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Six different titanium alloys are contained in the F-22 Raptor airframe, in both cast and wrought forms. Each alloy is chosen for its particular properties suite, usually with some dominant characteristic dictating the particular product form, alloy composition, and heat treatment.
The F-22 air superiority fighter utilizes more titanium than any other United States Air Force aircraft, over 9000 1b, or approximately 42% by weight (Fig. 1). Stringent F-- 22 performance requirements led to the extensive application of titanium alloys. For example, high cyclic loads under harsh environmental conditions require the airframe and critical systems to have outstanding resistance to fatigue, high temperatures, and environmental effects. Titanium alloys offer the best combination of properties to meet these requirements.
The F-22 product mix consists of cast and wrought versions of six different alloys and multiple heat treatments. The titanium alloys on the F22 include:
* Ti-6A1-4V (Ti-64), standard cast and wrought, and ELI wrought grades
* Ti-6A1-2Sn-2Zr-2Cr-2Mo-0.2Si (Ti-6--2-22)
* Ti-6A1-2Sn-4Zr-2Mo-0.1Si (Ti-6242)
* Ti-10V-2Fe-3A1 (Ti-10-2-3)
* Ti-15V-3Cr-3Sn-3A1 (Ti-15-3)
* Ti-3A1-2.5V (Ti-3-2.5)
The optimum Alloy /Product Form/ Heat Treatmerit combinations were selected based on design, producibility, and cost considerations for each application. Key factors were static and fatigue properties, formability, machinability, weldability, and thermal stability, all balanced against cost. The chemical composition ranges of the titanium alloys utilized for the F-22 airframe are typical for the industry. This article reviews their applications on the F-22 aircraft, and describes their properties and heat treatments.
Ti-6A14V
Three different grades of Ti-6-4 titanium are used, depending on application criticality and product form. The extra-low interstitial (ELI) grade of the Ti-6-4 alloy is the choice for most Safety of Flight structures, which are produced from wrought forms, plate, forged block, and die forgings. This grade has a maximum oxygen content of 0.13 wt%, and is preferred for its improved fracture properties over standard grades. For cast structures, the oxygen limits depend on the criticality of the structure. Safety of Flight castings are produced to an F-- 22-unique specification that confines the oxygen content to the 0.13 to 0.2 wt% range. The specification for non-Safety of Flight castings merely places a maximum on oxygen of 0.2 wt%.
Although the differences between these three grades of Ti-6-4 may seem subtle, oxygen is...