Metal Matrix Composites

Metal Matrix Composites (MMC's) are classified into three broad categories depending on the aspect ratio of the reinforced phase and are not new to the industry. Firstly, unidirectional aligned long fibers are embedded in a matrix. The fiber material could be carbon or a ceramic like silicon carbide, and the matrix can be a metal like titanium/copper/aluminum. Mechanical properties like strength and stiffness are higher in the longitudinal direction than those in the transverse direction of the fibers. Secondly, short fiber reinforced composite is formed, wherein the short fibers are oriented randomly in the matrix material.

Finally, particulate composite in which irregular-shaped particles of second phase are dispersed in a metal matrix. The particle ceramics like silicon carbide, graphite, magnesium oxide, fly ash or aluminum oxide and the matrix could be a metal like aluminum, copper, titanium or magnesium. These composites can be synthesized by vapor phase, liquid phase or solid phase processes.The focus of this work will be on liquid phase processes where the matrix, in the form of a liquid, is mixed with the reinforcements and allowed to solidify to form a composite. Fiber-reinforced composites, where the aspect ratio (ratio of length to diameter) of the second phase lies above 100, are used in applications where higher specific strength and specific modulus are required in a direction parallel to the fiber. In particulate composites, the aspect ratio of the second phase is close to one. This class of materials gives marginal improvement in strength, as compared to the matrix alloy and provides isotropic strength properties and improved wear properties. MMC's have proved to be an important class of materials, with the potential to replace a number of conventional materials being used in automotive, aerospace, defense, and leisure industries, where the demand for lightweight and higher strength components is increasing.

MMC-Characteristics of Composite Materials

The constituents of a composite are generally arranged so that one or more discontinuous phases are embedded in a continuous phase. The discontinuous phase is termed the reinforcement and the continuous phase is the matrix. In general the reinforcements are much stronger and stiffer than the matrix. The physical and mechanical properties of composites are dependent on the properties, geometry and concentration of these constituents. Increasing the volume content of reinforcements can increase the strength and stiffness of a composite to a point. If the volume content of reinforcements is too high there will not be enough matrix to keep them separate and they can be entangled. Similarly, the geometry of individual reinforcements and its arrangement within the matrix can affect the performance of a composite. There are many factors to be considered when working with composite materials. The type of reinforcement and matrix, the geometrical arrangement and volume fraction of each constituent, the anticipated mechanical loads, the operating environment for the composite, etc., must all be taken into

account.

Analysis of composites subjected to various mechanical and thermal conditions is the main thrust of this work. The constitutive relationship between stress and strain is established for homogeneous isotropic materials as Hooke's law. A composite material is analyzed in a similar manner by establishing a constitutive relationship between stress and strain. Isotropic, homogeneous materials (steel, aluminum, etc.) are assumed to be uniform throughout and have the same elastic properties in all the directions. Assuming a unit width and thickness for the specimen, the transverse in-plane and out-of-plane displacements are the same for these materials. Unlike these conventional engineering materials, a composite material is generally non-homogeneous and does not behave as an isotropic material. Most composites behave as anisotropic. There is typically a coupling of extension and shear deformation under conditions of uniaxial tension. There are varying degrees of anisotropic material behavior and the actual deformation resulting from applied loads depends on the nature of material.

Problems in utilizing the Composites in industries-MMC

The primary barrier to the use of composites materials is their high initial costs in some cases, as compared to traditional materials, regardless of how effective the material will be over its life cycle. Industry considers high upfront costs, particularly when the life-cycle cost is relatively uncertain. This cost barrier inhibits research into new materials. In general, the cost of processing composites is very high, especially in the

hand lay-up process where raw material cost is only a small fraction of the total cost of a finished product. The recycling of composite materials presents a problem when penetrating a high-volume market such as the automotive industry, where volume production is in the millions of parts per year. With the new government regulations and environmental awareness, the use of composites has become a concern and poses a big challenge for recycling. Although composite materials other many benefits, the following disadvantages too are there:

1. The cost for composite materials is almost 5 to 20 times high as compared to the conventional material like steel and aluminum.

2. In the past, composite materials have been used for the fabrication of large structures at low volume (one to three parts per day). The lack of high-volume production methods limits the widespread use of composite materials.

3. Classical ways of designing products with metals depend on the use of machinery and metals handbooks and design data handbooks.Large design databases are available for metals. Designing parts with composites lacks such books because of the lack of a database.

