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Solidification and cast structures

Which is the correct pairing of additions to a hypoeutectic Al-Si casting alloy and their main effects?

  1. ASmall amounts of Sr or Na change the eutectic Si from coarse plates to fine fibers, and Al-Ti-B master alloy refines the grains of primary α-Al
  2. BSmall amounts of Sr or Na refine the grains of primary α-Al, and Al-Ti-B master alloy changes the eutectic Si from coarse plates to fine fibers
  3. CBoth small amounts of Sr or Na and Al-Ti-B master alloy act as degassers that remove hydrogen from the melt, reducing gas porosity
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Answer: A. Small amounts of Sr or Na change the eutectic Si from coarse plates to fine fibers, and Al-Ti-B master alloy refines the grains of primary α-Al

Left alone, the eutectic Si in Al-Si alloys grows faceted into coarse plates (needles in section), which act as crack origins and lower the elongation. Adding tens to hundreds of ppm of Sr or Na introduces many twins into the growing Si so that it branches into a fine fibrous eutectic, raising elongation and toughness; this is called modification. Al-Ti-B master alloy (containing TiB₂ and Al₃Ti particles), on the other hand, is a grain refiner that provides sites for heterogeneous nucleation of primary α-Al, while dissolved Ti restricts growth, so the grains become finer. The option that swaps the two roles is wrong. Degassing is done by blowing argon and similar methods, and Sr tends, if anything, to increase hydrogen porosity. Phosphorus is used to refine primary Si in hypereutectic alloys.

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More Solidification and cast structures questions

  1. When a regular lamellar eutectic is directionally solidified, the lamellar spacing λ and growth rate V obey λ²V = constant. At a growth rate of 10 µm/s, λ = 2.0 µm. Which is closest to λ when the growth rate is raised to 160 µm/s?
  2. 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?
  3. Which combination of methods correctly reduces the columnar zone of an ingot and widens the central equiaxed zone, refining the grains?
  4. 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?
  5. 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?