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Metal Casting Technologies : September 2006
WHO'S WHO OF METALS -- ANNUAL 2006/7 51 hydride (TiH2) powder to molten metal. The titanium hydride decomposes below the aluminium melting temperature, releasing hydrogen gas to form bubbles in the foam. In the Shinko Wire process, calcium and titanium hydride are added to molten aluminum, which is then mixed with an impeller in a special casting chamber (4). Calcium increases the viscosity of the melt. Advantages of the direct foaming process are that large volume of foam can be continuously produced. Aluminum foams with open cells can be prepared by infiltrating polyurethane foam with a heat resistant material, removing the polyurethane by heating, casting liquid metal into the resulting form and then removing the heat resistant material (5). Foams can also be created by powder route. Metal powder and blowing agent are mixed and compacted (6). Later the matrix material is melted causing the blowing agent to decompose. The released gas forces the melting precursor material to expand forming a porous structure. METAL MATRIX COMPOSITES Composite refers to a material system composed of a reinforcement distributed in a matrix. Metal matrix composites (MMCs) usually consist of a low density metal like aluminum or magnesium, reinforced with particles/fibers/whiskers of a ceramic material. Hard reinforcements are embedded in the matrix. The reinforcements can be particles, fibres or whiskers. The fibres can be continuous or discontinuous. In many industrial applications, new materials are required for the production of light-weight structures. The need for materials with improved properties like lower density, improved strength, high temperature performance and increased wear resistance has led to the development of metal matrix composites (MMC's. The composites are classified by their matrix - polymer, ceramic and metal. The polymer based composites cannot be used in high temperature applications. Aluminum is the common matrix for the metal matrix composites (MMCs). The aluminium alloys are attractive due to their low density, good corrosion resistance, and high thermal and electrical conductivity. They show variety of mechanical properties depending on the chemical composition of the aluminium matrix. They are usually reinforced by SiC, Al2O3, SiO2, BN, B4C, TiO2, graphite, flyash etc. The general aluminum matrices used are Al-Si, 2xxx or 6xxx alloys. Properties of MMCs can be tailored to the demands of different industrial applications by suitable combinations of matrix, reinforcement and processing route. Aluminium matrix composite reinforced with ceramic particles possess properties like improved strength and wear resistance and has applications in automotive and aerospace industries (7). MMC finds applications in thermal management and electronic packaging areas because coefficient of thermal expansion (CTE) of the MMC can be adjusted by varying the SiC % while maintaining the thermal conductivity (8). MMC's can be prepared by powder metallurgy (pm) and liquid state methods. In PM process the matrix and reinforcement powders are mixed and pressed to form a compact. The compact is further sintered and then extruded. PM method avoids segregation effects and brittle reaction product formation prone to occur in liquid state processes. This method gives good properties of the particulate reinforced MMCs. The PM method is not suitable for large quantities. PM method is expensive and involves larger number of steps (9). The molten metal mixing method is attractive and economical as it uses conventional foundry equipments and furnaces. The common liquid state methods are stir casting, spray forming, infiltration, centrifugal casting and squeeze casting. Stir casting method is the most common method of production of MMCs. This approach involves mechanical mixing of the reinforcement particles into a molten metal and casting the slurry in metallic moulds. Compocasting is the modification of stir casting process. The reinforcing particulates are incorporated into vigorously agitated partially solid aluminum slurries (10). In the spray forming processes molten metal is sprayed together with the reinforcement and collected on a substrate where the solidification is completed (11,12). Squeeze Casting is a method by which molten metal solidifies under pressure press to produce void free castings. The ceramic preform is placed in a pre-heated die which is later filled with the liquid metal before applying the pressure (13). In the infiltration process liquid metal is infiltrated into the preform of reinforcement material by pressurized inert gas. (14). In the present article MMC's based on graphite and SiC reinforcements are prepared. Centrifugal casting is used to prepare composite pipe and graded casting. An attempt to prepare aluminium foam by casting route is tried. ALUMINIUM GRAPHITE MMC Graphite is a soft, crystalline form of carbon. Aluminium graphite particle composites are good bearing materials. Aluminium graphite composites possess gall resistance, improved wear resistance, damping capacity and good machinability properties (15). Gorbunov et al. (16) have reported that graphitic-aluminium alloys have good antifriction and anti-wear properties in wet media: better than bronze and the base alloy. The alloys can be recommended for the production of plain monolithic bearings which can be used under boundary lubrication conditions. Bruni et al (17) have shown that wear of graphitic aluminium when used as piston liner material is almost negligible and certainly lower than that of a conventional cast iron liner. Aluminium graphite composites possess lower friction coefficients and wear rates. B.P Krishnan et al. (18) found that Al --Si 3 % graphite particles piston resulted in 3 % reduction in specific fuel consumption, reduction in wear of piston rings and 9 % reduction in frictional horse power losses of the engine.