Joining Properties Joining metals structuraly by welding, brazing, or soldering, or by such mechanical means as riveting or bolting,  is  a  tremendous  help  in  design  and  fabrication. When all other properties are equal, material that can be welded has the advantage. Shock and Fatigue Properties Aircraft metals are subject to both shock and fatigue (vibrational) stresses. Fatigue occurs in materials that are   exposed   to   frequent   reversals   of   loading   or repeatedly applied loads, if the fatigue limit is reached or  exceeded.  Repeated  vibration  or  bending  will ultimately cause a minute crack to occur at the weakest point.  As  vibration  or  bending  continues,  the  crack lengthens until complete failure of the part occurs. This is termed “shock and fatigue failure. ” Resistance to this condition is known as shock and fatigue resistance. It is essential  that  materials  used  for  critical  parts  be  resistant to  these  stresses. The  preceding  discussion  of  the  properties  and qualities  of  metals  is  intended  to  show  why  you  must know which traits in metals are desirable and which are undesirable to do certain jobs. The more you know about a given material, the better you can handle airframe repairs. METAL  WORKING  PROCESSES When metal is not cast in a desired manner, it is formed  into  special  shapes  by  mechanical  working processes. Several factors must be considered when determining whether a desired shape is to be cast or formed by mechanical working. If the shape is very complicated,  casting  will  be  necessary  to  avoid expensive machining of mechanically formed parts. On the other hand, if strength and quality of material are the prime   factors   in   a   given   part,   a   cast   will   be unsatisfactory. For this reason, steel castings are seldom used in aircraft work. There are three basic methods of metal working. They are hot-working, cold-working, and extruding. The  process  chosen  for  a  particular  application  depends upon the metal involved and the part required, although in  some  instances  you  might  employ  both  hot-  and cold-working methods in making a single part. Hot-Working Almost  all  steel  is  hot-worked  from  the  ingot  into some  form  from  which  it  is  either  hot-  or  cold-worked to the finished shape. When an ingot is stripped from its mold, its surface is solid, but the interior is still molten. The ingot is then placed in a soaking pit, which retards loss of heat, and the molten interior gradually solidifies. After  soaking,  the  temperature  is  equalized  throughout the ingot, which is then reduced to intermediate size by rolling,  making  it  more  readily  handled. The rolled shape is called a bloom when its sectional dimensions are 6 x 6 inches or larger and approximately square.  The  section  is  called  a  billet  when  it  is approximately  square  and  less  than  6  x  6  inches. Rectangular sections that have width greater than twice the  thickness  are  called  “slabs.”  The  slab  is  the intermediate  shape  from  which  sheets  are  rolled. HOT-ROLLING.—Blooms, billets, or slabs are heated above the critical range and rolled into a variety of  shapes  of  uniform  cross  section.  The  more  common of these rolled shapes are sheets, bars, channels, angles, I-beams, and the like. In aircraft work, sheets, bars, and rods are the most commonly used items that are rolled from  steel.  As  discussed  later  in  this  chapter,  hot-rolled materials are frequently finished by cold-rolling or drawing to obtain accurate finish dimensions and a bright,  smooth  surface. FORGING.—Complicated sections that cannot be rolled, or sections of which only a small quantity is required,  are  usually  forged.  Forging  of  steel  is  a mechanical working of the metal above the critical range to shape the metal as desired. Forging is done either by pressing or hammering the heated steel until the desired shape is obtained. Pressing is used when the parts to be forged are large and  heavy,  and  this  process  also  replaces  hammering where high-grade steel is required. Since a press is slow acting, its force is uniformly transmitted to the center of the section, thus affecting the interior grain structure as well as the exterior to give the best possible structure throughout. Hammering can be used only on relatively small pieces. Since hammering transmits its force almost instantly, its effect is limited to a small depth. Thus, it is necessary to use a very heavy hammer or to subject the part  to  repeated  blows  to  ensure  complete  working  of the section. If the force applied is too weak to reach the center,  the  finished  forging  surface  will  be  concave.  If the center is properly worked, the surface will be convex 1-25