English
Menu

Solidification and cast structures

Take the densities of pure aluminum at its melting point (660 °C) as 2.375 g/cm³ for the liquid and 2.55 g/cm³ for the solid. Which is closest to the decrease in volume relative to the liquid volume (solidification shrinkage) when the liquid turns entirely into solid at the melting point?

  1. AAbout 2.3%
  2. BAbout 7.4%
  3. CAbout 6.9%
Show answer and explanation

Answer: C. About 6.9%

For a mass m, the volume is m/2.375 as a liquid and m/2.55 as a solid. The decrease relative to the liquid volume is 1 − 2.375/2.55 ≈ 0.069, about 6.9%. About 7.4% is the density difference divided by the liquid density, which corresponds to the decrease relative to the solid volume. About 2.3% converts the volume decrease into a linear contraction (1 − 0.931^(1/3)). Solidification shrinkage occurs during solidification, separately from the liquid contraction while the liquid cools and the solid contraction after it has solidified, so unless molten metal is fed from a riser it becomes a shrinkage cavity. In gray cast iron, by contrast, low-density graphite precipitates and expands during solidification, so the shrinkage is small and smaller risers can be used. Some substances, such as Si and Bi, even increase in volume on solidification.

Report an error

Practice this topic Back to topic

More Solidification and cast structures questions

  1. Which combination of methods correctly reduces the columnar zone of an ingot and widens the central equiaxed zone, refining the grains?
  2. In alloy solidification, the ratio k = Cs/CL of the solute concentrations in the solid and the liquid at the solid–liquid interface is called the equilibrium partition coefficient. Which statement is correct for a case such as Al-Cu alloys, where k is about 0.17?
  3. Most pure metals grow with a smooth, rounded solid–liquid interface, whereas Si and Bi tend to grow bounded by flat crystal faces (facets). Which is the correct guide to explain this difference?
  4. Which statement about constitutional supercooling, which causes an initially planar solid–liquid interface to become uneven in directional solidification of an alloy, is correct? Take an alloy with k < 1.
  5. The condition for solidification with a planar interface and no constitutional supercooling is G/V ≥ ΔT₀/D_L (G is the temperature gradient in the liquid at the interface, V the growth rate, D_L the solute diffusion coefficient in the liquid, and ΔT₀ = mC₀(1 − k)/k the freezing range). When an Al-2.0 wt%Cu alloy (magnitude of the liquidus slope m = 3.4 K/wt%, k = 0.17, D_L = 3.0×10⁻⁹ m²/s) is grown at V = 1.0×10⁻⁵ m/s, which is closest to the minimum temperature gradient needed to keep the interface planar?