Table of Contents

1. Introduction 3

2. Metal Matrix Composites (MMC) . 5

3. Aluminum Metal Matrix Composites 6

4. Types of Aluminum Metal Matrix Composites . 7

4.1. Particle reinforced aluminum matrix composites (PAMCs) ... 7

4.2. Short fiber- and whisker-reinforced aluminum matrix composites (SFAMCs) . 8

4.3. Continuous fiber-reinforced aluminum matrix composites (CFAMCs) . 8

4.4. Mono filament reinforced aluminum matrix composites (MFAMCs) ... 8

5. Al-MMC Application in Brake Rotors .. 9

5.1. Properties of The Ultilite Composites ... 10

5.2. Testing of Ultilite Drums .. 11

6. References 12

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1. Introduction

Metal composite materials have found application in many areas of daily life for quite some

time. Often it is not realized that the application makes use of composite materials. These

materials are produced in situfrom the conventional production and processing of metals. Here,

the Dalmatian sword with its meander structure, which results from welding two types of steel by

repeated forging, can be mentioned. Materials like cast iron with graphite or steel with a high

carbide content, as well as tungsten carbides, consisting of carbides and metallic binders, also

belong to this group of composite materials. 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 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 new materials are basically

determined by the properties of their single components.

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The reinforcement of metals can have many different objectives. The reinforcement of light

metals opens up the possibility of application of these materials in areas where weight reduction

has first priority. The precondition here is the improvement of the component properties. The

development objectives for light metal composite materials are:

Increase in yield strength and tensile strength at room temperature and above while

maintaining the minimum ductility or rather toughness,

Increase in creep resistance at higher temperatures compared to that of conventional

alloys,

Increase in fatigue strength, especially at higher temperatures,

Improvement of thermal shock resistance,

Improvement of corrosion resistance,

Increase in Youngs modulus,

Reduction of thermal elongation.

To summarize, an improvement in the weight specific properties can result, offering the

possibilities of extending the application area, substitution of common materials and

optimization of component properties. With functional materials there is another objective, the

precondition of maintaining the appropriate function of the material. Objectives are for example:

Increase in strength of conducting materials while maintaining the high conductivity,

Improvement in low temperature creep resistance (reactionless materials),

Improvement of burnout behavior (switching contact),

Improvement of wear behavior (sliding contact),

Increase in operating time of spot welding electrodes by reduction of burn outs,

Production of layer composite materials for electronic components,

Production of ductile composite superconductors,

Production of magnetic materials with special properties.

For other applications different development objectives are given, which differ from those

mentioned before. For example, in medical technology, mechanical properties, like extreme

corrosion resistance and low degradation as well as biocompatibility are expected.

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2. Metal Matrix Composites (MMC)

As the name implies, for metal-matrix composites (MMCs), the matrix is a ductile metal. These

materials may be utilized at higher service temperatures than their base metal counterparts;

furthermore, the reinforcement may improve specific stiffness, specific strength, abrasion

resistance, creep resistance, thermal conductivity, and dimensional stability. Some of the

advantages of these materials over the polymer-matrix composites include higher operating

temperatures, non-flammability, and greater resistance to degradation by organic fluids. Metal-

matrix composites are much more expensive than PMCs, and, therefore, their (MMC) use is

somewhat restricted.

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are employed

as matrix materials. The reinforcement may be in the form of particulates, both continuous and

discontinuous fibers, and whiskers; concentrations normally range between 10 and 60 vol%.

Continuous fiber materials include carbon, silicon carbide, boron, alumina, and the refractory

metals. On the other hand, discontinuous reinforcements consist primarily of silicon carbide

whiskers, chopped fibers of alumina and carbon, and particulates of silicon carbide and alumina.

In a sense, the cermets fall within this MMC scheme. In Table 15.6 are presented the properties

of several common metal-matrix, continuous and aligned fiber-reinforced composites.

Some matrixreinforcement combinations are highly reactive at elevated temperatures.

Consequently, composite degradation may be caused by high-temperature processing, or by

subjecting the MMC to elevated temperatures during service.

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This problem is commonly resolved either by applying a protective surface coating to the

reinforcement or by modifying the matrix alloy composition.

Normally the processing of MMCs involves at least two steps: consolidation or synthesis (i.e.,

introduction of reinforcement into the matrix), followed by a shaping operation. A host of

consolidation techniques are available, some of which are relatively sophisticated; discontinuous

fiber MMCs are amenable to shaping by standard metal-forming operations (e.g., forging,

extrusion, rolling).

3. Aluminum Metal Matrix Composites

In AMCs one of the constituent is aluminum/aluminum alloy, which forms percolating network

and is termed as matrix phase. The other constituent is embedded in this aluminum/aluminum

alloy matrix and serves as reinforcement, which is usually non-metallic and commonly ceramic

such as SiC and Al2O3. Properties of AMCs can be tailored by varying the nature of constituents

and their volume fraction.

