Preparation and Characterization of 7075 - Copy

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Preparation and Characterization of 7075Aluminum Alloy Reinforced with Nano

Particles of Alumina

A Thesis

Submitted To College of Engineering

Alnahrain University in a Partial Fulfillment

Of the Requirements for the Degree of Master Science

In

Mechanical Engineering

By:

Ibrahim Rahman Ibrahim

(B.Sc. 2006)

Thi AlQida 143

September 201


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Chapter One

Introduction

1.1 Background

The first focused efforts to develop Metal-Matrix Composites MMC's

originated in the 1950s and early 1960s. The principal motivation was to

dramatically extend the structural efficiency of metallic materials while retaining

their advantages, including high chemical inertness, high shear strength, and good

property retention at high temperatures. Early work on sintered aluminum powder

was a precursor to discontinuously reinforced MMCs. The development of high-

strength monofilamentsfirst boron and then silicon carbide (SiC)enabled

significant efforts on fiber-reinforced MMCs throughout the 1960s and early 1970s.

Issues associated with processing, fiber damage, and fiber-matrix interactions were

established and overcome to produce useful materials. Although these were very

expensive and had marginal reproducibility, important applications were

established, including 243 structural components on the space shuttle orbiters.

Recession in the early 1970s produced significant research and development

funding cuts, leading to an end of this phase of MMC discovery and development.

In the late 1970s, efforts were renewed on discontinuously reinforced MMCs using

SiC whisker reinforcements. The high cost of the whiskers and difficulty in

avoiding whisker damage during consolidation led to the concept of Particulate

reinforcements. The resulting materials provided nearly equivalent strength and

stiffness, but with much lower cost and easier processing. A renaissance in both

discontinuous and fiber-reinforced MMCs continued through the 1980s. Major

efforts included particle-reinforced, whisker-reinforced, and tow-based MMCs of

aluminum, magnesium, iron, and copper for applications in the automotive, thermal

management, tribology, and aerospace industries. In addition, monofilament-


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reinforced titanium MMCs were developed for high-temperature aeronautical

systems, including structures for high-mach airframes and critical rotating

components for advanced gas turbine engines. Significant improvements in

performance and materials quality were matched by an increasing number of mostly

small businesses that were specialized in the production of MMC components for

target markets.

In the early 1990s, a U.S. Air Force Title III program provided a significant

investment to establish an MMC technology base for the aerospace industry in the

United States. This program produced several landmark military and commercial

aerospace applications of discontinuously reinforced aluminum (DRA). In additionto these dramatic successes, new MMC insertions in the ground transportation,

industrial and thermal management electronic packaging industries far exceeded the

growth in the aerospace industry. Thus, the insertion of new materials in military

and commercial aircraft has actually lagged behind the industrial sector, reversing

the trend of earlier years for the insertion of new materials. The MMC market for

thermal management and electronic packaging alone was five times larger than the

aerospace market in 1999, and this gap is expected to increase in the coming years,

due to aggressive growth in the ground transportation and thermal management

markets [1].

For many researchers the term metal matrix composites is often equated with the

term light metal matrix composites (MMCs).Substantial progress in the

development of light metal matrix composites has been achieved in recent decades,

so that they could be introduced into the most important applications. In traffic

engineering, especially in the automotive industry, MMCs have been used

commercially in fiber reinforced pistons and aluminum crank cases with

strengthened cylinder surfaces as well as particle-strengthened brake disks. These

innovative materials open up unlimited possibilities for modern material

science and development; the characteristics of MMCs can be designed into the

material, custom-made, dependent on the application. From this potential, metal

matrix composites fulfill all the desired conceptions of the designer. This material


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group becomes interesting for use as constructional and functional materials, if the

property profile of conventional materials either does not reach the increased

standards of specific demands, or is the solution of the problem. However, the

technology of MMCs is in competition with other modern material technologies, for

example powder metallurgy. The advantages of the composite materials are only

realized when there is a reasonable cost performance relationship in the

component production. The use of a composite material is obligatory if a special

property profile can only be achieved by application of these materials. The

possibility of combining various material systems (metal ceramic nonmetal)

gives the opportunity for unlimited variation. The properties of these newmaterialsare basically determined by the properties of their single components [2].

1.2 Metal Matrix Nanocomposites

Metal matrix composites (MMCs) such as continuous carbon or boron fiber

reinforced aluminum and magnesium, and silicon carbide reinforced aluminum

have been used for aerospace applications due to their lightweight and tailorable

properties. There is much interest in producing metal matrix nanocomposites that

incorporate nanoparticles and nanotubes for structural applications, as these

materials exhibit even greater improvements in their physical, mechanical and

tribological properties as compared to composites with micron-sized

reinforcements. The incorporation of carbon nanotubes in particular, which have

much higher strength, stiffness, and electrical conductivity as compared to metals,

can significantly increase these properties of metal matrix composites.

Nanocomposites are being explored for structural applications in the defense,

aerospace and automotive sectors. Concurrent with the interest in producing novel

nanocomposites materials is the need to develop low cost means to produce these

materials. Most of the prior work in synthesizing nanocomposites involves the use

of powder metallurgy techniques, which are not only high cost, but also result in the


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presence of porosity and contamination. Solidification processing methods, such as

stir mixing, squeeze casting and pressure infiltration are advantageous over other

processes in rapidly and inexpensively producing large and complex near-net shape

components, however, this area remains relatively unexplored in the synthesis of

nanocomposites. Stir mixing techniques, widely utilized to mix micron size

particles in metallic melts, have recently been modified by dispersing small volume

percentages of nanosize reinforcement particles in metallic matrices. Although there

are some difficulties in mixing nanosize particles in metallic melts resulting from

their tendency to agglomerate, a research team in Japan has published research on

dispersing nanosize particles in aluminum alloys using a stir mixing technique.Researchers at the Polish Academy of Science have recently demonstrated the

incorporation of greater than 80 volume percent nanoparticles in metals using high-

pressure infiltration with pressures in the GPa range. Composites produced by this

method possess the unique properties of nanosize metallic grains.

Recently, metal matrix nanocomposites were synthesized at the University of

Wisconsin, Milwaukee using aluminum alloy A206 and nanoparticles of alumina

(Al2O3).TEM samples of the cast Al-A206/Al2O3clearly show nanoparticles present

within the metal matrix (Figure 1.1). SAD patterns show the pattern of the matrix as

well as the nanoparticles (Figure 1.2). EDX indicates that the grains are composed

of aluminum, which contains nanosize alumina particles. The distribution of

particles throughout the grains of the matrix with an absence of large concentrations

at the grain boundaries suggests wetting of the alumina by the liquid metal. In this

case the nanoparticles did not appear to act as nucleation sites for nanosized grains

[3].

1. 3 Material Used to Reinforce Aluminum Alloys

The most common DRMMC materials systems used for current aerospace structural

applications are silicon carbide (SiC) and boron carbide (B4C) particulate

reinforcement in an aluminum alloy matrix. Aluminum oxide particles are a lower


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cost alternative most commonly used for casting applications. Titanium carbide is

being investigated for high-temperature applications [1].

Among the ceramic reinforced materials, SiC is the most common used in MMCs.

The second most used reinforcement is Al2O3. Compared with SiC it is more stable

and inert and has better corrosion and high temperature resistance. The influence of

these reinforcements to aluminum alloys has been the subject of a significant

amount of research work [5].

Because of its extreme hardness and temperature resistant properties, Al2O3ceramic

particles are often used as reinforcement within the aluminum matrix. This type of

composite is more frequently used in the automotive industry today, particularly invarious engine components as well as brakes and rotors [2].

1.4 Stir Casting

Melting metallurgy for the production of MMCs is at present of greater technical

importance than powder metallurgy. It is more economical and has the advantage

of being able to use well proven casting processes for the production of MMCs.For

melting metallurgical processing of composite materials there are three procedures

mainly used :

compo-casting or melt stirring

gas pressure infiltration

squeeze casting or pressure casting.

Both the terms compo-casting and melt stirring are used for stirring particles into

a light alloy melt .The particles are often tend to form agglomerates, which can be

only dissolved by intense stirring[2]. Out of the available methods of producing

these composites, Stir casting route is most promising and economical for

synthesizing the particle reinforced AMCs and is not only simple, but is easy to

obtain shape castings[6].


