One of the hurdles that’s still being overcome in metal 3D printing is the ability to print with superalloys. The most common forms of metal printing that involve selectively melting metal powder with lasers or electron beams don’t work well with superalloys as they tend to crack during or after the printing process. But a research team at UC Santa Barbara recently published a paper outlining a new class of superalloys that are 3D printable, opening the door to new AM applications in aerospace and nuclear energy.
The research team was led by Tresa Pollock, a professor of materials and associate dean of UC Santa Barbara’s College of Engineering. Much of the funding came from a $3 million Vannevar Bush Faculty Fellowship (VBFF) that Pollock received from the US Department of Defense in 2017. In referring to the lasers used in metal AM, Pollock said “The highly focused beams provide exquisite control, enabling ‘tuning’ of properties in critical locations of the printed object. Unfortunately, many advanced metallic alloys used in extreme heat-intensive and chemically-corrosive environments encountered in energy, space, and nuclear applications are not compatible with the AM process.” Carpenter Technologies, Oak Ridge National Laboratory, and UCSB staff scientists also contributed to the project to find the key to printing with these advanced metals.
Most very-high-strength alloys that function in extreme environments cannot be printed, because they crack. They can crack in their liquid state, when an object is still being printed, or in the solid state, after the material is taken out and given some thermal treatments. This has prevented people from employing alloys that we use currently in applications such as aircraft engines to print new designs that could, for example, drastically increase performance or energy efficiency. Tresa Pollock, ALCOA Distinguished Professor of Materials at UC Santa Barbara
It’s important to note that the metals Pollock is speaking of are wildly expensive because they contain rare materials and require extensive processing and refining to create. So to then go and cut chunks off of such costly metals (that will go to waste) using traditional milling and CNC machining, that’s just plain inefficient. Being able to reduce that waste to practically nothing with 3D printing would generate huge cost savings across multiple industries. Add to that the material saved by printing more efficient geometries that can’t be produced with machining.
The new suite of superalloys developed by Pollock’s team are almost equal parts Cobalt and Nickel. Generally, superalloys are composed mostly of nickel, but cranking up the Cobalt content was an important ingredient in their formula. “The high percentage of cobalt allowed us to design features into the liquid and solid states of the alloy that make it compatible with a wide range of printing conditions,” explained Pollock. The CoNi-based superalloys are capable of maintaining their material integrity at temperatures up to 90% of their melting point, whereas most alloys start to crumble at 50% of their melting temperatures.
They’re also defect resistant, a nice feature when manufacturing parts such as turbine blades for nuclear reactors or wingtips for supersonic jets. These new high-strength CoNi superalloys are compatible with EBM (electron beam melting) and SLM (selective laser melting), the latter being a more accessible technology that has historically struggled more with superalloys, so expect to see more high-stress applications of metal AM.
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