page 302

-autobody panels

-covers

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,

page 304

Female Forming

Drape Forming

Male Forming

page 305

48.7.1 Diffusion Bonding

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

Step 1: The sheets have boron nitride placed on the sheets where no bonding is to occur. Sheets are put down in layers (with the boron nitride areas between), and heat and pressure are used to bond sheets together.

sheet 1 sheet 2

sheet 3 boron nitride

Step 2: the laminated sheets are put into a mold, and SPF is used to shape the outside. Pressure is applied by blowing air between the sheets. The boron nitride that stopped bonding before, now acts as a lubricant.