Abstract:
An anodization method includes steps of providing an object formed of an aluminum-copper alloy, providing an anodizing bath comprising a basic silicate solution, providing an AC power supply including a first electrode and a second electrode, placing the first electrode in contact with the anodizing bath, connecting the second electrode to the object, placing the object in the anodizing bath, applying a voltage to the first and second electrodes to maintain a current density of about 10 mA/cm 2  or less to form an anodized coating on the object, removing the object from the bath, and sealing the anodized coating on the object.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 60/823,728, filed Aug. 28, 2006, the entire contents of which are hereby incorporated by reference. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed toward a method of anodizing an aluminum-copper alloy in a basic silicate solution and, more specifically, toward a method of anodizing an aluminum-copper alloy in a basic silicate solution in a manner that reduces fatigue strength deficits in the anodized object. 
       BACKGROUND OF THE INVENTION 
       [0003]    Aluminum-copper alloys, including 2000 Series aluminum-copper alloys such as AA2219, AA2024 and AA2014, are desirable in many industries due to their high strength-to-weight ratios. However, these alloys tend to corrode if not protected in some manner. To enhance corrosion resistance, it is known to anodize these alloys in a sulfuric acid solution using a direct current to generate a protective oxide layer. It is also known that such anodization reduces the fatigue strength of the anodized objects to varying degrees. 
         [0004]    One of the present inventors has found that the amount of fatigue strength deficit for a given alloy depends on the nature of the product being anodized. Highly worked products such as bar stock lose relatively little fatigue strength after anodization, while lightly worked plate and similar objects suffer more significant deficits. This difference has been attributed to the number and size of copper rich areas of the metal surface. Dissolution of these copper rich areas during anodization significantly reduces fatigue strength. Working the alloy reduces the size of the copper rich areas, and therefore, while worked objects also experience a fatigue strength deficit, it is not as severe as the deficit that occurs in objects formed from more lightly worked alloy plate or large diameter bar. These findings are detailed in Rateick, R. G. et. al. “Relationship of Microstructure to Fatigue Strength Loss in Anodized Aluminium-Copper Alloys,”  Materials Science and Technology , vol. 21, no. 10, 2005 (1227-1235), the entire contents of which is hereby incorporated by reference. It would therefore be desirable to provide parts formed from such aluminum-copper alloys with acceptable corrosion resistance without adversely affecting their fatigue strength. 
       SUMMARY OF THE INVENTION 
       [0005]    These problems and others are addressed by embodiments of the present invention, a first aspect of which comprises an anodization method for anodizing an object formed of an aluminum-copper alloy in an anodizing bath containing a basic silicate solution. The method involves placing a first electrode of an AC power supply in the anodizing bath, connecting the object to be coated to a second electrode of the AC power supply and placing the object to be coated in the bath. A voltage is then applied to the first and second electrodes to maintain a current density of less than about 10 mA/cm 2  to form an anodized coating on the object. The object is then removed from the bath, and the coating on the object is sealed with, for example, a polymer sealant. 
         [0006]    Another aspect of the present invention is an anodization method for anodizing a 2000 series aluminum-copper alloy plate in an anodizing bath containing a sodium silicate solution and potassium hydroxide, the bath having a pH of about 13. The method involves placing a first electrode of an AC power supply in the anodizing bath, connecting the object to be coated to a second electrode of the AC power supply, and placing the object to be coated in the bath. A voltage is then applied to the first and second electrodes to maintain a current density of less than about 10 mA/cm 2  to form an anodized coating on the object. The object is then removed from the bath and the anodized coating on the object is sealed with Parylene HT. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These aspects and features of the invention and others will be better understood after a reading of the following detailed description together with the following drawings wherein: 
           [0008]      FIG. 1  is a graph of the anodization profile of AA2219 aluminum; 
           [0009]      FIG. 2  is a graph of the anodizing profile of AA2219 with varying anodizing bath silicate concentrations; and 
           [0010]      FIG. 3  is a graph of anodizing voltage over time for different anodizing bath pH&#39;s. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    These problems and others are addressed in the present invention by anodizing aluminum-copper alloys such as AA2219, AA2024 and AA2014 using a microarc oxidation process. The process employs a basic anodizing solution containing a metallic silicate and a combined AC/DC waveform and a relatively low current density. Beneficially, the anodization solution is cooled to about 4° C. at the beginning of the anodization process, and the current applied to the solution has a ratio of DC to AC components of about 5:1. As an oxide film begins to form on the object being anodized, the DC voltage is increased to maintain a current density of about 10 mA/cm 2  or less. The process is ended when the DC voltage reaches about 300 to 320 VDC. This process reduces the fatigue strength of the anodized object by a smaller amount than the above-described sulfuric acid anodization process. It is believed that this improvement is due to the fact that less copper is dissolved from aluminum alloy using the present process than is dissolved using prior anodization processes. 
