4D Porosity Evolution in Additively Manufactured 316L Stainless Steel through Tensile Testing and X-Ray Computed Tomography.

BACKGROUND

Many aspects of ductile failure through microvoid coalescence remain elusive due to the challenging spatial and temporal scales it operates on. Experimentally resolving all aspects of the process remains a significant goal of researchers. Much of the current understanding has been derived from post-mortem metallography, leaving key aspects of its evolution undocumented.

OBJECTIVE

This work builds on efforts using X-ray computed tomography (XCT) characterize voids and their evolution under loading.

METHODS

It employs XCT tensile testing on 316L Stainless Steel samples that were constructed by laser powder bed fusion that contain tailored, pre-existing voids with a spatial scale relevant to the growth and evolution stages of microvoid coalescence. Pre-existing voids extended the observation window for monitoring void growth and interaction under loading. They also enhanced fiducial correlation of voids during deformation.

RESULTS

Void populations were found to increase under loading as their deformed dimensions rendered them detectable by the XCT algorithm. Neighboring voids underwent interconnection events by a cleavage process when stress concentrations between them exceeded the macroscopic yield stress. Pores that did not undergo interconnection events were found to revert to their initial size and population after unloading. Finally, the porosity structure before failure was correlated to features on the fracture surface with high fidelity.

CONCLUSIONS

This unique combination of XCT tensile testing on samples with tailored void structure enabled new visualization and quantification of void evolution under load as well as strong correlation to the observed stress-strain behavior and post-mortem fracture characteristics.