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Equal amounts of a liquid metal at the same temperature are poured into three moulds made of steel, copper, and aluminium. The cavity is cylindrical with a diameter of 15 mm. The moulds are large enough that their outer surfaces do not rise appreciably above atmospheric temperature during solidification. The sequence of solidification from fastest to slowest is (thermal conductivities of steel, copper, and aluminium are 60.5, 401, and 237 W/m-K, respectively; specific heats are 434, 385, and 903 J/kg-K, respectively; densities are 7854, 8933, and 2700 kg/m3, respectively)

1. Copper - Steel - Aluminum
2. Aluminum - Steel - Copper
3. Copper - Aluminum - Steel
4. Steel - Copper - Aluminum

Correct answer: Steel - Copper - Aluminum

Solution

The solidification rate depends on how effectively the mould removes heat from the molten metal. For the given materials, steel has the lowest thermal diffusivity among the three, so it removes heat slowest; copper removes heat fastest, and aluminium is intermediate. Hence the solidification sequence from fastest to slowest is copper, aluminium, then steel.

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