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ABSTRACT
The alloy LM25 is a new entry to the family of LM series of heat
treatable Mg-Si-Cu aluminium alloys. The data available, while not sufficient to
establish design mechanical property values for LM25, indicate that tensile,
ultimate and yield strengths are about 40 percent higher than LM25 alloy. In
this study LM25 matrix alloy is chosen and SiC particles having average
particle size of 20µm with four volume fractions of 0, 5, 10 and 15% were
incorporated into the alloy at the liquid state stir casting followed by extrusion.
Tensile, impact, hardness test were carried out both alloy and composites. The
10% volume of SiCp aluminium matrix composites showed the maximum
tensile and yield strength. The results indicate that extrusion can substantially
improve the distributed homogeneity of the SiC particles in the matrix and help
to decrease the number of pores and improve interfacial bonding strength of the
composites. The yield strength and tensile strength of the composites decrease
with increasing the volume fraction of the SiC particles, while the hardness of
the composites increases with increasing the volume fraction of the SiC
particles.
Keywords: Matrix alloy, SiCp Aluminium Matrix Composites, Stir
Casting, Yield Strength.
CHAPTER 1
INTRODUCTION
A LM25 aluminium alloy, has been developed for application in hot and
cold extrusion and forging. It contains 2% Mg+Si, 1% Cu, 0.2% Cr, and 0.1%
V. This alloy also has favorable fatigue and corrosion fatigue properties. These
properties are attributable to a combination of composition, high solidification
rate, controlled homogenization, thermal and mechanical processing. Present
applications are high pressure seamless gas containers formed by either hot or
cold impact extrusion and tubing for light weight bicycle frames.
This study reports of the development of an aluminium-magnesium-silicon
alloy, LM25 that combines strength, extrudability, favorable corrosion
resistance with low cost and scrap compatibility. Six prospective alloy
compositions were studied and the composition of what is now designated
LM25 had the best properties. It will be demonstrated that relatively high
strength was anticipated by increasing Si, Mg, and Cu concentration, as these
are the principle basis of precipitation strengthening in alloys such as LM25.
The Mg and Si concentration exceed the solubility in Al.
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. Silicon carbide is not affected by any acids and molten salts up to
800ºc. In air sic forms a protective silicon oxide coating at 1200ºC and is able to
be used up to 1600ºC. The high thermal conductivity coupled with low thermal
expansion and high strength gives this material exceptional thermal shock
resistant quality.
Silicon carbide ceramics with little or no grain boundary impurities
maintain their strength to very high temperatures, approaching 1600ºC with no
strength loss. Chemical purity, resistance to chemical attack at temperature, and
strength retention at high temperatures has made this material very popular as
wafer tray supports and paddles in semiconductor furnaces. The electrical
conduction of the material ha lead to its use in resistance heating elements for
electric furnaces, and as a key component in thermistors (temperature variable
resistors) and in varistors (voltage variable resistors).
Composites are defined as the combination of two or more material, which
are having distinct phase and properties, superior to the base material.
Mechanical processing is an important step in the manufacturing of engineering
components and is used not only to achieve the required shape but also to
impart desirable changes in the micro structure and properties. Expensive and
time consuming trial and error techniques are generally adopted for this
purpose.
In most composites, reinforcement is added to the matrix of the bulk
material to increase strength and stiffness of the matrix [1-3].
A reduction in material density coupled with an increase in stiffness,
yield strength and ultimate strength can be translated to reductions to structural
weight. This led the aerospace industry to develop and examine new materials
having combination of low density, improved stiffness and high strength as
attractive alternatives to exiting high strength aluminium alloys and titanium
alloys. The high strength metal matrix composites combine the high strength
and hardness of the reinforcing phase with ductility and toughness of light
metals. Moreover the need for improved design procedures has arisen from an
attempt to achieve significant improvement in structural efficiency, reliability
and overall performance through either a reduction in absolute weight or
increase in strength to weight ratio. Recent research results have made it
possible to envision combining these effects through the development of
reinforced light weight alloys [1].
The metal matrix composites offer a spectrum of advantages that are
important for their selection and use as structural materials. A few such
advantages include the combination of high strength, high elastic modulus, high
toughness and impact resistance, low sensitivity to changes in temperature or
thermal shock, high electrical and thermal conductivity, minimum exposure to
the potential problem of moisture absorption resulting in environmental
degradation, and improved fabric ability with conventional metal working
equipment [2].With the exception of wires, which are metals, reinforcements are
generally ceramics. Typically these ceramics are oxides, carbides and nitrides,
which are used because of their excellent combinations of specific strength and
stiffness at both ambient and at elevated temperatures.
Silicon carbide, boron carbide and aluminium oxide are the key
particulate reinforcements that have been used. These can be obtained in
varying levels of purity and size distribution. The silicon carbide articulates are
also produced as a by-product of the processes used to make whiskers of these
materials [2].
The particulate reinforced metal matrix composites have emerged as
attractive candidates for use in a spectrum of applications to include industrial,
military and space related [1]. The renewed interest in metal matrix composites
has been aided by the development of reinforcement material, which provides
either improved properties or reduced cost when compared to the exciting
monolithic materials [2]. Particulate reinforced metal matrix have attracted
considerable attention on account of an availability of a spectrum of
reinforcements at competitive cost, successful development of manufacturing
processes to produce metal matrix composites with reproducible microstructures
and properties, and availability of standard and near standard metal working
methods, which can be utilized to form these materials.
Furthermore, use of discontinuous, reinforcements minimizes the
problems associated with fabrication of reinforced metal matrix composites
such as fiber damage, micro structural heterogeneity, fiber mismatch and inter-
facial reactions. For applications subjected to serve loads, extreme thermal
fluctuations, such as in automotive components, the discontinuously reinforced
metal matrix composites have been show to offer near isotropic properties with
substantial improvements in strength and stiffness, relative to those available for
the monolithic materials [3].
The most common particulate composite system in an aluminium alloy
reinforced with silicon carbide. So far most of the alloy that has been chosen as
matrices has been the A356, 2XXX alloys. Although few studies have been
reported on the LM25 series alloys reinforced with silicon carbide particulates,
much less attention has been given to the LM25. Al alloy matrix composites,
which show the highest strength of commercially available. Al alloys and
widely used for structural applications [3]. Stronger matrix alloys tend to
produce stronger composites. However within these composites system there
are variables, such as, ageing condition, weight/volume fraction of particulate,
particulate size, which exerts an influence on mechanical properties. The
objective of this study is to investigate the mechanical performance of silicon
carbide reinforced stir cast grain refined LM25 matrix composites.
1.1 METAL MATRIX COMOSITES
A combination of two or more materials reinforcing element, fillers, and
composite matrix binder is differing in form or composition on a macro scale.
The constituents retain their identities that are they do not dissolve or merge
completely into one another although they act in concert. Normally the
components can be physically identified and exhibit an interface between one
another.
Metal matrix composites can be classified depending on the nature and
number of reinforcements, type of metal matrix and functional behavior of
composites. Depending upon the nature of reinforcements used, metal matrix
composites can be classified as (a) Dispersion strengthened, (b) Discontinuously
reinforced and (c) Continuous fiber reinforced composites.
1.1.1 CONSTITUENTS OF MMC
The major constituents of MMC are the matrix and the reinforcements.
The interface between the matrix and the reinforcement is also considered as
one of the constituents as it plays a crucial role in determining the properties of
the composites.
The matrix is the continuous phase (i) binding and keeping the
reinforcement in position and orientation, (ii) transferring the load to and
between the reinforcements and (iii) protecting reinforcements from the
environment and handling. Further, the matrix determines the overall service
temperature limitations of composites as well as their resistance to
environment. Important requirements of a matrix alloy are compatibility with
reinforcements during composite production and product service, high strength
and sufficient and plasticity at room and elevated temperatures. The major
metal matrices used for the fabrication of composites include aluminium,
magnesium, titanium, and copper based alloys. The matrices studied are based
on zinc, tin, steel, super alloys and inter metallics. Among the various matrix
materials available, aluminium and its alloys are widely used for the fabrication
of MMC due to the fact that they are light in weight, economically viable,
amenable for production by various processing techniques and posses’ high
specific strength and good corrosion resistance.
The reinforcements are the second phase materials added to the matrix
alloys, which normally enhance strength, stiffness, wear and creep resistance of
the composites. The choice of reinforcements always depends on the final
property requirements of the composite system or component to be fabricated.
Certain dispersions normally import some special properties to composites
such as enhanced wear resistance and reduced density at the expense of
strength. Generally, the dispersions are refractory materials, such as oxides,
carbides, and nitrides of different elements. Basically, they are stable and non-
reactive in most of the matrix alloys. In addition, they do not mostly undergo
any change in phase or shape during composite synthesis or in use except those
produced by in-situ methods. At present, there are a wide range of
reinforcement materials, which can provide varying combinations of properties
to the composites synthesized.
Interface is the region that lies between the matrix and the
reinforcements. It plays a crucial role in determining the composites properties.
It may contain a simple row of atomic bonds (e.g. the interface between
alumina and pure aluminium), or reaction products between the matrix and the
reinforcement coatings (e.g. interface between aluminium carbide, aluminium
and carbon fibers), or reinforcement coatings (e.g. interfacial coatings between
SiC monofilaments and titanium matrices). In composites, (i) stiffening and
strengthening rely on load transfer across the interface; (ii) toughness is
influence by crack deflection/fiber pull-out and (iii) ductility id affected by
relaxation of peak stresses near the interface. The types of interface bonding
are mechanical, physical and chemical in nature.
Mechanical bonding is the simple mechanical keying or interlocking
effects between two surfaces which could lead to a considerable degree of
bonding. Any contraction of the matrix onto the fiber would result in a gripping
of the fiber by the matrix. The physical bonding is those involving weak,
secondary or van der Waals forces, dipolar interaction and hydrogen bonding.
Chemical bonding involves the atomic or molecular transport by diffusion
process. Solid solution and compound formation may
Occur at the interface, resulting in a reinforcement / matrix interfacial
reaction zone with a certain thickness. This encompasses all types of covalent,
ionic and metallic bonding.
1.2 TYPES OF MMC
i. Particle- reinforced MMC’s
ii. Dispersion strengthen MMC’s
iii. Fiber reinforced MMC’s
1.2.1 Particle – Reinforced MMC’s
Material consisting of one or more constituents suspended in a matrix
material. These particles or either metallic or non-metallic.
1.2.2 Dispersion Strengthen MMC’s
Continuous fibers are identified as one of important in the early stages of
MMC evolution because of their wide use in the fabrication of polymer matrix
composites. Their concentration in composites varies from a few to 80% of
volume. However, processing of continuous fibre reinforced composites is
expensive.
The reinforcement is incorporated in loose from into the metal matrix.
Because most metal reinforcements systems exhibit poor wetting, mechanical
force is required to combine the phases generally through stirring.
1.2.3 Fiber Reinforced MMC’s
Depending on the number of reinforcements, MMC are classified as
mono and hybrid metal composites. In mono composites, only one type of
reinforcement is used. In the case of hybrid MMC, more than one type and
shape of reinforcement are used. Similarly classification is made on the type of
matrix alloy chosen such as aluminium, magnesium , titanium, copper, and inter
metallics MMC. Based on the functional behaviour, they are classified as
‘functionally graded’ composites, ‘smart’ composites etc. based on the size of
reinforcements or matrix grain size they can be classified as macro, micro and
nano MMC.
