Special application 

Aluminum and other coin metals

Selection of metals and alloys for coins

The complex process of selecting a new metal or alloy for wide circulation coins is detailed in a report from the US Mint. [1], which was dedicated to finding new, cheaper materials for coins in denominations 1 cent.

Coin alloy selection factors

Factors, which must be considered when designing or choosing a new metal for making coins, are, including [1]:

  • Material availability, now and in the future
  • The cost of the required materials (table 1)
  • Manufacturing cost
  • Service life of stamping dies
  • Ability to perform electromagnetic signature
  • Wear resistance
  • Сorrosion resistance
  • Color and its change during the circulation of coins
  • Ability to perform minting (yield strength and ductility)
  • Strain hardening
  • Density
  • Impact on the environment
  • Toxicity
  • Health and safety of coin makers
  • Recyclability (recycling)
  • Coated coins, clad metal or monolithic
  • Security or inefficiency in issuing counterfeit coins
  • Compatible with vending machines and similar equipment
  • Recognition "by touch" for blind and partially sighted people
  • Possibility of simultaneous circulation of current and new coins

Potential metals for coins

Various metals, which can be considered, how candidates for materials for making coins are presented in the table 1 [1]. Excluded from this table are too reactionary, metals that are too rare and inaccessible for circulation. For comparison, traditional precious metals are included, gold and silver. Several expensive items are included because, that they can be considered for coating or alloying. A few impractical elements, such as uranium and tungsten are included to illustrate the limited choice for high density metallic elements.

Table 1 - Metals and alloys - potential candidates for making coins:
density and cost (in US dollars per pound)
(as of December 2011 year) [1]

5 real coin metals

Analysis of the advantages and disadvantages of metals and alloys tables 1 shows, that for various reasons most of them cannot be considered as metals for making coins, nor as metals for external coating of coins [1]:

  • Magnesium: corrodes too much in air
  • Beryllium, titanium, vanadium, zirconium: too high a price
  • Chromium: Cr ions+6 are carcinogenic
  • Tin: Expensive for Monolithic Coins
  • Manganese: Corrosion Problems
  • Niobium, cobalt, nickel, bismuth: too high a price
  • Molybdenum, silver, gold, tungsten: too high a price
  • Lead: toxic
  • Uranium: radioactive.

Therefore, only five are recognized as real coin metals [1]:

  • Aluminum
  • Zinc
  • Iron (steel)
  • Copper
  • Nickel

These five metals can be about 30 potential combinations of coinage metal alloys [1]:

  • Aluminum and aluminum alloys
  • Aluminized steel
  • Copper plated zinc. Applies to US 1 cent coins.
  • Low carbon steel:
    – multilayer coated (nickel + copper + nickel)
    – copper plated
    – nickel plated.
  • Stainless steel:
    – magnetic
    – non-magnetic.
  • Zinc:
    – tin plated
    – copper plated.
  • Copper alloys:
    – melhior ("cupernickel") (75 % Cu - 25 % Ni)
    – aluminum bronze (Cu - Al)
    – bronze (Cu - Sn)
    – brass (Cu - Zn))

Embossing stamps

One of the important criteria for the selection of suitable metals and alloys for the manufacture of coins is the level of effort of the minting equipment, which is required to make coins. The level of these forces is assessed by the yield strength of the metal and alloy.. The level of effort required for minting coins from various coin metals and alloys differs depending on the 4-5 times in the following descending order:

  • stainless steel 304
  • stainless steel 430
  • titanium
  • steel
  • aluminum bronze
  • Сu75Ni5Zn20
  • Сu70Ni12Zn18
  • DEFECATE
  • bronze
  • aluminum

Figure 1 - Embossing matrices [2]

Source:

  1. Alternative Metals – Final Report – 2012 – www.usmint.gov/