For many years aircraft designers proposed theoretical designs that could never be built. The need for the “unobtainium” materials – materials that were desired but not yet available – was always the roadblock. These “unobtainium” materials have since been discovered, and today both military and commercial aircraft manufacturers use them. They are composites and titanium.
Why Use Titanium?Composites are the most important materials to be adapted for aviation since the use of aluminum in the 1920s. They are materials that are combinations of two or more organic or inorganic components. One material serves as a matrix, which is the material that holds everything together, and the other material serves as a reinforcement in the form of fibers embedded in the matrix.
Composite materials are very valuable because they are lightweight. The heavier an aircraft is, the more fuel it burns. So, reducing weight is very important to aeronautical engineers. However, certain advanced structures of composites are neither not nearly as strong nor as temperature resistant as their metallic counterparts. Therefore, many of these structures must still be dependent upon metals. Aluminum and composites are galvanically incompatible, so these two materials should never be in contact with one another in any aerospace application. The only lightweight metal that is totally noble to composites is titanium.
TitaniumNamed after the Titans of Greek mythology, this lightweight metal was first purified in 1910 by Matthew Hunter. In 1946, Dr. Wilhelm Kroll developed the process, which is currently used for producing commercial titanium, since titanium is very soft and weak in its pure form. By alloying titanium, four distinct groups of metastable phases are formed. The four phases are alpha, alpha + beta, near alpha and beta. Each is differentiated by its distinct crystalline structure.
Titanium – besides being carbon/epoxy compatible – has a tremendous strength-to-weight ratio dependent upon the crystalline structure. In addition, another benefit of titanium is its elevated temperature capability (up to 1100oF). The superior resistance to oxidation, corrosion, fatigue and fracture is creating more and more applications for titanium in aerospace designs. Similar to composites, titanium is also critical to aviation and its future.
Titanium Use in the Aerospace IndustryTitanium today is used extensively in commercial and military applications and to some extent in space. The primary areas of application for aircraft are landing gear, landing-gear support structures, wing structures, vertical wing-actuation structures, engines, floor beams and seat-track architecture (Fig. 1).
The demand for titanium is projected to grow at least 40-50% over the next five years. Titanium’s superior properties and light weight allow aeronautical engineers to design planes that fly higher, faster and hotter.
On the commercial side, the new Boeing 787 Dreamliner and the Airbus A380 builds are driving the increase in titanium demand. For instance, the 787 alone will use more titanium (18% of the total aircraft weight) than all earlier Boeing models combined (Fig. 2). This percentage increase per ship-set, coupled with the market projections for the world fleet over the next 20 years, indicates that this relatively new lightweight metal has a very bright future (Fig. 3).
Heat Treating TitaniumHeating titanium products in an air atmosphere over 1100oF produces some very undesirable results. First, it produces a visible surface oxide scale and a diffused-in oxygen layer known in the industry as alpha case. This contaminated case could lead to fatigue or fracture in flight-critical parts. Therefore, this layer must be mechanically or chemically removed before use.
Secondly, titanium has a tremendous affinity for picking up hydrogen when heating in air atmospheres. This sensitivity of titanium to hydrogen absorption will cause hydrogen embrittlement, which detrimentally affects the macro-mechanical characteristics of the material. Current aerospace specifications limit hydrogen content to at most 100 ppm, depending upon the alloy and the mill form.
Since titanium is chemically active at elevated temperatures, the only atmosphere that will not have an effect on its properties is a vacuum atmosphere.
Vacuum Thermal Processing - VTPBefore being subjected to any VTP, titanium components should be cleaned and dried. Oil, fingerprints, grease, paint and other foreign matter should be removed from all surfaces. Thorough cleaning is required due to the chemical reactivity of the titanium at elevated temperatures, which can lead to its recontamination or embrittlement. After cleaning, parts must always be handled with clean, white gloves.
The following processes are typical when heat treating titanium and titanium alloys:
- Vacuum annealing – to produce the most acceptable combination of ductility, machinability, and dimensional and structural stability (Fig. 4)
SummaryThe new materials of choice for today’s commercial aeronautical engineer are the marriage of composites and titanium. These new materials were center stage to the world on July 8, 2007, when Boeing rolled out the first assembled 787 Dreamliner in Everitt, Wash. This company, which ushered in the age of affordable commercial flights in 1957, is once again revolutionizing the industry.
The only way Boeing could have sold over 550 ultra-efficient 787s to date is by designing an aircraft with newer, lighter-weight materials. Titanium alloys and their corresponding vacuum thermal processes play a critical role in making this new model the most popular airplane ever built. Together, titanium and composites will dominate the future of both military and commercial aerospace applications.IH
For more information:Bob Hill is president of Solar Atmospheres Inc., 1969 Clearview Rd., Souderton, PA 18964. tel: 215-721-1502; fax: 215-723-6460; e-mail: firstname.lastname@example.org; web: www.solaratm.com.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media SEARCH at www.industrialheating.com: strength-to-weight, galvanic, composite, noble metal, oxidation, hydrogen embrittlement, homogenization temperature