The most common metals in aircraft construction are aluminum, magnesium, titanium, steel, and their alloys. Aluminum alloy is widely used in modern aircraft construction. It is vital to the aviation industry because the alloy has a high strength-to-weight ratio. Aluminum alloys are corrosion-resistant and comparatively easy to fabricate. The outstanding characteristic of aluminum is its lightweight.

Magnesium, the worlds lightest structural metal, is a silvery-white material weighing only two-thirds as much as aluminum. Magnesium is used in the manufacture of helicopters. Magnesiums low resistance to corrosion has limited its use in conventional aircraft.

Titanium is a lightweight, strong, corrosion-resistant metal. It was discovered years ago, but only recently has it been made suitable for use in aircraft. Recent developments make titanium ideal for applications where aluminum alloys are too weak and stainless steel is too heavy. In addition, titanium is unaffected by long exposure to seawater and marine atmosphere.

An alloy is composed of two or more metals. The metal present in the alloy in the largest portion is called the base metal. All other metals added to the alloy are called alloying elements. Alloying elements, in either small or large amounts, may result in a marked change in the properties of the base metal. For example, pure aluminum is relatively soft and weak. When small amounts of other elements such as copper, manganese, and magnesium are added, aluminums strength is increased many times. An increase or a decrease in an alloys strength and hardness may be achieved through heat treatment of the alloy. Alloys are of great importance to the aircraft industry. Alloys provide materials with properties not possessed by a pure metal alone.

Alloy steels that are of much greater strength than those found in other fields of engineering have been developed. These steels contain small percentages of carbon, nickel, chromium, vanadium, and molybdenum. High-tensile steels will stand stresses of 50 to 150 tons per square inch without failing. Such steels are made into tubes, rods, and wires.

Another type of steel that is used extensively is stainless steel. This alloy resists corrosion and is particularly valuable for use in or near salt water.

In addition to metals, various types of plastic materials are found in aircraft construction. Transparent plastic is found in canopies, windshields, and other transparent enclosures. Handle transparent plastic surfaces with care, because this material is relatively soft and scratches easily. At approximately 225F, transparent plastic becomes soft and very pliable. Reinforced plastic is made for use in the con-struction of radomes, wing tips, stabilizer tips, antenna covers, and flight controls. Reinforced plastic has a high strength-to-weight ratio and is resistant to mildew and rot. Its ease of fabrication make it equally suitable for other parts of the aircraft.

Reinforced plastic is a sandwich-type material. See figure 1-22. It is made up of two outer facings and a center layer. The facings are made up of several layers of glass cloth, bonded together with a liquid resin. The core material (center layer) consists of a honeycomb structure made of glass cloth. Reinforced plastic is fabricated into a variety of cell sizes.

High-performance aircraft require an extra high strength-to-weight ratio material. Fabrication of composite materials satisfies the special requirement. This construction method uses several layers of bonding

materials (graphite epoxy or boron epoxy). These materials are mechanically fastened to conventional substructures. Another type of composite construction consists of thin graphite epoxy skins bonded to an aluminum honeycomb core.

Metallurgists have been working for more than a half century improving metals for aircraft construction. Each metal has certain properties and characteristics that make it desirable for a particular application, but it may have other qualities that are undesirable. For example, some metals are hard, others comparatively soft; some are brittle, some lough; some can be formed and shaped without fracture; and some are so heavy that weight alone makes them unsuitable for aircraft use. The metallurgists objectives are to improve the desirable qualities and tone down or eliminate the undesirable ones. This is done by alloying (combining) metals and by various heat-treating processes.

You do not have to be a metallurgist to be a good AM, but you should possess a knowledge and understanding of the uses, strengths, limitations, and other characteristics of aircraft structural metals. Such knowledge and understanding is vital to properly construct and maintain any equipment, especially airframes. In aircraft maintenance and repair, even a slight deviation from design specifications or the substitution of inferior materials may result in the loss of both lives and equipment. The use of unsuitable materials can readily erase the finest craftsmanship. The selection of the specific material for a specific repair job demands familiarity with the most common properties of various metals.