| Summary: | 碩士 === 逢甲大學 === 材料科學所 === 93 === Micro-arc oxidation(MAO) is a newly developed technology for metal surface treatment. It involves the usage of a liquid electrolyte by applying a voltage between metal workpiece and a counter-electrode, during which an electrical discharge occurs at the workpiece surface and leads to the formation of oxide layer.
The electrolyte normally used is low concentration alkaline solution without heavy ions and organic solvent and therefore is considered to be the environment friendly process. Micro-arc oxidation, the surface layers of MAO normally possess high hardness for wear resistance and good corrosion resistance, favorable of surface strengthening for protection purposes.
The electrolyte composition, current density, power input types, substrate materials in micro-arc oxidation process are known to effect the crystal structure, microstructure and mechanical properties, which have been reported extensively in literatures. Very little has been attempt for a systematic study to correlate different power input types to the crystal structure, microstructure, mechanical properties of oxide film. This motivated this study, that reveal the effect of using DC、unipolar、bipolar power mode to the microstructure and mechanical properties, surface hardness in particular of the oxide layer formed on AA1050 aluminum. Silicate and phosphate solutions were used as the electrolytes.
Experimental results show that the major crystal structure of the oxide layer synthesized in silicate electrolyte presents the ��-Al2O3、��-Al2O3、Al-Si-O and W, while that are synthesized in phosphate is ��-Al2O3 and ��-Al2O3 crystal structure mainly. The change in microstructure as affected by the process parameter is governed by the movement of anions and the micro-arc behavior in electrolyte.
In DC power mode, the growth rate increase monotonically with current density, to a maximum value of 1.41 �慆/min in silicate electrolyte, and 0.65 �慆/min in phosphate. In unipolar pulse power mode, the growth rate is affected by the frequency in both silicate and phosphate electrolyte. Oxide growth rate increase with frequency, and reach a maximum at 5 kHz then decrease.
In DC power mode, The crystal structure of oxide layers on AA1050 aluminum synthesized in silicate and phosphate electrolyte is ��-Al2O3 and Al-Si-O, respectively. The oxide layers on AA1050 aluminum synthesized in silicate and phosphate electrolyte exhibit maximum hardness of 1604 Hv and 1354 Hv, respectively.
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