Detection and Quantitation of Close Porosity in Additive Manufactured Parts Using Gas Displacement Pycnometry

Conference Abstract: Gas displacement pycnometry is often used to determine the skeletal density and volume of powders and formed materials. It can be used to detect the presence of significant closed porosity in formed pieces constructed from powders. Comparison of the skeletal density of the finished piece and the feedstock powder clearly demonstrates when closed porosity is present, and the volume within those closed pores. This method has recently been applied to parts made from Ti64 powder using Additive Manufacturing. This work was used to support development of an international standard by ISO/TC261 ASTM F42 joint committee on Additive Manufacturing.
Gas displacement pycnometry is an Archimedes method where the volume of an irregularly shaped object is determined based upon the volume of a fluid that it displaces. In this case, the fluid is a known inert gas such as helium or nitrogen, rather than a liquid, such as water, that is often associated with Archimedes methods. Gas displacement offers some true advantages, such as complete filling of the open porosity by the gas molecules from which liquids, such as water, might be unable to wet due to high contact angle and high liquid viscosity.
Data used to support the development of the International Standard, along with a description of the principle of operation, will be presented to demonstrate the utility of the method to detect the presence of closed porosity. Note that deliberate changes to the printing process were made to encourage the development of closed pores in some of the printed parts that were then detected using the gas displacement method.