Papers by Keyword: Mark Off

Abstract: Titanium is difficult to fabricate into complex aircraft configurations. There is several elevated temperature forming techniques that are available to produce titanium components for aircraft, two of which will be discussed here: Superplastic Forming (SPF) and hot forming. SPF is used when complex shapes are required, for example, tight radii, and uses a tool that contains the required configuration and seals around the periphery so inert gas pressure can be used to form the material. Since SPF is a process where the material is stretched, the part is not a uniform thickness when completed. A variation of the process combines SPF with diffusion bonding (SPF/DB) of two or more pieces of titanium together to produce integrally stiffened structure containing very few fasteners. The hot forming process uses matched metal tools, offset by the thickness of the starting material, are used to form the part contour at elevated temperature. The required part geometry usually contains no sharp features that have to be formed. Since the material is free to move as the die is closed, the part is fairly uniform in thickness when completed.

Abstract: In the past, engine aft fairing heat shields have typically been titanium castings. With a current single aisle airplane, these components were converted to sheet metal titanium 6Al-4V details fabricated by hot forming or Superplastic Forming (SPF). This conversion saved approximately 20% in both cost and weight per airplane. When heat shields for a twin aisle airplane were being developed, the engineers were interested in a sheet metal version of their heat shields hoping to achieve similar savings. However, the twin aisle configuration was different from the single aisle and did not allow the details to be simple pieces of formed sheet metal. Instead, these twin aisle heat shields are assemblies of details containing SPF components as well as Superplastically Formed and Diffusion Bonded (SPF/DB) panels. Some of the heat shield components are fabricated using the world's first applications of fine grain 6Al-4V titanium, which was developed to SPF at 775°C, covered by a U.S. patent [1], instead of 900°C, which is used for standard grain material. The SPF/DB technology being used contains innovative process developments that are covered by several patent applications [2-4]. The twin aisle heat shield assemblies were estimated to save approximately 15% in both cost and weight per airplane. Actual weight measurements of the first assembly showed an additional 5% savings over the calculated weight per engine resulting in a total weight savings of approximately 20% per airplane compared to titanium castings.

Abstract: 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.