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University POLITEHNICA of Bucharest Faculty of Materials Science and Engineering Department: Processing of Metallic Materials and Eco metallurgy No. Senate Decision in . .2017 Summary Thesis Research concerning the possibility to obtain Al alloy castings with Al-SiC p composite skin Author: Eng. MUNA N. ISMAEL ISMAEL Supervised by: Professor Dr. Eng. Mihai CHISAMERA DOCTORAL COMMITTEE Chairman Prof. Dr. Eng. Mihai BUZATU From University Politehnica of Bucharest Scientific Supervisor Prof. Dr. Eng. Mihai CHISAMERA From University Politehnica of Bucharest Reviewer Prof. Dr. Eng. Aurel CRISAN From University Transilvania of Brasov Reviewer Prof. Dr. Eng. Bela VARGA From University Transilvania of Brasov Reviewer Prof. Dr. Eng. Gigel NEAGU From University Politehnica of Bucharest Bucharest 2017

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  • University POLITEHNICA of Bucharest Faculty of Materials Science and Engineering

    Department: Processing of Metallic Materials and Eco metallurgy

    No. Senate Decision in . .2017

    Summary

    Thesis

    Research concerning the possibility to obtain Al alloy

    castings with Al-SiCp composite skin

    Author: Eng. MUNA N. ISMAEL ISMAEL

    Supervised by: Professor Dr. Eng. Mihai CHISAMERA

    DOCTORAL COMMITTEE

    Chairman Prof. Dr. Eng. Mihai BUZATU From University Politehnica of Bucharest

    Scientific Supervisor Prof. Dr. Eng. Mihai CHISAMERA From University Politehnica of Bucharest

    Reviewer Prof. Dr. Eng. Aurel CRISAN From University Transilvania of Brasov

    Reviewer Prof. Dr. Eng. Bela VARGA From University Transilvania of Brasov

    Reviewer Prof. Dr. Eng. Gigel NEAGU From University Politehnica of Bucharest

    Bucharest 2017

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    2

    Content

    INTRODUCTION 6

    CHAPTER 1

    SOME THEORETICAL ASPECTS CONCERNING Al-SiCp COMPOSITE PRODUCTION 7

    1.1. INTRODUCTION 7

    1.2. HISTORY OF COMPOSITE MATERIALS 8

    1.3. GENERAL DEFINITION OF COMPOSITE 8

    1.4. ADVANTAGE AND DISADVANTAGE OF COMPOSITES 9

    1.5. CLASSIFICATION OF COMPOSITES 9

    1.5.1. On the basic of the matrix 9

    1.5.2. On the basics of reinforcement 10

    1.6. STRENGTHENING MECHANISM OF PARTICULATE COMPOSITE 11

    1.6.1. Large-particle composite 11

    1.6.2. Dispersion strengthened composite 12

    1.7. ALUMINUM - SILICON CARBIDE PARTICLES COMPOSITES 12

    1.7.1. Silicon carbide prooerties 12

    1.7.2. Aluminum and Aluminum alloys properties 14

    1.8. RULE OF MIXTURE 16

    1.9. THEORETICAL ASPECTS OF AL-SiCP COMPOSITE PRODUCTION 18

    1.9.1. Oxidation behaviour of molten Aluminum and Aluminum alloy 18

    1.9.2. Factors influencing on particles incorporation into the Aluminum melt 18

    1.9.3.Analysis of the forces acting on particles through penetration into the liquid metals 21

    1.10. TECHNIQUES TO IMPROVE WETTABILITY 23

    1.10.1. Addition of alloying elements 24

    1.10.2. Coating the particles 25

    1.10.3. Preheating the particles 26

    1.11. INTERFACE PHENOMENA 27

    1.11.1. Reactions between silicon carbide particles and Aluminium melt 27

    1.11.2. Adsorption theory 28

    1.11.3. Chemical bonding theory 28

    1.11.4. Diffusion theory 29

    1.11.5. Fluidity 29

    1.11.6. Mechanical interlocking 30

    1.11.7. Distribution 31

    1.12. METHODS OF COMPOSITE CASTINGS PRODUCTION 32

    1.12.1. Stir casting method 32

    1.12.2. Squeeze casting or pressure infiltration method 34

    1.12.3. High pressure dies casting method 34

    1.12.4. Centrifugal casting method 34

    1.12.5. Gravity casting method 35

    1.13. SOLIDIFICATION PROCESS 35

    1.14. HARDNESS MEASURMENT 37

    1.15. GENERAL CONCEPTS OF WEAR PROCESS 39

    CHAPTER 2

    OBJECTIVES 41

    CHAPTER 3

    PROCEDURE OF EXPERIMENTAL RESEARCH 42

    3.1. EXPERIMENTAL METHODS TO PRODUCE Al CAST SAMPLES WITH AL-SiCP 42

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    3

    COMPOSIT SKIN

    3.2. MATERIALS 44

    3.3. DEVICES 44

    3.4. TECHNIQUE FOR PRODUCING OF ALUMINUM CASTINGS WITH Al- SiC COMPOSITE

    SUPERFICIAL LAYER SKIN BY GRAVITY CASTING 46

    3.4.1. Aluminum melt pouring in a SiC sieve coated mould 46

    3.4.2. Aluminum melt pouring over a SiC powder-polystyrene sandwich placed on the

    bottom of a metallic mould

    47

    3.4.3. Al/ Al alloy melt pouring over a SiC powder-Al foils sandwich placed on the bottom of

    a phenolic resin sand mould cup

    48

    3.5. AUTOMATIC IMAGES ANALYSISOF SiCP (BEFORE AND EMBEDDED PARTICLES) 49

    3.6. SOLIDIFICATION COOLING CURVES ANALYSIS 52

    3.7. MECHANICAL ANALYSIS 54

    3.8. SCANNING ELECTRON MICROSCOPE (SEM) ANALYSIS 55

    CHAPTER 4

    RESULTS AND DISCUSSION 56

    4.1. SILICON CARBIDE PARTICLES ANALYSIS 56

    4.1.1. Free silicon carbide particles 56

    4.1.2. Bonded silicon carbide particles (sieve) 59

    4.2. DETERMINE THE VOLUMETRIC COEFFICIENT OF SILICON CARBIDE PARTICLES

    AND ACCELERATION 60

    4.3. SETTLING VELOCITY OF SILICON CARBIDE PARTICLES 63

    4.4. ALUMINUM CAST SAMPLESWITH Al-SiC COMPOSITE SKIN OBTAINED BY Al MELT

    GRAVITY CASTING IN A SiC SIEVE COVERED SAND MOULD 69

    4.5. ALUMINUM-SIC COMPOSITE OBTAINED BY GRAVITY CASTING OVER SIC POWDER-

    POLYSTYRENE SANDWICHES METHOD 77

    4.5.1. Composite skin thickness 78

    4.5.2. SiC particles analysis in composite skin 79

    4.6. ALUMINUM CAST SAMPLE WITH Al - SiCP COMPOSITE SKIN OBTAINED BY Al MELT

    POURING OVER SiC POWDER-Al FOILS SANDWICHES METHOD 85

    4.6.1. SiC particles distribution 86

    4.6.2. SiC particles morphology 90

    4.6.3. Solidification process of samples 93

    4.7. BRINELLS HARDNESS TEST 113

    4.8. SCANING ELECTRON MICROSCOPE (SEM) ANALYSIS 116

    CHAPTER 5

    GENERAL CONCLUSIONS 123

    5.1. LITERATURE ANALYSIS 123

    5.2. EXPERIMENTAL PROPER RESEARCH 123

    5.3. ORIGINAL CONTRIBUTIONS AND FUTURE RESEARCH DEVELOPMENT THOUGHTS 126

    DISSEMINATION OF RESEARCH RESULTS 127

    REFRENCES 128

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    4

    INTRODUCTION

    A composite material is a naturally combination of at least two unique materials having an

    identifiable interface between them.

