269584792-Project-Report.pdf

“

FATIGUE CHARACTERIZATION OF A356-SiC p BASED METAL MATRIX COMPOSITES

”

A project report Submitted to

P.E.S COLLEGE OF ENGINEERING, MANDYA (AN AUTONOMOUS INSTITUTE AFFILIATED TO VTU, BELGAUM)

In partial fulfillment of the requirement for the award of the degree MASTER OF TECHNOLOGY In MECHANICAL ENGINEERING (COMPUTER INTEGRATED MANUFACTURING) 2013-2014

Submitted by

SATISH H S [4PS12MCM12]

Under the guidance of

Dr. S.L.AJIT PRASAD M.Tech, Ph.D. Professor Professor and Head of the Department Department of Mechanical Engineering,

P.E.S.C.E, MANDYA.

DEPARTMENT OF MECHANICAL ENGINEERING P.E.S COLLEGE OF ENGINEERING MANDYA-571401 (AN AUTONOMOUS INSTITUTE AFFILIATED TO VTU, BELGAUM )

Certified that, Mr. SATISH H S bearing university seat number 4PS12MCM12 has satisfactorily

completed

the

CHARACTERIZATION

project

OF

preliminary

A356-SiCp

report

entitled

BASED

METAL

“

FATIGUE MATRIX

COMPOSITES in partial fulfillment for the award of degree Master of Technology in ”

Mechanical Engineering, P.E.S.C.E, Mandya during the year 2013-2014. The Project has been approved as it satisfies the academic requirements in respect of project work prescribed for the Degree in Master of Technology.

Signature of the Guide

Signature of the HOD

Dr. S.L. AJIT PRASAD

Dr. S.L. AJIT PRASAD

Dr. V. SRIDHAR Principal, P.E.S.C.E, Mandya Details of Project Work Viva Voice Examination held Examiners Sl. No.

Date Name

1 2

Signature

DECLARATION I,

SATISH

H

S hereby

declare

that

this

dissertation

work

entitled

“

FATIGUE CHARATERIZATION OF A356-SiCp BASED METAL MATRIX

COMPOSITES

”

has been independently carried out by me under the guidance of

Dr. S.L.AJIT PRASAD, Professor and Head of the Department Mechanical

Engineering, P.E.S. College of Engineering, Mandya in the partial fulfillment of the requirement of the degree Master of Technology in Mechanical Engineering (Computer Integrated Manufacturing). I further declare that I have not submitted this dissertation either in part or full to any other university for the award of any degree or diploma.

Place: Mandya Date:

SATISH H S

ACKNOWLEDGEMENT

Before introducing my thesis work, I would like to thank the people without whom the success of this thesis would have been only a dream. I have a great pleasure in expressing my deep sense of gratitude and indebtedness to Dr. S.L.AJIT PRASAD, Professor and Head of the Department Mechanical Engineering, P.E.S. College of Engineering, Mandya for his guidance, constant supervision and his interest and precious help in the completion of my project work. I would like to extend my sincere thanks to Dr. V. SRIDHAR, Principal, P.E.S. College of Engineering, Mandya for permitting me to carry out this project work. I am sincerely thankful to Mohana kumar K.C, Abhinandan K.S, Ashok kumar M.S and Vikram C.K for their support and guidance to carry out this project.

I am thankful to Mr. G.C. Krishnappa Naik, Grindwell Norton LTD, for providing the SiC particles to carry out this project. I thank all the staff members of Mechanical, Industrial and production department, P.E.S. College of Engineering, Mandya for their co-operation in the timely completion of my project work. I thank Mr. Ravi, Mr. Nagaraju Foreman, Chandru, Chennegowda, Mahesh and Mr. Y.H. Nagaraju for their co-operation during the project work. Also I express my deep sense of gratitude to my parents and also to my friends, who have supported me during the project work.

SATISH H S

ABSTRACT

Composite materials are increasingly replacing traditional engineering materials because of their advantages over monolithic materials. The development of metal matrix composite has been one of the major innovations in the materials in the recent times. The Metal Matrix Composite is a material which consists of metal alloy reinforced with continuous fibers, whiskers, or particulates of ceramics. These MMCs are widely being used in the transport, aerospace, marine, automobile and mineral processing industries, owing to their improved strength, stiffness and wear resistance properties. Aluminium alloy is the most commonly used matrix for the metal matrix composites. The ceramic particles reinforced aluminium composites are termed as new generation material and these materials can be tailored and engineered with specific required properties for specific application requirements. Among metal-ceramic particle composite, aluminium-graphite, aluminium-alumina and aluminium-silicon carbide particles can possess improved wear resistance, high temperature hardness and strength. In the present study, A356 with 0%, 5% and 10% SiC p MMC material was fabricated using stir casting (vortex method) method. The vortex method is one of the better known approaches used to create and maintain a good distribution of the reinforcement material in the matrix alloy. The cast composites were carefully machined to prepare the test specimens for hardness, tensile tests, and fatigue test as well as for micro structural studies as per ASTM standards. Microstructural analysis of cast specimens has been carried out to investigate the influence of processing parameters. From the tests conducted for characterization of mechanical properties, composite material specimens have been found to possess enhanced hardness and tensile strengths compared to matrix alloy specimens, while at the same time, losing ductility as compared to matrix alloy. Also from the fatigue test performed it is found that fatigue life of the composite with 5% SiC p as reinforcement has longer fatigue life compared with 0% and 10% SiC p. Also fatigue life has increased with decrease in the neck diameter of composite with 5% SiC p at identical stress condition.

CONTENTS

Page no.

Acknowledgement Abstract List of Figures

I

List of Tables

III

Nomenclature

IV

CHAPTER 1: INTRODUCTION

1-4

CHAPTER 2: THEORY AND LITERATURE REVIEW

5-41

2.1 COMPOSITE MATERIAL

5

2.2 CLASSIFICATION OF COMPOSITE MATERIALS

6

2.2.1 Based on the form of reinforcement component

6

2.2.2 Based on the structure of the matrix materials

8

2.3 METAL MATRIX COMPOSITES

9

2.3.1 Merits of MMCs

11

2.3.2 Demerits of MMCs

11

2.4 ALUMINIUM MATRIX COMPOSITES

11

2.5 PROCESSING TECHNIQUES OF MMC

13

2.5.1 Solid state processing

13

2.5.2 Liquid state processing

14

2.6 FACTORS TO BE CONSIDER DURING STIR CASTING

17

2.6.1 Distribution of the reinforcement materials

17

2.6.2 Wettability of reinforcement

19

2.6.3 Porosity in cast metal matrix composites

20

2.7 MECHANICAL CHARACTERISTICS

21

2.8 FATIGUE CHARACTERIZATION

22

2.8.1 Mechanism of Fatigue failure

24

2.8.2 The Stress life approach and The Strain life approach

26

to determine the fatigue life 2.8.3 Factors affecting Fatigue behaviour

27

2.8.4 Establishing S-N curve

29

2.9 LITERATURE REVIEW

30

CHAPTER 3: OBJECTIVE AND METHODOLOGY

42-44

3.1 OBJECTIVE

42

3.2 WORK PLAN

43

3.2 METHODOLOGY

44

CHAPTER 4: EXPERIMENTAL DETAILS

45-55

4.1 WORK MATERIAL DETAILS

45

4.2 PROCESSING DETAILS

47

4.2.1 Fabrication of Al-SiC p metal matrix composites

47

4.2.2 Procedure to fabricate composites

49

4.3 MATERIAL CHARACTERISATION 4.3.1 Microscopy 4.4 MECHANICAL CHARACTERISATION

50 50 51

4.4.1 Rockwell Hardness Number (RHN)

51

4.4.2 Measurement of Tensile strength

52

4.4.3 Fatigue characterization

53

CHAPTER 5: RESULTS AND DISCUSSION

5.1 MICROSTRUCTURAL STUDY 5.1.1 Scanning Electron Microscopy (SEM) 5.2 MECHANICAL CHARACTERISATION

56-68

56 56 57

5.2.1 Hardness

57

5.2.2 Tensile strength

58

5.3 FATIGUE CHARATERIZATION

62

5.3.1 Stress Calculations

62

5.3.2 Fatigue life of the Composites with varying

63

the percentage of the reinforcement 5.3.3 Fatigue life of the Composites with 5%SiC

65

with varying the Neck diameter 5.3.4 Fatigue fractured surface SEM analysis

68

CHAPTER 6: CONCLUSIONS

69

SCOPE OF FUTURE WORK

70

REFERENCES

71

LIST OF FIGURES Fig

Page

No.

CAPTION

No.

2.1

Classification Based On The Form Of The Reinforcement

6

2.2

Types of reinforcement materials in composites

7

2.3

Classification of composite materials based on matrix materials.

8

2.4

Schematic representation of stir casting process

15

2.5

Different types of stirrer used in stir casting

18

2.6

A sketch of three degrees of wetting and the corresponding contact angles

20

2.7

S-N relationship for ferrous and non-ferrous alloys

25

2.8

Typical S-N relationship

26

3.1

Schematic diagram of work plan

43

4.1

Electrical heating furnace

47

4.2

Permanent spilt mould

48

4.3

Alumina- sodium silicate powder coated stirrer

48

4.4

Al Raw ingot material

49

4.5

Slag Remover

49

4.6

Degasser hexachloroethane C2Cl6 tablet

50

4.7

Cast Aluminium composites

50

4.8

Scanning Electron Microscope

51

4.9

Tensile testing machine

52

4.10

Tensile specimen according to ASTM B557 standard

53

4.11

Tensile test specimen

53

4.12

Rotary Bending machine

54

4.13

Fatigue test Specimen according to ASTM E446

54

4.14

Fatigue testing machine and loading diagram

55

5.1(a)

0%SiC cast-1000X

56

5.1(b)

5%SiC with 23μm cast-1000X

56

5.1(c)

10%SiC with 23μm cast-1000X

56

5.2

RHN of Base alloy and Composites

57

5.3(a)

Load v/s Displacement (elongation) of 0% SiC p

58

5.3(b)


58

5.3(c)


59

5.4(a)

Stress-Strain diagram of 0% SiC p

59

5.4(b)


60

5.4(c)


60

5.5

Proof Stress of base alloy and composites

61

5.6

Tensile strength of base alloy and composites

61

5.7

Strain to failure of base alloy and composites

61

5.8

Fatigue life of Base Alloy(0% SiC p)

63

5.9

Fatigue life of Composite with 5% SiC p

63

5.10

Fatigue life of Composite with 10% SiC p

64

5.11

Comparision of the Fatigue life of Composite with 0%SiC p, 5%SiC p & 10%

64

SiC p 5.12

Fatigue life of Composite with 5% SiC p having Neck dia 4mm

65

5.13


66

5.14


66

5.15


67

5.16

Comparision of the Fatigue life of Composite with 5%SiC p having varying

67

neck diameter 5.17

Fatigue Fractured Surface of 0% SiC reinforced in A356 Matrix (1000X)

68

5.18


68

5.19


68

LIST OF TABLES Table No.

Page CAPTION

No.

4.1

Mechanical properties of A356

45

4.2

Chemical composition of A356

46

4.3

Mechanical properties of SiC

46

4.4

Technical Specifications of Rotating Bending Fatigue Tester

55

5.1

RHN of as cast and extruded composites

57

NOMENCLATURE

ASTM

American Society for Testing Materials

Al

Aluminium

SiC

Silicon Carbide

PMC

Polymer matrix composite

MMC

Metal Matrix Composite

CMC

Ceramic Matrix Composite

AMC

Aluminium Metal Matrix

DRA

Discontinuously Reinforced Aluminium

N

Load in Newton

µ

Microns

m

Meter

σ

Stress

ε

Strain

E

Young’s Modulus

M

Bending moment

RHN

Rockwell Hardness Number

F

Imposed load in N

Kg

Kilo gram

d

Neck diameter of the Fatigue specimen

SEM

Scanning Electron Microscopy

1000X

1000 times magnification

“FATIGUE CHARACTERIZATION OF A356-SiCp BASED METAL MATRIX COMPOSITES”

CHAPTER 1 INTRODUCTION The engineering fraternity has always been on the lookout for wonder-materials which would fit the bills for all types of service conditions. It stem from the need to make progressive discoveries made by scientists, affordable. This affordability quotient has persuaded many researchers to develop such materials which would satisfy various hitherto unexplored conditions. In today’s world almost all generic materials have been tried for various uses and their limitations have been met. But the never ending quest of civilization requires that materials qualify for harsher environments. This unavoidable situation demands that new materials be created from various combinations of other compatible materials. It is to be noted here that this method is not new; it has been with mankind since ages. In every part of the world, various materials have been combined to achieve some intended properties, albeit each case differs from the others, i.e. one can create new materials with unique properties, which can be tailor-made and are different from their base ingredients. This concept holds true for a genre of materials called Composite materials where in, various types of matrices may be combined with reinforcements which contribute to the enhancement of the properties. A composite material is a combination of two or more chemically different materials with a distinct interface between them. The constituent materials maintain their separate identities in the composite, yet their combination produces properties and characteristics that are different from those of the constituents. One of these constituents’ forms a continuous phase and it is called as the matrix. The other major constituent is the reinforcement phase available in the form of fibers or as a particulate in general, added to the matrix to improve or alter the matrix properties. Reinforcement by a particulate forms a discontinuous phase uniformly distributed throughout the matrix. Therefore, composites have improved mechanical properties such as strength and toughness when compared with monolithic materials. Neither the matrices nor the reinforcements taken alone can stand up to the requirement, but the composite may be able to do so. This alteration in properties can be controlled by many ways, viz. controlling the matrix and reinforcement quality, their

