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University POLITEHNICA of Bucharest Faculty of Materials Science and Engineering Department:Processing of Metallic Materials and Ecometallurgy No. Senate Decision in . .2017 Summary Thesis Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts By Eng. Eng. HAZIM F. HASSAN AL-DULIAMI Supervised by Professor Dr. Eng. Florin ŞTEFĂNESCU DOCTORAL COMMITTEE Chairman Prof. Dr. Eng. Mihai BUZATU From University Politehnica of Bucharest Scientific Supervisor Prof. Dr. Eng. . Florin ŞTEFĂNESCU 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 Ecometallurgy

    No. Senate Decision in . .2017

    Summary

    Thesis

    Research on the possibility of obtaining Al-graphite

    (particles) composites in certain zones of parts

    By

    Eng. Eng. HAZIM F. HASSAN AL-DULIAMI

    Supervised by

    Professor Dr. Eng. Florin ŞTEFĂNESCU

    DOCTORAL COMMITTEE

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

    Scientific Supervisor Prof. Dr. Eng. . Florin ŞTEFĂNESCU 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 on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    2

    CONTENT

    Chapter I

    1.1. INTRODCTION 6

    1. 2. Composite materials 7

    1.2.1. Classification of composites 7

    1.2.2. Composites aluminium - graphite 10

    1.3. Wettability in the aluminium - graphite system 21

    1.3.1. Behaviour of graphite particles in the molten aluminium 21

    1.4. Casting methods to produce aluminium-graphite composite 36

    1.4.1. Gravitational casting 36

    1.4.2. Centrifugal casting 38

    1.4.3. Melt infiltration 39

    1.4.4. Liquid Metal Forging 39

    1. 5. Melting and deposition of components 40

    1.6. Solidification process 42

    1.7. Applications of aluminium-graphite composite 44

    1.7.1. Thermal properties for Al - graphite composites applications 45

    1.7.2. Friction and wear properties for Al-graphite applications 46

    Chapter II

    2. Objective of present investigation 49

    Chapter III

    3. Methods of practical research 51

    3.1. Materials 51

    3.1.1. Metal matrices 51

    3.1.2. Complementary materials 51

    3.2. Devices 52

    3.3. Gravity casting the aluminium - graphite particles composite 54

    3.3.1. Pure aluminium sieve - graphite powder method 55

    3.3.2. Layers of graphite powder fixed with pure aluminium foils prepared

    for gravity casting of Al alloy

    55

    3.4. Al alloy-graphite powder composite produced by electric arc 59

    3.4.1. Melting and deposition in electric arc of graphite powder inside

    channels in the surface of aluminium

    60

    3.4.2. The use of graphite powder inside surface made holes in aluminium

    alloy

    61

    3.4.3 Coating electrode by graphite powder layers 61

    3.5. Grinding and polishing processes 62

    3.6. Shape characterization 63

    3.7. Brinell hardness test 66

    3.8. Scanning electron microscope test 66

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    3

    Chapter IV

    4. Experimental data 68

    4.1. Floating velocity of graphite particles 68

    4.2. Analysis of graphite particles 74

    4.2.1. Particles size analysis 74

    4.2.2 Microscopy 74

    4.2.3. X - ray diffraction test 75

    4.3. Aluminium - pure aluminium sieve - graphite particles composite

    produced by gravity casting

    75

    4.4. Al alloy - layers of graphite powder - pure aluminium foil composite 81

    4.4.1. Optical micrograph of samples 82

    4.4.2. Thermal analysis of cast Al-graphite composites 86

    4.5. Melting and deposition in electric arc 99

    4. 5.1. Melting and deposition in electric arc of graphite inside holes made

    in aluminium

    99

    4.5. 2. Electric arc deposition by using an electrode coated with graphite

    powder

    105

    4.5.3. Electric arc deposition of aluminium alloy over graphite powder

    found inside a channel

    111

    4.6. Hardness of aluminium - graphite particles composite 111

    4.7. Surface layer thickness of graphite particle 114

    4.8. Scanning electron microscope test 115

    Chapter V

    5. Conclusions and personal contributions 120

    5.1. General conclusions 120

    5.2. Conclusions of experimental results 121

    5.3. Original contributions 125

    Dissemination of research results 126

    References 127

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    4

    General Introduction

    Composite materials are combinations of two or more materials aggregated at

    macroscopic level with the purpose of obtaining enhanced characteristics of the newly

    formed material.

    In the composition of metal matrix composites are included minimum two

    materials: the metal matrix and the reinforcement. The matrix is always a metal -

    generally an alloy is employed, and very rarely a pure metal.

    Ceramic is used as reinforcement and metal as matrix in typical MMCs, which

    results in suitable wear and structural resistance. However, due to thermal differences

    and volume increase under temperature factors, it is quite challenging to machine or

    weld these MMCs as increasing the temperature affects the properties of the material,

    which suffers from distortion and stress in these conditions. To prevent these effects,

    aluminium graphite (Al-Gr) MMCs used for new MMCs, generates composites that are

    light with high thermal conductivity, machinability and mechanical strength. Also, the

    thermal behaviour of these composites can be easily managed to obtain tailored

    characteristics based on requirements (thermal expansion) [15].

    An inhomogeneous distribution of the graphite particles in aluminium, unsuitable

    bonding between the aluminium and floating graphite particles in melt due to density

    difference between aluminium and graphite.

    In order to achieve a better penetration of the graphite particle into the

    aluminium melt, different measures can be taken to reduce the wetting angle.

    Moreover, wetting conditions improvement permits to include a bigger proportion of

    solid component into the matrix. Many techniques are used for this purpose.

    Melting and deposition process can be described as the melting of one

    component and deposition of the other component on it. This process can be achieved

    by melting one part and deposition of the second part together under pressure, perhaps

    with the application of heat, to form a composite material.

    Gravitational casting is the simplest method to achieve fabric from composite

    materials and consists of the introduction of metal and particles into a mould: sand

    mould or permanent mould.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    5

    Chapter 1

    Metal Matrix Composites (MMC):

    In the composite of metal matrix composites are included minimum two materials:

    the metal matrix and the reinforcement. The matrix is always a metal - generally an alloy

    is employed, and very rarely a pure metal. The composite is produced by incorporating

    the reinforcement into the matrix. In terms of the matrix composition, metal matrix

    composites are preferred to organic matrix composites due to their improved

    characteristics: they maintain their strength when the temperature increases, their

    transverse strength is superior, they have higher electrical and thermal conductivity and

    they resist erosion, etc. Metal matrix composites have higher densities when compared

    to polymer matrix composites, which represents a considerable drawback because their

    mechanical properties are also reduced. Another drawback is that for producing these

    matrix composites, high amounts of energy are needed [21].

    Metal matrix composite consists of a matrix metal (aluminium, iron, magnesium,

    and cobalt, copper) while the reinforcing material may be silicon carbide, boron carbide,

    graphite or alumina. Compared to the engineering metals, these composites offer high

    stiffness, high strength, and good electrical and thermal conductivity.

    1.2.2. Composites aluminium- graphite.

    Aluminium alloys reinforced by graphite particles have received considerable

    attention due to potential engineering materials for a variety of anti-friction

    applications [28]. In these composites, graphite is significant to improve tribological

    properties because graphite-rich film is formed between sliding surfaces [29].

