What is porosity in casting?

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.  Gas porosity may present itself on the surface of the casting as porosity or the pore may be trapped inside the metal, which reduces strength in that area.

Porosity is the most common casting defect.

Porosity typically accounts for an average of 5% of the total volume of the part. It is not realistic to eliminate porosity but rather to ensure that it’s contained in those areas where it’s not detrimental to part function or appearance.

Therefore when a design drawing or CAD file is produced, these areas should be clearly designated with a set of specifications and tolerances for allowable defects, just as would be the case with dimensional tolerances.

Surface porosity casting defects

Bronze Cylinder
Close up of porosity

Repairing casting defects with coatings

When an area of porosity is detrimental to part function because it doesn’t meet the required size, coatings can be applied to increase diameter to meet tolerance. The part above was repaired with phosphor bronze coating for a smooth finish using HVOF technology. 

After Coating

HVOF Coatings

HVOF applied coating is a technology which improves or restores the surface of a solid material. The process can be used to apply coatings to a wide range of materials and components, to provide resistance to: wear, erosion, cavitation, corrosion, abrasion or heat. It has huge scope to extend the life of new components or through the use of tried and tested techniques, to repair and re-engineer worn or damaged components.

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