“experimental determination and analysis of fracture toughness of mmc”

VISVESVARAYA TECHNOLOGICAL UNIVERSITY

Jnana Sangama, Belgaum-590018

A PROJECT REPORT ON

EXPERIMENTAL DETERMINATION AND ANALYSIS OF

FRACTURE TOUGHNESS OF MMC

Submitted in partial fulfillment of the requirement for the award of degree of

MASTER OF TECHNOLOGY

IN

DESIGN ENGINEERING

By:

SHIVARAJA.H.B

USN: 1DB12MDE12

Under the guidance of

Mr. B.S. PRAVEEN KUMAR

Associate professor Department of Mechanical Engineering Don Bosco Institute of Technology

Department of Mechanical Engineering

DON BOSCO INSTITUTE OF TECHNOLOGY

Kumbalagodu, Mysore Road, Bangalore - 560074

ACKNOWLEDGEMENT

I thank to Mr. B S PRAVEEN KUMAR, Associate Professor, Department of Mechanical Engineering, Don Bosco Institute of Technology, Bangalore without whom a project of this magnitude would not have been completed up to this extent.

I express my deep gratitude to my institute, DBIT, Bangalore which provided an opportunity and platform for fulfilling my dreams, and desire to reach my goal. I sincerely thank my respected Principal Dr. T SREENIVASAN who is the constant source of inspiration, throughout the academics.

I express my sincere gratitude Dr. V S RAMAMURTHY, Professor and Head of Department of Mechanical Engineering, Don Bosco Institute of Technology, Bangalore for extending all the support from the department for granting me the permission to carry out my project and his immense guidance, valuable inspiration, words of advice and unstinted support throughout the project.

I express my profound thanks and gratitude to all the faculty members and nonteaching staff, Department of Mechanical Engineering, Don Bosco Institute of Technology, Bangalore, for their guidance and need full help in carrying out the project.

ABSTRACT

Aluminium and its alloys have continued to maintain their mark as the matrix material most in demand for the development of Metal Matrix Composites (MMCs). This is primarily due to the broad spectrum of unique properties it offers at relatively low processing cost. Some of the attractive property combinations of Al based matrix composites are: high specific stiffness and strength, better high temperature properties (in comparison with its monolithic alloy), thermal conductivity, and low thermal expansion. The multifunctional nature of Al matrix composites has seen its application in aerospace technology, electronic heat sinks, solar panel substrates and antenna reflectors, automotive drive shaft fins, and explosion engine components.

Aluminium matrix composites with multiple reinforcements (hybrid MMCs) are finding increased applications because of improved mechanical and tribological properties and hence are better substitutes for single reinforced composites. Aluminum Silicon alloys are widely used in automotive applications. The present study focuses on the influence of addition of zirconium silicate (ZrSio4) particulates as a second reinforcement and study the influence on the mechanical properties of aluminium matrix composites reinforced with silicon carbide (SiC) particulates. The method employed for the development of castings was Stir casting. Experiments have been conducted under laboratory condition to assess the mechanical properties such as Fracture Toughness, Tensile, Compression and Hardness of the aluminium, zirconium silicate and silicon carbide composite. The Single Edge Notch Bend specimen is used in the present study to determine the Fracture Toughness of the fabricated composite. The composites were tested for the presence of reinforcement particles using metallurgical microscope.

TABLE OF CONTENTS

CHAPTER 1

INTRODUTIONPage No

1.1Preamble1

1.2Statement of the Problem1

1.3Aim and Objective of the Project2

1.4 Methodology3

CHAPTER 2

LITERATURE REVIEW4-10

CHAPTER 3

INTRODUCTION TO COMPOSITE

3.1Background11

3.2Composites12

3.2.1 Composite Definition13

3.2.2 Need for Developing Composite materials14

3.2.3 Characteristics of composites14

3.3Classification of Composite16

3.3.1 The Matrix Material16

3.3.2 The Reinforcing Material16

3.3.3 Classification (According to the Type of Reinforcement)17

3.3.3.1 Particulate Composites18

A. Non-Metallic in Metallic Composites18

B. Metallic in Non-Metallic Composites18

C. Non-Metallic in Metallic Composites19

3.3.3.2 Fibrous Composites19

3.3.3.3Laminated Composites19

3.4Applications of Composites19

3.5Fabrication techniques of MMCs20

3.5.1 Solid Phase Fabrication Method20

3.5.2 Diffusion Bonding21

3.5.3 Powder Metallurgy Technique21

3.5.4 Liquid Phase Fabrication Techniques21

3.5.5 Liquid Metal Infiltration22

3.5.6 Squeeze Casting22

3.5.7 Spray Co-deposition Method23

3.5.8 Stir Casting23

3.5.9 Compo Casting23

3.6Metal matrix composites24

3.7Examples27

3.7.1 Advantages and Disadvantages of MMCs27

3.8Scope of Present Investigation28

CHAPTER 4

SELECTION OF MATERIALS

4.1Matrix Material: Al 35629

4.1.1 Chemical Composition and Mechanical Properties

of Matrix Material Al35629

4.2Reinforcement Material (silicon carbide)30

4.2.1 Physical Properties of SiC31

4.2.2 Applications of SiC32

4.3Reinforcement Material (Zirconium Silicate)32

4.3.1 Properties of Zirconium silicate33

4.3.2 Applications of ZrSiO433

CHAPTER 5

FABRICATION OF COMPOSITES

5.1Stir Casting35

5.2Steps Involved in Stir Casting Method37

5.3 Composition of matrix and reinforcement39

CHAPTER 6

EXPERIMENTAL DETAILS

6.1Fracture Toughness40

6.2Specimen dimensions as per ASTM standards41

6.3Test for Fracture toughness41

6.4Tensile test43

6.5Hardness test46

6.6Compression test48

6.7 Microstructure48

6.8Etching49

6.9Optical metallurgical microscope50

6.10 Finite element analysis51

CHAPTER 7

RESULTS AND DISCUSSIONS

7.1Fracture toughness results53

7.2Comparison of the experimental and FEA results54

7.3Tensile test results55

7.4Hardness test results57

7.5Compression test results59

7.6 Microstructure60

CHAPTER 8

CONCLUSION63

CHAPTER 9

SCOPE FOR FUTURE WORK

REFERENCES

64

65-67

LIST OF TABLES

Table NoDescriptionPage No

4.1Chemical composition of Al35630

4.2Mechanical properties of matrix material Al35630

4.3Properties of Zircon sand33

5.1Different wt% ratios of matrix metal and reinforcement39

6.1ASTM codes for mechanical test and sample dimensions41

7.1Variation of Fracture toughness with different wt% reinforcements53

7.2Comparison of the experimental and FEA results54

7.3Tensile properties of the MMC55

7.4Variation of hardness with different wt% reinforcement57

7.5Compression strength for different wt% reinforcement59

LIST OF FIGURES

Fig NoDescriptionPage No

1.1Project Methodology3

3.1Classification of composites (based upon the matrix materials)16

3.2Classification of composites (based upon the reinforcing materials)16

3.3 Schematic Presentation of Three Shapes of Metal Matrix Composite

Materials18

4.1Ingot Structure of Al 35629

4.2Reinforcement Material (SiC)31

4.3Reinforcement material (ZrSiO4)32

5.1Flow chart of fabrication of Composite34

5.2Split type mould box36

5.3Pre heating the mould box36

5.4Electric furnace37

5.5Molten Metal in Furnace38

5.6Formation of Vortex38

5.7Pre heating of reinforcement38

5.8Poured molten metal in mould box38

5.9Cast Aluminium Composites38

6.1SENB specimen41

6.2Fracture toughness specimens42

6.3Dimension of Tensile Specimen43

6.4Specimens for Tensile test45

6.5Universal testing machine45

6.6Hardness test specimens47

6.7Brinell hardness testing machine47

6.8Compression test specimens48

6.9Polishing machine49

6.10Optical Metallurgical microscope50

6.11SENB specimen model51

6.12FE mesh model51

6.13Stress distribution from finite element simulations52

7.1Variation of Fracture toughness with different wt% reinforcement54

7.2 Variation of tensile strength and yield strength with different wt%

Reinforcement56

7.3Hardness value for different wt% reinforcement58

7.4Compression strength for different wt% reinforcement59

7.5Microstructure of Al356+0%SiC+8%ZrSiO460





Experimental Determination and Analysis of Fracture Toughness of MMC

CHAPTER 1

INTRODUTION

1.1 Preamble

New and high performance particle reinforced metal matrix composites (PRMMC) are expected to satisfy many requirements for a wide range of performance-driven, and price sensitive, applications in aerospace, automobiles, bicycles, golf clubs, and in other structural applications. In general, these materials exhibit higher strength and stiffness, in addition to isotropic behavior at a lower density, when compared to the un-reinforced matrix material. PRMMC benefits from the ceramics ability to withstand high velocity impacts, and the high toughness of the metal matrix, which helps in preventing total shattering. This contribution leads to an excellent balance between cost and mechanical properties, which are appealing for many applications.

