I'm sure this is a stupid question, but presumably the sheet is superplastic
(is it AA5083 or something?)? If not, you've not much chance, as the strain
rate sensitivity will be too low and it'll just neck at fairly low
Try anything between 400 and 500°C, at resonably low strain rates intially
(10-4, say). Now sure on pressures (depends on the thickness/temperature?).
As the other post alluded, this is all fairly standard stuff. Have a look
at any paper on AA5083 (there should be millions), or the numerous
superplastic conferences (e.g. the ICSAM series, and infact ICSAM 2006 is
currently running in China), or that superplastic book by TG Nieh et al.
Hope this helps.