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7 series aluminum alloy
The AI Zn Mg Cu series superhard aluminum alloy, a 7-series aluminum alloy, has been widely used in the aircraft manufacturing industry since the late 1940s and is still widely used in the aviation industry today. Its characteristics are good plasticity after solution treatment, particularly good heat treatment strengthening effect, high strength below 150 ℃, and particularly good low-temperature strength; Poor welding performance; There is a tendency for stress corrosion cracking; It needs to be treated with aluminum or other protective measures for use. Double stage aging can improve the ability of alloys to resist stress corrosion cracking. The plasticity in annealed and quenched states is slightly lower than that of 2A12 in the same state, slightly better than 7A04, and the static fatigue of the plate Gap sensitivity, stress corrosion resistance better than 7A04. Density of 2.85q/cm3.
Among the 7 series, 7075 aluminum alloy is the most commonly used. 7075 is commonly used in CNC cutting manufacturing of parts, suitable for aircraft structures and high-strength accessories. The 7-series aluminum alloy contains zinc and magnesium elements. Zinc is the main alloying element in this series, so its corrosion resistance is quite good. Combined with a small amount of magnesium alloy, the material can achieve very high strength after heat treatment.
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The anodizing of 7075 aluminum alloy is generally carried out in an acidic electrolyte, with aluminum as the anode. During the electrolysis process, oxygen anions react with aluminum to form an oxide film. When this film is initially formed, it is not dense enough. Although it has a certain resistance, negative oxygen ions in the electrolyte can still reach the aluminum surface and continue to form an oxide film. As the film thickness increases, the resistance also increases, resulting in a decrease in electrolysis current. At this point, the outer oxide film in contact with the electrolyte undergoes chemical dissolution. When the rate of oxide formation on the surface of aluminum gradually balances with the rate of chemical dissolution, this oxide film can reach its maximum thickness under this electrolysis parameter. The outer layer of the anodized film on aluminum is porous, which easily adsorbs dyes and colored substances, making it suitable for dyeing and enhancing its decorative properties. After being sealed with hot water, high-temperature steam or nickel salt, the corrosion resistance and wear resistance of the oxide film can be further improved.
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Adverse effects
However, due to the high content of zinc in 7075 aluminum alloy, zinc is the chemical component that has the greatest impact on the oxidation effect, which can easily cause adverse phenomena such as white edges, black spots, and uneven colors in the processed parts after oxidation. Generally, oxidation to the natural color and black color of aluminum alloy has better effects, while oxidation to bright colors such as red and orange red shows the most unstable performance.
The 7-series aluminum alloy (mainly doped with zinc) has higher hardness and strength. As a material for electronic product casings and frames, the 7 series aluminum alloy has the advantages of being lightweight and less prone to bending and deformation, and is increasingly favored by electronic product manufacturers. However, when using the traditional anodizing process of 6-series aluminum alloy (sulfuric acid anodizing) to oxidize 7-series aluminum alloy, due to the high composition of 7-series aluminum alloy, especially Al Zn Mg alloy, it is difficult to avoid uneven distribution of various components in the aluminum alloy, resulting in segregation. Aluminum segregation after sulfuric acid anodizing results in significant differences in the size of micropores on the surface of the oxide film, leading to variations in the quality and adhesion of the oxide film at different locations. In some areas, the adhesion of the oxide film is weak and prone to detachment.
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Oxidation method and precautions
According to the experiment, after anodizing, the thickness of the oxide film on the 7-series aluminum alloy can be 30um-50um, and the hardness can be 50-55HRC, which can meet the requirements of the aluminum material. (The electrolyte is oxalic acid, direct current, with an electrolysis temperature below 0 degrees, and the oxidation color is a metallic luster of dry grass yellow green to poop yellow).
① Preparation before oxidation
Oxidation should be done before the finished product after all machining is completed
② Oxidation process
01. The first step is acid washing. Before acid washing, it is important to understand whether any discharge type machining has been done before, such as electric discharge machining for splines, wire cutting machining, etc. If there is a hard oxide film formed on the machined surface, acid washing must corrode this layer. Otherwise, this non-conductive part will not be coated with an oxide film, but this will affect the surface smoothness and brightness of the workpiece, making it appear dark.
02. Calculate the surface area of the workpiece, which is proportional to the current and the oxalic acid content. Otherwise, problems such as slow coating and thin film layer may occur.
03. During the electrolysis process, do not frequently remove and check the thickness of the membrane, as there may be a problem of "no reaction" when it is put back into the oxidation tank.
04. Pay attention to controlling the current well. If it is too high, it will cause erosion, and if it is too low, it will not react.
