Version 1.0, August 31, 2001, Copyright, Hugh Jack 1993-2001

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,

  1. - bismuth-tin (200% elongation)
  2. - zinc-aluminum
  3. - titanium (Ti-6Al-N)
  4. - aluminum (2004, 2419, 7475)
  5. - aluminum-lithium (2090, 2091, 8090)
  6. - 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,

  1. - require multiple annealing and forming steps
  2. - have lower accuracy and repeatability
  3. - have springback
  4. - poorer surface finish

 

· For SPF of aluminum,

  1. - 70-90% of melting temperature
  2. - rate of 10**-4 to 10**-2 per second
  3. - typical time is 30-120 min.
  4. - temperature must be carefully maintained
  5. - 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