The recognition of the potential weight savings that can be achieved by using the advanced composites, which in turn means reduced cost and greater efficiency, was responsible for this growth in the technology of reinforcements, matrices and fabrication of composites. If the first two decades saw the improvements in the fabrication method, systematic study of properties and fracture mechanics was at the focal point in the 60s. Since then there has been an ever-increasing demand for new, strong, stiff and yet light-weight materials in fields such as aerospace, transportation, automobile and construction sectors. These materials have low specific gravity that makes their properties particularly superior in strength and modulus to many traditional engineering materials such as metals. As a result of intensive studies into the fundamental nature of materials and better understanding of their structure property relationship, it has become possible to develop new composite materials with improved physical and mechanical properties.

1.2 Statement of the Problem

Aluminium and its alloys have continued to maintain their mark as the matrix material most in demand for the development of Metal Matrix Composites (MMCs). This is primarily due to the broad spectrum of unique properties it offers at relatively low processing cost. Some of the attractive property combinations of Al based matrix composites are: high specific stiffness and strength, better high temperature properties (in comparison with its monolithic alloy), thermal conductivity, and low thermal expansion.

M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 1


The project is associated with the study of Fracture Toughness and mechanical properties of Aluminium, Zirconium Silicate and Silicon Carbide Metal Matrix Composite (MMC). Here we have used the Aluminium alloy of grade 356 with addition of varying weight percentage composition of Zirconium Silicate and Silicon Carbide particles by stir casting technique.

Finite element (FE) simulations for the proposed SENB geometry was carried out using ANSYS software package (v12) to investigate stress distribution around the notch and to validate the experimental results.

The mechanical properties were tested under laboratory conditions. The change in physical and mechanical properties was taken in to consideration. For the achievement of the above, an experimental set up was prepared to facilitate the preparation of the required specimen. The experiments were carried out to study the effect of variation of the percentage composition to predict the mechanical properties as well as to measure the micro hardness.

1.3 Aim and Objective of the Project

The aim of the project is to synthesize and characterize hybrid metal matrix composite by stir casting technique and to experimentally evaluate the fracture toughness and mechanical properties of the composite. Then finite element analysis is carried out to validate the obtained results. The objectives of the project are listed below.

1. Preparation of composite casting by liquid metallurgy route.

2. Preparation of specimen to required dimensions for the various tests.

3. The micro structural observations to evaluate the quality of the castings i.e., base alloy with Silicon Carbide and Zirconium Silicate (Al356+Sic+ZrSio4).

4. Tests are conducted to evaluate the Fracture toughness and mechanical properties such as tensile, hardness and compression.

5. Finite element (FE) simulation to validate the results.



1.4 Methodology

The methodology of the project in presented in figure 1.1

Literature review

Identification of the

problem

Development of MetalCasting and curingmatrix composites

Tensile

Fracture ToughnessTestingCompression

Hardness

Microstructure

FE analysis

Results and discussions

Conclusion

Fig 1.1 Project Methodology



CHAPTER 2

LITERATURE REVIEW

J.Jenix Rino, Dr.D.Sivalingappa, Halesh Koti, V.Daniel Jebin[1] The present study deals with the investigation of the mechanical behaviour of Aluminium6063 alloy composites reinforced by Zircon sand(ZrSiO4) and Alumina(Al2O3) particles were taken in to account for investigating the properties such as density tensile strength and hardness of the composites synthesized by Stir casting technique. The mechanical properties evaluation reveals variations in hardness and the tensile strength values with the composite combinations. From the experimental studies, the optimum volume fraction of hybrid reinforcement in Al 6063 alloy on the basis of microstructure and mechanical properties it is found that the (4+4) wt% combination.

The unique tailor ability of the composite materials for the specific requirements makes these materials more popular in a variety of applications such as aerospace, automotive (pistons, cylinder liners, bearings), and structural components, resulting in savings of material and energy. Discontinuous reinforced aluminum metal matrix composites (DRAMMCs) are a class of composite materials having desirable properties like low density, high specific stiffness, high specific strength, controlled co-efficient of thermal expansion, increased fatigue resistance and superior dimensional stability at elevated temperatures etc. The properties and behavior of various Al alloys and their composites are much explored in terms of microstructure, mechanical properties, loading conditions and applications.

K.K. Alaneme, A.O. Aluko [2] The tensile and fracture behavior of as-cast and age-hardened aluminium (6063), silicon carbide particulate composites produced, using borax additive and a two step stir casting method, was investigated. Al (6063), SiCp composites having 3, 6, 9, and 12 volume percent of SiC were produced, and sample representatives of each composition were subjected to age-hardening treatment at 1800 C for 3 hours. Tensile and Circumferential Notched Tensile (CNT) specimens were utilized for tension testing to evaluate, respectively, the tensile properties and fracture toughness of the composites. Experimental results show that the ageing treatment resulted in little improvement in the tensile strength of the composites. The tensile strength and yield strength increased to almost the same magnitude with an increase in SiC volume percent



for both as-cast and age-hardened conditions. The increase was, however, more significant for the 9 and 12 volume percent SiC reinforcement. The strain to fracture was less sensitive to volume percent SiC reinforcement and ageing treatment, with values less than 12% strain to fracture observed in all cases.

Aluminium and its alloys have continued to maintain their mark as the matrix material most in demand for the development of Metal Matrix Composites (MMCs). This is primarily due to the broad spectrum of unique properties it offers at relatively low processing cost. Some of the attractive property combinations of Al based matrix composites are: high specific stiffness and strength, better high temperature properties (in comparison with its monolithic alloy), thermal conductivity, and low thermal expansion. The multifunctional nature of Al matrix composites has seen its application in aerospace technology, electronic heat sinks, solar panelsubstrates and antenna reflectors, automotive drive shaft fins, and explosion engine components, among others.

Mohan Vanarotti, SA Kori, BR Sridhar, Shrishail B.Padasalgi [3] Aluminum alloy and silicon carbide metal matrix composites are finding applications in aerospace, automobile and general engineering industries owing to their favourable microstructure and improved mechanical behavior. Aluminium alloy A356 and silicon carbide composites were obtained by stir casting technique. Silicon carbide content in the alloy was fixed at 5 Weight % and 10 weight % during the casting. Microstructure revealed a uniform distribution of the silicon carbide throughout the matrix. Hardness and tensile properties of the composite showed an improvement as compared to the alloy without silicon carbide additions.The present paper highlights the salient features of casting technique and characterization of aluminum alloy A356 and silicon carbide metal matrix composite.

J.E. Perez Ipina, A.A. Yawny, R. Stukeb, C. Gonzalez Oliver[4] Metal matrix composites (MMC) are materials made from the dispersion of a ceramic phase, typically SiC or Al203 fibers or particles, in order to improve the mechanical and physical properties of the matrix. In the particular situation of Aluminum MMCs, both pure Al and alloys are employed. Continuous fibers (Continuous metal matrix composites CMMC) as well as short fibers and particles (Discontinuous Aluminum reinforced DAR) are employed. The production and use of composite materials is under intensive development because of the interesting physical and mechanical properties that these materials present and also due to the possibility to manipulate them by means of the variation of the type and proportion of



the reinforcement employed as well as the type of the metallic matrix. Materials with designed mechanical (yield stress, elastic modulus, etc) and physical (thermal expansion coefficient, resistivity, thermal conductivity, etc) properties can be produced in this way.