These type of MMC’s contains reinforcements having lengths much
greater than their cross sectional dimensions. Such a composite is considered to
be a discontinuous fiber or short fiber composite if its properties vary with fiber
length.
1.3 ENGINEERING PROPERTIES OF MMC’S
1.3.1 Stiffness Enhancement
Stiffness is nothing but the avoidance of excessive elastic deflection in
survive. Accordingly to the increase of reinforcement volume fraction. The
stiffness increases due to the increase in young modules.
1.3.2 Strength Enhancement
The enhancement of strength by addition of reinforcement experienced
in terms of yield stress or failure stress can be quite strength also increases.
1.3.2 Increased creep resistance
When fibers particularly long fibers are used as also reinforcement, creep
resistance of MMC’s would raise and give a high beneficial increase in stiffness
1.3.3 Increased wear resistance
While different wear applications require difference reinforcements
types to achieve optional wear rate redirection. Further more, it is often
advantages to control the distribution of high wear resistance is selected surface
areas, while other regions are suitably tough, strong, thermal conducting etc.
1.3.4 Density reduction
A relatively low density is an attractive feature of many MMC
materials. In many cases of interest, addition of the reinforcement raises the
density slightly, but the increase of strength, stiffness etc.
1.3.6 Thermal Expansion control
Thermal expansion control is as good as in MMCs. When the coefficient
of thermal expansion will be decreased, then MMCs are withstanding at high
temperatures.
1.4 STRESS-STRAIN DIAGRAM
In designing various parts of a machine, it is necessary to know how the
material will function in service. For this, certain characteristics or properties of
the material should be known. The mechanical properties mostly used in
mechanical engineering practice are commonly determined from a standard
tensile test. These tests consist of gradually loading a standard specimen of a
material and noting the corresponding values of load and elongation until the
specimen fracture. The load is applied and measure by attesting machine. The
stress is determined by dividing the load values by the original cross section
area of the specimen. The elongation is measures by determining the amount
that two reference points on the specimen are moved sprat by the action of the
machine.
The original distance between the two reference points is known distance
between the two reference points is known as gauge length. The strain is
determined by dividing the elongation values by the gauge length. The values of
the stress and corresponding strain are used to draw the stress-strain diagram of
the material tested. A stress-train diagram for mild steel under tensile test is
shown in Fig. The various properties of the material are discussed below.
Proportional limit:
We see from the diagram that from point 0 to A is a straight line, which
represents that the stress is proportional to strain. Beyond point A, the curve
slightly deviates from the straight line. It is thus obvious, that Hooke’s law
holds good up to point A and it is known as proportional limit. It is defined as
that stress at which the stress-strain curve begins to deviate from the straight
line.
Elastic limit:
It may be noted that even if the load increased beyond point A up to the
point B, the material will regain its shape and size when the load is removed.
This means that the material has elastic properties up to the point B. this point is
known as elastic limit. It is defined as the stress developed in the material
without any permanent set.
Yield point:
If the material is stressed beyond point B, the plastic stage will reach i.e.
on the removal of the load; the material will not be able to recover its original
size and shape. A little consideration will show that beyond point B, the strain
increase at a faster rate with any increase in the stress until the point C is
reached. At this point, the material yields before the load and there is an
appreciable strain without any increase in stress. In case of mild steel, it will be
seen that a small load drops to D, immediately after yielding commences. Hence
there are two yield points C and D are called the upper and lower yield points
respectively. The stress corresponding to yield point is known as yield point
stress.
Ultimate stress:
At S, the specimen regains some strength and higher values of stresses
are required for higher strains, than those between A and D. The stress goes on
increasing till the point E is reached. The gradual increase in the strain of the
specimen is followed with the uniform reduction of its cross sectional area.
The work done, during stretching the specimen, is transformed largely
into heat and the specimen becomes hot. At E, the stress, which attains its
maximum value, is known as ultimate stress. It is defined as the largest stress
obtained by dividing the largest value of the load reached in a test to the original
cross sectional area of the test piece.
Breaking stress:
After the specimen has reached the ultimate stress, a neck is formed,
which decreases the cross sectional area of the specimen. A little consideration
will show that the stress (load) necessary to break away the specimen is less
than the maximum stress. The stress is, therefore, reduced until the specimen
breaks away at point F. The stress corresponding to point F is known as
breaking stress.
Note:
The breaking stress (i.e. stress at F which is less than at E) appears to be
somewhat misleading. As the formation of a neck takes place at E which reduce
the cross sectional area, it causes the specimen suddenly to fail at F. If for each
value of the strain between E and F, the tensile load is divided by the reduced
cross sectional area at the narrowest part of the neck, then the true stress – strain
curve will follow the dotted line EG. However, it is an established practice, to
calculate strain on the basis of original cross sectional area of the specimen.
Percentage reduction in area:
It is the difference between the original cross sectional area and cross area
at the neck (i.e. where the fracture takes place). This difference is expressed as
percentage of the original cross sectional area. Percentage reduction in area =
(A – a/A) * 100
Let,
A = Original cross sectional area, a = cross sectional area at the neck.
A – a = reduction in area.
Percentage elongation:
It is the percentage increase in the standard gauge length (i.e. Original
length) obtained by measuring the fractured specimen after bringing the broken
parts together.
Let, l = Original length,
L = Length of specimen after fracture or final length,
L-l = Elongation,
(L-l)/l * 100 = Percentage elongation.
CHAPER 2
LITERATURE SURVEY
Kaczmar et al [21] stated that composite material are considered to be
more suitable than conventional materials as they have favourable mechanical,
thermal and frictional properties. Composites containing discontinuous
reinforcement, especially particulate metal matrix composites, have found
commercial use in some applications since they can be produced economically
by conventional processing techniques. Among these materials aluminium alloy
based composites are very attractive on account of there processing flexibility,
wide range, low density, high wear resistance, high thermal conductivity, heat
treatment capability and improved elastic modulus and strength. Various
methods have been developed for the production of metal matrix composites.
An economical way of producing metal matrix composites is the incorporation
of the particles into the liquid metal and casting.in as cast aluminium alloy
based composites, a moderate improvement in strength over the unreinforced
alloy is obtained. On the other hand when the particulate reinforcement is added
to improve stiffness, strength and tribological properties, a substantial decrease
in ductility is observed. Inferior ductility of these material limits their
performance and applications [22]. Kaczmar et al [21] and ghomashchi et al
[22] observed ductility is affected by various factors such as the matrix micro
structure, heterogeneous reinforcement distribution, porosity content and the
strength of the interfacial bond between matrix and the reinforcement. It has
been shown that some improvement in ductility as well as the strength is
observed by applying pressure during solidification [22]. Forming processes
alter the structural parameters. Which influence the properties of these material
[23] as these parameters, are sensitive to the type of reinforcement, the method
of production and fabrication processing of composite after the initial
production stage. Particulate Sic reinforced AlSi5 based composites are
produced under ordinary foundry conditions, and the cast ingots are hot
extruded to improve mechanical properties. The microstructures and mechanical
properties are studied in the cast of state and after the extrusion process. The
strength and ductility of the as cast extruded composites related to the
microstructures and to the volume fraction of reinforcement were concluded by
Ghomashchi et al[23].The influence of reinforcement phase on the properties of
the composites depends both on the manufacturing method and the
characteristics of ceramic particles (chemical composition, volume percent and
particle size). Soma Raju et al[24] were inferred that for instance, the wear
resistance of AL-Al2O3 composites was reported to be superior to those
containing SiC. The inferior performance of the SiC reinforced composites is
attributed to the reaction of interface between the metal matrix and the
reinforcement. The size and distribution of second phase particles also play an
important role in controlling the mechanical properties and wear resistance of
the composites, e.g. a composite with large size ceramic particles was reported
to exhibit superior wear resistance but inferior mechanical strength to that
containing small particles size [24,25]. On the other hand, the distribution of
reinforcement particles and the interfacial reaction between the metal matrix
and the reinforcement highly depends on the processing methodology. It is
known that the reaction between the matrix and particles is minimized by using
solid state processing, i.e. powder metallurgy route [24]. In liquid metallurgy
methods, the wettability of the reinforcement particles by molten metal is
improved by applying high pressure during casting [25]. The thermal mismatch
between the metal matrix and the reinforcement increases the dislocation
density of the matrix. Therefore, the aging kinetics of LM25 aluminium alloy
increases with addition of SiC particles, i.e. less time is required to obtain the
maximum hardness by T6 heat treatment. The aging kinetics is further increased
by decreasing the particle size of the reinforcement phase. The mechanical
strength of LM25-30 vol% SiC composite is superior to those of the
unreinforced material. Decreasing the particle size of the reinforcement phase
leads to better mechanical properties [24, 25].
Tan et al [26], who investigated the behaviour of powder processed
and extruded Al-Li alloy based SiCp reinforced composites and found that the
addition of over 20wt.% reinforcement causes a substantial decrease in yield
strength and ductility. In addition, Davies et al [27] reported that in advanced
LM25 series AL alloy based SiCp reinforced composites, the yield strength and
tensile strength values exhibited an increase with the addition of up to 10 and 15
vol.% SiCp. After the addition of reinforcement exceeding these volume
fraction values, the yield strength and tensile strength decreasing significantly.
The decrease in the strength of these composites after the addition of SiCp over
an optimal volume fraction value could be explained by the yearly void
formation at the reinforcing particles. Thus, higher volume fraction of the
reinforcement giving
Thus, higher volume fraction of the reinforcement giving smaller inter
particle spacing will make relaxation more difficult, and the build up of
dislocations leading void formation at reinforcing particles would occurs at
lower strains, as a result proof stress and the strain to failure are reduced.
In interpreting the decreased strength of high SiCp containing composites,
the interfacial reactions also be considered. The reaction takes place during the
incorporation process and may alter the matrix composition; as a result the
strength of the particulate containing composites is affected. Zhong et al. [28]
have reprted that in Al-Mg (5083) alloy based SiCp reinforced composites the
addition of the oxidized SiCp decreases the strength as a result of some
interfacial reactions in which MgO layer from at the particle-matrix interfaces at
the expense of Mg in matrix. The depletion of Mg in the matrix will induce
lower strength values due to the reduced solute strengthening. In the high SiCp
containing composites higher amount of Mg depletion take place and as a result
even lower strength levels will be observed. For the case of composite samples
of the present study, oxidized SiCp was used as reinforcement. Some formation
of Mg containing interfacial oxides was observed in the microstructures but the
age hardenability of the high reinforcement containing samples was not greatly
altered [27], and in the addition, no reduction in the strength of the extruded
samples of high reinforcement composites was obtained with increasing SiCp
content. The decrease in the strength partly be attributed to the formation of Mg
containing composites at the interfaces, giving raise to reduced cohesion
between the reinforcing particles and the matrix. In addition, the possible
formation of flaws in SiC particles during oxidation may induce early void
formation, and lower values of strength could be observed in the samples due to
the reduced load transfer to the particles [28]. The particles with flaws may
crack and separate during extrusion and may have no deteriorating effect on the
strength of the extruded samples. It is known that the forming process applied to
the a metal matrix composite affect the microstructure and mechanical
properties. The effect of such processes were outlined by McKimpson and Scott
[29]. They stated that achieving a uniform reinforcement distribution should
improve the strength and of the particulate containing composites.
