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Rhodes Interform (Wakefield, West Yorkshire, England) has developed a new process that enables large monocoque components, particularly those produced by super plastic forming (SPF) from very thin material, to more accurately retain their shape on cooling.

Super Plastic Forming (SPF)

One of two 2000-ton SPF presses, complete with semi-automatic tool and component loading equipment, manufacturing latest-generation aircraft panels.

SPF is a process where titanium or aluminum sheet is formed at elevated temperatures of 1000°C and 500°C respectively into complex shapes utilizing pressurized gas as the forming medium. At the super plastic phase of the material, very large extensions, up to 2000%, are possible. This allows certain complex shapes to be hot formed that would otherwise be impossible to achieve by modern cold forming techniques.

In circumstances dictated by the component design, superplastic forming occasionally operates in conjunction with diffusion bonding (DB), a process that utilizes similar elevated temperatures and pressures to create a fully homogenous molecular joint.

Overview of the Process

The manufacturing process for monocoque components involves diffusion bonding of multiple layers of titanium sheet at selected points, and then superplastically forming them using argon gas to inflate the sheets into the shape of a hollow die. This process is extremely temperature and pressure sensitive. At the point the ambient temperature argon forming-gas is admitted into the heated component, the gas expands, increasing its volume, and hence, its pressure, if not adequately controlled.

One of the most critical phases during the press cycle, is when the component has been formed by gas inflation and then needs to be cooled prior to extraction from the press. At this stage, very low-pressure argon gas is passed through the component (purging) to ensure no oxidization of the internal faces takes place while it is at elevated temperatures. If the purge gas pressure is too high, the component will over-inflate and lose its shape as soon as the dies are opened. If the purge gas pressure is too low, the ambient air pressure will implode the component.

Achieving the correct balance is further complicated because the cooling component reduces the gas temperature/volume, and thereby lowers the associated internal gas pressure. With operational experience, compensatory gas pressures can be fine-tuned to allow for such internal variables, although external factors have traditionally been more difficult to control.

One such external variable is ambient atmospheric air pressure, which can significantly affect the final shape of the component and take it out of tolerance. In the case of larger components with relatively thin membrane sheets, this can become a major operational problem.

Existing Technology — Applications and Challenges

Through 35 years of continuously working with blue-chip aerospace companies around the world and addressing their individual requirements, Rhodes Interform has developed a flexible, accurate, gas management system to improve its range of high-temperature superplastic forming presses.

Their current multiform line gas circuits rely on state-of-the-art, electronically controlled, gas pressure reducing valves. These valves, one in each form line, have onboard proportional integral derivative (PID) controllers to achieve extremely accurate pressure regulation and control to within 0.1 Bar. This level of control is more than adequate for the vast majority of superplastic formed components.

This 1200-ton SPF press manufactures aircraft panels for the Chinese aerospace sector.

More recently however, particularly to accommodate the continuous drive to lightweight aircraft, components are being designed with very thin membrane aerofoil skins and a large surface area. The skins are thus extremely susceptible to very small changes in forming pressure.

One recent example was an aerospace component comprised of four layers — one relatively thick lower layer to obtain structural stiffness, two intermediate layers to offer sectional stability, and a relatively thin upper layer that formed the aerofoil surface. The challenge was how to prevent the relatively thin upper layer from distorting during the cooling process.

Although the part appeared to form accurately and consistently, the structure always failed the final critical dimensional checks. By a process of elimination, mainly derived from checking the tooling and the press process parameters, it became apparent that the structure was unstable and moving during the cooling process. Investigation concluded that the thin upper aerofoil layer, being the weakest part of the structure, was subject to a myriad of varying pressures and forces, resulting in surface distortion. Realizing the problem was occurring during the component cool-down phase, the process elements at that point were investigated further.

When a component is cooling from the elevated forming temperature, normal procedure is to introduce a small positive argon gas pressure into the cavity of the component to prevent the ingress of atmospheric oxygen that could promote oxidization of the internal structure. If this argon pressure is too high it will tend to inflate the component, and if too low the component could, as previously indicated, implode.

The challenge was to control the pressure under varying situations, primarily including:

  • Fluctuating ambient air pressures.

  • The contraction of the cooling gas and the resultant reduction of pressure inside the component during the cooling process.

Finding a Solution

The team realized that to address this issue, it would be necessary to secure a very low gas supply pressure that would be directly related to the varying ambient atmospheric air pressure. It was noted that the low-pressure purge gas admitted into the component during the cooling process was normally exhausted to the atmosphere via an open bulkhead connection situated on the top of the gas control cabinet. It was therefore proposed that the exhaust line should be fed into a vertical pipe, which extended up and through the roof of the building and out to the atmosphere. The length of the vertical pipe, calculated to support a fixed column of gas, could therefore generate the very low pressure the team was striving for.

When applied in the field, not only did the vertical pipe length generate the pressure head required at the component, but it also served as a reservoir of gas to backfill the component when the internal gas volume reduced as the cooling gas contracted. Critically it was also able to compensate for fluctuating ambient air pressures that were negatively impacting the component tolerances.

In essence therefore, Rhodes Interform's solution uses a gas manometer principle in the form of a vertical, open-to-atmosphere vent pipe, for controlling the gas pressure. This ensures a constant low pressure gas supply, which self-compensates to changing ambient air pressures and keeps the material in a constant shape once formed in the mold.

This novel method of gas pressure control allows for the more accurate production of complex shapes and greater control when diffusion bonding multiple layers of thin titanium sheet.

This article was written by Peter Anderton, Technical Director of Rhodes Interform/Group Rhodes (Wakefield, West Yorkshire, England). For more information, visit here .