48.7 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,

48.7.1 Diffusion Bonding

Diffusion bonding is used with SPF to create more complicated shapes.