Open access peer-reviewed chapter

# Novel Applications of Aluminium Metal Matrix Composites

By Francis Nturanabo, Leonard Masu and John Baptist Kirabira

Submitted: November 22nd 2018Reviewed: April 5th 2019Published: May 10th 2019

DOI: 10.5772/intechopen.86225

## Abstract

Advanced materials have offered the materials designer a wide range of options in the specification and selection of materials for various applications. Material properties are continually being improved to meet safety and operational standards in line with prevailing technological developments. Modern technological requirements, together with the consumers’ demands for systems and machines that are more energy efficient, stronger, light-weight, cost-effective, etc., dictate that the search for new and advanced materials will remain a subject of interest all the time. The difficulty in designing materials for such stringent specifications cannot be overstated, owing to the conflicting nature of these specifications. Aluminium metal matrix composites (AlMMCs) are a class of materials that have proven successful in meeting most of the rigorous specifications in applications where light-weight, high stiffness and moderate strength are the requisite properties. With a variety of reinforcement materials and flexibility in their primary processing, AlMMCs offer great potential for the development of composites with the desired properties for certain applications. In this review, the development, utilisation and future potential of AlMMCs in various industrial and commercial applications is discussed, together with the existing challenges hindering their full market penetration.

### Keywords

• aluminium
• metal matrix composites
• novel applications
• light-weight
• high-temperature

## 1. Introduction

The choice of the right materials is an arduous engineering challenge to the materials engineer and, if done carefully, can be a springboard to the proper and successful implementation and subsequent operation of the design. There are a host of materials available to the designer, and making the right decision is a vital achievement in putting forth a successful design. Materials are required to perform according to the designer’s expectations and must possess and retain the right properties in the working environment throughout the working period.

Material selection is in most cases a contradictory decision-making process. Light-weight materials will most likely not possess sufficient strength, and brittle materials will not necessarily be good in fatigue resistance, stiffness or toughness. It is also almost impossible to find a single monolithic material with the required property profile for engineering applications. Moreover, material properties are greatly affected by the working environment (such as temperature, pressure, humidity, etc.) and the nature of loading (gradual, fluctuating, impact, fatigue, etc.). There is need, therefore, to combine two or more materials, as alloys or composites so as to utilise the different useful properties offered by the different materials. Most engineering materials appear in this configuration, and very few applications utilise pure monolithic materials [1]. This is true of aluminium, the most abundant metallic element in the Earth’s crust, accounting for 8% of the planet’s soil and rocks. Aluminium has been a metal of tremendous importance to the domestic and manufacturing industries from the mediaeval period (fifth–fifteenth century) and played an important role in the early years of the industrial revolution. The successful extraction and the first commercial applications of aluminium took place in the nineteenth century, the period in which the enthusiasm for new materials and their possible uses was immense [2].

The first mention of aluminium as a metal of industrial importance indicated the metal was first utilised in the manufacture of household and ornamental items before becoming an important material in the construction of large industrial structures and machine components. With the advent of alloying technology, the use of aluminium was developed farther and positioned aluminium as the most utilised industrial metal for decades. The popularity of aluminium grew due to its good attributes related to its unique properties, mainly of light-weight combined with good thermal/electrical conduction and reasonably good strength and resistance to corrosion. With alloying, aluminium has found more applications than previously envisioned, making aluminium a serious competitor with (and sometimes a preferred alternative to) the traditional “strong” metals iron and steel [3].

Aluminium alloys and composites have, in most applications, exhibited superior performance compared to their rival metals. The choice of aluminium alloys and composites derives from one important attribute of aluminium metal—light-weight. Light-weight translates into many important outcomes in engineering applications. In the automotive industry, it means less dead weight, lower fuel consumption, lower emissions, increased payload (for passengers and cargo) and easier handling. In the aerospace and aircraft industry, it translates into more payload (cargo), less fuel and lower emissions. There are similar advantages in all areas where aluminium is utilised—marine, rail, packaging, thermal management, building and construction, sports and recreation, etc. Aluminium’s good electrical and thermal conductivity have seen its increased use in electrical conductors, electronic packaging and thermal management. Nowadays, aluminium is viewed as an important material for energy conservation and environmental protection [4].