    Aluminium is the most popular matrix for the MMCs. Pure aluminium is relatively

    weak material. Therefore, for some applications a higher mechanical strength is

    required.

    1.7. Applications of aluminium-graphite composite

    The addition of graphite particles to the aluminium alloy matrices makes them

    attractive candidate materials in many tribological applications due to the fact that they

    act as solid lubricants and make a self-lubricating alloy. Initially, cast Al/Grp composites

    were developed through liquid metallurgy route for automotive antifriction applications.

    The essential advantages of this material were low cost, easy machinability and

    improved damping capacity which are essential for automotive applications, pistons,

    liners and bushings. The pistons of Al/Grp composites tested in diesel engines led to

    reduced wear, loss frictional in pistons and rings, freedom from seizure under lubrication

    conditions. Using Al-graphite composite caused decreasing in fuel consumption. Similar

    results have been obtained by using these pistons in gas-line engines [58,109].

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    6

    Chapter II

    2. Objectives of present investigation

    This study aims to overcome the floatation velocity of graphite particles (GrP) and

    to obtain improvements of the wettability between the graphite particles and aluminium

    melt.

    New methods of gravity casting and electrical arc process were used in this study

    to prepare the Al alloy-graphite particles composite without using the conventional

    methods to improve the wettability.

    Two methods of gravity casting were used for preparing Al-graphite particles

    composite in the first part of the experimental study:

    Table 1.14. Application of Al-graphite composite in power electronics

    Applications Properties Parts made from Al-graphite

    Base plates and coolers

    High thermal conductivity

    and low density

    Heat sinks and heat

    spreaders

    Low CTE (coefficient of

    thermal expansion) and a

    low density

    Discs and rings

    Low friction

    Electronic packaging

    applications

    Low density and high

    thermal conductivity

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    7

    The first method:

    Pure aluminium sieve was used in a new method for distribution of the graphite

    particles - casting the melt aluminium over the sieve, which coats the inner

    surface of mould.

    Microstructure and optical micrograph analysis were performed to measure the

    particles density and shapes factor of specimens.

    The second method:

    Samples of different Al alloy – layers of graphite particles and foil aluminium

    were produced by clay thermal mould casting. The effect of the alloy

    composition was studied.

    Microstructure and optical micrograph analysis were performed to measure the

    particles density and shapes factor of these samples

    The cooling curve – the graphite powder percentage is influenced by the time

    and temperature during the solidification process.

    The second part of the experimental study included producing Al alloy-

    graphite particle composite by superficial layering using electric arc to melt the

    aluminium. Three methods were used in this part:

    a)

    Deposition Al alloy - graphite particles in square channel; different particle sizes

    of graphite were used.

    Analysis of the optical micrograph and investigation of the particle density and

    shapes factor for the sample.

    b)

    Study the effect of electrodes type on Al alloy-graphite particle introduction in

    the hole with fixed amount of graphite particles.

    Different intensities (ampere) are used in this method in addition to different

    electrodes: pure Al (2.5 mm diameter), Al-5Si (2.5 mm diameter), Al-12Si (2.5

    mm diameter) and Al-12Si (4.2 mm diameter).

    Distribution and incorporation of the graphite particles are studied by

    microstructure graphs; the analysis of these graphs aims to determine the

    particles density and shapes factors.

    c)

    Preparation of the Al alloy-graphite particle composite by mixing with an electric

    arc. The graphite powder covered the electrode in different amounts.

    Using two types of electrodes: pure Al and Al-12Si (diameter 2.5 mm).

    Study all parameters (particles density, mean sphericity, mean convexity, mean

    aspect ratio, mean perimeter).

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    8

    All specimens of Al alloy- graphite composite were analysed by optical microscope to

    determine, microstructure, particle density, mean sphericity, mean aspect ratio, mean

    convexity, and mean perimeter.

    Chapter III

    3.3. Gravity casting the aluminium-graphite particles composite

    One of the most important methods for producing Al-graphite particles in this

    work is gravity casting by using many ways in order to get the best results, especially

    the mixture between the aluminium and the particles of graphite. The flow chart in

    Figure 3.5 is showing these ways.

    3.3.1. Pure aluminium sieve - graphite powder method

    Prepare the clay thermal mould.

    Coat the inner surface of the clay thermal mould by pure aluminium sieve

    which has the thickness of 2 mm

    Distribute the graphite powder (2, 3, 4 vol. %) at the sieve by using adhesive

    material to prepare different samples as shown in Figure 3.6.

    Fig. 3.5. Flowchart of gravity

    casting.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    9

    Melt the aluminium in crucible at 900 °C by gas furnace and cool it down to

    850°C.

    Pour the melt in sand mould and leave it to solidify.

    Cut and polish all the samples in order to prepare them for examination by

    microscope and to analyse them.

    3.3.2. Layers of (graphite powder – pure aluminium foils) –Al alloy composite

    prepared for gravity casting

    Prepare the package containing layers of Al foil - Grp – Al foil- Grp- Al foil

    respectively by cutting aluminium foil into pieces of 0.28 g weight and the area

    of each piece equal to the base area of the sand cup. One face of the foil has the

    adhesive material.

    Fix the thermocouple on the sand cup wall and centre, Figure 3.7.

    Arrange the layers respectively by using 0.25 wt. %, 0.5 wt. %, 0.75 wt. % of

    graphite and fix the layers on the base of cup by mechanical technique.

    Make some holes in the package of one sample with 0.75 g graphite powder,

    using the same procedure for other sample which is prepared in the previous

    steps. Three types of matrices were used: Al (aluminium), Al- 12Si, and Al-

    12Si – 1.5 Mg and different weights of graphite powder, available in Tables

    3.3, 3.4, 3.5.

    Fig. 3.6. Clay thermal mould and its section coated by

    sieve and graphite powder.

    Fig. 3.7. Clay thermal cup.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    10

    Melt the aluminium alloys in the furnace gas at 937°C for aluminium and 900-

    °C for Al-Si12 and Al-Si12-Mg alloys. Pour it over the layers which have been

    prepared inside the cups.

    Prepare three thermal analysis samples for each alloy and the temperature read

    by the thermocouple.

    Log the data on a computer.

    Analyse the data by drawing the cooling rate curve and the first derivative curve.

    Cut all the samples in centre for structural.

    Grind and polish the samples.

    Perform metallographic studies on all samples.

