Understanding Porosity to Prevent Defective Castings

Porosity is a naturally occurring phenomenon that is found in most materials, but it can affect the structural integrity and functionality of metal castings. Vacuum impregnation is an effective way to seal porosity in castings, but companies need to choose whether to perform this operation before or after machining the casting. The article “When to Vacuum Impregnate Castings” explains how companies can make the best decision.

Understanding the types of porosity that occur in castings can help companies decide when to vacuum impregnate castings. In metal castings, porosity is considered to be any void in the casting. Porosity comes in three types: blind, which is open in one surface; through, which is open in two surfaces; and fully enclosed, with no passages to the surface. Through and blind porosity pose immediate problems. Through porosity creates leak paths that enable gas and liquids to escape, while blind porosity can cause internal corrosion and defects if cleaning solutions leech out of the voids after surface finishing.

Machining can affect the porosity of a casting. Blind porosity can be opened on the opposite side to create through porosity, while enclosed porosity can be exposed and become either blind or through. Therefore, the best way to ensure leak-free castings is to perform vacuum impregnation on all of them after final machining. However, performing leak tests on individual parts and vacuum impregnating ones that fail is a good alternative. Manufacturers can also increase the amount of material removed during cubing to maximize porosity exposure and perform vacuum impregnation after. This reduces the opportunity to open porosity during final machining.

To learn more, read “When to Vacuum Impregnate Castings.”