Less manufacturing waste and fewer design constraints could enable a resurgence of metals in aircraft
“Metallics are dead, long live metallics!” This could be the rallying call for a manufacturing domain that is reinventing itself in the wake of a dramatic loss of ground to composites.
In their drive for lower weight and higher performance, where metal is used, aircraft manufacturers have moved away from traditional aluminum sheet-metal fabrication to computer-controlled machining of parts from lighter, stronger alloys. But both metals and machines are expensive. The finished part can weigh a fraction of the raw material from which it is cut, incurring high costs in machine time and waste metal.
So industry is developing ways to pre-form the raw material into shapes that are close to that of the final part, dramatically reducing the metal and machining needed, and significantly cutting manufacturing costs and lead times. Various methods are being developed to join simple shapes to create “near net-shape” pre-forms for complex components, including laser-beam, friction-stir and linear friction welding.
Additive manufacturing, where metal is deposited layer by layer to build up complex shapes, also is gaining ground, including in creating pre-forms that require minimal finishing by machine and in adding individual features to parts that would otherwise demand complex and costly machining from solid metal.
Investment in advanced metallic manufacturing technology reflects the reality that airframes and engines will use a mix of composites and metals for decades to come. “Our portfolio is split pretty evenly between composites and metallics, and between aerostructures and engine components,” says Rich Oldfield, director of technology at GKN Aerospace. “The drivers are similar across metallic products in airframe and engines, and the basic driver is the buy-to-fly ratio.”
Buy-to-fly is the weight ratio between material purchased and finished product. “In an extreme case, we do not use 90% of the metal,” he says. “We can recycle that, but these materials are expensive to buy and the value of the recycled material is lower, so we lose value. Also, the part spends a long time on the machine to remove all that material.”
Beginning with a shape that is closer to the final part can dramatically change the cost equation for metallics. “We get a double benefit if we go to near net-shape pre-forms. We buy less material and spend less time on the machine. As a result, there is a big drive to get to near net-shape manufacturing,” says Oldfield.
A second driver is the need to reduce mechanical assembly using advanced fabrication techniques such as fusion and friction welding. GKN acquired aeroengine component and subassembly supplier Volvo Aero last year, and advanced fabrication “is a core strategy in engine structures,” Oldfield says.
“The final driver is that advanced metallics promise a design freedom that is not constrained by the manufacturing process,” he says. “We can use materials in very different ways to produce design solutions with features that were not possible before, for weight, fluid flow, aerodynamics or loading.
“With additive manufacturing, we can put metal directly where it is needed. We can build parts inside out, to create geometric shapes that were not possible, or are very difficult, with other processes. Where the design philosophy was constrained by manufacturing, we now have the freedom to create features that provide greater benefit.”
GKN is pursuing several approaches to producing near net-shape parts for airframes and engines. One is to build up an engineered pre-form by depositing metal. Another is to join simple metal shapes together to approximate the final part, using either friction-stir or linear friction-welding (LFW)—both solid-state processes that fuse the metal without melting it, so preserving its mechanical properties.
“We are quite a long way down the road,” says Oldfield, noting that LFW is already used for some engine parts. So far GKN's focus is on titanium, use of which has grown dramatically as it is an enabler for composites in aircraft, avoiding the corrosion that results when carbon fiber is attached directly to aluminum.
The U.K.'s Thompson Friction Welding is working with GKN and others to move LFW into full-scale production. The process involves forcing parts together and oscillating them until the metal fuses. The technique is being used to attach blades to disks to produce titanium compressor blisks for aeroengines, and is being applied to wing ribs and other structural components, says Simon Jones, global aerospace sales manager.
“Titanium is expensive and takes a long time to machine,” he says, adding that it can take days to cut a blisk from solid metal. “We are now doing work on the engine hot end, with nickel- or Inconel-based alloys, where it is even worse. Inconel can take multiples of weeks to machine.” To produce a compressor blisk, near net-shape blades are joined to a disk by LFW. “The process takes 3-6 seconds, and machining takes minutes. We can join 22 blades in a couple of hours,” Jones says.