    Table 3.5. Weight and content of Al–12 Si–1.5 Mg – graphite materials in the clay thermal cup

    Table 3.3. Weight and content of aluminium – graphite materials in the clay thermal cup

    Weight of aluminium Temperature Cup no.1 Cup no.2 Cup no.3 Cup no.4

    4 kg 937⁰C 0.25 wt. %

    Gr +

    Al alloy

    0. 5 wt. %

    Gr +

    Al alloy

    0.75 wt. %

    Gr +

    Al alloy

    aluminium

    Table 3.4. Weight and content of Al -12Si – graphite material in the clay thermal cup

    Weight of AlSi12

    alloy Temperature Cup no.1 Cup no.2 Cup no.3 Cup no.4

    3.8 kg 900⁰C 0.25 wt. %

    Gr + Al -

    12Si alloy

    0.5 wt. %

    Gr + Al -

    12 Si alloy

    0.7 wt. %

    G r+ Al -

    12Si alloy

    Al -

    12Si alloy

    Weight of

    Al12Si -

    0.25 Mg

    Alloy

    Temperature Cup no.1 Cup no.2 Cup no.3 Cup no.4 Cup no.5

    3.8 kg 900⁰C 0.25 wt. %

    r+Al-12Si

    1.5 Mg alloy

    0.5 wt. %

    Gr+Al-12Si

    1.5 Mg alloy

    0.7 wt. %

    Gr+Al-12Si

    1.5 Mg alloy

    0.7 wt. %

    Gr+Al-12Si

    1.5 Mg alloy

    package

    with hole

    Al-12Si

    1.5 Mg alloy

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    11

    In Tables 3.3, 3.4, and 3.5 are presented the clay thermal cups which contain the

    samples of Al alloy-graphite powder-Al foil package. Each cup has a different material

    different than the other cup in terms of the graphite powder percentage and aluminium

    alloy type.

    In the third stage Al-12Si-1.5Mg alloy matrices are used, one of the prepared

    samples having some holes on the package. This sample has 0.75 wt. % graphite

    powder in two layers and three layers of the Al foils.

    3.4. Al alloy-graphite powder composite produced by electric arc

    Three methods are used for mixing aluminium alloy and graphite powder by

    electric arc process in this thesis. Flowchart 3.9 is showing these methods.

    3.4.1. Melting and deposition in electric arc of graphite powder inside

    channels in the surface of aluminium

    Cut the aluminium into pieces of 100 x 50 x 20 mm.

    Make two canals in each piece, having dimensions 100 x 9 x 5 mm, as in Figure

    3.10.

    Fig. 3.9. Flowchart of melting and mixing Al alloy and graphite powder by

    electric arc.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    12

    Weigh 2 g of graphite powder for three particle size 150, 212, 500 µm.

    Insert the powder into the canals.

    Use different conditions for electric arc process - two channels 20 V, 141 A and

    21.5 V and 202 A.

    Clean the samples to remove the layer of oxides and contaminants.

    Section the samples to prepare the test specimens by cutting the sample from

    centre along the piece.

    Cut 10 mm thick samples.

    Grind and polish the test specimens.

    3.4.2. The use of graphite powder inside surface made holes in aluminium

    alloy

    Sectioning the aluminium alloy bar which is mention in Table 3.2 in pieces of

    60 x 30 x 10 mm.

    Make a number of Ø=1mm diameter holes, 5 mm deep, arranging the holes as

    shown in Figure 3.13.

    Insert 0.1 g graphite inside these holes.

    Cover the samples by aluminium foil.

    Electric arc deposition of the graphite powder and melt aluminium alloy by

    using different electrodes: pure Al, Al-5Si and Al-12Si of 2.5 mm diameter,

    and Al-12Si electrode of 4.2 mm diameter.

    Fig. 3.10. Sketch of preparing sample to electric arc

    process. process.

    Fig. 3.13. Sketch of holes given in aluminium samples.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    13

    Perform the electric arc deposition process in two steps: high intensity of 70A,

    for Ø=2.5 mm, 100 A for Ø=4.2 mm, lower 55 A Ø=2.5 mm, and 90 A for

    Ø=4.2 mm.

    Clean all the samples and cut them into pieces of 30 x 10 x 10 mm.

    Grind and polish.

    3.4.3 Coating electrode by graphite powder layers

    Cut the aluminium alloy into 60 x 30 x 20 mm pieces.

    Use two types of electrodes: pure aluminium and Al-12Si, having 2.5 mm in

    diameter.

    Cover the electrodes used in the deposition process by graphite powder in

    different percentage weights 1.5, 3, and 4.5 in two layers.

    Coating process by adhesive aluminium foil weighing 0.4 g.

    Cover the electrode in copper wire, Figure 3.14.

    Use 60 amperes electric current for the deposition process.

    Clean the samples after completing the electric arc process.

    Cut the specimens to prepare 30 x 20 x 10 mm test samples.

    Grind and polish all the test samples.

    Chapter IV

    4. Experimental data

    4.3. Aluminium -pure aluminum sieve – graphite particles composite produced by

    gravity casting

    The graphite particles have the tendency to segregate because the graphite has a

    lower density and it is not wetted by the metallic melt. During the solidification of Al-

    Fig. 3.14. Electric arc process for Al alloy by electrode covered by graphite powder

    a- covered electrode; b- cross section of covered electrode.

    a

    b

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    14

    graphite composites, the graphite particles are pushed by the primary dendrites to the

    eutectic liquid, which solidifies the last. Graphite particles are not entrapped between

    the dendritic branches, because the distance between these branches is small (of about

    20 μm). Usually, graphite particles delay the solidification process due to the low

    thermal diffusivity. This fact determines the segregation and floating of particles [125].

    To reduce the floating of graphite particles, quick cooling is necessary.

    The microstructure of aluminum melt: Grp distributed on pure Al sieve samples

    in Figures 4.10-4.12 are presented.

    Fig. 4.10. Optical micrograph of composite aluminium- 2vol. % graphite particles on

    pure Al sieve [100X].

    Few particles of graphite appear in Figure 4.10.The low density of Grp compared

    to Al melt and the low of volume percentage led to this result.

    Fig. 4.11. Optical micrograph of composite aluminium- 3vol. % graphite particles on

    pure Al sieve [100X].

    The increasing in volume percentage of Grp to 3% leads to the formation of some

    Grp agglomerations. The distribution of these agglomerations depends on the site, as

    towards the top the agglomerations begin to increase.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    15

    0

    2

    4

    6

    8

    2 3

    4

    Par

    ticl

    e d

    en

    dit

    y ·

    10

    (n

    0.

    par

    ticl

    es/

    mm

    2)

    Vol. % graphite

    0.78

    0.8

    0.82

    0.84

    0.86

    0.88

    0.9

    2 3 4

    Me

    an c

    on

    vexi

    ty

    vol. % graphite

    Fig. 4.12 Optical micrograph of composite aluminium- 4vol. % graphite particles on pure Al sieve [100X].

    Fig. 4.13. Particle density in aluminuim, by using pure Al sieve with graphite.

    Particle density increased with increasing of the volume percentage of graphite

    over sieve. Many clusters and agglomerations occurred due to poor adhesion between

    the graphite and the pure aluminium sieve. The particle density increased with

    approximately 27 % between 2 and 4 vol. %. The increasing of the volume percentage

    of graphite led to increasing the graphite particles density into the matrix. Therefore,

    the particle density of graphite particles in the aluminium matrix becomes significantly

    higher.

    Fig. 4.14. Mean convexity of graphite particles in Al composite.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    16

    0.31

    0.315

    0.32

    0.325

    2 3 4

    Me

    an s

    ph

    eri

    city

    Vol. % graphite

    0

    0.5

    1

    1.5

    2

    2.5

    3

    2 3 4

    Me

    an a

    spe

    ct r

    atio

    Vol. % graphite

    Mean sphericity of Grp describes the roundness by using the central moment,

    while mean convexity measures the object area and the area of its convex hull (see

    Table 3.7). The maximum values of sphericity and the convexity are one.

