Abstract:
A chemical milling solution for lithium aluminum alloy is disclosed for manufacturing a surface finish having superior smoothness characteristics. The process is especially useful where material reduction or thinning is required wherein machining would otherwise be impossible or too expensive.

Description:
The government has rights in this invention pursuant to Contract No. F33657-81-C-2108 awarded by the Department of the Air Force. 
    
    
     BACKGROUND OF THE INVENTION 
     Lithium aluminum extruded alloys for utilization in the aerospace industry are available with the proper structural orientation and surface characteristics, but wherein the alloy is too thick requiring reduction in weight of the alloy. This is due to the fact that many extruded shapes cannot be manufactured as thin-walled extrusions. Those shapes which, due to their shape, must be extruded with a minimum thickness must subsequently be thinned. 
     Alloys other than lithium-aluminum have been successfully chemically milled. However, in the case of lithium-aluminum alloy, previous efforts at chemical milling have produced surface finishes which are unacceptable. 
     Surface roughness of metallic parts can be measured using a &#34;roughness height rating&#34; or RHR scale. RHR is an arithmetical average in microinches of the surface deviations from absolute smoothness. Standards in the aerospace industry dictate that the surface roughness of the lithium aluminum metal have a roughness height rating RHR of from less than 250 to 125. 
     Standard chemical milling processes in the aerospace industry for lithium aluminum alloy use a solution containing from 10% to 25% NaOH in water along with from 2 to 6 ounces elemental sulphur per gallon of the water - NaOH mixture. The mixture is heated to from 190 to 200 degrees Fahrenheit. This unsuccessful chemical milling mixture will produce a surface having a roughness height rating of not less than 500. Mechanical methods for producing a satisfactory surface roughness after such unsuccessful chemical milling to this reduced roughness standard are cost prohibitive. 
     SUMMARY OF THE INVENTION 
     The chemical milling solution of the present invention has produced a surface of satisfactory roughness having a roughness height rating of from 40 to 61. The process has been successfully practiced upon alloy 2090-T8ES1, a lithium aluminum extrusion. The process is also useful for reducing the thickness of the metal as desired and performs the reaction at room temperature. Chemical milling practiced using the solution of the present invention requires no heat, and desmutting of the metal surface is not required. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Alloy 2090-T8ES1, upon which the invention has been successfully practiced, has the following composition: 
     
         ______________________________________Component    Composition in alloy______________________________________Lithium      1.5-3.5%Magnesium    6.%Manganese    1.%Aluminum     91.5-89.5%______________________________________ 
    
     The solution used to successfully chemical mill the Lithium Aluminum extrusion described above is as follows: 
     
         ______________________________________Component      Composition in solution______________________________________Nitric Acid    8.9% to 12.0%Chromic Acid   4.7% to 6.0%Hydrofluoric Acid          approximately 1.0%Water          81.0% to 85.4%______________________________________ 
    
    
    
     EXAMPLE 
     The surface dimensions of a sample of alloy 2090-T8ES1, a lithium aluminum extrusion, were measured and recorded. The sample was then suspended in a horizontal position within a tank containing the chemical milling solution of the present invention. The roughness of the starting material ranged from an RHR rating of 30 to 60. The lithium aluminum sample was horizontally positioned in a manner to cause the most severe gas entrapment for a worst case test. The sample was an elongate metal member having a flattened plate-like base bisected into a forward and rear portion along its length by a planar surface member attached to the elongate metal member at an angle along the metal member&#39;s center. The sample was measured each morning for three days. On the third day the test was concluded. At that time all pre measured points on the sample were again measured and recorded. The following results were obtained: 
     RESULTS 
     Immersion time--74.5 hours 
     Average metal removed per surface--0.028&#34; 
     Chemical milling rate--0.00039&#34; per hour 
     Surface roughness--40-61 RHR (cross grain) 
     
         ______________________________________Thickness readings, before and after chemical milling(sample results in mils at three points in each area)______________________________________Base forwardLeftmost section         Center section                       Rightmost sectionBefore  After     Before  After   Before                                   After______________________________________349     288       347     287     345   290345     285       340     285     337   283340     283       343     284     342   287______________________________________Base rearwardLeftmost section         Center section                       Rightmost sectionBefore  After     Before  After   Before                                   After______________________________________342     286       342     285     340   286341     286       340     287     341   286348     292       345     290     347   292______________________________________Attached memberLeftmost section         Center section                       Rightmost sectionBefore  After     Before  After   Before                                   After______________________________________245     188       241     187     243   187244     184       239     185     241   186243     188       241     187     242   188______________________________________ 
    
     No other ingredients are required. During the chemical milling, the hydrofluoric acid is consumed in the reaction. Consequently, as the hydrofluoric acid is consumed, the etch rate will decrease. It is recommended to add hydrofluoric acid as needed to maintain a milling rate in the latter stages of chemical milling equivalent to about 50% of the initial milling rate. This step will assume greater or lesser importance depending upon the quantity and composition of the metal to be milled and the volume of solution in contact with the metal. 
     The foregoing was a description of a process for chemical milling of lithium aluminum alloy. The process results in a metal surface finish with superior smoothness characteristics. The process is also useful for controllably reducing the thickess of the metal. The rate of metal reduction can be readily controlled by controlling the strength of the hydrofluoric acid in the solution. The surface roughness reduction and reduction in thickness can, as recited in the example, be accomplished simultaneously. The chemical milling solution produces metal surfaces having a roughness height rating (RHR) of 40 to 61, and the surface of the metal after chemical milling does not require desmutting.