Modern technology aims at meeting the market whose standards are ever appreciating. The market demands faster, more comfortable and hassle-free transport, more compact and lighter machines and tools, more efficient methods of power generation, etc. Most engineered materials can easily meet or surpass design specifications that would not have been envisaged a few years back. Today’s materials are subjected to more critical loads, more stresses and more severe operating conditions in an environment never experienced before. In a spacecraft, for example, the operating conditions experienced are quite unique and require special types of materials to withstand the severe stresses imposed on the spacecraft during take-off and maintenance in the orbiting space. Traditional materials have been found wanting in meeting these operating conditions and hence the need to intensify research and development (R&D) efforts in new and advanced materials for specific applications and efficiency improvement. Among the advanced materials on the R&D, the menu is the metal matrix micro- and nanocomposites. Metal matrix composites (MMCs) are metals or metal alloys that incorporate particles, whiskers, fibres or hollow microballoons made of a different material and offer unique opportunities to tailor materials to specific design needs [5]. In automotive applications, for example, these materials can be tailored to be light-weight and with various other useful properties including high specific strength and specific stiffness, high hardness and wear resistance, high thermal conductivity, high energy absorption and a damping capacity and low coefficients of friction and thermal expansion.

MMCs, therefore, offer more possibilities for wider applications of materials by manipulating their processing to suit the requisite properties under different working environments. The design of composite materials with specific properties can, moreover, be accomplished with the use of finite element modelling techniques. It is possible to predict the properties of a certain material of specified composition by using these techniques. In the same way, it is possible to design materials to offer specified properties by the use of these techniques [1].

## 2. Types of metal matrix composites and their methods of production

### 2.1 An overview of metal matrix composites

A composite is a mixture of two or more constituents or phases which are chemically distinct on a microscopic scale, separated by a distinct interface, and can easily be specified. In addition, other criteria are normally satisfied before a material can be called a composite. The constituents have to be present in reasonable proportions, and the constituent phases should have distinctly different properties, such that the properties of the composite are noticeably different from the properties of the constituents [4]. The constituent which is continuous and in most cases available in larger quantities is termed the matrix. It is commonly viewed that it is the properties of the matrix that are improved upon in the process of producing a composite. The second constituent is known as the reinforcing phase, or reinforcement, as it enhances or reinforces the mechanical properties of the matrix. In most cases the reinforcement is harder, stronger and stiffer than the matrix, although there are some exceptions. The matrix may be in form of a ceramic material, metallic or polymeric, with each of these three classes of materials having considerably different /unique mechanical properties. Generally, polymers have low Young’s moduli and strengths; ceramics are strong, stiff and brittle; and metals have intermediate moduli, strengths and good ductility [6].

Composite materials are usually classified according to the physical or chemical nature of the matrix, e.g. metal matrix, polymer matrix and ceramic composites. Additionally, the emergence of the intermetallic matrix and carbon matrix composites as reported by [7] has broadened the scope of composites. Intermetallic compounds are metal-based systems centred on the fixed atomic compositions occurring in metallic systems of aluminium with nickel (Ni), titanium (Ti) and niobium (Nb), such as Ni3Al, Ti3Al, TiAl and Nb3Al. Intermetallic compounds are of interest because they often exhibit higher melting points and less ease of deformation due to the lattice arrangement of their atoms [8].

In certain applications, metal matrix composite materials, formed by combining two or more materials—one of which is a metal—exhibit a primary advantage over their counterpart organic matrix composites in regard to the maximum operating temperature. To support this point, [9] reports that the boron/aluminium composite offers useful mechanical properties up to a temperature of 510°C, whereas an equivalent boron/epoxy composite is limited to about 190°C. Furthermore, composites of graphite/aluminium, graphite/copper and graphite/magnesium exhibit higher thermal conductivity due to the significant contribution from the metallic matrix. A metal matrix composite retains the desirable properties of both the matrix and the reinforcement by combining the strength of its reinforcement with the ductility of its matrix [10]. The reinforcing constituent may be a particle, platelet, short fibre or continuous fibre and may range from sub-micrometre to millimetre in size. There is a difference between metal matrix composites and multiphase metallic alloys as the concept of MMCs introduces additional degrees of freedom into designing the microstructure. Materials with desirable properties not obtainable by conventional alloying and heat treatment can be created compositing. This can be achieved by altering the reinforcement type (metallic, ceramic or polymeric), content (volume fraction), size, shape, distribution and orientation [11].

