Analyses of Property-Graded Stainless Steel 316L Bulk Structures Processed by Selective Laser Melting

Abstract

The overarching goal of this research is to investigate the processing, structure, properties, and performance of (mechanical) property-graded bulk structures made from a single metallic alloy via a laser powder bed fusion (LPBF) process. With a vision to realize repeatable/reproducible functionally-graded additively manufactured (FGAM) bulk structures, this study aims to elucidate the underlying causes of the variations in macro- and microstructures in relation to process conditions, the resulting physical and mechanical property distributions, and its local and global mechanical performance. For this purpose, a systematic design of experiments that spanned the volumetric energy density (VED)-related process parameter design space was utilized to investigate the range of functionally-acceptable physical/mechanical properties achievable in stainless steel 316L via a pulsed selective laser melting (SLM) process. Results demonstrated the significant, but functionally-usable variations attainable in hardness, relative density, and modulus. Property variations resulted from a combination of porosity types/amounts, martensitic phase fractions, and grain sizes. Based on these, select process parameters were utilized for fabricating ASTM standard tensile/bending specimens with intended property differentials within its gauge volumes. Analyses of structure and property variations were performed, coupled with metallography, microscopy, and spectroscopy. Digital image correlation (DIC) was used to examine the evolution of spatial and temporal strains, especially at zonal interfaces. Altogether, this work helped lay the foundation for tailoring the structure, properties and performance of FGAM bulk parts.