Meaning
Microstructural gas evolution represents the formation of internal voids and gas bubbles within a printed metal component during post-process thermal treatment. This metallurgical defect governs the density and structural integrity of the finished part, with its boundary ending where the thermal energy applied is insufficient to trigger gas expansion or migration. The occurrence of thermal induced porosity is a significant concern when consolidating powder-metallurgy or additive-manufactured parts that contain trapped inert gases.
This porosity reduces the mechanical properties and fatigue life of the material, making it unsuitable for high-stress applications.
Mechanism of Formation
Small pockets of inert gas that are trapped within the powder particles can expand when the consolidated metal is heated to high temperatures. The surrounding metal becomes soft at elevated temperatures, allowing the gas to expand and coalesce into larger, spherical pores. This pore growth is driven by the pressure of the trapped gas and the surface energy of the voids.
Sourcing powders with low gas-entrapment levels is the most effective way to prevent the formation of these internal defects during subsequent heat treatment.
Impact on Properties
High internal porosity acts as a stress concentrator that can initiate and accelerate the propagation of fatigue cracks. This degradation of mechanical properties is particularly detrimental in dynamic structural applications like turbine blades or structural brackets. Sourcing parts that have been certified as having low internal porosity ensures more predictable performance and a longer service life.
This certification is typically achieved through non-destructive testing, such as x-ray computed tomography or density measurements.
Prevention and Control
Mitigating this defect requires careful selection of both the starting powder and the processing parameters used during consolidated printing. Sourcing gas-atomized powders with low satellite density and high internal cleanliness reduces the volume of trapped gas in the starting feedstock. Optimizing the laser scan strategy to minimize keyhole melting also prevents the formation of print-induced pores.
Post-processing steps like hot isostatic pressing can be used to close internal pores and increase the density of the component before final heat treatment.
