Nauka i Obrazovanie (Jan 2015)
Microarc Oxidation of the High-Silicon Aluminum AK12D Alloy
Abstract
The aim of work is to study how the high-silicon aluminum AK12D alloy microstructure and MAO-process modes influence on characteristics (microhardness, porosity and thickness of the oxide layer) of formed surface layer.Experimental methods of study:1) MAO processing of AK12D alloy disc-shaped samples. MAO modes features are concentration of electrolyte components – soluble water glass Na2SiO3 and potassium hydroxide (KOH). The content of two components both the soluble water glass and the potassium hydroxide was changed at once, with their concentration ratio remaining constant;2) metallographic analysis of AK12D alloy structure using an optical microscope «Olympus GX51»;3) image analysis of the system "alloy AK12D - MAO - layer" using a scanning electron microscope «JEOL JSM 6490LV»;4) hardness evaluation of the MAO-layers using a micro-hardness tester «Struers Duramin».The porosity, microhardness and thickness of MAO-layer formed on samples with different initial structures are analyzed in detail. Attention is paid to the influence of MAO process modes on the quality layer.It has been proved that the MAO processing allows reaching quality coverage with high microhardness values of 1200-1300HV and thickness up to 114 μm on high-silicon aluminum alloy. It has been found that the initial microstructure of alloy greatly affects the thickness of the MAO - layer. The paper explains the observed effect using the physical principles of MAO process and the nature of silicon particles distribution in the billet volume.It has been shown that increasing concentration of sodium silicate and potassium hydroxide in the electrolyte results in thicker coating and high microhardness.It has been revealed that high microhardness is observed in the thicker MAO-layers.Conclusions:1) The microstructure of aluminum AK12D alloy and concentration of electrolyte components - liquid glass Na2SiO3 and potassium hydroxide affect the quality of coating resulted from MAO-processing.2) Increasing concentration of the electrolyte leads to the thicker MAO-layer of higher microhardness and lower porosity.3) Deformation-heat treatment of AK12D alloy samples using a scheme "forging + quenching + aging" before the MAO leads to deteriorated quality of formed layer.
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