Super-forming a British solution for the F-35

Posted on 8 Oct 2012

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.

One of Joseph Rhodes' SPFDB machines. Metal is superheated and formed to precise, hollow shapes to save weight in the F-35 parts

Numerous advantages

Superheating and pressurising the metal tooling used is a destructive process. Thermal expansion of the tools was in excess of 40mm in length, a major source of damage to press and tooling. “Replacing platens [flat metal plates] and tooling was costing BAE Systems approximately £1 million every two years and many weeks of lost production time,” Price says. “The new press enables tools to be heated from cold without the problems associated with thermal expansion damage.”

The process is totally automated. This is a big advantage in the manufacture. Before the new press, incumbent equipment was incapable of automating many aspects of the process. “This resulted in a disproportionate demand for highly skilled engineering support during the manufacture of each door,” says Price. “The new press integrates and fully automates the entire process enabling manufacture to be scheduled on a 24-hour a day basis using a rota shift system, while eliminating inevitable variations in quality associated with complex human interventions. Cost, quality and rate are all enhanced.”

The SPFDB machine supplied to BAE is a highly developed and optimised version of previous machines. “The challenge we had with this application was a very large physical component using extremely thin sheets. It is essential not to over- or under-inflate the component,” says Group Rhodes’ technical director, Peter Anderton.

The press

The contract for the design and supply of the presses was subject to a formal competitive tender and other, non‑UK companies participated. BAE Systems selected Group Rhodes. “[We saw that] the final design would need to evolve to accommodate the production process, which was still maturing. There would also be opportunities to optimise both the press design and the final manufacturing process to make best use of the innovative solutions jointly being developed by both Rhodes and BAE Systems,” says Price. “The proximity of Wakefield to Samlesbury was an undoubted bonus in a complex contract such as this.”

Group Rhodes’s metal forming division, Joseph Rhodes, custom-manufactures all its diffusion bonding and SPFDB presses to meet specific customer needs. Titanium is the preferred material used by the aerospace industry, which consumes about 75% of Rhodes’ output. The company has supplied different presses to Samlesbury, a 3,000 tonne press to manufacture the canard wings for the Eurofighter Typhoon and two 2,000 tonne presses, one for the JSF parts and the other for Typhoon.

“Everything is designed and manufactured in-house, including all structural fabrications and machined items,” says Anderton. “We buy in some exotic materials for the working surface of the press patterns – when we’re applying 2,000 tonnes to a tool at 900°C, the metals are beginning to soften.”

Last year, Group Rhodes shipped the biggest press of its type to China, costing £2 million, and this year, they're shipping an even bigger one
BAE Samlesbury uses a Rhodes' SPFDB machine to form nozzle bay doors and heat shield for the F-35 fighter

Rhodes works closely with Cronite, a UK company in Crewkerne near Taunton, to make a nickel chrome alloy casting for the press. The hydraulic valves and pumps come from Bosch Rexroth, and Siemens supplies the computer controls – but the rest comes from UK companies, says Mr Anderton. The software is also now developed in-house by Rhodes’ ICT technicians with help from Siemens.

“The most tricky part of the manufacture is engineering the two platen press surfaces which apply the tonnage,” says Anderton. “They have to be flat and parallel within 0.2mm which is very difficult. Believe it or not standard machine tools are used here, but we rely on the expertise of the operators [to achieve the accuracy].”

Rhodes also works closely with Sheffield Hallam University to develop the best insulation materials to support the press platens.

Good for UK plc

The F-35 has been good for the UK supply chain. Group Rhodes has sold SPFDB presses worth £13 million to BAE Systems, which has boosted business for Rhodes’ supply chain.

And the company exports these presses all over the world. Rhodes has opened an office in Bangalore, India, and one Chinese customer is about to take delivery of its seventh and last SPFDB machine. More interest is on the horizon in Mexico. Does the F-35 contract help Rhodes with its overseas sales? “We cannot mention customers by name, but saying these presses are commissioned by large European defence contractors wins us some kudos.”

Orders for about 3,000 F-35s have been confirmed by Lockheed Martin and the work at Samlesbury will provide jobs, directly and indirectly, for the next two decades. Beyond that it is hard to forecast what the next military platform will require. For now the F-35 is a United States jet having a very profound effect on some British manufacturers, helping to fine-tune engineering processes with global markets.