E.G. Okafor , V.S.Aigbodion [5] The as-cast microstructure and properties of Al-4.5Cu/ZrSiO4 particulate composite synthesized via squeezed casting route was studied, varying the percentage ZrSiO4 in the range of 5-25wt%. The result obtained revealed that addition of ZrSiO4 reinforcements, increased the hardness value and apparent porosity by 107.65 and 34.23% respectively and decrease impact energy by 43.16 %. As the weight percent of ZrSiO4 increases in the matrix alloy, the yield and ultimate tensile strength increased by 156.52 and 155.81% up to a maximum of 15% ZrSiO4 addition respectively. The distribution of the brittle ZrSiO4 phase in the ductile matrix alloy led to increase strength and hardness values. These results had shown that, additions of ZrSiO4 particles to Al-4.5Cu matrix alloy improved properties. From the result of the investigation in this research work it could be concluded that addition of ZrSiO4 particles using Al-4.5%Cu alloy increased both the strength and hardness and an overall reduction in toughness and density. Also, little increase in the apparent porosity of the composite with percentage increase in ZrSiO4 addition was observed. From the result, maximum service performance of the Al- 4.5Cu/ZrSiO4 particulate composite synthesis via squeeze casting should not exceed 15% in order to develop balance in the necessary properties. Pronounce increase in hardness value was observed by reinforcing the matrix alloy with 5-25% zircon sand. Al-4.5Cu/15%ZrSiO4 particulate composite could be appreciable in automobile industries (brake drum, crankshafts, values and suspension arms), recreational products (golf club shaft and head, skating shoe, bicycle frames and base ball shaft) and in construction company (truss structure).

Khalid Mahmood Ghauri1, Liaqat Ali [6] The present work was mainly carried out to characterize the SiC/Al composite which was produced by reinforcing the various proportions of SiC (5, 10, 15, 25 and 30%) in aluminum matrix using stir casting technique. Mechanical properties of test specimens made from stir-casted Aluminum-Silicon Carbide composites have been studied using metallographic and mechanical testing techniques. However, beyond a level of 25-30 percent SiC, the results are not very consistent, and depend largely on the uniformity of distribution of SiC in the aluminum matrix It was observed that as the volume fraction of SiC in the composite is gradually



increased, the hardness and toughness increase. The experimental results showed that the composition of the composite for the optimized properties ranges between 70 to 80 percent aluminum and 20 to 30 percent silicon carbide. It is quite evident that deformation in metals is because of the movement of dislocations and if we block these dislocations by some means the strength which is resistance against the applied force of the material, sufficiently increases. There are numerous ways to block these dislocations like increasing the dislocation density, alloying and making composite in such a way that the newly reinforcing phase acts like a barrier against movement of these dislocations. In the current research work it is evident that as we increase the amount of silicon carbide in aluminum matrix, there is improvement in hardness and the impact properties, these are sufficiently high in the vicinity of mechanical mixture of 25% silicon carbide and 75% aluminum.

G. Hemath Kumar, M. Sreenivasan [7] The present work reports on mechanical properties and microstructure analysis of Al-SiC particulate composites with different wt. % of SiC. Al-SiC composite specimens with different weight % of SiC (viz. 5, 10, 15, 20, 25 and 30 wt. % of SiC) were fabricated through casting process. The induction furnace and open furnace were used for melting of Al-SiC particulate composites. The induction furnace gives the advantage of self stirring action on the introduction of SiC particles. Grinding and fine polishing was done using diamond paste to prepare different samples for microscopic study. The microstructure examination of the polished and carefully etched Al-SiC composite specimens showed that the structure consists of a network of silicon particles, which were formed in inter-dendritic aluminum silicon eutectic composition. These SEM micrographs clearly indicate that the SiC particulates are dispersed uniformly in the Al matrix even at higher percentage such as 20 weight % SiC. The SiC particulates were observed to be in irregular shape. The Al-SiCp composites were fabricated using induction melting show higher compressive strength values than those fabricated using open furnace melting. Al-SiC composites fabricated using induction melting exhibited better mechanical properties than the composites processed in open furnace. Al-SiC composite poppet valve guides with 5 to 30 wt. % of SiC were successfully fabricated by casting process. These guides possess very good surface finish. The compressive strength, density and hardness of Al- SiC composites increase with increase in wt. percent of SiC particulates for all the composites tested. The tensile strength decreases when the amount of reinforcement content exceed to 30 wt. % of SiC.



Mohammad M. Ranjbaran [8] This experimental investigation was initiated to study the low-toughness fracture in Al 356-SiCp (silicon carbide particles) with respect to the role of the various elements of the microstructure and their probable contribution. The fracture in this composite is studied experimentally, in terms of fracture toughness testing. The low-toughness fracture is believed to be an inherent property of this composite and is caused mainly by the differential elastic and thermal properties of the two constituents. These differentials degrade the matrix alloy near the interface by its strain hardening capacity and by stress intensification introduced by the SiC particle geometry. Consequently, the matrix near the interface is subjected to high localized damage leading to premature fracture. It is found that the matrix alloy controls both flow properties and fracture in the materials investigated. It is concluded that a higher toughness composite requires a proper choice of constituent properties which dominate the stress state at the interface. The measurement of valid plane strain fracture toughness, (KIC) values for particulate reinforced metal matrix composites is an important step in the process of developing useful products from these materials and increasing confidence in their properties and performance. The value of the KIC characterizes the fracture resistance of a material in the presence of a sharp crack under tensile loading,

This study shows that the failure is initiated by micro void nucleation at the different initiation sites. Void initiation is more pronounced in the matrix near the interface. The micro cracks can grow from these micro voids to absorb available strain energy. Crack propagation occurs by linking these micro cracks locating the crack path preferentially in the matrix adjacent to the interface. This study shows that this material must have adequate wettability with both Al and SiC to achieve good bonding. Moreover, the proposed material must have high value of tensile ductility and a low yield stress in order to accommodate the plastic strain developed during processing and relax stress concentrations introduced by particle geometry.

Shuyi Qin, Guoding Zhang [9] A structure-toughened SiC particle reinforced 6061 aluminum alloy matrix composite (SiCp-6061Al/6061Al) was designed and fabricated by vacuum infiltration processing. Its fracture toughness KQ was tested by three-point bending method and compared with a conventionally stirring-cast SiCp/6061Al composite's in case of same particle size and volume fraction. The fractography of the SiCp-6061Al/6061Al composite was observed on a Cambridge Instrument S360 Scanning Electron Microscopy (SEM). The results showed that SiCp-6061Al/6061Al



composite has higher fracture toughness KQ but lower yield strength and a comparative elastic modulus. The crack opening displacement (COD) vs load curve of the designed composite showed that the fracture procedure of SiCp-6061Al/6061Al composite is by three stages and the maximum load on it can maintain for a long time. The deformation of the unreinforced 6061Al matrix and the SiCp-6061Al/6061Al interface debonding toughen this composite cooperatively. The complete fracture procedure of the designed composite was schemed by a model. This kind of composite can avoid abrupt failure occurring in most other conventional composite.

Vignesh. S, Sanjeev. C [10] In this paper, turning experiments on machining of particle reinforced Hybrid Metal Matrix composite (MMC) have been carried out. The reinforcement particles selected are Silicon-Carbide of 10% by weight and Boron-Carbide of 5% by weight respectively. Stir casting method is followed to prepare cylindrical rods of specific length and diameter. Poly Crystalline Diamond (PCD) insert of grade 1600 is used for turning operations. Taguchis method of design of experiment is followed by using orthogonal array L9. Three level machining parameters selected are cutting speed, feed rate and depth of cut. The influence of these parameters on machined surface quality is determined by measuring the surface roughness of the workpiece by surface roughness tester. The optimal cutting conditions are arrived as feed rate 0.1 mm/rev, cutting speed as 70 m/min and depth of cut as 0.5mm. The S-N plot is drawn to show the characteristics of each parameter with respect to surface roughness. The results are validated by analysis of variance method (ANOVA) and the percentage of contribution of feed, speed and depth of cut are determined. Tool wear study also performed for a duration of 20 minutes. Hybrid metal matrix composites are economically cheaper in both raw materials and method of fabrication. Due to the reinforcement of ceramic materials, the machining of these metal matrix composites become significantly more difficult than those of conventional materials. The surface quality obtained in turning aluminium (Al 356) metal matrix composites with reinforcements of ceramic particles with 10% by weight of SiC and 5% by weight of

B4C under different cutting conditions with a PCD tool of 1600 grade, have been investigated using Taguchis orthogonal array (L9). The following conclusions are drawn based on the experimental and analytical results: 1. By using Taguchi method, the effect of machining parameters on the surface quality (Ra) has been evaluated and optimal machining conditions would be arrived to minimize the surface roughness.



