Properties of Self-Lubricating Silicon Carbide Particle Reinforced

Aluminium – Silicon – Magnesium Alloy Hybrid Composites

By

Mohammad Ismail

Student Number: 040811102F

This thesis is submitted to the Department of Materials and Metallurgical Engineering (MME)

in partial fulfillment of the requirements for the degree of Master of Philosophy in Material

Science (M.Phil (Mat. Sci.))

Department of Material and Metallurgical Engineering

Bangladesh University of Engineering and Technology, Dhaka

March 2015

ii

Candidate’s Declaration

It is hereby declared that this thesis paper or any part of it has not been submitted anywhere

else for the award of any degree.

Mohammad Ismail

iii

iv

Dedication

Hazrat Mohammad (SM)

v

Acknowledgments

I wishes to express his profound gratitude to his thesis supervisor Dr. A. K. M. Bazlur Rashid,

Professor, Department of Materials and Metallurgical Engineering of Bangladesh University

of Engineering and Technology, Dhaka for his guidance in carrying out this research work as

well as writing this thesis.

I would like to express his gratitude to Dr. Ahmed Sharif, Professor and Head, Department of

Materials and Metallurgical Engineering and Dr. Md. Fakhrul Islam, Professor and Head,

Department of Glass and Ceramic Engineering of Bangladesh University of Engineering and

Technology, Dhaka for providing him with the laboratory facilities for this work.

Sincere thanks are also due to Mr. Mehedi Hasan Rizvi, Mr. Md. Faruk Hossain and Mr. Md.

Abdullah Al Maksud, Mr. Wasim, Mr. ShamsulAlam and and Mr. Ahmedullah for their help

during conduction of experiments for this work.

Above all, I would like to thank all my family members (M. A. Jabber, Jobeda Akther, M.

Jalal Uddin, M. Mohiuddin Khan) and friends, family friends (Prof. Rezaul Karim Ph.D

Fellow UK, Mr Hedayt Islam DMD NICl, Mr. Iqbal Karim AMD IIBL) whose direct indirect

support helped me completing my thesis in time.

Last but not the least, I would like to thank my mother (Mrs. Nur Nahar Begum) for her

undying support, encouragement and endless love throughout my life.

Mohammad Ismail

vi

Abstract

A stir casting process was used to fabricate aluminum – 7% silicon – 0.4% magnesium alloy

(LM25) composites reinforced with various volume fractions of silicon carbide (SiC) and

graphite (G) particulates. Both optical and scanning electron microscopy were used to study

the microstructural characterization of the materials. Hardness, tensile and wear properties of

the unreinforced LM25 alloy and three hybrid composites were examined. It was observed

that the addition of both SiC and G reinforcing particles to LM25 alloys caused a general

increase in the properties, although the properties of the composites were predictably affected

mostly by the hard reinforcing SiC particles. The hardness, tensile strength and resistance to

wear were found to be the maximum in case of HC2 (LM25-5SiC-0G) composite. Addition of

G particles somewhat lowered the tensile properties, while a mixed result was obtained in

case of hardness. The wear resistance was decreased due to the addition of G particles. The

HC1 (LM25-2.5SiC-5G) hybrid composites had the intermediate properties of the two

composites HC2 and HC3 where SiC and G particles were used in isolation. The age

hardening response of all four materials was predictable. Hardness values were generally

increased in case of all materials.

vii

Table of Contents

Acknowledgements ................................................................................................................... v

Abstract ..................................................................................................................................... vi

1. Introduction ..................................................................................................................... 1

2. Theoretical background ................................................................................................. 2

2.1 Aluminum alloy ................................................................................................................ 3

2.1.1 Aluminium alloy systems designation .................................................................. 4

2.1.2 Temper designations ............................................................................................. 7

2.2 Aluminium-Silicon-Magnesiumalloys ............................................................................ 11

2.2.1 Introduction ......................................................................................................... 12

2.2.2 Properties of Al-7Si-Mg (LM25) alloys ............................................................. 18

2.2.3 Heat treatment of LM25alloys ............................................................................ 18

2.3 LM25-Silicon carbide-Graphite hybrid composites ....................................................... 22

2.3.1 Introduction ......................................................................................................... 22

2.3.2. Processing of LM25-SiC-G hybrid composites ................................................. 24

2.3.2.1 Stir casting fabrication method ........................................................... 25

2.3.2.2 Silicon carbide as reinforcement ........................................................ 28

2.3.2.3 Graphite as reinforcement .................................................................. 29

2.4 Literature review ............................................................................................................. 29

2.4.1 Summary of literature review ............................................................................. 33

2.5 Scope of present work ..................................................................................................... 34

viii

3. Experimental procedure ............................................................................................... 35

3.1 Introduction ..................................................................................................................... 36

3.2 Preparation of LM25 master alloy .................................................................................. 36

3.2.1 Raw materials ...................................................................................................... 36

3.2.2 Melting and casting ............................................................................................. 36

3.3 Preparation of LM25-SiC-G hybrid composite .............................................................. 37

3.4 Heat treatment of LM2S-SiC-G hybrid composites ....................................................... 39

3.5 Microstructural observations .......................................................................................... 39

3.6 Hardnessmeasurement .................................................................................................... 39

3.7 Tensile testing ................................................................................................................. 40

3.8 Wear testing .................................................................................................................... 40

4. Results and discussion .................................................................................................. 42

4.1 Manufacture of LM25-SiC-G hybrid composite ............................................................ 43

4.2 Microstructural observations .......................................................................................... 47

4.3 Hardness measurement ................................................................................................... 50

4.4 Tensile properties ............................................................................................................ 51

4.5 Wear properties ............................................................................................................... 52

4.6 Effect of heat treatment ................................................................................................... 55

5. Conclusions .................................................................................................................... 56

5.1 Conclusions ..................................................................................................................... 57

5.2 Suggestions for future work ............................................................................................ 57 List of Table…………………………………………………………………………..............58 List of Figure …………………………………………………………………………………59 References ............................................................................................................................... 60

1

1 Introduction

Aluminium based metal matrix composites (MMCs) have been a very valuable addition to the

field of newer materials for high performance tribological applications. Aluminium

composites based on 2024, 6061, and 7075 wrought alloys and A356 and A357 cast alloys are

being increasingly used in automobile, aerospace, marine and mineral processing industries

owing to their improved specific strength, good wear resistance, higher thermal conductivity

and lower coefficient of thermal expansion. The widely used reinforcing materials for these

composites are silicon carbide (SiC), aluminium oxide (Al2O3) and graphite (Gr) in the form

of particles or whiskers. Aluminium-matrix composites are normally fabricated by liquid

casting or powder metallurgy technique.

Generally lubricant is externally added to reduce wear. This poses problem when the material

needs periodic application of lubricant particularly to wear parts which are difficult to wear.

For such applications self-lubricating materials are preferred. Graphite is one of the most

commonly used lubricant materials, although a loss of strength due to the addition of solid

graphite in aluminium is often reported.

In the present work investigation aluminium-7Silicon-0.4Magnesium (A356 or LM25) alloys

based hybrid composites will be prepared using a hard reinforcing agent (silicon carbide

particles) and a soft reinforcing agent (graphite particles) reinforcing agents using stir casting

method. Then the mechanical and wear properties of the manufactured hybrid composites will

be determined and the effectiveness of the combined addition of these two reinforcement will

be investigated by comparing with the properties of LM25 master alloy.

2

2 Theoretical Background

2. Theoretical background ............................................................................................... 2

2.1 Aluminum alloy ............................................................................................................... 3

2.1.1 Aluminium alloy systems designation ................................................................. 4

2.1.2 Temper designations ............................................................................................ 7

2.2 Aluminium-Silicon-Magnesium alloys .......................................................................... 11

2.2.1 Introduction ........................................................................................................ 12

2.2.2 Properties of Al-7Si-Mg (LM25) alloys ............................................................ 18

2.2.3 Heat treatment of LM25 alloys .......................................................................... 18

2.3 LM25-Silicon carbide-Graphite hybrid composites ...................................................... 22

2.3.1 Introduction ........................................................................................................ 22

2.3.2 Processing of LM25-SiC-G hybrid composites ................................................. 24

2.3.2.1 Stir casting fabrication method .......................................................... 25

2.3.2.2 Silicon carbide as reinforcement ....................................................... 28

2.3.2.3 Graphite as reinforcement ................................................................. 29

2.4 Literature review ............................................................................................................ 29

2.4.1 Summary of literature review ............................................................................ 33

2.5 Scope of present work .................................................................................................... 34

3

2.1 Aluminium Alloys

Aluminium alloys are alloys in which aluminium is the predominant metal. Several properties

set aluminium alloys apart from other metals and alloys. First, aluminium is lighter than all

other engineering metals except magnesium and beryllium. It has a density of about 2700

kg/m3. A second important property of aluminium is its thermal and electrical conductivity. It

has about 60% of the conductivity of pure copper (IACS). Because of its lower density,

aluminium has a higher conductivity than copper per unit mass. The third property that is

responsible for the wide use of aluminium alloys is their corrosion resistance. Aluminium is

not widely used for chemical resistance, but for applications involving atmospheric corrosion

resistance it is probably the most widely used metallic material. Architectural applications of

aluminium are everywhere – railing, windows, frames, doors, flushing, and so on.

The mechanical, physical, and chemical properties of aluminum alloys depend upon

composition and microstructure of the alloy. The addition of selected elements to pure

aluminum greatly enhances its properties and usefulness. Because of this, most applications

for aluminum utilize alloys having one or more elemental additions. The major alloying

additions used with aluminum are copper, manganese, silicon, magnesium, and zinc. The total

amount of these elements can constitute up to 10 wt.% of the alloy composition. Impurity

elements are also present, but their total percentage is usually less than 0.15 wt.% in

aluminum alloys.

It is convenient to divide aluminum alloys into two major categories: wrought compositions

and cast compositions. A further differentiation for each category is based on the primary

mechanism of property development. Many alloys respond to thermal treatment based on

phase solubilities. These treatments include solution heat treatment, quenching, and

precipitation, or age, hardening. For either casting or wrought alloys, such alloys are

described as heat treatable. A large number of other wrought compositions rely instead on

work hardening through mechanical reduction, usually in combination with various annealing

procedures for property development. These alloys are referred to as work hardening. Some

casting alloys are essentially not heat treatable and are used only in as-cast or in thermally

modified conditions unrelated to solution or precipitation effects.

4

About 85% of aluminium is used for wrought products, for example rolled plate, foils and

extrusions. Cast aluminium alloys yield cost-effective products due to the low melting point,

although they generally have lower tensile strengths than wrought alloys. The most important

cast aluminium alloy system is Al–Si, where the high levels of silicon (4.0–13%) contribute to

give good casting characteristics. Aluminium alloys are widely used in engineering structures

and components where light weight or corrosion resistance is required.

Alloys composed mostly of aluminium have been very important in aerospace manufacturing

since the introduction of metal skinned aircraft. Aluminium-magnesium alloys are both lighter

than other aluminium alloys and much less flammable than alloys that contain a very high

percentage of magnesium.

