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 with Airbus and Boeing. 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's Comac C919 and Russia's Irkut 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.”