CHAPTER ONE
INTRODUCTION
A composite is considered to be any multi phase material that exhibits a significant proportion of the properties of both constituent phases such that a better combination of properties is realized. This is termed as the ‘principle of combined action’ (2). According to this principle, better combinations are fashioned by the judicious combination of two or more distinct materials. All composites generally have one thing in common: a matrix or binder combined with a reinforcing material, within which is a dispersion of one or more phases of another material.
Metal matrix composites, as we know today have evolved significantly during the past few years. The primary support of the composites has come from the aerospace industry for air frame and spacecraft structures. More recently the automotive, electronics and recreation industries have been working diligently with these composites. The driving force behind the development of most of the existing composites has been their capability to be designed to provide needed types of material behaviour.
The focus of research and development in the metal matrix composites (MMCs) area has recently shifted toward low-cost discontinuously reinforced composites which are targeted for automotive and aerospace applications. The optimum properties of MMCs and the enhanced performance from these materials however depend on the judicious selection of the metallic matrix material, reinforcing phase and the processing technique. The composite fabrication technique is an important consideration. For a given set of constituents, the fundamental link between properties and cost is determined by the fabrication method. Processing in general, is concerned with the introduction of reinforcement into the matrix with a uniform distribution. The major hurdle is the achievement of proper bonding between the matrix and the reinforcement in order to attain good load transfer between phases.
A wide variety of fabrication techniques have been explored for metal matrix composites. These include liquid phase methods, deposition of matrix from a semi solid or vapour phase, and solid state consolidation. Liquid phase processing has attractive economic aspects. Chopped fibres, porous ceramics compacts and particulates have been incorporated into matrix alloys. In some cases, pressure assistance has been used to infiltrate the reinforcement with the molten matrix. These methods result in micro structures dictated by the solidification of the molten metal. The green sand casting technique has been among the simplest and the most economical processes of fabricating the particulate metal matrix composites. However due to poor wetting of the ceramic particles by molten alloy, the introduction and uniform dispersion of the reinforcement into the liquid matrix is difficult.
The most common of the metal matrix composites is silicon carbide particulate, SiC, reinforced aluminium. When compared with its unreinforced matrix alloy, the metal matrix composite is characterized by significant increases in elastic modulus, tensile, shear and fatigue strength, wear properties and low thermal expansion along with high thermal conductivity.
Silicon carbide is the only chemical compound of carbon and silicon. It was originally produced by a high temperature electro-chemical reaction of sand and carbon. Silicon carbide (SiC) is an excellent abrasive and has been produced and made into grinding wheels and other abrasive products for over one hundred years. Today, the material has been developed into high quality technical grade ceramics with very good mechanical properties. It is used in abrasives, refractories, ceramics and numerous high performance applications including as reinforcement in composites.
The material can also be made an electrical conductor and has applications in resistance heating, flame igniters and electronic components. Structural and wear resistance applications are constantly developing. The high thermal conductivity coupled with low thermal expansion and high strength gives this material exceptional thermal shock resistant quality. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures, approaching 1600°C with no strength loss.
Aluminium on the other hand is the most abundant metallic element in the earth’s crust. It is light weight, silvery metal. The atomic weight is 26.9815, and its melting point is about 650°c. Aluminium is strongly electropositive metal and extremely reactive. In contact with air, it rapidly becomes covered with a tough transparent layer of aluminium oxide that resists further corrosive action. For this reason, materials made of aluminium do not tarnish or rust. Its light weight and corrosion resistant nature makes aluminium a good metal matrix for composites.
Aluminium matrix composites (AMC) offer superior combination of properties in such a manner that today no existing monolithic material can rival. Over the years, AMC have been tried and used in numerous structural, non-structural and functional applications in different engineering sector. Driving force for the utilisation 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 (6). Aluminium matrix composites are intended to substitute monolithic materials including aluminium alloys, ferrous alloys, titanium alloys and polymer based composite in several applications. Particle reinforced aluminium matrix composites (PAMCs) constitute the largest quantity of composite produced and utilized on volume and weight basis. They have been made into many automobile engine components. These applications attempt to advantage of the lower thermal expansion, the increased stiffness, the high thermal conductivity and the increased wear resistance of the composite. A partial list of the components tested to date include connecting rods, push rods, pistons, valve spring retainers and valve lifters(6). This composite material is being considered for connecting rods since the expansion is similar to steel, and this will reduce the large end, crankshaft, clearance problems encountered with aluminium alloys in this application.
1.1Â Â Â OBJECTIVE
To produce an automobile connecting rod using Al/SiC composite as opposed to the traditional material in use today.
1.2Â Â Â AIM OF STUDY
The aim of this project is to determine the mechanical properties of an Al/SiC composite connecting rod containing varying volume fractions of SiC.
1.3Â Â Â SCOPE OF STUDY
This experimental study is limited to connecting rods produced via green sand casting using aluminium matrix reinforced with Silicon carbide. Commercially pure Aluminium alloy (up to 99.1% Purity) was used as the matrix and Silicon Carbide reinforcements of 75, 125 and 300 microns sizes were used respectively. Metallographic test, tensile and hardness tests were carried out on the different samples produced by varying the volume fraction of SiC and calculations considered relevant to this work were included.
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