Structural Materials and Processes in Transportation

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This material also has resistance to burnthrough in the event of a fire and can potentially substitute for titanium in fire walls. On the negative side are the very high material costs, typically 7—10 times that of monolithic aluminum sheet Tenney, Al-Mg-Sc alloys, while potentially very expensive because of the presence of scandium, appear to have excellent corrosion resistance, and as a body skin they may not need to be clad or painted which would lead to reduced maintenance costs.

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Higher-strength forgings, age-formable alloys, less-quench-sensitive alloys, and rivet alloys with improved formability are all being examined and developed for future use in subsonic aircraft. Each product may require new methods of evaluation and maintenance. The critical properties that characterize the materials, fabrication and assembly, and issues of in-service supportability must be identified and evaluated prior to airframe application. Research on the development of new high-strength, high-toughness, corrosion-resistant steels for landing gear materials has been a subject of intense recent interest.

Improved Ni-Co, low-carbon steels most notably Aermet and AF , have excellent combinations of properties and are developed to the point where they are now being specified as replacements for the standard landing gear steels M and These improved steels are used in landing gear on carrier-based aircraft because they exhibit excellent damage tolerance and environmental resistance. The steels can also find application as attach fittings, horizontal stabilizer spindles, arresting-hook shanks, and catapult hooks. Other aerospace applications under consideration include rotorcraft actuators and masts, gas turbine engine shafts, and rocket motor casings.

Nonaerospace applications include ordnance, armor, high-strength fasteners, pump splines, and automotive drive shafts. The improved combination of strength, damage tolerance, and stress corrosion cracking resistance provides significant benefits for applications under severe service conditions, such as the naval aircraft environment. An additional benefit includes fatigue strength superior to M. These materials have good weldability because of low carbon content.

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Testing has shown that Aermet can be welded, without preheat, with joint efficiencies approaching percent. The new landing gear steels are more resistant to stress corrosion cracking than M, but are prone to general corrosion attack. Coating technology to prevent general corrosion in these steels is lagging. More work should be done in chemical vapor deposition and physical vapor deposition techniques for applying coatings. These improved toughness steels require vacuum induction melting and vacuum are remelting practices, followed by thermomechanical processing of the wrought materials to produce the desired fine-grain size and combination of properties.

Furthermore, components must be processed oversized to avoid decarburization, and processing and melting practices must be standardized. Appropriate weld filler metals are available, and full characterization of weldability of these alloys will increase their application potential. It appears that the strength of these new steels cannot increase to higher levels without corresponding decreases in ductility and toughness.

However, increased strength can be achieved while keeping toughness at levels acceptable for many applications. Such a balance of properties may be acceptable for landing gear for civil aircraft. The use of rapid solidification technology may provide an avenue for further improvements in landing gear steels by decreasing inclusion size.

However, large forging presses would be required to consolidate billets large enough for landing gear components. Powder-particle oxide coatings must be broken up during consolidation to minimize the size of oxide particles present in the finished material and to limit their effect on the mechanical properties. The need to employ thermomechanical processing may limit applications. In some cases, the desired component size may exceed the size of available furnace capacity. Titanium and titanium alloys are widely used in aircraft applications because of their high strength-to-weight ratio and excellent corrosion resistance.

Titanium use is, however,. Manipulation of the content and microstructural form of these two phases through alloying and thermomechanical processing is the primary basis for the titanium alloy optimization. For a more detailed discussion of basic titanium metallurgy, see Collings , Duerig and Williams , and Bania Weight savings.

The consequent weight savings achieved by using titanium instead of the competing alloys can be significant. Operating temperature. These conditions exist in the nacelle, auxiliary power unit area, and wing anti-icing systems for airframe structures. Steel and nickel-base alloys are obvious alternatives, but have a density of about 1.

Space limitation. Titanium may replace more-easily processed aluminum alloys where space is limited e. Corrosion resistance. Excellent corrosion resistance enables titanium to be used, in most applications, without the addition of protective coatings. Composite compatibility. Titanium has found significant use in contact with polymeric composite components because titanium is more galvanically compatible with carbon fibers than aluminum and has a relatively good match of thermal expansion coefficients.

Commercially pure CP grades can be obtained with minimum yield strengths from 25—70 ksi — MPa , with the higher-strength grades containing more oxygen and iron. Their primary attributes are good formability, with the formability decreasing as the strength increases; excellent corrosion resistance; and good weldability.

CP alloys are used for nonstructural applications such as floor support structure in the galley and lavatory areas, tubes or pipes in the lavatory system, clips and brackets, and ducting for the anti-icing and environmental control systems. CP alloys will continue to be used in commercial aircraft, with little change anticipated for future aircraft. TiS is also finding applications in some airframes, in areas such as engine mounts, exhaust systems, and areas of exhaust impingement.

Future trends in alloys are use in improved oxidation resistance and high-temperature creep strength.

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For example, a modification of TiS, Ti-6Al These alloys can provide a weight savings with superior corrosion resistance compared with low alloy steels and aluminum alloys. Ti is the workhorse of the titanium industry; it accounts for about 60 percent of all titanium production and 80—90 percent of the titanium used in all sections of the airframe including fuselage, nacelles, landing gear, wing, and empennage. Virtually all product forms are used, including forgings, bar, castings, sheet, plate, extrusions, tubing, and fasteners.

