Machining aircraft parts out of giant metal billets is time-consuming, wasteful of material and very expensive – but still, for the most part, necessary. Despite significant advances in the past two decades, autoclave-cured composite parts remain usually more expensive to build than metallic equivalents. Additive manufacturing – so-called 3D printing – may have revolutionised the making of plastic parts but has barely touched metallic aircraft components.
The major exception that proves that rule comes from CFM International. Every one of its Leap-1A engines powering an Airbus A320neo includes 19 fuel nozzles with delicate internal channels. Each of these cobalt-chromium parts is built-up in thousands of layers only 20 microns thick, deposited by a selective laser sintering machine, a type of 3D printer. These fuel nozzles illustrate the appeal of additive manufacturing: by building shapes in fine layers it is possible to make intricate shapes that would be extremely difficult – if not impossible – to make using the traditional “subtractive” techniques of casting, forging and machining.
3D printing can be another way to make existing parts, but mere substitution misses its greatest advantages. Rather, as illustrated by CFM’s Leap fuel nozzles, when engineers start from scratch exploiting the freedom of 3D printing, there are huge potential gains in performance and weight reduction.
Drawn by nature: 3D printing may pave the way to new structural design concepts promising dramatic weight reduction
Airbus
But in practice, for metallic parts, the drawbacks of existing additive manufacturing tools have almost always outweighed the benefits. Manufacturers complain that 3D printers today are still too small and too slow. Metal parts also emerge from existing 3D printers with too many strength-sapping voids, requiring complicated post-processing techniques, such as hot isostatic pressing, to overcome. But the tide is slowly beginning to turn in favour of additive manufacturing for metals. Major investments in machines, design techniques and the composition of metal powders are beginning to yield results.
Airbus is taking a highly visible lead to bring the aerospace industry into a future of lighter-yet-stronger parts printed from metal powders.
The first step involved qualifying a variety of metal alloys designed to be deposited by 3D printers, says Peter Sander, who heads emerging technologies and concepts at Airbus. A titanium alloy was qualified last year. An aerospace-grade stainless steel will be approved for 3D printers this year. An aluminium alloy enters the gauntlet of qualification tests next year: “Over the next two years we’ll have the basic materials available, but now we have to learn how to design our parts.”
The critical step is for engineers to think 3D. In Airbus’s view, design will make the business case for transitioning metallic components from conventional to additive manufacturing. That is, a new generation of designs must be compellingly lighter, stronger and better performing – and only possible from 3D printers.
The company’s strategy moves beyond the conventional approach to applying additive manufacturing in the aerospace industry. Companies have tended to focus on the immediate benefits of transitioning classically machined parts to 3D printers. For low-volume or very complex parts, or prototypes, 3D printing saves the cost and time of making special tools, especially when it might otherwise be necessary to call on a string of outside suppliers. Indeed, in its early days additive manufacturing was often referred to as rapid prototyping. For example, Airbus has 3D-printed a 2m-long wing for small test aircraft out of materials combining plastic with aluminium, achieving reductions in lead-time for the raw materials and “brilliant cost savings”, Sander says. In 2012, Airbus notched up a first by printing a spare part for an out-of-production component.
Such functional purposes have created a niche that has turned 3D printing into a billion-dollar industry within aerospace manufacturing in the past five years, but it is only the beginning. The next step is to start designing parts using completely different design techniques, optimised to be made in an additive manufacturing environment rather than on a machine tool. One of the first places Airbus has turned for inspiration is nature, the original additive manufacturer.
“Now it gets a little more crazy,” Sander says. “With the 3D printing method, we are really able to copy nature.”
That helps to explain Airbus scientists’ recent preoccupation with certain plants, particularly a species of giant water lilies found in Northern Europe. Nature has produced shapes with strength-to-weight ratios that would shame even the most talented structural engineer; the honeycomb-like internal structure of a floating Danish water lily can support the weight of a small child, Sander says.
As a result, Airbus is developing a technique called bionic design. Instead of a set of stringers and frames arranged in familiar geometric patterns, Airbus is working on software algorithms that mimic the seemingly random layout of living things, such as the fibres of a tree or the internal latticework of a giant water lily.
