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Metallurgical Applications Of
Metal Matrix Composites (MMCs)

By
Dr Thoguluva Raghavan Vijayaram
BE (Mechanical Engineering, Madurai Kamaraj University, India),
ME (Metallurgical Engineering, Bharathiyar University, India),
PhD (Mechanical Engineering, Universiti Putra Malaysia, Malaysia),
Rector Grant Researcher in Metallurgy (Genoa University, Italy),
Chartered Engineer (M123412-3, IIE, Calcutta, India)
MIIF, MISTE, MIIPE, MIE (Calcutta, India)
Senior Lecturer in Manufacturing Engineering and Researcher in Metallurgy,
Department of Manufacturing Process and System, Faculty of Manufacturing Engineering, UTeM, Universiti Teknikal Malaysia Melaka, Ayer Keroh, 75450 Melaka, Malaysia.
E-mail: thoguluva@utem.edu.my

Metal matrix composites (MMCs) are composed of an element or an alloy matrix in which a second phase is embedded and distributed to achieve some property improvement. Based on the size, shape and amount of the second phase, the composite property varies. Particulate reinforced composites, often called as discontinuously reinforced metal matrix composites, constitute 5 – 20 % of these new advanced materials. The microstructure of the processed composites influences and has a great effect on the mechanical properties. Generally, increasing the weight fraction of the reinforcement phase in the matrix leads to an increased stiffness, yield strength and ultimate tensile strength. However, the low ductility of particulate reinforced MMCs is the major drawback that prevents their usage as structural components in some applications.

Composite experts have carried out a detailed investigation on the strengthening mechanism of composites. They have found that the particle size and its weight fraction in metal matrix composites influences the generation of dislocations due to thermal mismatch and as well as the effect influenced by the developed residual and internal stresses. The researchers have predicted that the dislocation density is directly proportional to the weight fraction and due to the amount of thermal mismatch. The resulting strengthening effect is proportional to the square root of the dislocation density. Consequently, this effect would be significant for fine particles and for higher weight fractions.

Metal matrix composites have outstanding benefits due to the combined metallic and ceramic properties, thereby yielding improved physical and mechanical properties. Among the various types of MMCs, particulate-reinforced composites are the most versatile and economical one. During the past 40 years, materials design has shifted emphasis to pursue lightweight, environment friendliness, low cost, quality, and performance materials. Parallel to this trend, metal-matrix composites have been attracting growing interest. MMC attributes include alterations in mechanical behavior and physical properties by the reinforced filler phase. Apart from these advantages, MMCs have limitations on thermal fatigue, thermo chemical compatibility, and posses lower transverse creep resistance.

Fabrication of discontinuously reinforced Aluminium based MMCs can be achieved by standard metallurgical processing methods like powder metallurgy, direct casting, rolling, forging and extrusion, and further the products can be shaped, machined and drilled by using conventional machining facilities. Thus, they can be made available in suitable quantities particularly for automotive applications. Composite materials are characterized by good mechanical properties over a wide range of temperature. The choice of the processing method depends on the property requirements, cost factor consideration and future applications prospects. Composite materials with a metal or an alloy matrix can be produced either by casting or by powder metallurgy methods are considered as potential material candidates for a wide variety of structural application in the transportation, automobile and sport goods manufacturing industries due to the superior range of mechanical properties they possess.

MMCs represent a new generation of engineering materials in which a strong ceramic reinforcement is incorporated into a metal matrix to improve its properties including specific strength, specific stiffness, wear resistance, corrosion resistance and elastic modulus. MMCs combine metallic properties of matrix alloys (ductility and toughness) with ceramic properties of reinforcements (high strength and high modulus), leads to greater strength in shear and compression and higher service-temperature capabilities. Thus, they have significant scientific, technological and commercial importance. During the last decade, because of their improved properties, MMCs are being used extensively for high performance applications such as in aircraft engines and more recently in the automotive industries. Aluminium oxide and silicon carbide powders in the form of fibers and particulates are commonly used as reinforcements in MMCs and the addition of these reinforcements to aluminum alloys has been the subject of a considerable amount of research work. Aluminium oxide and silicon carbide reinforced aluminum alloy matrix composites are applied in the automotive and aircraft industries as engine pistons and cylinder heads, where the tribological properties of these materials are considered important. Therefore, the development of aluminum matrix composites is receiving considerable emphasis in meeting the requirements of various industries. Incorporation of hard second phase particles in the alloy matrices to produce MMCs has also been reported to be more beneficial and economical due to its high specific strength and corrosion resistance properties.

