Regulating gradient heterogeneous structure in additive manufactured 316 L stainless steel for optimizing mechanical properties

Abstract

This paper aims to produce and customize the gradient heterogeneous microstructure in the selective laser melting (SLM)-fabricated 316 L stainless steel through ultrasonic severe surface rolling (USSR), with an aim to optimize the strength-plasticity balance. The resultant gradient heterogeneous structure is characterized by the integration of a gradient surface layer and a heterogeneous core, thus demonstrating a high degree of microstructural heterogeneity. This microstructure imparts an extraordinary strength-ductility balance to the 316 L stainless steel. Furthermore, by controlling the static pressure, a vital processing parameter of the USSR technique, it is feasible to customize the microstructure as well as mechanical properties. Increasing the static pressure leads to a higher volume fraction of the gradient surface layer and promotes finer grain structures near the surface. This results in an enhancement of the tensile strength, despite a slight sacrifice in ductility. Consequently, the USSR sample processed at a static pressure of 0.6 MPa achieves an impressive combination of high yield strength of 758.2 ± 15.6 MPa and considerable ductility of 33.1 ± 2.0 %. Molecular dynamics simulation further reveals that dislocation slip and deformation twinning serve as the primary deformation mechanisms in 316 L stainless steel during USSR processing, with their complex interaction playing a crucial role in grain refinement and the formation of the gradient heterogeneous structures. Our work paves the way for engineering high-performance additively manufactured 316 L stainless steel parts through USSR.