Confined Discharge Of Microarc Oxidation For Large Area Aluminum Alloy Treatment And Preparation Of Thermal-Resistance Coatings

With the development of space and aircraft industries, more and more aluminum alloys have been used in the manufacturing aerocrafts. With the increase of aerocrafts'speed, the service environment becomes more and more harsh and temperature rise and thermal shock on aerocrafts surface are big problems. Consequently, aluminum alloys as frequent aerocraft structural material can not endure so high temperature circumstance, which substantially limits further their applications. Microarc oxidation (MAO), as a relatively new surface treatment method to produce thick, thermal stability, thermal protection and adherent coatings on rectifying metals has been developed rapidly in recent years. MAO is an anodizing process during which microdischarges generate dielectric breakdown of the anodic oxide film under high electric field. As a result, it is difficult to process large-scale workpiece in the suggested electrical regime. In this paper, as a novel technique, confined cathode microarc oxidation with shorter distance (SD-CCMAO) has been proposed to process workpiece with large area. The discharge characteristics of the SD-CCMAO have been investigated. In addition, in order to modulate the microstructure and then improve the thermal-resistance of MAO coatings fabricated on 2024 alloy, K₂ZrF₆ additive has been introduced into Na2SiO3-KOH based electrolyte. The modulation effect of K₂ZrF₆ addition on microstructure has been investigated by SEM, XRD, XPS and TEM respectively. Meanwhile, the thermal properties of MAO coatings formed on 2024 aluminum alloy in different electrolytes have been studied. Ultimately, a MAO system for processing workpiece with a large area has been designed and developed.A shorter electrode distance decreases the voltage drop, and eventually lowers the energy consumption. If the distance becomes too small, the MAO does not work. The novel technique using a grid cathode is employed to implement MAO with the shorter electrode distance. Both the voltage drop and the joule heat in the electrolyte are reduced. Consequently, the processing efficiency, from the viewpoint of energy consumption per unit coating volume, decreases by 25% if the electrode distance decreases from 50 mm to 5 mm. Meanwhile, the experimental result shows that the voltage changes non-linearly with the electrode distance. The non-linear behaviour is much dependent on relative dimension and electrode distance of anode-cathode system but slightly affected by applied anodizing current density and whether there is discharge on anode surface. This may be attributed to the divergence effect of electric field between two electrodes and well verified experimentally by the similar current density on the different parts of the anode if the distance is short. Our results also suggest that, a shorter electrode distance can reduce effectively anodizing voltage and, consequently, improve greatly the energy efficiency. This may speed up the industrial application of PEO technology.Confined cathode microarc oxidation (CC-MAO) has been investigated in Na2SiO3-KOH solution with industrial pure aluminums substrate materials. The effects of the electrode distance (ED) on working current, potential distribution, arcing voltage and so on have been focused on. The energy efficiency as a function of ED has also been evaluated. The results show the working current decreases with increasing ED with the same exposure area of confined cathode and anode. However, the working current increases with increasing ED if only the cathode is confined. It is attributed to different electrical field on the anode surface. With increasing ED, the electrical field beneath the confined cathode decreases while that far from the cathode increases. This leads to a different total working current. The arcing voltage increases with increasing ED while the effective voltage on anode surface nearly keeps constant. The thickness of the ceramic coating produced with a small ED is much higher than that with a larger ED. The energy consumption per unit volume of oxides by CCMAO indicates that smaller ED leads to higher energy efficiency. The influences of MAO voltage and treatment time during asynchronous MAO process on the oxide current, coating thickness and surface morphology have been studied. Experimental results show that, when the voltage keeps constant, the difference of MAO current between the naked sample and the local oxided sample decreases with treatment time increasing. For the voltage varying, at the beginning of MAO treatment, the difference of MAO current between the naked sample and the local oxided sample increases with the increase in voltage, and then decreases with the increase in treatment time. When the voltage is higher, the thicknesses of different regions are easy to get identical. Scanning electron microscopy shows that longer treatment time and higher voltage will contribute to more identical surface morphology of MAO coatings.Pure aluminum was treated by microarc oxidation in electrolyte containing Na2SiO3 and KOH at constant voltage mode. The effect of voltage on the properties of the coating prepared by discontinuous microarc oxidation was investigated. The phase composition and surface morphologies of MAO coatings were analyzed by XRD and SEM. The results show that, for the discontinuous MAO, the current drops to some extent at the interrupted point. MAO coatings fabricated by different modes show the similar thickness that increases with increasing oxide time. The processing mode has slight effect on the phase composition of MAO coatings, which are mainly composed ofγ-Al₂O₃ andα-Al₂O₃. The surface morphology of the MAO coatings is hardly affected by processing mode and no lamination is observed for the coatings prepared by discontinuous mode. The coatings fabricated by different modes possess the similar wear resistance and corrosion resistance. This discontinuous mode improves the control flexibility during micro-oxidation processes.Zr(OH)4 particles can be formed and negatively charged in alkaline solution with K₂ZrF₆ addition. Based on this, Zr-containing ceramic coatings were fabricated on 2024 aluminum alloy by microarc oxidation (MAO) using K₂ZrF₆ as a special additive in Na2SiO3-KOH base electrolyte. The results show that the K₂ZrF₆ addition can increase the micro-arc oxidation rate and significantly alter the structure of MAO coatings. Both the top surface and inner surface of MAO coatings fabricated in Zr-containing electrolyte become relatively smooth. Compared with the coating formed in Zr-free electrolyte, a large amount of Zr element is found in the coating formed in electrolyte with K₂ZrF₆ addition. Two main phases,γ-Al₂O₃ andα-Al₂O₃, are contained in Zr-free coating. In contrast, more amorphous phase is found in Zr-containing coating with reduced amount of crystalline alumina. Experimental results also demonstrate that Zr-containing coating exhibits higher heat resistance.To investigate the feasibility of industrial application, a MAO system for processing a 2024 alloy plate with the standard scale (2000 mm×1000 mm×1 mm) has been designed and manufactured. The Zr-containing coating on the whole large workpiece surface has been achieved successfully.