M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

1


proportion or the fabrication route. This flexibility in manufacturing allows one to develop composites with varying properties in a precisely controlled fashion. The main advantage of a composite material over conventional material is the combination of different properties which are not often found in the conventional materials. The extraordinary combination properties include high strength to weight ratio, higher stiffness to weight ratio, improved fatigue resistance, improved corrosion resistance, higher resistance to thermal expansion, higher wear resistance and fracture toughness etc. There are a number of situations in service that demand an unusual combination of properties. Further, the present day trend is to go in for light weight constructions for easy handling and reduced space, aesthetic appearance and high resistance to weathering attack. These factors have propelled the modern designers to develop newer composite materials up to the stage of large-scale production with exacting requirements. It is the superiority of properties that has triggered the penetration of composite materials into all fields of manufacturing. Metal Matrix Composites (MMCs) have emerged as a class of materials suitable for structural, aerospace, automotive, electronic, thermal and wear applications owing to their advantages over the conventional monoliths. They score over in terms of specific modulus, specific strength, high temperature stability, controlled coefficient of thermal expansion, wear resistance, chemical inertness, etc. But the down side is populated by inferior toughness and high cost of fabrication in comparison with Polymer Matrix Composites (PMCs). But MMCs supersede in terms of higher transverse strength and stiffness, shear strength and high temperature capabilities. The physical properties that attract are no moisture absorption, non-flammability, high electrical and thermal conductivities and resistance to most radiations. Compositionally, MMCs have at least two components, viz. the matrix and the reinforcement. The matrix is essentially a metal, but seldom a pure one. Except sparing cases, it is generally an alloy. The most common metal alloys in use are based on Aluminium and Titanium. Both of them are low density materials and are commercially available in a wide range of alloy compositions. Other alloys are also used for specific cases, because of their own advantages and disadvantages. Beryllium is the lightest of all structural materials and has a tensile modulus greater than that of steel, but it is extremely brittle, rendering it unsuitable for general purpose use. Magnesium is light, but is highly reactive to Oxygen. Nickel and Cobalt based super alloys have also found some use, but M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

2


some of the alloying elements present in the matrices have been found to have undesirable effect(promoting oxidation) on the reinforcing fibers at high temperatures. Aluminium oxide and silicon carbide powders in the form of fibers and particulates are commonly used as reinforcements in MMCs and the addition of these reinforcements to aluminium alloys has been the subject of a considerable amount of research work. Aluminium oxide and silicon carbide reinforced aluminium alloy matrix composites are applied in the automotive industries as engine pistons and cylinder heads, where the tribological properties of these materials are considered important. Therefore, the development of aluminium matrix composites is receiving considerable emphasis in meeting the requirements of various industries. Incorporation of hard second phase particles in the alloy matrices to produce MMCs has also been reported to be more beneficial and economical due to its high specific strength and corrosion resistance properties. Aluminium is the most popular matrix for the metal matrix composites. The aluminium alloys are quite attractive due to their low density, their capability to be strengthened by precipitation, their good corrosion resistance, high thermal and electrical conductivity, and their high vibration damping capacity. They offer a large variety of mechanical properties depending on the chemical composition of the aluminium matrix. They are usually reinforced by aluminium oxide, silicon carbide, silicon dioxide, graphite, boron nitride, boron carbide etc., Aluminium based composites, reinforced with ceramic particles, offer improvements over the matrix alloy: an elastic modulus higher than that of aluminium, a coefficient of thermal expansion which is closer to that of steel or of cast iron, a greater resistance to wear and an improvement in rupture stress especially at higher temperatures and possibly improved resistance to thermal fatigue. Following successful demonstration and qualification programmes, AMCs are now being used in the aerospace industry, which represents a major breakthrough in the growing acceptance of these composite materials in a market with exceptionally high levels of technical requirements. AMCs are also recognised as having an important role to play in high speed machinery applications where increased operating speeds of more than 50% have been achieved. Furthermore, their combination of lightness, fatigue resistance, and stiffness make them ideal for many sporting applications, such as road and mountain bicycles.


3


Applications in the defence sector are also varied and make use of some of the unique properties offered by AMCs. The strength of hoop-wound tubes is exciting, giving high burst and collapse strengths. Development is also expected for gun barrel overwraps, missile bodies, rocket blast pipes and submersibles. Other applications for AMCs will utilise their thermal and electrical properties, especially in dimensionally stable platforms and electronic packaging. The major methods to produce aluminium metal matrix composites are: stir casting, powder metallurgy, liquid metal infiltration, squeeze casting, rheocasting, and spray deposition technique. Liquid infiltration is a common process to produce metal infiltration, which involves a melt liquid infiltration into porous preform. However, the major problem for the production of these materials is to accomplish the wetting of reinforcement by the liquid metal, which is very poor and is favoured by strong chemistry bonding at the interface. The poor wetting is because of the presence of oxide film at the surface of the aluminium. The wettability is a complex phenomenon that depends on factors such as geometry of interface, process temperature, soaking time, and it determines the quality of bonding among the systems. The objective of developing the Al-SiCp metal matrix composite in the present study is to derive their potential application in the engineering fields. They are prepared by making use of stir casting technique. These Al-SiCp MMC is then analysed under SEM to study the SiC particle distribution in the matrix metal Al356 and also the porosity defects are being considered. Then an attempt has been made to study the mechanical properties viz. Hardness, Tensile strength and Fatigue life of the cast composite specimen.


4


CHAPTER 2 THEORY AND LITERATURE REVIEW 2.1 COMPOSITE MATERIAL A composite material is a bi-phase or multiphase material whose mechanical properties are superior to those of the constituent materials acting independently. One of the phases is usually discontinuous, stiffer and stronger and is called reinforcement where as the less stiff and weaker phase is continuous and is called matrix. Sometimes because of the chemical interactions or the other processing effects, an additional phase called interface exists between the reinforcement and the matrix. Literally the term composite means- a solid material that results when two or more different substances, each with its own characteristics, are combined to create a new substance whose properties are superior to those of the original components for any specific application. The term composite more specifically refers to a structural material within which a reinforcement material (such as silicon carbide) is embedded. And the engineering definition would also go alongside- A material system composed of a mixt ure or combination of two or more constituents that differ in form or material composition and are essentially insoluble in each other. In principle, composites can be fabricated out of any combination of two or more materials-metallic, organic, or inorganic; but the constituent forms are more restricted. The matrix is the body constituent, serving to enclose the composite and give it a bulk form. Major structural constituents are fibers, particulates, laminates or layers, flakes and fillers. They determine the internal structure of the composite. Usually, they are the additive phase. When two or more materials are interspersed, there is always a contiguous region. Simply this may be the common boundary of the two phases concerned, in which case it is called an interface. A composite having a single interface is feasibly fabricated when the matrix and the reinforcement are perfectly compatible. On the other end, there may an altogether separate phase present between the matrix phase and the reinforcement phase. This intermediate phase is called an inter-phase. In case there is an inter-phase present, there are two interfaces, one defining the boundary between the matrix and the inter phase, and the other between the inter-phase and the reinforcement. The strength of the composite in such a case is dependent upon the strength of the weakest of the two M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

5


interfaces. There are certain advantages of having a preferred inter-phase. Such a composite with an inter-phase is fabricated if the matrix and reinforcement are not chemically compatible or if the wettability of the pair is very poor, such a composite is materialized, by introducing a third material that has good bonding properties, individually with the matrix and the reinforcement, which would not be possible otherwise. More or less, the strength of a composite is a function of the strength of its interface between the matrix and the reinforcement. The failure of a functional composite is essentially a result of the failure of the interface. Hence the strengthening mechanism is the most dominant parameter in successful fabrication of a high strength composite. Composites differ by their matrix type, reinforcement type, size and form, composition, temper state, etc. With such a big window available for fabricating a composite from different constituent materials, it is not uncommon to experiment with materials with vividly different properties. There are three broadly classified groups of composites: Polymer Matrix Composite, Metal Matrix Composite and Ceramic Matrix Composite.

2.2 CLASSIFICATION OF COMPOSITE MATERIALS 2.2.1 Based on the form of the Reinforcement components

Reinforcing Material

Fiber

Particulate Or Whiskers

• •

Large particles Dispersions

• •

Continuous fibers Discontinuous (Short)  Aligned or  Random

Structural

• •

Laminates Sandwich Panels

Fig 2.1: Classification based on the form of the reinforcement M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

6


Fig 2.2: Types of reinforcement materials in composites 1) Particulates

Microstructures of metal and ceramics composites, which show particles of one phase strewn in the other, are known as particle reinforced composites. The shape of the reinforcements can be square, triangular, or random as shown in Fig 2.2. The size and volume concentration of the dispersoid distinguishes it from the dispersion. The dispersed size in particulate composites is of the order of a few microns. The reinforcement in the matrix materials reinforces the matrix alloy by arresting motion of dislocations and needs large forces to fracture the restriction created by dispersion. 2) Whiskers

Single crystals grown with nearly zero defects are termed whiskers. They are usually discontinuous and short fibers of different cross sections made from several materials like Graphite, Silicon Carbide, Copper, and Iron etc. Whiskers differ from particles in which, whiskers have a definite length to width ratio which is greater than one. Whiskers were grown quite incidentally in laboratories for the first time. Initially, their usefulness was overlooked as they were dismissed as incidental by-products of other structure. However, study of crystal structures and growth in metals sparked off an interest in them and also the study of defects that affect the strength of materials, led to their incorporation in the composites using several methods.


7


3) Fiber reinforcement

Fibers are the important class of reinforcements, as they satisfy the desired conditions and transfer strength to the matrix constituent influencing and enhancing their properties as desired. Glass fibers are the earliest known fibers used to reinforce materials. Ceramic and metal fibers were subsequently developed and put to extensive use, to render composites stiffer and more resistant to heat. Fibers fall short of ideal performance due to several factors. The performance of a fiber composite is judged by its length, shape, orientation, composition and the mechanical properties of the matrix. The different types of fibers in use are Glass fibers, Silicon Carbide fibers, High Silica and Quartz fibers, Alumina fibers, metal fibers and wires, Graphite fibers, Boron fibers, Aramid fibers and multiphase fibers. 2.2.2 Based on the Structure of the Matrix materials

Matrix Material

Polymer Matrix

Metal Matrix



Thermoplastic





Thermosets



Light metal &alloys (Al, Mg, Li &Ti) Refractory Metals (Co, W etc)

Ceramic Matrix





Ceramic (oxides, Carbide etc) Carbon

Fig 2.3: Classification of composite materials based on matrix materials 1) Polymer matrix composites (PMC) - Also known as FRP-Fiber reinforced

polymers(or plastic)-these materials use a polymer based resin as the matrix and variety of fibers such as glass, carbon, and aramid as the reinforcement. 2) Metal matrix composites (MMC) - Increasingly found in the automotive industry,

these materials use a metal such as aluminium as the matrix, and reinforce it with fibers/particles such as silicon carbide.


8


3) Ceramic matrix composites (CMC) - Used in very high temperature environments,

these materials use a ceramic as the matrix and reinforce it with short fibers or whiskers such as those made from silicon carbide and boron nitride.

2.3 METAL MATRIX COMPOSITES The sustained interest to develop engineering materials which could cope with the raised performance standards, resulted in emergence of a newer class of materials, called Metal Matrix Composites (MMCs). They constitute a family of customizable materials with customizable critical property relationships. Such materials are known for their exceptional high modulus, stiffness, wear resistance, fatigue life, strength-to-weight ratios, tailorable coefficient of thermal expansion, etc. With these enhancements in properties, they pose for strong candidature for replacing conventional structural materials. But what makes them stand apart is the ability to customize their properties to suit the service requirement. Such advantages have made this group of materials a nice pick for use in weight-sensitive and stiffness-critical components in transportation systems. MMCs can be described as a group of materials in which a continuous metallic phase (matrix) is combined with one or more reinforcement phases. The aim of such a composite material is to enhance the suitability of the end product by selectively enhancing the complimentary properties, and masking the detrimental properties of the matrix and the reinforcement. While fabricating the MMC, a solid material results when two or more substances are physically (not chemically) combined to create a new material whose properties are superior to those of the original substances for a specific application. The matrix may be a pure metal or any alloy suitable for the intended application. The reinforcement may be any other material in the form of particulates, whiskers, fibers, platelets, etc. The most common reinforcements are ceramics having nominal size in the range of 0.1 to 100 micrometers. But in fact, just about anything suitable for the application may be utilized as a potential reinforcement. Even though at times, the matrix and the reinforcement both can be metallic in nature, MMCs are not fabricated by conventional alloying methods suitable for metals; since, such a process would mar the essence of a composite. In alloys the phases are not chemically and physically distinct. But in a composite, such phases are intentionally kept distinct, to exploit the properties of the constituents to the fullest. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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The reinforcing phase is the nominal constituent of a composite. It is the principal load bearing component in the system. Hence the reinforcements with better mechanical properties than the matrix materials are chosen while designing a composite. The matrix is responsible for holding the load-carrying reinforcement together and retaining the bulk shape of the composite. It also shares some portion of the total load which is transferred to the reinforcement via the interface or vice versa. It is the effectiveness of the interface that decides how much load is transferred to and fr om the matrix. In MMCs a high degree of interaction between the matrix and the reinforcement is inherent. The resulting strength is a direct function of effectiveness of the interface between the matrix and the reinforcement. The character of the interface depends upon the chemical and mechanical compatibility of the two phases involved. The chemical incompatibility constraint can be overcome either by opting for a low-temperature processing route or by selecting stable constituents. The thermal mechanical incompatibility problem is sorted out by employing a ductile matrix that accommodates the strain generated by the thermal alterations. Also it helps to select a pair of matrix and reinforcement having matching coefficient of thermal expansion. However when it is chemically or thermo-mechanically not feasible to fabricate a composite from a pair of constituents, an intermediate phase which is compatible with the matrix and the reinforcement may be introduced in between the two that masks the incompatibility of the original pair. This interphase prevents the chemical reaction between the matrix and the reinforcement and/or aids the matrix in accommodating the strain generated due to any incongruous strain build-up. A soft precipitate-free layer around the reinforcing particulates limit the propagation of the crack generated at their surface by effectively reducing the stress value gradually, thereby increasing the ultimate strength. Metal matrix composites have been under constant development since the days of the World War-II. They were intended to be used in the aircrafts as structural materials. After the war ceased, no longer the purpose was the war, rather MMCs found interest in civilian uses. Today the composites are extensively used in all aspects of life, be it food packaging, medical

implants,

military armours,

automotive

applications, space

applications or just about anything else. This deep penetration of MMCs in a wide spectrum of application can be attributed to the previously mentioned advantages associated with them.