Aluminium alloy surfaces formulate a white, protective layer of aluminium oxide if left

unprotected by anodizing and/or correct painting procedures. In a wet environment, galvanic

corrosion can occur when an aluminium alloy is placed in electrical contact with other metals

with more negative corrosion potentials than aluminium, and an electrolyte is present that

allows ion exchange. Referred to as dissimilar metal corrosion this process can occur as

exfoliation or intergranular corrosion. Aluminium alloys can be improperly heat treated. This

causes internal element separation and the metal corrodes from the inside out. Aircraft

mechanics deal daily with aluminium alloy corrosion.

2.1.1 Aluminium alloy systems designation

Systems for designating aluminum and aluminum alloys that incorporate the product form

(wrought, casting, or foundry ingot), and its respective temper (with the exception of foundry

ingots, which have no temper classification) are covered by American National Standards

Institute (ANSI) standard H35.1. The Aluminum Association is the registrar under ANSI

H35.1 with respect to the designation and composition of aluminum alloys and tempers

registered in the United States.

Cast and wrought alloy nomenclatures have been developed. The Aluminum Association

system is most widely recognized in the United States. Their alloy identification system

employs different nomenclatures for wrought and cast alloys, but divides alloys into families

for simplification.

5

A four-digit numerical designation system is used to identify wrought aluminum and

aluminum alloys. As shown in Table 2.1 (ASM International, 1990), the first digit of the four-

digit designation indicates the group, while the second digit indicates mill control or lack of

same on specific elements. The last two digits have no significance, except in the 1xxx series

they coincide with aluminium content above 99% in hundredths. A 1040 alloy has, thus,

99.4% aluminium. The complete specification of wrought aluminium alloys involves a suffix

that indicates the degree of cold work or thermal treatments.

Table 2.1: Alloy designation system for wrought aluminium alloys

AA Series Description and typical uses of the alloy series

1xxx Controlled unalloyed (pure) composition (Al ≥ 99.00%), used primarily in the electrical and chemical industries

2xxx Alloys in which copper is the principal alloying element, although other elements, notably magnesium, may be specified. 2xxx-series alloys are widely used in aircraft where their high strength (yield strengths as high as 455 MPa, or 66 ksi) are valued.

3xxx Alloys in which manganese is the principal alloying element, used as general-purpose alloys for architectural applications and various products

4xxx Alloys in which silicon is the principal alloying element, used in welding rods and brazing sheet

5xxx Alloys in which magnesium is the principal alloying element, used in boat hulls, gangplanks, and other products exposed to marine environments

6xxx Alloys in which magnesium and silicon are the principal alloying elements, commonly used for architectural extrusions

7xxx Alloys in which zinc is the principal alloying element (although other elements, such as copper, magnesium, chromium, and zirconium, may be specified), used in aircraft structural components and other high-strength applications. The 7xxx series are the strongest aluminum alloys, with yield strengths 500 possible.

8xxx Alloys characterizing miscellaneous compositions. The 8xxx series alloys may contain appreciable amounts of tin, lithium, and/or iron.

9xxx Reserved for future use

Wrought alloys that constitute heat-treatable (precipitation-hardenable) aluminum alloys

include the 2xxx, 6xxx, 7xxx, and some of the 8xxx alloys. The various combinations of

alloying additions and strengthening mechanisms used for wrought aluminum alloys are

shown in Table 2.2 (ASM International, 1993).

6

Table 2.2: Classification of wrought aluminum alloys based on strengthening mechanism.

Work-hardenable alloys: Precipitation-hardenable alloys:

Pure Al 1xxx Al-Cu 2xxx

Al-Mn 3xxx Al-Cu-Mg 2xxx

Al-Si 4xxx Al-Cu-Li 2xxx

Al-Mg 5xxx Al-Mg-Si 6xxx

Al-Fe 8xxx Al-Zn 7xxx

Al-Fe-Ni 8xxx Al-Zn-Mg 7xxx

Al-Zn-Mg-Cu 7xxx

Al-Li-Cu-Mg 8xxx

Casting compositions are described by a three-digit system followed by a decimal value. The

first digit indicates the alloy group. The second and third digits identify an alloy within a

group and the digit after the decimal indicates the product form. The decimal .0 in all cases

pertains to casting alloy limits. Decimals .1, and .2 concern ingot compositions, which after

melting and processing should result in chemistries conforming to casting specification

requirements. Alloy families for casting compositions are given in Table 2.3 (ASM

International, 1993).

Table 2.3: Alloy designation system for cast aluminium alloys.

AA series Description and typical uses of the alloy series

1xx.x Controlled unalloyed (pure) compositions, especially for rotor manufacture

2xx.x Alloys in which copper is the principal alloying element. Other alloying elements may be specified.

3xx.x Alloys in which silicon is the principal alloying element. The other alloying elements such as copper and magnesium are specified. The 3xx.x series comprises nearly 90% of all shaped castings produced.

4xx.x Alloys in which silicon is the principal alloying element.

5xx.x Alloys in which magnesium is the principal alloying element.

6xx.x Unused

7xx.x Alloys in which zinc is the principal alloying element. Other alloying elements such as copper and magnesium may be specified.

8xx.x Alloys in which tin is the principal alloying element.

9xx.x Unused

7

A modification of an original alloy, or of the impurity limits for unalloyed aluminum, is

indicated by a serial letter preceding the numerical designation. The serial letters are assigned

in alphabetical sequence starting with A but omitting I, O, Q, and X, the X being reserved for

experimental alloys. Explicit rules have been established for determining whether a proposed

composition is a modification of an existing alloy or if it is a new alloy. Heat-treatable casting

alloys include the 2xx, 3xx, and 7xx series.

2.1.2 Temper designation

The temper designation system used in the United States for aluminum and aluminum alloys

is used for all product forms (both wrought and cast), with the exception of ingot. The system

is based on the sequences of mechanical or thermal treatments, or both, used to produce the

various tempers. The temper designation follows the alloy designation and is separated from it

by a hyphen. Basic temper designations consist of individual capital letters. Major

subdivisions of basic tempers, where required, are indicated by one or more digits following

the letter. These digits designate specific sequences of treatments that produce specific

combinations of characteristics in the product. Variations in treatment conditions within major

subdivisions are identified by additional digits. The conditions during heat treatment (such as

time, temperature, and quenching rate) used to produce a given temper in one alloy may differ

from those employed to produce the same temper in another alloy. Table 2.4 to 2.6 show the

temper designation systems used for aluminium alloys (ASM International, 1990).

Generally, the letter H is followed by one, two or three digits to indicate the type of cold

working (strain hardening), which is again followed by numbers 1 to 8 (where a 1 indicates

the smallest amount of cold work and an 8 indicates maximum cold work or full-hard

conditions). The letter T is followed by one, two or three digits to indicate various thermal

treatments. A complete temper designation systems used for wrought and cast aluminium

alloys is shown in Fig. 2.1.

8

Table 2.4: Basic temper designations.

F, As-Fabricated This is applied to products shaped by cold working, hot working, or casting processes in which no special control over thermal conditions or strain hardening is employed. For wrought products, there are no mechanical property limits.

O, Annealed O applies to wrought products that are annealed to obtain lowest-strength temper and to cast products that are annealed to improve ductility and dimensional stability. The O may be followed by a digit other than zero.

H, Strain-Hardened (wrought products only)

This indicates products that have been strengthened by strain hardening, with or without supplementary thermal treatment to produce some reduction in strength. The H is always followed by two or more digits.

W, Solution Heat-Treated (age hardens naturally)

This is an unstable temper applicable only to alloys whose strength naturally (spontaneously) changes at room temperature over a duration of months or even years after solution heat treatment. The designation is specific only when the period of natural aging is indicated (for example, W h).

T, Solution Heat-Treated

This applies to alloys whose strength is stable within a few weeks of solution heat treatment. The T is always followed by one or more digits

Table 2.5: Temper designation system for strain-hardened products.

H1, Strain-Hardened Only This applies to products that are strain hardened to obtain the desired strength without supplementary thermal treatment.

H2, Strain-Hardened and Partially Annealed

This pertains to products that are strain-hardened more than the desired final amount and then reduced in strength to the desired level by partial annealing

H3, Strain-Hardened and Stabilized

This applies to products that are strain-hardened and whose mechanical properties are stabilized by a low-temperature thermal treatment or as a result of heat introduced during fabrication

H4, Strain-Hardened and Lacquered or Painted

This applies to products that are strain-hardened and that are also subjected to some thermal operation during subsequent painting or lacquering

Additional temper designations:

Digit 1 to 8, following the designation H1, H2, H3, and H4, are used to indicate the degree of strain-hardening as identified by the minimum value of the ultimate tensile strength. The numeral 8 has been assigned to the hardest tempers normally produced.

xxxx-Hx2 Quarter-hard

xxxx-Hx4 Half-hard

xxxx-Hx6 Three-quarter-hard

xxxx-Hx8 Full-hard

9

Table 2.6: Temper designation system for heat-treatable alloys.

T1, Cooled from high-temperature shaping process and naturally aged

Products are not cold-worked after high-temperature shaping process (e.g., casting, extrusion) and mechanical properties are stabilized by natural ageing

T2, Cooled from high-temperature shaping process, cold-worked and naturally aged

Products are cold-worked to improve strength after high-temperature shaping process (e.g., rolling, extrusion) and mechanical properties are stabilized by natural ageing

T3, Solution treated, cold-worked and naturally aged

Products are cold-worked to improve strength after solution heat-treatment and mechanical properties are stabilized by natural ageing

T4, Solution treated and naturally aged

Products are not cold-worked after solution heat-treatment and mechanical properties are stabilized by natural ageing

T5, Cooled from high-temperature shaping process and artificially aged

Products are not cold-worked after high-temperature shaping process (e.g., casting, extrusion) and mechanical properties are substantially improved by precipitation heat-treatment

T6, Solution treated and artificially aged

Products are not cold-worked after solution heat-treatment and mechanical properties and/or dimensional stability are substantially improved by precipitation heat-treatment

T7, Solution treated and over-aged or stabilized

Applied to wrought products that are precipitation heat-treated beyond the point of maximum strength to provide special characteristic, such as enhanced resistance to stress-corrosion cracking

T8, Solution treated, cold-worked and artificially aged

Products are cold-worked to improve strength after solution heat-treatment and mechanical properties and/or dimensional stability are substantially improved by precipitation heat-treatment

T9, Solution treated artificially aged and cold-worked

Products are cold-worked specifically to improve strength after solution heat-treatment and precipitation heat-treatment

T10, Cooled from high-temperature shaping process, cold-worked and artificially aged

Products are not cold-worked specifically to improve strength after high-temperature shaping process (e.g., rolling, extrusion) and mechanical properties are substantially improved by precipitation heat-treatment

Additional temper designations:

When it is desirable to identify a variation of one of the ten major T tempers described above, additional digits, the first of which cannot be zero, may be added to the designation. Specific sets of additional digits have been assigned to the following wrought products that have been stress relieved by stretching, compressing, or a combination of stretching and compressing:

xxxx-T51 Stress relieved by stretching with no further straightening after stretching (applied to rolled products)

xxxx-T510 Stress relieved by stretching with no further straightening after stretching (applied to extruded products)

xxxx-T511 Stress relieved by stretching with minor straightening after stretching (applied to extruded products)

xxxx-T52 Stress relieved by compression to produce a permanent set of 1-5 %

xxxx-T54 Stress relieved by combined stretching and compression

10

Figure 2.1: Detail of aluminium alloy temper designation system.

