Superplastic forming/diffusion

A tracking shot on titanium alloys and their composites will close this section. Titanium is recognised as the most important metal in aerospace applications in the range between 200 C and 450 °C. Its position in the market has been further strengthened by the development of superplastic forming/diffusion bonding manufacturing techniques, which allow the production of complex shapes at reduced costs. 

One candidate is a titanium alioy that is reinforced with iarge diameter SiC/C filaments (see Chapter 4) and is fabricated by superplastic forming/ diffusion bonding. This MMC is suited to the fabrication of bladed compressor rings, shafts, ducks, fan components or structural rods for jet engines. Their use for parts submitted to still higher temperatures is limited by tiber/matrix reaction and environmental considerations. Titanium aluminide TisAI (or y-TiAl) matrices could permit an increase in the service temperature of the composites. 

A wing panel made of diffusion-bonded and superplastically formed titanium. 

For high-pressure applications in the hydrocarbon and chemical processing industries, a titanium compact heat exchanger has been developed by Rolls-Laval. This heat exchanger consists of diffusion-bonded channels that are created by superplastic forming of titanium plates (18). This heat exchanger can handle high pressure and corrosive fluids and is suitable for marine applications. 

Diffusion bonding is another process primarily used by the aircraft indu.stry to bond certain metals by using heat and pressure under controlled conditions. Hydraulic pres.ses are used to provide the bonding force with precise control over the pressing force, pressing velocity, and parallelism. As with superplastic forming, this process also usually involves thermal insulation, hot plates, temperature control, and a gas management system. 

Various jet engine and car engine components have been identified for application of TiAl-based alloys. It is particularly noteworthy that large sheets, which are suitable for superplastic forming and joining by diffusion bonding, can be produced from these intrinsically brittle intermetallic compound alloys. 

Superplastic Forming. The alloy displays superplastic properties between 725 and 775 C (1340 and 1430 F) 1000% ductility can be reached at strain rates as high as 8 x 10 s. Diffusion bonding is being studied. 

Superplastic diffusion bonding can integrate DFW with superplastic forming to produce complex fabrications (see 3.7). 

When accommodated by some of the mechanisms involving dislocation movement or the diffusion of point defects, GBS forms the basis of the structural superplastic behavior of these materials (see Section 15.2). By taking advantage of the processes involved in superplasticity, it is possible to join ceramics super-plastically. For example, when two pieces of the same ceramics in contact are deformed within a superplastic regime (i.e., as soon as GBS is activated), the grains of one part interpenetrate those of the other part. This produces a rapid and perfect junction of the two, in such a way that a shorter time and a lower temperature can be used than are commonly required in other conventional process for ceramics joining [90]. 

A polyciystalline material is said to be superplastic when it can support a deformation under tension of more than 100% without necking. This property, well known to metallurgists, is very widely used today for making complex architectural pieces by hot forming or diffusion welding. The revelation of superplasticity of... 

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