1.7 SUPERPLASTIC FORMING

 

• Basic process - some alloys can be slowly stretched well beyond their normal limitations at elevated temperatures. This allows very deep forming methods to be used that would normally rupture parts.

 

• Some materials developed for super plastic forming are,

- bismuth-tin (200% elongation)

- zinc-aluminum

- titanium (Ti-6Al-N)

- aluminum (2004, 2419, 7475)

- aluminum-lithium (2090, 2091, 8090)

- stainless steel (2205 series)

 

• In general the alloys should have a grain size below 5-8 microns and be equip-axed. The grain size must not increase if kept at temperatures 90% of melting for a few hours.

 

• Strain rates are generally low, approx. 10**-4/sec.

 

• Conventional forming techniques compared to SPF,

- require multiple annealing and forming steps

- have lower accuracy and repeatability

- have springback

- poorer surface finish

 

• For SPF of aluminum,

- 70-90% of melting temperature

- rate of 10**-4 to 10**-2 per second

- typical time is 30-120 min.

- temperature must be carefully maintained

- cavitation (voids) can occur in the aluminum if pressure is not applied to both sides of the sheet - a different pressure still causes motion.

 

• Parts are less expensive because only half of the tooling is required.

 

• The typical process is like,

 

 

• Various methods include,