Rolls Royce is unique for its patented manufacturing process of its hollow wide-chord titanium fan blades. To make these fan blades light and strong the fan blades are made from titanium 6-4 alloy with a hollow internal “Warren–Girder” structure. This is done using 2 processes namely Superplastic forming and Diffusion bonding. The manufacture process starts with 3 sheets of forged titanium. A material called stop-off is printed onto 2 of the sheets which will stop them from joining together in the diffusion bonding process.
The 3 sheets are then sandwiched together, and placed in a Furnace where the diffusion bonding takes place. A Flat sheet is produced and all layers are joined except in areas where stop-off has been applied. Thereafter, the Flat sheet is shaped at high temperatures, in a super plastic forming process. High pressure gas is blown between the sheets, very much like blowing up the balloon forming the fan blades aerofoil shape. The Central sheet is stretched and forms the girder structure.
After which the aerodynamic shape of the blade is cut out of the plate, the root fixings are machined and the blades surface is polished to make the surface smooth to ensure the engine is as efficient as possible by reducing the surface drag. Processes used in manufacturing Fan Blades * Application of stop-off Stop-off material is a chemical used to prevent diffusion bonding on areas where it is applied. It may be made of the following materials, such as yttria boron nitride, graphite or alumina.
In the application of stop-off for manufacturing, the chemical is printed onto the metal to form a pre-determined pattern such that, during super plastic forming, when the inert gas is blown through the gaps of the material where the metal is not joined, it will cause the Girder structure to be formed internally. * Diffusion Bonding Diffusion bonding is a method of joining metallic materials, based on the principles of the atomic diffusion of elements at the surfaces where they are joined.
The process of diffusion transports atoms through the lattice structure of a crystalline solid. This is done using several mechanisms, such as exchanging of places between adjacent atoms, motion of interstitial atoms or motion of vacancies (unoccupied sites in a lattice structure) in the crystal lattice structure. The preferred mechanism of diffusion is that of the motion of vacancies as a lower activation energy is required for the movement of atoms. The amount of contact between the surfaces of the metals is a very important factor in diffusion bonding.
This can be optimised, through mechanical machining and polishing, etching, cleaning, coating and creeping the material under high temperature or loading. In the industrial application of diffusion bonding, the metal pieces are clamped together and subjected to mild temperature heat and high pressure. The metals are usually heated to a temperature below their melting point, and placed under pressure by pressing them together for a long period of time in a furnace. Over time, atoms diffuse into the joint and fill in the gaps between the pieces thus bonding them together.
This is a chosen method for welding metal titanium alloys because the metal becomes extremely chemically reactive when exposed to high temperatures. Additionally, the alloy also oxidizes at low partial pressures of oxygen. Furthermore, Titanium alloys are able to react with oxygen and nitrogen from the air, if a normal welding process is carried out. Therefore diffusion bonding is preferred over other welding methods. * Superplastic Forming Superplastic forming is a process where a sheet of metal is heated up to a temperature whereby it begins to exhibit plastic properties, and becomes not very strong.
The metals are able to elongate and stretch several times more without rupture, allowing the metals to form into different shapes more accurately. In the superplastic forming process, the metal sheet is clamped and placed between die cavities. A pressurised inert gas is later applied to the metal to deform the sheet against the walls of the cavity in a controlled environment with vacuum. The process is always maintained and controlled under suitable stress and deformation rate, so as not to damage the metal.
This method is mainly employed because of difficulty in deforming titanium alloy due to its high tensile and yield strength, and moderate modulus of elasticity. Moreover, titanium alloy has an excessive spring-back frequency which necessitates the use of extreme temperatures (1200-1400 degree Fahrenheit) to overcome. However, there are limitations to the tools that can be used at these moderately high temperatures, which increases the cost of production further. Therefore superplastic forming is used, as it is more feasible to form the metal using a more cost effective means.
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