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Why is 7 series aluminum alloy not good for oxidation

July 24, 2023
7 series aluminum alloy belongs to Al-Zn-Mg-Cu series superhard aluminum, which has been applied to the aircraft manufacturing industry in the late 1940s and is still widely used in the aviation industry. It is characterized by good plasticity after solid solution treatment, particularly good heat treatment strengthening effect, high strength below 150℃, and particularly good low temperature strength; Poor welding performance; Prone to stress corrosion cracking; Need to be coated with aluminum or other protective treatment. Two-stage aging can improve the resistance to stress corrosion cracking. The plasticity in the annealing and hardening state is slightly lower than the same state of 2A12, slightly better than 7A04, the static fatigue of the plate. Notch sensitive, stress corrosion performance is better than 7A04. Density is 2.85g/cm3.
01 In the 7 series, 7075 aluminum alloy is the most commonly used. 7075 is widely used in CNC machining parts, suitable for aircraft frames and high-strength accessories. 7 series aluminum alloys contain zinc and magnesium. Zinc is the main alloying element in this series, so the corrosion resistance is quite good, and a little magnesium alloy can make the material can reach a very high strength after heat treatment.
7 series & Anodizing
The anodizing of 7075 aluminum alloy is generally carried out in an acidic electrolyte, with aluminum as the anode. During electrolysis, the anion of oxygen interacts with aluminum to produce an oxide film. When the film is initially formed, it is not fine enough, and although it has a certain resistance, the negative oxygen ions in the electrolyte can still reach the surface of aluminum and continue to form an oxide film. With the increase of film thickness, the resistance also increases, thus the electrolytic current decreases. At this time, the outer oxide film in contact with the electrolyte is chemically dissolved. When the rate of oxide formation on the aluminum surface is gradually balanced with the rate of chemical dissolution, the oxide film can reach the maximum thickness under this electrolytic parameter. The outer layer of the anodic oxidation film of aluminum is porous, which is easy to absorb dyes and colored substances, so it can be dyed and improve its decoration. The corrosion resistance and wear resistance of the oxide film can be further improved after it is closed with hot water, high temperature steam or nickel salt.
A bad effect?
However, because 7075 aluminum alloy contains a large amount of zinc, zinc element is the chemical composition that has the greatest impact on the oxidation effect, it is easy to make the processing parts after oxidation of white edges, black spots, uneven color and other undesirable phenomena, generally oxidized into aluminum alloy color and black effect is better, oxidized into red, orange and other bright color performance is the most unstable.
7 series aluminum alloys (mainly zinc doped aluminum alloys) have higher hardness and strength. As a material for the shell and frame of electronic products, 7 series aluminum alloy has the advantages of light weight and less bending and deformation, and is favored by more and more electronic product manufacturers. However, when the traditional anodizing 6-series aluminum alloy process (sulfuric acid anodizing) is used to oxidize 7-series aluminum alloy, because the 7-series aluminum alloy, especially the Al-Zn-Mg alloy, is relatively high, the components of the aluminum alloy will inevitably be uneven distribution, resulting in segregation. After the aluminum segregation is anodized by sulfuric acid, the size of micropores on the surface of the oxide film is quite different, and the quality and adhesion of the oxide film at different positions are different.
Oxidation method and precautions
According to the experiment, after the 7 series aluminum alloy is anodized, the oxide film thickness can be 30um-50um, and the hardness can be 50-55HRC to make the aluminum meet the requirements. (Electrolyte is oxalic acid, direct current, electrolytic temperature below 0 degrees, oxidation color is yellow green with metal luster subtilis - shit yellow).
1 Preparation before oxidation
Oxidation is done after all machining and before the finished product
2 oxidation process
01 The first step to pickling, before pickling must understand before do not do the discharge type of processing, such as electrical discharge machining spline, wire cutting processing, etc., if there is a processing surface will form a hard oxide film, pickling must corrosion this layer of film, otherwise this part is not conductive will not be attached to the oxygen flower film, but this will affect the surface smoothness and shine of the workpiece, Darken the work.
02 Calculate the surface area of the workpiece, which has a certain proportion of the current, and the oxalic acid content is also related, otherwise there will be problems such as slow film and thin film layer.
03 Do not take out frequently to check the thickness of the film during the electrolysis process, which will cause the problem of no reaction in the oxidation pool again.
04 Pay attention to control the current, too much ablation, too small does not react
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