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.”