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Metal Casting Technologies : September 2006
66 TECHNICAL FEATURE processing loss; therefore efforts are justified to recover as much aluminium as possible. Some of the metallic losses are recoverable within the system. (Figure 2) For example, trimmings, which are the waste portion of castings, unless they are contaminated or out of specification, often may be recycled directly through the melting furnaces on site. Dross is usually sent to specialist recyclers for processing. The final material losses are reduced then to the irredeemable fraction only. Aluminium lost by oxidation during the melting process is significant in the environmental context because of the high amount of energy invested in the metal during its conversion from bauxite, and the emissions associated with its production and reprocessing. Also, the irredeemable waste, (which may be contaminated by chemicals used for treatment), must be sent to landfill adding to the environmental impact of wastes. CASTING QUALITY The production of scrap castings is probably the largest single source of energy waste in foundries - scrap also wastes resources and lowers profit. Quality improvement has been a pre-occupation of UK industry for the last couple of decades. Product quality is directly related to energy efficiency since the one of the aims of quality management is to reduce scrap. Advanced techniques have been devised to avoid the generation of defects developed during mould filling, but these techniques are not fully effective unless the melt is free from gaseous and solid contaminants when it is transferred. Measures must be taken to correct the condition of the melt by controlling its temperature and preparing its metallurgical composition. Time and temperature above the liquids influence the loss of alloying elements that are characterized by low vapour pressure and high reactivity. The potential for depletion applies to elements such as sodium, calcium, strontium and magnesium. Deficiencies of these elements influence the physical properties of castings that then may be scrapped - creating further wastes of material and energy. Therefore it is important to: a) control the temperature of the melt to the lowest level compatible with the alloy, its chemical treatment and casting process, and b) to minimize the time that the metal is held in the liquid state. The adsorption of hydrogen into aluminium in the liquid state results in a dissolved hydrogen content which may rise up to the equilibrium value for the specific alloy composition and its temperature. The sources of hydrogen in molten aluminium are - moisture from the ambient air, products of combustion in flame heated furnaces, furnace and ladle linings, and the charge material itself. Dissolved hydrogen in the melt causes casting porosity, which for many products is unacceptable since the mechanical properties are impaired. As it is not often practicable to rework aluminium castings, the result of porosity is waste in the form of scrapped castings. Aluminium alloys oxidize readily. When aluminium is melted, a continuous oxide skin forms on the surface. When the skin is disturbed or removed, a further oxide skin forms. In this way a build up of oxide results which can later be trapped in the melt. Aluminium oxide films have almost the same specific weight as the melt itself; therefore they are suspended in the melt and are transferred to the finished casting unless action is taken to prevent it. It is important to reduce the formation of oxides for quality control and reduce metal loss. Devices for determining hydrogen levels in aluminium alloys are well established, but until recently a means of monitoring levels of oxides and other inclusions was not been readily available. Recent innovations in metal cleanliness measurement techniques can now be utilized to control this aspect of melt quality. Figure 3 shows the results of tests using a new measuring technique that demonstrates the effectiveness of rotary flux feeding when used with a suitable flux for the removal of inclusions. The bars on the chart show the total content of inclusions in one kilogram samples of metal on the surface of polished test piece samples measured as mm kg-1. The number of oxide films in the sample also can be counted in samples as indicated on the right axis of the chart. Both are a measure of 'metal cleanliness'. When used with appropriate chemicals, the rotary degassing and flux feeding system is effective for modification, silicon refinement and trace element removal. Much of the development work on fluxes currently taking place is aimed at reducing the environmental effect of fluxing, both Figure 2. Flow diagram of the melting & casting process with recycling loops Figure 3. Effect of process variables on metal cleanliness www.metals.rala.com.au