In the early development of MMCs, continuous ceramic fibres and single-crystal ceramic whiskers were the preferred reinforcements as they provided the most remarkable increase in strength and stiffness. Later, particulate and discontinuously reinforced MMCs then followed, registering substantial progress on many fronts especially in composites with aluminium as the metal matrix. In aluminium metal matrix composites (AlMMCs), aluminium or its alloy forms a percolating network and is the matrix phase, while the other constituent, which is embedded in this matrix, is the reinforcement. The reinforcement is usually ceramic such as silicon carbide (SiC) or aluminium oxide (Al2O3). The properties of AlMMCs can be varied by varying the nature of the constituent phases and their volume fractions [4].

Although the MMCs have been in existence since the 1960s, they have not been put to full commercial use due to their higher production costs and lack of proper understanding of their high-temperature behaviour [12]. The higher costs are mainly attributed to the machining processes requiring tool materials to have very high wear resistance because of the reinforcement component being extremely abrasive [13]. However, with the invention of functionally graded materials (FGMs), it is now possible to reduce the cost of secondary processing. FGMs are an emerging category of advanced materials that exhibit gradual microstructural transitions and/or the composition in a specific direction and hence different functional performances within a part [14, 15].

The rapid growth and development of AlMMCs happened in the years after the launch of the Aluminium Metal Matrix Composites Roadmap 2002, a policy document produced by the Aluminium Metal Matrix Composites Consortium with support from the Technology Research Corporation (TRC) of the United States and other stakeholders. The document spelt out a pathway for the AlMMCs’ growth in 20 years from 2002 and asserted the industry’s vision to position AlMMCs as the material of choice in a broad range of structural and nonstructural applications. This vision was to be achieved by addressing three strategic goals, namely:

1. To reduce the cost of discontinuously reinforced AlMMCs to be comparable to existing alternatives by 2010

2. To develop the necessary infrastructure to provide design confidence for AlMMCs

3. To increase the market size for AlMMCs

#### 3.1.2 Aerospace and aircraft industry

Aluminium alloys and composites have played a big role in the advancement of aircraft and rocket technology. Right from the Wright brothers’ utilisation of aluminium in the engine of their first biplane to NASA’s use of an aluminium-lithium alloy in the spacecraft, aluminium has created and enhanced the mankind’s potential to fly around the Earth and into the outer space.

Aluminium alloys and/or composites are the favoured choice for the fuselage, wing and supporting structures of commercial airliners and military or cargo aircraft. The airframe of a typical modern commercial transport aircraft is composed of 80% aluminium by weight. Attention is now focused towards aluminium casting technology, which offers lower manufacturing costs, the ability to form complex shapes and the flexibility to incorporate innovative design concepts.

Aluminium metal matrix composites have been the material of choice for space structures of all types ever since the launch of Sputnik 1 (October 4, 1957). Chosen for their light-weight and their ability to withstand the stresses that occur during launch and operation in space, AlMMCs and alloys have been used on Apollo spacecraft, the Skylab, the space shuttles and the International Space Station. Aluminium alloys/composites consistently exceed other metals in such areas as mechanical stability, dampening, thermal management and reduced weight [40].

#### 3.1.3 Rail transport

Aluminium railroad cars were pioneered for the railroad industry in the late 1950s and are still the material of choice for this mode of transportation. Rail cars, designed with aluminium-based extrusions, require one-third the number of components, have reduced welding needs and are two-thirds the weight of comparable steel cars. The higher carrying capacity of aluminium repays its higher initial cost in less than 2 years, and the life-cycle fuel costs are lower due to the lighter weight of the car [41]. Aluminium-based materials offer excellent resistance to corrosion and high salvage value.

Designing with aluminium results in light-weight cars that retain the strength of steel cars but can carry greater loads, hence saving money in increased freight and reduced fuel costs. The third generation of the French TGV Duplex high-speed train is a good example in this case. The train converted from steel to aluminium-based materials, resulting in a 20% weight saving, while at the same time converting to two decks and keeping the axle load below 17 tons. Similarly, the Japanese high-speed “bullet” train and the Washington DC Metro trains are also made with aluminium-based materials.

The durability of aluminium makes it a suitable material for the railroad environment. Extensive shaking tests and decades of use offer testimony to aluminium’s superiority for this application. A recent study shows that after 20 years of service, there is a negligible loss of metal thickness or surface defects on cars used to ship different materials an average of 110,000 miles per year. Metal loss on floors and sidewalls from corrosion and wear measured approximately 25% less than comparable steel cars [42].

