References

[1] Peel, C.J., Gregson, P.J., "Design Requirements for Aerospace Structural Materials", in High Performance Materials in Aerospace, Chapman & Hall, 1995, pp. 1-48.

[2] Ekvall, J.C., Rhodes, J.E., Wald, G.G., "Methodology of Evaluating Weight Savings From Basic Material Properties", in Design of Fatigue and Fracture Resistant Structures, ASTM STP 761, American Society for Testing and Materials, 1982, pp. 328-341.

[3] Barington, N., Black, M., "Aerospace Materials and Manufacturing Processes at the Millenium", in Aerospace Materials, Institute of Physics Publishing, 2002, pp. 3-14.

[4] Cotton, J.D., Clark, L.P., Phelps, H.R., "Titanium Alloys on the F-22 Fighter Aircraft", Advanced Materials & Processes, May 2002, pp. 25-28.

[5] Williams, J.C., Starke, E.A., "Progress in Structural Materials for Aerospace Systems", Acta Materialia, Vol. 51, 2003, pp. 5775-5799.

[6] Molloy, W.J., "Investment-Cast Superalloys - A Good Investment", Advanced Materials & Processes, October 1990, pp. 23-30.

[7] Freeman, W.T., "The Use of Composites in Aircraft Primary Structure", Composites Engineering, Vol. 3, Nos 7-8, 1993, pp. 767-775.

[8] Taylor, A., "RTM Material Developments for Improved Processability and Performance", SAMPE Journal, Vol. 36, No. 4, July/August 2000, pp. 1-24.

[9] Piellisch, R., "Composites Roll Sevens", Aerospace America, Vol. 30, No. 10, pp. 26-43.

[10] Buckley, J.D., "Carbon-Carbon Composites", in Handbook of Composites, Chapman & Hall, 1998, pp. 333-351.

[11] "Transport Construction Costs Studied", Aviation Week and Space Technology, Vol. 107, No. 24, pp. 68-70.

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Chapter 2

Aluminum

A typical material distribution for a modern commercial airliner, shown in Fig. 2.1, illustrates the heavy dominance of aluminum alloys. The attractiveness of aluminum is that it is a relatively low cost, light weight metal that can be heat treated to fairly high strength levels, and it is one of the more easily fabricated of the high performance materials, which usually results in lower costs. The advantages of aluminum as a high performance material can be summarized:

• High strength-to-weight ratio. The high strength 2XXX and 7XXX alloys are competitive on a strength-to-weight ratio with the higher strength but heavier titanium and steel alloys, and thus have traditionally been the predominate structural material in both commercial and military aircraft.

• Cryogenic properties. Aluminum alloys are not embrittled at low temperatures and become even stronger as the temperature is decreased without significant ductility losses, making them ideal for cryogenic fuel tanks for rockets and launch vehicles.

• Fabricability. Aluminum alloys are among the easiest of all metals to form and machine. The high strength 2XXX and 7XXX alloys can be formed in a relatively soft state and then heat treated to much higher strength levels after forming operations are completed.

Aluminum is also a consumer metal of great importance. In addition to the advantages cited above, other properties of commercial importance include corrosion resistance to natural atmospheres, suitability for food and beverage storage, high electrical and thermal conductivity, high reflectivity, and ease of recycling. As a result of a naturally occurring tenacious surface oxide

Other 1%

Other 1%

Fig. 2.1. Material Distribution for Boeing 777 Aircraft1

film (Al2O3), a great number of aluminum alloys provide exceptional resistance to corrosion in many atmospheric and chemical environments. Pure aluminum and some of its alloys have exceptionally high electrical conductivities, second only to copper for alloys commonly used as conductors. In addition, aluminum and its alloys are among the easiest to recycle of any of the structural materials.

Disadvantages of high strength aluminum alloys include a low modulus of elasticity, rather low elevated temperature capability, and susceptibility to corrosion. The modulus of elasticity of aluminum alloys is generally between 10 and 11msi, which is lower than competing metals, such as titanium (16msi) and steel (29msi). Although aluminum alloys can be used for short times at temperatures as high as 400-500° F, their long-term usage temperatures are usually restricted to 250-300° F. Finally, although commercially pure aluminum and many aluminum alloys are very corrosion resistant, corrosion can be a problem for the highly alloyed high strength aluminum alloys used in aerospace.

This chapter is organized as follows. First, some of the general metallurgical considerations for aluminum alloys are discussed along with an introduction to precipitation hardening, the main method of strengthening the aerospace structural alloys. The next section covers the designation system and tempers for the various series of aluminum alloys. Then, some of the specific alloys used for aerospace applications are discussed, primarily the 2XXX, 6XXX, 7XXX, and 8XXX aluminum-lithium alloys. Melting and primary fabrication, which are mill processes, are then briefly discussed. The remainder of the chapter then covers the main fabrication processes, i.e. heat treatment, forging, forming, casting, machining, and joining.

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