REVIEW ON EFFECT OF VARIOUS …PROPERTIES OF LM-25 AL ALLOY AND SOME COMMON LIGHT METAL ALLOYS Ishfaq Ahmad Ganaie, Adarsha Hiriyanniah, Kaushik Vijaya Prasad, V. Ravinarayan School

http://www.iaeme.com/IJMET/index.asp 143 [email protected]

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.


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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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Documents

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