    Fig. 4.15. Mean sphericity of graphite particles in Al composite.

    Fig. 4.16. Mean aspect ratio (ratio between maximum feret and minimum feret) of

    graphite particles in Al composite.

    The mean aspect ratio decreases when increasing the volume percentage of

    graphite particles. The percentage of decreasing in mean aspect ratio of graphite

    particles is 18 % between 2 vol. % and 4 vol. % Grp. When the shape of particles

    becomes sphere, the mean aspect ratio decreases, because the maximum ferret becomes

    nearly equal to minimum ferret.

    4.4. Al alloy-layers of graphite powder – pure aluminium foil composite

    The results of the computer simulation indicated that the procedures and

    techniques included in this program can be useful to study the course of the

    solidification process for different matrix and amount of graphite particles used as

    complementary material. The analysis of the results shows that the way of the

    composite solidification is closely related to the assumed thermal boundary conditions

    and physical parameters of the materials. Slow cooling of the cast promotes the

    segregation of the reinforcement particles during this process. The movement of the

    graphite particles is the result of balancing of forces, in which the density of graphite

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    17

    particles and matrix are taken into account. This proves a significant influence of

    graphite particles on the solidification process course and thereby, the disparate nature

    of the heterophase composite crystallization.

    From Figures 4.19, 4.20, and 4.21 some observations can be recorded about Al-

    Grp- pure Al foils:

    Optical micrographs revealed that a small amount of fine particles has a quite

    uniform distribution in Al-0.25 wt. % Grp-pure Al foils and Al-0.5 wt. % Grp-

    pure Al foils composites because the wettability between the graphite and

    aluminium melt is not good.

    Some large Gr particles and, in addition, very fine particle are appeared when the

    weight percentage of graphite was increased to 0.75. The fix package on the base of

    mould gives more time for this quantity to be incorporated in melt with some cluster

    of graphite particles.

    The aluminium foils layers are pressed on the graphite particles layers to

    grants allow more time for incorporation and to block the particles from moving

    upwards.

    The overheating of melt to 937 ⁰C improves the wetting between the graphite

    particles and aluminium melt.

    Fig.4.20. Optical micrograph of

    aluminum- 0.5 wt. % graphite – pure Al

    foils [50X].

    Fig. 4.19. Optical micrograph of

    aluminium- 0.25 wt. % graphite – pure Al

    foils [50X].

    Fig.4.21. Optical micrograph of

    aluminium - 0.75 wt. % graphite-pure Al

    foils [50X].

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    18

    The amount of Mg addition to aluminium melt enabled the uniform distribution

    of the graphite particles in the composites in limit amount 0.4-0.6 wt. %. If the Mg

    exceeded 1 wt. % caused agglomeration was [129]. Figures 4.25-4.27 represent

    microstructures of graphite particles in Al-Si12-Mg alloy composite.

    Fig. 4.22. Optical micrograph of Al - Si12 - 0.25 wt. % graphite – pure Al foils [50X].

    Fig. 4.23. Optical micrograph of Al - Si12 -

    0.5 wt. % graphite – pure Al foils [50X].

    Fig. 4.24. Optical micrograph of Al - Si12 - 0.75 wt. %

    graphite - pure Al foils [50X].

    Fig. 4.25. Optical micrograph of Al-Si12- Mg- 0.25 wt. % graphite – pure Al foils [50X].

    Fig. 4.26. Optical micrograph of Al-Si12-Mg -

    0.5 wt. % graphite – pure Al foils [50X].

    Fig. 4.27. Optical micrograph of Al - Si12-Mg -

    0.75 wt. % graphite – pure Al foils [50X].

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    19

    Analyzing the images presented in Figures 4.25, 4.26, and 4.27 some information

    could be obtained.

    - The images reveal the incorporation of graphite particles into the aluminium

    matrix, indicating that Mg is very effective in wettability improvement of

    graphite particles by molten aluminium.

    - Agglomerations took place in some zones. Also, porosity could be found in

    some places.

    - Residual porosity diminishes with decreasing particle size and with magnesium

    content; therefore, the increasing of graphite percentage determines more range

    of particle size incorporate in melt.

    - A weak bonding between graphite particles and aluminium, means the increase

    of graphite particles percentage incorporated into the melt.

    - By not using mechanical mixing, other ways to ensure interference between Grp

    and the molten matrix are needed.

    - The alloying elements such as Mg and Si are very important to increase the

    interface bond between Grp and aluminium melt.

    4.4.2. Thermal analysis and cooling rate

    Automatic analyses of the cooling rate for the samples were performed. Figures

    4.28-4.30 present the cooling rate of three samples, each Figure representing the

    following samples included in the experimental program; Al-0.25 wt. % Grp, Al-Si12-

    0.5 wt. % Grp, Al-Si12-Mg-0.75wt. % Grp.

    Fig. 4.28. Cooling curve for Al-0.25 Grp-Al foil composite.

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    20

    Fig. 4.29. Cooling curve for Al-Si12-0.5 Grp-Al foil composite.

    Fig. 4.30. Cooling curve for Al-Si12-Mg-0.75 Grp-Al foil composite.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    21

    540

    570

    600

    630

    660

    690

    720

    0 0.25 0.5 0.75

    T nu

    c. A

    l

    Wt. % graphite powder

    AL

    Al-Si12

    Al-Si12-1.5Mg

    520

    540

    560

    580

    600

    620

    640

    660

    0 0.25 0.5 0.75

    T n

    ucl

    . EU

    T.,

    ᵒC

    Wt. % graphite powder

    Al

    Al-Si12

    Al-Si12-1.5Mg

    Fig. 4.32. Dependence between the wt. % of Grp and

    T nuc. Al (the nucleation aluminium temperature).

    It can be noticed that nucleation which develops the eqaxed grain formation is

    well promoted as graphite particles are introduced into the melt.

    The graphite particles are more likely to be rejected by the solidification front due

    to the low relative velocity between the solid-liquid interface and the particles. This

    velocity is less than critical compared to velocity for pushing. Rejection occurred rather

    than immersion of particles due to the interaction of solid-liquid interface in the low

    velocity. The alteration in nucleation and growth mechanism of the micro constituent

    associated with the presence of graphite particles is very hard, therefore the quick

    solidification of melt reduces the particles rejection [130].

    Fig. 4.35. Dependence between the wt. % of Grp and Tnucl EUT

    (the nucleation at eutectic point temperature) of composite.

    The nucleation and growth is beginning opposite to the thermal gradient. The

    addition of alloying elements and graphite into aluminium changes the solidification

    process, this is shown in Figure 4.35.

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    22

    0

    100

    200

    300

    400

    500

    600

    700

    0 0.25 0.5 0.75

    T ES,

    C

    Wt. % graphite powder

    Al

    Al-Si12

    Al-Si12-1.5Mg

    0123456789

    0.25 0.5 0.75

    Par

    ticl

    es

    de

    nsi

    ty (

    no

    . p

    arti

    cle

    s/m

    m2 )

    Wt. % Grp

    AluminiumAl-Si12Al-Si12-Mg

    Fig. 4.38. Dependence between the wt. % of Grp and TES (the end solidification temperature) of composite.

