Effect of size and volume fraction of reinforcement on the properties of light metals

EFFECT OF SIZE AND VOLUME FRACTION OF
REINFORCEMENT ON THE PROPERTIES OF LIGHT
METALS

HO KAO FENG CALVIN
(B. Eng.(Hons.), NUS)

A THESIS SUBMITTED
FOR THE DEGREE OF MASTER OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2003


Acknowledgements
The author will like to express his sincere gratitude and appreciation to the
following people:

1. Dr. M. Gupta for his invaluable guidance and supervision.
2. Mr. Thomas, Mr. Maung Aye Thein, Hong Wei, Mr. Khalim, Mr.
Juraimi and Mdm. Zhong X L from the Materials Science Lab for their
technical support and assistance.
3. Mr. Chua Beng Wah, Miss Sharon Nai and Mr. Syed Fida Hassan for
their friendship, encouragement and valuable discussions.
4. All other research scholars for their advice and help given.
5. Mr. Lau P K for his advice and assistance in extrusion.
6. Mr. Chiam and Joe Low from the Experimental Mechanics Lab for his
assistance in tensile testing.
7. Mr. Lam from TSU for his assistance in machining.
8. N. Srikanth for his advice and assistance in dynamic modulus testing.

i


Table of Contents
ACKNOWLEDGEMENTS .................................................................................................................... i
TABLE OF CONTENTS .......................................................................................................................ii
SUMMARY ...............................................................................................................................................vi
LIST OF ILLUSTRATIONS .............................................................................................................viii
CHAPTER 1: INTRODUCTION........................................................................................................ 1
CHAPTER 2: LITERATURE RESEARCH.................................................................................... 4
2.1

PROCESSING M ETHODS..........................................................................................................5

2.1.1

Solid State Processing.................................................................................................... 5

2.1.1.1

Powder Metallurgy .........................................................................................................5

2.1.1.2

Diffusion Bonding..........................................................................................................6

2.1.1.3

Mechanical Alloying......................................................................................................7

2.1.2

Semi-solid Processing..................................................................................................... 7

2.1.3

Liquid State Processing.................................................................................................. 8

2.1.3.1

Dispersion Process.........................................................................................................8

2.1.3.2

Infiltration ......................................................................................................................9

2.1.3.3

Spraying.......................................................................................................................10

2.1.3.4

In-situ Fabrication........................................................................................................10

2.1.4
2.2

New Innovative Method................................................................................................11
TYPES OF REINFORCEMENT ................................................................................................12

2.2.1

Continuous fiber ............................................................................................................12

2.2.2

Metal wires.....................................................................................................................13

2.2.3

Whiskers..........................................................................................................................13

2.2.4

Short fibers .....................................................................................................................13

2.2.5

Particulates.....................................................................................................................14

2.3

THERMOMECHANICAL PROCESSES ....................................................................................15

2.4

INTERFACE CHARACTERISTICS...........................................................................................17

2.5

EFFECT OF REINFORCEMENT SIZE......................................................................................19

2.6

SUMMARY..............................................................................................................................20

ii


CHAPTER 3: MATERIALS AND EXPERIMENTAL PROCEDURES ..............................21
3.1

M ATERIALS............................................................................................................................22

3.2

SYNTHESIS.............................................................................................................................26

3.3

SPECIMEN PREPARATION.....................................................................................................29

3.4

EXTRUSION............................................................................................................................29

3.5

QUANTITATIVE A SSESSMENT OF REINFORCEMENT.........................................................30

3.6

QUANTITATIVE A SSESSMENT OF ALLOYING ELEMENT ..................................................30

3.7

DENSITY M EASUREMENT ....................................................................................................31

3.8

M ICROSTRUCTURE CHARACTERIZATION..........................................................................31

3.9

X-RAY DIFFRACTION STUDIES ...........................................................................................31

3.10

M ECHANICAL TESTING........................................................................................................32

3.10.1

Microhardness Measurement......................................................................................32

3.10.2

Macrohardness Measurement.....................................................................................32

3.10.3

Tensile Testing...............................................................................................................33

3.10.4

Dynamic Elastic Modulus Testing..............................................................................33

3.11

FRACTOGRAPHY....................................................................................................................34

3.12

THERMAL MECHANICAL ANALYSIS ..................................................................................34

CHAPTER 4: RESULTS .....................................................................................................................36
4.1

PHASE I & II RESULTS.........................................................................................................37