         [0012]    The oxide coating formed in this manner provides relatively little corrosion resistance and is not suitable for many applications. However, the present inventors have also found that the porous oxide coating provides an excellent base for various polymer sealants that would not bond well to untreated metal. Para-p-xylylene sealants, such as Parylene HT, have been found to bond particularly well to this coating and provide good levels of corrosion resistance. Thus the combination of a low current density oxidation process and a sealant provides an aluminum-copper alloy with greater corrosion resistance and greater fatigue strength than has heretofore been possible. 
         [0013]      FIG. 1  illustrates an anodization profile for AA2219. This anodization profile has four distinct stages labeled I, II, III and IV in the Figure. Stage I is an initiation stage and stage II represents sparkles oxidation formation. Sparking begins during Stage III, and intense sparking begins during Stage IV which may also be referred to as the microarc stage. 
         [0014]    As illustrated in  FIG. 2 , three baths, Bath  1 , Bath  2  and Bath  3  were tested to determine the effect of silicate concentration on anodizing time and final voltage. Each bath included 10 g/L of KOH and had a pH as measured with a pH tester, of 13. Bath  1  contained 10 g/L of the silicate solution, Bath  2  contained 25 g/L of the silicate solution, and Bath  3  contained 40 g/L of the silicate solution. The characteristic voltage vs. time plots for AA2219 samples are shown in  FIG. 2 . For ease of explanation, only the DC component is shown, although the AC component discussed above is present in this process with a DC:AC ratio of 5. The four distinct stages of AA2219 anodization can be seen in  FIG. 2 . The first stage (I) is a period of very slow voltage rise from an initial voltage of about 2 VDC to 6 VDC. The second stage (II) involves the most rapid increase in voltage, ranging from 6 to 60 VDC. It is possible that this stage involves the formation of a barrier oxide layer on the sample surface. The beginning of Stage III is demarcated by a reduction in slope that occurs at around 60 VDC and continues to about 280 VDC. The rate of voltage increase is fairly steady in this region of the plot. As sparks are visible by around 180 VDC, it is through that the change from Stage II to Stage III indicates the onset of sparking. The final stage, Stage IV, lasts from about 280 VDC until the selected ending voltage, and exhibits larger, longer lasting and less frequent sparks. 
         [0015]    The general shape of the V vs. t plots is independent of the anodizing bath composition and is comparable to the anodization profile of AA 2219 in  FIG. 1 . However, the total anodization time is significantly reduced by increasing the bath silicate concentration to 40 g/L. At this concentration, Stage I is not evident. It is assumed that the processes that occur in Stage I at low silicate concentrations still occur when baths containing higher silicate concentrations are used. The faster anodization rate that occurs with the higher concentrations, however, makes this first stage less evident. 
         [0016]    As Bath  3  offered the fastest anodization time, this bath was used in additional testing to determine optimal pH.  FIG. 3  illustrates the effect of pH on anodizing profile. As illustrated in this Figure, final anodizing voltage decreased with increasing pH. It was believed that pH&#39;s higher than 13 would excessively corrode the alloy being treated and these higher pH&#39;s were not tested for their affect on anodization rate. It is also believed that the higher pH would cause less copper to dissolve from the alloy than lower pH&#39;s would cause. 