    Aluminum/SiC is a standout amongst the most broadly concentrated on aluminum matrix

    composite (AMC). Aluminum alloy and AMCs have been utilized to supplant steel in car body

    boards, motor parts, and brake rotors to diminish structural weight, which has brought about

    expanded enthusiasm for these materials. Different reinforcement has been used in aluminium

    matrix composite such as: Particles, whiskers, short fibers, and contentious fibers.

    Aluminium alloy cast with Al -silicon carbide particulate composite skin is used within a

    variety of engineering applications due to their some excellent properties in comparison with non-

    reinforced alloys such as; harness and wear resistance.

    This thesis focuses on the development of new methods to obtain Al alloy castings with Al-

    SiCp composite skin by gravity casting.

    In the first chapter are presented the definition and classification the composites on basis the

    matrix and reinforcement and focus on the Al-SiC composite. The problems in wettability and

    incorporation between the SiCp and melt Aluminum and the methods to improvement these

    problems are presented.

    The second chapter presents the objective of this thesis.

    The third chapter deals the describing materials and the devices which used in all methods to

    produce the composites. In addition the procedure for each method, processing parameters, and

    sketch diagrams to for each way. Definition the measuring parameters of shape factors of SiCp and

    cooling curve.

    The fourth chapter is structured in two parts; In the first part is presented the acceleration and

    settling velocity for different shapes of particles.

    The second part presents the results for each method which describe affect the particle size,

    amount of SiCp on shape factors of SiCp, thickness of composite skin, and hardness of composite

    skin.

    Determine the heat losses and Al temperature decreasing in the cup during change the weight

    percentage of SiCp by making heat balance. The effect of wt. % SiCp on cooling rate and various

    temperatures during the solidification are presented. Microstructural analysis has been carried out

    using scanning electron microscope to examine the distribution of the reinforcements for Aluminum

    -sieve made from bonded SiC particles textile fibers are also presented in this chapter.

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    CHAPTER 1

    Introduction

    MMC is a (Ceramic particle reinforced metal matrix composite system), developed and

    utilized by a several of industrial applications. The most successful reinforcement materials for

    Aluminum was a (SiCp) the main reason, because it has a high stiffness and hardness, and in

    addition, its chemical compatibility with Aluminum alloys up to 500 °C [3]. The introduction of

    SiC-particles in Aluminum alloys. However, leads to a significant decline in their ductility and

    fracture solidity. The Aluminum-Silicon system forms a binary eutectic that displays limitations in

    solubility for both materials. The system is characterized as anomalous eutectic due to the

    composition of a metal and a non-metal. Silicon solubility in Aluminum shows a maximum at 1.5

    at% at eutectic temperature and an increase to 0.016% Si at 1190°C.

    1.9.2. Factors influencing on particles incorporation in to the

    Aluminium melt

    The particle penetration process can be divided into two parts: penetration of the particle

    through the melt surface and the movement of a particle in a liquid. A schematic diagram of the

    silicon carbide particle penetration process in Al melt is shown in Figure 1.4. The first part is

    basically a wetting problem, where the surface of the different interfaces in the system (solid-

    vapour, liquid-vapour and solid-liquid), change during penetration through the melt surface. The

    particle loses its kinetic energy when it penetrates through the surface, depending on the surface

    tensions (energies) of the different interfaces [46].

    Wettability is the property of liquids to spread on solid surfaces. Poor wettability can lead to

    the contamination of the end-material because it hinders the contact between the two phases. On the

    Figure 1.4. Penetration process of silicon carbide particle in Al melt.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    6

    surface of liquid Al there exists a layer of Al2O3 while on the SiC particles a SiO2 layer is formed

    that does not allow contact with the particle itself [47].

    The wetting between metal and ceramic could be the most crucial factor in the production of

    MMCS. However, the non-wetting nature of most ceramic materials urged numerous analysts to

    research the possibility of enhancing the wettability of ceramic materials. The most important

    characteristic to make adhesion between the ceramic materials and metal are casting processes,

    microelectronics, surface protection and coating technology [48].

    1.10. TECHNIQUES TO IMPROVE WETTABILITY

    Several methods have been applied to improve wetting of SiC particles with the Aluminum

    molten matrix:

    1- Addition of alloying elements to the molten metal such as, Cu, Mg, Li.

    2- Using of coatings, such as, Cu, Ni, Cr, Ag.

    3- Pre heating for reinforcing elements.

    1.12.5. Gravity casting

    The casting process can be defined the molten metal is poured under the gravitational force.

    Gravity casting and can be classified according to the mould type: sand casting and permanent

    mould casting.

    Sand casting

    Sand casting is a traditional method for creating parts by simply pouring the melt into a

    mould made of natural or synthetic sand or artificially blended material and it has no limitations in

    terms of casting material. Other benefits include low cost and small size operations and the method

    allows for complex parts to be casted.

    Permanent mould casting

    This casting method involves the use of reusable moulds made from hardened materials, usually

    metal, the main process uses gravity in order to fill the mould and it allows for repeated use in order

    to produce casts of the same form (Figure 1.9).

    The permanent mould casting is suitable for industrial productions of parts with high

    reproducibility, limited undercuts or intricate internal coring [100].

    The main advantages of the permanent mould casting are the production of uniform castings with

    surface finish and better mechanical properties but the main disadvantage is the cost of the moulds.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    7

    CHAPTER 2

    OBJECTIVES

    The main goal the thesis is to study the possibility to obtain Al /Al alloy castings with Al / Al

    alloy – SiC composite superficial layer (skin).

    The thesis is focusing on the development of a new possibilities to improve the Al melt-SiC

    particles compatibility to facilitate the producing of Al/ Al alloy castings with superficial layer

    (skin) of Al-SiCp composite in the same purpose, some special moulding techniques allow the Al-

    SiCp composite superficial layer producing as experimental laboratory research, were tested.

