L. D. Hefti, The Boeing Company, Seattle, WA
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 due to tolerance build-up in the tools and the titanium sheets. Boeing Commercial Airplanes is currently advancing the state of the SPF/DB process in several ways. One of these advances is 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, silk screening has been used to define the required pattern for the stop-off material. This process requires several pieces of equipment including a wash booth since the screen needs to be cleaned after each part. A paper maskant and laser scribing process has been developed for defining the stop-off pattern. Also, because diffusion bonding is performed first, when the component is superplastically formed, there is a tendency to form creases on the surface of the part. Methods have been developed to eliminate these surface creases on the unformed surface. Another advance in the SPF/DB process is in the titanium alloys being used for products. A fine grain 6Al-4V material has been developed that bonds and forms at 1450 °F. This use of this material will minimize wear on the tools and presses as well as significantly reducing the amount of alpha case on the part surface.
Summary: This paper will discuss the titanium Superplastic Forming and Diffusion Bonding (SPF/DB) process using stop-off and the innovations that have been made at Boeing Commercial Airplanes. One of the advances is using a paper maskant and laser scribing process instead of the traditional silk screening process for defining the stop-off pattern. This eliminates the need for several pieces of equipment that are required for the silk screening process as well as the associated floor space. Also, a process has been developed to eliminate mark-off on the outer surface caused by forming of the inner sheet after diffusion bonding. The final innovation is using a fine grain titanium alloy that can be bonded and formed at 1450 °F. This temperature will minimize wear on the tools and presses as well as significantly reducing the amount of alpha case on the part surface.