#### 3.1.4 Marine transport

Marine transport has also been revolutionised with the use of aluminium alloys and composites. The use of these materials has enabled an increase in the speed and size of boats, yachts, ferries and ships while improving their fuel efficiency, seaworthiness, safety and reliability and reducing maintenance costs. By substituting aluminium for steel, weight savings of 35–45% in hulls and 55–65% in superstructures can be achieved [42]. Higher vessel speeds and load capacities translate into extra traffic volume and profits for a ship or boat operator.

It is also possible to increase vessel volume and height without loss of stability. Passenger compartments can be larger, and more cabins can be located above sea level. The use of aluminium-based materials also ensures increased manoeuvrability and access to shallow draught ports.

Aluminium-intensive cargo ships with load capacities up to 3000 metric tons have been designed to operate at up to 60 knots, crossing the Atlantic in under 60 hours. Military requirements seek smaller, more agile vessel designs with a lower radar cross section and capable of 60–80 knots or more—another excellent fit for aluminium, which is made possible due to advances in manufacturing methods, such as friction-stir welding and structural bonding.

Aluminium-based materials satisfy the requirements of the International Maritime Organization high-speed code for vessel design, safety and control of fire risk. Compared to steel, aluminium performs better in handling the torsional, flexural, compression and impact loads of high-speed water travel [42].

#### 3.1.5 Building and construction industry

In 2009 the building and construction market constituted the third largest North American market for aluminium. Strength and stiffness are the two most important characteristics for structural applications of aluminium-based materials. The composites of aluminium such as the fibre-reinforced alloys of aluminium, discontinuously reinforced aluminium (DRA) and the conventional metals and graphite/epoxy composites provide the good uniaxial specific stiffness and specific strength and hence are the materials of choice for applications where maximum structural efficiency is the primary selection criterion [43].

Aluminium was first used in large quantities for building and construction in the 1920s, with the applications primarily oriented towards decorative detailing and art deco structures. Nowadays, aluminium-based materials are recognised as some of the most energy efficient and sustainable construction materials. Moreover, an estimated 85% of the aluminium used in modern buildings comes from recycled material. Bridge decks made from aluminium-based materials need minimal maintenance, are corrosion-resistant, require no painting and, unlike concrete, require no extension framework or cure time. Advanced aluminium alloys and composites can easily support the weight of heavy glass spans, thus maximising the building’s capability for using natural sunlight.

Aluminium has, over time, been viewed as a vital component of sustainable buildings since the metal is easily recycled and loses none of its properties during recycling. Moreover, the recycling process reduces energy consumption by more than 90% compared to the energy required to produce new aluminium [44]. Aluminium and its alloys are infinitely recyclable. More than 75% of all aluminium produced is still in use today.

#### 3.1.6 Offshore applications

Offshore platforms, helidecks and seawalls are other possible areas where aluminium-based materials can be effectively utilised. In water depths of 400 feet, a 1 ton weight saving in platform superstructure means weight savings of 6 tons in the supporting structure [42].

Aluminium-based materials are often used in the construction of helicopter decks (helidecks) for resupply of oil rigs. Here, marine-grade aluminium alloys offer maintenance-free service with remarkable corrosion resistance. Using aluminium components reduces handling and offshore lifting costs and speeds the task of assembly. Aluminium is safe to use as it does not burn and presents no thermite sparking risks. It requires minimal maintenance. Even in salty water applications, little or no protective coatings are required for aluminium seawalls.

Marine-grade aluminium alloys are used for helidecks, telescoping bridges, accommodation modules, stair towers, cable ladders, fire walls, mud mats, gratings and many other applications. Aluminium structures weigh 40–70% less than equivalent steel structures. Handling is made easier since larger, lighter aluminium structures can be handled and lifted with smaller, less expensive equipment. In marine environments, properly selected aluminium alloys/composites require no painting and require little or no maintenance.

Aluminium seawall shapes are generally extruded, achieving the most strength with the least material. Aluminium is easy to extrude and fabricate; hence, retrofitting of the offshore platforms and customisation become cost-effective. Installation is also easy since designers can create either a single-piece component, bolted connections or interlocking sections for fast and simple fit-up on site. Various proven mechanical methods joining can be applied to aluminium. Its weldability is good as it can be welded three times faster than steel, using inexpensive MIG machines. Aluminium offers excellent safety advantages as it is non-combustible and gives off no flammable vapour when heated—an important consideration when choosing materials for offshore applications such as helidecks [42].