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Among discontinuous metal matrix composites processes , stir casting is generally

accepted as a particularly promising route, currently practiced commercially. Its

advantages lie in its Simplicity, flexibility and applicability to large quantity

production. It is also attractive because, in principle, it allows a conventional metal

processing route to be used, and hence minimizes the final cost of the product. This

liquid metallurgy technique is the most economical of all the available routes for

metal matrix composite production, and allows very large sized components to be

fabricated. The cost of preparing composites material using a casting method is

about one-third to half that of competitive methods, and for high volume

production, it is projected that the cost will fall to one-tenth . In general, thesolidification synthesis of metal matrix composites involves producing a melt of the

selected matrix material followed by the introduction of a reinforcement material

into the melt, obtaining a suitable dispersion [7].

However, stir casting has inherent problems such as good wetting between the

particulate reinforcement and the liquid aluminum alloy melt. Moreover the

problem with finer reinforcement particles especially nano particles would be

agglomeration. If these challenges could be overcome, then stir casting would be a

commercially viable technology for producing AMCs especially nano particle

reinforced AMCs. A few researchers have made attempts to develop stir casting

setup that can overcome these problems and they have been quite successful. Most

of the researchers have focused on using micron sized reinforcement particles and

not much research has been done using nano size reinforcement particles.

Hashim et al., have identified four technical difficulties in stir casting: difficulty of

achieving a uniform distribution of the reinforcement material; wettability between

the two main substances; porosity in the cast metal matrix composites; and chemical

reactions between the reinforcement material and the matrix alloy. These difficulties

need to be overcome in order achieve a MMC with a broad range of mechanical

properties. They have also identified the important process variables that affect the

mechanical properties of MMC. The holding temperature, stirring speed, size of the


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impeller and the position of the impeller in the melt are to be considered in the

production of cast metal matrix composites (quoted in 8).

Figure 1-1Schematic showing the mechanical stirring device used in the vortex technique of

dispersing particles in melts [9].

Objective of this study

The aim of this study is to prepare and characterize the 7075alluminum alloy after

reinforcing it by alumina nano powder based on stir casting technique taking into

consideration the following :

1- Investigation into the improvement in mechanical properties of 7075 Aluminum

alloy reinforced with 20 nm alumina particles.

2- Obtaining best result from wt. % of reinforcement within 0.5%, 1.0% and 1.5%

wt. fraction of alumina particle as a reinforcement in Aluminum metal matrix.

3- Pointing out to utilization of addition of particles by capsules and how suitable it

is with stir casting technique to get regular distribution in the structure of the

AMC.


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Chapter Two

Literature Survey

2.1 Introduction

Many studies dealt with synthesizing and composing metal matrix composites .

Next studies which will be surveyed are focusing on preparation of MMC's,

especially AMC's reinforced by ceramic particles and nano particles . Different

techniques had been experimented as a route of formation and preparation of

MMC's For example: mechanical alloying, ultrasonic ,In situ ,stir casting and

powder metallurgy . The Following researches are surveyed:

2.2 Metal Matrix Composites

Tong and Ghosh, 2001[10] : evaluated high-strength Al-Si/TiC and the

elevated temperature-resistant Al-Fe(-V-Si)/TiC composites . The microstructural

characteristics of ingot metallurgy (IM) or rapid solidification (RS) Al-Si/TiC and

Al-Fe(-V-Si)/TiC composites could be thought of as a combination of the related

alloy matrix microstructures and the IM or RS Al/Tic composites. Included Orowan

strengthening, grain-size and substructure strengthening, and solid-solution

strengthening. Used RS technique, fine, uniform particle size distribution; a high

interfacial strength; control of particle shape; and a ductile matrix to maximize

strength and ductility for particular volume fraction, and influence the degree of

flexibility available.

Kim, et al, 2001[11]: described the optimum condition for synthesis of

Al2O3/Cu nanocomposites. Discussed the effects of Cu dispersion on the

microstructure and fracture toughness. Concluded that Al2O3/Cu nanocomposites

can be fabricated by a controlled powder preparation and sintering process with a

relatively simple and cost-effective method. Successfully synthesized the


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nanocomposites powders with crystallite size of about 25 nm by a high-energy ball

milling process. Sintered the composites at 1250C for 5 min which showed a

relative density above 97% and enhanced fracture toughness of 4.51 MPa.m1/2.

Microstructural observation of the sintered composites\revealed that the nano-sized

Cu particles are uniformly distributed and situated on the grain boundaries of Al 2O3

matrix.

Koch , 2003[12] :reviewed the 'top-down 'methods for synthesis of nano

structured or submicron grain size material which utilize sever plastic deformation

along with thermal process. Of the variety methods he discussed , appeared that

only ball milling ,high pressure torsion and accumulative roll bonding can regularly

produce average grain size below100nm .By examples of combining conventional

deformation methods with annealing ,showed the possibility of forming

nanocrystalline/submicrom grain sizes that can optimize mechanical properties in

practical ways .

Miyajima ndIwai ,2003[13] : investigated effects of reinforcements on

the wear behavior of aluminum matrix composites by pin-on-disk tests. The matrix

materials were 2024 and ADC12 aluminum alloys. The volume fractions of SiC

whiskers (Vfw) were 529% (MMCw), Al2O3fibers (Vff ) were 326% (MMCf),

and SiC particles (Vfp) were 210% (MMCp). The MMCs were rubbed against a

carbon steel pin under a load of 10N at a sliding velocity of 0.1ms1. The degree of

improvement were investigated By summarizing the results, they concluded that

particles are most beneficial for improving the wear resistance of MMC.

Bajaj ,2004[14]: His work focuses on the study of the behavior of Aluminum

cast alloy (LM6) with SiC and AL2O3 composites produced by the stir casting

technique .Different % age of reinforcement were used .Tensile test ,hardness test,

impact test were performed on the samples obtained by the stir casting process.X-


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ray diffraction was performed to find the presence of the phases of reinforcement

material .SEM was done to find the distribution of the SiC/Alumina particles in

Aluminum alloy . He concluded that stir casting ,stirrer design and position ,stirring

speed and time, particle preheating an temperature ,particle incorporation rate .etc.

are the important process parameters.

Xia, et al, 2005[15]: studied the structural evolution of an Al/BN mixture

during mechanical alloying (MA) using a planetary ball mill systematically. The X-

ray diffraction (XRD) patterns showed that the diffraction peaks of hexagonal boron

nitride (h-BN) disappeared. Characterization of the milled samples was also carried

out by means of differential scanning calorimetry (DSC), Fourier transform infrared

spectroscopy (FTIRS), and scanning electron microscopy (SEM). Experimental

results revealed that most h-BN gradually decompose into boron and nitrogen

atoms as MA proceedes . They conclude that no new phase is formed directly

during MA from Al/BNmixture in our experiments, but AlN and AlB2 are formed

when the milled samples were annealed at 773K for 2 h. AlN and AlB2 deposited

out from the solid solution of AlBN during annealing.

Hassan and Gupta, 2005 [16]: fabricated magnesium based composites

with three different volume percentages of nano-sized Al2O3 particulates

reinforcement using blend-press-sinter methodology avoiding ball milling.

Microstructural characterization of the materials revealed reasonably uniform

distribution of nano size Al2O3 reinforcement and presence of minimal porosity.

Mechanical properties characterization revealed that the presence of nano-Al2O3

particulates as reinforcement leads to a simultaneous increase in hardness, elastic

modulus, 0.2% yield strength, UTS and ductility of pure magnesium. The results

reveal that the 0.2% yield strength, UTS and ductility combination of the

magnesium nanocomposites containing 0.66 and 1.11 vol.% of alumina remained

higher when compared to high strength magnesium alloy AZ91 reinforced with


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much higher amount of micrometer size SiC particulates. An attempt is made in the

present study to correlate the effect of presence of nano-Al2O3as reinforcement and

its increasing amount with the microstructural and mechanical properties of

magnesium.

Kurzydlowski, 2006[17]: studied the microstructure and mechanical

properties of nanomaterials (Al, Al-alloys, Cu, Ni, Ti, and stainless steel) and

nanocomposites (Al2 O3 / Ni P) by the methods of transparent and scanning

electron microscopy, X-ray diffraction analysis, and micro hardness and tensile

tests. The experimental methods included the procedures of measuring the electric

and corrosion resistances. Prepared the materials a by using contemporary methods,

namely, by hydrostatic extrusion (nanometals) and by sintering ceramic powders

covered with NiP nanoparticles under high pressure by using the procedure of

nonelectric chemical metallization (Al2O3/ NiP nanocomposites).

