Imagine squeezing a tube of toothpaste - but instead of mint and fluoride, out comes a material made of metal particles and polymer binders, built up layer by layer into an intricate shape, such as a rocket nozzle, a radiation shield or an MRI component. Then bake it until it becomes a metal harder than steel and dense enough to withstand the unforgiving blast of cosmic radiation or nuclear activity.
This is not fantasy. The process to create these tungsten alloys is called extrusion-based sintering-assisted additive manufacturing - or more colloquially, 3D printing. A prestigious National Science Foundation CAREER grant awarded to Binghamton University Assistant Professor Fuda Ning will funnel almost $600,000 into research to innovate and improve this manufacturing method. It could lead to even stronger alloys and materials, used in industries from outer space to medicine.
"I hope that the fundamental understanding of these innovative processes and their underlying mechanisms also can be transferred to practical engineering solutions," said Ning, a faculty member at the Thomas J. Watson College of Engineering and Applied Science's School of Systems Science and Industrial Engineering and director of the Advanced Materials and Additive Manufacturing Lab. "Moving forward, that's also one of my future career goals: how to do the translation of this technology to industry applications for broader impacts."
Ning's interest in additive manufacturing stemmed from research he did while pursuing his doctorate, and the tungsten alloys at the center of his CAREER award are a combination of mostly tungsten, nickel and iron. This metal is ideal across many fields for its toughness, density and applicability.
Current manufacturing methods, however, can exacerbate its inherent weaknesses, from brittleness to temperature sensitivity. This makes it difficult to create components dense enough to survive in applications as demanding as nuclear shields or armor.
Ning is working on a novel method of 3D printing that could address these shortfalls. Simply put, there are three main steps to fabricate tungsten alloy components: Layer the material in a geometric pattern, remove its polymer binders to leave behind only a metal skeleton, and then essentially bake it into its final shapes.
For tungsten alloys, though, merely layering them isn't enough. Metal particles can sink and throw the structure off balance due to their fairly high density. Layers leave pores and spaces in between, which can become problematic when heating - or sintering - comes into play. Moreover, current sintering processes cause huge grain growth, resulting in a component that is neither stronger nor tougher.
Ning's unique solution to this density problem is to keep densification in mind from the very start. What happens if the metal isn't even? And how do you fill in the gaps and voids between the layers? His answer is to squeeze it more with a compression roller.
"During the extrusion of the material, we have a roller integrated with the printing nozzle so we can squeeze the deposited material a bit further," he said. "That means the interlayer porosity could be significantly reduced, and meanwhile, the metal particle distribution will be relatively more uniform."
Ensuring the metal particles are compacted so close together makes it easier down the road to control the binder removal and densification of the "green," or the initial sample after it's been printed.
Then how can you make the metal skeleton? Ning's solution for that is a unique method that removes water-soluble polymer binders without the need for hazardous acidic gases (like nitric acid) or other chemical solutions. This step is as straightforward as putting the green parts in a warm water bath. "We customize the 'toothpaste' formula in the lab with binder compositions that can be easily removed afterward," he said.
Finally, how about preventing the component from weakening while sintering? The last step can be the most challenging for tungsten alloys, in which iron and nickel components are liquified to leave behind pure tungsten particles. If the material is not dense enough already, it can crack and even collapse during sintering.
Using a hybrid process, Ning will first ensure the structure is densified with what is called solid-state sintering. "Let's say we have all Lego bricks and use glue to firmly attach all the bricks to each other for an assembly," he said. "It's quite similar to this process." Then, liquid-phase sintering can quickly take place after that: "The grains will have relatively little time to grow during this transient process for the strengthening purpose."
Ning joined Binghamton University in 2018 and has risen to stand among the world's top 2% of scientists since 2021, according to a study by Stanford University. With this CAREER grant, he also stands among the researchers tapped by NSF to become the future leaders and academics of their respective fields.
So far, his lab has graduated two doctoral students who have become tenure-track faculty in the United States. Ning credits both his students and previous mentors - including SUNY Distinguished Professor Mark Poliks and SUNY Empire Innovation Professor Peter Borgesen - for his continued success.
"This is not just my personal effort," he said. "The SSIE School's support, collaborations and, of course, my students' hard work also made this award happen."