Dinesh Kumar Koli, Geeta Agnihotri [11] This paper reviews the characterization of mechanical properties with production routes of powder metallurgy and castings for aluminium matrix- Al2O3 composites. Reinforcing aluminium matrix with much smaller particles, submicron or nano-sized range is one of the key factors in producing high-performance composites, which yields improved mechanical properties. Nearly 92% increase in the hardness and 57% increase in the tensile strength were obtained in the nano-composites as compared to the commercially pure aluminium. Ultrasonic assisted casting and powder metallurgy methods are becoming more common for the production of Al-Al2O3 composites. Agglomeration of the reinforcing particles along with the increasing volume percentage is still a challenging task in composites materials manufacturing. There are exciting opportunities for producing exceptionally strong, light weight, wear resistant metal matrix composites with acceptable ductility by solidification processing and powder metallurgy. In addition, processing methods must be developed to synthesize these materials in bulk, at lower cost, with little or no voids or defects, and with improved ductility, possibly as a result of bimodal and tri-modal microstructures. Metal matrix nanocomposites can lead to significant savings in materials and energy and reduce pollution through the use of ultra-strong materials that exhibit low friction coefficients, high wear resistance, low coefficient of thermal expansion and light weight.

Don-Hyun CHOI, Yong-Hwan KIM [12] Friction stir processing (FSP) was used to incorporate SiC particles into the matrix of A356 Al alloy to form composite material. Constant tool rotation speed of 1800 r/min and travel speed of 127 mm/min were used in this study. The base metal (BM) shows the hypoeutectic AlSi dendrite structure. The microstructure of the stir zone (SZ) is very different from that of the BM. The eutectic Si and SiC particles are dispersed homogeneously in primary Al solid solution. The hardness of the SZ shows higher value than that of the BM because some defects are remarkably reduced and the eutectic Si and SiC particles are dispersed over the SZ. The composite material of A356 with SiC particles was produced successfully by FSP. 2) In the SZ, the homogeneous distribution of SiC particles as well as the spherodization of Si needles and their spreading through the matrix are the dominant reasons for improvement of properties in the SZ. 3) The mechanical properties of the SZ with SiC particles, compared to the BM and SZ without SiC, were improved by the dispersed Si, SiC particles and the homogeneous microstructure.



CHAPTER 3

INTRODUCTION TO COMPOSITE

3.1 Background

New and high performance particle reinforced metal matrix composites (PRMMC) are expected to satisfy many requirements for a wide range of performance-driven, and price sensitive, applications in aerospace, automobiles, bicycles, golf clubs, and in other structural applications. A PRMMC consists of a uniform distribution of strengthening ceramic particles embedded within a metal matrix. In general, these materials exhibit higher strength and stiffness, in addition to isotropic behavior at a lower density, when compared to the un-reinforced matrix material. PRMMC benefits from the ceramics ability to withstand high velocity impacts, and the high toughness of the metal matrix, which helps in preventing total shattering. This contribution leads to an excellent balance between cost and mechanical properties, which are appealing for many applications.

The recognition of the potential weight savings that can be achieved by using the advanced composites, which in turn means reduced cost and greater efficiency, was responsible for this growth in the technology of reinforcements, matrices and fabrication of composites. If the first two decades saw the improvements in the fabrication method, systematic study of properties and fracture mechanics was at the focal point in the 60s. Since then there has been an ever-increasing demand for new, strong, stiff and yet light-weight materials in fields such as aerospace, transportation, automobile and construction sectors. Composite materials are emerging chiefly in response to unprecedented demands from technology due to rapidly advancing activities in aircrafts, aerospace and automotive industries. These materials have low specific gravity that makes their properties particularly superior in strength and modulus to many traditional engineering materials such as metals. As a result of intensive studies into the fundamental nature of materials and better understanding of their structure property relationship, it has become possible to develop new composite materials with improved physical and mechanical properties. Based on information now in the public domain, the following military applications for MMCs appear attractive: high-temperature fighter aircraft engines and structures; high-temperature missile structures; and spacecraft structures. Testing of a National Aerospace Plane (NASP) prototype is scheduled for the early to mid 1990s, which might be too early to include MMCs. However, it may be possible to incorporate MMCs in the structure or engines of the production vehicle.



3.2 Composites

Three decades of intensive research have provided wealth of new scientific knowledge on the intrinsic and extrinsic effects of ceramic reinforcement to metals and their alloys. The successes of these various researches have stimulated application of composite in the design of many engineering and non engineering component.

Further, the need of composite for lighter construction materials and more seismic resistant structures has placed high emphasis on the use of new and advanced materials that not only decreases dead weight but also absorbs the shock & vibration through tailored microstructures. Composites are now extensively being used for rehabilitation/ strengthening of pre-existing structures that have to be retrofitted to make them seismic resistant, or to repair damage caused by seismic activity.

While composites have already proven their worth as weight-saving materials, the current challenge is to make them cost effective. The efforts to produce economically attractive composite components have resulted in several innovative manufacturing techniques currently being used in the composites industry. It is obvious, especially for composites, that the improvement in manufacturing technology alone is not enough to overcome the cost hurdle. It is essential that there be an integrated effort in design, material, process, tooling, quality assurance, manufacturing, and even program management for composites to become competitive with metals. The composites industry has begun to recognize that the commercial applications of composites promise to offer much larger business opportunities than the aerospace sector due to the sheer size of transportation industry. Thus the shift of composite applications from aircraft to other commercial uses has become prominent in recent years.

Increasingly enabled by the introduction of newer polymer resin matrix materials and high performance reinforcement fibers of glass, carbon and aramid, the penetration of these advanced materials has witnessed a steady expansion in uses and volume. The increased volume has resulted in an expected reduction in costs. High performance FRP can now be found in such diverse applications as composite armoring designed to resist explosive impacts, fuel cylinders for natural gas vehicles, windmill blades, industrial drive shafts, support beams of highway bridges and even paper making rollers. For certain applications, the use of composites rather than metals has in fact resulted in savings of both cost and weight. Some examples are cascades for engines, curved fairing



and fillets, replacements for welded metallic parts, cylinders, tubes, ducts, blade containment bands etc.

Unlike conventional materials (e.g., steel), the properties of the composite material can be designed considering the structural aspects. The design of a structural component using composites involves both material and structural design. Composite properties (e.g. stiffness, thermal expansion etc.) can be varied continuously over a broad range of values under the control of the designer. Careful selection of reinforcement type enables finished product characteristics to be tailored to almost any specific engineering requirement.

The production and use of composite materials is under intensive development because of the interesting physical and mechanical properties that these materials present and also due to the possibility to manipulate them by means of the variation of the type and proportion of the reinforcement employed as well as the type of the metallic matrix. Materials with designed mechanical (yield stress, elastic modulus, etc) and physical (thermal expansion coefficient, resistivity, thermal conductivity, etc) properties can be produced in this way.

3.2.1 Composite Definition

A Composite material is defined as a structural material created synthetically or artificially by combining two or more materials having dissimilar characteristics. The constituents are combined at macroscopic level and are not soluble in each other. One constituent is called as Matrix phase and the other is called Reinforcing phase. Reinforcing phase is embedded in the matrix to give the desired characteristics.

Generally, a composite material is composed of reinforcement (fibers, particles, flakes, and/or fillers) embedded in a matrix (polymers, metals, or ceramics). The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. When designed properly, the new combined material exhibits better strength than would each individual material. As defined by Jartiz, Composites are multifunctional material systems that provide characteristics not obtainable from any discrete material. They are cohesive structures made by physically combining two or more compatible materials, different in composition and characteristics and sometimes in form.



Kelly very clearly stresses that the composites should not be regarded simple as a combination of two materials. In the broader significance; the combination has its own distinctive properties. In terms of strength or resistance to heat or some other desirable quality, it is better than either of the components alone or radically different from either of them.

3.2.2 Need for Developing Composite Materials

Composites with high specific stiffness and strength could be used in applications in which saving weight is an important factor. Included in this category are robots, high-speed machinery, and high-speed rotating shafts for ships or land vehicles. Good wear resistance, along with high specific strength, also favors MMC use in automotive engine and brake parts. Tailorable coefficient of thermal expansion and thermal conductivity make them good candidates for lasers, precision machinery, and electronic packaging. However, the current level of development effort appears to be inadequate to bring about commercialization of any of these in the next 5 years, with the possible exception of diesel engine pistons.

The increasing demand for lightweight, inexpensive, energy saving, stiff and strong materials in aircraft, space, defense and automotive applications have stimulated steadily growing efforts to develop composite materials. Lightweight composites are attracting a great deal of attention due to the possibility of weight saving in industrial applications. Automobile weight reduction can directly translate into reduced fuel consumption. Reduction in the weight of aircraft and marine vessels can lead to increased loading capacity.