Espinoza, et al 2007[18]:studiedthe microstructure, electrical conductivity

and hot softening resistance of two alloys (G-10 and H-20), projected to attain Cu

2.5 vol.% TiC2.5 vol.% Al2O3 nominal composition, and prepared by reaction

milling and hot extrusion. The alloys were characterized by many analysis

techniques. They concluded that the H-20 alloy is a good candidate

for engineering applications athigh temperature.

Tavoosi , et al 2007[19]: investigated fabrication and characterization of

alumina particles reinforced aluminum-based metal matrix nanocomposite by

mechanical alloying. Aluminum and zinc oxide powders mixture were milled by a

planetary ball mill in order to produce Al13.8 wt.% Zn/5 vol. % Al2O3

nanocomposite. The structural evaluation milled and annealed powders were

studied by X-ray diffraction, SEM observation and hardness measurement. The

aluminum crystallite size was estimated with broadening of XRD peaks by


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WilliamsonHall formula. The results show that milling of aluminum and zinc

oxide for 60 h leads to displacement reaction of the zinc oxide and aluminum to

produce Zn and Al2O3phases. The milled powder had a microstructure consisting of

nanosized Al2O3particles in an AlZn solid solution with a nanoscale grain size of

40 nm. Micro hardness of this nanocomposite was found to be about 190 HV.

Eschbach, et al 2007[20]:presented the fabrication and characterization of

nanocomposite materials based on crystalline nanoparticles dispersed in an

oligomer matrix . Two types of nanoparticles were used. They described the

nanocomposites fabrication process and underlined the dispersion step and the

problems inherent to clusters destruction. Explored Mechanical properties of the

nanocomposites based on Al2O3by Brillouin spectroscopy. An enhancement of the

Young's modulus was observed with a very weak mass percentage of nanoparticles

(3%). . This result is of great importance in order to achieve piezoelectric and

ferroelectric applications.

Ahmad, et al 2007[21]: report the effect of weight percentage on the

microstructure and properties of alumina particle reinforced aluminum metal matrix

composite and the microstructure, bulk density and hardness value of the

composites . They concluded that the production of Al2O3 particle reinforced

aluminum matrix composites in the form of net shape component can be achieved

by use of conventional P/M route, cold uniaxial pressing and sintering processing

technology. The hardness values and density of the composites increase with the

increasing percentage of Al2O3.

Rafaja, et al 2007[22]:illustrate the capability of the combination of the X-

ray diffraction and the transmission electron microscopy for the Microstructure

investigations on thin film and bulk nanocomposites of three experimental

examples: two Cr-Al-Si-N coatings with different chemical compositions and one


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BN bulk nanocomposites.They used a modified kinematical diffraction theory that

describes and explains the phenomenon of the partial crystallographic coherence of

crystallites, they could show that the analysis of the X-ray diffraction line

broadening is able to reveal nanocrystalline domains organized in semi-coherent

clusters, to determine the size of the nanocrystalline do mains and the clusters, and

to quantify the mutual orientation of the partially coherent crystallites within these

clusters.

Ren , 2007[23]: The aim of his project was to fabricate SiC (50nm)/7075

aluminium composites via a modified powder metallurgy and extrusion

route.Ageing treatment was used to increase the strength of the composites and

mechanical tests, including tensile test and abrasive wear test. He studied the

effects of nanometric silicon carbide particulates to the ageing behaviors and

mechanical properties of the composites by optical metallography, scanning

electron microscopy and transmission electron microscopy. He found that the

dispersion of nanometric silicon carbide is not homogeneous, but tends to dispersealong grain boundaries. He also found that of these nano-reinforcements cluster

within the grains. He concluded the addition of a small amount of SiC

nanoreinforcements has a high potential to further strengthen 7xxx aluminum alloy.

He suggested that by improving the dispersion of nanometric reinforcements, as

well as putting in reinforcements with different sizes, the mechanical properties and

wear resistance can both be increased.

Reddy, et al 2008[24]:preparedaluminum-based metal matrix composites

reinforced with in situ Al3Ni and Al2O3 hybrid reinforcements prepared by the

mechanically activated, self-propagating, high temperature synthesis (MASHS).

They demonstrated that the composite properties can be tailored by altering the

stoichiometry of reactants and concluded that the formation of different types of

nickel aluminides depending on the excess aluminum demonstrates the

compositional versatility of the MASHS process. Results they obtained have a


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Singla, et al 2009[7]: attempted to develop aluminum based silicon carbide

particulate MMCs with the objective to develop a conventional low cost method of

producing MMCs and to obtain homogenous dispersion of ceramic material.Twostep-mixing method of stir casting technique and subsequent property analysis had

been made to achieve these objectives. They chose aluminum (98.41% C.P) and SiC

(320-grit) as matrix and reinforcement material respectively. Experiments were

conducted by varying weight fraction of SiC (5%, 10%, 15%, 20%, 25%, and 30%),

while keeping all other parameters constant. Their results indicated that the

developed method is quite successful to obtain uniform dispersion of

reinforcement in the matrix. They observed an increasing trend of hardness and

impact strength with increase in weight percentage of SiC.

Flores-Campos, et al 2010[28]:prepared composites of 7075 aluminum

alloy (Al7075) with carbon-coated silver nanoparticles (Ag-C NP) by the

mechanical milling process and found that Ag-C NP has an effect on refining the

powder and in the crystal size. The Vickers microhardness (HVN) values were

higher at higher Ag-C NP contents and a saturation point exists where

microhardness does not present variation. They concluded that concentrations

higher than 2wt.% do not have an important effect on microhardness.

Wang, et al 2010[29]: fabricated In situ formation of Al2O3SiO2SnO2

composite ceramic coating on Al20%Sn alloy in aqueous Na2SiO3 electrolyte by

microarc oxidation technology successfully and studied in detail the compositions,

structure, mechanical and tribological properties of the composite by scanning

electron microscope, energy dispersive spectroscopy, X-ray diffraction, hardness

tester and ball-on-disc friction tester. They found that the species originating from

the Al20%Sn alloy substrate and the electrolyte solution both participate in

reaction and contribute to the composition of the coating, which results in the

generated coating firmly adherent to the substrate. They conclude that considering


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the convenience on preparation and attractive performance acquired in the final

products, the Al2O3SiO2SnO2 composite coating could be a promising way to

extend the applications of AlSn alloy in industry.

Khorshid, et al 2010[30]:prepared aluminum matrix composites reinforced

by two sizes of alumina particles (35 nm and 0.3 lm) wet attrition milling and hot

forward extrusion processes and evaluated the effect of the ratio of the nano- to

submicron-sized particles (2:8, 3:7, 4:6, 5:5, and 6:4 in weight percent) on

mechanical properties of the composites by micro-hardness and tensile tests. They

found that by increasing the nanoparticles content, the hardness and strength of the

composites first increase and then decrease when the amount of the nanoparticle

exceeds 4 wt.%. The tensile fracture surfaces are also observed by scanning electron

microscopy.

Monazzah , et al 2010[31]: produced alumina dispersion strengthened

aluminum powder by mechanical milling of a commercial gas-atomized aluminum

powder in a planetary ball mill under an argon atmosphere for 12 h. They explained

the creep behavior according to the invariant substructure model and dislocation

glide process and concluded that the nanometric alumina particles actively pin the

substructure and limit the deformation of the aluminum matrix.

Mirzadeh and Zomorodian , 2010[32]: modeled the effects of the

annealing temperature and time, cryomilling in liquid nitrogen, and the addition of

aluminum powder on the thermal stability and grain growth behavior of

nanocrystalline iron using the Artificial Neural Network (ANN) technique .The

developed model can be used as a guide for the quantification of the grain growth

by considering the effects of annealing temperature and time. The model also

quantifies the effect of Al on the thermal stability of cryomilled nanocrystalline Fe.


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The model results show that the cryomilling of Fe has a tangible effect on the

stabilization of the nanostructure.

Hoobi, et al 2011[33]:studied some of mechanical properties such as Brinell

hardness (BHN) and compression strength of aluminum matrix composite material

that reinforced was by (3, 6 , 9 , and 12 wt.% ) Al 2O3particles. Powder technology

technique was used in samples prepared. Their result show that an advancement in

the Brinell hardness (BHN) and compression strength especially at 12 wt.% - Al2O3.

The development in the Brinell hardness (BHN) and compression strength were

found to reach 89% and 54% respectively from the initial properties of unreinforced

aluminum samples.

