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Additive for Aerospace: Welcome to the New Frontier

Kip Hanson
By Kip Hanson Contributing Editor, SME Media

Of all the prospective applications for additive manufacturing (AM), it’s those in the aerospace and defense industries that present the greatest opportunities. Lighter and more aerodynamic structures with superior material properties are some of the more obvious ones. But the ability to reduce part counts and therefore simplify what were once complex assemblies has profound implications for the supply chain.

A Boeing CST-100 technician keeps an eye on the spacecraft Crew Module during a lifting process. Open ports are covered with a special low-tack tape to protect against foreign object damage.

Add to that the universe of advanced materials now available to AM adopters—many of which are difficult to work with via traditional manufacturing methods—and it becomes increasingly obvious that AM might literally take us where no one has gone before.

Youping Gao, the founder and president of AM research and development firm Castheon, hopes to do just that.

A tech fellow and AM technical lead at Aerojet Rocketdyne for longer than two decades, he decided to take AM matters into his own hands in 2018. Since then, he and his team have focused their efforts on the fundamentals of metal powder bed fusion (PBF), as well as the exploration of AM-generated microstructures and metallurgical properties.

Gao is particularly interested in the 3D printing of heat-resistant superalloys (HRSAs) and a special group of elements known as refractory metals. The first of these enjoy broad use in gas turbines and rocket engines, but it’s the latter that offers the greatest potential for changing the speed and manner in which humans propel aircraft, spacecraft, and weaponry from Point A to Point B.

That’s because, while HRSAs, such as Inconel 718 and Hastelloy X, are known for their extreme heat and wear-resistance, they don’t compare to the metals that Gao is targeting. Not even close. Where Inconels begin to lose strength at around 871°C (1,600 °F) and have recommended operating ranges far below this, refractory metals like tantalum and tungsten remain strong at temperatures that many metallurgists would consider hellish.

Youping Gao and his team at Aerojet Rocketdyne focus on the fundamentals of metal powder bed fusion, as well as the exploration of AM-generated microstructures and metallurgical properties.

For example, niobium—one of his favorite materials—has a melting point of 2,477°C (4,491°F), nearly twice that of Hastelloy X, while pure tungsten holds out to 3,422 °C (6,192 °F) before melting.

Why is that important? As any gas turbine engineer will attest, hotter operating temperatures are the holy grail of engine efficiency. And as Gao explained, laser-based PBF enhances the metallurgical properties of refractory metals, allowing them to perform in environments once considered off-limits.

RAMPTing up

“When you print these materials, they typically become both stronger and harder than their wrought or forged equivalents,” he said. “The laser promotes the creation of a supersaturated solid solution with fantastic properties, ones that cannot be achieved otherwise. When you combine this with AM’s ability to generate shapes that were previously impossible to manufacture, it presents some very exciting possibilities for the aerospace industry.”

Gao isn’t alone in his pursuit of novel 3D-printed parts and materials. Paul Gradl, a senior propulsion engineer at NASA’s Marshall Space Flight Center (MSFC), pointed to the agency’s use of GRCop, a copper-chrome-niobium alloy that boasts high conductivity and high strength, making it ideal for liquid rocket engine combustion devices and similar “high heat flux” applications.

As part of its RAMPT (Rapid Analysis and Manufacturing Propulsion Technology) project NASA has invested in a custom-built directed energy deposition (DED) machine able to print multiple metals in the same build.

One notable example of its use was the construction of a 65% scale RS-25 nozzle measuring 16 inches (40.64 cm) in diameter by 72 inches (182.88 cm) tall, one filled with integral cooling channels and composed of discrete layers of copper, Alloy 625, and other aerospace-grade alloys.

Building it required 90 days, an accomplishment that doesn’t seem terribly impressive until Gradl explained that, using conventional means, the test nozzle would have taken up to two years to complete. And while an abbreviated manufacturing cycle brings all manner of cost and lead-time savings, in this case, it’s the end product’s performance that is most relevant to people aiming for interplanetary travel.

One of Northrop Grumman’s latest breakthroughs is electrostatic dissipative polyetheretherketone with discontinuous carbon fibers, which is leading to exciting advancements in aircraft and spacecraft design.

“Between our DED and laser powder bed printers, we’ve built somewhere between 30 to 50 combustion chambers so far and have achieved roughly 30,000 seconds of hot-fire time, which in our world is a pretty big deal,” said Gradl. “We’ve tested in a wide range of environments, with all types of fuels and operating conditions. We’ve really put the technology through the wringer in terms of design complexity, and where and how far we can push it. It’s been a very successful program.”

