasing the aging time lengthens
the fatigue life of the alloy.

K. R
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REVIEW ON EFFECT OF VARIOUS REINFORCEMENTS ON THE THERMAL PROPERTIES OF LM-25 AL ALLOY AND SOME COMMON LIGHT METAL ALLOYS

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