11

2.2 Aluminium – Silicon – Magnesium alloys

Aluminum casting alloys are the most versatile of all common foundry alloys and generally

have the highest castability ratings. As casting materials, aluminum alloys have the following

favorable characteristics (ASM International, 1998):

• Good fluidity for filling thin sections

• Low melting point relative to those required for many other metals

• Rapid heat transfer from the molten aluminum to the mold, providing shorter casting

cycles

• Hydrogen is the only gas with appreciable solubility in aluminum and aluminum alloys,

and hydrogen solubility in aluminum can be readily controlled by processing methods.

• Many aluminum alloys are relatively free from hot-short cracking and tearing tendencies.

• Chemical stability

• Good as-cast surface finish with lustrous surfaces and little or no blemishes

Aluminum alloy castings are the cost-effective answer to many needs and problems in

construction of machines, equipment, appliances, vehicles and structures, usually serving a

primarily mechanical function, but often combining this with an appearance or decorative

function. This requires cast parts in a great variety of geometric configurations, frequently

combining several different basic forms in an integral or monolithic piece. These parts include

covers and housings, which may be ribbed for reinforcement or finned for heat conduction or

dissipation; frames and boxlike parts; cylindrical or spherical tanks for containment of gases

or fluids; brackets; pistons; wheels; disks; impellers; bulkheads; and clamps. The list of forms

produced is nearly endless, and many such parts have a multitude of cored holes in bosses for

fastening or as passage for fluids.

Aluminium casting is dominated by the automotive industry. Roughly two thirds of all

aluminium castings are used in automotive industries where the use of aluminum castings

continues to grow at the expense of iron castings. Although aluminium castings are

significantly more expensive than ferrous castings, there is a continuing market requirement

to reduce vehicle weight and to increase fuel efficiency. It is this requirement which drives the

replacement of ferrous parts by aluminium. Melting point of Al is 660°C and is light weight,

12

density is about 1/3 that of steel or copper alloys. Certain aluminium have a better strength to

weight ratio than that of high strength steel. Have good malleability and formability, high

corrosion resistance and high electrical and thermal conductivity.

The chemical composition and properties of common cast aluminium alloys are shown in

Table 2.7 (ASM International, 1993).

Commercial cast aluminum-silicon alloys are poly-phase materials of composed

microstructure belonging to the Aluminum Association classification series 3xx.x for

aluminum silicon plus copper and/or magnesium alloys and 4xx.x aluminum silicon alloys.

The silicon content in standardized commercial cast aluminum silicon alloys is in the range of

5 to 23 wt%. The equilibrium phase diagram for the aluminium – silicon system is shown in

Figure 2.2(a) (ASM International, 2004). These alloy system shows a eutectic at 577 C with

12.6 wt.% Si. A solid solution is formed in the alloys, which contains only 1.65 wt.% Si at

the eutectic temperature and about zero per cent at the room temperature. For all practical

purposes, aluminium is not soluble to silicon. Depending on the Si content, the structure of

the alloys can be hypoeutectic, hypereutectic, or eutectic. The properties of a specific Al-Si

alloy can be attributed to the individual physical properties of its main phase components (-

aluminum solid solution and silicon crystals) and to the volume fraction and morphology of

these components.

The microstructure of hypoeutectic Al-Si alloys contain primary aluminium dendrites with

fine eutectic particles, Figure 2.2(b) (ASM International, 2004). The near-eutectic 413.0 alloy

(~12% Si) contains a predominant eutectic phase containing acicular silicon. Hyper-eutectic

Al-Si alloys such as 390.0 and 393.0 containing 15 to 25% Si have chunky primary silicon

particles with eutectic (Figure 2.2(d)) and exhibit excellent wear resistance and low thermal

expansion.

The mechanical properties of cast aluminum-silicon alloys can be improved by cast

technology and heat treatment processes that:

• Increase the strength of soft matrix

• Decrease the brittle fracture risk in the polyphase regions

• Increase the degree of dispersion of the dendritic structure

13

Table 2.7(a): The chemical composition of common cast aluminium alloys.

14

Table 2.7(b): The mechanical properties of common cast aluminium alloys.

15

(a)

(b) (c) (d)

Figure 2.2: Commercial cast aluminum-silicon binary alloys. (a) Al-Si equilibrium diagram. (b) Microstructure of hypoeutectic alloy (<12.6 wt% Si). x150. (c) Microstructure of eutectic alloy (12.6% Si). x400. (d) Microstructure of hypereutectic alloy (>12.6% Si). x150.

An increase in strength of soft matrix of -aluminum solid solution can be achieved by its

hardening with point defects, such as substituted atoms and vacancies or by precipitation

hardening with dispersion particles of the second phase.

16

The ternary Aluminium–7Silicon–Magnesium alloys, commonly known as LM25 alloys (as

per British Standards) or 356 and 357 alloys (as per AA Standards) are mainly used where

good mechanical properties are required in castings of shape or dimensions requiring an alloy

of excellent castability in order to achieve the desired standard of soundness. Alloy 357.0

contains higher Mg level (0.6%) than alloy 356.0 (0.3% Mg). Consequently, heat treated

357.0 alloys have higher tensile strength than 356.0 alloys. A356.0 and A357.0 alloys are

higher purity versions of 356.0 and 357.0 alloys (low Fe levels). B356.0 and B357.0 are even

more purer. Addition of Be also improves properties.

When Mg is added to Al-Si alloys, Mg2Si phase is formed, which then form a somewhat

pseudo-binary phase diagram with the aluminium. Figure 2.3 shows aluminium-rich corner of

the pseudo-binary equilibrium phase diagram of Al-Mg2Si system. This diagram is important

in describing the heat treating behaviour of Al-Mg2Si system.

Figure 2.3: The pseudo-binary Al-Mg2Si phase diagram.

Addition of Mg to Al-Si alloy increases response to precipitation-hardening to yield higher

strengths. In Al-7Si-Mg alloys, Si imparts high fluidity, low shrinkage, good hot cracking

resistance and Mg is the major strengthening element. Addition of 0.2 – 0.4 wt.% Mg

strengthens the alloy by combining with Si to form Mg2Si precipitates in aluminum matrix.

17

Figure 2.4 and 2.5 show the microstructure of LM25 alloys containing 0.3 wt.% Mg (A356)

and 0.60 wt.% Mg (A357), both in unmodified and modified conditions (Gruzlesik, 1990).

The sharp, acicular silicon particles in the eutectics are seen in unmodified conditions. When

silicon presents in this form, they control the mechanical properties of the alloys. Addition of

minute amount of sodium or strontium modifies the silicon particles into round form and,

thereby, improves the mechanical properties.

(a) (b)

Figure 2.4: Microstructure of LM25 (A356) alloy. (a) As-cast (unmodified), (b) Modified with 0.008% Strontium. x100.

(a) (b)

Figure 2.5: Microstructure of LM25 (A357) alloy. (a) As-cast (unmodified), (b) Modified with 0.013% Strontium. x100.

18

2.2.2 Properties of Al-7Si-Mg (LM25) alloys

These alloys can be sand cast or permanent mould cast with excellent castability, pressure

tightness and corrosion resistance. Structure control through eutectic modification and heat

treatment provides a wide range of properties, however, an increased Fe level and slower

solidification rates have negative effect on the mechanical properties. This is attributed to the

formation of coarse iron intermetallics with high aspect ratio.

The alloy is also used where resistance to corrosion is an important consideration, particularly

where high strength is also required. It has good wettability. Consequently LM25 finds

applications in the food, chemical, marine, electrical and many other industries, and above all

- in the automotive industry where it is used for wheels, cylinder blocks and heads. Its

potential uses are increased by its 3 available levels of heat treatment in both sand and chill

castings. It is, in practice, the general purpose high strength gravity die-casting alloy.

Such alloys have good corrosion resistance. Alloys find applications in carburetor parts and

pump castings, automotive, aerospace and many electrical industries based on their good

electrical conductivity.

This alloy conforms to British Standard 1490 LM25. Castings are standardised in the

following conditions – as-cast, LM25-M precipitation treated LM25-TE; solution treated and

stabilized LM25-TB7; and fully heat-treated LM25-TF. Table 2.8–2.10 show typical

composition, properties and applications of these alloys (MRT Castings, 2015).

Table 2.8: Chemical composition range of LM25 alloys.

Copper 0.1 max. Magnesium 0.20-0.60 Silicon 6.50-7.50 Iron 0.5 max. Manganese 0.3 max. Nickel 0.1 max. Zinc 0.1 max. Lead 0.1 max. Tin 0.05 max. Titanium* 0.2 max. Aluminium Remainder

* 0.05% min. if Ti alone used for grain refining.

19

Table 2.9: Physical property range of LM25 alloys.

Coefficient of Thermal Expansion (per °C at 20 -100 °C) 0.000022

Thermal Conductivity (cal/cm2/cm/°C/s at 25°)* 0.36

Electrical Conductivity (% copper standard at 20 °C)* 39

Specific Gravity 2.68

Freezing Range (°C) Approx. 615-550

*Applies to fully heat treated sand castings; values are approximate and for castings in other conditions will depend on their thermal history.

Table 2.10: Mechanical properties of Aluminium-7Silicon-Magnesium alloys.

Properties LM25-M LM25-TE LM25-TB7 LM25-TF

SC* CC* SC CC SC CC SC CC

0.2% Proof Stress, MPa 80-100 80-100 120-150 130-200 80-110 90-110 200-250 220-260

Tensile Stress, MPa 130-150 160-200 150-180 190-250 160 230 230-280 280-320

Elongation, % 2 3 1 2 2.5 5 -- 2

Brinel hardness number 55-65 55-65 70-75 75-95 65-75 65-75 90-110 90-110

Endurance Limit, 5x108 Cycles + MPa 70-100 80-110 55 75 60 95 60 95

Modula’s of Elasticity, x103 MPa 71 71 71 71 75 71 71 71

*SC – Sand cast, CC – Chill cast

2.2.3 Heat treatment of LM25 alloys

Achievement of specified minimum tensile properties is dependent on the magnesium content

of this excellent heat-treatable alloy. Typically, the T6 heat treatment of LM 25 alloys

involved (1) solution treatment at 535°C for 2-6 hours, (2) quenching to room temperature,

and (3) artificial ageing at 150-180°C for 3-5 hours to obtain peak strength. In the ‘as cast

condition’ intermetallic’s appears as coarse precipitates. But after age-hardening treatment, it

becomes finer and evenly distributed and this provides maximum strength.