    A change in the melt composition has effects on the fluidity and the reactivity of

    the melts. The eutectic and primary silicon is agglomerated on the boundary of the

    graphite particles. The large quantity of eutectic silicon caused the rejection

    phenomenon of graphite at the interface of the eutectic-primary dendrites of

    aluminium, where the graphite delays the solidification process, due to its low thermal

    diffusivity compared to the aluminium. However, the end solidification temperature is

    not significantly affected (Figure 4.38).

    Particles density and shape factors

    The particles densities of samples are shown in Figure 4.49 and were calculated

    by optical graphs

    Fig. 4.42. Particles density of Grp in different Al alloys.

    One of the important factors which have effect on the particle behavior in Al

    alloy is the alloying elements which could improve the incorporation of Grp in the

    metallic bath. The value of particle density of Grp was increased with approximately

    12.7 % in Al-Si12-Mg, 4.5 % in Al-Si12 and 7.2 % in Al and Grp was increased from

    0.25 wt. % to 0.75 wt. %. The particle density in Al-Si12-1.5Mg alloy is higher than in

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    23

    0

    0.05

    0.1

    0.15

    0.2

    0.25

    0.3

    0.35

    0.4

    0.25 0.5 0.75

    Me

    an s

    ph

    eri

    city

    Wt. % Grp

    AluminiumAl-Si12Al-Si12-Mg

    other alloys due to the effect Mg on the improvement of wetting conditions between

    the Grp and the melt. (Addition of 3% of Mg to Al melt reduces its surface tension 0.62

    Nm-1

    at 993 Ko, a reduction of about 30% from its original value).

    Fig. 4.43. Mean sphericity of graphite particles in composite.

    The particle shape has a significant influence on the flotation effectiveness. Mean

    sphericity (see Table 3.8) was has increased by increasing graphite percentage

    especially for Grp in Al-Si12-Mg. The characteristic brittle of graphite boundary made

    it more exposed to the erosion property. The chemical reaction between the Grp and the

    melt could form some compounds on the boundary of the particles. These compounds

    can alter the behavior of the particles such as mean sphericity.

    4.5. Melting and deposition in electric arc

    4. 5. 1. Melting and deposition in electric arc of graphite inside holes made in

    aluminium

    Al alloy- graphite particles composite samples were prepared using different

    electrodes in composition and diameter by melting and deposition processes. Figure

    4.54 presents these final samples.

    Fig. 4.54. Al alloy-Grp composites obtained by using different electrodes.

    The composition of the electrodes was very important for incorporation the

    graphite particles. In addition, the size of electrode (cross-section) in contact with the

    material to be deposed defines the current density flowing through the electrode. In

    order to focus the arc energy at the interface, electrode sections have to be selected

    with a proper ratio. When it was applied the electric arc on the Al alloy having holes

    filled with Grp, both the rod and the melted Al alloy form a pool.

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    24

    Figure 4.48 shows that the distribution of graphite particles deposit on Al-alloy

    (by using Al-Si12 electrode with a diameter of 4.2 mm) is more regular and has low

    agglomeration tendency. When increasing Si in the electrode it resulted a high fluidity

    and more chance to retain the particles. The increasing in diameter led to increasing in

    heat and the matrix was melted in short time. In this case the distribution was

    improved.

    Fig. 4.48. Optical micrographs of Al alloy- graphite composite by low intensity of electric arc

    [50X]: A- Pure Al electrode in diameter of 2.5 mm; B- Al-Si5 electrode in diameter of 2.5 m;

    C-Al-Si12 electrode in diameter of 2.5 mm; D- Al-Si12 electrode in diameter of 4.2 mm.

    Fig. 4.49. Optical micrographs of A l alloy- graphite composite by high intensity of electric arc

    [50X]: A-Pure Al electrode in diameter of 2.5 mm; B- Al-Si5 electrode in diameter of 2.5 mm;

    C-Al-Si12 electrode in diameter of 2.5 mm; D- Al-Si12 electrode in diameter of 4.2 mm.

    In the case of high intensity, because that causes the diminution of melt viscosity,

    a negative effect on the distribution of graphite particles could appear. When the

    melting and depositing are made using Al-Si12 electrode of 4.2 mm diameter enough

    heat results and the incorporation of Grp in the molten aluminuim is improved.

    A

    C

    B

    D

    A B

    D C

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    0

    2

    4

    6

    8

    10

    12

    14

    16

    a b c d

    Par

    tica

    les

    de

    nsi

    ty ·

    no

    . par

    tica

    les/

    mm

    2

    Electrodes type

    Lowintensity

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    a b c d

    Me

    an s

    ph

    eri

    city

    Electrodes type

    Low intensity

    High intensty

    Fig. 4. 50. Particle density of particles of Al alloy with Grp in the holes. a-Pure Al

    electrode with diameter =2.5 mm; b- Al-Si5 electrode with diameter =2.5 mm; c-Al-Si12

    electrode with diameter =2.5 mm; d-Al-Si12 electrode with diameter =4.2 mm.

    In Fig. 4.50, it results that the particle densities of samples prepared by using

    high intensity is higher than for the samples prepared by using low intensity process.

    This refers to the interaction between the graphite particles and the molten aluminium.

    The electrode type Al-Si12 in diameter of 4.2 mm assures the incorporation of graphite

    particles in the melt of Al alloy more than other types of electrodes because of the

    large amount of heat. The particle densities for samples prepared by using this

    electrode are 2.283 and 1.856 no. particles /mm2 in the case of high intensity and low

    intensity respectively. The particle densities for samples prepared by using pure Al

    electrode are approximately. 1.81 and 1.737 no. particles / mm2. In this method the

    heat generation from electric arc is an important parameter which depends on cross

    section of electrode, voltage, and current. Below 1000°C, oxide layer have formed and

    the reactivity between Al and C was weak. Therefore the carbon was hardly

    incorporated into aluminium. The graphite particles will float to the melt surface in

    accordance with the formula: 0, ∑ 0.

    Fig. 4.51. Mean sphericity of Grp in Al alloy deposition by different electrodes a-pure Al

    electrode of diameter = 2.5 mm; b- Al-Si5 electrode of diameter = 2.5 mm; c-Al-Si12 electrode

    of diameter = 2.5 mm; d-Al-Si12 electrode of diameter=4.2 mm.

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    0

    0.1

    0.2

    0.3

    0.4

    0.5

    0.6

    0.7

    0.8

    0.9

    a b c d

    Me

    an c

    on

    vexi

    ty

    Electrodes type

    Low intensity

    High intensty

    Mean sphericity of graphite particles incorporated in Al by using pure Al

    electrode and Al-Si5 in the case of low intensity are higher than particles incorporation

    at high intensity of electric arc. The partial melting of Al occurred by this type of

    electrode. But when the other type of electrode of Al-Si12 in diameter of 2.5 mm is

    used, high melt for Al occurred, due to the increase in Si percentage at high intensity.

