"铸件气孔" can be translated into English as "casting porosity" or "porosity in castings".
"Casting porosity" is a commonly used term in the foundry and metallurgical industries to describe the presence of pores or voids within a cast component.
"Porosity in castings" is another straightforward way to express the same concept, emphasizing the occurrence of porosity specifically within castings.
Both expressions are accurate and widely understood in the relevant fields.