10


Generally MMCs are classified according to type of reinforcement and the geometric characteristics of the same. In particular, the main classification groups these composites into two basic categories: 1) Continuous reinforced composites, constituted by continuous fibers or filaments. 2) Discontinuous reinforced composites, containing short fibers, whiskers or particles. Both reinforcement and matrix are also selected on the basis of what will be the interface that unites them. This interface can be as a simple zone of chemical bonds (as the interface between the pure aluminium and alumina), but can also occur as a layer composed by reaction (matrix/reinforcement) products. 2.3.1 Merits of MMCS

1. Very high specific strength and specific modulus 2. Low thermal coefficient of expansion 3. Retention of properties at high temperatures 4. Higher operating temperature 5. Better capability to withstand compression and shear loading 2.3.2 Demerits of MMCS

1. Difficulty with processing 2. Reduction in ductility. However, MMCs are not without some drawbacks either. Their inadequate fracture toughness and damage tolerance, poor ductility, size limitations, inhomogeneity of properties, isotropy of properties stand as hindrance to their usability front. Continuous research works are underway to overcome these limitations and explore new possibilities.

2.4. ALUMINIUM MATRIX COMPOSITES Aluminium is the most popular matrix for the metal matrix composites. The aluminium alloys are quite attractive due to their low density, their capability to be strengthened by precipitation, their good corrosion resistance, high thermal and electrical conductivity, and their high vibration damping capacity. They offer a large variety of mechanical properties depending on the chemical composition of the aluminium matrix. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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They are usually reinforced by aluminium oxide, silicon carbide, silicon dioxide, graphite, boron nitride, boron carbide etc. In the 1980s, transportation industries began to develop discontinuously reinforced aluminium matrix composites. They are very attractive for their isotropic mechanical properties and their low costs. The properties are inevitably a compromise between the properties of the matrix and reinforcement phases. It is clear that the composition and properties of the matrix phase affect the properties of the composite both directly, by normal strengthening mechanisms, and indirectly, by chemical interactions at the reinforcement/matrix interface. Aluminium based composites, reinforced with ceramic particles, offer improvements over the matrix alloy: an elastic modulus higher than that of aluminium, a coefficient of thermal expansion which is closer to that of steel or of cast iron, a greater resistance to wear and an improvement in rupture stress especially at higher temperatures and possibly improved resistance to thermal fatigue. Research has shown that the addition of SiCp to Aluminium alloys would result in an increase of modulus, and may also be accompanied by an increase in yield stress depending upon the alloy composition, heat treatment, and manufacturing method. Furthermore it helps in increasing resistance to wear, corrosion and fatigue crack initiation as compared to the performance of the matrix alloy alone. It has been reported that addition of SiC particulate reinforcement to Aluminium alloys usually lowers the fracture toughness. However this drop in the fracture toughness has been found to be caused by the alterations in flow stress, fracture of SiC particulates, poor dispersion of SiC and a decrease in tensile ductility. Other factors such as the volume fraction of the reinforcement, matrix alloy chemistry and processing variables have also been found to affect the composite character. But the interactions of these parameters are yet to be quantified to an extent that they can be deciphered. Al-Si alloys are widely used for applications in the mechanical and tribological components of internal combustion engines, such as cylinder blocks, cylinder heads, pistons etc., owing to their good castability, high corrosion resistance and low density. However, they exhibit poor seizure resistance, which restrict their uses in such mechanical tribological environments. The wear resistance of these alloys can be enhanced by incorporation of a ceramic phase in the soft aluminium alloy matrix. Continuous-fiber-reinforced MMCs exhibit highly anisotropic properties, and this result in a higher cost for the metal working process. Discontinuous silicon carbide particles M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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reinforced MMCs are particularly attractive because they exhibit good specific properties and can be produced by conventional metal working processes. Hence they are being increasingly used in the automotive industry as materials for pistons, brake rotors, calipers and liner. 2.5. PROCESSING TECHNIQUES OF MMC’s

There is a multitude of fabrication techniques of metal matrix composites depending on whether they are aimed at continuously or discontinuously reinforced MMC production. The techniques can further be subdivided, according to whether they are primarily based on treating the metal matrix in a liquid or a solid form. The production factors have an important influence on the type of component to be produced, on the micro-structures, on the cost and the application of the MMCs. Processing methods of MMCs can be classified into two categories. 1. Solid state processing. 2. Liquid state processing. 2.5.1. SOLID STATE PROCESSING 1. Powder Blending and Consolidation

Blending of aluminium alloy powder with ceramic short fibre/whisker/particle is versatile technique for the production of AMCs. Blending can be carried out dry or in liquid suspension. Blending is usually followed by cold compaction, canning, degassing and high temperature consolidation stage such as hot is ostatic pressing (HIP) or extrusion. AMCs processed by this route contain reinforcement particles in the form of plate like particles of few tens of nanometers thick and in volume fractions ranging from 0.05 to 0.5 depending on powder history and processing conditions. These fine particles tend to act as dispersion – strengthening agent and often have strong influence on the matrix properties particularly during heat treatment. 2. Diffusion Bonding

The diffusion bonding employs the matrix in the solid phase, in the form of sheet or foil. Composite laminates are produced by consolidating alternate layers of precursor wires or fibre mats and metal matrix sheets or foils under temperature and pressure. The precursor wires are collimated filaments held together with a fugitive organic binder. This M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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is achieved either by winding binder-coated filaments onto a circular cylindrical mandrel or by spraying the binder on the filaments that are already wound on a mandrel. When the solvent is evaporated, the fibre-resin combination forms a rolled fibre mat on the mandrel surface. The binder resin in precursor wires and fibre mats decomposes at a high temperature without leaving any residue. Under temperature and pressure metal sheets or foils melt and diffuse through fibre layers to form a laminate. A multilayered laminate may have any desired stacking sequence. Several complex composite components can be fabricated by stacking monotapes as per design requirements. The temperature, pressure and their duration are very critical for making good quality composites. Carbon fibres have been successfully combined with matrices like aluminium, magnesium, copper, tin, lead and silver to make a wide range of carbon fibre reinforced metal composites. A number of products ranging from flat plates to curved engine blades have been fabricated using the diffusion bonding technique. 3. Physical Vapour Deposition

This process involves continuous passage of fibre through a region of high partial pressure of the metal to be deposited, where the condensation takes place and a relatively thick coating of aluminium on the fibre. Composite fabrication is usually completed by assembling the coated fibres into bundle or array and consolidating in a hot press or HIP process. Composites with uniform distribution of fibre and volume fraction as high as 80% can be produced by this technique. 2.5.2. LIQUID STATE PROCESSING 1. Stir Casting

This involves incorporation of ceramic particulate into liquid aluminium melt and allowing the mixture to solidify. Here, the crucial thing is to create good wetting between the particulate reinforcement and the liquid aluminium alloy melt. The simplest and most commercially used technique is known as vortex technique or stir-casting technique. The vortex technique involves the introduction of pre-treated ceramic particles into the vortex of molten alloy created by the rotating impeller (Fig. 2.4). Microstructural inhomogeneities can cause notably particle agglomeration and sedimentation in the melt and subsequently during solidification. Inhomogeneity in reinforcement distribution in these cast composites could also be a problem as a result of interaction between suspended ceramic particles and moving solid-liquid interface during M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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solidification. Generally it is possible to incorporate up to 30% ceramic particles in the size range 5 to 100 μm in a variety of mo lten aluminium alloys. The melt – ceramic particle slurry may be transferred directly to a shaped mould prior to complete solidification or it may be allowed to solidify in billet or rod shape so that it can be reheated to the slurry form for further processing by technique such as die casting, and investment casting. The process is not suitable for the incorporation of sub-micron size ceramic particles or whiskers. Another variant of stir casting process is compo-casting. Here, ceramic particles are incorporated into the alloy in the semi solid state.

Fig.2.4 Schematic representation of stir casting process

Major factors to be consider during stir casting 

Difficulty of achieving of uniform distribution of the reinforcement materials.



Wettability between the two main substances.



Porosity in cast metal matrix composites



Chemical reaction between the reinforcement material and matrix alloy.


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2. Infiltration Process

Liquid aluminium alloy is injected/infiltrated into the interstices of the porous preforms of continuous fibre/short fibre or whisker or particle to produce AMCs. Depending on the nature of reinforcement and its volume fraction preform can be infiltrated, with or without the application of pressure or vacuum. AMCs having reinforcement volume fraction ranging from 10 to 70% can be produced using a variety of infiltration techniques. In order for the preform to retain its integrity and shape, it is often necessary to use silica and alumina based mixtures as binder. Some level of porosity and local variations in the volume fractions of the reinforcement are often noticed in the AMCs processed by infiltration technique. The process is widely used to produce aluminium matrix composites having particle/whisker/short fibre/continuous fibre as reinforcement. 3. Spray Deposition

Spray deposition techniques fall into two distinct classes, depending whether the droplet stream is produced from a molten bath (Osprey process,) or by continuous feeding of cold metal into a zone of rapid heat injection (thermal spray process). The spray process has been extensively explored for the production of AMCs by injecting ceramic particle/whisker/short fibre into the spray. AMCs produced in this way often exhibit inhomogeneous distribution of ceramic particles. Porosity in the as sprayed state is typically about 5 – 10%. Depositions of this type are typically consolidated to full density by subsequent processing. Spray process also permit the production of continuous fibre reinforced aluminium matrix composites. For this, fibres are wrapped around a mandrel with controlled inter fibre spacing, and the matrix metal is sprayed onto the fibres. A composite monotype is thus formed; bulk composites are formed by hot pressing of composite monotypes. Fibre volume fraction and distribution is controlled by adjusting the fibre spacing and the number of fibre layers. AMCs processed by spray deposition technique are relatively inexpensive with cost that is usually intermediate between stir cast and PM processes. 4. In-situ Processing (Reactive Processing)

There are several different processes that would fall under this category including liquid-gas, liquid-solid, liquid-liquid and mixed salt reactions. In these processes refractory reinforcements are created in the aluminium alloy matrix. One of the examples is directional oxidation of aluminium also known as DIMOX process. In this process the M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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alloy of Al-Mg is placed on the top of ceramic preform in a crucible. The entire assembly is heated to a suitable temperature in the atmosphere of free flowing nitrogen bearing gas mixture. Al-Mg alloy soon after melting infiltrates into the preform and composite is formed.

2.6 FACTORS TO BE CONSIDER DURING STIR CASTING In order to achieve the optimum properties of the metal matrix composite, the distribution of the reinforcement material in the matrix alloy must be uniform, and the wettability or bonding between these substances should be optimised. The porosity levels need to be minimised, and chemical reactions between the reinforcement materials and the matrix alloy must be avoided. 2.6.1 Distribution of the reinforcement materials

One of the problems encountered in metal matrix composite processing is the settling of the reinforcement particles during melt holding or during casting. This arises as a result of density differences between the reinforcement particles and the matrix alloy melt. The reinforcement distribution is influenced during several stages including (a) distribution in the liquid as a result of mixing, (b) distribution in the liquid after mixing, but before solidification and (c) redistribution as a result of solidification. The mechanical stirrer used (usually during melt preparation or holding) during stirring, the melt temperature, and the type, amount and nature of the particles are some of the main factors to be considered when investigating these phenomena. Proper dispersion of the particles in a matrix is also affected by pouring rate, pouring temperature and gating systems. The method of the introduction of particles into the matrix melt is one of the most important aspects of the casting process. It helps in dispersing the reinforcement materials in the melt. There are a number of techniques for intr oducing and mixing the particles including 1. Injection of the particles entrained in an inert carrier gas into the melt with the help of an injection gun, wherein the particles are mixed into the melt as the bubbles rise through the melt; 2. Addition of particles into the molten stream as the mould is filled; 3. Pushing particles into the melt through the use of reciprocating rods;


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4. Spray casting of droplets of atomised molten metal along with particles onto a substrate; 5. Dispersion of fine particles in the melt by centrifugal action; 6. Pre-infiltrating a packed bed of particles to form pellets of a master alloy, and redispersing and diluting into a melt, followed by slow hand or mechanical stirring; 7. Injection of particles into the melt while the melt is irradiated continuously with high intensity ultrasound; 8. Zero gravity processing which involves utilising a s ynergism of ultra-high vacuum and elevated temperature for a prolonged period of time. The vortex method is one of the better known approaches used to create and maintain a good distribution of the reinforcement material in the matrix alloy. In this method, after the matrix material is melted, it is stirred vigorously to form a vortex at the surface of the melt, and the reinforcement material is then introduced at the side of the vortex. The stirring is continued for a few minutes before the slurry is cast. The different designs of mechanical stirrers are as shown in Fig.2.5. Among them, the turbine stirrer is quite popular. During stir casting for the synthesis of composites, stirring helps in two ways: (a) transferring particles into the liquid metal, and (b) maintaining the particles in a state of suspension.