4.1.1

Processing.......................................................................................................................37

4.1.2

Density Measurement ...................................................................................................37

4.1.3

Microstructural Characterization..............................................................................38

4.1.4

Mechanical Characterization......................................................................................42

4.1.4.1

Hardness.......................................................................................................................42

4.1.4.2

Tensile Properties.........................................................................................................42

4.1.5

Fractography..................................................................................................................43

4.1.6

Thermal Mechanical Analysis.....................................................................................46

4.2

PHASE III RESULTS...............................................................................................................47

4.2.1

Processing.......................................................................................................................47

4.2.2

Quantitative Assessment of Reinforcement ...............................................................47

iii


4.2.3

Quantitative Assessment of Alloying Element..........................................................48

4.2.4

Density Measurement ...................................................................................................48

4.2.5

Microstructural Characterization..............................................................................49

4.2.6

X-ray Diffraction Results.............................................................................................53

4.2.7

Mechanical Characterization......................................................................................53

4.2.7.1

Hardness.......................................................................................................................53

4.2.7.2

Tensile Properties.........................................................................................................54

4.2.8

Fractography..................................................................................................................54

4.2.9

Thermal Mechanical Analysis.....................................................................................57

4.3

PHASE IV RESULTS..............................................................................................................58

4.3.1

Processing.......................................................................................................................58

4.3.2

Quantitative Assessment of Reinforcement ...............................................................58

4.3.3

Quantitative Assessment of Alloying Element..........................................................59

4.3.4

Density Measurement ...................................................................................................59

4.3.5

Microstructural Characterization..............................................................................60

4.3.6

X-ray Diffraction Results.............................................................................................64

4.3.7

Mechanical Characterization......................................................................................65

4.3.7.1

Hardness.......................................................................................................................65

4.3.7.2

Tensile Properties.........................................................................................................65

4.3.8

Fractography..................................................................................................................66

4.3.9

Thermal Mechanical Analysis.....................................................................................69

CHAPTER 5: DISCUSSIO N ..............................................................................................................70
5.1

PHASE I & II DISCUSSION ....................................................................................................71

5.1.1

Processing.......................................................................................................................71

5.1.2

Secondary processing ...................................................................................................71

5.1.3

Density and Porosity.....................................................................................................72

5.1.4

Microstructure...............................................................................................................72

5.1.5

Mechanical Behavior....................................................................................................73

5.1.6

Fractography..................................................................................................................75

5.1.7

Thermal Mechanical Analysis.....................................................................................75

iv


5.1.8
5.2

Proceeding to Phase III................................................................................................75
PHASE III DISCUSSION .........................................................................................................76

5.2.1

Processing.......................................................................................................................76

5.2.2

Secondary processing ...................................................................................................77

5.2.3

Density.............................................................................................................................77

5.2.4

Microstructure...............................................................................................................78

5.2.5

Mechanical Behavior....................................................................................................80

5.2.6

Fractography..................................................................................................................82

5.2.7

Coefficient of Thermal Expansion (CTE)..................................................................82

5.2.8

Proceeding to Phase IV................................................................................................83

5.3

PHASE IV DISCUSSION.........................................................................................................84

5.3.1

Processing.......................................................................................................................84

5.3.2

Secondary Processing...................................................................................................84

5.3.3

Density.............................................................................................................................85

5.3.4

Microstructure...............................................................................................................85

5.3.5

Mechanical Behavior....................................................................................................87

5.3.6

Fractography..................................................................................................................90

5.3.7

Coefficient of Thermal Expansion (CTE)..................................................................90

CHAPTER 6: CONCLUSIONS .........................................................................................................91
CHAPTER 7: RECOMMENDATIONS..........................................................................................96
REFERENCES………………………………………………………………………………..98
APPENDICES ………………………………………………………………………………. 103

v


Summary
This study addresses the feasibility of synthesizing nanometric alumina
reinforced aluminium composites using the innovative disintegrated melt deposition
(DMD) technique and the effect of reinforcement size and volume fraction on the
microstructural and mechanical properties of aluminium.
In this study, pure aluminium material was fabricated in the Material Science

