Chemical polishing of aluminum

Abstract

A highly polished surface on an aluminum substrate is formed using any number of machining processes. During the machining process, intermetallic compounds are typically generated at a top surface area of the aluminum substrate caused by spot heat generated between the tool edge and the cut tip of the aluminum substrate during the cutting process. The intermetallic compounds can leave surface imperfections after conventional mechanical polishing operations that render the surface of the aluminum substrate difficult to obtain a desired high glossiness due to exfoliation of the intermetallic compounds from the top surface. In order to remove the effect of the intermetallic compounds, an acid etching solution is applied to the surface resulting in removal of intermetallic compounds across a surface portion of the aluminum substrate.

What is claimed is: 
     
         1 . A method of polishing a surface of an aluminum part, comprising:
 acid etching the aluminum part, comprising:
 supporting the aluminum part with a metal plate, 
 immersing the metal plate into a mixed acid bath of nitric and phosphoric acid at a temperature of about 60° C. to about 75° C., and 
 vibrating the metal plate in the acid bath for between about 5 minutes and about 15 minutes, 
 wherein the immersed metal plate acts as an anode creating a galvanic potential gap through the mixed acid bath to the aluminum part which acts as a cathode, resulting in electron concentration on a plurality of convex protrusions occurring across a surface portion of the aluminum part that are subsequently dissolved in the mixed acid bath, thereby improving surface quality of the aluminum part. 
   
     
     
         2 . The method as recited in  claim 1 , wherein the mixed acid bath comprises:
 66-71 percent by weight phosphoric acid (H 3 PO 4 ); and   5-9 percent by weight nitric acid (HNO 3 ).   
     
     
         3 . The method as recited in  claim 2 , wherein the metal plate is a titanium alloy plate. 
     
     
         4 . The method as recited in  claim 3 , wherein the galvanic potential gap is about a 0.8 Volts. 
     
     
         5 . The method as recited in  claim 3 , wherein an iron mesh in contact with both the surface of the aluminum part and the titanium alloy plate facilitates an increase in polishing performance. 
     
     
         6 . The method as recited in  claim 3 , wherein the acid etching removes intermetallic compounds along the surface of the aluminum part. 
     
     
         7 . The method as recited in  claim 1 , further comprising:
 rinsing residual acid from the mixed acid from off of the aluminum part by circulating water across the surface of the aluminum part in a neutralization tank.   
     
     
         8 . The method as recited in  claim 1 , further comprising:
 anodizing the surface of the aluminum part.   
     
     
         9 . The method as recited in  claim 1 , wherein the acid etching of the aluminum part results in a reduction in surface variation of the aluminum part of at least 50 percent. 
     
     
         10 . An acid etching assembly comprising:
 a plurality of metal plates, each metal plate configured to support a plurality of aluminum parts;   a plate holder configured to support a plurality of metal plates;   an acid etching tank containing a mixed acid bath, the mixed acid bath comprising:
 66-71 percent by weight phosphoric acid (H 3 PO 4 ), and 
 5-9 percent by weight nitric acid (HNO 3 ); 
   a heat exchanger configured to heat the mixed acid bath to a temperature of about 60° C. to about 75° C.; and   a vibration apparatus configured to vibrate the plate holder when the plate holder is positioned within the acid etching tank,   wherein a galvanic potential gap develops between the plurality of metal plates and the plurality of aluminum parts when immersed in the mixed acid bath causing in an electro-polishing operation to be applied to a surface portion of the plurality of aluminum parts.   
     
     
         11 . The acid etching assembly as recited in  claim 10 , wherein the plurality of metal plates are comprised of a metal selected from the group consisting of mild steel and titanium alloy. 
     
     
         12 . The acid etching assembly as recited in  claim 11 , further comprising:
 an acid neutralization tank configured to rinse residual acid from the mixed acid bath off of the plate holder subsequent to an acid etching operation.   
     
     
         13 . The acid etching assembly as recited in  claim 10 , further comprising:
 an iron mesh disposed between each of the plurality of aluminum parts and one of the plurality of titanium alloy plates.   
     
     
         14 . The acid etching assembly as recited in  claim 11 , wherein the plate holder has a number of fluid access openings configured to allow easy circulation of the mixed acid bath through the plate holder. 
     
     
         15 . The acid etching assembly as recited in  claim 10 , wherein the plurality of metal plates each include a plurality of perforations that ease circulation of the mixed acid bath across the plurality of aluminum parts. 
     
     
         16 . A non-transitory computer readable medium for storing computer instructions executed by a processor, the non-transitory computer readable medium comprising:
 computer code for preparing a mixed acid bath within an acid etching tank, the mixed acid bath comprising:
 66-71 percent by weight phosphoric acid (H 3 PO 4 ), and 
 5-9 percent by weight nitric acid (HNO 3 ); 
   computer code for setting a temperature of the mixed acid bath to a temperature of about 60° C. to about 75° C.;   computer code for immersing an aluminum part supported by a titanium plate within the mixed acid bath for a time of about 5 to 15 minutes; and   computer code for vibrating the titanium plate while it is immersed within the mixed acid bath.   
     
     
         17 . The non-transitory computer readable medium as recited in  claim 16 , wherein the immersing the aluminum part comprises immersing a plurality of aluminum parts in the mixed acid bath, the aluminum parts supported by a titanium plated and in direct contact with an iron mesh. 
     
     
         18 . The non-transitory computer readable medium as recited in  claim 17 , wherein the titanium plate acts as an anode during the acid immersion step causing oxidation to form on a surface portion of the plurality of aluminum parts. 
     
     
         19 . The non-transitory computer readable medium as recited in  claim 18 , further comprising:
 computer code for rinsing residual acid from the mixed acid bath from the plurality of aluminum parts in a neutralizing tank.   
     
     
         20 . The non-transitory computer readable medium as recited in  claim 17 , wherein the computer code for immersing the aluminum part comprises supporting the titanium plate by a plate holder while immersing the titanium plate and aluminum part.