    This improved the incorporation and increased the sphericity of incorporated particles.

    The negative effect on mean sphericity appeared in high intensity of deposition by Al-

    Si12 with diameter 4.2 mm. The amount of heat generated by Al-Si12 of 4.2 mm

    diameter can form more flux on the boundary of particles and reduce the sphericity of

    these particles.

    Fig. 4.52. Mean convexity of Grp in Al alloy deposition by using different electrodes. a- pure Al electrode in diameter = 2.5 mm; b- Al-Si5 electrode of diameter = 2.5 mm; c-Al-Si12

    electrode of diameter = 2.5 mm; d-Al-Si12 electrode of diameter = 4.2 mm.

    In Figure 4.52, the type of electrode has no effect on the mean convexity

    especially at low intensity of electric arc process. At high intensity of deposition, the

    mean convexity was different by using different electrodes, especially in electrode Al-

    Si12 in diameter of 4.2 mm. That means that Si content and the diameter of electrode

    could be important technological parameter in an electric arc deposition.

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    27

    1.83

    1.835

    1.84

    1.845

    1.85

    1.855

    1.86

    1.865

    1.87

    a b c d

    Me

    an a

    spe

    ct r

    atio

    Electrode type

    Low intensity

    High intensty

    0

    20

    40

    60

    80

    100

    120

    140

    a b c d

    Me

    an p

    eri

    me

    ter

    Electrode type

    Low intensity

    High intensty

    Fig.4.53. Mean aspect ratio of Grp in Al alloy deposition by using different electrodes. a- pure

    Al electrode of diameter = 2.5 mm; b- Al-Si5 electrode of diameter = 2.5 mm; c-Al-Si12

    electrode of diameter = 2.5 mm; d-Al-Si12 electrode in diameter = 4.2 mm.

    The mean aspect ratio of graphite particles (see Table 3.8) were was a little

    affected by the electric arc intensity in all types of electrodes. Only for Al-Si12

    electrode with diameter of 4.2 mm the effect was higher, because the large amount of

    generated heat was high for an electrode of big diameter. Also, in this case a lot brittle

    chemical compounds could be formed.

    Fig. 4.54. Mean perimeter of Grp in Al alloy deposition by different electrodes: a-pure Al

    electrode of diameter = 2.5 mm; b- Al-Si5 electrode of diameter =2.5 mm; c-Al-Si12 electrode

    of diameter = 2.5 mm; d-Al-Si12 electrode of diameter = 4.2 mm.

    Each pixel distance of particle graphite was classified according to both the size

    of the particle surrounded as well as the distance between adjacent particles. From

    Figure 4.54 it is obvious the decreasing in mean perimeter with increasing the

    percentage of Si in the electrode and the diameter.

    At high intensity of process the mean perimeter is less than in low intensity,

    because the number of small individual particles in high intensities is higher than in

    low intensity. The sum of pixel distances in the case of small individual particles was

    lower than in case of big particles.

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    4.5. 2. Electric arc deposition by using an electrode coated with graphite

    powder

    Optical micrographs of the cross-section of composites processed by different

    techniques are shown in Fig. 4. 56 - 4.61.

    From Fig. 4.56 it can be seen that the graphite particles are relatively large at 1.5

    wt. % addition. Higher amounts of graphite particles were incorporated and distributed

    in the melt of aluminium alloy when the percent of graphite particles covering the

    electrode is increased. The amount of graphite layers and copper wire with respect to

    the electrode type can also contribute to the distribution of particles. The amount of Grp

    which is incorporated in the aluminium melt is increasing with the increase of Grp

    percent on the electrode to 3 wt. %. Fine particles of Grp in uniform distribution can be

    seen in Figure 4.57.

    Fig. 4. 58. Optical micrograph of 4.5 wt. % graphite –Al alloy composite deposition by

    using an Al-Si12 electrode [50X].

    Fig. 4. 56. Optical micrograph of 1.5 wt. % graphite –

    Al alloy composite deposition by using an Al-Si12

    electrode [50X].

    Fig. 4. 57. Optical micrograph of 3 wt. % graphite –

    Al alloy composite deposition by using an Al-Si12

    electrode [50X].

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    Figure 4.58 shows higher amounts of graphite particles incorporated and

    distributed in the matrix. Alloying elements such as Si, Cu, and Mg affected the

    incorporation of Grp into the melt. The copper wire which was added over the electrode

    is an important factor for the improvement of wetting conditions. So, copper coating of

    Grp can enhance greatly the wettability and forms a good interface bonding between

    Grp and aluminium. It was revealed that when Cu appears in the matrix, it forms a

    liquid which flows into porous areas and helps in reducing the porosity.

    Small amount of Grp is deposited in aluminium by electric arc when using 1.5 wt.

    % Grp and covering pure Al electrode due to the absence of alloying elements.

    These alloying elements can reduce the melt temperature of the matrix and melt the

    matrix in short time.

    The distribution of graphite particles is improved in Al-Si12 electrode covered

    with graphite particles. The increasing of Grp percent which is covering the electrode

    appears to be increasing the particles density. This is illustrated in Figure 4.62.

    Fig. 4. 59. Optical micrograph of 1.5 wt. %

    graphite –Al alloy composite deposition by

    using a pure Al electrode [50X].

    Fig. 4. 60. Optical micrograph of 3 wt. % graphite –Al alloy composite deposition

    by using a pure Al electrode [50X].

    Fig. 4. 61. Optical micrograph of 4.5 wt. % graphite –Al alloy composite deposition by using a pure Al electrode [50X].

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    30

    0

    2

    4

    6

    8

    10

    12

    14

    16

    1.5 3 4.5

    Par

    ticl

    e d

    en

    sity

    (n

    o.

    par

    ticl

    e/m

    m2)

    Wt. % Grp

    Al-Si12 electrode

    Pure Al electrode

    0.74

    0.76

    0.78

    0.8

    0.82

    0.84

    1.5 3 4.5

    Me

    an c

    on

    vexi

    ty

    Wt. % Grp

    Al-Si12 electrode

    Pure Al electrode

    Usually, silicon forms a metastable phase with graphite or it is adsorbed on the

    surface of the graphite particles. In samples obtained by using an Al-Si12 electrode the

    particles density appears higher than in samples obtained by using a pure Al electrode

    of approximately 17%, 13.6%, and 10.6% at 1.5, 3, and 4.5 wt. % Grp respectively.

    Figures 4.63-4.66 show the effect of graphite percent on mean convexity, mean

    sphericity, mean aspect ratio, and mean perimeter of graphite particles respectively.

    Fig.4.63. Relationship between the Grp percent and mean convexity of Grp.

    The type of electrode does not affect very much the mean convexity. This

    geometrical parameter is increasing with approximately 8 % when the graphite content

    varies from 1.5 wt. % to 4.5 wt. %.

    Fig. 4.62. Particle density of Grp deposition with Al alloy.

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    31

    0

    0.1

    0.2

    0.3

    0.4

    0.5

    1.5 3 4.5

    Me

    an s

    ph

    eri

    city

    Wt. % Grp

    Al-Si12 electrode

    Pure Al electrode

    1.72

    1.74

    1.76

    1.78

    1.8

    1.82

    1.841.86

    1.88

    1.9

    1.5 3 4.5

    Me

    an a

    spe

    ct r

    atio

    n

    Wt. % Grp

    Al-Si12 electrode

    Pure Al electrode

    Fig.4.64. Dependence between the Grp percent and mean sphericity of Grp.