It was demonstrated by Harrigen et al. [30] that the application of the
hot rolling process of at least 80% reduction to the billets of SiCp reinforced
LM25 alloys improved the particulate distribution and increased strength and
ductility of these composites. The effect of hot forging on the properties of the
sand cast and squeeze cast LM25 alloy based 20 vol. % SiCp reinforced
composites was studied by Rozak et al. [31] who applied hot reduction of up to
95% to both unreinforced and reinforced materials. After the application of
forging they obtained the substantial improvement in the strength and ductility
of both group of materials. The improvement in properties, being more
pronounced in the sand cast composites with possessed coarse microstructures
and higher porosity content, was related to the refinement of the
microstructures, and to the reduction of the porosity content with the increased
amount of deformation.
As have been reviewed by Llyold [33], the tensile tensile elongation
decreases with the addition of reinforcing particles. It has been demonstrated
that the failure of the composite material is related to particle cracking and void
formation in the matrix within cluster of small particles. Particle fracture is
expected in larger particles as they will be loaded to higher levels and are more
likely to contain defects of critical size for initiating fracture. The failure related
to the particle cluster can be explained by higher stress triaxiality generated in
these regions. Attempts have been made to understand the fracture in
composites quantitatively [34] and it was concluded that the fracture of
composites was affected by particle shape and size, particle distribution
uniformity, interfacial strength and matrix ductility.
CHAPTER 3
WORK PLAN
3.1 FABRICATION PROCESSES
The evolution of different metal matrix composite system has led to the
development of newer processing techniques, in addition to conventional metal
processing techniques. The major criteria for the selection of a process rely on
the type of composite system to be fabricated, the properties to be achieved and
the component to be produced.
The processing methods are widely classified into primary and secondary
processes. The primary process combines matrix and reinforcements to produce
the basic composite system and their structures. The primary processing
techniques may be classified into liquid state, solid state and coating processes.
The major primary liquid state processes are stir casting or vortex method,
infiltration, in-situe and spray deposition processes and solid state processes are
powder metallurgy and diffusion bonding. The secondary process involves
processing of primary processed composites with the objective of improving
their mechanical properties by further consolidation (reduction or elimination of
porosity), break up of dispersion agglomerates, improved interfacial bonding,
generating dispersion alignment and / or forming into a required shape to obtain
semi-finished products. The most commonly used secondary processes for
MMCs are extrusion, rolling, forging, super plastic deformation, machining and
joining [4]
The processing of composites is accomplished by several techniques such
as stir casting, rheo-casting and powder metallurgy [5]. The powder metallurgy
method is viable only for critical applications due to the prohibitive cost of
production, and also the product size is limited. On the other hand, casting
processes lead to a number of processing limitations such as non uniform
distribution of particles, clustering of particles, several interfacial reactions,
large amount of porosity, and finally the inability of the process to incorporate
extremely fine size of ceramic particulates.
These processing limitations have been addressed in recent times by
adopting spray deposition/forming processing route for these composites. This
route is capable of low temperature processing and gives rise to uniformity in
particles distribution with minimum interfacial reaction products [6, 7].
Discontinuously reinforced metal matrix composites (MMCs) have been
considerable development in the recent years from the view point of both
primary as well as secondary processing. Such composites are attractive for
various structural applications in the aerospace, automobile industries and
electrical components due to their unique combination of mechanical, physical
and thermal properties [5-8]. The addition of particulates significantly enhances
the specific strength and stiffness, which make this material suitable for critical
structural components. However, these materials are difficult to shape using
conventional processing techniques because of intricate micro structural
features developed by rapid solidification and addition of ceramic
reinforcements. In addition, as their mechanical properties are strongly
dependent upon micro structural characteristics such as volume fraction and
aspect ratio of particulate, it is essential that manufacture of structural shapes to
be accomplished without fracturing particulate or changing their aspect ratio.
(a) External view of electrical resistance
furnace
(b) Degassing using Hexachloro-
ethane
(c) Mechanical stirrer for vortex method (d) Pouring of molten metal into the
mould
(e) Metallic Mould Used for Casting (f) Removal of Cast Composites from the
Mould
3.2 TENSILE TEST
Ultimate tensile strength (UTS), often shortened to tensile strength (TS)
or ultimate strength [9], is the maximum stress that a material can withstand
while being stretched or pulled before necking, which is when the specimen’s
cross sections starts to significantly contract. Tensile strength is the opposite of
compressive strength and the values can be quite different. The UTS is usually
found by performing a tensile test and recording the stress versus strain; the
highest point of the stress-strain curve is the UTS. It is an intensive property;
therefore its value does not depend on the size of the test specimen. However, it
is depended on the other factors, such as the preparation of the specimen, the
presence or otherwise of surface defects, and the temperature of the test
environment and material. Tensile strengths are rarely used in the design of
ductile members, but they are important in brittle members. They are tabulated
for common materials such as alloys, composite materials, ceramics, plastics,
and wood. Tensile strength is defined as a stress, which is measured as force per
unit area. In the SI system, the unit is Pascal (Pa) or, equivalently, Newton per
square meter (N/ ). The customary unit is pounds force per square inch
(lbf/i or psi), kilo – pounds per square inch (ksi), which is equal to 1000 psi;
kilo – pounds per square inch are commonly used for convenience when
measuring tensile strengths.
Many materials display linear elastic behavior, defined by a linear stress
strain relationship, in which deformations are completely recoverable upon
removal of the load, that is a specimen loaded elastically in tension will
elongate, but will return to its original shape and size when unloaded. Beyond
this linear region, for ductile materials, such as steel, deformations are plastic. A
plastically deformed specimen will not return to original shape and size when
unloaded. Note that there will be elastic recovery of a portion of the
deformation. For many applications, plastic deformation is unacceptable, and is
used as the design limitation. After the yield point, ductile metals will undergo a
period of strain hardening, in which the stress increases again with increase
strain, and they begin to neck, as the cross sectional area of the specimen
decreases due to plastic flow.
In a sufficiently ductile material when necking becomes substantial, it
causes a reversal of the engineering stress strain curve, this is because the
engineering stress is calculated assuming the original cross sectional area before
necking. The reversal point is the maximum stress on the engineering stress
strain curve, and the engineering stress coordinate of this point is the tensile
ultimate strength. The UTS is not used in the design of ductile static members
because design practices dictate the use of the yield stress. It is, however used to
for quality control, because of the case of testing. It is also used to roughly
determine material types for unknown sample [10]. Brittle materials, such as
concrete and carbon fiber, are characterized by failure at small strains. They
often fail while still behaving in a linear elastic manner, and thus do not have
defined yield point. Because strains are a low, there is negligible difference
between the engineering stress and the true stress. Testing several identical
specimens will result in different failure stresses; this is due to the Weibull
Modulus of the brittle material. The UTS is a common engineering parameter
when design brittle members, because there is no yield point [10].
The testing involves taking a small sample with a fixed cross section area,
and then pulling it with a controlled, gradually increasing force until the sample
changes shape or breaks. When testing metals, indentation hardness correlates
linearly with tensile strength. This important relation permits economically
important nondestructive testing of bulk metal deliveries with light weight, even
portable equipment, such as hand held Rockwell hardness testers.
Tensile test is gripped at either end by suitable apparatus in a testing
machine which slowly exerts an axial pull so that the material is stretched until
it breaks. The test provides information on proof stress, yield point, tensile
strength, elongation and reduction of area.
3.3 IMPACT TESTS
The charpy impact test, also known as the Charpy v-notch test, is a
standardized high strain rate test which determines the amount of energy
absorbed by a material during fracture. This absorbed energy is a measure of a
given materials toughness and acts as a tool to study temperature dependent
brittle ductile transition. It is widely applied in industry, since it is easy to
prepare and conduct and results can be obtained quickly and cheaply. But a
major disadvantage is that all results are only comparative [11]. The test was
developed in 1905 by the French scientist Georges Chary. It was pivotal in
understanding the fracture problems of ships during the Second World War.
Today it is used in many industries for testing building and construction
materials used in the construction of pressure vessels, bridges and to see how
storms will affect materials used in building [12]. The apparatus consists of a
pendulum axe swinging at a notched sample of material. The energy transferred
to the material can be inferred by comparing the difference in the height of the
hammer before and after a big fracture. The notch in the sample affects the
results of the impact test [13], thus it is necessary for the notch to be of regular
dimensions and geometry. The size of the sample can also affect results, since
the dimensions determine whether or not the material is in plane strain. This
difference can greatly affect conclusions made [14]. “The Standard methods for
Notched Bar Impact Testing of Metallic Materials” can be found in ASTM E23
[15], where all the aspects of the test and equipment used are described in detail.
The quantitative result of the impact test the energy needed to fracture a
material and can be used to measure the toughness of the material and yield
strength. Also, the strain rate may be studied and analyzed for its effect on
fracture.
The ductile – brittle transition temperature (DBTT) may be derived
from the temperature where the energy needed to fracture the material
drastically changes. However, in practice there is no sharp transition and so it is
difficult to obtain a precise transition temperature. An exact DBTT may be
empirically derived in many ways. A specific absorbed energy, change in aspect
of fracture (such as 50% of the area is cleavage), etc [1]. The qualitative results
of the impact test can be used to determine the ductility of a material. If the
material breaks on a flat plane, the fracture was brittle, and if the material
breaks with jagged edges or shear lips, then the fracture was ductile. Usually a
material does not break in just one way or the other, and thus comparing the
jagged to flat surface areas of the fracture will give an estimate of the
percentage of ductile and brittle fracture [11]. According to ASTM A370, the
standard specimen size for charpy impact testing is 10mm×10mm×55mm. Sub-
size specimen sizes are 10mm×7.5mm×55mm, 10mm×6.7mm×55mm,
10mm×5mm×55mm, 10mm×3.3mm×55mm, 10mm×2.5mm×55mm. Details of
specimens as per ASTM A370 (Standard Test Method and Definitions for
Mechanical Testing of Steel Products). According to EN 10045-1, standard
specimen sizes are 10mm×10mm×55mm. sub size specimens are
10mm×7.5mm×55mm and 10mm×5mm×55mm.
3.4 HARDNESS TEST
The metals handbook defines hardness as resistance of metal to plastic
deformation, usually by indentation. However, the term may also refer to
stiffness or temper or resistance to scratching, abrasion or cutting. It is the
property of a metal, which gives it the ability to resist being permanently,
deformed (bent, broken or have its shape changed ), when a load is applied. The
greater hardness o f the metal, the greater resistance it has to deformation. In
mineralogy the property of matter commonly described as the resistance of a
substance to being scratched by another substance. In metallurgy hardness is
defined as the ability of a material to plastic deformation. The dictionary of
metallurgy defines the indentation hardness as the resistance of a material to
indentation. This is the usual type of hardness test, in which a pointed or
rounded indenter is pressed into a surface under a substantially static load.
Hardness measurement can be defined as macro, micro or nano scale according
to the forces applied and displacements obtained [16]. Measurement of the
macro-hardness of materials is a quick and simple method of obtaining
mechanical property data for the bulk material from a small sample. It is also
widely used for the quality control of the surface treatments processes.
However, when concerned with coatings and surface properties of importance to
friction and wear processes for instance, the macro-indentation depth would be
too large relative to the surface-scale features. Where materials have a fine
microstructure, are multi-phase, non-homogeneous or prone to cracking, macro-
hardness measurements will be highly variable and will not identify individual
surface features. It is here that micro-hardness measurements are appropriate.
