- Practice · 6 marks
Solidification, shrinkage and casting defects
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).
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.
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, .
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
- Liquid contraction - temperature falls from pouring to liquidus; compensated by liquid metal from the riser.
- Solidification contraction - volume decrease on freezing (about 3-7 percent for most metals); shows as shrinkage cavities if the riser is inadequate.
- 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 |
✓ Answer checked by Claude Opus 5.5 (high effort)
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