    Experimental new methods to obtain Al castings with Al-SiC composites skin by gravitational

    casting are presented. This covers

    Analysis of the SiCp; X-ray diffraction, particle size and shape coefficient used in the preparation

    methods for Al-SiCp;

    Preparation of special moulds covered with silicon carbide sieve. Study on the effect of the SiCp

    particle size or the mesh number of the sieve on particles density and their distribution into the

    Aluminum matrix;

    Produce the thin layer of Al alloy-SiCp composite by pouring the Aluminum melt on SiCp-

    polystyrene sandwich;

    Producing of Al casting with Al-SiC composite skin by Al/Al alloy melt puring over a special

    prepared Al foil-SiC powder sandwich. Study the solidification process of Al-SiC composite

    samples by automatic analysis of the cooling curve;

    Examine the samples by optical microscope and analyse the micrograph to determine both of

    particles density, and their mean shape factors (sphericity, aspect ratio, shape factor, and

    elongation);

    Brinells hardness measurement in the samples composite skin;

    Measurment the Al-SiC composite skin thickness by optical microscope;

    SEM analysis of possible interactions in between Al matrix and SiC particles.

    CHAPTER 3

    PROCEDURE OF EXPERIMENTAL RESEARCH

    Three procedures to produce Al cast samples with Al-SiCp composites skin were used as

    casting methods (Figure 3.2):

    a) Aluminum-sieve made from bonded SiC powder (textile support);

    b) Aluminum-SiC particles-polystyrene (as volatilizable support);

    c) Aluminum-SiC powder-Al foils layers (to permit powder spreading).

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    8

    3.4.1. Aluminum melt pouring in a SiC sieve coated mould (experimental 1.a)

    The sand mould which was used for this method had a cylinder shape (45 diameter and

    101mm height). It was prepared by hand procedure and heated to 325 ºC to remove any moisture

    and reduce the temperature difference between the mould and melt. The sieve made from bonded

    SiC powder on textile support was used as reinforcement material. Four different sizes of bonded

    SiCp sieves (mesh 40, mesh 120, mesh 180, and mesh 220), were chosen as the reinforcements. The

    active surface of the mould was coated by one layer of sieve in cylindrical shape while the bottom of

    the mould was coated by a circle shape sieve (Figure 3.6).

    Fig. 3.6. Section of sand mould padded in bonded SiCp sieve.

    Fig. 3.2. Flowchart of Al-SiCp composite samples preparation and analyses.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    9

    Aluminum was melted in a gas furnace inside a graphite crucible by using a classical specific

    technique for Al melting, 900ºC superheating. Pouring aluminum melt in the mould was realized on

    900 ºC.

    3.4.2. Aluminum melt pouring over a SiC powder-polystyrene sandwich

    placed on the bottom of a metallic mould (experiment 1.b)

    SiC powder-polystyrene granules sandwiches were used in this method to obtain Al-SiCp composite

    by pouring of Al melt into metal mould (Figure 3.8). Three volume percentage of silicon carbide

    powder were used (1, 2, 3 vol. %) – (0.5 g poly-styrene granules) as sandwiches. The mould with

    dimension (height 170 x diameter 31) mm is used in this method. Two layers of silicon carbide and

    two layers of polystyrene were arranged as it is shown in Figure 3.8. The aluminum alloy was

    charged into the graphite crucible and heated to 900◦C until the entire melt in the crucible was

    homogenized. Melt aluminum infiltrates the layers after the pouring of alloy into the mould.

    3.4.3. Al/ Al alloy melt pouring over a SiC powder-Al foils sandwich placed

    on the bottom of a phenolic regime sand mould cup (experimental 1.c)

    Three types of matrices were used in this method: Aluminum, Al-Si12, Al-Si12- 1.5Mg. By

    varying weight of SiCp addition (1 g, 1.5 g, and 2 g), 106 µm particle size were added for each case.

    Aluminum foil (0.28 g) having adhesive face was cut into three pieces having the same area as the

    sand cup base. The package was made from three layers of aluminum foil and two layers of silicon

    carbide powder. In addition, for one sample of 2 g SiCp, the SiC powder – Al foil package was

    perforated in order to decrease its floating tendency.

    As mould typical quick-cup moulds were used for melt solidification thermo- analysis in order

    to study the possible effects of SiC powder addition on cooling curves parameters (Figure 3.9).

    Fig. 3.8. Metal mould containing SiC powder – polystyrene granules as a sandwich placed

    on the bottom side of the mould (cylindrical sample).

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    10

    CHAPTER 4

    RESULTS AND DISCUSSION

    4. 2. DETERMINE THE VOLUMETRIC COEFFICIENT AND CRITICAL

    ACCELERATION OF SILICON CARBIDE PARTICLES

    Although gravitational casting seems to be the simplest and cheapest method to produce

    aluminum-silicon carbide particles composite, a major problem appears to the achievement of the

    mixture by mechanical stirring due to the non-wetting conditions from system. To penetrate the

    aluminum melt, a silicon carbide particle needs to exceed a critical acceleration. For a spherical

    particle the value of critical acceleration can be determined by the relationship:

    ( )

    , (4.1)

    where: is the surface tension at the Aluminum melt-gas interface, - the wetting degree,

    - the radius of the particle, - the particle density; - the density of the liquid aluminium.

    For a spherical particle,

    [

    ]

    , (4.2)

    From relation 4.2, general relation of acceleration can be obtained:

    [

    ]

    , (4.3)

    where: is a volumetric coefficient of shape, - the particle mass for a spherical particle =

    3.896.

    Figure 3.9. The quick-cup regime mould- SiC powder-Al foils arrangement for experiments.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    11

    The calculated values of the volumetric coefficient shape for different types of particles are

    presented in Table 4.4.

    Table 4.4. Calculated values of volumetric coefficient shape

    Shape of particles Mathematical formula of

    Value of

    Sphere ( ) 3.896

    Cylinder, , *

    +

    5.106

    Conical, , , √

    *

    +

    3.972

    Conical frustum, ,

    ,

    ( √ ) *

    +

    4.423

    Cube, 6 6

    Prism triangle, , √

    ( √ )

    5.114

    Prism rectangle, ,

    ,

    5.04

    Pyramid quadrate, , (√ ) 4.667

    Regular pyramid (equilateral

    triangle base) , (√ √ ) (

    )

    3.802

    Pyramid (hexagonal base), ,

    (

    )

    (√ √ )

    6.889

    Truncated square pyramid,

    ,

    ,

    (

    )

    5.197

    In Fig. 4.6 and 4.7 the critical acceleration values for particles with different shapes are

    presented. The calculations were made for 973 K, and a value of contact angle for oxidised particles

    of SiC [64], Shen and Keene’s relationships for surface tension of aluminum melt, Luca’s equation

    for the density of aluminum liquid and a mass of particle mp = 1.34410-8

    kg (equivalent to a

    spherical particle with 100 m radius) [55, 111].

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    12

    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    1.4

    1.6

    Sphere Cylinder Conical Conicalfrustum

    Cri

    tica

    l acc

    ele

    rati

    on

    , acr

    ·10

    5

    [m/s

    ²]

    Shape of particle

    (Shen)

    (Keene)

    0

    1

    2

    3

    Cri

    tica

    l acc

    ele

    rati

    on

    , acr

    ·10

    ^5

    [m/s

    ²]

    Shape of particle

    (Shen)

    (Keene)

    Figure 4.6. Critical acceleration for round particles at 973K.

    Figure 4.7. Critical acceleration for polyhedral particles at 973K.

