Porosity is a sponge-like area in an otherwise sound metal part. It often occurs during the casting process when metals change from liquid to solid form and may be a result of:
- Shrinkage – often when areas within a casting are cooling at different rates
- Gas porosity when bubbles form when the metal is cooling
- Pouring defects where the liquid metal doesn’t fill the mold in the desired way, and
- Metallurgical defects caused when a casting is cooling
There are three basic types of porosity: enclosed, blind or through.
- Enclosed is completely isolated within the wall thickness of a part.
- Blind is connected to only one side of the part wall.
- Through completely extends through the wall thickness of a metal part, causing a leakage path.
- Size – what you see on the surface isn’t always what you get.
When castings are machined, both blind and enclosed porosity are often opened-up, which can them become continuous porosity and allowing leakage.
It can be found in any type of metal casting or part and is a particular problem in castings made from aluminium, zinc, bronze, iron, magnesium and other alloys. Porosity is always present in powdered or sintered metal parts.
In recent years the metal casting process has been greatly advanced, however, porosity, folds, inclusions and gas bubbles remain an inherent by-product of the casting process. These by-products, while not detrimental to the structural integrity of a casting, may lead to product failures based upon the components inability to retain gas or fluids under pressure.