The advantages of the reinforced matrix can be quantified for better appreciation. For example,

elastic modulus of pure aluminum can be enhanced from 70GPa to 240GPa by reinforcing with

60 vol.% continuous aluminum fiber. On the other hand incorporation of 60 vol% alumina fiber

in pure aluminum leads to decrease in the coefficient of expansion from 24 ppm/C to 7 ppm/C.

Similarly it is possible to process Al-9% Si-20 vol% SiCp composites having wear resistance

equivalent or better than that of grey cast iron. All these examples illustrate that it is possible to

alter several technological properties of aluminum/aluminum alloy by more than two three

orders of magnitude by incorporating appropriate reinforcement in suitable volume fraction.

AMC material systems offer superior combination of properties (profile of properties) in such a

manner that today no existing monolithic material can rival. Over the years, AMCs have been

tried and used in numerous structural, non-structural and functional applications in different

engineering sectors. Driving force for the utilization of AMCs in these sectors include

performance, economic and environmental benefits. The key benefits of AMCs in transportation

sector are lower fuel consumption, less noise and lower airborne emissions. With increasing

stringent environmental regulations and emphasis on improved fuel economy, use of AMCs in

transport sector will be inevitable and desirable in the coming years.

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AMCs are intended to substitute monolithic materials including aluminum alloys, ferrous alloys,

titanium alloys and polymer based composites in several applications. It is now recognized that

in order AMCs substitution for monolithic materials in engineering system to be wide spread,

there is a compelling need to redesign the whole system to gain additional weight and volume

savings. In fact according to the UK Advisory Council on Science and Technology, AMCs can

be viewed either as a replacement for existing materials, but with superior properties, or as a

means of enabling radical changes in system or product design. Moreover, by utilizing near-net

shape forming and selective-reinforcement techniques AMCs can offer economically viable

solutions for wide variety of commercial applications.

Recent success in commercial and military applications of AMCs is based partly on such

innovative changes made in the component design. Lack of knowledge and information about

utilization possibilities, service properties and material producers have hindered the wider usage

of AMCs. Recognizing these peripheral and extraneous difficulties, AMCs community in USA

and Europe are pursuing consortium and networking approaches to implement the applications of

AMCs in everyday societal use.

4. Types of AMCs

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

4.1.Particle reinforced aluminum matrix composites (PAMCs)

These composites generally contain equiaxed ceramic reinforcements with an aspect ratio less

than about 5. Ceramic reinforcements are generally oxides or carbides or borides (Al2O3 or SiC

or TiB2) and present in volume fraction less than 30% when used for structural and wear

resistance applications. However, in electronic packaging applications reinforcement volume

fraction could be as high as 70%. In general, PAMCs are manufactured either by solid state (PM

processing) or liquid state (stir casting, infiltration andin-situ) processes. PAMCs are less

expensive compared to CFAMCs. Mechanical properties of PAMCs are inferior compared to

whisker/short fiber/continuous fiber reinforced AMCs but far superior compared to unreinforced

aluminum alloys. These composites are isotropic in nature and can be subjected to a variety of

secondary forming operations including extrusion, rolling and forging. Figure 1a shows the

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microstructure of cast aluminum matrix composite having high volume fraction (40 vol%) SiC

particle reinforcements.

4.2.Short fiber- and whisker-reinforced aluminum matrix composites (SFAMCs)

These contain reinforcements with an aspect ratio of greater than 5, but are not continuous. Short

alumina fiber reinforced aluminum matrix composites is one of the first and most popular AMCs

to be developed and used in pistons. These were produced by squeeze infiltration process. Figure

1b shows the microstructure of short fiber reinforced AMCs. Whisker reinforced composites are

produced by either by PM processing or by infiltration route. Mechanical properties of whisker

reinforced composites are superior compared to particle or short fiber reinforced composites.

However, in the recent years usage of whiskers as reinforcements in AMCs is fading due to

perceived health hazards and, hence of late commercial exploitation of whisker reinforced

composites has been very limited. Short fiber reinforced AMCs display characteristics in

between that of continuous fiber and particle reinforced AMCs.

4.3.Continuous fiber-reinforced aluminum matrix composites (CFAMCs)

Here, the reinforcements are in the form of continuous fibers (of alumina, SiC or carbon) with a

diameter less than 20m. The fibers can either be parallel or pre woven, braided prior to the

production of the composite. AMCs having fiber volume fraction upto 40% are produced by

squeeze infiltration technique. More recently 3M Tm corporation has developed 60 vol%

alumina fiber (continuous fiber) reinforced composite having a tensile strength and elastic

stiffness of 1500 MPa and 240 GPa respectively. These composites are produced by pressure

infiltration route. Figure 1c shows the microstructure of continuous fiber (alumina) reinforced

AMCs.

4.4.Mono filament reinforced aluminum matrix composites (MFAMCs)

Monofilaments are large diameter (100 to 150m) fibers, usually produced by chemical vapor

deposition (CVD) of either SiC or B into a core of carbon fiber or W wire. Bending flexibility of

monofilaments is low compared to multifilaments. Monofilament reinforced aluminum matrix

composites are produced by diffusion bonding techniques, and is limited to super plastic forming

aluminum alloy matrices.

