Qualifying and testing additively manufactured parts for aircraft | Aerospace Testing International

Boeing’s use of additively manufactured parts already spans three decades. The company has 140,000 such parts in service today. Whilst most are polymer, recent growth has focused on metal additive processes.

“Additive manufacturing means delivering higher quality parts in shorter lead times at reduced cost,” says Melissa Orme, vice president for additive manufacturing, Boeing. “It enables geometries we couldn’t traditionally fabricate. We can redesign parts, make them smaller or integrate systems of parts. It is about making the aircraft or satellite a better end-product.”

Various additive manufacturing (AM) processes make parts directly from computer models by adding materials in layers. Fused deposition modeling (FDM) melts a polymer wire into layers while selective laser sintering (SLS) forms them from powder. Powder bed fusion (PBF) is most commonly used for metal parts, while directed energy deposition (DED) techniques like wire arc additive manufacturing (WAAM) make them from wire.

“AM processes have three fundamental ingredients,” says Filomeno Martina, CEO of WAAM3D. “A feedstock, which can be powder or wire, a heat source, which can be lasers, electron beams or electric arcs, then a motion system taking the heat source around the feedstock or vice versa.”

Benefits in aerospace that derive from AM include the freedom to design and manufacture otherwise impossible geometries, including biomimetic shapes. Parts that only use material where needed can reduce weight, reducing an aircraft’s emissions across its lifetime. Metrology company Hexagon helps customers like GKN Aerospace develop lightweight engine parts with its MSC Apex Generative Design software.

“Generative design combines topology optimization and manufacturability,” says Mathieu Pérennou, director of AM solutions, Hexagon. “We give the software boundary conditions for the part. We identify a design space, touch points with other parts and the forces it needs to sustain. It calculates an optimal, manufacturable geometry, often resulting in free-form, spaghetti-like shapes.”

Free-form geometries enable functional integration, whereby one part replaces several legacy parts. If one printer can also make any part, AM reduces potential supply-chain disruptions and can rapidly produce replacement parts.

“Parts can be repaired by machining away damaged sections and using AM to deposit new material,” Pérennou says. “AM is well-suited to rapid prototyping, its first application in many industries. It can also produce jigs, fixtures or tools to support production.”

Boeing uses AM to create better-quality parts. Previously, heat exchangers for fixed-wing UAVs were made by brazing hundreds of individual fins to a face sheet. Now, AM can produce one continuous part to which fins are already integral.

“Every secondary joining operation introduces the potential for a flaw, but AM removes all that,” says Orme. “When we put heritage brazed and additive heat exchangers in a vibratory test, only the heritage ones failed. Because heat exchangers with new, additive-only designs remove more heat, we can increase our electronics. We also use AM to replace castings with long lead times, where it produces better mechanical properties.”

Boeing’s earliest AM parts included polymer ducting for fighter aircraft. Polymer accounts for most of the 140,000 parts Boeing produces today, using relatively mature FDM technology from suppliers including Stratasys. Now senior director for aerospace and space at Stratasys, Foster Ferguson first encountered AM in the US Marine Corps.

“I was a battalion commander working on logistics,” Ferguson recalls. “We used Stratasys printers for maintenance and spares, producing impellers, fan wheels and radio and optics components. In defense, the localized manufacturing control inherent to additive is critically important.”

Stratasys counts Boeing, Lockheed Martin, Northrup Grumman, Space X and Blue Origin amongst its customers. Industrial processes for such companies require machine downtime to be minimized, while materials must be traceable.

“Stratasys worked with independent organizations to create a comprehensive testing matrix with data we publicly share,” says Ferguson. “We tested hundreds of coupons across different systems. We created a process control document, allowing customers to induct our process into their manufacturing. Our B-basis allowables dataset enables them to create build-files and manufacture parts then supports verification and certification testing.”

Boeing’s first steps in metal AM saw titanium wire technology used to produce spares for military aircraft, then commercial 787s. From 2016, it accelerated the adoption of powder bed fusion (PBF) to create components for drones, helicopters, fighter planes and satellites, including a small satellite product line built entirely by PBF.

But these parts are size-limited. PBF takes place in a chamber of metal powder which a laser irradiates to form cross-sectional layers around 50μm high. After each layer, the powder-bed descends by 50μm with most of the powder not used. Large amounts of surplus powder must be unloaded, loaded, kept clean and recycled to create a 500mm part.