Fig.2.5. Different types of stirrer used in stir casting M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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2.6.2 Wettability of reinforcement

Wettability is another significant problem when producing cast metal matrix composites. Wettability can be defined as the ability of a liquid to spread on a solid surface. It also describes the extent of intimate contact between a liquid and a solid. Successful incorporation of solid ceramic particles into casting requires that the melt should wet the solid ceramic phase. The problem of the wetting of the ceramic by molten metal is one of surface chemistry and surface tension. The chemistry of the particle surface, including any contamination, or oxidation, the melt surface and oxide layer must be considered. The basic means used to improve wetting are (a) increasing the surface energies of the solid, (b) decreasing the surface tension of the liquid matrix alloy, and (c) decreasing the solid-liquid interfacial energy at the particles-matrix interface. The magnitude of the contact angles ( ) in this equation is as shown in fig.2.6 describes the wettability, i.e. (a) - 0 o, perfect wettability, (b) -1800, no wetting and (c) 0 0 <  < 1800, partial wetting. Several approaches have been taken to promote the wetting of the reinforcement particles with a molten matrix alloy, including the coating of the particles, the addition of alloying elements to the molten matrix alloy, the treatment of the particles, and ultrasonic irradiation of the melt. In general, the surface of non-metallic particles is not wetted by the metallic metal, regardless of the cleaning techniques carried out. Wetting has been achieved by coating with a wettable metal. Metal coating on ceramic particles increases the overall surface energy of the solid, and improves wetting by enhancing the contacting interface to metal-metal instead of metal-ceramic. Nickel and copper are well wetted by many alloys, and have been used for a number of low melting alloys. In general, these coatings are applied for three purposes viz. to protect the reinforcement from damage in handling, to improve wetting, and to improve dispensability before addition to the matrix. The type of coating, in terms of wettability, can be divided into coating which reacts with the matrix, and coating which reacts with the oxide layer of the metal. The addition of certain alloying elements can modify the matrix metal alloy by producing a transient layer between the particles and the liquid matrix. This transient layer has a low wetting angle, decreases the surface tension of the liquid, and surrounds the particles with a structure that is similar to both the particle and the matrix alloy. The composites produced by liquid metallurgy techniques show excellent bonding between the ceramic and the metal when reactive elements, such as Mg, Ca, Ti, or Zr are added to M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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induce wettability . The addition of Mg to molten aluminium to promote the wetting of alumina is particularly successful and it has also been used widely as an addition agent to promote the wetting of different ceramic particles, such as silicon carbide and mica.

Fig.2.6 A sketch of three degrees of wetting and the corresponding contact angles 2.6.3 Porosity in cast metal matrix composites

The volume fraction of porosity, and its size and distribution in a cast metal matrix composite play an important role in controlling the material's mechanical properties. This kind of a composite defect can be detrimental also to the corrosion resistance of the casting. Porosity levels must therefore, be kept to a minimum. Porosity cannot be fully avoided during the casting process, but it can however, be controlled. In general, porosity arises from three causes: (a) Gas entrapment during mixing, (b) Hydrogen evolution, and (c) Shrinkage during solidification. According to Ghosh and Ray, the process parameters of holding times, stirring speed, and the size and position of the impeller will influence the development of porosity. Their experimental work showed that there is a decrease in the porosity level with an increase in the holding temperature. Structural defects such as porosity, particle cluster, oxide inclusions, and interfacial reaction are found to arise from unsatisfactory casting technology. It was observed that the amount of gas porosity in casting depends M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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more on the volume fraction of inclusions than on the amount of dissolved hydrogen. Composite casting will have a higher volume fraction of suspended non-metal solid than even the dirtiest conventional aluminium casting and hence the potential for the nucleation of gas bubbles is enormous. It has been observed that the porosity in cast composites increases almost linearly with particle content. The porosity of composite results primarily from air bubbles entering the slurry either independently or as an air envelope to the reinforcement particles. The air trapped in the cluster of particles also contributes to the porosity. Oxygen and hydrogen are both sources of difficulty in light alloy foundry. The affinity of aluminium for oxygen leads to a reduction of the surrounding water vapour and the formation of hydrogen, which is readily dissolved in liquid aluminium. There is a substantial drop in solubility as the metal solidifies, but because of a large energy barrier involved in the nucleation of bubbles, hydrogen usually stays in supersaturated solid solution after solidification.

2.7 MECHANICAL CHARACTERISTICS Mechanical properties of material like strength, hardness, elasticity are of vital importance in determining the type of fabrication and possible practical application. a) Strength:

The ability of a material to resist failure under the action of stresses caused by a load is known as its strength. The load to which a material is commonly subjected to are compression, tension, shear and bending. The corresponding strength is obtained by dividing the ultimate load with the cross-sectional area of the specimen. b) Hardness:

The ability of a material to resist penetration by a harder body is known as its hardness. It is a major factor in deciding the workability. The hardness bears a fairly constant relationship to the tensile strength of given material. c) Ductility:

It is the property of a material which permits a material to be drawn out longitudinally to a reduced section under the action of tensile force. A ductile material must be strong and plastic. The ductility is usually measured in terms of percentage of elongation or percentage of reduction in cross section area of the test specimen.


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d) Modulus of elasticity:

Hooke’s law states that when a material is loaded within elastic limit, the stress is directly proportional to the strain i.e. the ratio of stress to the strain is a constant with in elastic limit. This constant is known as Modulus of Elasticity or “Young’s Modulus”. Therefore, stress α strain i.e. i.e.

Stress Strain

ζ ε

= constant

=E

Where E = Young’s Modulus.

2.8 FATIGUE CHARACTERIZATION Fatigue is the condition where by a material fails due to the result of repeated loading (cyclic stresses) applied below the ultimate strength of the material. Fatigue failure is phenomenon in which a component fails due to repeated loading. Repeated loading condition in a compound arrives when the stresses in it due to the load applied vary or fluctuate between maximum and minimum values. In case of static loading conditions, the load is applied gradually, giving sufficient time for strain to develop. Whereas in case of repeated loading this does not hold good. Hence machine member subjected to repeated loading have them been found to fail at stresses which are very much below the ultimate strength and very often below the yield strength. Stress is defined as the intensity of distributed forces that tend to resist change in shape of a body. In most testing of those properties of materials that relate to the stressstrain diagram, the load is applied gradually to give sufficient time for the strain to fully develop. Furthermore, the specimen is tested to destruction and so the stresses are applied only once. Testing of this kind is applicable, then to what ar e known as “static conditions”. Such conditions only approximate the actual conditions to which many structural and machine members are subjected. Most failures in machinery are due to time varying loads rather than to static loads. These failures typically occur at stress levels significantly lower than the yield strengths of the materials. Thus using only the static failure theories can lead to unsafe designs when loads are dynamic. However, there are conditions wherein the stresses vary or fluctuate between levels. For example, surface on the rotating shaft subjected to the action of bending loads undergoes both tension and M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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compression for each revolution of the shaft. If, in addition, the shaft is also axially loaded, an axial component of the stresses is superposed upon the bending component. In this case, some stresses are always present, but the level of stress will be fluctuating. These and other kinds of loading occurring in machine members produce stresses which are called variable, repeated, alternating or fluctuating stresses. It has been found experimentally that when a material is subjected to repeated stresses, it fails at stresses below the yield point stresses, and such king of failure of a material is known as fatigue. Fatigue is the phenomenon of progressive, localized, permanent structural change occurring in a material subjected to conditions which produce fluctuating stresses and strains at some point or points and which may terminate in cracks or complete fracture after a sufficient number of fluctuation. Fatigue failure begins with a small crack. The initial crack is so minute that it cannot be detected by the naked eye and is even and is even quite difficult to locate in a magna flux of X-ray inspection. The crack will develop at a point of discontinuity in the material, such as change in cross sectional, a keyway or hole stamp marks, internal cracks or irregularities caused by machining. Once a crack is initiated, the stresses concentration effect becomes greater and the crack progresses more rapidly. As the stressed area decreases in size, the stress increases in magnitude until finally, the remaining area fails suddenly. A fatigue failure is characterized by two distinct regions. The first of these is due to the progressive development of the crack while the second is due to sudden fracture. The zone of sudden fracture is very similar in appearance to the fracture of a brittle material. When machine parts fail statically, they usually develop a very large deflection because the stress has exceeded the yield strength, and the part is replaced before fracture actually occurs. Thus many static failures give visible warning in advance. But a fatigue failure gives no warning. It is sudden and total and hence dangerous. Therefore the design of structural members is incomplete without fatigue considerations. Fatigue of materials is a well known situation whereby rupture can be caused by a large number of stress variations at a point even though the maximum stress is less than the proof or yield stress. The fracture is initiated by tensile stress at a macro or microscopic flaw. Once started the edge of the crack acts as a stress raiser and thus assists in propagation of the crack until the reduced section can no longer carry the imposed load. While it appears that fatigue failure may occur in all materials, there are marked M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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differences in the incidence of fatigue. For example, mild steel is known to have an endurance limit stress below which fatigue fracture does not occur, this is known as the fatigue limit. This does not occur with non-ferrous material, such as aluminum alloys, however, there is no such limit. To study and analyse the fatigue characteristic of different metals, rotating fatigue testing machine is used. Fatigue testing machines apply cyclic loads to test specimens. Fatigue testing is a dynamic testing mode and can be used to simulate how a component/material will behave/fail under real life loading/stress conditions. They can incorporate tensile, compressive, bending and/or torsion stresses and are often applied to springs, suspension components and biomedical implants. This machine is used to test the fatigue strength of materials and to draw S-N diagram by research institutes, laboratories, material manufacturers and various industries. This is a rotating beam type machine in which load is applied in reversed bending fashion. The standard 5 mm diameter specimen is held in special holders at its ends and loaded such that it experiences a uniform bending moment. Specimen acts as rotating beam subjected to bending moment. Therefore it is subjected to completely reversed stress cycle. Changing the bending moment by addition or removal of weights can vary the stress amplitude Basic fatigue testing involves the preparation of carefully polished test specimens (surface flaws are stress concentrators) which are cycled to failure at various values of constant amplitude alternating stress levels. The data are condensed into an alternating Stress (S) verses Number of cycles to failure (N), curve which is generally referred to as a material’s S-N curve. As one would expect, the curves clearly show that a low number of cycles are needed to cause fatigue failures at high stress levels while low stress levels can result in sudden, unexpected failures after a lar ge number of cycles. 2.8.1 MECHANISM OF FATIGUE FAILURE

Fatigue failure always begins with a crack. The crack may have been present in the material since its manufacture, or it may have developed with time due to cyclic straining around a stress concentration. Virtually all structural members contain discontinuities

ranging

from

microscopic

(<0.00254

mm),

introduced

in

the

manufacturing process. Fatigue cracks generally start at a notch or other stress concentration. The general term critical represents any geometric contour that increases M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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“FATIGUE CHARACTERIZATION OF A356-SiCp A356-SiCp BASED METAL MATRIX COMPOSITES”

local stress. Thus, it is critical that dynamically loaded parts be designed to minimize stress concentrations. There are three stages of fatigue failure; crack initiation, crack propagation and sudden fracture due to unstable crack growth. The first stage can be of short duration, the second stage involves most of life of the part, and the third stage is instantaneous. The figure (2.7-2.8) show typical stress life relationship. The ordinate of S-N diagram is called the fatigue strength and is always accompanied by a statement of the number of cycles N to which it corresponds. Endurance limit represents the largest value of fluctuating stress that will not cause failure for essentially an infinite number of cycles. In case of steels, a knee occurs in the graph and beyond this knee, failure will not occurs for any number of cycles. The endurance limit for steel is about 10 6 cycles. Most non ferrous alloys do not show knee and have no sharply defined endurance limit. Hence, limit of 108 cycles is taken to be the endurance limit. The body of knowledge available on fatigue failure from N=1 to N=1000 cycles is known as low cycles fatigue. High cycle fatigue is concerned with stress c ycles above 103 cycles.

