Nickel plated zinc alloys for coinage
A lower melting point metal substrate such as zinc is covered with a higher melting point metal layer such as nickel. In order to render the nickel layer suitable for coining without cracking, it is quickly annealed with a burst of induction heat. The induction heat is limited so that only the metal layer is heated without allowing substantial heat transfer to the metal substrate. This protects the metal substrate from any substantial melting, deformation or warping and maintains the metal substrate substantially intact and without a change in properties and shape.
1. A method of making a plated zinc planchet, comprising:
plating a higher melting point metal layer comprising nickel, bronze, or brass over a lower melting point metal substrate consisting of zinc or a zinc alloy; and
heating said higher melting point metal layer with a burst of predetermined amount of induction heat sufficient to anneal at least a portion of said higher melting point metal layer so as to limit heat transfer from said higher melting point metal layer to said lower melting point metal substrate so as to substantially avoid melting of said lower melting point metal substrate to form a plated zinc planchet that is suitable for a coining operation, wherein the burst of predetermined amount of induction heat is sufficient to anneal the at least a portion of said higher melting point metal layer to a temperature of more than 1000° F.
2. The method of claim 1 , wherein the burst of predetermined amount of induction heat is applied for a period of 0.1 to 2.5 seconds.
3. The method of claim 2 , further comprising plating the lower melting point metal substrate with a copper layer before plating the higher melting point metal layer.
4. The method of claim 1 , further comprising coining said plated zinc planchet without forming cracks in the higher melting point metal layer.
5. The method of claim 4 , further comprising burnishing said higher melting point metal layer prior to said coining.
6. The method of claim 1 , wherein the burst of predetermined amount of induction heat sufficient to anneal the at least a portion of said higher melting point metal layer is frequency tuned to impart energy only to the higher melting point metal layer.
7. The method of claim 1 , wherein the burst of predetermined amount of induction heat is applied for a period of 0.4 to 0.6 seconds.
8. The method of claim 1 , wherein the burst of predetermined amount of induction heat is applied at a current of 350 A to 500 A.
9. The method of claim 3 , wherein the copper layer has a thickness of from 1 μm to 25 μm.
10. The method of claim 3 , wherein the copper layer has a thickness that provides no pinholes for reaction of the lower melting point metal substrate when the higher melting point metal layer is plated.
11. The method of claim 3 , further comprising forming an interaction layer between said lower melting point metal substrate and said copper layer.
12. A plated zinc coin planchet formed by the method of claim 1 .
13. A coin formed by the method of claim 4 .
14. The method of claim 1 , wherein the higher melting point metal layer has a thickness of from 1 μm to 35 μm.