Elimination of porosity in additively manufactured 316L stainless steel by high-pressure torsion

Abstract

In metal additive manufacturing (AM), porosity is a major defect that could compromise their mechanical and functional performance. Very recently, high-pressure torsion (HPT) has emerged as an effective approach to significantly reduce porosity content in AM-fabricated metallic components, accompanied with considerably improved mechanical and functional properties. However, the mechanism behind the porosity elimination in additively manufactured metallic parts processed by HPT has not been fully understood yet. In this study, the porosity evolution in additively manufactured 316L SS before and after HPT processing is extensively investigated using microscopy techniques, including optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is revealed that significant reduction in porosity level and average pore size could be obtained even with a relatively small range of equivalent strain, εeq.-HPT ≤ 5 after 1/4 HPT revolution. Upon further straining to 1 HPT revolution (εeq.-HPT = 13.6), the pores are completely closed, and the bulk material can be considered pore-free. Based on OM, SEM, and TEM analysis, it can be inferred that the porosity closure mechanism is attributed to the creation of strong atomic bond by the release of internal sub-surfaces and formation of nano-scale grain (NG) microstructures that holds the bulk material, resulting from the HPT-induced combined hydrostatic pressure and extreme torsional strain.