Zhang, et al 2011[34]:fabricated in situ Al composites by selection of Al and

TiO2 powders via multiple pass friction stir processing (FSP) based on the

thermodynamic analysis. The microstructural investigations indicate FSP would

induce reaction between Al and TiO2. Al3Ti and Al2O3. Tensile tests indicate that

the in situ nanocomposites exhibits pronounced work hardening behavior and a

good combination of strength and ductility.

Choi, et al 2011[35]:usedan ultrasonic cavitation based dispersion technique

to fabricate Al-7Si- 0.3Mg alloyed with Cu and reinforced with 1 wt. pct. Al2O3

nanoparticles, in order to investigate their influence on the mechanical properties

and microstructures of Al-7Si-0.3Mg alloy. They concluded that that the

combination of 0.5 pct. Cu and 1 pct. Al2O3 nanoparticles results in a significant

increase in ultimate tensile strength (22 pct.). It is noticed that the ductility is

improved from 1.7 to 10.4 pct. (increased by 512 pct). The significant enhancement

of properties is attributed to the decrease in porosity (from 2.1 to 0.5 pct.) and

modification of eutectic Si phases due to the addition of Al2O3nanoparticles. The

enhancement of yield strength in T6 heat-treated Al-7Si-0.3Mg-0.5Cu-1 wt pct


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Al2O3 nanocomposite was limited by the reaction between Mg and Al2O3

nanoparticles during the heat treating process.

Law, et al 2011[36]: analyzed the mechanical response of aluminum matrix

reinforced with nanosized silicon carbide using plane strain, discrete dislocation

plasticity. In the simulations, plasticity arises from the collective motion of

dislocations within an elastic medium. They prescribed constitutive rules for

nucleation, motion and annihilation of the dislocations. Performed calibration of

various parameters or quantities which affect these processes .The numerical results

show improvements in the mechanical strength of the nanocomposite material with

increasing particle volume fraction and decreasing particle size.

Mazahery and Ostadshabani 2011[37]:incorporated 0.75, 1.5, 2.5, 3.5,

and 5 vol.% of alumina nanoparticles into the A356 aluminum alloy by a

mechanical stirrer and then, cylindrical specimens were cast at 800_C and 900_C.

It was observed a uniform distribution of reinforcement, grain refinement of

aluminum matrix, and presence of the minimal porosity by microstructural

characterization of the composite samples. They reveal that the presence of nano-

Al2O3 reinforcement leads to significant improvement in 0.2% yield strength and

ultimate tensile stress while the ductility of the aluminum matrix is retained.

Fractography examination shows relatively ductile fracture in tensile-fractured

samples.

Sajjadi, et al 2011[38]: used a novel three step mixing method in order to

improve the wettability and distribution of reinforcement particles within the

matrix. The process included heat treatment of micro and nano Al2O3 particles,

injection of heat-treated particles within the molten A356 aluminum alloy by inert

argon gas and stirring the melt at different speeds. Investigated The influence of

various processing parameters such as heat treatment of particles, injection process,


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stirring speed, reinforcement particle size and weight percentage of reinforcement

particles on the microstructure and mechanical properties of composites was studied

. The use of heat-treated particles, injection of particles and the stirring system

improved the wettability and distribution of the nano particles within the aluminum

melt. They revealed that the amount of hardness, compressive strength and porosity

increases as weight percentage of nano Al2O3particles increases.

Sivasankaran, et al,2011[39]: synthesized nanocrystalline AA 6061 alloy

reinforced with alumina (0, 4, 8, and 12 wt.%) in amorphized state composite powder by

mechanical alloying and consolidated by conventional powder metallurgy route. They

concluded that The lattice parameter of as milled powders is decreased with percentage of

reinforcement due to dissolution of some atoms in the Al lattice, whereas it is increased

with the percentage of reinforcement in the sintered pellets due to segregation of some

elements. elements. Their analysis can be used for understanding the stress and the strain

present in the nanocomposite through mechanical alloying.

Parvin

and Rahimian, 2011[5]: investigated the influence of Al2O3

particle size on the density, hardness, microstructure, yield stress, compression

strength, and elongation of the sintered AlAl2O3composites. They used 10 wt. %

of Al2O3 powder with three different particle sizes (3, 12 and 48 m) in the

production of the samples. Results show that the relative density of the composite is

initially increased with decreasing particle size and pointed out that the mechanical

properties of the specimens is increased with decreasing particle size. The grain size

and particle distribution homogeneity is decreased with raising the particle size.

Suresh, et al 2011[8]: produced LM25 aluminum alloy metal matrix

composites (MMCs) reinforced with weight fractions of micro and nano Al2O

3

particles up to 10 wt. % by stir casting. The produced composites for their

mechanical properties such as hardness and tensile strength as well as for the


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dispersion of the micro and nano Al2O

3particles were characterized. Their results

reveal that stir casting could be an economical route for the production of nano

composites. Nano particle reinforced MMCs exhibit better hardness and strength

when compared to micro particles reinforced composites. Scanning electron

microscopic observations of the microstructures reveal that the dispersion of the

micron size particles is more uniform while nano particles lead to agglomeration of

the particles.

Hosseini, et al 2012[40]: investigated wear behavior of nanostructured

Al6061alloyand Al6061Al2O3 nanocomposites produced by milling and hot

consolidation. They showed that Nanocomposites containing 3vol%Al2O3 a had

maximum hardness of 235HV and optimum wear rate of 4x10-3

mg/m. Increasing

the amount of Al2O3 up to5vol%resulted in decreaseing hardness values(112

HV)and a sharp rise in wear rate(18 x10-3

mg/m). It should be mentioned that in

best case, the relative density of nanostructured and nanocomposite samples was

9798% which is not high enough for industrial applications.

Murthy, et al 2012[41]:made an attempt to modify the micro sized fly ash

into nano structured fly ash using high energy ball mill. Produced Alfly ash nano

composites by ultrasonic cavitation route successfully. And found that as the

amount of nano fly ash is increasing the hardness of the composite also increass and

the nano fly ash addition leads to improvement in the compression strength of the

composites.

Flores-Campos,et al 2012[42]:producednanostructured composites

of 7075 aluminum alloy and carbon coated silver nanoparticles by mechanical

milling and indirect hot extrusion. The experimental results demonstrate that the

Al7075AgC NP's nanostructured composites can be produced by mechanical

milling and subsequent hot-extrusion processes. Both milling time and silver


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nanoparticles have a positive effect on the mechanical properties. It was observed

that the dispersion of the nanoparticles does not affect the ductility.

El-Daly, et al 2012[43]:used pulse echo overlap method (PEO) which is a

non-destructive technique for evaluating the mechanical properties of Al/SiC

nanocomposites. The nano-sized AI/SiC powders were prepared by mechanical

alloying method.It is concluded that mechanical alloying appears to be an ideal

method to synthesize nanocomposites in a variety of systems. And the elastic

moduli can be correlated well with the additions of SiC reinforcements in various

amounts to the Al matrix.

Deshmanya and Purohit, et al 2012[6]: presented the details of

developing a mathematical model to predict the tensile behavior like ultimate tensile

strength (UTS) and percentage elongation of the as-cast Al7075/Al2O3 in terms of

size and % fraction of Al2O3, holding temperature and holding time; using factorial

design of experiments. Adequacy of the models was tested using Fishers F -test.

UTS of the composite were increased by 20%compared to that of matrix and %

elongation which was reduced by around 30%.

2.3 Summary

From the previously surveyed researches, It is clear that the MMC's, specially

AMC's are promising field for research but still needs additional discovering and

experiments. Since no reliable standards had been recorded yet for nano AMCs'

properties and evaluation, the traditional routes and test are still used. Industrial

production of mentioned composites is still relatively coasty and almost restricted

to labor level. What is aimed in this study is contributing to making production of

AMC's possible by simpler method. This is, addition of particles to the molten


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metal using aluminum capsules. This technique has not been widely used. Yet, no

reliable standards had been recorded. From the researches that have been surveyed

the following point could be summarized:

1-Generally, ceramic particle reinforcement could enhance mechanical properties

of the metal matrix composites.

2- The main challenge in nano MMC's production is how to achieve regular

distribution of the particles throughout the material structure.

3- In production of the AMC's, best results are obtained by using in situ, stir

casting, powder metallurgy and ultrasonic techniques.

4-The mechanical properties increase with as used particle size decreases.5- Among properties enhanced using particle reinforcement, wear rate resistance

is the most enhanced property.