Laboratory of National University of Singapore (NUS). The material was successfully
synthesized using DMD technique followed by extrusion at different extrusion
temperatures (Phase I) and extrusion ratios (Phase II). The effect of different secondary
processing parameters on the microstructure and mechanical properties was
investigated to obtain optimum parameters for the subsequent processing of
aluminium-based composites.
Varying amounts of magnesium were added to the aluminium matrix to
investigate the wettability of nanometric (0.05µm) alumina particulates in Al-Mg
(Phase III). An attempt was made to correlate the magnesium content of the matrix to
the alumina incorporation and, microstructural and mechanical properties of the
composite. An optimum magnesium content was chosen for the matrix of composites
in the subsequent phase. Alumina particulates in the sizes of 0.3 µm, 1 µm and 10µm
were added to an Al-Mg matrix to investigate the effect of different reinforcement size
on the microstructural and mechanical properties of Al-Mg (Phase IV).
The first 2 phases of the study verified the effect of extrusion parameters on the
grain size of the matrix. Generally, grain size refinement was found to occur when the
extrusion temperature is reduced or the extrusion ratio is increased. Grain size
refinement subsequently led to an increase in hardness, 0.2% yield stress (YS),
ultimate tensile stress (UTS) and a decrease in ductility. The elastic modulus and
vi


coefficient of thermal expansion (CTE) was insensitive to changes in extrusion
parameters. Phases I & II revealed that the optimal extrusion temperature and ratio was
25°C and 26.45:1 respectively.
Phase III demonstrated the feasibility of fabricating nanometric alumina
reinforced aluminium composites when sufficient magnesium is added to the
aluminium matrix as a wetting agent. Alumina incorporation increased with higher
magnesium content in the matrix. The maximum volume fraction of nanometric
alumina was low but the presence of small volume fraction of uniformly distributed

nanometric reinforcement was sufficient to bring about a significant improvement in
mechanical properties. Phase III revealed that the addition of 5 wt.% magnesium to
aluminium holds the promise to realize the best mechanical properties.
Phase IV showed that larger alumina particulates are easier to incorporate in an
Al-Mg matrix. However, the larger sized particulates tended to form clusters. This
resulted in higher porosity and hence a reduction in mechanical properties. As a result,
properties such as hardness, elastic modulus, 0.2% YS and UTS decreased with
increasing reinforcement size. An improvement in ductility was observed as
reinforcement size decreased, until a threshold size of 0.3µm.
The results obtained in the present study revealed that a small addition of
nanometric alumina reinforcement has great potential in significantly enhancing the
mechanical properties of aluminium.

vii


List of Illustrations
FIGURE 3-1

FLOW CHART SHOWING THE OVERVIEW OF THE PROJECT .

24

FIGURE 3-2

RAW MATERIALS USED: A LUMINIUM GRANULES, MAGNESIUM TURNINGS AND ALUMINA.
25

FIGURE 3-3

SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SET -UP FOR THE DMD CASTING.

26

FIGURE 3-4

RESISTANCE FURNACE USED FOR DMD.

27

FIGURE 3-5

COMPONENTS OF MOLD AND METALLIC SUBSTRATE.

28

FIGURE 3-6

TOOLS USED IN FABRICATION (FROM BACK): GRAPHITE CRUCIBLE FITTED WITH
NOZZLE /PLUG AND MILD STEEL LID, CERAMIC TUBE , MILD STEEL STIRRER,
THERMOCOUPLE.

29

FIGURE 4-1

REPRESENTATIVE SEM MICROGRAPHS SHOWING GRAIN MORPHOLOGY OF AL
EXTRUDED WITH AN EXT RUSION RATIO OF 20.25:1 AND TEMPERATURE OF: (A) 25°C, (B )
85°C, (C ) 150°C, (D) 250°C AND (E) 350°C.
40

FIGURE 4-2

REPRESENTATIVE SEM MICROGRAPHS SHOWING GRAIN MORPHOLOGY OF AL
EXTRUDED AT 85°C WITH AN EXTRUSION RATIO OF: (A) 26.45:1 AND (B)12.96:1.

41

FIGURE 4-3

REPRESENTATIVE SEM MICROGRAPHS SHOWING THE FRACTURE SURFACE OF AL
EXTRUDED WITH AN EXTRUSION RATIO OF 20.25:1 AND TEMPERATURE OF: (A) 25°C, (B )
85°C, (C ) 150°C, (D) 250°C AND (E) 350°C.
44

FIGURE 4-4

REPRESENTATIVE SEM MICROGRAPHS SHOWING THE FRACTURE SURFACE OF AL
EXTRUDED AT 85°C WITH AN EXTRUSION RATIO OF: (A) 26.45:1 AND (B) 12.96:1.