Micro hardness is the hardness of a material as determined by forcing an
indenter such as Vickers or knoop indenter into the surface of a material under
15 to 1000 gf load; usually, the indentations are so small that they must be
measured with a microscope. Capable of determining hardness of different
micro constituents within structure, or measuring steep hardness gradients such
as those encountered in casehardening. Conversions from microhardness values
to tensile strength and other hardness scales (e.g Rockwell ) are available for
many metals and alloys [17]. Micro-indenters works by pressing a tip into a
sample and continuously measuring: applied load, penetration depth and cycle
time. Nano-indentation [18] tests
Measure hardness by indenting using very small, on the order of 1
nano-newton, indentation forces and measuring the depth of the indentation that
was made. These test are based on new technology that allows precise
measurement and control of the indenting forces and precise measurement of
the indentation depths. By measuring the depth of the indentation, progressive
levels of forcing are measurable on the same piece. This allows the tester to
determine the maximum indentation load that is possible before the hardness is
compromised and the film is no longer within the testing ranges. This also
allows a check to be completed to determine if the hardness remains constant
even after an indentation has been made. There are various mechanisms and
methods that have been designed to complete nano-indentation hardness tests.
One method of force application is using a coil and magnet assembly on a
loading column to drive the indenter downward. This method uses a capacitance
displacement gauge. Such gages detect displacements of 0.2 to 0.3 NM
(nanometer) at the time of force application. The loading column is suspended
by springs, which damps external motion and allows the load to be released
slightly to recover the elastic portion of deformation before measuring the
indentation depth. There are three types of tests used with accuracy by the metal
industry; they are the Brinell hardness test, the Rockwell hardness test, and the
Vickers hardness test. Since the definitions of metallurgic ultimate strength and
hardness are rather similar, it can generally be assumed a strong metal is also a
hard metal. The way the three of these hardness tests measure a metal’s
hardness is to determine the metal’s resistance to the penetration of a non-
deformable ball or cone.
CHAPTER 4
EXPERIMENTS
Alloy LM-25 is chosen as the matrix material. The key alloying element
is zinc. The second is magnesium, which is predominantly added to enhance
wetting between the matrix and the reinforcing phase. The composition of
aluminium LM-25 is tabulated in table 1.
Table 1 :
Composition (wt %) of LM-25 :
Cu Mg Si Ti V Fe Al
0.55-1.0 1.2-1.6 0.6-1.2 0-0.1 0.1-0.3 0-0.4 Bal
Stir casting technique was used to fabricate LM-25 alloy reinforced with
0%, 5%, 10%, 15% volume fraction of silicon carbide composites. Stir casting
set up is shown in fig. The matrix metal was LM-25 aluminium alloy and the
reinforcement was SiCp with an average size of 20 µm. The aluminium alloy
was melted by using an electric furnace. Preheated SiCp (250ºC) was added to
the melt and mixed usig a rotating impeller in Argon environment and poured
into a permanent mould. The cast billets were heated in a furnace to 400ºC and
for 30minutes in the same temperature and hot extruded
LM- Series for cast Aluminium Alloys
S.No Trade
Name
Si Cu Mg Mn others
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
LM-1
LM-2
LM-4
LM-5
LM-6
LM-8
LM-9
LM-10
LM-11
LM-12
LM-13
LM-14
LM-16
LM-17
LM-18
LM-23
LM-24
LM-25
3.0
10.0
5.0
-
11.5
5.5
11.5
-
-
-
12.0
0.3
5.0
11.5
5.0
2.5
8.5
7.0
7.0
2.0
3.0
-
-
-
-
-
4.5
10.0
0.9
4.0
1.2
-
-
1.8
3.5
-
-
-
-
5.0
-
0.6
0.4
10.5
-
0.25
1.2
1.5
0.5
-
-
0.2
-
0.3
-
-
0.5
0.5
-
0.5
0.5
-
-
-
-
-
-
-
-
-
-
-
3.0 Zn
-
0.8 Fe
-
-
0.15 Ti
-
0.2 Ti
0.15 Ti
-
2.5 Ni
2.0 Ni
0.3 Ni
3.0 Ni
-
1.2Ni
-
-
4.1 TENSILE TEST
Four types of composite alloys containing 0, 5, 10, 15 wt% SiC were
processed by stir cast an extruded to obtain the tensile test specimens. Only the
burrs were cleaned prior to mechanical tests, dimensions and shape of tensile
test specimens are given in fig. The tensile test specimens of dimensions, 10
mm in diameter and 120 mm in length were machined from the extruded rods
are shown in fig shows after tensile tested specimens. Tensile test were carried
out on a 10T servo controlled universal testing machine as shown in fig
The three specimens of tensile testing on all composite were tested. Load
(p) versus deflection (ᵹ) data was recorded during tensile test. Also the values of
ultimate tensile strength were evaluated. The recorded maximum loads are in
kilograms and were converted to maximum stress values (M pa). The cross
sectional areas of the tensile test samples were measured and lengths were
compared before and after fracture.
TENSILE TEST RESULTS
Materials Breaking
load Kn
Breaking
stress
N/mm^2
Ultimate
stress
N/mm^2
Yield
stress
N/mm^2
% of
Elongation
LM25/0%SiCp
composites
0.7 6.2 7.957 4.59 12%
LM25/5%SiCp
composites
0.82 7.25 8.66 5.128 10%
LM25/10%SiCp
composites
1.05 9.28 10.43 6.36 7.5%
LM25/15%SiCp
composites
0.9 7.95 9.01 5.39 8%
4.2 IMPACT TEST:
A quick and reproducible method is disclosed for evaluating the impact
properties of relatively stiff material such as plastic. A test specimen in the form
of a thin un-notched strand of known thickness is mounted in a holding in a
means and subjected to impact by a pendulum type striking means having
predetermined potential energy content. The excess type striking means after
breaking the specimen is measured and impact values determined by
mathematical formula as a function of specimen thickness and energy required
breaking the specimen. Four types of composite alloys containing 0, 5, 10, and
15 wt% Sic were processed by stir cast and extruded to obtain the impact test
specimens. Dimension and shape of impact test V notched specimen are given
fig The impact specimens of dimensions, According to ASTM A370, the
standard specimen sub size for Charpy impact testing is 10mm×7.5mm×55mm,
were machined form the extruded rods are as shown in fig ,Shows after impact
tested specimens. Impact tests carried out on a vintage impact test machine as
machine as shown in fig. The two were specimens of impact testing on alloy
and composites pair were tested. Impact test results were shown in table 3.
IMPACT TEST RESULTS
S .No Material Impact load in joules
1 LM25/0%SiCp composites 18
2 LM25/5%SiCp composites 40
3 LM25/10%SiCp composites 56
4 LM25/15%SiCpcomposites 24
IMPACT SPECIMEN
4.3 ROCKWELL HARDNESS TEST:
Hardness tests were carried out by Rockwell Hardness testing Machine .
To observe the effects of wt% addition of silicon carbide on LM 25 alloy
matrix.
Hardness is defined as resistance of metal to plastic deformation, usually by
indentation. However, the term may also refer to stiffness or temper or
resistance to scratching, abrasion or cutting. It is the property of a metal, which
gives it the ability to resist being permanently, deformed (bent, broken or have
its shape changed ), when a load is applied. The greater hardness of the metal,
the greater resistance it has to deformation.
Rockwell hardness testing are shown in Table 4. This is the usual type of
hardness test, in which a pointed or rounded indenter is pressed into a surface
under a substantially static load. For Aluminium 1/16” ball indenter is ued.
Reading should be taken on B scale (red dial) and load applied is 100kg.
In this experiment it was found that value of hardness of LM 25 in as-cast
condition is 67.57, 72.33, 77.37, 79.47.
HARDNESS TEST SPECIMEN
Before test After test
HARDNESS TEST RESULTS
S.No Material HRB (Rockwell No) Average
1 LM25/0%SiCp
composites
66 67.7 69.5 67.57
2 LM25/5%SiCp
composites
75.2 69.3 72.5 72.33
3 LM25/10%SiCp
composites
74.8 78.3 79 77.37
4
LM25/15%SiCp
composites
77.5 81.7 79.2 79.47
CHAPTER 5
RESULT AND DISCUSSION
In contrast to samples show a continuous increase in yield strength with
increasing reinforcement content. The effort of reinforcement content and hot
extrusion process on the tensile strength of the composites is shown in fig.
In the samples, the tensile strength increases continuously with the additions
of reinforcing SiC Particles up to 10 vol. %, showing an increase of about 50%,
but starts decreasing in the specimens containing over 10 vol. % SiCp. The
effort of reinforcement content on tensile strength is similar to that observed for
the yield strength thus the tensile strength of the extruded specimens increases
continuously with the volume fraction of SiC.
Vol.% SiCp
Fig: The effort of reinforced content and hot extrusion process on the tensile
strenth of the composite
0
2
4
6
8
10
12
0 5 10 15 20
Ultimate
tensile stress,
Mpa
Fig: The effect of the yield stregth of the matrix alloy and the composites as a
function of the reinforcement content.
The effect of SiC whisker and particle reinforcement on the
strength of discontinuously reinforced Al alloy materials was extensively
studied by Ghomashchi [39].He observed a substantial increase in yield and
ultimate tensile strengths, with the increasing volume fraction of reinforcement
, depending on the type of alloy and the matrix alloy temper. It was also
reported by Lloyd [33] that increasing the SiC content increases the yield
strength of the composites. Some of the reported results do not agree with the
observations.
Fig. it can be seen that the hardness of the composites increases with
increasing the volume fraction of the SiC particles, on the other hand. As seen
from Fig.. the tensile decreases with increasing the volume fraction of the SiC
particles. This phenomenon indicates the that particle cracking has no
significant influence on the hardness of the composites. The discrepancy
between the hardness and tensile strength of the composites is due to the
fundamental difference in the two types of loading [43]. During hardness testing
0
1
2
3
4
5
6
7
0 5 10 15 20
Series1
Yield
stress,
Mpa
Vol.% SiCp
the localized pressure causes the material directly under the indented is
predominantly compressive with the matrix under a severe triaxial pressure.
This stress state is totally different from deformation in tension, under which the
entire specimen is under nominal tensile loading. The pre-cracked particles
cannot undergo the tensile stress and thus will separate when the composite is
subject to the tensile loading. With increasing the tensile loading, the cracks
propagate and grow. During the hardness test, pre-cracked particles can still
endure the compressive traction across the cracking surfaces. plastics flow of
the matrix between the particles accommodates the deformation caused by the
indentation pressure, and thus the particles are "pushed" into the material [44].
Therefore, the material response under indentation is not significantly
influenced by the extrusion induced fractured particles .As a result of that, the
hardness of the composites increases with increasing the volume fraction or the
SiC particles.
Vol.% SiCp
Fig: The hardness values of the composites and the reinforced
material.
65
70
75
80
85
0 5 10 15 20
Hardness(Hv)
CHAPTER 6
CONCLUSIONS
In this study, effects of extrusion and reinforcement volume
fraction on the mechanical properties of SiC particle reinforced AL LM25
composites have been studied. The results indicate that the extrusion can
substantially improve the distributed homogeneity of the SiC particles in the
matrix, and help to decrease the number of pores and improve interfacial
bonding strength of the composites. The 10% volume of SiCp aluminium matrix
composites showed the maximum tensile and yield strength. The yield strength
and tensile of the composites decreases with increasing the volume fraction of
the SiC particles, while the hardness of the composites increase with volume
fraction of the SiC particles. The fracture mode of the composites during tensile
testing changes from particle "pullout” from the matrix before extrusion to
particle fracture after extrusion.