3.2.3 Characteristics of the Composite

Metal matrix composites are strong and tough and can be plastically deformed easily. The crystalline structures make the metals posses excellent properties like thermal and electrical conductivity, high malleability and ductility. Dislocations are critically important as they drastically reduce shear stress required to the slip process and hence make the metals to deform plastically.

Metals can be strengthened by a number of strengthening mechanisms namely, strain hardening, grain boundary strengthening, precipitation strengthening, strengthening due to phase transformation and dispersoid strengthening. By intentional addition of hard



dispersoid particles in the matrix it is possible to increase the strength as these particles retard the motion of the dislocation.

Properties of composites are strongly dependent on the properties of their constituent materials, their distribution and the interaction among them. The composite properties may be the volume fraction sum of the properties of the constituents or the constituents may interact in a synergistic way resulting in improved or better properties. Apart from the nature of the constituent materials, the geometry of the reinforcement (shape, size and size distribution) influences the properties of the composite to a great extent. The concentration distribution and orientation of the reinforcement also affect the properties.

The shape of the discontinuous phase (which may by spherical, cylindrical, or rectangular cross-sanctioned prisms or platelets), the size and size distribution (which controls the texture of the material) and volume fraction determine the interfacial area, which plays an important role in determining the extent of the interaction between the reinforcement and the matrix.

Concentration, usually measured as volume or weight fraction, determines the contribution of a single constituent to the overall properties of the composites. It is not only the single most important parameter influencing the properties of the composites, but also an easily controllable manufacturing variable used to alter its properties. The orientation of the reinforcement affects the isotropy of the system.



3.3 CLASSIFICATION OF COMPOSITE

3.3.1 The Matrix Material

Matrix Material

Polymer MatrixMetal MatrixCeramic Matrix

Thermo-Light metalsCeramics

plastics& alloys(Al,Carbon

Thermo setsMg, Li & Ti)Glass

Refractory

metals

Fig 3.1 Classification of composites (based upon the matrix materials)

3.3.2 The Reinforcing Material

Reinforcing Material

Particulate reinforcedFiber reinforcedStructural composites

Large particlesContinuous fibersLaminates

DispersoidsDiscontinuesSandwich

(short)panels

Aligned or

random

Fig 3.2 Classification of composites (based upon the reinforcing materials)

The classification of composites based upon the Reinforcing materials is as shown in figure. 1.2



Systematic combinations of different constituents, conventional monolithic materials have limitations in terms of achievable combinations of strength, stiffness, coefficient of expansion, and density engineered MMCs consisting of continuous or is continuous fiber,

Whiskers or particle in a metal results in combination of very high specific strength and specific modulus. Furthermore, systematic design and syntheses procedures can be developed to achieve unique combination of engineering properties such as high elevated-temperature strengths, fatigue strength, damping properties, electrical conductivity, thermal conductivity, coefficient of thermal expansion.

A variety of methods for producing MMCs, including foundry techniques, have recently become available. The potential advantage of preparing these composite materials by foundry technique is near-neat shape fabrication in a simple and cost-effective manner. In addition, foundry processes lend themselves to the manufacture of large number of complexly shaped components at higher production rates, which is required by automotive and other consumer oriented industries.

Structurally, as cast MMCs consist of continuous or discontinuous fibers, whiskers, or particles in an alloy matrix that solidifies the restricted spaces between the reinforcing phases to form the bulk of the bulk of matrix. By carefully controlling the relative amounts and distributions of the ingredients constituting a composite and by controlling the solidification conditions, MMCs can be imparted a tailoring set of use full engineering properties that cannot be realized with conventional monolithic materials. In addition, the solidification microstructure of the matrix is refined and particles, indicating the possibility of controlling macro segregation, and grain size in the matrix. This represents an opportunity to develop new matrix alloys.

3.3.3 Classification (According to the Type of Reinforcement)

1. Particulate Composites (Compose of Particle in a Composite)

2. Fibrous Composites (Consists of Fibers in Matrix)

3. Laminated Composites (Consists of Layers of Various Materials)



3.3.3.1 Particulate Composites

This consists of particulates of one or more material suspended in a matrix of another material. The particle can be metallic or non-metallic, as can the matrix following are the types. Fiber composite materials can be further classified into continuous fiber composite materials (multi- and monofilament) and short fibers or, rather, whisker composite materials as shown in figure. 1.3.

A. Non-Metallic in Metallic Composites

The most common example in this case is concrete. Flakes of non-metallic materials such as mica or glass can form an effective composite material when suspended in a glass or plastic respectively. Mica in glass composite is used in electrical application because of good insulating and machining qualities.

`

Figure 3.3 Schematic Presentation of Three Shapes of Metal Matrix Composite

Materials

B. Metallic in Non-Metallic Composites

The most common example is rocket propellants, which consists of inorganic particles such as Al powder and per chlorate oxidizer in a flexible organic binder such as polyurethane or Poly-sulphide rubber. Metal flakes in a suspension are also common. Aluminium paint is actually Aluminum flakes suspended in paint. Upon application the flakes orient themselves parallel to the surface giving good coverage. Similarly, similar flakes can be applied to give good electrical conductivity. Cold solder is metal powder suspended in thermosetting resin. The composite is strong, hard and conducts heat and electricity. Inclusion of copper in an epoxy resin increases the conductivity immensely.



C. Non-Metallic in Metallic Composites

Non-metallic particles such as ceramic can be suspended in metallic matrix. The resulting component is called cermets. Two common classes of cermets are; oxide based and carbide based composites.

1. Oxide Based Cermets can be either oxide particles in a metal matrix or vice- versa. These basically used in tool and high temperature application where erosion resistance is required. 2. Carbide Cement has particles of carbide of tungsten, chromium, and titanium in metal matrix, generally cobalt matrix. These are used in dies, valves, turbine parts. 3. Cermets are also used as nuclear reactor fuel element and control rods.

3.3.3.2 Fibrous Composites

A fiber is defined with respect to its length to diameter ratio and its near crystal diameter. A whisker has essentially the same near crystal size diameter as a fiber, but generally very short and stubby, although the length to diameter ratio can be in hundreds. Fibers and whiskers are of little use unless they are bounded together to take the form of a structural element, which can take loads.

3.3.3.3 Laminated Composites

Laminated composites consist of layers of at least two different materials that are bonded together. They are of following types:

Bimetals

Clad metals

Laminated glass

These are hybrid class of composite involving both fibrous composite and laminate technique. A more common is laminated fiber reinforced composite. Here layers of fiber-reinforced materials are built up with the fiber direction of each layer typically oriented in different direction to give stiffness and strength to fiber.

3.4 Applications of Composites

a. Aerospace and Space-craft applications.

b. Automotive



c. Defense

d. Space hardware

e. Marine application

f. Electrical and electronic devices

3.5 Fabrication techniques of MMCs

There are several fabrication techniques available to manufacture the MMC materials: there is no unique route in this respect. Due to the choice of material and reinforcement and of the types of reinforcement, the fabrication techniques can vary considerably. The processing methods used to manufacture particulate reinforced MMCs can be grouped as follows.1. Solid-phase fabrication methods: diffusion bonding, hot rolling, extrusion, drawing, explosive welding, PM route, pneumatic impaction, etc.

2. Liquid-phase fabrication methods: liquid-metal infiltration, squeeze casting, compo casting, pressure casting, spray co deposition, stir casting etc.

3. Two phase (solid/liquid) processes: Which include Rheocasting and Spray atomization.

Normally the liquid-phase fabrication method is more efficient than the solid-phase fabrication method because solid-phase processing requires a longer time. The matrix metal is used in various forms in different fabrication methods. Generally powder is used in pneumatic impaction and the powder metallurgy technique, and a liquid matrix is used in liquid-metal infiltration, plasma spray, spray casting, squeeze casting, pressure casting, gravity casting, stir casting, investment casting, etc. A molecular form of the matrix is used in electroforming; vapor deposition and metal foils are used in diffusion bonding, rolling, extrusion, etc.

There are certain main manufacturing processes which are used presently in laboratories as well as in industries are diffusion bonding, the powder metallurgy route, liquid-metal infiltration, squeeze casting, spray co-deposition, stir casting and compo casting. Brief Description of these processes is given below.

