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Metal Casting Technologies : September 2006
64 TECHNICAL FEATURE he global warming theory has been discussed in the wider context of the potential impacts on the Earth's climate. To address this, international action has been taken to reduce greenhouse gas emissions by developed countries. To meet the commitments made by the international community, all sectors of the world's industries must reduce energy wastage. This is seen as a problem by industries that use large amounts of energy for processing. For example, it presents a particular problem for foundries where metal melting uses the major proportion of the energy needed to produce castings. This article deals specifically with the aluminium casting sector, but many aspects of it apply to other sectors of the foundry industry. The production of aluminium castings is highly dependent on the output of the automotive sector and the sector's use of the metal. The average content of aluminium castings in cars and light trucks is now in the order of 120 kilograms per vehicle. Other major markets for aluminium castings are also expanding with global economic growth. About two thirds of aluminium castings are for the automotive market. However, the increasing demand for domestic appliances, other consumer goods, and office machines, and the greater use of aluminium castings in electrical and civil engineering, will increase further the output significantly over the next decade. As an energy intensive industry, the non-ferrous metals sector has a relatively low profile, unlike iron and steel, chemicals and paper for example. The aluminium casting sector's environmental externality may seem even benign compared to others. However, this is not so. Furthermore, the environmental impacts from aluminium casting will increase with growth in demand. Against a background of growth, benefits from environmental and energy improvements will become more significant. Around 60% of the energy used in the production of raw (as-cast) aluminium castings is for melting and then holding in the molten state prior to casting. Research in European industry indicates that in many aluminium foundries there is an average metal loss in the order of 5% incurred in the molten state. With this metal loss, there is the associated energy waste and CO2 emissions to air. Using existing technologies and modest investments can make some improvements in the aluminium casting industry's performance. In the longer term, more efficient melting technology and different operating practices will be needed to minimize waste of both energy and materials. There are opportunities for the industry to reduce its specific energy consumption, but there is no single model melting and holding system that could be replicated throughout the sector. CASTING PROCESSES There are two basic reasons for melting aluminium, either to change its composition - alloying, or to change its form by processing in its liquid state - casting. The main aluminium casting routes are: diecasting -- casting in metal dies or 'permanent' moulds sand casting -- casting in expendable sand moulds investment casting -- casting in expendable ceramic moulds. There are variants of the first two of these processes: diecasting -- gravity filling - high pressure filling - low pressure filling sand casting - gravity filling - low pressure Sand casting is the oldest and probably the simplest process for producing complex cast metal forms. Various methods are used to make sand moulds; selection depends upon several factors including casting complexity, accuracy, size, and production requirements. Gravity diecasting utilizes a metal mould into which molten metal is poured either from a manual or mechanical ladle, or directly from a pressurized dispensing furnace. The moulds are used repeatedly for medium to high production volumes of moderately complex shapes. Low-pressure diecasting employs a ferrous alloy mould into which the liquid metal is counter-gravity fed from a pressurized holding furnace under the machine. This process is widely used for producing car wheels and other high quality complex shapes. High-pressure diecasting uses steel moulds mounted in machines in which the metal is injected under high pressure by a piston in the machine. The machines require high capital investment that is justified for the manufacture of high volume, dimensionally accurate, near-net-shape castings. Investment casting is a technique that is mainly used for producing small, complex shapes with good surface finish and to near net shape. Wax patterns are progressively coated with ceramic slurry to form a shell. The wax is melted out to leave a ceramic mould into which liquid metal is poured. Gravity diecastings account for approximately 40% aluminium castings produced in foundries. 40 to 50% are high pressure diecastings, about 10% are sand cast. The balance is produced by other processes such as low pressure diecasting, 'lost foam' and investment casting. Energy & Aluminium Casting T By Dr Phil Ramsell www.metals.rala.com.au