45

REPRESENTATIVE SEM MICROGRAPHS SHOWING GRAIN MORPHOLOGY OF: (A) A L1.6M G/A L2 O3 , (B) A L-2.9M G/A L2 O3 AND (C) AL-3.4M G/A L2 O3 .

50

FIGURE 4-5
FIGURE 4-6

REPRESENTATIVE SEM MICROGRAPHS SHOWING UNIFORM DISTRIBUTION OF

INTERMETALLIC AL12 M G17 (INDICATED BY ARROWS) IN: (A) A L-1.6M G/A L2 O3 , (B) A L2.9M G/A L2 O3 AND (C) A L-3.4M G/A L2 O3 .
51

FIGURE 4-7

REPRESENTATIVE FESEM MICROGRAPHS SHOWING DISTRIBUTION OF ALUMINA IN: (A)
A L-1.6M G/A L2 O3 , (B) A L-2.9M G/A L2 O3 AND (C) A L-3.4M G/A L2 O3 .
52

FIGURE 4-8

REPRESENTATIVE SEM MICROGRAPHS SHOWING THE FRACTURE SURFACE OF: (A) A L1.6M G/A L2 O3 , (B) A L-2.9M G/A L2 O3 AND (C) AL-3.4M G/A L2 O3 .
55

FIGURE 4-9

REPRESENTATIVE SEM MICROGRAPHS AT HIGH MAGNIFICATION SHOWING THE
DEFORMATION CHARACTERISTICS OF: (A) A L-1.6M G/A L2 O3 , (B) A L-2.9M G/A L2 O3 AND
(C) A L-3.4M G/A L2 O3 .
56

FIGURE 4-10

REPRESENTATIVE SEM MICROGRAPHS SHOWING GRAIN MORPHOLOGY OF: (A) A L-M G,
(B) A L-M G-0.3µM A L2 O3 , (C) A L-M G-1µM A L2 O3 AND (D) AL-M G-10µM A L2 O3
SAMPLES.
61

FIGURE 4-11

REPRESENTATIVE SEM MICROGRAPHS SHOWING UNIFORM DISTRIBUTION OF
INTERMETALLIC AL12 M G17 (INDICATED BY ARROWS) IN: (A) A L-M G, (B) A L-M G-0.3µM
A L2 O3 , (C) A L-M G-1µM A L2 O3 AND (D) A L-M G-10µM A L2 O3 SAMPLES.
62

FIGURE 4-12

REPRESENTATIVE SEM MICROGRAPHS SHOWING DISTRIBUTION OF ALUMINA IN : (A) A LM G-0.3µM A L2 O3 , (B) A L-M G-1µM A L2 O3 AND (C) A L-M G-10µM A L2 O3 SAMPLES. 63

FIGURE 4-13

REPRESENTATIVE SEM MICROGRAPH SHOWING ALUMINA CLUSTER AND GOOD
INTERFACIAL INTEGRIT Y BETWEEN INDIVIDUAL AL2 O3 PARTICULATES AND MATRIX IN
THE CASE OF A L-M G-10µM A L2 O3 .
64

FIGURE 4-14

REPRESENTATIVE SEM MICROGRAPHS SHOWING THE FRACTURE SURFACE OF: (A) A LM G, (B ) A L-M G-0.3µM A L2 O3 , (C) A L-M G-1µM A L2 O3 AND (D) A L-M G-10µM A L2 O3
SAMPLES.
67

viii


FIGURE 4-15

REPRESENTATIVE SEM MICROGRAPHS AT HIGH MAGNIFICATION SHOWING THE
DEFORMATION CHARACTERISTICS OF: (A) A L-M G, (B) A L-M G-0.3µM A L2 O3 , (C) A LM G-1µM A L2 O3 AND (D) A L-M G-10µM A L2 O3 SAMPLES.
68

FIGURE 5-1

GRAPH OF EXTRUSION LOAD VS. CUMULATIVE MAGNESIUM AND ALUMINA
REINFORCEMENT WEIGHT PERCENT .

77

GRAPH OF 0.2% YS VS. CUMULATIVE MAGNESIUM AND ALUMINA REINFORCEMENT
WEIGHT PERCENT .

81

FIGURE 5-2
FIGURE 5-3

GRAPH OF UTS VS. CUMULATIVE MAGNESIUM AND ALUMINA REINFORCEMENT WEIGHT
PERCENT .
82

FIGURE 5-4

GRAPH OF 0.2% YS, UTS AND DUCTILITY VS. ALUMINA PARTICULATE SIZE.

