---
title: "Solidification, shrinkage and casting defects"
subject: "Manufacturing and Production Processes"
chapter: 4
key: "semester_4/manufacturing_and_production_processes:solidification-shrinkage-casting-defects"
source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/solidification-shrinkage-casting-defects
site: BCT Engineering
---

# Solidification, shrinkage and casting defects

Manufacturing and Production Processes · Chapter 4: Solidification Process and Powder Metallurgy

- Asked in: year not given, 6 marks
- Video: [Casting Defects and Remedies | How to prevent casting defects | Casting Definition | Defects Types](https://www.youtube.com/watch?v=pNsZ4-So4n4) — Engg Concepts
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/solidification-shrinkage-casting-defects
- Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-4

## Question

Explain the solidification of a pure metal and of an alloy in a mould. Describe the three stages of shrinkage in a casting. List and explain any four casting defects with their remedies.

## Short answer

A pure metal solidifies at a constant temperature with columnar grains; an alloy solidifies over a temperature range, creating a mushy zone and segregation. Shrinkage occurs in three stages: liquid contraction, solidification contraction (cavity risk), and solid contraction. Defects: shrinkage cavity (proper riser), gas porosity (degas, vent), misrun (raise pouring temperature), hot tear (fillets).

## Answer

#### Solidification of a pure metal

A pure metal solidifies at a single temperature. Heat flows into the mould wall, a thin **chill zone** of small equiaxed grains forms at the wall, then grains grow inward against the heat-flow direction as columnar (dendritic) crystals. At the centre, equiaxed grains may form. Total solidification time is given by Chvorinov's rule, $t = C(V/A)^2$.

#### Solidification of an alloy

An alloy freezes over a range between **liquidus** and **solidus** temperatures. A mushy zone of dendrites and liquid exists, so the casting can be segregated (coring) and the feeding of liquid shrinkage is difficult. A wide freezing range gives more porosity and a longer mushy zone.

#### Shrinkage in three stages

1. **Liquid contraction** - temperature falls from pouring to liquidus; compensated by liquid metal from the riser.
2. **Solidification contraction** - volume decrease on freezing (about 3-7 percent for most metals); shows as shrinkage cavities if the riser is inadequate.
3. **Solid contraction** - cooling from solidus to room temperature; allowed for by the pattern shrinkage allowance.

#### Casting defects

| Defect | Cause | Remedy |
|---|---|---|
| Shrinkage cavity | Insufficient feeding | Properly designed riser, directional solidification, chills |
| Gas porosity | Dissolved gas, damp sand, poor venting | Degas melt, dry sand, add vents |
| Misrun / cold shut | Low pouring temperature, thin section | Higher pouring temperature, larger gates |
| Hot tear | Restrained contraction | Fillets, collapsible core, uniform sections |
| Sand inclusion / blow | Eroded sand, high moisture | Strong sand, correct gating, lower moisture |
| Core shift | Weak core prints | Better core support |

## Check yourself

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

1. How does a pure metal solidify in a mould?
   A. Over a range between liquidus and solidus
   B. At a single temperature, with a chill zone at the wall then columnar grains growing inward
   C. Only as equiaxed grains throughout
   D. From the centre outward
   Answer: B. A pure metal freezes at one temperature: chill zone of small grains, then columnar crystals inward, possibly equiaxed at centre. (Key point: Pure metal solidification)

2. Why is feeding difficult for an alloy with a wide freezing range?
   A. It has a mushy zone of dendrites and liquid, giving more porosity
   B. It solidifies instantly
   C. It has no solidification contraction
   D. It has no chill zone
   Answer: A. An alloy freezes between liquidus and solidus with a mushy zone, so segregation and porosity are more likely. (Key point: Alloy freezing range)

3. Which is the correct shrinkage stage compensated by a pattern shrinkage allowance?
   A. Liquid contraction
   B. Solidification contraction
   C. Solid contraction from solidus to room temperature
   D. None of them
   Answer: C. Solid contraction on cooling to room temperature is allowed for by pattern shrinkage allowance; the riser handles liquid and solidification shrinkage. (Key point: Stage 2 shrinkage)

4. A shrinkage cavity is caused by insufficient feeding. A remedy is
   A. Dry the sand
   B. Properly designed riser, directional solidification, chills
   C. Raise moisture
   D. Lower the pouring temperature
   Answer: B. Risers, directional solidification and chills ensure liquid metal feeds the last-freezing regions. (Key point: Shrinkage cavity remedy)

## Related in Manufacturing and Production Processes

- [Sand casting steps and mould parts](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Asand-casting-process-steps-mould-components) — Manufacturing and Production Processes, chapter 4
- [EDM principle and ECM metal removal rate](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Aedm-ecm-principle-mrr-numerical) — Manufacturing and Production Processes, chapter 8

## Related in other subjects

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