Copper-Boron Master Alloy And Its Use In Making Silver-Copper Alloys

Abstract

A master metal composition adapted for alloying with silver to give an alloy containing at least 77 wt % Ag and at least 0.5 wt % Ge comprises Cu, Ge and boron together with any further ingredients for said alloy and any impurities. It further provides a process for making silver alloy containing silver in an amount of at least 77 wt % Ag, 1-7.2 wt % Cu copper, at least 0.5 wt % Ge and B together with any further ingredients for said alloy and any impurities, comprising the step of melting together fine silver and a master metal composition which is at least a ternary alloy of copper, germanium and boron, e.g. 92.5-92.8 wt % Ag, 6.0-6.3 wt % Cu, about 1.2 wt % Ge and 1-15 ppm boron as grain refiner. The resulting silver alloy exhibits good tarnish and firestain resistance and can exhibit significant precipitation hardening on gradual air cooling. In a further aspect the invention relates to method for manufacturing a copper-based master alloy for use in the manufacture of silver alloy products, wherein the molten master alloy prior to solidification is treated with a decomposable boron compound e.g. a boron hydride or metal borohydride.

1 . A master metal composition adapted for alloying with silver to give a silver alloy containing at least 77 wt % Ag and at least 0.5 wt % Ge, said master metal comprising Cu, Ge, optionally 0-30 wt % Ag and 0.001-0.3 wt % boron together with any further ingredients for said alloy and any impurities. 
     
     
         2 . The metal of  claim 1 , comprising 80-95 wt % Cu (or of Cu together with further ingredients for said alloy) and 20-5 wt % Ge. 
     
     
         3 . The metal of  claim 1 , comprising 80-86.7 wt % Cu (or of Cu together with further ingredients for said alloy) and 20-13.3 wt % Ge. 
     
     
         4 . The metal of  claim 1 , comprising 82-84.5 wt % Cu and any further ingredients for said alloy and 15.5-18 wt % Ge. 
     
     
         5 . The metal of any preceding claim, comprising about 0.03 wt % B. 
     
     
         6 . The metal of any preceding claim, which comprises only copper, germanium, boron and impurities. 
     
     
         7 . A process for making silver alloy containing silver in an amount of at least 77 wt % Ag, 1-7.2 wt % Cu copper, at least 0.5 wt % Ge and 0.005-0.3 wt % B together with any further ingredients for said alloy and any impurities, comprising the step of melting together fine silver and a master metal composition which is at least a ternary alloy comprising copper, germanium and boron. 
     
     
         8 . The process of  claim 7 , wherein the silver alloy is an alloy of silver, copper and germanium. 
     
     
         9 . The process of  claim 8 , wherein the alloy consists, apart from impurities, incidental ingredients and any grain refiner, of 80-96% silver, 0.1-5% germanium and 1-19.9% copper, by weight of the alloy. 
     
     
         10 . The process of  claim 8 , wherein the alloy consists, apart from impurities, incidental ingredients and grain refiner, of 92.5-98% silver, 0.3-3% germanium, and 1-7.2% copper, by weight of the alloy, together with 1-40 ppm boron as grain refiner. 
     
     
         11 . The process of  claim 8 , wherein the alloy consists, apart from impurities, incidental ingredients and grain refiner, of 92.5-96% silver, 0.5-2% germanium, and 1-7% copper, by weight of the alloy, together with 1-20 ppm boron as grain refiner. 
     
     
         12 . The process of  claim 8 , wherein the alloy comprises 92.5-92.8 wt % Ag, 6.0-6.3 wt % Cu, about 1.2 wt % Ge and 1-15 ppm boron as grain refiner. 
     
     
         13 . The process of any of  claims 7 - 12 , further comprising the steps of annealing and/or brazing a shaped article of the silver alloy in a furnace, and hardening by subsequent air cooling. 
     
     
         14 . The process of  claim 13 , wherein the silver alloy is annealed and/or brazed by heating in a furnace at 600-680° C. 
     
     
         15 . The process of  claim 13 , wherein the silver alloy is annealed and/or brazed by heating in a furnace at 600-660° C. 
     
     
         16 . The process of  claims 13 - 15 , wherein the silver alloy is annealed and/or brazed at a temperature of from 600-650° C. 
     
     
         17 . The process of  claim 13 , wherein annealing is during investment casting, and hardening is by air-cooling the investment or allowing it to air cool. 
     
