Heavy section size is generally defined as forged
or rolled thickness that exceeds four inches. Titanium alloys have been
successfully used for heavy sections thickness, in both airframe parts,
and in rotating components such as heavy section fan disks for PWA and
G.E. high bypass jet engines, and Sikorsky helicopter rotor forgings.
The primary alloys that have been involved are
Ti-6AI-4V, in the annealed or STOA (Solution Treated and Overaged) condition;
the near-beta Ti-17 (Ti-5AI-2Sn-2Zr-4Mo-4Cr), Ti-lOV-2Fe-3AI and the Ti-6AI-2Sn-4Zr-6Mo
compositions in the STA (Solution Treated Aged) condition; and the beta
alloys Ti-13V-11Cr-3Al and Ti-3AI-8V-6Cr-4Mo-4Zr, also in the STA condition.
Certainly the most extensive heavy section applications in one project
to date featured the Ti-13V-11Cr-3AI alloy in the SR-71 Blackbird (fuselage
frames, wing beams and landing gears). In this program, Lockheed engineers
stated that while only titanium and steel had the ability to withstand
the operating temperatures encountered, aged Ti-13V-11Cr-3AI titanium
weighed one-half as much as stainless steel per cubic inch and it's ultimate
strength was about equal to stainless. Using "conventional" fabrication
techniques, fewer parts were needed with Ti-13V-11Cr-3AI than with steel.
For a given process and heat treatment condition,
titanium alloys such as these demonstrate superior fatigue and fracture
toughness properties, not only in the absolute sense, but also from the
stand point of uniformity throughout the heavy section thickness, and
as the section thickness increases from 4" to 6", or even to 8". Titanium
alloys offer a useful and in many cases, a superior alternative to steel
alloys for heavy section application.
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