3.5.1 Solid Phase Fabrication Method

There are several ways to fabricate MMC using solid-phase materials but among them diffusion bonding and the powder metallurgy route are used widely.M-Tech, MDE, Dept of Mech Engg., DBIT, BangalorePage 20


3.5.2 Diffusion Bonding

This method is normally used to manufacture fiber reinforced MMC with sheets or foils of matrix material.

Here primarily the metal or metal alloys in the form of sheets and the reinforcement material in the form of fiber are chemically surface treated for the effectiveness of interdiffusion. Then fibers are placed on the metal foil in pre-determined orientation and bonding takes place by press forming directly, as shown by the dotted line. However sometimes the fibers are coated by plasma spraying or ion plating for enhancing the bonding strength before diffusion bonding, the solid line shows this. After bonding, secondary machining work is carried out. The applied pressure and temperature as well as their durations for diffusion bonding to develop, vary with the composite systems. However, this is the most expensive method of fabricating MMC materials.

3.5.3 Powder Metallurgy Technique

The PM technique is the most commonly used method for the preparation of discontinuous reinforced MMCs. This technique is used to manufacture MMCs using either particulates or whiskers as the reinforcement materials. In general process the powders of matrix materials and reinforcement are first blended and fed into a mould of the desired shape. Pressure is then applied to further compact the powder (cold pressing). In order to facilitate the bonding between the powder particles, the compact is then heated to a temperature that is below the melting point but sufficiently high to develop significant solid-state diffusion (sintering). The consolidated product is then used as a MMC material after some secondary operation.

This method is popular because it is reliable compared with other alternative methods, but it has also some demerits. The blending step is a time consuming, expensive and potentially dangerous operation. In addition, it is difficult to achieve an even distribution of particulate throughout the product and the use of powders requires a high level of cleanliness, otherwise inclusions will be incorporated into the product with a deleterious effect on fracture toughness, fatigue life, etc.

3.5.4 Liquid Phase Fabrication Techniques

Most of the MMCs are produced by this technique. In this technique, the ceramic particles are incorporated into liquid metal using various processes. The liquid composite



slurry is subsequently cast into various shapes by conventional casting techniques or cast into ingots for secondary processing. The process has major advantage that the production costs of MMCs are very low. The major difficulty in such processes is the non-wettability of the particles by liquid aluminium and the consequent rejection of the particles from the melt, non-uniform distribution of particles due to their preferential segregation and extensive interfacial reaction.

3.5.5 Liquid Metal Infiltration

This process can also be called fiber-tow infiltration. Fibers tows can be infiltrated by passing through a bath of molten metal. Usually the fibers must be coated in line to promote wetting. Once the infiltrated wires are produced, they must be assembled into a preform and given a secondary consolidation process to produce a component. Secondary consolidation is generally accomplished through diffusion bonding or hot molding in the two-phase liquid and solid region.

In this technique, as the first step, FP yarn is made into a handle able FP tape with a fugitive organic binder in a manner similar to producing a resin matrix composite preparation. Fibre FP tapes are then laid-up in the desired orientation, fiber volume loading, and shape, and are then inserted into a casting mold of steel or other suitable material. The fugitive organic binder is burned away, and the mold is infiltrated with molten metal and allowed to solidify. Metals such as Aluminium, magnesium, silver and copper have been used as the matrix materials in this liquid infiltration process because of their relatively lower melting points. This method is desirable in producing relatively small-size composite specimens having unidirectional properties.

3.5.6 Squeeze Casting

Squeeze casting is a one-step metal forming process in which a metered quantity of liquid metal in a reusable die is subjected to a rapid solidification under high pressures (50 to 100 MPa) to produce close-tolerance, high-integrity finished shapes. The fabrication process of MMC by squeeze casting, the preform of the ceramic fiber is pre-heated to several hundred degrees centigrade below the melting temperature of the matrix and then set into a metal die. The Al or Mg alloy is heated to just above its melting temperature and is then squeezed into the fiber preform by a hydraulic press to form a mixture of fiber and molten metal.



This process can be used for large scale manufacturing but it requires careful control of the process variables, including the fiber and liquid metal preheat temperature, the metal alloying elements, external cooling, the melt quality, the tooling temperature, the time lag between die closure and pressurization, the pressure levels and duration and the plunger speed. Imperfect control of these process variables results in various defects, including freeze chocking, preform deformation, fiber degradation, oxide inclusions and other common casting defects. However, in practical use, squeeze casting is the most effective method of constructing a machine parts with a complex shape in a short time.

3.5.7 Spray Co-deposition Method

Spray co-deposition method is an economical method of producing a particulate composite. The alloy to be sprayed is melted in a crucible by induction heating. The crucible is pressurized and the metal is ejected through a nozzle into an atomizer where, at the same time, particles (reinforcement) are injected into the atomized metal and deposited on a preheated substrate placed in the line of flight. A solid deposit is built up on the collector. The deposited strip, when cold, is moved from the substrate for subsequent rolling. The shape of the final product depends on the atomizing condition and the shape and the motion of the collector.

3.5.8 Stir Casting

This approach involves mechanical mixing of the reinforcement particulate into a molten metal bath and transferred the mixture directly to a shaped mould prior to complete solidification. In this process, the crucial thing is to create good wetting between the particulate reinforcement and the molten metal.

Micro structural in homogeneties can cause notably particle agglomeration and sedimentation in the melt and subsequently during solidification. In homogeneity 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 solidification. This process has major advantage that the production costs of MMCs are very low.

3.5.9 Compo Casting

Other than PM, thermal spraying, diffusion bonding and high-pressure squeeze casting, this is the most economical method of fabricating a composite with discontinuous fibers



(chopped fiber, whisker and particulate). This process is the improved process of slush- or stir-casting.

The apparatus consists of an induction power supply (50 kW, 3000 Hz), a water-cooled vacuum chamber with its associated mechanical and diffusion pumps and a crucible and mixing assembly for agitation of the composites.

First, a metal alloy is placed in the system with the blade assembly in place. Then the chamber is evacuated and the alloy is superheated above its melting temperature and stirring is initiated by the DC motor to homogenize the temperature. The induction power is lowered gradually until the alloy is 40 to 50% solid, at which point the nonmetallic particles are added to the slurry, However, the temperature is raised during adding in such a way that the total amount of solid, which consists of fibers and solid globules of the slurry, does not exceed 50%. Stirring is continued until interface interactions between the particulates and the matrix promote wetting.

The melt is then superheated to above its liquid temperature and bottom poured into the graphite mould by raising the blade assembly. The melt containing the nonmetallic particles is then transferred into the lower die-half of the press and the top die is brought down to shape and solidify the Composite by applying the pressure. This is using to make the composite of the highest values of volume fractions of reinforcement.

Literature in general, suggests that MMCs will be less forgiving in terms of processing practice than unreinforced alloys, but if the appropriate practice is employed, useful combinations of mechanical and physical properties can be obtained

3.6 Metal matrix composites

Metal matrix composites are materials with metals as the base and distinct, typically ceramic phases added as reinforcements to improve the properties. The reinforcements can be in the form of fibers, whiskers and particulates. Properties of the metal matrix composites can be tailored by varying the nature of constituents and their volume fraction. They offer superior combination of properties in such a manner that today no existing monolithic material can rival and hence are increasingly being used in the aerospace and automobile industries. The principal advantage MMCs enjoy over other materials lies in the improved strength and hardness on a unit weight basis.



The family of materials classified as metal-matrix composites (MMCs) comprises a very broad range of advanced composites of great importance to both automobile, aerospace & defense applications. However, the development and use of MMCs are still in their infancy when compared to monolithic materials or even polymer resin-matrix composite systems. The family of metal-matrix composites is made up of many varieties of materials, which can be categorized based on their matrix composition, fabrication process, or reinforcement type.

Metal matrix composites have a metal matrix usually of lighter metal such as (Al, Mg or Ti) or a super alloy (Ni based or Co based super alloy). The reinforcement materials include Boron, Silicon carbide, carbon, graphite, alumina, Boron carbide, Boron nitride or metallic system like tungsten, beryllium or steel. The form of reinforcement material can be either fiber or whisker or particulate. Metals are reinforced either to increase certain properties like elastic modulus and tensile strength or decrease certain properties like coefficient of thermal expansion and thermal conductivities.

In recent years, the development of metal matrix composite (MMCs) has been receiving worldwide attention on account of their superior strength and stiffness in addition to high wear resistance and creep resistance comparison to their corresponding wrought alloys. The ductile matrix permits the blunting of cracks and stress concentrations by plastic deformation and provides a material with improved fracture toughness.

