Strength and durability are crucial for nuclear power plants, cargo ships, airplanes, and other critical technologies. For this reason, many are made of the remarkably durable alloy 17-4 precipitation hardened (PH) stainless steel. For the first time, 17-4 PH steel can be 3D printed in a reliable way while still keeping its useful properties.
Researchers from the University of Wisconsin-Madison, Argonne National Laboratory, and the National Institute of Standards and Technology (NIST) have identified specific 17-4 steel compositions that, when printed, have properties that are comparable to those of the conventionally produced version. The researchers used high-energy X-rays from a particle accelerator to get fast information about the printing process. They used this information to come up with their strategy, which was published in the journal Additive Manufacturing.
The latest research might make 3D printing more cost-effective and flexible for manufacturers of 17-4 PH parts. The method used to investigate the substance in this study may also lay the groundwork for a better understanding of how to print other kinds of substances and forecast their properties and performance.
Despite its advantages over traditional manufacturing, some materials can produce 3D prints that are too erratic for some uses. The complexity of printing metal is largely due to how quickly temperatures change while the process is being carried out.
According to NIST physicist Fan Zhang, a study co-author, “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.” But the rate of cooling is fast, sometimes more than one million degrees Celsius per second. This extreme state of not being in equilibrium makes it hard to measure a lot of things.
The arrangement, or crystal structure, of the atoms within the material shifts quickly and is challenging to pinpoint because the material heats and cools so quickly, according to Zhang. Researchers have struggled for years to 3D-print 17-4 PH, a material whose crystal structure must be precisely correct — a type called martensite — for the material to exhibit its highly desired properties. This is because they do not understand what is happening to the steel’s crystal structure as it is printed.
The goal of the new study was to find out what happens when temperatures change quickly and come up with a way to speed up the internal structure’s change to martensite.
Just as a high-speed camera is required to capture a hummingbird’s flapping wings, specialized equipment was required by the researchers to capture rapid structural changes that take place in milliseconds. They discovered synchrotron X-ray diffraction, or XRD, to be the ideal tool for the job.
Lianyi Chen, a mechanical engineering professor at the University of Wisconsin–Madison and one of the study’s co-authors, explained that when X-rays interact with a material, they create a signal that is like a fingerprint and matches the crystal structure of the material.
At Argonne National Lab’s Advanced Photon Source (APS), a particle accelerator that is 1,100 meters long, the authors printed steel samples with high-energy X-rays.
The authors charted the evolution of the crystal structure during a print, showing how variables under their control, like the metal-powder composition, affected the entire process.
Although iron is the main element in 17-4 PH steel, the composition of the alloy can contain up to a dozen different chemical elements in varying amounts. The authors were able to fine-tune the composition of the steel until they found a set of only iron, nickel, copper, niobium, and chromium that worked. This was possible because they now had a good idea of how the structure changed during the printing process.
“The secret to 3D printing alloys is actually composition control. We can control how it solidifies by adjusting the composition. “We also demonstrated that our compositions consistently produce fully martensitic 17-4 PH steel over a broad range of cooling rates, say between 1,000 and 10 million degrees Celsius per second,” said Zhang.
In addition, some compositions produced nanoparticles that increased strength instead of the traditional method, which involves cooling and then reheating the steel. In other words, 3D printing might make it possible for manufacturers to skip a step that requires special tools, extra work, and money.
Due to its martensite structure and strength-increasing nanoparticles, the 3D-printed steel was just as strong as steel that was made in the traditional way.
The recent study might be influential beyond 17-4 PH steel as well. The information obtained from the XRD-based method could be used to develop and test computer models intended to forecast the quality of printed parts, in addition to optimizing other alloys for 3D printing.
“Because our 17-4 is repeatable and dependable, there are fewer restrictions on its use in commerce. Wang said that manufacturers should be able to print 17-4 structures that are just as good as traditionally manufactured parts if they adhere to this composition.
