Nanostructuring of Additively Manufactured 316L Stainless Steel via High Pressure Torsion: Microstructural and Hardness Inhomogeneities at Low Torsional Strain Levels

Abstract

High-pressure torsion (HPT) is a severe plastic deformation (SPD) process that produces ultrafine-grained (UFG) and/or nano-grained (NG) microstructures in bulk disk-shaped metallic materials to enhance their mechanical properties, accompanied by inhomogeneous microstructures and hardness distributions across the disk radius. Although such inhomogeneity due to the radial dependency of HPT process is well-documented in conventionally wrought and cast HPT-processed metals and alloys, it has not been studied in their additively manufactured counterparts. Thus, this study aims to investigate the inhomogeneous distribution of microstructures and hardness values across HPT-processed disk-shaped 316L stainless steel (316L SS) samples additively manufactured by laser powder bed fusion (L-PBF).  The disk samples are subjected to 1/4 and 1 HPT revolution, followed by extensive microstructural characterisation and Vickers hardness (HV) measurements. The results show: (i) fine micron (18±17 µm) and UFG (118±25 nm) microstructures at the central (0<r<2.5 mm) and peripheral (r>3 m) disk regions, respectively after 1/4 HPT revolution, and (ii) UFG (980±500 nm) and true NG (68±15 nm) microstructures at the central and peripheral disk regions, respectively after 1/4 HPT revolution. Unsurprisingly, a 2- to 3-fold hardness increase (430 – 550 HV) is observed after HPT processing compared to the as-received L-PBF AM 316L SS (200 – 250 HV) with the central disk area consistently exhibits relatively lower hardness values compared to the disk peripheries. The results present an exciting potential avenue to tailor or enhance the properties of additively manufactured metals and alloys such as strength-ductility synergy by introducing gradient nanostructures through the microstructural inhomogeneity attained via HPT.