Anodic coating behavior of x7091 aluminum p/m alloy
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Aluminum powder metallurgy (P/M) alloys are being considered for applications to airframe structural components by the aircraft industry because of their potential advantages over ingot metallurgy (I/M) alloys in terms of a superior combination of mechanical and physical properties. One of the recent P/M alloys known as X7091 is being considered for various airframe applications by several of the aircraft companies. Industry is now using X7091 as a baseline material to scale up aluminum powder making facilities, as well as to scale up large billet production and large size sheet, plate, and extrusion fabrication techniques. In order to be able to use X7091 or similar P/M alloys for airframe applications, it is important to determine how well the surface of P/M alloys can be protected for corrosion resistance or prepared for adhesive bonding by using the standard anodizing procedures already established for I/M alloys such as 7075. Bare 7075 aluminum is currently being anodized with chromic and sulfuric acids for exterior corrosion protection. The most durable pretreatment currently being used in industry for adhesive bonding of aluminum is the phosphoric acid anodize. The objective of this work was to determine if the chemical and microstructural differences between P/M alloys and I/M alloys would have an adverse effect on the formation of an effective anodic coating on the P/M alloys. The X7091 alloy was selected as being representative of a typical powder metallurgy alloy. This alloy contains 0.4 pct cobalt and uniformly distributed aluminum oxides which are created during the powder making process and are not present in the 7075 I/M alloy. The composition of these alloys is shown in Table I. Samples of X7091-T651 were machined from extrusions to a thickness of 0.040 inch (1 mm). Aluminum alloy 7075-T6 bare 0.063-inch (1.6 mm) sheet was used for control samples. The machined surfaces of the X7091 specimens were hand sanded using 400-grit SiC paper to obtain a surface smoothness equal to that of the 7075 sheet. Samples of both alloys were anodized using the phosphoric acid (10V), sulfuric acid (13V), and chromic acid (40V) treatments. A diagram of the process flow and processing
parameters for these three anodic surface treatments is shown in Figure 1. Scanning Electron Microscopy (SEM) photomicrographs were taken of the X7091 and 7075 specimens after they were anodized by the chromic, sulfuric, and phosphoric acid processes. The photomicrographs are shown in Figures 2, 3, and 4. These figures show that the morphology and thicknesses of the anodic oxides for each anodizing process were similar for the two alloys. In addition, SEM photomicrographs were taken of the two alloys prior to the anodizing treatment to confirm that both alloys had been completely deoxidized. In addition to visual evaluation of the three types of anodized layers, the weight of the anodized layer was determined in accordance with Military Specification MIL-A8625C for the chromic acid and the sulfuric acid anodized specimens. The results in
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