Simultaneously enhanced strength and ductility for 3D-printed stainless steel 316L by selective laser melting

Abstract

Laser-based powder-bed fusion additive manufacturing or three-dimensional printing technology has gained tremendous attention due to its controllable, digital, and automated manufacturing process, which can afford a refined microstructure and superior strength. However, it is a major challenge to additively manufacture metal parts with satisfactory ductility and toughness. Here we report a novel selective laser melting process to simultaneously enhance the strength and ductility of stainless steel 316L by in-process engineering its microstructure into a <011> crystallographic texture. We find that the tensile strength and ductility of SLM-built stainless steel 316L samples could be enhanced by ~16% and ~40% respectively, with the engineered <011> textured microstructure compared to the common <001> textured microstructure. This is because the favorable nano-twinning mechanism was significantly more activated in the <011> textured stainless steel 316L samples during plastic deformation. In addition, kinetic simulations were performed to unveil the relationship between the melt pool geometry and crystallographic texture. The new additive manufacturing strategy of engineering the crystallographic texture can be applied to other metals and alloys with twinning-induced plasticity. This work paves the way to additively manufacture metal parts with high strength and high ductility. A steel alloy with both high tensile strength and ductility has been three-dimensional (3D) printed by researchers in Singapore. Additive manufacturing builds 3D objects by adding materials layer by layer, a relatively simple process for plastics. However, this manufacturing process is much more difficult for metals, which are susceptible to defects and internal pores. This is particularly problematic when the final product needs excellent mechanical properties, such as hardness or strength. Xipeng Tan and co-workers from Nanyang Technological University used a specific laser scanning strategy to melt metallic powders and form a stainless steel alloy with a zig-zag crystallographic microstructure. The tensile strength and ductility of their stainless steel samples were increased by approximately 16% and 40%, respectively, compared to an alloy with the typical microstructure. A creative approach to substantially enhance both the strength and ductility of SLM-printed metal parts was successfully demonstrated on the ubiquitous marine-grade stainless steel 316L. The new discovery improves the strength and ductility of stainless steel parts by ~16% and 40% compared with the typical 3D printing process and conventional manufacturing methods. Control of the crystallographic texture is key for this breakthrough, which was achieved by tailoring the geometrical features of the melt pool involved in the laser-based 3D printing process. The desired <011> crystallographic texture favors the activation of the nano-twinning mechanism, which simultaneously enhances the strength and ductility.