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)
- 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
- 70-90% of melting temperature
- rate of 10**-4 to 10**-2 per second
- 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,