CHAPTER 7
REFERENCES
1. Karnezis PA, Durrant G, Cantor B. Characterization of reinforcement
distribution in cast Al-alloy/SiC composited. Mater Charac;1998;0:97-
109
2. Lee Kon Bae, Kwon Hoon. Strength of Al-Zn-Mg-Cu matrix
composite reinforced with SiC particles. Metall Mater Trans A,
2002;33A:455-65
3. Zhu H, Xia X, Mc Queen HJ. Fracture behavior of particle reinforced
metal matrix composites. Appl Compos Mater, 2002:17-31
4. Cerri E, Spigarelli S, Evangelista E, Cavaliere P. Hot deformation and
processing maps of a particulate reinforced LM25 + 20% Al2o3
composite. Mater Sci Eng A, 2002;324:157-612
5. Schneider A, Srivastava V.C, Uhlenwinkel V, Bauckhage K, Metall Z,
2004;95:763-768
6. Schneider A, Srivastava V.C, Uhlenwinkel V, Bauckhage K, Mater.
Sci Eng., 2005;412:19-102
7. Schneider A, Srivastava V.C, Uhlenwinkel V, Bauckhage K., Trans.
Ind. Inst. Met., 2005;58:91-102
8. Zambon A, Badan B, Maddalena A, Mater. Sci. Eng.2004;377:645-650
9. Degarmo, Black & Kosher Materials and processes in manufacturing,
2003;9:31
10.Meyers Marc A, Chawla Krishnan Kumar Mechanical Behaviors of
materials Prentice Hall. ISBN 9780132628174.1998.
11.Jacobs James A, Kilduff Thomas F. Engineering Materials
Technology. Pearson Prentice Hall. pp. ISBN
9780130481856,2005;5:153-155
12.Kurishita H, Kayano H, Narui M, Yamazaki M, Kano Y, Shibahara I.
“Effects of V notch dimensions on Charpy impact test results for
differently sized miniature specimens of ferritic steel” Materials
Transaction –JIM (Japan Institute of Metals). ISSN 0916-1821,
1993;34(11):1042-1052
13.Mills NJ “The mechanism of brittle fracture in notched impact tests on
polycarbonate”. Journal of Materials Science, 1976;11:363-75
14.ASTM E23 Standard Test Methods for notched Bar Impact Testing of
Metallic Materials.
15.Ref: http://www.plint.co.uk/at2/leaflet/te76.htm:
16.Ref: http://www.mee-inc.com/microhar.html:
17.Tobolski E.L. & A. Fee, “Macro indentation Hardness Testing”, ASM
Handbook, Mechanical Testing and Evaluation, ASM International,
ISBN 0-87170-389-0:2000;8:203-211
18.Correlation of Yield strength and Tensile Strength with Hardness for
Steels, E.J. Pavlina and C.J. Van Tyne, Journal of Materials
Engineering and Performance, 2008:17:20
19.Kaczmar JW, Pietrzak K, Wlosinski W. The production and
application of metal matrix composite materials. J Mater Process
Techn, 2000;106:58-67
20.Ghomashchi MR, Vikgrow A. Squeeze casting an overview. J Mater
Process Techno1101,2000;101:1-9
21.Ozdemir I, Cocen U, Onel K. The effect of forging on the properties of
particulate SiC reinforced aluminium alloy composites. Compos Sci
Technol, 2000;60:9-411
22.Soma Raju K, Bhanu Prasad VV, Rodrakshi GB, Ojha N. PM
Processing of Al-Al2o3 composites and their characterization. Powder
Metall, 2003;46(3):219

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Final project.report

  • 1. ABSTRACT The alloy LM25 is a new entry to the family of LM series of heat treatable Mg-Si-Cu aluminium alloys. The data available, while not sufficient to establish design mechanical property values for LM25, indicate that tensile, ultimate and yield strengths are about 40 percent higher than LM25 alloy. In this study LM25 matrix alloy is chosen and SiC particles having average particle size of 20µm with four volume fractions of 0, 5, 10 and 15% were incorporated into the alloy at the liquid state stir casting followed by extrusion. Tensile, impact, hardness test were carried out both alloy and composites. The 10% volume of SiCp aluminium matrix composites showed the maximum tensile and yield strength. The results indicate that extrusion can substantially improve the distributed homogeneity of the SiC particles in the matrix and help to decrease the number of pores and improve interfacial bonding strength of the composites. The yield strength and tensile strength of the composites decrease with increasing the volume fraction of the SiC particles, while the hardness of the composites increases with increasing the volume fraction of the SiC particles. Keywords: Matrix alloy, SiCp Aluminium Matrix Composites, Stir Casting, Yield Strength.
  • 2. CHAPTER 1 INTRODUCTION A LM25 aluminium alloy, has been developed for application in hot and cold extrusion and forging. It contains 2% Mg+Si, 1% Cu, 0.2% Cr, and 0.1% V. This alloy also has favorable fatigue and corrosion fatigue properties. These properties are attributable to a combination of composition, high solidification rate, controlled homogenization, thermal and mechanical processing. Present applications are high pressure seamless gas containers formed by either hot or cold impact extrusion and tubing for light weight bicycle frames. This study reports of the development of an aluminium-magnesium-silicon alloy, LM25 that combines strength, extrudability, favorable corrosion resistance with low cost and scrap compatibility. Six prospective alloy compositions were studied and the composition of what is now designated LM25 had the best properties. It will be demonstrated that relatively high strength was anticipated by increasing Si, Mg, and Cu concentration, as these are the principle basis of precipitation strengthening in alloys such as LM25. The Mg and Si concentration exceed the solubility in Al. 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. Silicon carbide is not affected by any acids and molten salts up to 800ºc. In air sic forms a protective silicon oxide coating at 1200ºC and is able to be used up to 1600ºC. The high thermal conductivity coupled with low thermal expansion and high strength gives this material exceptional thermal shock resistant quality. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures, approaching 1600ºC with no
  • 3. strength loss. Chemical purity, resistance to chemical attack at temperature, and strength retention at high temperatures has made this material very popular as wafer tray supports and paddles in semiconductor furnaces. The electrical conduction of the material ha lead to its use in resistance heating elements for electric furnaces, and as a key component in thermistors (temperature variable resistors) and in varistors (voltage variable resistors). Composites are defined as the combination of two or more material, which are having distinct phase and properties, superior to the base material. Mechanical processing is an important step in the manufacturing of engineering components and is used not only to achieve the required shape but also to impart desirable changes in the micro structure and properties. Expensive and time consuming trial and error techniques are generally adopted for this purpose. In most composites, reinforcement is added to the matrix of the bulk material to increase strength and stiffness of the matrix [1-3]. A reduction in material density coupled with an increase in stiffness, yield strength and ultimate strength can be translated to reductions to structural weight. This led the aerospace industry to develop and examine new materials having combination of low density, improved stiffness and high strength as attractive alternatives to exiting high strength aluminium alloys and titanium alloys. The high strength metal matrix composites combine the high strength and hardness of the reinforcing phase with ductility and toughness of light metals. Moreover the need for improved design procedures has arisen from an attempt to achieve significant improvement in structural efficiency, reliability and overall performance through either a reduction in absolute weight or increase in strength to weight ratio. Recent research results have made it
  • 4. possible to envision combining these effects through the development of reinforced light weight alloys [1]. The metal matrix composites offer a spectrum of advantages that are important for their selection and use as structural materials. A few such advantages include the combination of high strength, high elastic modulus, high toughness and impact resistance, low sensitivity to changes in temperature or thermal shock, high electrical and thermal conductivity, minimum exposure to the potential problem of moisture absorption resulting in environmental degradation, and improved fabric ability with conventional metal working equipment [2].With the exception of wires, which are metals, reinforcements are generally ceramics. Typically these ceramics are oxides, carbides and nitrides, which are used because of their excellent combinations of specific strength and stiffness at both ambient and at elevated temperatures. Silicon carbide, boron carbide and aluminium oxide are the key particulate reinforcements that have been used. These can be obtained in varying levels of purity and size distribution. The silicon carbide articulates are also produced as a by-product of the processes used to make whiskers of these materials [2]. The particulate reinforced metal matrix composites have emerged as attractive candidates for use in a spectrum of applications to include industrial, military and space related [1]. The renewed interest in metal matrix composites has been aided by the development of reinforcement material, which provides either improved properties or reduced cost when compared to the exciting monolithic materials [2]. Particulate reinforced metal matrix have attracted considerable attention on account of an availability of a spectrum of reinforcements at competitive cost, successful development of manufacturing processes to produce metal matrix composites with reproducible microstructures
  • 5. and properties, and availability of standard and near standard metal working methods, which can be utilized to form these materials. Furthermore, use of discontinuous, reinforcements minimizes the problems associated with fabrication of reinforced metal matrix composites such as fiber damage, micro structural heterogeneity, fiber mismatch and inter- facial reactions. For applications subjected to serve loads, extreme thermal fluctuations, such as in automotive components, the discontinuously reinforced metal matrix composites have been show to offer near isotropic properties with substantial improvements in strength and stiffness, relative to those available for the monolithic materials [3]. The most common particulate composite system in an aluminium alloy reinforced with silicon carbide. So far most of the alloy that has been chosen as matrices has been the A356, 2XXX alloys. Although few studies have been reported on the LM25 series alloys reinforced with silicon carbide particulates, much less attention has been given to the LM25. Al alloy matrix composites, which show the highest strength of commercially available. Al alloys and widely used for structural applications [3]. Stronger matrix alloys tend to produce stronger composites. However within these composites system there are variables, such as, ageing condition, weight/volume fraction of particulate, particulate size, which exerts an influence on mechanical properties. The objective of this study is to investigate the mechanical performance of silicon carbide reinforced stir cast grain refined LM25 matrix composites. 1.1 METAL MATRIX COMOSITES A combination of two or more materials reinforcing element, fillers, and composite matrix binder is differing in form or composition on a macro scale. The constituents retain their identities that are they do not dissolve or merge completely into one another although they act in concert. Normally the
  • 6. components can be physically identified and exhibit an interface between one another. Metal matrix composites can be classified depending on the nature and number of reinforcements, type of metal matrix and functional behavior of composites. Depending upon the nature of reinforcements used, metal matrix composites can be classified as (a) Dispersion strengthened, (b) Discontinuously reinforced and (c) Continuous fiber reinforced composites. 1.1.1 CONSTITUENTS OF MMC The major constituents of MMC are the matrix and the reinforcements. The interface between the matrix and the reinforcement is also considered as one of the constituents as it plays a crucial role in determining the properties of the composites. The matrix is the continuous phase (i) binding and keeping the reinforcement in position and orientation, (ii) transferring the load to and between the reinforcements and (iii) protecting reinforcements from the environment and handling. Further, the matrix determines the overall service temperature limitations of composites as well as their resistance to environment. Important requirements of a matrix alloy are compatibility with reinforcements during composite production and product service, high strength and sufficient and plasticity at room and elevated temperatures. The major metal matrices used for the fabrication of composites include aluminium, magnesium, titanium, and copper based alloys. The matrices studied are based on zinc, tin, steel, super alloys and inter metallics. Among the various matrix materials available, aluminium and its alloys are widely used for the fabrication of MMC due to the fact that they are light in weight, economically viable, amenable for production by various processing techniques and posses’ high specific strength and good corrosion resistance.
