Internal Porosity

Formation Source

Origins of the voids are often traced back to the physical phenomena that occur during the rapid cooling of molten metal. In gas atomization, internal porosity frequently happens when the atomizing gas becomes trapped inside the liquid droplets as they solidify into powder. This gas remains inside the particles and can be carried over into the final manufactured part.

In powder bed fusion, porosity might be caused by a melt pool that is too shallow or by the use of laser parameters that do not provide enough energy to fully melt the material. Other sources include the evaporation of volatile elements or the presence of moisture on the surface of the powder.

Performance Risk

Impact of the cavities on the longevity of a mechanical system depends on their size, shape, and distribution throughout the material. Large and irregular pores are particularly dangerous because they are more likely to initiate cracks than small spherical voids. If internal porosity is located near the surface of a part, it can severely degrade the surface finish and make the component more susceptible to environmental corrosion.

In critical aerospace parts, even a low percentage of porosity can be grounds for rejecting the entire batch. Engineers use non destructive testing methods like X ray computed tomography to inspect parts for hidden defects before they are placed into service.

Inspection Methodology

Detection and quantification of the voids require advanced imaging techniques that can look inside the solid metal without damaging it. Optical microscopy of polished cross sections is a common way to measure internal porosity in a laboratory setting, but it only provides a two dimensional view. X ray computed tomography offers a more complete three dimensional reconstruction of the pore network, allowing for the measurement of total volume fraction and pore size distribution.

This data is essential for validating the manufacturing process and for ensuring that the parts meet the required safety standards. Archimedes density testing is another simple method used to estimate the total porosity by comparing the actual weight of the part to its theoretical maximum density.