    In Figure 4.6 and Figure 4.7, the results show that triangular pyramid particles have the

    minimum values of critical acceleration compared to other shapes, but in spite of that, it could be not

    enough for the penetration of the particle. Also, spherical and triangle pyramid have nearly the same

    value.

    4.4. ALUMINUM CAST SAMPLES WITH Al-SiC COMPOSITE SKIN OBTAINED BY Al

    MELT GRAVITY CASTING IN A SiC SIEVE COVERED SAND MOULD

    (experiment 1.a)

    Al-SiC composite skin thickness analysis.

    The grain size of SiC particles extracted from the sieves and their ability to incorporation with

    the aluminum melt would be determine the thickness of Al-SiCp composite skin which appears in

    cross section of the sample, Figure 4.17.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    13

    Figure 4.17. The cross section of Al- SiCp composite skin sample obtained by 1.a experiment method.

    Table 4.7 shows the thickness values of SiCp thin layers for different samples.

    Table 4.7. The influence of sieve mesh on the thickness of Al casting composite skin

    Where:

    A1: Al-SiCp extracted from sieve (40 mesh), A2: Al-SiCp extracted from sieve (120 mesh),

    A3: Al-SiCp extracted from sieve (180 mesh), A4: Al-SiCp extracted from sieve (220 mesh).

    As from the Table 4.7 results, the thickness of the Al-SiCp composite skin in sample A1 is

    higher than the other samples which means a better compatibility between particles and Al melt in

    this range of SiC particles size. The samples A2, A3and A4 have SiCp with high settling velocity

    more than SiCp in A1 due to the low mean spherecity value in A1.

    For sample A4, the most particles have spherical shape; this led to a high settling velocity of

    these particles more than other samples and the weak incorporation between the particles and the

    melt.

    As from Figure 4.19 results, it seems to be an optimum size of SiC particles (40 mesh) which

    assures a higher particles density on the sample section. This could be explained by the fact that the

    settlement of particles in the melt is already starting to occur when the melt is still in the mould. This

    Symbol Sieve mesh Skin thickness

    (μm)

    A1 40 7021

    A2 120 6692

    A3 180 6354

    A4 220 5491

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    14

    is because density of the SiCp is higher than that of the molten aluminium and high mean sphericity

    value of particles, which leads to settling or sedimentation of the particle reinforcements. Therefore,

    the higher sedimentation tendency of the spherical particles (120, 1800 mesh and 220) occurred

    during the alloy in a liquid alloy state.

    and

    Figure 4.19. Particles density of silicon carbide in Aluminum matrix.

    Although the big difficulties in quantification of the particles shape especially of irregular

    particles, the analysis of the several parameters proposed so far to describe both the SiCp and Al-

    embedded SiC particles shapes have been generally accepted [114]. In this content, the results of

    shape coefficients analysis both of individual SiC particles (SiC powder) and Al-embedded SiC

    particles are presented in Figures below as comparing.

    Figure 4.20. Mean of particle aspect ratio.

    As from Fig. 4.20 results, mean aspect ratio decreases with particle size decreasing both for

    free and embedded particles practically in the same manner (11 % decreasing for free particles and 6

    0

    2

    4

    6

    8

    40 120

    180 220

    Par

    ticl

    e d

    en

    sity

    (n

    o. o

    f p

    arti

    cle

    s)1

    /m

    m2

    Sieve mesh

    0

    0.5

    1

    1.5

    2

    2.5

    Mesh 40 Mesh120

    Mesh180

    Mesh220

    Me

    an a

    spe

    ct r

    atio

    Sample

    SiC powder

    Al-SiCp composite

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    15

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    Mesh40

    Mesh120

    Mesh180

    Mesh220M

    ean

    of

    sph

    eri

    city

    Sample

    SiC powder

    Al-SiCp composite

    0

    0.5

    1

    1.5

    2

    2.5

    3

    Mesh40

    Mesh120

    Mesh180

    Mesh220

    Me

    an e

    lon

    gati

    on

    Sample

    SiC powder

    Al-SiCp composite

    % decreasing for embedded particles respectively). Usually the shape of SiCp become more regular

    and closer to the spherical shape in small size, therefore the mean aspect ratio decrease. In all cases

    the mean aspect ratio of free particles is smaller than embedded particles, the difference being of

    about 17 % [15 % for the biggest particles (40 Mesh) and 19.6 % for the smallest one (120 Mesh)].

    This case refer to some chemical reaction could be occur on the boundary of SiCp

    The mean sphericity for SiCp (powder) and embedded SiCp in Aluminum are presented in

    Figure 4.21.

    Figure 4.21. Mean of particle sphericity.

    The SiCp (powder) extracted from the sieves has a higher mean sphericity range

    (4.441…0.493) compare to the mean sphericity range of embedded SiCp in melt Aluminum

    (0.255…0.374). Usually, the small particle size has more uniformity shape and sphericity values, as

    shown in Figure 4.21.

    Figure 4. 22. Mean of particle elongation.

    Usually, the mean elongation of particles is inversely proportional with the mean sphericity. In

    Figure 4.22 is clear that the mean elongation at large particles is higher than at small particles. It is

    found that mean elongation of SiCp (powder) is much higher from Al-embedded SiCp. This is

    explained by some chemical reactions that may occur between the SiC particles and the Aluminum

    alloy melt and the formation of another compound which remains in the melt, such as:

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    16

    4Al + 3SiC Al4C3+ 3Si (4.8)

    The continuous reaction layer of Al4C3 was occurred at 900 ºC between the melt aluminum

    and silicon carbide [115].

    The Al4C3 forms at the interface while the Si dissolves in the Al matrix and caused the

    reduction on the values of mean elongation.

    4.5. ALUMINUM CAST SAMPLES WITH SiC COMPOSITE SKIN OBTAINED BY Al

    MELT GRAVITY CASTING OVER SiC POWDER- POLYSTYRENE SANDWICHES METHOD

    (EXPERIMENT 1.b)

    The polystyrene is a synthetic aromatic polymer (C8H8)n with density of 0.96 – 1.04 g/cm3 and

    melting temperature 240°C. It contains fixed carbon which does not burn off even at 1000 ºC. Since

    voids of particle−porosity clusters can be formed after the polystyrene burns and hydrogen is

    released. These porosities can be filled with melt aluminum and improve the incorporation between

    the melt and the particles. Hydrogen was liberated from polystyrene and it helps to raise

    some particle. This state reduced the settled particles and improved the incorporation between the

    particles and the melt.

    Figure 4.27 demonstrates the SiCp distribution across the sample section as a function of

    volume percentage of silicon carbide. Usually the polystyrene has the ability to swell. During the

    swelling, the material changes from a rigid solid into a soft highly swollen hydrogel. The silicon

    carbide particles were interfered with polystyrene and there could be incorporation with melt

    Aluminum.

    Figure 4.27. Optical micrographs of Al-SiCp composite skin -experiment 1.b (unetched).

