http://en.wikipedia.org/wiki/Casting_defect

A casting defect is an irregularity in the metal casting process that is undesired. Some defects can be tolerated while others can be repaired otherwise they must be eliminated. They are broken down into five main categories: gas porosity, shrinkage defects, mold material defects, pouring metal defects, and metallurgical defects.[1]

Gas porosity is the formation of BUBBLES within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools.[6] Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal,[7] which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most encountered gases in cases of gas porosity.[5] In aluminum castings, hydrogen is the only gas that dissolves in significant quantity, which can result in hydrogen gas porosity.[8] For casting that are a few kilograms in weight the pores are usually 0.01 to 0.5 mm (0.00039 to 0.020 in) in size. In larger casting they can be up to a millimeter (0.040 in) in diameter.[7]

See also plenty of examples (approx. 153 thousand Google hits) of how "bubbles" is used in this context:

http://www.google.com/search?q=bubbles ; "metal casting"&hl=...