Researchers Uncover How to 3D-Print One of the Strongest Stainless Steels

September 30, 2022


NIST-Stainless-lpbfA team of researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison and Argonne National Laboratory has identified particular 17-4 steel compositions that, when printed, match the properties of the conventionally manufactured versions. This research, they say, demonstrates for the first time that 17-4 PH steel can be consistently 3D-printed while retaining its favorable characteristics, and could help producers of 17-4 PH parts use 3D printing to cut costs and increase their manufacturing flexibility. 

The researchers’ strategy, described in the research journal Additive Manufacturing, is based on high-speed data about the printing process they obtained using high-energy X-rays from a particle accelerator. 

“When you think about additive manufacturing of metals, we essentially are welding millions of tiny, powdered particles into one piece with a high-powered source such as a laser, melting them into a liquid and cooling them into a solid,” says NIST physicist Fan Zhang, a study co-author. “But the cooling rate is high, sometimes higher than 1 million C/sec., and this extreme nonequilibrium condition creates a set of extraordinary measurement challenges.”

Because the material heats and cools so hastily, the arrangement, or crystal structure, of the atoms within the material shifts rapidly and is difficult to pin down, Zhang explains. Without understanding what is happening to the crystal structure of steel as it is printed, researchers have struggled for years to 3D-print 17-4 PH, in which the crystal structure must be just right—martensite—for the material to exhibit its highly sought-after properties. 

The authors of the new study aimed to shed light on what happens during the fast temperature changes and find a way to drive the internal structure toward martensite. Using an observation technique called synchrotron X-ray diffraction, or XRD, they were able to map out how the crystal structure changed over the course of a print, revealing how certain factors they had control over, such as the composition of the powdered metal, influenced the process throughout. 

While iron is the primary component of 17-4 PH steel, the composition of the alloy can contain differing amounts of as many as a dozen different chemical elements. The study authors, now equipped with a clear picture of the structural dynamics during printing as a guide, were able to fine-tune the makeup of the steel to find a set of compositions including just iron, nickel, copper, niobium and chromium that did the trick. 

“Composition control is truly the key to 3D printing alloys,” Zhang says. “By controlling the composition, we are able to control how it solidifies. We also showed that, over a wide range of cooling rates, say between 1000 and 10 million C/sec., our compositions consistently result in fully martensitic 17-4 PH steel.” 

As a bonus, some compositions resulted in the formation of strength-inducing nanoparticles that, with traditional methods, require the steel to be cooled and then reheated. In other words, 3D printing could allow manufacturers to skip a step that requires special equipment, additional time and production cost. 

Mechanical testing showed that the 3D-printed steel, with its martensite structure and strength-inducing nanoparticles, matched the strength of steel produced through conventional means. 

The new study could make a splash beyond 17-4 PH steel as well. Not only could the XRD-based approach be used to optimize other alloys for 3D printing, but the information it reveals could be useful for building and testing computer models meant to predict the quality of printed parts. 

Industry-Related Terms: Additive manufacturing
View Glossary of 3D Metal Printing Terms

Technologies: Metal Powders

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