10/08/12 Excerpt from The Manfucturer
The best engineering solutions for specific parts of the world’s most advanced fighter aircraft have inadvertently created a very British supply chain. Will Stirling reports.
When plans for the F-35 Joint Strike Fighter (JSF) were first drawn up about 20-years ago, the prime contractor Lockheed Martin had a multitude of engineering riddles to solve.
To suck weight out of certain components designed to operate in extreme temperatures and pressures, Lockheed turned to BAE Systems. BAE’s Samlesbury facility in turn sourced a manufacturing solution from a British metal forming specialist.
Both the structures for the F-35 JSF made at Samlesbury, the nozzle bay doors and heat shield, need to be very light, strong and heat resistant. On the short take-off and vertical landing, or STOVL, variant of the F-35, the nozzle both directs exhaust gases from the jet engine to provide vertical propulsion, and helps the aircraft perform tight manoeuvres by changing the direction of exhaust in flight.
The operational environment is harsh, as exhaust gases are several hundred degrees Celsius.
About 12-years ago BAE Systems did an exhaustive investigation to select the most appropriate technology for making these parts. The winner was super-plastic forming and diffusion bonding, a technique used in the aerospace and luxury automotive industry to create very light, very strong monolithic structures. It is not a brand new technique – SPFDB was developed through the 1990s and 2000s. But engineers at BAE Systems Samlesbury worked with the supplier, Wakefield-based Group Rhodes, to design and manufacture optimised presses that could perform a three-stage process perfect for this application.
Perfection in the process
Stage one is to diffusion bond two inner sheets of titanium to each other to form what is called a core pack. The sheets are heated in an inert argon atmosphere to in excess of 900°C and then pressed together at high pressure. “By a process of solid state atomic diffusion the sheets join and achieve a homogeneous bond of parent metal strength’” says Metallic Materials Technologist at BAE Systems Samlesbury, Howard Price.
Prior to bonding, an yttria stop‑off – a chemical buffer – is placed at the interface between the two sheets by a silk screen printing technique. “This stop-off layer enables a hollow structure to be subsequently formed by inflation and superplastic forming of the core – think of inflating a lilo – within a gas‑tight sandwich of external skins,” adds Mr Price. The ‘superplastic’ in the name is the forming of the structure by inflation using gas. The process requires precise control of the pressure, flow rates and gas purity in three separate argon gas delivery lines. “For the process to be successful the argon gas used must be extremely pure (very low levels of oxygen and nitrogen). There is also the need to integrate a vacuum system and to achieve very exacting temperature control within SPF tools of up to 20 tonnes in weight,” adds Mr Price.