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Cast structure and solidification defects

Short answer

Cast ingot shows chill zone (fine grains at mould wall), columnar zone (elongated dendrites), and equiaxed central zone. Common defects: shrink age cavity/pipe, gas porosity, segreation (coring), hot tears, and inclusions/cold shuts.

Describe the typical structure of a cast ingot and the common solidification defects in castings.

Cast structure (three zones)

 +---------------------------+
 | chill zone (fine, random) |
 |  +---------------------+  |
 |  | columnar zone       |  |
 |  |   +-------------+   |  |
 |  |   | equiaxed    |   |  |
 |  |   | central zone|   |  |
 |  |   +-------------+   |  |
 |  +---------------------+  |
 +---------------------------+
  1. Chill zone: at the cold mould wall, rapid cooling gives many small randomly oriented grains.
  2. Columnar zone: grains grow inwards, opposite to the heat flow, as long parallel dendrites.
  3. Equiaxed zone: in the centre, grains nucleate in the undercooled melt and grow roughly equal in all directions.

Solidification defects

  • Shrinkage cavity / pipe: metal contracts on solidifying (3-7%), leaving a cavity at the last-to-freeze region; reduced by risers.
  • Porosity: gas (H2_2 in aluminium, for example) is rejected on freezing and trapped as bubbles.
  • Segregation: non-uniform composition, as in coring (dendrite cores differ from edges) and macrosegregation.
  • Hot tears / cracks: restrained contraction cracks the weak, partly solid metal.
  • Inclusions and cold shuts: oxide/slag particles, or incomplete fusion of two metal streams.

✓ Answer checked by Claude Opus 5.5 (high effort)

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