The sequence of precipitation in Al-Si-Mg alloys can be described as follows (Mohamed and

Samuel, 2012):

20

i. Precipitation of GP zones (needles of about 10 nm long)

ii. Intermediate phase ’’ – Mg2Si (homogeneous precipitation)

iii. Intermetallic phase ’ – Mg2Si (heterogeneous precipitation)

iv. Equilibrium phase – Mg2Si, FCC structure (a = 0.639), rod or plate-shaped

The maximum alloy strength (peak-ageing) is achieved just before the precipitation of the

incoherent -platelets. Apelian et al (1989) studied the ageing behaviour of Al-Si-Mg alloys

and observed that the precipitation of very fine ’–Mg2Si during ageing leads to a pronounced

improvement in strength properties. Both ageing time and temperature determine the final

properties. Their study also established that increasing the ageing temperature by 10 C is

equivalent to increasing the ageing time by a factor of two.

The ageing effect and properties of Al-7Si-Mg alloys are very much dependent on the ratio

between magnesium and silicon in the supersaturated solid solution. The excess silicon (with

respect to the stoichiometry of the Mg2Si phase) can considerably change the kinetics of

precipitation and the phase composition.

The decomposition of the supersaturated solid solution results in precipitation of the

(Mg2Si) phase and free Si. The coherent ’’ (monoclinic) phase is an efficient hardener and

participates in processes of natural and artificial ageing. In the stage of softening, it gives

place to the semicoherent ’ (hexagonal or orthorhombic) phase, which is considerably stable.

The decomposition starts directly with the formation of ’ particles at temperatures above 300

C, and the sole equilibrium (cubic) phase precipitates upon annealing above 400 C (Eskin et

al, 1999).

The composition of the metastable phases, i.e., Mg:Si ratio, is different from that of Mg2Si

(Mg:Si = 2 at.%). The Mg:Si ratio continuously increases in the series GPZ, ’’ , ’, and ’. In

other words, metastable phases are enriched in silicon. The maximum strength is achieved in

alloys with an excess of silicon and in the stage of ’’ precipitation. The silicon-vacancy

complexes acts as nucleation sites for the coherent phase. The transition between the

metastable modifications is likely to occur separately. In other words, coherent precipitates

dissolve, semicoherent particles are formed at dislocations, and the coherent phase may

precipitate on dislocations or grain boundaries.

21

Zones are precipitated very intensely, the precipitation density being 3x1015 per mm3 and the

size, 6x20-100 nm. The coherent ’’ phase has the same precipitation density with the size

somewhat larger, 5x16-200 nm (Mould and Mapother, 1962). The zone stage lasts from 5 h to

8 min at 150-200 C, respectively, while the transition to the phase requires 200 to 8 h in the

same temperature range (Stroganov et al, 1977).

Alloys with an excess of silicon has one major difference from alloys with Mg:Si ratio lower

than 1.73 wt.%. In these alloys, free silicon precipitates and affects the entire situation. Silicon

accelerates the formation and increases the precipitation density of spherical (Al, Mg, Si)

zones. These zones act like nuclei for the coherent ’’ phase. It is found that the Mg:Si ratio in

the semicoherent ’ phase is lower in the alloys with an excess of silicon, as compared with

balanced alloys (1.21 and 1.75 at.%, respectively).

The sequence of precipitation in the Al-Mg2Si-Si alloys is as follows (in parentheses the

temperature range according to DSC analysis) (Kanno et al, 1978):

Formation of magnesium and silicon segregates in the matrix crystal lattice (below 100 C)

Nucleation of needle-like zones onto these segregates (200-250 C)

Nucleation of free silicon with rapidly lost coherency. The (Al, Mg, Si) zones remain

unchanged (240-320 C)

Formation of the coherent ’’ phase onto (Al, Mg, Si) zones (below 450 C)

Formation of the semicoherent ’ phase (below 450 C)

Formation of the equilibrium phase (above 400 C)

The delay between quenching and artificial ageing in Al-Mg2Si-Si alloys decreases the

hardening effect. However, small copper additions can minimize this effect. Cadmium,

indium, and lead also diminish this effect, but only upon ageing at temperatures below 160-

170 C.

Some Al-Si-Mg alloys contain Sb, Pb or Sn that are added either to modify the Al-Si eutectics

or to improve the machinability of alloys. These elements can considerably worsen the

mechanical properties, binding part of magnesium in insoluble, brittle particles of the Mg2X

type. In this case, the efficient concentration of magnesium in the supersaturated solid

solution is decreased.

22

2.3 LM25-Silicon carbide – Graphite hybrid composite

2.3.1 Introduction

The possibility of taking advantage of particular properties of the constituent materials to

meet specific demands is the most important motivation for the development of composites. A

composite is a material made with several different constituents intimately bonded. This

definition is very large, and includes a lot of materials such as the Roman ways (constituted of

different layers of stones, chalk and sand), wood, human body etc... A more restrictive

definition is used by industries and materials scientists: a composite is a material that consists

of constituents produced via a physical combination of pre-existing ingredient materials to

obtain a new material with unique properties when compared to the monolithic material

properties. This definition distinguishes a composite from other multiphase materials which

are produced by bulk processes where one or more phases result from phase transformation

("in-situ" composites).

The terms matrix and reinforcement are often used. The matrix is a percolating “soft” phase

(with in general excellent ductility, formability and thermal conductivity) in which are

embedded the “hard” reinforcements (high stiffness and low thermal expansion). The

reinforcements can be continuous or discontinuous, orientated or disorientated. The

composites are classified by: (1) their matrix (polymer, ceramic, metal), (2) their

reinforcement, which includes the chemical nature (oxides, carbides, nitrides), shape

(continuous fibers, short fibers, whiskers, particulates) and orientation, (3) their processing

routes.

Metal matrix composites (MMCs) possess significantly improved properties including high

specific strength, specific modulus, damping capacity and good wear resistance compared to

the unreinforced alloys. Aluminum is the most popular matrix for the metal matrix composites

(MMCs). Aluminum alloys are quite attractive for many engineering applications due to their

low density, their capability to be strengthened by precipitation, their good corrosion

resistance, high thermal and electrical conductivity, and their high damping capacity.

However, low hardness and consequently low wear resistance limit their use in some

applications. Aluminum metal matrix composites (Al-MMCs) containing particulate and other

23

reinforcements are considered as the promising solution for imparting better wear resistance

to aluminum alloys.

Aluminum matrix composites (AMCs) have been widely studied since the 1920s and are now

used in sporting goods, electronic packaging, armours and automotive industries. They offer a

large variety of mechanical properties depending on the chemical composition of the Al-

matrix. Aluminium-based metal-matrix composites (MMCs) basically consist of a

nonmetallic reinforcement incorporated into aluminium metallic matrix. The combination of

light weight, corrosion resistance, and useful mechanical properties, which has made

aluminum alloys so popular, lends itself well to aluminum MMCs. The melting point of

aluminum is high enough to satisfy many application requirements, yet is low enough to

render composite processing reasonably convenient.

Aluminum can also accommodate a variety of reinforcing agents. Reinforcements,

characterized as either continuous or discontinuous fibers, typically constitute 20 vol% or

more of the composite. Aluminum matrix composites (AMCs) are usually reinforced by

Al2O3, SiC, C but SiO2, B, BN, B4C, AlN may also be considered. In the 1980s, transportation

industries began to develop discontinuously reinforced AMCs. They are very attractive for

their isotropic mechanical properties (higher than their unreinforced alloys) and their low

costs (cheap processing routes and low prices of some of the discontinuous reinforcement

such as SiC particles or Al2O3 short fibers).

Nowadays, the use of agro/industrial wastes as a secondary reinforcement in the fabrication of

composites is gaining more importance. The advantages of using these wastes are production

of low cost by-products, reduction in the cost of aluminum products (Prasad, 2011), readily

available with less cost and often lower densities in comparison with most technical ceramics.

Many researches have reported the potentials and limitations of the use of wastes as

reinforcements (Ramachandra et al, 2005 and Das et al, 2006).

Hybrid metal matrix composites (HMMCs) are second-generation composites where more

than one type, shape, and sizes of reinforcements are used to obtain better properties

(Matsunaga, 1996). Hybrid composites possess better properties compared with single

reinforced composites as they combine the advantages of their constituent reinforcements

(Thakur, 2007). The addition of silicon carbide particles to aluminum alloys as hard

reinforcing agent (to improve their wear resistance due to high hardness and strength of the

24

reinforcement SiC phase) is often complemented by the addition of graphite as soft

reinforcing agent to improve lubricating and damping properties (Prasad and Asthana, 2006).

2.3.2 Processing of LM25-SiC-G hybrid composites

There are many processes viable to fabricate AMCs; they can be classified in: solid-state,

liquid-state and solid-liquid state (deposition) processes.

In solid-state processes, the most spread method is powder metallurgy PM; it is usually used

for high melting point matrices and avoids segregation effects and brittle reaction product

formation prone to occur in liquid state processes. This method permits to obtain

discontinuously particle reinforced AMCs with the highest mechanical properties. These

AMCs are used for military applications but remain limited for large scale productions.

In liquid-state processes, one can distinguish the infiltration processes where the

reinforcements form a preform which is infiltrated by the alloy melt (1) with pressure applied

by a piston (squeeze-casting) or by an inert gas (gas pressure infiltration) and (2) without

pressure. In the last case, one can distinguish (a) the reactive infiltration processes using the

wetting between reinforcement and melt obtained by reactive atmosphere, elevated

temperature, alloy modification or reinforcement coating (reactive infiltration) and (b) the

dispersion processes, such as stir-casting, where the reinforcements are particles stirred into

the liquid alloy. Process parameters and alloys are to be adjusted to avoid reaction with

particles.

In deposition processes, droplets of molten metal are sprayed together with the reinforcing

phase and collected on a substrate where the metal solidification is completed. This technique

has the main advantage that the matrix microstructure exhibits very fine grain sizes and low

segregation, but has several drawbacks: the technique can only be used with discontinuous

reinforcements, the costs are high, and the products are limited to the simple shapes that by

obtained by extrusion, rolling or forging.

25

2.3.2.1 Stir casting fabrication method

Stir casting is a liquid state method of composite materials fabrication, in which the dispersed

phase(s) is(are) mixed with a molten matrix metal by means of mechanical stirring as shown

in Figure 2.6. The liquid composite material is then cast by conventional casting methods and

may also be processed by conventional metal forming technologies, if necessary. Distribution

of dispersed phase throughout the matrix is not perfectly homogeneous. There are local clouds

(clusters) of the dispersed particles. There may be gravity segregation of the dispersed phase

due to a difference in the densities of the dispersed and matrix phase. The technology is

relatively simple and low cost (Das et al, 2006).

Process variables and their effects on properties

Speed of rotation. The control of speed is very important for successful production of casting.