    Usually, the particles tend to sediment in the bottom (due to high density) more than float

    when the gas layers are destroyed, therefore the fast solidification was very important to prevent

    2vol. % SiCp 1vol. % SiCp

    3vol. % SiCp

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    17

    0

    10

    20

    30

    1% 2%

    3% Par

    ticl

    es

    die

    nsi

    ty (

    no

    . p

    arti

    cle

    s), (

    1/m

    m2 )

    Vol. % of SiCp addition

    y = -25x2 + 3.85x + 0.196

    0

    0.05

    0.1

    0.15

    0.2

    0.25

    0.3

    0.35

    0% 1% 2% 3%

    Me

    an s

    ph

    eri

    city

    Vol. %of SiCp

    sedimentation of the particles. The results in Figure. 4.27 at 3 vol. % show that the number of

    particles in melt Aluminum is clearly increased by the volume percent of SiCp addition.

    The effect of reinforcement amount on the particle density of the composite material is shown

    in Figure 4.28. The result shows that as the silicon carbide particles content was gradually increased,

    the particles density of composite increased. Slight increase was observed in the particle density

    because silicon carbide particles have a slight higher density value than Aluminum (the density of

    Al is 2.7 g/cm3 and of SiCp is 3.21 g/cm

    3). The void fraction of the material decreases with

    increasing reinforcement content [116].

    Figure 4.28. SiC particles density in the composite skin (experiment 1.b)

    Different SiCp particles density in the composite bar head where obtained after sample

    analysis. It can be observed that the mean area of the particles increased from 312.461 μm2 at 1vol.

    % SiCp in composite to 606.833 μm2 when 3vol. % SiCp thus showing a more than twice times

    increase. The particle density is increased with increase of SiCp percent addition. This is referring to

    the increase of the mean area which enables more distribution in the melt.

    The different measurements of shape coefficients are plotted vs. the added weight percent of

    SiCp powder in Figures 4.30 and 4.31. The curve points for these figures were fitted by polynomial

    equation of 2nd

    order.

    4.30. Effect of SiCp percentage addition on mean sphericity of SiCp.

    Mean sphericity increased from 0.232 at 1vol. % SiCp to 0.289 at 3vol. % SiCp. Increasing the

    volume fraction of SiC particles above reduces the spiral length considerably and the reduction is

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    18

    y = -2155x2 + 58.25x + 2.104

    0

    0.5

    1

    1.5

    2

    2.5

    3

    0% 1% 2% 3%

    Me

    an e

    lon

    gati

    on

    Vol. % SiCp

    0

    1

    2

    3

    4

    5

    6

    7

    8

    9

    1 1.5 2

    Par

    ticl

    esd

    en

    sity

    (n

    o. p

    arti

    cle

    s),

    (1/m

    m2)

    Wt.% SiCp

    commercial Al

    Al-Si12

    Al-Si12-Mg

    probably due to an increase in viscosity. Some elements such as Si and Mg which are found in the

    alloy could be an interface with SiCp and cause growth in sphericity due to some compounds.

    Figure 4.31. Effect of SiCp percentage addition on mean elongation of SiCp.

    In Figure 4.31, the decrease in mean elongation at increasing the volume percent of SiCp is a

    result of the form of granules approaching the spherical shape, which is a result of the friction and

    erosion process between the particles.

    4.6. ALUMINUM CAST SAMPLE WITH Al - SiCP COMPOSITE SKIN OBTAINED BY Al

    MELT POURING OVER SiC POWDER-Al FOILS SANDWICHES METHOD

    (EXPERIMENT 1.c)

    4.6.1. SiC particles distribution

    Figure 4.37. Particles density for silicon carbide particles in different alloys.

    Some comments could be made concerning these techniques of Al-SiC composite: As the

    results Figure 4.37 show, the increase of SiC powder addition in the cup leads to a slow increasing

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    19

    of particle density in the sample. This could be explained by the decreasing of the melt temperature

    because Al foil assimilation needs a high heat consumption which leads to melt temperature

    decreasing. By this way the Al foils are not melted and as a result the SiC powder-Al foil sandwich

    on will be float without assimilation. So, good correlation is necessary between the melt temperature

    and the sandwich weight. Much more, the sandwich must be fixed at the mould bottom by

    mechanical means;

    - Both silicon and especially Mg alloying gave a better assimilation of the SiC powder and as

    result a higher SiC particles density was obtained;

    - As it is known, the addition of magnesium improves the wetting characteristics of

    Aluminum alloy- SiCp composites because of its lower surface tension. Magnesium also acts as a

    scavenger of oxygen, thereby increasing the surface energy of the particles. However, magnesium

    must not be added in excess, as this will form low melting compounds, which will have negative

    effect on the mechanical properties of the composite [57]. Silicon addition could avoid the reaction

    between Al and SiC. The addition of Si to Aluminum prevents formation of Al4C3, but does not

    affect wettability between SiC and Aluminum. In reaction bonded SiCp also free Si prevents

    formation of Al4C3 and is effective in promoting wetting by liquid Aluminum in the temperature

    range 700-1040 oC.

    4.6.3. Solidification process of samples

    Adding silicon carbide particles or alloying elements such as Mg, Si into Al alloy usually

    affects both morphology and parameters of cooling curves solidification of Al alloys-SiCP

    composites.

    Automatical analysis for all samples at solidification process was conducted. Cooling curve

    aspect for the three alloys is represented in Figures 4. 42 to 4.44.

    Figure 4.42. Cooling curve and its first derivative for Al -2wt.% SiCp addition (see also Table 3.5).

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    Figure 4. 43. Cooling curve and its first derivative for sample Al –Si12-2wt.% SiCp addition

    (see also Table 3.5).

    Figure 4. 44. Cooling curve and its derivative for sample Al –Si12-1.5Mg-2.0wt.% SiCp addition

    (see also Table 3.5).

    Adding Mg to the Al-SiC composites has beneficial impacts on the wettability characteristics

    of the matrix due to its low surface tension. The optimum Mg amount is approximately 1-1.5% in

    order to obtain the best distribution and mechanical properties. The addition also reduces the

    temperatures in the solidification process.

    Calculus of recalescence on Al or Al-Si eutectic solidification

    Heat balance

    The specific heat of SiCp is higher than of the Aluminum alloy melt, therefore the increasing

    of SiCp amount in Al alloy caused decreasing the released latent heat. In addition the sensible heat

    of Al-SiCp composite increased due to increased of specific heat and this led to decreased heat

    amount.

    Table 4.24 is present the values of Al melt temperature decreasing under different SiC powder

    addition.

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    120

    125

    130

    135

    140

    0 0.5 1 1.5 2 2.5

    ΔT

    tota

    l lo

    ss, °C

    Wt. % SiCp

    Table 4. 24. Values of Qassim without losses and Tm Al matrix

    As Table 4. 24 it seems to be the Al melt temperature decreasing (ΔTassim) increase when increase

    the content of silicon carbide particles in the composite. It is seen the specific heat value of SiCp is

    higher than that specific heat of Al melt, therefore the increasing of SiCp content led to released

    latent heat by solidification decreases and reduce the liquid amount of Aluminum.

    Figure 4. 48 show the total losses of temperature in the cup.

