Oct 31, 2017 | By Benedict

Researchers at Lawrence Livermore National Laboratory (LLNL), joined on their work by scientists from Ames National Laboratory, Georgia Tech University, and Oregon State University, have achieved a “breakthrough” in the 3D printing of 316L, a common form of “marine grade” stainless steel with a low-carbon composition. Marine grade steels, used in places like oil pipelines, engine parts, and kitchen equipment, typically have a low corrosiveness and high ductility. Excitingly, tests showed that under certain conditions the final 3D printed stainless steels were up to three times stronger than steels made by conventional techniques.

These expert scientists believe their 3D printed 316L stainless steel could offer higher levels of strength and ductility than other forms of the steel, making it useful for chemical equipment, medical implants, engine parts, and various other applications that require superior physical properties from their equipment.

The researchers were well aware that simply 3D printing some stainless steel would be worthless if its quality were lesser than existing metallurgical options. So when the parts came out rugged and ductile, LLNL materials scientist and lead author Morris Wang had good reason to celebrate.

“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,” Wang said. “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.”

The LLNL breakthrough could have big consequences not just for the CVs of Wang and his associates, but for a wide range of companies involved in the production of marine grade stainless steel parts.

The researchers think that those in the aerospace, automotive, and oil and gas industries are in a particularly strong position to adopt the additive manufacturing of 316L in order to improve the physical properties of their metal parts. Because these industries often need parts that are resistant to extreme weather conditions, the added strength and ductility of the 3D printed stainless steel could prove invaluable.

“[The breakthrough is] really a big jump,” Wang added. “It makes additive manufacturing very attractive and fills a major gap.”

Of course, being able to 3D print the stainless steel at the required quality was no easy task, and required some “steely” determination on the part of the joint research group. Perhaps the biggest hurdle of all was the porosity of the 3D printed metal caused by the laser melting process.

Porosity is great for some materials—activewear, for example, would be hard to use without a bit of breathability—but for building engines, pipelines, and such, it’s obviously a bit of a hindrance. Porous parts are liable to degrade and fracture, making them potentially unsafe for critical-use applications.

To overcome the problem of laser-induced porosity, the LLNL researchers used a density optimization process, manipulating the underlying microstructure of the steel using computer modeling.

“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.”

Wang and the team used two different laser 3D printers to fabricate thin plates of 316L for mechanical testing. The laser melting process produced hierarchical cell-like structures, which could be tuned by the researchers to manipulate the physical properties of the 3D printed plates.

“The key was doing all the characterization and looking at the properties we were getting,” said LLNL scientist Alex Hamza. “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.”

(Images: Kate Hunts/LLNL)

Amazingly, discovering this grain structure ultimately allowed the researchers to create 3D printed stainless steel parts that were superior to existing alternatives.

“We didn't set out to make something better than traditional manufacturing,” Hamza admitted. “It just worked out that way.”

The research will have consequences not just for big companies making steel parts, but for other researchers looking to build upon the discoveries made by the LLNL group. It could, for example, enable scientists to better understand the relationship between the structure of 3D printed metal parts and their physical properties.

“Deformation of metals is mainly controlled by how nanoscale defects move and interact in the microstructure," said LLNL postdoc researcher Thomas Voisin. “Interestingly, we found that this cellular structure acts…as a filter, allowing some defects to move freely and thus provide the necessary ductility while blocking some others to provide the strength.”

Voisin added that being able to observe this phenomenon ultimately helped the research group to better understand the properties of 3D printed materials (and how to control them).

The methodologies used could even be replicated on other 3D printable metals besides stainless steels, but the big aim is to someday devise a system in which high-performance computing can be used to both validate and predict the performance of 3D printed stainless steel by tuning the underlying microstructure. This would enable the researchers to create tailored high-performance steels that have, for example, a high level of corrosion resistance.

The research paper, “Additively manufactured hierarchical stainless steels with high strength and ductility,” has been published in Nature Materials.

Other contributors to the paper include Joe McKeown, Jianchao Ye, Nicholas Calta, Zan Li, Wen Chen, Tien Tran Roehling, Phil Depond, and Ibo Matthews.



Posted in 3D Printing Application



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