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Chapter Three

Theoretical Aspects

3.1 Aluminum-based Metal Matrix Composites

Aluminum-based metal matrix composites (AMCs) are ideal materials for

structural applications in the aerospace and automotive industries due to their high

strength-to-weight ratio. Reinforcing the ductile aluminum matrix with stronger and

stiffer second-phase reinforcements such as oxides, carbides, borides, and nitrides

provides a combination of properties of both the metallic matrix and the ceramic

reinforcement components. This may result in improvement of physical and

mechanical properties of the composite. In recent years, many researchers have paid

attention to new fabrication techniques for making aluminum matrix composite.

High-energy ball milling can be named as one of the most important of such

techniques. Mechanical alloying (MA) is a solid-state powder processing technique

involving repeated deformation, welding and fracturing of powder particle. MA has

been widely used to synthesize a variety of materials, such as amorphous alloys ,

Nano crystalline materials , intermetallic compounds , borides, carbides , nitrides

and composites. In almost all cases, the final product has nanosized structure which

exhibits better properties and performance compared to the conventional coarse-

grain materials[19].

3.2 Mechanisms of Strengthening in MMCs

The characteristics of metal matrix composite materials are determined by their

microstructure and internal interfaces, which are affected by their production and

thermal mechanical prehistory. The microstructure covers the structure of the matrix

and the reinforced phase. The chemical composition, grain and/or sub-grain size,

texture, precipitation behavior and lattice defects are of importance to the matrix.

The second phase is characterised by its volume percentage, its kind, size,


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distribution and orientation. Local varying internal tension due to the different

thermal expansion behavior of the two phases is an additional influencing factor.

With knowledge of the characteristics of the components, the volume percentages,

the distribution and orientation it might be possible to estimate the characteristics of

metallic composite materials. The approximations usually proceed from ideal

conditions, i.e. optimal boundary surface formation, ideal distribution (very small

number of contacts of the reinforcements among themselves) and no influence of

the component on the matrix (comparable structures and precipitation

behavior)[44].

Early materials studies led to the computation of the theoretical strengths of perfectcrystals, which were many times greater than those actually measured. During the

1930s it was theorized that this discrepancy in mechanical strengths could be

explained by a type of linear crystalline defect that has since come to be known as a

dislocation. It was not until the 1950s, however, that the existence of such

dislocation defects was established by direct observation with the electron

microscope. Since then, a theory of dislocations has evolved that explains many of

the physical and mechanical phenomena in metals.

Metallurgical and materials engineers are often called on to design alloys having

high strengths yet some ductility and toughness; ordinarily, ductility is sacrificed

when an alloy is strengthened. Several hardening techniques are at the disposal of

an engineer, and frequently alloy selection depends on the capacity of a material to

be tailored with the mechanical characteristics required for a particular application.

Important to the understanding of strengthening mechanisms is the relation between

dislocation motion and mechanical behavior of metals. Because macroscopic plastic

deformation corresponds to the motion of large numbers of dislocations, the ability

of a metal to plastically deform depends on the ability of dislocations to move.

Since hardness and strength (both yield and tensile) are related to the ease with

which plastic deformation can be made to occur, by reducing the mobility of

dislocations, the mechanical strength may be enhanced; that is, greater mechanical

forces will be required to initiate plastic deformation. Virtually, all strengthening


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techniques rely on this simple principle: restricting or hindering dislocation motion

renders a material harder and stronger [45].

3.3 Strengthening by Particles

Metals and metal alloys may be strengthened and hardened by the uniform

dispersion of several volume percents of fine particles of a very hard and inert

material. The dispersed phase may be metallic or nonmetallic; oxide materials are

often used. the strengthening mechanism involves interactions between the particles

and dislocations within the matrix[45].The strengthening of particulate MMCs may

be due to different mechanisms[46]:

* Orowan strengthening

* grain and sub-structure strengthening

* quench strengthening

* work hardening

Orowan strengthening theory could explain the strengthening mechanism of metal

matrix composites. The mechanism is shown schematically in Figure (3-1) below.

According to this mechanism the yield stress is determined by the stress required for

a dislocation line to pass by the two particles shown. The dislocation line is bowed

around the two particles as the applied stress is increased until the dislocation line

reaches a critical curvature (stage 2). When this critical curvature is reached the

dislocation line can then move forward without increasing its curvature (stage 3).

The segments of dislocation line on either side of each particle then join, and a

dislocation loop is left around each particle. As each dislocation line moves past a

particle the dislocation cell structure around the particle builds up.


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Fig. 3-1 Orowan Strengthening Theory[46]

By scaling down the particle size from micro-meter to nano-meter, better material

properties could be obtained. It has been reported that with a small fraction of nano-

sized reinforcements, nano-size dispersion composite could obtain comparable or

even far superior mechanical properties than those of micro-metric dispersion

strengthened MMCs. For example, the tensile strength of 1 vol% Si3N4(10nm)-Al

composite which is fabricated by powder metallurgy method, is comparable to that

of a 15 vol% SiCp(3.5 m)-Al composite fabricated by the same technique[24]. The

yield strength of the nano-metric dispersion composite is much higher than that of

the micro-metric dispersion reinforced composite [45].

Summary of characteristic properties of particle reinforcements is given below [47]:

1 when strength is not main objective

2 improved stiffness

3 reduced wear

4 controlled thermal expansion

5 increased service temperature

3.4 Liquid State Route of AMMCs

Liquid state route involves the incorporation of reinforcement into a liquid

aluminum alloy or the infiltration of a preform (e.g. pressure or vacuum infiltration,


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squeeze casting). These routes are very attractive because they are simple, cheap

and can be applied for the production of complex three-dimensional compounds. It

is also feasible to produce parts with local reinforcements. Basic foundry

technologies can easily be adapted for the fabrication of discontinuously reinforced

aluminum composites. The main drawbacks of the liquid state routes are the lack of

wetting of the reinforcements (mainly ceramics) by liquid aluminum, the

development of casting defects (shrinkage, gas holes) in the final product, the

insufficient bonding between reinforcement and the matrix or/and the degradation

of the reinforcement by excessive reaction[44].


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Fig. 3-2 Liquid state Process [44]

3.5 Stir Casting Process

One of significance is the difficulty of achieving a homogeneous distribution of

reinforcement in the matrix, essential for optimum mechanical properties. This

problem is common to most production routes, including the stir casting process

which, using particulate reinforcement, offers the possibility of producing relatively

complex shaped composites. It is important to identify and control casting process

parameters relevant to reinforcement distribution in order to achieve a good qualitycomposite.

There are however problems associated with the production of reinforced

composites, one of significance being the difficulty of achieving a homogeneous

distribution of reinforcement in the matrix, essential for optimum mechanical

properties . This problem is common to most production routes, including the stir

casting process which, using particulate reinforcement, offers the possibility of

producing relatively complex shaped composites. It is important to identify and

control casting process parameters relevant to reinforcement distribution in order to

achieve a good quality composite. In general stir casting of MMCs involves

producing a melt of the selected matrix material, followed by the introduction of a

reinforcement material into the melt, obtaining a suitable dispersion through

stirring. The next step is the solidification of the melt containing suspended particles

under selected conditions to obtain the desired distribution of the dispersed phase in

the cast matrix. The emphasis has been given to MMCs fabricated by the stir casting

technique. In order to design a process to give a homogeneous distribution of

reinforcement the following factors need to be understood [48]:

1- Particle density, size, shape and volume fraction will influence the reinforcement

settling rate,

2- Surface properties of particles will affect the ease or difficulty with which

wetting is achieved.


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3- The rheological behavior of the matrix alloy melt and reinforcement slurry is

influenced by the reaction of particles with the liquid, and with each other.

4- Gas entrapment during particle incorporation or mixing will lead to poor

distribution as particles attach to gas bubbles, and porosity is also increased.

5-Mixing parameters must be such as to produce a uniform particle distribution in

the radial and axial directions.

6- During solidification settling time must be minimized.

7- In general the reinforcement particles occupy inter-dendritic or between

secondary dendrite arm spacing; therefore the finer the spacing or the finer the

matrix grain size, the better the particle distribution.