    Figure 4. 48. Effect the wt. % SiCp on Total losses of temperature in the cups.

    Analysis of cooling curve parameters (see also table 3.5 and table 4.26)

    The analysis of cooling curve events during cup samples solidification was made by help of

    the first derivitave of which showes the event position on the cooling curve. The analysis results are

    presented in Table 4.26.

    Wt. % SiCp Qassim, J ΔTassim, °C

    1 1109.73 10.3

    1.5 1513.95 14.1

    2 1918.2 17.8

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    Table 4. 26. Analysis of cooling curves A

    L –S

    iCp

    Wt.% SiCp

    TM °C /t(s)

    T nucl. Al °C /t(s)

    Tmin. Al cry °C/t(s)

    Tmax Alcry °C/t(s)

    Tnucl. EUT °C/t(s)

    T min EUT °C/t(s)

    T max EUT °C/t(s)

    TES °C/t(s)

    ΔTr (Al cr)

    ΔTr (EUT)

    0

    875 696.31 650.58 652.42 638.75 636.56 635.95 583.48 1.84 -0.61

    196.6 243.3 345.9 297.2 445 457.5 470.3 570.4 --- ---

    1.0

    874 687.1 651.69 652.2 638.09 636.37 635.98 580.7 0.51 -0.38

    199.2 236 323.8 281.1 433.5 443.8 454.9 561.3 --- ---

    1.5

    865.19 686.18 651.68 651.78 637.66 635.8 635.13 575.68 0.10 0.66

    190.7 225.1 298.2 262.3 409.2 420 431.8 541.3 --- ---

    2.0

    858.4 685.08 651.24 651.75 636.06 635.79 635.75 588.62 0.51 0.01

    183.1 213.2 246.9 251.3 404.9 426.1 428.3 515.5 --- ---

    AL-

    Si1

    2-S

    iCP

    0

    785.58 604.62 590.67 594.37 578.24 565.4 566.06 470.98 3.7 0.66

    263.9 337.8 353.9 373.5 425.3 443.5 461.5 902.7 --- ---

    1.0

    781.6 602.89 582.15 584.22 578.81 565.57 566.06 473.85 2.07 0.49

    342.3 353.1 463.3 397.5 446.9 508.9 530.5 946 --- ---

    1.5

    784.16 597.03 581.89 584.42 577.55 565.09 565.69 462.82 2.53 0.6

    268.9 324.7 338.6 355.2 383.8 429.3 446.8 912.8 --- ---

    2.0

    784.47 600.02 581.84 583.91 577.02 564.99 565.58 469.26 2.08 0.6

    251.8 300.2 329.3 346.1 380.3 430 451 804.5 --- ---

    AL-

    S1i2

    -1.5

    Mg-

    SiC

    P

    0

    800.19 635.5 580.99 583.93 580.31 557.56 560.33 469.06 2.94 2.77

    60.5 149.8 170.1 194.2 330.5 359.6 447.9 852.2 --- ---

    1.0

    790.37 629.67 580.95 583.04 579.96 557.23 559.95 464.73 2.09 2.72

    47.8 83.2 73.5 112.9 243.8 230.6 293.5 633.2 --- ---

    1.5

    779.61 623.15 580.34 582.73 579.01 556.79 559.52 471.36 2.39 2.73

    44.9 69.1 30.8 145.6 219.1 215.9 274.7 574.7 --- ---

    2.0

    784.56 623 580.17 582.92 568.66 557.84 560.05 469.23 2.76 2.21

    36 71.9 117.6 137.6 208.2 269.5 328.6 600 --- ---

    2.0 penetrated

    Al foil

    778.05 624.53 580.64 582.73 577.10 557.85 560.49 462.63 2.08 2.64

    63.4 96.5 123.5 138.7 163 229 268.5 588.2 --- ---

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    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    0 0.5 1 1.5 2 2.5

    ΔT/

    Δt

    (°C

    /s)

    Wt. % SiCp

    cr1

    cr2

    0

    0.5

    1

    1.5

    0 0.5 1 1.5 2 2.5

    ΔT/

    Δt

    (°C

    /s)

    Wt. % SiCp

    cr1

    cr2

    The cooling rate at crystallization Al area and crystallization in eutectic increased when

    adding silicon carbide particles. Increasing the percentage of SiCp results in shorter solidification

    time and retains the SiCp to heat within the particles. The molten is chill and solidifies in area

    faraway the particles as result of the reduction of thermal conductivity and effective thermal

    diffusivity of the SiCp. Adding Mg to the Al alloy increases the wetting and improves the heat

    transfer rate. Thermal conductivity of SiCp usually was low compared to aluminum alloy. This

    affects the cooling rate. Figures 4.62 to 4. 64, show the cooling rate with different SiCp percentages

    with different Al alloys.

    Figure 4. 62. Effect of (wt.%) SiCp on the cooling rate at crystallization and eutectic regions of Al (Cr1=TM-

    Tmin EUT / tmin EUT – tM alloy, Cr2= Tmin EUT - TES / tEs – tmin EUT).

    Silicon carbide particle plays a dominant role by increasing the cooling rates. From Figure

    4.62 results a cooling rate increasing with SiCp amount increasing in the Al melt. The cooling rate

    resulted from the cooling curves shows that the increase of SiC particles in the Al melt lowered the

    liquids temperature (see Table 4.26). This can be attributed to the unfavourable primary aluminum

    nucleation condition predominant at the reinforcement surface and the depression in the freezing

    point due to the presence of reinforcement.

    Figure 4. 63. Effect of (wt.%) SiCp on the cooling rate at crystallization in eutectic region

    of Al –Si12 alloy(Cr1=TM-Tmin EUT / tmin EUT – tM alloy, Cr2= Tmin EUT - TES / tEs – tmin EUT) .

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    0

    0.2

    0.4

    0.6

    0.8

    1

    1.2

    1.4

    0 0.5 1 1.5 2 2.5

    ΔT/

    Δt

    (°C

    /s)

    Wt. % SiCp

    cr1cr2

    During solidification, the concentration of Si atoms in the liquid phase near the solid-liquid

    interface increases the cooling rate. This is clear in Figure 4.64.

    Fig. 4. 64. Effect of (wt.%) SiCp on the cooling rate at crystallization in eutectic regions

    of Al Si12-Mg alloy(Cr1=TM-Tmin EUT / tmin EUT – tM alloy, Cr2= Tmin EUT - TES / tEs – tmin EUT) .

    The specific heat of SiCp is higher than that of Al alloy. The increasing of SiCp addition to Al

    alloy led to released latent heat by the solidification process and decreased the liquid amount in unit

    volume. On the other hand, the sensible heat of composite melts increases since the increase of

    specific heat. The rate of cooling in Al-Si12-Mg-SiCp composite is more than Al-SiCp composite

    and Al-Si12-SiCp composite because the magnesium is play the important role. The alteration in

    nucleation and growth mechanism of the micro constituent associated with the presence of Mg in

    alloy acting as a nucleating agent has affected the cooling rate of Al-Si12-Mg-SiCp composite.

