COLD-ROLLING.—Cold-rolling usually refers to the working of metal at room temperature. In this operation, the materials that have been hot-rolled to approximate sizes are pickled to remove any scale, after which they are passed through chilled finished rolls. This action gives a smooth surface and also brings the pieces to accurate dimensions. The principal forms of cold-rolled stocks are sheets, bars, and rods. COLD-DRAWING.—Cold-drawing is used in making seamless tubing, wire, streamline tie rods, and other forms of stock. Wire is made from hot-rolled rods of various diameters. These rods are pickled in acid to remove scale, dipped in lime water, and then dried in a steam room, where they remain until ready for drawing. The lime coating adhering to the metal serves as a lubricant during the drawing operation. Figure 1-23 shows the drawing of rod, tubing, and wire. The size of the rod used for drawing depends upon the diameter wanted in the finished wire. To reduce the rod to the desired wire size, it is drawn cold through a die. One end of the rod is filed or hammered to a point and slipped through the die opening, where it is gripped by the jaws of the draw, then pulled through the die. This series of operations is done by a mechanism known as the drawbench, as shown in figure 1-23. To reduce the rod gradually to the desired size, it is necessary to draw the wire through successively smaller dies. Because each of these drawings reduces the ductility of the wire, it must be annealed from time to time before further drawings can be accomplished. Although cold-working reduces the ductility, it increases the tensile strength of the wire enormously. In making seamless steel aircraft tubing, the tubing is cold-drawn through a ring-shaped die with a mandrel or metal bar inside the tubing to support it while the drawing operations are being performed. This forces the metal to flow between the die and the mandrel and affords a means of controlling the wall thickness and the inside and outside diameters. and other favorable properties, can be economically extruded to more intricate shapes and larger sizes than is practicable with many other metals. Extruded shapes are produced in very simple as well as extremely complex sections. A cylinder of aluminum, for instance, is heated to 750°F to 850°F, and is then forced through the opening of a die by a hydraulic ram. Many structural parts, such as stringers, are formed by the extrusion process. ALLOYING OF METALS 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. Extruding FERROUS AIRCRAFT METALS The extrusion process involves the forcing of metal through an opening in a die, thus causing the metal to take the shape of the die opening. Some metals such as lead, tin, and aluminum may be extruded cold; but generally, metals are heated before the operation is begun. The principal advantage of the extrusion process is in its flexibility. Aluminum, because of its workability 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. 1-27