  • 7. The reinforcements are the second phase materials added to the matrix alloys, which normally enhance strength, stiffness, wear and creep resistance of the composites. The choice of reinforcements always depends on the final property requirements of the composite system or component to be fabricated. Certain dispersions normally import some special properties to composites such as enhanced wear resistance and reduced density at the expense of strength. Generally, the dispersions are refractory materials, such as oxides, carbides, and nitrides of different elements. Basically, they are stable and non- reactive in most of the matrix alloys. In addition, they do not mostly undergo any change in phase or shape during composite synthesis or in use except those produced by in-situ methods. At present, there are a wide range of reinforcement materials, which can provide varying combinations of properties to the composites synthesized. Interface is the region that lies between the matrix and the reinforcements. It plays a crucial role in determining the composites properties. It may contain a simple row of atomic bonds (e.g. the interface between alumina and pure aluminium), or reaction products between the matrix and the reinforcement coatings (e.g. interface between aluminium carbide, aluminium and carbon fibers), or reinforcement coatings (e.g. interfacial coatings between SiC monofilaments and titanium matrices). In composites, (i) stiffening and strengthening rely on load transfer across the interface; (ii) toughness is influence by crack deflection/fiber pull-out and (iii) ductility id affected by relaxation of peak stresses near the interface. The types of interface bonding are mechanical, physical and chemical in nature. Mechanical bonding is the simple mechanical keying or interlocking effects between two surfaces which could lead to a considerable degree of bonding. Any contraction of the matrix onto the fiber would result in a gripping
  • 8. of the fiber by the matrix. The physical bonding is those involving weak, secondary or van der Waals forces, dipolar interaction and hydrogen bonding. Chemical bonding involves the atomic or molecular transport by diffusion process. Solid solution and compound formation may Occur at the interface, resulting in a reinforcement / matrix interfacial reaction zone with a certain thickness. This encompasses all types of covalent, ionic and metallic bonding. 1.2 TYPES OF MMC i. Particle- reinforced MMC’s ii. Dispersion strengthen MMC’s iii. Fiber reinforced MMC’s 1.2.1 Particle – Reinforced MMC’s Material consisting of one or more constituents suspended in a matrix material. These particles or either metallic or non-metallic. 1.2.2 Dispersion Strengthen MMC’s Continuous fibers are identified as one of important in the early stages of MMC evolution because of their wide use in the fabrication of polymer matrix composites. Their concentration in composites varies from a few to 80% of volume. However, processing of continuous fibre reinforced composites is expensive. The reinforcement is incorporated in loose from into the metal matrix. Because most metal reinforcements systems exhibit poor wetting, mechanical force is required to combine the phases generally through stirring.
  • 9. 1.2.3 Fiber Reinforced MMC’s Depending on the number of reinforcements, MMC are classified as mono and hybrid metal composites. In mono composites, only one type of reinforcement is used. In the case of hybrid MMC, more than one type and shape of reinforcement are used. Similarly classification is made on the type of matrix alloy chosen such as aluminium, magnesium , titanium, copper, and inter metallics MMC. Based on the functional behaviour, they are classified as ‘functionally graded’ composites, ‘smart’ composites etc. based on the size of reinforcements or matrix grain size they can be classified as macro, micro and nano MMC. These type of MMC’s contains reinforcements having lengths much greater than their cross sectional dimensions. Such a composite is considered to be a discontinuous fiber or short fiber composite if its properties vary with fiber length. 1.3 ENGINEERING PROPERTIES OF MMC’S 1.3.1 Stiffness Enhancement Stiffness is nothing but the avoidance of excessive elastic deflection in survive. Accordingly to the increase of reinforcement volume fraction. The stiffness increases due to the increase in young modules. 1.3.2 Strength Enhancement The enhancement of strength by addition of reinforcement experienced in terms of yield stress or failure stress can be quite strength also increases. 1.3.2 Increased creep resistance When fibers particularly long fibers are used as also reinforcement, creep resistance of MMC’s would raise and give a high beneficial increase in stiffness
  • 10. 1.3.3 Increased wear resistance While different wear applications require difference reinforcements types to achieve optional wear rate redirection. Further more, it is often advantages to control the distribution of high wear resistance is selected surface areas, while other regions are suitably tough, strong, thermal conducting etc. 1.3.4 Density reduction A relatively low density is an attractive feature of many MMC materials. In many cases of interest, addition of the reinforcement raises the density slightly, but the increase of strength, stiffness etc. 1.3.6 Thermal Expansion control Thermal expansion control is as good as in MMCs. When the coefficient of thermal expansion will be decreased, then MMCs are withstanding at high temperatures. 1.4 STRESS-STRAIN DIAGRAM In designing various parts of a machine, it is necessary to know how the material will function in service. For this, certain characteristics or properties of the material should be known. The mechanical properties mostly used in mechanical engineering practice are commonly determined from a standard tensile test. These tests consist of gradually loading a standard specimen of a material and noting the corresponding values of load and elongation until the specimen fracture. The load is applied and measure by attesting machine. The stress is determined by dividing the load values by the original cross section area of the specimen. The elongation is measures by determining the amount that two reference points on the specimen are moved sprat by the action of the machine.
  • 11. The original distance between the two reference points is known distance between the two reference points is known as gauge length. The strain is determined by dividing the elongation values by the gauge length. The values of the stress and corresponding strain are used to draw the stress-strain diagram of the material tested. A stress-train diagram for mild steel under tensile test is shown in Fig. The various properties of the material are discussed below. Proportional limit: We see from the diagram that from point 0 to A is a straight line, which represents that the stress is proportional to strain. Beyond point A, the curve slightly deviates from the straight line. It is thus obvious, that Hooke’s law holds good up to point A and it is known as proportional limit. It is defined as
  • 12. that stress at which the stress-strain curve begins to deviate from the straight line. Elastic limit: It may be noted that even if the load increased beyond point A up to the point B, the material will regain its shape and size when the load is removed. This means that the material has elastic properties up to the point B. this point is known as elastic limit. It is defined as the stress developed in the material without any permanent set. Yield point: If the material is stressed beyond point B, the plastic stage will reach i.e. on the removal of the load; the material will not be able to recover its original size and shape. A little consideration will show that beyond point B, the strain increase at a faster rate with any increase in the stress until the point C is reached. At this point, the material yields before the load and there is an appreciable strain without any increase in stress. In case of mild steel, it will be seen that a small load drops to D, immediately after yielding commences. Hence there are two yield points C and D are called the upper and lower yield points respectively. The stress corresponding to yield point is known as yield point stress. Ultimate stress: At S, the specimen regains some strength and higher values of stresses are required for higher strains, than those between A and D. The stress goes on increasing till the point E is reached. The gradual increase in the strain of the specimen is followed with the uniform reduction of its cross sectional area. The work done, during stretching the specimen, is transformed largely into heat and the specimen becomes hot. At E, the stress, which attains its
  • 13. maximum value, is known as ultimate stress. It is defined as the largest stress obtained by dividing the largest value of the load reached in a test to the original cross sectional area of the test piece. Breaking stress: After the specimen has reached the ultimate stress, a neck is formed, which decreases the cross sectional area of the specimen. A little consideration will show that the stress (load) necessary to break away the specimen is less than the maximum stress. The stress is, therefore, reduced until the specimen breaks away at point F. The stress corresponding to point F is known as breaking stress. Note: The breaking stress (i.e. stress at F which is less than at E) appears to be somewhat misleading. As the formation of a neck takes place at E which reduce the cross sectional area, it causes the specimen suddenly to fail at F. If for each value of the strain between E and F, the tensile load is divided by the reduced cross sectional area at the narrowest part of the neck, then the true stress – strain curve will follow the dotted line EG. However, it is an established practice, to calculate strain on the basis of original cross sectional area of the specimen. Percentage reduction in area: It is the difference between the original cross sectional area and cross area at the neck (i.e. where the fracture takes place). This difference is expressed as percentage of the original cross sectional area. Percentage reduction in area = (A – a/A) * 100 Let,
  • 14. A = Original cross sectional area, a = cross sectional area at the neck. A – a = reduction in area. Percentage elongation: It is the percentage increase in the standard gauge length (i.e. Original length) obtained by measuring the fractured specimen after bringing the broken parts together. Let, l = Original length, L = Length of specimen after fracture or final length, L-l = Elongation, (L-l)/l * 100 = Percentage elongation.
  • 15. CHAPER 2 LITERATURE SURVEY Kaczmar et al [21] stated that composite material are considered to be more suitable than conventional materials as they have favourable mechanical, thermal and frictional properties. Composites containing discontinuous reinforcement, especially particulate metal matrix composites, have found commercial use in some applications since they can be produced economically by conventional processing techniques. Among these materials aluminium alloy based composites are very attractive on account of there processing flexibility, wide range, low density, high wear resistance, high thermal conductivity, heat treatment capability and improved elastic modulus and strength. Various methods have been developed for the production of metal matrix composites. An economical way of producing metal matrix composites is the incorporation of the particles into the liquid metal and casting.in as cast aluminium alloy based composites, a moderate improvement in strength over the unreinforced alloy is obtained. On the other hand when the particulate reinforcement is added to improve stiffness, strength and tribological properties, a substantial decrease in ductility is observed. Inferior ductility of these material limits their performance and applications [22]. Kaczmar et al [21] and ghomashchi et al [22] observed ductility is affected by various factors such as the matrix micro structure, heterogeneous reinforcement distribution, porosity content and the strength of the interfacial bond between matrix and the reinforcement. It has been shown that some improvement in ductility as well as the strength is observed by applying pressure during solidification [22]. Forming processes alter the structural parameters. Which influence the properties of these material [23] as these parameters, are sensitive to the type of reinforcement, the method of production and fabrication processing of composite after the initial production stage. Particulate Sic reinforced AlSi5 based composites are
  • 16. produced under ordinary foundry conditions, and the cast ingots are hot extruded to improve mechanical properties. The microstructures and mechanical properties are studied in the cast of state and after the extrusion process. The strength and ductility of the as cast extruded composites related to the microstructures and to the volume fraction of reinforcement were concluded by Ghomashchi et al[23].The influence of reinforcement phase on the properties of the composites depends both on the manufacturing method and the characteristics of ceramic particles (chemical composition, volume percent and particle size). Soma Raju et al[24] were inferred that for instance, the wear resistance of AL-Al2O3 composites was reported to be superior to those containing SiC. The inferior performance of the SiC reinforced composites is attributed to the reaction of interface between the metal matrix and the reinforcement. The size and distribution of second phase particles also play an important role in controlling the mechanical properties and wear resistance of the composites, e.g. a composite with large size ceramic particles was reported to exhibit superior wear resistance but inferior mechanical strength to that containing small particles size [24,25]. On the other hand, the distribution of reinforcement particles and the interfacial reaction between the metal matrix and the reinforcement highly depends on the processing methodology. It is known that the reaction between the matrix and particles is minimized by using solid state processing, i.e. powder metallurgy route [24]. In liquid metallurgy methods, the wettability of the reinforcement particles by molten metal is improved by applying high pressure during casting [25]. The thermal mismatch between the metal matrix and the reinforcement increases the dislocation density of the matrix. Therefore, the aging kinetics of LM25 aluminium alloy increases with addition of SiC particles, i.e. less time is required to obtain the maximum hardness by T6 heat treatment. The aging kinetics is further increased by decreasing the particle size of the reinforcement phase. The mechanical strength of LM25-30 vol% SiC composite is superior to those of the
