Method for adding boron to metal alloys

Abstract

A method to grain refine and deoxidize a precious metal alloy or a master alloy includes the steps of (a) forming a precursor melt consisting essentially of constituents of the precious metal alloy or master alloy and inevitable impurities; (b) dispersing a compound selected from the group consisting of boron containing metal hydrides, boron containing metal fluorides and mixtures thereof throughout the precursor melt; and (c) solidifying the boron containing precious melt alloy or master alloy. One suitable compound is solid sodium borohydride (sodium tetrahydroborate). To minimize evaporation of the boron on contact with the precursor alloy melt, the sodium borohydride may be wrapped in a metal foil formed from constituents of the precious metal alloy or master alloy. The cast precious metal alloy or master alloy has been found to have a reduced number of hard spots and reduced silicon contamination when compared to conventional casting methods.

1 . A method to cast a precious metal alloy or master alloy, comprising the steps of: 
 (a). forming a precursor alloy melt consisting essentially of constituents of said precious metal alloy or said master alloy and inevitable impurities;    (b). dispersing a compound selected from the group consisting of boron containing metal hydrides, boron containing metal fluorides and mixtures thereof throughout said master melt; and    (c). solidifying said boron containing precious melt alloy or master alloy.    
   
   
       2 . The method of  claim 1  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.  
   
   
       3 . The method of  claim 2  wherein said compound is selected to be solid sodium borohydride (sodium tetrahydroborate).  
   
   
       4 . The method of  claim 2  wherein said boron containing metal hydride or said boron containing metal fluoride is wrapped in a metal foil selected to be one of said constituents of said precious metal alloy or master alloy prior to being dispersed in said precursor alloy melt.  
   
   
       5 . The method of  claim 4  wherein said metal foil is selected to have a thickness of between 0.01 millimeter and 0.3 millimeter.  
   
   
       6 . The method of  claim 4  wherein said precious metal alloy or master alloy contains silver and said metal foil is selected to be silver or a silver-base alloy.  
   
   
       7 . The method of  claim 4  wherein said precious metal alloy contains gold and said metal foil is selected to be copper or a copper-base alloy.  
   
   
       8 . The method of  claim 4  wherein said dispersing step (b) includes stirring for a time effective to disperse boron throughout said precious metal alloy or said master alloy.  
   
   
       9 . The method of  claim 1  wherein said precious metal alloy or master alloy is transferred to a grain box.  
   
   
       10 . The method of  claim 1  wherein said precious metal alloy is transferred to a continuous casting die and withdrawn following said solidifying step (c) as an extended length of desired cross-sectional shape.  
   
   
       11 . The method of  claim 10  wherein said dispersing step (b) is repeated multiple times to maintain a desired boron content.  
   
   
       12 . The method of  claim 4  wherein sufficient boron is added to obtain a precious metal alloy or master alloy having, by weight, from 1 ppm to 1600 ppm of boron.  
   
   
       13 . The method of  claim 12  wherein said boron content, by weight, is from 100 ppm to 1600 ppm for said master alloy and from 1 ppm to 100 ppm for said precious metal alloy.  
   
   
       14 . A silver- or gold-base alloy or master alloy containing, by weight, from 1 ppm to 1600 ppm of boron and being substantially free of both silicon and copper.  
   
   
       15 . The silver- or gold-base alloy of  claim 14  wherein said boron content is from 100 ppm to 1600 ppm for said master alloy and from 1 ppm to 100 ppm for said precious metal alloy.  
   
   
       16 . Casting grain formed from the silver- or gold-base alloy or master alloy of  claim 14 .  
   
   
       17 . Casting grain having a nominal composition, by weight, selected from the group consisting of 93% silver, 5.7% copper and 1.3% germanium; 74.8% gold, 12.2% nickel; 9.9% copper and 3.1% zinc; and 81.4% copper and 18.6% germanium, all plus inevitable impurities.  
   
   
       18 . An extended length of desired cross-sectional area formed from the silver- or gold-base alloy of  claim 14 .  
   
   
       19 . The extended length of  claim 18  formed from an alloy having a nominal composition by weight of 93% silver, 5.7% copper, 1.3% germanium and inevitable impurities.