Control Principle Of Fabrication And Characterization For Micro-structure And Properties Of Microarc Oxidation Coatings On AZ91D Magnesium Alloys

The effects of MAO technical parameters on characteristic parameters and properties of MAO coatings were researched by invariable small current density process and rapid forming MAO coatings process: thickness increased with the increase of oxidation time, current density and interval time of enhancive voltage, but it first increased and then decreased with the increase of increase extent of voltage, average growth rate was the efficiency of MAO; roughness had a increased trend; surface porosity first increasedand then decreased; surface morphology reflected qualitatively the size and quantity of surface pores and granules of coating;MAO coating was mainly composed of Mg₂SiO₄ and MgO. Adhesion stress first increased and then decreased with the increase of oxidation time, current density and increase extent of voltage, but it increased with the increase of interval time of enhancive voltage; compared with the matrix, corrosion resistance and wear resistance were obviously increased.Control principles of AZ91D magnesium alloys prepared by MAO were gained by analyzed the relationship between technical parameters of MAO, characteristic parameters and properties of MAO coatings: adjusting the current density and time to control the thickness; adjusting the technical parameters of the reaction as mitigation to reduce the roughness; surface porosity can be controlled in a larger extent by adjusted the technical parameters wer moderate or the greater electrical parameters or the longer oxidation time; adjustting the smaller electrical parameters or the longer oxidation time to increase the adhesion stress; adjusting the technical parameters wer moderate or the greater electrical parameters or the longer oxidation time to enhance the corrosion resistance and improve the wear resistance.Control principles of AZ91D magnesium alloys prepared by MAO were verified by invariable voltage process: Characteristic parameters and properties of MAO coatings under the invariable voltage craftwork agrees well with control principles prepared by MAO,indicating that control principles is accurate, consistent with the mechanism of MAO and has universality.Thickness of MAO coating was characterized rapidly by eddy current thickness measurement, and accurate characterization by cross-section method; roughness was characterized rapidly by surface profiling instrument, but it demanded sample had a good parallel; surface morphology gained by SEM; the thickness, surface porosity and pore size were characterized with general image analysis software on the clear surface and section morphologies photos; the volumetric porosities were estimation calculated by Sub-sampling methods with the assumption that pores were cylindrical and hemisphere. The test conditions of adhesion stress characterized by scratch test: Loading speed was 50 N/m, End loading was 20 N, with a double loading mode, to judge adhesion stress combined with the sscratch morphology, the end loading adjusted to 30N if did not detect the adhesion stress when the end loading was 20 N in the test; to determine the holding temperature and holding time according to the concretely environment as the adhesion stress characterized by thermal shock method. The test conditions of hardness characterized by micro Vickers hardness after the suface of coating painted: holding time was 5 s, the surface coating applied load at least 1000 g, while the density layer applied load was at least 25 g, the middle layer and porous layer applied load not more than 10 g.