    The mean sphericity increased with approximately 23 % when graphite varied

    from 1.5 wt. % to 4.5 wt. for the sample prepared by using a pure Al electrode and

    with approximately 19 % when it was used the Al-Si12 electrode. The brittle

    characteristic of graphite particles contributed to turn their shape to sphere because of

    the friction between particles.

    Fig.4.65. Relationship between the Grp percent and the mean aspect ratio of Grp.

    During the development of the electric arc process, fluidity of molten aluminium

    is increasing. This causes low porosity and the decrease of the aspect ratio parameter.

    For samples obtained by using Al electrode the mean aspect ratio decreased with

    approximately 3.8 %, while in the case of an Al-Si12 electrode the aspect ratio

    decreased with approximately 4.2%, respectively. The silicon plays an important role

    by some chemical compounds which could be found at the boundary of particles.

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    32

    0

    20

    40

    60

    80

    100

    1.5 3 4.5

    Me

    an p

    eri

    me

    ter

    Wt. % Grp

    Al-Si12 electrode

    Pure Al electrode

    Fig.4.66. Dependence between the Grp percent and the mean perimeter of Grp.

    Figure 4.66 present the effect of the type of electrode on the mean perimeter. The

    increase of the graphite particle percent from 1.5 wt. % to 4.5 wt. % leads to the

    decrease of mean perimeter with about 17% for samples obtained with an Al-Si12

    electrode and with about 23% for sample prepared by using a pure Al electrode.

    4.8. Scanning electron microscope test

    Some graphite particles which were identified in the Al-graphite

    composite structure were choosing for SEM analysis (Figure 4.74 A and Figure 4.76

    A). Both EDAX (spectrum and chemical composition) and mapping techniques were

    applied in order to characterize the graphite particles features. The results are presented

    in Figure 4.74B, Table 4.9 and Figure 4.75, for the particles in Figure 4.74A. Figure

    4.76B, Table 4.10 and Figure 4.77 appear the results for the particles in Figure 4.76A

    respectively.

    From the SE images presented in Figures 4.74A and 4.74A it can be observe that

    amount of graphite is distributed in many places the distribution of Grp particles being

    different from an area to another. The high C level in the selected area is a proof of

    graphite presence despite of many other companion elements such as Al, O, Cu, Si,

    Mg. The presence of various elements such as Cu and Mg around graphite particles can

    considered as a benefit factor for their restrain in Al matrix because of favorable effect

    on the wettability between the graphite particles and melt aluminum. The high Al level

    in graphite particle composition can be considered as a result of Al matrix influence

    while the oxygen comes from surroundings (Al oxidation).

    The high level of graphite particles are clear in EDAX analysis, this a good

    signal for the sufficient conditions to make incorporations between the graphite

    particles and aluminum melt.

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    Fig. 4. 74. Graphite particle embedded in Al matrix: A - Secondary electron image; B -

    X-ray spectrum in selected area 1.

    Table 4. 9. Chemical composition of graphite particle selected area 1(Fig. 4.74 A)

    Element

    Weight %

    Atomic %

    C 44.41 61.97

    O 9.49 9.95

    Cu 1.61 0.43

    Mg 0.72 0.5

    Al 42.92 26.66

    Si 0.85 0.51

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    34

    Fig.4.75. Overlaying of X – ray maps of elements distribution in graphite particle area (see Fig.

    4.81A).

    Chapter V

    5. Conclusions and personal contributions

    5.2. Conclusions of experimental results

    Aluminium-pure aluminum sieve - Grp composite produced by gravitational casting

    - The particle density increased with approximately 27 % when the volume

    percentage of graphite deposed over a sieve increased from 2 to 4 vol. %.

    - More agglomerations besides some individual particles appeared by increasing the

    graphite particles percent from 2 to 4 vol. %.

    - Increasing in graphite percentage from 2 to 4 vol. % led to the increase of mean

    sphericity from 0.315 to 0.323.

    Cu

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    35

    - Mean convexity of graphite particles increases from 0.813 to 0.876 when the

    graphite particles content increases from 2 to 4 vol. %.

    Al alloy-layers of graphite powder - pure aluminium foil composite

    - Little fine particles had an approximately uniform distribution in Al by using 0.25

    wt. % Grp-pure Al foils and - 0.5 wt. % Grp-pure Al foils composites, but some

    large Grp particles appeared when the weight percentage of graphite increased to

    0.75.

    - The graphite particles incorporated in Al-Si12 melt are increasing with the increase

    of percentage of Grp added in package, in comparison with Al matrix.

    - Residual porosity diminishes with the decrease of particle size and with the

    magnesium content.

    Thermal analysis

    - The increase of graphite percentage and addition of alloying elements, Mg and Si,

    are reducing the temperatures TM, T nucl Al, TU Al cry, T max Al cry, T nucl EUT, TU EUT, T

    max EUT, and TES due to the retention of heat around the graphite particles. This fact

    was confirmed by the thermal analysis.

    - It was also observed that the introduction of graphite particles in the Al melt

    decreases the temperature of solidification.

    - Usually, the cooling rate begins with low rate, during the interval of transition from

    nucleation to crystallization, and the starts to increase due to the transmission of

    heat.

    - The value of particles density of Grp increased with approximately 12.7 % in Al-

    Si12-Mg and only with 4.5 % in Al-Si12 and 7.2 % in Al, when Grp content

    increased from 0.25 wt. % to 0.75 wt. %.

    - Mean sphericity and convexity increased by increasing the graphite percentage,

    especially for Grp in Al-Si12-Mg.

    Melting and deposition in electric arc of composite inside holes made an

    aluminium alloy

    - Using low intensity of electric arc, the distribution of graphite particles inside holes

    made an Al alloy by using an electrode of Al-Si12, having the diameter of 4. 2 mm,

    was better than for other type of electrodes.

    - The particle density of samples prepared by using high intensity is higher than of

    the samples prepared by using low intensity current. Thus, the particle densities for

    samples prepared by using an Al-Si12 of 4.2 mm diameter electrode have big

    values in the case of high intensity while the particle densities for sample prepared

    by using pure Al electrode and low intensities are significantly lower.

    - Mean sphericity of graphite deposition in Al alloy at low intensity of electric arc

    obtained with pure Al, Al-Si12, and Al-Si12 electrodes, with the diameter of 2.5

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    36

    mm, was more than the same property for samples melted and deposed in high

    intensity of current.

    - The electrodes type does not affect essentially the mean convexity of graphite

    particles deposition in Al alloy at low intensity. However, this property increases in

    the melt and deposition processes unfolded by using electrodes of Al-Si5 and Al-

    Si12 with the diameter of 2.5 mm, as well as for the Al-Si12 electrode with 4.2 mm

    diameter, at high intensity of current.

    - The mean aspect ratio of graphite particles was not affected very much when the

    electric arc intensity was changed (for all types of electrodes, except the Al-Si12

    electrode with the diameter of 4.2 mm).

