Researchers from several US universities have pioneered a project to 3D print marine-grade steel, creating a material that is stronger and more flexible
The corrosive effects of saltwater naturally call for the careful choice of materials in offshore projects. In the case of stainless steels, specific “marine-grade” alloys are used, typically characterised by higher levels of molybdenum to protect against the harsh environment.
These marine-grade steels also have high ductility – the ability to bend without breaking under stress – meaning they are a preferred choice in oil pipelines, welding, chemical equipment and other applications. Yet while it is possible to strengthen these marine grade steels for added protection, this usually reduces ductility.
A new alternative approach has been suggested by a consortium of US universities. By 3D-printing a common marine-grade alloy – a low-carbon grade known as 316L – the group has succeeded in producing steel which is both stronger and more flexible than its conventional form. The team, led by Lawrence Livermore National Laboratory (LLNL), with the aid of Ames National Laboratory, Georgia Tech University and Oregon State University), believes the breakthrough could aid applications in aerospace, automotive and oil and gas, and recently published its work in Nature Materials.
LLNL materials scientist and lead author Morris Wang noted: “We were able to 3D-print real components in the lab with 316L stainless steel, and the material's performance was actually better than those made with the traditional approach. That's really a big jump. It makes additive manufacturing very attractive and fills a major gap.”
Just around the bend
Achieving this meant first adapting the printing process for this particular alloy. Until now, the melting of metal powders during the additive manufacturing process – in this case powder bed fusion (PBF), which uses either a laser or electron beam to melt and fuse material powder together – resulted in an overly porous material which could degrade and fracture easily. This had been regarded as a serious drawback to using this technology. To overcome this, the team experimented and modelled the material as part of a density optimisation process, and also examined the effects of manipulating the underlying microstructure of the metals.
Over several years, the group built up a more detailed understanding across the campuses. Ames Lab used X-ray diffraction to understand material performance, Georgia Tech used computer modelling to understand the materials' strength and ductility, and Oregon State performed characterisation and composition analysis.
"This microstructure we developed breaks the traditional strength-ductility trade-off barrier,” Wang explained. “For steel, you want to make it stronger, but you lose ductility essentially; you can't have both. But with 3D printing, we're able to move this boundary beyond the current trade-off.”
Using two different laser PBF machines, the researchers printed thin plates of 316L stainless steel for mechanical testing. Their laser melting technique produced “hierarchical cell-like structures” that could be tweaked to alter the mechanical properties of the final material.
Not only is the material stronger, but, as Wang told InnovOil via email, “There is potential evidence from other groups suggesting that 3D printed stainless steels actually have better corrosion resistance.”
LLNL’s Alex Hamza added: “When you additively manufacture 316L it creates an interesting grain structure, sort of like 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 seem to be controlling the properties. This was the discovery. We didn't set out to make something better than traditional manufacturing; it just worked out that way.”
"Deformation of metals is mainly controlled by how nanoscale defects move and interact in the microstructure," added LLNL postdoctoral researcher Thomas Voisin, another key contributor to the paper. “Interestingly, we found that this cellular structure acts [like] a filter, allowing some defects to move freely and thus provide the necessary ductility while blocking some others to provide the strength. Observing these mechanisms and understanding their complexity now allows us to think of new ways to control the mechanical properties of these 3D-printed materials."
For the team, the goal now is to apply similar methodologies and computer modelling to predict the performance and composition of other stainless steels and lighter-weight alloys which may be brittle or crack prone.