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In CFAMCs and MFAMCs, the reinforcement is the principal load-bearing constituent, and role

of the aluminum matrix is to bond the reinforcement and transfer and distribute load. These

composites exhibit directionality. Low strength in the direction perpendicular to the fiber

orientation is characteristic of CFAMCs and MFAMCs. In particle and whisker reinforced

AMCs, the matrix is the major load-bearing constituent. The role of the reinforcement is to

strengthen and stiffen the composite by preventing matrix deformation by mechanical restraint.

5. Al-MMC Application in

Brake Rotors

ULTALITE is a low cost aluminum

metal matrix composite (AL-MMC)

that uses ceramic particles extracted

from flyash as the reinforcement

particulate. Australian developed

ULTALITE composites use ceramic

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particles to increase the wear resistance, provide high thermal conductivity, and give good

machinability for metal components, which is very desirable for brake rotors.

ULTALITE composites typically contain between 15% and 30% spherical ceramic particles.

However, unlike other more expensive composites, ULTALITE uses inexpensive ceramic

particles derived from flyash, a by-product of the power generation industry. It consists of

inorganic, incombustible matter formed during the combustion of powdered coal in electric

power generating plants, and consists mainly of SiO2 and Al2O3, along with other extraneous

impurities.

The flyash material solidifies while suspended in the exhaust gasses, and forms as amorphous,

spherical particles between 0.5 m and 100 m in size. Both hollow (cenospheres) and solid

(precipitator) flyash particles are formed, but only the solid particles are used in the ULTALITE

AL-MMC brake applications.

5.1.Properties of The Ultilite Composites

Table 1 shows typical properties of squeeze cast ULTALITE composites, and compares them to

hand book data for permanent mould A356-T6 and die cast 380. The figures show that

ULTALITE composites have strengths similar to die cast 380, but significantly higher than the

A356. Elongation of the ULTALITE composites is slightly lower than the die cast alloy, due to

the ceramic reinforcement particles in the composite.

Table 1: Typical mechanical properties of ULTALITE composites, along with handbook data

for permanent mould cast A356 and die cast 380

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5.2.Testing of Ultilite Drums

Recently dynamometer has been performed. Dynamometer testing of 8inch diameter ULTALITE

brake drums, manufactured by the squeeze casting process, was performed at Brake Testing

International (BTI), located in Hinckley, England. The test regime followed procedures laid out

in SAE specification J25222. Standard off the shelf brake linings (Don #8259 and Nisshimbo)

were used in all the tests.

A series of fade segment 6.9 tests for three lining

combinations (cast iron + Nisshimbo,

ULTALITE+ Nisshimbo and ULTALITE+ Don

#8259) were conducted 7. The plots in the top

graph, which show brake factor, temperature and

applied pressure variations across 15 stops for an

OE cast iron + Nisshimbo combination, were used

as benchmarks.

The data shows that the ULTALITE brake drum

has higher brake factors. The average brake factors

for all ULTALITE brake lining combinations was

about 1.6 or above. The average brake factor for

the cast iron drum was around 1.2. The temperature

graph illustrates maximum temperature in the

ULTALITE brake drums never exceeded 185oC

with either lining, compared to a maximum

temperature of over 300oC for the cast iron +

Nisshimbo combination. This is due to the

significantly higher thermal conductivity of

Aluminum compared to cast iron, providing

significantly better heat dissipation characteristics.

Dynamometer testing also revealed that overall

lining wear was actually a little lower for the

ULTALITE drums, even when compared to

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conventional cast iron drums. This testing with ULTALITE brake drums has shown that special

brake linings are not required. In the past, excessive lining wear has been a problem with other

composites brake drums, where lining wear has been so high as to require the use of special

linings.

The extremely low lining wear of ULTALITE drums is probably a consequence of the spherical

nature of the ceramic reinforcement particles used with ULTALITE composites. Other AL-

MMC materials generally use angular ceramic reinforcement particles that can tear up the

linings.

In addition, due to superior heat dissipation, the

operating temperature of the ULTALITE brake

drums was more than 100C lower than that of

the cast iron drums, thereby reducing brake

fade 8. This data shows that the ULTALITE

brake drum lining wear characteristics are

equivalent to, or better than, those of a cast iron

brake drum.

6. REFERENCES

[1] William D. Callister Jr, |Materials science and Engineering, an introduction. 7th Ed, Wiley and Sons Publishing.

[2] M. K. Surappa, Aluminum metal matrix composites: challenges and opportunities, Sadhana, vol. 28, no. 1-2, pp. 319334, 2003.

[3] Karl U. Kainer, Metal Matrix Composites: Custom-Made Materials for Automotive and Aerospace Engineering, Wiley and Sons Publishing, 2003

[4] U.S. Department of Defense, Composite Materials Handbook Vol. 4: Metal Matrix Composites

[5] Rupa Dasgupta, Aluminum Alloy-Based Metal Matrix Composites: A Potential Material for Wear Resistant Applications, ISRN Metallurgy, Volume 2012, Article ID 594573.

[6] Graham Withers, and Dr. Ren Zheng, ULTALITE A low cost, lightweight aluminum metal matrix composite for braking applications.