It takes a special kind of material to be able to hold up in a marine setting, with its harsh conditions and corrosive environment. So-called marine grade stainless steel is commonly used in maritime applications because it can handle those conditions. Its resistance to corrosion and its ductility, or ability to bend without breaking, make it a valued material in not only that industry but several others, including oil and gas, medical equipment and more. There’s one issue, though – the techniques used to strengthen the material typically reduce its ductility.
A group of researchers from Lawrence Livermore National Laboratory (LLNL), Ames National Laboratory, Georgia Tech University, and Oregon State University have successfully 3D printed one of the most common forms of marine grade stainless steel, called 316L, in such a way that both strength and ductility are retained. The research was published in an article entitled “Additively manufactured hierarchical stainless steels with high strength and ductility,” which you can access here.
“In order to make all the components you’re trying to print useful, you need to have this material property at least the same as those made by traditional metallurgy,” said LLNL materials scientist and lead author Morris Wang. “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.”
Before they could successfully 3D print the stainless steel, the researchers had to overcome one major obstacle of metal 3D printing: porosity. Porosity is a common issue that occurs during laser melting and that can cause parts to easily degrade and fracture. To overcome this, the scientists created a density optimization process through experimentation and computer modeling, and worked to manipulate the materials’ underlying microstructure.
“This microstructure we developed breaks the traditional strength-ductility tradeoff barrier,” Wang said. “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 tradeoff.”
The team used two different laser powder bed fusion 3D printers to print thin plates of the stainless steel for mechanical testing. This created hierarchical cell-like structures that could be tuned to alter the mechanical properties of the steel.
“The key was doing all the characterization and looking at the properties we were getting,” said LLNL scientist Alex Hamza, who oversaw production of some additively manufactured components. “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.”
According to LLNL postdoc researcher Thomas Voisin, the work could shine new light on the structure-property relationship of 3D printed materials.
“Deformation of metals is mainly controlled by how nanoscale defects move and interact in the microstructure,” Voisin said. “Interestingly, we found that this cellular structure acts such as 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.”
Wang said that years of simulation, modeling and experimentation went into the research in order to understand the link between microstructure and mechanical properties. The stainless steel they worked with, he said, can be considered a “surrogate material” system that could be applied to other types of metals. Eventually, they want to use high-performance computing to validate and predict future performance of stainless steel, using models to control the underlying microstructure. They hope to learn how to make high-performance, corrosion-resistant steels and then use similar methods with other lighter-weight alloys that are prone to cracking and brittleness.
Contributors to the paper include Y. Morris Wang, Thomas Voisin, Joseph T. McKeown, Jianchao Ye, Nicholas P. Calta, Zan Li, Zhi Zeng, Yin Zhang, Wen Chen, Tien Tran Roehling, Ryan T. Ott, Melissa K. Santala, Philip J. Depond, Manyalibo J. Matthews, Alex V. Hamza and Ting Zhu.
Discuss this and other 3D printing topics at 3DPrintBoard.com or share your thoughts below.[Source: LLNL / Images: Kate Hunts/LLNL]