3D-printed parts made from refractory metals can handle the heat
According to the American Society of Metals and their ASM Metals Reference Book, refractory metals are naturally occurring metallic elements with a melting point above 2,200 ° C. These metals – molybdenum, niobium, tantalum, tungsten and rhenium – are not only extremely heat-resistant. and corrosion resistant, they retain their structural integrity even at elevated temperatures. This makes them an excellent choice for a variety of demanding aerospace, industrial, and scientific applications.
One problem is that refractory metals are extremely difficult to process using traditional manufacturing methods such as machining and forming. Between these and their relatively high cost, the use of refractory metals has long been limited to applications where workpiece shapes are relatively simple, stock removal is minimal, and simpler-to-make superalloys would not perform well.
However, thanks to additive manufacturing (AM) and the efforts of researchers like Youping Gao, that is set to change.
Gao is President of Castheon Inc., an AM research, development, and manufacturing company based in Thousand Oaks, California. He explained that laser powder bed fusion (LPBF) solves most of the manufacturability problems associated with refractory metals and alloys. And as with all metallic AM technologies, topologically optimized lightweight components with pore or lattice structures can also be realized that would otherwise not be practical or even impossible to produce.
Low-cost manufacturing of metal components with melting points nearly double that of INCONEL, Hastelloy, and other popular heat-resistant superalloys (HRSAs) opens the door to some exciting possibilities. Higher temperatures mean more fuel efficient and longer lasting gas turbine engines, which are of paramount importance to the commercial aerospace and power generation industries.
But there’s also the potential for hypersonic air travel, one of the many projects Gao and his team have been working on since the company was founded in 2016.
“NASA, the US Air Force and other agencies have long had an interest in sustained hypersonic flights for commercial and military purposes,” said Gao. “Until recently, however, it was not possible to get refractory metals into the sophisticated shapes required to support speeds of Mach 5 and above.”
He and his colleagues found that not only can metal AM make these shapes, but niobium-based alloys are also far more stable than their forged equivalents. At temperatures of 1,300 degrees C they have a tensile strength that is 1.8 times higher. Other refractory metals such as tungsten and rhenium also have similar advantages.
Hard to do
Gao encountered challenges on his path to success in this niche market. He described the production of refractory metal alloys and powders as a “nightmare”, which led to high costs and material shortages. And 3D printing is also quite a challenge because of a narrow operating window and “unique grain control mechanism,” he said.
Regardless, these unique metals are vastly superior to their HRSA alternatives, and Gao is confident that their use will increase as raw material prices drop and more people acquire the knowledge needed to print.
Faith Oehlerking agrees with each of these points. As an additive manufacturing R&D engineer at refractory metals maker and parts maker HC Starck Solutions, Coldwater, Michigan, she spends her days unraveling the secrets of 3D printing tungsten, tantalum, and other elements. Like Gao, Oehlerking and the HC Starck team are using Renishaw’s LPBF technology to help her with this. Starck also builds refractory metal parts and test coupons on a Binder-Jet 3D printer provided in collaboration with ExOne.
Some of the challenges for molybdenum, tungsten, tantalum, and niobium arise from their body-centered cubic atomic structure. They have a ductile-brittle transition temperature (DBTT), explained Oehlerking. Metals such as molybdenum and tungsten have very high DBTTs, which can lead to stress build-up and microcracks in the finished component.
With forged materials, it is possible to mitigate these types of defects through thermomechanical processes such as cold forming, but this is not practical with 3D printed components. The solution is to alloy the refractory base metals with elements like rhenium, nickel, and iron to lower the metal’s DBTT and reduce stresses.
“We do a lot of research in this area, but we also evaluate various printing strategies and technologies,” says Oehlerking. “For example, heating the build plate is a common strategy to reduce cracking, with some machines reaching 500 degrees C or more. This has the potential to ease the transition from liquid to solid, especially with molybdenum and tungsten, which have a much higher DBTT. “
It’s also important to note that oxygen in the build chamber or in the powder is harmful as too much oxygen can further increase the DBTT and microcracks of the metal, Oehlerking said. “A good quality feedstock and good atmospheric control in your metal AM machine are essential for success in printing refractory metals.”
Even if it is impossible to eliminate microcracks and the resulting loss of structural integrity, refractory metals still play an important role. For example, tungsten is often used for anti-scatter collimators in X-ray and CT scanners, which do not experience the same mechanical stress as in aerospace and military applications. Refractory metals also have excellent thermal conductivity and a low coefficient of thermal expansion, which makes them well suited for heat exchangers and crucibles for growing sapphire.
One project that Starck is involved in that requires the superior structural integrity possible with most refractory metals is to print a Resistojet nozzle segment for the UK space agency. The project is called Super High Temperature Additive Manufactured Resistojets or STAR. (A resistojet is a simple electrical propulsion system that develops thrust by heating a liquid.-Ed.)
Because of its low DBTT and high temperature resistance, tantalum and a tantalum-tungsten alloy (Ta10W) were selected for the satellite component, which has constant electrical conductivity, extreme tensile strength and minimal warpage at temperatures above 3,000 degrees C. must have.
With the Renishaw AM400 LPBF printer from the 3D printing partner HiETA Technologies, various construction parameters were tested in order to achieve 99.95% tight coupons from both materials. These settings were then used to prototype the thin-walled nozzle segments. These parts are currently being evaluated internally by HC Starck Solutions and at the University of Southampton, England.
Metal AM offers incredible flexibility for refractory metals development projects like this, as well as the ability to create custom alloy blends. Unlike most manufacturers who have to mix different metal powders together to achieve the desired properties, Starck can atomize its own metal powders.
According to Oehlerking, these “pre-alloyed” metals offer better consistency in 3D printing. “We have a number of different metals available and we are always developing more,” she said. “For example, there is titanium-zirconium-molybdenum, a popular medical alloy. But there is also molybdenum-lanthanum, tungsten-rhenium and the niobium-based alloy C-103, which Youping Gao uses for much of his work.
“Between refractory metals and 3D printing, the potential here is enormous, especially in the aerospace and defense sectors. We’re just getting started, ”said Oehlerking.