Published online by Cambridge University Press: 27 November 2015
M. Mohaghegh and
Titanium and titanium alloys did not become production materials until the 1950s, under significant government support. Similarly to 2024 aluminum, Ti-6Al-4V was one of the first titanium alloys developed and remains the predominant titanium alloy in the aerospace industry, because of its balanced and robust property set. (Numbers in the alloy name indicate the weight percentages of each alloying addition.) In addition, numerous other titanium alloys have been developed over the yearsthat offer a wide range of properties. Ti-6Al-4V has an ultimate strength level of ∼900 MPa with toughness ranging from ∼55 MPa m1/2 to well over 100 MPa m1/2, depending on the annealing temperature. Ti-6Al-2Sn-2Zr-2Mo-2Cr used at a strength level of about 1100 MPa has a toughness of about 100 MPa m1/2, and Ti-10V-2Fe-3Al at about ∼1240 MPa has a typical toughness of ∼55 MPa m1/2.
At present, most alloy development for airframe materials is focused on cost reduction, with relatively few dollars going toward performance improvements. An effort that has been pursued successfully at Boeing is the development of fine-grain Ti-6Al-4V to enable a reduction of the superplastic-forming (SPF) temperature by about 110°C to about 775°C, and a reduction of the SPF/diffusion-bond temperature as well. The resulting reductions of the allowable processing temperatures has several significant advantages, such as a large increase in die life, a decrease in surface contamination, and much greater comfort for the operators who must transfer the sheets into and out of the press upon completion of forming.Titanium is the only structural material with an alloy such as Ti-6Al-4V that is superplastically formable in sheets and, to a lesser extent, plates using standard production methods. Other alloy systems require special chemistries or special processing, increasing costs, and do not have the formability of Ti-6Al-4V.
Another area being studied is additive manufacturing, again to reduce component costs.Both powder and wire input stocks are being evaluated utilizing laser-beam, electron-beam, and plasma-transferred-arc energy sources. Because input stock is significantly more expensive than wrought forms, the key savings would result from reducing the buy-to-fly ratio.
Some suppliers have estimated that quite significant cost savings could be achieved using this technology. However, one serious challenge is the nondestructive testing (NDT) of additively manufactured shapes. Initial applications will likely be for components with large fatigue and crack growth design margins. These would not be flight-critical and would provide the opportunity for suppliers to demonstrate that they can provide a product of consistent quality with on-time deliveries. As development proceeds, suppliers could develop sufficient fatigue and NDT data to provide customers the confidence they need to consider this technology for more critical applications. Current studies on additively manufactured parts are primarily focused on Ti-6Al-4V.
Another potential benefit of additive manufacturing is the opportunity to vary the material composition at different locations within a part. If higher strength is required in a given location, for example, but is not desirable over the entire part because of a corresponding loss in fracture toughness, one could modestly increase the oxygen or iron content in that location without changing the properties through the rest of the part.
Titanium alloys have excellent corrosion resistance for aerospace applications. They have a very thin, tough oxide surface that provides this corrosion resistance. However, corrosion/hydrogen embrittlement can occur if hot hydraulic fluid in commercial aircraft comes into contact with titanium. The problem is due to an additive used for commercial aircraft to raise the flash point of the hydraulic fluid; military aircraft do not use this additive, so they do not encounter this problem. Hydrogen accumulation can occur at temperatures in excess of ∼130°C. Therefore, most titanium alloys are not used in areas of potential hydraulic fluid leaks in hot structures, such as struts, unless it can be shielded. The exception is β-21S, which is the only titanium alloy used in the aerospace industry that is not affected by this problem.
Titanium alloys are used from subzero temperatures to as high as ∼600°C. Titanium is unique in that some sheet alloys, such as Ti-6Al-4V, are superplastically formable using standard manufacturing procedures. For the other alloy systems, special alloys or processing have been developed to enable this capability, but they cannot achieve the same elongations observed with Ti-6Al-4V sheet.
Titanium alloys are generally difficult to machine, costing about 10 times as much as the machining of aluminum alloys. Stiff machines with high horsepower are required. The cutters must be kept sharp: Their lives are usually measured in minutes, as opposed to hours for aluminum. It is very difficult to grind titanium without inducing high residual stresses in the parts, which are detrimental to fatigue performance. Sanding should also be done with care. During sanding, extensive sparks can be thrown up. This must be minimized because, if one or more hot sparks land back on the titanium, they bond back in and are contaminated with interstitial elements, also resulting in a substantial fatigue debit. Care must also be taken with regard to the motion of a contacting surface against titanium, because titanium galls very easily. Some type of lubricant or coating must be used to eliminate this problem.
The increased use of CFRP composites has played a key role in titanium usage. The fact that titanium has a low coefficient of thermal expansion and is compatible with the graphite fibers in the composite in the presence of moisture, in conjunction with its low density and high strength, make it an ideal material for interfacing with composites.