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