MMCs are materials that are attractive for a large range of engineering applications. They are a family of new materials, which are attracting considerable industrial interest and investment worldwide. They are defined as materials whose microstructures compromise a continuous metallic phase (the matrix) into which a second phase, or phases, have been artificially introduced. This is contrast to conventional alloys whose microstructures are produced during processing by naturally occurring phase transformations. Metal matrix composites are distinguished from the more extensively developed resin matrix composites by virtue of their metallic nature in terms of physical and mechanical properties and by their ability to lend themselves to conventional metallurgical processing operations. Electrical conductivity, thermal conductivity and non-inflammability, matrix shear strength, ductility and abrasion resistance, ability to be coated, joined, formed and heat treated are some of the properties that differentiate metal matrix composites from resin matrix composites. They are a class of advanced materials, which have been developed for weight-critical applications in the aerospace industry. Discontinuously reinforced aluminium composites, composed of high strength aluminium alloys reinforced with silicon carbide particles or whiskers, are a sub class of MMCs. Their combination of properties and fabricability makes them attractive candidates for many structural components requiring high stiffness, high strength and low weight. Since the reinforcement is discontinuous, discontinuously reinforced composites can be made with properties that are isotropic in three dimensions or in a plane.

Conventional secondary fabrication methods can be used to produce a wide range of composite product forms, making them relatively inexpensive compared to other advanced composites reinforced with continuous filaments. The benefit of using composite materials and the cause of their increasing adoption is to be looked for in the advantage of attaining property combinations that can result in a number of service benefits. Among these are increased strength, decreased weight, higher service temperature, improved wear resistance and higher elastic module. The main advantage of composites lies in the tailorability of their mechanical and physical properties to meet specific design criteria.  

Composite materials are continuously displacing traditional engineering materials because of their advantages of high stiffness and strength over homogeneous materials formulations. The type, shape and spatial arrangement of the reinforcing phase in metal matrix composites are key parameters in determining their mechanical behavior. The hard ceramic component that increases the mechanical characteristics of metal matrix composites causes quick wear and premature tool failure in the machining operations. Metal matrix composites have been investigated since the early 1960s with the impetus at that time being the high potential structural properties that would be achievable with materials engineered to specific applications.

In the processing of metal matrix composite, one of the subjects of interest is to choose a suitable matrix and a reinforcement material. In some cases, chemical reactions that occur at the interface between the matrix and its reinforcement materials have been considered harmful to the final mechanical properties and are usually avoided. Sometimes, the interfacial reactions are intentionally induced, because, the new layer formed at the interface acts as a strong bond between the phases.

During the production of metal matrix composites, several oxides have been used as reinforcements, in the form of particulates, fibers or as whiskers. For example, alumina, zirconium oxide and thorium oxide particulates are used as reinforcements in aluminium, magnesium and other metallic matrices. Very few researchers have reported on the use of quartz as a secondary phase reinforcement particulate in an aluminium or aluminium alloy matrix, due to its aggressive reactivity between these materials. Preliminary studies showed that the contact between molten aluminum and silica-based ceramic particulates have destroyed completely the second phase microstructure, due to the reduction reaction which provokes the infiltration of liquid metal phase into the ceramic. Previous works carried out by using continuous silica fibers as reinforcement phases in aluminium matrix showed that even at temperatures nearer to 400 0C, silica and aluminium can react and produce a transformed layer on the original fiber surface as a result of solid diffusion between the phases and due to the aluminium-silicon liquid phase formation. Metal matrix composites are composites with a metal or alloy matrix. It has higher elastic modulus, resistance to elevated temperatures, higher toughness and ductility. The limitations are higher density and greater difficulty in processing parts. Matrix materials used in these composites are usually aluminium, aluminium-lithium, magnesium, copper, titanium and super alloys. Fiber materials used in MMCs are graphite, aluminium oxide, silicon carbide, boron, molybdenum and tungsten. The elastic modulus of nonmetallic fibers ranges between 200GPa and 400GPa, with tensile strengths being in the range from 2000MPa to 3000MPa. Because of their high specific stiffness, lightweight, and high thermal conductivity, boron fibers in an aluminium matrix have been used for structural tubular supports in the space shuttle orbiter. MMCs having silicon carbide fibers and a titanium matrix are being used for the skin, beams, stiffeners and frames of the hypersonic aircraft under development. Other applications are in bicycle frames and sporting goods.

