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Metal Casting Technologies : September 2006
54 www.metals.rala.com.au TECHNICAL FEATURE the vortex produced in the aluminium melt. Figure 4 shows the castings. Figure 5 shows the distribution of graphite particles in aluminium matrix. Figure 6 shows a machined aluminium graphite bush prepared by casting route. Also MMC's were prepared with varying graphite % and their properties were determined. Magnesium piece was used as a wetting agent in this case. Figure 7 shows the strip fluidity of aluminium and Al/Gr MMC. CENTRIFUGAL CASTING Centrifugal casting consists of producing castings by pouring molten metal into rotating moulds. Molten metal is poured into the spinning mold cavity and the metal is held against the wall of the mold by centrifugal force. The principle of centrifugal casting is long established, dating originally from a patent taken out by A. Eckhardt in 1809. All centrifugal casting machines consist of an integral mould, a mechanism to rotate the mould and a metal pouring tundish for introducing molten metal into the mould. Low density non metallic impurities separate towards the bore. The yield of this process is high. The inside diameter has impurities and inclusions which are machined later. They are relatively free from gas and shrinkage porosity. Little metal is lost in tundish compared to gating and risering of the conventional sand casting process. Typical materials cast with this process are steel, aluminum, copper and nickel. Typical parts made by this process are pipes, boilers, pressure vessels, cylinder liners, etc. Composite pipes are a modification of centrifugal casting process. The centrifugal casting can be used to produce two cylinder layers fused together at the annular surface. With this optimum properties can be selected for both the inside and outside layers. The centrifugal casting process can be employed to cast two layer/three layer composite pipes. Bimetal casting can be thought of a better idea depending on the application. If some application demands outer layer with improved properties, then in such case bimetal may be a substitute to a full base metal casting. The inner layer of this casting can be cast with a lower cost metal. This can reduce the cost of the final part. Bimetal casting gives combinations of properties which are not available from one single base composition. Parts can be cast of two/three dissimilar metals with a sound metallurgical bond. In chemical processing area the application demands high strength and chemical inertness. If a single metal is used then it will have a bigger wall thickness and it will lead to a higher cost. In such case bimetal casting is a better option. Typical Bimetallic Combinations (31) Outer Layer Inner Layer M.S Martensic white iron S.S Grey iron 5% Cr steel S.S Low alloy steel 27% Cr C.I S.S M.S Corrosion resisting rolls for the textile or paper industry can be made in bimetal combination. Slush pump cylinder liners are operated in abrasive and corrosive conditions. Employing a bimetal liner will serve the purpose. Functionally graded casting is another application which can be produced by the centrifugal casting process. Recent advances in materials processing and engineering have led to a new class of materials called functionally graded materials (FGMs). Graphite is a soft material and it acts as solid lubricant. Centrifugal casting of aluminium melt with graphite particles will result in the formation of two zones in the material. As graphite is lighter it will segregate in the inner periphery due to the density difference. The casting with segregated graphite particles near the inner periphery can be used for bearing applications (32). Figure 5. Figure 6. Figure 7.