Non-Ferrous Metals and Alloys

Aluminium Alloys

  • Pure aluminium is very soft and malleable, making it quite unusable for most aircraft structures.
  • This is why it is usually alloyed or hot rolled to the surface of other metals as a skin.
    • This is because Aluminium forms a corrosion resistant layer.
    • This use is usually called Alclad covering.
  • Aluminium alloys can be separated into two groups:
    • Wrought Alloys: These are alloys that can be treated.
      • These are again split into 2 categories: Heat-treatable/Non-heat treatable
    • Casting Alloys: These are alloys that are used in casting (i.e. lower melting points)
  • Aluminium alloys have a numbering system which explains the main alloying component and other information. This is shown below
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Aluminium Copper (2XXX)

  • These are usually used in their Wrought state - Heat Treatable.
  • Contains 2.5-5% Copper
  • A famous alloy would be Duralumin (2017- 4% Cu)
  • Enhances ductility and malleability of Aluminium
  • High tensile strengths
  • Prevents stress cracks and makes Aluminium more shock resistant
  • Affected by Age/Precipitation Hardening (explained later)
  • Used in aircraft structures (e.g. frames)

Aluminium Silicon (4XXX)

  • These can be used in both Wrought and Casting forms, but mostly Casting.
  • Usually contains up to 9-13% Silicon
  • Increases hardness of Aluminium without increasing brittleness.
  • Corrosion Resistant
  • Has a low melting point and other factors that make it a great casting material, making it suitable for die-casting.
  • When used in the Wrought state, has great forgability and shape stability.
  • Used in gearboxes, etc.

Aluminium Magnesium (5XXX)

  • These are used in Wrought forms - Non-heat treatable
  • Usually mixed with other materials, up to about 3% Mg
  • A famous allow would be 5052 (2.5% Mg, 0.2% Cu)
  • Hardness and Tensile Strength are increased
  • Often used as a sheet(e.g. surfaces)
  • 5056 can be used in rivets to prevent a galvanic reaction.

Aluminium Magnesium and Silicon (6XXX)

  • Used in both Wrought and Casting forms - Heat-treatable.
  • Magnesium Sillicide ($Mg_2Si$) is formed with Magnesium and Silicon.
  • Wrought forms have great corrosion resistance and more workability
  • Casting forms have good castability, strength, corrosion resistance and pressure-tightness
  • For aircrafts, the casting form is often used in aircraft instruments.

Copper Alloys

  • Pure copper finds use as an excellent conductor (See Telecommunications) but other than that it is usually undesirable.
  • Copper and its alloys have been used since 3000 BC

Brass

  • Brass implies any sort of Copper - Zinc Alloy
    • This can be even up to 45% Zinc, but most don't go beyond 37% as it becomes brittle and often unusable.
    • However alloys with less than 37% have a single phase structure that is consisted of only of alpha grains.
  • Its typical properties are tough, ductle and easily cold worked
  • Common brass include:
    • Catridge Brass (Cu 70%, Zn 30%)
      • It is very ductile, and perfect for deep drawing.
      • It got its name from being used in cartridges for bullets.
    • Standard Brass (Cu 75%. Zn 25%)
      • A high quality brass used for cold working
      • It's used for stamping and limited drawing

Heat treatment

Precipitation hardening

  • This is a method that aims to:
    • Controllingly segregate the different elements into uniform distributions.
    • Warp the lattice structure and thus cause an increase in hardness and strength.
  • This is done in 3 steps:
    • Solution treatment: The alloy is heated to a temperature so that a homogenous structure is formed in the alloy.
      • Note that this merely means a uniform structure is present
      • However, this is when the elements are in random places, just uniformly distributed.
    • Quenching: The alloy is then rapidly cooled, preventing the solution from segregating.
      • This is to retain the homogenous structure formed at high temperature
    • Ageing/Precipitation: This can be done in 2 ways:
      • Natural Ageing: The alloy is allowed to stand at room temperature for a long time (Around a week)
      • Artificial Ageing/Precipitation Hardening: The alloy is slightly heated (200C) and left for several hours
    • Both these methods produce the same result: similar particles gather in groups, warping the bonds of the lattice structure.
    • Over-ageing can break necessary bonds and reduce the quality of the material.