         [0017]    To further reduce the dissolution of copper, it is beneficial to begin the anodization process in a chilled water bath so that the starting temperature of the bath is about 4° C. While the bath temperature increases during the anodization process, it generally does not exceed about 30° C. Higher starting temperatures were found to result in a greater loss of copper from the alloy and consequently a greater reduction in fatigue strength. 
         [0018]    As noted above, stage IV begins at approximately 280 VDC and continues until an equilibrium is reached and no further increases in VDC occur. Tests have been conducted up to a voltage of about 320 VDC. When the fatigue strength deficit of AA2219 samples tested using Bath  10  and a pH of 13 was analyzed, it was found to be about 12 percent as compared to about 38 percent for samples anodized in a traditional acid anodization bath. Additional tests were conducted on specimens wherein the anodization process was stopped at a final voltage of about 300 VDC. This shorter anodization reduced the amount of copper dissolved and also produced a less durable protective oxidation layer. However, this method produced almost no fatigue strength deficit. Moreover, the coating produced by stopping the process at 300 VDC was porous and readily accepted a parylene coating. Two examples of the above-described process are discussed below. 
       EXAMPLE 1 
       [0019]    An anodizing bath was formed of potassium hydroxide and a sodium silicate solution (Fisher-Scientific SS338) which contained 29.2% amorphous silica, 9.1% sodium oxide and 61.7% water. The bath contained 5 g/L KOH and 40 g/L of the silicate solution to produce a pH of about 13. The bath was contained in a 60 mL cylindrical cell with a stainless steel cathode fabricated such that it lined the sides and bottom of the cell. The cell was cooled in an ice-water bath to about 4° C. 
         [0020]    The objects to be coated comprised disc-shaped samples of AA2219 that were 600 grit polished and had a diameter of 1 cm and a height of 0.5 cm. The flat surfaces of the samples were cut in the long transverse/short transverse plane from AA2219-T851 plate. 
         [0021]    AC and DC power supplies were used in series to produce a combined DC/AC waveform with a ratio of about 5:1. The object to be coated or anode was connected to the power supply using a steel rod wrapped in PTFE tape and placed into the solution which in turn was chilled in an ice bath to about 4° C. The initial DC voltage was 0 and was raised gradually to 320 VDC as the spark anodization process proceeded to maintain a current density at the anode of 10 mA/cm 2 . The process was complete in about 30 minutes at which point the further increase in voltage with time was minimal. In addition, the pitting of Parylene coated samples was less severe. The pits were much smaller and fewer in number. 
         [0022]    The anode was thereafter removed from the electrical circuit and coated with Parylene HT. Subsequent testing of the sample showed an improved corrosion resistance over uncoated samples. Uncoated samples subjected to a 5% NaCl solution typically exhibited pitting after approximately 24 hours while coated samples resisted pitting for from 4 to 7 days. 
         [0023]    Fatigue strength of the above sample was also improved, being about 12 percent in the untreated sample as opposed to about 38 percent in samples anodized using a sulfuric acid bath. 
       EXAMPLE 2 
       [0024]    The anodizing bath and sample preparation in the second example was identical to the first example. AC and DC power supplies were used in series to produce a combined DC/AC waveform with a ratio of about 5:1, and a water bath was used to cool the anodization bath to 4° C. The initial DC voltage was 0 and was raised gradually to maintain a current density at the anode of 10 mA/cm 2 . However, in the second example, the anodization process was halted when the applied voltage reached 300 VDC. This required less than the 30 minute process time of Example 1. 
         [0025]    The anode was thereafter removed from the electrical circuit and coated with Parylene HT. Subsequent testing of the sample showed an improved corrosion resistance over uncoated samples, similar to that exhibited by the samples of Example 1. However, in this case, almost no fatigue strength deficit was noted. 
         [0026]    The present invention has been described herein in terms of a presently preferred embodiment. Obvious modifications and additions to this embodiment will become apparent to those skilled in the relevant arts upon a reading and understanding of this disclosure. It is intended that all such modifications and additions comprise a part of the present invention to the extent they fall within the scope of the several claims appended hereto.