1.1 Composite Materials
A composite material is one which is composed of at least two phases combined together to produce new material properties that are different to the properties of those elements on their own. Practically, most composites composed of a bulk material termed as the ‘matrix’ and reinforcement added essentially to increase the overall strength and stiffness of the matrix.
Metal matrix composite (MMC) is engineered combination of the metal (Matrix) and hard particle/ceramic (Reinforcement) to get modified properties. Metal matrix composites increasingly found their application in aerospace and automotive sectors because of their properties like high strength, high stiffness, low density, wear resistance, elastic modulus, and tensile strength. Most commonly used metal matrices are Aluminum, Magnesium, Titanium and their alloys. The reasons for using these matrices are less weight, low cost and availability. Similarly reinforcements are also classified as fibers, whiskers and particulates. Particulates are extensively used in weight critical application in aerospace, automotive parts fabrication. Hybrid metal matrixes are engineering material that constitutes two or more reinforcement in order to obtain the combined advantage compare to individual constituent.
Particulate reinforced Aluminium matrix composites (PRAMC) have received considerable observation in twenty first century because of their high specific strength and good wear resistance. The PRAMCs were usually produced by Casting technique and Powder metallurgy.
In recent years many research have been investigated by combining Aluminium matrix with SiC, Al2O3, TiC reinforcements to gain better understanding of the mechanical behavior of these composites and their excellent wear resistance.
1.2 Classification of Composites:
The composites can be classified as two types on the basis of matrix constituent and reinforcement form.
The first level of classification with respect to the matrix constituent includes:
1. Polymer Matrix Composites
2. Metal Matrix Composites
3. Ceramic Matrix Composites
The second level of classification with respect to the reinforcement form includes:
1. Fiber reinforced composites
2. Laminar composites
3. Particulate composites.
1.2.1 Classification Based on Matrices
The first level of classification is usually made with respect to the matrix constituent. The figure 1.1 indicates classification of MMC’s based on matrices.
Fig 1.1: Classification of MMC’s based on matrices
Polymer Matrix Composites (PMC’s): Polymer based composites are also known as Fiber reinforced polymers (FRP). These material use polymer based resin as the matrix and a different types of fibers such as glass, carbon as the reinforcement.
Metal Matrix Composites (MMC’s): Metal matrix composites are increasingly found in the aerospace and automotive industry. These materials use a metal such as Aluminium as the matrix and reinforce it with fiber, particulates or whiskers such as Silicon carbide, Boron carbide etc to get tailored properties.
Ceramic Matrix Composites (CMC’s): Ceramic matrix composites are used in very high temperature environment. These materials use a ceramic as the matrix and reinforce it with short fibers or whiskers such as those made from silicon carbide and boron nitride.
1.2.2 Classification Based on Reinforcements
The figure 1.2 indicates classification of MMC’s based on Reinforcements.
Fig 1.2: Classification of MMC’s based on reinforcements
1.3 Metal Matrix Composites (MMC):
Metal Matrix Composites (MMCs) provide remarkably enhanced properties over regular conventional materials, such as good strength, weight savings and stiffness. Metal matrix composites are used in a wide range of high performance applications today. Most of their current applications are in aviation, ground transportation, electronics and sports industries. The applications of metal matrix composites in aeronautics have been established in the aero-structural, aero-propulsion and subsystem categories. The Aluminium alloys are very attractive because of their low cost and light weight and can be heat treated to fairly high-strength levels. Also Aluminium is one of the most easily fabricated of the high-performance materials, which usually correlates directly with lower costs.
1.3.1 Advantages of MMC’s
• Higher temperature capability.
• Fire resistance.
• Higher transverse stiffness and strength.
• No moisture absorption.
• Higher electrical and thermal conductivities.
• Better radiation resistance.
1.3.2 Disadvantages of MMC’s
• Higher cost of some material systems.
• Relatively immature technology.
• Complex fabrication methods for fiber-reinforced systems (except for casting).
• Limited service experience.
1.3.3 Applications of Metal Matrix Composites
Space: The space shuttle uses Boron/Aluminium tubes to support its fuel usage frame. In addition to decreasing the mass of the space shuttle by more than 145 kg, Boron/ Aluminium also reduced the thermal insulation requirements because of its low thermal conductivity.
