Main causes of the porosity of alloy castings are:
1) Solidification defects
2) Trapped gases
3) Residual air
1. Solidification defects
Solidification defects cold lead to two different manifestations of porosity;
localised shrinkage porosity and Microporosity.
Localised shrinkage porosity is caused by insufficient feeding of the alloy
during solidification.
Microporosity is also caused by solidification shrinkage, but generally happens
in fine grain alloys when the solidification is too rapid for the microvoids to
segregate. This in turn is caused the mould or casting temperature being too
low.
2. Trapped gases
Many metals dissolve or occlude gases when they are molten. On solidification,
these gases are forced out of the casting causing what is usually called pinhole
porosity. These voids are rather small.
3. Incomplete casting
If the molten alloy is prevented from fully or partially filling the mould and
incomplete or even no casting at all can result.
