Porosity and Microstructure

Wrought materials (rolled, extruded or forged alloys) are 100 percent dense and the mechanical deformation and recrystallization in these “wrought” processes refine the microstructure or grain structure. A good microstructure in an alloy provides more boundaries to stop dislocations and cracks when the metal is bent or stressed. Castings, powdered metal parts and certain additive manufactured-components are not typically 100 percent dense. For example, printed stainless steels are usually highly porous on a microscopic scale, which makes them weak and prone to fracture. Pores in a material provide initiation points for crack formation.

Yinmin “Morris” Wang, a materials scientist at Lawrence Livermore National Laboratory (LLNL) in California, said, “The performance has been awful.” The static tensile properties of cast, PM or additive parts are usually equivalent to wrought metal parts, but fatigue and creep properties are sensitive to porosity levels and cleanliness or inclusions.
International Materials Reviews, fabricated additive manufactured-metal parts (especially powder bed processes) typically have columnar, oriented microstructures. Equiaxed growth in EBM additive processes can occur at low temperature gradients with high liquid-solid interface velocities. However, in-situ aging and extensive grain growth can occur in EBM processes as the part cools. Equiaxed or acicular structures tend to have better fatigue characteristics, at least in titanium alloys.

Additive manufactured-properties vary with orientation because directional solidification, residual stress, grain orientation and cracks vary with direction. Every metallurgical process can produce a fiber texture or preferred orientation of the crystal grains within a microstructure. Properties tend to vary with crystallographic direction due to directional solidification, so particular properties could be enhanced or reduced depending on the specific texture. Fatigue crack growth in single-crystal nickel-based alloys is very sensitive to the orientation of the crystals relative to the loading axis, so developing the ability to control microstructure and grain orientation is important for critical aerospace components.


In “AM Research and Development at the NASA Glenn Research Center” report, Materials Research Engineer Robert Carter found mechanical properties of EBM Ti6Al4V titanium equivalent or superior to MMPDS handbook data values. He also noted that processing parameters can alter texture and impact properties. Lot 1 and lot 2 (different “builds”) showed different mechanical strengths correlated with fiber texture variation observed by X-ray diffraction pole figures (figure 9). Figure 10 below shows the fatigue strength variations for AM samples taken from the X, Y and Z directions. Z is perpendicular to the deposit surface, X is in-line with the traversing beam and Y is in-plane and perpendicular to Y. Properties change with direction due to the directional solidification occurring during AM.