Addressing the strength-corrosion tradeoff in 316 L stainless steel by introducing cellular ferrite via directed energy deposition

Typical cellular structure composed of high-density dislocation tangle can lead to high strength in directed energy deposition 316L stainless steel, but with reduced corrosion resistance in acid solution due to accompanied element segregation. Herein, a unique cellular structure consisting of a ferrite phase is manufactured to address the strength-corrosion tradeoff via a direct energy deposition process. The relationship between strength and microstructural features, such as grain type, ferrite phases, and dislocations, as well as the relationships between corrosion resistance and these microstructural characteristics, are identified. Cellular ferrite retains the beneficial effect of the cellular structure on increasing strength and changes the material from a pure austenitic phase to an austenite-ferrite biphasic structure. The ferrite phase has a lower corrosion potential, which promotes the rapid formation of Cr- and Mo-rich corrosion product layers. It can avoid the unfavorable effects of element segregation and suppress the corrosion rate in acid solution. This study shows that directed energy deposition has the potential to customize the microstructure through a printing process that introduces a cellular ferrite phase to improve its corrosion resistance in acidic solutions as well as its strength and ductility.