Machinability of Nonferrous Metals

Following is a summary of the machinability of nonferrous metals and alloys, in alphabetic order:

  • Aluminum is very easy to machine, although the softer grades tend to form a built-up edge, resulting in a poor surface finish. Thus, high cutting speeds, high rake angles, and high relief angles are recommended. Wrought aluminum alloys with high silicon content and cast aluminum alloys are abrasive; hence, they require harder tool materials.
  • Controlling of dimensional tolerance may be a problem in machining of aluminum because it has a high thermal expansion coefficient and a relatively low elastic modulus.
  • Beryllium is machinable, but because the fine particles produced during machining are toxic, it requires machining in a controlled environment.
  • Cobalt-based alloys are abrasive and highly work hardening. They require fine, abrasion-resistant tool materials, small feeds, and speeds.
  • The wrought condition of copper can be difficult to machine because of built-up edge formation, although cast copper alloys are easy to machine. Brasses are easy to machine, in particular with the addition of lead (leaded free-machining brass). Note, however, the toxicity of lead and associated environmental concerns. Bronzes are more difficult to machine than brass.
  • It is very easy to machine magnesium, with good surface finish and prolonged tool life. However, care needs to be taken because of its high rate of oxidation (pyrophoric) and the danger of fire.
  • Molybdenum is ductile and works hardening. It can produce a poor surface finish, to produce good surface finish sharp tools are essential.
  • Nickel-based alloys and superalloys are work hardening, abrasive, and strong at high temperatures. Their machinability depends on their condition and improves with annealing.
  • Tantalum is very work hardening, ductile, and soft. It produces a poor surface finish, and tool wear is high.
  • Titanium and its alloys have very poor thermal conductivity (the lowest of all metals), causing a significant temperature rise and built-up edge. They are highly reactive and can be difficult to machine.
  • Tungsten is brittle, strong, and very abrasive; hence, its machinability is low, although it improves significantly at elevated temperatures.
  • Zirconium has excellent machinability, but it requires a coolant-type cutting fluid because of the danger of explosion and fire.
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