**6.1 Equipment cost**

12 Recent Trends in Processing and Degradation of Aluminium Alloys

Due to the intrinsic ability of the casting technique to produce cleaner castings, it is more adaptable to the use of scraps and foundry returns. These types of foundry feedstock contain significant admixed impurities like moulding sand and oxide inclusions. Such scrap

The process of taking the melt can actually be used to pump clean metal below the melt surface. Figures 10 and 11 respectively show the micrographs of gravity-pour ceramic mould cast samples and countergravity cast samples of scraps of aluminium foundry returns. The microstructure shows more segregation of melt impurities in the gravity-pour ceramic mould, while the vacuum cast specimen shows significant reduction in impurities.

The countergravity technique is well suited for producing net-shape cast products. It is especially suited for thin-walled sections and intricate details due to its excellent mould filling. This is possible due to the virtual elimination of shrinkage defects in the countergravity casting technique. Near net-shape castings of even higher temperature alloys, such as steels are possible. Such has been reported by Chandely *et al* (1997) in the

Countergravity cast products have improved strength over green sand and ceramic mould specimens. The technique may be thus deployed in the production of high strength parts hitherto produced by forging. High Counter-Pressure Moulding, a proprietary variant, has been reported to exhibit the same strength characteristics as forging in alloy wheel production, at little more than the price of cast wheel (Alexander, 2002). Countergravity techniques are increasingly becoming the preferred choice for the production of alloy wheels because of the added advantage of design flexibility over forging processes. Furthermore, the Cosworth process, which achieves countergravity melt flow by means of an electromagnetic pump, has been successfully used for high strength structural components for air frames, gun cradles, and air tanker re-fuelling

Griffiths *et al* (2007) observed that countergravity filling method produced higher values of the Weibull modulus than conventional gravity mould filling methods. This is a pointer to

There are often considerable wastages of melt in more conventional casting techniques due

This also results in considerable fettling time and costs. Such wastages are virtually eliminated in countergravity casting since there is no need for risers and complex in-gates

Use of the countergravity casting technique is gradually branching into novel materials production. An emerging field of application is the production cast Metal Matrix

the reduced variability of strength achievable in the countergravity technique.

to provisions made for risering and complicated in-gates.

**5.5 Production of metal matrix composites** 

**5.1 Scrap reduction and scrap usage** 

**5.2 Net-shape casting** 

**5.3 Improved strength** 

manifolds (Bray, 1989).

**5.4 Economical use of melt** 

are not necessary.

metals produce significant slag which float on the melt surface.

production of thin-walled steel exhaust manifolds.

Spada (1998) reported the cost of countergravity mould and handling equipment to be typically between \$50,000 to \$1.25 million depending on complexity. Present day prices would naturally be much higher. This is so because the proper utilisation of a countergravity casting equipment requires an ecosystem of support facilities. These include high-temperature mould pre-heating ovens, mould and moulding flask positioning units, and sophisticated vacuum control systems. These added facilities add to the cost of setting up and operation of the technique. In some instances, licensing fees may also apply, further raising up the cost.