Graphite fibers reinforced in aluminium and magnesium matrices are applied in satellites, missiles and in helicopter structures. Lead matrix composites having graphite fibers are used to make storage-battery plates. Graphite fibers embedded in copper matrix are used to fabricate electrical contacts and bearings. Boron fibers in aluminium are used as compressor blades and structural supports. The same fibers in magnesium are used to make antenna structures. Titanium-boron fiber composites are used as jet-engine fan blades. Molybdenum and tungsten fibers are dispersed in cobalt-base super alloy matrices to make high temperature engine components. Squeeze cast MMCs generally have much better reinforcement distribution than compo cast materials. This is because a ceramic preform is used contains the desired weight fraction of reinforcement rigidly attached to one another so that movement is inhibited. Consequently, clumping and dendritic segregation are eliminated. Porosity is also minimized, since pressure is used to force the metal into interfiber channels, displacing the gases. Grain size and shape can vary throughout the infiltrated preform because of heat flow patterns. Secondary phases typically form at the fiber-matrix interface, since the lower freezing solute-rich regions diffuse toward the fiber ahead of the solidifying matrix. In recent years, the aerospace, military and automotive industries have been promoting the technological development of composite materials to achieve good mechanical strength/density and stiffness/density ratio. Modern fiber-reinforced or particulate reinforced metal matrix composites are produced by casting techniques; find a wide variety of applications due to the low cost of fabrication and achievable engineering properties and shown in Table-1.

Table-1 Features and Applications of Metal Matrix Composites (MMCs)

MMC Types	Industrial Applications	Special Features
Graphite reinforced in aluminium	Bearings	Cheaper, lighter, self-lubricating, conserves Copper, lead, tin, Zinc
Graphite reinforced in  aluminium, Silicon carbide reinforced in aluminium, aluminium oxide reinforced in aluminium	Automobile pistons, Cylinder liners, Piston rings, Connecting rods	Reduced wear, anti seizing, cold start, lighter, conserves fuel, Improved efficiency.
Graphite reinforced in copper	Sliding electrical contacts	Excellent conductivity and anti seizing properties.
Silicon carbide reinforced in  aluminium	Turbocharger impellers	High temperature use
Glass or Carbon bubbles reinforced in aluminium		Ultra light Material.
Cast Carbon fiber reinforced magnesium fiber composites	Tubular composites for space structures	Zero thermal expansion, high temperature strength, good Specific strength and Specific stiffness.
Zircon reinforced in aluminium-silicon alloy, aluminium silicate reinforced in aluminium	Cutting tool,  Machine Shrouds, Impellers	Hard, Abrasion- resistant materials.

Some of the properties are high longitudinal and transverse strengths at normal and elevated temperatures, near-zero coefficients of thermal expansion, good electrical and thermal conductivities and excellent antifriction, anti abrasion, damping and machinability properties. The application of composite materials is well established in aircraft technology and they are now applied in fuselage-production technologies as well as in jet engine technologies. Application in car production technology is growing very fast, although it is still not as common as in aircraft technology. Due to mechanical, electrical and heat resistant properties, their application in the electronics industries are also growing considerably. Composite material parts are applied in electronic sub-assemblies, lasers and computer parts can work at higher temperature and function with better efficiency when compared to conventional electronic materials.

