Titanium Springs
Titanium is one of the strongest metallic elements, having the highest strength-to-weight ratio of any metal. With its high strength-to-weight ratio, Titanium is as strong as some steels but is almost 50% lighter. With these characteristics, Titanium springs are one of the strongest and most weight-sensitive springs available. Not only are titanium springs exceptionally strong, but they are also corrosion-resistant. Titanium springs are used in many industries and in many applications, from motorbikes and race cars to aircraft.
The primary attributes that make titanium an attractive material include an excellent strength-to-weight ratio, providing weight savings attractive to the aerospace and petrochemical industries; corrosion resistance, particularly appealing to the aerospace, chemical, petrochemical, and architectural industries; and biological compatibility, of interest to the medical industry. The chemical industry is the largest user of titanium due to its excellent corrosion resistance, particularly in the presence of oxidizing acids. The aerospace industry is the next largest user, primarily due to its elevated (and cryogenic) temperature capabilities and weight savings due to its high strength and low density; with the increased use of polymeric graphite fiber reinforced composites on aircraft, the low coefficient of thermal expansion is also an important factor. The ballistic properties of titanium are also excellent on a density-normalized basis. Highlights of titanium applications in other areas will be briefly discussed.
Why Use Titanium?
Weight Savings
The high strength and low density of titanium (~40% lower than that of steel) provide many opportunities for weight savings. The best example of this is its use on the landing gear of the Boeing 777 and 787 aircraft and the Airbus A380. Figure 1 shows the landing gear on the 777 aircraft.1 All of the labeled parts are fabricated from Ti-10V-2Fe- 3Al. This alloy is used at a minimum tensile strength of 1,193 MPa; it is used in replacement of high-strength low-alloy steel, 4340M, which is used at 1,930 MPa. This substitution resulted in a weight savings of over 580 kg.1 The Boeing 787 used the next-generation high-strength titanium alloy, Ti-5Al- 5V-5Mo-3Cr, which has slightly higher strength and some processing advantages use of titanium in landing gear structure should also significantly reduce the landing gear maintenance costs due to its corrosion resistance. The low density and high strength make it very attractive for reciprocating parts, such as connecting rods for automotive applications. Again, the price is too high for family vehicles but the U.S. Department of Energy is investing in a substantial effort to make titanium components for automobiles and trucks affordable. (Titanium is successfully utilized for high-end racing cars, where cost is not that much of an issue.)
Space Limitations
This application does not come up often, but it is an important one. The best example for this is the landing gear beam used on the 737, 747, and 757. This component, running between the wing and fuselage, supports the landing gear. Other Boeing aircraft utilize an aluminum alloy for this application, but for the above aircraft the loading is higher and the aluminum structure will not fit within the envelope of the wing. An aluminum alloy would be the preferred option as it is much lower in cost. Steel would be another option, but that would be a higher weight.
Operating Temperature
The structure in the engine and exhaust areas operates at elevated temperature, so the primary options are titanium- or nickel-base alloys; again, the nickel alloys would add significant weight. Titanium engine alloys are used up to about 600°C. There are applications, such as the plug and nozzle (Figure 2), which experience temperatures higher than this for short times during certain operating conditions. The temperature limitation for titanium alloys, other than specialized engine alloys, is about 540°C. Above this temperature, oxygen contamination becomes an issue, embrittling the surface. Titanium is also used at cryogenic temperatures for structures such as impellors for rocket engines.
Corrosion Resistance
Titanium has a very tenacious nascent oxide which forms instantly upon exposure to air. This oxide is the reason for the excellent corrosion resistance. Corrosion is not a factor for titanium in an aerospace environment. Titanium does not pit, which in the author’s opinion is the rationale for the excellent service experience. In service, aluminum and steel alloys will eventually form corrosion pits, which serve as stress risers which will then initiate stress corrosion or fatigue cracks. This does not happen with titanium. This corrosion resistance carries through to the chemical, petrochemical, pulp, paper, and architectural industries. Titanium and its alloys have excellent resistance under most oxidizing, neutral, and inhibited reducing conditions. It is also corrosion-resistant within the human body. Biocompatibility is also excellent; it is used for prosthetic devices and bone will grow into properly designed titanium structures. Commercially pure titanium is also being used for exterior architectural applications, a practice started in Japan. It is used for exterior surfaces as it will never require any maintenance. The most famous of these is its use on the exterior of the Guggenheim Art Museum in Bilbao, Spain.
Composites Compatibility
Titanium is compatible with the graphite fibers in the polymeric composites. There is high galvanic potential between aluminum and graphite, and if the aluminum comes into contact with the graphite in the presence of moisture the aluminum would be corroded away. It can be isolated from the composite by methods such as a layer of fiberglass, but in areas that are difficult to inspect and difficult to replace, titanium is used as a conservative approach. In addition, the coefficient of thermal expansion (CTE) of titanium, while higher than that of graphite, is much lower than that of aluminum. Even in the operating temperature range of fuselage structure, about –60°C at cruise to +55°C on a hot day, the difference in CTE using aluminum structure attached to the composite would result in very high loading. This is not an issue with titanium structure. Obviously, the longer the component, the bigger the issue would be for utilizing aluminum.
Low Modulus
The primary area where this is important is in the replacement of steel springs. With the modulus being about half that of steel, only half the number of coils are required. That in conjunction with the high strength and density being about 60% of that of steel could ideally result in a weight savings of about 70% of that of a steel spring. In addition, the titanium offers much superior corrosion resistance, reducing maintenance costs.
