•  Alloy 5052 is used where good workability, very  good corrosion resistance, high fatigue strength,  weldability, and moderate static strength are desired.  This alloy is used for fuel, hydraulic, and oil lines. •  Alloy 5056 is used for making rivets and cable  sheeting and in applications where aluminum comes  into contact with magnesium alloys. Alloy 5056  is generally resistant to the most common forms  of corrosion. •  Cast aluminum alloys are used for cylinder  heads, crankcases, fuel injectors, carburetors, and  landing wheels. •  Various alloys, including 3003, 5052, and 1100  aluminum, are hardened by cold working rather than  by heat treatment. Other alloys, including 2017 and  2024, are hardened by heat treatment, cold working,  or a combination of the two. Various casting alloys  are hardened by heat treatment. •  Alloy 6061 is generally weldable by all commercial  procedures and methods. It also maintains acceptable  toughness in many cryogenic applications. Alloy 6061  is easily extruded and is commonly used for hydraulic  and pneumatic tubing. •  Although higher in strength than 2024, alloy 7075  has a lower fracture toughness and is generally used  in tension applications where fatigue is not critical.  The T6 temper of 7075 should be avoided in corrosive  environments. However, the T7351 temper of 7075  has excellent stress corrosion resistance and better  fracture toughness than the T6 temper. The T76 temper  is often used to improve the resistance of 7075 to  exfoliate corrosion. Rivet Head Shape Structural Fasteners  Description Solid Shank Rivet  Structural fasteners, used to join sheet metal structures  securely, come in thousands of shapes and sizes with many  of them specialized and specific to certain aircraft. Since  some structural fasteners are common to all aircraft, this  section focuses on the more frequently used fasteners. For  the purposes of this discussion, fasteners are divided into two  main groups: solid shank rivets and special purpose fasteners  that include blind rivets. The solid shank rivet is the most common type of rivet used  in aircraft construction. Used to join aircraft structures, solid  shank rivets are one of the oldest and most reliable types of  fastener. Widely used in the aircraft manufacturing industry,  solid shank rivets are relatively low-cost, permanently  installed fasteners. They are faster to install than bolts  and nuts since they adapt well to automatic, high-speed  installation tools. Rivets should not be used in thick materials  or in tensile applications, as their tensile strengths are quite  low relative to their shear strength. The longer the total grip  length (the total thickness of sheets being joined), the more  difficult it becomes to lock the rivet.  Riveted joints are neither airtight nor watertight unless special  seals or coatings are used. Since rivets are permanently  installed, they must be removed by drilling them out, a  laborious task. Before installation, the rivet consists of a smooth cylindrical  shaft with a factory head on one end. The opposite end is  called the bucktail. To secure two or more pieces of sheet  metal together, the rivet is placed into a hole cut just a bit  larger in diameter than the rivet itself. Once placed in this  predrilled hole, the bucktail is upset or deformed by any of  several methods from hand-held hammers to pneumatically  driven squeezing tools. This action causes the rivet to expand  about 1 1 ⁄ 2 times the original shaft diameter, forming a second  head that firmly holds the material in place. Solid rivets are available in several head shapes, but the  universal and the 100° countersunk head are the most  commonly used in aircraft structures. Universal head rivets  were developed specifically for the aircraft industry and  designed as a replacement for both the round and brazier head  rivets. These rivets replaced all protruding head rivets and are  used primarily where the protruding head has no aerodynamic  significant. They have a flat area on the head, a head diameter  twice the shank diameter, and a head height approximately  42.5 percent of the shank diameter.  [Figure 4-74] The countersunk head angle can vary from 60° to 120°, but  the 100° has been adopted as standard because this head style  provides the best possible compromise between tension/ shear strength and flushness requirements. This rivet is used  where flushness is required because the rivet is flat-topped  and undercut to allow the head to fit into a countersunk or  dimpled hole. The countersunk rivet is primarily intended  Countersunk head Universal head Figure 4-74.  Solid shank rivet styles.  4-31