Super plastic forming method with heat treated seals

A method of making metallic sheet structure by superplastic forming, which comprises:

• providing one or more metallic sheets, where at least one of said sheets possesses superplastic properties;
• providing first and second forming members which sandwich said one or more sheets, said first forming member having a cavity therein which defines a surface similar to the shape of said sheet structure desired to be formed;
• positioning one seal between said one or more sheets and said first forming member, and positioning a second seal between said one or more sheets and said second forming member, said seals being made of a seal material that is free of substances that will volatilize at superplastic forming conditions for said at least one sheet, said seal material being nonmetallic and maintaining structural integrity at high temperaturees and pressures of said forming conditions;
• applying compressive forces across said forming members to seal said cavity;
• bringing said at least one sheet to within a temperature range at which it exhibits superplastic characteristics;
• and applying pressure to said at least one sheet to superplastically form said at least one sheet into said cavity wherein said seal material is flaked graphite that has been heat treated to a temperature in excess of the superplastic forming temperature to be used for said at least one sheet to remove volatile contaminants therefrom.