Armor
Titanium has excellent ballistic resistance and provides a 15–35% weight savings when compared to steel or aluminum armor for the same ballistic protection at areal densities of interest, which has resulted in substantial weight savings on military ground combat vehicles. Lighter vehicles have better transportability and maneuverability. The excellent corrosion resistance, low ferromagnetism, and compatibility with composites also provide significant benefits. Two programs that use titanium in upgraded vehicles are the Bradley Infantry Fighting Vehicle (Figure 3) and Abrams Main Battle Tank.2 The relatively high cost of titanium has been successfully mitigated by using plate produced from electron beam, cold hearth, single melt ingot.
Attributes of Titanium
General corrosion resistance has already been discussed. With regard to stress-corrosion cracking (SCC), commercially pure and most titanium alloys are virtually immune unless there is a fresh, sharp crack in the presence of stress. If the titanium is cracked in air, the protective oxide will immediately re-form, and SCC may not occur. If the crack is initiated in seawater, for instance, then SCC could occur on certain high-strength alloys or high oxygen grades of commercially pure titanium. Even here, the SCC may be mitigated if the part is not loaded immediately. Dawson and Pelloux4 showed that fatigue crack growth of Ti-6Al-6V-2Sn can be reduced when tested at a low frequency as long as the stress intensity is below that of the stress corrosion threshold. This is attributed to re-passivation (re-formation of the oxide) in the seawater at the lower frequency whereas there is insufficient time for this to occur at higher frequencies. The modulus of ß-alloys can be altered significantly. Ti-15V-3Cr-3Al-3Sn with 60% cold work had a tensile strength of ~1,070 MPa with a modulus of ~76–83 GPa. When aged at 480°C the strength and modulus were ~1,515 MPa and 103 GPa, respectively. Titanium alloys containing Nb, Zr, and Ta, referred to as gum metal, developed for the medical industry, have elastic moduli as low as 40–50 GPa depending on orientation and processing. These moduli are close to that of bone, making it ideal for prosthetic applications. Cold work decreases the modulus while increasing strength.
The crystallographic texture of the hexagonal close-packed (HCP) a-phase can have a very significant effect on properties in different directions. Larson6 modeled the modulus of a single crystal of commercially pure titanium and determined that when stressed along the basal pole the modulus is ~144 GPa, but when stressed orthogonal to the basal pole it is ~ 96 GPa. Differences in ultimate tensile strength, which are also an indicator of crystallographic texture, between the longitudinal and transverse direction of about 205 MPa have recently been observed for a rolled strip, with continuous rolling in one direction which can result in a strong texture.
The Bauschinger effect, while not necessarily unique, seems to have a stronger effect in titanium alloys than other alloy systems. It is attributed to the limited number of slip systems in hexagonal close-packed (HCP) low-temperature α-phase. If a tensile specimen is pulled in tension and the test is stopped prior to failure, and a compression specimen is taken from the gage length of the tensile specimen, a significant drop in the yield strength is observed. A tensile strain of 0.5% at room temperature can reduce the compression yield by 30%. This is attributed to the dislocations in the material going in the reverse direction following the same slip path, meaning dislocation barriers do not have to be overcome in the early stages of deformation. The same phenomenon is observed if one strains a compression specimen and then pulls a tensile from its gage length. This effect can be eliminated or mitigated by forming at elevated temperature, or subsequent annealing. Consequently, at least in the aerospace industry, when a titanium part is formed, it is subsequently annealed to avoid this large yield reduction. It does not affect the ultimate tensile strength.
Solid metal embrittlement has been a problem with titanium and its alloys, with the most prominent example being cadmium. Intimate contact (forcing the titanium into the cadmium or vice-versa) and high tensile stresses are required for this to occur.
Cost of Titanium
As many are aware, the primary factor limiting more extensive use of titanium is its cost. With a significantly higher cost than aluminum and steel alloys, titanium utilization must be justified for each application.
There are several factors contributing to this. High energy is required for separation of the metal from the ore. Ingot melting is also energy-intensive; in addition, its high reactivity requires melting in an inert atmosphere using a water-cooled copper retort or hearth, depending on the melting technique.
Machining is also very high cost, on the order of 10–100 times slower than the machining of aluminum alloys. It was recently pointed out by Froes7 that a kilogram of the aluminum sheet could be purchased for a lower cost than that of a kilogram of titanium sponge, the starting material. This sponge still must be multiple-melted with a master alloy addition, forged or forged and rolled to a size appropriate for sheet bar, put into a pack with multiple sheet bars, rolled to the appropriated thickness and etched and ground to the final thickness to obtain the titanium sheet.
With these factors in mind, much of the research and development at Boeing and other original equipment manufacturers and fabricators is being devoted to a reduction of the buy-to-fly ratio of titanium components. For instance, a 40 kg plate may be used to machine out a 5 kg part, meaning almost 90% of the titanium is turned into chips (scrap).
Reduction of that buy-to-fly ratio then means one is procuring a reduced weight of very expensive material, and also reducing the amount of machining being done on that material. Several technologies are being pursued to accomplish this. These include welding, greater use of extrusions where appropriate, superplastic forming and superplastic forming with diffusion bonding, hot stretch forming to obtain more precise formed shapes, and even powder metallurgy. With regard to welding, both fusion and solid-state welding are being investigated.