For commercial aircraft, cargo ships, nuclear power plants and other critical technologies, strength and durability are critical. For this reason, many contain a remarkably strong and corrosion-resistant alloy called 17-4 precipitation hardening (PH) stainless steel. Now, for the first time, 17-4 PH steel can be 3D printed throughout while retaining its beneficial properties.
A team of researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison and Argonne National Laboratory have identified certain 17-4 steel compositions that, when printed, match the properties of the conventionally produced version. The researchers’ strategy, described in the journal additive manufacturing, is based on high-speed data about the printing process, which they obtained using high-energy X-rays from a particle accelerator.
The new findings could help manufacturers of 17-4 PH parts to use 3D printing to reduce costs and increase their manufacturing flexibility. The approach used to study the material in this study can also set the table for a better understanding of how other types of materials are printed and their properties and performance predicted.
Despite its advantages over conventional manufacturing, 3D printing of some materials can produce results that are too inconsistent for certain applications. Metal printing is particularly complex, in part due to the rapid temperature changes during the process.
“If you think about additive manufacturing of metals, we’re essentially welding millions of tiny, powdered particles into one piece with a high-power source like a laser, melting them into a liquid, and cooling them into a solid,” said NIST physicist Fan Zhang, co-author of the study. “But the cooling rate is rapid, sometimes greater than a million degrees Celsius per second, and this extreme non-equilibrium condition creates a number of extraordinary measurement challenges.”
Because the material heats and cools so quickly, the arrangement, or crystal structure, of atoms within the material shifts rapidly and is difficult to pin down, Zhang said. Without understanding what happens to the crystal structure of steel when printed, researchers have struggled for years to 3D print 17-4 PH, which requires the crystal structure to be just right — a type called martensite — for the material to have its coveted properties to show.
The authors of the new study wanted to shed light on what happens during the rapid changes in temperature and find a way to push the internal structure towards martensite.
Just as a high-speed camera is needed to see a hummingbird’s wing beats, researchers needed specialized equipment to observe rapid structural changes 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 form a signal that’s like a fingerprint that corresponds to the material’s specific crystal structure,” said Lianyi Chen, professor of mechanical engineering at UW-Madison and co-author of the study.
At the Advanced Photon Source (APS), a 1,100-meter-long particle accelerator at Argonne National Lab, the authors smashed high-energy X-rays into steel samples during printing.
The authors mapped out how the crystal structure changed over the course of a print and revealed how certain factors over which they had control – such as the composition of the powdered metal – affected the overall process.
While iron is the primary component of 17-4 PH steel, the alloy’s composition can contain variable amounts of up to a dozen different chemical elements. Now armed with a clear picture of the structural dynamics during printing as a guide, the authors were able to fine-tune the composition of the steel to find a range of compositions containing only iron, nickel, copper, niobium and chromium that did this did trick.
“Composition control is really the key to 3D printing alloys. By controlling the composition, we can control how it solidifies. We have also shown that this is possible 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, 3D printing could allow manufacturers to skip a step that requires specialized equipment, additional time, and production costs.
Mechanical tests showed that the 3D printed steel, with its martensite structure and strength-inducing nanoparticles, matched the strength of steel made by traditional means.
The new study could also cause a sensation beyond 17-4 PH steel. Not only could the XRD-based approach be used to optimize other alloys for 3D printing, but the information gleaned from it could also be useful for building and testing computer models designed to predict the quality of printed parts.
“Our 17-4 is reliable and repeatable, lowering the barrier to commercial deployment. By following this composition, manufacturers should be able to print 17-4 structures that perform as well as traditionally manufactured parts,” Chen said.