3.6 Differentiation between Microscale and Nanoscale

Reinforcement Particles

Large-particle and dispersion-strengthened composite are the two sub classifications

of particle-reinforced composites. The distinction between these is based upon

reinforcement or strengthening mechanism. The term is used to indicate that

particlematrix interactions cannot be treated on the atomic or molecular level;

rather, continuum mechanics is used. For most of these composites, the particulate

phase is harder and stiffer than the matrix. These reinforcing particles tend to

restrain movement of the matrix phase in the vicinity of each particle. In essence,

the matrix transfers some of the applied stress to the particles, which bear a fraction

of the load. The degree of reinforcement or improvement of mechanical behavior

depends on strong bonding at the matrixparticle interface.

For dispersion-strengthened composites, particles are normally much smaller,

having diameters between 0.01 and 0.1 m (10 and 100 nm). Particlematrix

interactions that lead to strengthening occur on the atomic or molecular level. The

mechanism of strengthening is similar to that for precipitation hardening. Whereas


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the matrix bears the major portion of an applied load, the small dispersed particles

hinder or impede the motion of dislocations. Thus, plastic deformation is restricted

such that yield and tensile strengths, as well as hardness, improve [45].

3.7 WeightVolume Conversion

Most of the rules that deal with composites are in term of volume fraction Vfrather

than weight fraction Wf., it is worthy to estimate volume in term of weight. It should

be known that:

Density = W/ V, So:

V = W/ .. (3-1)

3.8 Method Used to Determine the Volume Fraction of the

Matrix and Reinforcement

Many researchers carried out studies about this type of reinforcement and they all

agreed about the methods to calculate the moduli of elasticity and rigidity [49],

assuming that:

1- The particles are randomly distributed in the matrix.

2- The particles are of the same size.

3- The particles are bonded with the matrix strongly.

4- Each of the particles and the matrix is isotropic.The law of mixture [50]: -

The mass (Mc) of composite is made up of the masses of the matrix (m) and the

filler particle Mr

Mc= Mm+ Mr . (3-2)

Since the mass is volume times density then equation can be written as below: -

Vcc=Vmm+ Vrr


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... (3-3)

where

is the volume fraction of the matrix and

volume fraction of fiber

c= mVm+ rVr ...(3 -4)

noted that sinceVm= VcVrit must have:

Vm = 1-Vf.. (3-5)

Also the mass of the matrix and the mass of reinforcement material can be

calculated as follows: -

SinceVf =

Then Mr= Vc VrPr

And Mm= Vc Pm(1-Vr) .(3-6)

3.9 Law of Mixture

The mass (Mc) of composite is made up of the masses of the matrix (Mm) and the

filler particle (Mr),

Mc= Mm+ Mr .. (3-2)

Since the mass is volume time's density then

equation (3-3),for density , can be written as follows[46,49]: -

c= mVm+ rVr .(3-4)

3.10 Principles and Properties of Dislocation Reinforcement

Composite materials technologies offer a unique opportunity to tailor the properties

of aluminum. This could include increased strength, decreased weight, higher

service temperature, improved wear resistance, higher elastic modulus, controlled

coefficient of thermal expansion, improved fatigue properties, etc.


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It is of utmost importance to have rules or models in order to predict or to calculate

the expected properties of the composite. As a first estimation, the rule of mixtures

can be helpful. That is[46]:

Pc = PmVm + PrVr (3-10)

with P = property

V = volume fraction

and subscript c, m and r indicate resp. composite material, matrix and

reinforcement. Youngsmodulus (E) is an elastic property which is well bracketed

by two models. Figure (3-3)illustrates the evolution of the Young's modulus of an

Al-SiCcomposite as a function of the volume fraction of SiC.

Fig. 3-3 The evolution of the Young's modulus of an Al-SiC[46]

Linear bound in fig (3-3) is defined by the simple rule of mixtures:

Ec = Em Vm + ErVr ,,,,,,,,.(3-11)

The nonlinear bound (see dotted line in 3-3) is given by a more complex expression

(valid for discontinuously reinforced composite with spherical particles as

reinforcement) :


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(3 -12)

3.11 Basics of Wettability

Basically the wettability of reinforcement with a metal melt can be shown by the

edge angle adjustment of a molten droplet on a solid base as the degree of

wettabilityaccording to Young:

SALS = LA cos

where LA is the surface energy of the liquid phase,SA the surface energy of the

solid phase, LS the interface energy between the liquid and solid phases and is

the edge angle.

Figure (3-4) shows the edge angle adjustment of a molten droplet on a solid base for

different values of the interface energy. At an angle of > /2 a nonwettable system

is described and for an angle limit of < /2 a wettable system. With decreasing

angle the wettability improves. In Table 1.3 the surface and interface stresses of

selected metal ceramic systems at different temperatures are summarized. Of

special relevance is the system Al/SiC, since it is the basis for the melting

metallurgy of particle reinforced aluminum composite materials.

As the contact develops, for example at the beginning of an infiltration, adhesion

occurs. The adhesion work WA for separation is [46]:

WA = SALA =LS

WA = LA (1 + cos)


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In the case of immersion the interface between the solid and the atmosphere

disappears, while the interface between the solid and the liquid forms. The

immersing work WI is:

WI = LSSA

In the case of spreading the liquid is spread out on a solid surface. During this

procedure the solid surface is reduced as well as a new liquid surface being formed

and hence a new solid/liquid interface is formed. The spreading work WS is:

WS = SALS LA

The wetting procedure is kinetic and is dependent on time and temperature.

Therefore, the kinetics can be affected by the temperature.

Fig. 3-4 Edge angle adjustment of a melt drop on a solid base for various values of the interface energy.

3.12 7000 Serious Aluminum Alloys as Matrix Reinforced with

Ceramic Particles

Among all the commercial aluminum alloys, the 7000 series aluminum alloys

exhibit the highest mechanical properties. They are used for high strength structural


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applications such as aircraftparts and sporting goods. The desirable properties of

these alloys are: low density, high stiffness, specific strength, good wear resistance

and creep resistance [42].

Aluminum metal matrix composites (MMCs) with 7075alloy as matrix reinforced

with ceramic particles are finding application in aerospace, automobile and farm

machinery equipment because of their improved mechanical and tribological

properties . In particular, Al2O3particles mixed with Al7075 matrix in appropriate

proportions are reported to exhibit improved tensile properties because of the higher

modulus of elasticity and strength of the alumina [6]. However, it is difficult to

reach the same strength in the composite as in the monolithic alloy, for 2xxx, 6xxxand 7xxx based composites[46,47].Existence of Mg in 7075 composition make it

good candidate for particle reinforcement since Mg is considered a wetting

agent[51,52]

3.13 Features of Al

Al2O3 System

The system shows high stability at elevated temperatures. It resists softening at

high temperatures. Alumina does not react with matrix at high temperature and does

not create any undesired phase and the system has superior properties such as

hardness, fracture toughness as compared to the native material. High thermal

stability of alumina particles prevents the particles from ripening and

dissolving[55].

3.14 Addition of Particles using Aluminum Capsules

One of the common challenges in preparing AMC's is the addition of the second

phase of the composite, i.e. the reinforcement. The basic issue is how to get regular

distribution. In practical work of this thesis, an Aluminum capsulated Al2O3powder


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Chapter Four

Experimental Work

4.1 Introduction

In this chapter, the Experimental procedure is explained. These steps are based

on previously investigated researches and studies.

4.2 Preparations of Basic Materials

A 7075 aluminum shafts were chosen. Their surface was cleaned by Iron brush to

remove any undesired oxides and containments. It was washed with alcohol then

with Acetone. The alloy chemical composition was examined using SPECTRO

metal analyze. The composition is given in table (4-1).

Table 4-1 Chemical composition

NiCr Mg Mn Cu Fe Si

0.00360.1902.110.02441.730.295

0.0756

Al Zr V PbBeTiZn

balance0.00140.0101 0.00930.00420.02935.53

The shaft was cut to smaller pieces for easier melting and to meet crucible size.

The pieces were weighted to know melting charge according to desired weight

fraction of each cast as in figure (4-1).


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Fig. 4-1 7075 alloy scrap

The Al2O3 powder is Gamma white 20 nm in size with specific surface area of 230-

400 m2/g ( SSNano,USA). The powder was charged in known weight commercial

aluminum foil with known weight. In first trial the capsule weight was 1g and filled

with 1g of alumina powder. A 0.01 g accuracy balance was used to prepare alumina

capsules as in figure (4-2).

Fig. 4-2 Capsule preparation


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4.3 Furnace Description

The furnace used in experiment is constructed from graphite crucible and heater

element controlled by Ktype thermal controller + stirring system.(figure 4-3).The

stirrer has four blades with 45o

inclination [56] rotated by hand drill electrical

motor and its speed set at 300 rpm [15, 53, 56]. A K-type thermocouple is attached

to the crucible base to give accurate temperature feedback to thermal controller .the

temperature was set at 700 CO [57, 37].

