Titanium is a difficult material to fabricate into complex configurations. But it can be done.
Several elevated-temperature forming processes are used to produce titanium components for aerospace applications. Among them are superplastic forming (SPF), hot forming, and creep forming. Each process is used for different applications based on the limitations as to how complex of a configuration can be achieved. In addition, each process involves temperatures that are above the annealing point of the material, which is about 700°C, and so there is little to no spring-back after the part cools.
Of the three processes, SPF is the one that can produce the most complex shapes containing the tightest radii. A variation of the process combines an SPF operation with diffusion bonding (DB) of two or more pieces of titanium to produce integrally stiffened structure containing very few fasteners. SPF uses a tool that contains the required configuration and seals around the periphery so inert gas pressure can be used to form the material.
Typically, the SPF process uses standard titanium alpha-beta alloys, such as 6Al-4V, and the forming temperature is between 900° and 925°C. This forming temperature causes tool wear and oxidation. It also shortens the life of press components, such as heaters and platens. A recent innovation in this technology is the introduction of fine-grain alloys that allow for a forming temperature of about 775°C. The lower temperature range allows the tool to maintain a smooth surface and generate less alpha case on the part surface that has to be removed. Production hardware is currently being fabricated at Boeing using this material, and the company received a patent in 2009 for SPF and SPF/DB processing using this alloy.
Parts are fabricated without excessive material thinning or rupture, and typical elongations are usually 300% or less. The SPF temperature for standard titanium 6Al-4V with a grain size of about 8 µm, and other standard-grain alpha-beta alloys such as 6Al-2Sn-4Zr-2Mo, is between 900° to 925°C. At these temperatures, die life and surface finish are issues that have not found cost-effective solutions.
Stainless steel alloys have been developed that successfully withstand oxidation at 900°C in air; however, it is apparent that under the SPF conditions of combining titanium, graphite, and boron lubricants along with a low partial pressure of oxygen, as well as other factors, the die surface is preferentially attacked, so dies have to be regularly cleaned in order to maintain the required surface finish. Eventually, the prolonged thermal cycling of the dies results in degradation of the tool surface that normal clean-up will not be able to remove.
In addition, the design of an SPF die inevitably has both thick and thin sections, and the stresses caused by repeated thermal cycling to 900°C eventually cause the die to develop cracks. These conditions result in having to replace the dies at specific intervals.
To lower the forming temperature of the 6-4 titanium alloy, collaborative research between Boeing and Verknaya Salda Metallurgical Production Association (VSMPO) in Russia yielded a version of the material with a grain size of about 1 µm and the capability to be superplastically formed at about 775°C. The chemistry of the alloy was kept within the limits of the AMS4911 specification to keep the designation familiar to engineers and manufacturing. Also, this assisted in minimizing any mechanical property testing that would be required to qualify the material after SPF since the properties already existed for standard grain material. Forming at this lower temperature reduces the amount of alpha case, a brittle, oxygen rich layer on the surface that affects fatigue, which has to be removed by chemical milling.
Two or more sheets of clean titanium are capable of being diffusion-bonded together, so it was also of interest to the researchers to have a material that would diffusion-bond at the lower temperature of 775°C. Testing showed that the fine-grain 6Al-4V material would diffusion-bond to itself at this temperature using the same times and pressures typical for standard-grain sheet. It was also discovered that this material would diffusion-bond to standard-grain alpha-beta titanium alloys at this same temperature using the same time and pressure profiles. This is an important innovation since standard-grain alpha-beta materials require temperatures around 900° to 925°C to fully diffusion-bond.
This article is based on SAE technical paper 2010-01-1834 by Larry D. Hefti of Boeing.