Elevated Temperature Forming Processes for Fabricating Titanium Aircraft Hardware

Titanium is difficult to fabricate into complex aircraft configurations.  There is several elevated temperature forming techniques that are available, two of which will be discussed here: Superplastic Forming (SPF) and hot forming.  SPF is used when complex shapes are required.  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 SPF process uses alpha-beta alloys, like 6Al-4V, and the forming temperature is between 900° and 925°C (1650° to 1700°F).  This temperature causes tool wear and oxidation as well as reduces the life of press components, such as heaters and platens.  A recent innovation has been the introduction of a fine grain version of 6-4 that allows the forming temperature to be reduced to 775°C (1425°F).  This allows the tool to maintain a smooth surface and generates less alpha case on the part that has to be removed.  Production hardware is being fabricated at Boeing using this material and the company received a patent in 2009 for SPF and SPF/DB processing using this alloy. Although the current material is produced by VSMPO in Russia, several US titanium suppliers are working on fabricating fine grain 6-4 as well as alloys that are similar to 6-4 that can be formed at lower temperatures.   The other process to be discussed for shaping titanium is hot forming which occurs around 730°C (1350°F).  In this process, matched metal tools, offset by the thickness of the starting material, are used to form the part contour.  One application of this process is to pre-form plates of titanium that will be subsequently machined.  The forming process allows the starting material to be thinner which lowers the buy-to-fly ratio, how much material starts to how much material is contained in the final part, as well as reduces the amount of time for machining which leads to lower cost products.  Since these processes occur above the annealing point of the material, around 700°C (1300°F), there is little to no springback after the part cools down which is important for fabrication of aircraft hardware.