Effect of In-Situ Layer-by-layer Rolling on the Microstructure, Mechanical Properties, and Corrosion Resistance of a Directed Energy Deposited 316L Stainless Steel

In this study a novel micro-rolling set-up was installed on a directed energy deposition additive manufacturing system to achieve in-situ grain refinement and porosity closure. A 316 L stainless steel was investigated and it was found that even low micro-rolling loads were capable of increasing the part density from 99.7% to 99.98%. Martensitic transformations as a result of micro-rolling were not detected. Additionally, using a combination of X-ray diffraction and optical, scanning, and transmission electron microscopy techniques, it was found that micro-rolling resulted in a finer internal grain substructure, twinning, an increase in dislocation density, a reduction of grain sizes, a refinement of the cellular structures, and a randomization of the crystallographic orientation distribution. This resulted in an increase in hardness by up to - 23% and in yield strength by up to 31%. More importantly, due to pore closure, improved mechanical properties were achieved without compromising ductility. As the segregation of solute elements to cell boundaries was also not affected by micro-rolling, the excellent corrosion resistance of the alloy was generally retained. Thus, this approach allows the fabrication of denser, stronger and harder 316 L parts without the need for additional post-processing steps such as hot isostatic pressing or thermo-mechanical treatments, while also maintaining ductility and corrosion performance.