Fig. 2.7 - S-N relationship for ferrous and non-ferrous non-ferrous alloys


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Fig. 2.8 – 2.8 – Typical Typical S-N relationship 2.8.2 THE STRESS LIFE APPROACH AND THE STRAIN LIFE APPROACH TO DETERMINE THE FATIGUE LIFE 1. The Stress Life Approach

This is oldest of the three models and is mostly used for high cycle fatigue (HCF) application where the assembly is expected to last for more than about 10 3 cycles of stress. It works best when the load amplitudes are predictable and consistent over the life of the part. It is the stress based model, which seeks to determine the fatigue strength and or endurance limit for the material so that the cyclic stress can be kept below that level and failure avoided for the required number of cycles. The part is then designed based on the materials fatigue strength (or endurance limit) and a safety factor. In the effect, this approach attempts to keep local stress in notches so low that the crack initiation stage never begins. The assumption (and design) is that stress and strains everywhere remains in the elastic region and local yielding occurs to initiate a crack. This approach is fairly easy to implement, and large amounts of relevant strength data are available due to its long-time use. However, it is the most empirical and least accurate of the three models in terms of defining the true local stress/strain states in the part, especially for low cycle (LCF) finite life situation where the total number cycle is expected to be less than about 10 3 and the stresses will be high enough to cause local yielding. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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2. The Strain Life Approach

The initiation of crack involves yielding; a stress based approach cannot adequately model this stage of the process. A strain based model gives a reasonable accurate picture of the crack initiation stage. It can be also account for cumulative damage due to variation in the cyclic load over the life of the part, such as overloads that may introduce favourable or unfavourable residual stresses to the failure zone. Combinations of fatigue loading and high temperature are better handled by this method, because the creep effect can be included. i ncluded. This method is most often applied to LCF, finite life problems where the cyclic stresses are high enough to cause local yielding. It is the most complicated of the three models to use and requires a computer solution. Test data are still being developed on the cyclic strain behaviour of various engineering materials. 2.8.3 FACTORS AFFECTING FATIGUE BEHAVIOUR

Variables affecting fatigue behaviour are conveniently classified as variations in, 1. Variation in the specimen 2. Surface defects 3. Design factors 4. Surface treatments 5. Size effect 6. Operating temperature 7. Corrosion 8. Stress concentration 9. Overload / Under load 10. Residual stress 1. Variation in the Specimen: The history and geometry of a specimen of a given

material will affect its fatigue behaviour, as will the different processing method (resulting in variation in grain size, residual stress and surface finish). In general treatments, which raise the yield strength or tensile strength of a composition, also raise the fatigue resistance. Thus fine-grained structures have higher fatigue resistance than corresponding coarse-grained structure.


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2. Surface Defects: Since maximum stress within a component or a structure occurs at its

surface. Consequently most cracks leading to fatigue failures originate at surface positions, specifically at stress amplification sites. Therefore it has been observed that fatigue life is especially sensitive to condition and configuration of component surface. 3. Design Factors: Design of component can have significant influence on its fatigue

characteristics. Any notch or geometric discontinuity can acts as a stress raiser and fatigue crack initiation site. These design factors include grooves, holes, keyways, threads etc. The sharper the discontinuities the more severe are the stress concentration. The probabilities of fatigue failure may be reduced by avoiding (whenever possible) these structural irregularities. 4. Surface Factors: During machining operations small scratches and grooves are

invariably introduced into the work surface by cutting tool acti on. These surfaces marking can limit the fatigue life. An important method of increasing fatigue performance is by imposing residual compressive stress within a thin outer surface. 5. Size Effect: Larger specimens and machine parts are observed to exhibit poor fatigue

strength then smaller specimens or machine parts, especially when subjected to cyclic bending stress. This may be due to the fact that larger specimens have greater volume and surface area which in turn will have more number of defects when compared to smaller specimens. 6. Operating Temperature: The temperature of operation has a significant influence on

the fatigue strength. The fatigue strength is enhanced at temperature below room temperature and diminished at temperature above room temperatures. 7. Corrosion: A corrosive environment tends to lower the fatigue strength of the

engineering material, often by large amount. The use of certain solvents or the presence of distilled water results in lowering the fatigue strength, especially when the specimens are operated at elevated temperature. 8. Stress Concentration: The existence of irregularities or discontinuities, such as holes,

grooves, or notches, in a part increase the magnitude of stresses significantly in the immediate vicinity of the discontinuity due to higher stress concentration. Fatigue failure mostly originates from such places. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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9. Overload/ Underload: The fatigue crack growth life decrease with increasing

overload stress and the fatigue life decreases with compressive underload stress. 10. Residual Stress: The fatigue crack growth behaviour of various types of alloy is

significantly affected by the presence of residual stress induced by manufacturing and post-manufacturing processes. Residual stress is often a cause of premature failure of critical components. 2.8.4 ESTABLISHING S-N CURVE

To determine the strength of materials under the action of fatigue loads, specimens are subjected to repeated or varying forces of specified magnitudes while cycle of stress reversals are counted to destruction. The most widely used fatigue testing device is the R.R.Moore high speed rotating beam machine. This machine subjects the specimens to pure bending by means of weights. The specimens are very carefully machined and polished, with a final polishing in an axial direction to avoid circumferential scratches, other fatigue machine are available for applying fluctuating or reversed stresses, torsional stresses, or combined stresses to the test specimens. To establish the fatigue strength of a material, quite a number of tests are necessary because of the statistical nature of fatigue. For rotating beam test, a constant bending load is applied, and the number of revolution (stress reversals) of the beam required for failure is recorded. The first test is made at a stress which is somewhat under the ultimate strength of the material. The second test is made at a stress which is less than that used for first. The processed is continued, and the results are plotted on the S-N diagram. In case of the ferrous metal and alloys, the graph become horizontal after the material has been stressed for a certain number of cycles. Plotting on log paper emphasizes the bend in the curve, which might not be apparent if the results were plotted by using Cartesian coordinates. The ordinate of the S-N diagram is called the fatigue strength S f , a statement of this strength must always be accomplished by a statement of number of cycles N to which it correspond. The abscissa of the S-N diagram is life i.e. the number of cycles of stress reversals required to cause the fatigue of the specimen.


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2.9 LITERATURE REVIEW Yunhui et.al [1] studied the tilt-blade mechanical stirring of A356-2.5vol%SiCp liquid which was conducted in a cylindrical crucible to solve the problem of nonhomogeneous radial distribution of SiC particles in conventional straight-blade mechanical stirring. In this paper, a specially-designed mechanical stirrer with the tilt blade was used to stir A356-2.5vol% SiCp liquid. In straight-blade mechanical stirring of A356-SiCp liquid, SiC particles can move from the centre to the periphery of the crucible under the action of centrifugal force and thus resulting in a non-homogeneous distribution of SiC particles in A356 liquid along the radial of crucible. In this experimental equipment, a tilt-blade stirrer which can generate an inward movement of SiC particles is used. The radial distribution of SiC particles in A356 liquid was studied under the conditions of 25 ° for horizontal tilt angle α of the blade, 200 RPM for rotating speed of stirrer and 10 mm/s for speed of moving up and down of stirrer. The results show that the non-homogeneous radial distribution of SiC particles in conventional straight-blade mechanical stirring can be eliminated in tilt-blade mechanical stirring of A356-SiCp liquid by adjusting the circumferential tilt angle β of tilt-blade. The reasonable tilt-blade mechanical stirring parameters of A356-2.5vol%SiCp liquid are 26 ° for circumferential tilt angle β of blade, 25 ° for horizontal tilt angle α of blade, 200 RPM for rotating speed of stirrer and 10 mm/s for speed of moving up and down of stirrer. Sakthivel et.al [2] studied 2618 aluminium alloy metal matrix composites(MMCs) reinforced with two different sizes and weight fractions of SiCp particles up to 10% weight were fabricated by stir cast method and subsequent forging operation. The effects of SiCp particle content and size of the particles on the mechanical properties of the composites such as hardness, tensile strength, hot tensile strength (at 1200C),and impact strength were investigated. The density measurements showed that the samples contained little porosity with increasing weight fraction. Optical microscopic observations of the microstructures revealed uniform distribution of particles and at some locations agglomeration of particles and porosity. The results shows that hardness and tensile strength of the composites increased with decreasing size and increasing weight fraction of the particles. The hardness and tensile strength of the forged composites were higher than those of the cast samples.


30


Abdel Jaber [3] et.al, in this study has aimed to investigate solidification and mechanical behaviour of Al- Si alloy against both the molding conditions and silicon content (3%- 15% Si). The pure aluminium matrix and pure silicon with a purity of 99.793% have supplied by the aluminium company of Egypt. The alloy were prepared by melting the pure aluminium in an oil fired crucible furnace and the required amount of silicon was added to the molten aluminium in powder form with a particle size about 300µm to 500µm. Five sets of the casting alloys were prepared with different silicon content, (3%, 6%, 8%, 12%, and 15%Si). From the results author concluded that with the increase in silicon content the cooling rate decreased and also a decrease of the liquidus temperature was observed up to 12% and then increased with increasing Si%. But with the increase of silicon content the ultimate tensile strength and hardiness increased, and high coefficient of friction and high wear resistance was produced. The change of mold thickness affected on the cooling rate of aluminium-silicon casting alloys so on the microstructure. A pronounced change in the mechanical and tribological properties by the change of mold thickness was obtained. Neelima Devi [4] et.al, have studied the mechanical characteriz ation of aluminium silicon carbide composite. In this paper tensile strength experiments have been conducted by varying mass fraction of SiC (5%, 10%, 15%, and 20%) with Aluminium. The tension test is conducted on a universal testing machine model TUE600(C) at room temperature. Aluminium silicon carbide alloy composite material is two times less in weight than the aluminium of the same dimensions. From the results obtained it is found that the maximum tensile strength has been obtained at 15% SiC ratio. This indicates that the Aluminium silicon carbide composite material is having less weight and more strength. The corrosion behaviour of 6061 Al alloy-SiC composites (in as cast and extruded form) have also been studied in sea water and acid media. The effects of temperature of both the media and concentration of the acid medium were also investigated. The corrosion behaviour was evaluated using electrochemical technique. The studies revealed that corrosion damage of composites exposed to sea water medium was mainly localized in contrast to uniform corrosion observed for base alloy. Further, composites were found to corrode faster than the base alloy even though the attack was mainly confined to the interface, resulting in crevices or pits.


31


Bahul Saroya [5] et.al, studied about the aluminium (Al-6063)/SiC Silicon carbide reinforced particles metal matrix composites which are fabricated by melt-stirring technique. The MMCs bars and circular plates are prepared with varying the reinforced particles by weight fraction ranging from 5%, 10%, 15% and 20%. The average reinforced particles size of SiC are 220 mesh, 300 mesh, 400 mesh respectively. The stirring process was carried out at 200 rev/min rotating speed by graphite impeller for 15 min. The microstructure and mechanical properties like Tensile strength upper yield point (MPa), Tensile strength lower yield point (MPa), Ultimate tensile strength (MPa), Breaking strength(MPa), % Elongation, % Reduction in area, Hardness (HRB) Density (gm/cc), Impact Strength (Nm) are investigated on prepared specimens of MMCs. From the result it is observed that the hardness of the composite is increased with increasing of reinforced particle weight fraction and Tensile strength upper yield point (MPa), Tensile strength lower yield point (MPa), Ultimate tensile strength (MPa) and Breaking strength (MPa) increases with the increase in reinforced particulate size (220 mesh, 300 mesh, 400 mesh) and weight fraction (5%, 10%, 15%, 20%) of SiC particles. % Elongation and % Reduction in area decreases with the increase in reinforced particulate size (220 mesh, 300 mesh, and 400 mesh) and weight fraction (5%, 10%, 15%, and 20%) of SiC particles. Manjunath [6] et.al, in the present study have studied silicon carbide particulate reinforced LM6 alloy matrix composites which were produced by gravity die casting process by varying the percentage of the reinforcement added (5% and 15%). The reinforcement was silicon carbide particles in powder form of size 150 microns was used as a dispersoid. Mechanical properties such as tensile, impact and wear test studies were conducted to determine the tensile strength, ductility, and toughness and wear characteristics of cast MMC’s. The results of the study suggest that with the increase in weight percentage of SiC, an increase in tensile strength has observed. Ductility of the prepared MMC decreased with increase in weight percentage of SiC in base alloy. Impact strength of the base alloy LM6 is high. When the reinforcing material SiC (with 5% & 15%) is added in LM6 alloy, the impact strength is reduced notably . The abrasive wear resistance of MMC has increased with increase in SiC content. But wear has increased with increase in sliding velocity and normal load.