Rotational speed also influences the structure, the most common effect of increase in speed

being to promote refinement and instability of the liquid mass at very low speed. It is logical

to use the highest speed consistent with the avoidance of tearing. (Das et al, 2006).

Figure 2.6: The schematics of the mixing machine with other setup to prepare the hybrid composite.

Pouring temperature. Pouring temperature exerts a major role on the mode of solidification

and needs to determine partly in relation to type of structure required. Low temperature is

associated with maximum grain refinement and equiaxed structures while higher temperature

1. Motor 2. Shaft 3. Molten aluminium 4. Thermocouple 5. Particle injection chamber 6. Insulation hard board 7. Furnace 8. Graphite crucible

26

promotes columnar growth in many alloys. However practical consideration limits the range.

The pouring temperature must be sufficiently high to ensure satisfactory metal flow and

freedom from cold laps whilst avoiding coarse structures (Das et al, 2006).

Pouring speed. This is governed primarily by the need to finish casting before the metal

become sluggish. Although too high a rate can cause excessive turbulence and rejection. In

practice slow pouring offers number advantages. Directional solidification and feeding are

promoted whilst the slow development of full centrifugal pressure on the other solidification

skin reduces and risk of tearing. Excessive slow pouring rate and low pouring temperature

would lead to form surface lap (Rajan et al, 2007).

Mould temperature. The use of metal die produces marked refinement when compared with

sand cast but mould temperature is only of secondary importance in relation to the structure

formation. Its principal signification lies in the degree of expansion of the die with preheating.

Expansion diminishes the risk of tearing in casting. In nonferrous castings, the mould

temperature should neither be too low or too high. The mould should be at least 25 mm thick

with the thickness increasing with size and weight of casting (Rajan et al, 2007).

Mould coatings. Various types of coating materials are used. The coating material is sprayed

on the inside of the metal mould. The purpose of the coating is to reduce the heat transfer to

the mould. Defects like shrinkage and cracking that are likely to occur in metal moulds can be

eliminated, thus increasing the die life. The role of coating and solidification can be adjusted

to the optimum value for a particular alloy by varying the thickness of coating layer. For

aluminium alloys, the coating is a mixture of silicate and graphite in water.

Mould life. Metal mould in casting is subjected to thermal stresses due to continuous

operation. This may lead to failure of the mould. The magnitude of the stresses depends on

the mould thickness and thickness of the coating layer, both of which influence the production

rate. Deterioration takes place faster in cast iron mould than in steel mould (Rajan et al, 2007).

27

Wettability between reinforcement and matrix alloy

Wettability can be defined as the ability of a liquid to spread on a solid surface. It also

describes the extent of intimate contact between a liquid and a solid. Successful incorporation

of solid ceramic particles into casting requires that the melt should wet the solid ceramic

phase. The basic means used to improve wetting are

(a) Increasing the surface energies of the solid,

(b) Decreasing the surface tension of the liquid matrix alloy, and

(c) Decreasing the solid-liquid interfacial energy at the particles-matrix

Several approaches have been taken to promote the wetting of the reinforcement particles

with a molten matrix alloy, including the coating of the particles, the addition of alloying

elements to the molten matrix alloy, the treatment of the particles, and ultrasonic irradiation of

the melt. In general, the surface of non-metallic particles is not wetted by the metallic metal,

regardless of the cleaning techniques carried out. Wetting has been achieved by coating with a

wet table metal. Metal coating on ceramic particles increases the overall surface energy of the

solid, and improves wetting by enhancing the contacting interface to metal-metal instead of

metal-ceramic (Epstein, 2001).

Nickel and copper are well wetted by many alloys and have been used for a number of low

melting alloys. In general, these coatings are applied for three purposes, viz., to protect the

reinforcement from damage in handling, to improve wetting, and to improve dispensability

before addition to the matrix.

Heat treatment of the particles before dispersion into the melt aids their transfer by causing

desorption of adsorbed gases from the particle surface. Heating silicon carbide particles to

900 C, for example, assists in removing surface impurities and in the desorption of gases, and

alters the surface composition by forming an oxide layer on the surface. Hence a clean surface

provides a better opportunity for melt-particles interaction, and thus, enhances wetting. Thus

results in strong interfacial bonding (Ray and Kerketta, 2010).

28

2.3.2.2 Silicon carbide as reinforcement

Silicon carbide is the only chemical compound of carbon and silicon. It was originally

produced by a high temperature electro-chemical reaction of sand and carbon. Silicon carbide

is an excellent abrasive and has been produced and made into grinding wheels and other

abrasive products for over one hundred years. Today SiC has been developed into a high

quality technical grade ceramic with very good mechanical properties (Divecha et al, 1981).

It is used in abrasives, refractoriness, ceramics, and numerous high-performance applications.

The material can also be made an electrical conductor and has applications in resistance

heating, flame igniters and electronic components. Silicon carbide is composed of tetrahedral

of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard

and strong material (Divecha et al, 1981).

Properties of silicon carbide include:

• Low density

• High strength and high hardness

• High elastic modulus

• Low thermal expansion and thermal conductivity

• Excellent thermal shock resistance

• Superior chemical inertness

Important physical and mechanical properties of silicon carbide are given in Table 2.11

(Accuratus, 2015).

Table 2.11: Properties of silicon carbide.

Property Commercial graphite

Melting point, C 2200 - 2730

Density, kg/m3 3210

Hardness, Mohs 9

Modulus of elasticity, GPa 220 – 250

Compressive strength, MPa 3900

Fracture toughness K1C, MPa.m1/2 4.6

Thermal conductivity, W/m K 120

Specific heat, J/kg K 750

29

2.3.2.3 Graphite as reinforcing materials

There are two main types of graphite, natural and synthetic. Graphite derives its name from

the Greek word “graphein” to write. The material is generally greyish black, opaque and has a

lustrous black sheen. It is unique that it has properties of both a metal and non-metal. It is

flexible but not elastic, and has a high thermal and electrical cconductivity. It is highly

refractory and chemically inert. Table 2.12 shows properties of commercial graphite (Azo

material, 2015). Shape of particles has an important effect on the graphite content to be

incorporated in the alloy.

Table 2.12: Properties of commercial graphite.

Property Commercial graphite

Bulk density, g/cc 1.30 – 1.95

Porosity, % 0.7 – 53

Hardness, Mohs 1 – 2

Modulus of elasticity, GPa 8 – 15

Compressive strength, MPa 20 – 200

Flexural strength, MPa 6.9 – 100

Coef. of thermal expansion, x10– 6 C 1.2 – 8.2

Thermal conductivity, W/m K 25 – 470

Specific heat capacity, J/kg K 710 - 830

2.4 Literature Review

The increased demand of lightweight materials with high specific strength in the aerospace

and automotive industries has led to the development and use of Al alloy-based composites

(mainly Al alloy/SiC composites). The metal matrix composites (MMCs) are slowly replacing

the general light metal alloys such as aluminium alloy in different industrial applications

where strength, low mass and energy savings are the most important criteria. The combination

of various properties like electrical, mechanical, and even chemical can be achieved by the

use of different types of reinforcements, i.e., continuous, discontinuous, short, whiskers, etc.,

with the MMCs.

30

The MMCs are attractive materials for use in structural applications because they combine

favorable mechanical properties, good wear resistance, and low thermal expansion. Particle-

reinforced metal matrix composites (PMMCs) are very promising heterogeneous materials

for structural applications due to their isotropic material properties, low cost, and ability to

be formed using conventional metal forming processes such as rolling, forging, and extrusion

to produce the finished products. However, the indentation characteristics of heterogeneous

material systems in various forms of composites, precipitation- hardened alloys, and

dispersion-strengthened alloys are not known well. Their macroscopic indentation responses

are affected by the mechanical properties of the matrix material and reinforcement as well as

the type, shape, dimension, geometric arrangement, and volume fraction of the reinforcement.

Particulate-reinforced metal matrix composites have paved a new path to produce high

strength and high wear-resistant materials by introducing hard ceramic particles and solid

lubricant in the metal matrix. Hybrid metal matrix composites (HMMCs) are second-

generation composites where more than one type, shape, and sizes of reinforcements are used

to obtain better properties.

Rajan et al (2007) studied the effect of three different stir casting routes on the structure and

properties of fine fly ash particles reinforced Al-7Si-0.35Mg alloy composite is evaluated.

Among liquid metal stir casting, compo casting (semi solid processing), modified compo

casting and modified compo casting followed by squeeze casting routes evaluated, the latter

has resulted in a well-dispersed and relatively agglomerate and porosity free fly ash particle

dispersed campsites. Interfacial reactions between the fly ash particle and the matrix leading

to the formation of MgAl2O4 spinel and iron intermetallics are more in liquid metal stir cast

composites than in compo cast composites.

Garg et al (2012) observed based on the literature review, the machining of hybrid AMMCs

(Al/SiC/Gr and Al/Si10Mg/Fly ash/Gr) composites. These hybrid MMCs can easily be

machined by EDM and a good surface quality can be obtained by controlling the machining

parameters. These AMMCs with multiple reinforcements are finding increased applications

because of improved mechanical and tribological properties and hence are better substitutes

for single reinforced composites. These materials are developed for bushes, bearings and

cylinder liners in cast aluminium engine blocks. The problems encountered during machining

of hybrid MMCs and their amendments by the use of EDM are discussed.

31

Surappa (2003) studied aluminium matrix composites (AMCs) manufactured by different

routes and reinforcements. This paper presents an overview of AMC material systems on

aspects relating to processing, microstructure, properties and applications.

Shorowordi et al (2003) observed the aluminium metal matrix composites containing

reinforcing particles of B4C, SiC and Al2O3 (0-20 vol.%). The stir-casting manufacturing

route followed by hot extrusion was utilized, being one of the cost-effective industrial

methods. A clear interfacial reaction product/layer was found at Al-SiC interface for

composites held for a relatively long processing time (>30 minutes). No reaction product was

observed at Al- B4C and Al-Al2O3 interfaces at the resolution limit of the SEM used. On the

other hand, two secondary phases (alumina and another phase containing aluminium, boron

and carbon) were found in the aluminium matrix away from the interface in Al-B4C

composites. From the fracture surface analysis, B4C reinforced Al composite seemed to

exhibit a better interface bonding compared to the other two composites.

Naher (2004) observed the examination of the liquid and semi-solid stir casting method to

produce Al-SiC composites. In this work stainless steel was chosen as the main crucible and

stirrer material. The machine consisted of a four 45 flat bladed stirrer and a crucible in a

resistance heated furnace chamber. A linear actuator was integrated to this rig to allow the

crucible to be quickly extracted from the furnace for quenching. Stirring speed ranging from

200 to 500 rpm and different shear periods were investigated. Ten percentage volumes of 30

μm sized SiC particles were used. The main research challenge was to get a uniform

distribution of SiC in the aluminium matrix. In the compo casting experiments it was found

that the uniformity of SiC particles in the aluminium matrix were dependent on shear rate,

shear period, cooling rate and volume fraction of primary solid. The quick quench compo

caster system was successful in producing cast MMC samples. The use of clean heat-treated

SiC particles and the quick quench method was sufficient to produce homogeneous

composites. Castings from the liquids condition were found to result in poor incorporation of

SiC particles whereas castings from the semi-solid condition were found to produce a uniform

distribution of SiC particles. However, quicker solidification, after cessation of mixing, was

found to improve the uniformity of the SiC distribution significantly. Characterization of the

MMC samples produced included microstructure recording and image analysis thereof. The

matrix phase size, morphology and distribution of SiC particles throughout the stir castings

were examined.