### 3.2 High-temperature applications

#### 3.2.1 Automotive industry

The high-temperature applications in the automotive industry are mainly concerned with the engine, transmission and braking components. These experience temperatures up to about 300°C. The AlMMCs suitable for use under these circumstances must be able to retain the desired properties of the part/component operating under these conditions [1].

The major automotive components that have been successfully manufactured from AlMMCs are the following:

Pistons and cylinder liners. The University of Wisconsin-Milwaukee (UWM) reportedly developed aluminium alloy pistons and cylinder liners containing dispersed graphite particles that provide solid lubrication [5]. The graphite-containing aluminium has a lower friction coefficient and wear rate and does not seize under boundary lubrication. Aluminium/graphite pistons and liners were tested in gas and diesel engines and in race cars, and the results showed reduced friction coefficients and wear rates. The friction coefficient of Al-graphite composites was measured and found to be as low as 0.2 [45]. This makes it a suitable material for cylinder liners in light-weight aluminium-engine blocks, for its ability to enable engines reach operating temperatures more quickly while providing superior wear resistance, improved cold start emissions and reduced weight [46]. Aluminium-based composite liners can be cast in situ using conventional methods, including sand, permanent mould, die casting and centrifugal casting.

Main bearings. Lead-free aluminium or copper matrix composites containing graphite particles, as developed at UWM [5], can replace the copper-lead bearings used in crankshaft main-bearing caps. The bearings also improve wear characteristics because deformation of the graphite particles results in the formation of a continuous graphite film, which provides self-lubrication of the component, allowing for improved component longevity. Virtually all journal bearings in the power train could benefit from these materials. Selectively reinforced functionally gradient bearings of aluminium-graphite and copper-graphite alloys can be manufactured in a single step by centrifugal casting of metal-graphite suspensions [47].

Connecting rods. For components requiring high strength at high temperatures, such as connecting rods, cast aluminium matrix nanocomposites may be ideal to produce near-net-shape components to replace steel, forged aluminium and titanium components while reducing reciprocating mass.

Accessories. For components not exposed to extreme loading, further cost and weight reductions can be realised by incorporating fly ash in the aluminium matrix. Components such as A/C pump brackets, timing belt/chain covers, alternator housings, transmission housing, valve covers and intake manifolds can be replaced with aluminium-fly ash composites, reducing the vehicle cost and weight, thereby improving emissions and saving energy. Adding fly ash to aluminium also reduces its coefficient of thermal expansion and increases its wear resistance along with making lighter and less expensive material [46].

Suspension. Although many automakers use aluminium and light gauge steel for suspension components to reduce unsprung weight and improve vehicle dynamics, a big number of components are still being made from cast iron. Components such as control arms or wheel hubs made of strong silicon carbide (SiC)-reinforced aluminium or aluminium nanocomposites can further improve aluminium alloy designs by enhancing strength while using less material than similar aluminium arms [31].

Brakes. Automotive disc brakes and brake callipers, typically made of cast iron, are an area where significant weight reduction can be realised. SiC-reinforced aluminium brake rotors have been embraced by a number of prominent vehicle manufacturers [47]. High cost and machinability issues need to be addressed for widespread use of aluminium composite brake rotors. UWM developed aluminium-silicon carbide-graphite composites, aluminium alumina-graphite and hypereutectic aluminium-silicon graphite alloys with reduced silicon carbide to help overcome cost and machinability barriers. Aluminium-fly ash composites developed at UWM have been explored to make prototype brake rotors in Australia [31]. Strength improvements seen in aluminium nanocomposites being developed at UWM can provide significant improvements in component rigidity without adding a significant amount of material, resulting in lower-weight components.

#### 3.2.2 Applications in aerospace and aircraft industry

Aerospace propulsion and power systems are ever placing increasing demands on load bearing materials. The quest to propel bigger payloads into space and provide electrical power for space experiments while at the same time meeting the demands of manned and unmanned spacecraft flying at hypersonic velocities requires the right materials. The materials must be light-weight and be able to withstand high temperatures for long periods of time in hostile environments.

Metal matrix composites have the potential to meet the wide variety of these requirements. By selection of the proper high-temperature fibre and combining the fibres with an appropriate matrix, a high temperature, light-weight MMC can be produced. Extensive research is needed on advanced fibres and matrices. Since the fibres provide the characteristics that dominate the strength, stiffness and conductivity of a composite, superior fibres need to be developed. Fibres having high melting points and coefficients of thermal expansion matching those of the matrices need to be evaluated for high-temperature strength, modulus and compatibility with various matrices. In case of matrices, intermetallic compounds offer higher melting points, light-weight and (in the case of aluminides and silicides) good oxidation resistance for aerospace propulsion systems [48].

