Boom Supersonic prepares for flight with metal additive manufacturing

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The XB-1 is Boom Supersonic’s demonstrator aircraft, unveiled in October 2020, that brings the return of supersonic air travel closer to reality. Scheduled to begin flight testing in 2021, XB-1 is a one-third-scale demonstrator for the full-size Overture airliner slated for launch in 2025.

At the start of the project, Boom Supersonic’s design and engineering team studied additive manufacturing (AM) to produce some of their more complex part designs.

“There are many reasons to choose AM technology over others,” says Boom engineer Byron Young. “A lot of the time and effort required to design an aircraft is spent on the joints, on the interfaces between components. By designing directly for AM, we can reduce the number of parts and gaskets, which also reduces time and effort. And the consolidation of parts significantly reduces the weight.

Geometric freedom

Many of Boom’s 3D printed parts channel air using intricate vanes, ducts and louvers. Some of the air flowing through these parts exceeds 500 ° F.

“If fast moving air hits it, we care about that surface from an efficiency and performance standpoint,” says Byron. “So when designing these parts you usually start with airfoils and then cut, round and thicken the surfaces to create the solid part. The resulting parts are very complex – they absolutely had to be made by AM.

BIKE3d Applications Engineer Gene Miller worked closely with design engineers from Boom Supersonic and Duncan Machine Products (DMP), the supply chain partner who handled printing and post-processing.

“The unique types of boom geometries created to direct the flow, with an emphasis on weight savings, could not be achieved with sheet metal or cast or any other way,” says Gene. “To reap the benefits of complex design and weight reduction, the only option was to do it with metal AM.”

Having worked with VELO3d on test parts in 2019, the Boom Supersonic team chose the company’s next-generation powder bed laser fusion (LPBF) technology to produce:

  • Printed titanium manifolds for the Variable Bypass Valve (VBV) system that routes air released by the engine compressor to the aircraft’s external molding line (OML)
  • Exit louvers for the environmental control system (ECS) which cools the cockpit and systems bay
  • Louvers directing the secondary purge flow from the central inlet to the OML
  • NACA conduits and two pieces of bypass flange. NACA ducts are often used in high speed aircraft to capture outside air and channel it into the aircraft to cool engine compartments.

All parts have been printed on the BIKE3d Sapphire system. In almost all cases, the Sapphire was able to print parts directly from Boom’s CAD data, preserving the original design intent.

“We used our system’s Flow pre-print software to add structural ribs to the thinner walls of the NACA ducts that needed to be stressed,” says Gene. “But for the most part, the other components are all printed as is, without compromising on design.”

Byron de Boom was impressed with Sapphire’s ability to accurately produce extremely thin-walled parts. “The Sapphire system allowed us to print walls as thin as 20,000 (0.02″ or 750 µm), with a surface finish that did not require additional machining in most cases.

The high aspect ratio made possible by the VELO3d the machine’s non-contact coating system – which dispenses each new layer of powdered metal to be fused by two lasers – was another plus. To eliminate mass, the vanes of the center inlet purge louvers were debossed and the parts were designed with thin walls along long spans.

“Because our technology can print this very high aspect ratio in this type of design, we didn’t need excess material for strength inside the structures, and we were able to develop these vanes from drives very high without any interference from the coater, ”says Gene.

Titanium

One of the big challenges of the project was to work with the titanium material chosen by Boom for the 3D printed parts.

“One positive aspect of using titanium is the temperature permissible material,” says Aaron Miller, DMP additive manufacturing engineer (unrelated to Gene). “There is less loss of strength at high temperature compared to aluminum or carbon fiber, and its strength-to-weight ratio is higher. ”

But lightweight, extremely heat-resistant titanium can be delicate and difficult to work with, no matter how it’s made. If titanium is cooled too quickly, it becomes brittle and tends to crack.

Titanium parts can be made in the conventional way by casting, which has a slower cooling rate to prevent cracking, notes Gene. But the extremely thin walls of the aircraft’s hardware designs would have been nearly impossible to sink.

“It’s really one of the driving forces behind using 3D printing for these parts, because we can print large sections of thin-walled titanium without the high scrap rate of cracked castings,” says Gene.

He recognizes the learning curve. “Boom designed a family of parts that was new to us, really pushing the boundaries of weight reduction and thin wall geometries, and we had a lot to learn about printing these components from titanium. and what to expect from the physics of printing. How will it move? What can you print without supports? What areas needed help? ”

Two 1kW lasers trace the geometry and melt the titanium powder into an almost neat-shaped piece, in this case, for the XB-1’s Variable Bleed Valve (VBV) fuselage manifolds.

Images courtesy of DMP

Three parts for the VBV system ready to be unpacked in an inert sapphire fabrication chamber.

Process control is essential

BIKE3dThe AM process optimizes print parameters and sequences to produce tough titanium parts.

“This reduces the amount of internal stress in the substrate when material is deposited in the Z construction direction,” explains Gene. “It decreases the possibility of cracking by alleviating the internal stresses formed during cooling.”

Quality control is integrated throughout construction, starting with Flow pre-printing software, executed through the Sapphire system and validated with Assure quality assurance. Pre-construction machine calibration is done with one click, automatically checking laser alignment, beam stability and powder bed quality. Ongoing metrology monitors key measurements and reports anomalies. Complete construction reports for all parts are compiled for future reference.

After Boom’s titanium parts were 3D printed, they were cut from the build plate with sawing or wire EDM. DMP machinists found post-processing to be relatively straightforward.

“After we cut the build plate, we didn’t have much to do in terms of post-machining other than minimal support shrinkage,” says Aaron. “You don’t have small supports in small crevices or hard-to-reach places because SupportFree technology eliminates the need for them. The parts come out of the Sapphire system almost finished, requiring just a little manual work with a screwdriver or grinder. We also ream pilot holes (on larger pieces to be assembled) with a milling cutter to make sure they are the correct size. It depends on the part, but probably just half an hour of machining per part, which is okay.

Close-up of airfoils, as printed.

Image courtesy of VELO3d

The part geometries presented an additional challenge when creating devices to hold the parts during finishing. Aaron adds, “There are almost no perfectly flat or round surfaces on an airplane, which makes them difficult to grip. But we used the CAD models of the quickly designed and 3D printed custom plastic parts and fixtures (on a separate FDM printer) that were properly squared or rounded so that we could grab them with our finishing tools.

The partial finish of the machine was tested with a profilometer, recording around 250Ra on average.

“If the customer wanted to upgrade to 125Ra, it would only take a few minutes with a vapor barrier to get there,” says Aaron.

The finished parts have been heat treated and / or hot isostatic press (HIP) to improve fatigue resistance. “Supersonic flight introduces a number of different phenomena and stresses that you don’t typically see with conventional air travel,” notes Gene.

“The main applied forces are usually not pressure loads resulting from breaking the sound barrier,” adds Byron. “In many cases, this is an induced stress caused by the overall structure of the aircraft flexing around your part. Parts with different coefficients of thermal expansion mounted to each other can produce significant stresses. Designing these 3D printed parts to be very thin and flexible can actually alleviate some of these issues. ”

Try, learn and repeat

The companies that have successfully produced the 3D printed parts for Boom Supersonic’s XB-1 supersonic demonstrator have learned a lot from their collaboration. The Boom team found that AM was more complex than they imagined, but it could also meet its original design intent. Duncan Machine Products significantly developed its expertise in 3D printing and eventually purchased a third Sapphire machine. “We get a lot of new business because of our additive manufacturing capabilities,” says Aaron.

Supersonic boom

Duncan Machinery Products

BIKE3d


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