Method of superplastically forming and braze bonding a structure

A process for producing a superplastically formed trusscore structure which comprises:

• providing first and second core elements, each of said elements formed of a pair of sheets made from a metal alloy having superplastic characteristics, at least one of said sheets in each core element being associated with a brazing alloy capable of brazing said sheets, said sheets being welded together along a plurality of pairs of spaced parallel weld lines, leaving space to permit free passage of gas between the sheets throughout the core element;
• providing a slot between the weld lines of each of said pairs of weld lines;
• placing said core elements together in face to face contact, with the pairs of weld lines in each of said core elements in parallel relation and the pairs of weld lines in said second core element displaced laterally from the pairs of weld lines of said first core panel;
• applying a first face sheet in contact with the outer sheet of said first core element, and a second face sheet in contact with the outer sheet of said second core element;
• sealing the perimeter of the resulting assembly of core elements and face sheets, while providing means for the admission of pressurized gas between the sheets of said core elements and in the space between each of said face sheets and the respective adjacent outer sheets of said first and second core elements;
• providing a forming die having at least one wall and defining a cavity therein;
• placing the resulting assembly of sealed core elements and face sheets in said cavity;
• and
• heating said last mentioned assembly to a temperature suitable for superplastic forming and applying gas pressure at said means for admission of pressurized gas, causing superplastic forming and expansion of said sheets of each said core element, between adjacent pairs of weld lines, and expansion of said face sheets into contact with the at least one wall of said cavity;
• continuing to apply said pressurized gas to cause further expansion of said sheets of each core element and formation of a plurality of successive substantially similar trapezoids having a pair of non-parallel sides from the expanded core sheets of said first and second core elements between said face sheets, adjacent trapezoids being in contact with each other and with said face sheets, and
• ensuring that the die temperature is at or above the minimum brazing temperature for said brazing alloy for a specified period of time, thereby braze bonding adjacent contacting surfaces of said trapezoids, and the contacting surfaces of said trapezoids and said face sheets.