Fig. 1. Primary areas of titanium application for aircraft1


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.

Titanium

Named 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.

Fig. 2. Titanium usage in aircraft over time1

Titanium Use in the Aerospace Industry

Titanium 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.

Fig. 3. Market projections for the world fleet over the next 20 years

Recent designs in military aviation include the new F22 Raptor fighter jet, which is composed of 60,000 pounds of titanium per ship-set. The F35 Joint Strike Fighter has 80,000 pounds of titanium on board. When compared to the older F4 Fighter, the new-age jet fighter increases the titanium usage from 9% to 35%.

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 Titanium

Heating 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.

Fig. 4. A beta-anneal treatment of 6AL4V titanium forgings is used on the wing fold for the new F35 Joint Strike Fighter.

Vacuum Thermal Processing - VTP

Before 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)


Fig. 5. This titanium nosecone is stress relieved to minimize or eliminate forming and electron-beam welding stresses.

  • Vacuum stress relieving – to reduce residual stresses developed during fabrication (Fig. 5)


    • Fig. 6. Pictured are critical airframe structural forgings made from Ti 5553.

  • Vacuum solution treating and aging – to increase strength and optimize special properties such as fracture toughness, fatigue strength and high-temperature creep strength (Fig. 6)


    • Fig. 7. These critical landing-gear forgings must be below 50 ppm of hydrogen. The parts will typically come in at 400 ppm H2, and after VTP of 12x10-6 Torr the forgings will test at less than 10 ppm H2.

  • Vacuum degassing – to reduce the H2 levels, which can cause embrittlement (Fig. 7)


    • Fig. 8. Fixtures, weights and temperatures allow the aeronautical engine to creep form titanium very easily.

  • Vacuum creep forming – to flatten or form a near-net shape to eliminate scrap with pressure and heat (Fig. 8)


    • Fig. 9. Each 10,000-pound billet of titanium aluminide will be used as stock for rotating components in jet engines. Homogenizing temperatures are 2400°F with a 24-hour hold.

  • Vacuum homogenizing – to eliminate or decrease chemical segregation by diffusion (Fig. 9)
  • Vacuum brazing – metal is formed when braze alloy creates a diffusion bond between the metal components
  • Vacuum hydriding – the addition of hydrogen to embrittle the metal for pulverizing for scrap reclamation


      • Summary

      The 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: info@solaratm.com; 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