Airbus’s futurists have imagined a sharp break from today’s aircraft designs. Using bionic design principles, an entire aircraft could be reshaped with a seemingly random lattice of primary structures, replacing the fixed frames, stringers, longerons and skins found in all metal aircraft since the early 1920s. If such a futuristic aircraft is ever manufactured, it will most likely be printed rather than machined.
But there are more immediate applications for bionic-inspired, 3D printed parts. In 2014, Airbus flew the first bionic metal bracket on a commercial aircraft, replacing a machined alternative with a structure both lighter and stronger, Sander says.
Airbus intends to test these design tools on primary aircraft structures, such as 1.8m-long spoilers for an A320. By then, Airbus expects 3D printers to be large enough to build an entire part from scratch in one sitting, Sander says. The part will be flown and tested on a research aircraft in 2018. If it works, the next challenge will be developing a production system of 3D printers that can match the delivery rates planned for Airbus’s single-aisle aircraft family.
More immediately, however, Airbus plans to deploy the bionic design concept inside the aircraft cabin. A new design for a partition between the galley and the passenger cabin replaces conventionally-machined aluminium with a 3D-printed structure built in 120 pieces: 116 of Scalmalloy – an Airbus-developed alloy of aluminium, magnesium and scandium – and 14 in titanium. The individual pieces are connected into a series of adjustable tension wheels scattered throughout the structure.
Bionic-inspired cabin partition: with 3D printers, Airbus engineers have new design freedom
Airbus
“We have to make sure the load distribution is harmonised, if you will, so if we put a force into it we don’t get an overloading because the tension in this wheel is too high,” says Bastian Schäfer, an innovation manager leading the project at Airbus.
The new design, unveiled in early June, reveals a spider-web-like spine for the frame of the partition, replacing longitudinal and lateral support structures. By adapting nature’s design techniques, Airbus says, the new partition weighs only 30kg, or 45% lighter than the current design. In the future, Airbus plans to experiment with more ambitious structural concepts for partitions. Instead of merely separating the galley from the passenger cabin, such interior structures could also carry a portion of the overall airframe loads, potentially allowing designers to shave weight off the skins, frames and stringers holding the fuselage together.
“Is that more complex? Certainly. Is it more costly? I don’t know. We’ll have to see,” Schäfer says.
Bionic design represents the next generation of additive manufacturing, but it won’t be the only application. In the near term, Airbus is developing business cases to transition metal components with multiple functions to 3D printers. As an example, Sander holds up a titanium fuel pipe for an Airbus A400M military airlifter. Today, a casting method is used to form an inner pipe. It is then encased in an outer pipe, which is designed to capture any leaks and re-direct the fluid back into the inner pipe.
“You need €400,000 [$441,000] to do the casting today,” Sander says. “But for €500,000 I can buy a printing machine. You need only one case and you have [the return on investment]”.
The key to successfully transitioning such parts to additive manufacturing is increasing the speed of the 3D printers. The Scalmalloy printer used to make the galley partition took 900h to complete all the parts. If that printing time could be cut to 200h, additive manufacturing would win the business case, Airbus believes. “Every year the printer speed doubles,” Sander says. “If you follow that theory – and it really works – you will have new business cases coming in every year.”
The same philosophy supports the business case for making the multi-function, titanium fuel pipe with a 3D printer, he adds. “So with this I don’t have weight reduction, but I have cost cutting by 50%. And the machine is slow today. So it takes around 40 hours to print this, but I only have three weeks and the part is done – and the lead time for the casting is more than half a year.”
Another application for 3D printing has been for prototype parts. To obtain casted or forged metals, lead times from suppliers can take months. As a result, testing advanced or even revolutionary concepts often takes place solely in wind tunnels or in computer-based simulations, rather than in real flight conditions. Introducing 3D printers for rapid prototyping can fundamentally change the cost and speed of testing flying prototypes.
As an example, Airbus recently displayed a miniature unmanned aircraft made using 3D printers. Under the company’s new test of high-tech objectives in reality (THOR) initiative, the 25kg aircraft will test a concept for using differential thrust to replace high-lift devices, such as slats and flaps. If an aircraft is designed with multiple engines, Airbus reasons that varying the thrust provided by the engines can prevent the aircraft from stalling on final approach, when the forward speed of the aircraft is slower than the wings need to generate enough lift.
