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Metal Casting Technologies : September 2006
50 INTRODUCTION asting is one of the oldest manufacturing processes which dates back to approximately 4000 B.C. Human beings have been producing castings for thousands of years by pouring molten metal into sand molds. Casting is one of the many forming processes and it is one of the cheapest methods of giving finished shape. The molten metal is poured into a mould to get the final finished part. Metal castings form important components of machines that perform useful functions for human beings. The metal casting process is the simplest way to produce a near net shape product. The other methods of making a finished product require a cast ingot. The cast ingot is given final shape by hot or cold working. The other way of making finished part is by powder metallurgy (PM) process which involves die pressing and sintering of the powdered parts. Casting is used in mass production work and large sized parts can be produced. The cast metal industry has advanced with time due to the applications of sciences of physics, chemisty, metallurgical engineering and ceramics. Some examples are development of high strength steels, new casting methods, mould materials and refining methods. The foundry metal can be classified as ferrous (iron base) and non ferrous. Lot of research has been done in the fields of ferrous and non ferrous castings. Weight reduction of the vehicles is an important need of the new generation. Non ferrous metal like aluminium is lighter than steel and has density one third of that of steel. Engineers tried to focus more research on aluminium alloys since aluminium has a lower density of 2.7 gm/cc compared to 7.8 gm/cc of steel. Aluminum and aluminum alloys are lightweight with good corrosion resistance, ductility and strength. The greater use of aluminum can decrease vehicle weight, improve its performance and reduce fuel costs. To cut the emissions there is a need to improve the fuel efficiency and make the vehicle lighter in weight. Castings are produced by a wide variety of processes. The present article will describe two light weight materials 1) Metal foam 2) Metal Matrix Composites (MMC's). These two materials have important applications in the current and future engineering sectors. These materials can be prepared by casting route as it is more economical. In the present article brief discussion on MMC's and metal foam is given. METAL FOAM The interest in metallic foam has increased. The main reason for this development is new concepts for light weight vehicles. When discussing about foams a common idea that will come in people's mind is the foam used in cushions. Polymer foams are widely used in domestic and industrial applications. However, structural applications of polymer foams generally have been limited to low-stress components because of the relatively low strength compared with other engineering materials. It is interestng to know that even a foam can be made of metal. The metal can be foamed to achieve lower density. A great reduction in the weight of metals can be achieved by a metal foam. Metal foams are lower density materials with high porosity levels. Metallic foams have become an attractive area for research in all academic institutes. The existing porous materials led to the development of cellular materials made from polymers, ceramics and glass which have applications for damping, insulating and cushioning. Hence it can be thought to make porous structures from metals as it will give it the metallic properties and characteristics of foam. Aluminium is commonly used material for metal foam production. Metal foam combines the advantages of a metal (metal is strong, tough, and conductive to electricity and heat) with the structural advantages of foam (foam is lighter and stiff) that makes it attractive in industrial applications. Aluminum foam is used as a material for sound absorption, and impact energy absorption (1). Other applications include light weight panels for transport and in buildings, cores for sandwich structures in marine or aircraft industries, heat exchangers, filters and crash energy absorption (2). During the last ten years several methods had been developed for the production of metallic foams or similar porous metal structures. There are many ways to manufacture these porous metallic materials. Foams can be prepared by molten metal route and powder metallurgy method. Foam can be prepared by molten route by bubbling gas through molten metal. In the Alcan process gas is injected into a molten aluminum containing ceramic particles like SiC or Al2O3. The bubbles rise to the melt surface forming liquid foam. The liquid foam is then removed from surface and allowed to cool. Aluminum foams have been produced using this technique (3). Foams can be prepared by adding titanium Cast Metal Engineering www.metals.rala.com.au By Amit M Joshi