Military: Precision components of missile guidance systems require dimensional stability that is the geometries of the composites cannot alter during use. MMC’s such as SiC composites satisfies this requirement because they have high micro yield strength. In addition, the volume fraction of reinforcements can be varied to have a suitable co-efficient of thermal expansion suitable with other parts of the system.
Transportation: MMC’s are finding use now in automotive engines that are lighter than their metal counter parts. Also, because of their high strength and low weight, MMC’s are the material of choice for gas turbine engines.
1.4 Fabrication Process:
1.4.1 Stir Casting Method
Stir casting is an economical liquid phase method of composite materials fabrication. Here a dispersed phase (like short fibers, ceramic particles) is mixed with a molten metal by means of mechanical stirring. Then the liquid composite material is poured into an appropriate crucible and cast by conventional metal forming technologies. However, major challenge in the liquid phase processing is to achieve uniform distribution of reinforcement and to obtain strong interfacial bonding between the reinforcement and the matrix. The stir casting set up is shown in figure 1.3.
Fig 1.3 Stir casting
1.4.2 Characterization of Stir Casting
1. Percentage of dispersed phase is limited (usually not more than 30% by volume)
2. Dispersed phase distribution is not perfectly homogeneous throughout the matrix.
• Gravity segregation of the dispersed phase can occur due to a difference in the densities of the dispersed and matrix phase.
• Clusters of the dispersed particles (fibers) may form.
3. The technology is relatively simple and low cost.
1.5 Aluminium Alloys:
Aluminium alloy is a composition consisting mainly of aluminum to which other elements have been added. The typical alloying elements are Magnesium, Manganese, Copper, Silicon and Zinc. There are two primary classifications namely casting alloys and wrought alloys, both of which are further subdivided into the categories heat-treatable and non-heat-treatable.
About eighty-five percentage of Aluminium is used for wrought products, for example foils, rolled plate and extrusions. Cast Aluminium alloys yield cost-effective products due to the low melting point, although they generally have lower tensile strengths than wrought alloys. Alloys comprised mostly of Aluminium plays crucial role in aerospace manufacturing since the introduction of metal skinned aircraft.
Aluminium alloys are extensively used in engineering structures and components where light weight or corrosion resistance is required. Wrought Aluminium alloys are used in the shaping processes: stamping, rolling, forging, extrusion, pressing. Cast Aluminium alloys comes after sand casting, permanent mould casting, die casting, investment casting, centrifugal casting, squeeze casting and continuous casting.
1.5.1 Cast Aluminium Alloys
Aluminium and its alloys are used in a variety of cast and wrought form and conditions of heat treatment. Forgings, sections, extrusions, sheets, plate, strip, foils and wire are some of the examples of wrought form while castings are available as sand, pressure and gravity die-castings e.g. Al-Si and Al-Mg alloys. The designation of Cast Aluminium alloy is shown in Table 1.1.
Table 1.1: Designation of Cast Aluminium alloys
Alloy Designation Details
1XX.X 99% pure Aluminium
2XX.X Cu containing alloy
3XX.X Si, Cu/Mg containing alloy
4XX.X Si containing alloy
5XX.X Mg containing alloy
7XX.X Zn containing alloy
8XX.X Tin containing alloy
6XX.X Unused series
1.5.2 Wrought Aluminium Alloys
To meet various requirements, Aluminium is alloyed with Copper, Manganese, Magnesium, Zinc and Silicon as major alloying elements. A four-digit numerical designation system is used to identify wrought Aluminium alloys. As shown in table 1.2. below, the first digit of the four-digit designation indicates the group.
Table 1.2: Designation of Wrought Aluminium alloys
Alloy designation Details
1XXX 99% pure Aluminium
2XXX Cu containing alloy
3XXX Mn containing alloy
4XXX Si containing alloy
5XXX Mg containing alloy
6XXX Mg and Si containing alloy
7XXX Zn containing alloy
8XXX Other alloys
1.5.3 Designation of Aluminium Alloys
The Aluminium Association of America has classified the wrought Aluminium alloys according to a four-digit system. The classification is adopted by the International Alloy Development System (IADS). Table 1.1 & 1.2 gives the basis of designation of wrought and cast Aluminium alloys in the four-digit system. The first digit identifies the alloy type the second digit shows the specific alloy modification. The last two digits indicate the specific Aluminium alloy group.