A substance that possesses metallic properties and is composed of two or more chemical elements, of which at least one is a metal, is called an "alloy." The metal present in the alloy in the largest proportion is called the "base metal." All other metals and/or elements added to the alloy are called "alloying elements." The metals are dissolved in each other while molten, and they do not separate into layers when the solution solidifies. Practically all the metals used in aircraft are made up of a number of alloying elements.

Alloying elements, either in small or in large amounts, may result in a marked change in the properties of the base metal. For example, pure aluminum is a relatively soft and weak metal, but by adding small amounts of other elements such as copper, manganese, magnesium, and zinc, its strength can be increased many times. Aluminum containing such other elements purposely added during manufacture is called an aluminum alloy.

In addition to increasing the strength, alloying may change the heat-resistant qualities of a metal, its corrosion resistance, electrical conductivity, or magnetic properties. It may cause an increase or decrease in the degree to which hardening occurs after cold-working. Alloying may also make possible an increase or decrease in strength and hardness by heat treatment. Alloys are of great importance to the aircraft industry in providing materials with properties that pure metals alone do not possess.

A wide variety of materials is required in the repair of aircraft. This is a result of the varying needs with respect to strength, weight, durability, and resistance to deterioration of specific structures or parts. In addition, the particular shape or form of the material plays an important role. In selecting materials for aircraft repair, these factors, plus many others, are considered in relation to their mechanical and physical properties.

Among the common materials used are ferrous metals. The term ferrous applies to the group of metals having iron as their principal constituent.

If carbon is added to iron, in percentages ranging up to approximately 1.00 percent, the product will be vastly superior to iron alone and is classified as carbon steel. Carbon steel forms the base of those alloy steels produced by combining carbon with other elements known to improve the properties of steel. A base metal (such as iron) to which small quantities of other metals have been added is called an alloy. The addition of other metals is to change or improve the chemical or physical properties of the base metal.

SAE NUMERICAL INDEX. -The steel classification of the Society of Automotive Engineers (SAE) is used in specifications for all high-grade steels used in automotive and aircraft construction. A numerical index system identifies the composition of SAE steels.

Each SAE number consists of a group of digits, the first of which represents the type of steel; the second, the percentage of the principal alloying element; and usually the last two or three digits, the percentage, in hundredths of 1 percent, of carbon in the alloy. For example, the SAE number 4150 indicates a molybdenum steel containing 1 percent molybdenum and 50 hundredths of 1 percent of carbon. Refer to the SAE numerical index, shown in table 1-1, to see how the various types of steel are classified into four-digit classification numbers.

HARDNESS TESTING METHODS. -Hardness testing is a factor in the determination of the results of heat treatment as well as the condition of the metal before heat treatment. There are two commonly used methods of hardness testing, the Brinell and the Rockwell tests. These tests require the use of specific machines and are covered later in this chapter. An additional, and somewhat indirect, method known as spark testing is used in identifying ferrous metals. This type of identification gives an indication of the hardness of the metal.

Spark testing is a common means of identifying ferrous metals that have become mixed. In this test, the piece of iron or steel is held against a revolving stone, and the metal is identified by the sparks thrown off. Each ferrous metal has its own peculiar spark characteristics. The spark streams vary from a few tiny shafts to a shower of sparks several feet in length. Few nonferrous metals give off sparks when touched to a grinding stone. Therefore, these metals cannot be successfully identified by the spark test.

Wrought iron produces long shafts that are a duIl red color as they leave the stone, and they end up a white color. Cast iron sparks are red as they leave the stone, but turn to a straw color. Low-carbon steels give off long, straight shafts that have a few white sprigs. As the carbon content of the steel increases, the number of sprigs along each shaft increases, and the stream becomes whiter in color. Nickel steel causes the spark stream to contain small white blocks of light within the main burst.