4. The temperature resistance of composite parts depends on the temperature resistance of the matrix materials. Some composites absorb moisture also which may affect the properties and dimensional stability of the composites.

Metal Matrix Composites - Why Composites??????

Composites have been normally designed and manufactured for applications in which high performance and light weight are needed. There other several advantages over traditional engineering materials as discussed below.

1. Composite materials provide capabilities for part integration. Several metallic components can be replaced by a single composite component. This others greater exibility for design changes in this competitive market where product lifetime is continuously reducing and eliminates several machining operations and thus reduces process cycle time and cost.

2. Composite materials other greater feasibility for employing Design for Manufacturing (DFM) and Design for Assembly (DFA) techniques. These techniques help minimize the number of parts in a product and thus reduce assembly and joining time. Noise, Vibration and Hardness (NVH) characteristics are better for composite materials than metals.

3. Composite structures provide in-service monitoring or online process Monitoring with the help of embedded sensors. This feature is used to monitor fatigue damage in aircraft structures or can be utilized to monitor the resin ow in Resin Transfer Molding (RTM) process. Materials with embedded sensors are known as "smart" materials, can also be fabricated using composite structures.

4. Composite materials have a high specific stiffness (stiffness-to-density ratio) and the specific strength (strength-to-density ratio). The fatigue strength (endurance limit) is much higher for composite materials.

5. Composite materials other high corrosion resistance. Iron and aluminum/ aluminum alloys, however, corrode in the presence of water and air and require special coatings.

6. The Coefficient of Thermal Expansion (CTE) of composite structures can be made zero by selecting suitable materials and lay-up Sequence. As the CTE for composites is much lower than for metals, Composite structures provide good dimensional stability and manufacturing feasibility.

Metal matrix Composites- COMPOSITES

The concept of composites is found to be naturally occurring. Scientists have found that the fibers taken from a spider's web are stronger than synthetic fibers. In India, Greece and other countries, husks or straws mixed with clay have been used to build houses for several hundred years. The main concept of a composite is that it contains matrix materials (metals, plastics, or ceramics) and the reinforcements (fibers, particulate, or whiskers can be reinforcement and the matrix materials can be metals, plastics, or ceramics). The continuous, long or short fibers made with a polymer matrix have become more common, and are widely used in various applications. The important thing about composites is that the fibers/particulate/whiskers carry the load and its strength is greatest along the axis. Depending upon the type of application (structural or nonstructural) and manufacturing method, the form of reinforcement is selected. For structural applications, continuous fibers or long fibers are recommended; whereas for nonstructural applications, particulate and short fibers are recommended.

Metal Matrix Composites- General Introduction

There are more than 50,000 materials available to engineers for the design and manufacturing of products for various applications. These materials range from ordinary metals(copper, cast iron, brass) that have been available for several hundred years, to the more recently developed, advanced materials e.g., composites, ceramics and other high performance materials. Due to the wide choice of materials, today's engineers are posed with a big challenge for the right selection of a material and the right choice of a manufacturing process for an application. It is difficult to study all of these materials individually; therefore, a broad classification is necessary for simplification and characterization. These materials, depending on their major characteristics (stiffness, strength, density and melting temperature) can be broadly divided into four main categories: (1) metals, (2) plastics, (3) ceramics and (4) composite . Each class contains large number of materials with a range of properties which to some extent results in an overlap of properties with other classes.

A ceramic matrix is usually brittle. Carbon, ceramic, metal and glass fibers are typically used with ceramic matrices in areas where extreme environments (high temperatures, etc.) are anticipated. Glass and glass-ceramic composites usually have elastic modulus much lower than that of the reinforcement. Carbon and metal oxide fibers are the most common reinforcements with glass matrix composites. The best characteristics of glass/ceramic matrix composites are their strength at high service temperatures. The primary applications of glass matrix composites are for heat-resistant parts in engines exhaust systems and electrical components.

Increasing quantities of metal matrix composites (MMCs) are being used to replace conventional materials in many applications, especially in the automobile and recreational industries. The MMCs are aluminum alloys reinforced with ceramic particles and these low-cost composites provide higher strength, stiffness and fatigue resistance with a minimal enhancement in density over the base alloy. The major advantages of Aluminum Matrix Composites (AMCs) include greater strength, improved high temperature properties, controlled thermal expansion coefficient, thermal/heat management, enhanced/tailored electrical performance, improved abrasion/wear resistance,improved damping capabilities, low induced radioactivity under nuclear environments, low stiffness and weight, saving in materials and energy .

