New research could help manufacturers avoid the 3D printing trap
A research team has discovered that a method commonly used to circumvent one of the biggest problems in metal 3D printing may be far from a silver bullet.
For manufacturers, 3D printing, or additive manufacturing, makes it possible to build complex shaped parts that are more durable, lighter and more environmentally friendly than those made using traditional methods. The industry is booming, with some predicting that it will double in size every three years, but growth often goes hand in hand with growing pains.
Residual stress, a by-product of the repeated heating and cooling inherent in metal printing processes, can introduce defects into parts and in some cases damage printers. To better understand how residual stress forms and how it might be restrained, researchers from the National Institute of Standards and Technology (NIST), Lawrence Livermore National Laboratory, Los Alamos National Laboratory, and other institutions took a close look at the effects of different printing patterns on titanium. alloy parts manufactured using a common laser method.
Their results, published in Additive manufacturing, show that a printing model often used in industry to decrease residual stresses, called island scanning, had the worst result among the approaches studied, defying the team’s expectations. The data they produced could help manufacturers test and improve predictive models for 3D printing, which, if accurate, could move them away from destructive levels of residual stress.
“It was very surprising and it underscores the complexity of the problem,” said Thien Phan, materials research engineer at NIST, co-author of the study. “It shows that while island digitization can work in a lot of cases, it didn’t work in ours, which really underscores the fact that we need precise modeling.
The team’s research focused on a common additive manufacturing method called laser powder bed fusion (LPBF), in which a laser scans a layer of metallic powder in a predetermined pattern, melting and fusing particles on the surface. . As the molten metal cools into a solid, a material supporting step lowers and the printer adds a new layer of powder on top, allowing the laser to continue building the part layer by layer.
Once the second layer of a build begins, residual tension begins to rise up its nasty head. The metals used in LPBF cool quickly, which means that by the time a printer’s laser begins to heat a new layer, the metal in the previous layer is already solid. The molten layers contract inward as they cool, pulling on the solid metal below and creating stress. And the greater the temperature difference, the more the molten layer pulls. This process is repeated for each layer until the part is complete, locking the stresses into the solid metal.
“You end up with an incredible amount of residual stress inside your room,” Phan said. “So he’s sitting there tearing himself apart. The residual stress could crack the part and lift it up during construction, which could actually crash the machine.”
The simplest printing pattern in LPBF is continuous scanning, where the laser sweeps back and forth from one end of the room to the other. But an alternative option called islet scanning has emerged as a way to alleviate stress. The idea behind this approach is that fusing small sections, or islands, of metal one at a time rather than an entire layer would result in less contraction of the metal at the same time, reducing the overall stress.
The digitization of islands has gained traction with manufacturers, but previous studies of the technique have been inconsistent. And more broadly, the relationship between scanning strategies and residual stress remains largely a mystery. To begin to fill these gaps, the multi-agency team set out to analyze in detail the effects of island sweeping on stress.
The authors of the new study printed four titanium alloy bridges that were just over 2 centimeters (0.8 inches) in length. Samples were constructed via continuous or island scanning, with lasers along their length and width or at a 45 degree angle.
At a glance, the bridges looked similar when they came out of the printer, but rather than take them at face value, the researchers took a close look at them.
They emitted high-energy X-rays, generated by a powerful tool called a synchrotron, deep into the samples. By measuring the wavelengths of x-rays reflected from the metal, the team extracted the distances between metal atoms with great precision. From there, the researchers calculated the stress. The greater the distances, the more stressed the metal. With this critical information in hand, they generated maps showing the location and degree of stress in the samples.
All samples contained stresses near the yield strength of the titanium alloy – the point at which a material undergoes permanent deformation. But the maps revealed something else that surprised researchers.
“Island scan samples have these very large stresses on their sides and on top, which are absent or much less pronounced in continuous scan samples,” said physicist and NIST coauthor Lyle Levine. “If the digitization of the islands is one way the industry is trying to alleviate these tensions, I would say that for this particular case it is far from successful.”
In another test, they detached one leg of each bridge from the metal base plates it was glued to. The study authors measured the distance the legs sprang upward, obtaining another indicator of the amount of residual stress stored inside the arch of each bridge. Again, the island sweep samples performed poorly, with their legs deforming more than twice as much as the other samples.
The authors suggest that sweeping the islands could be a double-edged sword. While the small size of the islands can reduce contraction, the islands could also cool much faster than larger melt pools, creating larger temperature differences and therefore greater stress.
While island scanning is not well suited for the particular room, material and equipment used in the study, it could still be a good choice in different circumstances, Phan said. The results indicate, however, that it is not a panacea for residual stress. To avoid stress, manufacturers may need to tailor the scanning strategy and other parameters to their specific build – an effort that would be greatly facilitated by computer models.
Rather than optimizing a printout through trial and error, manufacturers could use models to quickly and inexpensively identify the best parameters, if their predictions are correct. Modelers could build confidence in their tools by testing them against rigorously produced benchmarks, much like the data obtained in the new study, Levine said.
This work offers a new perspective on a popular printing strategy, adding a key piece to the puzzle of residual stress formation and ultimately bringing 3D printing closer to its full potential.