Marine-grade stainless steel is valued for its performance under corrosive environments, and for its high ductility — the ability to bend without breaking under stress. But conventional techniques for strengthening this class of stainless steels typically come at the expense of ductility. A method of 3D printing one of the most common forms of marine grade stainless steel — a low-carbon type called 316L — promises an unparalleled combination of high-strength and high-ductility properties for the ubiquitous alloy.
Components were 3D-printed with 316L stainless steel, and the material's performance was better than those made with the traditional approach. The methodology could lead to widespread 3D printing of such stainless steel components, particularly in the aerospace, automotive, and oil and gas industries, where strong and tough materials are needed to tolerate extreme force in harsh environments.
To successfully meet and exceed the necessary performance requirements for 316L stainless steel, researchers first had to overcome a major bottleneck limiting the potential for 3D printing high-quality metals: the porosity caused during the laser melting (or fusion) of metal powders that can cause parts to degrade and fracture easily. Researchers addressed this through a density optimization process involving experiments and computer modeling, and by manipulating the materials’ underlying microstructure.
Using two different laser powder bed fusion machines, researchers printed thin plates of stainless steel 316L for mechanical testing. The laser melting technique inherently resulted in hierarchical celllike structures that could be tuned to alter the mechanical properties. When 316L is additively manufactured, it creates a grain structure that resembles a stained-glass window. The grains are not very small, but the cellular structures and other defects inside the grains that are commonly seen in welding control the properties.
Deformation of metals is mainly controlled by how nanoscale defects move and interact in the microstructure. This cellular structure acts as a filter, allowing some defects to move freely, and thus providing the necessary ductility while blocking some others to provide the strength. The stainless steel is a “surrogate material” system that could be used for other types of metals.
The eventual goal is to use high-performance computing to validate and predict future performance of stainless steel using models to control the underlying microstructure, and discover how to make high-performance steels including corrosion resistance. Researchers will then look at employing a similar strategy with other lighter-weight alloys that are more brittle and prone to cracking.