### 3.3 Other novel applications of AlMMCs

#### 3.3.1 Electronic packaging and thermal management

Heat sinks play two key roles in electronic packaging: thermal management and mechanical support. Heat sinks support electronic devices and provide a path for heat dissipation. They are used in packages and with printed circuit boards (PCBs). Traditional heat sinks were primarily aluminium, copper or unalloyed blends of two metals, such as copper-tungsten or copper-molybdenum. The traditional heat sinks have exhibited a number of shortcomings, which has necessitated designing of new improved materials, primarily composites reinforced with fibres and particles. The new materials exhibit better properties including high thermal conductivities; low, controllable coefficients of thermal expansion; weight reductions; high strength and stiffness; and availability of net-shape fabrication processes.

The packaging density is ever on the increase, which has resulted in the demand for materials with high thermal conductivities. In addition, to minimise thermal stresses that can cause component or solder failure, it is desirable that the packaging material should have a coefficient of thermal expansion (CTE) matching that of the ceramic component it supports. Utilisation of composite materials is not a new phenomenon in electronic packaging. For example, polymer matrix composites (PMCs) in the form of E-glass fibre-reinforced polymer PCBs are well-established packaging materials.

Aluminium metal matrix composites with the high volume fraction of reinforcement are attractive materials for thermal management. This is in view of the possibility to further enhance the thermal conductivity (TC) of the composite material by the use of high TC reinforcements and the flexibility to adjust the CTE by controlling the volume fraction of the reinforcement. Aluminium and copper were usually used as matrices due to their high TCs, and the reinforcements involved SiC, carbon and diamond. However, owing to the fact that the specific thermal conductivity of aluminium-based composites was higher than that of Cu-based composites, aluminium-based composites are more desirable in avionic applications where light-weight is demanded [49].

#### 3.3.2 Packaging and containerisation

In 2009, containers and packaging regained their position as the top market for aluminium-based materials. The aluminium industry shipped 4.73 billion pounds for packaging applications or 26.5% of all shipments [42]. Aluminium-based materials are used in products such as beverage cans and bottles, food containers and household and institutional foil. Manufacturers and consumers appreciate foil for its impermeability to light, water and air—making it a preferred packaging material for food, beverage and pharmaceutical products. Moreover, aluminium’s light-weight gives it a competitive advantage over other materials with regard to shipping costs and volume.

Regarding containerisation, it is difficult to discuss rail transport of freight and commercial goods without reference to the ubiquitous container. The cargo can be packed into large containers and conveniently shipped to their destinations interchangeably by rail, road, sea or air. The container has greatly simplified the transport of goods and has been adapted to the different modes of transport. With a backbone of aluminium extrusions and with considerable use of aluminium-based sheet material, the growth of containerisation has greatly facilitated the transportation industry.

#### 3.3.3 Electrical transmission

Aluminium-based materials have many advantages for electrical applications. Properties such as light-weight, strength, corrosion resistance and high efficiency in electrical conduction (aluminium has twice the conductivity of copper) render these materials the best choice for transmitting power from generating stations to homes and businesses. Their ease of recyclability makes them a perfect fit for today’s environment.

In 2010, electrical market applications rose by 13.1%, and shipments of aluminium conductor steel-reinforced (ACSR) cable, bare cable, insulated wire and cable products soared to 631 million pounds, an increase of 11 million pounds from the previous year. The North American electrical market was the fourth largest for aluminium worldwide, accounting for 7.3% of all aluminium shipments during the year [42].

#### 3.3.4 Sports and recreation

The sporting goods industry is not left behind as far as utilisation of AlMMCs is concerned. Aluminium metal matrix composites are very attractive as materials for sporting goods applications. The material used generally consists of an aluminium matrix reinforced with particles of silicone carbide or boron carbide. The specific strength and modulus of these materials can offer design advantages not possible with steel or carbon/epoxy composites. In addition, they have a tremendous marketing appeal for the high-end sporting goods consumer as they are a new phenomenon [50]. Recreational products, including those used in golf, cycling, baseball, skiing and other leisure as well as competitive sporting activities, have always offered profitable opportunities for high-performance materials due to the focus on performance over cost. Although AlMMCs have been used in niche applications, more widespread opportunities are available if an improved combination of performance, manufacturability and cost can be achieved through specific R&D activities.