32


Mahendra Boopathi [7] et.al, in this study focused on the formation of Al-Sic-fly ash hybrid metal matrix composites. The present study was aimed at evaluating the physical properties of Aluminium 2024 in the presence of silicon carbide, fly ash and its combinations. The compositions were added up to the ultimate level and stir casting method was used for the fabrication of aluminium metal matrix composites, Al-SiC, Alfly ash, Al-SiC-fly ash composites with various concentrations like aluminium 2024 with 5%SiC, 10%SiC, 5%fly ash, 10%fly ash, 5%SiC+5%fly ash, 5%SiC+10%fly ash, 10%SiC+5%fly ash, and 10%SiC+10%fly ash were prepared. The mechanical behaviours of composites like density, tensile strength, yield strength, elongation and hardness tests were conducted. Based on the experimental observations the following conclusions have been drawn: Density of the composites decreased by increasing the content of the reinforcement. Hence, it was found that, instead of Al-SiC and Al-fly ash composites, AlSiC-fly ash composites show better performance. Increase in area fraction of reinforcement in matrix result in improved tensile strength, yield strength and hardness. With the addition of SiC and fly ash with higher percentage the rate of elongation of the hybrid MMCs is decreased significantly. From the above results they conclude that instead of Al-SiC or Al-fly ash composites, the Al-SiC-fly ash composites could be considered as an exceptional material in sectors where lightweight and enhanced mechanical properties are essential. Ajay Singh [8] et.al, in this present work focused on the study of behaviour of Aluminium Cast Alloy (6063) with alumina (Al 2O3) composite produced by the stir casting technique. Different percentage of alumina powder is used as reinforcement phase in this AMMC. Various mechanical tests like tensile test, Hardness Test, Impact test are performed on the samples of AMMC to evaluate the mechanical properties of this aluminium based metal matrix composite. These tests are done on various mechanical testing machines like universal testing machine, Vickers hardness testing machine, impact testing machine. The results confirmed that Al alloy 6063 with Al 2O3 reinforced composites is clearly superior to base Al alloy 6063 in the comparison of tensile strength, Impact strength as well as Hardness. Dispersion of Al2O3 particles in aluminium matrix improves the hardness of the matrix material. It is found that elongation tends to decrease with increasing particles weight percentage, which confirms that alumina addition increases brittleness. It appears from this study that UTS and Yield strength trend starts increases with increase in weight percentage of Al2O3 in the matrix. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

33


Basavaraju [9] et.al, Studied on Mechanical properties and Tribological characteristics of LM25 Graphite-Silicon Carbide and LM25-Flyash-Silicon Carbide Hybrid MMC’s. The studies were done using graphite and fly ash by varying the percentage of Silicon Carbide and aluminium LM25 as base metal, composite were produced by the stir casting technique. Aluminium LM25 ingot was weighed and melted in electric furnace up to 800°C, and the 2% Graphite (2% of aluminium weight) was mixed using a mechanical stirrer for uniform dispersion. The Silicon carbide was added for 2, 4, 6, and 8 % weight of aluminium. The same method was prepared for 2% Fly ash and Silicon carbide in varying proportions as 2, 4, 6, and 8 %. The cast was poured into the mould and the specimens were machined according to ASTME standards required for testing. Tensile, compression and hardness tests were conducted and the results were derived. From the results it is observed that Graphite and Fly ash mixed with SiC makes the material harder up to a certain limit. Prepared MMC’s provide excellent wear characteristics up to a limit load. The tensile strength improves for 2% addition of SiC and 4% of SiC in Al+Graphite. The hardness of the material increases with the combination of 2% addition of SiC and Graphite. The compressive strength is ideal at 2% and 4% addition of SiC graphite and Fly ash. Harun [10] et.al, studied the behaviour of Aluminium alloy metal matrix with reinforcement of 10 % & 15% Fly ash and Sic with weight Percentage of 10% and they concluded that increase in the fly ash content increased the porosity in the composites, with matrix alloy was found with 15% weight % of Fly ash particulate having the highest porosity and lower hardness. Hardness of the Aluminium alloy increased with the addition of Sic particulate in composites. Lokesh [11] et.al, studied the mechanical properties of Aluminium metal matrix with 5 wt% of Cu, Fly ash and Silicon carbide. Author fabricated specimens with Al-4.5 Wt. % Cu and reinforcement Fly ash 49-60 μm particle size and Sic with 65 μm particle size fly ash weight percentage was 4% and Sic with 6 %. Author concluded that with the addition of 4 % Fly ash and 6% SiC hardness was improved and similarly tensile, wear resistance enhanced.


34


Uvaraja [12] et.al compared Al6061 and Al7075 with Sic, B4C reinforcements (5 %, 10 % and 15 % weight ration) respectively for both alloys. The hybrid composite specimen specimens were fabricated and author experimentally concluded that hardness for both the composition was found to be maximum at 15% weight ratio, wear resistance was also found to be improved. Author also concluded that Al7075 with Sic and B4C was performing better that Al6061. Sujan [13] et.al, in this paper presented a study on the performance of stir cast Al2O3 SiC reinforced metal matrix composite materials. The composites used in the experiments are produced by the stir casting method. For Al-Al 2O3 composite material, Al356 alloy powders are mixed with Aluminium oxide (Al 2O3) particles of uniform size (400 μm) in the weight fraction of 5%, 10%, and 15%. For Al -SiC composite material, Al356 alloy powders are mixed with SiC in the weight fraction of 5%, 10%, and 15%. From the results obtained it is observed that aluminium metal matrix composites with aluminium oxide (Al2O3) as particle reinforcements have higher density values compared to Aluminium metal matrix composites with Silicon Carbide (SiC) as particle reinforcements. The composite materials achieve significant improvement in hardness and tensile strength compared to Al 356 alloy. For instance, the tensile strength of Al with 15% SiC is 23.68% more than that of pure Al. The composite materials show significantly higher strength to weight ratios compared to pure Al. For instance, Al with 15% SiC exhibits strength to weight ratio of 1.74. The corresponding values of strength to weight ratios for pure Al and cast iron are 1.54 and 0.765 respectively. It is found experimentally that the wear rate decreases significantly with the addition of reinforcement particles. Al-SiC composites exhibit lower wear rate compared to Al-Al 2O3 composites. Viswanatha [14] et.al, in the present investigation studies on microstructure and mechanical properties of Aluminium Matrix Composites (AMCs) reinforced with silicon carbide (SiCp) and graphite (Gr) particles. A356 alloy is used as the matrix material with varying the reinforcement of SiCp from 0 to 9 wt% in steps of 3 wt% and fixed quantity of 3 wt% of graphite. The composites were fabricated by liquid metallurgy stir casting method which is considered to be the most potential method for engineering applications in terms of production capacity and cost efficiency. The prepared composites were examined for microstructure to know the particle distribution in the matrix material. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

35


Hardness and tensile properties were studied and compared with the alloy. The hardness tests were conducted using Vickers macro hardness testing system as per ASTM E-92 standard. Tensile tests were carried out using computerized universal tensile testing machine. The tensile specimens were machined according to the ASTM E8 standard. From the experimental investigation the following conclusions were drawn on the mechanical properties SiCp and graphite particles reinforced A356 aluminium alloy composites. The hardness of composites increased significantly with addition of SiCp, while maximum hardness was obtained for 9% of SiCp. The addition of low weight percentage of SiCp to A356 leads to increase in tensile strength and decrease in percentage elongation with increase in SiCp particles. The tensile strength of the composite material improved by 5%, with an addition of 3 wt% of SiC and graphite particles. Arivukkarasan [15] et.al, have studied the fatigue behaviour of Aluminium alloy (LM4)-Alumina silicate (Al2O3SiO2) particulate composite in comparison with unreinforced LM4 aluminium alloy in this work. Four different volume fractions (0.05, 0.15 and 0.20) of Alumina silicate particulates of size 10 µm are introduced into the melt. The fabrication of specimen is carried out by stirring followed by squeeze casting. After the production of composite billets size of 160×100×30mm, specimens are cut and machined from these billets. The test specimens (ASTM B 557M) were cut from the billet. The fatigue strength tests are conducted on these specimens with a stress ratio (R) of 0.1, fatigue tests are conducted at three different stress levels: 100, 75 and 50Mpa. The results found that the composite specimens have longer fatigue lives than matrix alloy in lower stress state and exhibited a reduced fatigue lives at elevated stress state irrespective of their reinforcement volume fraction. Ana Garcia Romero [16] et.al, have studied the fatigue behaviour of an aluminium alloy matrix composite locally reinforced with 15% short δ-alumina fibres has been analysed. Prototype pistons were industrially produced by squeeze casting infiltration of a preform of fibres with the molten Al-12%Si-1%Mg-1%Cu-1%Ni alloy into a cylinder shaped mould. Samples were in the T5 condition that is just aged for 12 hours at 160º C. Cylindrical 6.4 mm diameter and 25.4 mm gauge length smooth specimens were machined from the composite zone of the pistons. Two of these specimens were used to perform tensile tests at room temperature according to ASTM D3552 standard. The M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

36


remaining ones were used for conducting uniaxial constant stress amplitude fatigue tests according to ASTM E466 standard. These fatigue tests were carried out also at room temperature with a stress ratio R=0.1 and a frequency of 10 Hz. The stress-life (S-N) diagram was obtained by plotting the maximum stress of the cycle versus the fatigue life. The results showed a large scatter in the fatigue lives, Fractographic examination indicates that the presence of a cluster of fibres at periphery of the component could significantly decrease its fatigue performances. Analysis of the data revealed that crack closure played a significant role in the fatigue crack growth in this condition. Gonzalo M. Dominguez Almaraz [17] et.al, conducted Rotating bending fatigue tests on the aluminium alloy 6063-T5 for pre-corroded and non corroded specimens. Special attention was devoted to fatigue endurance reduction induced by controlled surface corrosion on testing specimens. Corrosion attack was obtained by immersion of specimens in an acid solution for: two, four and six minutes in order to induce three degrees of surface corrosion. The corrosion agent was a solution of hydrochloric acid with a pH close to 0.8 and solution concentration of 38%. Rotating bending fatigue tests at room temperature and without environmental humidity control were carried out on the pre-corroded and non corroded specimens in order to investigate the corrosion effect on the fatigue endurance. Additionally, numerical crack propagation investigation was carried out using a 2D model for testing specimen and the Displacements Correlation Method (DCM).The results showed Rotating bending fatigue endurance decreases with time of corrosion attack in pre-corroded specimens. The compared fatigue life reduction between pre-corroded specimens increases when the loading rate decreases (for 60% and 70% of yield stress of this material).

Corrosion pits size and depth increase with

corrosion time; corrosion pits density on specimen surface increases with corrosion time. Analysis on fracture surfaces shows that corrosion pitting holes were the sites for crack initiation and propagation Eleichea [18] et.al, have performed Low-cycle fatigue tests of rotating cantilevered notched beams under deflection controlled conditions on four grades of high strength medium alloy steel of hardness 280, 310, 350 and 380 HB. The cyclic stress – strain response and strain-life fatigue curves are determined. The effects of hardness, prestraining and machine allowance after heat treatment, on the fatigue life have also been studied. In the present work the notched specimens have been investigated in the M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

37


low-cycle fatigue regime under conditions of rotating bending. The results showed in notched fatigue tests under rotating bending, the actual measured lives in notched specimens for the four investigated materials were five times greater on the average, than the predicted lives based on crack initiation model. This indicates that the notched specimens have extremely longer proportion of lives during the crack propagation phase than those in the initiation phase. The notch fatigue resistance for the investigated materials were found to increase with the increase in hardness. This may indicate that a significant fraction of the total life is involved in crack propagation region, where the applied nominal stresses are taken into consideration. In this region, the crack is no longer affected by the notch. Abolishing the machine allowance after heat treatment for the notched specimens was found to lower fatigue life under rotating bending. Yuki Nakamura [19] et.al,has studied the effect of alumite surface treatments on long life fatigue behaviour of cast aluminium in rotating bending. Alumite treatment is one of surface treatments for aluminium alloy and it provides an oxide layer (Al 2O3) on the surface by anodizing. This method has been widely used for aluminium alloy because the alumite possesses characteristics such as high hardness, wear resistance, and electrical resistance. In order to examine the effect of alumite layer on the fatigue behaviour, fatigue tests were carried out on aluminium alloy specimens with two different alumite treatments by means of a dual-spindle rotating bending fatigue testing machine. Tests were also conducted on untreated specimens for comparison. Fracture surfaces of all the failed specimens were examined in a scanning electronic microscope (SEM). As results of fatigue tests, significant deteriorations of the fatigue strength for alumite-treated specimens having each thickness of the alumite layer were observed in comparison with the results for untreated specimen in the short life regime. However, in long-life regime of >107, the fatigue strength of alumite-treated specimen having 3 µm alumite layer becomes a little higher than the results for untreated specimen. Shin and Chen [20] in their work have evaluated the fatigue crack propagation behaviour using surface crack growth in a rotating bending rod. Nine different rod geometries have been tested. Cylindrical rod specimens with lengths 43mm and diameters

φ

L

= 86, 56 and

= 12, 8 and 6 mm, giving a total of nine different size

combinations, were machined from 12.5mm diameter SK4 carbon steel rod stock. Cyclic testing was carried out on a rotating bending machine. Suitable loading was applied M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

38


through the use of dead weights in the form of four-point bending. Consequently, a speed of 7000 rpm, which can be comfortably achieved in all cases, was chosen for the tests. From the results it is observed that Crack growth behaviour in the rod specimens under rotating bending is not sensitive to rod length except for the shortest (43mm long) specimens with the smallest diameter (6 mm), which exhibited a slower rate than the longer rods with the same diameter. Arun [21] et.al, have studied the dynamic behaviour of hybrid aluminium6061 metal matrix reinforced with sic and fly ash particulates. The Aluminium alloy composite are made by mixing particulate in a molten alloy. The present work is concentrated in a view to produce such type of enhanced composites. The cheaper method of fabricating Aluminium metal matrix composite is by stir casting method and by proper reinforcement selection. Silicon Carbide (SiC) is ceramic material which is hard and can enhance aluminium strength, fly ash is a power plant debris and easily available. The composition is selected on weight percentage basis SiC with 6 % and 9% and fly ash with constant 15 % and AL6061 T6 alloy as matrix material. Mechanical characteristics like tensile test and fatigue test were conducted; specimens were prepared according to ASTM standards. Tensile test is conducted with Universal testing machine and fatigue test is carried out on rotating bending machine with predetermined value of loads by considering 0.5UTS,0.7UTS and 0.9UTS and for which required stress level and cycles up to failure were documented which is used in plotting S-N curve. From results it is observed that Tensile strength of composite has enhanced and at 15% fly ash it is higher when compared with Al6061 T6 alloy, fatigue strength of the composite with 6% SiC and 15% fly ash reinforcement is having good fatigue performance compared to the monolithic AL6061 T6 Alloy. Achutha [22] et.al, has investigated the fatigue and mechanical properties of composites of A6061 (LM 25) aluminium alloy reinforced with silicon carbide and graphite particles. Author fabricated the composites by gravity die casting technique in which the reinforcement particles were dispersed in the vortex created in the molten matrix alloy. Initially he evaluated the fatigue properties of the composites through experimental setup and he found that the data are scattered and to plot S-N curve it requires more number of test specimens, hence he used Monte Carlo simulation technique to plot the S-N curve the composites. Initially he validated the software results with M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