32

Uthayakumar et al (2013) fabricated the hybrid metal matrix composites consist of a metal or

an alloy matrix with strongly embedded multiple hard reinforcements to enhance the wear

resistance properties. This research study emphasizes on the dry sliding wear behaviour of

aluminium reinforced with 5% SiC and 5% B4C hybrid composite using a pin on disc

tribometer. Wear performance of the hybrid composites was evaluated over a load range of

20-100 N, at the sliding velocities from 1 to 5 m/s. Detailed metallurgical examination and

energy dispersive analysis were carried out to assess the effect of SiC and B4C particles on

the wear mechanisms. The Focused Ion Beam (FIB) technique is used to characterize the

Tribo layers that have been formed at the worn surfaces of composites. The experimental

results show that the hybrid composites retain the wear resistance properties up to 60 N loads

and sliding speed ranges 1-4 m/s. The enhancement of wear resistance with small amount of

SiC and B4C is achieved by the cooperating effect of reinforcement particles.

Suresha et al (2010) focused on the influence of addition of graphite particulates as a second

reinforcement on the tribological behavior of aluminium matrix composites reinforced with

SiC particulates. Dry sliding wear tests have been performed to study the influence of Gr

particulates, load, sliding speed and sliding distance on the wear of hybrid composite

specimens with combined % reinforcement of 2.5%, 5%, 7.5% and 10% with equal weight %

of SiC and Gr particulates. Experiments are also conducted on composites with %

reinforcement of SiC similar to hybrid composites for the sake of comparison. Parametric

studies based on DOE techniques indicate that the wear of hybrid composites decreases from

0.0234 g to 0.0221 g as the % reinforcement increases from 3% to 7.5%. But the wear has a

tendency to increase beyond % reinforcement of 7.5% as its value is 0.0225 g at %

reinforcement of 10%. This trend is absent in case of composites reinforced with SiC alone.

The values of wear of these composites are 0.0323 g, 0.0252 g and 0.0223 g, respectively, at

% reinforcement of 3%, 7.5% and 10% clearly indicating that hybrid composites exhibit

better wear characteristics compared to composites reinforced with SiC alone. Load and

sliding distance show a positive influence on wear implying increase of wear with increase of

either load or sliding distance or both. Whereas speed shows a negative influence on wear

indicating decrease of wear with increase of speed. Interactions among load, sliding speed and

sliding distance are noticed in hybrid composites and this may be attributed to the addition of

Gr particulates. Such interactions are not present in composite reinforced with SiC alone.

Mathematical models are formulated to predict the wear of the composites.

33

Ravindran et al (2012) studied the wear and sliding friction response of a hybrid aluminium

metal matrix composite reinforced with hard ceramic (5 wt. % of SiC) and soft solid lubricant

(0, 5 and 10 wt. % of graphite) fabricated by powder metallurgy. The influence of the

percentages of reinforcement, load, sliding speed and sliding distance on both the wear and

friction coefficient were studied using the pin-on-disc method with tests based on the DOE.

ANOVA was used to investigate the influence of the parameters on both the wear rate and the

coefficient of friction. The hardness of the composites decreases as the % of graphite (Gr)

increases. The wear and friction coefficient were mainly influenced by both the sliding

distance and the load applied. The morphology of the worn out surfaces and the wear debris

was analyzed to understand the wear mechanisms. The wear resistance of the hybrid

composite containing 5 wt. % SiC and 5 wt. % graphite is superior to that of the graphite free

composites and the other hybrid composites. This study reveals that the addition of both and

reinforcement like SiC and soft reinforcement like graphite improves the wear resistance of

aluminium composites significantly.

Basavarajappa et al (2007) investigated the influence of sliding speed on dry sliding wear

behavior and the extent of subsurface deformation in aluminium metal matrix composites,

namely Al 2219/15SiCp and Al 2219/15SiCp-3graphite all fabricated by the liquid metallurgy

route. Dry sliding wear tests were conducted using a pin-on-disc machine. The subsurface

deformation was assessed as a measure of variation in micro-hardness along the depth normal

to the cross-section of the worn surface. The results reveal that with increasing sliding speeds

in the mild wear region the degree of subsurface deformation was also increasing. The

graphite composite exhibited a less degree of subsurface deformation in comparison to the

graphite free composite.

2.4.1 Summary of literature review

The extensive review of literature carried out for the present study reveals that a lot of work

has been reported on Aluminium metal matrix composites through stir casting or by any other

process. A number aluminium alloys, either wrought (6063, 6067, etc.) or cast (LM6, LM24,

LM25, etc.), have been selected as base materials in these research works. Also, the

reinforcing media also varied to a wide extent, both in chemical composition (SiC, Al2O3,

B4C, G, etc.) and form (hard and soft, fibres and particulate). The properties of interest

34

included mechanical (hardness, strength and ductility, fracture toughness) and wear

properties. Little works, however, on heat treated behaviour of aluminium-based composites

are reported.

2.5 Scope of Present Work

In this work, the mechanical behaviour of aluminium-based metal matrix composite (MMC)

with addition of varying percentage composition of SiC particles and Graphite particles to

Al-7Si-Mg (LM25) aluminium alloy will be made. Attempts will be made to manufacture

good quality LM25-SiC-G hybrid composites with a uniform dispersion of reinforcing

particles using stir casting methods. The raw materials will be collected from local market and

then be processed and used to produce hybrid composites using departmental facilities.

The aim of the experiment is to study the effect of variation of the percentage composition of

SiC and G particles, in isolation or in combination, to determine the mechanical properties of

LM25-SiC-G composites. The present work emphasizes the literature review of Al-Si alloys

and its composites and the stir casting technique is selected to prepare the composites. One of

the most important characteristics of LM25 alloys is the ability to enhance their properties

after heat treatment. In this work, the behaviour of LM25-SiC-G after heat treatment will also

be determined.

The objectives of the present work are as follows:

1. To prepare the cost-effective LM25-SiC-G hybrid composites using stir casting technique

by taking LM25 alloy as the matrix, silicon carbide particles as the hard reinforcing phase

and graphite as the soft reinforcing phase.

2. To analyze the microstructural characteristics of the hybrid composite.

3. To determine the mechanical properties (hardness, tensile strength and elongation) and

the wear behaviour of the manufactured hybrid composites.

4. To understand the effect of heat treatment on the mechanical properties of the

composites.

35

3 Experimental Procedure

3.1 Introduction .................................................................................................................... 36

3.2 Preparation of LM25 master alloy ................................................................................. 36

3.2.1 Raw materials ..................................................................................................... 36

3.2.2 Melting and casting ............................................................................................ 36

3.3 Preparation of LM25-SiC-G hybrid composite ............................................................. 37

3.4 Heat treatment of LM2S-SiC-G hybrid composites ...................................................... 39

3.5 Microstructural observations ......................................................................................... 39

3.6 Hardness measurement ................................................................................................... 39

3. 7 Tensile testing ................................................................................................................ 40

3.8 Wear testing ................................................................................................................... 40

36

3.1 Introduction

In this investigation, microstructural and mechanical characterization of aluminium-based

hybrid composites were carried out. Silicon carbide (0 – 5 wt%) and graphite particles (0 – 5

wt%) were incorporated into Aluminium-7Silicon-Magnesium master alloy to form a hybrid

composite, where SiC particles were the hard reinforcing particles used to improve the

mechanical properties, while G particles were used as a soft reinforcing particles to improve

the lubricating properties.

The master alloy, conforming the composition of LM25 was prepared using piston scrap and

pure magnesium ribbon. Both reinforcing particles were ground to -270 mesh and then added

to the master alloy using stir-casting process and cast into metal moulds.

Metallographic examinations of each category of sample were carried out using both optical

and scanning electron microscope. The tensile properties of the specimen were measured by

using an electronic tensile testing machine at room temperature based on ASTM standard.

The polished composite specimens were tested for their Brinell hardness. Finally, a pin-on-

disc wear testing machine was used to evaluate the sliding wear behavior of the composites.

The wear tests were carried out under un-lubricated dry sliding conditions against a cast iron

disc as per ASTM standard.

3.2 Preparation of LM25 master alloy

3.2.1 Raw materials

In the present study, aluminium piston scrap was used as the basic aluminium source. These

were collected from local market, and melted to form a homogenized composition. Pure

magnesium ribbon was used to increase the magnesium content in the melt.

3.2.2 Melting and casting

Calculated amount of scrap pistons and pure aluminium ingot were melted in a gas-fired clay-

bonded graphite crucible and heated to about 750⁰ C till the entire metal in the crucible was

37

melted. Pure magnesium ribbon was added at the bottom of the melt in the requisite amount

to have a target composition of about 0.3% Mg. The liquid was properly degassed and

defluxed using FOSECO Degasser before pouring into sand moulds. Table 3.1 shows the

composition of the master alloy.

Table 3.1: Chemical composition of the raw materials and the master alloy.

Material Chemical composition, wt.%

Si Mg Cu Ni Mn Fe

Piston scrap 7.42 0.11 1.52 0.30 0.15 0.50

LM25 master alloy 7.77 0.44 1.53 0.32 0.18 0.48

3.3 Preparation of LM25-SiC-G hybrid composite

Semi-solid casting process was used to prepare the LM25-SiC-G hybrid composites. Silicon

carbide and graphite particles were used as hard and soft reinforcing agents, respectively, to

prepare the hybrid composites.

The SiC particles have an average composition of 49.8% C and 49.69 % Si. Both SiC and

pure G particles were collected from local market and then crushed and screened to have an

average particles size of -270 mesh.

Three hybrid composites of compositions shown in Table 3.2 were prepared. The master alloy

was charged in about 1.5 kg batches into the graphite crucible and heated to 750⁰ C till the

entire metal in the crucible was melted. The reinforcement SiC particles were pre-heated to

700⁰–800⁰ C for 1 h before incorporation into the melt to remove moisture and to increase the

wetting efficiency. The graphite particles were kept near the melting furnace to have

sufficient temperature.

After complete melting, FOSECO degassing tablet was added to reduce the hydrogen level

and skim off the slags. The crucible with the clean liquid was then placed into a mixing

machine (Fig. 3.1) to incorporate the reinforcing particles. The mixing chamber was

maintained at a constant temperature of about 500⁰ C.

38

Table 3.2: Chemical make-up of the LM25-SiC-G hybrid composites.

