Evolution of Porosity and Failure in Additively Manufactured Steel AISI 316L Under Tension via X-Ray Computed Tomography

Abstract This work uses staged X-ray computed tomography to track porosity evolution and fracture initiation in AISI 316L specimens produced by laser powder bed fusion under low global energy density conditions, resulting in porous material dominated by lack-of-fusion (LOF) defects. Specimens were imaged in the undeformed state, after loading to peak force, and after fracture under interrupted monotonic tensile loading. Half of the specimens were machined to remove the as-built surface crust to isolate surface-condition effects. Global porosity statistics were similar between as-manufactured and machined groups, indicating that machining neither altered existing porosity nor introduced new detectable voids. Unsupervised K-means grouping of pore morphology separated gas-, LOF-, and keyhole-like populations and confirmed LOF-like porosity as the dominant contribution. Analysis of pore shapes indicates pronounced elongation and reorientation during loading, shifting from predominantly flat/transverse features in the undeformed state toward pores aligned with the loading direction at peak force and after fracture, while the largest pores grew markedly. In parallel, running-average effective cross-sectional area profiles highlight regions that later neck and/or contain large pore clusters. Crack initiation mechanisms differed by surface condition: in as-manufactured specimens cracks initiated from deep surface defects, while in machined specimens cracks initiated when large subsurface LOF pores linked to the external surface. Overall, failure initiation is governed by the combined effects of surface condition, defect size, and defect-to-surface distance, underscoring that surface finishing and bulk defect mitigation must be addressed jointly.