    4.8. SCANING ELECTRON MICROSCOPE (SEM) ANALYSIS (experiment 1.a – 220 Mesh SiC

    sieve sample)

    Two kinds of particles were identified by SEM technique:

    a- Non disintegrated SiC sieve wire containing bonded on textile support SiC particles;

    b- Embedded in Al matrix SiC particles coming from disintegrated SiC sieve wires.

    Both the analysed areas positions and their chemical composition are presented in table 4.35 while

    the X-ray mapping is presented in Fig. 4.70 and 4.71 respectively. Some comments could be made

    after analysing these results as follows.

    As from table 4.35 can be seen some non-disintegrated SiC sieve remainders were found as wires

    with bonded SiC particles. The polygonal shapes were assimilated to be bonded SiC particles

    because of their high silicon content and carbon presence despite of lower content of the last.

    The very high oxygen content (much more than with silicon equilibrium) both in SiC particles area

    and in the space among them suggests on the SiO2 presence as a result of SiC particles oxidation and

    /or other external sources. Part of this oxygen could be bonded by the Al which is present especially

    in the area among SiC particles. Much more complex chemical composition was recorded in the

    space among SiC particles that is wire matrix. This could be explained if the textile support and its

    adhesive are considered.

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    25

    The presence of non-disintegrated SiC sieve wires in the aluminium matrix suggests on the

    insufficient Al melt temperature caused by a low pouring temperature or a low poring rate. In this

    respect much more experiments are necessary to establish the optimum values of Al melt pouring

    parameters.

    The embedded in Al matrix SiC particles have a specific SiC composition (higher silicon and lower

    carbon levels) but also with relative high oxygen content which suggests on the SiC oxidation. The

    presence of some Al in SiC particles composition could be attributed to the possibility of Al4C3

    compound formation as a result of the reaction between melt Al and SiC particles on high

    temperature (see rel. 1.16). Al signal could be also come from the Al matrix if the SiC particle

    thickness is very small.. The lower level of carbon in SiC particles composition is attributed to the

    instrument lower accuracy for the light elements such as carbon determination. However a better

    overlaying of the carbon given oxygen maps on the silicon map can be seen from fig. 4.71. Oxygen

    signal comes especially from SiC particle surrounding area.

    The EDAX analysis on the Al matrix area shows a specific commercial aluminium composition

    having some Mg and Fe in composition. As it is known, Mg is considered as a benefit element

    because of its favourable influence on SiC particles inglobation while Fe is considered as a harmful

    element because precipitation of the brittle iron phases on aluminium solidification.

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    26

    Non disintegrated SiC sieve wire with bonded SiC particles Embeded in Al matrix SiC particles

    Al matrix area Pos. 1- SiC particle

    Pos. 2-wire matrix

    Pos.3-wire matrix

    Singular SiC particle

    Aglomerated particles

    Chemical composition

    Wt. % A. wt. % Wt. % A. wt. % Wt. % A. wt. % Wt. % A. wt. % Wt. % A. wt. % Wt. % A. wt. %

    Al Al Al Al Al Al

    0.38 0.26 28.19 18.84 10.95 7.37 5.69 4.11 2.58 1.51 98.71 97.99

    C C C C C C

    8.06 12.52 20.08 30.13 17.60 26.62 6.65 10.80 48.72 64.14 --- ---

    Si Si Si Si Si Si

    38.99 25.91 11.00 7.06 18.86 12.20 41.35 28.69 31.62 17.80 0.87 0.84

    Mg Mg Mg Mg Mg Mg

    --- --- --- --- 0.31 0.23 --- --- --- --- 0.77 0.86

    O O O O O O

    52.56 61.31 38.40 43.28 44.65 50.71 15.06 20.05 15.94 15.75 --- ---

    Table 4. 35. Chemical composition of different interest areas

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    27

    Ca Ca Ca Ca Ca Ca

    --- --- --- --- 1.00 0.45 --- --- --- --- --- ---

    Fe Fe Fe Fe Fe Fe

    --- --- 0.99 0.32 3.40 1.11 1.32 0.50 --- --- 0.64 0.31

    Ti Ti Ti Ti Ti Ti

    --- --- --- --- 1.81 0.69 --- --- --- --- --- ---

    Na Na Na Na Na Na

    --- --- --- --- --- --- --- --- 1.15 0.79 --- ---

    Zn Zn Zn Zn Zn Zn

    --- --- 1.34 0.37 0.22 0.06 --- --- --- --- --- ---

    S S S S S S

    --- --- --- --- 0.04 0.02 --- --- --- --- --- ---

    K K K K K K

    --- --- --- --- 1.16 0.54 --- --- --- --- --- ---

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    28

    Fig. 4. 70. Compo image and mapping of SiC sieve wire embedded in Al matrix and the elements

    distribution.

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    29

    Fig. 4. 71. Compo image and mapping of embedded in Al matrix SiCp and the elements distribution.

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    30

    Conclusions

    5.2. EXPERIMENTAL PROPER RESEARCH

    Determine the volumetric coefficient of silicon carbide particles, shape factor, acceleration,

    and settling velocity

    The triangular pyramid particles have the minimum values of critical acceleration comparing

    with other shapes, with nearly the same value for spherical particles.

    Pyramid (hexagonal base) particles have a bigger value of volumetric coefficient shape than

    other shapes which have a straight edge such as prism triangle, prism rectangle, pyramid

    quadrate etc.

    Volumetric coefficients shape of cylinder is bigger than other values of volumetric

    coefficient shape for other shapes which have a round shape.

    Settling velocity for all shapes of SiCp in Al alloy is lower than in Al-4Mg and Al-2Si alloys,

    because the fluidity of these alloys is higher than the fluidity of Al as a result of the existence

    of Si and Mg.

    Aluminum castsamples with Al-SiC composite skin obtained by Al melt gravity casting in a

    SiC sieve covered sand mould (experiment 1.a)

    The optimum size of SiC particles is 40 mesh, which assures higher particles density on the

    aluminum melt.

    The mean aspect ratio in small particle size of SiCp (220 mesh) in aluminum melt is smaller

    than of big particles because the ratio of length to width decreases in SiCp as a result the

    particles shape approaching to sphere shape.

    It was found that the mean elongation of free SiCp (powder) is much higher by comparing with

    embedded SiCp in Aluminum matrix.

    The values of free SiCp (powder) shape factor are constant bigger than the similar values for

    embedded SiC particles in Al matrix.

    The thickness of composite skin obtained by using SiC particles extracted from sieve (40

    mesh) is bigger than other sieves sizes.

    Aluminum cast samples with Al –SiC composite skin obtained by Al melt gravity casting

    over a SiC powder-polystyrene sandwich method (experiment 1.b).

    By pouring of Al melt over the SiCp-polystyrene sandwich system, the polystyrene changes

    from a rigid solid state into a soft highly swollen hydrogel. The silicon carbide particles were

    interfered with polystyrene and they could be incorporated by Aluminum melt.

    Slight increase was observed in the particle density when the silicon carbide particles content

    was gradually increased.