Aluminium matrix composite (AMCs) have shown high mechanical properties such as high strength, high stiffness, wear resistance and good elevated temperature properties when compared to the unreinforced matrix alloy, which has lead to the use of aluminium matrix composite in the following; electronic heat sinks, automotive drive shaft, ground vehicles brake rotors, jet fighters, air craft firms, electronic instrument racks, satellite struts, crankshafts, gear parts brake drum cylinder block and suspension arms. New researches on metal matrix composite have focus on particle reinforcement due to low cost of the ceramic reinforcement and less complex fabrication technique. Stirring casting route has been used successfully to synthesis metal matrix composite.

It has been proved that particle reinforced aluminum matrix composites can improve considerably the strength and hardness of aluminum and its alloys. However, at the same time, the plasticity and ductility can substantially reduced. This will severely affect the safety and reliability of components fabricated from Al matrix composites (AMCs).



In aluminum matrix composites system (AMCs), one of the constituent is aluminum/ aluminum alloy, which forms percolating network and is termed as matrix phase. The other constituent is embedded in this aluminum/ aluminum alloy matrix and serves as reinforcement, which is usually non-metallic and commonly ceramic such as SiC and Al2O3. Properties of AMCs can be tailored by varying the nature of constituents and their volume fraction.

Aluminum matrix composites system (AMCs) offer superior combination of properties in such manner that today no existing monolithic material can rival. Over the years, AMCs have been tried and used in numerous structural, non-structural and functional applications in different engineering sectors. Driving force for the utilization of AMCs in these sectors include performance, economic and environmental benefits. The key benefits of AMCs in transportation sectors are lower fuel consumption, less noise and lower airborne emissions.

AMCs can be classified into four types depending on the type of reinforcement

1. Particle reinforced AMCs (PRAMCs)

2. Whiskers or short fiber reinforced AMCs (SFAMCs)

3. Continuous fiber reinforced AMCs (CFAMCs)

4. Mono filament-reinforced AMCs (MFAMCs)

New and high performance particle reinforced AMCs (PRAMCs) are expected to satisfy many requirements for a wide range of performance-driven, and price sensitive, applications in aerospace, automobiles, bicycles, golf clubs, and in other structural applications. A PRAMCs consists of a uniform distribution of strengthening ceramic particles embedded within aluminum matrix. In general, these materials exhibit higher strength and stiffness, in addition to isotropic behavior at a lower density, when compared to the unreinforced aluminum matrix. PRAMCs benefits from the ceramic's ability to withstand high velocity impacts, and the high toughness of the metal matrix, which helps in preventing total shattering. This contribution leads to an excellent balance between cost and mechanical properties, which are appealing for much application; the main drawback of these materials is their low ductility, which is caused by the nucleation, growth, and coalescence of voids created by the ceramic reinforcement.

The main contribution to the strengthening of PRAMCs is particle addition, which affects most of the mechanical properties of PRAMCs. Several particle parameters, which are



critical in determining the mechanical properties of PRAMCs, include the volume fraction (vf), size, shape, and distribution of reinforced particles within the metal matrix.

3.7 Examples

Continuous carbon fiber reinforced Al alloy likewise has been used in most structures (vertical support structures) in the Hubble telescope.

SiC continuous fiber reinforced Al alloy has been used vertical section of advanced fighter aircrafts.

SiC continuous fiber reinforced Ti alloy has been used for hypersonic aircraft.

Precision components of missile guidance system demand very high dimensional stability i.e. geometries should not change with temperature excursions during use. Al alloy with 20% SiC continuous fiber satisfy this requirement.

Discontinuous SiC fibers 1 to 3mm in diameter and 50 to 200mm long are mixed with Al powders consolidated by hot pressing and then extruded or forged to the desired shape. With 20% SiC whiskers, the tensile strength increased from 310Mpa to 480Mpa and the tensile modulus can be increased from 69 to 115 Gpa.

Hybrid composites of 12% by volume fraction of alumina particles (for high strength) and 9% volume fraction of graphite fibers (for self lubrication) in alloys have been developed by Honda for Engine blocks, connecting rods, piston rods etc., for

automobiles, which helps in reducing weight of automobile and enhanced engine life. SiC coated on inter-metallic compound Ti3Al fibers in Ti alloy matrix have been found to be very effective for high temperatures resistance. These composites find

applications in compressor discs and blades in aero-engines.

A relatively new technique called rapid solidification rate processing has been developed to obtain metallic glass ribbons which can be effective reinforcing material in MMCs.

3.7.1 Advantages and Disadvantages of MMCs

Advantages

Very high specific strength and specific modulus. Low thermal coefficient of thermal expansion. Retention of properties at higher temperatures.



Higher operating temperatures. Insensitivity to moisture.

Non-flammability.

Better capability to withstand compression and shear loading.

Disadvantages

Higher densities as compared to PMCs.

MMCs demand higher processing temperatures. Processing methods are expensive.

MMCs are expensive as compared to PMCs.

3.8 Scope of Present Investigation

Due to the advancement in the material technology to produce desired materials from various industrial applications and fast changing scenario in the production of lighter and stronger materials, composite materials are gaining wide acceptance due to their unusual characteristics of behavior with their high strength to weight ratios. The most widely used material in these industries is aluminum and their alloys because of their light weight property.

To make these alloys of aluminum further versatile and flexible for varieties of application, during which these materials is expected to behave as expected and provide a long life under different environments, the composites have emerged as the single most material, which can provide a better service and better quality.

Therefore in the present investigation, a study had been conducted to evaluate the various mechanical properties such as tensile, hardness, compression, microstructure and Fracture Toughness of Al356 with Silicon Carbide (SiC) and Zirconium silicate (ZrSio4) composite castings.



CHAPTER 4

SELECTION OF MATERIALS

Cast Al356 is one of the most widely used commercial Al-Si-Mg alloys in the aircraft and automotive industries due to its good castability and the fact that it can be strengthened by artificial aging. However, the mechanical properties of Al356 are significantly affected by micro structural features such as microporosity, intermetallics, eutectic silicon particles and heat treatments. Aluminium Metal matrix composites (AMMC), where hard ceramic particles are distributed in a relatively ductile matrix, have widespread applications in aerospace, automobiles and other engineering industries because of their excellent physical, mechanical and tribological properties.

4.1 Matrix Material: Al 356

Fig 4.1 Ingot Structure of Al 356

4.1.1 Chemical Composition and Mechanical Properties of Matrix

Material Al356

Chemical composition and mechanical properties of matrix material Al356 is as shown in Table 4.1 and 4.2.



ElementSiFeCuMnMgNiZnTiPbAluminium

Wt%7.5%0.2%0.25%0.35%0.45%0.1%0.35%0.2%0.1%Rem

Table 4.1 Chemical composition of Al356

Density(*1000 Kg/m3 )2.685

Poissons ratio0.33

Tensile Strength, Ultimate (MPa)228

Tensile Strength, Yield (MPa)165

Elongation (%)3.5%

Shear Strength (MPa)180

Thermal Conductivity (W/m-K)151

Melting Temperature5550C

Fatigue Strength(MPa)60

Table 4.2 Mechanical properties of matrix material Al356

4.2 Reinforcement Material (silicon carbide)

Silicon carbide (SiC), also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Grains of silicon carbide can be bonded together by sintering to form very hard ceramics which are widely used in applications requiring high endurance, such as car brakes and ceramic plates in bulletproof vests.

Silicon carbide, is a high-temperature structural material, offering many advantages such as high melting temperatures, low density, high elastic modulus and strength, and good resistance to creep, oxidation and wear. These properties make SiC suitable for use in applications such as gas turbines, piston engines and heat exchangers, and where load-bearing components are required to operate at temperatures up to 1500oC. Silicon carbide does not melt at any known pressure. It is also highly inert chemically.

Silicon Carbide is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon. Silicon



carbide is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today the material has been developed into a high quality technical grade ceramic with very good mechanical properties. It is used in abrasives, refractories, ceramics, and numerous high-performance applications. The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear applications are constantly developing. The reinforcement material (SiC) is as shown in figure. 4.2.

Fig 4.2 Reinforcement Material (SiC)

4.2.1 Physical Properties of Sic

Low density High strength

Low thermal expansion

High thermal conductivity High hardness

High elastic modulus

Excellent thermal shock resistance



Superior chemical resistance.