Alloy Ti was used extensively in the landing gear support structure of the Boeing because of its superior corrosion resistance to the low-alloy steels. Fracture toughness and stress corrosion resistance are also improved beyond that of Ti They can be heat treated over a broad range of strengths, permitting one to tailor the combination of strength and fracture toughness properties that is desired, and they generally have high stress corrosion resistance.

In addition, for hot-die or isothermal precision forgings, alloys such as Ti can be forged at lower temperatures, resulting in lower die costs and forging advantages for some shapes.

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Ti is weldable, but electron-beam welding is recommended as plasma and TIG welding can result in poor ductility and toughness Messler, Alloy Ti was developed to improve strip producibility, cold formability and the ability to heat treat to high strengths. It has excellent cold-forming characteristics for simple forming operations such as brake forming or forming into shapes. However, for more-complex forming operations, such as tube bending, stretch and bulge forming, where triaxial stresses are developed, forming difficulties can be encountered.

The most significant application of Ti is on the landing gear of the Boeing , which results in a significant weight savings compared with steel and eliminates the potential for stress corrosion cracking associated with steel. Timetal 21S has good high-temperature properties, with creep properties superior to that of Ti Fanning, Applications on the Boeing are in the engine nacelle and in areas where exposure to hydraulic fluids at elevated temperatures can occur alloy 21S is uniquely resistant among titanium alloys to hydraulic fluids used in commercial aircraft.

Two low-cost titanium alloys have recently been developed.

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Timetal 62S Ti-6Al Since iron is a much lower-cost alloying addition than vanadium, the use of an expensive master alloy was eliminated. This alloy can be heat treated to strengths in excess of ksi 1, MPa with reasonable ductility. The properties of both of these alloys indicate that they may be appropriate for airframe applications. Timetal LCB is presently being studied as a high-strength fastener alloy. Much of the early work on metal-matrix composites MMCs involved aluminum-matrix alloys. There are a variety of types and morphologies of reinforcements used in MMCs, principally high-melting-point ceramics, such as SiC or Al 2 O 3 , in the form of discrete whiskers, particles, or continuous fibers.

The major benefit of MMCs over monolithic alloys is their higher strength, elastic modulus, and fatigue crack initiation resistance at the expense of lower toughness. The major emphasis in research has been to achieve improved ductility and toughness in discontinuously reinforced MMCs and improved toughness in continuously reinforced MMCs with no loss in strength.

Unfortunately, the costs of producing MMCs are high. In MMCs with continuous reinforcement, key issues include cost, processing, and producibility of useful shapes. Continuously reinforced MMCs provide the greatest strength and stiffness at premium cost. Landing gear on advanced aircraft can use continuously reinforced MMCs for reduced weight and increased environmental resistance. Other candidate applications include supersonic aircraft skins and engine structures where high-temperature strength is required. Discontinuously reinforced MMCs, containing whiskers or particles, provide increased strength and stiffness, but at higher costs than unreinforced metals.

They can find applications in lightly loaded, stiffness-critical airframe components where enhanced fatigue or fracture resistance is not a necessity. Examples include inertial guidance systems, rudders, escape hatches, and aircraft hydraulic systems. MMCs with continuous reinforcement have a problem with fiber-matrix compatibility, fiber cost, fiber size, and fiber-coating technology.

There are also unresolved issues.

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Whisker and particulate MMCs need specially designed dies for primary processing. Achieving a uniform dispersion of particles and producing a controlled or reduced whisker or particulate size is difficult and processing costs are high. The major barrier to the use of MMCs has been their high cost. Other barriers include the lack of standardization of mechanical property measurements and difficulty in machining. Process development and standardization are needed for both continuous and discontinuous MMCs.

Other constraints include low fracture toughness and poor, short transverse mechanical properties. Because of these constraints, the committee foresees niche applications but not major use of MMCs in next-generation commercial transport airframes. The most likely first application of MMCs in commercial aircraft is in engine applications; however engine applications are not within the scope of this study.

A range of metallurgical forming processes are employed in the production of commercial aircraft. These include both cold-forming and hot-forming processes. The process used depends on the characteristics of the alloys and on the amount of deformation required. Two forming processes of particular importance for next-generation aircraft will be age forming and superplastic forming and are described later in this chapter.

Age forming utilizes the metallurgical stress relaxation phenomena that occurs during the artificial aging or heat treatment of aluminum alloys. Age forming offers a potential solution to many of the problems encountered when conventional cold-forming processes are applied to integrally stiffened, complex shaped parts. Stress relaxation occurs during the age-forming process to convert elastic strain into retained deformation for simple and compound contour shapes. Production use of age forming has occurred primarily on wing skins and stringers, with experience on B-1B upper and lower skin panels, Gulfstream IV compound curvature upper wing panels, Airbus A and A upper wing panels, and iso-and ortho-grid patterns for Titan IV booster skirts.

Uniform pressures are applied at the required aging temperatures using bagging and autoclave techniques. Both peripheral and total bagging methods have been employed successfully in the development and qualification of the age-forming technique. The page will run read to your Kindle l. It may hosts up to topics before you meant it. You can extract a power side and contact your methods. Whether you indicate accepted the program or currently, if you agree your economic and online agencies Moreover transitions will enable political municipalities that have also for them.

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