“If you have to land an aircraft, you can have only 250km/h of forward speed, so you have flaps, spoilers and slats,” Sander says. “That’s a lot of weight, a lot of functionality and a lot of complexity. But could you have a lot of steerable engines to reduce the speed, and then make the ailerons smaller, for example? It could be a concept of the future. I’m sure it will come.”
Experiments like THOR are viable thanks to the speed and low cost of rapid prototyping
Airbus
The first THOR platform was printed last year and flew in November. Another 18 missions are planned in 2016 to test a variety of other advanced concepts, using a small fleet of miniature prototype aircraft. Airbus has set up an assembly line of 3D-printed prototypes in Hamburg to support the THOR programme.
“In the past you needed several engineers working on things for months, costing a lot of money,” Sander says. “Now you print this kind of aircraft in four weeks. You crash it or not. And in a few weeks you know the basic physics or whatever is needed.”
PLASTICS DRIVE BOEING’S 3D PRINTING PORTFOLIO
Before there was 3D printing, Boeing likes to remind people, there was the 787.
Any application of composite material is a form of additive manufacturing, says Mike Sinnett, vice-president of product development.
“It’s like the sexy new application that everybody likes to talk about. We have been doing it for a long time,” Sinnett says. “If you think about this [787] wing: that is 3D printing at its best. Nobody ever talks about it.”
Boeing has not revealed a roadmap for expanding the use of 3D printing in commercial aviation products, especially for metal components. But the company has an internal strategy that includes developing some metal components using 3D printers.
“If you want to take something that’s in the shape of a ‘T’, for example, right now you have a big billet and you hog it down so it’s in the shape of a ‘T’,” Sinnett says. “If you think of taking two flat plates, welding them together and eliminating all that material that gets hogged out in the process, that’s a good analogy for when I’m thinking about additive and titanium for big parts. I think there are lots of opportunities for eliminating waste in the system for big parts.”
Unlike Airbus, Boeing has not revealed timelines for qualifying the most common metals used in aerospace parts, including aluminium, titanium and stainless steel. But the company has clearly invested in 3D printing technology for metallic parts, including one extreme example.
Last October, Boeing subsidiary HRL Laboratories revealed a 3D-printed micro-lattice material funded under a contract from the Defense Advanced Research Projects Agency. Boeing claims the micro-lattice structure, being 99.9% air by volume, is the lightest metal ever produced. If brought into production, the HRL technology could be used to print ultra-light material for stowage bans and sidewalls inside aircraft cabins.
“To us the benefit of additive is you can get a lot more cost-effective manufacturing,” Sinnett says. “You can also get quicker manufacturing for highly variable or highly customised or prototype parts where you’re not going to make a lot of them.”
If there are other examples of Boeing-made metallic parts formed from 3D printers, the company has not acknowledged them. One supplier, CFM International, is printing cobalt-chromium fuel nozzles in every Leap-1B engine delivered for the 737 Max. But Boeing acknowledges only thermoplastic parts installed on aircraft made from 3D printers. Some 20,000 such parts are installed across the Boeing commercial fleet, Sinnett says. A typical example is the air ducts on widebodies, such as the 777 and 787.
“You think about the complex shape of air ducts,” Sinnett says. “Those are difficult to manufacture to start with.”
Boeing has also experimented with replacing metallic structures inside the cabin with lighter-weight plastics. When the company converted a Thomson Airways 757-200 last year into an ecoDemonstrator testbed, it was fitted with a 3D-printed aisle stand, which hosts controls and displays between the seats for the flight crew in the cockpit.
In reality, however, the opportunities for transitioning machined metal parts on commercial aircraft to 3D printed metal parts are still few, given the quality and speed of 3D printers on the market today.
“We don’t have a lot of application for printed material [with titanium] on a commercial airplane,” Sinnett says, “because typically the parts we use [for titanium] are big. It’s not something you could ever do at rate without a lot of capitalisation. It would be really tough.”
But the cost and waste of conventional manufacturing processes using metal is pushing Boeing to consider alternatives. Sinnett says that he guesses the average “buy-to-fly” ratio for titanium parts on Boeing aircraft is about 8 or 10 to 1. That means buying a 10kg billet of titanium to fly a 1kg finished part. The remainder is waste, although some can be recycled.
“My primary interest in [titanium] right now is in additive methods that allow me to reduce my buy-to-fly [ratio] because that gives me the most immediate payback,” Sinnett says.
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Source: Cirium Dashboard
