Manufacturing Processes - Superplastic Forming
Superplastic Forming Process
Superplastic forming as the name suggests falls in the manufacturing processes category of forming. The process works under closely controlled conditions of temperature and strain rates which allow certain metallic microstructures to be deformed to large strains using relatively low forces.
The process can be used to produce complex 3D shapes with constant section thickness's, no re-entrant angles and with large deformations from any metal with a stable superplastic microstructure. The most common metals utilised for this process are titanium and aluminium alloys.
The different types of superplastic forming:
- Female forming, a graphic coated blank is put into a heated hydraulic press. Air pressure is then used to force the sheet into close contact with the mould.
- Female drape forming, a graphic coated blank is clamped over a 'tray' containing a heated male mould. Air pressure forces the metal into close contact with the mould. The difference between this and the female forming process is that the mould is, as stated, male and the metal is forced over the protruding form. For the female forming the mould is female and the metal is forced into the cavity.
- Plug assisted snap back male forming, a graphic coated blank is placed into a heated press. Air pressure is used to force the metal into a bubble shape before the male mould is pushed into the underside of the bubble to make an initial impression. Air pressure is then used from the other direction to final form the metal around the male mould.
This process has long cycle times because the superplastic strain rates are low. Product also suffers from poor creep performance due to the small grain sizes and there can be cavitation porosity in some alloys. Surface texture is generally good however. With dedicated tooling, dies and machines are costly.
The main advantage of the process is that it can be used to produce large complex components in one operation. This can be useful for keeping the mass down and avoiding the need for assembly work, a particular advantage for aerospace products.
"Superplasticity is the term used to describe materials capable of being formed to high strains without the formation of unstable tensile necks. The term is normally associated with specific metal alloys, most commonly in the material groups of aluminium and titanium, but there are also glasses and a number of thermoplastics that exhibit the characteristic 'superplasticity' when a high enough temperature is achieved."
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