88

FIGURE 5-5

BAR CHART SHOWING A SUMMARY OF MECHANICAL PROPERTIES OF MATERIALS
FABRICATED IN THE PRESENT STUDY.

89

ix


Chapter 1: Introduction

Chapter 1


Introduction

Metal-matrix composites (MMCs) combine the metallic properties such as
ductility, toughness and environmental resistance with the ceramic properties such as
high strength and high modulus. They offer several advantages in applications where
high strength, high modulus and good thermal conductivities are desirable. One of the
main advantages of MMCs is the ability to tailor the composite behavior for the
intended application. Aluminium-based MMCs reinforced with particulate ceramics
have attracted the interest of many researchers recently owing to their low density,
wide alloy range, heat treatment capability, processing flexibility and low cost.
Particulates such as SiC, Al2O3, TiC, B4C and TiB2 have commonly been used
to reinforce aluminium alloys. The size of the ceramic particulates in commercial
MMCs generally ranges from a few micrometers to several hundred micrometers.
Several studies have indicated that the strengths of particulate composites tend to
increase with decreasing particle size [1]. However, little work has been reported in
open literature concerning the properties of aluminium-based MMCs reinforced with
nanometric particulates. Most of the work involving the use of nanometric particulates
was based on the powder metallurgy route.
The benefits of nanometric reinforcement can be seen from secondary phases

formed during heat treatment of precipitation-hardened aluminium alloys. The fine
particles formed during heat treatment of such alloys are able to impede dislocation
motion and produce significant improvement in mechanical properties. However, most
precipitates are not chemically stable at elevated temperature, leading to their
dissolution or coarsening [2], and subsequently a decrease in strength.
The introduction of chemically stable, insoluble reinforcement has the potential
to overcome this problem. In experiments with sintered aluminium powder materials,
fine alumina reinforcement formed from the densification of oxidized aluminium

2


Introduction

powder has shown to be capable of pinning grain boundaries during recrystallization,
resulting in stable fine grains and additional grain boundary strengthening.
Accordingly, the primary aim of the present study was to synthesize aluminium
based materials reinforced with different sizes of alumina particulates using an
innovative disintegrated melt deposition technique followed by hot extrusion.
Phase I & II of the present project involves varying the extrusion temperature
and ratio of aluminium respectively. The purpose was to obtain as fine matrix
microstructure as possible. The samples obtained from both phases were characterized
for their microstructural, physical and mechanical properties with respect to the effect
of different extrusion temperatures and ratios used. The optimal and feasible extrusion
parameters were used to process the composites containing different length scales of
reinforcement.
Alumina is not wetted by aluminium in normal foundry temperatures. The
problem of wettability of ceramic particulates is even more severe for the nanosized
particulates used in the present study. Phase III investigates the effect of the addition of
magnesium as a wetting reagent on the incorporation of alumina in an aluminium melt.

The samples obtained were characterized for their microstructural, physical and
mechanical properties with respect to the effect of different magnesium contents in the
aluminium matrix. The optimal magnesium weight fraction was determined and used
to process the composites in Phase IV.
Phase IV investigates the effect of the size of alumina reinforcement on the
properties of the composite. The samples obtained were characterized for their
microstructural, physical and mechanical properties with respect to the effect of
different alumina reinforcement size in the aluminium matrix.

3


Chapter 2: Literature Research

Chapter 2


Literature Research

2.1

Processing Methods
Processing of MMCs can be done using a number of techniques which can be

grouped under: (a) solid state, (b) semi-solid state and (c) liquid state categories [3].
The selection of the processing technique is very important since the resultant
microstructural features are highly dependent on it. With such a wide range of
processing routes available, the appropriate choice will depend on the application and
acceptable cost.