Material Chemical composition, wt.%

Silicon carbide (SiC) Graphite (G) LM25 master alloy

MA (Master alloy) -- -- Balance

HC1 (LM25–2.5SiC–5G) 2.50 5.00 Balance

HC2 (LM25–5SiC) 5.00 -- Balance

HC3 (LM25–5G) -- 5.00 Balance

Figure 3.1: The mixing machine with other setup to prepare the hybrid composite.

A stirrer made up of graphite was lowered into the melt slowly to stir the molten metal at a

speed of 700 rpm. The speed of the stirrer can be controlled by means of a regulator provided

on the furnace. During stirring, preheated SiC and the G particles were added at a constant

rate to the vortex created in the melt due to the stirring of the liquid metal. Stirring was

continued for another 5–10 min after the completion of particle feeding. Then the stirrer was

withdrawn and the temperature of the liquid was raised very quickly to about 700⁰ C and the

liquid was poured into a pre-heated metal mould (at about 500⁰ C) to cast a rectangular block.

39

3.4 Heat treatment of LM25-SiC-G hybrid composites

A few composite samples were heat treated to see the effect on mechanical properties. The

heat treatment process involved a solution treatment at 540⁰ C for 8 h, followed by quench in

water at about 30⁰ C, and then artificial aging at 175⁰C for heat treatment, the samples were

kept in a freezer of -15⁰ C till testing.

3.5 Microstructural observation

The microstructure of LM25 and the hybrid composites was examined using an optical

microscope and a Field Emission Scanning Electron Microscope (FESEM) (model JEOL JSM

7600F), equipped with EDS. The metallographic samples were cut from the broken tensile

specimens and ground and polished using standard procedures, and then etched using Keller’s

reagent.

3.6 Hardness measurement

Brinell hardness test was conducted for measuring the hardness of the master alloy and the

composites. The 10 mm steel ball was used as indenter and 500 kg pressure was maintained

for a dwell period of 15 seconds. The hardness values were measured in five locations over

the sample and the average values were taken for analysis. The results obtained from the

hardness test of LM25 master alloy and the three composites, before and after heat treatment,

are tabulated in Table 3.3.

Table 3.3: Hardness test data.

Material Hardness value before

heat treatment, BHN

Hardness value after heat

treatment, BHN

LM25 master alloy 80 82

HC1 (LM25–2.5SiC–5G) 100 96

HC2 (LM25–5SiC) 93 96

HC3 (LM25–5G) 83 94

40

3.7 Tensile properties

Tensile testing of the samples was conducted in accordance with the ASTM Standard

B557M-10. (A=100,B=10,C=32,D=6,E=5 in mm.) The cast composite specimens were

machined using lathe to get the standard dimensions. Tensile samples were prepared from the

separately cast LM25 master alloy and three composites blocks and then tested using an

automated servo-hydraulic testing machine, with cross-head speed set at 3.0 mm/min. The

results obtained from the tensile test of LM25 master alloy and the three composites are

tabulated in Table 3.4.

Figure 3.1 A: Tensile Sample

Table 3.4: Tensile test data.

Material Ultimate Tensile

Strength, MPa Elongation, %

LM25 master alloy 158.1 2.66

HC1 (LM25–2.5SiC–5G) 174.3 3.40

HC2 (LM25–5SiC–0G) 178.4 3.75

HC3 (LM25–0SiC–5G) 156.6 3.15

3.8 Wear testing A pin-on-disc wear testing machine (manufactured locally, Figure 3.2) was used to evaluate the dry

sliding wear behavior of the composites. The wear tests were carried out under un-lubricated dry

sliding conditions (temperature 30±3 °C, RH 55±5%) as per the ASTM standard. The pin (the sample)

of 6 mm x 8 mm in dimension was initially cleaned with acetone and weighed accurately using a

digital electronic balance. During the wear test, the pin was pressed using a load against the

counterpart (100 mm diameter cast iron disc, hardness 63 HRC) rotating at a fixed speed of 200 rpm.

The track radius was fixed at 30 mm and the applied load (F) was varied between 1.0 to 2.5 kg during

the tests. After running through a fixed sliding time of 1 h (equivalent to about 1130 m sliding

distance), the specimen was removed, cleaned with acetone, dried, and weighed to determine the

weight loss due to wear. The wear test data of all samples are reported in Table 3.5.

41

The wear rate was calculated using the equation

Wr = Weight loss, g

Sliding distance, m

and the wear resistance was calculated as the reciprocal of wear rate (m/N). The worn

surfaces of both the sample and the counterpart were investigated after the test using SEM to

understand the mechanism of wear in the sample. he worn surfaces and worn debris of the

unreinforced A356.2 alloy and hybrid composites were examined using scanning electron

micro- scope (SEM) equipped with energy dispersive X-ray analysis (EDX).

Figure 3.2: Wear testing set-up.

Table 3.5: Wear test data.

Material Weight loss at different applied load, g

1.0 kg 1.5 kg 2.0 kg 2.5 kg

LM25 master alloy 0.72 1.05 1.24 1.43

HC1 (LM25–2.5SiC–5G) 0.62 0.72 0.94 0.93

HC2 (LM25–5SiC–0G) 0.99 0.83 0.63 0.78

HC3 (LM25–0SiC–5G) 0.84 0.86 0.90 0.93

42

4 Results and Discussion

4. Results and discussion ................................................................................................. 42

4.1 Manufacture of LM25-SiC-G hybrid composite ........................................................... 43

4.2 Microstructural observations ......................................................................................... 47

4.3 Hardness measurement .................................................................................................. 50

4.4 Tensile properties ........................................................................................................... 51

4.5 Wear properties .............................................................................................................. 52

4.6 Effect of heat treatment .................................................................................................. 55

43

4.1 Manufacture of LM25-SiC-G Hybrid Composite

In the present work, LM25 based hybrid composites were prepared using SiC and G particles

as the reinforcing media by stir casting methods. Silicon carbide and graphite particles were

successfully incorporated into the LM25 matrix after several attempts. The scanning electron

micrographs and the EDX analysis indicating the presence of SiC and G particles are shown

in Figures 4.1 to 4.3.

Figure 4.1: Scanning electron micrograph and EDX analysis of HC1 (LM25-2.5SiC-5G).

44

Figure 4.2: Scanning electron micrograph and EDX analysis of HC2 (LM25-5SiC-0G).

45

Figure 4.3: Scanning electron micrograph and EDX analysis of HC3 (LM25-0SiC-5G).

46

The distribution of reinforcing particles into the matrix, however, was noticeably not uniform.

Further, these figures reveal the some degree of inhomogeneity of the cast composites.

Although homogeneous distribution of the reinforcement in the matrix is essential to form a

composite with uniform mechanical properties, this was not achieved in the present work.

Large agglomeration of particles, particularly graphite particles, along with porosity was seen,

Figure 4.4.

Figure 4.4: Inhomogeneity and agglomeration of silicon carbide and graphite particles

in LM25-SiC-G hybrid composite.

These are attributed to many of the limitations encountered at the foundry shop. Firstly, there

was no heating furnace available at the foundry shop. SiC particles were preheated at the

chemical analysis laboratory and, because of this, the temperature of the particles was

decreased considerably during transport of the pre-heated particles from the chemical analysis

laboratory to the foundry shop. No facility to control atmosphere during stirring of liquid

aluminium was available. As a result formation of oxide and incorporation of hydrogen gas

into the melt can not be prevented. Lack of technical skill and human error were also play

some part in attaining these defects.

Because of these deficiencies, the degree of incorporation of either or the reinforcing agents

was not achieved up to the marks, which significantly affected the ultimate properties of the

composites.

47

4.2 Microstructural Observations

The examination of microstructures is one of the principal means of evaluating alloys and

products to determine the effects of various fabrication and thermal treatments. Figures 4.5 to

4.8 show the optical micrographs of LM25 and the three composites, respectively, while

Figures 4.9 – 4.10 show the scanning electron micrographs of LM25 and the three

composites, respectively. The silicon eutectic particles with iron bearing intermetallics are

visible in both the optical and scanning electron micrographs of all samples. SiC and G

particles can also be identified in these micrographs of the three hybrid composites.

(a) (b)

Figure 4.5: Optical micrograph of LM25 master alloy. (a) x200, (b) x500.

(a) (b)

Figure 4.6: Optical micrograph of HC1 (LM25-2.5SiC-5G) hybrid composite. (a) x200, (b) x500.

48

(a) (b)

Figure 4.7: Optical micrograph of HC2 (LM25-5SiC-0G) hybrid composite. (a) x200, (b) x500.

(a) (b)

Figure 4.8: Optical micrograph of HC3 (LM25-0SiC-5G) hybrid composite. (a) x200, (b) x500.

49

(a) (b)

Figure 4.9: Scanning electron micrograph of (a) LM25 master alloy and

(b) HC1 (LM25-2.5SiC-5G) hybrid composite.

(a) (b)

Figure 4.10: Scanning electron micrograph of (a) HC2 (LM25-5SiC-0G) and

(b) HC3 (LM25-0SiC-5G) hybrid composites.

50

4.3 Hardness measurement

The results obtained from the Brinell hardness test of LM25 master alloy and the three

composites are tabulated earlier in Table 3.3 and cross-plotted here in Figure 4.11. It is clear

form the figure that, the addition of reinforcing agents generally improve the hardness value

of the LM25 master alloy. Composite HC2 contained 5 wt.% hard SiC particles and showed a

maximum of 25% increase the hardness value (from 80 to 100 BHN) of the original LM25

master alloy. This was predicted and also reported by earlier examiners as SiC particles were

used as the hard reinforcing material to improve the mechanical properties.

Figure 4.11: Hardness value of LM25 master alloy and other composites.

Addition of graphite particles also improves the hardness of the aluminium alloy matrix but

not as pronounced as the hard SiC particles. Although supported by many, this is contrary to

the findings of Pandy and Muthusamy (2012) and Suresha and Sridhara (2010) who observed

a decrease in hardness with the addition of graphite particles in 6061 aluminium matrix. They

concluded that this was due to the increase in the porosity level in the matrix with the addition

of graphite particles. The HC3 composite, containing 5 wt.% graphite showed only a minor

increment in hardness (from 80 to 83 BHN). The HC1 hybrid composite containing 2.5% SiC

and 5 wt.% graphite particles showed about 16 % increase in hardness (from 80 to 93 BHN).

The increment was not high because this composite contained only about 2.5 wt.% SiC.

0

20

40

60

80

100

120

LM25 HC 1 HC 2 HC 3

Har

dnes

s Val

ue, B

HN

51

4.3 Tensile properties

The results obtained from the tensile test of LM25 master alloy and the three composites are

tabulated earlier in Table 3.4 and cross-plotted here in Figure 4.12.

A minor increment in tensile strength (of about 10%) was reported due to the addition of 2.5

wt.% SiC and 5 wt.% G in the LM25 master alloy. Tensile strength was increased further (an

another 2 %, 12 % all together) when SiC alone was added to 5wt.%. Decrease in tensile

strength of LM25 due to the addition of 5 wt.% graphite particles was found to be

insignificant (only by about 1 %) and well within the experimental error.

