| Qualities of Metal HARDNESS. The quality that permits relative resistance to abrasion, penetration, cutting action, or permanent distortion is hardness. This property can be increased by working the metal and, in the case of steel and certain aluminum alloys, by heat treatment and/or cold working. Structural parts are often formed from metals in their soft state and are then heat treated to harden them enough to develop the strength necessary to retain their finished shape. Hardness and strength are closely associated metal properties. BRITTLENESS. The quality in a metal that permits breaking or cracking when the metal is stressed is brittleness. Brittle metal can break or crack without changing shape. Because structural metals are often heavily stressed, brittleness is an undesirable property. Cast iron, cast aluminum, and very hard steels are examples of brittle metals. MALLEABILITY. The property in a metal that permits it to be hammered, rolled, or pressed into various shapes without the metal cracking or breaking is malleability. This property is required in sheet metal to be worked into curved shapes such as cowlings, fairings, and contoured skin. Copper is an example of a malleable metal. DUCTILITY. When a metal can be permanently drawn, bent, or twisted into various shapes without breaking, it has ductility. This property is essential in metals used to make wire or tubing. Ductile metals are much preferred for aircraft because they are easily formed and resist failure under loads. For this reason, aluminum alloys are used for cowl rings, fuselage, wing skin, and formed or extruded parts such as ribs, spars, and bulkheads. Chrome-molybdenum steel is easily formed into the desired shapes. Although malleable and ductile are frequently shown as synonymous, the two differ in meaning. Note: Malleable metals are generally shaped by compressive methods, and ductile metals are shaped by expansive methods. ELASTICITY. The characteristic that enables a metal to return to its original shape when stresses are removed is elasticity. This quality, or property, is valuable because it is highly undesirable to have a part permanently distorted after an applied load is removed. However, each metal has a point, its elastic limit, beyond which it cannot be loaded without permanent distortion. Members and parts used in aircraft construction are designed so that the maximum loads to which they are subjected never stress them beyond their elastic limits. An example of a metal with a high elasticity limit is spring steel. CONDUCTIVITY. The characteristic of a metal that enables it to transfer heat or electricity is conductivity. Heat conductivity in a metal is especially important in welding because it governs the amount of heat required for proper fusion. To a certain extent, a metal's conductivity determines the type of jig to be used to control expansion and contraction during repairs. Metals vary in how they conduct heat and electricity; to eliminate radio interference, careful thought should go into selecting metals to be bound together. For example, copper has a relatively high degree of heat conductivity, and its low resistance makes it a good electrical conductor. Aluminum, on the other hand, is a good heat conductor, but it is a poor electrical conductor. DENSITY. The mass per unit volume of a substance is its density. In aircraft work the preferred unit of volume is the cubic inch. This unit is convenient in determining the weight of a part before manufacture. Give careful thought to the density of material to be used because it affects the weight and balance of an aircraft. CONTRACTION AND EXPANSION. Heat applied to a metal causes it to expand or become larger, and cooling metal causes it to contract or shrink. Contraction and expansion affect the design of welding jigs, castings, and tolerances necessary for hot-rolled metal. STRENGTH AND TOUGHNESS. When a metal can hold loads or withstand an applied force without breaking, it has strength. This is a property that encompasses many of the desirable qualities of metals. Strength with toughness is the most important combination of properties a metal can possess. Metals with this combination are used for vital structural members that may become overloaded in service. Toughness describes the resistance of a metal to tearing or shearing and permits the metal to be stretched or otherwise deformed without breaking.
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