The road to certification

Additive manufacturing is cool, but what does all this research and development mean to those engaged in spaceflight?

Alison Park, Deputy Tech Fellow for materials and additive manufacturing at NASA’s Engineering and Safety Center (NESC), suggested that the agency’s work produces a very tangible trickle-down effect, most notably with NASA’s recently released AM Standard, NASA-STD-6030 “Additive Manufacturing Requirements for Spaceflight Systems.”

“NASA has been motivated to develop internal standards for AM to provide for a complete and common foundation while industry standards and standards of practice evolve,” Park said. “We also stay close to AM Standards Development Organizations (SDOs) to help them develop consensus within the AM community regarding minimum requirements, as NASA is intently interested in standardization of AM. NASA is in a unique position to both collaborate with and support the aerospace industry overall.”

This is welcome news to Eric Barnes, a fellow of advanced and additive manufacturing at Northrop Grumman, who indicated that the current certification process can be frustrating.

“One of the main benefits of additive manufacturing is its speed, in that it allows you to develop products very rapidly, but this is hampered somewhat by delays in the product or material qualification,” he said. “Still, it’s often much faster than the alternatives—procurement of a large forging might take a year, for example, never mind the tooling costs that come with it.”

Crossing the trough

Like his colleagues, Barnes has been in the additive arena since its inception longer than three decades ago.

He has watched as 3D printing grew from a rapid-prototyping-only technology that used admittedly low-performance resins to one that now produces highly functional, end-use parts in both metals and polymers, and has therefore become the go-to manufacturing solution for a wide range of aerospace applications.

In his opinion, AM is about to enter a golden age. “To put it in the context of the Gartner hype cycle, we are past the trough of disillusionment and well on our way to the plateau of productivity,” Barnes said. “Northrop Grumman and its customers are now in a position to more readily adopt additive manufacturing and prepare to enter that plateau of productivity because we have spent the past few years collecting the required data and generating the statistical information needed to ensure long term use of additive manufacturing in an aeronautical environment.”

Some of that work involves the development of more advanced in-process monitoring systems. These will provide the kinds of detailed build data needed for post-mortem analyses in the unlikely event of a product failure, but will also reduce the need for costly CT (computed tomography) and other NDT (non-destructive testing) currently performed on flight-critical components.

Said Barnes: “In the future, you may be able to eliminate NDT completely. Comprehensive build data will also serve to reduce qualification timelines, and if you’re able to understand all that’s going on inside the build chamber in real-time, machine learning and AI systems might be able to adjust process parameters such that you never have a bad part.”

Spares to repairs

Dan Braley brings up another consideration, one that is often overlooked in the race to bring aerospace products to market quickly, safely and cost-effectively: sustainment.

Boeing team members push the CST-100 Starliner to an area for fueling prior to the launch of Orbital Flight Test-2 at NASA’s Kennedy Space Center in Cape Canaveral, Fla.

An associate technical fellow and additive manufacturing technical focal at Boeing Global Services, Braley’s responsible for ensuring that everything from commercial airliners to fighter jets and vertical lift helicopters remains operational post-deployment through AM. It’s a job that he refers to as “additive manufacturing for spares and repairs.”

“It’s all about helping Boeing’s various programs and customer to keep their aircraft in the air,” Braley said. “If there are components that they can’t get, for instance, or a key supplier has gone out of business, or a part on an aircraft is in need of repair, we find ways to get them out of these tough situations.

“And in order to achieve that in the most expedient manner, advanced manufacturing of all kinds needs to be examined, whether that’s hybrid CNC machining, AFP (automated fiber placement), cold spray (CS) technology, and of course, metal and polymer-based 3D printing.”

Braley noted that, in at least one case, these additively manufactured parts outperformed the legacy parts they were replacing.

In its FY 2017 Additive Manufacturing Report to the U.S. Congress, the Department of Defense listed a host of AM-related activities, among them its first official certification of what was then a nearly unheard of technology.

In 2003, the USAF was unable to find conventionally manufactured replacements for roughly a dozen F-15 pylon ribs that had failed due to corrosion fatigue. They decided to put DED technology to the test. According to the report, “those parts are still flying today with no reported field issues.”

The supplier in this case? Boeing.

“Those pylons are critical safety items that are heavily loaded,” Braley said. “The Air Force couldn’t get hold of any replacements and had multiple aircraft that were grounded. We worked with them to prove out the solution using additive manufacturing, producing titanium components in far less time than it took for the titanium forging supplier to deliver a workable solution. It’s a great example of the role that AM can play in getting downed aircraft back in service quickly, but even more so, it’s a great example of the immense value that AM and other types of advanced manufacturing bring to the industry.”

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