2.1.1 Solid State Processing
Solid state processes are generally used to obtain the highest strength properties
in MMCs because segregation effects and brittle reaction product formation are at a
minimum for these processes when compared to liquid state processes. The main solid
state processes are:
•

Powder metallurgy

•

Diffusion bonding

•

Mechanical Alloying

2.1.1.1 Powder Metallurgy
Powder metallurgy is the most commonly used method in solid state processing
[4]. Metal powder is first blended with reinforcement particulates. A cold isostatic
pressing is utilized to obtain a green compact that is then thoroughly outgassed and
forged or extruded. When hot isostatic pressing is required, the powder blend must
first be outgassed. The main advantages of the powder metallurgy process are: (a) it

5


Literature Research

allows any alloy to be used as the matrix, (b) it allows any type of reinforcement to be

used because reaction between matrix and reinforcement can be minimized by working
below the matrix solidus temperature, and (c) high volume fractions of reinforcement
are possible, thus maximizing the improvement of the properties of the matrix. The
main difficulty in this process is the removal of the binder used to hold the powder
particles together. These organic binders often leave residual contamination that causes
deterioration of the mechanical properties of the composite. The other disadvantages
are the: (a) inherent danger when handling large quantities of highly reactive powders,
(b) the complexity of the manufacturing route and (c) the limitation of the initial
products forms it can produce [3]. As a result, the product is expensive in comparison
with liquid state methods.

2.1.1.2 Diffusion Bonding
Diffusion bonding is used for consolidating alternate layers of foils and fibers
to create single or multiple-ply composites. This is a solid state creep deformation
process. After creep flow of the matrix occurs between the fibers to make complete
metal-to-metal contact, diffusion occurs across the foil interfaces. This process
combines the advantages of ease of processing a wide variety of matrix metals, and the
control of orientation and volume fraction of the fibers. The main problems associated
with this process are fiber degradation and thermal expansion mismatch. Fiber-matrix
interfacial reactions during diffusion bonding can cause degradation of the fiber
interface region and reduce its load carrying ability. Thermal expansion mismatch
between fiber and matrix can often cause tensile stresses and matrix cracking when
cooling from the diffusion bonding temperature.

6


Literature Research

2.1.1.3 Mechanical Alloying

In this method, a high-energy impact mill is used to continuously fragment and
reweld powder particles as fresh internal surfaces are exposed. Frictional heating at the
particle interface causes local melting and consolidation, and rapid heat extraction by
the cooler particle interior causes rapid solidification. Hence, composites produced by
this method are often strong due to high dislocation density and homogenous due to
the thorough mixing of the constituents. The main disadvantage of this process is the
reduced ductility of its products.

2.1.2 Semi-solid Processing
Semi-solid processes were investigated in the early 1970s [5]. One such
process is known as compocasting. It differs from conventional casting in that semisolid alloys are used. In this method, liquid alloy at a temperature slightly above the
liquidus is vigorously agitated and allowed to slowly cool to the semi-solid range. The
alloy exhibits thixotropic behavior, that is, its viscosity decreases when agitated. The
continued agitation prevents a rise in viscosity and breaks up the solidifying dendrites
into fine, spheroidal particles. Reinforcement particles are added to the slurry during
the stirring stage. The viscosity of the slurry inhibits ceramic particle settling and
floating, and can be used to retain particles in the melt. Compocasting has been shown
to be an effective processing scheme with acceptable economics but it faces the
quality-related problems of uneven distribution of particles and high levels of porosity
[6].

7


Literature Research

2.1.3 Liquid State Processing
Liquid state processes can be classified by the method used to physically
combine the matrix and reinforcement. They can be divided into four major categories:
(a) dispersion, (b) infiltration, (c) spraying and (d) in-situ fabrication.

2.1.3.1 Dispersion Process
In dispersion processes, the metallic matrix is superheated in the molten range
and the reinforcement is subsequently incorporated in loose form. Most metalreinforcement systems exhibit poor wetting so mechanical agitation is required to
combine the 2 phases. Surface modification of reinforcement particulates or the
addition of wetting agents to the melt can also help the incorporation and retention of
reinforcement in the matrix. There are several techniques used to incorporate the
reinforcement particulates into the matrix melt [7]. These often proprietary methods
include:
•

Injection of particulates entrained in an inert carrier gas into the melt
with the help of an injection gun. The particulates mix into the melt as
the bubbles ascend through the melt.

•

Addition of particulates into the molten stream as it fills the mold.

•

Addition of particulates into the melt via a vortex introduced by
mechanical agitation.

•

Addition of small briquettes, co-pressed aggregates of matrix alloy
powder and reinforcement particulates, into the melts while stirring.

•

Dispersion of the fine particulates in the melt using centrifugal
acceleration.

8


Literature Research

•

Pushing the particulates into the melt using reciprocating rods.

•

Injection of the particulates into the melt while the melt is continuously
irradiated with high intensity ultrasound.

•

Zero-gravity processing, which involves utilizing a synergism of ultrahigh vacuum and elevated temperature for prolonged periods of time.

