Flaw type and build orientation dependent tensile and creep strength of 316L stainless steel fabricated via laser powder bed fusion

Laser powder bed fusion (LPBF) with varying laser scan speed under constant power was utilized to manufacture specimens of 316L stainless steel containing keyhole (KH) and lack of fusion (LoF) flaws. Optimal fully dense (FD) specimens were also manufactured. Statistical distributions of size and aspect ratio of the flaws were determined via analyses of micro-X-ray computed tomography data. Moreover, microstructural characterization of the materials was performed using electron backscattered diffraction (EBSD) along with fractography of fractured surfaces using secondary electrons imaging. The role of flaws in the flow behavior of the materials in simple tension was assessed first. While the LoF flaws containing specimens showed an inferior static strength and ductility behavior compared to the other materials, the KH defects containing specimens exhibited a shorter nonuniform deformation after reaching the ultimate tensile strength compared to the FD specimens. Subsequently, a set of specimens underwent creep strength testing to establish the relationship between the flaw characteristics and creep behavior of the materials under several constant stress levels at several temperatures. The measurements showed that the LoF specimens had the lowest strength, least resistance to creep, and shortest rupture life owing to the defects and their high-volume fraction. While the fully dense specimens showed the highest creep strength in general, the KH specimens demonstrated the highest creep resistance during the initial stages of creep especially at lower temperatures and stresses. Microstructural origins including the defects of such behavior are elucidated and discussed along with the fractography findings. Finally, the creep parameters were estimated using the measured creep data and then refined by a subsequent data fitting using the Mukherjee - Bird - Dorn equation. The quality of fits and established parameters for the creep behavior for all material categories are presented and discussed.