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Solidification and cast structures

Which combination of methods correctly reduces the columnar zone of an ingot and widens the central equiaxed zone, refining the grains?

  1. ALowering the pouring temperature to reduce the superheat, adding a grain refiner and stirring the melt electromagnetically during solidification
  2. BRaising the pouring temperature to increase the superheat, steepening the temperature gradient with a water-cooled copper mold and keeping the melt still
  3. CReducing alloying elements to raise the purity, narrowing the freezing range and extracting heat in one direction from the mold wall
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Answer: A. Lowering the pouring temperature to reduce the superheat, adding a grain refiner and stirring the melt electromagnetically during solidification

The equiaxed zone spreads when crystals form and grow in the liquid ahead of the columnar grains and block their growth. A small superheat helps crystals formed at the mold wall, or left undissolved, survive in the liquid. Grain refiners add sites for heterogeneous nucleation, and stirring breaks off dendrite arms and carries them away, increasing the seeds for new crystals. Conversely, with a large superheat and a steep temperature gradient the liquid ahead is barely undercooled and the seeds remelt, so the columnar grains grow easily to the center; directional solidification makes use of this. Reducing solute to narrow the freezing range lowers the constitutional supercooling, which also favors columnar growth.

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More Solidification and cast structures questions

  1. 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?
  2. 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?
  3. 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.
  4. 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?
  5. Which statement about how the shape of the solid–liquid interface and the fineness of the structure change with the temperature gradient G and growth rate V in alloy solidification is correct?