    Electric arc deposition by using an electrode coated with graphite

    - The rate of increase in particle densities of samples with 1.5, 3.0, and 4.5 wt. %

    Grp are coated an Al-Si12 electrodes in comparison with samples obtained by

    using a pure Al electrodes are 16.9 %, 13.6 %, and 10.6 %, respectively.

    - The type of electrode has not an important effect on the mean convexity,

    especially for a graphite content of 3 wt. % and 4.5 wt. %. At smaller content of

    1.5 wt. %, the mean convexity for sample obtained by using Al-Si12 electrode

    coated with graphite was greater than samples obtained by using a pure Al

    electrode.

    - Mean convexity increased with about 5-7 % when the graphite content increased

    from 1.5 wt. % to 4.5 wt. %.

    - When particles tend to a spherical shape, the value of aspect ratio is diminishing

    and there was a decrease of this parameter with the increase of the Grp percent.

    - The increase in the graphite particle percent from 1.5 wt. % to 4.5 wt. % leads to

    the decrease in mean perimeter with about 18 % for samples by using an Al-Si12

    electrode and with 23 % samples obtained by using a pure Al electrode.

    Scanning electron microscope test

    - SEM analyses pointed out the presence of graphite particles embedded in Al matrix;

    - Many other elements such as Al, O, Cu, Mg and Cu were identified as C companion;

    - The presence of different elements such as Cu, Mg in the surface layer of Al-

    Grp composite is considered as a benefit because of wettability between

    graphite and Al melt improvement;

    - The high Al level in graphite particle composition can be considered as a result

    of Al matrix influence while the oxygen comes from surroundings (Al

    oxidation);

    - The graphite particles are non- uniform distributed in the Al melt due to its

    floating tendency.

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    37

    5.3. Original contributions

    The main original contributions of this research are the following:

    Calculation of shape factors for different shape of particles: cylinder, conical,

    conical truncated, and ellipsoid.

    Obtaining a surface layer of gravity cast Al-Grp composite by using a pure

    aluminium sieve covered with graphite particles, the sieve being inserted on the

    internal surface of a mould.

    Production of surface layer of Al-Grp composite making packages with layers

    of graphite powder and pure aluminium foil fixed mechanically on the base of

    the mould, before the pouring of molten aluminium by gravity casting.

    Deposition of aluminium alloy and graphite particles on aluminium to produce

    surface layer by using an electric arc. Introduction of Grp in some holes made

    on the surface of aluminium alloy and the use of different types of electrodes

    for melting and deposition.

    Preparation of thin layers of Al alloy–graphite particle composites by the

    electric deposition method. The use of a covered electrode with graphite, pure

    Al foil, and copper wire.

    Insertion of the graphite particles inside a channel made on the surface of an

    aluminium alloy, before the melt and deposition under an electric arc to prepare

    thin layers of Al-graphite composites on the Al alloy surface.

    Dissemination of research results

    Scientific papers published

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

    Applications of Aluminium - Graphite Composites, Advanced Materials

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

    2. Muna NOORI, Hazim FALEH, Mihai CHISAMERA, Florin STEFANESCU ,

    and Gigel NEAGU, Wettability of Silicon Carbide Particles by the Aluminium

    Melts in Producing Al-SiC Composites, Advanced Materials Research,

    ISSN:1662-8985, VOL. 1128, pp. 149-155

    Scientific papers accepted to be published

    1. Hazim FALEH, Muna NOORI, Florin STEFANESCU, Gigel NEAGU, and

    Eduard Marius STEFAN, Wettability in aluminium-graphite particles

    composite (review) (poster), "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. Muna Noori, Hazim Faleh, Mihai Chisamera, Gigel Neagu, Florin Stefanescu

    and Eduard Marius Stefan, Properties of Al-SiCp composites (review)

    (poster), "Dunarea de Jos" University of Galati, International Conference on

  • Research on the possibility of obtaining Al-graphite (particles) composites in certain zones of parts

    38

    Material Science, 7th

    Edition, 19th

    -21st May 2016. Paper published in

    advanced Material Research-ISI journal /proceeding.

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

    EXPERIMENTAL RESEARCH CONCERNING A NEW METHOD TO

    PRODUCE ALUMINIUM ALLOY-GRAPHITE PARTICLE COMPOSITE

    IN SURFACE LAYERS, U.P.B. Scientific Bulletin, Series B, Chemistry and

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

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

    Stefan Eduard Marius, SOME ASPECTS CONCERNING A NEW

    POSSIBILITY OF Al-SiCP COMPOSITE PRODUCTION, U.P.B. Scientific

    Bulletin, Series B, Chemistry and Materials Science,Vol….., pp……, ISSN

    1454-2331.

    References

    [15] J.A. Cornie et al., “Discontinuous Graphite Reinforced Aluminum, and Copper Alloys for

    High Thermal Conductivity-Thermal Expansion Matched Thermal Management

    Applications," www.mmccinc.com (2003).

    [21] N.J. Parrat, “Reinforcement effects of Silicon Nitride in Silver and Resin Matrices”,

    Powder Metallurgy, Vol. 7(14), 1964, pp 152-167.

    [28] Z. Liuy, G. Zu, H. Luo, Y. Liu and G. Yao, Influence of Mg Addition on Graphite Particle

    Distribution in the Al Alloy Matrix Composites, J. Mater. Sci. Technol., 2010, 26(3), 244-

    250.

    [29] J. B. Yang, C. B. Lin, T. C. Wang and H. Y. Chu, "The tribological characteristic of

    A356.2Al alloy-Gr (p) composite", wear 257(2004), pp.941-952 pp-51-56.

    [58] P. Shanmughasundaram and R. Subramanian, "Wear Behaviour of Eutectic Al-Si Alloy-

    Graphite Composites Fabricated by Combined Modified Two-Stage Stir Casting and

    Squeeze Casting Methods", Advances in Materials Science and Engineering, Volume

    2013, Article ID 216536, 2013, pp. 8.

    [109] R. Mahadevan and R Gopal,” Selectively reinforced squeeze cast pistons”, 68th WFC -

    World Foundry Congress 7th - 10th February, 2008, pp. 379-384.

    [118] Hazim FALEH, Muna NOORI, and Florin STEFANESCU, "Properties and Applications

    of Aluminium - Graphite Composites", Advanced Materials Research, ISSN: 1662-8985,

    VOL. 1128, pp.134-143.

    [125] D.G. Thomas, “Transport characteristics of suspension: VIII. a note on the viscosity of

    Newtonian suspensions of uniform spherical particles”, Journal of Colloid Science, Vol.

    20, Issue 3, 1965, p. 267-277.

    [129] Zhengang Liu , Guoyin Zu, Hongjie Luo, Yihan Liu and Guangchun Yao, "Influence of

    Mg Addition on Graphite Particle Distribution in the Al Alloy Matrix Composites", J.

    Mater. Sci. Technol., 2010, 26(3), 244-250.

    [130] P. Rohatgi and B. Schultz, "Solidification During Casting of Metal-Matrix Composites",

    ASM Handbook, Volume 15: Casting, ASM Handbook Committee, p 390-397.