The first airframe structural components produced using linear friction welding are expected to fly within the next 6-12 months. LFW is used to join simple shapes, cut by waterjet from aluminum or titanium billet, to produce near-net pre-forms called tailored blanks. These are then machined to final shape. Buy-to-fly ratio can be cut to 2:1 from an average of 10:1 and an extreme of 100:1, he says.
Final part cost can be reduced significantly. “What we see in titanium is that what you save in material cost you save in post-processing as well. With nickel-based superalloy, for $100 saved on material you save $500-plus on machining,” he says. Because machining time is so expensive, Jones argues, “you don't need to make as many parts as you might think to justify the cost of an LFW machine.”
At Volvo, meanwhile, GKN is using laser-beam welding to build up complex engine structures by joining together simpler components. “The benefit from a cost point of view is that we can bring simple castings and forgings together at near-net shape and avoid huge, expensive castings from which we have to remove a lot of material,” says Oldfield. “A key strategy is to develop a flexible manufacturing approach where it is all done robotically.”
Efforts to apply similar fabrication technology to airframes are in early stages, with research work underway on a welded engine pylon. “Our objective is to leverage capabilities developed on the engine side into airframe applications, and so bring another level of automation and integration to fabrication,” he says.
Another near-net-shape technology with the potential to reduce mechanical assembly involves the explosive forming of complex curved components. This enables contoured panels previously fabricated from sheet metal pieces to be replaced by a single part machined from a solid plate that has been formed to shape by explosive force.
Netherlands-based 3D-Metal Forming is commercializing explosive forming technology originally developed by Dutch R&D organization TNO. Initial applications have been in architecture and the energy industry, but the company is moving into aerospace through projects withand . A subscale nose panel, with structure and cockpit windows, has been produced for Airbus. A full-scale part will be produced this year.
“It's amazingly simple,” says Marcel Oud, managing director. The metal plate is placed on a one-sided die and a charge of high-energy material is positioned above it. The package is lowered into a water tank and ignited at a depth of 4-5 meters (13-16 ft.). “The energy of the explosion transfers to the water, which pushes the metal into the die,” he says.
The key has been perfecting the ability to simulate the explosion, determine the spring-back of the plate and correct for it in the die. The formed double-curvature, near-net-shape plate is then machined on both sides to produce a single part at “significantly lower cost” than an assembled component, Oud says. 3D-Metal Forming is looking at developing the process to form complex sheet metal parts and produce lighter formed, rather than machined, Invar tooling for composite parts.
Where sheet metal is still used, technology to replace hazardous chemical milling with machining of fuselage skins to reduce weight is gaining acceptance. France's Dufieux Industrie has supplied six of its milling mirror systems (MMS) to Airbus, with a seventh in construction, and further contracts have been signed in Italy and Russia. Developed with Airbus, the MMS comprises two horizontal high-speed machining centers mounted face to face. One side supports the part and provides the reference while the other side removes the metal. The part is clamped around its periphery to a palletized frame that provides access from both sides and allows one part to be prepared while another is being machined, says commercial director Jeff Line.
Machining can cut cost and cycle time by at least 50% compared with chemical milling. Dufieux has conducted trials with titanium panels, Line says, and is developing the capability to machine smaller parts, such as wing leading edges. Spain's MTorres, meanwhile, has supplied one surface milling machine to Airbus in Germany, with a second to be delivered in a few weeks. Three more are in backlog for other customers to machine fuselage panels for China'sand Russia's MS-21.
But the technology that perhaps has the aerospace industry most excited by its potential is additive manufacturing (AM)—building parts layer by layer rather than removing metal cut by cut. “We are really just entering the phase where additive manufacturing is beginning to be viewed as a real capability for production parts,” says Oldfield. “It has been a long time in its creation, but is becoming a credible production solution.”
