Fittings must be made of carefully selected materials because of their importance of holding the aircraft together under expected stress and loading. The same holds true for important fasteners such as bolts and rivets. It is essential that these parts not fail under stress. It is also essential that these parts not weaken with exposure to stress and weather elements. UFOS have been observed that seem to have seamless, rivetless hulls which could give such a craft high structural integrity.
Corrosion is also a consideration.
A fitting made of one metal cannot be secured to the structure with a bolt or fastener made of another metal. For example, while in straight and level flight the engine power and propeller are pulling the airplane forward. The wings, tail section and fuselage, however, resist that movement because of the airflow around them. The result is a stretching effect on the airframe. Bracing wires in an aircraft are usually in tension.