Advances in the Superplastically Formed and Diffusion Bonded Process
The Superplastically Formed and Diffusion Bonded (SPF/DB) titanium structure in production today for Boeing products, not including engines, are all diffusion bonded using matched metal tooling and are all fabricated using the common 6Al-4V alloy. The matched metal tooling concept presents a challenge in obtaining high quality bonds over large areas where direct tool pressure is being used to place the titanium sheets into contact with each other. This is due to tolerance build-up in the tools and in the titanium sheets that are used to fabricate the components. Also, because the parts are partially formed before bonding begins, material has been pulled away from the bonding area and the thickness in that location is now less than what the tool was designed for which makes achieving a good quality bond even more challenging. Boeing Commercial Airplanes (BCA) is currently advancing the state of the SPF/DB process in several ways. One of these advances is using a different approach for diffusion bonding. The process includes using stop-off between the sheets and diffusion bonding the pack first and then superplastically forming the stiffening features. This generates a component that is very well bonded in the required locations. However, this process also has its challenges. One of these involves how to apply the stop-off material in the proper location using the most cost effective process. Historically, the application method has been silk screening. A new method has been developed for applying the stop-off that eliminates the need for several pieces of equipment that are required for the silk screening process as well as the associated floor space. Another advancement has been in the development of a different titanium alloy for use in SPF/DB structure.
L. D. Hefti "Advances in the Superplastically Formed and Diffusion Bonded Process", Materials Science Forum, Vols. 551-552, pp. 87-93, 2007