The alloy designation for Al is based on four digits corresponding to the principal alloying elements. The most important alloying elements in aluminum alloy systems are copper (2xxx), manganese (3xxx), silicon (4xxx), magnesium (5xxx) and zinc (7xxx). The most commonly used route for fabrication of these composites has been through infiltration of molten metal into preformed and porous ceramic bodies. For example, Al-alloys have been successfully infliltrated with hydraulic or gas pressure into fly-ash,SiC, Al2O3, MgO and AlN performs. It is very interesting to emphasize the influence of volume fraction of reinforcing particle on the mechanical properties of MMCs .

EXPERIMENTAL STUDIES ON REINFORCED ALUMINUM ALLOYS (MMCs)-ABSTRACT

The energy crisis and global inclination to reduce green house gas emissions have been catalytic in directing the attention of researchers/scientists to look for good physical and mechanical properties light weight materials. Aluminum (Al) is an important engineering material being used in a number of engineering applications. It is the most abundant metal in nature around 8% by weight of the earth's crust. Al is a good electrical and thermal conductor with Face-Centered Cubic structure. Al/Al alloys have been attracting much attention as light weight materials due to their high specific strength, good castability, good machinability, high damping capacity, and its availability as a natural mineral. Al/Al alloys are also 35% lighter than iron based materials. The main focus currently is to develop new generation of composite materials capable of exhibiting good combination of thermal, mechanical and other properties. New materials such as Metal Matrix Composites are necessary to fulfill the requirements in applications for instance in sports, aerospace and automobile sectors.

The most commonly used and economically viable techniques for fabrication of MMC's are solidification processing and stir casting. Another important technique used for the purpose, is infiltration of liquid metal through narrow crevices between fibers or particulate reinforcements that are arranged in a perform. In solidification process, liquid metal is combined with the reinforcement phase and solidified in a mold. However in stir casting, the molten metal is stirred with the help of either a mechanical stirrer or high intensity ultrasonic waves. This action disperses the reinforcing phase, which is added to the surface of the melt in the molten metal and solidifies the composite melt, containing reinforcements suspended in it. Stir Casting route is now used for large-scale production of Metal Matrix Particulate Composites. Various metals such as Al, Mg, Ni, and Cu have been used as the matrix and a wide variety of reinforcements like ZrSiO4, SiC, graphite, SiO2, Si3O4 and Al2O3 have been used in the available literature using the aforesaid techniques. In this work, new composites based on SiC, MgO and Al2O3 are developed to address the industries problems.

In the first phase of the present work, the reinforced MMC's of Al/Al alloy-Al2O3 system, with nominal composition (A384.1)(1-x)[(Al2O3)p]x were fabricated under extruded and peak aged conditions by using A384.1 Al Alloy as matrix and Al2O3 with 0.220, 0.106 and 0.053 microns particle sizes as reinforcement in varying amounts. The modified stir casting method is employed to produce twenty four composite samples from 0.0 to 0.20 and then characterized along with the unreinforced alloy.

The results revealed reasonably uniform distribution of Al2O3 particulate, good adherence of reinforcement with the matrix, and presence of minimal porosity suggesting the suitability of processing methodology adopted in the present study. The microstructure of the reinforced samples is found to have lower amount of overall porosity as compared to any other similar composite system. Along with reduction in porosity, a better distribution of particles was achieved in all alumina composites.

No interaction layer or any other reaction product was found at the interface that appeared clean in the as-cast Al-Al2O3 materials. The EDX analysis, however, revealed the presence of the elements Al and O at the interface layer. It is believed that this is probably coming due to an oxide layer formed during sample preparation. With increasing amount of alumina, the MMCs showed a significant increase in tensile failure strain (from 5.6% to 29.5%), compressive strengths (0.2% proof stress and compressive strength) and tensile strengths (0.2% proof stress and ultimate tensile strength) and hardness reduced during ageing. Moreover, the composite of (A384.1)(1-x)[(Al2O3)p]x exhibited a good combination of porosity, density, tensile strength, compressive strength and age hardening properties.

