I. Causes of Fine Lines in 6061 hard oxidation
The fine lines produced during the hard oxidation process of 6061 aluminum alloy are the result of the combined effect of multiple factors.
(1) Regarding the stress of the oxide film
1. Stress generation mechanism
During the hard oxidation process, a large amount of heat is generated and gas is released. For instance, when current passes through the electrolyte, ions in the electrolyte react on the electrode surface, and this process is accompanied by the release of heat. For 6061 aluminum alloy, during the process of forming an oxide film on its surface, due to the ongoing oxidation reaction, a large amount of stress will be generated inside the film layer. The generation of this kind of stress is like constantly inflating the interior of a closed container. As the reaction proceeds, the stress accumulates continuously. When the oxide film is formed, its volume will expand. Since the oxide film is attached to the 6061 aluminum alloy substrate, when the oxide film expands, it will be restricted by the substrate, thereby generating stress within the film layer. If this kind of stress cannot be effectively released, it will cause the oxide film to deform and subsequently produce fine lines.
2. Relationship between Stress Release and Fine Lines
When the stress exceeds the load-bearing capacity of the oxide film itself, the film layer will release the stress by generating fine lines. Just like a taut rubber band, if the tension is too great, the rubber band will develop tiny cracks to relieve the tension. In the 6061 hard oxide film, these fine lines are the manifestation of stress release.
(2) Uneven growth of the oxide film
The influence of alloying elements
The distribution of alloying elements in 6061 aluminum alloy may be uneven. The main alloying elements of 6061 aluminum alloy are magnesium and silicon, and it may also contain small amounts of manganese, chromium, copper, zinc and other elements. The reactivity of these alloying elements during the oxidation process may vary. For instance, silicon is only slightly oxidized during the oxidation process and exists in the alloy in the form of eutectics, which may lead to a significant unevenness in the thickness of the anode coating. When the oxide film grows unevenly, the thicker and thinner parts are prone to fine lines under stress.
2. Factors related to electrolyte
The concentration, temperature and fluidity of the electrolyte and other factors will affect the uniformity of the growth of the oxide film. Take sulfuric acid electrolyte as an example. If the concentration of sulfuric acid is not uniform, the oxidation reaction rate in different areas on the surface of 6061 aluminum alloy will be different. In high-concentration areas, the growth rate of the oxide film may be faster, while in low-concentration areas, it is slower. This difference in growth rate can lead to uneven stress within the oxide film, thereby generating fine lines. Similarly, when the electrolyte temperature is uneven, for instance, if the local temperature is too high or too low, it will also affect the growth rate and quality of the oxide film, promoting the formation of fine lines. Moreover, if the fluidity of the electrolyte is poor, the gases generated during the reaction cannot be discharged in time, which will also interfere with the uniform growth of the oxide film and subsequently cause fine lines.
(3) Characteristics of the base material
The original state of the material
The original state of 6061 aluminum alloy material before hard oxidation will affect the oxidation result. If there are internal stresses in the material during processing, for instance, if it is deformed after cold working without proper annealing treatment, these internal stresses will superimpose with the stress generated by the oxide film during the oxidation process. For instance, in the extruded 6061 aluminum alloy profiles, if the extrusion process parameters are not appropriate, there may be considerable residual stress inside the profiles. In the subsequent hard oxidation process, this superimposed stress is more likely to cause fine lines to form in the oxide film.
2. The purity and microstructure of the material
The purity of the material is also crucial. If 6061 aluminum alloy contains impurities, these impurities may affect the normal growth of the oxide film during the oxidation process. For instance, the oxidation reactions around impurity particles may differ from those of the base material, leading to abnormal local oxide film structures. From a microstructure perspective, the grain size and orientation of 6061 aluminum alloy also affect the growth of the oxide film. The reactivity of surfaces with different grain orientations during oxidation may vary, thereby affecting the uniformity of the oxide film and subsequently causing fine lines.
