Conductivity Conductivity is the property that enables a metal to carry heat or electricity. The heat conductivity  of a metal is  especially  important  in  welding,  because  it  governs the amount of heat that will be required for proper fusion.  Conductivity  of  the  metal,  to  a  certain  extent, determines the type of jig to be used to control expansion and contraction. In aircraft, electrical conductivity must also  be  considered  in  conjunction  with  bonding,  which is used to eliminate radio interference. Metals vary in their capacity to conduct heat. Copper, for instance, has a relatively high rate of heat conductivity and is a good electrical  conductor. Contraction  and  Expansion Contraction  and  expansion  are  reactions  produced in metals as the result of heating or cooling. A high degree of heat applied to a metal will cause it to expand or  become  larger.  Cooling  hot  metal  will  shrink  or contract  it.  Contraction  and  expansion  affect  the  design of  welding  jigs,  castings,  and  tolerances  necessary  for hot-rolled  material. QUALITIES  OF  METALS The  selection  of  proper  materials  is  a  primary consideration in the development of an airframe and in the proper maintenance and repair of aircraft. Keeping in mind the general properties of metals, it is now possible  to  consider  the  specific  requirements  that metals must meet to be suitable for aircraft purposes. Strength,  weight,  and  reliability  determine  the requirements to be met by any material used in airframe construction and repair. Airframes must be strong and as light in weight as possible. There are very definite limits to which increases in strength can be accompanied by increase in weight. An aircraft so heavy that it could not  support  more  than  a  few  hundred  pounds  of additional weight would be of little use. All metals, in addition to having a good strength/weight ratio, must be thoroughly  reliable,  thus  minimizing  the  possibility  of dangerous and unexpected failures. In addition to these general properties, the material selected for definite application must possess specific qualities suitable for the purpose. These specific qualities are discussed in the following  text. Strength The material must possess the strength required by the  demands  of  dimensions,  weight,  and  use.  There  are five  basic  stresses  that  metals  may  be  required  to withstand.  These  are  tension,  compression,  shear, bending,  and  torsion.  Each  was  discussed  previously  in this  chapter. Weight The relationship between the strength of a material and its weight per cubic inch, expressed as a ratio, is known as the strength/weight ratio. This ratio forms the basis  of  comparing  the  desirability  of  various  materials for use in airframe construction and repair. Neither strength nor weight alone can be used as a means of true comparison.  In  some  applications,  such  as  the  skin  of monocoque structures, thickness is more important than strength; and in this instance, the material with the lightest weight for a given thickness or gauge is best. Thickness  or  bulk  is  necessary  to  prevent  buckling  or damage  caused  by  careless  handling. Corrosive Properties Corrosion is the eating away or pitting of the surface or the internal structure of metals. Because of the thin sections  and  the  safety  factors  used  in  aircraft  design  and construction, it would be dangerous to select a material subject to severe corrosion if it were not possible to reduce  or  eliminate  the  hazard.  Corrosion  can  be reduced or prevented by using better grades of base metals; by coating the surfaces with a thin coating of paint,   tin,   chromium,   or   cadmium;   or   by   an electrochemical  process  called  “anodizing.”  Corrosion control is discussed at length in  Aviation  Maintenance Ratings  Fundamentals,  and it is not covered in detail in this  TRAMAN. Working  Properties Another  significant  factor  to  consider  in  the selection of metals for aircraft maintenance and repair is the ability of material to be formed, bent, or machined to  required  shapes.  The  hardening  of  metals  by cold-working or forming is called  work hardening. If a piece of metal is formed (shaped or bent) while cold, it is said to be cold-worked. Practically all the work you do on metal is cold-work. While this is convenient, it causes the metal to become harder and more brittle. If the metal is cold-worked too much (that is, if it is bent back and forth or hammered at the same place too often),  it  will  crack  or  break.  Usually,  the  more malleable and ductile a metal is, the more cold-working it can withstand. 1-24