  • 17. unreinforced material. Decreasing the particle size of the reinforcement phase leads to better mechanical properties [24, 25]. Tan et al [26], who investigated the behaviour of powder processed and extruded Al-Li alloy based SiCp reinforced composites and found that the addition of over 20wt.% reinforcement causes a substantial decrease in yield strength and ductility. In addition, Davies et al [27] reported that in advanced LM25 series AL alloy based SiCp reinforced composites, the yield strength and tensile strength values exhibited an increase with the addition of up to 10 and 15 vol.% SiCp. After the addition of reinforcement exceeding these volume fraction values, the yield strength and tensile strength decreasing significantly. The decrease in the strength of these composites after the addition of SiCp over an optimal volume fraction value could be explained by the yearly void formation at the reinforcing particles. Thus, higher volume fraction of the reinforcement giving Thus, higher volume fraction of the reinforcement giving smaller inter particle spacing will make relaxation more difficult, and the build up of dislocations leading void formation at reinforcing particles would occurs at lower strains, as a result proof stress and the strain to failure are reduced. In interpreting the decreased strength of high SiCp containing composites, the interfacial reactions also be considered. The reaction takes place during the incorporation process and may alter the matrix composition; as a result the strength of the particulate containing composites is affected. Zhong et al. [28] have reprted that in Al-Mg (5083) alloy based SiCp reinforced composites the addition of the oxidized SiCp decreases the strength as a result of some interfacial reactions in which MgO layer from at the particle-matrix interfaces at
  • 18. the expense of Mg in matrix. The depletion of Mg in the matrix will induce lower strength values due to the reduced solute strengthening. In the high SiCp containing composites higher amount of Mg depletion take place and as a result even lower strength levels will be observed. For the case of composite samples of the present study, oxidized SiCp was used as reinforcement. Some formation of Mg containing interfacial oxides was observed in the microstructures but the age hardenability of the high reinforcement containing samples was not greatly altered [27], and in the addition, no reduction in the strength of the extruded samples of high reinforcement composites was obtained with increasing SiCp content. The decrease in the strength partly be attributed to the formation of Mg containing composites at the interfaces, giving raise to reduced cohesion between the reinforcing particles and the matrix. In addition, the possible formation of flaws in SiC particles during oxidation may induce early void formation, and lower values of strength could be observed in the samples due to the reduced load transfer to the particles [28]. The particles with flaws may crack and separate during extrusion and may have no deteriorating effect on the strength of the extruded samples. It is known that the forming process applied to the a metal matrix composite affect the microstructure and mechanical properties. The effect of such processes were outlined by McKimpson and Scott [29]. They stated that achieving a uniform reinforcement distribution should improve the strength and of the particulate containing composites. It was demonstrated by Harrigen et al. [30] that the application of the hot rolling process of at least 80% reduction to the billets of SiCp reinforced LM25 alloys improved the particulate distribution and increased strength and ductility of these composites. The effect of hot forging on the properties of the sand cast and squeeze cast LM25 alloy based 20 vol. % SiCp reinforced composites was studied by Rozak et al. [31] who applied hot reduction of up to 95% to both unreinforced and reinforced materials. After the application of
  • 19. forging they obtained the substantial improvement in the strength and ductility of both group of materials. The improvement in properties, being more pronounced in the sand cast composites with possessed coarse microstructures and higher porosity content, was related to the refinement of the microstructures, and to the reduction of the porosity content with the increased amount of deformation. As have been reviewed by Llyold [33], the tensile tensile elongation decreases with the addition of reinforcing particles. It has been demonstrated that the failure of the composite material is related to particle cracking and void formation in the matrix within cluster of small particles. Particle fracture is expected in larger particles as they will be loaded to higher levels and are more likely to contain defects of critical size for initiating fracture. The failure related to the particle cluster can be explained by higher stress triaxiality generated in these regions. Attempts have been made to understand the fracture in composites quantitatively [34] and it was concluded that the fracture of composites was affected by particle shape and size, particle distribution uniformity, interfacial strength and matrix ductility.
  • 21. 3.1 FABRICATION PROCESSES The evolution of different metal matrix composite system has led to the development of newer processing techniques, in addition to conventional metal processing techniques. The major criteria for the selection of a process rely on the type of composite system to be fabricated, the properties to be achieved and the component to be produced. The processing methods are widely classified into primary and secondary processes. The primary process combines matrix and reinforcements to produce the basic composite system and their structures. The primary processing techniques may be classified into liquid state, solid state and coating processes. The major primary liquid state processes are stir casting or vortex method, infiltration, in-situe and spray deposition processes and solid state processes are powder metallurgy and diffusion bonding. The secondary process involves processing of primary processed composites with the objective of improving their mechanical properties by further consolidation (reduction or elimination of porosity), break up of dispersion agglomerates, improved interfacial bonding, generating dispersion alignment and / or forming into a required shape to obtain semi-finished products. The most commonly used secondary processes for MMCs are extrusion, rolling, forging, super plastic deformation, machining and joining [4] The processing of composites is accomplished by several techniques such as stir casting, rheo-casting and powder metallurgy [5]. The powder metallurgy method is viable only for critical applications due to the prohibitive cost of production, and also the product size is limited. On the other hand, casting processes lead to a number of processing limitations such as non uniform distribution of particles, clustering of particles, several interfacial reactions,
  • 22. large amount of porosity, and finally the inability of the process to incorporate extremely fine size of ceramic particulates. These processing limitations have been addressed in recent times by adopting spray deposition/forming processing route for these composites. This route is capable of low temperature processing and gives rise to uniformity in particles distribution with minimum interfacial reaction products [6, 7]. Discontinuously reinforced metal matrix composites (MMCs) have been considerable development in the recent years from the view point of both primary as well as secondary processing. Such composites are attractive for various structural applications in the aerospace, automobile industries and electrical components due to their unique combination of mechanical, physical and thermal properties [5-8]. The addition of particulates significantly enhances the specific strength and stiffness, which make this material suitable for critical structural components. However, these materials are difficult to shape using conventional processing techniques because of intricate micro structural features developed by rapid solidification and addition of ceramic reinforcements. In addition, as their mechanical properties are strongly dependent upon micro structural characteristics such as volume fraction and aspect ratio of particulate, it is essential that manufacture of structural shapes to be accomplished without fracturing particulate or changing their aspect ratio. (a) External view of electrical resistance furnace (b) Degassing using Hexachloro-
  • 23. ethane (c) Mechanical stirrer for vortex method (d) Pouring of molten metal into the mould (e) Metallic Mould Used for Casting (f) Removal of Cast Composites from the Mould
  • 24. 3.2 TENSILE TEST Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength [9], is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen’s cross sections starts to significantly contract. Tensile strength is the opposite of compressive strength and the values can be quite different. The UTS is usually found by performing a tensile test and recording the stress versus strain; the highest point of the stress-strain curve is the UTS. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is depended on the other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material. Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood. Tensile strength is defined as a stress, which is measured as force per unit area. In the SI system, the unit is Pascal (Pa) or, equivalently, Newton per square meter (N/ ). The customary unit is pounds force per square inch (lbf/i or psi), kilo – pounds per square inch (ksi), which is equal to 1000 psi; kilo – pounds per square inch are commonly used for convenience when measuring tensile strengths.
  • 25. Many materials display linear elastic behavior, defined by a linear stress strain relationship, in which deformations are completely recoverable upon removal of the load, that is a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this linear region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen will not return to original shape and size when unloaded. Note that there will be elastic recovery of a portion of the deformation. For many applications, plastic deformation is unacceptable, and is used as the design limitation. After the yield point, ductile metals will undergo a period of strain hardening, in which the stress increases again with increase strain, and they begin to neck, as the cross sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material when necking becomes substantial, it causes a reversal of the engineering stress strain curve, this is because the engineering stress is calculated assuming the original cross sectional area before
  • 26. necking. The reversal point is the maximum stress on the engineering stress strain curve, and the engineering stress coordinate of this point is the tensile ultimate strength. The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however used to for quality control, because of the case of testing. It is also used to roughly determine material types for unknown sample [10]. Brittle materials, such as concrete and carbon fiber, are characterized by failure at small strains. They often fail while still behaving in a linear elastic manner, and thus do not have defined yield point. Because strains are a low, there is negligible difference between the engineering stress and the true stress. Testing several identical specimens will result in different failure stresses; this is due to the Weibull Modulus of the brittle material. The UTS is a common engineering parameter when design brittle members, because there is no yield point [10]. The testing involves taking a small sample with a fixed cross section area, and then pulling it with a controlled, gradually increasing force until the sample changes shape or breaks. When testing metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with light weight, even portable equipment, such as hand held Rockwell hardness testers. Tensile test is gripped at either end by suitable apparatus in a testing machine which slowly exerts an axial pull so that the material is stretched until it breaks. The test provides information on proof stress, yield point, tensile strength, elongation and reduction of area. 3.3 IMPACT TESTS The charpy impact test, also known as the Charpy v-notch test, is a standardized high strain rate test which determines the amount of energy absorbed by a material during fracture. This absorbed energy is a measure of a
  • 27. given materials toughness and acts as a tool to study temperature dependent brittle ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply. But a major disadvantage is that all results are only comparative [11]. The test was developed in 1905 by the French scientist Georges Chary. It was pivotal in understanding the fracture problems of ships during the Second World War. Today it is used in many industries for testing building and construction materials used in the construction of pressure vessels, bridges and to see how storms will affect materials used in building [12]. The apparatus consists of a pendulum axe swinging at a notched sample of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after a big fracture. The notch in the sample affects the results of the impact test [13], thus it is necessary for the notch to be of regular dimensions and geometry. The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made [14]. “The Standard methods for Notched Bar Impact Testing of Metallic Materials” can be found in ASTM E23 [15], where all the aspects of the test and equipment used are described in detail. The quantitative result of the impact test the energy needed to fracture a material and can be used to measure the toughness of the material and yield strength. Also, the strain rate may be studied and analyzed for its effect on fracture.