“For larger parts, the powder-handling just gets too difficult,” says Orme. “That’s why we are continuing to invest in DED techniques including wire arc additive manufacturing. I’m betting on DED for some of our larger parts in the future.”

WAAM3D has produced two 6m (20ft) components – an aluminum alloy aircraft structure and an architectural cantilever beam. WAAM3D’s RoboWAAM machine incorporates the hardware and software needed to implement an end-to-end process and enables two WAAM techniques: plasma transferred arc (PTA) and metal inert gas (MIG).

“PTA establishes an arc between an electrode and the workpiece, then the wire comes from outside,” Martina explains. “MIG establishes an arc between the wire tip and workpiece, so the wire is the electrode.

“Typically, MIG offers higher productivity at the expense of control. PTA allows us to deposit multiple wires and combine materials in one geometry, or mix materials in situ to create new chemistries.”

PBF creates elaborate, highly accurate geometries, but is costly and slow. WAAM deposits material a hundred times faster, sacrificing fine accuracy to achieve macroscopic size. PBF can ingeniously lightweight parts providing lifetime CO2 savings, whereas WAAM reduces manufacturing emissions. While PBF wastes plentiful portions of powder, WAAM consumes only the wire it needs to deposit.

“We can save 90% of raw materials versus incumbent technologies and reduce emissions by up to 70%,” says Martina. “We eliminate all seven forms of waste according to the elegant Japanese concept of Kaizen. We save time with flexible capability which avoids

supply-chain congestion. We don’t over-produce to keep parts on shelves, because we can make spares quickly. We can repair damaged components to avoid scrapping and extend their lives.”

WAAM3D has produced proof-of-concept components including an aft pylon bracket mount in titanium 64 for the Airbus A320. This could support Airbus delivery rates hampered by titanium forging supply-chain challenges and halve waste versus conventional, subtractive manufacturing. A titanium 64 pressure vessel produced for Thales Alenia Space addresses different production imperatives.

“That project demonstrated efficient, cost-effective production with small batch-sizes,” says Martina. “We reduced manufacturing waste by 200kg per pressure-vessel with 40% cost-saving and 60% lead-time reduction, which reduces the potential for complex space projects to run over schedule. Conventional pressure vessels have three sections welded together, but we delivered the whole thing in one shot.”

Both components demonstrated viability and will enable the respective customers to advance to qualification to implement WAAM production. Meanwhile, WAAM3D is developing novel means to manufacture landing gear in the Airbus-led I-Break project. Martina sees growing use of AM for secondary and interior aircraft parts versus more limited adoption for safety-critical structures.

“We feel comfortably equipped to produce tertiary or secondary, non-structural components,” Ferguson says of polymer FDM technology. “Structural, load-bearing parts are a different conversation.

“We need to provide confidence to engineers, improve strengths between layer heights and demonstrate that built parts correspond to intended specification values. We want to test the actual article with in situ monitoring.”

In the US Department of Defense (DoD) where suppliers of 50-year-old platforms cannot always respond quickly to demand, Stratasys technology typically produces spare parts. In aviation, limited aftermarket opportunities mean future specifications offer the best chance of growth.

“Flying a WAAM fuselage part on the Eurofighter Typhoon would mean recertifying the entire airframe,” says Martina. “Because the cost of requalification prevents AM replacement parts being used, we hope WAAM will be considered for new platforms from the beginning.”

Martina describes a qualification pyramid for WAAM parts proceeding from baseline coupon tests to tests of representative geometries, components then assemblies of components. Fundamentally, AM differs from conventional manufacturing in making both a part and a new material, so that an imperfect process can produce imperfect materials.

“Every safety-critical part produced must be systematically inspected,” says Pérennou. “It means both dimensional inspection and inspecting for material defects like porosities, using an optic system with laser or structured light, or computed tomography to inspect the inner channels of bionic geometries.

“Of course, expensive parts which consolidate 70 conventional components need inspecting using non-destructive testing.”

Pérennou believes statistical process control could reduce the need for costly, time-consuming testing and inspection. It means monitoring process variables, for instance, humidity or oxygen levels in the powder chamber, which may correlate to deteriorations in material quality.

“Statistical analysis aims to anticipate problems,” he says. “In a well-controlled process where those values stay within your process window, you know parts are good without inspecting each one. But small AM production runs with fewer parts make accurate analysis harder.”

AM parts can differ from their designed geometries due to distortions during printing. Hexagon’s compensated mesh software compares the nominal and actual geometry of a printed part, then pre-deforms the tessellated STL or mesh sent to the printer to compensate for process-associated distortion. Since distortions differ between prints, pre-compensation is iteratively tuned through simulated prints before a physical part is produced.