Fig. 4-3 furnace

4.4 Mold

The mold was manufactured of stainless steel with ability of disassemble to prevent

cast sticking or entrapment .Its dimensions are 120 x 100x 14 mm vertically stand

with open upper side to be a raiser as in fig(4-4) .


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Fig. 4-4 Mold

4.5 Casting Procedure

1 -First Trial

The furnace with the crucible was preheated for 1 hour with aluminum scrap charge

to remove undesired moisture, then temperature was raised up to 700 Co

.When the

all scrub melt first alumina capsule was introduced and stirrer was lowered for

stirring .Introducing of other capsules was in rate of 1 capsule per min. until the

desired amount was completely introduced .The stirring continued for total time of

5 min [57] and AlCl3was usedto degas the melt .Then the melt was poured slowly

in the mold .

After the cast was cooled, it was machined to examine interior places of the cast.

Agglomerations of alumina powder were observed by macro examination.

2 -Second Trial

First trial procedures were repeated but with capsules of thicker foil and less powder

weight .The stirring time was 20 min.[37] machining the sample showed no

agglomeration of particles on macro level.

In the first trial capsules were: 1g Al + 1g alumina powder

In the second trial capsules were 3g Al + 0.5 alumina powder


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3- Last Trial

In the last trials the charges (Aluminum scrub +alumina capsules ) were prepared as

following in table (4-2) .Each capsule contained 0.5g of alumina.

Table 4-2 sample charges

Aluminum Capsules

weight x No.

Alumina weight7075 alloy scrub weightSample

4 x 3g =12 g2g386 g0.5 %

8 x 3g = 24g4g372 g1%

12 x 3g = 36g6g358g1.5%

The final cast specimen was machine cut to visually examine porosity and

agglomeration as in figure (4-5).


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Fig. 4-5 Sample machining and cut

Then the samples were heat treated at 200o

for 12 hour and then quenched in cold

water from about 200COcooling rate for internal stress relief and grain refinement

[58].

4.6 Preparing samples and tests

4.6.1Impact test:

Samples were prepared for impact test by machining to dimensions of 10 x10 x 50

mm with thin saw was cut in the middle with 8m height under the saw cut

(charpy)[45] . The tests were carried out using BROOKS charpy Impact test

machine as shown in figure (4-6). Two readings were taken for each sample.


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Fig. 4-6 Impact test specimen [45] and Impact test instrument

4.6.2 Wear Rate Test:

Dry sliding wear resistance test was carried out on all samples according to ASTM

G99[59]with sample of 10mm in diameter and 16.5 mm in length, taking the load as

the alternative parameter with (500,1000 and 1500 gram) [60]and a rotating speed

of disc was set to 500 RPM. The pin was located at a radial distance of 60 mm from

the rotating axis, for each 8000 revolution the sample was taken out, cleaned and

weighted with 0.0001 g accuracy balance.

Two readings were taken for each case to plot the mass loss against the load. Figure

(4-7)shows the pin on disc and testing of samples.

Fig. 4-7: Sample in pin on disc device


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4.6.3 Tensile Test:

Tensiletest samples were prepared according to ASTM -E 8M sub sized specimen

[61].Thespecimens were made using wire cut CNC machine.The test was carried

out in Al-Nahrain University-Applied Mechanics Laboratory using GUNT WP310

instrument at speed of 0.1mm/s as shown in figure(4-8).

(a)

(b)


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Fig.4-8 Tension test, (a) samplespreparing,(b) apparatus

4.6.4Hardness Test:

Brinel hardness testing device which is located in State Company for Inspection and

Engineering Rehabilitation (SIER) had been used, shown in figure (4-9) with a 50

gram load. The tests were carried out on all samples with a holding time of 2

second. Six readings of hardness were taken for each sample to get the mean value.

The values were then converted to Vickers hardness by using standard chart of the

device.

Fig. 4-9 Hardness instrument

4.7 Physical Tests

Porosity of each sample was calculated by comparing theoretical and apparent

density .Theoretical density was calculated using law of mixture and apparent

density was measured using Archimedes law.

Three similar samples were prepared for X-ray diffraction (XRD), Optical

Microscope, Atomic Force Microscope (AFM) and scanning electron microscope

(SEM). To prepare the samples for the test, samples of (SEM) and Optical

Microscope were cut and mounted using acrylic material for easy handling in

grinding and polishing stage.


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An etching process was done to reveal the grain boundary using two kinds etching

solutions which were HNO3+H2O and NaOH [62].

Scanning Electron Microscope (SEM) and X-Ray Differaction (XRD) tests were

carried out in the Ministry of Sciences and Technology / Material Science

department. 40 kV with Cu K radiation (=0.15406 nm) was used for XRD

measurements. The XRD patterns were recorded in the 2 range of 3080 (step

size 0.05 and speed 5 degree/min.). Microstructure Image analysis and (AFM)

were carried out In Al-Nahrain University /Mechanical engineering Department

Laboratories . Microstrucure images were obtained using Olympic type optical

microscope with a max magnification of 1000X. Fig (4-10) shows the optical

microscope.

Atomic Force Microscopy (AFM) was used to capture the surface topography and

the surface and measure the materials AFM topographic images were obtained in

air at room temperature 25 and 27% relative humidity. Images were acquired and

processed with the NanoScope 531r software.

Fig. 4-10 Optical microscope


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Chapter Five

Results and Discussion

5.1 Introduction

In this chapter results of physical and mechanical tests results of all samples will

be illustrated and discussed.

5.2 Physical Results

The results of Physical properties and their discussion can be given as follows:

5.2.1 Density and Porosity Measurement

The experimental density of the composites was obtained by the archimedean

method by weighing small pieces cut from the composite cylinder first in air andthen in water. In addition, the theoretical densities were calculated using the mixture

rule.

Density of Al203 =3.92g/ cm3[3]

1 g of Al203 = 1/ 3.92 = 0.2551 cm3

Porosity = (Theoretical DensityApparent Density)/Theoretical Density

The Al 7075 and the reinforcement Al2O3particles have the densities of 2.7 and 3.9

g/cm3, respectively [22]. In order to determine the porosity content, density

measurements were conducted on unreinforced and composites reinforced with 0.5,

1 and 1.5 wt. % nano-Al2O3 particles. The difference between the theoretical

densities and calculated densities which were obtained are given in table (5-1) .


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Table 5-1 porosity

Sample Theoretical

density(g/cm3)

Apparent or practical

density (g/cm3)

Porosity %

0.5 % 2.7880 2.7810 0.251

1 % 2.7925 2.7768 0.562

1.5 % 2.7921 2.7748 0.6196

Fig. 5-1 The variation of porosity

The variation of porosity with volume percent with nano alumina volume percent is

shown in fgure (5-1) The figure shows that increasing porosity volume percent has

occurred with increasing alumina weight fraction . This is due to the effect of low

wettability and agglomeration at high content of reinforcement and pore nucleation

at the matrixAl2O3 interfaces. Moreover, decreasing liquid metal flow associated

with the particle clusters leads to the formation of porosity , and also the hydrogen

content of molten increasing viscosity due to particle addition and entrap gassed

inside the melt and prevent them from floating before it solidify. Porosity highly

effect the mechanical properties by decreasing them.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1 2 3


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Fig, 5-2 Variation of porosity with the nano alumina content [38]

5.2.2X-ray Diffraction Results

The XRD spectrum of Al 7075 with 0.5wt.%, 1% and 1.5%Al2O3 nanocomposite

powder is shown in figure (5-3) The diffraction patterns of the nanocomposite

exhibit various peaks corresponding to the face centered cubic phase of Al. It

appears that the addition of ultra-fine Al2O3 resulted in further broadening of Al

peaks. X-ray diffraction peaks, which increased with an increase of fine Al2O3

content. The peak width of 1.5 wt.% Al2O3nanocomposite was increased by around

25% higher than unreinforced nanocrystalline alloy. This indicated the formation of

fine grain. Further, a minor shift (towards smaller angle) in the position of the XRD

peaks was also noticed and this could be related to the dissolution of little oxygen

atoms from alumina and other atoms like Mg, Cu etc. related to minor matrixalloying elements in the lattice of aluminum.