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    31

    The mean area of the SiCp particles penetration in Al-SiCp-polystyrene sandwich system

    could be increased from 312.461 μm2 at 1vol. % SiCp addition to 606.833 μm2 for 3 vol. %

    SiCp addition.

    The mean aspect ratio and the mean elongation are decreased when the content of SiCp

    increased in Al- SiCp-polystyrene sandwich composites while mean sphericity and mean

    shape factor of SiCp are increased.

    Aluminum cast samples with Al-SiC composite skin obtained by Al melt pouring over a

    SiC powder-Al foils sandwich method (experimen 1.c)

    Both silicon and especially Mg alloying gave a better assimilation of the SiC powder and

    as result; a higher SiC particles density is obtained.

    The mean elongation and aspect ratio of SiCp decrease with increases in weight percent

    of SiCp.

    Mean aspect ratio of SiCp in Al composite is less than in samples whereAl-Si12 and Al-

    Si12-1.5 Mg is used as matrix.

    The increasing in SiCp percent usually enhances the shape factor and the mean sphericity

    of SiCp.

    The roundness increasing of SiCp can be observed in Al-Si12-Mg melt more than in

    other matrices.

    A lot of heat losses by convection, radiation, and wall of mould could occur during

    casting and solidification process.

    The SiCp presence produces increase in the nucleation Al phase due to release latent heat

    during solidification and the difference of specific heat between SiCp and Al melt.

    A slight decreasing in maximum temperature crystallization and minimum temperature

    at eutectic was observed when SiCp addition for all types of Aluminum alloys. This is

    caused increasing in cooling rate during solidification process.

    Values of Δt1, Δt2, Δt3, Δt4, Δtr EUT, and Δtt lowered when increasing wt. % SiCp for all

    types of alloys used for preparation of the composites.

    The cooling rate at crystallization Al area and crystallization in eutectic increased when

    increasing the SiCp percentage.

    Increases in wt. % SiCp are promoting composite skin thickness to increase.

    SEC analysis

    High level of silicon and some lower level of carbon in SiC particles coming both from SiC

    sieve remainders and embedded in Al matrix particles were recorded. This is a proof of SiC

    particles identification both in SiC sieve wire and Al-SiC composite skin matrix;

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    32

    The high oxygen level (much more than with silicon equilibrium) both in SiC particles area

    and in the space among them suggests on the SiO2 presence as a result of SiC particles

    oxidation and /or other external sources presence;

    The presence of some Al in SiC particles composition could be attributed to the possibility

    of Al4C3 compound formation as a result of the reaction between melt Al and SiC particles

    on high temperature (see rel. 1.16);

    The presence of non-disintegrated SiC sieve remainders in the aluminium matrix can be

    considered as a defect in the Al-Sic composite skin structure and it suggests on the

    insufficient Al melt temperature caused by a low pouring temperature or a low poring rate.

    In this respect much more experiments are necessary to establish the optimum values of Al

    melt pouring parameters.

    5.3. ORIGINAL CONTRIBUTIONS AND FUTURE RESEARCH

    DEVELOPMENT THOUGHTS

    The main original contributions in this thesis are the following:

    Updating the technical literature analysis in Al alloy- SiCp composites field;

    Make theoretical calculations both on the volumetric coefficient shape and shape factor

    for different shapes of silicon carbide particles in order to correlate them with the SiC

    particles behavior in the Al alloy melt taking into account the new proposed technique for

    Al-SiC composite obtaining; ;

    Determine by theoretical calculus both of settling velocity and acceleration parameters

    for different shape of silicon carbide particles both in Aluminum and Aluminum alloys.

    Obtaining in laboratory conditions of Al cast samples with Al-SiC composite skin by

    using the in mold addition technique. Three ways of SiC particles addition in Al alloy

    melt were used in this reason:

    a) SiC particles addition as bonded particles on textile support;

    b) SiC particles addition as SiC powder-polystyrene sandwiches;

    c) SiC particles addition as SiC powder- Al foils sandwiches.

    Characterization of superficial layer Al-SiC composites both from metallographic and

    mechanical point of view;

    Experimental measurements on the cooling curves parameters of Al castings with Al-SiC

    composite skin

    Taking into account the originality of the new technique of Al-SiC composite

    obtaining, some specific research is needed in the future in order to finish both the

    theoretical and technological aspects of this technique such as:

    a) Research on the mechanism of SiC particles inclusion into the Al alloy

    matrix;

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    33

    b) Study on the possibilities to improve the Al-SiC particles compatibility in

    the conditions of drastically decreasing of Al melt temperature during mold

    filling;

    c) Study on the correlation between the particles morphology and their behavior

    in the Al melt ;

    d) Study on the technological aspects of this technique implementation on

    industrial level.

    DISSEMINATION OF RESEARCH RESULTS

    Scientific papers published

    1. Muna NOORI, Hazim FALEH, Mihai CHISAMERA, Florin STEFANESCU and

    Gigel NEAGU, Wettability of Silicon Carbide Particles by the Aluminum Melts in

    Producing Al-SiC Composites, Advanced Materials Research, ISSN:1662-8985,

    VOL. 1128, pp. 149-155.

    2. Hazim FALEH, Muna NOORI and Florin STEFANESCU, Properties and

    Applications of Aluminum - Graphite Composites, Advanced Materials Research,

    ISSN: 1662-8985, VOL. 1128, pp.134-143.

    Scientific papers accepted to published

    1. Muna Noori, Hazim Faleh, Mihai Chisamera, Gigel Neagu, Florin Stefanescu and

    Eduard Marius Stefan, Properties of Al-SiCp composites (review), "Dunarea de Jos"

    University of Galati, International Conference on Material Science, 7th

    Edition,

    19th

    -21st May 2016. Paper published in advanced material research-ISI

    journal/proceeding.

    2. Hazim FALEH, Muna NOORI, Florin STEFANESCU, Gigel NEAGU and Eduard

    Marius STEFAN, Wettability in Aluminum-graphite particles composite (review),

    "Dunarea de Jos" University of Galati, International Conference on Material

    Science, 7th

    Edition, 19th

    -21st May 2016. Paper published in advanced material

    research-ISI journal/proceeding.

    3. Muna Noori, Hazim Faleh, Mihai Chisamera, Neagu, Stefanescu Florin, Stefan

    Eduard Marius, SOME ASPECTS CONCERNING A NEW POSSIBILITY OF Al-

    SiCP COMPOSITE PRODUCTION, U.P.B. Scientific Bulletin, Series B,

    Chemistry and Mterials Science, Vol. …., pp…….., ISSN 1454-2331.

    4. Hazim Faleh, Muna Noori, Florin Stefanescu, Mihai Chisamera, Gigel Neagu,

    EXPERIMENTAL RESEARCH CONCERNING A NEW METHOD TO

    PRODUCE ALUMINIUM ALLOY-GRAPHITE PARTICLE COMPOSITE IN

    SUPERFICIAL LAYERS, U.P.B. Scientific Bulletin, Series B, Chemistry and

    Mterials Science, Vol. …., pp…….., ISSN 1454-2331.

  • Research concerning the possibility to obtain Al alloy castings with Al-SiCp composite skin

    34

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