Application of composites in the automotive, transportation and construction industries depends on the choice of cost affordable factor. Apart from the emerging and economical processing techniques that combine quality and ease of operations, researchers are at the same time turning to particulate-reinforced aluminum-metal matrix composites because of their relatively low cost and isotropic properties especially in those applications not requiring extreme loading or restricted thermal conditions in the case of automotive components. The presence of aluminium alloys as matrix materials are due to its comparative advantages, including low cost and ease of handling. The space shuttle uses boron reinforced aluminum tubes to support its fuselage frame, which decreases the mass of the space shuttle by more than 145 kg. It has also reduced the thermal insulation requirements because of its lower thermal conductivity. The mast of the Hubble telescope uses carbon-reinforced aluminum matrix composites.

Precision components in the missile guidance systems demands dimensional stability and the geometries of the components cannot change during usage. Metal matrix composites such as silicon carbide reinforced aluminum composite satisfy this requirement since they have high micro-yield strength. In addition, the weight fraction of silicon carbide can be varied to have a coefficient of thermal expansion compatible with other parts of the system assembly. Metal matrix composites are now used in automotive engines, which are lighter than their metal component parts. In addition, metal matrix composites are the unique materials of choice for gas turbine engines due to their high strength and low weight.

The range of MMCs applications is very large. Some of the important metal matrix composite components are applied and used as insulation materials for electrical construction, supports for circuit breakers and printed circuits, armors, boxes and covers, antennas, radomes, tops of television covers, cable tracks, wind mills, housing cells, chimneys, concrete molds, domes, windows, facade panels, partitions, doors, and furniture. Automotive engineering parts like automotive body parts, wheels, shields, radiator grills, transmission shafts, suspension springs, chassis, suspension arms, casings, highway tankers, isothermal trucks, trailers, wagons, doors, seats, interior panels, and ventilation housings. In marine transports, it is used to fabricate hovercrafts, rescue crafts, patrol boats, trawlers, landing gears, anti-mine ships, racing boats and canoes. In air transports, MMCs are used as passenger aircrafts, composite gliders, leading edges, ailerons, vertical stabilizers, helicopter blades, propellers, transmission shafts and aircraft brake discs. For space transports, it is used to make rocket boosters, reservoirs, nozzles and shields for atmosphere reentrance. Some of the general mechanical applications are as gears, bearings, housing and casings, jack body, robot arms, fly wheels, weaving machine rods, pipes, components for drawing table, compressed gas bottles, tubes for offshore platforms, and pneumatics for radial frames. It is widely applied in sports and recreation industries to manufacture tennis and squash rackets, fishing poles, skis, poles used in jumping, sails, surfboards, roller skates, bows and arrows, javelins, protection helmets, bicycle frames, golf balls and golf sticks, and oars.

About the Author

Dr.Thoguluva Raghavan Vijayaram, currently working as Senior Lecturer in the Faculty of Manufacturing Engineering at UTeM, Universiti Teknikal Malaysia Melaka, Malaysia. He hails from India and he has completed his PhD Research Degree in Mechanical Engineering (Metal Matrix Composites: Materials Engineering) from the Faculty of engineering, Universiti Putra Malaysia. He has published quality research papers in reputed International journals, National journals, International conference proceedings and in the Malaysian broadsheet. He has a wide range of work experience, both in academics and as well as in industry, consultancy and teaching and especially in research and development work. His areas of expertise include: Metallurgical Engineering, Mechanical Engineering and Manufacturing Engineering and his special areas of research interests are in the field of advanced casting technology and techniques, composite materials and processing, powder metallurgy, Ferrous and Non-Ferrous foundry metallurgy, solidification science and technology, solidification processing of metals, alloys and composites, microgravity solidification, squeeze casting, die casting die design, heat treatment, Metallography, microstructure-property correlation ship, new materials and process development, aerospace engineering materials, computer simulation of casting solidification, FEM analysis and advanced engineering mathematics. Besides, he is a prominent writer and possesses wider experience in editing technical papers, theses and dissertations.

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