“For Boeing, a stable process is the crux of everything we do,” says Orme. “It means the part we get today is the same as the part tomorrow, or in five years’ time. We’ve doubled down on understanding processes, repeatability and the reliability of machines. Consequently, we find the performance of our parts exceeds conventional manufacturing.”

Last year, Boeing established an internal Qualification Review Board comprised of experts on different aircraft types and additive manufacturing. It must approve a qualification and certification plan for design, analysis and manufacturing to validate that all external regulatory requirements are met.

“Qualification and certification requirements are different for every part,” says Orme. “It depends on the part and the regulator, which may be the FAA, the Navy or NASA,” says Orme. “Defense products follow the processes for any structural part, then additional documents focused on a controlled additive process.”

While most proficient in aluminum, titanium and nickel-based alloys, Boeing would like to add a high-temperature, high-strength material to the narrow palette offered by AM today. Contrary to early hype, Pérennou believes AM will never completely replace conventional manufacturing.

“It’s simply another process with its own specificities,” he says. “For customized parts, or complex functional integration, AM is a good fit. Where weight is critical, you can really leverage its design freedom. But if you need to make a million screws, you should just cold-form them as usual.”

In 2022, Boeing opened the Boeing Additive Manufacturing Fabrication Center at the heart of its Fabrication Division in Auburn, Washington.

“People assume it’s a print shop where designers send drawings for us to print,” says Boeing’s Melissa Orme. “But it’s much more than that. It’s a center of engineering, design, analysis, manufacturing, testing, post-processing, assembly and delivery. Our multi-disciplinary team does everything there.”

A 32,000ft² metal powder-bed print room houses eleven metal printers, each dedicated to a specific titanium or aluminum alloy material system to avoid cross-contamination. A 4,500ft² polymer print room contains thirteen fused filament fabrication machines that print both tooling and parts.

“Our Large-Scale Additive Manufacturing machine makes tools and fixtures – for instance, a wing spar lay up mandrel,” says Orme. “It’s an enormous machine with a 20ft print bed, which is exciting for people to see.”

The Boeing Additive Manufacturing Fabrication Center (BAM) also features traditional machine assets, since every AM part must be machined at points of attachment to other parts. BAM has mechanical and metallurgical laboratories and comprehensive testing equipment.

“Every production part comes with witness coupons,” says Orme. “Each satellite antenna we make will have three fatigue and three tensile coupons. We measure for strength and dynamic durability. We have tensile and fatigue fixtures and microscopes to examine microstructures.”

The resulting data updates a database that enables Boeing engineers to monitor the health and stability of systems. It uses technology from several suppliers and watches the market for machines offering new capabilities.

“Creating the database for a new machine is expensive, so we have to ensure it provides value by addressing a specific problem where the current technology lacks,” she says.

Finally, AM parts are assembled with fasteners, helicoils or nut plates ready to attach to end-products.

“It’s an end-to-end process,” Orme concludes. “But the most important part is the up front engineering design and analysis and creating a test plan.”

In 2022, the NASA’s Artemis 1 mission saw the Orion spacecraft complete an uncrewed lunar orbit as a prelude to crewed Moon missions. Onboard Orion were 300 additively manufactured parts.

“Space applications use low-volume, unique designs and a variety of iterations,” says Foster Ferguson, senior aerospace and space director, Stratasys. “We give designers the freedom to create geometries which consolidate housings, reduce weight or protect electronics.”

Orion features a 1m diameter outer docking-hatch developed by Stratasys and prime contractor, Lockheed Martin. It uses Antero, a Stratasys-developed polyetherketoneketone (PEKK) high-performance polymer with low outgassing properties and electrostatic dissipative (ESD) capability, meaning it requires no additional ESD coating.

“The Antero is filled with carbon nanotubes to provide an ESD component,” Ferguson explains. “Other ESD materials do exist. But additive enables much faster design, integration and manufacturing.”

NASA’s Commercial Lunar Payload Services 19B mission will test Antero-based coupons on the lunar surface. These are the ESD Antero 840CN03 used on Orion and Antero 800NA, which shields against gamma and x-ray radiation.

“Antero 800NA is a PEKK-based polymer filled with tungsten to provide radiation shielding,” says Ferguson. “Sending materials to the lunar surface will help us understand their performance in an environment with dramatic temperature swings, no atmosphere and pressures which can lead to outgassing.”