In the second stage of this work, the (A384.1)(1-x)[(MgO)p]x composite system has been investigated by taking MgO, with grain sizes of 0.220, 0.106 and 0.053 microns, as reinforcement and Al alloy (A384.1) as matrix and composite samples with x=0.0 to 0.20. In general densification is obtained in the base matrix (alloy) and all composites samples (having reinforcements) are fabricated under extruded and peak aged conditions.

But the composites having reinforcement MgO, in the extruded condition, observed some amount of porosity, probably due to dissolved gases i.e. hydrogen, nitrogen etc. A characteristic change is observed in porosity and density of the Al alloy and MMCs from extrusion to peak aged as a function of reinforcement from x=0 to x=0.20. It noticed that the value of porosity from unreinfroced to reinforced conditions increases with the increase in reinforcement. The change in porosity observed from unreinfroced to reinforced conditions is 5.47% for x=0.0 sample, but this is found to be 25.6%, 29% and 32% for x=0.10, 0.15 and 0.20 samples respectively. It is further noticed that density exhibits an increase of 13.2% from from unreinfroced to reinforced conditions, which increases to 18.2% as x varies from 0.0 to 0.20. The micro-hardness and macro-hardness measurements on unreinforced alloy and the MMCs in extruded and peak aged conditions showed that the composite with x=0.10 has higher macro-hardness than the unreinforced alloy (x=0.0) in both extruded conditions and peak aged conditions. On the other hand, composites x=0.15 and 0.020 show lower hardness than the alloy in the extruded condition. This is due to the porosity present in these two composites. Due to extrusion the pores are closed and as a result these two composites exhibit higher hardness than the unreinforced alloy in the extruded condition. The results show that increasing the content of MgO led to a decrease in amount of MgAl2O4 phase, an increase in the amount of Mg,Al phase and the significant reinforcement of matrix grain size. The increasing percentage of reinforcement with change in particle size also led to a noticeable improvement in ageing properties, tensile and compressive strengths.

In the third stage of this work, the (A384.1)(1 - x)[(SiC)p]x composite system has been investigated by taking SiC, with grain sizes of 0.220, 0.106 and 0.053 microns, as reinforcement and Al alloy (A384.1) as matrix. The study revealed that SiC can be used for production of in-situ reinforced composites. Depending upon the particle size, SiC up to 10-20% by weight can be successfully added to Al alloy matrix. Twenty four composite samples with 0x0 from 0.0 to 0.20 have been fabricated. The micro-structural characterization of MMCs and unreinforced alloy exhibited lower amount of porosity as compared to any other similar composite system. SiC particles are found to get wetted by Al Alloys melt in a better way as compared to other ceramic reinforcements and thus the interfacial structure bonding is comparatively strong in this system as evidenced by the improved mechanical properties of these materials. The observed strengthening of the composite can be explained in terms of dispersion strengthening due to SiC particle reinforcement. A change in density from as-cast to extruded conditions is found to be 5.546%, 6.444%, 8.086% and 9.304% for x values of 0.0, 0.10, 0.15 and 0.20 respectively. Accordingly, the reduction in porosity from unreinforced to reinforced samples was also observed for the different values of 0x0 (94.4% for x = 0 and 67.05% for x=0.20). Moreover, the values of macro- and micro-hardness of the samples exhibited a trend of variation. The higher values of micro-hardness observed in case of peak-aged composites can be explained in terms of higher dislocation density near the particle-matrix interface due to the large different in thermal expansion coefficient between the matrix and reinforcement as compared to unreinforced alloys. The mechanical testing of the fabricated composites was also done under compressive and tensile stresses.

To evaluate the performance of the fabricated MMC samples under actual service conditions and hence to elucidate the application potential of the fabricated Al/Al Alloys matrix-ceramics particulate composite in technological applications, the Diffraction Scanning Calorimetric (DSC) was performed on the different composites of Al2O3, MgO and SiC at 2_C/min to determine the phase stability and transformations and to study precipitation in aluminum alloys. Quantitative interpretation of DSC experiments on aluminum based alloys had been done. All the materials were found to have phase stability and no thermal evidence of formation of any undesirable phase. Overall, the developed composites show a different range of physical and mechanical properties and thus show a great potential in diverse engineering applications such as in sports industry, aerospace, automobile , military applications and transportation. Fuzzy model of the system is also developed for density and porosity of the reinforced MMCs, using adaptive neuro-fuzzy inference system (ANFIS) and performance of experimental values is evaluated by comparing it with fuzzy model and good correlation is achieved between them.