Electrochemical grain refining of a metal

Abstract

A method for surface and subsurface grain refining of a bulk hydrogen-absorbing metal includes the steps of cathodically charging the bulk hydrogen-absorbing metal with an electric current in the presence of a source of hydrogen to hydride the hydrogen-absorbing metal, and, changing polarity of the electric current to dehydride the hydrogen-absorbing metal. The method results in improvement to hardness and/or wear resistance of the metal, particularly titanium alloys such as Ti-6Al-4V. Metals treated with this method are particularly useful for medical implants and vehicle parts in which improved hardness and/or wear resistance is required.

1 . A method for surface and subsurface grain refining of a bulk hydrogen-absorbing metal comprising: 
 (a) cathodically charging the bulk hydrogen-absorbing metal with an electric current in the presence of a source of hydrogen to thereby hydride the hydrogen-absorbing metal; and,    (b) changing polarity of the electric current to thereby dehydride the hydrogen-absorbing metal.    
     
     
         2 . The method of  claim 1 , wherein the electric current is AC.  
     
     
         3 . The method of  claim 1 , wherein the electric current is DC.  
     
     
         4 . The method of  claim 1 , wherein the hydrogen-absorbing metal is nickel-free.  
     
     
         5 . The method of  claim 1 , wherein the hydrogen-absorbing metal comprises a titanium alloy.  
     
     
         6 . The method of  claim 5 , wherein the titanium alloy is Ti-6Al-4V.  
     
     
         7 . The method of  claim 1 , wherein the source of hydrogen is an aqueous acid or base.  
     
     
         8 . The method of  claim 1 , wherein the source of hydrogen is an aqueous inorganic acid or an aqueous inorganic base.  
     
     
         9 . The method of  claim 8 , wherein the aqueous inorganic acid has a concentration in a range of from 0.1 M to 10 M, and the aqueous inorganic base has a concentration in a range of from 0.05 M to 6 M.  
     
     
         10 . The method of  claim 1 , wherein the source of hydrogen is aqueous sulfuric acid or aqueous potassium hydroxide.  
     
     
         11 . The method of  claim 2 , wherein 
 the AC during hydriding has a current density (I 1 ) in a range of from 0.01 to 100 mA/cm 2  and a pulse period (t 1 ) in a range of from 2 to 120 seconds,    the AC during dehydriding has a current density (I 2 ) in a range of from 0.01 to 100 mA/cm 2  and a pulse period (t 2 ) in a range of from about 2 to about 120 seconds, and    total time for hydriding/dehydriding is in a range of from 1 hour to 50 hours,    temperature is in a range of from 0° C. to 100° C.,    and wherein    (a) in an acidic environment, the AC has a pulse potential for hydriding (E 1 ) of from −1.3 to −0.5 V, a pulse potential for dehydriding (E 2 ) of from about −0.5 to −0.1 V, or,    (b) in a basic environment, the AC has a pulse potential for hydriding (E 1 ) of from −1.9 to −1.4 V a pulse potential for dehydriding (E 2 ) of from −1.1 to −0.5 V.    
     
     
         12 . The method of  claim 3 , wherein the metal is hydrided at a current density in a range of from 0.01 to 20 mA/cm 2  for a period of time of from 1 to 200 hours per cycle at a temperature in a range of from 0° C. to 100° C., and the metal is dehydrided at a current density in a range of from 0.01 to 1 mA/cm 2  for a period of time in a range of from 1 to 400 hours per cycle at a temperature in a range of from 0° C. to 100° C.  
     
     
         13 . The method of  claim 1 , wherein dehydriding is conducted initially at a first rate and then subsequently at a second rate, the second rate being lower than the first rate.  
     
     
         14 . A method for surface and subsurface grain refining of a bulk titanium alloy comprising: 
 (a) cathodically charging the bulk titanium alloy with an electric current in an aqueous inorganic acid or an aqueous inorganic base to thereby hydride the titanium alloy; and,    (b) changing polarity of the electric current to thereby dehydride the titanium alloy.    
     
     
         15 . The method of  claim 14 , wherein the titanium alloy is Ti-6Al-4V.  
     
     
         16 . The method of  claim 14 , wherein the electric current is AC.  
     
     
         17 . The method of  claim 16 , wherein the aqueous inorganic acid comprises sulfuric acid having a concentration in a range of from 0.1 M to 10 M, and the aqueous inorganic base comprises potassium or sodium hydroxide having a concentration in a range of from 0.05 M to 6 M.  
     
     
         18 . The method of  claim 17 , wherein 
 the AC during hydriding has a current density (I 1 ) in a range of from 0.01 to 100 mA/cm 2  and a pulse period (t 1 ) in a range of from 2 to 120 seconds,    the AC during dehydriding has a current density (I 2 ) in a range of from 0.01 to 100 mA/cm 2  and a pulse period (t 2 ) in a range of from about 2 to about 120 seconds, and    total time for hydriding/dehydriding is in a range of from 1 hour to 50 hours,    temperature is in a range of from 0° C. to 100° C.,    and wherein    (a) in the aqueous inorganic acid, the AC has a pulse potential for hydriding (E 1 ) of from −1.3 to −0.5 V, a pulse potential for dehydriding (E 2 ) of from about −0.5 to −0.1 V; or,    (b) in the aqueous inorganic base, the AC has a pulse potential for hydriding (E 1 ) of from −1.9 to −1.4 V a pulse potential for dehydriding (E 2 ) of from −1.1 to −0.5 V.    
     
     
         19 . The method of  claim 18 , wherein dehydriding is conducted initially at a first rate and then subsequently at a second rate, the second rate being lower than the first rate.