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Metal Casting Technologies : September 2006
56 www.metals.rala.com.au TECHNICAL FEATURE EXPERIMENTAL WORK 1. In the present case zinc lead bimetallic casting, aluminium casting and graded casting are produced by the centrifugal casting process. A bimetallic casting is prepared by centrifugal casting process. Zinc and lead are considered for the present case. Zinc has a higher melting point compared to that of lead. Both the metals are melted in two different electric resistance furnaces. The set up has a cast iron mould having two grooves on its outer surface, which rest on two sets or rollers, one of which is driven by a motor, while the other rotates by friction. The mould ends are closed by removable flanges. The flanges have hole at the centre to allow the passage of pouring lip of tundish. To guard against any accident or splashing of molten metal a metallic cover was attached above the mould and roller assembly. A refractory slurry of zircon was applied on the inner surface of the mould and on the flanges. The pouring lip of tundish is inserted in the hole of the front flange. The mould was preheated by oxyacetylene flame. First molten zinc is poured into the rotating mould via a tundish. After a determined waiting period, the second metal (molten lead), which is to form the inner zone of the casting was poured in the rotating mould. When the freezing is complete the tube has an annular weld or diffusion zone joining the two metals. The flanges were removed and the bimetallic casting was stripped out. The second metal should be poured when the first metal surface has lost fluidity. If lead is poured earlier then both metals can be mixed due to which the composition of metal will be altered. If lead is poured late, then there won't be good bonding between zinc and lead layers. This will lead to some discontinuity between the zinc and lead layers. The castings were cut on power hacksaw to observe the bond at different locations. The bimetallic castings were observed visually for proper bonding. 2. Aluminium was melted in a clay graphite crucible in an electrical resistance furnace and further poured in a rotating mould of the centrifugal casting machine. Two speeds 500 and 700 rpm were used. The remaining melts were poured in strip fluidity moulds coated with (a) carbon and (b) ceramic based coatings. The strip fluidity mould consists of thin rectangular channels. The sum of the all the fingers is an index of fluidity. For carbon coating oxyacetylene flame was used. The moulds were preheated before metal is poured. 3. Aluminium was melted in graphite crucible. The temperature was set at 8000C. After the desired temperature the stirrer was introduced in the melt to create a vortex. Graphite particles were added in the melt vortex and melt was stirred some minutes. The melt was hand stirred with a graphite rod and poured in the centrifugal casting machine. The tundish was preheated with oxy acetylene flame. The casting was later sectioned to observe the graphite distribution. Hardness values were taken on the casting. Figure 8. Figure 9. Figure 10. Figure 11.