NIST
Microscopic image of 3D printed 17-4 PH stainless steel
A strong and corrosion-resistant alloy called 17-4 precipitation hardening (PH) stainless teel is used on many airliners, cargo ships, nuclear power plants and other critical technologies, where strength and durability are essential. A team of researchers have developed a way for the material to be consistently 3D printed while retaining its key characteristics.
Researchers from the National Institute of Standards and Technology (NIST), the University of Wisconsin-Madison and Argonne National Laboratory say they have identified particular 17-4 steel competitions that, when printed, match the properties of the conventionally manufactured version.
The researchers’ strategy is based on high-speed data about the printing process they obtained using high-energy X-rays from a particle accelerator. They believe that the new findings could help producers of 17-4 PH parts to cut costs whilst increasing manufacturing flexibility.
“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, co-author on the study. “But the cooling rate is high, sometimes higher than one million degrees Celsius per second, and this extreme non-equilibrium condition creates a set of extraordinary measurement challenges.”
Zhang said that 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. It has been a struggle for many years to 3D print the material, as the crystal structure has to be so perfect.
The researchers needed special equipment to observe rapid shifts in structure that occur in milliseconds. They found the 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.
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 team, now equipped with a clearer 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 worked.
“Composition control is truly the key to 3D 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,” said Zhang.
The researchers hope that the study will also make an impact beyond 17-4 PH steel. The XRD-based approach could be used to optimise other alloys for 3D printing as well as the information it reveals being useful for building and testing computer models meant to predict the quality of printed parts.