Dispersion processes are the most inexpensive way to produce large quantities
of MMCs. The main disadvantages include: (a) the settling of reinforcement
particulates and (b) the limited volume fraction of reinforcement that can be
incorporated. Settling of reinforcement occurs as a result of density difference between
reinforcement particulates and the matrix melt. The volume fraction of reinforcement
is limited because the viscosity of the melt increases with particle incorporation and
becomes non-Newtonian [3]. As a result, the power requirements necessary for mixing
limits the amount of reinforcement that can be incorporated.

2.1.3.2 Infiltration
Infiltration processes involve holding a porous body of the reinforcing phase
within a mold and infiltrating it with molten metal that flows through the interstices to
fill the pores and produce a composite. Depending on the external forces applied to the
metal, the infiltration process can have several variations. In pressureless infiltration,
the metal is allowed to spontaneously infiltrate the reinforcement. However, pressure is
usually applied for benefits such as increased processing speed, control over chemical
reactions, refined matrix microstructures and better soundness of the product. For some
matrix-reinforcement systems, creating a vacuum around the reinforcement provides
the necessary pressure difference to drive infiltration. Pressure can also be applied by a
9


Literature Research

gas, mechanical means, vibrations, centrifugal forces and electromagnetic body forces.
The advantages of infiltration processes include: (a) near-net shape production of parts
and (b) its ability to selectively reinforce metallic matrix with a variety of materials. If
cold dies and reinforcements are used, or if high pressures are maintained during
solidification, matrix-reinforcement chemical reactions can be minimized and defectfree matrix microstructures can be achieved. A limitation is the need for the
reinforcement to be self-supporting. The application of pressure also has the tendency
to induce preform breakage during infiltration, resulting in heterogeneity.

2.1.3.3 Spraying
In spraying processes, droplets of molten metal are sprayed together with the
reinforcing phase and collected on a substrate where metal solidification is completed
[8]. Alternatively, the reinforcement may be placed on the substrate and molten metal
is sprayed onto it. The main advantage of spray processes is the high solidification rate
of droplets produces materials with little segregation and a refined grain structure [9].

Contact time between the melt and the reinforcing particles is also brief, so reaction
between the two is limited and a wider range of reinforcements are possible. The major
disadvantages include limitation of product to simple shapes and the presence of
residual porosity. The process is also not economical due to the high cost of gases used
and the large amounts of waste powder to be collected and disposed.

2.1.3.4 In-situ Fabrication
In-situ processes involve the synthesis of composites such that the
reinforcement is formed in the melt during the processing. An example of an in-situ

10


Literature Research

process is the XD process. The XD process is a patented composite manufacturing
method in which ceramic particles, such as TiB2 and TiC, are produced in situ in a
melt. The process consists of adding compounds to a solvent metal to generate an
exothermic reaction and produce the required reinforcing particles. Alternative
methods are Self-propagation High temperature Synthesis (SHS) and gas injection
[10]. A major advantage of in-situ composite materials is that the reinforcing phase is
homogenously distributed, and the spacing or size of the reinforcement may be
adjusted by the solidification or reaction time. The particles produced are inherently
wetted by the matrix and therefore possess high interfacial strength [11]. The main
disadvantages of this method are the high viscosity of melts, limitation in choice of
reinforcement-matrix systems, and difficulty in controlling the kinetics of the process.

2.1.4 New Innovative Method
In recent years, spraying processes have begun to attract attention among
researchers. The benefits of microstructural refinement, reduced segregation and

minimal interfacial reactions give it great potential as a fabrication technique for
MMCs. A new variant of spray processing was developed recently, which brings
together the cost-effectiveness of conventional foundry processes and the advantages
of spraying processes. This method is known as the Disintegrated Melt Deposition
(DMD) technique [12]. It involves incorporating the ceramic particulates by vortex
mixing. The resulting slurry is then disintegrated by jets of inert gas and subsequently
deposited on a metallic substrate. Unlike conventional spray processes, the DMD
technique employs higher superheat temperatures and lower impinging gas jet velocity.
This process produces only bulk composite material and avoids the formation of overspray powders. It offers both the features of finer grain size and low segregation of
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reinforcement of spray processes, and the simplicity and cost effectiveness of
conventional foundry processes. This technique was thus chosen as the primary
process in the present study to fabricate the composites.

