Research On Composite Hard Anodic Oxidation Of Aluminum Alloys And Mechanism

In this paper, hard anodizing and composite anodic oxidation on 2024 Al-Cu alloy and heat treatment technology of the oxide film were investigated to obtain a novel technique of preparing anodizing film with excellent wear properties and low friction coefficient, which settle experimental and theoretical foundation of application of composite anodic oxidation technique.Basing on sulfuric acid, oxalic acid and sulfamic acid as basic electrolyte, the effect of bath solution components, anodizing temperature and four kinds of power waveforms on hardness and thickness of the hard anodizing layer on 2024 Al-Cu alloy was investigated. This experiment indicated that hardness of the pulse power oxide film was higher than that of the single-direct-current power, especially a hard film with hardness of more than 400HV could be obtained for double pulse power at 10℃.The hard anodizing film with hardness of about 400HV was fabricated by employing direct current power and treated it under protection of hydrogen atmosphere in a temperature range of 100-400℃.The results showed that a trend of great increase of hardness and the decrease of toughness with increasing heat-treatment temperature, the optimized temperature was 250℃, the samples with hardness of 606HV and minimum wear loss of 11mg possessed high hardness and toughness of the surface and good anti-wear properties. It had been found that structure units of the oxide film became rather tightness and some void flaws declined and thin-long pipes in the cross-section ranged close with increasing temperature via analysis of micro-images of the anodizing layer. In order to improve the hardness and wear properties, composite hard anodic oxidation and co-depositing secondary phase in the film were applied. The experimental results proved that the film was strengthened by direct current composite hard anodic oxidation with adding SiC particles. When adding content of SiC reached to 12g/L, the hardness of oxide film was enhanced from about 400HV to 440HV, the wore loss of oxide declined to 8mg from 25mg; the hardness of the composite hard anodizing film obtained by direct current composite anodic oxidation with adding into PTFE solid lubricating material had been changed a little, hardness of composite hard anode oxide film with adding into PTFE 25ml/L was still about 400HV, whereas average friction coefficient was less than 0.1 and the lowest wore loss was 7mg. Nano-SiC inlayed into an oxide film were exposed gradually in the process of friction, owing to the ability to bear most loads, they played an important role in improvement of wear properties of the oxide film, PTFE solid lubricating material composite in the oxide film contacting with a counter-body could form an lubricating layer, which decreased the friction coefficient and played a self-lubricating part to protect the anodizing film.