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(a-0.5%)

(b-1%)


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(c-1.5%)

Fig 5-3 a, b and c shows XRD for different wt. fractions

It is shown that difference in intensity not in 2dfferaction this is perhaps due to

small additives from Al2O3 and alloying elements which are not clear affected in the

differaction of X-ray.

5.2.3 Microstructure Observation

A-Optical Microscope

Fig (5- 4) shows Micro structures of different wt. fractions as well as the scale.


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(a)

(b)

(c)

Fig 5-4: Microstructures, (a) 1.5%, (b) 1%, (c) 0.5 %

Optical micro structures show that there are several areas in the cast for three samples. It

could be said that practically, porosity appears even with smallest fraction of alumina

addition. 7075 AL alloy is very sensitive to cooling stage in heat treatment [58].It is clear

that the grain size range is about 70-200 m .Considering it is not cold worked and

assuming there is no porosity ,this grain size range indicates good mechanical properties ,


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However ,mechanical properties do not depend on grain size only, but also on the bonding

between them[45].Grain boundaries tend to enlarge with increasing Al2O3 wt. fraction due

to movement of particles (if it is dealt with as impurities out of the grain) during

solidification stage [63].

B- Scanning Electron Microscope

Figure( 5-5) shows the (SEM) images for the three samples

(a)

(b)


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(c)

Fig.5-5 :( SEM) micrographs,(a)0.5%,(b)1%,(c)1.5%

It is clear from micrographs above that the alumina particles are distributed. The

particles cannot be considered as distributed regularly all over the matrix since these

micrographs represent a small portion of the material. However, as stir casting

technique is used , it can be successful technique applicable to industrial purposes

and mass product . For more regular distributed and efficient particles, ultrasonic

technique showed better result [27]. This technique is still costly high to be

applicable in industrial and mass productions. Figure (5-4) shows SEM micrographs

of the as-cast Al2%Al2O3nanocomposites synthesized using ultrasonic technique.

In 0.5% a pitting like point appears .It is expected to be formed during

solidification process as a result of clustering of alumina and entrap of gasses by

it,as well as the deference of thermal expansion between it and Al matrix . In 1.5%

image it is clear that there are eutectic zones caused by precipitation of alumina

particles on grain boundary during grain growth stage.


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The morphological changes of the metal nanocomposite investigated by From Fig.

1, it can be observed that the morphology Surface roughness is also an important

parameter composite application. surface roughness of the composite increase with

increasing the volume fraction of the reinforcement. These results are further

confirmed by AFM images, which show a increases in RMS roughness of the

nanocomposite It is widely established that addition of a nanocrystalline phase

affects the rod-like structure and crystallite size, which determine the surface

morphologies of the nanocomposite [63].Figure (5-7) for Aluminum with 0.5%

20nm alumina, figure (5-8) pure Aluminum and figure (5-9) Aluminum reinforce

with micro size Alumina.

Fig. 5-8 Atomic force microscopy (AFM) images of aluminum


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Fig.5-9 (AFM) images of aluminum reinforce with micro alumina [57]

5.3 Mechanical Results

Mechanical results and their discussion can be given as follows:

5.3.1 Hardness

Table (5-2) shows hardness results of the original alloy and different wt. fractions. This

change can be stated as the particle was reinforcing the structure by preventing the lattice

movement and also it indicates the good binding force between the particle and the

structure. This is not to be explained for 1.5% which, as previously mentioned, has weak

grain boundary .These results are a little higher than 100 micron SiC /LM6 Al alloy but

with higher wt. % [14] .

Table (5-2):Vickers hardness for all samples

sample Hv

0.5% 134.33

1% 105.6


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1.5%


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Table 5-3 mass loss against applied load

Sample load 500g 1000g 1500g

0.5% 0.0225 0.0404 0.8715

1% 0.085 0.0206 0.0225

1.5% 0.0053 0.0100 0.015

(a)

(b)

0

0.2

0.4

0.6

0.8

1

1 2 3

0

0.02

0.04

0.06

0.08

0.1

1 2 3


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(c)

Fig. 5-11mass loss against load ,(a)0.5%,(b)1%,(c) 1,5%

Above results agree with those of CNT reinforced 6061 Al alloy [64](figure 5-12)

Fig.5-12 1%CNT/6061 wear rate[65]

5.3.3 Fracture Toughness Results

Table (5-4) shows the fracture toughness of the samples. It is clear that there is

improvement in 1 and 1.5 wt fractions. But for 1.5 sample ,again, the voids caused a

reduction in fracture toughness .Brittleness behavior increased with increasing wt. fraction.

0

0.005

0.01

0.015

0.02

1 2 3


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If the high porosity is kept in mind ,above results agree with those of 35nm

Al2o3/pure Aluminum 2,3,4 wt.%[29] .As mentioned before the initial strength of

7075 Al alloy cannot be easily enhanced[52,46].

Tensile test specimens showed Brittle Failure .Therefore, there is no clear

measurement for yield strength. The Results showed a small increase upon o-

tempered 7075 Al alloy[3].

Generally , when talking about enhancement effect of particle reinforcement and

comparing different researches, It is noticed that there is an efficiency for

strengthening .That is the finer the particles the higher enhancement . It is also

noted that there is some kind of relation between different particle sizes and

increase in mechanical property . For example , the effect of 2.5 wt% of 100 micron

Al2O3 is very close or equivalent to 1wt.% of 20 nm[14].

Chapter Six

Conclusions and Recommendations

6.1 Conclusions

It is clear that the addition of ceramic nanoparticles to the Aluminum alloy

generally enhances its properties. However, the main challenge is How to achieve

regular or good distribution of particles across the matrix (The Aluminum

alloy).Not only this, But also how to produce final composite. The routes ofproduction Is very similar to those in ordinary composites. For example, achieving

good porosity for the material which has an important role in production of any

MMC. For this thesis, conclusions could be summarized with following points:

1- The best results regarding enhancement of mechanical properties are wear

resistance, fracture toughness and micro hardness but not tensile strength.

2- Increasing stirring time can reduce agglomeration of added particles.


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3- Heat treatment refines grain size and consequently enhances mechanical

properties

4- Reducing capsule particle content and increasing its thickness lead to reduction

in particles agglomerations.

6.2 Recommendations for Future Work

it is obviously known that subject of reinforcement of Aluminum using ceramic

nanoparticle is very promising and can be applied in wide fields of application

Although It is not standardized yet . It is very fertile subject for researches; however

it is divergent, especially in routes and materials. May be in future new routes will

be applied. Industrial application and mass production of AMCs reinforced with

ceramic nanoparticles are still limited. So, it is more useful for researchers to find or

Investigate methods and techniques that are easier to be utilized in industrial level

of production. There should be an optimization between production quality and its

cost. Regarding present thesis and starting from the availability of initial materials

following recommendations are put forward:

1- Repeat the stir casting under inert gas flow, for example: Argon.

2- Enhancement capsule addition method by focusing and improvement of used

capsules.

3- Using more than one particle size for same fraction.

4-Designing and modeling more molds and cast techniques.

5- Reinforcement of more than Aluminum alloy for same particle kind and size.

6- Study the effect of work hardening on produced AMC .


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And for far future standardization the following flow chart is designed to clarify the

recommendations for Industrial applications:

AMC using

Nanopoweder

Matrix alloy Reinforcement

Production method

Liquid state

or casting

Powder

metallurgy

Size

kind

Squeez casting

Sand

Introduction of

reinforcement


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Fig (6-1) recommendationsflow chart

References

[1]. Daniel B. Miracle and Steven L. Donaldson "Composites Handbook

,Vol.21. ASM International (2001).

[2] Karl U. Kainer"Metal Matrix Composites. Custom-made Materials for

Automotive and Aerospace Engineering" , WILEY-VCH Verlag GmbH &

Co. KGaA, Weinheim ISBN: 3-527-31360-5(2006).

[3]. Pradeep K. Rohatgi; Ben Schultz, J.B. Ferguson, ChemFiles

"Nanomaterials for Advanced Applications,Vol. 5 No. 3(2005).

[4]. J. Gilbert KaufmanIntroduction to Aluminum Alloys and Tempers"

,ASM International(2000).

Mechanical

alloying

Capsules

Direct and

preheating


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[5]. N. Parvinand M. Rahimian" The Characteristics of Alumina Particle

Reinforced Pure Al Matrix Composite", ACTA PHYSICA POLONICA, Vol.

121 No. 1 (2012).

[6]. Indumati B. Deshmanya, Dr. GK Purohit " Modelling T

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