39


experimental results and then he plotted for composites. Author concluded that the presence of the reinforcements increases the fatigue strength of the composite materials. Chitoshi Masuda [23] et.al, has studied the fatigue properties and fatigue fracture behaviour of A357 alloy reinforced with Sic whiskers and Sic particles by using rotating bending fatigue test method. Author added reinforcement up to volume fraction of 20%. Author stated that the fatigue strength of the Sic whiskers composites are 60% higher than the unreinforced alloy and also the fatigue strength of Sic particulates are superior to the base alloy materials. Author also observed that the fatigue crack propagation is different for both whiskers and particulate composites and both composites would have very significant effect on the fatigue crack initiation and crack propagation near the fatigue limit. Victor H. Mercado Lemus [24] et.al, have worked with rotating bending fatigue tests on aluminium alloy 6061-T6, under loading condition close to elastic limit of material. One or two artificial pitting holes are machined at the narrow section of the hourglass shape specimen. Special attention was focused on the stress concentration factors caused by the artificial pitting holes and the relationship to experimental fatigue endurance. Results have been obtained for three types of specimens: without artificial pitting, specimens with one artificial pitting hole and specimens with two close artificial pitting holes. Results show that fatigue endurance under rotating bending fatigue tests of aluminium alloy 6061-T6 decreases with the presence of one artificial pitting hole and dramatically with two close artificial pitting holes. In order to explain this behaviour, numerical analysis by FE were carried out to determine the stress concentrations for the three types of specimens. It is found that the stress concentration for two close pitting holes is an exponential function of the separation between the two holes, under uniaxial loading. Raja Thimmarayan and Thanigaiyarasu [25] have studied the effect of particle size, forging and ageing on the mechanical and fatigue properties of the cast, forged and age-hardened aluminium 6082 (AI6082) reinforced with SiC p. Al6082 reinforced with three different particle sizes of SiC p (average particles size of 22, 12 and 3 µm) in the forged and ageing conditions were studied. The samples were characterised by optical microscopy, hardness, tensile and fatigue tests. The forged microstructure shows a more uniform distribution of SiC p in the aluminium matrix. The addition of SiC p results in M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

40


improved tensile strength, yield strength and elastic constants of the composites with reduction in ductility. It also increases the fatigue strength of the composites by increasing the number of cycles required for fatigue failure of the composites for the given value of stress. The results also show considerable improvements in mechanical fatigue properties due to forging and ageing heat treatment of the metal matrix composites.


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CHAPTER 3 OBJECTIVE AND METHODOLOGY 3.1 OBJECTIVE In the view of the large scope available for investigation, the present work is taken up to study the influence of SiC reinforcement particulates in A356 matrix alloy, on the mechanical and fatigue characteristics of the composites. The following are the main objectives of the present work. 

Fabrication of Al-SiC p Metal Matrix Composite using stir casting method with 0% 5% and 10% SiC particulates.



Micro structural analysis of the cast composite.



Mechanical characterization of cast Al- SiCP Metal Matrix Composite.



Fatigue characterization of cast Al-SiC p Metal Matrix Composites.


42


3.2 WORK PLAN The schematic diagram of work plan for the present studies is as shown in Fig.3.1

Matrix Material

Reinforcement Material

A356

SiC (23µm)

Electric Resistance Furnace Casting by Stir Casting

Composites Prepared with varying percentage of reinforcement content A356-SiCp (0%, 5%, 10%)

Fatigue Characterization

•

•

Mechanical properties

Fatigue life evaluation of the composite casted (0%, 5%, 10% SiC p) with 5mm neck dia. Fatigue life evaluation for composite with 5% SiC p by varying the neck dia (4mm, 5mm, 6mm, and 7mm).

• •

Hardness Tensile strength

Micro structural study of the composite casted

Fig.3.1 Schematic diagram of work plan


43


3.3 METHODOLOGY In order to fulfill the objectives selected for the present work, the following methodology has been adopted. a) Casting

In the present work Al-SiC p composites specimens have been fabricated by stir casting method, with composition of 0%, 5% and 10% SiC particulates. Specimens of two different diameters 30mm and 10mm are cast to facilitate fabrication of specimens for mechanical and Fatigue characterization. The details of fabrication are discussed in the next section. b) Micro Structural Analysis

The cast specimens are subjected to micro structural analysis using scanning electron microscope to study the structural features like grain structure, porosity, casting defects, secondary phases etc., c) Mechanical Characterization

The cast specimens are subjected to mechanical characterization tests as per ASTM standards in order to evaluate the hardness, tensile strength, ductility and percentage of deformation of the material. d) Fatigue Characterization

The cast specimens are also subjected to fatigue characterization tests as per ASTM standards in order to evaluate the fatigue life of the cast composites with 0%, 5%, 10% SiC p as reinforcement with same neck diameter and to evaluate the fatigue life for composites with 5% SiC p by varying the neck diameter.


44


CHAPTER 4 EXPERIMENTAL DETAILS The details of experimental investigation planned, to fulfill the objectives set for the present work are discussed in the following section.

4.1 WORK MATERIAL DETAILS The details of the material selected for present investigation are as discussed below. Aluminium (A356) based metal matrix composite with varying volume fraction (0%, 5% and10%) of reinforcement of particulate silicon carbide particles of 23µm size has been selected for the present investigation. a) Aluminium A356

Among several series of aluminium alloys, A356 is one of the most extensively used alloys for its excellent properties. Basically A356 is an alloy of Aluminium, Magnesium and Silicon, which is highly resistant to corrosion, has excellent extrudability and exhibit moderate strength. A356 alloy is of much use in the fields of construction, automotive and marine applications. The MMCs consisting of A356 matrix alloy reinforced with SiC p have found extensive industrial applications. The A356 ceramic particulate reinforced composites exhibit improved mechanical characteristics and they find applications as cylinder blocks, pistons, piston insert rings, brake disks, calipers, connecting rod, microwave filters, vibrator component, contactors, impellers and space structures. Some of the properties of A356 are represented in Table.4.1. Table 4.1 Mechanical properties of A356 Properties

Values

Elastic Modulus (Gpa)

70-80

Density (g/cc)

2.7

Poisson’s Ratio

0.33

Brinell Hardness (HB500)

75

Tensile Strength in Mpa

220

Melting Temperature in C

660


45


Chemical composition of matrix alloy selected

The chemical composition of the A356 aluminium alloy is mentioned in Table.4.2. Table 4.2 Chemical composition of A356 Elements

Percentage of Elements

Al

91.1-93.3

Cu

<=0.2

Iron

<=0.2

Mg

0.25-0.45

Mn

<=0.1

Other each

<0.05

Silicon

6.5-7.5

Titanium

<=0.2

Zinc

<=0.1

b) Silicon Carbide (SiC)

Silicon Carbide is the only chemical compound of Carbon and Silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon. SiC is an excellent abrasive which is used in grinding wheels and other abrasive products. Some of the properties of SiC are represented in Table.4.3. Table 4.3 Mechanical properties of SiC

Properties

Values

Elastic Modulus (Gpa)

410

Density (g/cc)

3.1

Poisson’s Ratio

0.14

Hardness (HB500)

2800

Compressive Strength (Mpa)

3900

Melting Temperature in C

3100


46


4.2 PROCESSING DETAILS 4.2.1 Fabrication of Al-SiCp Metal Matrix Composites

The casting unit consists of a graphite crucible of about 5kg capacity, which is heated by electrical resistance type heating coils. The temperature level of the heating unit is controlled by thermocouple activated controlling unit. Duration of heating is determined based on the quantity of material to be melted. The furnace used in the present work is of bottom pouring type, which is regulated using a valve operated from the bottom. A motor operated stirrer is provided at the top, for mixing the particulate reinforcement with the molten metal. Arrangement is made at the bottom of the crucible for exact positioning of the mould below the valve as shown in Fig 4.1.

Electric motor

Stirrer

Control panel Heating unit

Permanent mould with Band heater

Fig 4.1: Electrical heating furnace


47


a) Permanent mould box details:

Permanent split type of mould made of Mild Steel is used for casting the composites in the present study as shown in Fig 4.2.

Split mould

mould assembly Fig 4.2: Permanent spilt mould

b) Mechanical stirrer:

The mechanical stirrer used for stirring the molten alloy during fabrication of composites is made of steel blades coated with Alumina powder and sodium silicate mixture to withstand high temperature and to avoid iron pickup by the melt. Ceramiccoated impeller can be immersed up to ¾ of molten metal from top and rotated at a speed of about 800 rpm to create the vortex. Fig 4.3 shows the Al 2O3 coated stirrer.

Fig 4.3: Alumina- sodium silicate powder coated stirrer M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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4.2.2 Procedure to Fabricate Composites: 

Cleaned A356 ingot (Fig 4.4) of required quantity is to be placed in the melting crucible. The furnace top is to be closed by refractory material and heater is to be switched on and set to the required temperature (900 0C). Heating is to be continued for about 2 hrs and stabilize it for 20 minutes af ter reaching 900 0C.



The SiC reinforcement particulates of about 23 microns size are to be heated to 4500C for about 1 hr in another closed furnace.

 

Add the Slag remover (fig4.5) to the molten metal to remove the sla g. Chlorine based solid degassing tablet hexachloroethane – C 2Cl6 Tablet (fig4.6) is to be added to remove gasses entrapped during melting.



Magnesium of about 0.5% is to be added to the melt to improve the wettability.



Stirrer is to be immersed up to ¾ of the molten metal and stirring action to be carried for about 2 minutes while heated SiC material is to be added slowly.



Heat the mixture for 15 min after stirring.



After stirring the molten composite metal is poured into pre heated mould by opening the bottom valve of the furnace.



After allowing the mould to cool at room temperature, the cast material (fig4.7) is taken out, by opening the mould halves.



Aluminium alloy specimens (0% Sic) as well as Aluminium metal matrix composites with SiC reinforcement of 5% and 10% by weight were cast, by the above mentioned procedure.

Fig 4.4: Al Raw ingot material

Fig 4.5: Slag Remover


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Fig 4.6: Degasser hexachloroethane

Fig 4.7: Cast Aluminium composites

C2Cl6 tablet

4.3 MATERIAL CHARACTERIZATION 4.3.1 MICROSCOPY

Microstructure analyses of the specimens is carried out to study the grain structure, secondary phases, voids, surface defects, cracks and the types of deformations that take place during strength and tribological tests. Specimens for micro structural analysis are prepared as discussed below. 1) Cut the cast specimen in the shape of cube with dimensions 15×15 mm approximately. Also all the surfaces must be flat and parallel to the opposite surface and must be perpendicular to the adjacent surfaces. 2) Polish the specimen’s surface using rough emery paper and wash the specimen. 3) Then the specimen are polished using fine emery paper of grade 0,1,2,3. 4) After polishing with emery paper, wash the specimen with water. Polishing is done again using a polishing machine with different grades of alumina paste like alumina-grade1, alumina- grade 2 and alumina- grade 3 one after the other. 5) After polishing in polishing machine, wash the specimen with water and etching has to be done by adding few drops of Hydro fluoric acid in to water; approximately 3 drops of acid into 9 drops of water (i.e. 1:3 ratio). 6) After etching, the specimen is observed under Scanning Electron Microscope with different magnification.


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Scanning Electron Microscope (SEM)

The Scanning Electron Microscope (SEM) uses electron beam rather than light beam for analysis. The SEM produces images of high resolution. The Microstructure images for polished faces of casted specimen and fractured surfaces of the fatigue specimen were examined using a Scanning Electron Microscope. The scanning electron microscope is as shown in the fig 4.8.

Fig 4.8: Scanning Electron Microscope

4.4. MECHANICAL CHARACTERISATION 4.4.1 ROCKWELL HARDNESS NUMBER (RHN)

The Rockwell Hardness number is obtained by applying load of 100 Kg for 3060sec with ball indenter of 1.58mm diameter on flat composite specimens. Procedure to find RHN of composite specimens 1) Flatten two faces of composite specimens whose hardness is to be found. 2) The flat surface should not contain any scale, rust, oil.


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3) The load is applied on the composite and RHN values are noted obtained from the display unit. In display unit C-scale is to be referred for RHN value (C-scale is used for non-ferrous materials and B-scale is used in ferrous materials). 4.4.2. MEASUREMENT OF TENSILE STRENGTH

The tensile tests were conducted using standard computerized Universal Testing Machine. The machine is of 20 KN capacities with a loading rate of 0.02 mm/sec. The equipment is best suited for tensile, compression, and Shear, Flexural properties of different materials. The tensile test was performed in accordance with ASTM – B557 for standard Aluminium alloy. From the tensile strength tests, the effect of reinforcement on the tensile strength and ductility of composite materials can be studied. Fig4.9 shows the tensile testing machine and fig.4.10 shows the specimen geometry as per ASTM B557 Standard.