Finally, AlMMCs have been considered for specialised applications in which the combination of properties makes them especially well suited. Examples of these applications include robotics, medical, biomedical and nuclear shielding. These applications may require specific R&D activities to be carried out and technical problems solved before substantial use can occur but may represent high-value market opportunities for the industry if successful [16].

## 4. Challenges and barriers in the development of AlMMCs

Several challenges must be overcome in order to intensify the engineering usage of AlMMCs. Design, research and product development efforts and business development skills are required to overcome these challenges. Surappa [4] emphasised the need to address the following issues:

1. A more and thorough understanding of the science of primary processing, especially the factors affecting the microstructural integrity including agglomerates in AlMMCs.

2. Need to improve the damage tolerant properties particularly fracture toughness and ductility in AlMMCs.

3. Need for work to be done towards the production of high-quality and low-cost reinforcements from industrial wastes and by-products.

4. An urgent need to develop simple, economical and portable non-destructive kits to quantify undesirable defects in AlMMCs.

5. Work in developing less expensive secondary processing tools for machining and cutting AlMMCs.

6. Work must be done to develop recycling technology for AlMMCs.

The challenges and barriers listed above are echoed by [16]. Further penetration of AlMMCs will largely depend on their primary production processes and secondary machining processes being affordable. Generally, the cost of aluminium is around 4–5 times that of steel. In addition, the manufacturability of these composites is cumbersome. These challenges are being addressed through R&D activities. In early development of AlMMCs, the industry was modelled on the roadmap drawn by the Aluminium Metal Matrix Composites Roadmap 2002, which spelt out a pathway for the AlMMCs growth in 20 years from 2002 and asserted the industry’s vision to position AlMMCs as the material of choice in a broad range of structural and nonstructural applications [1]. During the workshop that gave birth to the AlMMCs Roadmap 2002, a number of critical barriers hampering the market penetration of AlMMCs were identified, and common themes agreed on how to mitigate these barriers and realise their vision [16].

## 5. Conclusion

AlMMCs present a great opportunity and a host of possibilities for the materials/design engineer. There are now many possibilities for manipulation of properties/property combinations to suit specific requirements of material and component properties in order to enhance performance and reliability. New and emerging technological developments point to increased utilisation of AlMMCs in current and future industrial developments. Some of the existing barriers and challenges are being addressed through various R&D efforts to find a lasting solution.

From the foregoing review, it is evident that the future of AlMMCs in various industrial and commercial applications is very bright. Advanced technological developments in primary and secondary processing of AlMMCs will continue to give them a competitive edge over the alternative materials such as Mg, AHSS and polymer composites. The main challenges and barriers that have been identified include lack of property modelling (especially the high-temperature behaviour of AlMMCs), lack of design data and high costs of primary and secondary processes. However, there are promising signs of technological breakthroughs by various research efforts dedicated to finding solutions to these challenges. New developments in CNT and nanotechnology have, for example, offered possibilities of production of AlMMCs with enhanced properties for high-temperature applications and improved wear and corrosion resistance. Other developments such as the novel rheocasting process of semi-solid alloys [e.g. see [51]] and FGMs have also offered new possibilities of cost reduction in primary production and secondary processing of AlMMCs, respectively. New alloys of aluminium have been developed for application in such areas as crash management (crash alloy)—an area previously dominated by steel. These alloys offer new R&D opportunities for further development of AlMMCs and will redefine new roles and potential of AlMMCs in automotive applications. Various researchers are also coming up with innovative cost-reduction techniques to bring down the cost of replacing conventional ferrous materials with aluminium metal matrix composites.

## Acknowledgments

The authors would like to sincerely acknowledge the material and financial support extended by the Vaal University of Technology, Department of Mechanical Engineering and the collaborating institutions—The Council for Scientific and Industrial Research, Pretoria and Makerere University, Kampala Uganda.

## Conflict of interest

The authors envisage no conflict of interest.

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Francis Nturanabo, Leonard Masu and John Baptist Kirabira (May 10th 2019). Novel Applications of Aluminium Metal Matrix Composites, Aluminium Alloys and Composites, Kavian Omar Cooke, IntechOpen, DOI: 10.5772/intechopen.86225. Available from:

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