  • 28. The ductile – brittle transition temperature (DBTT) may be derived from the temperature where the energy needed to fracture the material drastically changes. However, in practice there is no sharp transition and so it is difficult to obtain a precise transition temperature. An exact DBTT may be empirically derived in many ways. A specific absorbed energy, change in aspect of fracture (such as 50% of the area is cleavage), etc [1]. The qualitative results of the impact test can be used to determine the ductility of a material. If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture [11]. According to ASTM A370, the
  • 29. standard specimen size for charpy impact testing is 10mm×10mm×55mm. Sub- size specimen sizes are 10mm×7.5mm×55mm, 10mm×6.7mm×55mm, 10mm×5mm×55mm, 10mm×3.3mm×55mm, 10mm×2.5mm×55mm. Details of specimens as per ASTM A370 (Standard Test Method and Definitions for Mechanical Testing of Steel Products). According to EN 10045-1, standard specimen sizes are 10mm×10mm×55mm. sub size specimens are 10mm×7.5mm×55mm and 10mm×5mm×55mm. 3.4 HARDNESS TEST The metals handbook defines hardness as resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper or resistance to scratching, abrasion or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken or have its shape changed ), when a load is applied. The greater hardness o f the metal, the greater resistance it has to deformation. In mineralogy the property of matter commonly described as the resistance of a substance to being scratched by another substance. In metallurgy hardness is defined as the ability of a material to plastic deformation. The dictionary of metallurgy defines the indentation hardness as the resistance of a material to indentation. This is the usual type of hardness test, in which a pointed or rounded indenter is pressed into a surface under a substantially static load. Hardness measurement can be defined as macro, micro or nano scale according to the forces applied and displacements obtained [16]. Measurement of the macro-hardness of materials is a quick and simple method of obtaining mechanical property data for the bulk material from a small sample. It is also widely used for the quality control of the surface treatments processes. However, when concerned with coatings and surface properties of importance to
  • 30. friction and wear processes for instance, the macro-indentation depth would be too large relative to the surface-scale features. Where materials have a fine microstructure, are multi-phase, non-homogeneous or prone to cracking, macro- hardness measurements will be highly variable and will not identify individual surface features. It is here that micro-hardness measurements are appropriate. Micro hardness is the hardness of a material as determined by forcing an indenter such as Vickers or knoop indenter into the surface of a material under 15 to 1000 gf load; usually, the indentations are so small that they must be measured with a microscope. Capable of determining hardness of different micro constituents within structure, or measuring steep hardness gradients such as those encountered in casehardening. Conversions from microhardness values to tensile strength and other hardness scales (e.g Rockwell ) are available for many metals and alloys [17]. Micro-indenters works by pressing a tip into a sample and continuously measuring: applied load, penetration depth and cycle time. Nano-indentation [18] tests Measure hardness by indenting using very small, on the order of 1 nano-newton, indentation forces and measuring the depth of the indentation that was made. These test are based on new technology that allows precise measurement and control of the indenting forces and precise measurement of the indentation depths. By measuring the depth of the indentation, progressive levels of forcing are measurable on the same piece. This allows the tester to determine the maximum indentation load that is possible before the hardness is compromised and the film is no longer within the testing ranges. This also allows a check to be completed to determine if the hardness remains constant even after an indentation has been made. There are various mechanisms and methods that have been designed to complete nano-indentation hardness tests. One method of force application is using a coil and magnet assembly on a loading column to drive the indenter downward. This method uses a capacitance
  • 31. displacement gauge. Such gages detect displacements of 0.2 to 0.3 NM (nanometer) at the time of force application. The loading column is suspended by springs, which damps external motion and allows the load to be released slightly to recover the elastic portion of deformation before measuring the indentation depth. There are three types of tests used with accuracy by the metal industry; they are the Brinell hardness test, the Rockwell hardness test, and the Vickers hardness test. Since the definitions of metallurgic ultimate strength and hardness are rather similar, it can generally be assumed a strong metal is also a hard metal. The way the three of these hardness tests measure a metal’s hardness is to determine the metal’s resistance to the penetration of a non- deformable ball or cone.
  • 32. CHAPTER 4 EXPERIMENTS Alloy LM-25 is chosen as the matrix material. The key alloying element is zinc. The second is magnesium, which is predominantly added to enhance wetting between the matrix and the reinforcing phase. The composition of aluminium LM-25 is tabulated in table 1. Table 1 : Composition (wt %) of LM-25 : Cu Mg Si Ti V Fe Al 0.55-1.0 1.2-1.6 0.6-1.2 0-0.1 0.1-0.3 0-0.4 Bal Stir casting technique was used to fabricate LM-25 alloy reinforced with 0%, 5%, 10%, 15% volume fraction of silicon carbide composites. Stir casting set up is shown in fig. The matrix metal was LM-25 aluminium alloy and the reinforcement was SiCp with an average size of 20 µm. The aluminium alloy was melted by using an electric furnace. Preheated SiCp (250ºC) was added to the melt and mixed usig a rotating impeller in Argon environment and poured into a permanent mould. The cast billets were heated in a furnace to 400ºC and for 30minutes in the same temperature and hot extruded
  • 33. LM- Series for cast Aluminium Alloys S.No Trade Name Si Cu Mg Mn others 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. LM-1 LM-2 LM-4 LM-5 LM-6 LM-8 LM-9 LM-10 LM-11 LM-12 LM-13 LM-14 LM-16 LM-17 LM-18 LM-23 LM-24 LM-25 3.0 10.0 5.0 - 11.5 5.5 11.5 - - - 12.0 0.3 5.0 11.5 5.0 2.5 8.5 7.0 7.0 2.0 3.0 - - - - - 4.5 10.0 0.9 4.0 1.2 - - 1.8 3.5 - - - - 5.0 - 0.6 0.4 10.5 - 0.25 1.2 1.5 0.5 - - 0.2 - 0.3 - - 0.5 0.5 - 0.5 0.5 - - - - - - - - - - - 3.0 Zn - 0.8 Fe - - 0.15 Ti - 0.2 Ti 0.15 Ti - 2.5 Ni 2.0 Ni 0.3 Ni 3.0 Ni - 1.2Ni - -
  • 34. 4.1 TENSILE TEST Four types of composite alloys containing 0, 5, 10, 15 wt% SiC were processed by stir cast an extruded to obtain the tensile test specimens. Only the burrs were cleaned prior to mechanical tests, dimensions and shape of tensile test specimens are given in fig. The tensile test specimens of dimensions, 10 mm in diameter and 120 mm in length were machined from the extruded rods are shown in fig shows after tensile tested specimens. Tensile test were carried out on a 10T servo controlled universal testing machine as shown in fig The three specimens of tensile testing on all composite were tested. Load (p) versus deflection (ᵹ) data was recorded during tensile test. Also the values of ultimate tensile strength were evaluated. The recorded maximum loads are in kilograms and were converted to maximum stress values (M pa). The cross sectional areas of the tensile test samples were measured and lengths were compared before and after fracture.
  • 35. TENSILE TEST RESULTS Materials Breaking load Kn Breaking stress N/mm^2 Ultimate stress N/mm^2 Yield stress N/mm^2 % of Elongation LM25/0%SiCp composites 0.7 6.2 7.957 4.59 12% LM25/5%SiCp composites 0.82 7.25 8.66 5.128 10% LM25/10%SiCp composites 1.05 9.28 10.43 6.36 7.5% LM25/15%SiCp composites 0.9 7.95 9.01 5.39 8% 4.2 IMPACT TEST: A quick and reproducible method is disclosed for evaluating the impact properties of relatively stiff material such as plastic. A test specimen in the form of a thin un-notched strand of known thickness is mounted in a holding in a means and subjected to impact by a pendulum type striking means having predetermined potential energy content. The excess type striking means after breaking the specimen is measured and impact values determined by mathematical formula as a function of specimen thickness and energy required breaking the specimen. Four types of composite alloys containing 0, 5, 10, and 15 wt% Sic were processed by stir cast and extruded to obtain the impact test specimens. Dimension and shape of impact test V notched specimen are given fig The impact specimens of dimensions, According to ASTM A370, the standard specimen sub size for Charpy impact testing is 10mm×7.5mm×55mm,
  • 36. were machined form the extruded rods are as shown in fig ,Shows after impact tested specimens. Impact tests carried out on a vintage impact test machine as machine as shown in fig. The two were specimens of impact testing on alloy and composites pair were tested. Impact test results were shown in table 3. IMPACT TEST RESULTS S .No Material Impact load in joules 1 LM25/0%SiCp composites 18 2 LM25/5%SiCp composites 40 3 LM25/10%SiCp composites 56 4 LM25/15%SiCpcomposites 24 IMPACT SPECIMEN
  • 37. 4.3 ROCKWELL HARDNESS TEST: Hardness tests were carried out by Rockwell Hardness testing Machine . To observe the effects of wt% addition of silicon carbide on LM 25 alloy matrix. Hardness is defined as resistance of metal to plastic deformation, usually by indentation. However, the term may also refer to stiffness or temper or resistance to scratching, abrasion or cutting. It is the property of a metal, which gives it the ability to resist being permanently, deformed (bent, broken or have its shape changed ), when a load is applied. The greater hardness of the metal, the greater resistance it has to deformation. Rockwell hardness testing are shown in Table 4. This is the usual type of hardness test, in which a pointed or rounded indenter is pressed into a surface under a substantially static load. For Aluminium 1/16” ball indenter is ued. Reading should be taken on B scale (red dial) and load applied is 100kg. In this experiment it was found that value of hardness of LM 25 in as-cast condition is 67.57, 72.33, 77.37, 79.47. HARDNESS TEST SPECIMEN Before test After test
  • 38. HARDNESS TEST RESULTS S.No Material HRB (Rockwell No) Average 1 LM25/0%SiCp composites 66 67.7 69.5 67.57 2 LM25/5%SiCp composites 75.2 69.3 72.5 72.33 3 LM25/10%SiCp composites 74.8 78.3 79 77.37 4 LM25/15%SiCp composites 77.5 81.7 79.2 79.47
  • 39. CHAPTER 5 RESULT AND DISCUSSION In contrast to samples show a continuous increase in yield strength with increasing reinforcement content. The effort of reinforcement content and hot extrusion process on the tensile strength of the composites is shown in fig. In the samples, the tensile strength increases continuously with the additions of reinforcing SiC Particles up to 10 vol. %, showing an increase of about 50%, but starts decreasing in the specimens containing over 10 vol. % SiCp. The effort of reinforcement content on tensile strength is similar to that observed for the yield strength thus the tensile strength of the extruded specimens increases continuously with the volume fraction of SiC. Vol.% SiCp Fig: The effort of reinforced content and hot extrusion process on the tensile strenth of the composite 0 2 4 6 8 10 12 0 5 10 15 20 Ultimate tensile stress, Mpa
  • 40. Fig: The effect of the yield stregth of the matrix alloy and the composites as a function of the reinforcement content. The effect of SiC whisker and particle reinforcement on the strength of discontinuously reinforced Al alloy materials was extensively studied by Ghomashchi [39].He observed a substantial increase in yield and ultimate tensile strengths, with the increasing volume fraction of reinforcement , depending on the type of alloy and the matrix alloy temper. It was also reported by Lloyd [33] that increasing the SiC content increases the yield strength of the composites. Some of the reported results do not agree with the observations. Fig. it can be seen that the hardness of the composites increases with increasing the volume fraction of the SiC particles, on the other hand. As seen from Fig.. the tensile decreases with increasing the volume fraction of the SiC particles. This phenomenon indicates the that particle cracking has no significant influence on the hardness of the composites. The discrepancy between the hardness and tensile strength of the composites is due to the fundamental difference in the two types of loading [43]. During hardness testing 0 1 2 3 4 5 6 7 0 5 10 15 20 Series1 Yield stress, Mpa Vol.% SiCp
  • 41. the localized pressure causes the material directly under the indented is predominantly compressive with the matrix under a severe triaxial pressure. This stress state is totally different from deformation in tension, under which the entire specimen is under nominal tensile loading. The pre-cracked particles cannot undergo the tensile stress and thus will separate when the composite is subject to the tensile loading. With increasing the tensile loading, the cracks propagate and grow. During the hardness test, pre-cracked particles can still endure the compressive traction across the cracking surfaces. plastics flow of the matrix between the particles accommodates the deformation caused by the indentation pressure, and thus the particles are "pushed" into the material [44]. Therefore, the material response under indentation is not significantly influenced by the extrusion induced fractured particles .As a result of that, the hardness of the composites increases with increasing the volume fraction or the SiC particles. Vol.% SiCp Fig: The hardness values of the composites and the reinforced material. 65 70 75 80 85 0 5 10 15 20 Hardness(Hv)
  • 42. CHAPTER 6 CONCLUSIONS In this study, effects of extrusion and reinforcement volume fraction on the mechanical properties of SiC particle reinforced AL LM25 composites have been studied. The results indicate that the extrusion can substantially improve the distributed homogeneity of the SiC particles in the matrix, and help to decrease the number of pores and improve interfacial bonding strength of the composites. The 10% volume of SiCp aluminium matrix composites showed the maximum tensile and yield strength. The yield strength and tensile of the composites decreases with increasing the volume fraction of the SiC particles, while the hardness of the composites increase with volume fraction of the SiC particles. The fracture mode of the composites during tensile testing changes from particle "pullout” from the matrix before extrusion to particle fracture after extrusion.
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