---
title: "Cast structure and solidification defects"
subject: "Material Science"
chapter: 5
key: "semester_3/material_science:cast-structure-and-solidification-defects"
source: https://www.bctengineer.com/q/semester_3/material_science/cast-structure-and-solidification-defects
site: BCT Engineering
---

# Cast structure and solidification defects

Material Science · Chapter 5: Solidification, Phase Relations and Strengthening Mechanism

- Asked in: year not given, 4 marks
- Video: [THERMODYNAMICS EXPLAINED: Metal Casting, Solidification, Dendrites & Mushy Zone Science](https://www.youtube.com/watch?v=enNqnnot4Mo) — Ibham Veza
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/material_science/cast-structure-and-solidification-defects
- Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-5

## Question

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

## 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.

## Answer

#### Cast structure (three zones)

```text
 +---------------------------+
 | 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 (H$_2$ 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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Chill zone is:
   A. Large columnar grains
   B. Fine random grains at mould wall
   C. Equiaxed centre
   D. Porosity
   Answer: B. Rapid cooling gives many small grains. (Key point: Chill zone)

2. Columnar grains grow:
   A. Along heat flow direction
   B. Randomly
   C. Opposite to heat flow, inward
   D. Only at centre
   Answer: C. They grow opposite to heat flow. (Key point: Columnar zone)

3. How reduce shrinkage cavities?
   A. Risers
   B. Faster pouring
   C. More gas
   D. Cold shuts
   Answer: A. Risers feed the last-to-freeze region. (Key point: Shrinkage)

4. Cause of porosity?
   A. Contraction
   B. Slag
   C. Gas rejected on freezing and trapped
   D. Segregation
   Answer: C. Dissolved gas such as H2 in aluminium forms bubbles. (Key point: Porosity)

## Related in Material Science

- [Cooling of 3% C cast iron](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Ahypoeutectic-cast-iron-3-carbon) — Material Science, chapter 6

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