Selection of hard anodizing materials
The choice of hard anodizing materials directly affects the quality and performance of the oxide film. Not all metals and alloys are suitable for hard anodizing. At present, the most widely used materials are aluminum and aluminum alloys, followed by magnesium alloys and titanium alloys. These metals can generate hard oxide films with excellent performance under specific process conditions. Factors such as the chemical composition, microstructure and mechanical properties of the material will have a significant impact on the hard anodizing process and the performance of the oxide film. Therefore, when selecting materials, it is necessary to comprehensively consider factors such as usage requirements, process adaptability and cost to ensure the ideal treatment effect.
Aluminum and aluminum alloys are the preferred materials for hard anodizing. Both wrought and cast aluminum alloys can be hard anodized, but wrought aluminum alloys offer the best results. Among wrought aluminum alloys, Series 2 (aluminum-copper alloys), Series 6 (aluminum-magnesium-silicon alloys), and Series 7 (aluminum-zinc-magnesium alloys) are well-suited for hard anodizing, producing thick, hard oxide films. Series 2 aluminum alloys can achieve oxide film hardnesses of 400-600 HV after hard anodizing, making them suitable for parts requiring high strength and wear resistance. Series 6 aluminum alloys have oxide film hardnesses of 300-500 HV, combining excellent corrosion resistance and processability for widespread use in the construction, automotive, and other fields. Series 7 aluminum alloys achieve oxide film hardnesses of 500-700 HV, making them suitable for high-end applications such as aerospace. However, aluminum alloys with high silicon contents (such as cast aluminum alloys) are prone to developing loose oxide films and insufficient hardness during hard anodizing, requiring adjustment of process parameters to improve these issues.
Magnesium alloy is also one of the materials that can be hard anodized, but its process is more difficult than that of aluminum alloy. Magnesium alloys have high chemical activity and are prone to excessive corrosion during the anodic oxidation process, resulting in a decrease in the quality of the oxide film. Therefore, magnesium alloys suitable for hard anodizing usually need to contain appropriate amounts of alloying elements (such as aluminum, zinc, manganese, etc.) to improve their chemical stability and ability to form oxide films. For example, AZ series magnesium alloys (such as AZ91D) are hard anodized in a specific electrolyte to produce an oxide film with a thickness of 10-30 microns and a hardness of 200-400HV, which significantly improves its corrosion resistance and wear resistance. Magnesium alloy hard anodized film is mainly used in aerospace, automotive parts and other fields to reduce weight and increase service life.
Hard anodizing technology for titanium and titanium alloys is also evolving. Titanium alloys inherently possess excellent corrosion resistance and high-temperature performance, and hard anodizing can further enhance their surface hardness and wear resistance. Titanium alloys suitable for hard anodizing primarily include α-titanium alloys, β-titanium alloys, and α+β-titanium alloys. For example, TC4 (Ti-6Al-4V) titanium alloy, after hard anodizing, can achieve an oxide film hardness of 500-800 HV and exhibit excellent high-temperature stability, making it suitable for high-end applications such as aircraft engine components and medical devices. The formation of a hard anodized film on titanium alloys is closely related to the composition of the electrolyte, typically containing sulfuric acid, phosphoric acid, or oxalic acid. Adjusting process parameters can control the thickness and properties of the oxide film.
When selecting hard anodizing materials, the processing state and surface quality must also be considered. Aluminum and titanium alloys that have undergone heat treatment (such as quenching and aging) have a more uniform microstructure and produce a more uniform and dense oxide film. However, residual stress on the surface of cold-worked materials can cause cracks in the oxide film, requiring annealing to relieve the stress. Furthermore, surface roughness affects the quality of the oxide film. A smoother surface results in a more uniform oxide film, while a smoother surface is more prone to porosity and defects. Therefore, in practical applications, it is important to select the appropriate material based on the part’s intended use and perform necessary pretreatment to ensure optimal hard anodizing results. With the advancement of materials science, the development of new alloy materials will expand the application space for hard anodizing technology and further expand its application in high-end manufacturing.
