For airliners, cargo ships, nuclear power plants and other critical technologies, strength and durability are essential. This is why many contain a remarkably strong and corrosion-resistant alloy called 17-4 precipitation hardening (PH) stainless steel. Now, for the first time ever, 17-4 PH steel can be consistently 3-D-printed while retaining its favorable characteristics. A team of researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison, the Missouri University of Science and Technology, and Argonne National Laboratory has identified particular 17-4 steel compositions that, when printed, match the properties of the conventionally manufactured version. The researchers’ strategy, described in the journal Additive Manufacturing, is based on high-speed data about the printing process they obtained using high-energy x-rays from the U.S. Department of Energy’s Advanced Photon Source (APS).
The new findings could help producers of 17-4 PH parts use 3-D printing to cut costs and increase their manufacturing flexibility. The approach used to examine the material in this study may also set the table for a better understanding of how to print other types of materials and predict their properties and performance.
Despite its advantages over conventional manufacturing, 3-D printing of some materials can produce results that are too inconsistent for certain applications. Printing metal is particularly complex, in part because of how quickly temperatures shift during the process.
“When you think about additive manufacturing of metals, we are essentially 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,” said NIST physicist Fan Zhang, a study co-author. “But the cooling rate is high, sometimes higher than one million degrees Celsius per second, 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 said. Without understanding what is happening to the crystal structure of steel as it is printed, researchers have struggled for years to 3-D print 17-4 PH, in which the crystal structure must be just right — a type called 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.
Just as a high-speed camera is needed to see a hummingbird’s flapping wings, the researchers needed special equipment to observe rapid shifts in structure that occur in milliseconds. They found the right tool for the job in synchrotron x-ray diffraction, or XRD.
“In XRD, x-rays interact with a material and will form a signal that is like a fingerprint corresponding to the material’s specific crystal structure,” said Lianyi Chen, a professor of mechanical engineering at UW-Madison and study co-author.
At the APS, a U.S. Department of Energy Office of Science user facility at Argonne, the authors focused high-energy x-rays onto steel samples during printing at the APS X-ray Science Division (XSD) Materials Physics & Engineering Group’s 1-ID-E beamline to probe the phase transformation dynamics in the 17–4 PH stainless steel (Fig. 1). “The APS is a premier national research facility that provides ultra-bright, high-energy x-ray beams. When equipped with special optics and the-first-of-its-kind additive manufacturing setup, we were able to use the high-energy x-rays to monitor the phase transformation during laser additive manufacturing process,” said Andrew Chuang, a beamline scientist at the APS and study co-author.
They followed that study with small-angle x-ray scattering measurements at the XSD Chemical & Materials Science Group’s ultra-small-angle x-ray scattering beamline 9-ID-C to determine the nanoscopic microstructural features in the as-printed 17–4 steel.
The authors mapped 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 up to a dozen different chemical elements. The 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 3-D-printing alloys. 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 1,000 and 10 million degrees Celsius per second, our compositions consistently result in fully martensitic 17-4 PH steel,” Zhang said.
As a bonus, some compositions resulted in the formation of strength-inducing nanoparticles that, with the traditional method, require the steel to be cooled and then reheated. In other words, 3-D printing could allow manufacturers to skip a step that requires special equipment, additional time and production cost.
Mechanical testing showed that the 3-D-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 3-D printing, but the information it reveals could be useful for building and testing computer models meant to predict the quality of printed parts.
“Our 17-4 is reliable and reproducible, which lowers the barrier for commercial use. If they follow this composition, manufacturers should be able to print out 17-4 structures that are just as good as conventionally manufactured parts,” Chen said.
While the current study was able to monitor alloy solidification with good time resolution, the APS Upgrade promises to improve future measurements. “APS-U intensities are expected to be up to 100x higher, meaning we can see the dynamics of primary phases proportionally faster as well as minor phases – which often control properties – with greater sensitivity”, noted co-author Jonathan Almer.
See: Qilin Guo1, Minglei Qu1, Chihpin Andrew Chuang2, Lianghua Xiong3, Ali Nabaa1, Zachary A. Young1, Yang Ren2, Peter Kenesei2, Fan Zhang4*, and Lianyi Chen1**, “Phase transformation dynamics guided alloy development for additive manufacturing,” Additive Manu. 59 103068 (2022). DOI: 10.1016/j.addma.2022.103068
Author affiliations: 1University of Wisconsin–Madison, 2Argonne National Laboratory, 3Missouri University of Science and Technology, 4National Institute of Standards and Technology
We thank Dr. Jan Ilavsky (APS/Argonne) for his assistance with the SAXS measurement. This work is funded by the National Science Foundation (NSF) CMMI- 2011354 and the University of Wisconsin-Madison Startup Fund. The authors acknowledge use of facilities and instrumentation at the UW-Madison Wisconsin Centers for Nanoscale Technology (wcnt.wisc.edu) partially supported by the NSF through the University of Wisconsin Materials Research Science and Engineering Center (DMR-1720415). Atom-probe tomography was performed at the Northwestern University Center for Atom-Probe Tomography (NUCAPT). The LEAP tomograph at NUCAPT was purchased and upgraded with grants from the NSF-MRI (DMR-0420532) and ONR-DURIP (N00014-0400798, N00014-0610539, N00014-0910781, N00014-1712870) programs. NUCAPT received support from the MRSEC program (NSF DMR-1720139) at the Materials Research Center, the SHyNE Resource (NSF ECCS-2025633), and the Initiative for Sustainability and Energy (ISEN) at Northwestern University. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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