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Fluid Reservoir^®

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Rubber Diaphragm

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Bottom Die

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Rubber Diaphragm t

Hydro Forming Fig. 2.19. Rubber Pad and Hydro Forming t

Hydro Forming Fig. 2.19. Rubber Pad and Hydro Forming superplasticity occurs in some materials with a fine grain size, usually less than 10 ^m, when they are deformed in the strain range of 0.00005-0.01/s at temperatures greater than 0.5 Tm, where Tm is the melting point in degrees Kelvin. Although superplastic behavior can produce strains in excess of 1000%, superplastic forming (SPF) processes are generally limited to about 100-300%. The advantages of SPF include the ability to make part shapes not possible with conventional forming, reduced forming stresses, improved formability with essentially no springback and reduced machining costs. The disadvantages are that the process is rather slow and the equipment and tooling can be relatively expensive.

The main requirement for superplasticity is a high strain rate sensitivity. In other words, the strain rate sensitivity "m" should be high where m is defined as:15

m = d(ln o-)/d(ln S) where m = strain rate sensitivity a = flow stress

S = strain rate

The strain rate sensitivity describes the ability of a material to resist plastic instability or necking. For superplasticity, m is usually greater than 0.5 with the majority of superplastic materials having an m value in the range of 0.40.8, where a value of 1.0 would indicate a perfectly superplastic material. The presence of a neck in a material undergoing a tensile strain results in a locally high strain rate and, for a high value of m, to a sharp increase in the flow stress within the necked region, i.e. the neck undergoes strain hardening which restricts its further development. Therefore, a high strain rate sensitivity resists neck formation and leads to the high tensile elongations observed in superplastic materials. The flow stress decreases and the strain rate sensitivity increases with increasing temperature and decreasing grain size. The elongation to failure tends to increase with increasing m.

Superplasticity depends on microstructure and exists only over certain temperature and strain rate ranges. A fine grain structure is a prerequisite since super-plasticity results from grain rotation and grain boundary sliding, and increasing grain size results in increases in flow stress. Equiaxed grains are desirable because they contribute to grain boundary sliding and grain rotation. A duplex structure also contributes to superplasticity by inhibiting grain growth at elevated temperature. Grain growth inhibits superplasticity by increasing the flow stress and decreasing m.

The Ashby and Verrall model for superplasticity, based on grain boundary sliding with diffusional accommodation, is shown in Fig. 2.20 in which grains switch places with their neighbors to facilitate elongation. However, in real metals, since the grains are not all the same size, some rotation must also take place. Slow strain rates are necessary to allow the diffusion mechanisms time to allow this rearrangement. Since a fine grain size is a prerequisite for superplasticity, a fine dispersion of the metastable cubic phase Al3Zr can be used in aluminum alloys to help prevent grain growth during SPF at temperatures up to 930° F. The other option is to use thermomechanical processing to achieve

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