Machinability is optimum with the T 851 precipitation heat treated condition. The heat treatment is as follows: the alloy is "solution annealed at 920 F followed by water quench. Precipitation hardening is done at 375 F for 12 hours and air cooling - this produces the T 851 temper." (Metal Suppliers, 2006).Aluminum alloy 2124 has been selected for the main use of our aircraft. The reasons for this are: although aluminum 2124 has a lower strength and lower hardness than 7475, the fracture toughness is higher in aluminum 2124. This will contribute to a lower maintenance cost for aluminum 2124, all other things held equal.Other mechanical properties of aluminum 2124 and aluminum 7475 are virtually the same. The shear strength and shear modulus of aluminum alloys 2124 and 7475 are very similar. Their densities are almost exactly the same, as well as their moduli of elasticities.The composite selected for our aircraft is Kevlar. The reason for using Kevlar is its high ultimate tensile strength, which is almost twice as high as CFRPs. Kevlar is the lightest material that was studied here; according to table 2, Kevlar has the lowest density. Kevlar also has a relatively low cost; according to the research done for table 1, Kevlar was found to be cheaper than the aluminum alloys.The locations of the materials in our aircraft design are as follows: the aluminum alloy 2124 will be used for the fuselage skin; and Kevlar will be used for the empennage and for the ailerons, elevators and rudder of our aircraft design. Composite materials such as Kevlar can be used in these aircraft areas in order to save weight. This is similar to the Boeing Business Jet 2 (BBJ. Materials for a Business Jet.
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