2.2

Types of Reinforcement
The main role of reinforcement in composites used in most engineering

applications is to carry load. Appropriately reinforced materials tend to have higher
strength, stiffness and temperature resistance capabilities when compared to their
monolithic counterparts. The reinforcements for MMCs can be broadly divided into
five categories:
•

Continuous fibers

•

Metal wires

•

Whiskers

•

Short fibers

•

Particulates

2.2.1 Continuous fiber
Continuous fiber reinforcements frequently used in composites include boron
in tungsten and silicon carbide fibers in tungsten [13]. The basic requirements for fiber
reinforcement are high strength, high elastic modulus and low density. In addition, the
melting temperature of the fibers has to be higher than that of the matrix, and the fibers
are expected to be compatible with the matrix from the points of view of technology
and lifetime. Continuous fiber MMCs exhibit very good directional properties.

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However, the high cost of continuous fiber reinforcement and labor-intensive
fabrication routes put them at disadvantage for most commercial applications [14].

2.2.2 Metal wires
Metal wires that have been investigated by researchers include tungsten and
steel [15]. The good ductility of metal wires makes them an ideal reinforcement for
composites carrying tensile load. However, their high density and significant metal-tometal reaction at elevated temperatures lead to fabrication problems, thus limiting their
use.

2.2.3 Whiskers
Whiskers are characterized by their fibrous, single-crystal structures which
have almost no crystalline defects. They are needle-like crystals, having an aspect ratio
ranging from 100 to 15000 [16]. Due to their small size, whiskers are either free of
dislocations or the dislocations they contain do not significantly affect their strength,
which approaches the theoretical strength of the material. Numerous materials,
including metals, oxides, carbides, halides and organic compounds have been prepared
under controlled conditions in the form of whiskers. Silicon carbide whiskers are most
commonly used as reinforcement in composites.

2.2.4 Short fibers
Short fibers are longer than whiskers. Their length is longer than the critical
length Lc (Lc = dσf/σm where d is the fiber diameter, σf is the reinforcement strength

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and σm is the matrix strength) and their aspect ratio ranges from 20 to 60. In a given
fiber, if the mechanical properties improve as a result of increasing the fiber length,

then it is denoted a short fiber. The oxide fibers, Saffil and Kaowool, are used mainly
for the reinforcement of automobile engine components. While short fibers are cheaper
than both whiskers and continuous fibers, they are also less effective as reinforcement.

2.2.5 Particulates
Particulate reinforced metals, with their advantages of low cost, high modulus
and strength, high wear resistance, are more viable for commercial use. The lower
fabrication costs come about due to the low cost and easy availability of reinforcement,
and the ability to utilize existing conventional metallurgical processes such as forging,
rolling, extrusion, etc. Particulate reinforced materials are also attractive because they
exhibit more isotropic properties [17] as a result of their random distribution and low
aspect ratio of the particulates. Particulates such as oxides, carbides, nitrides, borides
and elemental materials have been extensively investigated as reinforcement by
researchers worldwide.

With all the advantages of using particulate reinforcement, it can be expected
that much of the current research and commercialization will lean toward this area of
study. From the above description, the possibilities of particulate reinforced MMCs
might seem boundless. In reality, the choice of reinforcement is dictated by several
factors [3]:
1. The application – If the composite is to be used in a structural
application, a low density and high modulus reinforcement should be
used.
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2. Particle size and shape – Angular particles can act as local stress raisers,
reducing the ductility of the composite. For composites processed in the

molten state, coarser particles are generally easier to be incorporated but
are more susceptible to gravity settling, leading to segregation in the
casting. Finer particles increase the viscosity of the melt, making
processing difficult.
3. Coefficient of thermal expansion (CTE) – If the composite is to be used
in thermal applications, the CTE and thermal conductivity are
important. The CTE also influences the strength of the composite.
4. Compatibility with matrix material – Reaction of the reinforcement
with the matrix can severely degrade the properties of the resultant
composite.
5. Cost – A major reason for using particles as reinforcement is to reduce
the cost of the composite. To keep in line with that objective, the
reinforcement has to be readily and cheaply available.

From the considerations above, Al2O3 and SiC particulates have emerged as the
most widely used reinforcements in aluminium-based MMCs. Due to the wide range
of grades available, Al2O3 particulates were selected as the reinforcement phase in the
present study.

2.3

Thermomechanical Processes
One of the advantages of particulate MMCs is that raw ingots obtained from

primary processing can be processed into usable shapes and sizes by employing
conventional metal forming technologies. Control of parameters of these
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