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GATE 2016 ME (ME3) – Question 31

Casting, Forming and Joining Processes · Casting: patterns, moulds, cores, solidification, riser and gating design, defects · 1 mark · Multiple choice

Equal amounts of a liquid metal at the same temperature are poured into three moulds made of steel, copper and aluminum. The shape of the cavity is a cylinder with 15 mm diameter. The size of the moulds are such that the outside temperature of the moulds do not increase appreciably beyond the atmospheric temperature during solidification. The sequence of solidification in the mould from the fastest to slowest is (Thermal conductivities of steel, copper and aluminum are 60.5, 401 and 237 W/m-K, respectively. Specific heats of steel, copper and aluminum are 434, 385 and 903 J/kg-K, respectively. Densities of steel, copper and aluminum are 7854, 8933 and 2700 kg/m3, respectively.)

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

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Correct answer: (C) Copper - Aluminum - Steel

Explanation

For transient contact, the relevant thermal effusivity is √(kρc), not conductivity alone. Substitution of the tabulated properties ranks copper above aluminium above steel. Therefore the fastest-to-slowest order is **Cu,Al,steel, C**. Higher effusivity draws heat from the casting more rapidly under comparable contact conditions.