4.2.2 Application of SiC

Fixed and moving turbine components Suction box covers

Seals, bearings Ball valve parts

Hot gas flow liners Heat exchanger

Semiconductor process equipment.

4.3 Reinforcement Material (Zirconium Silicate)

Zircon silicate is naturally occurring sand. Zirconium silicate contains mainly zirconium oxide and silicon oxide with a minor amount of potassium, gold and calcium oxide. It possesses properties such as high temperatures up to 2400C, High density, Low thermal conductivity (20% that of alumina), Chemical inertness, Resistance to molten metals, Ionic electrical conduction, Wear resistance, High fracture toughness and High hardness. This has made it a good reinforce for the production of MMCs for engineering applications.

Fig 4.3 Reinforcement material (ZrSiO4)



4.3.1 Properties of Zirconium Silicate

Good strength

Corrosion resistant

Low thermal conductivity

Good thermal shock resistance Excellent Thermal resistance High flexural strength

High hardness

4.3.2 Applications of ZrSiO4

Nuclear reactors.

Zirconium with Aluminium, iron and titanium are used in vacuum tubes. Zirconium is used in satellites as reflective surface agent.

Super conductive magnets.

Zirconium is used in optical glasses and for glass toughening. Foundry/investment casting

PropertiesZircon Sand

M.P. (0C)2500

Limit of application (0C)1870

Hardness7.5

Density (g/cm3)4.5-4.70

Linear coefficient of expansion (m/m 0C)4.5

Fracture toughness (MPa-m1/2)5

Crystal structureTetragonal

Table 4.3 Properties of Zircon sand



CHAPTER 5

FABRICATION OF COMPOSITES

Flow chart of composite fabrication is as shown in figure 5.1

Aluminium matirx (Al356)

Metal matrix composite

Al356-Sic-ZrSio4

Reinforcement

(Silicon Carbide+

Zirconium Silicate)

MatrixFurnace

Degassing+

Scum powder

Stir

ReinforcePouring

Casting

Fig 5.1 Flow chart of fabrication of Composite



5.1 Stir Casting

The large ingots of matrix material were cut into small pieces for accommodating into the crucible. Composites were produced by Stir casting process. Melting was carried out in a cast iron crucible in a resistance furnace. Degassing was carried out with hexa-chloroethane tablets and Scum powder. Cut pieces of alloy A356 were heated at 840 C for 3 to 4 hours before melting, and before mixing the SiC and ZrSio4 particles were preheated for 1 to 3 hours to make their surfaces oxidized. Furnace temperature was first raised above the liquidus to melt the alloy scraps completely and was then cooled down just below the liquidus to keep the slurry in a semi-solid state. At this stage the preheated SiC and ZrSio4 particles were added and mixed manually according to the required proportions. Due to difficulties of mixing in semi solid state, initially manual mixing was used for the synthesis of A356 and SiC and ZrSio4. After this, the composite slurry was re-heated to a fully liquid state and then automatic mechanical mixing was carried out for about 5 minutes at an average stirring rate of 300 rpm. In the final mixing processes, the furnace temperature was controlled to be within 84010 C.

To ensure the homogeneity of the added reinforcement particles through molten aluminum, electrical stirrer was inserted into the crucible. Molten aluminum was stirred at (300 r.p.m.) to get suitable vortex. Later reinforcement particles were added to molten metal. This process was followed to modify reinforcement particles distribution through the molten aluminum.

Due to the vortex effect, reinforcement particles were pulled inside the molten metal and uniformly distributed. Molten aluminum was stirred for (1- 5 min.) until the molten aluminum becomes slurry. Later molten aluminum was poured into suitable cast iron mould, which is preheated at 350C to prevent sudden cooling for molten aluminum.

The pouring temperature was controlled to be around 820 C. A preheated permanent cast iron mould with diameters in the range of 10 mm to 25 mm was used to prepare cast bars. Finally the super heated melt was poured into the cast iron mould. The preheating temperature 350 C for Cast Iron moulds was maintained for slower cooling.



5.2 Pre-heating of Mould Box

A mould is an assembly of two or more metal blocks. The mould cavity holds the liquid material and essential acts as a negative of the desired product. A permanent mould box which is prepared according to required dimensions of the casting is used. Permanent split type mould box is used for casting the composites in the present study as shown in the figure 5.2. Pre heating of the mould box is shown in figure 5.3. The mould box is tightened with the help of screws and is checked for any gaps in the mould box. The mould is then heated to a temperature of about 300-3500c to prevent the sudden cooling of the molten aluminium.

Fig 5.2 Split type mould box

Fig 5.3 Pre heating the mould box



5.2 Steps Involved in Stir Casting Method

Aluminum (Al356) 3kg was melted in the furnace to a temperature of 850 degree centigrade as shown in figure 5.4.

Addition of scum powder. Formation of slag.

Slag removal.

After 10 mins titanium dioxide was added to remove the entrapped gases (degasification) and Stirrer was introduced.

Stirrer was rotated at a speed of 0 to 300 rpm to create a vortex in the liquid metal. Reinforcement material Sic and ZrSiO4 powder was added according to the

required proportions to molten metal in steps while stirring.

After 15 mins molten metal is poured to the pre-heated mould and left for solidification.

The mould box is opened and cast components are obtained.

Fig 5.4 Electric furnace



Fig 5.5 Molten Metal in FurnaceFig 5.6 Formation of Vortex

Fig 5.7 Pre heating of reinforcementFig 5.8 Poured molten metal in mould box

Fig 5.9 Cast Aluminium Composites



5.3 Composition of matrix and reinforcement

SamplesAl356 (kg)Sic (%)ZrSio4 (%)

13-8

2362

3326

4344

538-

Table 5.1 Different wt% ratios of matrix metal and reinforcement

The composition of the matrix metal and the reinforcement in different wt% ratios is shown in the above table. The casting samples with different wt% reinforcements were prepared respectively as shown below.

Casting 1: Al356+0%SiC+8%ZrSiO4







CHAPTER 6

EXPERIMENTAL DETAILS

6.1 Fracture Toughness

The measurement of valid plane strain fracture toughness, (KIC) values for particulate reinforced metal matrix composites is an important step in the process of developing useful products from these materials and increasing confidence in their properties and performance.

Fracture toughness is a material property that characterizes the materials resistance to crack propagation when under load or stress. In more precise terms, it refers to the resistance of a preexisting crack to extend either under unstable (i.e., brittle fracture) or by stable tearing means (i.e., ductile fracture). Experimental methods for characterizing fracture toughness play a critical role in applying fracture mechanics to integrity assessment, fitness-for-service evaluation, and limit state analyses for a wide variety of engineering structures. Fracture toughness properties are frequently used as a basis for material selection, material qualification programs, and quality assurance for critical structures such as high-pressure gas and liquid transmission pipelines, pressure vessels, nuclear reactor components, petrochemical processing vessels, and aircraft. Fracture toughness is a quantitative way of expressing a material's resistance to brittle fracture when a crack is present. If a material has much fracture toughness it will probably undergo ductile fracture. Brittle fracture is very characteristic of materials with less fracture toughness. Fracture mechanics, which leads to the concept of fracture toughness, was broadly based on the work of A. A. Griffith who, among other things, studied the behavior of cracks in brittle materials.

The measurement procedure of fracture toughness is based on the principle of linear-elastic fracture mechanics (LEFM) and contains three main steps: generation of cracks in the test specimen, measurement of the load at failure stress respectively, and crack depth. In the case of ideally brittle materials, the fracture toughness is independent of the crack extension. The crack growth resistance increases with the increasing crack extension. Some structural ceramics show an increase of fracture resistance with crack extension under stable crack growth. The Single-Edge-Notched Beam (SENB) method was developed as a simple and inexpensive alternative, but the results can be influenced by the tip radius of the sawed notch.



6.2 Specimen dimensions as per ASTM standards

The samples were cut to the dimensions as per ASTM standards ASTM C393-62 for Testing; ASTM standards are given in Table.

Sl. NoASTM CodeMechanical TestSampleSpan length

Dimensions(mm)

(mm)

1ASTM-D790Flexural127 x 13 x 665

Table 6.1 ASTM codes for mechanical test and sample dimensions

6.3 Test for Fracture toughness

The Fracture toughness of the specimens were determined as per ASTM-D790. The specimens (127 X 13 X 6 mm) were tested with a span length of 65mm using three point bend setup with 10 ton capac

“experimental determination and analysis of fracture toughness of mmc”

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