Fig.4.9 Tensile testing machine


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Fig4.10. Tensile specimen ASTM B557 standard, all dimensions are in mm

Fig 4.11: Tensile test specimen 4.4.3 FATIGUE CHARACTERIZATION

The experiments were conducted on the rotating beam fatigue testing machine (shown in fig 4.12). Technical Specifications of Rotating Bending Fatigue Tester is shown in table 4.4. The specimen used for the fatigue testing is prepared as per ASTM E446 and is as shown in the fig 4.13. The specimen is loaded as a simply supported beam and the loading diagram is as shown in the fig 4.14. The Rotary Bending Machine have been operated at a constant speed of 1750 rpm, then loads are added on and the specimens were tested till failure. For these loads the stress is calculated, the number of cycles required for failure is noted down and S-N curve is plotted.


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Procedure for Fatigue Test •

Measure the dimensions of the specimen

•

Fix the specimen in its position.

•

Apply the loads in the pan attached to the spring.

•

Note the initial reading on the counters and switch on the motor.

•

Note down the number of revolution to break the specimen.

•

Repeat the test on number of specimens for plotting S-N curve.

Fig 4.12: Rotary Bending machine

Fig 4.13: Fatigue test Specimen according to ASTM E446


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Fig 4.14: Fatigue testing machine and loading diagram

Table 4.4 Technical Specifications of Rotating Bending Fatigue Tester

Maximum bending moment

Up to 200 kg-cm

Speed

Min 1000rpm Max 3000rpm

Preset counter with speed

6 digits Indicator

Maximum load applied

Up to 20kg

Specimen dimensions

Length 70mm-90mm with neck Diameter 4mm-8mm

Distance between load bearing point to

100mm

hinge bearing point


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CHAPTER 5 RESULTS AND DISCUSSION Al-SiC p composite of different composition were prepared by stir casting process. The cast specimens were subjected to mechanical characterization and fatigue characterization. The result of the characterization tests are discussed in the following section.

5.1. MICROSTRUCTURAL STUDY 5.1.1. SCANNING ELECTRON MICROSCOPY

Fig 5.1(a): 0%SiC cast-1000X

Fig 5.1(b): 5%SiC with 23μm cast-1000X

Fig 5.1(c): 10%SiC with 23μm cast-1000X M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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Fig5.1 (a), (b) & (c) show the SEM micrographs of the cross section of cast specimens with 0%, 5% &10% SiC particulates respectively. Shrinkage porosity can be observed in fig5.1 (a) as well as some oxide particles of aluminium, which might have formed during the process of casting. Distribution of secondary silicon phases can be observed in fig5.1 (b) &(c). Secondary silicon phases appear largely at grain boundaries in fig5.1 (b), whereas they are most uniformly distributed in fig5.1 (c).

5.2MECHANICAL CHARATERIZATION 5.2.1 HARDNESS Rockwell Hardness Number

It can be observed from the results that there is an increasing trend in the hardness values of the composites. The increase in hardness can be attributed to the uniform distribution of SiC particulate reinforcement in A356 matrix, forming strong interfacial bond between the matrix and the reinforcement. Table.5.1 RHN of as cast and extruded composites Specimen

RHN as cast composite

A356+ 0%SiC p

70

A356+ 5%SiC p

73

A356+10%SiC p

78

Fig 5.2: RHN of Base alloy and Composites M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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5.2.2

TENSILE STRENGTH

Fig 5.3 (a), (b) & (c) show the load-displacement curves and fig 5.4 (a), (b) & (c) shows the stress-strain diagram of as cast specimens with 0%, 5% & 10% SiC p as reinforcement respectively. Form the load-displacement curves and stress-strain diagram Proof stress and Tensile strength and strain to failure of the cast composites have been calculated and are shown in Fig5.5, 5.6 & 5.7 respectively.

Fig 5.3(a): Load v/s Displacement (elongation) of 0% SiC p

Fig 5.3(b): Load v/s Displacement (elongation) of 5% SiC p M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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Fig 5.3(c): Load v/s Displacement (elongation) of 10% SiC p

Fig 5.4(a): Stress-Strain diagram of 0% SiC p


59


Fig 5.4(b): Stress-Strain diagram of 5% SiC p

Fig 5.4(c): Stress-Strain diagram of 10% SiC p


60


Fig 5.5: Proof Stress of base alloy and composites

Fig 5.6: Tensile strength of base alloy and composites

Fig 5.7: Strain to failure of base alloy and composites

From the test results, it can be observed that there is an increase in the proof stress values (fig 5.5) of the composite material with increase in the percentage composition of SiC particles. This indicates that by increasing the percentage composition of SiC particle in the aluminium matrix increases the strength of the cast composites. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

61


Increase in proof stress is a clear indication that the tensile strength values (fig 5.7) of the cast composites also increases by increasing percentage composition of SiC p. This increased strength can be attributed to the uniform distribution of the SiC particulate reinforcement in the aluminium matrix alloy and better interfacial bonding between the matrix and the reinforcement phases. SiC reinforcement with its higher tensile strength imparts better mechanical properties to composite structure, when good interfacial bond is formed between the matrix and reinforcement phases.

5.3 FATIGUE CHARACTERIZATION 5.3.1 Stress Calculations

  ,  =  ×  N-mm Where, F= Load applied in N L= Distance between load bearing point to hinge bearing point= 100mm

,  =

   



Where, M= Bending moment in N-mm d= Neck diameter in mm


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5.3.2 Fatigue life of the Composites with varying the percentage of the reinforcement 250

200

a150 P M s s e r t100 S

50

0 0

50000

100000

150000

No. of Cycles

200000

250000

Fig 5.8: Fatigue life of Base Alloy(0% SiC p)

250

200

a150 P M s s e r t S100

50

0 0

50000

100000

150000

200000

250000

No. of Cycles

Fig 5.9: Fatigue life of Composite with 5% SiC p M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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250

200

a150 P M s s e r t S100

50

0 0

20000

40000

60000

80000

100000

120000

140000

160000

180000

No. of Cycles

Fig 5.10: Fatigue life of Composite with 10% SiC p 250

0% SiC As Cast 5% SiC As Cast

200

a150 P M s s e r t S100

50

0 0

50000

100000

150000

No. of Cycles

200000

250000

Fig 5.11: Comparision of the Fatigue life of Composite with 0%SiCp, 5%SiCp & 10% SiCp M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

64


Fatigue test is done to evaluate the fatigue life of the cast composites and the fatigue test is performed on the Rotating Bending fatigue testing machine. Fatigue test is performed on the cast composites with 0% SiC p, 5% SiC p and 10% SiC p with neck diameter 5mm and at identical stress condition. Also at each loading point 3 specimens were tested and the average value is taken to plot the S-N curve. From the results (fig 5.8fig 5.11) it can be seen that fatigue life of the composite with 5% SiC p is more at lower stress levels and at higher stress levels when compared with composites with 0%SiC p and 10%SiC p. 5.3.3 Fatigue life of the Composites with 5%SiC p with varying the Neck diameter 450 400 350 300

a P 250 M s s e r t200 S 150 100 50 0 0

50000

100000

150000

200000

250000

300000

350000

400000

No. of Cycles Fig 5.12: Fatigue life of Composite with 5% SiCp having Neck dia 4mm


65


250

200

a 150 P M s s e r t 100 S

50

0 0

50000

100000

150000

200000

250000

300000

350000

400000

No. of Cycles

Fig 5.13: Fatigue life of Composite with 5% SiCp having Neck dia 5mm

250

200

a150 P M s s e r t S100

50

0 0

50000

100000

150000

200000

250000

300000

350000

No. of Cycles

Fig 5.14: Fatigue life of Composite with 5% SiCp having Neck dia 6mm


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250

200

150 a P M s s e r t 100 S

50

0 0

50000

100000 150000 200000 250000 300000 350000 400000 450000 500000

No. of Cycles

Fig 5.15: Fatigue life of Composite with 5% SiCp having Neck dia 7mm 450 4mm neck dia 5mm neck dia

400

6mm neck dia 350

7mm neck dia

300

a P 250 M s s e r t200 S 150 100 50 0 0

50000

100000 150000 200000 250000 300000 350000 400000 450000 500000

No. of Cycles

Fig 5.16: Comparision of the Fatigue life of Composite with 5%SiC p having varying neck diameter


67


Fatigue test were also performed on the composite with 5%SiC p reinforcement added to the base alloy A356. Here in the composites, test were performed by varying the neck diameter (4mm, 5mm, 6mm, 7mm). From the graphs (fig 5.12-fig 5.16) it is clearly shown that the fatigue life of the cast composite with 5% SiC p has increased by reducing the neck diameter of the specimen i.e, cast composites with 4mm neck diameter is having higher fatigue life compared with composites with 5mm, 6mm and 7mm neck diameter at identical stress conditions. 5.3.4 Fatigue fractured surface SEM analysis

Fig 5.17: Fatigue Fractured Surface of 0% SiC reinforced in A356 Matrix (1000X)



After the fracture of the specimen, the fractured surface of the specimen is analysed under SEM for its microstructure study. The SEM analysis shows that there are sharp edges at the crack surface which can be concluded that fatigue fracture is a brittle fracture. M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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CHAPTER 6 CONCLUSIONS Experimental investigations conducted in the present work to study the influence of percentage composition of SiC particulates in A356 Aluminium matrix alloy on the microstructural, mechanical and fatigue characteristics have provided the following conclusions. 

SEM analysis shows that the SiC particles are evenly distributed.



An improvement of about 5-12% in hardness of composite specimen is observed as compared to matrix alloy specimens.



An improvement of about 12-37% in tensile strength of composite specimen is observed as compared to matrix alloy specimens.



The fatigue strength of the composites with neck diameter of 5mm was found to increase with increase in reinforcement content. And it was found that maximum life was observed for composites with 5%SiC p as reinforcement at identical stress condition.



Also fatigue life of the cast composite with 5% SiC p has increased by reducing the neck diameter of the specimen at identical stress condition.


69


SCOPE FOR FUTURE WORK Experience from the present study has shown avenues for future investigation some of the possible studies are: 

To investigate the Fatigue life by extruding the composites cas ted.



To investigate the Fatigue life by increasing the reinforcement content.



To investigate the Fatigue life at various surface conditions


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9. Basavaraju.S, Arasukumar.K,

Dr.Chandrashekhar Bendigeri,

Dr.C.K.Umesh

“Studies on Mechanical Properties and Tribological Characteristics of LM25 Graphite- Silicon Carbide and LM25-Flyash- Silicon Carbide - Hybrid MMC’s” International Journal of Innovative Research in Science, Engineering and Technology Vol. 1, Issue 1, November 2012 10. M.B.Harun, “ Effect Of Fly ash Particulate Reinforced On Microstructure, Porosity and Hardness In AL-(Si-Mg)”, AJSTD Vol. 23 Issues 1&2 pp. 113 -122 (2006) 11. G.N.Lokesh, “ Effect of Hardness, Tensile and Wear Behaviour of Al -4.5wt.% Cu Alloy/Fly ash/ SiC metal matrix composite”. International Journal of Modern Engineering Research (IJMER) Vol.3.Issue.1 Jan-Feb.2013 pp-381-385 ISSN: 2249-6645. 12. V.C.Uvaraja, “Comparision on Al6061 and Al7075 Alloy with Sic and B4C Reinforcement hybrid metal matrix composites”. International Journal of Advance Research In Technology (IJART), Vol.2 Issue 2,2012,1-12 ISSN NO:66023127 13. D. Sujan, Z. Oo, M. E. Rahman, M. A. Maleque, C. K. Tan “Physio -mechanical Properties of Aluminium Metal Matrix Composites Reinforced with Al2O3 and SiC” World Academy of Science, Engineering and Technology Vol:6 2012-08-25 14. B. M. Viswanatha, M. Prasanna Kumar, S. Basavarajappa, T. S. Kiran “Mechanical Property Evaluation Of A356/Sicp/Gr Metal Matrix Composites” Journal of Engineering Science and Technology Vol. 8, No. 6 (2013) 754 – 763 © School of Engineering, Taylor’s University 15. S. Arivukkarasan, V. Dhanalakshmi, A. Suresh babu and M. Aruna “ Performance Study on Fatigue Behaviour in Aluminium Alloy and Alumina Silicate Particulate Composites” Journal of Applied Science and Engineering, Vol. 16, No. 2, pp. 127134 (2013) DOI: 10.6180/jase.2013.16.2.03 16. Ana garcia romero, M. Anglada and A. M. Irisarri “Fatigue behaviour of an aluminium alloy matrix composite” F. INASMET, Mikeletegi p.2 20009 San Sebastian (Spain). 17. Gonzalo M. Dominguez Almaraz , Jorge L. Avila Ambriz, Erasmo Cadenas Calderon “Fatigue endurance and crack propagation under rotating bending fatigue tests on aluminum alloy AISI 6063-T5

with controlled corrosion attack”

Engineering Fracture Mechanics 93 (2012) 119 –131 M.Tech, Thesis 2013-2014 Dept. of Mechanical Engg, P.E.S.C.E, Mandya

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269584792-Project-Report.pdf

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