Figure 4.12: Tensile properties of LM25 master alloy and other composites.

The ductility data, measured as the percent elongation at failure, was not clear, as they

followed a similar trend of those of tensile strength data. All of these tensile data

complements the hardness values and the change in mechanical properties that was expected

beforehand due to the addition of hard and soft reinforcing agents, either in isolation and in

combination.

0

2

4

6

8

0

50

100

150

200

LM25 HC 1 HC 2 HC 3

Elo

ngat

ion

at F

ailu

re, %

Ulti

mat

e Te

nsile

Str

engt

h, M

Pa

Tensile strengthElongation

52

4.4 Wear properties

The results obtained from the wear test of LM25 master alloy and the three composites are

tabulated earlier in Table 3.5. Figures 4.13 and 4.14, respectively, show the variation of wear

(weight) loss and wear resistance (reciprocal of wear rate) as a function of normal load for the

unreinforced LM25 alloy and hybrid composites. In general, addition of reinforcing particles

of both SiC and G was found to reduce wear of the matrix material.

It is seen from the plots that, with the addition of SiC reinforcement particles alone in HC2

composites, the wear rate of the composite is decreased. Also, the wear rate (mass loss)

increases as the normal load increases. The increase in wear resistance due to SiC addition

can be attributed to the strengthening of the matrix due to the reinforcement, which results

from an increase in the dislocation density as the percentage of reinforcement’s increases.

The addition of G particles only in HC3 composites was found to slightly increase the rate of

wear. Although this phenomena was also found by Pandi and Muthusamy (2012), this is

contrary to the general behaviour of reducing wear as reported by a majority of researcher.

This is also seen in Figure 4.14, when the wear resistance of the composite against the applied

load is considered. However, Balasubramanya et al (2014) reported that the function of the

graphite particles in decreasing wear is usually become more effective at a higher sliding

speed due to quick formation of lubricating layer which avoids the direct contact between

specimen and disc.

In the HC1 hybrid composites, the wear resistance was increased by the combined effect of

both SiC and G particles. As SiC particles come in contact with sliding disc, that avoids

further wear and graphite particles help to run smoothly on the disc. SiC particles help avoid

cease of the material that occur during the test period. The SiC particles act as load bearing

member and graphite acts as a solid lubricant in LM25 matrix material. G particles also

reduce the subsurface deformation of the matrix thereby reducing the wear rate. It is also clear

from these plots that, the effect of silicon carbide particles in improving the wear resistance is

more pronounced than the graphite particles. The improvement of wear rate for the addition of

SiC is much higher than the reduction in wear rate due to the same amount (i.e., 5 wt.%) of

graphite addition. Thus, the addition of both SiC and G particles to LM25 leads mild wear

regime at higher load thereby inhibiting severe wear, which is seen in LM25 master alloy.

53

Figure 4.13: Variation of wear loss in LM25 master alloy and other composites.

Figure 4.14: Variation of wear resistance in LM25 master alloy and other composites.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0.018

0.02

1 1.5 2 2.5

Wea

r L

oss,

g

Applied Load, kg

LM25 HC1 HC2 HC3

0

5000

10000

15000

20000

1 1.5 2 2.5

Wea

r R

esis

tanc

e, g

/m/N

Applied Load, kg

LM25 HC1 HC2 HC3

54

Figure 4.15 shows the optical micrographs of the worn-out surface of the LM25 master alloys

and the composites. The microstructure of the master alloy shows large amount of plastic

deformation on the surface, indicating adhesive wear. On the other hand, the microstructures

of composites reveal grooves formed by the hard SiC particles. The graphite particles clearly

show interaction with the SiC particles (Figures 4.15b and 4.15c) and form the lubricant layer.

(a) (b)

(c) (d)

Figure 4.15: Optical micrograph of worn surface of (a) LM25 master alloy, (b) HC1 (LM25-2.5SiC-

5G) composite, (c) HC2 (LM25-5SiC-0G) composite, and (d) HC3 (LM25-0SiC-5G) composite

55

Concerning the dry sliding wear of aluminium-silicon alloys, however, the wear mechanism

varies as a function of applied load, with low wear rates being recorded at low loads (Deuis et

al, 1996). Conflicting theories, however, have been published regarding the wear mechanism

within this low load range and this is perhaps due to the fact that varying speeds and different

counterface materials were used.

4.5 Effect of Heat Treatment

Brinell hardness measurements were used to ascertain the age hardening behavior of the

LM25 master alloy and the composites in the present study. The hardness data was reported

in Table 3.3 and cross-plotted here in Figure 4.16. A general trend in the increase in hardness

was observed after heat treatment. This is attributed to the precipitation hardening effect of

LM25 alloys and rounding up the silicon particles.

Figure 4.16: Variation in hardness before and after heat treatment

in LM25 master alloy and other composites.

0

20

40

60

80

100

120

LM25 HC 1 HC 2 HC 3

Bri

nell

Har

dnes

s Num

ber

Before HT After HT

56

5 Conclusions

5. Conclusions ................................................................................................................... 56

5.1 Conclusions .................................................................................................................... 57

5.2 Suggestions for future work ........................................................................................... 57

57

5.1 Conclusions

From this study the following conclusions are drawn:

1. Manufacture of Aluminium–7Silicon–Magnesium alloys (LM25) based hybrid

composites using silicon carbide and graphite particles as reinforcing agents was partly

successful using stir casting process. The distribution of reinforcing particles in the

composites were not uniform and each sample contained a varying degree of porosity.

2. Addition of both SiC and G reinforcing particles to LM25 alloys caused a general

increase in the properties, although the properties of the composites were predictably

affected mostly by the hard reinforcing SiC particles.

3. The hardness, tensile strength and resistance to wear were found to be the maximum in

case of HC2 (LM25-5SiC-0G) composite. Addition of G particles somewhat lowered the

tensile properties, while a mixed result was obtained in case of hardness. The wear

resistance was decreased due to the addition of G particles.

4. The HC1 (LM25-2.5SiC-5G) hybrid composites had the intermediate properties of the

two composites HC2 and HC3 where SiC and G particles were used in isolation. The

hardness, tensile strength and resistance to wear were found to be higher but lower than

those obtained for HC2.

5. The age hardening response of all four materials was predictable. Hardness values were

generally increased in case of all materials.

5.2 Suggestions for future works

Preparation of homogeneous composite with well dispersed reinforcing agent and minimal

porosity is always mandatory to conduct investigation like those mentioned in this work. For

the production of good quality composites, proper equipment and facilities are required.

Use of variable amounts of reinforcing agents and consideration of an increased number of

test parameters (e.g., variable sliding speed, sliding load, sliding time in case or wear) will

help deducing authentic and accurate conclusions.

58

List of table

Page Number

Table 2.1: Alloy designation system for wrought aluminium alloys 5

Table 2.2: Classification of wrought aluminum alloys based on strengthening mechanism. 6

Table 2.3: Alloy designation system for cast aluminium alloys. 7

Table 2.4: Basic temper designations. 8

Table 2.5: Temper designation system for strain-hardened products. 8

Table 2.6: Temper designation system for heat-treatable alloys. 9

Table 2.7(a): The chemical composition of common cast aluminium alloys. 13

Table 2.7(b): The mechanical properties of common cast aluminium alloys. 14

Table 2.8: Chemical composition range of LM25 alloys. 18

Table 2.9: Physical property range of LM25 alloys. 19

Table 2.10: Mechanical properties of Aluminium-7Silicon-Magnesium alloys. 19

Table 2.11: Properties of silicon carbide. 28

Table 2.12: Properties of commercial graphite. 29

Table 3.1: Chemical composition of the raw materials and the master alloy. 37

Table 3.2: Chemical make-up of the LM25-SiC-G hybrid composites. 38

Table 3.3: Hardness test data. 39

Table 3.4: Tensile test data. 40

Table 3.5: Wear test data. 41

59

List of Figure Page number

Figure 2.1: Detail of aluminium alloy temper designation system. 10

Figure 2.2: Commercial cast aluminum-silicon binary alloys. (a) Al-Si equilibrium diagram.

(b) Microstructure of hypoeutectic alloy (<12.6 wt% Si). x150. (c) Microstructure of eutectic alloy

(12.6% Si). x400. (d) Microstructure of hypereutectic alloy (>12.6% Si). x150. 15 Figure 2.3: The pseudo-binary Al-Mg2Si phase diagram. 16

Figure 2.4: Microstructure of LM25 (A356) alloy. (a) As-cast (unmodified), (b) Modified with

0.008% Strontium. x100. 17

Figure 2.5: Microstructure of LM25 (A357) alloy. (a) As-cast (unmodified), (b) Modified with

0.013% Strontium. x100. 17

Figure 2.6: The schematics of the mixing machine with other setup to prepare the hybrid composite. 25 Figure 3.1: The mixing machine with other setup to prepare the hybrid composite. 36 Figure 3.1 A: Tensile Sample 37

Figure 3.2: Wear testing set-up. 38 Figure 4.1: Scanning electron micrograph and EDX analysis of HC1 (LM25-2.5SiC-5G). 43

Figure 4.2: Scanning electron micrograph and EDX analysis of HC2 (LM25-5SiC-0G). 44

Figure 4.3: Scanning electron micrograph and EDX analysis of HC3 (LM25-0SiC-5G). 45

Figure 4.4: Inhomogeneity and agglomeration of silicon carbide and graphite particles

in LM25-SiC-G hybrid composite. 46

Figure 4.5: Optical micrograph of LM25 master alloy. (a) x200, (b) x500. 47

Figure 4.6: Optical micrograph of HC1 (LM25-2.5SiC-5G) hybrid composite. (a) x200, (b) x500. 47 Figure 4.7: Optical micrograph of HC2 (LM25-5SiC-0G) hybrid composite. (a) x200, (b) x500. 48 Figure 4.8: Optical micrograph of HC3 (LM25-0SiC-5G) hybrid composite. (a) x200, (b) x500. 48

Figure 4.9: Scanning electron micrograph of (a) LM25 master alloy and

(b) HC1 (LM25-2.5SiC-5G) hybrid composite. 49

Figure 4.10: Scanning electron micrograph of (a) HC2 (LM25-5SiC-0G) and

(b) HC3 (LM25-0SiC-5G) hybrid composites. 49

Figure 4.11: Hardness value of LM25 master alloy and other composites. 50

Figure 4.12: Tensile properties of LM25 master alloy and other composites. 51

Figure 4.13: Variation of wear loss in LM25 master alloy and other composites. 53

Figure 4.14: Variation of wear resistance in LM25 master alloy and other composites. 53

Figure 4.15: Optical micrograph of worn surface of (a) LM25 master alloy, (b) HC1 (LM25-2.5SiC-

5G) composite, (c) HC2 (LM25-5SiC-0G) composite, and (d) HC3 (LM25-0SiC-5G) composite. 54

Figure 4.16: Variation in hardness before and after heat treatment

in LM25 master alloy and other composites. 55

60

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