     
         18 . The process of  claim 17 , wherein the article is of jewellery or giftware. 
     
     
         19 . A master metal composition adapted for alloying with silver to give a silver alloy containing at least 77 wt % Ag and at least 0.5 wt % Ge, said master metal comprising Cu, Ge and 0.005-0.5 wt % boron together with any further ingredients for said alloy and any impurities. 
     
     
         20 . A method for manufacturing a master alloy for use in the manufacture of precious metal products, wherein the molten master alloy prior to solidification is treated with a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof. 
     
     
         21 . The method of  claim 20 , wherein the master alloy is based on Cu, Cu—Ge, Cu—Ag or Cu—Ag—Ge optionally containing one or more incidental ingredient elements selected from Al, Ba, Be, Cd, Co, Cr, Er, Ga, In, Mg, Mn, Ni, Pb, Pd, Pt, Si, Sn, Ti, V, Y, Yb, Zn and Zr. 
     
     
         22 . The method of  claim 20  or  21 , wherein the master alloy is treated with a borane that is solid at ambient temperatures. 
     
     
         23 . The method of  claim 20  or  21 , wherein the master alloy prior to solidification is treated with a metal borohydride. 
     
     
         24 . The method of  claim 20  or  21 , wherein the master alloy prior to solidification is treated with sodium borohydride. 
     
     
         25 . The method of any of  claims 20 - 25 , wherein the master alloy prior to solidification is treated by wrapping in copper or silver foil a borane that is solid at ambient temperatures or a metal borohydride and plunging the wrapped borane or metal borohydride into the molten precious metal. 
     
     
         26 . The method of any of  claims 20 - 25 , further comprising solidifying the molten metal into casting grain. 
     
     
         27 . A method for casting a master alloy containing at least Cu and B, including the steps of:
 (a) forming a precursor master melt containing at least Cu;   (b) dispersing throughout said master melt a compound selecting from the group consisting of alkyl boron compounds, boron hydrides, boron halides, boron-containing metal hydrides, boron-containing metal halides and mixtures thereof; and   (c) allowing the melt to solidify.   
     
     
         28 . The method of  claim 27 , comprising dispersing said boron compound into said precursor melt by bubbling an inert carrier gas containing a gaseous hydride or halide of boron through said melt. 
     
     
         29 . The method of  claim 28 , wherein said boron compound is one or more selected from boron trifluoride, diborane and trimethylboron. 
     
     
         30 . The method of  claim 27 , wherein said boron compound is introduced into said precursor melt in the liquid phase optionally in an inert organic solvent and sealed into one or more containers of silver or copper foil or of an inert thermally decomposible material. 
     
     
         31 . The method of  claim 30 , wherein said boron compound is selected from the group consisting of triethylborane, tripropylborane, tri-n-butylborane, methoxydiethylborane and dispersions of any of them in hexane or ThF. 
     
     
         32 . The method of  claim 27 , wherein said boron compound is a higher borane that is solid at ambient temperatures. 
     
     
         33 . The method of  claim 32 , wherein said boron compound is decaborane. 
     
     
         34 . The method of  claim 32 , wherein said metal constituent of said boron containing metal hydride is selected from the group consisting of sodium, lithium, potassium, calcium, zinc and mixtures thereof and said metal constituent of said boron containing metal fluoride is sodium. 
     
     
         35 . The method of  claim 32 , wherein said compound is selected to be sodium borohydride. 
     
     
         36 . The method of  claim 32 , further comprising the step of:
 wrapping said boron hydride, boron-containing metal hydride, or boron-containing metal halide in copper or silver foil prior to dispersion in said precursor alloy melt.   
     
     
         37 . The method of  claim 36 , wherein said metal foil is selected to have a thickness of between 0.01 mm and 0.3 mm. 
     
     
         38 . The method of  claim 32 , wherein said dispersing step (b) includes stirring for a time effective to disperse boron throughout said precious metal alloy or said master alloy. 
     
     
         39 . The method of  claim 27 , further comprising the step of transferring said precious metal alloy or master alloy to a grain box. 
     
     
         40 . The method of  claim 27 , wherein the precursor master alloy melt comprises 0-30 wt % Ag, 0-20 wt % Ge, 0-2 wt % Si the balance being copper or being a mixture of copper and zinc in which the ratio by weight of zinc to copper is not more than 1:1.