GKN sees applications for three approaches to AM. One is large deposition technology to create pre-forms by feeding wire into a laser or electron beam to melt the metal and build up the part. Another is fine deposition technology to add local features such as bosses or pads to a simple forging using wire-fed laser melting. “This is in production in Sweden for parts of the [Trent] engine,” Oldfield says.
The third approach is powder-bed additive manufacturing, which enables complex parts to be “printed” using a laser or electron beam to melt layer after layer of metal powder. The three approaches have different speeds, accuracies and scales. “Large deposition can produce parts on a big scale,” says Oldfield. 'With powder bed, you can pick up the part, it is good for high-value, high-performance parts that you want to optimize to a high degree.”
Powder-bed machines are restricted in size, and there are issues with ensuring consistency, but they are “completely flexible,” Oldfield says. Ultimately, this freedom will result in radical new approaches to designing parts. “Imagine a manifold with material only where there is flow—and no housing, just attachment points. There will be massive cost savings, and weight savings at the same time,” he says.
is stepping up its additive manufacturing efforts to support future engines including the GE9X and . The effort is led by Morris Technologies, a small prototyping company specializing in AM that was acquired by GE last year. “We want to take it out of the model shop and into the production shop as quick as we can,” says GE Aircraft Engines President David Joyce.
Initial parts have been made using direct metal laser melting for the first Leap-1, assembly of which began in April. Laser melting is used to build up intricate swirl passages in the combustor's fuel nozzles. “To get the low nitrous oxide [emissions], these passages are very byzantine, and complicated if you have to braze them,” says Joyce, adding that laser melting can create a more architecturally complex part that is lighter and easier to build.
A new joint venture with Parker Aerospace will focus on AM processes for fuel systems. Beyond this, Joyce says, obvious targets for applications include turbine blades and tip repairs. “The big challenge is what material systems we can use and how we can use them,” he says. “I don't see it being used for big disks, but definitely for blades and repairs, particularly on blisks, as well as the manufacture of fuel nozzles and brackets.”
But in the long term, Leap chief project engineer Gareth Richards believes, the concept could be used for far larger parts. “Additive manufacturing is the downfall of subtraction manufacturing. Ever since the Industrial Revolution, we've had to begin with a big block of metal, and in the future it won't be like that,” he says. “We're already considering how to make compressor cases additively. It might take weeks to build up, but that's still shorter than the current 18-month cycle.”
Manufacture of such components traditionally includes a lot of “white space” between times when the part is being worked on. “With additive, you don't have that. You don't have any forgings, sheets, bars and tubes. You don't have to carry all these parts. This is the vision,” says Richards, who adds that “25 percent-plus of the engine could be made using this process within the next number of years.”
Engine manufacturer Pratt & Whitney also is moving quickly to embrace AM, partnering with the University of Connecticut to set up a research center with an initial $4.5 million investment and another $3.5 million to follow over the next five years. “We are looking at additive manufacturing to wring out development lead time,” says Engineering Vice President Tom Prete. “We see it offering a great improvement in cost and time to market. It's fast, lean, efficient and green—because its use of materials is less wasteful.”
AM is already being used to make parts for theseries of geared turbofan engines, says Prete, without specifying what they are. He describes them as “simple” but says in the future the process will be used to make more complex parts such as manifolds and, in some cases, to design them differently.
“It opens up a whole new game plan in terms of design,” says Eli Liechty, manager of GE's Additive Development Center. “We're getting our engineers to think about designs that are more organic-like lattice structures. We're looking at things like coral and bones, and how we can model components after nature in some cases.”
AM will be a key technology for metallics in future aircraft, but it is not a panacea, cautions GKN's Oldfield. “It will be good for complex components with high function where we want to make a big performance step with geometries and features we cannot do in parts today.”
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